Impact of water regime and fish predation on zooplankton resting egg production and emergence

The emergence of zooplankton from ephemeral areas upon re-wettinghas been suggested as a significant contributor to billabongproductivity. Sixteen experimental billabongs were designedto test two hypotheses (i) that changing the pattern of floodingalters zooplankton abundance within billabongs, and (ii) thepresence of small planktivorous fish alters the zooplanktoncommunity structure and diversity within billabongs. Resultsindicated that flooding caused microcrustaceans to increasein abundance but not rotifers. This response was not modifiedby changing the time of flooding. The presence of the planktivorousfish had only a small influence on the structure of rotiferassemblages, whereas microcrustacean assemblages became dominatedby juveniles. Emergence of rotifers and microcrustaceans fromsediment taken from the experimental billabongs and incubatedunder controlled conditions indicated the existence of a largereservoir of resting stages. The number of resting stages inthe sediment had not been modified by changing the season offlooding in the experimental billabongs, but it was modifiedby permanent inundation, which may have removed environmentalcues. The presence of planktivorous fish also appeared to influencethe number of resting stages that accumulated in the sediment.Emergence under stable laboratory conditions was rapid. Thiswas not the case under more variable natural conditions.     

Effects of hydrological variation and planktivorous competition on macroinvertebrate community structure in experimental billabongs

1. To study two factors which are predicted as causing changes to community structure in cut-off meanders (colloquially known in Australia as billabongs, a term of aboriginal origin), 16 experimental billabongs were constructed. These were designed to test two hypotheses: (a) that the structure of macrophyte and invertebrate communities within billabongs is altered by changing the pattern of flooding; and (b) that the presence of small planktivorous fish alters invertebrate community structure and diversity within billabongs.
2. An increase in the duration of flooding seems to favour animals better adapted to a greater availability of macrophyte habitat. Changing the seasonality of flooding resulted in prolonging of the time water was available over the summer months.
3. The presence of a planktivorous fish appears to affect macroinvertebrate communities through competition with other planktivores. Variable top-down pressure may create differing successional patterns and ultimately different communities at lower trophic levels.     

Relative impacts of Daphnia grazing and direct stimulation by fish on phytoplankton abundance in mesocosm communities

• 1 Planktivorous fish were hypothesised to influence the abundance of algal biomass in lakes by changing zooplankton grazing, affecting zooplankton nutrient recycling and by direct recycling of nutrients to phytoplankton. The relative roles of direct fish effects vs. zooplankton grazing were tested in mesocosm experiments by adding to natural communities large grazing zooplankton (Daphnia carinata) and small planktivorous fish (mosquitofish or juveniles of Australian golden perch).
• 2 The addition of Daphnia to natural communities reduced the numbers of all phytoplankton less than 30 µm in size, but did not affect total biomass of phytoplankton as large Volvox colonies predominated.
• 3 The addition of Daphnia also reduced the abundance of some small (Moina, Bosmina, Keratella) and large (adult Boeckella) zooplankton, suggesting competitive interactions within zooplankton.
• 4 The addition of mosquitofish to communities containing Daphnia further reduced the abundance of some small zooplankton (Moina, Keratella), but increased the numbers of Daphnia and adult Boeckella. In spite of the likely increase in grazing due to Daphnia, the abundance of total phytoplankton and dominant alga Volvox did not decline in the presence of mosquitofish but was maintained at a significantly higher level than in control.
• 5 The addition of juveniles of golden perch to communities containing Daphnia reduced the abundance of small zooplankton (Moina), increased the abundance of large zooplankton (adult Boeckella) but had no significant effect on Daphnia and total phytoplankton abundance.
• 6 The results of the present study suggest that some planktivorous fish can promote the growth of phytoplankton in a direct way, probably by recycling nutrients, and even in the presence of large grazers. However, the manifestation of the direct effect of fish can vary with fish species.

     

Field and experimental evidence of the effect of Jenynsia multidentata, a small omnivorous–planktivorous fish, on the size distribution of zooplankton in subtropical lakes

1. Small cladocerans, copepod nauplii and rotifers often dominate the zooplankton community in tropical and subtropical lakes. This is probably because of high predation pressure by small omnivorous–planktivorous fish, but experimental evidence is scarce.
2. This study used two approaches to test the effect of the small omnivorous–planktivorous fish species Jenynsia multidentata , which is frequently abundant in (sub)tropical eutrophic lakes in South America, on the size distribution of zooplankton. In Lake Blanca (Uruguay), which lacks any piscivores, we sampled seasonally for both fish and zooplankton. We also conducted an outdoor mesocosm experiment with treatments containing or lacking J. multidentata .
3. Together, the empirical and experimental data suggest that J. multidentata predation plays an important role in modulating the size structure of the zooplankton community in subtropical lakes. In the absence of J. multidentata , stocked large-sized zooplankters like Daphnia obtusa were abundant in the experiments, while small-sized zooplankton dominated in the presence of fish, as they did in the lake itself from spring to the end of the season.     

Do the effects of piscivorous largemouth bass cascade to the plankton?

Ecologists have hypothesized that an increase in the biomass of piscivorous fish in lakes will cause a decrease in populations of planktivorous fish, an increase in the size of herbivorous zooplankton and a decrease in the biomass of phytoplankton. Here we present an experimental test of whether the effects of largemouth bass (Micropterus salmoides) cascade to the planktivorous fish, zooplankton and phytoplankton of a 15-ha water storage reservoir. A pilot study indicated that the reservoir was eutrophic with dense populations of planktivorous fish dominated by threadfin shad (Dorosoma petenense). No piscovorous fish were present in the reservoir. We conducted a one-month mesocosm experiment using water and plankton from the reservoir showing that the presence of threadfin shad reduced large-sized zooplankton and increased the productivity and biomass of phytoplankton. To test whether the effects of piscivorous fish could cascade to the plankton, we assessed the effects of the addition of piscivorous largemouth bass on the planktivorous fish, zooplankton and biomass of phytoplankton of the reservoir by monitoring the reservoir during the year before and the two years after largemouth bass were stocked. In the second year after the addition of largemouth bass, the number of planktivorous fish decreased and the relative abundance of threadfin shad declined. Although the abundance of cladocerans increased after the addition of largemouth bass, the average size of zooplankton did not change. We did not detect changes in chlorophyll a, Secchi depth, or concentrations of total phosphorus and total nitrogen as a result of the addition of largemouth bass.     

Gill raker structure and selective predation on zooplankton by particulate feeding fish

The relationship between the gill raker structure of planktivorous fish (number, distance between gill rakers and length) and selective feeding on different species and size classes of zooplankton was studied. Gill raker structure was measured for brown trout Salmo trutta , Arctic charr Salvelinus alpinus , whitefish Coregonus lavaretus , roach Rutilus rutilus , bleak Alburnus alburnus , and three-spined stickleback Gasterosteus aculeatus . All species are facultative planktivorous fish and occur commonly in Scandinavian lakes. The effect of gill raker structure was studied by comparing prey found in fish stomachs with the availability of zooplankton from several lakes. Gill raker length and distance were significantly correlated with fish length. Although gill raker structure differed among species, all fish species selected the larger zooplankters. The minimum size of cladoceran species found in fish stomachs was much smaller than the distance between gill rakers. Despite great differences in gill raker spacing, the minimum size ingested of Daphnia galeata and Bosmina longispina was similar for all predators. The hypothesis that small zooplankton are strained and retained by the gill rakers in particulate feeding planktivorous fish, particularly in salmonids and roach, is rejected.     

Differential zooplankton feeding behaviors,selectivities, and community impacts of two planktivorous fishes

Synopsis We examined the feeding behaviors and selectives of two common planktivorous fishes, pumpkinseeds Lepomis gibbosa and fathead minnows Pimephales promelas in the laboratory. Ingestion rates for both pumpkinseeds and fathead minnows feeding on zooplankton increased as a function of fish length. Pumpkinseeds fed on zooplankton strictly as particulate feeders, with preferences increasing as a function of zooplankton body size regardless of taxonomic identity. Preferences were highest for large Daphnia, intermediate for intermediate-sized copepods, and lowest for small Ceriodaphnia. Fathead minnows displayed the ability to use both particulate-feeding and filter-feeding behaviors. Differential preferences tended to reflect both zooplankton size and taxon, being highest for large, slow-swimming Daphnia, intermediate for small Ceriodaphnia, and lowest for faster-swimming copepods. These differences in prey capture behaviors and preferences of the two fishes are reflected in the zooplankton taxonomic composition of small ponds containing each fish type. The crustacean zooplankton assemblages in ponds containing both pumpkinseeds and fathead minnows were dominated by copepods. Cladocerans were rare. In ponds containing pumpkinseeds, but no fathead minnows, cladocerans were abundant, generally accounting for up to 80% of total crustacean zooplankton biomass. These results suggest that the type of planktivore, and not simply the presence or abundance of planktivores in a system, can determine zooplankton community structure.     

Omnivory by Planktivores Stabilizes Plankton Dynamics, but May Either Promote or Reduce Algal Biomass

Classical models of phytoplankton–zooplankton interaction show that with nutrient enrichment such systems may abruptly shift from limit cycles to stable phytoplankton domination due to zooplankton predation by planktivorous fish. Such models assume that planktivorous fish eat only zooplankton, but there are various species of filter-feeding fish that may also feed on phytoplankton. Here, we extend these classical models to systematically explore the effects of omnivory by planktivorous fish. Our analysis indicates that if fish forage on phytoplankton in addition to zooplankton, the alternative attractors predicted by the classical models disappear for all realistic parameter settings, even if omnivorous fish have a strong preference for zooplankton. Our model also shows that the level of fish biomass above which zooplankton collapse should be higher when fish are omnivorous than when fish are zooplanktivorous. We also used the model to explore the potential effects of the now increasingly common practice of stocking lakes with filter-feeding fish to control cyanobacteria. Because omnivorous filter-feeding fish forage on phytoplankton as well as on the main grazers of phytoplankton, the net effect of such fish on the phytoplankton biomass is not obvious. Our model suggests that there may be a unimodal relationship between the biomass of omnivorous filter-feeding fish and the biomass of phytoplankton. This implies that to manage for reductions in phytoplankton biomass, heavy stocking or strong reduction of such fish is best.     

Seasonal variation in the interactions between piscivorous fish,planktivorous fish and zooplankton in a shallow eutrophic lake

The interactions between the higher trophic levels in a shallow eutrophic lake were studied during the course of a year. Three fish species determined the main pathways of organic matter flow within the system: the predominantly planktivorous bream (Abramis brama), the obligate planktivorous smelt (Osmerus eperlanus), and the piscivorous pikeperch (Stizostedion lucioperca). Of the thirteen common zooplankton taxa Daphnia hyalina and cyclopoid copepods were utilized most by the planktivorous fish, while the large production of small cladocerans is almost left unutilized.The seasonal variations of production and consumption are large. This is mainly affected by seasonal variation of the water temperature. The production of O + smelt is efficiently utilized by the pikeperch. Being the most important zooplankton consumer, as well as the most important prey group, O + fish plays a key role in the Tjeukemeer food web.     

Changes in the fish and zooplankton communities of Ringsjön,a Swedish lake undergoing man-made eutrophication

During the 20th century Lake Ringsjön has developed from a mesotrophic to a eutrophic lake, and the phytoplankton community has changed from a rather diverse community to a monoculture of blue-green algae. The eutrophication process has accelerated during the last decade. The most important external nutrient loading of today comes from agriculture.Although phosphorus has been shown to be the primary nutrient leading to excessive algal growth in fresh water, several biotic factors — such as interactions between nutrients, phytoplankton, zooplankton and planktivorous fish — may play a decisive role in the occurrence and maintenance of large algal blooms.The aim of this investigation was to study the changes in the fish community of Lake Ringsjön, especially the most dominant planktivores, and the state of the zooplankton community during the seventies. The fish fauna is dominated by cyprinids, especially roach, and there are relatively few predatory fish. During the seventies the mean size of roach decreased, and measurements of the zooplankton community indicated that the predation pressure on zooplankton had increased. The mean sizes of cladocerans such as Daphnia and Bosmina, which were selected for by the planktivorous fish, decreased; the size of the calanoid Diaptomus, which was not preyed upon by the dominating fish species, did not change. The growth of zooplankton-feeding stages of several fish species was retarded, which meant that the growth of young perch decreased, while older roach were mainly affected. In the prevailing situation, planktivorous roach can maintain a numerous population of small individuals, whereas the predatory perch is at a disadvantage, and predation on zooplankton is intense.     

Community structure and diel migration of zooplankton in shallow brackish lakes: role of salinity and predators

Diel horizontal migration (DHM), where zooplankton moves towards macrophytes during daytime to avoid planktivorous fish, has been reported as a common migration pattern of zooplankton in shallow temperate freshwater lakes. However, in shallow eutrophic brackish lakes, macrophytes seem not to have the same refuge effect, as these lakes may remain turbid even at relatively high macrophyte abundances. To investigate the extent to which macrophytes serve as a refuge for zooplankton at different salinities, we introduced artificial plants mimicking submerged macrophytes in the littoral zone of four shallow lakes, with salinities ranging from almost freshwater (0.3) to oligohaline waters (3.8). Furthermore, we examined the effects of different salinities on the community structure. Diel samples of zooplankton were taken from artificial plants, from areas where macrophytes had been removed (intermediate areas) and, in two of the lakes, also in open water. Fish and macroinvertebrates were sampled amongst the artificial plants and in intermediate areas to investigate their influence on zooplankton migration. Our results indicated that diel vertical migration (DVM) was the most frequent migration pattern of zooplankton groups, suggesting that submerged macrophytes were a poor refuge against predation at all salinities under study. Presumably, this pattern was the result of the relatively high densities of small planktivorous fish and macroinvertebrate predators within the submerged plants. In addition, we found major differences in the composition of zooplankton, fish and macroinvertebrate communities at the different salinities and species richness and diversity of zooplankton decreased with increasing salinity. At low salinities both planktonic/free-swimming and benthic/plant-associated cladocerans occurred, whilst only benthic ones occurred at the highest salinity. The low zooplankton biomass and overall smaller-bodied zooplankton specimens may result in a lower grazing capacity on phytoplankton, and enhance the turbid state in nutrient rich shallow brackish lakes.     

The influence of planktivorous fish on zooplankton communities in riverine slackwaters

1. Contrary to that for lakes and ponds, our knowledge of the influence of planktivorous fish on zooplankton communities in rivers is slight, largely because of the general assumption that such communities are overwhelmingly regulated by physical conditions. 2. In two separate but concurrent in situ enclosure experiments, we investigated the effects of carp gudgeon (Hypseleotris spp.) and Eastern Gambusia (Gambusia holbrooki) on zooplankton communities in slackwaters of a temperate Australian floodplain river. 3. A high biomass of Hypseleotris suppressed the density of daphniid microcrustaceans, but enhanced the total density of rotifers. A high biomass of Gambusia, on the other hand, suppressed the total density of both microcrustaceans and rotifers. 4. A high biomass of planktivorous fish also reduced the density of many of the ovigerous (egg‐carrying) zooplanktonic taxa. Indeed, ovigerous cyclopoid copepods were suppressed in the presence of a high biomass of Hypseleotris, even though there was no significant effect on overall (ovigerous plus non‐ovigerous) density. 5. Our results imply that a high biomass of planktivorous fish can potentially influence zooplankton communities in riverine slackwaters, as in many lakes and ponds.     

The Impact of Fish Predation and Cyanobacteria on Zooplankton Size Structure in 96 Subtropical Lakes

Zooplankton are relatively small in size in the subtropical regions. This characteristic has been attributed to intense predation pressure, high nutrient loading and cyanobacterial biomass. To provide further information on the effect of predation and cyanobacteria on zooplankton size structure, we analyzed data from 96 shallow aquaculture lakes along the Yangtze River. Contrary to former studies, both principal components analysis and multiple regression analysis showed that the mean zooplankton size was positively related to fish yield. The studied lakes were grouped into three types, namely, natural fishing lakes with low nutrient loading (Type1), planktivorous fish-dominated lakes (Type 2), and eutrophic lakes with high cyanobacterial biomass (Type 3). A marked difference in zooplankton size structure was found among these groups. The greatest mean zooplankton size was observed in Type 2 lakes, but zooplankton density was the lowest. Zooplankton abundance was highest in Type 3 lakes and increased with increasing cyanobacterial biomass. Zooplankton mean size was negatively correlated with cyanobacterial biomass. No obvious trends were found in Type 1 lakes. These results were reflected by the normalized biomass size spectrum, which showed a unimodal shape with a peak at medium sizes in Type 2 lakes and a peak at small sizes in Type 3 lakes. These results indicated a relative increase in medium-sized and small-sized species in Types 2 and 3 lakes, respectively. Our results suggested that fish predation might have a negative effect on zooplankton abundance but a positive effect on zooplankton size structure. High cyanobacterial biomass most likely caused a decline in the zooplankton size and encouraged the proliferation of small zooplankton. We suggest that both planktivorous fish and cyanobacteria have substantial effects on the shaping of zooplankton community, particularly in the lakes in the eastern plain along the Yangtze River where aquaculture is widespread and nutrient loading is high.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

The impact of planktivorous flsh on the structure of a plankton community

SUMMARY. 1. The abundance of pianktivorous juvenile yellow perch, Perca flavescens , was manipulated in three 750 m³ enclosures in a eutrophic lake.
2. There was a significant negative relationship between fish and zoopiankton biomasses. At high fish densities the zooplankton community was dominated by small filter-feeding cladocera. primarily bosmi- nids. At low fish densities the zooplankton community was dominated by large filter-feeding cladocera, primarily daphnids.
3. There was no significant relationship between zooplankton and phytoplankton biomasses when considered over the whole experiment but there was a trend towards lower phytoplankton biomass in the enclosure dominated by daphnids during mid-summer.
4. We conclude that although planktivorous fish have a strong negative impact on zooplankton community biomass and size structure, the relationship at the next lower trophic level, zooplankton and phytoplankton, is much weaker. Therefore, the biomanipulation of planktivorous fish populations as a management technique to control phytoplankton abundance is largely ineffective.     

Near-bottom depletion of zooplankton over coral reefs: III: vertical gradient of predation pressure

Many diurnal planktivorous fish in coral reefs efficiently consume zooplankton drifting in the overlying water column. Our survey, carried out at two coral reefs in the Red Sea, showed that most of the diurnal planktivorous fish foraged near the bottom, close to the shelters from piscivores. The planktivorous fish were order of magnitude more abundant near (<1.5 m) the bottom than higher in the water column. The predation pressure exerted by these fish was assessed by measuring the consumption of brine shrimps tethered at different heights above the bottom on a vertical line which was pulled over the reef. Below 1.5 m above bottom, the shrimps survival probability sharply decreased toward the bottom. Higher in the water column, survivorship was nearly 100% with little vertical variation. Our results indicate that near-bottom depletion of zooplankton in coral reefs is likely due to intense predation at that boundary layer. Risk of predation by piscivorous fish apparently restricts planktivorous fish to forage near the bottom, with a distribution pattern greatly deviating from ideal-free distribution.     

Differential responses of size‐based functional groups to bottom–up and top–down perturbations in pelagic food webs: a meta‐analysis

We performed a meta‐analysis of 31 lake mesocosm experiments to investigate differences in the responses of pelagic food chains and food webs to nutrient enrichment and fish presence. Trophic levels were divided into size‐based functional groups (phytoplankton into highly edible and poorly edible algae, and zooplankton into small herbivores, large herbivores and omnivorous zooplankton) in the food webs. Our meta‐analysis shows that 1) nutrient enrichment has a positive effect on phytoplankton and zooplankton, while fish presence has a positive effect on phytoplankton and a negative effect on zooplankton in the food chains; 2) nutrient enrichment has a positive effect on highly edible algae and small herbivores, but no effect on poorly edible algae, large herbivores and omnivorous zooplankton in the food webs. Planktivorous fish have a positive effect on highly edible algae and small herbivores, a negative effect on large herbivores and omnivorous zooplankton, and no effect on poorly edible algae. Our meta‐analysis confirms that nutrient enrichment and planktivorous fish affect functional groups differentially within trophic levels, revealing important changes in the functioning of food webs. The analysis of fish effects shows the well‐described trophic cascade in the food chain and reveals two trophic cascades in the food web: one transmitted by large herbivores that benefit highly edible phytoplankton, and one transmitted by omnivorous zooplankton that benefit small herbivores. Comparison between the responses of food webs and simple food chains also shows consistent biomass compensation between functional groups within trophic levels.     

Experimental evaluation of diversity–productivity relationships in a coral reef fish assemblage

The global decline in biodiversity is causing increasing concern about the effects of biodiversity loss on ecosystem services such as productivity. Biodiversity has been hypothesised to be important in maintaining productivity of biological assemblages because niche complementarity and facilitation among the constituent species can result in more efficient use of resources. However, these conclusions are primarily based on studies with plant communities, and the relationship between diversity and productivity at higher trophic levels is largely unknown, especially in the marine environment. Here, we used a manipulative field experiment to test the effects of species richness and species identity on biomass accumulation in coral reef fish assemblages at Lizard Island. Small patch reefs were stocked with a total of 30 juveniles belonging to three planktivorous damselfish (genus Pomacentrus) according to three different levels of fish species richness (one, two and three species) and seven different combinations of fish species. Species richness had no effect on the relative growth in this assemblage after 18 days, but relative growth differed among individual fish species and the different combinations of species. Patterns of increase in biomass were best explained by species-specific differences and variable effects of intra- and interspecific competition on growth. These results suggest that niche complementarity and facilitation are not the most influential drivers of total productivity within this guild of planktivorous fishes. Total productivity may be resilient to declining reef fish biodiversity, but this will depend on which species are lost and on the life-history traits of remaining species.     

Abiotic and biotic processes in lakes recovering from acidification: the relative roles of metal toxicity and fish predation as barriers to zooplankton re‐establishment

1. Recovery of acidified aquatic systems may be affected by both abiotic and biotic processes. However, the relative roles of these factors in regulating recovery may be difficult to determine. Lakes around the smelting complexes near Sudbury, Ontario, Canada, formerly affected by acidification and metal exploration, provide an excellent opportunity to examine the factors regulating the recovery of aquatic communities. 2. Substantial recovery of zooplankton communities has occurred in these lakes following declines in acidity and metal concentrations, although toxicity by residual metals still appears to limit survival for many species. Metal bioavailability, not simply total metal concentrations, was very important in determining effects on zooplankton and was associated with a decrease in the relative abundance of cyclopoids and Daphnia spp., resulting in communities dominated by Holopedium gibberum. 3. As chemical habitat quality has improved and fish, initially yellow perch and later piscivores (e.g. smallmouth bass, walleye), have invaded, biotic effects on the zooplankton are also becoming apparent. Simple fish assemblages dominated by perch appear to limit the survival of some zooplankton species, particularly Daphnia mendotae. 4. Both abiotic (residual metal contamination) and biotic (predation from planktivorous fish) processes have very important effects on zooplankton recovery. The re‐establishment of the zooplankton in lakes recovering from stress will require both improvements in habitat quality and the restoration of aquatic food webs.     

Top-down control in pelagic systems: a role for invertebrate predation

Limnologists have long recognized the importance of predation in freshwater communities. The majority of study of predator effects has involved vertebrate predators, with emphasis on planktivorous fish. Documented effects of planktivorous fish have been so dramatic that manipulations of their populations are seen by many as potential tools in lake management. However, the success of such manipulations is often less than desired due to the ubiquitous complexity of food webs and the pervasiveness of compensatory responses to food web manipulation. Recently, enormous effort has been applied to the Lake Kinneret pelagic food web in effort to reduced the abundance of the planktivorous Kinneret bleak Acanthobrama terraesanctae and thereby increase the biomass of herbivorous zooplankton in the hopes of increasing water clarity. We compared potential predation pressure on Lake Kinneret herbivorous zooplankton by bleak and the other major zooplankton predators in the lake, the cyclopoid copepods Mesocyclops ogunnus and Thermocyclops dybowskii. We found that, despite having much lower biomass, cyclopoid copepods accounted for a greater portion of the predation mortality on herbivorous zooplankton than bleak. Our results suggest that reductions in predation pressure by bleak will not yield subsequent increases in herbivorous zooplankton biomass. Rather, reductions in bleak predation pressure may allow for increases in cyclopoid copepod abundance and thereby a net increase in predation pressure on herbivorous zooplankton.