The Ecology of the Attached Diatoms and Other Algae in a Small Stony Stream

The Ecology of the Attached Diatoms and Other Algae in a Small Stony StreamAuthor(s): Barbara DouglasSource: Journal of Ecology, Vol. 46, No. 2 (Jul., 1958), pp. 295-322Published by: British Ecological SocietyStable URL: http://www.jstor.org/stable/2257397 .

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[2951

THE ECOLOGY OF THE ATTACHED DIATOMS AND OTHER ALGAE IN A SMALL STONY STREAM

BY BARBARA DOUGLAS*

Freshwater Biological Association, The Ferry House, Ambleside, Westmorland, England

(With eight Figures in the Text)

CONTENTS PAGE

I. INTRODUCTION 295 II. SAMPLING METHODS 296

(a) The sampling of stones from the bed of the beck 296 (b) The sampling of permanent rock under water 297 (c) The sampling of epiphytes on aquatic bryophytes 298 (d) The estimation of samples collected by the above techniques 300 (e) Discussion of the methods 301

III. THE INVESTIGATION OF BELLE GRANGE BECK, LANCASHIRE 301 (a) The course of the stream and the sampling stations 301 (b) Identifications 303 (c) Environmental factors 304

Chemistry 304 Light 304 Temperature 305 Water level and flow conditions . . . . 305 Stream fauna . . . . . . 306

IV. GENERAL FEATURES OF THE HABITAT AND ITS ALGAL FLORA . 306 V. THE DISTRIBUTION AND PERIODICITY OF THE Achnanthes SPECIES

GROUP . . . . . . . . 307 (a) The populations of Achnanthes species and their distribu-

tion . . . . . . . . 307 (b) The factors controlling the Achnainthes populations . 307

Light intensity . . . . . 307 Temperature . . . . . . . . 308 Water level and flow conditions . . . 308 Grazing . . . . . . . . . 310 Notes on the animal populations at stations I and II,

and their gut contents . . . . . 310 (c) The periodicity of the Achnanthes species group . 313

VI. OTHER DIATOM SPECIES AND SPECIES GROUPS . . . 315 (a) Principal species . . . . . . . 315 (b) Species occurring in less abundance . . . . 317

VII. DISCUSSION. . . . . . . . . . 318 SUMMARY . . . . . . . . . . 320 REFERENCES . . . . . . . . 321

I. INTRODUCTION

A recent review (Blum 1956) has shown that most ecological investigations of attached algae in rivers and streams have been based on qualitative surveys. These may be expressed semiquantitatively by scoring each species according to its degree of abundance (e.g. Scheele 1952), but this is subjective and does not assess the actual population densities. Butcher (1932, and for further references see Butcher 1946) estimated the localized growth developing on glass slides immersed in the water, but the relation of this to the growth on natural substrata is as yet

* This paper forms part of a thesis accepted for the degree of Ph.D. by the University of London.



296 Attached Diatoms in a Small Stony Stream uncertain. Blum (1957) has given a quantitative account of the growths in a Michigan stream, from the weight of algae growing on rocks related to the weight of rock, and from the frequency of occurrence of different species in transects across the stream bed. Margalef's method (1948) of peeling the attached algae off a stone surface in a celloidin film is cumbersome for periodic sampling, and can only be used on stones removed from the water.

The present work was undertaken to develop methods for the estimation of populations growing on stones, rock and moss; to follow population changes for several years at a number of places in one small stream; and to determine as far as possible the factors controlling the populations. The methods developed are limited in their use to unicellular or shortly filamentous forms with a more or less random distribution, and are not suitable for streams with gravelly or muddy beds. This paper does not give a detailed account of the whole algal flora of the stream, but describes and interprets only such results as could be obtained with the described sampling methods. Since diatoms are the predominant group in the flora, and the sampling methods are particularly suitable for their estimation, consideration is restricted to them, apart from a few general comments on other forms in Section III.

II. SAMPLING METHODS

(a) The sampling of stones from the bed of the beck This consists in scrubbing the algae off a known area of stone surface with a brush, and washing them into a collecting bottle. The brush (Fig. la) is a piece of steel rod with a hole about 5 mm. deep drilled into one end, into which nylon hairbrush bristles are cemented. The cement consists of 2 parts of resin dissolved by heat in 1 part of castor oil. It is heated to melting point for use and sets hard again on cooling. Nylon hairbrush bristles are used as they are coarse enough for entangled algae to be washed out of the brush, stiff enough to withstand considerable pressure, and hard wearing. Wire bristles abrade the rock more, increasing the amount of detritus in the sample. The bristles are about 0 5 mm. in diameter and the length protruding from the handle between 3 and 6 mm. If they are longer, they bend when pressure is put on them; if shorter, the cracks and roughnesses on the stones are not cleaned sufficiently.

The area brushed is delimited by the neck of a 50 ml. polythene bottle with the bottom sawn off. This is stiff enough to be. held tightly in the hand without being crushed, and yet soft enough to grip the surface of a wet stone without slipping. The neck (2-5 cm. diameter) is large enough to cover a representative area of a stone without being so large as to preclude it from use on small stones, or small smooth areas of very irregular ones.

A sampling station is a more or less uniform stretch of stream with a stony bed, up to 10 m. long. Stones are selected at random by throwing in a 6 in. nail (15 cm.) and taking that stone against which the point of the nail comes to rest. Each stone is removed from the stream. The polythene bottle is pressed, neck downwards, on to the upper surface; the brush is inserted at the other end; and the area delimited by the neck scrubbed clean. The polythene bottle is removed, and the detached algae are washed into a wide-mouthed collecting bottle with a fine- jet pipette. The algae from each stone are washed into the same bottle. All the



BARBARA DOUGLAS 297

brushing and washing are carried out over an enamel sorting tray so that none of the sample is lost, and finally the tray, polythene bottle and brush are washed, and the washing added to the sample. From the area of the polythene bottle neck and the number of stones treated, the total area of stone surface cleaned may be calculated.

polythene haltf-

u steel ~ ~ brush-.

(b

~~ Oiji;f ag J lubing 3t_ b~~~~~~~~~~~~~~ubing A

nylon bristles

(a) (urbt)

glass tube (replacing brush

famarewtgIk

, i\. ? t (/og rg

dimt s)

sp5 bmen tube (ssuction bottti/

tubular sponge rubber pad shoft of th (of pressure

(scale X 0 2) and technique for sampling moss.~~~~~~~~~~~tbing

ptof ctive rubber tubing

steel csng (for cdesli mtng

__________ ~ ~~~~~~~~~~~~XN1 a rea. of rock moss bororbe cleaned)

nylon bristles sponge rubber pad

Fig. 1. Sampling apparatus and techniques. (a) The apparatus (scale x 0.33) and technique for sampling a stone removed from the water. (b) The apparatus (scale x 0.4) for sampling submerged rock, and (bl) the replacement for the brush for very thick growths. (c) The apparatus (scale x 0G2) and technique for sampling moss.

(b) The sampiliisg of Permansent rock under water This is, in principle, the same as the sampling of stones which can be removed

from the water, but it is elaborated to prevent the algae from being washed away. The brush (Fig. I b) is a narrow-bore tube with nylon hairbrush bristles surrounding the hole at one end. At the other end to the bristles a piece of rubber tubing (tubing A) connects the brush to a tightly bunged specimen tube. The air can be sucked out of this with a second piece (tubing B). For very thick growths which

G J. E.



298 Attached Diatoms in a Small Stony Stream become so entangled with the bristles that the hole in the brush is blocked, the brush is replaced by a length of thick narrow-bore glass tubing with a short piece of rubber pressure tubing on the end (Fig. lb'), and the algae are detached by rubbing with the pressure tubing.

The area brushed is delimited by a steel tube (Fig. lb inner casing) surrounded by a wider steel tube (outer casing) which shields the inner from the direct flow of water and prevents the algae from being washed away. The casing is narrowed off at the top and a length of wide rubber tubing attached, through which the brush is passed, to prevent too much water from washing in at the top. Sufficient space must be left, however, to allow the free movement of the brush. Between the two casings is a ring of thick sponge-rubber, which acts as a grip on the surface of the rock while allowing water to seep through.

A sampling station is a rock face about 2-3 sq. m. in area under a shallow flow of water; or a series of submerged boulders and small rock faces along a 10-20 m. stretch of stream. Several small smooth areas, each more or less uniform in itself, are selected within such a station, and one sample taken from each. The steel casing is pressed firmly down on the submerged rock; the brush is inserted; and the area delimited by the inner casing is scrubbed. During the scrubbing the air is sucked out of the specimen tube, and the detached algae are drawn up into it through the hole in the brush. Finally, air is sucked into the specimen tube to prevent the sample from siphoning back through the brush. The sample is transferred to a collecting bottle by blowing through tubing B and allowing it to siphon back. Several samples are taken in this way, one from each of the small areas selected, and combined. From the area covered by the inner casing and the number of such areas cleaned, the total area sampled can be calculated.

(c) The sampling of epiphytes on aquatic bryophytes This consists in taking a series of very small samples from an area of moss-

covered rock, measuring the volume of moss obtained, and grinding it up to pro- duce a suspension of epiphytes.

The sampling apparatus (Fig. lc) is a hardened steel tube sharpened off at one end (borer), with a piece of stiff wire (ramrod) which can be pushed through it. A sampling station is a more or less uniform expanse of submerged, moss-covered rock, 2-3 sq. m. in area. Very small sections of moss are cut out with the borer at more or less equal intervals over the surface of such a station (Fig. 1c). The moss is pushed out of the borer with the ramrod into a collecting bottle. This method is preferable to pulling small pieces away by hand, partly because a better section through the moss sheet from surface to rock is obtained, and partly because smaller pieces are taken and less damage is done to the habitat. In the laboratory, the moss is shaken up with water; sieved through netting to remove the silt; sorted over in a petri dish to remove small pieces of rock that are invariably present (see below); squeezed in the netting to remove excess water; and the volume measured by the displacement of water in a 10 ml. measuring cylinder. All the water, silt and any moss leaves that slip through the netting are kept and added to the final sample, so that none is lost. The moss is chopped up into smaller pieces and passed through the grinding machine to detach the algae.

This machine (Fig. 2) consists of two concentric ground glass tubes, one of which is rotating inside the other, with a space of about 05 mm. between them. The



BARBARA DOUGLAS 299

and siphon

moss put cotton reel becaring in here

driving rod

rubber tubing connection

(b) ~~~~resin cement (a)

I cm. -inner gloss tube

outer glass tube

rubber lining pivot support

wire sling support metal ring of outer tube

| | \ / ~~collecting funnel

final samnple Of ground moss a epiphytes in suspension

Fig. 2. (a) Machine for grinding moss to obtain a suspension of the epiphytes. (Scale x 0-25; length of inner glass tube 30 cm.; outer glass tube 32 cm.) (b) Detail of top part of machine.



300 Attached Diatoms in a Small Stony Stream tubes are ground by rubbing with coarse carborundum powder; the inner on its outer surface and the outer on its inner surface. The inner tube is cemented to the steel driving rod with the resin-castor oil cement described on p. 296. By holding the driving rod at the top in a fixed bearing (a cotton reel with an enlarged hole) and pivoting the outer tube at the bottom in a rubber-lined metal ring slung on wires, the rigidity of the machine is decreased and the risk of the tubes cracking during use is reduced. A continual dripping of water is siphoned into the tin funnel on the top of the outer tube, at a rate of about 25 drips a minute. The small polythene funnel held underneath the machine encloses the bottom of the tubes. The following precautions are necessary to ensure the passage of the moss into the space between the tubes (Fig. 2b). The neck of the tin funnel must be cut to the same diameter as that of the tube to which it is attached. If it is smaller its overlapping edge blocks the inlet to the space between the tubes, while if it is larger the moss sticks on the ledge formed by the top of the tube. The top of the inner tube is smoothed off on to the driving rod with resin-castor oil cement, and an oblique screw-thread is filed round the driving rod and this cement, in the same direction as that of the rotation. The outer tube is cut a little longer than the inner so that the top of the inner is about 1 cm. below the end of the tin funnel.

The moss is put into the tin funnel and gradually washed down the space between the tubes by the dripping water, supplemented by occasional douches of water to prevent it from sticking to the tubes or in the neck of the funnel. As it passes down, the moss is torn up into small pieces against the ground glass surfaces, while the shearing forces set up by the rotation of the inner tube strip the algae off it. The importance of removing all small pieces of rock from the sample arises here, as these may crack the glass tubes if they pass down with the moss. The water must not be allowed to flow through the machine too fast, as the moss is then washed through without being ground up. Although it is effective in grinding the moss up, this machine causes no apparent damage to the algae for which it has been used, probably because of the slowness of its action. As it can be left to operate with only occasional attention, this slowness is no disadvantage. Also, as its effective- ness seems to be due more to the shearing effect than the actual grinding, the moss does not need to be finely ground. It was found impossible to grind the moss up by hand into sufficiently small pieces to detach the algae without damaging them.

(d) The estimation of samples collected by the above techniques This is done with a counting cell of the Sedgewick-Rafter type (Whipple 1927)

with a circular chamber formed by a hole in a brass plate cemented to a slide. The chamber is 10 mm. in diameter' and 0-5 mm. deep. The chamber is filled by immersing the slide in a beaker of a sample, diluted as required (see below), with a cover-slip held over one end; and, after stirring to ensure an evenly distributed suspension, pushing the cover-slip over the chamber with the thumb and quickly withdrawing the whole thing. Excess water is removed and a small piece of thick, wet blotting paper is laid across each end of the cover-slip to prevent the cell from drying-up. The numbers of individuals of each species in the whole chamber are counted, and the number per 1 sq. cm. of stone or rock surface, or per 10 mm.3 of -moss, are calculated.



BARBARA DOUGLAS 301 (e) Discussion of the methods

Errors arising from faults in the techniques (such as failure to collect all the individuals from a point sampled, or inclusion in the sample of individuals from outside it) were assessed as far as possible. They appear to be small, particularly compared with the sizes of populations involved.

Series of test samples from populations of different densities were analysed statistically to determine the error arising from lack of randomness in the distribution of the populations. Tests were also made with samples of different sizes. Diatom populations were generally randomly distributed, but those of other forms (e.g. Lyngbya purpurascens, Rhodochorton violaceum) had more bunched distributions and estimations of them are less reliable. The counting method was also tested. Most of its error was due to the obscuring of cells by non-algal material, and to prevent this, samples had to be diluted to an equivalent of 50 sq. cm. of stone or rock surface in 700 ml. of water, or I 0 c.c. of moss in 1500 ml. Random distribution was then found as long as no species exceeded 100 cells per count.

In all these tests occasional discrepant results arose, as may be expected on statistical grounds. Such discrepancies may also occur in regular sampling.

A full account of these tests is available in typescript from the author or the Librarian of the Freshwater Biological Association.

III. THE INVESTIGATION OF BELLE GRANGE BECK, LANCASHIRE

Belle Grange Beck is a small stony stream flowing into the north basin of Winder- mere from Claife Heights, a small hill range running along the western shore of this part of the lake. The beck is approximately 2000 m. long, rises at a height of 206 m. above sea level, and enters the lake at about 40 m. above sea level. At the mouth it is approximately 3-5 m. wide at periods of high water level. The drainage area is of hard non-calcareous rocks of the Coniston Grits series (Upper Silurian), with a shallow soil cover and peat in places.

(a) The course of the stream and the sampling stations The beck is divisible into three main zones according to the gradient, as shown

in Fig. 3. It rises in a small peat hollow with Sphagnum and passes into a coniferous plantation. The bed is narrow, with overhanging sides and a gravel bottom. It then passes through a swampy area, principally of Nardus stricta and Sphagnum, with Pteridium aquilinum on the higher ground, and an increase in Molinia caerulea as the beck descends (upper flat zone, open part, Fig. 3). The bed here varies between gravel, stones and solid rock, with the banks cut straight and vertical in places for drainage purposes. It is unshaded apart from small trees here and there along the banks. It then passes into the shaded part of the upperflat zone (Fig. 3). It passes into a Larix plantation, where the bed is of solid rock with low waterfalls. It goes through clefts in the rock and is shaded by steep banks as well as by the trees. The next stretch is through mixed wood with Quercus and Corylus dominant on the north side and coniferous plantation on the south. The bed is of flat stony reaches with low falls of solid rock. It then passes into the middle steep zone, with the mixed wood on both sides. The bed is of solid rock with steep falls and a few short flat stony stretches, and is shaded by steep banks as well as by



302 Attached Diatoms in a Small Stony Stream trees. The upper part of the lower fiat zone has the same type of drainage area as the middle zone, with a bed of solid rock but with lower falls and more flat stony stretches. The lower part of the loweer flat zone is between two pasture fields. The bed is fairly uniform, flat and stony with very low falls over boulders, and is shaded by large Fagus lining the banks. Finally, the beck passes under the Ferry- Wray road, through a small thicket of Corylus and Rubus fruticosus, and into the lake. The bed is flat and stony and is shaded by the bushes.

Sampling stations were selected to represent stones, rock and moss in each of these zones (Fig. 3). They are as follows:

Stones station I: 6 m. stretch of stony bed just below Ferry-Wray road, width approx. 2-5 m.; shaded by small Corylus along banks.

N

plantation.'

fXp1/ --'i; ;.i~

-- .~~~ ~ ~~ ~ ~ JINAKE RANCE

UPPER FLAT ZONE (OPEN) UPPER FLAT ZONE (SHADED) MIDDLE LOWER FLAT STEEP ZONE

course of beck

sampling station. height ar m

coniterous Plantation. level in m.

4x 3 5mixed wood. b0

contours at III m. intervals

-- track

of themiddle st;ee000,withaprox.25.;h l 800 600 400 2 e

0

aistance from mouth in m.

Fig. 3. Map of Belle Grange Beck, Lancashire, showing the zones of its course, and the positions of the sampling stations; and (below, right) a profile of the stretch regularly sampled to show the variations in gradient.

Stones station VI: 10 m. stretch of stony bed at the top of the pasture field, width approx. 3z5 in.; shaded by Fagus.

Stones station III: 8 m. stretch of stony bed forming a short ledge at the bottom of the middle steep zone, width apprcix. 2-5 in.; heavily shaded by high steep banks and overhanging trees.

Stones station IV: 5 m. stretch of stony bed in the middle of the shiaded Part of the uppfer flat zone, width approx. 2 in.; moderately shaded by Corylus and small Betula.

Stones station V. 6 m. stretch of stony bed at the bottom of the opien piart of the ufifer flat zone, width approx. 1P5 in.; quite unshaded with low banks and no trees.

Rock station a.: 9 m. stretch with a series of large boulders and small rock faces, right at the bottom of the shaded Part of the lower fia zone; moderately shaded by sparse wood of large trees on south bank, open to north.

Rock station P: small smooth rock face, sloping at about 30?, along the direction of flow (i.e. forming the south bank), approx. 2-5 m. long and 1 m. deep; heavily



BARBARA DOUGLAS 303

shaded by north-facing position, steep overhanging rock face forming the opposite bank, and trees.

Rock station y: short steep fall across the direction of flow in the Larix plantation, approx. 3 5 m. wide and 2 m. deep; uneven rock with small smooth areas, and partly covered with bryophytes, principally Scapania sp. and Eurhynchium riparioides; fairly heavily shaded by small Larix lining the banks and a steep rock face forming the south bank.

Moss station A: long shallow fall at the top of the lower fiat zone covered with Eurhynchiium riparioides; a series of sloping rock faces along the direction of flow, approx. 3 5 m. long and 1 m. wide; moderately shaded by the leaf canopy of fairly large trees, and a steep bank on north side.

Moss station B: two opposite rock faces at the top of the middle steep zone, covered with E. riparioides; the north face vertical and the south sloping at about 400, along the direction of flow; approx. 3-5 m. long and 1 m. deep on sloping side and 0 5 m. deep on vertical side.

Moss station C: the same fall as rock station y. Moss station C'. similar to C and replacing it at the end of 1954 owing to the

depletion of the bryophytes from C; steep fall across the direction of flow, immediately below the Larix plantation; shaded by small Quercus, Corylus and Betula along the banks, but rather more open than C.

Samples consisted of 10 stones from a stones station, on each of which one area covered by the neck of the polythene bottle was cleaned, 5 'brushings'* from a rock station, and 25 'bores't from a moss station, as decided from the test sampling. They were taken at intervals of a week to a fortnight, or occasionally longer. Each sample was diluted appropriately, and one count of all the species made. The regular sampling of all the stones stations and moss stations B and C was started in January 1953, of rock stations a and p in December 1953, and of moss station A and rock station y in March 1954. The upper three stones stations (III, IV and V) were stopped in August 1955, the moss stations in November 1955, the lower two stones stations (I and II) in July 1956, and the rock stations in October 1956. Occasional samples were also taken at other places in the beck.

(b) Identifications Identifications of the diatoms were made with Hustedt (1930), with the assist-

anice of Dr Hustedt during a visit to England and of Mr R. Ross of the British Museum (Natural History), for which I am most grateful. It has been found impossible to separate different species of certain genera quantitatively. The magnification used for counting ( x 150) is too low. Attempts have been n-iade to determine the proportions of the different species from prepared slides, after incinerating samples or boiling them with acid. But when mounted, many of the cells concerned come to rest on their girdle surfaces, from which they are unidentifiable. For this reason the following have to be considered together:

Achnanthes minutissima A. minutissima var. cryptocephala I A. microcephala } Achnanthes species group A. microcephala A. linearis

* A 'brushing' is the cleaning of one small area delimited by the inner casing. t A 'bore' is one small section cut out with the moss borer.



304 Attached Diatoms in a Small Stony Stream Gomphonema parvulum G. parvulum var. subelliptica | G. parvulum var. micropus Gomphonema species group G. intricatum var. pumila G. olivaceoides Hust

Synedra vaucheriae s S. vaucheriae var. capitellata Synedra S. rumpens S. amphicephala J

Blue-green algae were identified with Geitler (1930-32). The identification of Rhodochorton violaceum was checked for me by the late Mrs K. M. Drew-Baker of Manchester University.

(Nomenclature authorities are only given when they differ from, or are not included in, the standard floras used for identification.)

(c) Environmental factors In addition to algal sampling the following factors were assessed. Chemistry. Water samples were taken at approximately monthly intervals in the

first year, and occasionally in subsequent years (principally in relation to the water level in the beck), and analysed by Mr J. Heron, analyst of the Freshwater Biologi- cal Association, for the following:

Nitrate: by colorimetric estimation after treatment with phenol di-sulphonic acid (see American Public Health Association 1946).

Phosphate: by colorimetric estimation after treatment with Deniges' reagent (Atkins 1925a).

Silica: by colorimetric comparison with a standard picric acid solution after treatment with ammonium molybdate (Atkins 1925b).

Total ions, chloride and sulphate: by titration of the effluent from water passed through a column of hydroxyl ion exchange resin (Mackereth 1955).

Calcium and magnesium: by titration with standard sodium versenate solution (Heron & Mackereth 1955).

Sodium and potassium: by flame photometer. Alkalinity -(calculated as p.p.m. CaCO3 expressed as m.e./l. HCO3'); by titration

with N/100 HCl. pH: by glass electrode pH meter. Samples were at first taken along the length of the beck, and later at stations I

and V only. These analyses are summarized in Table 1. Light. Continuous measurements of solar radiation passed by glass are taken

with a Moll-solarimeter recorder at The Ferry House. The daily energy totals.in g. cals. cm.2 were calculated, and are plotted in Fig. 4 together with Auren's theoretical curve for clear skies in lat. 550 N. (Auren 1934).

An accurate comparison of the light at the different stations is difficult. Some idea of the shading was obtained from the comparison of the light intensity in the open field with that at each station, at midday on cloudless days. The light meter was a VA 26T vacuum photo-cell manufactured by Cinema-Television Co. Ltd., connected to a one-valve battery-operated amplifier. Measurements were taken on three occasions under different leaf cover, in midsummer, in autumn and in winter. Several galvanometer readings were taken at each station, avoiding



BARBARA DOUGLAS 305 Table 1. Summary of chemical analyses of water samples

Radical, etc. Variation Remarks min. max.

Alkalinity as HCO3' in m.e./l. 0-082 0-448 Varying greatly with water level, being high at low water level and vice versa; but generally between 0-2 and 0-3 m.e./l.

Nitrate in m.e./l. 0-0014 0-0057 Generally about 0003 or 0-004; tending to be higher during a period of dry weather and lower during wet.

Phosphate (as P04") in m.e./l. <0000003 0-000006 GenerallyaboutO-000003, andonlyoccasionally higher.

Chloride as m.e./l. 0-150 0-250 Generally about 0-210-0-230. Sulphate as m.e./l. 0-170 0-260 Generally about 0-200 to 0-230; lower in wet

weather, higher in dry. Silica in p.p.m. 2-6 3-6 Generally about 3-0. Calcium as m.e./l. 0-315 1-305 Variation rather wide; and not apparently

connected with flow conditions. Magnesium as m.e./l. 0 007 0250 Variation rather wide; and not apparently

connected with flow conditions. Sodium as m.e./l. 0-165 0-235 Potassium as m.e./l. 0-004 0-020 Total ions as m.e.11. 0-615 1-555

sunflecks, the mean calculated, and expressed as the percentage of that measured in the open. These values are given in Table 2.

Temperature. No continuous temperature records were made. The temperature was. taken at each station with a mercury thermometer during sampling visits. The range of temperature measured in the beck on each occasion is plotted in Fig.4. This does not give a complete picture of the temperature conditions, which Macan has shown to fluctuate rapidly (diurnally and seasonally) in small streams of this type (Macan, in preparation). However, it gives an indication of the temperatures prevailing in different parts of the beck under certain conditions, which is sufficient for the present purpose.

Water level and flow conditions. This is deduced indirectly from rainfall measurements. In an area like the English Lake District, where the soil is shallow, gradients often steep and run-off rapid, rainfall records are a good index of the flow conditions in the small streams. Fig. 4 shows the daily rainfall as recorded at The Ferry House, approximately 3 miles (5 km.) south of Belle Grange Beck on the same side of Claife Heights. Observation has shown that under average weather conditions over 1 in. (25 mm.) of rain in a day will cause a flood in the becks; after a prolonged drought up to 2 in. (50 mm.) may be needed to give that effect; while

Table 2. Approximate estimates of the degree of shading at the sampling stations. For method of estimation see text

Sampling Light intensity at station as per cent of that in open field station June 1954 September 1956 December 1955

Stones I 22 32 90 II 11 60 93 III 5 15 75 IV 23 65 93 V 100 100 100

Rock a 18 18 89 p 13 13 86 y 33 84 93

Moss A 35 33 86 B 58 33 93 C 33 84 93



306 Attached Diatoms in a Small Stony Stream during continual wet weather, such as occurred at the end of 1954, I in. (13 mm.) may be sufficient.

Stream fauna. Quantitative samples of the fauna at stations I and II were taken at roughly monthly intervals from 1954 to 1956, with occasional ones at stations III, IV and V. The sampling was done by walking slowly upstream, washing the stones and gravel into a phytoplankton net (180 threads to the inch) for 5 minutes. The contents of the net were washed into a wide-mouthed jar, sorted through in the laboratory, and the animals counted. The fore-gut contents of some of the specimens were noted at intervals. A full discussion of this method is given by Macan (1958). It does not determine the actual populations present, and its accuracy must vary with the flow conditions, but it has been sufficient to give comparisons of the fauna at the two stones stations involved, which is all that is required here. The fauna related with the rock and moss populations was not considered, because of sampling difficulties.

Identifications were made with the help of Mrs F. J. Mackereth, using for Ephemeroptera, Macan (1955); for Plecoptera, Hynes (1941); and for Trichoptera (as far as possible) Hickin (1950). As only the relations of the animals to the algal populations were being considered, identifications were not usually carried to the species. The genera of most forms were identified, others (including all the Chironomids) being grouped according to their order or family. Only the caddis Agapetus fuscipes Curt. is discussed in detail; its data are included in Fig. 4. Other forms are given in Table 3 and mentioned in the text.

IV. GENERAL FEATURES OF THE HABITAT AND ITS ALGAL FLORA

Diatoms are the dominant group in the beck. Certain blue-green algae, particularly Lyngbya purpurascens, and Phormidium uncinatum and Ph. autumnale (which Geitler 1932, notes as difficult to distinguish from one another), formed considerable growths on the permanent rock at times. Chamaesiphon curvatus and Nostoc sphaericum also occur attached to the bryophytes. Small growths of Lemanea sp. and considerable ones of Rhodochorton violaceum developed at times, particularly on the rock in the uipperflat zone, and patches of Hildenbrandia rivularis were present in places in the middle steep zone. Chlorophyceae were very poorly developed, only occasional sparse growths of Ulothrix and Mougeotia being found.

There is very little growth in the uipper flat zone of the beck, particularly on the stones. There is a black deposit, probably of a peaty origin, on the stones and rock in this zone, and this may make the surface unsuitable for colonization, particularly diatom colonization. This is discussed further on p. 318. This lack of growth in the u.pperflat zone was the reason why no sampling stations were sited above stones station V.

No regular sequence of chemical changes was observed. The variations of the measured constituents, except the alkalinity, were fairly small, and the results are not given in detail but are summarized in Table 1. Alkalinity varied considerably with the flow conditions, being high in low flow, and low during floods. Similarly, there was very little difference in samples taken at any one time from different parts of the beck between station I and V. It was not possible to correlate any changes in the algal populations at different times of the year, or at different places in the beck, with changes in the chemistry of the water, and no further considera-



POPULATION CHANGES OF 15

ACHNANTHES SPR GROUP AND FACTORS OF ECOLOGICAL INFLUENCE

see text for fuller explanation of keys, etc. POUAIN

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KY ,collected in a no. of larv6e 5 minute sample , no. of pupaeJ from each station

o none in sample



BARBARA DOUGLAS 307

tion, therefore, is given to these factors. Miss Q. E. Hobbs showed rapid chemical changes in the upper stretches of a near-by stream, due to the oxidation of organic matter and the production of soluble nitrate, and correlated the distribution of certain algae, notably Batrachospermum, with them (Annual Reports of the Freshwater Biological Association for 1933, 1934; no details are available). The equivalent region in Belle Grange Beck, that above station V, is not included in this paper, however.

The sampling methods are only really suitable for diatoms, and any discussion of the other algae would be purely speculative at present. Further consideration is therefore restricted to the diatoms, and the dominant Achnanthes species group is first considered in detail.

In considering the changes in the epiphytic populations, the picture is com- plicated by the changes of the moss itself. This makes it difficult to draw con- clusions about, for example, the growth rate of Achnanthes when both it and the moss are growing, or make comparisons of populations at different times of the year. The results are therefore less valuable than those from stones or rock, but I include them because of certain comparisons that can be made with the other populations.

V. THE DISTRIBUTION AND PERIODICITY OF THE ACHNANTHES SPECIES GROUP

(a) The populations of Achnanthes species and their distribution The population densities attained on the permanent rock were much higher than those on the stones. Populations of 105 to 5 x 105 cells per sq. cm. were quite common on the rock, and over 106 occurred several times. The maximum was 5-1 x 106 (station a on 24 May 1955). On the stones the populations occasionally reached 105 cells per sq. cm., but only at stations I and II, and the maximum recorded was 2v7 x 105 (station II, 29 May 1953). This is because (for reasons which are discussed on p. 309) a reduction to a few hundred cells per sq. cm. was common on the stones, but the rock populations seldom fell below 5000 cells, and never below 1000. Whenever high populations began to build up, therefore, those on the rock started from a higher initial level than those on the stones, and, the same growth rate gave a proportionately higher final population. The epiphytic populations cannot be compared, quantitatively, with those on stones and rock, as they are related to the volume of moss and not to the surface area. Nevertheless, the range from a few thousand cells per 10 mm.3 to several hundred thousand, represents a growth similar to that on stones and on rock.

When population increases occurred on the stones (Fig. 4, for example, May- June 1953, April-May 1954, March-June 1955) station II had, a higher population than station III, station III than station IV, and station IV than station V; that is, the population increased with the descent of the beck. This may be due to the decrease as the beck descends of the black deposit on the stones mentioned above, but it may also be due to the washing-off of cells in the upper reaches and their deposition lower down. This is discussed further on p. 318.

(b) The factors controlling the Achnanthes populations Light intensity. The increases in April to June at station II in 1953, 1954 and 1955,

at station I and all the rock stations in 1955, and at all the moss stations each year,



308 Attached Diatoms in a Small Stony Stream might be correlated with the spring increase in light intensity. But a similar increase also occurred under low light intensity, at station II in November 1955, at all the rock stations in November to December 1954 and in November 1955, and station P in February to March 1956. These were periods of low light intensity (Fig. 4). Also, although stations P and II are among the most shaded (see Table 2), the former often showed as good growth as its fellows, or even better, and the latter showed consistently the best growth on the stones. Also, the spring and summer months were brighter in 1955 than in the other years (see Fig. 4), but this had no apparent effect on the growth.

There is sufficient light energy for the growth of Achnanthes species group at all the stones and rock stations throughout the year, therefore. Knudson (1957) has similarly shown that attached populations of Tabellaria flocculosa on lake reed- beds are capable of growth during months of minimum illumination. As she remarks, this difference from planktonic algae such as Asterionella is because the latter receive little light during the winter months when the lake is in full circula- tion, while cells attached near a water surface are in a position to make the maximum possible use of the light available.

The April-June maxima of the epiphytic populations may possibly be correlated with the increased light intensity. Since the overlapping leaves of a moss probably do reduce the light penetration, this difference between epiphytic and epilithic populations is quite reasonable. Increases were restricted to these months, except for that in November 1956. But as this was one of a series of very irregular results it may be unwise to take it as definite evidence of epiphytic growth at low light intensity.

There is no evidence of inhibition by high light intensities. It would be most likely at station V, as the others are all shaded to some degree. The absence of growth at station V during the April-June increases elsewhere is unlikely to be due to such an inhibition, as a similar lack of growth occurred at the shaded station IV at the same time, and at station V at the lower light intensities in other months.

Temperature. The temperature records plotted in Fig. 4 show occasions with a temperature range of several degrees in the beck. Such a range occurred on hot sunny- days during periods of low flow, when the water was warmed in the open part of the upper flat zone and cooled rapidly after entering the wood. Most of the temperature difference was measured between stations V and IV. In cloudy weather the difference was much less. With high water levels the temperature was almost, if not quite, the same throughout the beck.

Changes in A chnanthes populations cannot justifiably be correlated with changes in temperature, just as they cannot be correlated with the annual variation in light intensity. Growth occurred at one station or another at temperatures ranging from near freezing point (e.g. station P in February-March 1956) to the maximum recorded (e.g. station y and stations I and II in June and August 1955). There is no reason, therefore, to consider the April to June increases as due to the rising temperature of the water. Nor is there any evidence of population decreases being related to temperature.

Water level andflozw conditions. The flow of the water in the beck was the most important factor controlling the populations, although sometimes difficult to explain. Attempts to show a statistical correlation between rainfall and population changes were unsuccessful as the estimation of the water level from rainfall



BARBARA DOUGLAS 309

records is only approximate. The effect of a certain amount of rain on a stream depends on the time over which it falls, and on the wetness of the soil of the drainage area. Also, in comparing rainfall and population changes, different lengths of time must be considered for different periods during the four years, and for different stations, and this makes a concise statistical treatment difficult.

The stones populations frequently showed marked decreases after a day of heavy rain, particularly at stations I and II. These are marked D in Fig. 4. The most striking example was a violent storm at the end of June 1953. At station II, of the samples taken after a day with 1 in. or more of rain (2-5 cm.), 66 per cent showed a drop in population, 47 per cent being a decrease to a half or less. Not every day of heavy rain was followed by such a decrease. On some occasions the populations were already low, and no flood effect could show; and on others the rain came after a period of dry weather, and much of it soaked into the ground and caused little flooding in the beck. At station I the flood effects were less frequent than at station II, the corresponding proportions being 60 per cent and 38 per cent. This is discussed below. At the three other stones stations the populations were generally too low for flood effects to show. Floods affected the rock populations less (also marked D). Taking all three populations together, 46 per cent showed a decrease after a day with 1 in. or more of rain (2-5 cm.), only 25 per cent showing a decrease to a half or less. With the epiphytic populations, such flood effects hardly occurred at all. Only the storm of 26 June 1953 caused such a decrease, and then only at one station.

Since the decreases are fewer and smaller with the rock than the stones populations, the washing-off of cells from the substratum, although it does occur, is probably less than one might expect. The greater decreases on the stones must be through the actual disturbance of the bed of the beck. This gives a third reason why a sample from the stones does not always show a decrease after a flood. There is always a chance, in selecting the stones at random, of picking a number which, although shifted and disturbed, still carry a high population on their upper surfaces. It also explains why the flood effect is apparently less at station I than at station II, since some of the disturbed stones from higher up are likely to be deposited at station I with their populations still attached. It also explains the higher populations initiating increases on the rock, which was discussed on p. 307. The absence of flood effects on the epiphytic populations is probably due to the protection provided for the cells by the overlapping leaves of the moss.

A period of wet weather maintained low populations on the stones. During September-December 1954 the populations on the stones were reduced to a low level and remained there. The population at station II remained level in February- March 1954 in contrast to those months in 1953 and 1955, and this was probably due to the higher rainfall in 1954. The rock populations, however, behaved in the opposite way. During the wet period at the end of 1954 they were built up, with some irregular fluctuations, at all three rock stations. Similarly, the populations which were built up in October and November 1955 continued to increase during the wet weather at the beginning of December, although declining at the end of that month. The epiphytic populations on the moss were hardly affected by wet weather. During that at the end of 1954, for instance, they remained more or less steady.

High populations on the stones were built up during periods of low flow in the



310 Attached Diatoms in a Small Stony Stream beck. The Achnanthes maxima at station II occurred during periods of dry weather when little or no rain fell for several weeks (e.g. April-June 1953 and 1954, April- May and July 1955). Smaller maxima at the same station coincided with short periods when, although successive days were wet, the rain did not generally exceed W in. (12 mm.) a day (e.g. July to mid-August 1953, mid-September to Nov- ember 1955). Similarly, during relatively dry periods in February-March fairly high populations at station II were maintained in 1953 and built up in 1955. The prolonged drought in 1955, however, eventually caused a sharp decline at station II. The upper three stones stations showed slightly higher populations during these dry periods, but they always remain low. (Station I, and station II in 1956, which are ignored for the moment in this context, are discussed in the next sub- section.) On the rock a dry period sometimes caused increases in populations at first (e.g. March-May 1955, station a in July 1955, and station P in February 1956), but if it continued the populations declined. Thus, in May and June 1955 they were declining on the rock before the flood of 13 June (which caused the decrease on the stones at stations I and II); this decline continued through the drought in July and August; and after March 1956 they decreased steadily until the onset of slightly wetter weather in June. In April and May 1954, and at stations a and y in February 1956, there was not even an increase at the beginning of the dry spell. This is presumably similar to the decline of the stones population at station It in August 1955. The April-May-June increases of the epiphytic populations have been discussed in connection with the increasing light at this time of the year. As they coincide with those on the stones and reck, it is impossible to say how far they are correlated with increasing light and how far with the low water levels in the beck. The increase in October-November 1955 suggests a correlation with low water level. During the drought of July and August 1955 they declined slowly (with much irregular fluctuation at station C), along with the rock populations. But, apart from these occasions, the moss populations were more steady than those on stones or rock.

Grazing. The faunas at stations I and II were sampled to determine the effects of grazing on the algal populations. The animals found and their gut contents are noted below, and the populations of the principal algal-feeding forms are given in Table 3. A correlation diagram was made for each algal-feeder of the number of specimens in a sample to the number of Achnanthes estimated at the same time, restricting it to the samples taken during low-water periods when the Achnanthes populations were not reduced by flooding. On these diagrams distinction was also made between the samples collected in the spring and summer months, and in the rest of the year.

NOTES ON THE ANIMAL POPULATIOiNS AT STATIONS I AND II, AND THEIR GUT CONTENTS

Certain forms which are taxonomically related and have similar diets are grouped together for convenience in enumerating them in Table 3.

'Algal material' in the guts consisted largely of the epilithic diatoms of the sampling stations, often with a certain amount of blue-green filaments and cells. 'Detritus' consisted of higher plant remains and bits of moss leaves as well as mineral and unidentifiable organic material.

EPHEMEROPTERA (MAY-FLIES). The Ecdyonurids, Ecdyonurus torrentis, Rithrogena semicolorata and Heptagenia lateralis, are enumerated together in Table 3. The guts generally contained some algal material together with detritus.

Baetis pumilus and B. rhodani are enumerated together. The guts usually contained considerable amounts of algal material, with a lower proportion of detritus than the Ecdyonurids.



BARBARA DOUGLAS 311

PLECOPTERA (STONE-FLIES). Chloroperla torrentium, C. tripunctata and Brachyptera risi are noted by Hynes (1941) as feeding on algae more than the other stone-flies do; they are therefore enumerated together. The guts when examined were generally empty, although occasional diatoms were seen.

Leuctra spp., Nemoura spp., Amphinemura sp. and Protonemura sp. are noted by Hynes as algal- feeders, but to a less extent than Chloroperla and Brachyptera; they are therefore enumerated together. The guts usually contained a rather high proportion of detritus with some algal material.

Perla carlukiana and Isoperla grammatica were occasionally seen, but as they are noted as carnivorous by Hynes they are not considered here.

TRICHOPTERA (CADDIS-FLIES). Agapetus fuscipes (with occasional Glossosoma sp.) was the only caddis occurring in large numbers. The guts nearly always contained a high proportion of algal material. It is fully discussed in the text, and its populations plotted in Fig. 4. The following genera were also occasionally found but never in sufficient numbers to affect the algal populations even when they are herbivorous: Limnophilus, Stenophylax, Leptocerus, Odontocerum, Hydropsyche, Plectrocnemia, Polycentropus, Philopotamus, Rhyacophila. These genera are not enumerated.

OTHER FORMS. In addition to the three insect groups above the following were found: Gammarus pulex; frequently in large numbers, but as it is a detritus feeder it is not enumerated. Turbellarians (Flatworms); principally Polycelis cornuta; frequently in large numbers, but as it is

not an algal feeder it is not enumerated. Chironomids; frequently in large numbers, but not identifiable; the diets are uncertain, and they

are not enumerated here. A ncylastrum fiuviatile (freshwater limpet) Diptera larvae, particularly Simulium sp. Unidentified Oligochaete worms Palpicorn beetles occasional specimens Helodid larvae Gordius sp. Hydracarina (water mites) J

Only the caddis Agafetusfuscipes showed any numerical relation to the Achnan- thes populations, although several of the insect larvae grazed on them. There was generally a higher proportion of algae in the guts of Agafetus than of the may- flies and stone-flies. This may be because Agapetus spends more of its time on the tops of the stones, presumably moving round and scraping off what grows there. Agafetus larvae generally contained a high proportion of Achnanthes cells (among other algae) during the spring and summer months. Grazing is progressively reduced after the onset of pupation in May. Certain insect larvae are thought to stop feeding a few days before they pupate but it is not certain whether this is true of Agapetus fuscipes. The larvae of the new generation collected in the autumn contained proportionately less epilithic algae and more detritus than is usual in the spring and summer months, even when much Achnanthes was available as in October 1955.

During the Achnanthes peak at station II in May and June 1953 the population at station I slowly declined. I noticed that, while nearly absent from station II, large numbers of Agafetus were present at station I, and it was this that suggested the advisability of estimating the fauna in the following years. In 1954 the Achnanthes population increased rapidly at station II from April to June in the almost complete absence of Agapetus (Fig. 4), reaching a higher level than at station I, where larger numbers of Agapetus were present. In 1955 the population at station II slowly increased from the beginning of the dry period in February, again in the virtual absence of Agafetuws. At station I there was a large number of Agafetuws at first, and the population remained low. At the beginning of March the farmer on whose land the beck lies, cleared it out under the road and down to the lake (that is, at station I), straightening the sides, throwing out all the big stones, and levelling the bed. This reduced the Achnanthes population as a bad flood would. It also cleared out the Agapetus, and although they were replaced to some extent in the following months (presumably by migration up from the lake), the usual numbers were not re-established. The A chnanthes population immediately



312 Attached Diatoms in a Small Stony Stream began to increase, and paralleled that at station II in the following months. After the flood reduction in June and July, Agapetus was nearly absent from both stations, and the Achnanthes increases were very rapid and very close. The usual distribution of Agapetus was re-established in September with the hatching of the new generation. (The December sample was taken in high water and was a gross underestimate of the Agafpetus at station I.) Despite the difference in Agapetus numbers between them, the Achnanthes populations at both stations increased during the October-November dry period. It has been noted that the larvae

Table 3. Populations of principal algal-feeding insect larvae at stations I and II, excluding A. fuscipes

Species and/or genera grouped together as discussed in the text. Numbers per 5 minute net sample.

Ephemeroptera Plecoptera Chloroperla and Nemouridae and

Ecdyonurids Baetis spp. Brachyptera Leuctra Station Station Station Station Station Station Station Station

I II I II I II I II 1953 November 3 0 5 1 1 0 0 0 0 1954 January 21 0 0 0 0 0 0 0 0

February 2 0 6 1 6 0 0 0 0 February 15 1 4 1 2 0 0 0 6 March 8 0 5 2 6 0 0 2 March 30 3 4 4 6 1 0 6 4 April 13 6 17 20 19 1 2 5 1 May 4 8 7 31 13 4 3 14 3 May 18 4 2 2 2 7 2 11 8 June 5 3 10 1 9 1 1 6 1 August 3 0 2 5 2 5 1 7 0 September 7 0 4 2 10 1 1 0 1 October 12 0 3 3 26 1 3 5 6 December 7 0 4 8 59 0 9 2 18

1955 January 24 0 2 19 20 7 1 2 March 15 0 19 12 112 4 10 1 7 April 26 0 6 12 25 3 5 3 6 May 23 0 6 12 52 3 4 4 9 June15 2 6 2 10 0 2 0 5 August 2 0 2 0 1 0 0 0 1 September 6 0 3 0 0 0 0 1 0 September 19 2 7 1 17 0 3 6 13 October 26 1 7 4 13 0 0 10 17 December 12 2 26 4 55 2 9 24 12

1956 January 24 64 6 115 13 17 4 46 37 February 28 66 21 85 39 9 5 44 16 March 22 8 125 63 201 8 2 14 27 April 23 11 23 34 80 14 23 8 29 May 28 19 6 17 49 1 0 4 3

collected at this time had proportionately fewer diatoms than usual in their guts. In their early instars the larvae are minute and possibly graze less than when they are older and larger. I doubt if it is due to a reduction in the grazing due to the lower temperatures, as specimens collected in January 1956 contained many diatoms. During the dry period in 1956 there were large numbers of Agapetus at station I, with the usual maintenance of a low Achnanthes population. More larvae than usual also occurred at station II, apparently through their slow migration upstream throughout the long dry period, and the Achnanthes population also failed to build up there. At the end of June, by which time pupation had



BARBARA DOUGLAS 313

progressed considerably, the Achnanthes made a sudden increase at both stations, but this was cut short by a flood on 4 July and the onset of wetter weather.

Fig. 5 shows the correlation between the numbers of Agapetus and Achnanthes in the samples collected in the spring and summer months in periods of low water. The negative correlation strongly suggests that in spring and summer, water level conditions being favourable, a high population of Achnanthes only develops where there are few Agapetus, and that where there are many, the Achnanthes populations remain low.

Occasional samples were made at the other three stones stations and showed

xi * STATION I d O STATIONIE CP 0 0

0 0

0

0

O O

o

0 *0 ~ ~ 0

E 10 0 0. O

0

0 ci

o3

z03 zI 0

l10b 20 30 4.)

0

Nos. of Agapetus fuscipes per 5 min. sample

Fig. 5. Correlation diagram to show the relation between populations of Achnanthes species group and the larvae of Agapetus fuscipes at the lower two stones stations.

similar animal populations to station II, as far as the -algal-feeders were concerned. There is no evidence that the low algal populations at these stations were due to grazing.

(c) The periodicity of the Achnanthes species group The periodicity of Achnanthes can now be considered in relation to the factors

just described. Stones. In the first months of 1953 the population at station II remained steady

until its rapid increase in the April-June dry period. I cannot explain why this population did not increase earlier, when the weather was equally dry; nor why, having reached 300 000 cells per sq. cm. it did not continue to increase. At station

H J.E.



314 Attached Diatoms in a Small Stony Stream I both the apparent fluctuation of the population in March and April (which implies a patchy distribution), and the decline after the end of April, were probably due to the grazing of Agapetus. The populations at the other three stations remained low, although showing their slight increase with the descent of the beck. The heavy storm at the end of June sharply reduced the stones populations. Two periods of only moderately wet weather, in July and August, and in October and November, allowed small increases at stations I and II, but both were reduced by floods and continued wet weather kept the populations low until the following April. With the drier weather from April to June the population at station II increased to 200 000 cells per sq. cm., but was prevented at station I by the Agapetus. On the resumption of sampling at the end of August all the populations were at a lower level, presumably due to flooding on 15 June. The heavy rainfall throughout the rest of the year kept them low. With drier weather after January 1955 the population at station II was built up to its June peak, that at station I following once the Agapetus were removed. Even station III showed a small peak. These high populations were sharply reduced by flooding in mid-June and early July, but both recovered in the continued dry weather and the absence of Agapetus. The drought reduced -them again in August, by the drying-up of station I, and by the unexplained low-flow effect at station II. The moderate rainfall of the following months allowed both populations to increase to another peak of 200 000 cells per sq. cm., the minute Agapetus larvae at station I having no effect at this time. A flood reduction of the population at station II in October enabled that at station I to exceed it throughout the remaining months of sampling. Both populations were reduced by flood in December. Neither was built up again in the first six months of 1956, owing to the presence of Agapetus. With the onset of pupation of the Agapetus, sharp increases occurred, but these were immediately cut short by a flood.

Rock. The rock populations appeared to fluctuate irregularly during the first months of their sampling in 1954, particularly that at station a. How far this was caused by a personal sampling error and how far to a patchy distribution of the population is difficult to say. The population at station p showed a definite increase during the wet weather of January, February and March, and probably a decrease in the drier weather of April and May, both these effects being contrary to those shown by the stones population at station II. All three populations increased irregularly during the exceptionally wet weather of the last four months of the year, and this increase continued on into the drier weather after January 1955. The continuation of this drier period, however, caused them to decline, at station y at the beginning of May, and at station a at the beginning of June. The reductions at station p appeared to be due to floods. The decline at station y continued right on to the end of August. At the other two stations the small increases in July paralleled those at stations I and II, but were shortlived, the drought reducing them again, With wetter weather in the autumn months all three increased, slightly at first and rapidly after the end of October, and these increases were not curtailed by the December floods. With low flow in the beck after Febru- ary 1956 the population at station a decreased almost steadily, that at station P increased to a large peak and then rapidly declined, and that at station y remained fairly steady up to the end of March, after which it also declined. I cannot explain the increases at the lower two stations in May and June; the water level in the beck



BARBARA DOUGLAS 315

was only slightly higher than in the preceding months; and there was no reason to be seen connecting them with the parallel increase on the stones, which was correlated with the cessation of grazing by Agapetus. The later increases in September and October were probably due to the moderate rainfall in these months.

Moss. Less can be said of the epiphytic populations. Their May-June peaks have already been discussed, and apart from these they remain, on the whole, fairly steady. During the drought period of 1955 they showed irregular fluctuations which imply a patchy distribution.

VI. OTHER DIATOM SPECIES AND SPECIES GROUPS (a) Principal species

Certain other diatoms regularly formed considerable populations. When, as frequently happened, only one or two cells of a species were seen in a count, that species was recorded as 'present'.

1) May to June 1953 2)AprIt3May1954 June 1954 4) May 1955 5) June 1955 stones moss rock stones moss rock moss stones

stations stations stations stations stations stations stations stations

5X106 I CB n I C B A 6 0 C B A III %I06

U ? 4 U 1 L ;Xo 1

6) July - 7) November 8)December 9)February - 10) July 1956 August 55 1955 1955 March 1956

stones stones moss rock rock stones stations stations stations stations stations stations flI Ii C B A io Of IE I

Fig. 6. Comparison of the maxima of Achnanthes (white), Gomphonema (stippled) and Synedra (black) species groups at times of high population densities.

Gomphonema species group. The population changes of this group closely resenmbled those of Achnanthes. They seem to be governed by the same factors, although both maxima and minima of Gomphonema tend to be lower (Fig. 6).

Synedra species group. Measurable 'populations, showing similar changes, occurred at the same times as the high populations of A chnantles and Gomphonema. The maxima were generally lower on the stones than those of A chnanthes or Gomphonema, although quite high on the moss and occasionally also on the rock (Fig. 6). Between these small maxima it was either absent or recorded as 'present'. Although conditions were less suitable, therefore, its populations seemed to be governed by the same factors as Achnanthes.

Cocconeis placentula and Eunotia pectinalis var. minor. In Figs. 7 and 8 an isolated measurable amount is plotted as a single point on the graphs, using different symbols for different stations. Two consecutive measurable samples from



316 Attached Diatoms in a Small Stony Stream one station are joined by a line. When recorded as 'present', it is shown below the abscissa of the year's graph at the appropriate date, using the appropriate symbol for the station.

These species only occurred constantly and to a measurable extent as epiphytes on the moss. Eunotiapectinalis var. minor occurred at all three stations throughout the period, but Cocconeis placenlula only at the lower two. The latter was frequently 'present' at station C and elsewhere on the moss in the upfiper flat zone, but never grew to any extent.

STATIONS

,maoss l o'M A O,M,J,J,A,s,ODi1

mA 1953 . .. 0 4

stlo2 _JAS,ONes D Io CRSET -* - -

+ 1z ae

o -

r ck k e^ a

Ontesoe nte oe ttos(n onc ttemdl roc sttonlo

small w oa

ao2 j A F M A M J J A E. S ec0 iN D 0o

wasalsosihlmoecmoatteae tie Thi is conry to Buce'

PRESENT which MA + a

0 954

2 z ~ ~ ~ ~ ~ ~ L * ,

Fiag.e7 Thecsugsethtihr populations of Cocconeis placentuxla atalsatosoFouuleplntolsetx.

deeopn the stoesine then lwere satiosparndlrsn oncea the middlesrc. sainlo smalero wths ofcsoa hr rpsi h ppyi Cocconeis placentuaocurdintelaesmmradapoutumn

apaenl irepcieo1h ethrcniin.Enta etnlsvr io was also slightly more common at the same time. This is contrary to Butcher's

'sme nrsigcmuiy Bthr191 hc nldsCcoeslcnua an Mhc eeoe nsmensie immrse intewte0nanubro diffren stram. I fe sAted obevtoswtAmere ldsi el

Th nere wer ocsnAl shr rp nteeihtcCcoespaetloua



BARBARA DOUGLAS 317

tions, which may have been due to flooding; and periods of apparent sharp fluctuations of both species which denote a patchy distribution. But on the whole, the populations of both species remained more or less level throughout the year.

Ceratoneis arcus (with var. amphioxys and var. linearis). This was 'present' sporadically throughout the period, increasing to small maxima between March and June (see Table 4). In 1955 this maximum was attained at all but stations III, IV and V and unusually high numbers were reached. In 1956 it was lacking, but there was a small growth from January to March. These maxima are not apparently connected with the water level of the beck.

STATIONS u 1953 Key to symbols ?

Ai p Ingraph Of r % C. .. ac.t o A0 . ~~~~~~~~~~~ ~~~~~~~~~0

I olO ;; X .R S-v?.oo

%

m0 % o A~~. .V -~O. /O. 0 l 0

J 5 R ff.o. ,o* 'od .

[o s Jr , F f

o.

02 J F M A M J J A S 0 N D

0L + 00

o. P~~~~~0.3

PRSNW 1o J, * M | A, 4. ? , D

4RESENTist?nCs * - - * * .4 . * + 0*

t k 4 0 O

0 8. A..

(b Spce ocrig inls bnac

10 . v. c a wo

0 .

Cymbell sinuaa wa4cainlyrcre s'rsn'a l ttos ml

u10'

PRESEN ~~~~~~~~~~~~~~~~~~~ * t + ~ ~ ~ ~ ~ ~ ~ ~ A A -

jo_ ___ __

allEtheTstatos n atclrytoeo h oe itadmdlones. Bewee

1954.~~~ 15



318 Attached Diatoms in a Small Stony Stream Table 4. Population maxima (in cells per sq. cm. or per 10 cu. mm.) attained-by Ceratoneis arcus, excluding the ufpper three stones stations where no increases

occurred (P 'present'; see text) Stones stations Rock stations Moss stations II I y , a C B A

May 1953 2100 1300 not sampled 17 000 5600 not sampled

May 1954 1500 1000 38 500 2000 2000 5000 2500 3000 November 1954 P P 6000 P 10 000 P P P May 1955 12 700 5000 230 000 47 000 230 000 19 000 19 500 32 000 February 1956 P P P 22 000 18 000 not sampled

Synedra ulna was frequently recorded as 'present' at all stations, and was occasionally found in measurable amounts, up -to 4000 cells per sq. cm., at stations I and II. It also invariably colonized slides immersed in the water for a few weeks at station II.

Occasional specimens were also noted of the following:

Achnanthes lanceolata Cymbella ventricosa Diatoma hiemale var. mesodon Didymosphenia geminata-at stations I and II Eunotia lunaris-frequently recorded as 'present', particularly from the mosses Gomphonema constrictum Meridion circulare Navicula spp.-unidentified cells frequently recorded as 'present', particularly from the mosses, the

following were identified: N. cryptocephala, N. cari, N. vitrea and N. radiosa var. subrostrata Cl.

Nitzschia palea agg. Pinnularia spp. Surirella sp. Tabellaria flocculosa

VII. DISCUSSION

This survey, although preliminary and of limited scope, suggests the factors controlling certain populations, and opens several lines of inquiry. The algae in a stream vary in habit and distribution, and a range of methods is necessary to gain reliable quantitative information on all the species. The methods used here are suitable for unicellular forms with more or less random distributions, but for filamentous or encrusting types with bunched distributions entirely different methods would be required. The observations were restricted to one small stream, poor in species, so that the detailed data on population changes would be simpler to interpret.

It was suggested on p. 306 that the lack of growth in the ufipper flat zone is caused by the peaty deposit on the stones there. Whether this is due to a chemical inhibition, or to a physical effect making the substratum unsuitable for attach- ment, or to some other factor, is not known. The lack of epiphytic growth of Cocconeis placentula in this zone may be connected with this. Round (1953) showed that certain benthic diatoms in Malham Tarn did not grow on a peat sediment, although flourishing on a calcareous one near by. Diatoms grew on the permanent rock in one or two places (one of which was selected as station y), although partly as epiphytic populations on Rhodochorton violaceum.

It was suggested on p. 307 that the increase down the beck in numbers of A chnanthes on the stones may be partly due to the washing-off of cells in the upper



BARBARA DOUGLAS 319

reaches and their deposition lower down. The Gomphonema and Synedra species groups showed the same general distribution. It is reasonable to suppose that this washing-off and subsequent deposition of cells does take place. The 'plankton' of small rivers and streams consists of cells of the benthic species which have been detached (Butcher 1932). Water samples from Belle Grange Beck always contained cells of the attached diatoms, and since slides immersed in the water are rapidly colonized by epilithic algae, the subsequent deposition of sulch cells is also likely. Over a hypothetical uniform stretch of stream, therefore, where conditions are sufficiently uniform for the growth rate-to be uniform, the population will increase downstream. How far this holds true in practice will depend on the other ecological conditions. Thus if the growth rate is so high that the maximum possible population for the available space is developed in the upper regions, the situation would not arise. But it may be an important baXsic factor to bear in mind in considering the distribution of a species within one drainage basin. Quennerstedt has found (Round 1953) that the numbers of diatoms in Swedish streams decreased with increasing altitude. This might possibly be due, not so much to a decrease in growth rate with altitude, as to transport downwards by the stream. Eddy (1934) showed that the plankton of a river develops and increases in abundance with the passage downstream of the water, and related the stage of development of the plankton to the age of the water. While not exactly comparable to the detachment and subsequent deposition of attached organisms, this puts the distribution within one river on a similar basis.

There is no regular periodicity such as is found with certain planktonic species. The periodicity depends mainly on the flow of water, and this depends on the weather, which tends to be irregular in the British Isles. The increases in population in May and June are due to the dry weather that generally occurs then, and it is a coincidence that they happen during the period of increased light intensity. If the populations are affected by seasonal variations in light and temperature these effects are masked by the more violent ones due to changes in the water level. In a habitat like this it.is the rate of population increase, not the growth rate, which is estimated. The continued, and probably varying, loss of cells from the population probably means that the relation of the rate of population increase to the growth rate is different at equivalent times of different years.

I suggest that the basic pattern of the periodicity of A chnanthes species group (and by implication, Gomphonema and Synedra species groups also) is as follows. Conditions are only adverse to growth when the flow is low. With high flow the rock populations increase, and the stones populations are only prevented by the instability of the substratum. As the, flow is reduced in a dry period, the bed of the beck becomes stable, and populations increase on the stones, and sometimes also on the rock. With continued reduction in flow the rock populations decline, and with a very severe reduction due to a prolonged drought, the stones populations decline as well. I cannot explain why low flow should be adverse. Geitler (1927) suggested that slow-flowing water is less 'physiologically active' than fast, since a less rapid renewal of dissolved nutrients is given to a cell immersed in it. This could explain a slower rate of growth under low flow, but it is difficult to see how it can explain an actual decrease in population, which requires an increased loss of cells from it.

Budde (1928) suggested that the rapidity with which diatom populations are



320 Attached Diatoms in a Small Stony Stream

re-established on dried rocks in a stream once it starts flowing again, implies a capacity for drought resistance. This is known for some soil diatoms (Petit 1877, Lund 1945). Rough experiments were made by drying stones from the bed of the beck, and petri dishes on the bottoms of which A chnanthes minutissima and mixed cultures of Belle Grange Beck diatoms had been grown, and then re-immersing them in culture solution (Chu 1942 No. 10 with soil extract). Both stones and petri dishes were dried for a few minutes, for 24 hours, and for several days. On no occasion was any subsequent growth obtained, even after several weeks. The cell contents, although rather pale looked in quite good condition in cells dried for a short time, but the desiccation must have killed them.

The grazing of animals has been rather neglected as a factor controlling algal populations. Certain workers on attached algae have mentioned it in passing; e.g. Godward (1937) noted the grazing of ChironomXids on the algae growing on slides suspended in Windermere. More attention has been paid by zoologists, since many entomologists have noted algae as part of the diet of aquatic insects. As far as I know B3rook has given the only descriptions of a relation between attached algal and animal populations. He found the grazing of insect larvae (Chironomidae, Trichop- tera, Ephemeroptera) a prime factor in controlling the algal populations on slow sand filter beds (Brook 1954, 1955a), and followed these observations up with similar ones on epiphytic populations on higher plants in Scottish lochs (Brook 1955b).

How far the substratum affects the attached populations is uncertain. The failure of growth in the 4pper flat zone, and the apparently greater growth of Cocconeis placentula on slides immersed in the water than on the stones, both point to an effect of the substratum. It is possible that the attached algae are able to derive part of their mineral supply from the substratum, and that the decline of rock populations before those on the stones during periods of low flow may be connected with changes in the substratum.

SUMMARY

Methods are described for estimating populations of attached algae, particularly diatoms, on stones, rock and bryophytes in a stream. The errors of these methods are considered.

Populations at eleven stations in a small stony stream were followed for four years by periodic sampling. Standard chemical analyses; temperature measurements at the sampling stations; total light in the open, and the approximate shading at each station; daily rainfall records; and comparisons of the fauna at two stones stations, are included.

The distribution and the factors controlling the periodicity of Achnanthes species group are given in detail. Populations reached several million cells per sq. cm. on the permanent rock, and several hundred thousand on the stones. High populations developed at all the rock and moss stations, but only the lower two stones stations. Poor growth in the upper reaches may be due to a peaty deposit on the substratum. An increase down the stream of the stones populations may be due to the washing-off of cells in the upper reaches and their deposition lower down.

Populations on the stones and rock increased at all times of the year. There is no evidence of differences in light intensity or temperature having any effect, except possibly on the populations on the moss.



BARBARA DOUGLAS 321 Stones populations were sharply reduced by floods in the stream, rock popula-

tions to a less extent, and epiphytic populations only with very severe flooding. In high-water periods stones populations remain low owing mainly to the instability of the substratum, and rock populations increased. In low-water periods stones populations increased; rock populations declined, sometimes after an initial increase. Severe drought caused a stones population to decline also. Epiphytic populations remained fairly steady, apart from increases in late spring and early summer.

A negative correlation, suggesting a grazing effect, was found between the populations of Achnanthes and the larvae of the caddis Agapetus f'scipes at the lower two stones stations.

Similar population changes were shown by Gomphonema and Synedra species groups, although their populations were lower. Eunotia pectinalis var. minor and Cocconeis placentula were mainly epiphytic, the latter not growing in the upper zone of the stream. Other diatom species which formed small growths, or were occasionally present, are mentioned.

The principal diatom species were unable to withstand desiccation. Certain blue-green and red algae also formed considerable growths.

My thanks are due to the Director and staff of the Freshwater Biological Association, particularly to Dr J. W. G. Lund for his suggestion and direction of this work; to Miss C. Kipling for her guidance of the statistics; to Mr W. Findlay and Mr J. Anderson for making most of the apparatus; to Mr G. J. Thompson, and members of the laboratory staff who have assisted with the field work; and to those mentioned in the text. I should also like to acknowledge the interest and encour- agement which I have received from Mrs T. H. Kipling (Dr B. M. Knudson). The early part of this work was financed by The Nature Conservancy, for which I am grateful.

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(Received 28 September 1957)



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The Ecology of the Attached Diatoms and Other Algae in a Small Stony Stream