Effects of acidic pH and phytoplankton on survival and condition of Bosmina longirostris and Daphnia pulex

The effects of pH, algal composition and algal biomass on abundance, size, reproduction and condition of Daphnia pulex and Bosmina longirostris were tested in a field experiment using water and natural phytoplankton assemblages from a circumneutral (pH 6.43) and a moderately acidic (pH 5.75) lake in south-central Ontario. Both species were affected by pH and phytoplankton composition, with decreased egg production, lipid reserves, body size or abundance in treatments containing algae and/or water from the more acidic lake compared to treatments containing water and phytoplankton from the circumneutral lake. This result was unexpected for Bosmina, which often increases in relative and/or absolute abundance in acidified lakes. The negative effect of acidic conditions on Bosmina suggests that the population increase observed in most acidified lakes is not due to a positive response to low pH or ambient phytoplankton, but to altered biotic interactions possibly involving reduced competition.     

Inorganic carbon fluxes at the water–sediment interface in five littoral systems in Spain (southern Europe)

The assemblage of littoral oligochaetes in six crater lakes in Central Mexico, was studied throughout a yearly cycle. To establish species composition, richness, density and biomass, 14 localities were sampled in the lakes. A total of eight species belonging to the families Naididae (five species), Tubificidae (two species), and Enchytraeidae (one species) were found. The dominant species, Limnodrilus hoffmeisteri, contributed with up to 99% in both abundance and biomass. Sediment organic matter is the most important environmental variable explaining the differences in density and biomass. Seasonal (dry and rainy seasons) changes were not significant for density and biomass. Higher density and lower biomass values characterized these lakes in contrast to other tropical and subtropical lakes worldwide. The small size of the dominant species L. hoffmeisteri was recorded in all lakes and explained the low biomass recorded in the area of study. The correlation between L. hoffmeisteri and four other species (Dero (Dero) nivea, D. (D.) digitata, Nais variabilis and Tubifex tubifex) was negative. The naidid species were positively correlated (>0.5) to each other.     

Susceptibility to freshwater acidification by two species of loon: Red-throated Loon (Gavia stellata) and Arctic Loon (Gavia arctica) in southwest Sweden

In southwest Sweden, the two species of loon, Gavia stellata and G. arctica, have shown different trends in population size and production of young during the last decades. Both species fish in oligotrophic freshwaters, susceptible to acidification. The number of breeding sites occupied by G. stellata has been reduced by almost 50% during the last 40–50 years. For G. arctica, there are no indications of significant declines in population size or reproductive success during the last 20 years. The different trends in numbers and production of young might reflect different susceptibility to the ecological changes in acidified lakes. G. stellata prefer fishing lakes with high abundance of Acerina cernua and salmonid and cyprinid fish, such as Coregonus albula and Rutilus rutilus. They also feed their prefledged chicks almost entirely on cyprinid and salmonid fish. G. arctica prefer fishing lakes with high transparency and, when feeding in groups, high abundance of Perca fluviatilis. Their young can be fed on aquatic insects as a supplement to the fish diet. Thus, G. stellata to a higher degree than G. arctica relies on fish which are susceptible to low pH, and G. arctica may also benefit from the increased abundance of aquatic insects in lakes with reduced predation from fish. Furthermore, high water transparency is important for the selection of lakes by G. arctica but not by G. stellata. In G. stellata, high contents of mercury in eggs can be related to the intake of fish in lakes affected by acidification.     

Seasonality of planktonic ciliated protozoa in 20 subtropical Florida lakes of varying trophic state

The planktonic ciliate populations of 20 Florida lakes ranging from oligotrophic to hypereutrophic were examined monthly for one year. Oligotrophic lakes displayed abundance peaks during fall mixis and biomass peaks in late winter and fall. Mesotrophic systems exhibited a spring-fall bimodality in ciliate abundance with a biomass maxima occurring during fall. Eutrophic/hypereutrophic lakes had pronounced abundance and biomass maxima during summer, with the large ciliates Plagiopyla nasuta and Paramecium trichium often contributing heavily to the midsummer biomass peak. Members of the Oligotrichida numerically dominated abundance and biomass peaks in oligotrophic lakes while the Scuticociliatida dominated the communities of higher trophic states. Total ciliate abundance and biomass were strongly correlated with chlorophyll a concentrations as were various ciliate taxonomic groups. The relationship between ciliate seasonal distribution in these subtropical lakes with lake thermal regimes and trophic state is discussed.     

Zooplankton biomass rarely improves predictions of chlorophyll concentration in Canadian Shield lakes that vary in pH

To determine the frequency with which zooplankton influence chlorophyll a (Chla) levels, we explored annually-averaged data from oligotrophic and mesotrophic lakes that differed in morphometry, total phosphorus (TP) concentrations, and zooplankton community composition due to pH. The data were divided into two sets according to the type of filter used to collect chlorophyll. Residuals of the Chla: TP regressions were not related to lake morphometry, TN content, water clarity or pH. In the first data set there were no consistent relationships between residuals in Chla and twelve grazer biomass variables for 37 of the 38 lakes. The single exception had a very large population of Daphnia dubiaand low concentrations of Chla for its TP. In the second data set, 3 of 25 lakes had exceptionally low Chla concentrations for their TP. These lakes were acidic (pH < 6) and had very large biomasses of Holopedium gibberumcorrelated with negative Chla residuals, indicating significant grazing. At pH > 6, Daphnia spp. strongly influenced the significant correlations. We conclude that zooplankton contribute to the prediction of Chla beyond that possible by TP alone in acidic and non-acidic Canadian Shield Lakes, but evidence for strong suppression of chlorophyll by grazers was relatively rare (4 of 63 cases) on annual time steps.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

The Seasonal Dynamics and Composition of Photosynthetic Picoplankton Communities in Temperate Lakes in Ontario,Canada

The seasonal abundance and composition of photosynthetic picoplankton (0.2-2 μm) was compared among five oligotrophic to mesotrophic lakes in Ontario. Epilimnetic picocyanobacteria abundance followed a similar pattern in all lakes; maximum abundance (2-4 × 10⁵ cells · ml⁻¹) occurred in late summer following a period of rapid, often exponential increase after epilimnetic temperatures reached 20 °C. In half of the lakes picocyanobacteria abundance was significantly correlated with temperature, while in other lakes the presence of a small spring peak resulted in a poor correlation with temperature. In all lakes there was a significant correlation between epilimnetic abundance and day of the year. Correlations with water chemistry parameters (soluble reactive phosphorus, total phosphorus, particulate C: P and C: N) were generally weaker or insignificant. However, in the three lakes with the highest spring nitrate concentrations, a significant negative correlation with nitrate was observed. During summer stratification, picocyanobacteria abundance reached a maximum within the metalimnion and at or above the euphotic zone (1% of incident light) in all lakes. These peaks were not related to nutrient gradients. The average total phytoplankton biomass ranged from 0.5 g m⁻³ (wet weight) in the most oligotrophic lake to 1.4 g m⁻³ for the most mesotrophic with picoplankton biomass ranging from 0.01 g m⁻³ to 0.3 g m⁻³. Picocyanobacteria biomass comprised 1 to 9 % of total phytoplankton biomass in late summer, but in one year for one lake represented a maximum of 56%. Other photosynthetic picoplankton (unidentified eukaryotes, Chlorella spp. Nannochloris spp.), although less abundant (10³ cells · ml⁻¹) than picocyanobacteria, represented biomass equal or greater than that of the picocyanobacteria in spring and early summer. On average, half of the photosynthetic picoplankton biomass was eukaryotic in the more coloured lakes, while in the clear lakes less than 20% was eukaryotic. Among the lakes there was a significant positive correlation between the average light extinction coefficient and the proportion of eukaryotic biomass of the picoplankton. In mesotrophic Jack's Lake, the contribution of picoplankton to the maximum photosynthetic rate ranged from 10 to 47% with the highest values in the spring (47%) and late summer (33%), as a result of eukaryotic picoplankton and picocyanobacteria respectively. Picocyanobacteria cell specific growth rates were high during July (0.6-0.8 day⁻¹) and losses were close to 80% of the growth rate. Thus, despite low biomass, photosynthetic picoplankton populations appeared to turn over rapidly and potentially contributed significantly to planktonic food webs in early spring and late summer.     

Rapid and unexpected effects of piscivore introduction on trophic position and diet of perch (Perca flavescens) in lakes recovering from acidification and metal contamination

1. Yellow perch (Perca flavescens) are often the only surviving fish species in acidified lakes. We studied four lakes along a gradient of recovery from acidification and that had different food web complexities. All had abundant yellow perch, two had low piscivore abundance, one had a well‐established piscivore population and one was manipulated by introducing piscivorous smallmouth bass (Micropterus dolomieu). We hypothesised that there would be strong effects on perch abundance, behaviour and diet induced by the presence of piscivores. 2. In the manipulated lake, the bass reduced yellow perch abundance by 75% over a 2‐year period. Concomitantly, perch use of the pelagic habitat fell from 48 to 40%. 3. In contrast to findings from less disturbed systems, yellow perch in the littoral zone of the manipulated lake did not strongly shift from zooplankton to benthic food sources after the arrival of piscivores. Diet analysis using stable carbon isotopes revealed a strong continued reliance on zooplankton in all lakes, independent of the degree of piscivory. The failure to switch to benthos in the refuge area of the littoral zone is most likely related to the depauperate benthos communities in these formerly acidified lakes. 4. Yellow perch in lakes recovering from acidification face a considerable ecological challenge as the necessary switch to benthic diet is hindered by a low abundance of benthos. The arrival of piscivores in these recovering lakes imposes further restrictions on perch access to food items. We infer that future recovery of perch populations (and higher trophic levels) will have to be preceded by the re‐establishment of diverse benthic macroinvertebrate communities in these lakes.     

Changes in phytoplankton,benthic invertebrate and fish assemblages of boreal lakes following invasion by Gonyostomum semen

1. During recent decades, Gonyostomum semen populations have spread in northern temperate regions forming dense blooms that may dominate the phytoplankton assemblage for extended periods. In this study, we investigate the effects of G. semen blooms in boreal brown water lakes with special emphasis on phytoplankton, fish and benthic invertebrate assemblages using data from 10 boreal lakes sampled annually over a 10‐year period. 2. Significant differences in phytoplankton and benthic invertebrate assemblages were found between lakes with high (3.01 mm³ L^?1; >80% phytoplankton biomass) and lakes with low G. semen biomass (0.03 mm³ L^?1; <5% phytoplankton biomass). In particular, high G. semen lakes had lower biomass of smaller, edible phytoplankton and a higher abundance and biomass of benthic invertebrates, especially Chaoborus flavicans, and perch than low G. semen lakes. 3. The length distribution of fish also suggested a tendency towards large and older fish and a lower recruitment success in high G. semen lakes, as denoted by lower abundances of fish shorter than 10 cm and higher biomass and abundance of fish longer than 15 cm in high G. semen lakes. 4. This study shows that high G. semen lakes are characterised by less edible phytoplankton, dominance by a few species and enhanced benthic secondary and fish production. Hence, the conjecture that high biomasses of G. semen create a bottleneck in the energy transfer to higher trophic levels seems less likely in boreal lakes.     

Bird abundance and species richness on Florida lakes: influence of trophic status,lake morphology,and aquatic macrophytes

Data from 46 Florida lakes were used to examine relationships between bird abundance (numbers and biomass) and species richness, and lake trophic status, lake morphology and aquatic macrophyte abundance. Average annual bird numbers ranged from 7 to 800 birds km^–2 and bird biomass ranged from 1 to 465 kg km^–2. Total species richness ranged from 1 to 30 species per lake. Annual average bird numbers and biomass were positively correlated to lake trophic status as assessed by total phosphorus (r = 0.61), total nitrogen (r = 0.60) and chlorophyll a (r = 0.56) concentrations. Species richness was positively correlated to lake area (r = 0.86) and trophic status (r = 0.64 for total phosphorus concentrations). The percentage of the total annual phosphorus load contributed to 14 Florida lakes by bird populations was low averaging 2.4%. Bird populations using Florida lakes, therefore, do not significantly impact the trophic status of the lakes under natural situations, but lake trophic status is a major factor influencing bird abundance and species richness on lakes. Bird abundance and species richness were not significantly correlated to other lake morphology or aquatic macrophyte parameters after the effects of lake area and trophic status were accounted for using stepwise multiple regression. The lack of significant relations between annual average bird abundance and species richness and macrophyte abundance seems to be related to changes in bird species composition. Bird abundance and species richness remain relatively stable as macrophyte abundance increases, but birds that use open-water habitats (e.g., double-crested cormorant, Phalacrocorax auritus) are replaced by species that use macrophyte communities (e.g., ring-necked duck, Aythya collaris).     

Habitats and Littoral Zone Fish Community Structure of Two Natural Lakes in Southeast Brazil

The fish community and habitat structure in the littoral zone of lakes Pedra and Hortência, middle Rio Doce Valley, Brazil, were investigated in three sampling periods from August 1992 to May 1993. A total of 9106 fishes were collected, including 11 species in Lake Hortência and 12 species in Lake Pedra. Diversity was higher in Lake Hortência, although total fish abundance was the same for both lakes, with no significant temporal differences. The length distribution of the majority of fish species was similar between lakes. Geophagus brasiliensis was the dominant species (number/biomass) in all habitats of the two lakes in all three sampling periods. A striking difference in total fish abundance in relation to habitat types, which we attribute to physical variables, especially the abundance of macrophytes was observed. Negative impacts attributable to the local Eucalyptus sp. plantations were not detected in the fish community structure of Lake Hortência, which is probably due to the maintenance of the riparian vegetation contributing to similar physical environmental conditions in the littoral zone and high similarity in fish community structure between the two lakes. However, further studies on fish community structure and potential Eucalyptus impacts are suggested as necessary to improve conservation measures of the lakes and their fish faunas.     

Spring phytoplankton of 54 small lakes in southern Finland

The abundance and species composition of phytoplankton communities were studied rapidly following the spring ice-melt in 54 small Finnish lakes that form a unique mosaic of water bodies. Phytoplankton biomass and cell density varied among the study lakes with a factor 100 between the lowest and highest values. Highest biomass and densities of phytoplankton characterized small ( < 0.05 km²) lakes with moderate or high water colour (> 80 mg Pt l^–1). In contrast, biomass was low in clear-water lakes and lakes where water throughflow was strong. Typically one species dominated most phytoplankton communities, and usually comprised up to about 45% of the total phytoplankton biomass. Two-thirds of the 103 taxa observed were Chrysophyceans and Chlorophyceans. The most common taxa wereChlamydomonas spp. (Chlorophyceae) andCryptomonas ovata (Cryptophyceae).     

Reduction of benthic macroinvertebrates due to waterfowl foraging on submerged vegetation during autumn migration

If present in large numbers, as during migration, herbivorous waterfowlmay reduce the amount of submerged vegetation. Because the vegetation is a keyfactor in shallow eutrophic lakes, removal of the green biomass can be expectedto affect also other biota that depend on the vegetation. We conducted anexperiment to determine how the abundance of chironomids andPisidium sp. were affected by intense foraging ofwaterfowlon the submerged plant Potamogeton pectinatus. This wasdone in Lake Ringsjön in southern Sweden, during the autumn migration ofthe birds. Three treatments, replicated six times, were used: (i) closed cagesthat excluded all waterfowl, (ii) semi-open cages that excluded only largewaterfowl (geese and swans), and (iii) open plots where all waterfowl couldfreely enter. Waterfowl densities were monitored during the experiment. Theresults suggest that the foraging of large waterfowl (swans) had a clearlynegative effect on macroinvertebrate abundance and aboveground biomass ofP. pectinatus. At the end of the experiment, the densityofchironomids was about 46% lower in the open than in the closed cages. Ingeneral, the density of Pisidium sp. tended to be lower inthe open plots. Small waterfowl alone did not seem to affect either thevegetation or macroinvertebrates. We suggest that thePisidium sp. was influenced at an early stage of grazing,when waterfowl foraged on aboveground biomass, whereas chironomids wereaffectedat a later stage, when swans were digging for below-ground tubers.     

The effect of acidification in enclosures on the biomass and population size structure of Bosmina longirostris

An in situ enclosure experiment was performed from June to September, 1982, in Lake O'Woods, West Virginia (pH7.0), to examine the effects of acidification on the Bosmina longirostris population. Treatments were two control enclosures and two enclosures acidified from an initial pH of 7.2 to pH 4.2 over a 44 day period, using H₂SO₄. B. longirostris was not adversely affected by the acidification. In fact, its abundance, biomass, and mean body size increased in the acid treatment, as compared withe control. This positive response was probably not a direct consequence of the altered water chemistry, but instead may have occured due to reduced competition for food, and also reduced predation pressure. Acidification resulted in a decreased abundance of other herbivores, and elimination of the dominant carnivore, Mesocyclops edax.     

Reciprocal Relationships Between Exotic Rusty Crayfish,Macrophytes, and <Emphasis Type="Italic">Lepomis</Emphasis> Species in Northern Wisconsin Lakes

Abstract The non-native rusty crayfish (Orconectes rusticus) has invaded many lakes of northern Wisconsin, profoundly changing littoral zones in the process. There are other lakes that have been invaded, but do not exhibit these changes. We hypothesized that endogenous feedbacks could form involving rusty crayfish, the macrophytes they destroy, and Lepomis species whose abundance is positively related to macrophyte abundance and also consume juvenile crayfish. We assessed this proposal with long-term data from one lake, a regional comparative study, and a case study of Lepomis predation on crayfish. Through time and across lakes, abundances of rusty crayfish, littoral macrophytes and species of the genus Lepomis were related in a fashion that indicated a set of feedbacks that regulate the abundance of all three. Intense predation on juvenile crayfish by abundant Lepomis is capable of maintaining some crayfish populations at low abundance. Thus, some lakes display profound ecological changes where crayfish achieve high abundance, and others sustain crayfish at low abundance. Consequently, lakes invaded by rusty crayfish may take on the appearance of alternative ecological regimes. Direct experimentation is necessary to determine if, and under what conditions, a lake can exist in either regime. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Invasive Predator, <Emphasis Type="Italic">Bythotrephes</Emphasis>, has Varied Effects on Ecosystem Function in Freshwater Lakes

Bythotrephes longimanus is an invertebrate predator that has invaded the North American Great Lakes and a number of inland lakes, where it preys on crustacean zooplankton. We examined the effect of Bythotrephes on two measures of ecosystem function during a four-month observational study of freshwater lakes on the boreal shield. Bythotrephes-invaded lakes had significantly lower epilimnetic zooplankton abundance and production compared to reference lakes. On average, Bythotrephes consumed 34% of zooplankton production when it was present in lakes. There was some evidence of changes in the timing of zooplankton production, as well as shifts to cooler, less productive habitats, which may lessen the overall effect of the invader on the transfer of energy to higher trophic levels. We experimentally demonstrated a weak trophic cascade where invader predation reduced zooplankton biomass, and subsequently increased phytoplankton growth. However, the response was small in magnitude and not biologically relevant at the whole lake-scale. The most conspicuous effect of Bythotrephes that we measured was a diversion of energy away from native predators at higher trophic levels. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Seasonality of chlorophyll and nutrients in Lake Erken – effects of weather conditions

The effect of submerged macrophytes on interactions among epilimnetic phosphorus, phytoplankton, and heterotrophic bacterioplankton has been acknowledged, but remains poorly understood. Here, we test the hypotheses that the mean summer phytoplankton biomass (chlorophyll a): phosphorus ratios decrease with increased macrophyte cover in a series of nine lakes. Further, we test that both planktonic respiration and bacterioplankton production increase with respect to phytoplankton biomass along the same gradient of increasing macrophyte cover. Increased macrophyte cover was associated with a lower fraction of particulate phosphorus in epilimnia, with total particulate phosphorus declining from over 80% of total phosphorus in a macrophyte free lake to less than 50% in a macrophyte rich lake. Phytoplankton biomass (chlorophyll a) too was lower in macrophyte dominated lakes, despite relatively high levels of total dissolved phosphorus. Planktonic respiration and bacterioplankton production were higher in macrophyte rich lakes than would be expected from phytoplankton biomass alone, pointing to a subsidy of bacterioplankton metabolism by macrophyte beds at the whole lake scale. The results suggest that the classical view of pelagic interactions, which proposes phosphorus determines phytoplankton abundance, which in turn determines bacterial abundance through the production of organic carbon, becomes less relevant as macrophyte cover increases.     

Perch,Perca fluviatilis L., in Small Lakes: Relations between Population Characteristics and Lake Acidity

Perch population characteristics of small lakes were examined with a special reference to lakes which have acidified recently due to atmospheric deposition of air pollutants. The population density and biomass of perch were higher in an acid (pH 4.5) clear water lake with low aluminium concentration than in recently acidified (pH 4.3–4.7) clear water lakes with higher aluminium levels. The structure of perch population in an acid (pH 4.4) humic lake was similar to recently acidified clear water lakes. The population density and the biomass of perch were significantly higher in 6 lakes with pH>5.0 than in 6 lakes of pH<5.0 whereas the mean age and length at a given age were higher in the more acid lakes.     

Effects of a rapidly increasing population of common carp on vegetative cover and waterfowl in a recently restored Midwestern shallow lake

Although the common carp (Cyprinus carpio), an invasive benthic fish from Eurasia, has long been strongly implicated in the disappearance of vegetative cover and reduced waterfowl abundance in North American shallow lakes, the details of this relationship are obscure. This study documented ecological changes in a recently restored shallow lake (Hennepin and Hopper Lakes, IL, USA) at a time that it was experiencing a large increase in its carp population. We estimated the abundance and biomass of carp 7 years after this lake had been restored and then back-calculated carp population size across time while examining changes in the lake’s plant and waterfowl communities. We found that the biomass of carp remained below ~30 kg/ha for 5 years following restoration, but then increased to ~100 kg/ha in the sixth year following a strong recruitment event. Although a carp biomass of <30 kg/ha had no discernible effects on vegetative cover (which exceeded 90%) or waterfowl (which exceeded 150,000 individuals during fall censuses), the increase to 100 kg/ha was associated with a ~50% decrease in both vegetative cover and waterfowl. A further increase in carp biomass to over 250 kg/ha during the seventh year coincided with a decrease in the vegetative cover to 17% of the lake’s surface and a decline in waterfowl use to ~10% of its original value. These data suggest that the common carp is extremely damaging to the ecological integrity of shallow lakes when its density exceeds ~100 kg/ha. Since the biomass of carp in Midwestern shallow lakes commonly exceeds this value by 3–4 times, it seems likely that carp are responsible for the large-scale habitat deterioration described in many of these ecosystems. Handling editor: J. Cambray     

Analysis of the community structure of planktonic ciliated protozoa relative to trophic state in Florida lakes

The planktonic ciliate populations of 30 Florida lakes constituting a broad trophic gradient were examined to determine the response of protozoan community structure to increasing eutrophication. Both ciliate abundance and biomass were strongly related to lake trophic state. Comparison of the Florida data base with a comparable north temperate lake group indicated that subtropical lakes generally possess higher ciliate abundance and biomass at a given trophic state than temperate lakes. However the equations derived for each data base were not significantly different. Community diversity and species richness increased with increasing lake productivity. Highly acidic lakes displayed significantly reduced diversity and numbers of species when contrasted with nonacidic oligotrophic lakes. Small-bodied (< 30 um) ciliates dominated all lakes but were proportionally less important in oligotrophic lakes. Presence-absence data produced three assemblages: an ubiquitous association of primarily small ciliate taxa, a group of large ciliates mainly restricted to eutrophic-hypereutrophic lakes, and a very large ciliate,Stentor niger, which dominated the protozoan communities of acidic oligotrophic lakes.