Primary production and phytoplankton in two small,polyhumic forest lakes in southern Finland

Primary production and phytoplankton in polyhumic lakes showed a very distinct seasonal succession. A vigorous spring maximum produced by Chlamydomonas green algae at the beginning of the growing season and two summer maxima composed mainly of Mallomonas caudata Iwanoff were typical. The annual primary production was ca. 6 g org. C · m^–2 in both lakes. The mean epilimnetic biomass was 1.1 in the first lake and 2.2 g · m^–2 (ww) in the second one. The maximum phytoplankton biomass, 14 g · m^–2, was observed during the vernal peak in May.     

Light-limited algal growth in Lake Loosdrecht: steady state studies in laboratory scale enclosures

Phytoplankton growth in the shallow, turbid Lake Loosdrecht (The Netherlands) is importantly influenced by light availability, and thus the concentrations of the various light-attenuating materials. The system is highly eutrophic and supports an algal biomass of ca. 160 mg Chl m^–3. A model is proposed here which predicts algal growth in the lake as a function of the light received and subsequent attenuation in the water column by phytoplankton, tripton and background colour. The model is based on an energy balance which relates growth rate to the true growth yield on light energy and the energy demand for cell maintenance. The coefficients for energy conversion (Y = 0.002 gDW kJ^–1) and cell maintenance (µ_e = 0.031 day^–1) were determined from steady state growth kinetics of Prochlorothrix hollandica in light-limited laboratory flow systems with the same depth as the lake and receiving summer average conditions of irradiance. Light attenuation by phytoplankton and tripton were quantified using specific attenuation coefficients: 0.011 m² mg^–1 Chl for the phytoplankton and 0.23 m² g^–1 DW for tripton.The growth studies demonstrated that Lake Loosdrecht can support a much higher algal biomass in the absence of non-algal particulate matter. The proposed model is used to predict chlorophyll a concentrations in dependence on growth rate and levels of tripton. Since approximately 75% of the sestonic dry weight in Lake Loosdrecht may be attributed to tripton, it is concluded that the algal biomass is markedly lowered by the abundance of tripton in the water column. A knowledge of the sources and fate of tripton in the lake is thus of fundamental importance in modelling phytoplankton dynamics.     

Species composition and seasonal cycles of phytoplankton with special reference to the nanoplankton of Lake Mikri Prespa

The phytoplankton of Lake Mikri Prespa was studied atmonthly or biweekly intervals during the period May1990–September 1992. Its species composition,consisting of a great number of cyanophytes and a verysmall number of chrysophytes and desmids, may reflectthe eutrophic character of the lake. Moreover, themean annual biomass values (15.0 and 3.2 g m^–3 inthe two years, respectively) and the maximum biomass(38.1, 6.4 and 9.6 g m^–3), classify Mikri Prespaas a eutrophic lake. A tendency towards adouble-peaked pattern of biomass distribution in timewith one peak in autumn, composed mainly ofcyanophytes, and another in spring made up of diatoms,was observed. This pattern contrasts with the standardpattern in eutrophic, stratified temperate lakes,which exhibit a third biomass maximum in summer.Cyanophytes were the most important group in terms ofbiomass and were dominated by the species Microcystis aeruginosa, Microcystis wesenbergii,Anabaena lemmermannii var. minor and Aphanocapsa elachista var. conferta. Diatomsconstituted the second most important group, with main representative the species Cyclotellaocellata. Cyanophytes, diatoms, chlorophytes anddinophytes revealed annual periodicity whereas theother algal groups did not show any seasonality atall.The nanoplankton constituted an important part ofalgal biomass (38.9 and 49.9% in the two years,respectively) and revealed annual periodicity withmaximum values in winter and spring, mainly composedof diatoms and cryptophytes. Low temperature,increased rainfall and high DIN concentrations seemedto be the main factors influencing the seasonality.Although the percentage contribution of nanoplanktondecreased with the increase in total biomass,justifying the classification of Lake Mikri Prespaamong the eutrophic lakes, the nanoplankton biomassdid not correlate significantly with totalphytoplankton biomass.     

Factors regulating summer phytoplankton in a highly eutrophic Antarctic lake

Lakes from Maritime Antarctica are regarded as systems generally inhabited by metazoan plankton capable of imposing a top-down control on the phytoplankton during short periods, while lakes from Continental Antarctica lacking these communities would be typically controlled by scarcity of nutrients, following a bottom-up model. Otero Lake is a highly eutrophic small lake located on the NW of the Antarctic Peninsula, which has no metazoan plankton. During summer 1996, we studied the density, composition and vertical distribution of the phytoplankton community of this lake with respect to various abiotic variables, yet our results demonstrated neither light nor nutrient limitation of the phytoplankton biomass. Densities of heterotrophic nanoflagellates (HNAN) and ciliates from three different size categories were also studied. Extremely low densities of HNAN (0–155 ind. ml^–1) could be due to feeding competition by bacterivore nanociliates and/or predation by large ciliates. A summer bloom of the phytoflagellate Chlamydomonas aff. celerrima Pascher reached densities tenfold those of previous years (158.10³ ind. ml^–1), though apparently curtailed by a strong peak of large ciliates (107 ind. ml^–1) which would heavily graze on PNAN (phototrophic nanoflagellates). Top-down control can thus occur in this lake during short periods of long hydrologic residence time.     

Primary productivity and trophic status of Lakes Sorell and Crescent,Tasmania

The primary productivity of two turbid, shallow lakes on the Tasmanian Central Plateau was determined by the C¹⁴ technique from half-light day incubations in situ. Graphical integration of depth-rate curves gave estimates of areal day rates of production and of annual rates.The 2 lakes are closely adjacent and very similar physically and chemically, but have very different phytoplankton populations. Lake Crescent has ten times the standing crop biomass of Lake Sorell but its greater turbidity restricts light penetration, and production per unit of surface per day and per year is only 2.6 times that of Sorell.With day rates of 25-(44)-93 mgCm^–2 and annual production of 16.9 gCm^–2 Lake Sorell could be regarded as oligotrophic. Consideration of standing crop biomass and morphometry however indicates oligo-mesotrophy. Lake Crescent with day rates of 35-(115)-250 mgCm^–2 and annual production of 45 gCm^–2 is moderately eutrophic.Incubations in constant light demonstrated considerable variation in production rates in different parts of Lake Crescent.     

Interactions between sediment and water in a shallow and hypertrophic lake: a study on phytoplankton collapses in Lake Søbygård,Denmark

Short-term changes in phytoplankton and zooplankton biomass have occurred 1–3 times every summer for the past 5 years in the shallow and hypertrophic Lake Søbygård, Denmark. These changes markedly affected lake water characteristics as well as the sediment/water interaction. Thus during a collapse of the phytoplankton biomass in 1985, lasting for about 2 weeks, the lake water became almost anoxic, followed by rapid increase in nitrogen and phosphorus at rates of 100–400 mg N M^–2 day^–1 and 100–200 mg P m^–1 day^–1. Average external loading during this period was about 350 mg N m^–2 day^–1 and 5 mg P m^–2 day^–1, respectively.Due to high phytoplankton biomass and subsequently a high sedimentation and recycling of nutrients, gross release rates of phosphorus and nitrogen were several times higher than net release rates. The net summer sediment release of phosphorus was usually about 40 mg P m^–2 day^–1, corresponding to a 2–3 fold increase in the net phosphorus release during the collapse. The nitrogen and phosphorus increase during the collapse is considered to be due primarily to a decreased sedimentation because of low algal biomass. The nutrient interactions between sediment and lake water during phytoplankton collapse, therefore, were changed from being dominated by both a large input and a large sedimentation of nutrients to a dominance of only a large input. Nitrogen was derived from both the inlet and sediment, whereas phosphorus was preferentially derived from the sediment. Different temperature levels may be a main reason for the different release rates from year to year.     

Can suspension feeding by bivalves regulate phytoplankton biomass in Lake Waccamaw, North Carolina?

Suspension feeding by bivalves has been hypothesized to control phytoplankton biomass in shallow aquatic ecosystems. Lake Waccamaw, North Carolina, USA is a shallow lake with a diverse bivalve assemblage and low to moderate phytoplankton biomass levels. Filtration and ingestion rates of two relatively abundant species in the lake, the endemic unionid, Elliptio waccamawensis, and an introduced species, Corbicula fluminea, were measured in experiments using natural phytoplankton for durations of 1 to 6 days. Measured filtration and ingestion rates averaged 1.78 and 1.121 ind.^–1 d^–1, much too low to control phytoplankton at the observed phytoplankton biomass levels and growth rates. Measured ingestion rates averaged 4.80 and 1.50 µg chlorophyll a ind.^–1 d^–1, too low to support individuals of either species. The abundance of benthic microalgae in Lake Waccamaw reaches 200 mg chlorophyll a m^–2 in the littoral zone and averages almost an order of magnitude higher than depth-integrated phytoplankton chlorophyll a. Total microalgal biomass in the lake is therefore not controlled by suspension feeding by bivalves.     

Distribution and abundance of littoral benthic fauna in Canadian prairie saline lakes

The littoral benthos of 18 lakes in Alberta and Saskatchewan ranging in salinity from 3 to 126 (g1^–1 TDS) were investigated twice, in the spring and in the summer of 1986. Multiple Ekman dredge samples were taken at water depths of about 0.5, 1.0 and 2 metres in each transect. Two to three transects were used in each lake according to its estimated limnological diversity for a total of 114 stations. A total of 76 species was present varying from 29–31 species in the three lakes of lowest salinity (means of 3.1–5.55) to only 2 species in lakes exceeding 100. Species richness decreased rapidly in salinities greater than 15.Biomass maximum mean of 10.91 g m^–2 dry weight (maximum 63.0 g m^–2) occurred in culturally eutrophic Humboldt Lake (3.1) but one third as great in other low salinity lakes. However, biomass again increased to about 4.5 gm^–2 in two lakes of 15 As the salinity increased still further biomass declined steadily until a minimum of 0.0212 g m^–2 was recorded in most saline Aroma Lake (mean 119). Summer biomass (11 lakes) was greater than spring biomass (4 lakes) because some groups such as amphipods, corixids and ostracods became more abundant in summer. Wet weight biomass averaged 15.8 of dry weight biomass.Seasonality (spring or summer), sediment texture and organic matter content, water depth, pH, salinity (TDS) and the presence of aquatic plants ( plant cover) were considered in the matrix involving species dry weight biomass at each of 117 stations. TWINSPAN classification of the samples yielded a dendrogram with 18 indicator species. Successive dichotomies divided these indicator species into four main lake groups based on salinity, i.e., Group I: 3–10 (Gammarus, Glyptotendipes I, Chironomus cf. plumosus), Group II: 10–38%. (Hyalella, Enallagma,Bezzia), Group III: 38–63 (Hygrotus salinarius, Cricotopus ornatus), Group IV: >63 (Dolichopodidae, Ephydra hians). Each of these main groups was subdivided into smaller groups of lakes based on factors such as pH, seasonality (spring or summer species dominance), organic matter and plant cover. Depth of samples played no apparent role.     

Submerged aquatic vegetation correlations with depth and light attenuating materials in a shallow subtropical lake

A 3-year study was done to quantify the biomass of submerged aquatic vegetation (SAV) and its relationship with environmental attributes in Lake Okeechobee, the largest lake in the southeastern United States. Plants were sampled on 21 occasions at sites located along 15 fixed transects around the shoreline, giving rise to 721 observations of SAV species (Chara spp., Vallisneria americana, Hydrilla verticillata, Potamogeton illinoinensis) dry weight biomass. Environmental sampling focused on factors that attenuate light, including phytoplankton chlorophyll a (chl a), total suspended solids (TSS), non-volatile suspended solids (NVSS) and color. Depth and Secchi transparency also were measured. Based on regression analysis, NVSS was considerably more important in attenuating light than chl a or color. Total biomass of SAV varied from 0 to 271 g dw m^–2, with a mean of 4.7 g dw m^–2, and strong dominance by Chara. The SAV biomass was lower than average for Florida lakes, and may reflect the influence of suspended solids on underwater irradiance, as well as high water level in the late 1990s. Dense SAV was found only where depth was < 2 m and TSS < 20–30 mg l^–1. At locations where high biomass of SAV occurred, the plants may have influenced water quality, because concentrations of TSS, NVSS, and chl a were 2–3 fold lower than at sites with no plants. The potential effects of SAV also were apparent at a regional scale. The shoreline region of the lake displayed a pattern of rising and falling chl a and NVSS with water depth. This occurred both at sites with and without plants, suggesting that it may be driven by physical processes, such as water circulation patterns, which are influenced by depth. However, the pattern was dampened at sites with SAV, indicating a potential to influence these attributes of water quality.     

Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration

Lake Pamvotis is a moderately sized (22 km²) shallow (z _avg=4 m) lake with a polymictic stratification regime located in northwest Greece. The lake has undergone cultural eutrophication over the past 40 years and is currently eutrophic (annual averages of FRP=0.07 mg P l^-1, TP=0.11 mg P l^-1, NH₄ ⁺=0.25 mg N l^-1, NO₃ ^–=0.56 mg N l^-1). FRP and NH₄ ⁺ levels are correlated to external loading from streams during the winter and spring, and to internal loading during multi-day periods of summer stratification. Algal blooms occurred in summer (July–August green algae, August–September blue-green algae), autumn (October blue-green algae and diatoms), and winter (February diatoms), but not in the spring (March–June). The phytoplankton underwent brief periods of N- and P-limitation, though persistent low transparency (secchi depth of 60–80 cm) also suggests periods of light limitation. Rotifers counts were highest from mid-summer to early autumn whereas copepods were high in the spring and cladocerans were low in the summer. Removal of industrial and sewage point sources a decade ago resulted in a decrease in FRP. A phosphorus mass balance identified further reductions in external loading from the predominately agricultural catchment will decrease FRP levels further. The commercial fishery and lake hatchery also provides opportunities to control algal biomass through biomanipulation measures.     

Interactions between phytoplankton,zooplankton and fish in a shallow,hypertrophic lake: a study of phytoplankton collapses in Lake Søbygård,Denmark

Since 1983 severe phytoplankton collapses have occurred 1–4 times every summer in the shallow and hypertrophic Lake Søbygård, which is recovering after a ten-fold decrease of the external phosphorus loading in 1982. In July 1985, for example, chlorophyll a changed from 650 µg l^–1 to about 12 µg 1^–1 within 3–5 days. Simultaneously, oxygen concentration dropped from 20–25 mg O₂l^–1 to less than 1 mg O₂l^–1, and pH decreased from 10.7 to 8.9. Less than 10 days later the phytoplankton biomass had fully recovered. During all phytoplankton collapses the density of filter-feeding zooplankton increased markedly, and a clear-water period followed. Due to marked changes in age structure of the fish stock, different zooplankton species were responsible for the density increase in different years, and consequently different collapse patterns and frequencies were observed.The sudden increase in density of filter-feeding zooplankton from a generally low summer level to extremely high levels during algae collapses, which occurred three times from July 1984 to June 1986, could neither be explained by changes in regulation from below (food) nor from above (predation). The density increase was found after a period with high N/P ratios in phytoplankton or nitrate depletion in the lake. During that period phytoplankton biomass, primary production and thus pH decreased, the latter from 10.8–11.0 to 10.5. We hypothesize that direct or indirect effects of high pH are important in controlling the filter-feeding zooplankton in this hypertrophic lake. Secondarily, this situation affects the trophic interactions in the lake water and the net internal loading of nutrients. Consequently, not only a high content of planktivorous fish but also a high pH may promote uncoupling of the grazing food-web in highly eutrophic shallow lakes, and thereby enhance eutrophication.A tentative model is presented for the occurrence of collapses, and their pattern in hypertrophic lakes with various fish densities.     

Vertical structure and photosynthetic activity of Lake Shira phytoplankton

This study was conducted to analyse vertical dynamics of phytoplankton distribution in Shira Lake during the summer stratification regime. From late June to September phytoplankton in Shira Lake were stratified with the maximum in the lower part of the thermocline, at a depth of 8–12 m, with a chlorophyll concentration up to 23 g and biomass up to 5 mg l^–1. Maxima of chlorophyll and biomass of cyanobacteria and green algae were in different layers. From June to September a major part of chlorophyll a was in green algae, while under ice – in cyanobacteria. The variable fluorescence proves high photosynthetic activity of algae in the depth assemblage. Epifluorescent analysis disclosed that additional light-harvesting pigments were better developed in cells from the depth maximum. The maximum of gross primary production calculated from fluorescence corresponded to the depth maximum of phytoplankton. Primary production over a season was 2.7 gO₂ m^–2. Formation mechanisms of the depth maximum of phytoplankton are discussed in this paper.     

Large-scale patterns of Nodularia spumigena blooms in Pyramid Lake,Nevada, determined from Landsat imagery: 1972–1986

Magnitude and long-term periodicity of summer-autumn blooms of the nitrogen-fixing cyanobacterium, Nodularia spumigena, were characterized for hyposaline Pyramid Lake, Nevada, from Landsat MSS band 3 film negatives. Predicted lakewide mean chlorophyll a concentrations for Landsat overpasses during the July–October Nodularia bloom season ranged from 27 to 72 mg m^–3 with an overall average concentration of 32 ± 7 mg m^–3 between 1972 and 1986. Nodularia blooms were usually annual events. Blooms were not observed on Landsat images in only three of 15 years (1973, 1980, 1982) and midsummer calcium carbonate whitings occurred in two of these years (1973, 1980). Magnitude of Nodularia blooms was highly variable among years and very large blooms, where maximum mean chlorophyll a concentration exceeded one standard deviation of the 15 year overall mean (> 39 mg m^–3) appeared in 1974, 1975, 1977, 1979, 1984, 1985 and 1986. Very large early-July blooms always occurred during or following years of above average fluvial discharge to Pyramid Lake (1984–1986) and were associated with meromixis produced by the large influx of freshwater.Several problems arise using Landsat remote sensing to estimate magnitude and periodicity of scum-forming blue-green algal blooms. These complications may reduce accuracy and precision of phytoplankton biomass estimates made from Landsat images. Nevertheless, Landsat remote sensing enabled us to quantify relative bloom magnitude with limited collection of ground-based data and at a large-scale temporal and spatial resolution not possible using alternative methodologies.     

Oligochaeta of Lake Taimyr: a preliminary survey

Lake Taimyr in Siberia is northernmost among the world's large lakes: 73°40–75°20N, 99–106°E. The lake area is up to 4650 km² in summer, with a maximum depth of 26 m and a mean depth of only 2.8 m. The ice-free period lasts about three months. The water level sinks 5.5–6 m during winter, so that 85% of the bottom surface is frozen into ice for some time and subjected to negative temperatures, probably down to –20 °C. In artificially melted sediment samples, 75–92% of animals survived. The average summer biomass of zoobenthos is about 1 g m^–2 wet weight, a half of this being formed by Oligochaeta. Altogether 76 samples with 3742 oligochaete specimens collected by V. N. Grëze in 1943–1944 were studied. At least 14 taxa of Tubificidae, Lumbriculidae, and Enchytraeidae were found in the lake, and some more enchytraeids in an adjacent river. Many immature animals could not be identified to species. Naididae were completely lacking probably due to the absence of macrovegetation. The shallow freezing zone is inhabited mostly by Alexandrovia ringulata. The profundal fauna is dominated by Lamprodrilus isoporus, Stylodrilus sp., and Isochaetides sp.     

Phytoplankton pigments and adenosine triphosphate (ATP) in Lake Peipsi-Pihkva

The material for pigment analysis was collected 1–3 times a year from Lake Peipsi-Pihkva in 1983, 1987, 1988, 1991 and 1992–1995. Concentrations of chlorophyll a, b and c (Chla, Chlb, Chlc), pheopigment (Pheo) and adenosine triphosphate (ATP) were measured biweekly in 1985–1986. The mean of all Chla values was 20.2 mg m^–1 (median 13.3 mg m^–1) indicating the eutrophic state of the lake. Average Chlb, Chlc, Pheo and carotenoid (Car) contents were 3.7 mg m^–3, 4.1 mg m^–3, 3.0 mg m^–3 and 4.8 mg m^–3, respectively. The average Chlb/Chla ratio was 22.9%, Chlc/Chla 23.4%, Pheo/Chla 38%, Car/Chla 37% and ATP/Chla 3%, the medians being 14.3, 13.6, 17.5, 39.4 and 1.9%, respectively. The proportion of Chla in phytoplankton biomass was 0.41%, median 0.32%. There were no significant differences in temperature, oxygen concentration, Chla, and ATP between the surface and bottom water; the lake was polymictic during the vegetation period. The Chla concentration had its first peak in May followed by a decrease in June and July. In late summer Chla increased again achieving its seasonal maximum in late autumn. The ATP concentration was the highest during spring and early summer, decreasing drastically in autumn together with the decline of primary production. ATP/Chla was the highest during the clear water period in June and early July, which coincided also with the high proportion of Chla in phytoplankton biomass. The highest Chla occurred in November (average 37.2 mg m^–3) when Secchi transparency was the lowest (1.05 m). Concentrations of Chlb, Chlc and carotenoids were the highest in August, that of Pheo in June. Concentrations of Chla and other pigments were the lowest in the northern part of Lake Peipsi (mean 14.7 mg m^–3, median 12.5 mg m^–3) and the highest in the southern part of Lake Pihkva (mean 47.9 mg m^–3, median 16.3 mg m^–3). An increase of Chla and decrease of Secchi depth could be noticed in 1983–1988, while in 1988–1994 the tendency was opposite.     

Phytoplankton productivity and chlorophyll-A concentration of Oguta Lake in Southeastern Nigeria

The phytoplankton productivity and chlorphyll-a concentration of Oguta Lake, the largest natural lake in south-eastern Nigeria, are presented (Dec. 1983. Nov. 1984). The gross productivity ranged from 1.3 to 3.77 g C.m^–2.day^–1 for the water column, dropped during the period of heavy rainfall and varied with depth. The chlorophyll-a concentration had monthly means ranging from 2.31 to 4.00 mg.m^–3, with a drop during the rains, but little depth variation. Both productivity and chlorophyll-a showed non-significant correlation with the physico-chemical features of the water. The values of the biological parameters showed the lake as mesotrophic. The values are compared with those of other African lakes.     

Photosynthetic activity of phytoplankton in a high altitude lake (Emerald Lake,Sierra Nevada,California)

Photosynthetic activity by phytoplankton was measured during the ice-free seasons of 1984, 1985 and 1987 using the ¹⁴C radioassay in high altitude Emerald Lake (California). Relative quantum yield (^B) and light-saturated chlorophyll-specific carbon uptake (P_m ^B) were calculated from the relationship of light and photosynthesis fitted to a hyperbolic tangent function. Temporal changes in P_m ^B showed no regular pattern. Seasonal patterns of ^B generally had peaks in the summer and autumn. Phytoplankton biomass (as measured by chlorophyll a) and light-saturated carbon uptake (P_m) had peaks in the summer and autumn which were associated with vertical mixing. Estimates of mean daily carbon production were similar among the three years: 57 mg C m^–2 2 d^–1 in 1984, 70 mg C m^–2 2 d^–1 in 1985 and 60 mg C m^–2 d^–1 in 1987. Primary productivity in Emerald Lake is low compared to other montane lakes of California and similar to high-altitude or high-latitude lakes in other regions.     

Measurement and verification of rates of sediment phosphorus release for a hypereutrophic urban lake

The contribution of sediment release to the phosphorus budget of hypereutrophic Onondaga Lake was determined through laboratory measurements made on intact cores. Rates ranged from 9–21 mg P m^–2 d^–1 with a mean of 13 mg P m^–2 d^–1, values similar to those observed in other lakes of comparable trophic state. There was no statistically significant trend in rates in time (July versus September) or in space (location along the major N/S axis of the lake). Rates of sediment phosphorus release measured in the laboratory compared favorably with the observed rate of soluble reactive phosphorus accumulation in the lake's hypolimnion. The sediments are the second largest source of phosphorus for Onondaga Lake, contributing 24% of the overall phosphorus load to the system.     

Underwater light penetration,phytoplankton biomass and photosynthetic activity in Lake Xolotlán (Managua)

We measured underwater light penetration, phytoplankton biomass and photosynthetic activity during three years (1987–1990) in Lake Xolotlán (L. Managua), Nicaragua. Phytoplankton biomass governed the light climate of the photic zone, but as biomass also was composed of a varying proportion of dead algae, light availability for the potential biomass of actively photosynthesizing algae (170 mg Chl-a.m^–2) was reduced. The concentration of chlorophyll-a within the photic zone was thus lower and ranged between 58 and 141 mg Chl-a.m^–2. Still, photosynthetic activity was high (2,162 mg 02.m-2.h^–1) due to an extremely high specific rate of photosynthesis; light was the only factor that limited growth. As also other conditions in Lake Xolotlán, beside light limitation, met with the requirements of the models that have been used to analyse production and photosynthetic characteristics in tropical lakes there was a striking agreement between observed and predicted values.     

Zooplankton, phytoplankton and the microbial food web in two turbid and two clearwater shallow lakes in Belgium

Components of the pelagic food web in four eutrophic shallow lakes in two wetland reserves in Belgium (Blankaart and De Maten) were monitored during the course of 1998–1999. In each wetland reserve, a clearwater and a turbid lake were sampled. The two lakes in each wetland reserve had similar nutrient loadings and occurred in close proximity of each other. In accordance with the alternative stable states theory, food web structure differed strongly between the clearwater and turbid lakes. Phytoplankton biomass was higher in the turbid than the clearwater lakes. Whereas chlorophytes dominated the phytoplankton in the turbid lakes, cryptophytes were the most important phytoplankton group in the clearwater lakes. The biomass of microheterotrophs (bacteria, heterotrophic nanoflagellates and ciliates) was higher in the turbid than the clearwater lakes. Biomass and community composition of micro- and macrozooplankton was not clearly related to water clarity. The ratio of macrozooplankton to phytoplankton biomass – an indicator of zooplankton grazing pressure on phytoplankton – was higher in the clearwater when compared to the turbid lakes. The factors potentially regulating water clarity, phytoplankton, microheterotrophs and macrozooplankton are discussed. Implications for the management of these lakes are discussed.