Effects of fish density on planktonic communities and water quality in a manipulated forest pond

We examined the effects of fish on lower trophic levels in a small pond in eastern Finland. The pond was divided into four sections with plastic curtains and stocked with crucian carp (Carassius carassius); two sections had low (4.4–5.5 g m^–3) and two high (10.4–13.7 g m^–3) densities of fish. In the summer of 1987, the pond was sampled weekly for phyto- and zooplankton until the fish were removed by rotenone in September after a three month experiment. Fish density as well as the extent of macrophyte cover had a considerable impact on planktonic communities and water quality. Mean zooplankton biomass was significantly lower and phytoplankton biomass higher, at high fish density. Water transparency was correlated negatively with chlorophyll-a at low fish density but turbidity appeared to reduce transparency at high fish density. The composition and dynamics of the plankton also differed at different fish densities. The mechanisms behind these effects, and the influences of habitat and fish behaviour on the results, are discussed.     

Experimental study of the impacts of silver carp on plankton communities of eutrophic Villerest reservoir (France)

We examined the impact of five silver carp biomass levels (0, 8, 16, 20, and 32 g m⁻³) on plankton communities and water quality of Villerest eutrophic reservoir (France). We realized the experiments using outdoor mesocosms. The presence of silver carp led to changes in zooplankton and phytoplankton assemblages. High fish biomass strongly reduced cladoceran abundance (through predation). Silver carp inefficiently grazed down particles < 20 μm. More importantly, however, the suppression of herbivorous cladocerans resulted in the increase of small size algae which were relieved from grazing and benefit from high nutrient concentrations. In contrast, in mesocosms without fish, the dominance of cladocerans (mainly Daphnia) controlled small size algae and probably also larger size algae (colonial chlorophytes, cyanobacteria). Thus, the Secchi disc transparency increased markedly. Through cascade effects, the modification of grazers communities led to changes in the utilization patterns of the added nutrients by phytoplankton communities. In high fish biomass treatments, nutrients were more efficiently accumulated into particulate fractions compared with no-fish and low-fish biomass treatments that were characterized by higher dissolved nutrients concentrations. Zooplankton was an essential source of food for silver carp. The productivity of zooplankton sustained a moderate silver carp biomass (up to 16 g m⁻³). In the presence of the highest fish biomass, the productivity of zooplankton was not large enough and silver carps fed on additional phytoplankton. Although mesocosms with high fish biomass were characterized by a slight cyanobacteria development compared with other fish mesocosms, silver carp was not effective in reducing cyanobacteria dominance. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of omnivorous filter‐feeding fish and nutrient enrichment on the plankton community and water transparency of a tropical reservoir

1. The major aim of this study was to test the hypothesis that nutrient enrichment and the introduction of the Nile tilapia (Oreochromis niloticus), an exotic omnivorous filter‐feeding fish, operate interdependently to regulate plankton communities and water transparency of a tropical reservoir in the semi‐arid northeastern Brazil. 2. A field experiment was performed for 5 weeks in 20 enclosures (9.8 m³) to which four treatments were randomly allocated: tilapia addition (F), nutrient addition (N), tilapia and nutrient addition (F + N) and a control treatment with no tilapia or nutrient addition (C). A two‐way repeated measures anova was undertaken to test for time, tilapia and nutrient effects and their interactions on water transparency, total phosphorus and total nitrogen concentrations, phytoplankton biovolume and zooplankton biomass. 3. Nutrient addition had no effect except on rotifer biomass, but there were significant fish effects on the biomass of total zooplankton, copepod nauplii, rotifers, cladocerans and calanoid copepods and on the biovolume of total phytoplankton, large algae (GALD ≥ 50 μm), Bacillariophyta and Zygnemaphyceae and on Secchi depth. In addition, we found significant interaction effects between tilapia and nutrients on Secchi depth and rotifers. Overall, tilapia decreased the biomass of most zooplankton taxa and large algae (diatoms) and decreased water transparency, while nutrient enrichment increased the biomass of rotifers, but only in the absence of tilapia. 4. In conclusion, the influence of fish on the reservoir plankton community and water transparency was significant and even greater than that of nutrient loading. This suggests that biomanipulation of filter‐feeding tilapias may be of importance for water quality management of eutrophic reservoirs in tropical semi‐arid regions.     

Invasion of a Top Predator into an Epipelagic Ecosystem can bring a Paradoxical Top-Down Trophic Control

We apply mathematical modeling to explore different scenarios of invasion of a top predator (carnivorous zooplankton or planktivorous fish) into an epipelagic plankton ecosystem. We use a ‘minimal’ model of three nonlinear ordinary differential equations (nutrient–phytoplankton–herbivores) with the top predator density as a time-dependent parameter. The ecosystem shows different types of response, which can be described in terms of top-down trophic control. Our investigation indicates that under certain conditions the plankton ecosystem model demonstrates a surprising kind of response: in a wide range of realistic ecosystem parameters the invasion of the top predator leads to a prominent increase in the average density of zooplankton and to a resulting decrease of phytoplankton density. This phenomenon is opposite to the ‘typical’ top–down control when the carnivore pressure decreases zooplankton density which, in turn, increases phytoplankton biomass. We call the revealed type of top-down control ‘paradoxical’. Examples of such a response in natural aquatic ecosystems were reported earlier but no clear explanation has been provided hitherto. In this paper, we analyze possible mechanisms of ‘paradoxical top–down control’ and show that it can occur in eutrophic epipelagic ecosystems subject to high rate of cross-pycnocline exchange.     

Predator-induced bottom-up effects in oligotrophic systems

Five treatments (replication n=2) were applied to mesocosms in an oligotrophic lake (TP=6–10 µg 1^-1) to assess the effects of fish on planktonic communities. The treatments were: (1) high fish (30 kg ha^–1 Lepomis auritus, Linnaeus), (2) low fish (10 kg ha^–1), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. Total phosphorus, chlorophyll a, zooplankton biomass, and species richness decreased from high fish > low fish > control > low removal > high removal treatments. The fish treatments were dominated by crustacean zooplankton, while rotifers outnumbered the other zooplankters in the removal treatments. Calculations of zooplankton grazing rates suggested that clearance rates seldom exceeded 2% of the enclosure volume d^–1 and were unlikely to have had much influence on phytoplankton biomass. Calculations from a phosphorus bioenergetics model revealed that when fish were present, their excretion rates were higher than the rates ascribed to zooplankton. Diet analysis showed that the fish derived most of their energy from the benthos and periphyton, and that fish excretion and egestion made significant contributions to the very oligotrophic pelagic phosphorus pool. In the absence of fish, zooplankton excretion was highest in the control treatments and lowest in the zooplankton removal treatments. Our results suggest that in oligotrophic systems, planktivorous fish can be significant sources of phosphorus and that fish and zooplankton induced nutrient cycling have significant impacts on planktonic community structure.     

A simple method of feeding coregonid larvae on frozen zooplankton

7 days old larvae of Coregonus lavaretus L. (local name: ‘Blaufelchen’) were fed on zooplankton that had been frozen at—18°C for 1 year. 4 times per day the pieces of frozen plankton were added into the preaeration tank thus providing that freshly thawed zooplankters were offered to the fish. Within 10 weeks the fish reached a total length of 30.5 ± 5.5 ram and an average dry weight of 22 mg. Within this period the survival was 45 % of the initial number. After a period of 7 weeks feeding on long-term frozen pankton 400 fish were switched to living zooplankton and after 9 weeks 400 fish were switched to short-term frozen plankton. The growth of those fish that were fed on living plankton was significantly faster than it was when frozen zooplankton was given. A switch of coregonids which were prefed on frozen zooplankton to dry diet proved to be successful as soon as the fish had reached a body length of 40 mm.     

A 2-year experimental study on nutrient and predator influences on food web constituents in a shallow lake of north-west Spain

1. A 2‐year study was carried out on the roles of nutrients and fish in determining the plankton communities of a shallow lake in north‐west Spain. Outcomes were different each year depending on the initial conditions, especially of macrophyte biomass. In 1998 estimated initial ‘per cent water volume inhabited’ (PVI) by submerged macrophytes was about 35%. Phytoplankton biomass estimated as chlorophyll a was strongly controlled by fish, whereas effects of nutrient enrichment were not significant. In 1999 estimated PVI was 80%, no fish effect was observed on phytoplankton biomass, but nutrients had significant effects. Water temperatures were higher in 1998 than in 1999. 2. In the 1998 experiment, cladoceran populations were controlled by fish and cyanobacteria were the dominant phytoplankton group. There were no differences between effects of low (4 g fresh mass m^?2) and high (20 g fresh mass m^?2) fish density on total zooplankton biomass, but zooplankton biomass was higher in the absence of fish. With the high plant density in 1999, fish failed to control any group of the zooplankton community. 3. Total biovolume of phytoplankton strongly decreased with increased nutrient concentrations in 1998, although chlorophyll a concentrations did not significantly change. At higher nutrient concentrations, flagellate algae became more abundant with likely growth rates that could have overcompensated cladoceran feeding rates. This change in phytoplankton community composition may have been because of increases in the DIN : SRP ratio. Both chlorophyll a concentration and total phytoplankton biovolume increased significantly with nutrients in the 1999 experiment. 4. A strong decline of submerged macrophytes was observed in both years as nutrients increased, resulting in shading by periphyton. This shading effect could account for the plant decline despite lower water turbidity at the very high nutrient levels in 1998.     

A review of planktivorous fishes: Their evolution,feeding behaviours,selectivities, and impacts

The classical approach of limnologists has been to consider the interactions between lake ecosystem components as an unidirectional flow of influence from nutrients to the phytoplankton, to the zooplankton, and finally to the fish, through successive controls by physical, chemical, and biological processes (Strakraba, 1967). The effect of planktivorous fishes on zooplankton and phytoplankton communities was not recognized until the studies of Hrbáek et al. (1961), Hrbáek (1962), Brooks & Dodson (1965) and Strakraba (1965). They showed that (1) in ponds and lakes in the presence of planktivorous fishes the zooplankton communities were composed of smaller bodied species than in those lacking planktivores, and (2) the resulting small-bodied zooplankton communities affected the phytoplankton communities. Although the variability of the phytoplankton response to fish predation showed the importance of other factors (such as nutrient limitation and interspecific competition of algae), these studies emphasized that zooplankton and phytoplankton communities can be affected by the feeding selectivity of planktivorous fishes. During the last two decades, many limnological studies have focused on this dramatic impact of fish on plankton communities. The direct response of zooplankton communities to visual fish predation (i.e. particulate feeding) has been of major interest, whereas the multilevel effects of filter-feeding fish (predation on zooplankton plus grazing on phytoplankton) have been neglected. The objectives of this review are to document fish-plankton interrelationships in order to (1) provide insights into the impact of fish on plankton communities, and (2) outline mechanistic models of planktivory according to the feeding repertory and the selectivity of the fish, the adaptive responses of the plankton, and the environmental conditions.The approach adopted here is based on field and laboratory experimental results derived from the literature on tropical and temperate freshwater (occasionally marine) systems. Four types of planktivorous fish are distinguished: the gape-limited larvae and small fish species, the particulate feeders, the pump filter feeders, and the tow-net filter feeders. For each type of planktivore, the mechanisms of prey selection are analyzed from the point of view of both the predator and the prey. To investigate the main determinants of the predator feeding selectivity, and to discuss its potential effects on prey communities, the predation-act is divided into a sequence of successive events (Holling, 1966): detection, pursuit, capture, retention, and digestion for particulate feeders; and capture, retention, and digestion for filter feeders. The strengths and weaknesses of various measures of selectivity (i.e. electivity indices), as well as their appropriate usages are considered. Available prey selection models and optimal foraging theories are analyzed for the different planktivore feeding modes. Mechanistic models based on Holling's (loc. cit.) approach are proposed for each feeding mode to determine differential prey vulnerabilities and optimal diet breadth.This review has application to several fields, including general ecology, limnology, fisheries management (for example, utilization of planktonic resources, stocking, introduction, or maintenance of natural fish populations), and biological control of the eutrophication processes (biomanipulation approaches). It emphasizes the real need for more knowledge of the feeding selectivity and food utilization of planktivores. It concludes that predator and prey are mutually adapted. Thus, in most cases, study of plankton dynamics and water quality should include the assessment of fish predation and grazing pressures.     

Zooplankton response to climate warming: a mesocosm experiment at contrasting temperatures and nutrient levels

Zooplankton community response to the combined effects of nutrients and fish (hereafter N + F) at contrasting temperatures was studied in a long-term experiment conducted in 24 shallow lake mesocosms with low and high nutrient levels. We found a positive effect of N + F on zooplankton biomass, chlorophyll-a and turbidity. In contrast, zooplankton species and size diversity decreased with added N + F, as did submerged macrophyte plant volume inhabited (PVI). The community composition of zooplankton in high N + F mesocosms was related to chlorophyll-a and turbidity and to macrophyte PVI in the low N + F mesocosms. Macrophytes can protect zooplankton from fish predation. Compared to N + F effects, temperature appeared to have little effect on the zooplankton community. Yet analysis of community heterogeneity among treatments indicated a significant temperature effect at high N + F levels. The results indicate an indirect temperature effect at high N + F levels that can be attributed to temperature-dependent variation in fish density and/or chlorophyll-a concentration.     

Cascading effects of larval Crucian carp introduction on phytoplankton and microbial communities in a paddy field: top-down and bottom-up controls

Effects of fish predation propagate through aquatic food webs, where the classical grazing food chain and microbial loop are interwoven by trophic interactions. The overall impact on aquatic food webs is further complicated because fish may also exert bottom-up controls through nutrient regeneration. Yet, we still have limited information about cascading effects among fish, zooplankton, phytoplankton, and microbes. In this study, we performed a mesocosm experiment to evaluate effects of fish introduction on plankton communities. Six plots were set in factorial combination with fish introduction and rice straw plowing in a paddy field, and the experiment was continued for 4 weeks. Introduction of fish significantly increased chlorophyll a concentrations in smaller size fractions (<15 μm) and abundances of filamentous bacteria (>5 μm in length) and heterotrophic nanoflagellates in 3–15 μm fraction. Microbes in 0.8–3 μm fraction showed increasing but not significant trends in response to fish introduction. These results indicate cascading effects of fish predation operating via two pathways, one through grazing food chain and the other through microbial food web. Phytoplankton community compositions shifted in similar fashion in all plots until 1 week after fish introduction, and then diverged between plots with and without fish thereafter. Bottom-up effects of fish introduction were suggested by increases of total chlorophyll a and inedible phytoplankton species in response to fish introduction. This study provides an example of how fish predation regulates biomass and structure of phytoplankton and microbial communities.     

Effects of Biomanipulation on Fish and Plankton Communities in Ten Eutrophic Lakes of Southern Finland

The effects of biomanipulation were studied in ten Finnish lakes to determine responses in fish and plankton communities and water quality after mass removal of cyprinids. From 1997 to 2001, the fish communities shifted from the dominance of large cyprinids to an explosion of small cyprinids and a higher proportion of piscivores in effectively biomanipulated lakes (>200 kg ha⁻¹ 3 yr⁻¹). The biomass of cyanobacteria decreased, and the duration of the blooms shortened and shifted towards the autumn. Decreased concentrations and slower cycling of nutrients and increased grazing by cladocerans probably affected the declined biomass of cyanobacteria. Less intensive sediment disturbance and increased phosphorus-retention in fast growing fish biomass may have turned the role of the fish assemblage from ‘nutrient recycler’ to ‘nutrient storage’. Increased potential grazing pressure, higher proportion of edible algae, and lower chlorophyll a:total phosphorus ratio indicated strengthened herbivore control. A high mass removal catch in relation to trophic state, low background turbidity, and bearable external loading favoured the successful biomanipulation, whereas intensive cyprinid reproduction, high nutrient loading and non-algal turbidity hindered the recovery. Three important issues should be noticed before biomanipulation in Finland: (1) careful selection of target lake, (2) well-planned, effective and long-lasting biomanipulation and (3) sustainable management of piscivores. An erratum to this article is available at .     

Impact of mass mortality of a mosquito fish,Gambusia affinison the ecology of a fresh water eutrophic lake (Lake Naini Tal,India)

Between 28th March and 4th April, 2000 a fungal infection killed >80% of the most abundant planktivorous fish, Gambusia affinis in Lake Naini Tal, Uttaranchal, India. In response to this mortality, planktonic communities and some eutrophication-related parameters viz., primary productivity, phosphate–phosphorus, nitrate–nitrogen and transparency of the water, were considerably changed. Total zooplankton number more than doubled, phytoplankton number reduced nearly to half, primary production and phosphate-phosphorus was dramatically reduced, while nitrate–nitrogen and water clarity increased. The phytoplankton decline was caused by increased zooplankton grazing (top-down control) rather than phosphorus deficiency (bottom-up control). After 3 months, Gambusia and planktonic communities and nutrient levels reverted back almost to their pre-mortality state. Thus removal of G. affinis could improve water quality of Lake Naini Tal.     

Impact of cyprinids on zooplankton and algae in ten drainable ponds

To study the impact of cyprinids on algae, zooplankton and physical and chemical water quality, ten drainable ponds of 0.1 ha (depth 1.3 m) were each divided into two equal parts. One half of each pond was stocked with 0 + cyprinids (bream, carp and roach of 10–15 mm), the other was free of fish. The average biomass of the 0 + fish at draining of the ponds was 466 kg ha^–1, to which carp contributed about 80%.The fish and non-fish compartments showed significant differences. In the non-fish compartments the density of Daphnia hyalina was 10–30 ind. l^–1 and that of Daphnia magna 2–4 ind. l-^–1, whereas in the fish compartments densities were c. 1 ind. l^–1. Cyclopoid copepods and Bosmina longirostris, however, showed higher densities in the fish compartments. The composition of algae in the two compartments differed only slightly, but the densities were lower in the non-fish compartments. The significant difference in turbidity was probably caused by resuspension of sediment by carp. No significant difference in nutrient concentration between the compartments was found.     

Nutrient enrichment and zooplankton effects on the phytoplankton community in microcosms from El Andino reservoir (Venezuela)

To quantify the effects of nutrient enrichment (N and P) and zooplankton grazing on the phytoplankton community structure of El Andino reservoir (Venezuela), in situ microcosms were installed for 6–7 days. Microcosms consisted of polyethylene bags (42 cm × 71 cm, non-cylindrical shaped) filled with 10 l of filtered epilimnetic water. Experiments were carried out on a monthly basis from January to December 1993. The lack/addition of nutrients was cross-classified with the absence/presence of zooplankton, resulting in an experimental design of four treatment levels: (1) no nutrient addition, zooplankton absent (C); (2) nutrient addition (150 NH₄Cl mol ml^–1 and 10 KH₂PO₄ mol ml^–1; 1 ml per l of sample), zooplankton absent (N); (3) no nutrient addition, zooplankton present (collected from the reservoir water column using a 6-m vertical tow with a 80-m plankton net) (Z); and (4) nutrient addition (as in [2]), zooplankton present (as in [3]) (NZ). Treatments were triplicated, and samples were collected at the start and end of each experiment. Significant differences between treatments were determined using a two-way ANOVA at p<0.05. Nutrient enrichment caused an increase in phytoplankton biomass, with the increase of all algal groups, except Pyrrhophyta. In spite of this, relative proportions of Cyanobacteria decreased in most cases. Chlorophyta and Bacillariophyta increased, probably due to their greater competitive abilities for phosphorus. After enrichment, Scenedesmus was the dominant species from January to June, while from July to December, Dactylococcopsis and Lyngbya dominated in the enriched microcosms. Zooplankton affected the phytoplankton community in microcoms through grazing and nutrient (mainly P) regeneration. Cladocerans (Ceriodaphnia cornuta, Moina micrura and Diaphanosoma sp.) mainly grazed on diatoms, although particulate material was present in almost all the gut contents analyzed. Particulate material probably consisted of micro-algae, detritus, bacteria, triturated algae and mineral particles. Ostracoda mainly fed on Peridinium and particulate material, whereas Thermocyclops sp. and rotifers (Brachionus spp. and Keratella spp.) mainly ingested particulate material. On the other hand, zooplankton excretion caused a slight increase in phytoplankton biomass and P concentrations in microcosms with the animals present. The effects of nutrient and zooplankton did not interact in most cases. Experimental results suggest that, at the initial stages of a eutrophication process, phytoplankton could increase their abundance and biomass, but might not change its community structure. Since there was a strong correlation between phosphorus and chlorophyll-a (bottom-up control), it is suggested that eutrophication could be avoided by controlling P input to the reservoir.     

Climate Change and Eutrophication Induced Shifts in Northern Summer Plankton Communities

Marine ecosystems are undergoing substantial changes due to human-induced pressures. Analysis of long-term data series is a valuable tool for understanding naturally and anthropogenically induced changes in plankton communities. In the present study, seasonal monitoring data were collected in three sub-basins of the northern Baltic Sea between 1979 and 2011 and statistically analysed for trends and interactions between surface water hydrography, inorganic nutrient concentrations and phyto- and zooplankton community composition. The most conspicuous hydrographic change was a significant increase in late summer surface water temperatures over the study period. In addition, salinity decreased and dissolved inorganic nutrient concentrations increased in some basins. Based on redundancy analysis (RDA), warming was the key environmental factor explaining the observed changes in plankton communities: the general increase in total phytoplankton biomass, Cyanophyceae, Prymnesiophyceae and Chrysophyceae, and decrease in Cryptophyceae throughout the study area, as well as increase in rotifers and decrease in total zooplankton, cladoceran and copepod abundances in some basins. We conclude that the plankton communities in the Baltic Sea have shifted towards a food web structure with smaller sized organisms, leading to decreased energy available for grazing zooplankton and planktivorous fish. The shift is most probably due to complex interactions between warming, eutrophication and increased top-down pressure due to overexploitation of resources, and the resulting trophic cascades.     

The nature of zooplankton spatial heterogeneity in a nonriverine impoundment

Knowledge of spatial heterogeneity characteristic of reservoir plankton communities is fundamental to a variety of ecological studies. Degree of spatial heterogeneity in the zooplankton community of Center Hill Reservoir, with water residence times of 50–250 days, was positively correlated with rate of water influx. Important spatial differences resulted from the contrast between zooplankton associated with new and longer-impounded water. The nature of spatial heterogeneity differed fundamentally from the more riverine impoundments where spatial differences are often persistent and characterized by gradual change (as opposed to contrast) in plankton assemblages with respect to location. Magnitude of plankton spatial heterogeneity in nonriverine impoundments may be predictable from inflow rates. Areas, between which major differences in plankton communities exist, may also be definable from knowledge of inflow dispersal patterns in these impoundments.     

Near- bottom depletion of zooplankton over coral reefs: I: diurnal dynamics and size distribution

Changes in the near-bottom abundance of zooplankton on scales of centimeters to meters and hours to seasons are of great importance to corals and other benthic zooplanktivores. Our objective was to characterize such spatio-temporal changes over several coral reefs in the Gulf of Aqaba, Red Sea. Using arrays of underwater pumps, we found a substantial depletion of zooplankton near the bottom. Vertical gradients in zooplankton abundance were steeper during the night than day, mostly due to a greater nocturnal increase in zooplankton biomass higher in the water column. On average, the layer <1 m above bottom (mab) was depleted by 2.6±2.2 mg m^–3 (46±35%) and 1.4±1.4 mg m^–3 (37±43%) during night and day, respectively. A long time series of bi-weekly samples at 0.5 mab, lasting 1.5 years, indicated a doubling of the biomass during the night with no apparent seasonality. The diel change was due to an increase in the abundance of only large (>200 m) zooplankters around dusk and their disappearance in the morning. Diurnal predation by zooplanktivorous fish, sediment resuspension by benthivorous fish and zooplankton behavior appear to control the dynamics of suspended particles over the reef, creating sharp vertical gradients and a remarkable diel cycle in the ratio between nutritious plankton and inorganic particles.     

The impact of zooplankton status on the management of Lake Kinneret (Israel)

Monthly averages of standing stock wet biomass of zooplankton in Lake Kinneret (Israel) varied between 11 and 76 g m^–2 during 1969–1981, with the exception of two months. Averaged contributions of different groups were: Cladocera 58%, Copepoda 35% and Rotifera 7%. Total standing crop wet biomass is highest during January–June, averages varied between 35 and 50 g m^–2, and decreases during summer–fall (23–36 g m^–2). The winter biomass of Cladocera fluctuated between 22 and 35 g m^–2 and dropped to a range of 9–23 g m^–2 in summer, whereas copepod biomass varied very little around an average of 18 g (ww) m^–2 with the exception of low values from April to June. The stock biomass of Rotifera is relatively high during winter floods season (December-March) whilst in summer it is very low.Young stages of fish in Lake Kinneret feed mostly on zooplankton and zoobenthic forms. The most abundant fish in the Kinneret ecosystem, Mirogrex terraesanctae terraesanctae, also feed on zooplankton at the adult stage throughout the year, and herbivorous fish consume zooplankton during the summer when lake plankton resources are limited.The summer ecosystem of Lake Kinneret is characterised as a steady state type, in which the impact of the zooplankton-chain is of great importance. Increase of predation pressure on zooplankton by fish can disequilibrate the balanced trophic relations existing between nannoplankton production and zooplankton grazing capacity. Such a situation can lead to organics accumulation as nannoplankton blooms, resulting in water quality deterioration. Management options aimed at preventing collapse of zooplankton populations are discussed.     

The role of light for fish–zooplankton–phytoplankton interactions during winter in shallow lakes – a climate change perspective

1. Variations in the light regime can affect the availability and quality of food for zooplankton grazers as well as their exposure to fish predation. In northern lakes light is particularly low in winter and, with increasing warming, the northern limit of some present-day plankton communities may move further north and the plankton will thus receive less winter light.
2. We followed the changes in the biomass and community structure of zooplankton and phytoplankton in a clear and a turbid shallow lake during winter (November–March) in enclosures both with and without fish and with four different light treatments (100%, 55%, 7% and <1% of incoming light).
3. In both lakes total zooplankton biomass and chlorophyll- a were influenced by light availability and the presence of fish. Presence of fish irrespective of the light level led to low crustacean biomass, high rotifer biomass and changes in the life history of copepods. The strength of the fish effect on zooplankton biomass diminished with declining light and the effect of light was strongest in the presence of fish.
4. When fish were present, reduced light led to a shift from rotifers to calanoid copepods in the clear lake and from rotifers to cyclopoid copepods in the turbid lake. Light affected the phytoplankton biomass and, to a lesser extent, the phytoplankton community composition and size. However, the fish effect on phytoplankton was overall weak.
5. Our results from typical Danish shallow eutrophic lakes suggest that major changes in winter light conditions are needed in order to have a significant effect on the plankton community. The change in light occurring when such plankton communities move northwards in response to global warming will mostly be of modest importance for this lake type, at least for the rest of this century in an IPCC A2 scenario, while stronger effects may be observed in deep lakes.     

Quantifying Ecosystem Controlsand Their Contextual Interactionson Nutrient Export fromDeveloping Forest Mesocosms

The complexity of natural ecosystems makes it difficult to compare the relative importance of abiotic and biotic factors and to assess the effects of their interactions on ecosystem development. To improve our understanding of ecosystem complexity, we initiated an experiment designed to quantify the main effects and interactions of several factors that are thought to affect nutrient export from developing forest ecosystems. Using a replicated 2 × 2 × 4 factorial experiment, we quantified the main effects of these factors and the factor interactions on annual calcium, magnesium, and potassium export from field mesocosms over 4 years for two Vermont locations, two soils, and four different tree seedling communities. We found that the main effects explained 56%–97% of total variation in nutrient export. Abiotic factors (location and soil) accounted for a greater percentage of the total variation in nutrient export (47%–94%) than the biotic factor (plant community) (2%–15%). However, biotic control over nutrient export was significant, even when biomass was minimal. Factor interactions were often significant, but they explained less of the variation in nutrient export (1%–33%) than the main effects. Year-to-year fluctuations influenced the relative importance of the main effects in determining nutrient export and created factor interactions between most of the explanatory variables. Our study suggests that when research is focused on typically used main effects, such as location and soil, and interactions are aggregated into overall error terms, important information about the factors controlling ecosystem processes can be lost.