Gradient responses of epilithic diatom communities in the ...proper in spring when seasonal diatom abundance is high. Di-atom taxa vary in size over several orders of magnitude. Large

Estuarine, Coastal and Shelf Science 68 (2006) 661e674www.elsevier.com/locate/ecss

Gradient responses of epilithic diatom communitiesin the Baltic Sea proper

Anna Ulanova a,b, Pauli Snoeijs a,*

a Department of Plant Ecology, Evolutionary Biology Centre, Uppsala University, Villavägen 14, SE-752 36 Uppsala, Swedenb Department of Botany, Biology and Soil Sciences Faculty, Sankt Petersburg State University, Universitetskaya emb. 7/9, St. Petersburg 199034, Russia

Received 10 November 2005; accepted 28 March 2006

Available online 26 May 2006

Abstract

Diatom communities and assemblages are widely used as indicators of ecological change in aquatic environments and for reconstructingpalaeo-environments. Good calibration data sets, directly linking changes in diatom composition to environmental factors, are needed for build-ing reliable gradient models with high indicative value. Such models have a broad applicability because most diatom species have cosmopolitandistributions. This paper presents community analyses of brackish-water diatoms living on submerged stones in four areas of the Baltic Seaproper along the Swedish coast. The communities on the stones were composed of epilithic, epiphytic, epipelic, epipsammic and occasionallysome pelagic species. Altogether, 158 quantitative samples were taken at 41 sites between 23 April and 11 May, 1990, and the data set contained300 diatom taxa belonging to 75 genera. Species data were analysed with principal components analysis (PCA) and environmental factors werefitted passively by multiple regression analysis on the ordination results. Differences in community composition could be explained by variationin salinity (which was correlated with N:P and Si:P ratios and the occurrence of macroalgae on the stones), nutrient concentrations and variationin exposure to wave action (which was correlated to the occurrence of soft bottoms around the stones, water temperature and the occurrence ofsand grains and macroalgae on the stones). Separate analyses for small taxa (cell biovolume

662 A. Ulanova, P. Snoeijs / Estuarine, Coastal and Shelf Science 68 (2006) 661e674

in the Bothnian Bay, 5 in the Bothnian Sea, 5e10 in the BalticSea proper, 10e15 in the western Baltic Sea, 15e25 in theKattegat to 25e34 in the Skagerrak (Snoeijs, 1999).

The present study is a continuation of our previous work ondiatom communities along the Swedish Baltic Sea coast,which was started in 1990. Snoeijs (1994, 1995) found that sa-linity was the major environmental factor structuring epiphyticdiatom communities between salinity 4 and 12 with a sharptransition at salinity 5e6 of communities with >95% diatomswith freshwater affinities to >95% diatoms with marine affin-ities. In two previous studies of epilithic communities in theBothnian Bay (salinity 0.4e3.3) and the Bothnian Sea (salinity5), Busse and Snoeijs (2002, 2003), found that epilithic dia-toms were more abundant in lower salinity, while macroalgae,and thereby epiphytic diatoms, were more abundant in highersalinity. They also showed that ‘‘small’’ taxa (biovolume

663A. Ulanova, P. Snoeijs / Estuarine, Coastal and Shelf Science 68 (2006) 661e674

Fig. 1. Map of the Baltic Sea area, showing the sampling areas (Areas 2, 3, 4 and 5) in the Baltic Sea proper, and isohalines of surface salinity.

5 (Himmerjärden, 12 sites), following the area numbering sys-tem used in a larger sampling programme (Snoeijs, 1994,1995; Busse and Snoeijs, 2002, 2003). All samples were col-lected between 23 April and 11 May 1990. One sampling sitewas defined as a 10 m long stretch of waterline at 0.2e0.7 mof depth. The stones sampled were ca. 25e30 cm in diameterand were taken randomly within the given limits for samplingsite and stone size. Within each area, the sites represented gra-dients from the mainland to the outer archipelagos. The siteswere selected to include a maximum variation in exposureto wave action. Quantitative samples were taken from thestones according to the procedure described by Snoeijs andSnoeijs (1993). The samples, each representing all biomassfrom a 55.4 cm2 circular area, were removed from the stonesby brushing with distilled water. They were transferred topolyethylene containers and fixed with formalin in the field.Altogether, 158 quantitative samples were taken from sub-merged stones in the upper hydrolittoral zone. Normally fourreplicate samples were collected at each site, but from threesites only two replicates were taken (Sites 27, 28, 43). Sam-pling sites 25, 26, 30 (Area 3), 34, 38e40, 42, 43 (Area 4)were situated on small archipelago islands, and the rest ofthe sites on the mainland.

Salinity and water temperature were measured in situ witha Yellow Springs Instruments TCS-meter, Model 33 (YellowSprings, OH, USA). Salinity was measured using the PracticalSalinity Scale. Exposure to wave action, beach type (the ter-restrial part not covered by water) and soft bottom coverage(between the stones below the water line) were recorded on or-dinal scales as estimates based on observations in the field(Table 2). Estimation of macroalgal cover in the field wasnot possible because small filamentous algae, especially thebrown alga Pilayella littoralis could not be distinguishedfrom diatom colonies by eye. Water samples were taken atall sites, about 0.5 m above the algal vegetation, frozen imme-diately, and later analysed for concentrations of dissolvednitrogen (DIN: NO2-N þ NO3-N), dissolved phosphorus(DIP: PO4-P) and dissolved silicate (DSi: SiO2-Si) at the lab-oratory of the Swedish Environmental Protection Board in Up-psala, according to the methods described in Anonymous(1965, 1991) and Schuster (1969).

2.3. Subsampling procedure

The samples were processed in 2002. They contained vary-ing amounts of macroalgae, microalgae, zoobenthos, silt and


sand that created the microenvironment for the diatom com-munities. After 24e36 h of sedimentation in the polyethylenesampling containers, as much as possible of the supernatantwas removed. The containers with the samples were vigor-ously shaken to remove as many as possible of the epipsammicdiatoms from the sand grains, poured through a 1 mm meshsieve, and rinsed with distilled water. From the fraction inthe sieve, zoobenthos >1 mm was sorted out. Macroalgaeand filamentous cyanobacteria in the same fraction were cutfinely with a pair of scissors. Finally, all material was returnedto the suspended fraction, which consisted mainly of unicellu-lar algae, silt and sand grains. The sand grains were separatedfrom the rest of the sample as well as possible by vigorousshaking and repeated 5 s sedimentation, until the sedimenta-tion of sand grains became negligible. The sand fraction wasdried at 85 �C, incinerated in a muffle oven at 550 �C for24 h, kept in a desiccator while cooling, and weighed. Theash weight of this fraction was used as an environmental factorin the data analysis, to illustrate the amount of sand grains asthe habitat availability for epipsammic diatoms.

The algal biomass fraction of the samples was divided intosubsamples with a quantitative subsampling device (Busse andSnoeijs, 2003). The device was made of a cylindrical polysty-rene container of 140 mm height and a volume of 1000 mlwith a watertight screw lid. The container had an inner diam-eter of 9.9 cm and contained five small (30 ml) polystyrenecylinders that were 6.5 cm high and open on the top, attachedto the bottom of the container with epoxy glue. The smallcylinders had an inner diameter of 2.6 cm at the top. Eachof the five subsamples sedimenting inside the open cylinderscontained 6.9% of the original sample. A subsample thusrepresented a stone surface of 3.82 cm2. The algal biomassfraction of the samples was transferred to the subsampling de-vice with distilled water and a drop of detergent; it was thenshaken vigorously to allow random distribution of biomassin the container before being kept still for 36 h to sedimentevenly. After sedimentation, most of the water column wasremoved from the subsampler through a hole of 0.6 mm in di-ameter, situated 2 cm from the bottom of the container. Thesmall inner cylinders were simultaneously emptied throughsimilar holes in their outer walls (Busse and Snoeijs, 2003).

Table 2

Ordinal scales (estimates based on observations in the field) used for exposure

to wave action, beach type and soft bottom coverage

Exposure to wave

action

Beach type (the

terrestrial part not

covered by water)

Soft bottom coverage

(between the stones

below the waterline)

1 ¼ Stagnant water 1 ¼ >90% sand 1 ¼ 25% sand

and >25% stones

2 ¼ 1e10%

3 ¼ Little exposed 3 ¼ >90% stones< 50 cm in size

3 ¼ 11e25%

4 ¼Medium exposed 4 ¼ >90% stones>50 cm in size

4 ¼ 26e75%

5 ¼ Highly exposed 5 ¼ >25% stonesand >25% solid rock

5 ¼ 76e90%

6 ¼ >90% solid rock 6 ¼ >90%

The algal biomass, together with a small water residue, wastaken out with a plastic Pasteur pipette with a wide opening.Of the five subsamples obtained, one was dried on a filterand is kept in the algal herbarium of the Department of PlantEcology, Uppsala University, three were used for biomassmeasurements, and one for diatom species identification.

2.4. Biomass measurements

For dry weight (DW) measurements, three subsamples weretransferred to porcelain weighing trays, dried in a ventilatedoven at 85 �C, kept in a desiccator while cooling, and weighed.Thereafter, the subsamples were incinerated in a muffle oven at550 �C for 24 h, kept in a desiccator while cooling, andweighed again. The ash weight was subtracted from the DWto obtain the ash-free dry weight (ADW). From DW andADW, a relative ash-free dry weight measure was derived,also known as relative ignition loss (ADW% ¼ ADW �100%/DW). DW is a measure of biomass including organicand inorganic matter, and ADW refers to the organic fractionof the DW, so that ADW% provides a relative measure of theorganic content of the DW. ADW% is assumed to be high fordiatom-poor algal communities (containing more macroalgae)and low for communities dominated by diatoms (containingless macroalgae), because silica, the main component of the di-atom frustules, persists at 550 �C. Thus, DW, ADWand ADW%reflect different aspects of the algal standing crop on the stones.

2.5. Species identification and counting of valves

One subsample per sample was dried in a ventilated oven at85 �C, oxidized with 30% H2O2 and a pinch of K2Cr2O7, andwashed four times with distilled water after sedimentation in-tervals of 24 h. A dilution series of each diatom valve suspen-sion was made, and drops of nine different concentrationswere dried on cover glasses and mounted in Hyrax�. The rel-ative abundance of diatom species was recorded at �1000magnification by light microscopy with a Nikon �100 Pla-nApo oil immersion objective. Taxonomy and identificationin the present study follow Snoeijs (1993), Snoeijs and Vil-baste (1994), Snoeijs and Potapova (1995), Snoeijs andKasperovi�cien _e (1996), and Snoeijs and Balashova (1998).For species not treated by these authors, the freshwater floraby Krammer and Lange-Bertalot (1986, 1988, 1991a,b) andthe brackish-water treatise by Witkowski et al. (2000) weremainly used. Taxonomy follows the system suggested byRound et al. (1990). The vast majority of the taxa recordedwere species, but some nominal forms and varieties were re-corded separately and some closely related species that weredifficult to distinguish from each other were merged. Theterm ‘‘species’’ is used flexibly throughout this paper to denotedifferent taxa, including these varieties and merged species.

For identification and counting of diatom valves, one out ofthe nine slides per sample was selected, in which there were ca.10e20 valves per microscopic field. For classification as smallor large diatom species, the mean biovolumes of Snoeijs et al.(2002) for the Baltic Sea diatoms were used. In one count


(Count 1a), a total of 250 valves were enumerated, identifiedand recorded as either small (


Tab

le4

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0.0

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P<

0.0

01

Ex

po

sure

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chS

oft

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pS

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ity

DIN

DIP

DS

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N:P

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NS

and

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AD

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%R

ich

SR

ich

SL

Ric

hL

(rS)

(rS)

(rS)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

(rP)

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0.5

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0.4

1**
and more rocky coasts, less sand grains occurred on the stones
(Table 4). Exposure to wave action was not associated with anynutrient concentration or nutrient ratios (rS: P > 0.05).

3.2. Biomass

The algal DW on the stones varied between 3 and60 mg cm�2 among the 41 sampling sites and ADW between0.5 and 8.3 mg cm�2 (Fig. 2A,B). DW, but not ADW, variedsignificantly among the sampling areas (ANOVA: P ¼ 0.045),with mean DW being highest in Area 5 (21.5 mg cm�2; Table3). DW, but not ADW, was weakly negatively correlated to sa-linity and exposure to wave action (rP ¼ �0.30 and rS ¼ �0.35,respectively), indicating that biomass was lower with highersalinity and higher exposure to wave action.

The indicator for the presence of macroalgae, ADW%, var-ied between 10% and 67% among the 41 sampling sites(Fig. 2C). ADW% also varied significantly among the samplingareas (ANOVA; P < 0.001), being lowest in Area 5 (25%, indi-cating lower occurrence of macroalgae in this area) and highestin Area 3 (53%, indicating higher occurrence of macroalgae inthis area). ADW% was positively correlated to salinity and ex-posure to wave action (rP ¼ 0.40 and rS ¼ 0.45, respectively),indicating that the occurrence of macroalgae in the sampleswas higher with higher salinity and higher wave action.

The occurrence of sand grains on the stones was positivelycorrelated to biomass, both to DW and ADW, and negativelyto ADW%, salinity and exposure to wave action (Table 4).This indicates that sites exposed to wave action had lesssand grains and relatively more macroalgae on the stones,but lower biomass.

3.3. Diversity and species composition

Altogether, 300 taxa belonging to 75 genera, 165 small and135 large taxa, were identified in the 158 samples analysed.The 46 most abundant diatom taxa encountered are listed inTable 5. The most abundant small species were Nitzschia in-conspicua (14.7% of all valves counted in Count 1a), Naviculaperminuta (11.8%) and Rhoicosphenia curvata (10.6%). Themost abundant large species were Tabularia fasciculata(62.7% of all valves counted in Count 2), Cocconeis pediculus(4.9%) and Navicula lanceolata (4.4%).

The sample with the highest species richness (52A, Area 5,Count 1a) included 28 small and 10 large taxa. This count re-corded 170 small and 80 large specimens, which gives a smallto large (S:L) ratio of 2.1. Small species in sample 52A weredominated by Nitzschia inconspicua (16.4%) and largespecies by Tabularia fasciculata (24%). Site 52A had expo-sure level 4, beach type 3 and soft-bottom coverage 3. It hadthe lowest concentration of DIN of all 41 sampling sites(0.21 mmol L�1), and also a rather low concentration ofDIP (0.5 mmol L�1), but the fourth highest concentration ofDSi (7.5 mmol L�1), the maximum of all 41 sites being8.6 mmol L�1. Samples 15B and 18A (Area 2, count 1a)had the lowest species richness (14). Sample 15B contained10 small and four large species and was completely


0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 55 56 57

DW

(g c

m-2

)

0

1

2

3

4

5

6

7

8

Sand

(g c

m-2

)Sa

nd (g

cm

-2)

Sand

(g c

m-2

)

A

0.000

0.002

0.004

0.006

0.008

0.010

0.012

15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 55 56 57

ADW

(g c

m-2

)

0

1

2

3

4

5

6

7

8B

0

10

20

30

40

50

60

70

80

15 16 17 18 19 20 21 22 23 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 55 56 57

Sampling site

ADW

%

0

1

2

3

4

5

6

7

8C

Fig. 2. Site means of (A) DW (bars) and sand weight (dots); (B) ADW (bars) and sand weight (dots); and (C) ADW% (bars) and sand weight (dots). Error

bars ¼ Standard error of the mean. White bars, Area 2; light grey bars, Area 3; dark grey bars, Area 4; black bars, Area 5.

dominated by Tabularia fasciculata (41.2%). Among thesmall species in sample 15B Navicula perminuta was mostabundant (19.2%). This count recorded 139 small and 111large specimens, which gives a S:L of 1.25. Site 15 had ex-posure level 5, beach type 6 and soft-bottom coverage 1.Sample 18A contained 11 small and three large species.The most abundant large species was Tabularia fasciculata(40.8%) and the most abundant small species was Rhoicos-phenia curvata (27.2%). This count recorded 144 small and106 large specimens, which gives a S:L of 1.35. Site 18had exposure level 4, but a completely reverse situation com-pared with Site 15 with regard to beach type (1) and soft-bot-tom coverage (6). Sites 15 and 18 had similar nutrientconditions with low DIN concentrations (0.35 and

0.43 mmol L�1, respectively), medium-high DIP concentra-tions (0.23 and 0.65 mmol L�1, respectively) and low DSiconcentrations (1.0 and 2.9 mmol L�1, respectively).

Diatom species richness (RichSL) was weakly positively cor-related to soft bottom coverage (rS ¼ 0.34) and water temperature(rP ¼ 0.31), but not to exposure to wave action directly. RichSLwas negatively correlated to salinity (rP ¼�0.41) and positivelyto DSi (rP ¼ 0.43) and the amount of sand grains on the stones(rP ¼ 0.45). RichSL increased with higher biomass (DW,rP ¼ 0.45) and decreased with higher presence of macroalgae(ADW%, rP ¼ �0.56). These correlations of diversity with envi-ronmental factors and algal occurrence and composition weremainly regulated by the small species (RichSL � RichS:rP ¼ 0.92; RichSL � RichL: rP not significant).


Table 5

List of the 46 most abundant diatom taxa identified in the 158 samples. S ¼ small species with biovolume 0.5% in data set ‘‘small species only’’ or ‘‘small and large species together’’ or ‘‘large species only’’ are includedSpecies name Abbreviation Biovolume group

Amphora pediculus (Kützing) Grunow in A. Schmidt et al. Amp pedi S

Berkeleya rutilans (Trentepohl) Grunow Ber ruti SCocconeis pediculus Ehrenberg Coc pedi L

Cocconeis placentula Ehrenberg Coc plac S

Cocconeis scutellum Ehrenberg Coc scut S

Cocconeis stauroneiformis (W. Smith) Okuno Coc stau SCtenophora pulchella (Ralfs ex Kützing) Willams and Round Cte pulc L

Diatoma moniliformis Kützing Dia moni S

Epithemia sorex Kützing Epi sore LEpithemia turgida (Ehrenberg) Kützing Epi turg L

Epithemia turgida var. westermannii (Ehrenberg) Grunow Epi tuwe L

Fragilaria hyalina var. durietzii Cleve-Euler Fra hydu S

Gomphonema olivaceum (Hornemann) Brébisson Gom oliv LGomphonemopsis exigua (Kützing) Medlin Gos exig S

Grammatophora oceanica Ehrenberg Gra ocea L

Licmophora gracilis var. anglica (Kützing) H. and M. Peragallo Lic gran L

Martyana atomus (Hustedt) Snoeijs Mar atom SMelosira lineata (Dillwyn) C.A. Agardh Mel line L

Melosira moniliformis (O.F. Müller) C.A. Agardh Mel moni L

Melosira nummuloides C.A. Agardh Mel numm L

Navicula bottnica Grunow in Cleve and Möller Nav bott LNavicula gregaria Donkin Nav greg S

Navicula lanceolata (C.A. Agardh) Ehrenberg Nav lanc L

Navicula perminuta Grunow in Van Heurck Nav perm SNavicula phyllepta Kützing Nav phyl S

Navicula ramosissima (C.A. Agardh) Cleve Nav ramo S

Navicula rhynchotella Lange-Bertalot Nav rhyn L

Navicula sjoersii Busse and Snoeijs Nav sjoe SNitzschia frustulum (Kützing) Grunow in Cleve and Grunow Nit frus S

Nitzschia inconspicua Grunow Nit inco S

Nitzschia microcephala Grunow in Cleve and Möller Nit micr S

Opephora krumbeinii Witkowski, Witak and Stachura Ope krum SOpephora mutabilis (Grunow) Sabbe and Vyverman Ope muta S

Planothidium delicatulum (Kützing) Round and Buktiyarova Pln deli S

Pseudostaurosira brevistriata ‘‘small’’ (Grunow) Williams and Round Pss br01 SPseudostaurosira zeillerii (Héribaud) Williams and Round Pss zeil S

Pteroncola inane (Giffen) Round in Round et al. Pte inan S

Rhoicosphenia curvata (Kützing) Grunow Rho curv S

Stauronella indubitabilis Lange-Bertalot and Genkal Stn indu LStaurosira elliptica (Shuman) Williams and Round Sts elli S

Staurosira punctiformis Witkowski, Metzeltin and

Lange-Bertalot in Witkowski et al.

Sts punc S

Staurosira subsalina (Grunow) Williams and Round Sts subs SStaurosirella pinnata (Ehrenberg) Williams and Round Str pinn S

Surirella brebissonii Krammer and Lange-Bertalot Sur breb L

Tabularia fasciculata (C.A. Agardh) Willams and Round Tab fasc L

Tabularia waernii Snoeijs Tab waer S

The ratio of small to large specimens (S:L) was correlated toDIN (rP ¼ 0.82, Table 4). However, this was caused by the factthat large species were nearly absent from a few sites with ex-tremely high DIN concentrations. S:L was also positively cor-related to RichL (rP ¼ 0.41, Table 4), which indicates thatspecies richness of large diatoms is higher when small diatomsdominate. In only 6 out of the 158 samples (4%) large diatomsdominated or were equally abundant as were small diatoms(S:L 0.5e1.0), in 61% of the samples S:L was 1.1e5.0, in16% of the samples it was 5.1e10.0 and in 19% of the samplesit was above 10.0. Site 27, situated close to a sewage plant, was

very different from all other sites with extremely high S:L of124 and 249 (only two replicate stones were taken at thissite). The large species dominating here was Melosira lineataand the small species were dominated by Martyana atomus(9e20%) and Amicula speculum (14e20%).

3.4. Principal components analysis

The eigenvalues of the first four PCA ordination axes were0.28, 0.20, 0.10 and 0.08, respectively, for the small speciesdata set (Data set S, Fig. 3), 0.26, 0.23, 0.11 and 0.06 for


-3.0

3.0

-2.9 3.5

Area 2

Area 3

Area 4

Area 5

Axis 2A

Axis 1

RichSSi:NTemp

DSi

S:L

Beach

ADW%DIN

Sed DW

ADW

Si:PSand

N:PExp

DIPSalin

AREA 5

AREA 3

AREA 2

-1.4

1.4

-1.4 1.7

AREA 4

C

Axis 1

Axis 2

B

Str pinn

Ber ruti

Rho curv

Nav phyl

Pln deliNit frus

Gos exigCoc plac

Nav perm

Dia moni

Sts elli

Sts punc

Nit micr

Ope krumMar atom

Pss zeil Pss br01

Ope muta

Fra hydu

Amp pedi

Sts subs

Tab waer

Coc stau

Nav ramo

Coc scut

Pte inan

Nav gregNav sjoe

Nit inco

-0.9

0.9

-0.9 1.1

Axis 1

Axis 2

Motile

Epipsammic

Epiphytic

Fig. 3. Small species (cell biovolume 0.5% in Data set S (for abbreviations see Table 3); and (C) centroids for nominal-scale variables and biplot scores (arrows) for interval-scale

and ordinal-scale variables tested passively on the results of the PCA by multiple regression analysis. The arrows indicate the directions and rates of change of the

variables.

the data set of small and large species together (Data set SL,Fig. 4), and 0.68, 0.08, 0.05 and 0.03 for the large speciesdata set (Data set L, Fig. 5). As shown by these eigenvalues,all analyses yielded four ordination axes, which subsequentlydecreased in importance from Axis 1 to Axis 4. For Data set L,Axis 1 dominated completely while for Data set SL, Axes 1and 2 were of almost equal importance, suggesting a largervariability in the response patterns of the diatom communitiesto environmental variables.

The sample scores were clearly separated into groups of sitesaccording to the different sampling areas and the salinity

gradient, for Data sets S and L mainly along Axis 1 (Figs. 3A,5A), and for Data set SL mainly along Axis 2 (Fig. 4A). The bi-plot scores (arrows) fitted passively on the results of the PCA ex-press the direction and relative importance (arrow length) ofthese factors in the analysis. Salinity was the strongest factorin Data set S, relatively strong in Data set SL, but weak inData set L (Figs. 3C, 4C, 5C). In all three data sets, the biplotscores of the environmental factors showed similar patternswith salinity and the amount of sand grains on the stones in op-posite directions and DSi at a ca. 90 � angle to salinity (Figs. 3C,4C, 5C). Similarly, exposure to wave action was always opposite


-1.9

4.1

-2.5 2.8

Area 2

Area 3

Area 4

Area 5

AAxis 2

Axis 1

AREA 4

AREA 3

AREA 5

AREA 2

RichLS

Beach

ADWSalin

Sand

N:P

Si:P

SedExp

DIP

S:LDSi

DIN

ADW%

DW

TempSi:N

-1.3

1.4

-0.7 1.2

CAxis 2

Axis 1

B

Pln deliNav sjoe

Nav lancNav perm Nit micr

Nit frusNav greg

Dia moni

Ber ruti Sts subs

Pss br01

Nit inco

Cte pulc

Tab waer Ope muta

Mar atomOpe krum

Pss zeil

Str pinn Sts elliSts punc

Coc pediNav ramoCoc stau

Coc scut

Rho curv Pte inan

Tab fasc

Nav phyl

-0.5

1.1

-0.9 0.8

Axis 1

Axis 2Motile

Epipsammic

Epiphytic

Fig. 4. Small and large species together (cell biovolume 0.5% in Data set SL (for abbreviations see Table 3); and (C) centroids for nominal-scale variables and biplot

scores (arrows) for interval-scale and ordinal-scale variables tested passively on the results of the PCA by multiple regression analysis. The arrows indicate the

directions and rates of change of the variables.

to DIN and ADW%. However, exposure was in the same direc-tion as sand for Data set L (Fig. 5C) but not for Data sets S and SL(Figs. 3C, 4C), while water temperature was at different anglesto DIN for the three data sets. Species richness of small speciesand large and small species together was fitted opposite to salin-ity (Figs. 3C, 4C), but species richness of the large species wasalmost in the same direction as salinity (Fig. 5C).

The species scores of Berkeleya rutilans, Cocconeis pedicu-lus, Fragilaria hyalina var. durietzii, Stauronella indubitabilisand Tabularia waernii were associated with higher salinityand those of Gomphonema olivaceum and Nitzschia

inconspicua, Planothidium delicatulum with lower salinity(Figs. 3B,C, 5B,C). Small epipsammic diatoms like Opephorakrumbeinii, Martyana atomus, Staurosira elliptica, Staurosirapunctiformis and large epipelic diatoms like Navicula rhyncho-tella and Surirella brebissonii were associated with low expo-sure to wave action and high DIN and DSi concentrations andthe epiphyte Gomphonema olivaceum and some small Naviculaspecies, Navicula sjoersii and Navicula gregaria, with highexposure and low concentrations of DIN and DSi (Figs. 3B,C,4B,C, 5B,C). Typical epiphytes like Cocconeis pediculus,C. scutellum, C. stauroneiformis, Ctenophora pulchella,


-3.7

4.3

-1.5 2.8

Area 2

Area 3

Area 4

Area 5

AAxis 2

Axis 1

Salin

ADW%

DW

Temp

N:PADW

RichL

DINDSi

S:L

AREA 5

AREA 2

AREA 4AREA 3

Si:N

SandExp

Beach

DIPSi:P

Sed

-1.0

1.2

-0.6 1.1

C

Axis 1

Axis 2

B

Gom oliv

Mel numm

Sur breb

Nav lanc

Epi sore

Nav rhyn

Nav bottStn indu

Epi tuweLic granEpi turg

Gra ocea

Mel moni

Coc pedi

Cte pulc

Tab fasc

Mel line

-0.7

0.8

-1.1 0.7

Axis 1

Axis 2

Motile

Epipsammic

Epiphytic

Fig. 5. Large species (cell biovolume �1000 mm3): PCA ordination diagrams for the first two axes, showing (A) sample scores; (B) scores of the 17 species withrelative abundance >0.5% in Data set L (for abbreviations see Table 3); and (C) centroids for nominal-scale variables and biplot scores (arrows) for interval-scale

and ordinal-scale variables tested passively on the results of the PCA by multiple regression analysis. The arrows indicate the directions and rates of change of the

variables.

Rhoicosphenia curvata, Tabularia fasciculata and Tabulariawaernii were placed in the direction of higher salinity (Figs.3B,C, 4B,C), except for the data set L (Fig. 5B,C) in which largeepiphytic diatoms dominated.

4. Discussion

4.1. Gradient responses of biomass

In certain years during the spring bloom in April, epilithicmicrophytobenthic DW (mainly diatoms) can be as high as

60 mg cm�2 in the southern Bothnian Sea (Snoeijs, 1990).For comparison, this is about 10% of the maximum DW ofthe perennial macroalga Fucus vesiculosus L. in the northernBaltic Sea proper (Kautsky et al., 1992). However, exceptfor the northernmost Area 5 (mean ADW% ¼ 26%), our epi-lithic communities had a higher proportion of filamentousmacroalgae already in AprileMay (mean ADW% 36%,42%, 53% in three areas) compared to the Bothnian Sea(mean ADW% 26%, 33%, 35% in three areas; Busse andSnoeijs, 2003) and the Bothnian Bay (mean ADW% 11%,20%, 20%, 28% in four areas; Busse and Snoeijs, 2002).


Snoeijs (1994) reported ADW% values of ca. 40% as typicalfor a pure sample of epilithic colonial diatoms in mucilagetubes (mainly Berkeleya rutilans) and ca. 80% for a pure sam-ple of filamentous macroalgae. Sommer (1998) provideda mean value for diatoms in plankton of 45%. However, for ep-ilithic diatom communities (including many more small spe-cies than colonial diatoms) values are generally lower, ca.21% for the spring blooms in March-May of the years1983e1989 in the southern Bothnian Sea (Snoeijs, 1990). Inepilithic samples small inorganic sediment grains and dead di-atom frustules may also lower ADW%. We have interpreted anincrease in ADW% as an increase in macroalgal cover becausemacroalgae visibly occurred in many samples. This measure isnot commonly used for benthic algae, although it is rathercommon in studies of zoobenthos (e.g. Lappalainen and Kan-gas, 1975; Andersin, 1986) and sediments (e.g. Underwood,1997; Heider and Scharf, 2001).

We found that the epilithic communities contained more di-atoms and less macroalgae when biomass was high and thatsites more exposed to wave action had less sand grains and rel-atively more macroalgae on the stones, but lower biomass. Thesame was found by Busse and Snoeijs (2003) for three areas inthe Bothnian Sea and our results confirm that this is a generaltrend for epilithic communities in the Baltic Sea in spring.Negative relationships between microphytobenthic biomassand water movement have previously been described (Fieldinget al., 1988; Delgado et al., 1991; Kendrick et al., 1996).Hoagland (1983) showed that water turbulence can lead to bio-mass losses of up to 80% and Shaffer and Sullivan (1988) sug-gested that high water-column primary productivity at wave-exposed sites can be largely caused by resuspension of thebenthic diatom flora as 90% of the species in the water columnwere benthic pennates.

We also found that with higher salinity relatively moremacroalgae were found on the stones. This is a general trendalong the whole Swedish east coast stretching from ca. 56 �

N (salinity 7e8) to 66 � N (salinity below 1). In the north(Bothnian Bay) the epilithic communities in the end of Mayconsist mainly of diatoms (Busse and Snoeijs, 2002) whereasin the south filamentous macroalgae such as Pilayella littora-lis, Dictyosiphon foeniculaceus (Huds.) Grev. Cladophoraglomerata (L.) Kütz. and Ceramium gobii Wærn are alreadywell established in the hydrolittoral zone in April. This isa combined result of higher water temperature in the southand ice scouring, which is heavier in the north. The ice scrapesoff the basal parts of the filamentous macroalgae from thestones so that they will have to colonise anew each year inthe north while in the south they can start growing from lastyear’s basal parts immediately in spring when conditions be-come favourable (Snoeijs and Prentice, 1989; Snoeijs, 1999).Thus, the effect of salinity cannot be separated from climaticforces along the northesouth Baltic Sea gradient.

4.2. Gradient responses of species diversity

At first glance, species richness seems low in our samples,area means 16e22 for small diatoms and 7e12 for large ones.

However, it should be kept in mind that our values are for 250(Data sets S and SL) or 125 (Data set L) valves counted. Infloristic studies, including selective searching for ‘‘rare’’ spe-cies, one would certainly detect more diatom taxa, comparableto e.g. Simonsen findings in the brackish Schlei estuary in thewestern Baltic Sea of on average 80 species per sampling site(Simonsen, 1962). In previous studies Busse and Snoeijs(2002, 2003) found area means of 22e38 for small diatomsand 16e29 for large ones in epilithic diatom communities.We suspect that this is an effect of the amount of macroalgaeon the stones, given the negative correlations between ADW%and species richness (Table 4).

The observed decrease in diatom species richness with in-creasing salinity towards the south is probably also an effectof the increasing abundance of macroalgae and therefore in-creased dominance of a few epiphytic diatom species and lowerspecies richness in our counts. Snoeijs (1994, 1995) did not finddifferences in species richness of epiphytes between the Both-nian Bay, Bothnian Sea and the Baltic Sea proper and thereforeconcluded that there does not seem to be a minimum in diatomspecies richness in the brackish Baltic Sea as observed in manyother groups of organisms, e.g. macroalgae (Snoeijs, 1999).This absence of a minimum in diatom species richness alsoseems to be valid for epilithic diatoms; altogether we identified300 taxa in 158 samples, which is comparable to 290 taxa in151 samples in the Bothnian Bay (Busse and Snoeijs, 2002)and 218 taxa in 120 samples in the Bothnian Sea.

Especially Area 2 had low species richness due to the largedominance of the epiphytes Tabularia fasciculata and Rhoi-cosphenia curvata. Similar observations were made by Busseand Snoeijs (2002) in the Bothnian Bay where decreasingspecies richness from north to south was explained by largerdominance of Diatoma spp., which usually grows as an epi-phyte (Snoeijs and Potapova, 1998). Diatom species richnessincreased when the amount of sand grains on the stonesincreased and the abundance of macroalgae decreased becauseof the general higher diversity of sediment-living species com-pared to epiphytic species. We found that species richness oflarge diatoms was higher when small diatoms dominated thecommunities. This reflects the higher diversity of large epi-pelic species when small epipsammic diatoms dominate incontrast to the lower diversity of large epiphytic specieswhen epiphytes dominate.

4.3. Gradient responses of community composition

Snoeijs et al. (2002) put forward the idea that the small spe-cies in a diatom community may respond differently to envi-ronmental variation than the large species in the samecommunity because of the enormous size range in diatom cells(21 mm3 to 14�106 mm3 for 515 Baltic Sea taxa, Snoeijs et al.,2002) and size-related differences in growth rates and lifeforms. For the Bothnian Bay, which has a pronounced salinitygradient, salinity was found to be the major environmentalfactor affecting diatoms �1000 mm3 (with exposure to waveaction as second factor), while exposure was the major factoraffecting diatoms


(Busse and Snoeijs, 2002). In the present study, the hypothesisthat small and large diatoms respond differently to environ-mental variables was confirmed. However, opposite to theBothnian Bay study we found that the small diatoms re-sponded more to salinity and less to exposure to wave actionthan the large diatoms. Exposure to wave action and factorscorrelated to this (water temperature, amount of sand grainsand macroalgae on the stones) were most important for thelarge diatoms. The major differences between the BothnianBay data set of Busse and Snoeijs (2002) and the BalticSea proper data set (this study) are the salinity gradients,0.4e3.3 and 3.5e7.8, respectively, and the amount of macro-algae on the stones. Busse and Snoeijs (2002) recorded 36%non-raphid or monoraphid epiphytes among the most abundantlarge diatoms while we recorded 53%. This illustrates thehigher amount of attached epiphytes in relation to motile spe-cies in the epilithic diatom communities in the Baltic Seaproper. The epiphytes in the Baltic Sea show strong host de-pendency (Snoeijs, 1994; Snoeijs et al., 2002) and the abun-dance of macroalgae is highly dependent on exposure towave action, as can be seen from the opposite arrows for ex-posure and ADW% in our ordinations (lower macroalgal coverwith a higher degree of exposure). This may explain why ourlarge diatoms mainly responded to exposure to wave actionand less to salinity. Our results suggest that it is useful to an-alyse small and large diatoms separately because they showdifferent ecological responses, which may provide a deeperunderstanding of community processes.

4.4. A calibration data set for gradient models

A practical application for which our data can be used isthat of building a diatom-based salinity inference model(¼transfer functions) for reconstructing palaeo-environmentsin the Baltic Sea basin. This area has experienced dramatic sa-linity changes with several lacustrine and marine stages duringthe past 12,000 years (Björck, 1995). However, diatom com-munities and assemblages are naturally affected by a multitudeof environmental factors simultaneously and statistically sig-nificant salinity transfer functions do not guarantee that thereis an underlying causal mechanism and it is important toalso consider alternative explanations. Previous studies haveshown that salinity, pH and phosphorus yield transfer func-tions with higher significance than transfer functions for e.g.temperature (Wilson et al., 1996; Anderson, 2000; Bloomet al., 2003). In our study we found a strong relationship be-tween salinity and community changes, but also climaticforces, macroalgal cover and exposure to wave action contrib-uted to these changes. To develop transfer functions with ro-bust and reliable estimates of species optima and tolerancesfor the Baltic Sea area, our data must be complemented withdata representing more sites and a longer salinity gradient.

Acknowledgements

We are grateful to Annette Axén who provided excellenttechnical assistance in the laboratory and to Erik Borén for

help with maps and geographical coordinates. The researchpresented here was financially supported by the Swedish Re-search Council (VR), the Swedish Environmental ProtectionAgency and the Royal Swedish Academy of Sciences (KVA,Grant for Cooperation between Sweden and the former SovietUnion). A.U. wishes to thank the staff and students at theDepartment of Plant Ecology, Uppsala University, for supplyingresearch facilities and creating an inspirational atmosphereduring her stay in Uppsala.

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Gradient responses of epilithic diatom communities in the Baltic Sea properIntroductionMaterial and methodsArea descriptionSamplingSubsampling procedureBiomass measurementsSpecies identification and counting of valvesData analysis

ResultsEnvironmental variablesBiomassDiversity and species compositionPrincipal components analysis

DiscussionGradient responses of biomassGradient responses of species diversityGradient responses of community compositionA calibration data set for gradient models

AcknowledgementsReferences

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Gradient responses of epilithic diatom communities in the ...proper in spring when seasonal diatom abundance is high. Di-atom taxa vary in size over several orders of magnitude. Large