Eutrophication and its control by biomanipulation: an enclosure experiment

Biomanipulation experiments were carried out in small enclosures in a small, shallow eutrophic lake in eastern England. The effects of fish removal on water quality and planktonic communities were examined in relation to nutrients and macrophytes. Exclusion of fish resulted in enhanced densities of the large-bodied Daphnia hyalina which, in turn, led to improvements in water quality. The provision of refuges from fish predation also enhanced Daphnia populations enough to suggest this strategy as feasible for the restoration of this lake. The development of a mysid population in enclosures without fish indicated that some indirect effects might occur if fish are completely removed.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

The zooplankton-phytoplankton interface in lakes of contrasting trophic status: an experimental comparison

We report here the results of an experimental study designed to compare algal responses to short-term manipulations of zooplankton in three California lakes which encompass a broad range of productivity (ultra-oligotrophic Lake Tahoe, mesotrophic Castle Lake, and strongly eutrophic Clear Lake). To assess the potential strength of grazing in each lake, we evaluated algal responses to a 16-fold range of zooplankton biomass. To better compare algal responses among lakes, we determined algal responses to grazing by a common grazer (Daphnia sp.) over a range ofDaphnia densities from 1 to 16 animals per liter. Effects of both ambient grazers andDaphnia were strong in Castle Lake. However, neither ambient zooplankton norDaphnia had much impact on phytoplankton in Clear Lake. In Lake Tahoe, no grazing impacts could be demonstrated for the ambient zooplankton butDaphnia grazing had dramatic effects. These results indicate weak coupling between phytoplankton and zooplankton in Clear Lake and Lake Tahoe, two lakes which lie near opposite extremes of lake trophic status for most lakes. These observations, along with work reported by other researchers, suggest that linkages between zooplankton and phytoplankton may be weak in lakes with either extremely low or high productivity. Biomanipulation approaches to recover hypereutrophic lakes which aim only to alter zooplankton size structure may be less effective if algal communities are dominated by large, inedible phytoplankton taxa.     

Controlling Harmful Cyanobacteria: Taxa-Specific Responses of Cyanobacteria to Grazing by Large-Bodied Daphnia in a Biomanipulation Scenario

Lake restoration practices based on reducing fish predation and promoting the dominance of large-bodied Daphnia grazers (i.e., biomanipulation) have been the focus of much debate due to inconsistent success in suppressing harmful cyanobacterial blooms. While most studies have explored effects of large-bodied Daphnia on cyanobacterial growth at the community level and/or on few dominant species, predictions of such restoration practices demand further understanding on taxa-specific responses in diverse cyanobacterial communities. In order to address these questions, we conducted three grazing experiments during summer in a eutrophic lake where the natural phytoplankton community was exposed to an increasing gradient in biomass of the large-bodied Daphnia magna. This allowed evaluating taxa-specific responses of cyanobacteria to Daphnia grazing throughout the growing season in a desired biomanipulation scenario with limited fish predation. Total cyanobacterial and phytoplankton biomasses responded negatively to Daphnia grazing both in early and late summer, regardless of different cyanobacterial densities. Large-bodied Daphnia were capable of suppressing the abundance of Aphanizomenon, Dolichospermum, Microcystis and Planktothrix bloom-forming cyanobacteria. However, the growth of the filamentous Dolichospermum crassum was positively affected by grazing during a period when this cyanobacterium dominated the community. The eutrophic lake was subjected to biomanipulation since 2005 and nineteen years of lake monitoring data (1996–2014) revealed that reducing fish predation increased the mean abundance (50%) and body-size (20%) of Daphnia, as well as suppressed the total amount of nutrients and the growth of the dominant cyanobacterial taxa, Microcystis and Planktothrix. Altogether our results suggest that lake restoration practices solely based on grazer control by large-bodied Daphnia can be effective, but may not be sufficient to control the overgrowth of all cyanobacterial diversity. Although controlling harmful cyanobacterial blooms should preferably include other measures, such as nutrient reductions, our experimental assessment of taxa-specific cyanobacterial responses to large-bodied Daphnia and long-term monitoring data highlights the potential of such biomanipulations to enhance the ecological and societal value of eutrophic water bodies.     

High predation is of key importance for dominance of small-bodied zooplankton in warm shallow lakes: evidence from lakes, fish exclosures and surface sediments

The mean body size of limnetic cladocerans decreases from cold temperate to tropical regions, in both the northern and the southern hemisphere. This size shift has been attributed to both direct (e.g. physiological) or indirect (especially increased predation) impacts. To provide further information on the role of predation, we compiled results from several studies of subtropical Uruguayan lakes using three different approaches: (i) field observations from two lakes with contrasting fish abundance, Lakes Rivera and Rodó, (ii) fish exclusion experiments conducted in in-lake mesocosms in three lakes, and (iii) analyses of the Daphnia egg bank in the surface sediment of eighteen lakes. When fish predation pressure was low due to fish kills in Lake Rivera, large-bodied Daphnia appeared. In contrast, small-sized cladocerans were abundant in Lake Rodó, which exhibited a typical high abundance of fish. Likewise, relatively large cladocerans (e.g. Daphnia and Simocephalus) appeared in fishless mesocosms after only 2 weeks, most likely hatched from resting egg banks stored in the surface sediment, but their abundance declined again after fish stocking. Moreover, field studies showed that 9 out of 18 Uruguayan shallow lakes had resting eggs of Daphnia in their surface sediment despite that this genus was only recorded in three of the lakes in summer water samples, indicating that Daphnia might be able to build up populations at low risk of predation. Our results show that medium and large-sized zooplankton can occur in subtropical lakes when fish predation is removed. The evidence provided here collectively confirms the hypothesis that predation, rather than high-temperature induced physiological constraints, is the key factor determining the dominance of small-sized zooplankton in warm lakes.     

Cyanobacterial chemical warfare affects zooplankton community composition

1. Toxic algal blooms widely affect our use of water resources both with respect to drinking water and recreation. However, it is not only humans, but also organisms living in freshwater and marine ecosystems that may be affected by algal toxins. 2. In order to assess if cyanobacterial toxins affect the composition of natural zooplankton communities, we quantified the temporal fluctuations in microcystin concentration and zooplankton community composition in six lakes. 3. Microcystin concentrations generally showed a bimodal pattern with peaks in early summer and in autumn, and total zooplankton biomass was negatively correlated with microcystin concentrations. Separating the zooplankton assemblages into finer taxonomic groups revealed that high microcystin concentrations were negatively correlated with Daphnia and calanoid copepods, but positively correlated with small, relatively inefficient phytoplankton feeders, such as cyclopoid copepods, Bosmina and rotifers. 4. In a complementary, mechanistic laboratory experiment using the natural phytoplankton communities from the six lakes, we showed that changes in in situ levels of microcystin were coupled with reduced adult size and diminished juvenile biomass in Daphnia. 5. We argue that in eutrophic lakes, large unselective herbivores, such as Daphnia, are ‘sandwiched’ between high fish predation and toxic food (cyanobacteria). In combination, these two mechanisms may explain why the zooplankton community in eutrophic lakes generally comprise small forms (e.g. rotifers and Bosmina) and selective raptorial feeders, such as cyclopoid copepods, whereas large, unselective herbivores, such as Daphnia, are rare. Hence, this cyanobacterial chemical warfare against herbivores may add to our knowledge on population and community dynamics among zooplankton in eutrophic systems.     

Habitat selection and diel distribution of the crustacean zooplankton from a shallow Mediterranean lake during the turbid and clear water phases

1. The fish fauna of many shallow Mediterranean Lakes is dominated by small‐bodied exotic omnivores, with potential implications for fish–zooplankton interactions still largely unknown. Here we studied diel variation in the vertical and horizontal distribution of the crustacean plankton in Lake Vela, a shallow polymictic and eutrophic lake. Diel sampling was carried out on three consecutive days along a horizontal transect, including an open‐water station and a macrophyte (Nymphaea alba) bed. Since transparency is a key determinant of the predation risk posed by fish, the zooplankton sampling campaigns were conducted in both the turbid (autumn) and clear water (spring) phases. 2. In the turbid phase, most taxa were homogeneously distributed along the vertical and horizontal axes in the three consecutive days. The only exception was for copepod nauplii, which showed vertical heterogeneity, possibly as a response to invertebrate predators. 3. In the clear water phase, most zooplankton taxa displayed habitat selection. Vertically, the general response consisted of a daily vertical migration (DVM), despite the limited depth (1.6 m). Horizontally, zooplankters showed an overall preference for the pelagic zone, independent of the time of the day. Such evidence is contrary to the postulated role of macrophytes as an anti‐predator refuge for the zooplankton. 4. These vertical (DVM) and horizontal (macrophyte‐avoidance) patterns were particularly conspicuous for large Daphnia, suggesting that predation risk from size‐selective predators (fish) was the main factor behind the spatial heterogeneity of zooplankton in the spring. Thus, the difference in the zooplankton spatial distribution pattern and habitat selection among seasons (turbid and clear water phases) seems to be mediated the predation risk from fish, which is directly related to water transparency. 5. The zooplankton in Lake Vela have anti‐predator behaviour that minimises predation from fish. We hypothesise that, due to the distinct fish community of shallow Mediterranean lakes, aquatic macrophytes may not provide adequate refuge to zooplankters, as seen in northern temperate lakes.     

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.     

An experimental study of habitat choice by Daphnia: plants signal danger more than refuge in subtropical lakes

1. In shallow temperate lakes, submerged plants often provide refuge for pelagic zooplankton against fish predation, a mechanism with potential strong cascading effects on water transparency and on the entire ecosystem. In (sub)tropical lakes, however, the interaction between aquatic plants and predation may be more complex, particularly because fish density is high within the plant beds in such systems. 2. Using laboratory ‘habitat choice’ experiments, we determined the effects of three (sub)tropical free‐floating plants, Eichhornia crassipes, Pistia stratiotes and Salvinia auriculata and the cosmopolitan submerged Ceratophyllum demersum, on horizontal movement by the water flea Daphnia obtusa. We tested for avoidance of plants in the absence and presence of alarm signals from crushed conspecifics and chemical cues from the fish Cnesterodon decemmaculatus, the fish have been subjected to different feeding regimes. 3. In the absence of other stimuli, D. obtusa strongly avoided the plants and the crushed conspecifics, as expected. However, the response to fish was insignificant regardless of their previous feeding regime. The avoidance of free‐floating plants was more pronounced than that of the submerged plant. Contrary to predictions based on research in temperate lakes, Daphnia did not take refuge among the plants but rather swam away from them when exposed simultaneously to plants and alarm signals. 4. We hypothesise that the avoidance of plants by D. obtusa may ultimately be attributable to an expectedly higher predation risk within the plants than in the pelagic, because of a high density of associated zooplanktivorous fish in the former. In the (sub)tropics, therefore, aquatic plants and particularly the free‐floating ones, may not promote cascading effects via Daphnia grazing on phytoplankton as seen in temperate eutrophic lakes.     

Restoring lakes by using artificial plant beds: habitat selection of zooplankton in a clear and a turbid shallow lake

1. Return of large‐bodied zooplankton populations is of key importance for creating a shift from a turbid to a clear‐water state in shallow lakes after a nutrient loading reduction. In temperate lakes, recovery is promoted by submerged macrophytes which function as a daytime refuge for large zooplankton. However, recovery of macrophytes is often delayed and use of artificial plant beds (APB) has been suggested as a tool to enhance zooplankton refuges, thereby reinforcing the shift to a clear‐water state and, eventually, colonisation of natural plants. 2. To further evaluate the potential of APB in lake restoration, we followed the day–night habitat choices of zooplankton throughout summer in a clear and a turbid lake. Observations were made in the pelagic and littoral zones and in APB in the littoral representing three different plant densities (coverage 0%, 40% and 80%). 3. In the clear lake, the zooplankton (primarily Daphnia) were mainly found in the pelagic area in spring, but from mid‐May they were particularly abundant in the APB and almost exclusively so in mid‐June and July, where they appeared in extremely high densities during day (up to 2600 ind. L⁻¹). During night Daphnia densities were overall more equally distributed between the five habitats. Ceriodaphnia was proportionally more abundant in the APB during most of the season. Cyclopoids were more abundant in the high APB during day but were equally distributed between the five habitats during night. 4. In the turbid lake, however, no clear aggregation was observed in the APB for either of the pelagic genera (Daphnia and Bosmina). This may reflect a higher refuge effect in the open water due to the higher turbidity, reduced ability to orient to plant beds and a significantly higher fish density (mainly of roach, Rutilus rutilus, and perch, Perca fluviatilis) in the plant beds than in the clear lake. Chydorus was found in much higher proportions among the plants, while cyclopoids, particularly the pelagic Cyclops vicinus, dominated in the pelagic during day and in the pelagic and high density plants during night. 5. Our results suggest that water clarity is decisive for the habitat choice of large‐bodied zooplankton and that introduction of APB as a restoration measure to enhance zooplankton survival is only a useful tool when water clarity increases following loading reduction. Our results indicate that dense APB will be the most efficient.     

Sediments,not plants,offer the preferred refuge for Daphnia against fish predation in Mediterranean shallow lakes: an experimental demonstration

1. Different behavioural responses of planktonic animals to their main predators, fish, have been reported from shallow lakes. In north temperate lakes, large‐bodied zooplankton may seek refuge from predation among macrophytes, whereas in subtropical lakes, avoidance of macrophytes has been observed. The prevalent behaviour probably depends on the characteristics of the fish community, which in Mediterranean lakes is typically dispersed in both the open water zone and in the littoral, as in temperate lakes, and is dominated by small size classes, as in subtropical lakes. 2. We performed ‘habitat choice’ experiments to test the response of Daphnia magna to predation cues at both the horizontal and vertical level by mimicking a ‘shallow littoral’ zone with plants and a ‘deeper pelagic’ zone with sediments. 3. Initial separate response experiments showed that natural plants, artificial plants and predation cues all repelled D. magna in the absence of other stimuli, while sediments alone did not trigger any significant response by D. magna. 4. The habitat choice experiments showed that, in the presence of predation cues and absence of plants, Daphnia moved towards areas with sediment. In the presence of both plants and sediments, Daphnia moved away from the plants towards the sediments under both shallow and deep water treatment conditions. 5. Based on these results, we suggest that Daphnia in Mediterranean shallow lakes avoid submerged macrophytes and instead prefer to hide near the sediment when exposed to predation risk, as also observed in subtropical shallow lakes. This pattern is not likely to change with water level alterations, a common feature of lakes in the region, even if the effectiveness of the refuge may be reduced.     

Biodiversity on the brink: an assessment of conservation strategies for North American freshwater mussels

Fish and zooplankton populations of nine Ethiopian freshwater lakes were quantitatively sampled along a North–South gradient. Differences in altitude and latitude resulted in a temperature gradient from North to South. We tested three hypotheses: (1) the degree of zooplanktivory decreases with water temperature, i.e. from North to South; (2) the degree of zooplanktivory increases with the abundance of large-bodied zooplankton; and (3) the pattern of zooplanktivory in eutrophic Ethiopian water bodies differs from other tropical and temperate water bodies. Proportions of zooplanktivory in the fish communities did not show a geographical trend, but mainly depended on fish species, zooplankton density and the availability of large-bodied cladocerans. The degree of zooplanktivory in eutrophic Ethiopian water bodies differs from other eutrophic water bodies, both temperate and tropical. In Ethiopia, the degree of zooplanktivory can be both low and high, in contrast with other tropical water bodies where zooplanktivory is generally low and with temperate eutrophic water bodies where it is generally high. As a result, predation pressure on zooplankton by fish varies dramatically amongst Ethiopian water bodies.     

Effects of habitat complexity on community structure and predator avoidance behaviour of littoral zooplankton in temperate versus subtropical shallow lakes

1. Structural complexity may stabilise predator–prey interactions and affect the outcome of trophic cascades by providing prey refuges. In deep lakes, vulnerable zooplankton move vertically to avoid fish predation. In contrast, submerged plants often provide a diel refuge against fish predation for large‐bodied zooplankton in shallow temperate lakes, with consequences for the whole ecosystem. 2. To test the extent to which macrophytes serve as refuges for zooplankton in temperate and subtropical lakes, we introduced artificial plant beds into the littoral area of five pairs of shallow lakes in Uruguay (30°–35°S) and Denmark (55°–57°N). We used plants of different architecture (submerged and free‐floating) along a gradient of turbidity over which the lakes were paired. 3. We found remarkable differences in the structure (taxon‐richness at the genus level, composition and density) of the zooplankton communities in the littoral area between climate zones. Richer communities of larger‐bodied taxa (frequently including Daphnia spp.) occurred in the temperate lakes, whereas small‐bodied taxa characterised the subtropical lakes. More genera and a higher density of benthic/plant‐associated cladocerans also occurred in the temperate lakes. The density of all crustaceans, except calanoid copepods, was significantly higher in the temperate lakes (c. 5.5‐fold higher). 4. Fish and shrimps (genus Palaemonetes) seemed to exert a stronger predation pressure on zooplankton in the plant beds in the subtropical lakes, while the pelagic invertebrate Chaoborus sp. was slightly more abundant than in the temperate lakes. In contrast, plant‐associated predatory macroinvertebrates were eight times more abundant in the temperate than in the subtropical lakes. 5. The artificial submerged plants hosted significantly more cladocerans than the free‐floating plants, which were particularly avoided in the subtropical lakes. Patterns indicating diel horizontal migration were frequently observed for both overall zooplankton density and individual taxa in the temperate, but not the subtropical, lakes. In contrast, patterns of diel vertical migration prevailed for both the overall zooplankton and for most individual taxa in the subtropics, irrespective of water turbidity. 6. Higher fish predation probably shapes the general structure and dynamics of cladoceran communities in the subtropical lakes. Our results support the hypothesis that horizontal migration is less prevalent in the subtropics than in temperate lakes, and that no predator‐avoidance behaviour effectively counteracts predation pressure in the subtropics. Positive effects of aquatic plants on water transparency, via their acting as a refuge for zooplankton, may be generally weak or rare in warm lakes.     

Clay-Turbid Interactions May Not Cascade—A Reminder for Lake Managers

Food web management is a frequently used lake restoration method, which aims to reduce phytoplankton biomass by strengthening herbivorous zooplankton through reduction of planktivorous fish. However, in clay‐turbid lakes several factors may reduce the effectivity of food web management. Increasing turbidity reduces the effectivity of fish predation and weakens the link between zooplankton and phytoplankton. Therefore, the effects of fish stock manipulations may not cascade to lower trophic levels as expected. Additionally, in clay‐turbid conditions invertebrate predators may coexist in high densities with planktivorous fish and negate the effects of fish reductions. For instance, in the stratifying regions of the clay‐turbid Lake Hiidenvesi, Chaoborus flavicans is the main regulator of cladocerans and occupies the water column throughout the day, although planktivorous Osmerus eperlanus is very abundant. The coexistence of chaoborids and fish is facilitated by a metalimnetic turbidity peak, which prevents efficient predation by fish. In the shallow parts of the lake, chaoborids are absent despite high water turbidity. We suggest that, generally, the importance of invertebrate predators in relation to vertebrate predators may change along turbidity and depth gradients. The importance of fish predation is highest in shallow waters with low turbidity. When water depth increases, the importance of fish in the top‐down regulation of zooplankton declines, whereas that of chaoborids increases, the change along the depth gradient being moderate in clear‐water lakes and steep in highly turbid lakes. Thus, especially deep clay‐turbid lakes may be problematic for implementing food web management as a restoration tool.     

Zooplankton structure in the Loosdrecht lakes in relation to trophic status and recent restoration measures

A five-year zooplankton study (1982–86) on three shallow and highly eutrophic lakes in the Loosdrecht area (The Netherlands) did not reveal any significant changes following the considerable reduction in external P-loading (from about 1.0 g to 0.3 g P m^–2 year^–1) since mid-1984.The recent annual fluctuations in the rotifer and crustacean densities are within the range of those found before the restoration measure became operative. A decrease in the average size of the crustaceans and an absence of large-bodied forms reflects an increased fish predation rather than a change in the quality or quantity of their sestonic food ( < 150 µm) which continues to be dominated by filamentous cyanobacteria and Prochlorothrix hollandica, a prochlorophyte discovered in these lakes recently.     

Pathways of Increased Water Clarity After Fish Removal from Ventura Marsh; a Shallow, Eutrophic Wetland

We investigated the pathways by which water clarity increases following fish removal by evaluating the effects of a benthivorous fish reduction in a large, shallow, eutrophic, wetland in a predominately agricultural watershed in Iowa, U.S.A. Phytoplankton was phosphorus limited prior to manipulation. After a substantial fish removal was obtained, water clarity increased as a result of decreased suspended sediment and phytoplankton biomass. Trophic cascading, mitigated by release from fish predation and decreased physical interference from suspended sediments, appears to determine water clarity. Inorganic suspended solids declined immediately after fish were removed but the biomass of Daphnia and Ceriodaphnia did not increase until a few weeks after fish removal. High grazing by zooplankton likely reduced phytoplankton biomass during the height of the clear-water phase. Phytoplankton appeared to be limited by zooplankton grazing for approximately two months before reverting to bottom-up control. An increase in suspended sediment and/or increased predation pressure on zooplankton, due to the return of juvenile carp, appears to account for the decline of larger-bodied zooplankters and the switch back to bottom-up control. Macrophyte diversity and density increased substantially after the initiation of the clear-water phase.     

Trophic structure in the pelagial of 25 shallow New Zealand lakes: changes along nutrient and fish gradients

To gain better insight into the importance of predator and resourcecontrol in New Zealand lakes we surveyed the late summer trophicstructure of 25 shallow South Island lakes with contrastingnutrient levels (6–603 µg TP l^–1) and fishdensities. Total catch of fish per net (CPUE) in multi-meshgillnets placed in the open water and the littoral zones waspositively related with the nutrient level. Trout CPUE was negativelycorrelated with total phosphorus (TP) and total nitrogen (TN).Zooplankton seemed largely influenced by fish, as high fishCPUE coincided with low zooplankton and Daphnia biomass, lowaverage weight of cladocerans, low contribution of Daphnia tototal cladoceran biomass, low ratio of calanoids to total copepodbiomass and low ratio of zooplankton biomass to phytoplanktonbiomass. However, chlorophyll a was only slightly negativelyrelated to Daphnia biomass and not to zooplankton biomass ina multiple regression that included TN and TP. Ciliate abundancewas positively related to chlorophyll a and negatively to Daphniabiomass, but not to total zooplankton biomass, while no relationshipswere found between heterotrophic nanoflagellates and zooplankton.The relationships between fish abundance and nutrients and fishabundance and zooplankton:phytoplankton ratio and between chlorophylla and TP largely followed the pattern obtained for 42 northtemperate Danish lakes. We conclude that fish, including trout,have a major effect on the zooplankton community structure andbiomass in the pelagial of the shallow oligotrophic to slightlyeutrophic New Zealand lakes, but that the cascading effectson phytoplankton and protist are apparently modest.     

Evidence that fish structure the zooplankton communities of turbid lakes and reservoirs

1. Visually foraging fish typically exclude large zooplankton from clear‐water lakes and reservoirs. Do fish have the same effect in turbid waters, or does turbidity provide a refuge from visual predation? 2. To test the hypothesis that fish exclude large zooplankton species from turbid sites, I searched for populations of medium or large Daphnia species in turbid, fish‐containing reservoirs of south‐central Oklahoma and north‐central Texas, U.S.A., and surveyed the literature for accounts of Daphnia species in turbid habitats worldwide. 3. Only small Daphnia species and the exuberantly spined Daphnia lumholtzi were detected in the turbid reservoirs. The Daphnia species in the reservoirs are smaller than other Daphnia species that occur in the area but were not detected. An extensive survey of the literature suggests that large Daphnia may be found in the lakes of extreme turbidity [Secchi disk depth (SD) < 0.2 m] but that only small and spiny Daphnia are likely to occur in more typical turbid locations (1.0 m > SD > 0.2 m) unless some additional factor reduces the influence of fish predation in such sites. 4. The field samples from Texas and Oklahoma together with the literature review suggest that the effect of visually foraging planktivorous fish on the size structure of turbid‐water zooplankton communities may often be as strong or even stronger than the effect of fish on clear‐water zooplankton communities.     

Winter ecology of shallow lakes: strongest effect of fish on water clarity at high nutrient levels

While the structuring role of fish in lakes is well studied for the summer season in North temperate lakes, little is known about their role in winter when fish activity and light irradiance potentially are lower. This is unfortunate as the progressing climate change may have strong effects on lake winter temperature and possibly on trophic dynamics too. We conducted an enclosure experiment with and without the presence of fish throughout winter in two shallow lakes with contrasting phosphorus concentrations. In hypertrophic Lake Søbygård, absence of fish led to higher biomass of zooplankton, higher grazing potential (zooplankton:phytoplankton ratio) and, accordingly, lower biomass of phytoplankton and chlorophyll a (Chl a), while the concentrations of total nitrogen (TN), total phosphorus (TP), oxygen and pH decreased. The average size of egg-bearing Daphnia and Bosmina and the minimum size of egg-bearing specimens of the two genera rose. In the less eutrophic Lake Stigsholm, zooplankton and their grazing potential were also markedly affected by fish. However, the decrease in Chl a was slight, and phytoplankton biovolume, pH and the oxygen concentration were not affected. TN was higher when fish were absent. Our results indicate that: (i) there is a notable effect of fish on zooplankton community structure and size during winter in both eutrophic and hypertrophic North temperate lakes, (ii) Chl a can be high in winter in such lakes, despite low light irradiance, if fish are abundant, and (iii) the cascading effects on phytoplankton and nutrients in winter may be more pronounced in hypertrophic lakes. Climate warming supposedly leading to reduced winter mortality and dominance of small fish may enhance the risk of turbid state conditions in nutrient-enriched shallow lakes, not only during the summer season, but also during winter.     

The influence of plant-associated filter feeders on phytoplankton biomass: a mesocosm study

Low phytoplankton biomass usually occurs in the presence of submerged macrophytes, possibly because submerged macrophytes enhance top-down control of phytoplankton by offering a refuge for efficient grazers like Daphnia against fish predation. However, other field studies also suggest that submerged macrophytes suppress phytoplankton in the absence of Daphnia. In order to investigate these mechanisms further, we conducted an outdoor mesocosm experiment to study the effect of submerged macrophytes (Elodea nuttallii) on phytoplankton and zooplankton biomass. The experiment combined four nutrient addition levels (0, 10, 100, and 1000 μg P l⁻¹; N/P ratio: 16) with three macrophyte levels (no macrophytes, artificial macrophytes, and real macrophytes). We inoculated the tanks with species-rich inocula of phytoplankton and zooplankton but excluded fish or macro-invertebrates. Probably due to the lack of predators in the mesocosms, potential grazing rates of pelagic zooplankton (estimated from zooplankton biomass) did not differ between the macrophyte treatment combinations. Compared to the treatment combinations without macrophytes, lower phytoplankton biomass occurred in the treatment combinations with real macrophytes at all the nutrient addition levels and in those with artificial macrophytes at all the nutrient levels except the highest. Significantly, higher abundances of plant-associated filter feeders (Simocephalus vetulus and Ceriodaphnia spp.) occurred in the treatment combinations with real and artificial macrophytes. The estimated potential grazing rate of these plant-associated filter feeders indicated that these filter feeders could be responsible for the lower phytoplankton biomass in the presence of real and artificial macrophytes. Our results suggest that the plant-associated filter feeders may be significant grazers in vegetated shallow lakes.