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At depths smaller than those of the daytime bathypelagic layers, the vertically migrating perch Perca fluviatilis fry form a benthic community. Modified Breder traps were used for sampling these deep benthic perch fry during their daylight stay in the benthic habitat of Řimov Reservoir, Czech Republic, in spring 2007 and 2009. The size and condition of perch caught in the traps (the catch was removed in late afternoon) was compared to that of fish caught by trawl in the bathypelagic habitat at midday and by electrofishing in the littoral habitat at midday. Perch from the traps were in significantly worse condition compared to those from the trawl and electrofishing. All fish left in the traps overnight were dead. It is likely that the evening migration to the epilimnion is essential for perch fry performing diel vertical migrations in stratified canyon-shaped reservoirs. The sampling efficiency of traps is strongly affected by the abundance of targeted fish and their swimming activity in a cold and dark benthic habitat.

Organisms and processes in the epilimnion of lakes determine the rate of sedimentation. To investigate the impact of phyto- and zooplankton on the sedimentation rate, we sampled the sedimenting material in Lake Stechlin. Sedimenting matter was collected using a high-resolution multi-trap in three-day intervals during the thermally stratified seasons in 2011 (at 65 m depth) and 2012 (at 20 m depth). Dry weight of the sedimented material was related to chemical, physical, and biological data collected from the water column, as well as to meteorological data. The high-resolution trap showed two mass sedimentation peaks in 2011 and one in 2012. We found that diatom blooms in spring were followed by the highest sedimentation rate in 2011, but not in 2012. The sedimentation rates significantly correlated to low wind speed, followed by a rapid formation of the thermocline, as well as to high calcite concentrations in 2011. Our results suggest that the presence of some aggregation factors like calcite crystal exopolymers or fecal pellets support the sinking process. Furthermore, the high resolution of the trap used here allowed for obtaining precise correlations between sedimentation and the measured parameters indicating relevance of temporal coincidence of multiple environmental variables.

In lakes, spatial and temporal variability of water chemistry and phytoplankton are characteristic phenomena although often difficult to link together. This motivated us to study their interplay in Lake Vanajanselka, a eutrophic lake in Finland. We hypothesized that in summer spatial and temporal differences in phytoplankton and water chemistry can be extended in comparison to spring and autumn. Therefore, chlorophyll a and water chemistry was examined by six sampling campaigns with 15 sampling sites over the lake in May–October 2009–2010. In summer, chlorophyll, pH, and oxygen were horizontally and vertically unevenly distributed in the lake, and in the epilimnion pH and oxygen showed a distinct diurnal variability suggesting high photosynthesis during the day. Daily >1 pH unit difference between the sites and 2.5 pH unit difference between the epi- and hypolimnion were found. In agreement with pH and oxygen, NO3-N and NH4-N could be unevenly distributed in the epilimnion. In autumn no spatial differences were found, however. The results emphasized that algae and cyanobacteria were responsible, at least partly, for the variability in water chemistry in the surface layer, and short- and long-term gradients in space and time need to be considered when productive lakes are studied.

The classic evolutionary theory of senescence predicts that long lifespan evolves under low risk of extrinsic mortality. As lakes present planktonic animals with vertical gradients of mortality risk associated with fish predation, we expected the individual lifespan of Daphnia of the “hypolymnetic” clones to be longer than that of “epilimnetic” Daphnia. In order to test this prediction, we performed a laboratory study on 14 clones from the D. longispina species complex, taken during the daytime either from epilimnion or hypolimnion of three mesotrophic lakes. “Epilimnetic” Daphnia started reproduction earlier, aged faster, and lived shorter than their “hypolimnetic” conspecifics. Our results indirectly support the predation-avoidance hypothesis as the ultimate explanation for depth-selection behavior in Daphnia.

Pigment content in particles accumulated in sediment traps are often not directly correlated with phytoplankton abundance, but are rather indicative of transformations phytoplankton underwent on its downward move and following resuspension. We argue that the variability in temporal and spatial sedimentation patterns of different phytoplankton groups is not only an outcome of pigment persistence, but is also associated with dissimilarity in life strategies and dependent on the physical conditions of the water column. Pigment concentrations were measured on weekly–biweekly basis in the water column and in five sets of traps positioned in Lake Kinneret, Israel. Highly degradable peridinin and chlorophyll c reached the deep traps in minute quantities indicating that dinoflagellates mostly recycled in the epilimnion; these migrating algae dominated plankton community under low turbulence and high light. When fast-sinking diatoms persisted in the water column during holomixis they could reach the bottom intact, and fucoxanthin was found in equal proportions in water and traps, chlorophytes rarely dominated phytoplankton, but lutein and chlorophyll b harbored by this group were often the most abundant signature pigments in traps, reflecting the effect of high accumulation rates of these stable compounds in resuspensed particles from the bottom.

Diatoms typically dominate during April and May in Ford Lake, a eutrophic urban impoundment in southeastern Michigan, USA, whereas cyanobacteria dominate during summer. To alter this, whole lake experiments were conducted during 2006 and 2008 in alternation with control years 2005 and 2007. In experimental years, water was withdrawn from the hypolimnion during summer to destabilize the stratified water column, promote deeper mixing, and reduce anoxia. In 2006, withdrawals occurred over a 6-week period with three discrete intervals, releasing 3 × 105 m3 d−1 for the first and third intervals, and 1.5 × 105 m3 d−1 for the second. In 2008, withdrawal occurred continuously for 6 weeks at a rate of 3 × 105 m3 d−1. In control years, water was released only from the epilimnion. Nuisance cyanobacteria replaced diatoms in control years when the lake became thermally stratified. Diatom response to experimentally induced mixing was examined with focus on 2008 as a model for future management. Selective withdrawal of hypolimnetic water destabilized the water column and promoted diatom summer blooms. Diatom success was greatest in 2008, with abundance reaching 10 mm3 L−1 and persisting through spring and summer. Dominance peaks of diatoms were punctuated when Daphnia populations approached or exceeded 3 × 105 individuals m−2.

The sinusoidal swimming of fish, previously interpreted as foraging behaviour, was studied with respect to season, density of large zooplankton, fish length, time of the day, weather condition and solar radiation in Řimov Reservoir, Czech Republic, using a bottom-mounted, split-beam transducer (7°, nominal angle; frequency 120 kHz). The proportion of sinusoidally swimming fish increased from April to August while this behaviour was absent in October. The occurrence of sinusoidal swimming showed an apparent pattern throughout the day; it increased sharply around sunrise, was highest within 5–6 h around solar noon, and sharply decreased around sunset. Significantly less frequent occurrence of sinusoidal swimming was recorded during cloudy days compared to sunny days. The vast majority of records came from fish of standard length ranging from 100 to 400 mm, which represents the typical size range of common bream Abramis brama and roach Rutilus rutilus of age >1+, the main zooplanktivores in the reservoir. The presence of these larger fish in the open water of the reservoir, as well as the presence of sinusoidal swimming, apparently correlates with the presence of large zooplankton (Daphnia, Leptodora and Cyclops vicinus) in the epilimnion. The increase of sinusoidal swimming between April, June and finally August resulted in an increase of zooplankton component in fish guts. It appears that high values of solar radiation, and stable calm weather during high pressure periods, result in optimal optical conditions for sinusoidal swimming, making this foraging behaviour more efficient and widely used in fishes exploiting the zooplankton production in the reservoir.

In order to adequately assess the ecological status of thermally stratified lakes based on chlorophyll, the sampling must cover all productive layers of the water column. Missing the deep chlorophyll maxima (DCM) that often occur in the meta- or hypolimnion of transparent lakes supported by sufficient illumination and good nutrient availability may cause serious underestimation of the productivity and lead to misclassification of the lake ecological status. There is no commonly accepted sampling design for stratified lakes, and various monitoring guides suggest controversial designs. Our aim was to find some robust criteria to assess the probability of occurrence of a DCM and estimate the differences in measured mean chlorophyll concentrations caused by various sampling designs. Our theoretical model showed that the probability of occurrence of a DCM increases with increasing water transparency and decreasing lake size. Empirical data from Italian and Estonian stratified lakes confirmed the results. Testing of different sampling designs on lakes with full measured chlorophyll profiles available showed that taking only surface layer samples will lead with a high probability to an underestimation of the chlorophyll concentration in the trophogenic layer. In order not to miss the Chl peak in stratified lakes, in most cases it would be more precautious not to limit the sampling to the well-mixed epilimnion but to extend it to the whole euphotic layer. Sampling the epilimnion instead of the euphotic zone could cause up to a 70% underestimation of the chlorophyll concentration, an error that would cause a misclassification of the lake by one or even two status classes in a 5-class assessment system. In most cases, the 2.5 * Secchi depths proved a suitable criterion of the sampling depth and only in the case of surface scums, would sampling of a 3 * Secchi depth layer be recommended in order not to miss the deep chlorophyll maximum.

We analysed the long-term dynamics (1980–2007) of hypolimnetic and epilimnetic bacterial abundances and organic carbon concentrations, both dissolved (DOC) and particulate (POC), in the deep holo-oligomictic Lake Maggiore, included in the Southern Alpine Lakes Long-Term Ecological Research (LTER) site. During the 28 years of investigation, bacterial abundance and POC concentrations did not decrease with declining phosphorus concentrations, while DOC concentrations showed a pronounced decrease in the epi- and hypolimnion. We used the annual mean total lake heat content and total annual precipitation as climate-related variables, and in-lake total phosphorus as a proxy for trophic state. The model (forward stepwise regression, FSR) showed that reduced anthropogenic pressure was more significant than climate change in driving the trend in DOC concentrations. Bacterial dynamics in the hypolimnion mirrored the fluctuations observed in the epilimnion, but average cell abundance was three times lower. The FSR model indicates that bacterial number variability was dependent on POC in the epilimnion and DOC in the hypolimnion. In the hypolimnion, cell biovolumes for rod and coccal morphotypes were significantly larger than in the epilimnion.

The present study was carried out in Faxinal Reservoir, a warm monomictic, meso-eutrophic reservoir in subtropical southern Brazil, with a long-standing, well-stratified condition, low epilimnetic nutrient concentrations, and a relatively clear epilimnion. In this study, we analyzed the dynamics of the phytoplankton functional groups, recognizing their driving forces in Faxinal Reservoir. Samples were taken at monthly intervals from January 2004 to January 2005 in surface waters. According to the reservoir’s mixing regime, three periods were identified during the study: stratification 1 (January–May 2004); mixing period (June–August 2004); and stratification 2 (September 2004–January 2005). The nutrient dynamics were driven by the mixing regime. The H1, F, and C phytoplankton functional groups were the most important in biomass, mainly represented by the N-fixing cyanobacterium Anabaena crassa, the colonial green alga with thick mucilaginous sheaths Nephrocytium sp., and the diatom Asterionella formosa, respectively. Tendencies pointed out by redundancy analysis (RDA) indicated that the mixing regime was the main determining factor of the seasonal dynamics of the phytoplankton community. The dominant functional groups showed a close relationship with the relative water-column stability (RWCS), and also, as a consequence of the mixing regime, with nutrient availability. The study also revealed the important role of physical processes in the seasonal gradient, in selecting for phytoplankton functional groups and, consequently, in the assessment of ecological status. Q index (assemblage index) of water quality based on functional groups revealed ecological status varying from very poor to tolerable in the stratification 1 period and from tolerable to medium in the mixing and stratification 2 periods.

The abundance of two main pelagic fish species in Lake Tanganyika (Stolothrissa tanganicae and Lates stappersii) has always been observed to fluctuate considerably at different time scales. The inverse correlation between the abundance of these species has often been interpreted as the consequence of predator-prey relations (avoidance behaviour by the prey). However, currently the two species often appear spatially segregated in the lake, S. tanganicae dominating in the north while L. stappersii is generally abundant in the south where it feeds mostly on shrimps. A fluctuating abundance of the species is nevertheless observed. As these fish species have a major importance for the fisheries, we investigated the limnological variability in relation to the short-term variability of fish catches. The abundance of S. tanganicae was positively correlated to plankton biomass (r = 0.65), while water transparency (r = 0.56), depth of mixed layer (r = -0.70) and oxygenated water appeared important drivers for the abundance of L. stappersii. Alternating "mixing" and "stable" states of the epilimnion related to seasonal and internal waves variability are probably determinant for the short-term variability in abundance of S. tanganicae and L. stappersii. In the framework of this study, remote sensing has shown a potentially interesting application for fisheries research at Lake Tanganyika. We observed a close correspondence between phytoplankton blooms at the time of trade winds changes and increased catches of S. tanganicae in the south. The anti-correlated abundance of S. tanganicae and L. stappersii probably mainly reflects the underlying fluctuating limnological environment. Fisheries studies need to integrate limnological and planktonic monitoring to better understand large and complex ecosystems such as Lake Tanganyika.

Daphnia galeata is traditionally regarded to be a non-migratory species, which lives in warm epilimnetic waters. Depth segregation or vertical migration is usually attributed to other Daphnia species such as D. hyalina or D. longispina. In a two-year study, we found that in a deep, dammed-valley reservoir (Řimov Reservoir, Czechia) the majority of the population of D. galeata lives in the warm epilimnetic waters during the summer months, but some specimens of this species could be always found in the deep strata as well. This hypolimnetic subpopulation stays in the cold hypolimnetic water and does not migrate. The abundance of hypolimnetic D.galeata does not exceed one specimen per litre and usually shows seasonal variation (minimal densities in early spring, maximal in late summer). Using allozyme electrophoresis, we found that the subpopulation from the deep hypolimnion was clearly genetically differentiated from the population in the epilimnion. We found significant differences in both allele and multilocus genotype frequencies; the FST values at most sampling dates exceeded 0.05. However, the spatial segregation between the epilimnetic and hypolimnetic subpopulations is not permanent. The reservoir is dimictic and hence, at least twice per year, all vertically segregated parts of the population are mixed together. Our results suggest that the deep hypolimnetic subpopulation is repeatedly re-established in spring by deepwater “colonists”, at least some of which seem to be ecologically specialised for the hypolimnetic conditions, and dominate the hypolimnion by the end of the season. The genetic differentiation is likely the result of both the different depth preferences of various D. galeata clones and different selective pressures in the epilimnion and hypolimnion.

Nitrogen (N2) fixation has been identified as possibly an important source of “new” nitrogen (N) to the epilimnion of Lake Malawi but studies in 1999–2000 and 2002 (September–December) estimated that the contribution of N2-fixation by heterocystous Anabaena filaments to the N budget of the lake’s epilimnion is only 3–4% of total N input. N2-fixation rates in Lake Malawi, as estimated by the acetylene reduction assay were higher during the stratified season (September–March) than during the rest of the year. Planktonic N2-fixation in Lake Malawi can be monitored by measuring heterocyst biovolume concentrations because a significant correlation (r2 = 0.945, P < 0.0001) was observed between the two parameters. Heterocyst density cannot be used to estimate N2-fixation because heterocyst sizes in the lake change continuously as at least two Anabaena species with different heterocyst dimensions are present. During September–October 2002, a species similar to A. maxima, contained larger heterocysts (16.34 ± 2.46 and 13.25 ± 1.89 μm in cross and apical section dimensions, respectively). In November–December of 2002, A. discoidea dominated and had smaller sized heterocysts (8.92 ± 1.13 and 7.25 ± 0.95 μm in cross and apical section dimensions). Since planktonic N2-fixation is higher near shore than offshore, its contribution to the N budget in the littoral zone where high densities of grazing benthic fish occur may be more critical to maintaining the high rates of benthic algal productivity observed.

Crater Lake covers the floor of the Mount Mazama caldera that formed 7700 years ago. The lake has a surface area of 53 km2 and a maximum depth of 594 m. There is no outlet stream and surface inflow is limited to small streams and springs. Owing to its great volume and heat, the lake is not covered by snow and ice in winter unlike other lakes in the Cascade Range. The lake is isothermal in winter except for a slight increase in temperature in the deep lake from hyperadiabatic processes and inflow of hydrothermal fluids. During winter and spring the water column mixes to a depth of about 200–250 m from wind energy and convection. Circulation of the deep lake occurs periodically in winter and spring when cold, near-surface waters sink to the lake bottom; a process that results in the upwelling of nutrients, especially nitrate-N, into the upper strata of the lake. Thermal stratification occurs in late summer and fall. The maximum thickness of the epilimnion is about 20 m and the metalimnion extends to a depth of about 100 m. Thus, most of the lake volume is a cold hypolimnion. The year-round near-bottom temperature is about 3.5°C. Overall, hydrothermal fluids define and temporally maintain the basic water quality characteristics of the lake (e.g., pH, alkalinity and conductivity). Total phosphorus and orthophosphate-P concentrations are fairly uniform throughout the water column, where as total Kjeldahl-N and ammonia-N are highest in concentration in the upper lake. Concentrations of nitrate-N increase with depth below 200 m. No long-term changes in water quality have been detected. Secchi disk (20-cm) clarity varied seasonally and annually, but was typically highest in June and lowest in August. During the current study, August Secchi disk clarity readings averaged about 30 m. The maximum individual clarity reading was 41.5 m in June 1997. The lowest reading was 18.1 m in July 1995. From 1896 (white-dinner plate) to 2003, the average August Secchi disk reading was about 30 m. No long-term changes in the Secchi disk clarity were observed. Average turbidity of the water column (2–550 m) between June and September from 1991 to 2000 as measured by a transmissometer ranged between 88.8% and 90.7%. The depth of 1% of the incident solar radiation during thermal stratification varied annually between 80 m and 100 m. Both of these measurements provided additional evidence about the exceptional clarity of Crater Lake.

Chemical changers associated with seasonal stratification are described from three English lake basins of widely differing depth and algal production. They are biologically driven, linked to the acid–base system (pH, titration alkalinity, CO2 distribution) and to oxidation–reduction reactions (O2 depletion, CO2 reduction and generation, HCO 3 − generation). The development of anoxia, in the shallowest and most productive basin, is associated with the generation of HCO 3 − , increase of electrical conductance, and some elevation of pH. This connection is lacking in the deeper basins with less complete depletion of O2. Vertical gradients of pH, some exceeding 2.5 units, develop during summer stratification in all three basins. The largest are due to the photosynthetic depletion of CO2 in the epilimnion by dense concentrations of phytoplankton. Long-term records indicate an accentuation during lake eutrophication. There is also some increase of pH in hypolimnetic water when anoxic, with free CO2 partly converted to HCO 3 − . Depth-profiles of conductance reflect the production of HCO 3 − in deep anoxic regions. They can also indicate, as by minima, the descent of relatively cool flood-water.

The spatio-temporal distribution of the heterotrophic nanoflagellates (HNF) and ciliates was monitored in the reservoir of Esch-sur-Sure during the year 1999. Three main periods of protozoan development were observed, in early April, early May, and in July. On the basis of the seasonal dynamics, it appeared that the early spring development of protozoa was probably not controlled by resources or predators. The second protozoan development was progressively controlled by the increase of metazooplankton density that led to the clear water phase characterised by very low protozoan densities and biomasses. A summer development of protozoa was possible thanks to the development of bacteria and moderate metazooplankton densities due to the appearance of non-edible algae. Prorodontida, Halteriida and Strombidiida were the dominant ciliates in the upper part of the water column. A development of Tintinnida was moreover observed in spring whereas Philasterida and Sessilida developed in winter and summer. Ciliates occupied the entire water column in spring and were concentrated in the epilimnion and the metalimnion during the summer period where they fed on bacteria and algae.

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 103 ind m−3; in the epilimnion 2320 × 103 ind m−3, in the metalimnion 2178 × 103 ind m−3, and in the hypolimnion 237 × 103 ind m−3. The average biomass of metazooplankton was 1.75 g m−3; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m−3. The highest abundances — 19,136 × 103 ind m−3 and 12,008 × 103 ind m−3 — were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer Keratella cochlearis f. typica (Gosse, 1851) was the dominating species in abundance. In biomass, Asplanchna priodonta Gosse, 1850, among the rotifers, and Eudiaptomus graciloides (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000’2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, Chaoborus larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.

The potential of periphyton for phosphorus removal from lakes has been investigated using a novel method involving polypropylene (PP) substrate carriers submerged in the pelagial. The study area Lake 'Fuhlinger See' in Cologne (Germany) is a complex of mesoeutrophic gravel pit lakes. The whole site is intensively used as a recreation area. Visitors are thought to be the most important single contributors to lake eutrophication. Carriers were exposed at different depths (2, 3.5, 5 m), for different time intervals (1–8 months) and from March to November PP-sheets were readily colonised by periphyton and a biofilm consisting mainly of benthic diatoms developed. Seasonal variability of periphyton on substrates was observed since filamentous green algae colonised the artificial substrates mainly between July and November. Chlorophyll a content of periphyton on the PP-fleece was up to 240-fold higher than chlorophyll a concentrations in the same volume in the epilimnion. Up to around 100 mg of total phosphorus per m2 PP-fleece was bound and can be eliminated from the lake by removal of the substrate carriers together with the periphyton after four months of exposure. Though large-scale validations are needed, this method may be applicable as a technique to harvest phosphorus from the water column in larger-scale settings.

This study documents for the first time both vertical and horizontal distribution patterns of the zooplankton community in Lake Kinneret during the period of thermal stratification. The zooplankton distribution patterns were explored in relation to abiotic (temperature, oxygen) and biotic (picocyanobacteria, ciliates, flagellates, phytoplankton, fish) environmental gradients. Sampling was carried out on 6–7 July 1992 at five stations and six depths from nearshore to offshore. Zooplankton abundance and biomass varied from 5 to 267 ind. l−1(mean: 95 ind. l−1), and from 0.1 to 65 d.w. mg m−3(mean: 24 d.w. mg m−3). Zooplankton taxonomic groups (Rotifera, Cladocera, Cyclopoida, Calanoida) and size classes (micro-, meso- and macrozooplankton) showed peaks of maximal density and biomass in the epilimnetic and metalimnetic strata (5 and 14 m). Depth, accounting for 31–39% of total spatial variation, reflected the vertical distribution of zooplankton in relation to temperature and oxygen declines, and the higher concentration of food resources (protists and phytoplankton) in the epilimnion and metalimnion. Onshore–offshore distance, accounting for 17–22% of the total spatial variance, reflected different distribution patterns shown among zooplankton groups and size classes. The macrozooplankton (Copepoda, Cladocera) was more abundant offshore, whereas microzooplankton (Rotifera and nauplii) predominated nearshore. These horizontal distribution patterns were related to small increases in temperature and phytoplankton biomass, and higher concentrations of fish in the littoral zone. Although limited to a short temporal scale, our study indicated that zooplankton spatial distribution in Lake Kinneret during the period of thermal stratification was related to physicochemical, food and predation factors, manifested differently along the vertical and nearshore–offshore gradients.

Allozyme diversity of a population of D. pulex was studied over two seasons in a small lake (Grafenhain, 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.

A baseline study on a temperate, oligotrophic North Patagonian lake (Lake Chapo, Southern Chile) was made prior to the installation of a hydroelectric power station. Throughout one year (September 1986–October 1987) the physical and chemical properties of the lake were investigated monthly from the surface to a depth of 40 m. Lake Chapo is a deep, transparent (Secchi depth: 17–25 m), glacial lake located at 41° 27.5′ S and 72° 30′ W. It has a maximum depth of 298 m, mean depth of 183 m, surface area of 45.3 km2 and water volume of 8.296 km3. The theoretical residence time of the water was 5.5 years. The temperature regime is monomictic with the mixed temperature between 8.1–8.8 °C. Maximum temperature at the surface was 18.7 °C during thermal stratification in summer when the epilimnion had a thickness of about 20 m. The conductivity was low (20.3–23.8 μS cm−1) as was the buffering capacity of a predominantly CO2-carbonate system. The predominant cations were Ca+2? Na+?Mg+2?K+. The phosphorous and nitrogen contents were very low (soluble reactive ortophosphate: 0–1.5 μg P l−1, total phosphorus: 0.3–4 μg P l−1 and nitrate: 0–35 μg N l−1), which is typical of North Patagonian lakes.

Total and abiotic P-uptake rates and P-uptake capacities of seston suspended in the water column and of entrapped settling seston were measured by a radiotracer method in the moderately eutrophic Řimov Reservoir. The study was carried out over one seasonal cycle to investigate whether the settling seston was a sink or a source of dissolved orthophosphate. Total P-uptake rate and P-uptake capacity of both types of seston increased in the epilimnion during the summer months when dissolved reactive phosphorus (DRP) concentrations 180 (molar) indicated P-deficiency of phytoplankton. The P-uptake capacity within the flux of particles sinking from the epilimnion in this period corresponded to the removal of 1-4 mg P m -2 d -1 from the lower, DRP-rich layers. During the remaining parts of the year and at other depths, the P-uptake rates and capacities were low but always positive. Abiotic uptake represented

Long term monitoring of the Řimov Reservoir provided a data set for the analysis of phytoplankton composition and biomass in coincidence with physical, chemical, hydrological and biotic factors. Data from 15 years of monitoring (1984 and 1998) were statistically processed using the CANOCO software. The distribution of dominant species and whole assemblages among three ordination axes were analyzed indirectly by detrended correspondence analysis (DCA). The relationships between the species and the environmental factors were studied by canonical correspondence analysis (CCA) with special attention to the seasons. Statistical analysis specified only two types of phytoplankton assemblages interchanging each year: a cold assemblage in winter and spring and a warm assemblage in summer and fall. The transition period was a `clear-water' period. Species composition and phytoplankton biomass of the warm assemblage are the most important variables for drinking water reservoirs, especially for the Řimov reservoir. The appearance of cyanobacteria of the potentially toxic genus Microcystis was related the highest water temperature, higher concentrations of Na+ and an undisturbed epilimnion. The appearance of the green desmid alga Staurastrum planctonicum and several species of the cyanobacterial genera Aphanizomenonand Anabaenawere connected with lower summer temperature and an undisturbed epilimnion but higher water level. The third type of summer dominance, Fragilaria and/or dinoflagellates was typical for years with summer disturbances (high discharge from the epilimnion) irrespective of the temperature.

Patterns of estimates of oxygen flux (J) across the air–water interface of hypereutrophic Onondaga Lake, NY, U.S.A., are characterized for time scales ranging from diel to seasonal for an 8-month period. The analysis is supported by a high frequency (most often hourly) monitoring program, conducted with a robotic buoy, that included measurements of dissolved oxygen (DO), temperature, and fluorometric chlorophyll a in the lake's surface waters, vertical profiles of DO through the epilimnion, and wind speed and solar radiance. The magnitude and direction of J is demonstrated to vary dramatically at diel, day-to-day, and seasonal time scales. Thus, large errors in estimates of J may result from extrapolating flux calculations made from short-term data to longer time periods. The variations in J were driven by variations in metabolic activity and meteorology, and were mediated by departures from equilibrium DO concentrations and wind-driven turbulence. Extended periods of high J values are shown to coincide with intervals of large departures from equilibrium DO concentrations, but day-to-day differences are driven mostly by variations in wind. A distinct diel pattern of J estimates is manifested for average conditions, with substantially higher J values during daylight hours. This pattern reflects the common diel patterns of the drivers of both higher DO oversaturation and wind speed over those hours. It is demonstrated that the magnitude of J is substantial relative to net changes in the epilimnetic DO pool, and thus must be accommodated accurately in estimates of primary production and community respiration that are to be based on diel monitoring of DO in the water columns of productive lakes.

The influence of calcite precipitation on the phosphorus cycle in stratified hardwater lake was studied before and during experiments with a new restoration technique. Surveys of the chemical composition of water column and monitoring of settling particles of Lake Luzin (North–East) showed that calcite precipitation occurs each year over 2–3 periods during spring and summer. The change of the phosphorus content influenced the calcite precipitation intensity. The sedimentation fluxes of phorphorus and the calcite precipitation were closely associated. Based on the hypothesis that calcite precipitation acts as an improvement to the trophic state by enhancing the internal phosphorus sink, this new technology for lake restoration was developed. The hypolimnetic Ca(OH)2 addition during summer stratification in 1996–1997 induced the calcite precipitation in the deep water layer of Basin Carwitz of Lake Schmaler Luzin. The treatment also supported the natural calcite precipitation in the epilimnion. The annual total phosphorus content decreased from 0.46 tons in 1995 to 0.35 tons in 1997. The annual SRP content decreased from 0.02 tons in 1996 to 0.01 tons in 1997 after beginning the artificial calcite precipitation in 1996. The decrease of the annual Chl-a concentration in 1998 on 38% compared with that in 1996 pointed out the lake recovering. According to the one box model, the artificial calcite precipitation affected the P cycle in the lake by suppressing the P release from the sediments.

We collected data and samples, from June to November of 1999 and 2000, to in situ quantify the abundance of phytoplankton in Boadella reservoir. Samples were taken at different stations along the reservoir and diatoms were persistent in the epilimnion and were the main phytoplankton component, with a peak of abundance up to ∼5 105 particles ml−1. The diatom growth during the initiation of the summer bloom was high in the reservoir upstream concurrent with maximum Chlorophyll a concentrations. A delay in the onset of the diatom bloom, together with lower values of the volume concentration, was found in 1999 compared to the results in 2000 and was attributed to the different stratification of the reservoir as a result of meteorological conditions. The diatom population sank from the epilimnion by sedimentation and formed aggregates that accumulated at the bottom of the epilimnion. Sedimentation to the hypolimnion occurred at the end of the bloom, where peaks of Chlorophyll a were found.

Lake Sempach, located in the central part of Switzerland, has a surface area of 14 km2, a maximum depth of 87 m and a water residence time of 15 years. Restoration measures to correct historic eutrophication, including artificial mixing and oxygenation of the hypolimnion, were implemented in 1984. By means of the combination of external and internal load reductions, total phosphorus concentrations decreased in the period 1984–2000 from 160 to 42 mg P m−3. Starting from 1997, hypolimnion oxygenation with pure oxygen was replaced by aeration with fine air bubbles. The reaction of the plankton has been investigated as part of a long-term monitoring program. Taxa numbers, evenness and biodiversity of phytoplankton increased significantly during the last 15 years, concomitant with a marked decline of phosphorus concentration in the lake. Seasonal development of phytoplankton seems to be strongly influenced by the artificial mixing during winter and spring and by changes of the trophic state. Dominance of nitrogen fixing cyanobacteria (Aphanizomenon sp.), causing a severe fish kill in 1984, has been correlated with lower N/P-ratio in the epilimnion. Buoyant algae such as Planktothrix rubescens (syn. Oscillatoriarubescens) increased in abundance due to enlargement of the trophogenic layer and extended mixing depth during winter. The interactions between zoo- and phytoplankton seemed to be depressed as a result of restoration measures. Zooplankton composition changed to more carnivorous and less herbivorous species. Oxygenation of the hypolimnion induced bioturbation of sediments, mainly by oligochaetae worms, and stimulated germination of spores and cysts and hatching of resting eggs.

Lake Naivasha is a shallow freshwater lake in the Rift Valley of Kenya. Since the 1980s, when the lake showed a seasonal shift between diatom and cyanobacterial dominance it has become moderately eutrophic. Its algal assemblage is now dominated by a persistent Aulacoseira italica population both numerically and in terms of contribution to overall primary production. Algal and cyanobacterial counts were used to derive Simpson’s diversity, succession rate and total community succession, focusing on the 10 most numerically abundant taxa. 170 species were identified, 43 of which were in common with the 143 found in 1979–80, before the increase in trophic state. Most diatoms are indicators of moderate to high nutrient conditions. There is little horizontal or vertical variation in successional processes throughout the lake and although the absolute abundance of cells varies widely, proportional composition is relatively stable. In Crescent Island lagoon, the only regularly stratified site, hypolimnetic succession rates are lower than those in the epilimnion. Overall, community composition is controlled by mixing (and hence light regime) and nutrient availability. With ‘endless summer’ conditions and full mixing, there is a successional pattern of ‘muted seasonality’ adapted to physical instability and environmental stability.

The cell-associated mucilage and soluble extracellular polysaccharides (EPS) were investigated in a eutrophic freshwater lake (Rostherne Mere, Cheshire, U.K.) over up to 2 year annual cycles. Five particular lake algae (Anabaena spiroides Klebahn, Anabaena flos-aquae Brebisson ex Bornet & Flahault,Anabaena circinalis Rabenhorst, Microcystis aeruginosa Kutzing emend. Elenkin and Eudorina elegans Ehrenberg) were found to be the major contributors to cell-associated mucilage, particularly M. aeruginosa. Calculation of the total amount of cell-associated mucilage in the phytoplankton samples showed that it occupied 0.0001–0.007% (the latter during a bloom of Microcystis) of lake water volume within the epilimnion. Seasonal changes in the total volume of associated mucilage reflected the succession of mucilage-producing algal species in Rostherne Mere, which was closely correlated with the physico-chemical (temperature, oxygen, pH, nutrients) and biological (Secchi depth, phytoplankton) parameters within the lake. High levels of cell-associated mucilage present in the lake may have potential for binding metals or other ions in the aquatic environment. Colourimetric determination of the concentration of soluble EPS revealed concentrations of between 2.5 and 60 mg l−1, with peak levels during the bacillariophyceaen bloom and late clear water phase. The second phase did not appear to relate directly to changes in algal population, and may result from bacterial activity, algal lysis or zooplankton activity. As soluble EPS forms a major component of the total amount of dissolved carbon in lakes, the study of the soluble EPS is important to understand the carbon cycle in freshwaters. No direct correlation occurred between algal-associated mucilage and soluble EPS over a single annual cycle.

Grazing rates of zooplankton were analysed in the summer of 1999 in Yellow Belly Lake, an oligotrophic system in the Sawtooth Mountains of Idaho (U.S.A.). The colonial rotifer Conochilus unicornis was a dominant species in the epilimnion, with densities reaching 20 colonies 1−1 (ca. 400 ind. 1−1). Clearance rates were measured with an in situ Haney Grazing chamber and synthetic microspheres 5, 9 and 23/μm in diameter. At epilimnetic temperatures of around 14 °C, mean clearance rates for 5µm particles ranged from 30 to 65 µ1 ind.−1 h −1. Clearance rates were 2–9 times higher on the 5µm spheres than on the 9 µm spheres, and C. unicornis almost never fed on the 23 µm spheres. Grazing rates did not change over the diel cycle. Clearance rates declined more than 10-fold as temperatures declined from 14 °C in the epilimnion to 7 °C in the metalimnion. In the epilimnion, grazing by C. unicornis was more important than grazing by crustaceans in the community, at least on particles ≤9µm. The results show the importance of grazing by rotifers in lakes, and the significance of spatial variations that influence grazing rates.

Colonization and proliferation of zebra mussel (Dreissena polymorpha) population in Hargus lake, a small thermally stratified reservoir in Ohio, U.S.A., caused a significant increase in water clarity and a remarkable decrease in phytoplankton biomass during the period from 1993 to 1995. Increased light penetration and reduced organic matter loading to the meta-and hypolimnion were reflected in the lake stratification patterns, particularly in the temperature and oxygen profiles in the metalimnion. The meta- and hypolimnetic water temperature increased significantly over three years, irrespective of variation in surface water temperature. The epilimnion depth (mixing depth) increased by about the same magnitude as did the average Secchi depth. However, the total heat content of the lake did not show a consistent trend to increasing zebra mussel abundance, as it was largely influenced by the temperature of the large water volumes near the surface, which were in turn affected by weather conditions. Concurrent with the thermal structure change, the dissolved oxygen structure also changed over three years, though to a lesser extent. The changes in oxygen stratification pattern were reflected by increased oxygen concentrations in the metalimnion and a lowered depth of 3 mg l−1 DO isopleth. These observed changes were likely attributed to increased water mixing depth, metalimnion photosynthesis and reduced oxygen consumption by organic matter. With increased epilimnion thickness and improved oxygen conditions in the metalimnion, the habitable space for aquatic macro-organisms (including fish) expanded substantially. Our results suggest that the indirect impacts of zebra mussels on small lake stratification patterns may have much broader implications than do the direct trophic interactions to the whole ecosystem.

The diurnal vertical migrations of smelt (Osmerus eperlanus), larvae of phantom midge (Chaoborus flavicans) and cladoceran zooplankton in eutrophic Lake Hiidenvesi were studied in order to clarify the factors behind the low zooplankton biomass. In the study area, an oxygen minimum occurred in the metalimnion in the 10–15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above the oxygen minimum. C. flavicansconducted a diurnal migration. During the day, the majority of the population inhabited the 12 – 15 m depth just in the oxygen minimum, while during darkness they were found in the uppermost 8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around 23:00. During daytime, the majority of smelts inhabited the depth of 7–9 m, where the water temperature was unfavourably high for them (18 °C). Smelts thus probably avoided the steep oxygen gradient in the metalimnion, whereas Chaoborusused the oxygen minimum as a refuge against predation. Those smelts that were found in the same water layers as Chaoborusused the larvae as their main prey. The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladoceran zooplankton.

The vertical distribution of planktonic ciliates in eight strongly stratified temperate lakes was studied in summer 1998. Ciliate abundance and biomass were highest (mean 39.9 cells ml−1 and 181.9 μg C l−1) in the epi-, and lowest (mean 8.2 cells ml−1 and 97.6 μg C l−1) in the hypolimnion. The community of ciliates was dominated by five orders: Oligotrichida, Haptorida, Prostomatida, Scuticociliatida and Peritrichida. The community composition varied greatly with depth. In the epilimnion, the ciliate numbers were dominated by oligotrichs but small algivorous prostomatids, peritrichs and haptorids were also numerous. In the metalimnion, these groups were replaced by scuticociliates and mixotrophic prostomatids. In the hypolimnion species known as benthic migrants appeared. We found a positive significant correlation (p < 0.05) between ciliate numbers and Chl a and bacterial densities. Only in the hypolimnion, the correlation between ciliates numbers and Chl a was not significant.

Daphnia lumholtzi has invaded the U.S. from its original distribution of Africa, southern Asia and Australia. We examined the factors that affected the abundance and distribution of D. lumholtzi during 1994–1995 in Lake Texoma, a large southern plains reservoir. Daphnia lumholtzi was most abundant during summer months when temperatures were highest. The population exhibited a distributional shift in mid July from the upstream, more riverine portion of the reservoir to the cooler area near the dam when epilimnion temperatures upreservoir exceeded 30 °C. The pattern of variation in distribution and abundance did not correspond to the spatio-temporal patterns of most other resident zooplankton, nor did it correspond to changes in the phytoplankton assemblage. We conclude that the ability of D. lumholtzi to withstand the physiological effects of high temperature on reproduction and survival have a strong influence on the distribution and abundance of this organism in Lake Texoma. Furthermore, because D. lumholtzi became abundant only in the summer, after the decline of many of the native zooplankton, we suggest that it probably has little competitive impact on the other members of the zooplankton assemblage.

The relationships between long-term surface average concentrations of humic acids measured as water colour, dissolved organic carbon (DOC) or Secchi disk transparency and trophic state variables were studied with literature data from more than 600 freshwater lakes. The geometric means of summer surface average nutrient (phosphorus and nitrogen) concentration, phytoplankton biomass (chlorophyll concentration), and hypolimnetic anoxia (anoxic factor) were significantly higher in coloured than in clear lakes. The regressions of colour or DOC on these trophic state variables were positive and significant throughout a range of three orders of magnitude. Phytoplankton or primary productivity was higher in coloured lakes, when expressed per volume of epilimnion. Annual integral primary productivity expressed on an areal basis was smaller in coloured lakes, probably a reflection of shallower phototrophic depths in these lakes. There is evidence that annual integral bacteria productivity is much higher in coloured lakes for two reasons: first, epilimnetic bacteria production was ca. four times higher in coloured lakes, second, other studies have shown that hypolimnetic bacteria production is commonly higher than epilimnetic production, especially in anoxic hypolimnia that are frequent in coloured lakes. All volumetrically expressed variables indicated higher productivity in coloured lakes. In addition, high bacteria productivity reflects a different food chain involving mixotrophs, possibly compensating for low light conditions. Our analyses indicate that primary and secondary productivity is as high as or higher than in clear lakes.

Phosphorus fluxes in phytoplankton, between intracellular andexternal compartments have been measured, in the epilimnion ofatemperate eutrophic reservoir, by means of phosphorusdepletionexperiments performed in situ. Flow rates betweencompartments were plotted against the phosphorus concentrationofthe compartments' origin of the flows and fitted to saturationfunctions.A value of phosphorus cell subsistence quota for the wholecommunity has been computed from the X-intercept values ofeveryfunction. The obtained figures are 10–100 times higher,dependingon the reference value, than those measured running theclassicalsingle species growth response experiments in chemostats.We propose a model using five compartments and a series offlowrates between them in order to simulate the general phosphoruscycling in freshwater phytoplankton. Short term simulation ofphosphorus content in every compartment shows a fluctuatingpatternaround rates of equilibrium, in agreement with the pattern ofvariation observed in situ for the selectedcompartments.

The surface level of Lake Kinneret is regulated to remain between −209 m and −213 m msl. During the stratified period, soluble reactive phosphorus (SRP), ammonium (NH inf4 sup+ ) and dissolved sulphide (H2S) accumulate in the hypolimnion. The concentration of these solutes, which are direct and indirect products of the decomposition of organic matter, increase considerably in summers with lower lake levels. A numerical model describing depth-averaged hypolimnion and epilimnion current velocities for high and low lake levels was adapted for Lake Kinneret. Simulated hypolimnetic currents were shown to be stronger for low lake levels as a result of the fact that low lake levels are characterized by a thinner hypolimnion while the thickness of the epilimnion remains unchanged. We suggest that the stronger hypolimnetic currents have the following consequences: 1. turbulence is induced, 2. the enhanced turbulence results in higher resuspension, 3. because SO4= is available to bacteria on resuspended particles, mineralization rates are enhanced, and 4. focusing of fine sediments and associated organic matter to the pelagic zone is enhanced.

In order to study the N and P balances in the Driss I reservoir, we measured concentrations in the water of these elements, their sedimentation rates, and their input by the river Inaouen and output through the dam. Supply and loss of N and P were calculated from samples collected every 48 hours and were for N 15 and 27. 5 mg m−2 d−1 and for P 33.6 and 1.1 mg m−2 d−1 respectively. The sedimentation rates, determined using sediment traps, were high, about 75 mg m−2 d−1 for N and 34 mg m−2 d−1 for P. Phosphate input came from point sources and was mainly in particulate form. A large fraction of the phosphate sedimented in the form of calcium bound phosphate, with some iron bound phosphate. Bioavailability was, however, low, as the particulate phosphate disappeared rapidly from the epilimnion because of the high sedimentation rate. The ratio Tot-N/Tot-P of the sedimented particulate matter varied between 0.05 and 7.74 and depended on primary production and watergate management.

The availability and importance of food sources for growth of Daphnia longispina L. from a highly coloured fishless lake with anoxic hypolimnion were assessed by combining in situ and laboratory experiments. In in situ experiments populations were enclosed in tubes with natural temperature stratification and with or without anoxic hypolimnion. In the laboratory experiments the importance of food source (littoral zone vs pelagic epilimnion) was assessed by enclosing moss thalli and a natural zooplankton population in a large-scale flow-through system supplying food for experimental Daphnia. Growth of juveniles of Daphnia in epilimnetic water was determined in batch culture experiments and the importance of increasing concentrations of bacteria and algae for their growth and development was investigated with a small-scale flow-through system. Access to the anoxic hypolimnion enhanced the growth of Daphnia by 23–24%. Growth rates in the tubes with anoxic hypolimnion were 0.36 and 0.16 d−1 in July and August respectively. In tubes without anoxia the corresponding values were 0.29 and 0.13. In batch-cultures the highest growth rate determined was 0.16 and the overall rates were lower than in in situ experiments. In batch culture Daphnia was able to grow in darkness for 10 days with a rate of 0.16. In the large-scale flow-through system Daphnia population fed with littoral water reproduced well despite the low concentration of algae and increased its number by a factor of c. 32 in 10 days. However, the animals were small and net production of Daphnia population thus lower under the littoral influence than in the control treatment. Population could survive and grew slowly on pelagial water processed by a natural zooplankton community and with very little algae left. It is thus possible that bacteria serve as a ‘life-support system’ enabling the population survival over periods of algal shortage. Small-scale flow-through experiments revealed that Daphnia longispina is able to mature and reproduce on a bacterial diet if the food concentration is high enough and Daphnia on bacterial food could achieve growth rates similar to those on an algal diet. The threshold food level for Daphnia longispina was estimated to be c. 18.5 μg C 1−1. Detrital material is of limited value in nutrition of Daphnia even in a lake where more than 75% of carbon is bound in particulate detritus.

Convective water exchange patterns, determined from water temperature variations, were examined in the Minky Creek embayment of Guntersville Reservoir, Alabama (USA), during the month of September, 1990. During periods of differential cooling, cooler water originating from shallow stations moved as an underflow current toward the center of the embayment, while warmer water moved as an overflow current toward the shore. During periods of differential heating, convective exchange was much more shallow, occurring in the upper 3 m of the water column. As warmer water from shallow regions moved out as an overrflow current, this water was replaced by a return flow of cooler water originating from the pelagic epilimnion. Wind speed appeared to influence convective exchange patterns during differential heating phases by affecting the depth of the surface mixed layer. The potential for convective exchanges during periods of differential cooling and heating occurred on 54% and 74% of the days, respectively, in September. This high potential for horizontal water exchanges in Guntersville Reservoir has strong implications for the lateral transport of dissolved constituents.

Sediment traps were used to investigate the settling, resuspension, and decomposition of particulate organic matter in Lake Itasca, MN (USA). Traps were deployed in the epilimnion and hypolimnion of the deepest basin during June, 1988, sampled twice during stratified conditions (August, September) and once after the lake had mixed (October). The downward flux of particulate material increased from summer to fall. The net sedimentation of organic matter ranged from 0.6 to 2.3 g m−2 d−1 at 4 m and increased to 2.1 to 3.2 g m−2 d−1 two meters above the bottom sediment indicating that resuspended sediment was at least 33% of the settling mass during all periods. The C:N ratios of captured particles (6.8–9.5) were between the ratios of plankton (5.8 to 6.8) and the sediments (9.9 to 10.2) but smaller than the ratios of terrestrial organic materials (13.5 to 222). The monosaccharide compositions of the entrapped particles were similar to plankton samples and different from the distinct composition of the sediments. Capture of rebound particles similar to the primary flux and not decomposition may have been responsible for this similarity. Total monosaccharide concentrations were lower in the sediments than in entrapped particles. Individual sugars exhibited different patterns of accumulation in the sediments. Glucose was lowest in sediments when the relative concentrations were compared to those in source materials and entrapped particles. In contrast, sediments had the highest rhamnose and fucose concentrations. Bacterial biomass could only account for small portions of these sugars in the sediment. The distinct monosaccharide composition of resuspended sediments was not strongly recorded in materials captured by the sediment traps even after the lake had mixed.

A detailed record of suspended particulate matter (SPM) concentrations in the benthic boundary layer (BBL) 1.5 m above an accumulation bottom and 13.5 m below the surface was obtained from frequent (30 min interval) beam attenuation measurements made with a Sea Tech transmissometer in the main basin of Lake Erken, a moderately deep (mean depth 9 m, maximum depth 21 m) dimictic lake in central Sweden. Concentrations of SPM (g m−3) were not as strongly correlated to the beam attenuation coefficient (c, [m−1]), as were concentrations of the inorganic SPM fraction. Apparently, this was caused by large optically inactive organic particles which significantly affected the measurements of SPM, but had little effect on the attenuation of light. When the water column was thermally stratified, SPM concentrations in the BBL showed a seasonal increase which was related to an increase in the thermocline depth. As the epilimnion deepened, there was also a marked increase in the occurrence of rapid and large changes in SPM concentration. After the loss of stratification, the amount of SPM and the temporal variability in its concentration was reduced. Since surface waves could not influence sediment resuspension at the depth of measurement, these data show the importance of internal waves in promoting sediment resuspension in areas of sediment accumulation. The relatively short period in each summer, when the thermocline reaches a sufficient depth to allow for resuspension over accumulation bottoms, can have important consequences for both the redistribution of lake sediments and the internal loading of phosphorus.

In this paper we describe the main features of planktonic size structure in a stratified reservoir during a summer, deep bloom due to the dinoflagellate Ceratium hirundinella (O. F. Muller) Bergh. As one might expect from the eutrophic character of the ecosystem, the complete size-biomass spectrum is dominated by large phytoplankton and small zooplankton. The relation between numerical abundance (N, organisms per ml) and body size (V μm3) for the overall size spectrum is described by the model log N = 7.25–0.99 log V (r2 = 0.93). However, the existence of tendencies in the points around the regression line suggest limiting linear scaling to the size range of unicells (log N = 6.34–0.51 log V, r2 = 0.85). The size structure shows the less negative slope at the level of the thermocline and when the bloom of C. hirundinella reaches its maximum. In the epilimnion, important taxonomic changes have no effect on the size structure, which appears to be a conservative character of the planktonic community at this level.

Seasonal changes in the phytoplankton community of a small tropical reservoir were monitored over a four year period comprising of an initial two seasonal cycles during which the water column stratified strongly for extended periods each year, and two further seasonal cycles after installation of a mechanical aeration system to induce artificial destratification. In the unmanaged reservoir, the concentration of chlorophyll a at 0.5 m reached maximum values (on one occasion > 90 mg m−3) when the water column was stratified and the epilimnion was very shallow (ca 2 m depth). The hypolimnion at this time was anoxic (less than 2% oxygen saturation) and had a high concentration of bacteriochlorophyll (100–200 mg m−3). The phytoplankton community of the unmanaged reservoir was generally dominated by cyanobacteria (Cylindrospermopsis raciborskii, Anabaena tenericaulis) during the warmer months of the year (November–March) (but replaced by chlorophyta, dinophyceae and euglenophyceae after periods of intense rain) and by bacillariophyceae (Synedra ulna var. chaseana, S. tenera) during the cooler, dry months. In the artificially destratified reservoir (8 h aeration day−1), the phytoplankton community was largely dominated by diatoms except after depletion of the silica content of the water column which caused diatoms to be replaced by cyanobacteria (dominated by A. tenericaulis) and a range of chlorophytes. The changing pattern of stratification and circulation of the water column in the unmanaged reservoir caused repeated disruption of the established phytoplankton assemblage with peaks of high biomass associated with transient cyanobacterial blooms. Continuous aeration and the consequent increase in the ratio mixed: euphotic depth provided conditions suitable for dominance of the phytoplankton by diatoms, as long as silica was available, and resulted in average chlorophyll levels higher than in the unmanaged reservoir (120 ± 10 v. 64 ± 9 mg m−2). Hierarchical fusion analysis based on the biomass of species differentiated the phytoplankton samples into cluster groups that could be related primarily to stratification or mixing of the water column.

The report presents studies on temporal and spatial variations of kinetics (Vmax and Km) of bacterial ectoenzyme activity (β-glucosidase - βGlc, leucine aminopeptidase - Leu-amp) in the naturally eutrophic Pluβsee. βGlc and Leu-amp activity were positively correlated with the flux of polymeric materials (polysaccharides, proteins) in the lake. βGlc activity was low when algal populations grew actively, but during the algal bloom breakdown βGlc activity increased rapidly. Leu-amp displayed the highest rates of activity in the epilimnion and was tightly coupled to bacterial production. The synthesis of studied ectoenzymes was under control of a repression/derepression mechanism. The significance of ectoenzymes for the transformation and bacterial utilization of organic matter, and their role in the microbial loop in aquatic environments is discussed.

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.

Abundances of white sucker, 100–500 mm FL, were not significantly different among the epilimnia, metalimnia and portions of the hypolimnia shallower than 20 m in each of two lakes. However, small suckers < 200 mm were captured most frequently in the epilimnion and no white suckers were captured in the deepest region, 20–38 m, of the two lakes. White suckers consumed prey from all three temperature zones in each lake. Prominent food items were Hyalella azteca and the chironomid larvae Heterotrissocladius, Djalmabatista and Procladius. Despite differences in relative densities of benthic invertebrates among thermal zones of the two lakes, suckers in neither lake foraged exclusively on prey of epilimnetic origin. Suckers captured in the metalimnia foraged on invertebrates that were common to all three thermal zones. And, only 0–4% of the suckers captured in the hypolimnia of the two lakes contained prey that were unique to the epilimnia. Suckers caught in the hypolimnia mainly consumed deep water invertebrates; 83% of the suckers foraged in the metalimnion and hypolimnion of Islets Lake and 45% foraged in the hypolimnion in Burnt Island Lake. Consequently there was little evidence of a massive inshore feeding migration followed by a post-feeding return to the hypolimnion. Northern pike and lake trout rarely fed on white suckers in these lakes and thus piscivory was an unlikely factor in the observed distribution of suckers.

Eau Galle Reservoir, Wisconsin, was treated with a hypolimnetic dose of aluminum sulfate (alum) in 1986 to diminish excessive phytoplankton production associated with high phosphorus loading from anoxic, profundal sediments. Prior to treatment, internal total phosphorus (TP) loading was 3 to 6 times greater than external TP loading during summer stratification. Periodic increases in epilimnetic TP mass and chlorophylla concentrations closely corresponded with elevated internal TP loading. For one year following treatment, internal TP loading and concentrations of soluble reactive phosphorus (SRP) in the hypolimnion were substantially reduced. However, abnormally high external TP loading during the stratified period of 1986 resulted in high TP mass and chlorophylla in the epilimnion. During the summers of 1987 and 1988, effects of alum treatment on internal TP loading were essentially negated, and epilimnetic TP mass and chlorophylla remained unchanged from pretreatment years. Multiple potential sources of P input to this reservoir make it difficult to reduce epilimnetic P and phytoplankton growth.

A study of the biogeochemistry of organic matter in Lake Geneva at a central station (SHL2), was carried out at key periods of the biological cycles from November 1985 to September 1986. The seasonal variability of particulate hydrocarbons was studied in the epilimnion and the hypolimnion, focusing on specific periods such as high phytoplankton productivity and rising herbivorous zooplankton activity (May 1986), clearness of waters (June 1986) and intense mixing of waters (February 1986). Analysis of non-aromatic hydrocarbons (NAH) indicated that their major source is the autochtonous phytoplankton. Year 1986 was characterized by a spring bloom of nanoplankton (chlorophyceae and cryptophyceae) which remained predominant during summer and fall. These biological events were correlated with an increase of biogenic NAH, particulary n-alkane and alkene concentrations; the latter increased by one order of magnitude in May. n-Alkane distributions maximized at n-C17 or n-C29, and the n-C17/n-C29 ratio varied in surface water from 2.8 in May to 0.06 in November of the preceding year, reflecting phytoplankton dominant species. Values of NAH/POC, n-alkanes/POC, alkenes/POC and pristane/phytane ratios underwent seasonal and vertical variations which could be related to grazing and/or settling of particles. NAH, n-alkane and alkene enrichment in hypolimnion particles was interpreted to be a result of zooplankton migrations. A series of 37 monoenes and polyenes having from 15 to 31 carbon atoms, originating from planktonic sources, were recognized in the particulate matter by gas chromatography/mass spectrometry (GC/MS). Their distribution was described in 3 sized particles (> 200 µm, 200-60 µm and < 60 µm, identified as adult zooplankton, young zooplankton and nanoplankton respectively) at two different periods of the biological cycle, May and June 1986. Low level inputs originating from terrestrial higher plants were evident in particles; the value of the Carbon Preference Index (CPI) in the carbon range 23–35 was less than 2.8. In all samples analyzed by GC, an unresolved complex mixture of hydrocarbons (UCM) was present and was interpreted, at least in May, as partly originating from the degradation of organic matter by microorganisms. Typical anthropogenic PAH, such as benzofluoranthene and dibenzo (a) anthracene, were found at a low level (9 and 3 ng l−1 respectively). The absence of hopane-type hydrocarbons, except in November 1985 and in September 1986, and typical combustion PAH associated with particles indicated high temperature pyrolytic contaminants rather than petroleum-derived compounds.

The seasonal distribution of benthic species in the water column above and below the thermocline in a small eutrophic lake is described. During summer stratification populations of Spirostomum spp, Loxodes spp., Plagiopyla and Deltopylum become established in the plankton on or below the oxycline/thermocline. At shallow sites no migration occurred and populations of the migratory species in the benthos were sparse, with the exception of Plagiopyla which occurred in high densities in the sediment. Two distinct planktonic populations are established during stratification: an epilimnetic community of obligate planktonic ciliates and a hypolimnetic community of benthic migrants.