Comparison of Plankton-Water Chemistry Relationships in Three Acid Stressed Lakes in Nova Scotia,Canada

A field program was conducted in 1980 and 1981 on three lakes in Kejimkujik National Park, Nova Scotia, to study plankton-water chemistry relationships in a region of potential acid precipitation stress. Among lake and between year comparisons were made. The three study sites represent two different types of lakes. Kejimkujik Lake (pH 4.8) and Pebbleloggitch Lake (pH 4.5) are both high-colored, dystrophic acid lakes. Beaverskin Lake is a clear, oligotrophic lake which is not as acidic (pH 5.4). The phytoplankton of Kejimkujik Lake was dominated by diatoms, while Pebbleloggitch Lake was dominated by chlorophytes and chrysophytes, and Beaverskin Lake was dominated by cyanophytes. Kejimkujik Lake had the highest total algal cell volume per liter, and Pebbleloggitch Lake the lowest. Rotifer populations composed the majority of the zooplankton communities, while the crustacean zooplankton were dominated by the acid-tolerant copepod Diaptomus minutus. Kejimkujik Lake had the lowest zooplankton biomass and Pebbleloggitch Lake the highest zooplankton biomass per m³. A pattern of highest algal density in Beaverskin Lake, but highest zooplankton density in Pebbleloggitch Lake emerged in both years. This is likely an effect of abundant, but unsuitable food (blue-green algae) in Beaverskin Lake and important detrital food resources in Pebbleloggitch Lake in both years. Multiple regression analysis of water chemistry variables with plankton species produced many significant effects, but failed to show clear patterns. Cause and effect relationships in aquatic ecosystems are poorly delineated by such techniques.     

Cladocerans respond to differences in trophic state in deeper nutrient poor lakes from Southern Norway

We investigated the role of trophic state in structuring the cladoceran assemblages in 35 relatively deep lakes of low nutrient levels in Southern Norway. The lakes cover gradients of altitude, latitude, longitude, area and total phosphorus concentration. The environmental control of the cladoceran assemblages was analyzed using canonical correspondence analysis. Total phosphorus was the most important variable in the minimal adequate model. Hence, the cladoceran assemblage changed with trophic state. Typical clear water species were displaced by indicators of eutrofication with increasing trophic levels. Littoral species constituted the majority of the species recorded. Littoral and pelagic species richness showed a unimodal relationship with trophic state. Along with the change in trophic state and shift in cladoceran assemblage, we observed an increase in the ratio of pelagic to littoral species as well as in the ratio of pelagic efficient bacterial feeders to total pelagic filter feeders. The study indicates that zooplankton could provide a valuable indicator of ecosystem’s structure and function in deeper nutrient poor lakes if included in water quality assessments according to the EU Water Framework Directive. Including sampling of the littoral zooplankton would improve diversity estimates and incorporate the response of the littoral zone to eutrophication.     

Cladocera assemblages from the surface sediments of south-central Ontario (Canada) lakes and their relationships to measured environmental variables

Fossil cladoceran remains preserved in surface sediment samples from 44 oligotrophic lakes in south-central Ontario were examined to evaluate the relationships between species assemblages and measured environmental variables. Differences in cladoceran assemblages were related to physical and chemical variables using multivariate techniques. Redundancy Analysis (RDA) identified five environmental variables as significantly influencing assemblage composition: sulphate (SO₄²⁻), calcium (Ca²⁺), pH, maximum lake depth (Z _max) and dissolved organic carbon (DOC). There was a distinct separation of lakes and taxa along the ion gradient based on SO₄, Ca and pH. Additionally, cladoceran communities in coloured, shallow lakes had relatively higher abundances of littoral chydorid species and the pelagic taxa Holopedium spp., and the Daphnia pulex complex. Deep, clear lakes had relatively higher abundances of other pelagic taxa. Predation by fish (measured as presence–absence) and Chaoborus (measured as density) were less significant than some of the physico-chemical variables in influencing cladoceran assemblage structure. However, this could be due to the limited resolution of the predation data that was available at the time of this study. The distribution of cladocerans in the surface sediment, and their relation to these important environmental variables, suggests that there is considerable potential for the use of sedimentary cladoceran remains as environmental indicators in south-central Ontario lakes. Handling editor: J. Saros     

Long-term lake acidification trends in high- and low-sulphate deposition regions from Nova Scotia,Canada

Diatom-based paleolimnological techniques were used to study 14 lakes from two regions of Nova Scotia which represent regions of high and low sulphate deposition. Using decadal scale intervals, changes in diatom assemblages and diatom-inferred pH were tracked in relation to deposition of anthropogenic-sourced strong acids. Eight study lakes were located in Kejimkujik National Park in the southwestern part of the province, which receives an annual sulphate deposition (2000–2002) of ~10.5 kg ha⁻¹ yr⁻¹. These lakes showed significant changes in diatom assemblages with overall diatom-inferred acidification of ~0.5 pH units starting between 1925 and 1940, with the timing of acidification related to pre-industrial (or pre−1850) lakewater pH. Six study lakes were located in Cape Breton Highlands National Park, in northern Nova Scotia, a region of lower sulphate deposition. These lakes did not show any consistent trends in diatom assemblages or inferred pH values consistent with recent acidic deposition, but rather variations that may be related to climatic influences. Nova Scotia lakes that have been most impacted by acidic deposition had the lowest pre-industrial lakewater pH values and were in an area of relatively high sulphate deposition. Handling editor: K. Martens     

Correlation of surface sediment diatoms with the present lake water pH in 28 Algoma lakes,Ontario, Canada

The surface sediment diatom analysis of 28 Algoma lakes (pH 4.40–8.13) indicates that even though each lake has a widely different aquatic environment and characteristic diatom assemblage, a definite relationship exists between the lake water pH and their diatom assemblages. In the acidic lakes acidobiontic and acidophilous diatom species predominate whereas in circumneutral and alkaline lakes circumneutral and alkaliphilous diatoms were most common. Cluster analysis of the pH indicator diatom assemblages grouped the study lakes into three distinct cluster groups. These groups also closely corresponded to lake water pH. On the basis of published ecological information as well as their presence in our study lakes, the pH indicator status of a number of diatom taxa have been discussed. A detailed listing of the diatom taxa identified and their pH indicator status is provided in order to facilitate their use in future diatom-inferred pH studies.     

Patterns in the Distribution of Cladocerans (Crustacea: Branchiopoda) in Lakes Across a North–south Transect in Alaska, USA

The remains of cladocerans were examined from the surface sediments of 51 freshwater sites along a north–south transect spanning Alaska. We identified 27 cladoceran taxa from the sediments, consisting primarily of littoral chydorid species. Variations in cladoceran assemblages were related to measured physical and chemical variables using multivariate techniques. Redundancy analysis (RDA) indicated that lake depth, total phosphorus (TP), and altitude all had a significant influence in determining the composition of cladoceran assemblages. Cladoceran communities in tundra and forest-tundra lakes, which were relatively shallow and nutrient-poor, had relatively low abundances of pelagic Cladocera, and were primarily composed of several littoral chydorid species. Among pelagic cladoceran species, there was a distinct shift in dominance from the Bosminidae in lakes in the southern boreal forest region to Daphniidae in lakes in the northern boreal forest. Daphnia dominated lakes had significantly higher total phosphorus, specific conductivity, and calcium concentrations than lakes dominated by Eubosmina. Overall, the relative importance of physical and chemical factors in structuring cladocerans is similar to other previously studied regions, and suggests the Cladocera may be useful as ecological and paleoenvironmental indicators in this region.     

Distribution of cladoceran assemblages across environmental gradients in Nova Scotia (Canada) lakes

Cladocera sub-fossils have widely been recognized as useful environmental indicators. Nevertheless, investigations into the distribution and environmental controls on Cladocera are scarce in North America, hindering their use in paleolimnological studies. Here, we examine cladoceran assemblage and size structure from the surface sediments of 49 soft-water Nova Scotia (Canada) lakes to provide ecological data on this key taxonomic group, the first study of its kind for the northern Atlantic coast of North America. We found that Nova Scotia lakes contain a diverse chydorid assemblage, but were generally dominated by the smaller pelagic herbivore Eubosmina longispina. Daphnia, a larger pelagic herbivore, was relatively uncommon in these lakes. Redundancy analyses (RDA) identified maximum lake depth and dissolved organic carbon (DOC) as the environmental variables that best explained the structuring of these cladoceran assemblages. Generalized linear models were then used to better characterize the ecological associations for individual taxa. Body size of the bosminids in these lakes was significantly correlated only to total nitrogen (TN), with larger bosminids in lakes with lower TN values. Bosminid mucro length, an indication of invertebrate and fish predation pressure, was significantly related to TN, maximum lake depth, and lake surface area. The ecological information provided by this study should assist the interpretation of paleolimnological assessments of environmental change in these and other similar lakes.     

Littoral benthic macroinvertebrates of mountain lakes in the Tatra Mountains (Slovakia,Poland)

Littoral benthic macroinvertebrates of 45 mountain lakes in the Tatra Mountains were sampled using a semi-quantitative method in September 2000. A total of 32,852 specimens were identified to 93 taxa belonging to 14 higher taxonomic groups. Multivariate statistics (CCA, RDA) and nine biotic metrics (AQEM/STAR) were used to explain relationships between macroinvertebrate assemblages and environmental variables. Up to 57% of the ecological position of littoral macroinvertebrate assemblages were explained by variance of environmental variables divided into chemical, trophic, physical, catchment and location. Five types of Tatra lakes were recognized using CCA: A — strongly acidified lakes (small catchment, low pH, high concentration of TP, DOC, highest amount of POM in littoral); B — alpine acidified lakes (low amount of POM, low values of biotic metrics); C — alpine non-acidified lakes (high value of diversity index, predominance of Diptera); D — subalpine acidified lakes (high values of biotic metrics: number of families, proportion of crenal and rhithral taxa/total taxa); E — subalpine non-acidified lakes (high values of biotic metrics: number of families, number of genera, BMWP score, number of taxa and abundance of EPT taxa). RDA was used to design five levels of macroinvertebrate taxa acidification tolerance. The Tatra Acidification Index (TAI) was established to assess the acidification status of the lakes in the Tatra Mts.     

Recovery of copepod, but not cladoceran, zooplankton from severe and chronic effects of multiple stressors

In the mid‐twentieth century, many lakes near Sudbury, Canada, were severely contaminated by acid and metal emissions from local smelters. For example, in the early 1970s, Middle Lake had pH of 4.2, and Cu and Ni levels both >0.5 mg L^?1 . To determine if crustacean zooplankton could recover from such severe and chronic damage, Middle Lake was neutralized in 1973. A comparison of its zooplankton with that of 22 reference (pH > 6) lakes indicates that the planktonic Copepoda completely recovered by 2001. In contrast, the cladoceran assemblage improved but did not recover. Colonist sources existed – Cladocera and Copepoda occurred with equal frequency in area lakes – but six separate colonizations by cladoceran species failed. We argue that local factors, metal toxicity and predation by yellow perch, have, to date, prevented cladoceran recovery. Nonetheless, the complete copepod recovery is encouraging, given the severity and duration of pre‐neutralization stress.     

Elevated metal concentrations inhibit biological recovery of Cladocera in previously acidified boreal lakes

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RELATIONSHIPS BETWEEN LITTORAL DIATOMS AND THEIR CHEMICAL ENVIRONMENT IN NORTHEASTERN GERMAN LAKES AND RIVERS1

We explored statistical relationships between the composition of littoral diatom assemblages and 21 chemical and physical environmental variables in 69 lakes and 15 river sites in the lowland of northeastern Germany. Canonical correspondence analysis with single treatment and with forward selection of environmental variables was used to detect 11 important ecological variables (dissolved inorganic carbon [DIC], Na ⁺ , total phosphorus [TP], dissolved organic carbon [DOC], total nitrogen [TN], pH, oxygen saturation, dissolved iron, SO₄^{2 ?} , NH₄ ⁺ , soluble reactive silicium) and maximum water depth or Ca^{2 +} or soluble reactive phosphorus that most independently explain major proportions of the total diatom variance among the habitats. Monte Carlo permutation tests showed that each contributed a significant additional proportion (P < 0.05) of the variance in species composition. Together, these 11 most important environmental variables explained 34% of the total variance in species composition among the sites and captured 73% of the explained variance from the full 21 parameters model. Weighted‐averaging regression and calibration of 304 indicator taxa with tolerance down‐weighting and classic deshrinking was used to develop transfer functions between littoral diatoms and DIC, pH, TP, TN, and Cl ^? . The DOC:TP ratio was introduced and a weighted‐averaging model was developed to infer allochthonous DOC effects in freshwater ecosystems. This diatom‐DOC/TP model was significant (P < 0.001) and explained 7.6% of the total diatom variance among the sites, surpassing the inferential power of the diatom‐TP‐transfer function (7.3% explained variance). The root‐mean‐square errors of prediction of the models were estimated by jack‐knifing and were comparable with published data sets from surface sediment diatom samples. The data set of littoral diatoms and environmental variables allows use of the diatom‐environmental transfer functions in biomonitoring and paleolimnological approaches across a broad array of natural water resources (such as floodplains, flushed lakes, estuaries, shallow lakes) in the central European lowland ecoregion.     

Shifts in taxonomical and guild composition of littoral diatom assemblages along environmental gradients

Diatoms are commonly and frequently used as water quality indicators, but only a few studies have been done to evaluate the importance of littoral, contemporary diatoms as bioindicators. This study aims to determine the main predictors of diatom community composition from 73 Swedish lakes. Canonical correspondence analysis (CCA) revealed pH, phosphate, nitrite/nitrate levels, longitude and percentage of forest in the catchment to be the most important factors of 51 environmental variables for structuring diatom assemblages. Cluster analysis separated the lakes into three groups based on the diatom community composition. Lakes belonging to these groups were characterised as: (1) acidic, nutrient-poor; (2) circumneutral, nutrient-poor and (3) alkaline, nutrient-rich, according to the results of a discriminant function analysis and dominant diatom taxa revealed by similarity percentage analysis. Ecological guilds according to growth morphology and the ability of nitrogen-fixation were assigned to all diatom taxa. All three lake groups exhibited a distinct guild composition. Nitrogen-fixing diatoms were found in nutrient-rich lakes, only. Our results indicate that taxonomical composition of littoral diatom assemblages can be applied in the assessment of nutrient and acidity status of Swedish lakes. Differences in distribution of the ecological guilds were connected to several environmental factors such as nutrients, light and grazing; their application in assessment of trophic status of lakes is therefore precarious.     

Diatom assemblages and their relationship to environmental variables in lakes from the boreal forest-tundra ecotone near Yellowknife,Northwest Territories,Canada

The relationship between diatom (Bacillariophyceae) taxa preserved in surface lake sediments and measured limnological and environmental variables in 22 lakes near Yellowknife (N.W.T.) was explored using multivariate statistical methods. The study sites are distributed along a latitudinal gradient that includes a strong vegetational gradient of boreal forests in the south to arctic tundra conditions in the north. Canonical correspondence analysis (CCA) revealed that lakewater concentrations of dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) each accounted for independent and statistically significant proportions of variation in the distribution of diatom taxa. Weighted-averaging (WA) models were developed to infer DIC and DOC from the relative abundances of the 76 most common diatom taxa. These models can now be used to infer past DIC and DOC concentrations from diatom assemblages preserved in sediment cores of lakes in the Yellowknife area, which may provide quantitative estimates of changes in lakewater chemistry related to past vegetational shifts at treeline.     

Littoral macroinvertebrates of acidified lakes in the Bohemian Forest

Mountain lakes in the Bohemian Forest, on both the Czech and German sides, were atmospherically acidified mainly in the 1960s–1980s and have since been recovering from acidification. In 2007, we performed the first complete study on littoral macroinvertebrates in all eight lakes. The goals of the study were to 1) compare macroinvertebrates in the lakes during the process of recovery and 2) investigate relations between the occurrence of taxa and water chemistry. Lake water pH varied from 4.6 to 5.7, concentrations of dissolved reactive Al and labile Al ranged from 118–601 and 11–470 μg L^?1, respectively, and DOC concentrations were < 6 mg L^?1. Altogether 73 taxa were identified from all lakes; a positive relationship was found between pH and the number of macroinvertebrate taxa. The highest number of taxa was found in the least acidic lakes Laka and Grosser Arbersee, including the mollusk Pisidium casertanum. In contrast, the lowest diversity was found in the most acidified ?ertovo jezero. Cluster analyses of macroinvertebrates and water chemistry suggested pH as the key factor influencing the occurrence of macroinvertebrate taxa. An interesting finding was the occurrence of the boreo-montane water beetle Nebrioporus assimilis in Prá?ilské record of this species in the Czech Republic since 1960.     

Paleolimnological investigation of three manipulated lakes from Sudbury,Canada

Stratigraphic changes in diatoms and chrysophytes from three manipulated Sudbury lakes were explored in an attempt to examine the influence of fertilization and/or neutralization on algal microfossil assemblages. Both diatom- and chrysophyte-inferred pH profiles indicate that the pH of Labelle Lake has remained fairly stable in the past. The study of Labelle and Middle lakes indicates that the addition of nutrients to acidic and non-acidic oligotrophic lakes did not directly influence diatom and chrysophyte species composition, perhaps because pH remained stable. The diatom and chrysophyte assemblages of Middle Lake only changed when the pH was raised. In Mountaintop Lake the recent shift in chrysophyte species composition and the resulting inferred pH decline is most likely related to a decline in mid-summer epilimnetic pH. Reliable paleolimnological inferences are difficult in lakes such as these because it is difficult to track limnological conditions in the absence of modern analogues.     

The role of cladocerans in tracking long-term change in shallow lake trophic status

Shallow lakes have been affected by a variety of human activities profoundly altering their ecological structure and function. Cladocerans have been used to track change resulting from a variety of drivers at a number of time scales. Aquatic macrophytes are well recognised as reflecting the ecological condition of a lake. Here, we compare the plant macrofossils with the sub-fossil cladoceran assemblages from 20 dated sediment cores. Co-correspondence analysis was used to determine the degree of commonality of change in community composition of the two biological groups through time. This analysis revealed very high levels of agreement in the nature and timing of change at all the sites examined with very high correlation coefficients between the axis 1 scores for macrofossils and cladocerans. Furthermore, at all sites a high proportion of the variance (min 20%, max 54%) in the macrofossil data was explained by the change in the cladoceran assemblage. Sub-fossil macrofossil and cladoceran assemblages, from at least from 1700 AD onwards, were examined in more detail at three sites: Ormesby Great Broad, Felbrigg Lake and Lake Søbygaard. There was very good accord in the main shifts of the cladoceran and macrofossil assemblages at all three sites. This may reflect the long-term shift in the principal focus of primary production from the benthic to the pelagic habitat. We suggest that the combination of their central position in the food-web and the presence of both pelagic and benthic taxa make cladocerans a strong candidate as the single best indicator of (palaeo) ecological condition related to changing trophic status and alteration in food-web structure in shallow lakes.     

Responses of zooplankton and zoobenthos to experimental acidification in a high-elevation lake (Sierra Nevada, California, U.S.A.)

SUMMARY. 1. During the summer of 1987 we conducted an acidification experiment using large enclosure at Emerald Lake, a dilute, high-elevation lake in the Sierra Nevada, California, U.S.A. The experiment was designed to examine the effects of acidification on the zooplankton and zoobenthos assemblages of Sierran lakes.
2. Treatments consisted of a control (pH 6.3) and pH levels of 5.8, 5.4, 5.3, 5.0 and 4.7; each treatment was run in triplicate. The experiment lasted 35 days.
3. The zooplankton assemblage was sensitive to acidification. Daphnia rosea Sars emend. Richard and Diaptomns signicauda Lilljeborg decreased in abundance below pH 5.5–5.8, and virtually disappeared below pH 5.0. Bosmina longirostris (Müller) and Keratella taurocephala Ahlstrom became more abundant with decreasing pH. although B. longirostris was rare in the pH 4.7 treatment. These species might serve as reliable indicators of early acidification in lakes such as Emerald Lake.
4. The elimination of D. rosea in acidified treatments probably allowed the more acid-tolerant taxa to increase in abundance because interspecific competition was reduced. Even slight acidification can therefore alter the structure of the zooplankton assemblage.
5. In contrast to the zooplankton, there was no evidence that the zoobenthos in the enclosures was affected by acidification.     

Long-term change of the littoral Cladocera in the Tatra Mountain lakes through a major acidification event

In our study, we focused on littoral Cladocera living and feeding in shallow shore parts of 46 mountain lakes in the Tatra Mountains (Slovakia and Poland). The studied lakes underwent a major acidification event in the 1980s and are now in the process of recovery. Lakes were divided into three categories based on their sensitivity to acidification: 5 extremely sensitive (ES), 11 acid sensitive (AS), and 30 non-sensitive (NS) lakes. In our study, we included historical data from the literature, and data from sediment core and littoral samples, which together represent the evolution of the littoral communities from a pre-industrial period up to the present. In total, 11 littoral species were found belonging to three cladoceran families. Most of the species were members of the family Chydoridae: Alona affinis, A. quadrangularis, A. rectangula, A. guttata, Acroperus harpae, Alonella excisa, A. nana, Chydorus sphaericus, and Eurycercus lamellatus. One species belonged to each family Daphniidae (Ceriodaphnia quadrangula) and Polyphemidae (Polyphemus pediculus). The most numerous littoral taxa were Alona affinis, Acroperus harpae, and Chydorus sphaericus. All species reacted to decreased pH levels during peak acidification in the 1980s by disappearing from most of the lakes of all categories; the only persisting species was Chydorus sphaericus. Most species returned to the lakes when pH started to increase in the 1990s, although their return was noticeably slower in AS lakes. Alona quadrangularis decreased its distribution range over the studied period; Polyphemus pediculus was mostly detected in the 1910s only. The number of species was highest in all lake categories when dwarf pine was present in the lake catchment. On the whole, the littoral community was richest in NS lakes.     

Distribution patterns of Cladocera in subarctic Fennoscandian lakes and their potential in environmental reconstruction

Multivariate statistical techniques were used to examine the relationships between surface-sediment cladoceran assemblages and 28 physical and chemical variables in 53 small subarctic lakes from northern Fennoscandia. The lakes were distributed along a steep eco-climatic gradient, spanning boreal corniferous forest to treeless tundra. In general, the sites were small, oligotrophic, and bathymetrically simple, with little or no disturbance in their catchments. From the initial 53 localities, only 36 contained a sufficient number of cladoceran remains for reasonable quantification. From these, a total of 29 cladoceran taxa representing 19 genera were identified, comprising predominantly littoral chydorid species. A constrained redundancy analysis (RDA) and associated Monte Carlo permutation tests indicated that maximum lake depth, sediment organic content, epilimnetic summer temperature, lake perimeter, and lake catchment area made statistically significant (p le; 0.05) contributions to explaining the variance in the cladoceran taxon data. These five variables together accounted for 67.7% of the explained variance, and made a unique contribution of 26.8% to the total variance: all physical determinants independently captured 33.2% of the total variance. The significance of the most powerful explanatory variables is discussed in the paper in detail, and autecological information regarding the most common cladoceran taxa is given. To assess the potential of cladoceran assemblages in environmental reconstruction, quantitative inference models for mean July water and air temperatures were developed for the cladoceran assemblage using partial least squares (PLS) regression. The final prediction model yielded a root mean squared error of prediction (RMSEP). as assessed by jackknifing, of 1.19°C for Cladocera-water temperature data-set, whereas the cladoceran assemblages showed only very weak relationships to mean July air temperature. The overall results emphasize the role of physical factors in regulating species abundance and distributions in these environmentally sensitive ecotonal lakes.     

Declines in littoral species richness across both spatial and temporal nutrient gradients: a palaeolimnological study of two taxonomic groups

1. Using a palaeolimnological approach in shallow lakes, we quantified the species richness responses of diatoms and Cladocera to phosphorus enrichment. We also examined differences in species richness responses between littoral and pelagic assemblages of our focal communities. To address both spatial and temporal relationships, our study includes an analysis of both surface sediments from 40 lakes and of a lake sediment record spanning c. 120 years. The objective of our study was to determine whether similar species richness patterns occurred across trophic levels, as well as along spatial and temporal gradients. 2. We found that both diatom and Cladocera species richness estimates significantly declined with increasing phosphorus across space and through time. When the assemblages were subdivided according to known habitat preferences, littoral biodiversity maintained a negative trend, whereas pelagic species richness tended to show no relationship with phosphorus. 3. Negative productivity–diversity patterns have been observed across almost all palaeolimnological studies that span large productivity gradients. This congruence in patterns is most likely due to the similarity in data collection methods and in focal communities studied. The contrasting responses between littoral and pelagic assemblages may be explained by the differences in physical habitat and the pool of taxa in each of these environments. Consistent with the literature, we found statistical support for the idea that littoral diversity declines could be explained by an interaction between macrophytes and nutrients along strong trophic gradients. The general lack of a diversity response in our pelagic assemblages could be attributable to the limited pool of subfossil taxa. The response of the pelagic diatom could also be related to their broad range of nutrient tolerances. 4. The observed negative response of species richness to phosphorus enrichment, particularly in the littoral assemblages, has implications for ecosystems functioning because communities with reduced biodiversity often are less resilient to anthropogenic change.