Increase In Lake Productivity Research Articles

Sedimentologic and palynologic records of the last deglaciation and Holocene from Ballston Lake, New York

Continuous pollen and sediment records from two ∼8.5-m-long cores document late Pleistocene and Holocene sedimentation and vegetation change in the Ballston Lake basin, eastern New York State. Pebbles at the base of both cores and the geomorphology of the watershed reflect the presence of the Mohawk River in the basin prior to ∼12,900 ± 70 cal yr B.P. Ballston Lake formed at the onset of the Younger Dryas (YD) by an avulsion of the Mohawk River. The transition from clay to gyttja with low magnetic susceptibility (MS), low bulk density, and high organic carbon indicates rapid warming and increased lake productivity beginning 11,020 cal yr B.P. MS measurements reveal that the influx of magnetic particles, associated with pre-Holocene clastic sedimentation, ceased after ∼10,780 cal yr B.P. The pollen record is subdivided into six zones: BL1 (12,920 to 11,020 cal yr B.P.) is dominated by boreal forest pollen; BL2 (11,020 to 10,780 cal yr B.P.) by pine ( Pinus) forest pollen; BL3 (10,780 to 5290 cal yr B.P.) by hemlock ( Tsuga) and mixed hardwood pollen; BL4 (5290 to 2680 cal yr B.P.) by mixed hardwood pollen; BL5a (2680 cal yr B.P. to 1030 cal yr B.P.) by conifer and mixed hardwood pollen; and BL5b (1030 cal B.P. to present) by increasing ragweed ( Ambrosia) pollen. A 62% decrease in spruce ( Picea) pollen in <320 cal years during BL1 reflects rapid warming at the end of the YD. Holocene pollen zones record more subtle climatic shifts than occurred at the end of the YD. One of the largest changes in the Holocene pollen spectra began ∼5300 cal yr B.P., and is characterized by a marked decline in hemlock pollen. This has been noted in other pollen records from the region and may record preferential selection of hemlock by a pathogen or parasites.

Late Quaternary Glacial and Environmental History of the Burstall Pass Area, Kananaskis Country, Alberta, Canada

We used integrated multiproxy analysis of a lake sediment core and glacial geomorphology to reconstruct the late Pleistocene and Holocene climate and geomorphic evolution of the Burstall Pass area, Kananaskis Country, Alberta, Canada. Analysis of macrofossils, pollen, sedimentology, and sediment geochemistry from a lake sediment core and geomorphology and tephrochronology of glacial moraines provide evidence for multiple modes of climate during the last ca. 11,000 yr. An advance of the Robertson Glacier prior to ca. 9200 14C BP is correlated to the Crowfoot advance and was the largest of the postglacial period. Immediately following this event, increased lake productivity and the deposition of marl as well as increased arboreal/nonarboreal pollen (AP/NAP) ratios suggest that the climate warmed, possibly accompanied by increased aridity. Decreased turbidity and clastic sediment flux in Lower Burstall Lake during the early Holocene suggest reduced glacial runoff and may indicate the complete ablation of the Robertson Glacier shortly after 10,000 14C BP. Clastic sediment flux to Lower Burstall Lake remained minimal until after ca. 3500 14C BP, when decreasing LOI (loss-on-ignition) organic carbon levels in lake sediments signaled the return of glacial runoff to the lake system. The largest Neoglacial advance in the Burstall Pass area appears to have been the most recent and was followed by rapid recession during the 20th century.

Role of Methane Cycling in Organic Matter Turnover in Different Types of Lakes

The rates of microbiological processes in CH4 cycling and organic matter destruction in the bottom sediment and water of different types of lakes in the Upper Volga basin and Baltic countries are determined. The rates of CH4 removal from silts and the peculiarities of its distribution in water mass as a function of the lake productivity and the character of water mixing in it are studied. The role of processes in CH4 cycling in destruction is shown to increase with increasing lake productivity.

Late-Glacial and Early Holocene Environmental and Climatic Change at Lake Tambichozero, Southeastern Russian Karelia

High-resolution lithostratigraphy, mineral magnetic, carbon, pollen, and macrofossil analyses, and accelerator mass spectrometry 14C measurements were performed in the study of a sediment sequence from Lake Tambichozero, southeastern Russian Karelia, to reconstruct late-glacial and early Holocene aquatic and terrestrial environmental changes. The lake formed ca. 14,000 cal yr B.P. and the area around the lake was subsequently colonized by arctic plants, forming patches of pioneer communities surrounded by areas of exposed soil. A minor rise in lake productivity and the immigration of Betula pubescens occurred ca. 11,500 cal yr B.P. The rise in summer temperatures probably led to increased melting of remnant ice and enhanced erosion. The distinct increase in lake productivity and the development of open Betula-Populus forests, which are reconstructed based on plant macrofossil remains, indicate stable soils from 10,600 cal yr B.P. onward. Pinus and Picea probably became established ca. 9900 cal yr B.P.

Holocene Environmental Variability in Southern Greenland Inferred from Lake Sediments

Sediments from Qipisarqo Lake provide a continuous Holocene paleoenvironmental record from southern Greenland. Following deglaciation and glacio-isostatic emergence of the basin from the sea ∼9100 cal yr B.P., proxies of lake paleoproductivity, including biogenic silica and organic matter, increased markedly until 6000 cal yr B.P. and thereafter remained stable over the ensuing warm three millennia. Subsequent decreases in these proxies, most dramatically between 3000 and 2000 cal yr B.P., show the lake's responses to initial Neoglacial cooling. Intervals of ameliorated limnological conditions occurred between 1300 and 900 and between 500 and 280 cal yr B.P., briefly interrupting the decreasing trend in productivity that culminated in the Little Ice Age. Increased lake productivity during the latter half of the 20th century, which reflects the limnological response to circum-arctic warming, still has not reached peak Holocene values. The Neoglacial climate of the last 2000 yr includes the most rapid high-amplitude environmental changes of the past nine millennia. The Norse thus migrated around the North Atlantic Ocean region in the most environmentally unstable period since deglaciation. Lacustrine sediment records provide a context with which to consider future environmental changes in the Labrador Sea region. In particular, any future warming will be superposed on a regional climate system that is currently exhibiting highly unstable behavior at submillennial timescales.

Pre-industrial atmospheric pollution: was it important for the pH of acid-sensitive Swedish lakes?

Acid rain has caused extensive surface water acidification in Sweden since the mid-20th century. Sulfur emissions from fossil-fuel burning and metal production were the main sources of acid deposition. In the public consciousness, acid deposition is strongly associated with the industrial period, in particular the last 50 years. However, studies of lake-water pH development and atmospheric pollution, based on analyses of lake sediment deposits, have shown the importance of a long-term perspective. Here, we present a conceptual argument, using the sediment record, that large-scale atmospheric acid deposition has impacted the environment since at least Medieval times. Sulfur sources were the pre-industrial mining and metal industries that produced silver, lead and other metals from sulfide ores. This early excess sulfur deposition in southern Sweden did not cause surface water acidification; on the contrary, it contributed to alkalization, i.e. increased pH and productivity of the lakes. Suggested mechanisms are that the excess sulfur caused enhanced cation exchange in catchment soils, and that it altered iron-phosphorus cycling in the lakes, which released phosphorus and increased lake productivity.

MATERNAL AND ENVIRONMENTAL GRADIENTS IN THE EGG SIZE OF AN ITEROPAROUS FISH

Life history theory predicts that organisms inhabiting environments with relatively poor conditions for the growth and survival of their offspring should produce fewer and larger offspring. We examined egg size (an index of offspring size) of an iter- oparous, broadcast-spawning, freshwater fish-the walleye (Stizostedion vitreum)-from 34 populations across 260 of latitude in order to determine whether egg size varied with respect to environmental indices of juvenile habitat quality. Variation among populations (among environments) was compared to variation within populations (among females). Within populations, egg size generally increased with maternal size and age. Slopes of these relationships were much more variable among populations (cv > 100%) than inter- annually within populations (cv 10%) than interannually within populations (cv < 5%), but only slightly more variable than among females within populations (mean cv = 8.5%). Nev- ertheless, among-population variability in egg size was related to environmental conditions. Mean egg size decreased with increasing latitude/decreasing mean annual temperature, contrary to our predictions. However, as predicted, egg size decreased with increasing lake productivity following adjustment for the latitudinal/temperature effect. These results sug- gest that egg size in fishes may be influenced by multiple environmental factors across populations, as well as by maternal effects within populations.

Maternal and Environmental Gradients in the Egg Size of an Iteroparous Fish

Life history theory predicts that organisms inhabiting environments with relatively poor conditions for the growth and survival of their offspring should produce fewer and larger offspring. We examined egg size (an index of offspring size) of an iteroparous, broadcast-spawning, freshwater fish—the walleye (Stizostedion vitreum)—from 34 populations across 26° of latitude in order to determine whether egg size varied with respect to environmental indices of juvenile habitat quality. Variation among populations (among environments) was compared to variation within populations (among females). Within populations, egg size generally increased with maternal size and age. Slopes of these relationships were much more variable among populations (cv > 100%) than interannually within populations (cv < 50%). Egg size vs. female size/age correlations were stronger in populations closer to the northern and southern limits of the walleye range. Egg size was also related to maternal growth history, but the effects of recent growth (previous year) were inconsistent. Egg size varied much less than fecundity among females of the same population. For a standard size/age of female, predicted egg size was more variable among populations (cv > 10%) than interannually within populations (cv < 5%), but only slightly more variable than among females within populations (mean cv = 8.5%). Nevertheless, among-population variability in egg size was related to environmental conditions. Mean egg size decreased with increasing latitude/decreasing mean annual temperature, contrary to our predictions. However, as predicted, egg size decreased with increasing lake productivity following adjustment for the latitudinal/temperature effect. These results suggest that egg size in fishes may be influenced by multiple environmental factors across populations, as well as by maternal effects within populations.

Effects of nutrient enrichment on recruitment of age-0 fathead minnows (Pimephales promelas): potential impacts of environmental change on the Boreal Plains

Eutrophication in lakes on the Canadian Boreal Plains is predicted to increase because of climate and land-use changes. The resulting increase in lake productivity might then increase recruitment of young fish via increased food availability, growth, and survival. To assess this hypothesis, we manipulated nutrient concentrations in experimental ponds and examined mechanisms influencing production and survival of age-0 fathead minnows (Pimephales promelas). Nutrient enrichment increased phytoplankton biomass (chlorophyll a) sevenfold in treatment compared to reference systems. In response, fish laid more eggs and survival of age-0 fish was enhanced, both of which contributed to a more than fivefold increase in total number of age-0 fish that survived to the end of the growing season in treatment versus reference systems. A complementary enclosure experiment suggested that enhanced growth and decreased susceptibility to starvation contributes to the greater survival of age-0 fish when food resources are increased. Furthermore, overwinter mortality of age-0 fathead minnows in experimental ponds was strongly size-selective; no fish smaller than 20 mm survived winter. Because of these effects on egg production and growth and survival of age-0 fish, environmental changes predicted for the Boreal Plains could significantly alter the dynamics of fish populations.

A multi-proxy analysis of climate impacts on the recent development of subarctic Lake Saanajärvi in Finnish Lapland

Responses to recent climatic changes in the sediment of subarctic Lake Saanajarvi in northwestern Finnish Lapland are studied by comparison of various biological and sedimentological proxies with the 200-year long climate record, specifically reconstructed for the site using a data-set of European-wide meteorological data. The multi-proxy evidence of simultaneously changing diatom, Cladocera, and chrysophyte assemblages along with the increased rates of organic matter accumulation and pigment concentrations suggest that the lake has undergone a distinct typological change starting from the turn of the 20th century. This change, indicating an increase in lake productivity, parallels a pronounced rise in the meteorologically reconstructed mean annual and summer temperatures in the region between ca. 1850 and 1930's. We postulate that, during the Little Ice Age, the lake was not, or was only weakly, thermally stratified during summer, whereas the subsequent increase in air and hence epilimnetic water temperatures resulted in the development of the present summer stratification. The increased thermal stability of the lake created more suitable conditions for the growth of phyto- and zooplankton and changed the overall primary production from benthos to plankton. Mineral magnetic and carbonaceous particle records suggest long-distance pollution, particularly since the 1920's, yet the observed changes in lake biota and productivity can hardly be explained by this very minor background pollution; the 20th century species configurations are typical of neutral waters and do not indicate any response to pollution.

Effects of epilimnetic versus metalimnetic fertilization on the phytoplankton and periphyton of a mountain lake with a deep chlorophyll maxima

Nutrients can load directly to either the epilimnion or metalimnion of lakes via either differential inflow depths of tributaries or intentional fertilization of discrete strata. We evaluated the differential effects of epilimnetic versus metalimnetic nutrient loading using 17-m-deep mesocosms that extended into the deep chlorophyll layer of oligotrophic Pettit Lake in the Sawtooth Mountains of Idaho. Addition of nitrogen plus phosphorus stimulated primary production nearly identically (2.4- to 4-fold on different dates) in both treatments, with the production peaks occurring in the strata where nutrients were added. The metalimnetic fertilization, however, resulted in equal or greater stimulation of chlorophyll a and phytoplankton biovolume than when nutrients were added directly to the epilimnion. Periphyton growth was stimulated 10–100 times more by epilimnetic fertilization than by metalimnetic fertilization and diverted nutrients from the planktonic autotrophs. These results suggest that the development of deep chlorophyll layers may be influenced by plunging river inflows that carry nutrients to the metalimnion and that metalimnetic lake fertilization may be useful as a tool for increasing lake productivity while reducing the impact on water quality.

Recent trends in chemistry and mass budget of a high altitude lake in the southern Alps (Laghetto Inferiore, Canton Ticino, Switzerland)

Since 1980, surveys have been performed every three years on about 50 small high-altitude Alpine lakes located in the Swiss part of the Lago Maggiore watershed (Southern Central Alps). Since 1996 one of these lakes, Laghetto Inferiore (LI) has been included in the Mountain Lakes Research (MOLAR) program, an extensive European co-operative research project with 23 partners. Laghetto Inferiore, situated at 2074 m a.s.l., has a watershed mainly composed of crystalline silicic rocks, a maximum depth of 33 m and a theoretical renewal time of 41 days. The watershed includes a second lake, Laghetto Superiore (LS), located at 2128 m a.s.l. The surface water pH of LI is around 6.6, while alkalinity is about 30 μeq l-1, increasing to 90 μeq l-1 at the maximum depth. Calculated inputs from atmospheric deposition and the main tributary streams were compared with changes in the lake concentration, and outflow fluxes on a monthly basis. Weathering ranged between 90-130 meq m-2 of alkalinity using a mass budget approach, of which 50% was consumed by the actual (H+) and potential (NH4 +) atmospheric acidity. Chemical trends over the period 1985-1998 show an increase in alkalinity and a decrease in sulphate, due to a reduction in the deposition of atmospheric acidity, and a decrease in nitrate, probably because of increased lake productivity.

Paleolimnological evidence of limnetic nutrient concentration equilibrium in a shallow, macrophyte-dominated lake

Paleolimnological techniques were used to infer long-term changes in trophic status of Lake Opinicon (Ontario), a mesotrophic, shallow, macrophyte-dominated lake, and to assess whether these patterns support the hypothesis of 'alternative equilibria' proposed for shallow lake systems. Analysis of the diatom assemblages indicated cold, oligotrophic to mesotrophic, alkaline conditions during the period shortly following deglaciation (>11000 yr BP). With the establishment of an open spruce-woodland (ca. 11000 yr BP), benthic diatoms dominated, indicating shallow waters and the presence of macrophytes. Pieces of aquatic macrophytes also became common in the core. An increase in lake productivity to mesotrophic levels (diatom-inferred [TP] of 23 μg/L) occurred following the transition to a deciduous forest in the catchment ca. 7500 yr BP. Paleolimnological data did not indicate any significant change in limnetic nutrient concentrations resulting from the hemlock decline (ca. 4800 yr BP) or from extensive flooding with the construction of the Rideau Canal (1828-1832). Modest nutrient enrichment of the lake has occurred more recently with increasing agricultural and residential/recreational activities, with a diatom-inferred [TP] increase to 32 μg/L. However, compared to other, similarly-disturbed deep lakes in the area, Lake Opinicon showed much smaller changes in lakewater conditions, still maintaining a clear-water, mesotrophic state and a large macrophyte community.¶The apparent inertia of Lake Opinicon to changes in trophic status, as compared to other deep, near-by, stratifying lakes in southeastern Ontario, supports the 'alternative equilibria' hypothesis. Paleolimnological evidence suggests that Lake Opinicon has been in a clear-water, equilibrium state dominated by macrophytes since early in its development, and has displayed hysteresis to nutrient enrichment. These results may provide lake managers with important information for effective management strategies of shallow water systems in southeastern Ontario.

Significance and fate of bacterial production in oligotrophic lakes in British Columbia

We measured bacterial abundance, biomass, production rates, and grazing losses to protozoans in three oligotrophic British Columbia lakes and analyzed bacteria abundance and lake productivity data from 11 other lakes, most fertilized with inorganic nutrients to increase their productivity for juvenile salmon. Bacteria contributed about 24% to the phytoplankton-bacteria carbon biomass in the most ultraoligotrophic lakes, and their relative contribution declined to &lt;11% with increasing lake productivity. At increasingly high nutrient loadings to the lakes, bacteria abundance increased and was closely correlated with phytoplankton biomass and productivity. Heterotrophic nanoflagellate (HNF) abundance was positively correlated with bacterial numbers. Grazing experiments revealed that HNF were the primary pathway for moving bacterial production to higher trophic levels in oligotrophic British Columbia lakes, and predation by the ciliate and rotifer microzooplankton community appeared to exert top-down control over the abundance of HNF and the transfer of carbon from bacteria. The HNF and microzooplankton, in turn, were affected by the abundance of crustacean mesozooplankton, principally copepods in our experiments.

Concordance of taxonomic composition patterns across multiple lake assemblages: effects of scale, body size, and land use

We assessed environmental gradients and the extent to which they induced concordant patterns of taxonomic composition among benthic macroinvertebrate, riparian bird, sedimentary diatom, fish, and pelagic zooplankton assemblages in 186 northeastern U.S.A. lakes. Human population density showed a close correspondence to this region's dominant environmental gradient. This reflected the constraints imposed by climate and geomorphology on land use and, in turn, the effects of land use on the environment (e.g., increasing lake productivity). For the region as a whole, concordance was highest among assemblages whose taxa were relatively similar in body size. The larger-bodied assemblages (benthos, birds, fish) were correlated most strongly with factors of broader scale (climate, forest composition) than the diatoms and zooplankton (pH, lake depth). Assemblage concordance showed little or no relationship to body size when upland and lowland subregions were examined separately. This was presumably because differences in the scales at which each assemblage integrated the environment were obscured more locally. The larger-bodied assemblages showed stronger associations with land use than the diatoms and zooplankton. This occurred, in part, because they responded more strongly to broad-scale, nonanthropogenic factors that also affected land use. We argue, however, that the larger-bodied assemblages have also been more severely affected by human activities.

Concordance of taxonomic composition patterns across multiple lake assemblages: effects of scale, body size, and land use

We assessed environmental gradients and the extent to which they induced concordant patterns of taxonomic composition among benthic macroinvertebrate, riparian bird, sedimentary diatom, fish, and pelagic zooplankton assemblages in 186 northeastern U.S.A. lakes. Human population density showed a close correspondence to this region's dominant environmental gradient. This reflected the constraints imposed by climate and geomorphology on land use and, in turn, the effects of land use on the environment (e.g., increasing lake productivity). For the region as a whole, concordance was highest among assemblages whose taxa were relatively similar in body size. The larger-bodied assemblages (benthos, birds, fish) were correlated most strongly with factors of broader scale (climate, forest composition) than the diatoms and zooplankton (pH, lake depth). Assemblage concordance showed little or no relationship to body size when upland and lowland subregions were examined separately. This was presumably because differences in the scales at which each assemblage integrated the environment were obscured more locally. The larger-bodied assemblages showed stronger associations with land use than the diatoms and zooplankton. This occurred, in part, because they responded more strongly to broad-scale, nonanthropogenic factors that also affected land use. We argue, however, that the larger-bodied assemblages have also been more severely affected by human activities.

How does the vertical distribution of chlorophyll vary in littoral sediments of small lakes?

1. In this study, sediment chlorophyll profiles at twenty littoral stations in three oligo‐mesotrophic lakes were compared to test whether the vertical distribution of chlorophyll is related to site characteristics (light availability, temperature, physical disturbances) and whether these profiles differ between shallow and deep portions of the littoral zone.2. The magnitude of chlorophyll peaks at the sediment surface did not vary with light intensity. Chlorophyll peaks in the shallow littoral zone had a weak tendency to decrease with increasing effective fetch. The magnitude of chlorophyll peaks at deeper sites was more closely related to water temperature than to substrate slope.3. High chlorophyll concentrations were measured down to 1–3 cm in the sediments, both at shallow (&lt; 2.5 m) and deep (4–10 m) stations. The depth to which high chlorophyll was found in sediments did not vary with effective fetch or sediment water content, two indices of wave disturbance in the shallow littoral zone, or with substrate slope, an index of sediment stability in the deep littoral zone. Sediment mixing is apparently not related to common indices of physical disturbances.4. Between 8 and 100% of sediment surface chlorophyll was ‘retained’ 4–5 cm into the sediments. The proportion of chlorophyll ‘retained’ in littoral sediments increased with increasing depth, increasing lake productivity (total phosphorus concentration) and increasing lake pH.5. Among‐core variability (standard error/mean) in chlorophyll concentration at the sediment surface ranged from less than 1% to 33% at different stations and was highest at shallow, exposed sites. These levels of variability are similar to those found in other periphytic communities.

A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district

Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing.

Biostratigraphic Evidence of the Allerød-Younger Dryas-Preboreal Oscillation in Northern Iceland

Basal sediments of Lake Torfadalsvatn, northern Iceland, record changes in terrestrial and limnic environments in the period 11,300-9000 14C yr B.P. These changes were probably forced by climate and connected with displacements of the marine polar front and sea-ice margin. Pollen, spores, green algae (Pediastrum), saturation isothermal remanent magnetization, and carbon content of the basal sediments provide the first detailed biostratigraphic record of the last glacial-interglacial transition in Iceland. During the first pioneer phase, beginning at ca. 11,300 14C yr B.P., grasses and fell-field herbs became established, and lake productivity was very low. At ca. 10,900 14C yr B.P., climatic and soil conditions became favorable for shrubs and dwarf shrubs. This change, together with increased limnic productivity, clearly indicates long seasons without ice-cover in the sea immediately north of Iceland. A return to a colder climate (Younger Dryas), probably in connection with a southward displacement of the marine polar front, occurred by 10,600 14C yr B.P. Shrub and dwarf-shrub vegetation disappeared, and limnic productivity diminished. A second pioneer vegetation phase, dominated by Oxyria/Rumex and grasses, was initiated by a change to longer seasons without sea ice at ca. 9900 14C yr B.P. This warming is also evident as a contemporaneous increase in lake productivity. After ca. 9400 14C yr B.P. the reestablishment of dwarf-shrub heaths and very high limnic productivity indicate further warming.

Replacement of herbivore zooplankton species along gradients of ecosystem productivity and fish predation pressure

In general the pooling of major taxa did not provide a basis for classifying zooplankton communities in 342 large Norwegian lakes, as neither cladocerans nor calanoids varied systematically with lake productivity or fish predation pressure. At the species level, most herbivorous cladocerans and calanoids, which constituted three quarters of the metazoan zooplankton biomass, differed in their preference for lake productivity and fish community and could be distinctly grouped according to these variables by canonical correspondance analysis. The analysis pointed out one oligotrophic and one eutrophic specialist among the herbivorous cladocerans, while two of the calanoids were oligotrophic specialists. The biomasses of cladocerans, calanoids, or daphnids were poorly correlated with both lake productivity and fish predation, whereas shifts in average size and species distribution could be attributed to these variables. At low lake productivity, chemical variables such as pH and Ca, as well as the species' physiological adaptations, appear as the main determinants for the competitive advantage and relative success of herbivorous species. Fish community composition changes with increasing lake productivity, but only at very high fish predation intensity (cyprinid communities) did the effects of predation become the main determinant of the zooplankton community, superimposed on the lake productivity.