Seasonal and spatial variability in Lake Michigan sediment small-subunit rRNA concentrations

We have used molecular biological methods to study the distribution of microbial small-subunit rRNAs (SSU rRNAs), in relation to chemical profiles, in offshore Lake Michigan sediments. The sampling site is at a depth of 100 m, with temperatures of 2 to 4 degrees C year-round. RNA extracted from sediment was probed with radiolabeled oligonucleotides targeting bacterial, archaeal, and eukaryotic SSU rRNAs, as well as with a universal probe. The coverage of these probes in relation to the present sequence database is discussed. Because ribosome production is growth rate regulated, rRNA concentrations are an indicator of the microbial populations active in situ. Over a 1-year period, changes in sedimentary SSU rRNA concentrations followed seasonal changes in surface water temperature and SSU rRNA concentration. Sedimentary depth profiles of oxygen, reduced manganese and iron, and sulfate changed relatively little from season to season, but the nitrate concentration was approximately fivefold higher in April and June 1997 than at the other times sampling was done. We propose that sediment microbial SSU rRNA concentrations at our sampling site are influenced by seasonal inputs from the water column, particularly the settling of the spring diatom bloom, and that the timing of this input may be modulated by grazers, such that ammonia becomes available to sediment microbes sooner than fresh organic carbon. Nitrate production from ammonia by autotrophic nitrifying bacteria, combined with low activity of heterotrophic denitrifying bacteria in the absence of readily degradable organic carbon, could account for the cooccurrence of high nitrate and low SSU rRNA concentrations.     

Spatial and Temporal Variability in Sediment Denitrification Within an Agriculturally Influenced Reservoir

Reservoirs are intrinsically linked to the rivers that feed them, creating a river–reservoir continuum in which water and sediment inputs are a function of the surrounding watershed land use. We examined the spatial and temporal variability of sediment denitrification rates by sampling longitudinally along an agriculturally influenced river–reservoir continuum monthly for 13 months. Sediment denitrification rates ranged from 0 to 63 μg N₂O g ash free dry mass of sediments (AFDM)⁻¹ h⁻¹ or 0–2.7 μg N₂O g dry mass of sediments (DM)⁻¹ h⁻¹ at reservoir sites, vs. 0–12 μg N₂O gAFDM⁻¹ h⁻¹ or 0–0.27 μg N₂O gDM⁻¹ h⁻¹ at riverine sites. Temporally, highest denitrification activity traveled through the reservoir from upper reservoir sites to the dam, following the load of high nitrate (NO₃⁻-N) water associated with spring runoff. Annual mean sediment denitrification rates at different reservoir sites were consistently higher than at riverine sites, yet significant relationships among theses sites differed when denitrification rates were expressed per gDM vs. per gAFDM. There was a significant positive relationship between sediment denitrification rates and NO₃⁻-N concentration up to a threshold of 0.88 mg NO₃⁻ -N l⁻¹, above which it appeared NO₃⁻-N was no longer limiting. Denitrification assays were amended seasonally with NO₃⁻-N and an organic carbon source (glucose) to determine nutrient limitation of sediment denitrification. While organic carbon never limited sediment denitrification, all sites were significantly limited by NO₃⁻-N during fall and winter when ambient NO ₃⁻-N was low.     

Spatial and temporal variation in phosphorus budgets for 24 watersheds in the Lake Erie and Lake Michigan basins

We estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for 1974, 1978, 1982, 1987, and 1992. NAPI quantifies all anthropogenic inputs of P (fertilizer use, atmospheric deposition, and detergents) as well as trade of P in food and feed, which can be a net input or output. Fertilizer was the dominant input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed. NAPI increased in relation to area of disturbed land (R² = 0.90) and decreased with forested and wetland area (R² = 0.90). Export of P by rivers varied with NAPI, especially for the 18 watersheds of LM (R² = 0.93), whereas the relationship was more variable among the six LE watersheds (R² = 0.59). On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. A comparison of our results with others as well as nitrogen (N) budgets suggests that fractional export of P may vary regionally, as has been reported for N, and the proportion of P inputs exported by rivers appears lower than comparable findings with N.     

Physical-Biological Coupling in Southern Lake Michigan: Influence of Episodic Sediment Resuspension on Phytoplankton

The influence of episodic, sediment resuspension on phytoplankton abundance/volume and composition, the photosynthetic maximum rate (P^B _max) and efficiency (^B), and chlorophyll-specific growth (_Chl) was evaluated during the spring isothermal period in southern Lake Michigan (Laurentian Great Lakes, USA). Resuspension altered the nutrient and light climate of nearshore waters; light attenuation (K_d) and phosphorus concentrations corresponded (p 0.0001 and p 0.001, respectively) with concentrations of suspended particulate matter (SPM). Phytoplankton cell volume and diatom cell abundance and volume were not associated with SPM concentrations (p > 0.05). Diatom composition displayed spatial dissimilarities corresponding with resuspension (p 0.001); small centric diatoms exhibiting meroplanktonic life histories and pennate diatoms considered benthic in origin were most abundant within SPM-impacted, nearshore waters whereas taxa typically comprising assemblages in optically-clear, offshore waters and the basin-wide, spring bloom were not. Values of P^B _max and ^B corresponded (p 0.0001) with both K_d coefficients and SPM concentrations, potentially reflecting increased light harvesting/utilization within impacted assemblages. However, integral production was inversely associated with K_d coefficients and SPM concentrations (p < 0.0001) and photosynthesis was light-limited (or nearly so) for most assemblages. Although _Chl values corresponded with K_d coefficients (p 0.05), values were quite low (x ± S.E., 0.10 ± 0.004 d^-1) throughout the study. Most likely, distinct rate processes between SPM- and non-impacted assemblages reflected short-term compositional (and corresponding physiological) variations due to infusion of meroplankton and/or tributary-derived phytoplankton. Overall, resuspension appears to have little, if any, long-term impact upon the structure and function of the lakes phytoplankton.     

Spatial distribution, biomass and population dynamics of Mysis relicta in Lake Michigan

The abundance, biomass, and life history traits of Mysis relicta were evaluated in the spring, summer, and autumn of 2000 at 8 nearshore (45 m) and 8 offshore (75–110 m) stations throughout central and southern Lake Michigan. Abundance was also evaluated on a smaller scale during June 2000 at 4 nearshore and 4 offshore stations in southeastern Lake Michigan. For large-scale sampling, the abundance of M. relicta did not differ among locations in the spring. In the summer and autumn the abundance of M. relicta was similar among offshore stations with the exception of one station each season; for nearshore stations, abundance was generally highest off Pentwater, Michigan. The abundance of mysids was not consistently high for central or southern basin sites, although overall biomass was higher in the southern basin each season. Abundance of Mysis was positively correlated with bottom depth, but not with bottom water temperature, surface water temperature, or mean chlorophyll concentration. Within the smaller region in southeast Lake Michigan, the abundance of M. relicta differed among locations for both nearshore and offshore stations. Brood size and size of reproductive females did not differ among lake wide locations, but the proportion of females with broods and the size distribution of M. relicta did.     

Seasonal Variations in Chlorophyll a Concentrations in Relation to Potentials of Sediment Phosphate Release by Different Mechanisms in a Large Chinese Shallow Eutrophic Lake (Lake Taihu)

The relationship between chlorophyll a and fractionation of sediment phosphorus, inorganic phosphate-solubilizing bacteria (IPB), and organic phosphate-mineralizing bacteria (OPB) was evaluated in a large Chinese shallow eutrophic lake (Lake Taihu) and its embayment (Wuli Bay). At the three study sites, the increase of chlorophyll a concentrations in April paralleled those of the iron bound phosphate accounting for major portion of sediment inorganic phosphate, and in June significantly higher OPB and IPB numbers (especially OPB) in sediment were main contributors to the peaks of chlorophyll a concentration. Even though IPB peaked from February to June, it should serve as an unimportant P source due to the irrelevancy with chlorophyll a and soluble reactive phosphorus (SRP). By contrast, at the other site in the embayment, the calcium-bound phosphate was predominant and solid, which was difficult to be released, and neither IPB nor OPB were detectable in the sediment, indicating weak potential for phosphorus release from the sediment, which was reflected in the small seasonal variation in SRP concentration in water column. Hence, the extents to which the three general mechanisms behind phosphate release from sediment (desorption of iron bound phosphate, solubilization by IPB and enzymatic hydrolysis by OPB) operated were different depending on seasons and sites in Lake Taihu, they may jointly drive phosphate release and accelerate the eutrophication processes.     

Seasonal and Spatial Variations in Mercury Methylation and Demethylation in an Oligotrophic Lake

Microbial mercury methylation and methylmercury decomposition were examined in Lake Clara, an oligotrophic northern Wisconsin seepage lake, using radioisotopic tracers. Methylation activity was near background in the water column, was greatest in the profundal surficial sediments, and decreased with depth in sediment cores. Active demethylation occurred in the water column but was variable. Demethylation was greatest in the surficial sediments and decreased slightly with sediment depth. The methylation/demethylation ratio (M/D) was >1 in the water column, exhibited a sharp peak in surface sediments, and decreased in deeper sediments. Methylation and demethylation activity varied in surface sediments collected along a lake transect. The M/D ratio in surface sediments ranged from 1.4 to 5.8. Methylation in attached microbial communities was near background, while demethylation was high. The M/D ratios in the attached communities were all <0.20. Methylation activity in surface sediments incubated at in situ temperature increased from spring to late summer and decreased in the fall. Demethylation increased from early to midsummer and then declined. The M/D ratio in surface sediments increased from mid- to late summer, and decreased in the fall. These results indicate that the greatest potential for methylation in Lake Clara occurs in the surficial sediments and that methylation in surficial sediments is greatest from mid-July through September. In addition, the net rate of methylmercury production may be significantly affected by demethylation.     

Spatial and temporal variability of internal and external phosphorus loads in Mona Lake,Michigan

A study was conducted in Mona Lake, a small eutrophic lake located in western Michigan (USA) to address the temporal and spatial variability of external and internal phosphorus loading. External P load varied among subbasins, which was mostly related to discharge, but also to land use. Black Creek, which drains lands with natural cover and agriculture, accounted for the majority of flow, and total phosphorus (TP) and soluble reactive phosphorus (SRP) load, to Mona Lake. However, the relative contribution of SRP load was greater in Little Black Creek, which flows through a mostly urbanized subbasin, than in Black Creek. The relative importance of internal loading was strongly related to season, as internal TP loads contributed only ∼9% of the overall P load in April 2005, but ∼68–82% of the overall P load in the summer and early fall seasons. Internal TP and SRP loading was greater under anaerobic than aerobic conditions. Mean anaerobic TP release rates ranged from 0.80 to 15.56 mg P m⁻² d⁻¹, varying with site and season. Spatial variability in both internal phosphorus loading and sediment P concentration was also evident. By taking into account the spatial and temporal variability of different loading sources, management practices can be targeted to optimize nutrient source control strategies.     

The Small-Subunit rRNA Gene Sequences of Venerids and the Phylogeny of Bivalvia

The complete nucleotide sequence of the 18S subunit of ribosomal DNA (rDNA) was determined for the venerid clams Callista chione (Pitarinae) and Venus verrucosa (Venerinae). Comparison of the new sequences with the published sequences of 1 annelid, 2 gastropods, 2 polyplacophorans, and 19 bivalves showed that when the annelids are used as outgroup the gastropods diverge from the bivalves, which form a cluster including the polyplacophorans. When the gastropods alone were compared with the bivalves, the latter split in two groups corresponding to the two subclasses of Heterodonta and Pteriomorpha. The former include two taxa that diverged early, Galeomma and Tridacna, while the Veneridae and Mactridae form two sister groups. In contrast to previous reports and in line with morphological data, the Ostreidae are included in the Pteriomorphia and form a monophyletic group. Received: 16 May 1998 / Accepted: 11 August 1998     

Seasonal Variability of Extracellular Enzymes Involved in Carbon Mineralization in Sediment of a Subtropical Mangrove Wetland

This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyperalkaline, chromium-contaminated soil-water system representative of the conditions at a chromite ore processing residue (COPR) site. Soil from the former surface layer that has been buried beneath a COPR tip for over 100 years was shown to have an active microbial population despite a pH value of 10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter. With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ～300 μM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occured over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH of 10.5 on a time scale compatible with engineering intervention at COPR-contaminated sites.     

Biogeochemical silica mass balances in Lake Michigan and Lake Superior

Silica budgets for Lake Michigan and Lake Superior differ in several respects. Mass balance calculations for both lakes agree with previous studies in that permanent burial of biogenic silica in sediments may be only about 5% of the biogenic silica produced by diatoms. Because dissolution rates are large, good estimates of permanent burial of diatoms can not be obtained indirectly from the internal cycle of silica (silica uptake by diatoms and subsequent dissolution) but must be obtained from the sediment stratigraphy. The annual net production of biogenic silica in Lake Michigan requires 71% of the winter maximum silica reservoir which must be maintained primarily by internal cycling in this large lake whereas the comparable silica demand in Lake Superior is only 8.3%. The greater silica demand in Lake Michigan is the result of phosphorus enrichment which has increased diatom production. It is hypothesized that steady-state silica dynamics in Lake Michigan were disrupted by increased diatom production between 1955 and 1970 and that a new steady state based on silica-limited diatom production developed after 1970. Mass balance calculations for Lake Michigan show in contrast with previous work that the hypothesized water column silica depletion of 3.0 g · m^–3 could have occurred even though 90% or more of the biogenic silica production is recycled.     

Seasonal Variation of Cadmium, Copper, and Lead Concentrations in Fish from a Freshwater Lake

This paper aims to compare the Cd, Cu, and Pb concentration in the bone, gills, and muscle of the seven fish species sampled during normal and dry seasons. Cadmium, Cu, and Pb concentrations varied significantly depending on the type of the tissue and season. Bone samples of the Osteochilus hasseltii showed the highest concentrations of Pb (6.08 μg/g dw) during September (the dry season), whereas bone samples of the Puntioplites bulu showed the lowest concentrations of Cd (0.08 μg/g dw) during September. Muscle samples of the P. bulu indicated the highest concentrations of Cu (2.58 μg/g dw) during March (the normal season). On the other hand, muscle samples of Channa straitus sowed the lowest concentrations of Cd (0.04 μg/g dw) during July (the dry season). Gills samples of the O. hasseltii showed the highest concentration of Pb (6.56 μg/g dw) during March (the normal season), while gills samples of C. straitus indicated the lowest concentration of Cd (0.06 μg/g dw) during July (the dry season).     

Sediment formation in Lake Baikal

The following stages in the formation of sediments in Lake Baikal have been distinguished. In the Middle Miocene lacustrine deposits accumulated in the southern depression. In Early and Middle Pliocene, as well as in Early-Middle Pleistocene the bottom of the southern and middle Baikal depressions was occupied by large lakes, but the northern basin was mainly dry with a system of small lakes, streams and rivers. This accounts for the great facies variety and the genetic heterogeneity of sediments in the northern depression. At the end of Middle and in the Late Pleistocene an abrupt warping of the depression took place, and a shore-line similar to the present one was formed. Sands in the upper part of the sediments are considered to be the product of intensive erosion which probably occurred in the pluvial epoch in the Late Pleistocene period.     

Seasonal feeding ecology of co-existing native and invasive benthic fish along a nearshore to offshore gradient in Lake Michigan

Relative abundance, diet composition and feeding strategy were determined for three benthic fish, the native deepwater sculpin Myoxocephalus thompsonii (Girard, 1851) and slimy sculpin Cottus cognatus (Richardson, 1836), and the invasive round goby Neogobius melanostomus (Pallas, 1814), along a nearshore to offshore gradient in southeastern Lake Michigan during March–December 2010, 2015, and 2016. Round goby were most abundant in the nearshore (<25 m), slimy sculpin were most abundant in the transitional zone (35–65 m), and deepwater sculpin were most abundant in the offshore zone (>75 m). Despite a large degree of spatial separation, some species did overlap, with slimy and deepwater sculpin occurring in sympatry throughout the year in the offshore and transitional zones, and round goby overlapping with both sculpin species seasonally in the transitional zone. Deepwater sculpin exhibited specialization on Mysis diluviana in all depth regions. Slimy sculpin in the offshore reduced diet overlap with deepwater sculpin by specializing on fish eggs during spring and fall, whereas in the transitional depth zone, there was considerable overlap between sculpin species due to the high importance of Mysis in diets. The invasive round goby had a mixed diet, with some diet overlap with native sculpin, especially slimy sculpin, in the transitional zone. In the nearshore zone, round goby displayed a generalized diet with many prey contributing to the diet, but the average contribution of any prey was generally low. Spatial separation and variable feeding strategies help reduce, but not eliminate shared resource use amongst these benthic fish in Lake Michigan.     

Seasonal and Spatial Variability of Bacterial and Archaeal Assemblages in the Coastal Waters near Anvers Island, Antarctica

A previous report of high levels of members of the domain Archaea in Antarctic coastal waters prompted us to investigate the ecology of Antarctic planktonic prokaryotes. rRNA hybridization techniques and denaturing gradient gel electrophoresis (DGGE) analysis of the bacterial V3 region were used to study variation in Antarctic picoplankton assemblages. In Anvers Island nearshore waters during late winter to early spring, the amounts of archaeal rRNA ranged from 17.1 to 3.6% of the total picoplankton rRNA in 1996 and from 16.0 to 1.0% of the total rRNA in 1995. Offshore in the Palmer Basin, the levels of archaeal rRNA throughout the water column were higher (average, 24% of the total rRNA) during the same period in 1996. The archaeal rRNA levels in nearshore waters followed a highly seasonal pattern and markedly decreased during the austral summer at two stations. There was a significant negative correlation between archaeal rRNA levels and phytoplankton levels (as inferred from chlorophyll a concentrations) in nearshore surface waters during the early spring of 1995 and during an 8-month period in 1996 and 1997. In situ hybridization experiments revealed that 5 to 14% of DAPI (4′,6-diamidino-2-phenylindole)-stained cells were archaeal, corresponding to 0.9 × 10⁴ to 2.7 × 10⁴ archaeal cells per ml, in late winter 1996 samples. Analysis of bacterial ribosomal DNA fragments by DGGE revealed that the assemblage composition may reflect changes in water column stability, depth, or season. The data indicate that changes in Antarctic seasons are accompanied by significant shifts in the species composition of bacterioplankton assemblages and by large decreases in the relative proportion of archaeal rRNA in the nearshore water column.     

Depuration of PCBs in the Lake Michigan Ecosystem

A suite of physical processes controls the rate at which polychlorinated biphenyl (PCB) concentrations change in Lake Michigan fauna. To the extent that these processes are homogenous in space, predator and forage fish species are exposed to the same water column and sediment concentrations of PCB in Lake Michigan, whereas exposure due to diet varies with trophic position. We can think of observed PCB concentrations in a particular species as realizations of an unknown process consisting of many sources of variability. Because all species are utilizing the resources of the same ecosystem, a component of variability of PCB concentration is shared by all organisms in the system, whereas other components are not. Individual species were modeled with parameters that were estimated as the overall, or reference mean and a shared rate of change, plus an individual species offset from that reference mean and shared rate of change. This approach allows species-specific differences to be manifest via the offset, whereas the shared components of variability are known with greater precision than would be possible if we considered only a single species. The objective of this work is to determine the extent to which observed declines in PCB concentrations in various species in the Lake Michigan ecosystem are similar and synchronous. A high degree of similarity and synchrony between species would imply a high degree of spatial homogeneity of the physical processes controlling PCB concentrations in Lake Michigan fauna. A lack of synchrony suggests that other factors, such as food web interactions or spatial patterns, may be important. Received 9 March 1999; accepted 29 February 2000.     

Seasonal Development and Morphological Variability of Cyclotella ocellata (Bacillariophyceae) in the Eutrophic Lake Dagow (Germany)

The development of Cyclotella ocellata Pantocsek was studied systematically in the eutrophic, dimictic hardwater Lake Dagow from March to October, 1994. Cyclotella ocellata was the most important centric diatom in the lake with a maximum cell density of 6 × 10⁶ cells 1⁻¹. The seasonal development, characterized by a spring and a summer maximum, is considered in relation to environmental factors and the succession of the phytoplankton community. The amount of Cyclotella ocellata biomass as a proportion of the total phytoplankton biomass varied from 0.5 to 35%. In addition, seasonal changes in cell size and feature associated with sexual reproduction of Cyclotella ocellata were documented. Light and electron microscopic investigations demonstrate an extremely wide range of morphological variability of this natural population.     

Spatial variation in density, mean size and physiological condition of the holarctic amphipod Diporeia spp. in Lake Michigan

• 1 We examined spatial patterns in population characteristics (density, biomass, mean body length) and physiological condition (lipid content, length‐weight) of the amphipod Diporeia spp. in Lake Michigan by collecting samples at up to 85 sites in late summer 1994 and 1995. Variables were examined relative to water depth and three lake regions: south, central and north. Most major river systems are found in the south, and this region is more nutrient‐enriched compared to the north.
• 2 Over all sites, mean density was 5240^‐2, biomass was 4.1g dry wt m^‐2, and mean body length was 5 mm. While maximum densities were related to depth, with a peak at 30–70 m, greatest densities occurred on the west side of the lake, and low densities were found in the south‐east, north‐east and lower Green Bay. High densities in the west probably resulted from upwelling, and reduced densities in the south‐east may reflect food competition with Dreissena polymorpha (zebra mussel).
• 3 Lipid content, weight per unit length, and mean length declined with increased water depth, but depth‐related trends were most evident in the south. Overall, mean lipid content and weight per unit length were significantly lower in the south (16.6% dry wt, 0.59 mg at 5 mm body length) compared to the north (23.7% dry wt, 0.78 mg at 5 mm body length). These regional differences may have resulted from greater diatom availability in the north and competition from D. polymorpha in the south. Triacylglycerols and phospholipids were the dominant lipid classes in all three regions. Although the mean proportion of triacylglycerols, the energy‐storage lipid, was lower in the south than in the north, regional differences in proportions of lipid classes were not significant.
• 4 Mean lipid content and weight per unit length of Diporeia in the south were lower than values found in the late 1980s prior to the establishment of Dreissena. Mean lipid content of mature individuals is now at levels considered a minimum for successful reproduction.

     

Spatial and Temporal Variability in the Ecosystem Metabolism of a High-elevation Lake: Integrating Benthic and Pelagic Habitats

We characterized spatial and temporal variability in net ecosystem production (NEP), community respiration (CR), and gross primary production (GPP) over an ice-free season in an oligotrophic high-elevation lake using high-frequency measurements of dissolved oxygen. We combined the use of free-water and incubation chamber measurements to compare pelagic and benthic habitats and estimate their relative contributions to whole-lake metabolism. Despite a brief period of predominant heterotrophy after snowmelt, both free-water and incubation chamber measurements confirmed autotrophy of the epilimnion in all habitats throughout the ice-free season. In contrast, benthic incubation chambers showed the benthos to be consistently heterotrophic. Although temperature was the strongest seasonal driver of benthic metabolism, bacterioplankton density and indexes of organic matter quality explained the most variability in pelagic metabolism. Driven largely by benthic metabolism, free-water measurements of GPP and CR were twice as high in littoral than pelagic habitats. However, rates of water column primary production overlying the littoral benthos were high enough to overcome net benthic heterotrophy, and seasonal mean NEP in littoral habitats remained positive and not significantly different from pelagic habitats. Benthic rates averaged about 25% of whole-lake metabolism. Pelagic metabolism measurements were affected by littoral rates about half the time, with the degree of isolation between the two a function of advection and water column stability. These results emphasize the importance of characterizing spatial and temporal variability in metabolism within the context of physical dynamics and challenge the notion that benthic metabolism will necessarily be larger than pelagic metabolism in oligotrophic lakes.     

Low Genotypic Diversity in a Daphnia Pulex Population in a Biomanipulated Lake: The Lack of Vertical and Seasonal Variability

Allozyme diversity of a population of D. pulex was studied over two seasons in a small lake (Gräfenhain, Saxony, Germany). In this eutrophic lake, accumulation of hydrogen sulfide during winter ice covers leads to an annual killing-off of cladocerans. Hence, the population must be re-established every year by hatchlings from sexual eggs. The absence of planktivorous fish due to strong biomanipulation ensures high abundances of Daphnia from June until September. Besides the investigation of seasonal changes of genotype diversity, vertical differences between individuals from the epilimnion and from a deep chlorophyll maximum (DCM) were studied. Specimens were classified by allozyme electrophoresis using six allozyme loci. Except for aldehyde oxidase (AO) all tested allozyme loci were monomorphic. Two alleles were found at the AO-locus in the overall study, allowing the population to be subdivided into three clonegroups (ss, fs, ff). The proportion of each clonegroup both in the epilimnion and in the DCM did not vary over the two seasons and no differences were found in the year-to-year comparison. Although two completely different habitats (epilimnion, DCM) were found to be used by D. pulex, clonal structure in both depth strata was similar at all times indicating no intraspecific segregation of the population. In spite of a non-permanent population with periodical sexual reproduction, a constant deviation from Hardy-Weinberg-equilibrium was detected in the overall study. This remarkable genetic structure is interpreted to be the result of both founder effects and a strong and long-lasting selection pressure caused by 15 years of stationary conditions due to intense biomanipulation.