Structural and grazing responses of zooplankton community to biomanipulation of some Dutch water bodies

Abstract

Structure and grazing activities of crustacean zooplankton were compared in five lakes undergoing manipulation with several unmanipulated eutrophic (shallow) and mesotrophic (deep) lakes in The Netherlands. The biomanipulated lakes had lesser number of species and their abundance, both of rotifers and crustaceans, and had much larger mean animal size (3–11 μg C ind.⁻¹) than in the unmanipulated eutrophic lakes (0.65 μg C ind.⁻¹). Whereas D. hyalina ( = D. galeata) and D. cucullata generally co-occurred in the unmanipulated lakes, in the manipulated lakes both D. hyalina and other large-bodied daphnids, D. magna, D. pulex ( = D. pulicaria), were the important grazers. In the biomanipulated lakes an increase in the individual crustacean size and of zooplankton mass were reflected in a decrease in seston concentration, higher Secchi-disc depth and a marked decrease in the share in phytoplankton biovolume of cyanobacteria. Biomass relationship between seston (150 μm) and zooplankton indicated a Monod type relationship, with an initial part of the curve in which the zooplankton responds linearly to the seston increase up to about ca. 2 mg C l⁻¹, followed by a saturation of zooplankton mass (0.39 mg C l⁻¹) at 3–4 mg C l⁻¹ seston, and an inhibitory effect on zooplankton mass at seston levels > 4 mg C l⁻¹. This latter is related to predominance in the seston of cyanobacteria.

In the biomanipulated lakes, the zooplankton grazing rates often exceeded 100% d ⁻¹, during the spring, and food levels generally dropped to < 0.5 mg C l⁻¹. The computed specific clearance rate (SCR) of zooplankton of 1.9 1 mg⁻¹ Zoop C is well within the range of SCR values (1.7–2.2 1 mg⁻¹ Zoop C) from deep and mesotrophic waters, but about an order of magnitude higher than in the eutrophic lakes, with the food levels 10-fold higher. For 25% d ⁻¹ clearance of lake seston between 35 and 60 ind. 1⁻¹ are needed in the biomanipulated lakes against 1200–1300 ind. 1⁻¹ in eutrophic lakes. Similarly, about 10 to 15 times more crustacean grazers are required to eliminate the daily primary production in the eutrophic lakes than in the biomanipulated lakes. These numbers are inversely related to the differences in animal size. The corresponding biomass values of zooplankton needed to clear the daily primary production in the eutrophic waters were 0.1–0.2 mg C l⁻¹ in the biomanipulated lakes, but about 0.45 mg C l⁻¹ in the unmanipulated eutrophic waters.

Only if the water was kept persistently clear by zooplankton was there a balanced seston budget between the input via primary production and elimination by zooplankton. Mostly, however, the input exceeded the assimilatory removal by zooplankton, such that the estimated seston loss could be attributed to sedimentation and mineralization.