P-load,phytoplankton, zooplankton and fish stock in Loosdrecht Lake and Tjeukemeer: confounding effects of predation and food availability

The fish community in the Loosdrecht lakes is dominated by bream, pikeperch and smelt and is characteristic of shallow eutrophic lakes in The Netherlands. The biomasses of the respective fish species amount to ca. 250, 25 and 10 kg ha^–1 and correspond to those in Tjeukemeer, another lake in The Netherlands. The average size of bream, however, is much smaller in the Loosdrecht lakes as a consequence of poorer feeding conditions. The zooplankton community in the Loosdrecht lakes is predominantly composed of relatively small species such as Daphnia cucullata, Bosmina coregoni and cyclopoid copepods, whereas in Tjeukemeer, Daphnia hyalina is permanently present in relatively high densities and the other species show a larger mean length. In the Loosdrecht lakes, the absence of D. hyalina and the smaller sizes of the other zooplankton species could be the consequence of a higher predation pressure, in combination with unfavourable feeding conditions for the zooplankton including the low density of green algae and the high density of filamentous cyanobacteria. A biomanipulation experiment in Lake Breukeleveen, one of the Loosdrecht lakes, indicated that feeding conditions were too unfavourable for large zooplankton to develop in spring, when the reduced fish biomass was not yet supplemented by natural recruitment and immigration.     

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.     

Shifts in water quality in a drinking water reservoir during and after the removal of cyprinids

Lake ülemiste, the drinking water reservoir of Estonia’s capital city Tallinn, was biomanipulated by manual removal of cyprinids in 2004–2006 and its impact on water quality in the vegetation period was studied. A total biomass of 156 tonnes corresponding to 160 kg ha⁻¹ of fish, predominantly cyprinids, were removed. A decline in the unit catches of fishing was observed. The removed fish biomass versus phosphorus concentration of the lake was considered sufficient to reduce the impact of cyprinids on water quality. The phosphorus removed within fish biomass corresponded to 38 μg l⁻¹ and 21% of the external phosphorus load of the fishing period. The mean total phosphorus concentration dropped from >50 to ≤36 μg l⁻¹. However, the densities of planktivorous young-of-the-year percids remained high and the role of zooplankton grazing in improving water quality was found non-significant or transient. The cladocerans biomass decreased and the small-sized Daphnia cucullata remained almost the only daphnid in Lake ülemiste during and after the manipulation. Predomination of filamentous cyanobacteria was replaced by a more diverse phytoplankton composition and co-domination of micro- and pico-sized colonial cyanobacteria during summer. Mean phytoplankton biomass decreased from 15 to 6 mg l⁻¹ primarily as a result of decreased in-lake TP availability. The Secchi disc transparency increased only in May 2005–2007. The effects of coincidental events, a decline of external loading of phosphorus and a simultaneous flushing induced by heavy rainfall, on lake water quality are discussed with some implications to the future management of the reservoir.     

Fish manipulation as a lake restoration tool in shallow,eutrophic, temperate lakes 2: threshold levels,long-term stability and conclusions

In order to evaluate short-term and long-term effects of fish manipulation in shallow, eutrophic lakes, empirical studies on relationships between lake water concentration of total phosphorus (P) and the occurrence of phytoplankton, submerged macrophytes and fish in Danish lakes are combined with results from three whole-lake fish manipulation experiments. After removal of less than 80 per cent of the planktivorous fish stock a short-term trophic cascade was obtained in the nutrient regimes, where large cyanobacteria were not strongly dominant and persistent. In shallow Danish lakes cyanobacteria were the most often dominating phytoplankton class in the P-range between 200 and 1 000μg P l⁻¹. Long-term effects are suggested to be closely related to the ability of the lake to establish a permanent and wide distribution of submerged macrophytes and to create self-perpetuating increases in the ratio of piscivorous to planktivorous fish. The maximum depth at which submerged macrophytes occurred, decreased exponentially with increasing P concentration. Submerged macrophytes were absent in lakes>10 ha and with P levels above 250–300μg P l⁻¹, but still abundant in some lakes<3 ha at 650μg P l⁻¹. Lakes with high cover of submerged macrophytes showed higher transparencies than lakes with low cover aboveca. 50μg P l⁻¹. These results support the alternative stable state hypothesis (clear or turbid water stages). Planktivorous fish>10 cm numerically contributed more than 80 per cent of the total planktivorous and piscivorous fish (>10 cm) in the pelagical of lakes with concentrations above 100μg P l⁻¹. Below this threshold level the proportion of planktivores decreased markedly toca. 50 per cent at 22μg P l⁻¹. The extent of the shift in depth colonization of submerged macrophytes and fish stock composition in the three whole-lake fish manipulations follows closely the predictions from the relationships derived from the empirical study. We conclude that a long-term effect of a reduction in the density of planktivorous fish can be expected only when the external phosphorus loading is reduced to below 0.5–2.0 g m⁻² y⁻¹. This loading is equivalent to an in-lake summer concentration below 80–150μg P l⁻¹. Furthermore, fish manipulation as a restoration tool seems most efficient in shallow lakes.     

Fish manipulation as a lake restoration tool in shallow,eutrophic temperate lakes 1: cross-analysis of three Danish case-studies

The use of fish manipulation as a tool for lake restoration in eutrophic lakes has been investigated since 1986 in three shallow, eutrophic Danish lakes. The lakes differ with respect to nutrient loading and nutrient levels (130–1000 μg P l⁻¹, 1–6 mg N l⁻¹). A 50% removal of planktivorous fish in the less eutrophic cyanobacteria-diatom dominated Lake V?ng caused marked changes in lower trophic levels, phosphorus concentration and transparency. Only minor changes occurred after a 78% removal of planktivorous fish in eutrophic cyanobacteria dominated Frederiksborg Castle Lake. In the hypertrophic, green algae dominated Lake S?byg?rd a low recruitment of all fish species and a 16% removal of fish biomass created substantial changes in trophic structure, but no decrease in phosphorus concentration. The different response pattern is interpreted as (1) a difference in density and persistence of bloomforming cyanobacteria caused by between-lake variations in nutrient levels and probably also mixing- and flushing rates, (2) a difference in specific loss rates through sedimentation of the algal community prevaling after the fish manipulation, (3) a decreased impact of planktivorous fish with increasing mean depth and (4) a lake specific difference in ability to create a self-increasing reduction in the phosphorus level in the lake water. This in turn seems related to the phosphorus loading.     

Feeding in Euchlanis dilatata lucksiana Hauer on filamentous cyanobacteria and a prochlorophyte

Ingestion and assimilation rates of Euchlanis dilatata lucksiana Hauer, isolated from Lake Loosdrecht (The Netherlands) and cultured on lake water (seston < 33 µm), were measured in the laboratory using the ¹⁴C-tracer technique. The five taxa used as tracer foods, together with 6–7 mg C l^–1 of lake seston in each case, included four species of filamentous cyanobacteria (Oscillatoria redekei, O. limnetica, Aphanizomenon flos-aquae, Anabaena PCC 7120) and a prochlorophyte (Prochlorothrix hollandica). Except Anabaena, they are all commonly encountered in eutrophic Loosdrecht lakes, including Lake Loosdrecht, and their dimensions ranged between 150 and 250 µm in length and 2 and 3.5 µm in width. The small and large Euchlanis used as experimental animals had mean lengths of 217–223 µm and 276–305 µm, respectively. Euchlanis fed on all the taxa offered as food. Clearance rates ranged from 51 to 99 µl ind^–1 d^–1 for the large animals and from 22 to 41 µl ind^–1 d^–1 for the small animals. The highest ingestion rate observed, 1.7 µg ind^–1 d^–1, was for the large animals feeding on Aphanizomenon. The daily ration for both size classes far exceeded 100% of body weight, reaching up to 690% for the small animals feeding on Aphanizomenon. The small animals generally appeared to assimilate the ingested food more efficiently (assimilation efficiencies: 37–100%) than the large animals (34–77%). Compared with an earlier study in which only lake seston (<33 µm) was used as food, the specific clearance rates of Euchlanis on the cyanobacteria and Prochlorothrix were generally somewhat lower.     

Observations on cyanobacterial population collapse in eutrophic lake water

In two laboratory-scale enclosures of water from the shallow, eutrophic Lake Loosdrecht (the Netherlands), the predominating filamentous cyanobacteria grew vigorously for 2 weeks, but then their populations simultaneously collapsed, whereas coccoid cyanobacteria and algae persisted . The collapse coincided with a short peak in the counts of virus-like particles. Transmission electron microscopy showed the morphotype Myoviridae phages, with isometric heads of about 90 nm outer diameter and >100-nm long tails, that occurred free, attached to and emerging from cyanobacterial cells. Also observed were other virus-like particles of various morphology. Similar mass mortality of the filamentous cyanobacteria occurred in later experiments, but not in Lake Loosdrecht. As applies to lakes in general, this lake exhibits high abundance of virus-like particles. The share and dynamics of infectious cyanophages remain to be established, and it is as yet unknown which factors primarily stabilize the host–cyanophage relationship. Observations on shallow, eutrophic lakes elsewhere indicate that the cyanophage control may also fail in natural water bodies exhibiting predominance of filamentous cyanobacteria. Rapid supply of nutrients appeared to be a common history of mass mortality of cyanobacteria and algae in laboratory and outdoor enclosures as well as in highly eutrophic lakes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Spatial variability in the abundance of pelagic invertebrate predators in relation to depth and turbidity

The abundance of pelagic invertebrate predators in relation to turbidity and depth gradients in Lake Hiidenvesi (southern Finland) were studied. In the shallow (<5 m) and the most turbid (up to 75 NTU) part of the lake, the community of invertebrate predators consisted of cyclopoid copepods (max biomass >500 μg dw l⁻¹) and Leptodora kindtiii (Focke) (17 μg dw l⁻¹), while in the less turbid (10–40 NTU) stratifying area Chaoborus flavicans (Meigen) dominated (max 146 μg dw l⁻¹). In the temporarily stratifying and moderately turbid basin Chaoborus and small-bodied invertebrate predators co-existed. Mysis relicta (Lovén) occurred only in the stratifying area (max 15 μg dw l⁻¹). The results suggested that both water depth and turbidity contributed to the community structure of Chaoborus flavicans. Depth great enough for stratification was of special importance and its effect was amplified by elevated turbidity, while high turbidity alone could not maintain chaoborid populations. Mysis relicta also requires a hypolimnetic refuge but is more sensitive to low oxygen concentrations and may therefore be forced to the epilimnion where it is vulnerable to fish predation. Cyclopoids as rapid swimmers can take advantage at elevated turbidity levels and coexist in high biomass with fish even in shallow water. Leptodora kindtii can form high biomass despite planktivorous fish providing that turbidity exceeds 20 NTU. The results demonstrated that depth and water turbidity can strongly regulate the abundance and species composition of invertebrate predators. These factors must thus be taken into account when applying food web management, which aims to reduce phytoplankton biomass by depressing planktivorous fish.     

Scaling food chains in aquatic mesocosms: do the effects of depth override the effects of planktivory?

To assess the effects of physical dimension and planktivorous fish on phytoplankton standing crop, we repeated an experiment at different scales in plastic enclosures during summer 1995 in Lake Créteil, France. Enclosures were scaled for a constant surface (1.5 × 1.5 m) as depth was increased from 2.5 to 4.5 m. Even-link (zooplankton and phytoplankton) and odd-link (planktivorous fish, zooplankton and phytoplankton) food webs were established in both shallow and deep enclosures. Fish densities in the deep enclosures were scaled to allow comparisons with shallow ones for both in individuals m⁻² or individuals m⁻³. We explicitly designed this experiment to examine the scale-dependent behavior of the top-down mechanism of algal biomass control in lakes, and in particular to test the hypothesis of stronger cascading effects of fish on lower trophic levels at reduced depth. Both fish and enclosure size had highly significant effects on phytoplankton biomass over the duration of the experiment. No depth × fish interaction effects were observed. The presence of planktivorous fish enhanced phytoplankton biomass in both shallow and deep enclosures, although the reduction in depth generally produced a stronger effect. The mean concentration of chlorophyll a in the deep odd-link systems (ca 5 mg m⁻³) was lower than in the shallow even-link systems (ca 17 mg m⁻³). Statistical interpretation did not change when data were expressed as phytoplankton biomass per unit of surface area. Light limitation and zooplankton grazing are the most probable mechanisms explaining our results in these nutrient-enriched systems. Moreover, we found that the strength of the cascading effect of fish on plankton was not a function of depth. We believe that further studies on scaling effects should be conducted in order to improve our understanding of ecological patterns and to extrapolate results from micro/mesocosms to natural ecosystems. Received: 18 January 1999 / Accepted: 7 June 1999     

Whole-lake food-web manipulation as a means to study community interactions in a small ecosystem

Whole-lake food-web manipulation was carried out in the hypertrophic Lake Zwemlust (The Netherlands), with the aim of studying the effects on the lake's trophic status and to gain an insight into complex interactions among lake communities. Before manipulation this small (1.5 ha) and shallow (1.5 m) lake was characterized byMicrocystis blooms in summer and high chlorophyll-a concentrations were common (ca. 250 μg 1⁻¹). In March 1987 the planktivorous and benthivorous fish species in the lake were completely removed (ca. 1000 kg ha⁻¹), a new simple fish community (pike and rudd) was introduced and artificial refuges were created. The effects of this manipulation on the light climate, nutrient concentrations, phytoplankton, zooplankton, fish, macrophytes, and macrofauna were monitored during 1987, 1988 and 1989. Community interactions were investigated in phytoplankton bioassays and zooplankton grazing experiments. After the manipulation, despite the still high P and N loads to the lake (ca. 2.2 g P m⁻² y⁻¹ andca. 5.3 g N m⁻² y⁻¹), the phytoplankton density was low (Chl-a<5μg l⁻¹), due to control by large-sized zooplankton in spring and N-limitation in summer and autumn. A marked increase in the abundance of macrophytes and filamentous green algae in 1988 and 1989, as well as N loss due to denitrification, contributed to the N limitation of the phytoplankton. Before manipulation no submerged macro-vegetation was present but in 1988, the second year after manipulation, about 50% of the lake bottom was covered by macrophytes increasing to 80% in 1989. This led to substantial accumulation of both N and P, namely 76% and 73% respectively of the total nutrients in the lake in particulate matter. Undesirable features of the increase in macrophytes were: 1) direct nuisance to swimmers; and, 2) the large scale development of snails, especiallyL. peregra, which may harbour the parasite causing ‘swimmers' itch’. But harvesting of only about 3% of the total macrophyte biomass from the swimmers' area, twice a year, reduced the nuisance for swimmers without adversely affecting the water clarity.