Structure and feeding activities of zooplankton community in Lake Zwemlust,in the two years after biomanipulation

The biomanipulation study in Lake Zwemlust (area 1.5 ha; mean depth 1.5 m) is among the series of such investigations initiated recently in The Netherlands. The effects of the lake's reffilling (after it was first completely drained empty) almost entirely the nutrient-rich seepage water from the River Vecht flowing nearby and of removal of the planktivorous bream (Abramis brama), on zooplankton community structure and feeding activities of crustaceans were monitored for two successive years (1987, 1988). In these years a classical pattern of succession occurred, with the rotifer spring maximum preceding the crustacean maximum by about 3 weeks. Among the fiveDaphnia species, which appeared in quick succession during May–July, two were large-bodied forms (D. magna; D. pulex). OnlyD. pulex persisted and was the important grazer species in the second year, especially in spring. In the first year the crustacean grazing, with several values >100%.d^–1, contributed significantly to the lake's improved water clarity, with Secchi-disc transparencies of 1.5 m and more almost throughout the summer, compared with 0.3 m before the biomanipulation. Even though the water clarity climate in the second year was quite similar to that in the first, the causal factor was high macrophytic vegetation, rather than zooplankton grazing. The lake developed a rich littoral flora and fauna in the second year in response to the optimal light and nutrient conditions. Apparently, the predation by the introduced planktivorous rudd (Scardinius erythrophthalmus) on zooplankton was an important factor in the changes in zooplankton structure, and in the reduced role of zooplankton in seston elimination during the second year. There is some evidence from bioassay work that, simultaneously with the littoral development, nitrogen limitation of the phytoplankton also contributed to the improved light situation in the second year.     

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.     

Ecological consequences of a manual reduction of roach and bream in a eutrophic,temperate lake

Hydrobiologia - A biomanipulation experiment was carried out in the eutrophic lake, Frederiksborg Slotssø (Denmark). During 1987 and 1988, densities of roach (Rutilus rutilus) and bream...     

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.     

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.     

Three years of experience in biomanipulating a small eutrophic lake: Lago di Candia (Northern Italy)

Results obtained from a step by step approach to the biomanipulation of a natural lacustrine environment (Lago di Candia, Northern Italy) are presented. Since the diversion of the municipal sewage of the small town of Candia, runoff and precipitation have been the sole contributors of nutrient to the lake. Fish population is mainly characterized by rudd (Scardinius erythrophthalmus) overstocking and by a low density of large-mouth-bass (Micropterus salmoides) and pike (Esox lucius). During 1986 about 12t of rudd (1–2 year old) were removed from the lake. Considering 1986 as ‘control year’, average Secchi disc transparency improved from 2.3 m in 1986 to 3.3 m in 1988; phytoplankton biovolume decreased from 114 to 58 mm³ l⁻¹ but zooplankton biovolume increased from 8 to 11.5 mm³ l⁻¹. The results achieved show that a grodual biomanipulation treatment can have a satisfactory outcome, and has the advantage of not producing catastrophic situations either in the biotic or in the abiotic compartments of the lake.     

Comparison of losses of planktivorous fish by predation and seine-fishing in a lake undergoing long-term biomanipulation

SUMMARY 1. Piscivore stocking at artificially high densities and fishing are the two common approaches to reduce the amount of planktivorous and benthivorous fish in lake biomanipulation programmes. Both measures have advantages and disadvantages, but their relative efficacy has not previously been directly compared.
2. We calculated the average annual catch of roach and bream in a lake undergoing long-term biomanipulation (Feldberger Haussee, Germany) by seining each year between 1992 and 1998. We compared this value with a bioenergetics estimate of annual consumption rates of the dominant cohorts of piscivores, pikeperch and pike, in 1997 and 1998. We also determined species composition and length distribution of prey fish in stomachs of the piscivores.
3. Roach was the dominant prey species of both pikeperch and pike, whereas bream was rarely taken by either piscivorous species. Seining removed on average larger specimens of roach than were found in the stomachs of the piscivores.
4. Based on stocking densities of the piscivores, published mortality rates, and individual consumption rates, feeding of pikeperch and pike on roach exceeded the manual removal of roach by seining by a factor of 4–15 (biomass) in 1997 and 1998.
5. Based on these results, a combination of fishing and piscivore enhancement is recommended. Whereas the stocks of adult roach and bream have to be reduced mainly by fishing, the predation of piscivores should be directed predominantly towards the juvenile zooplanktivorous fish. Therefore, small size-classes of piscivorous fish should be promoted by fisheries management, including stocking and harvest regulations.     

Food web manipulation in Lake Zwemlust: Positive and negative effects during the first two years

The hypertrophic Lake Zwemlust, a small water body used as a swimming pool, was characterized by algal blooms in summer, reducing the Secchi disk transparency to less than 0.3 m. Since in The Netherlands a Secchi disk transparency of 1 m is obligatory for swimming waters, corrective measures were called for to improve the light climate of the lake. In March, 1987, as an experiment, the lake was drained by pumping out the water to facilitate fish elimination. Planktivorous and benthivorous fish species, which were predominant, were removed by seine- and electro-fishing. After the lake had refilled by seepage it was restocked by a new simple fish community comprising pike (Esox lucius) and rudd (Scardinius erythrophthalmus) only. Stacks of willow twigs (Salix) and macrophytes (roots ofNuphar lutea and seedlings ofChara globularis) were introduced into the lake as spawning grounds and refuges for the pike against cannibalism and as shelter for the zooplankton. The effects of this food web manipulation on the light climate, phytoplankton, zooplankton, fish, macrophytes, macrofauna and on the nutrient concentrations were monitored during 1987 and 1988. In summer 1987, despite of high nutrient concentrations, the phytoplankton density was low, due to control by zooplankton, causing a Secchi disk transparency of 2.5 m, the maximum depth. Chlorophyll-a concentrations were low (<5 g Chl.l^–1), blooms of cyanobacteria did not occur and a shift from rotifers to cladocerans took place. In 1988, however, also some negative effects were noticed. Macrophytes and filamentous green algae reached a much higher biomass (50–60% cover of the lake bottom) than in 1987; some species, growing through the entire water column, interfered with the lake's recreational use. Associated with the macro-vegetation and possibly with the absence of larger cyprinids, the diet of which also comprises snails, a large scale development of the snail population, among themLymnaea peregra var.ovata took place. This species is known to act as an intermediate host of the bird-parasitizing trematodeTrichobilharzia ocellata, the cercariae of which cause an itching sensation at the spot of penetration of the human skin, accompanied by rash (schistosome dermatitis or swimmers' itch); in July, 1988, about 40% of the bathers complained about this itching. A positive effect of the macrophytes and filamentous green algae was the high uptake of nitrogen, resulting in a low nitrogen concentration in the lake and growth limitation of the phytoplankton population by nitrogen in the summer of 1988. In 1988 the cladocerans were abundant in April only; and unlike in 1987, in the summer of 1988 there was a shift from cladocerans to rotifers. Therefore, only in early spring (April) zooplankton grazing controlled phytoplankton growth and in summer nitrogen limitation was the major controlling factor, keeping chlorophyll-a concentrations low.     

Biomanipulation and food-web dynamics — the importance of seasonal stability

Responses of phytoplankton biomass were monitored in pelagic enclosures subjected to manipulations with nutrients (+N/P), planktivore roach (Rutilus rutilus) and large grazers (Daphnia) in 18 bags during spring, summer and autumn in mesotrophic Lake Gjersjøen. In general, the seasonal effects on phytoplankton biomass were more marked than the effects of biomanipulation. Primary top-down effects of fish on zooplankton were conspicuous in all bags, whereas control of phytoplankton growth by grazing was observed only in the nutrient-limited summer situation. The effect of nutrient additions was pronounced in summer, less in spring and autumn; additions of fish gave the most pronounced effect in spring. The phytoplankton/zooplankton biomass ratio remained high (10–100) in bags with fish, with the highest ratios in combination with fertilization. The ratio decreased in bags without fish to<2 in most bags, but a real grazing control was only observed in bags with addition ofDaphnia. No direct grazing effects could be observed on the absolute or relative biomass of cyanobacteria (mainlyOscillatoria agardhii). The share of cyanobacteria in total phytoplankton biomass was lowest in summer (7–26%), higher in spring (39–63%) and more than 90% in the autumn experiment. The development of the cyanobacterial biomass was rather synchronous in all bags in all the three experiments. A high biomass ofDaphnia gave no increase in the pool of dissolved nutrients in spring, a slight increase in summer and a pronounced increase in autumn. While a strong decrease in the P/C-cell quota of the phytoplankton was observed from spring to autumn, no effect of grazing or nutrient release could be related to this P/C-status. The experiments indicate that such systems, with high and stable densities of inedible cyanobacteria, are rather insensitive to short-term (3–4 weeks) biomanipulation efforts. This is supported by observations on the long-term development of the lake.