Applicability of planktonic biomanipulation for managing eutrophication in the subtropics

Although large-bodied cladocerans such asDaphnia can reduce algal biomass significantly in temperate lakes if freed from fish predation, the applicability of such biomanipulation techniques for eutrophication management in the subtropics and tropics has been examined only recently. Subtropical cladoceran assemblages differ from those of temperate lakes by their low species richness, early summer gameogenesis, and greatly reduced body size. Eutrophic Florida lakes are dominated by pump-filter feeding fish rather than by size selective planktivores as a temperate lakes. Cladocerans in Florida lakes can increase in abundance significantly if freed from fish but fail to have an impact on algal biomass or composition. The greatest potential for using biomanipulation to manage phytoplankton-dominated lakes in the subtropics and tropics lies with phytophagous fish. Future research should concentrate on defining the role of individual fish taxa on phytoplankton composition and community structure, nutrient cycling, and planktonic productivity before embarking on whole lake manipulation projects.     

Restoration by biomanipulation in a small hypertrophic lake: first-year results

Biomanipulation was carried out in order to improve the water quality of the small hypertrophic Lake Zwemlust (1.5 ha; mean depth 1.5 m). In March 1987 the lake was drained to facilitate the elimination of fish. Fish populations were dominated by planktivorous and benthivorous species (total stock c. 1500 kg) and were collected by seine- and electro-fishing. The lake was subsequently re-stocked with 1500 northern pike fingerlings (Esox lucius L.) and a low density of adult rudd (Scardinius erythrophthalmus). The offspring of the rudd served as food for the predator pike. Stacks of Salix twigs, roots of Nuphar lutea and plantlets of Chara globularis were brought in as refuge and spawning grounds for the pike, as well as shelter for the zooplankton.The impact of this biomanipulation on the light penetration, phytoplankton density, macrophytes, zooplankton and fish communities and on nutrient concentrations was monitored from March 1987 onwards. This paper presents the results in the first year after biomanipulation.The abundance of phytoplankton in the first summer (1987) after this biomanipulation was very low, and consequently accompanied by increase of Secchi-disc transparency and drastic decline of chlorophyll a concentration.The submerged vegetation remained scarce, with only 5 % of the bottom covered by macrophytes at the end of the season.Zooplankters became more abundant and there was a shift from rotifers to cladocerans, comprised mainly of Daphnia and Bosmina species, the former including at least 3 species.The offspring of the stocked rudd was present in the lake from the end of August 1987. Only 19% of the stocked pike survived the first year.Bioassays and experiments with zooplankton community grazing showed that the grazing pressure imposed by the zooplankton community was able to keep chlorophyll a concentrations and algal abundance to low levels, even in the presence of very high concentrations of inorganic N and P. The total nutrient level increased after biomanipulation, probably due to increased release from the sediment by bioturbation, the biomass of chironomids being high.At the end of 1987 Lake Zwemlust was still in an unstable stage. A new fish population dominated by piscivores, intended to control the planktivorous and benthivorous fish, and the submerged macrophytes did not yet stabilize.     

Fisheries management as an additional lake restoration measure: biomanipulation scaling-up problems

Restoration of the highly eutrophic Reeuwijk lakes (ca. 700 ha) started in 1986 by reducing the external phosphorus loading. As an additional measure to improve the quality of the lake water, the structure of the fish population in Lake Klein Vogelenzang (18 ha) was altered in 1989 by the removal of ca. 100 kg ha^–1 bream from the lake in April and December. This constituted about 50% of the total bream biomass in the lake.The fish-stock reduction in April, 1989, was initially followed by high phosphorus concentrations, probably the result of considerable phosphorus release from the sediments. The resulting heavy algal blooms that occurred reduced the transparency to very low values. During the summer the zooplankton population increased markedly in numbers coinciding with reductions in total suspended matter including (blue-green) algae. A great improvement in Secchi-disc transparency was observed and by the end of December, 1989, the bottom of the lake (1.5–2.0 m) was visible. After heavy storms in January and February 1990, transparency dropped to < 1 m as a result of resuspension of high concentrations of suspended matter from the bottom sediments. Although transparency over the rest of 1990 was higher than in 1988, i.e. the year preceding the removal of fish (biomanipulation), it was lower than expected, based on the results of 1989. The study shows that technical and biological factors can cause serious management problems for the implementation of biomanipulation in larger water bodies.     

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.     

Linking Cascading Effects of Fish Predation and Zooplankton Grazing to Reduced Cyanobacterial Biomass and Toxin Levels Following Biomanipulation

Eutrophication has been one of the largest environmental problems in aquatic ecosystems during the past decades, leading to dense, and often toxic, cyanobacterial blooms. In a way to counteract these problems many lakes have been subject to restoration through biomanipulation. Here we combine 13 years of monitoring data with experimental assessment of grazing efficiency of a naturally occurring zooplankton community and a, from a human perspective, desired community of large Daphnia to assess the effects of an altered trophic cascade associated with biomanipulation. Lake monitoring data show that the relative proportion of Daphnia spp. grazers in June has increased following years of biomanipulation and that this increase coincides with a drop in cyanobacterial biomass and lowered microcystin concentrations compared to before the biomanipulation. In June, the proportion of Daphnia spp. (on a biomass basis) went from around 3% in 2005 (the first year of biomanipulation) up to around 58% in 2012. During months when the proportion of Daphnia spp. remained unchanged (July and August) no effect on lower trophic levels was observed. Our field grazing experiment revealed that Daphnia were more efficient in controlling the standing biomass of cyanobacteria, as grazing by the natural zooplankton community never even compensated for the algal growth during the experiment and sometimes even promoted cyanobacterial growth. Furthermore, although the total cyanobacterial toxin levels remained unaffected by both grazer communities in the experimental study, the Daphnia dominated community promoted the transfer of toxins to the extracellular, dissolved phase, likely through feeding on cyanobacteria. Our results show that biomanipulation by fish removal is a useful tool for lake management, leading to a top-down mediated trophic cascade, through alterations in the grazer community, to reduced cyanobacterial biomass and lowered cyanobacterial toxin levels. This improved water quality enhances both the ecological and societal value of lakes as units for ecosystem services.     

Stocking piscivores to improve fishing and water clarity: a synthesis of the Lake Mendota biomanipulation project

SUMMARYY 1. A total of 2.7 × 10⁶ walleye fingerlings and 1.7 × 10⁵ northern pike fingerlings were stocked during 1987–99 in eutrophic Lake Mendota. The objectives of the biomanipulation were to improve sport fishing and to increase piscivory to levels that would reduce planktivore biomass, increase Daphnia grazing and ultimately reduce algal densities in the lake. The combined biomass of the two piscivore species in the lake increased rapidly from < 1 kg ha^?1 and stabilised at 4–6 kg ha^?1 throughout the evaluation period. 2. Restrictive harvest regulations (i.e. increase in minimum size limit and reduction in bag limit) were implemented in 1988 to protect the stocked piscivores. Further restrictions were added in 1991 and 1996 for walleye and northern pike, respectively. These restrictions were essential because fishing pressure on both species (especially walleye) increased dramatically during biomanipulation. 3. Commencing in 1987 with a massive natural die‐off of cisco and declining yellow perch populations, total planktivore biomass dropped from about 300–600 kg ha^?1 prior to the die‐off and the fish stocking, to about 20–40 kg ha^?1 in subsequent years. These low planktivore biomasses lasted until a resurgence in the perch population in 1999. 4. During the period prior to biomanipulation when cisco were very abundant, the dominant Daphnia species was the smaller‐bodied D. galeata mendotae, which usually reached a biomass maximum in June and then crashed shortly thereafter. Beginning in 1988, the larger‐bodied D. pulicaria dominated, with relatively high biomasses occurring earlier in the spring and lasting well past mid‐summer of many years. 5. In many years dominated by D. pulicaria, Secchi disc readings were greater during the spring and summer months when compared with years dominated by D. galeata mendotae. During the biomanipulation evaluation period, phosphorus (P) levels also changed dramatically thus complicating our analysis. Earlier research on Lake Mendota had shown that Daphnia grazing increased summer Secchi disc readings, but P concentrations linked to agricultural and urban runoff and to climate‐controlled internal mixing processes were also important factors affecting summer readings. 6. The Lake Mendota biomanipulation project has been a success given that high densities of the large‐bodied D. pulicaria have continued to dominate for over a decade, and the diversity of fishing opportunities have improved for walleye, northern pike and, more recently, yellow perch. 7. Massive stocking coupled with very restrictive fishing regulations produced moderate increases in piscivore densities. Larger increases could be realised by more drastic restrictions on sport fishing, but these regulations would be very controversial to anglers. 8. If the lake's food web remains in a favourable biomanipulation state (i.e. high herbivory), further improvements in water clarity are possible with future reductions in P loadings from a recently initiated non‐point pollution abatement programme in the lake's drainage basin.     

Is reduction of the benthivorous fish an important cause of high transparency following biomanipulation in shallow lakes?

Experimental reduction of the fish stock in two shallow lakes in The Netherlands shows that such a biomanipulation can lead to a substantial increase in transparency, which is caused not only by a decrease in algal biomass, but also by a decrease in resuspended sediment and detritus. A model was developed to describe transparency in relation to chlorophyll-a and inorganic, suspended solids (resuspended sediment). With the use of this model it is shown that more than 50% of the turbidity in these shallow lakes before biomanipulation was determined by the sediment resuspension, mainly caused by benthivorous fish. Another analysis reveals that the concentration of inorganic suspended solids and the biomass of benthivorous fish are positively correlated, and that even in the absence of algae a benthivorous fish biomass of 600 kg ha⁻¹ can reduce the Secchi depth to 0.4 m in shallow lakes. In addition, it is argued that algal biomass is also indirectly reduced by removal of benthivorous fish. Reduction of benthivorous fish is necessary to get macrophytes and macrophytes seem to be necessary to keep the algal biomass low in nutrient-rich shallow lakes. It is concluded that the impact of benthivorous fish on the turbidity can be large, especially in shallow lakes.     

Zooplankton abundance and composition in the hypertrophic Rietvlei Dam,South Africa,negate prospects for its remedial ‘top-down’ biomanipulation

Crustacean zooplankton abundance and composition were determined at one offshore and three nearshore sites in the hypertrophic Rietvlei Dam on 19 dates between July 2009 and December 2011. Total biomass fluctuated seasonally, generally declining from spring to winter through the annual cycle, but also appeared to decline progressively through the study. On overall average, total biomass was high (0.51 mg l^–1 or 2.39 g m^–2 DW), with Daphnia accounting for ～40%. Total volumetric biomass was invariably higher in shallow nearshore than offshore locations (average time-paired ratio = 8.1), with comparably large-bodied Daphnia (geometric mean ～1.2 mm, largest individuals ～1.75 mm) in both habitats, contra-indicating substantive opportunistic zooplanktivory by juvenile fish. Zooplankton was dominated numerically (ind. l^–1, overall average values) by cyclopoid copepods, mostly Thermocyclops (47.1 nauplii and 86.5 copepodites), plausibly favoured by their selective raptorial feeding mode. Cladoceran densities were lower — Daphnia (25.0), Bosmina (7.1), Ceriodaphnia (2.9) and sporadically occurring Moina and Chydorus (<0.5). Seasonal replacement of Daphnia by small-bodied cladocerans during late summer and into autumn was evident in near-monthly samples between July 2009 and June 2010. The findings reflect negligible zooplanktivory in Rietvlei, ruling out top-down biomanipulation prospects for its remedial management.     

Restoration by biomanipulation of lake Bleiswijkse Zoom (The Netherlands): First results

In 1987, the Bleiswijkse Zoom, a small, shallow lake in The Netherlands, was divided into two compartments to investigate the possible use of biomanipulation as a tool for restoring the water quality of hypertrophic lakes. The density of the fish stock before restoration was about 650 kg.ha^–1, composed mainly of bream, white bream and carp. Pikeperch was the main fish predator in the lake. In April, 1987, in one compartment (Galgje) all planktivorous bream and white bream and about 85% of the benthivorous bream and carp were removed. Advanced pikeperch fry were introduced as predator during the transient period. The other compartment (Zeeltje) was used as a reference. Removal of the fish in Galgje resulted in low concentrations of chlorophyll-a, total phosphorus, nitrogen and suspended solids. The absence of bottom-stirring activity by benthivorous fish and the low chlorophyll-a concentrations led to an increase in the Secchi disk transparency from 20 to 110 cm. Within two months after removal of the fish, macrophytes, mainly Characeae, became abundant. Until July the high density of large zooplankton species caused low algal biomass. From June onwards, the zooplankton densities decreased, but the algal concentrations remained low. This is probably because of nutrient limitation or depression of algal growth by macrophytes or both. Compared with the non-treated compartment the number of fish species in the treated compartment was higher. Perch, rudd and roach, i.e. the species associated with aquatic vegetation, were found in the samples. The survival of the O⁺ pikeperch was poor. The pikeperch could not prevent the growth of young cyprinids. Within two months after the removal of the fish a habitat for northern pike was created.     

Why biomanipulation can be effective in peaty lakes

The effects of fish stock reduction (biomanipulation) was studied in an 85 ha shallow peaty turbid lake. The lake cleared in a 4-week period in April–May 2004, which demonstrated that biomanipulation can be effective in peaty lakes. We demonstrated that it is possible to reduce the fish stock to <25 kg ha⁻¹ benthivorous fish and <15 kg ha⁻¹ planktivorous fish, sufficiently low to switch the lake from a turbid to a clear state. Knowledge of lake morphology, fish stock, fish behaviour, and a variety of fishing methods was necessary to achieve this goal. It is expected that continuation of fisheries to remove young of the year planktivorous species is needed for several years, until macrophytes provide sufficient cover for zooplankton and can compete with phytoplankton. Cladocerans developed strongly after fish removal. The clearing of the lake coincided with a sudden decrease of filamentous cyanobacteria and suspended detritus, and a strong increase of Bosmina. We assume that Bosmina was able to reduce filamentous prokaryotes and detritus. After the disappearance of the cyanobacteria, Bosmina disappeared too. After the clearing of the lake Daphnia dominated in zooplankton and apparently was able to keep phytoplankton levels low. In our case, wind resuspension did not prevent biomanipulation from being successful. No correlation between windspeed and turbidity was found, neither in an 85 ha nor in a 230 ha shallow peaty lake. Regression analysis showed that on average 50% of the amount of suspended detritus can be explained by resuspension by fish and 50% by phytoplankton decomposition. The main goal of this biomanipulation experiment, clear water and increased submerged plant cover in a shallow peaty lake, was reached.     

A review: limnological management and biomanipulation in the London reservoirs

Low algal biomasses and high water transparencies are a feature of the storage reservoirs that supply most of London's treated water. This is a result of knowledgeable limnological management and biomanipulation and despite the eutrophic nature of the River Thames with its high nutrients (7 gN m⁻³; 1 gP m⁻³) and particulate organic carbon (2 gC m⁻³). Built-in possibilities of jetting input water are managed to prevent stratification, to ensure isothermy, to mix chemicals and plankton vertically and horizontally and to manipulate the mixed-depth of the algal populations such that their potential for biomass growth is reduced by light-energy limitation. Spring algal growth is delayed and the spring peak is reduced and curtailed by the grazing impact of considerable biomasses of large-bodied daphnid populations (Daphnia magna, pulicaria & hyalina) whose development is also supported by the continuous input of high riverine algal crops. The existence of a large-bodied daphnid zooplankton in the reservoirs is associated with low levels of fish predation since the late 1960s. Variations in the intensity and nature of this vertebrate predation during the subsequent twenty years (1968–88) are illustrated by the changes that have occurred in the relationship between the phytoplankton and zooplankton biomasses of the April-May-June quarter of the year. This example of the London reservoirs serves to illustrate biomanipulation in deep water bodies by bottom-up as well as top-down effects.     

Strong interactions between stoichiometric constraints and algal defenses: evidence from population dynamics of Daphnia and algae in phosphorus-limited microcosms

The dynamic interactions among nutrients, algae and grazers were tested in a 2 × 3 factorial microcosm experiment that manipulated grazers (Daphnia present or absent) and algal composition (single species cultures and mixtures of an undefended and a digestion-resistant green alga). The experiment was run for 25 days in 10-L carboys under mesotrophic conditions that quickly led to strong phosphorus limitation of algal growth (TP ? 0.5 μM, N:P 40:1). Four-day Daphnia juvenile growth assays tested for Daphnia P-limitation and nutrient-dependent or grazer-induced algal defenses. The maximal algal growth rate of undefended Ankistrodesmus (mean ± SE for three replicate microcosms; 0.92 ± 0.02 day^?1) was higher than for defended Oocystis (0.62 ± 0.03 day^?1), but by day 6, algal growth was strongly P-limited in all six treatments (molar C:P ratio >900). The P-deficient algae were poor quality resources in all three algal treatments. However, Daphnia population growth, reproduction, and survival were much lower in the digestion-resistant treatment even though growth assays provided evidence for Daphnia P-limitation in only the undefended and mixed treatments. Growth assays provided little or no support for simple threshold element ratio (TER) models that fail to consider algae defenses that result in viable gut passage. Our results show that strong P-limitation of algal growth enhances the defenses of a digestion-resistant alga, favoring high abundance of well-defended algae and energy limitation of zooplankton growth.     

Impacts of environmental scenarios on chlorophyll-a in the management of shallow, eutrophic lakes following biomanipulation: An application of a numerical model

Accurate prediction of species changes in lake ecosystems following biomanipulation measures is of paramount importance in view of water quality management. The temporal variation of phytoplankton biomass as chlorophyll-a and transparency as Secchi depth measurements are studied in the Lake Bleiswijkse Zoom, The Netherlands, with a comprehensive structural dynamic model. In the formulation of the biological model, phytoplankton as several species, zooplankton, detritus, planktivores and benthivores, and piscivores are considered to be major contributing state variables for the model. The primary goal of this paper is to describe the possible impacts of several environmental scenarios on chlorophyll-a biomass qualitatively as it would help lake and environmental managers and relevant authorities elucidate the processes of eutrophication and biomanipulation in a broad way. Some of the scenarios that have been studied by this model are: (1) The effect of fixed stoichiometry in terms of internal nitrogen and phosphorus that are tied up within algal cells; (2) the effects of external phosphorus limitation; (3) light limitation and external nitrogen limitation on algal growth; (4) probable consequences that have taken place within the chlorophyll-a biomass due to change in biomasses of various aquatic organisms; and (5) possible changes of chlorophyll-a biomass due to higher temperatures caused by global warming.     

Cladoceran community responses to biomanipulation and re‐oligotrophication in Lake Vesijärvi,Finland, as inferred from remains in annually laminated sediment

1. We studied the role of zooplankton in biomanipulation and the subsequent recovery phase in the Enonselkä basin of Lake Vesijärvi, using subfossil cladocerans in annually laminated sediment. Measures to restore the Enonselkä basin included reduction in external nutrient loading and mass removal of plankti‐ and benthivorous fish. Water clarity increased and the lake changed from a eutrophic to a mesotrophic state. However, some signs of increased turbidity were observed after 5–10 years of successful recovery. 2. Annual laminae in a freeze core sample were identified and sliced, based on the seasonal succession of diatoms. Cladoceran remains and rotifer eggs were counted, and Daphnia ephippia and Eubosmina and Bosmina ephippia and carapaces were measured. Annual changes in pelagic species composition were studied with principal component analysis. Individual species abundance, size measurements and various cladoceran‐based indices or ratios (commonly used to reconstruct changes in trophic state and fish predation) were tested for change between four distinct periods: I (1985–1988) dense fish stocks, poor water quality; II (1989–1992) fish removal; III (1993–1997) low fish density, improved water quality; IV (1998–2002) slightly increased fish density and poorer water quality. 3. After the removal of fish, the mean size of Daphnia ephippia and Eubosmina crassicornis ephippia and carapaces increased significantly. In contrast, the percentage of Daphnia did not increase. When based on ephippia, the ratio Daphnia/(Daphnia + E. crassicornis) increased, but the interpretation was obscured by the tolerance of fish predation by small Daphnia and by the fact that bosminids were the preferred food of roach. Moreover, ephippial production by E. crassicornis decreased in recent years. 4. The abundance of Diaphanosoma brachyurum and Limnosida frontosa increased significantly after the fish population was reduced, while that of Ceriodaphnia and rotifers decreased. 5. The expanding littoral vegetation along with improved water clarity was clearly reflected in the concentration of littoral species in the deep sediment core. The species diversity index for the entire subfossil community also increased. 6. The period of faltering recovery was characterised by greater interannual variability and an increased percentage of rotifers. Nevertheless, the mean sizes of Daphnia ephippia and E. crassicornis ephippia and carapaces indicated a low density of fish. The deteriorating water quality was apparently related to multiple stressors in the catchment after rehabilitation, such as intensified lakeshore building, as well as to exceptional weather conditions, challenging the management methods in use.     

Fish community assemblages changed but biomass remained similar after lake restoration by biomanipulation in a Chinese tropical eutrophic lake

While the effects of lake restoration by fish manipulation are well-studied in the temperate zone, comparatively little information is available on this issue from tropical lakes. It may be expected that fish removal leads to faster recovery of the fish stock here than in temperate lakes due to more frequent and earlier reproduction, which may, in turn, delay positive effects of the restoration. We studied the community composition, feeding type and abundance of fish in three basins of a tropical shallow lake: one unrestored basin (UR) and two basins restored by fish manipulation and transplantation of submerged macrophytes. While omni-benthivorous fish dominated the biomass in the restored basins 3 and 5 years after restoration, planktivores were most abundant in the UR, although total fish biomass remained similar. One-way analyses of similarities based on fish species presence/absence, abundance and biomass data revealed significant differences in fish community composition among the restored basins and UR, and redundancy analyses further indicated that submerged macrophytes were a key driver behind this difference. Our results indicate that active implantation of submerged macrophytes to stabilise the fish community is a tool to consider when planning lake restoration by biomanipulation in the tropics.     

Early responses of plankton and turbidity to biomanipulation in a shallow prairie lake

We evaluated the effect of a fish removal from a shallow, turbid, eutrophic lake. By late May (following an October fish removal), the cladoceran community shifted from small-bodiedBosmina andChydorus (less than 100 l⁻¹) to largerDaphnia (over 100 l⁻¹). During the periods of peak daphnid abundance (late May–June) chlorophyll-a concentrations and edible diatoms were reduced and water transparency improved dramatically. Total phosphorus was not significantly lowered during this period. Although this clear-water phase was short-lived (May, June and early July), it corresponded to the critical period of plant growth and allowed dramatic increases in submergent macrophytes.     

Depth-selection behavior and longevity in Daphnia: an evolutionary test for the predation-avoidance hypothesis

The classic evolutionary theory of senescence predicts that long lifespan evolves under low risk of extrinsic mortality. As lakes present planktonic animals with vertical gradients of mortality risk associated with fish predation, we expected the individual lifespan of Daphnia of the “hypolymnetic” clones to be longer than that of “epilimnetic” Daphnia. In order to test this prediction, we performed a laboratory study on 14 clones from the D. longispina species complex, taken during the daytime either from epilimnion or hypolimnion of three mesotrophic lakes. “Epilimnetic” Daphnia started reproduction earlier, aged faster, and lived shorter than their “hypolimnetic” conspecifics. Our results indirectly support the predation-avoidance hypothesis as the ultimate explanation for depth-selection behavior in Daphnia.     

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.

     

Effect of a Zooplankton Community on Seston Elimination in a Restored Pond in Japan

The rates of seston elimination by zooplankton and primary production were measured in Funada-ike Pond, typical of human-made impoundments in Japan, from April to September in order to evaluate various treatments of the pond aimed at improving water quality by reducing seston abundance. The treatments included draining the pond water, dredging the bottom mud, eliminating the wastewater inflow, and biomanipulation through removal of all fish. After the treatment, seston abundance was reduced from more than 10 to 0.4–2.5 mg C/liter, and large daphnid species, Daphnia similis and D. magna, occurred and predominated in the zooplankton community. Seston abundance remained at a relatively low level from June to August but increased markedly in late August, while the biomass of zooplankton became high from June to mid-August and then decreased. A decrease in seston abundance was found when the elimination rate exceeded the primary production rate. The results indicate that the development of daphnid populations was effective in keeping seston abundance at a low level. The relationship between the rate of primary production and the zoo-plankton biomass required to offset this rate, however, suggests that biomanipulation aimed at increasing zooplankton biomass alone is less effective in a pond with a high primary production. The success in improving water quality in this pond seems to depend not only on the increase in biomass of large daphnid species that resulted mainly from the removal of fish, but also on the decrease in nutrient load that was realized by the other treatments.     

The evaluation of the role of pelagic invertebrate versus vertebrate predators on the seasonal dynamics of filtering Cladocera in a shallow, eutrophic reservoir

An evaluation of the role of invertebrate (Leptodora kindtii) versus vertebrate (juvenile fish) predators in the dynamics of filtering zooplankton (Daphnia sp.) in the pelagic zone of the Sulejow Reservoir was conducted during summer from 1994 to 1996. Leptodora kindtii appeared in high densities (mid-July: 2–6 ind. l^?1) in the pelagic zone. Its predation impact on the dominant filtering zooplankter population Daphnia cucullata, expressed by consumption, was significant and accounted for a reduction of 10 to 51% of Daphnia biomass. Although D. cucullata was a preferred food of juvenile pikeperch in June, due to low biomass, these fish were able to eliminate daily only 0.1–5.4% of Daphnia biomass in the pelagic zone. Thus, the role of juvenile fish in the midsummer decline of D. cucullata density in the pelagic zone seems to be much smaller than that of Leptodora kindtii.