Temperate Freshwater Lakes Research Articles

Dietary niche partitioning in a piscivorous fish guild in response to stocking of an additional competitor – The role of diet specialisation

Species belonging to the same ecological guild can coexist only if there are differences in their responses to limiting resources. Thus, competitor introductions may modify the niches of the resident competitors, but empirical studies are rare. The extent of niche modification might further depend on the species’ trophic specialisation along the specialists-to-generalists axis. In this study, we performed a semi-natural experiment by introducing a trophically specialised piscivorous fish (pikeperch) to the resident predator guild of a trophic generalist (perch) and a trophic specialist (pike) in a temperate freshwater lake. Individual diet data were used to calculate total niche hypervolume and inter- and intraspecific dietary niche overlap to evaluate strength of competitive interactions before and after pikeperch introduction. In both pike and perch, there was a moderate niche compression in response to pikeperch stocking. Furthermore, whereas perch diet hypervolume did not overlap at all with that of pikeperch, the intersection of niche space between the two trophic specialists was highly asymmetrical in favour of pike due to its less specialised diet composition in comparison with pikeperch. The intraspecific dietary niche overlap between perch size classes reflected an enhanced partitioning, whereas that of pike size classes was found almost unchanged in response to pikeperch stocking. These empirical results corroborate that competitive interactions and the degree of specialisation may strongly shape the success of introductions of top-predators in freshwater ecosystems. The most specialised species may persist only if the competitors belong rather to the generalists along the specialisation continuum.

Metagenomic Insights into the Evolution, Function, and Complexity of the Planktonic Microbial Community of Lake Lanier, a Temperate Freshwater Ecosystem

Lake Lanier is an important freshwater lake for the southeast United States, as it represents the main source of drinking water for the Atlanta metropolitan area and is popular for recreational activities. Temperate freshwater lakes such as Lake Lanier are underrepresented among the growing number of environmental metagenomic data sets, and little is known about how functional gene content in freshwater communities relates to that of other ecosystems. To better characterize the gene content and variability of this freshwater planktonic microbial community, we sequenced several samples obtained around a strong summer storm event and during the fall water mixing using a random whole-genome shotgun (WGS) approach. Comparative metagenomics revealed that the gene content was relatively stable over time and more related to that of another freshwater lake and the surface ocean than to soil. However, the phylogenetic diversity of Lake Lanier communities was distinct from that of soil and marine communities. We identified several important genomic adaptations that account for these findings, such as the use of potassium (as opposed to sodium) osmoregulators by freshwater organisms and differences in the community average genome size. We show that the lake community is predominantly composed of sequence-discrete populations and describe a simple method to assess community complexity based on population richness and evenness and to determine the sequencing effort required to cover diversity in a sample. This study provides the first comprehensive analysis of the genetic diversity and metabolic potential of a temperate planktonic freshwater community and advances approaches for comparative metagenomics.

High-throughput single-cell sequencing identifies photoheterotrophs and chemoautotrophs in freshwater bacterioplankton

Recent discoveries suggest that photoheterotrophs (rhodopsin-containing bacteria (RBs) and aerobic anoxygenic phototrophs (AAPs)) and chemoautotrophs may be significant for marine and freshwater ecosystem productivity. However, their abundance and taxonomic identities remain largely unknown. We used a combination of single-cell and metagenomic DNA sequencing to study the predominant photoheterotrophs and chemoautotrophs inhabiting the euphotic zone of temperate, physicochemically diverse freshwater lakes. Multi-locus sequencing of 712 single amplified genomes, generated by fluorescence-activated cell sorting and whole genome multiple displacement amplification, showed that most of the cosmopolitan freshwater clusters contain photoheterotrophs. These comprised at least 10-23% of bacterioplankton, and RBs were the dominant fraction. Our data demonstrate that Actinobacteria, including clusters acI, Luna and acSTL, are the predominant freshwater RBs. We significantly broaden the known taxonomic range of freshwater RBs, to include Alpha-, Beta-, Gamma- and Deltaproteobacteria, Verrucomicrobia and Sphingobacteria. By sequencing single cells, we found evidence for inter-phyla horizontal gene transfer and recombination of rhodopsin genes and identified specific taxonomic groups involved in these evolutionary processes. Our data suggest that members of the ubiquitous betaproteobacteria Polynucleobacter spp. are the dominant AAPs in temperate freshwater lakes. Furthermore, the RuBisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) gene was found in several single cells of Betaproteobacteria, Bacteroidetes and Gammaproteobacteria, suggesting that chemoautotrophs may be more prevalent among aerobic bacterioplankton than previously thought. This study demonstrates the power of single-cell DNA sequencing addressing previously unresolved questions about the metabolic potential and evolutionary histories of uncultured microorganisms, which dominate most natural environments.

Relationships between body size and trophic position of consumers in temperate freshwater lakes

Animal body size is a driving force behind trophic interactions within biological communities, yet few studies have explored relationships between body size and trophic position (based on δ15N) at a broad-scale in freshwater lakes. Therefore, our goals were to (1) determine whether body size is a good predictor of trophic position for multiple pelagic zooplankton taxa and fish communities, and (2) examine how body size-trophic position relationships at the community level compare to species level for fish. Zooplankton and fish were collected from 12 and 7 lakes, respectively, located in the Kawarthas, southern Ontario, Canada. The results indicated that for zooplankton, significant positive but different relationships were found between body size and trophic position for cladocerans, in general, and Daphnia, but not Holopedium. For fish, at the lake community level six out of seven relationships were positive and significant, but again, different among lakes. In contrast, at the species level only three of eight species-specific relationships were significant. Furthermore, for two widespread species, Perca flavescens and Micropterus dolomieu, significant differences were found between community- and lake-specific species relationships. Our community-level models and most species-level models provide evidence that trophic interactions in freshwater lakes are size-based. These results demonstrate that general species models should be applied with caution when using body size to predict trophic position. Additionally, the predictive power of some relationships found here is questionable since, albeit significant, their strengths are generally low. Together, our results suggest that body size may have limited use in predicting trophic position of some biota in temperate freshwater lakes.

Ecological impacts of excessive water level fluctuations in stratified freshwater lakes

Water levels of lakes fluctuate naturally in response to climatic and hydrological forcing. Human over-exploitation of water resources leads to increased annual and interannual fluctuations of water levels, at times far beyond natural amplitudes and/or at altered time schedules. Climate change models predict increased occurrence of extreme events (flooding, extended droughts), which will further magnify the seasonal and multiannual amplitude of water level fluctuations in lakes. A relatively wide literature base already exists for shallow lakes, demonstrating that excessive water level fluctuations impair ecosystem functioning, ultimately leading to shifts between clear-water and turbid states. Evidence is gradually building in the published literature demonstrating that deep (stratified) freshwater lakes also respond adversely to excessive water level fluctuations. Analysis of existing data suggests that at moderate disturbance levels littoral habitats are affected, and hence their biota is also impacted. At further disturbance levels, ecosystem destabilization symptoms are observed, including weakening of keystone species, proliferation of nuisance and invasive species, loss of biodiversity, and increased internal nutrient loading. Ultimately, eutrophication symptoms are manifested, especially large and more frequent cyanobacterial blooms, without increased external nutrient loading. Examples from a range of subtropic and temperate freshwater lakes and reservoirs demonstrate that both top-down and bottom-up processes promote those symptoms. The response of aquatic ecosystems, particularly deep lakes, to water level fluctuations is an under-studied field of crucial importance to the management of water resources, where limnologists have a leading role to play in the near future.

Reproductive divergence between growth forms of Lake Winnipeg walleye (Sander vitreus)

Moles MD, Johnston TA, Robinson BW, Bernard AM, Wilson CC, Wiegand MD, Leggett WC. Reproductive divergence between growth forms of Lake Winnipeg walleye (Sander vitreus). Ecology of Freshwater Fish 2011: 20: 52–66. © 2010 John Wiley & Sons A/SAbstract – Growth polymorphisms occur in many fish species, particularly in northern temperate freshwater lakes. The only known growth polymorphism in walleye (Sander vitreus) is in Lake Winnipeg, Canada, where slow‐growing (dwarf) and fast‐growing (normal) morphotypes co‐exist. We examined differences in reproductive traits between forms to determine whether divergence in growth rates was accompanied by divergence in reproductive strategies and also assessed the genetic composition of both forms using mitochondrial and microsatellite DNA to determine their origin and potential reproductive isolation. Both sexes of the dwarf form produced relatively larger gonads than the normal form. Female dwarfs also had a lower liver lipid concentration, a higher ovary lipid concentration, and produced relatively larger eggs compared to the normal form. There was also some indication that dwarf and normal females differed in the fatty acid profiles of their egg lipids. Population genetic analysis based on partial sequencing of the mitochondrial control region and genotyping nine microsatellite loci indicated a single interbreeding population and sympatric origin for both morphotypes. The dwarf form appears to make a relatively larger reproductive investment at a given body size, and the differences between the two forms may result from plastic developmental responses to subtle differences in niche use.

Community structure and diel migration of zooplankton in shallow brackish lakes: role of salinity and predators

Diel horizontal migration (DHM), where zooplankton moves towards macrophytes during daytime to avoid planktivorous fish, has been reported as a common migration pattern of zooplankton in shallow temperate freshwater lakes. However, in shallow eutrophic brackish lakes, macrophytes seem not to have the same refuge effect, as these lakes may remain turbid even at relatively high macrophyte abundances. To investigate the extent to which macrophytes serve as a refuge for zooplankton at different salinities, we introduced artificial plants mimicking submerged macrophytes in the littoral zone of four shallow lakes, with salinities ranging from almost freshwater (0.3) to oligohaline waters (3.8). Furthermore, we examined the effects of different salinities on the community structure. Diel samples of zooplankton were taken from artificial plants, from areas where macrophytes had been removed (intermediate areas) and, in two of the lakes, also in open water. Fish and macroinvertebrates were sampled amongst the artificial plants and in intermediate areas to investigate their influence on zooplankton migration. Our results indicated that diel vertical migration (DVM) was the most frequent migration pattern of zooplankton groups, suggesting that submerged macrophytes were a poor refuge against predation at all salinities under study. Presumably, this pattern was the result of the relatively high densities of small planktivorous fish and macroinvertebrate predators within the submerged plants. In addition, we found major differences in the composition of zooplankton, fish and macroinvertebrate communities at the different salinities and species richness and diversity of zooplankton decreased with increasing salinity. At low salinities both planktonic/free-swimming and benthic/plant-associated cladocerans occurred, whilst only benthic ones occurred at the highest salinity. The low zooplankton biomass and overall smaller-bodied zooplankton specimens may result in a lower grazing capacity on phytoplankton, and enhance the turbid state in nutrient rich shallow brackish lakes.

Permanent lake stratification caused by a small tributary - the unusual case of Lej da San Murezzan

Meromixis in temperate freshwater lakes is in most cases caused by (1) a morphometric predisposition combined with a high productivity that leads to anoxic conditions in the hypolimnion, (2) saline subsurface inflows, or (3) turbidity currents. Here we report an unusual case where meromixis is caused by the surface inflow of a small tributary. Lej da San Murezzan has been known for several decades to be salinity-stratified during winter, contrary to its neighbouring lakes Lej da Silvaplauna and Lej da Segl. To determine the cause of this stratification, vertical profiles of conductivity were measured on ten occasions between 27 September 2006 and 23 March 2007. An evaluation of these profiles showed that the salinity stratification was continuously built up by a salt source of approximately 5 t d-1. Ovel dal Mulin, a small tributary that contributes only about 1% to the total water inflows of the lake, was identified as the source of this salt. To our knowledge this is the first reported case of a lake where such a small unpolluted surface inflow leads to almost permanent salinity stratification. The salinity stratification has important consequences: it causes the lake to be meromictic with only occasional complete mixing instead of dimictic as the neighbouring lakes and thus supports the build-up of an anoxic bottom layer. Furthermore, it allows restratifying the cold water discharge from a recently installed heat pump within the hypolimnion without the risk of disturbing the ice formation at the lake surface by a rising plume.

RESPONSE OF COMPLEX FOOD WEBS TO REALISTIC EXTINCTION SEQUENCES

Although an ecosystem's response to biodiversity loss depends on the order in which species are lost, the extinction sequences generally used to explore such responses in food webs have been ecologically unrealistic. We investigate how several extinction orders affect the minimum number of secondary extinctions expected within pelagic food webs from 34 temperate freshwater lakes. An ecologically plausible extinction order is derived from the geographically nested pattern of species composition among the lakes and is corroborated by species' pH tolerances. Simulations suggest that lake communities are remarkably robust to this realistic extinction order and highly sensitive to the reverse sequence of species loss. This sensitivity is not well explained by the known sensitivity of networks to the loss of highly connected species but appears to be better explained by our observation that trophic specialists preferentially consume widely distributed species at low risk of extinction. Our results highlight an important aspect of community organization that may help to maintain biodiversity amidst changing environments.

Transfer and early diagenesis of biogenic silica oxygen isotope signals during settling and sedimentation of diatoms in a temperate freshwater lake (Lake Holzmaar, Germany)

We have investigated the transfer of oxygen isotope signals of diatomaceous silica ( δ 18O diatom) from the epilimnion (0–7 m) through the hypolimnion to the lake bottom (∼20 m) in freshwater Lake Holzmaar, Germany. Sediment-traps were deployed in 2001 at depths of 7 and 16 m to harvest fresh diatoms every 28 days. The 7 m trap collected diatoms from the epilimnion being the main zone of primary production, while the 16 m trap collected material already settled through the hypolimnion. Also a bottom sediment sample was taken containing diatom frustules from approximately the last 25 years. The δ 18O diatom values of the 7 m trap varied from 29.4‰ in spring/autumn to 26.2‰ in summer according to the temperature dependence of oxygen isotope fractionation and represent the initial isotope signal in this study. Remarkably, despite the short settling distance δ 18O diatom values of the 7 and the 16 m trap were identical only during spring and autumn seasons while from April to September δ 18O diatom values of the 16 m trap were roughly ∼1.5‰ enriched in 18O compared to those of the 7 m trap. Isotopic exchange with the isotopically lighter water of the hypolimnion would shift the δ 18O diatom value to lower values during settling from 7 to 16 m excluding this process as a cause for the deviation. Dissolution of opal during settling with intact organic coatings of the diatom cells and near neutral pH of the water should only cause a minor enrichment of the 16 m values. Nevertheless, opal from the bottom sediment was found to be 2.5‰ enriched in 18O compared to the weighted average of the opal from the 7 m trap. Thus, resuspension of bottom material must have contributed to the intermediate δ 18O diatom signal of the 16 m trap during summer. Dissolution experiments allowed further investigation of the cause for the remarkably enriched δ 18O diatom value of the bottom sediment. Experiments with different fresh diatomaceous materials show an increase of opaline 18O at high pH values which is remarkably reduced when organic coatings of the cells still exist or at near neutral pH. In contrast, high pH conditions do not affect the δ 18O diatom values of sub-fossil and even fossil opal. IR analyses show that the 18O enrichment of the sedimentary silica is associated with a decrease in Si–OH groups and the formation of Si–O–Si linkages. This indicates a silica dehydroxylation process as cause for the isotopic enrichment of the bottom sediment. Silica dissolution and dehydroxylation clearly induce a maturation process of the diatom oxygen isotope signal presumably following an exponential behaviour with a rapid initial phase of signal alteration. The dynamics of this process is of particular importance for the quantitative interpretation of sedimentary δ 18O diatom values in terms of palaeothermometry.

Gross Photoreduction Kinetics of Mercury in Temperate Freshwater Lakes and Rivers: Application to a General Model of DGM Dynamics

Previous published measurements of mercury photoreduction are for net-photoreduction, since photooxidation processes occur simultaneously. In this research we combine continuous dissolved-gaseous mercury (DGM) analysis with a photoreactor and a quartz sparger in order to derive mercury gross photoreduction rate constants for UVB and UVA irradiations. The DGM concentration in each filter-sterilized freshwater was measured at 5 min intervals over a period of 23 h. Photoreduction proceeded for the initial 200 min, after which, reducible mercury was depleted in the sample. Substantial losses in DOC fluorescence were observed during the incubations for UVA radiation but not for UVB; therefore, UVB photoreduction dynamics are not linked to a loss in DOC fluorescence. Pseudo first-order reaction kinetics fit the data well (r2 > 0.87). The rate constants appear divided between lakes and rivers with the mean lake UVB rate constant (kUVB = 8.91 x 10(-5) s(-1)), significantly less than the mean rate constant (kUVB = 1.81 x 10(-4) s(-1)) for the river samples. However, while there were differences for the UVB rates between lakes and rivers, the mean and median rate constants for UVA in lakes (kUVA = 7.76 x 10(-5) s(-1)) did not differ significantly from the mean rate constant forthe river sites (kUVA = 1.78 x 10(-4) s(-1)). Here, we propose a model for mercury photoredox dynamics for both temperate lake and river systems. The lake model was validated using principal axis analysis to compare observed and predicted DGM data (n=279) from a variety of lake sites in Nova Scotia and Central Quebec. Principal axis analysis found a linear fit (correlation = 0.81; slope = 2.13) between predicted and observed environmental DGM values when log-normalized. The constant bias on the predicted values was attributed to estimates of available reducible mercury and the effect of DGM volatilization on observed data.

Why the limiting nutrient differs between temperate coastal seas and freshwater lakes: A matter of salt

Comparison of bottom‐water chemistry in the marine‐limnic habitat gradient shows greater phosphorus availability in marine waters, primarily because of enhanced iron sequestration by sulfide. In the oxidative hydrolysis of iron and the concomitant precipitation of phosphate, a minimum of two iron atoms are needed to precipitate one phosphate molecule (Fe : P = 2). However, dissolved Fe : P < 2 predominates in anoxic marine waters, therefore leaving some phosphate in solution after oxygenation because of a shortage of dissolved iron for phosphate coprecipitation by iron oxyhydroxide. In contrast, anoxic bottom waters in most freshwater lakes show Fe : P > 2, allowing almost complete phosphate removal on oxygenation. This difference is a consequence of the high sulfate content of sea salt, and a main reason why nitrogen normally limits net primary production in temperate coastal waters, in contrast to the predominant phosphorus limitation of near‐neutral lakes.

The social organization of free‐ranging fish shoals

The phenotypic composition of social groups can affect the foraging efficiency and predation risk of their individual members. In shoaling fishes, active choice of shoal companions on the basis of phenotypic characters such as body length has been reported. In this study, we investigated the phenotypic composition of free‐ranging, multi‐species fish shoals in a temperate freshwater lake. We collected 21 entire shoals, each consisting of up to four species and comprising 3622 fish in total. Shoals were strongly assorted by species, body length, parasite prevalence and parasite load. Using a mark‐recapture procedure involving 785 banded killifish (Fundulusdiaphanus), we found no evidence for shoal fidelity or site fidelity, as individuals from different shoals mixed extensively after just 24 h and ranged all over the study site irrespective of initial capture locations. Killifish showed strong changes in shoal‐size choice over 24 h, and no evidence was found that shoals might break up into sub‐units of individuals that are more phenotypically assorted than their original shoals. The mechanisms by which assortative groups may arise and the consequences of low group fidelity for the evolution of cooperative behaviour are discussed.

Phylogenetic differentiation of two closely related Nitrosomonas spp. That inhabit different sediment environments in an oligotrophic freshwater lake.

The population of ammonia-oxidizing bacteria in a temperate oligotrophic freshwater lake was analyzed by recovering 16S ribosomal DNA (rDNA) from lakewater and sediment samples taken throughout a seasonal cycle. Nitrosospira and Nitrosomonas 16S rRNA genes were amplified in a nested PCR, and the identity of the products was confirmed by oligonucleotide hybridization. Nitrosospira DNA was readily identified in all samples, and nitrosomonad DNA of the Nitrosomonas europaea-Nitrosomonas eutropha lineage was also directly detected, but during the summer months only. Phylogenetic delineation with partial (345 bp) 16S rRNA gene sequences of clones obtained from sediments confirmed the fidelity of the amplified nitrosomonad DNA and identified two sequence clusters closely related to either N. europaea or N. eutropha that were equated with the littoral and profundal sediment sites, respectively. Determination of 701-bp sequences for 16S rDNA clones representing each cluster confirmed this delineation. A PCR-restriction fragment length polymorphism (RFLP) system was developed that enabled identification of clones containing N. europaea and N. eutropha 16S rDNA sequences, including subclasses therein. It proved possible to analyze 16S rDNA amplified directly from sediment samples to determine the relative abundance of each species compared with that of the other. N. europaea and N. eutropha are very closely related, and direct evidence for their presence in lake systems is limited. The correlation of each species with a distinct spatial location in sediment is an unusual example of niche adaptation by two genotypically similar bacteria. Their occurrence and relative distribution can now be routinely monitored in relation to environmental variation by the application of PCR-RFLP analysis.

Divergent members of the bacterial division Verrucomicrobiales in a temperate freshwater lake

Divergent members of the bacterial division Verrucomicrobiales in a temperate freshwater lake

Divergent members of the bacterial division Verrucomicrobiales in a temperate freshwater lake

Bacterial diversity in the water column of a freshwater lake in the Netherlands was investigated by analysis of 16S rRNA gene sequences recovered through PCR amplification from total community DNA. Among 23 unique cloned sequences, two appeared to belong to the recently described bacterial division Verrucomicrobiales. One of the two sequences was most similar to a group of environmental clones that form a distinct lineage within the division. The other sequence was divergent (less than 85% similarity) from all 16S rRNA gene sequences, both from cultivated species and from environmental clones, known in this division to date. Analysis by denaturing gradient gel electrophoresis (DGGE) and sequencing of DNA recovered through excision from the DGGE gel showed that the two sequence types were present in the lake throughout the year.

Temperature-dependent allometric models to estimate zooplankton production in temperate freshwater lakes

Empirical relationships between body size and temperature, and daily P/B of temperate freshwater zooplankton species were derived from the literature. Body size explained 81% of the variation in daily P/B of zooplankton at temperatures <<= 10°C and 48% at temperatures >10°C. The two relationships did not differ in slope but significantly differed in elevation. The regression models were then applied to zooplankton samples collected from a midlake station in Lake Ontario to predict zooplankton seasonal production. Production estimates from the regression models, when compared with estimates from the egg ratio method for the same data set, were similar for cladoceran species, four and 10 times higher for two copepod species, and two times higher for total zooplankton production. In general, these differences are within the range expected based on previous studies that compared different production models. The regression models can provide quick, independent estimates of zooplankton production to complement estimates based on traditional, species-specific methods. Furthermore, the models can be used in conjunction with automated synoptic instruments, such as optical plankton counters and temperature probes, to provide production estimates that better represent large, complex systems such as the Great Lakes.

Temperature-dependent allometric models to estimate zooplankton production in temperate freshwater lakes

Temperature-dependent allometric models to estimate zooplankton production in temperate freshwater lakes

TEMPERATURE‐DEPENDENT CHANGES IN COLONY SIZE OF THE FRESHWATER PENNATE DIATOM ASTERIONELLA FORMOSA (BACILLARIOPHYCEAE) AND THEIR POSSIBLE ECOLOGICAL IMPLICATIONS1

ABSTRACTChanges in colony size (cell number per colony) of Asterionella Formosa Hass. were experimentally evaluated in relation to water temperature using two types of clones having colony sizes of four or eight cells. The clones were isolated from two different temperate freshwater lakes. Both clones showed the same general trend with changing temperature. Most of the colonies were normal in size at low temperatures, but colony size was twice as large at high temperatures. Variable colony sizes were present at low percentages. Colony separation occurred at the oldest connection within the colony after cell division. Culture experiments showed that the rates of specific growth and colony separation were balanced except for a rather short period of time when the temperature was changed. Optical and scanning electron micrography did not show any distinctive morphological structure at the point of connection except for porelli and mucilage pads. Seasonal changes in colony size of A. formosa observed in a freshwater lake are discussed based on these temperature results.

Seasonal occurrence of mesophilic Aeromonas spp. as a function of biotype and water quality in temperate freshwater lakes

Densities of mesophilic aeromonads were compared in Virginia freshwater lakes with trophic classifications ranging from mesotrophic to hypereutrophic. Aeromonad concentrations were independent of trophic status and did not correlate statistically with water quality parameters used to measure trophic state (total phosphorus, chlorophyll a and Secchi depth). Bacterial indicators (fecal coliforms and Escherichia coli) used to assess the public health safety of recreational waters correlated with mesophilic aeromonads only when sewage pollution occurred. Overall ( n = 101–107) there were small negative correlations of aeromonad densities with dissolved oxygen ( r = −0.30, P = <0.05), and total dissolved nitrogen ( r = −0.30, P = 0.05). During frequent sampling from April through October 1991, mean mesophilic aeromonad concentrations ranged from 10 3–4 cells 100 ml −1 with the lowest values measured during mid-summer. Over the course of twelve months, 273 aeromonad isolates were identified as belonging to the following biotypes: A. sobria (54%), A. hydrophila (29%), A. caviae (10%), and undetermined (7%). Seasonally, the frequency of isolation of A. sobria, considered the most pathogenic biotype, increased from 17 to 69% when water temperatures exceeded 20°C in a hypereutrophic lake. These temperatures typically occur during periods of peak recreational usage. Phenotypic characteristics associated with virulence were positive in 70% (hemolysis) and 42% (autoagglutination) of glycerol-stored strains ( n = 85), and in 15% (autoagglutination) of strains immediately processed ( n = 73). S-layer proteins associated with invasiveness were not found in 14 strains which autoagglutinated. To evaluate the public health significance of aeromonads in nutrient enriched lakes, our results demonstrate a need to couple investigations of mesophilic aeromonad ecology with virulence and biotype assessments.