Changes in the Zooplankton community off the coast of Northumberland between 1969 and 1988, with notes on changes in the phytoplankton and benthos

J. E.\/I. iMar. Biol. Ecol.. 172 (1993) 1 l-29 0 I993 Elsevier Science Publishers B.V. All rights reserved 0022-098 I :93/$06.00

JEMBE 02024

Changes in the zooplankton community off the coast of Northumberland between 1969 and 1988, with notes on

changes in the phytoplankton and benthos

(Received 6 August 1992; revision received 18 November 1992; accepted 2 December 1992)

.4bstract: Species abundance data for the holoplanktonic fraction of zooplankton samples taken monthly from 1969-1988 at a station off Northumberland on the north-cast coast of England (western North Sea) have been combined to give annual gcomctric mean abundances and subjected to multi-dimensional scal- mg (MDS) ordination. The years arc found to fall into two groups (1970s and 1980s) with a distinct transition in community structure between 1979 and 1980. The macrobcnthos at a nearby benthic station sub- ject to long-term annual sampling also falls into two groups but with a transition in community structure

occurring a year later between 1980 and 198 1, and similar results have been reported for pelagos and bcnthos

in the eastern North Sea. The monthly and mean annual abundances of the more numerically important spccics of the zooplankton arc examined to see if consistent patterns of change are discernible. The changes

seen are found to bc complex and varied, and not the result of changca in abundance of a few dommant apccics. The results are examined in the context of long-term climatic changes in the north-east Atlantic and

changes in the phytoplankton off Northumberland.

Key words: Annual variations: Climate; North Sea; Phytoplankton: Zooplankton

Austen et al. (1991) used multi-dimensional scaling ordination to examine long-term

changes in community structure at two benthic stations in the North Sea. At both a

western North Sea station off Northumberland (80 m deep, sample series from 1971

to 1986) and an eastern one in the Skagerrak (100 m deep, sample series from 1973

to 1988) they found a clear transition in community structure between 1980 and 198 1.

They also examined plankton data collected over the same period by the Continuous

Plankton Recorder (CPR) from the adjacent areas of the North Sea, areas C2 and Cl

respectively (Colebrook, 1978). For the eastern North Sea area Cl there appeared to

be a transition in zooplankton community structure, though less clear than that seen

in the benthic communities, at the end of the 1970s but the pelagic data from the

western North Sea area C2 showed no discernible pattern.

Correspondence address: A. Edwards, Department of Marine Scicnccs and Coastal Management. LJni- versity of Newcastle upon Tyne, Newcastle upon Tync NE1 7RU. UK

12 F. EVANS AND A. EDWARDS

Both benthic stations lie on the coastal peripheries of the extensive (about

185000 km*) CPR areas and it is possible that data averaged over these large areas,

may not truly reflect conditions at the inshore peripheries. Thus, if there were some

coupling between planktonic and benthic communities and, by inference, similar pat-

terns of change in community structure, this might not be apparent in comparisons of

plankton community changes averaged over wide areas with changes in benthic com-

munities at discrete points. Detailed zooplankton data for a site 5 nautical miles off

the coast of Northumberland (Fig. 1) and 6 nautical miles from the western North Sea

benthic station discussed by Austen et al. (1991) have been collected by the senior

author since 1968 (Evans, 1977, 1981, 1985; Roff et al., 1988). These data have been

analyscd to discover whether the zooplankton community in the immediate vicinity of

the benthic station shows any transition in structure between the 1970s and 1980s and

the nature of any such changes.

a ZOOPLANKTON STATION

North Sea

I 1 10 35’ I” 30’ 1” 25’ 1” 20’ 1” 15’

Fig. 1. The zooplankton sampling station in relation to the Northumbcrland coast and CPR arc;1 C2.

LONG-TERM CHANGES IN ZOOPLANKTON 13

METHODS

The nets employed in the present series of sampling were those designed to be used

as a standard by the plankton workers of a UNESCO working party (UNESCO,

1968), namely the WP2 net and WP3 net. The WP2 net is a medium plankton net of

mesh aperture 200 pm, having a mouth diameter of 57 cm and hence a mouth area of

0.25 rn’. The WP3 net is a coarse net of mesh aperture 1 mm, having a diameter of

1.13 m and hence a mouth area of 1 m2. The Unesco report recommended that to

supply a single standard of comparison, metered samples of 50 m3 be taken. To this

end, for the present research a permanent plankton station was established in 1969 at

approximately 55’ 07’ N 1’ 20’ W, about five naulical miles east of the port of Blyth

in Northumberland, at a depth of 54 m. The procedure for fishing the WP2 net involved

hauling it vertically four times on 50 m of wire, bulking the samples together and thus

filtering a water column of a nominal 200 m length and of close to the desired volume.

Effectively, the whole water column was sampled. With the WP3 net a horizontal haul

of 10 minutes duration (about 700 m) at a net depth of about 30 m was taken. Both

nets were fitted with a flow meter to record the distance traversed (Evans, 1985).

Between 1969 and 1988 samples were taken monthly when weather allowed. The

average number of months on which the station was sampled each year over the pe-

riod exceeded eleven and was never less than ten.

Catches from the WP2 net were subsampled with a stempel pipette of a capacity

appropriate to the size of the sample, while the Huntsman technique using beakers (Van

Guelpen et al., 1982) was employed for the bulkier WP3 catches. Replicate test counts

and the subsampling of uni- and multispecific control samples prepared in the labo-

ratory with known numbers of animals in them have indicated acceptable standards

of accuracy.

For analysis, abundance data (numbers per m’) from both WP2 (200 pm mesh) and

WP3 (1 mm mesh) nets were used. For most entities sampled there was a one to several

orders of magnitude difference between nets in catch rates and it was clear which net

was the better sampling device. For a few entities abundances (usually low) were similar

in both nets; in such cases data from the net with the higher overall catch rate was

always used in analyses. The total number of entities in the combined data set amounted

to 102, of which 35 were meroplankton. The latter were excluded from the analysis

which focused on the holoplankton. For several species of holoplankton different en-

tities (females, males, juveniles, stage V copepodites, etc.) were enumerated separately;

these entities and the taxa into which they were combined for various analyses are

shown in Table I.

For multivariate analysis the geometric mean of the abundance of each entity in each

year’s set of monthly samples was calculated. Additional analysis was also carried out

on a data set where infra-specific entities (i.e. stages of a given species) had been

combined to find out whether this affected the results of the analysis. Nonparametric

multi-dimensional scaling (MDS) ordination using a Bray-Curtis similarity index was


TABLE I

Holoplankton taxa subjected to multivariate analyses. For several species, sexes or stages were counted separately in samples (F = females, M = males, V = copepodite stage V, J = juveniles). The net used to sample

different taxa is also indicated.

Genus/taxon Species

Copepoda Acmtiu

Acartiu

Accrrtia

A no??~u/ocevu

CUIUWS

Calanus

CUlUillAS

Calamu

Candacia

Centropages

Centropages

C0rJMeUS

Metridiu

Microcalanus

Microsetella

Oithoncl

Oithorm

Purcrcalamu

Pseudocalanuv

PseudoiParalMicrocala}lus

Rhincalunus

Temoru

clausi

longirerizis

SPP. putersoni

jinmcrrchicus

helgolandicus

linmarchicuslhelg~~lt~dicus

SPP. armata

hamatus

t?picus

anglicus

Iucens

pusillus

norvegica

plutmfera

sirnilis

parvus

elongotus

SPP. nasutus

tongicorniv

Euphausiacea Megun)~etiphanes

N~jrtiphanes

Thwatwessa

Thyanoessa

Euphausiacea Euphausiacca

norvegica

couchi

inerniis

raschi

Cladocera Ewdne

Podon

nordmarmi

SPP.

Amphipoda Hyperoche

Themisto

medusarunz

compressa

ornatu

Cnidaria unidentified

Siphonophora Nanomicr

Scyphozoa A ureliu

CJzrnea

aurita

VP.

Entities

F/M F/M

J F/M/V

F/V

F/V

M J

F/M F/M/J F/M/J F/M/J F/MiJ

F/M/J F,‘M F,‘M

J

F/M/J

late larvae nauplii

medusae

Net

WP2 WP2

WP2 WP3 WP3 WP3 WP3 WP2

WP3 WP2 WP2 WP2 W P2

WP2 WP2 \VP2 WP2 WP2 WP2

WP2 WP3

WP2

WP3 WP3

WP3 WP3 WP2 WP’

WP2 U’P2

WP3

WP3

WP3

WP3

wp3

WP3 WP3

Ctenophora SPP.

“PP. helgolandicu

Pteropoda Clione

Chaetognatha Su,yittcr elegans

Appendicularia Frifilluricr

Oikopleurcr horeulis

SPP.

Thaliacea S&c1 SPP.

LONG-TERM CHANGES IN ZOOPLANKTON

WP3

WP3

WP3

WP2

WP2 WP2

WP3

15

used to examine between year changes in community structure (Clarke & Green, 1988).

A posteriori significance testing for differences between groups of years identified by

MDS ordination was performed using the ANOSIM simulation/permutation test, a

multivariate equivalent of ANOVA which does not make the assumption of multivari-

ate normality in the data (Clarke, 1988). The species mainly responsible for the Bray-

Curtis dissimilarity between groups of years were determined using the program SIM-

PER (Warwick et al., 1990).

RESULTS

Inter-annual variation in geometric mean abundances for each year for total hol-

oplankton is shown in Fig. 2 for both the WP2 and WP3 nets. The pattern for the

coarse-mesh WP3 net is largely determined by abundances of Calanusjnmarchicus and

C. helgolandicus both of which were rare during the most of the 1970s but compara-

tively abundant during the 1980s apart from 1987 (Fig. 3). Interestingly the abundance

of C.$nmarchicus (adapted to cold boreal water) appeared to decline about two years

later and to increase two years earlier than that of C. helgolandicus (an inhabitant of

warmer water masses, Fransz et al., 1991) in the early and late 1970s. Overall hol-

oplankton abundance follows the WP2 pattern because the smaller zooplankton domi-

nates numerically; there was a peak of abundance in 1973-1975, a marked decline from

1978-1980 and a recovery in the 1980s interrupted by low abundances in 1985-1986.

The 15 most abundant groups of entities in the holoplanktonic zooplankton samples

were, in decreasing order of abundance: Oithona similis (F + M + J), Pseudo/Paw/

Microcalanus spp. (J), Acartia spp. (J), A cartia clausi (F + M), Temora longicornis

(F + M + J), Pseudocalanus elongatus (F + M), Acartia longiremis (F + M), Oikopleura

F. EVANS AND A. EDWARDS

3.0

2.0

c

E 1.0 E

‘_ ,’ ‘,

E ? “, 0.0

5 1 z 8

.p!

5 D -1.0 ,.__.~........-.

-2.0

-3.0

Fig. 2. Annual geometric mean abundances of holoplanktonic fraction of the zooplankton collected by WP2 and WP3 nets expressed as standard deviations from the mean abundance over the whole period.

To examine inter-annual changes in community structure the geometric mean abundances of each entity and also of each group of entities (see Table I) were subjected to MDS ordination after double square-root transformation to reduce effects of the few numerically dominant species. The MDS ordinations (Fig. 4a,b) show a transition between 1979 and 1980; this transition is equally clear in log~~rithmically tratlsforl~ed data, slightly less clear in square-root transformed data and not apparent in untrans- formed data. For tr~sformed grouped entity data, the A~~SIM test (using random samples of 5000 permutations) indicates that the two groups of years, 1970-1979 and 1980- 1988, had si~ificantly different (a -=x 0.0 1) community structure (sample statis- tic T= 0.412, probability that the two groups of years are not significantly different p = O.lOu/, for double square-root transformed data; T= 0.446, p = 0.120/;, for logarith- mically transformed data; T= 0.226, p = 0.567; for square root transformed data). There was no significant difference if abundances were untr~sformed (T= 0.078; p = 9.92:~). Ungrouped entity data yielded a very similar set of resuIts.

To discover which zooplankton species contributed most to dissimilarities between the two groups of years, the SIMPER program was used. Table II summarises the results of this analysis and shows that Sag&a ekgaru and Fyjt~~lffri~ bore&s contributed most to the Bray-Curtis dissimilarity between 1970-1979 and 1980-1988 group- ings of years using log transformed abundances (to reduce effects of dominant species) of 45 entities listed in Table I. The annual and monthly changes in abundances in the zooplankton of the majority of the taxa in Table II during the period 1969/1970- 1988


are now examined to discover what clues these give to the reasons for the transition

in community structure between 1979 and 1980

Calanus finmarchicus (F + VI

69 70 ?I 72 73 74 75 76 77 78 79 80 81 82 83 64 85 66 67 88 89

Calanus helgolandicus (F + V)

40 _

35 - .’

30 - i . : : ..’ *~._

;25- -”

2

‘,I.., ‘. ‘.

2 20 - .I__ .-_____._..__._~~~“~~~~....___.-~’

s z 15 -

5-

-I’, , ,-,-: , + 69 70 71 72 73 74 75 76 77 78 79 80 61 82 63 84 85 86 87 88 89

Fig. 3. Monthly abundances of females and stage V copepodites of Culu,lus,finn~avchicus and C. helg&~~dicus (left-hand vertical axis) for 1969-1988 and standard deviations of annual geometric mean abundances from

long-term mean (right-hand vertical axis).


b

Fig. 4. MDS ordinations of double square-root transformed abundances of (a) all entities listed in Table 1.

and (b) the 45 taxa (grouped entities) in Table I.

The predatory chaetognath Sagitta elegans was relatively uncommon in samples

during the 1970s and showed a marked increase in both persistence and abundance

in the plankton during the 1980s (Fig. 5), closely following the annual abundance trends

of the two Calanus species (Fig. 3).

Sag&a elegans

._: .’

,:“. ,’ ..: :

_..____.. .. ;’ ‘. .,’

‘.,’

1 69 70 71 72 73 74 75 76 77 7% 79 80 81 82 83 94 95 86 97 98 89

Fig, 5. Monthly and annual abundances of Sugiftu elegms for 1969-1958 (axes as Fig. 3 but month11 abundances square root transformed for clarity of presentation).


TABLE 11

Percentage contributions of various holoplankton entities to Bray-Curtis dissimilarity between years 1970-

1979 and 1980-1988. Entities are listed in decreasing order of importance with a cut-off at a cumulative

percentage of 90:,. Analysis was done on log,,, transformed abundances to minimise weighting of numerically dominant species. + denotes abundance in 1980-88> than in 1970-79; - denotes the revcrsc.

Entity Percentage contribution to

dissimilarity

Direction of difference

Sagitta elegans Fritillaria bore& Ternora longicornis (F + M + J) Acurtia clausi (F + M)

Paracalanus parvus (F t M)

Pseudo-/Para-/Microcalarrus (J)

Centropages hamatus (F + M + J) Acurtia spp. (J) Calanus helgolandicus (F + V)

Oikopleuru spp. Acartia longiremis (F + M) Podon spp. Pseudocalanus elongatus (F f M) Oithorm similis (F + M + J) Calanus spp. (J)

Euphausiacea (nauplii) Microcalarms pu.siNus Evadize nordmarmi Microsetella norvegica Euphausiacea (late larvae)

Centropages typicus (F + M + J)

Ca1unu.r j&march&u (F + V)

8.02 6.42 5.29 5.00 4.92 4.82 4.69 4.64 4.47 4.25 3.96 3.14 3.58 3.42 3.37 3.33 3.15 2.19 2.66 2.57 2.38 2.26

+

_ +

+ + _ + + + + _ + _

_ +

The cyclopoid copepod Oithona similis (a boreal, eurythermal, euryhaline species

characteristic of the central North Sea), the coastal neritic calanoid copepod Temora

longicornis and the appendicularian Oikopleura spp. showed broadly similar patterns of

annual abundance changes with a decline from 1973 to a low in 1979/80 (particularly

pronounced for Oithona and Oikopleura) and a recovery in numbers during the 1980s

with poor years in 1985186 (Fig. 6). Overall Oithona similis and Oikopleura spp. were

more abundant in 1980- 1988 than in 1970- 1979 but T. longicornis was more abundant

in the 1970s (Table II).

The two common coastal neritic calanoids Acartia clausi and A. longiremis were both

more abundant overall during the 1970s than the 1980s primarily due to high abun-

dances in 1974/75 (Fig. 7). A. clausi is remarkable for the constancy of the annual

geometric means of monthly abundances from 1976 to 1988, and no long-term trend

in annual abundances is apparent for A. longiremis over the period of sampling.

Two other common calanoid copepods, Pseudocalanus elongatus and Paracalanus

parvus, show some similarity in their year-to-year changes in abundances off Northum-

berland but no clear long-term trend over the two decades (Fig. 8). The first species

(. ‘.

‘. ‘. ;’

:. - 1 - . .;

‘.., ‘...

‘,.

-., . .._ “.-.....____

3: ,:’

__...‘. ._.‘.

‘k: . . .

‘. ., ‘.._.‘-- -..._

:’ _ ‘..

( . . .

_.. :.’

. . ‘.

,:’

I ‘.. :

_,..:._ -:::. . . . . ..____

. . . ,_:

.:’

: : ‘..,

. . . . . ..-’ ::.,

. . .

‘....,

“.._._

:.’ .,TY

zi .,.

,,..


Acartia clausi

80 1

60

21

-4

-- 3

--2 5 _.

--1 2 : __: 2

.1’ 0 - a

-’ 3 3

-- -2 5

-- -3

I

v 1 I

69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

Acartia longiremis

69 70 71 72 73 74 75 76 77 78 79 80 81 82 63 84 85 86 87 88 89

Fig. 7. Monthly and annual abundances of Acarricr clausi and A. hgi~mis for 1969-1988 (ZIX~S as Fig. 5).

was overall more abundant from 1980-1988 whilst the second was more abundant

from 1970-1979 (Table II).

The congeneric calanoids Centropages hamatus and C. typicus, the first of which is


Pseudocalanus elongatus

50

40

“E 2

L 30

b

fi

z” 20

10

0

25

20

“E 15 ;i ;i

lI

2 2 10

5

0

; .< : . . . .

. :. . : . . _.. : : ‘. : ‘ . . .’

1 . I . . .’

69 70 71 72 73 74 75 76 77 70 79 60 67 82 63 8.4 66 66 87 88 89

Paracalanus parvus

,’

,’ : .,.... I’ : ,’ ’ : ,’

: _’ : /

69 70 71 72 73 74 75 76 77 78 73 80 81 82 83 84 85 86 87 88 89

Fig. 8. Monthly and annual abundances of Pseudocaiatm e/orzprm and Purmr/mu.r pmw for 196% 19% (am as Fig. 5).

usually the commoner off Northumberland, show very difl’erent trends (Fig. 9). C&z-

tropuges hamatus is a neritic/estuarine species which is usually associated with cooler

less saline water than C. tpiczq which is considered to be primarily a temperate At-


- Centropages hamatus

.....-. Centropages typicus

@J -1 :.....................

-- D

-2 -

/

-3 I

69 70 71 72 73 74 75 76 77 78 79 a0 ai a2 a3 a4 a5 a6 a7 aa a9

Fig. 9. Annual geometric mean abundances of Crrzrroyqes hammus and C. typicus expressed as standard

deviations from the long-term mean over the period 1969-1988.

lantic oceanic species (Fransz et al., 1991). Annual abundances of C. hamatus have

increased since 1979 with particularly high numbers (up to 250-550 mm”) being re-

corded in the summer months of 1987-1988. By contrast C. typicus, which was occa-

sionally commoner than C. hamafus in the early 1970s has not been recorded since

1983.

Two copepods which are considered indicators of Atlantic water inflow into the

North Sea are Cunduciu armata and Rhincalanus nasutlrs (Fransz et al., 1991). Both

were very rare or absent from samples in the 1970s. C. urnzatu appeared off Northum-

berland in small numbers in October in 1980 and 1982-1984 and both species were

present in unusually high numbers in autumn samples (September-November) in 1985

and 1988 (Fig. 10). However, they were still uncommon with C. armuta reaching a

maximum density of only 7 me3 and R. nustltus only 0.6 m-‘. Metridiu kens is con-

sidered an indicator of inflow of mixed oceanic and coastal water from the north and

is near its southerly limit in the North Sea off Northumberland. It is rare in samples

but was seen in low numbers in 1970-1972 and 1986-1988 in winter and was unusually

common from February to May 1983 reaching densities of up to 25 m-s. A possible

indicator of temperate oceanic water is Anomalocera patersoni which in most years is

a rare constituent of hauls made in summer months, usually May-June. This species

was more abundant than usual in 1971-1972, 1984 and 1988 reaching densities of up

to 4 in+.


__ Candacia armata

-..-. Rhincalanus nasutus

4-

3 --

70 71 72 73 74 75 76 77 78 79 00 01 62 83 04 85 86

Fig. 10. Annual geometric mean abundances of Carzdidrr anmrfr~ and Rhitmlmuu rms~rt~rs

standard deviations from the long-term mean over the period 1970-1988.

87 88 89

expressed as

The appendicularian Fritillariu borealis which was a major contributor to the Bray-

Curtis dissimilarity between the 1970s and 1980s (Table II) was commoner during the

former decade. It was relatively abundant from 1973 to 1977, then declined in abun-

dance until 1982-1984 when it was not found in samples and finally showed some

recovery in numbers from 1985-1988 (Fig. 11). The highest abundances were seen in

May 1973 and 1975 when densities reached almost 2000 m -3.

DISCUSSION

In contrast to Austen et al. (1991) who found no evidence from MDS ordination of

a change in community structure in the plankton in the west-central North Sea between

the 1970s and 1980s we found clear evidence of such a change in the zooplankton with

a transition in community structure between 1979 and 1980. Their analysis used CPR

data from area C2 (LR and LE routes of ships of opportunity; Colebrook, 1986) and

included abundances of only nine entities, namely: Chaetognatha, Clione sp., decapod

larvae, Euphausiacea, Evudne sp;, Hyperiidae, phytoplankton colour, Podon sp. and

total copepods. Our analysis was restricted to the holoplanktonic fraction of the zoo-

plankton sampled at a single station 5 nautical miles off the Northumberland coast and

included abundances of 45 entities. Either the differences in geographic focus or in the

combination of entities included in the analysis may account for the divergent results

of the MDS ordinations.


69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

Fig. 11. Annual geometric mean abundances of Frifilhria hoveah expressed as standard deviations from the

long-term mean over the period 1969-1988.

Investigations of patterns of change of individual taxa indicated a complex pattern

with some species increasing and some decreasing in abundance between the 1970s and

1980s whilst others showed no obvious trend in mean annual abundances. ‘This

complexity is reflected in the fact that MDS ordination showed a clear transition in

community structure only using transformed abundance data, indicating that changes

were subtle (involving a wide range of species) and not just a result of fluctuations in

abundance in a few dominant species.

At the Northumberland benthic station (Buchanan & Moore, 1986) the change in

macrobenthos community structure reported by Austen et al. (1991) occurred between

1980 and 1981, one year after the community structure change reported here for the

zooplankton station. Likely reasons for such a lag are discussed by Austen et al. in the

context of some food-chain relationship between pelagos and benthos. However, Roff

et al. (1988) did not find any correspondence between benthic abundances and plank-

tonic copepod abundance or production off Northumberland and concluded that their

inter-annual cycles were independent of each other. Before discussing this further it is

useful to put the results in a wider context.

Although the 1969-1988 zooplankton datasct discussed here is a “long-term” one.

in comparison to data on long-term trends in weather patterns (e.g. Dickson et al.,

1975; Lamb, 1969) or to the CPR data (Colebrook, 1985, 1986) its period is short, not

even fully covering two sun-spot cycles. For the biological significance of the observed

changes in species abundances or community structure to be interpreted, they must be

viewed within the broader temporal and spatial scales of such longer datasets. For

example, Pseudoocahrzus elmgatus shows no clear trend in the 20-year segment of the


present dataset but was shown by Colebrook (I 985) to have declined in the North Sea

by 1982 to about 15% of numbers in 1950.

The key observation which must be considered is that in the eastern North Atlan-

tic (including area C2) there was a long-term decline in zooplankton abundance from

1950 to 1980 with some recovery during the 1980s (Colebrook, 1986). The present study

thus straddles a turning-point in the long-term trend in zooplankton abundance, with

its first decade at the end of a 30-year decline and its second during a period of re-

covery. In order to evaluate the present study’s results we must first briefly explore

hypotheses of the underlying causes of the long-term trend.

Dickson et al. (1988) have shown that during the period 1950-1980 there was an

increasing tendency for cold northerly winds to blow over the eastern North Atlantic

in winter and spring. This long-term trend appeared to reverse around 1980. This same

trend was recognised by Lamb (1969) as a complementary decrease in the frequency

of westerly weather over the British Isles from a peak in 1950; this trend also reversed

in 1980 (Fransz et al., 1991). Increased winter and spring storminess has been sug-

gested as likely to lead to a greater likelihood of mixing of phytoplankton below critical

depth and thus to delay in the onset of the spring phytoplankton bloom (cf. Glover

et al., 1972). Area C2 in particular suffered a marked increase in the frequency of strong

winds (lForce 8) between the 19.50s and 1970s (Lamb & Weiss, 1979) which was

correlated with a progressive decline in the amplitude of the spring phytoplankton

abundance-peak and a shift in its timing from April in the 1950s to May in the latter

part of the 1970s (Dickson et al., 1988). This trend was reversed during the 1980s.

Dickson et al. hypothesised that the delayed and decreased production of phytoplank-

ton in spring led to a reduction in both the carrying capacity of the surface waters for

zooplankton and the length of the growing season, and was thus a causal factor in the

long-term zooplankton decline.

CPR records of the phytoplankton index (Colebrook & Robinson, 1965) for area C2

for the period for which we have zooplankton data (Fig. 12) show a declining trend

with low amplitude abundance-peaks from 1969 to 1978 succeeded by an increasing

trend with generally high amplitude abundance-peaks from 1979 to 1988. Davies &

Payne (1984) discussed the flux of organic matter from pelagos to benthos during a

spring phytoplankton bloom in the North Sea. This flux was in the order of 20-25”~~

of overlying production before and after the spring bloom but rose to 35:; of produc-

tion during the bloom. Organic fluxes to the benthos may thus be disproportionately

enhanced by the occurrence of high-amplitude phytoplankton abundance-peaks (due

to greater underexploitation of the phytoplankton, cf. Colebrook, 1984), and the con-

verse may be true in periods of low-amplitude abundance-peaks such as occurred from

1969 to 1978 in area C2. On this basis one might infer a significant increase in the supply

rate of fresh organic carbon and nitrogen to the benthos in area C2 from 1979 onwards

when the monthly records of phytoplankton index moved from a “quiet” phase to a

“noisy” one. In the mid- to late-1970s this flux is likely to have been not only reduced

but also delayed (see above).


CPR data from area C2

27

69 70 71 72 73 74 75 76 77 78 79 80 ai a2 a3 a4 a5 a6 a7 aa a9

Fig. 12. Monthly measurements of phytoplankton index from CPR area C2 and annual mean index ex-

pressed as standard deviations from the long-term mean over the period 1969-1988. (Data courtesy of The Sir Alister Hardy Foundation for Ocean Science).

Given this background, a change in zooplankton community structure pivoted around

1980 is not entirely unexpected. The trend in overall abundance of holoplankton

sampled off Northumberland follows the trends seen for North Sea zooplankton gen-

erally (Fransz et al., 1991) with a nadir being reached around 1980. Austen et al. (1991)

tentatively favoured a water quality change explanation (increased organic pollutant

loadings) as opposed to a climatic one to account for their observations of a transi-

tion in community structures of Northumberland and Skagerrak macrobenthos and

eastern-central North Sea (area Cl) plankton in 1980-1981 and 1979-1980, respec-

tively. Part of the reason for this was that the plankton off the Northumberland coast

(area C2), which they considered primarily influenced by Atlantic water quality, as

opposed to that in the Skagerrak area which is influenced both by discharges of

north-west Europe’s rivers and relatively nutrient rich Baltic water (Elmgren, 1989),

showed no transition in community structure. They suggested that the Northumberland

benthic station, being relatively close inshore, might be affected by locally-induced

changes in water quality. Our results indicate a transition in inshore zooplankton

community structure in 1979-1980 and Fig. 12 shows a marked change in amplitude

of phytoplankton abundance-peaks in the wider, Atlantic water quality influenced, C2

area in 1978-1979. In the light of this we consider that a transition in community

structure ultimately related to long-term changes in North-East Atlantic weather pat-

terns is the more parsimonious explanation. Water quality changes such as eutrophi-

cation may be having effects as well, and it may be difficult to disentangle these effects

from those related to long-term climate trends where both factors act in concert.


ACKNOWLEDGEMENTS

We wish to thank the numerous people who have over 20 years helped with the

collection, sorting, identification and enumeration of the zooplankton samples. We are

also grateful to M. Austen and A. Lindley for helpful discussions and to M. Carr for

advice on implementation of the PRIMER software.

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Documents

Changes in the Zooplankton community off the coast of Northumberland between 1969 and 1988, with notes on changes in the phytoplankton and benthos