Changes in phosphorus cycling in a shallow lake due to food web manipulations

SUMMARY. 1. Food web manipulation, by removal of planktivorous or benthivorous fish, is a promising method for reducing phytoplankton concentrations in shallow lakes. The part that nutrients may play in the success of such a measure is not well documented.
2. In this study, we analysed the flow of phosphorus through the food web of the shallow, eutrophic Lake Wolderwijd/Nuldernauw. Our studies occurred in the years 1981 (when a bloom of cyanobacteria occurred) and 1987 (no bloom); a hypothetical situation was also examined in which most of the bream are assumed to be removed.
3. The analysis shows that the success of biomanipulation is probably due not only to an increased grazing pressure on the phytoplankton, but also to a decreased availability of phosphorus. The reason for this is the removal of detrital phosphorus by increased sedimentation as a result of a predicted increase in growth of macrophytes after biomanipulation.     

Biomanipulation and its feasibility for water quality management in shallow eutrophic water bodies in The Netherlands

Biomanipulation as a tool for lake restoration is discussed mainly using literature data. It is based on the exploitation of the interactions both within and between the trophic levels in an aquatic ecosystem. Important among the interactions are: competition for light and nutrients between aquatic macrophytes and phytoplankton and among different phytoplankton species; grazing by planktonic and benthic filter feeders; and size-selective predation by fish. In several case studies biomanipulation has proved to be successful in restorating mildly eutrophic small waterbodies. However, for long-term stability of the restored ecosystems supplementary measures like reducing the external nutrient loadings are needed. The feasibility of the different biomanipulation measures to improve the water quality in shallow Dutch lakes is discussed. Preliminary results on biomanipulation experiments in enclosures withOscillatoria agardhii and the benthic filter feederDreissena polymorpha are given.     

Lime treatment and its effects on the chemistry and biota of hardwater eutrophic lakes

1. The main focus of this study was to investigate the effects of single and multiple moderate doses of lime (slaked lime, Ca(OH)₂, and/or calcite, CaCO₃) on eutrophic hardwater lakes. This information would contribute to strategies to manage phytoplankton and macrophyte biomass in eutrophic lakes.
2. Water chemistry and biota were monitored for up to 7 years after initial lime treatment and results were compared with reference systems.
3. Complementary studies investigated the effect of lime on macrophytes in ponds, irrigation canals and microcosm experiments.
4. When water pH was kept within its natural range (≤ 10), single and multiple lime applications to lakes and ponds controlled macrophyte biomass, without negatively affecting invertebrate communities.
5. Single lime treatments at moderate dosages of lakes and ponds resulted in variable and mostly temporary changes in chlorophyll a (chl a ) and phosphorus (P) concentration. Although sediment P release was reduced in single-dose lakes during the first winter following treatment, reductions appeared temporary.
6. Multiple treatments of lakes and ponds were effective at reducing both chl a and P concentrations over longer periods. Mean winter P release rate was also reduced after initial treatment.
7. In laboratory studies, sediment cores were incubated with eight different treatments to assess P release. Redox-sensitive treatments were no more effective at lowering total P concentration in overlying water than some redox-insensitive treatments. Lime reduced total P concentrations, but was not as effective as treatments with alum.
8. The use of lime in managing macrophyte and phytoplankton biomass in shallow, hardwater lakes and ponds may be preferable over other treatments, because lime is economical and non-toxic as long as pH is kept within a natural range.     

Major changes in CO<Subscript>2</Subscript> efflux when shallow lakes shift from a turbid to a clear water state

Lakes can be sources or sinks of carbon, depending on local conditions. Recent studies have shown that the CO₂ efflux increases when lakes recover from eutrophication, mainly as a result of a reduction in phytoplankton biomass, leading to less uptake of CO₂ by producers. We hypothesised that lake restoration by removal of coarse fish (biomanipulation) or invasion of mussels would have a similar effect. We studied 14–22 year time series of five temperate Danish lakes and found profound effects on the calculated CO₂ efflux of major shifts in ecosystem structure. In two lakes, where limited colonisation of submerged macrophytes occurred after biomanipulation or invasion of zebra mussels (Dreissena polymorpha), the efflux increased significantly with decreasing phytoplankton chlorophyll a. In three lakes with major interannual variation in macrophyte abundance, the efflux declined with increasing macrophyte abundance in two of the lakes, while no relation to macrophytes or chlorophyll a was found in the third lake, likely due to high groundwater input to this lake. We conclude that clearing water through invasive mussels or lake restoration by biomanipulation may increase the CO₂ efflux from lakes. However, if submerged macrophytes establish and form dense beds, the CO₂ efflux may decline again.     

Biomanipulation of lake ecosystems: an introduction

SUMMARY 1. This paper is an introduction to a special issue of Freshwater Biology containing selected papers from an international symposium on Food Web Effects of Fish in Lake Ecosystems: Research Progress, Water Quality and Fisheries Management held from 31 May to 3 June 2000 in Rheinsberg, Germany. The primary goal of the workshop was to enlarge the current view of fish-induced effects on lake ecosystems. An additional goal was to promote biomanipulation as a multiple-use tool for managing freshwater ecosystems.
2. The three main topics addressed at the workshop were: (i) mechanisms involved in biomanipulation, (ii) whole-lake case studies and (iii) management aspects in water quality and fisheries.
3. Mortality of Daphnia , nutrient recycling, habitat selection and fish predation are reported as important mechanisms governing food-web effects as a result of biomanipulation.
4. Whole-lake case studies indicate that repeated fish removal can help improve water quality of shallow lakes, but successful biomanipulation of deep, thermally stratifying lakes remains difficult.
5. In many cases, biomanipulation of lakes has proved to provide benefits in addition to improving water quality. As all lake users are potentially affected when biomanipulation is used as a lake management tool, their concerns need to be clearly recognised if biomanipulation is to be successful in practice.     

Stabilizing the clear-water state in eutrophic ponds after biomanipulation: submerged vegetation versus fish recolonization

Biomanipulation through fish removal is a tool commonly used to restore a clear-water state in lakes. Biomanipulation of ponds is, however, less well documented, although their importance for biodiversity conservation and public amenities is undisputed. In ponds, a more complete fish removal can be carried out as compared to lakes and therefore a stronger response is expected. Fish recolonization can, however, potentially compromise the longer term success of biomanipulation. Therefore, we investigated the impact of fish recolonization on zooplankton, phytoplankton, and nutrients for several years after complete drawdown and fish removal in function of submerged vegetation cover in 12 peri-urban eutrophic ponds situated in Brussels (Belgium). Fish recolonization after biomanipulation had a considerable impact on zooplankton grazers, reducing their size and density substantially, independent of the extent of submerged vegetation cover. Only ponds with <30% cover of submerged vegetation shifted back to a turbid state after fish recolonization, coinciding with an increase in density of small cladocerans, rotifers, and cyclopoid copepods. In ponds with >30% submerged vegetation cover, macrophytes prevented an increase in phytoplankton growth despite the disappearance of large zooplankton grazers. Our results suggest that macrophytes, rather than by providing a refuge for zooplankton grazers, control phytoplankton through other associated mechanisms and confirm that the recovery of submerged macrophytes is essential for biomanipulation success. Although the longer term effect of biomanipulation is disputable, increased ecological quality could be maintained for several years, which is particularly interesting in an urban area where nutrient loading reduction is often not feasible.     

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

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

Hypolimnetic anoxia hampers top–down food-web manipulation in a eutrophic lake

SUMMARY 1. A biomanipulation experiment was carried out in a small (10 ha), but relatively deep (17 m) and highly eutrophic lake in northern Poland. The lake had been stocked in 1996, 1997 and 1998 with a variety of piscivorous fish (pike, catfish, trout and pikeperch), in order to reduce numbers of cyprinid planktivores.
2. Piscivore stocking was associated with a threefold decrease in the offshore fish density (night echosounding). Despite this reduction, the large planktonic cladoceran, Daphnia hyalina , remained scarce, whereas the density of small-sized zooplankton increased greatly.
3. The lack of demographic response in D. hyalina was probably due to the anoxia in the hypolimnetic refuge of this vertically migrating species. The anoxic hypolimnion, below 3–4 m depth, was inhabited day and night by numerous Chaoborus flavicans larvae.
4. Changes in zooplankton were associated with shifts in the taxonomic composition (from single-cell green algae to filamentous cyanobacteria), size structure (from nano- to net phytoplankton) and seasonal dynamics of phytoplankton, but not in the average biomass of planktonic algae. A clear-water phase, which was absent in the prestocking years, developed in spring, with Secchi depth reaching 2.5 m, a value which had never been recorded in the 20 years preceding the biomanipulation. In general, the lake's status was switched from hypertrophic to eutrophic.
5. Deteriorating food conditions, resulting from qualitative changes in the phytoplankton community, combined with predation pressure by the remaining fish and Chaoborus larvae were associated with the ultimate elimination of D. hyalina from the lake.     

Ceratophyllum demersum hampers phytoplankton development in some small Norwegian lakes over a wide range of phosphorus concentrations and geographical latitude

1. Data on submerged and floating-leafed macrophytes, phytoplankton, nutrients (N, P) and calcium were collected from twenty-four small lakes ( 1 km²) over a wide range of latitudes in Norway. The majority of the investigated lakes were mesotrophic or eutrophic, and most of the lakes were markedly affected by diffuse and point-source runoff from agriculture. According to their macrophyte species composition, the majority of the lakes can be classified as Potamogeton lakes or Chara lakes, or a combination of these.
2. This study is consistent with the 'two alternative stable states' hypothesis. We observed clearwater lakes with dense macrophyte cover over a wider range of total P concentration than has been reported previously: from 30 to more than 700 mg P m^–3. The clearwater state was only observed in lakes with mean depths of less than 1.9 m.
3. Most clear lakes with high cover of submerged vegetation showed indications of N limitation.
4. In this study nearly all the macrophyte-dominated lakes with P concentrations above 30 mg m^–3 had dense stands of Ceratophyllum demersum (L.). This indicates that Ceratophyllum may also play an important role in stabilizing and maintaining a clearwater state at high P concentrations.     

Ceratophyllum demersum hampers phytoplankton development in some small Norwegian lakes over a wide range of phosphorus concentrations and geographical latitude

1. Data on submerged and floating-leafed macrophytes, phytoplankton, nutrients (N, P) and calcium were collected from twenty-four small lakes ( 1 km²) over a wide range of latitudes in Norway. The majority of the investigated lakes were mesotrophic or eutrophic, and most of the lakes were markedly affected by diffuse and point-source runoff from agriculture. According to their macrophyte species composition, the majority of the lakes can be classified as Potamogeton lakes or Chara lakes, or a combination of these.
2. This study is consistent with the 'two alternative stable states' hypothesis. We observed clearwater lakes with dense macrophyte cover over a wider range of total P concentration than has been reported previously: from 30 to more than 700 mg P m^–3. The clearwater state was only observed in lakes with mean depths of less than 1.9 m.
3. Most clear lakes with high cover of submerged vegetation showed indications of N limitation.
4. In this study nearly all the macrophyte-dominated lakes with P concentrations above 30 mg m^–3 had dense stands of Ceratophyllum demersum (L.). This indicates that Ceratophyllum may also play an important role in stabilizing and maintaining a clearwater state at high P concentrations.