Relative importance of grazing on algae by plant-associated and open-water microcrustacea (Cladocera)

Abstract

Little Mere is a small and shallow fertile lake, in which floating and submerged macrophytes are abundant. Phytoplankton biomass and water transparency are often regulated by planktonic herbivores, mainly Daphnia hyalina. On occasion during the growing season, however, low chlorophyll-a coincides with few or no Daphnia, when inorganic nutrients are not likely to be limiting. Grazing rates (% lake volume filtered per day) of both Daphnia and plant-associated grazers (Sida crystallina and Simocephalus vetulus) were estimated during two growing seasons. Densities [ind. (g DW plant)' or ind. L -1 ] were determined at five sites in the lake, three of them within lily beds (Nymphaea alba and Nuphar lutea), the other two over submerged plant beds of mixed composition. Total populations were estimated by extrapolation, using values of areal plant biomass. Clearance rates (mL ind. -1 d -1 ) from in situ dual-isotope experiments were used, in conjunction with estimated animal population sizes, to calculate total grazing by the main filterers in the lake (% lakevolume d -1 ). A comparison between Daphnia hyalina and the main plant-associated filterers (Simocephalus vetulus and Sida crystallina) showed that the former was the main phytoplankton grazer during most of the growing season, with grazing rates often above 50% lake volume d -1 . Grazing rates were higher in lily beds than in open waters. Sida crystallina was a significant filter-feeder both at the beginning and end of the growing season, however, and was sometimes the main grazer in the lake, with a grazing rate occasionally exceeding 100 % d -1 . These impacts were short-lived, although the duration of peaks was much larger in 1999 than in 1998, and localized in space. Very low chlorophyll-a concentrations (<10 μg L -1 ) and high water transparency were found during these episodes, when nutrients (N and P) were not limiting (234 μg L -1 NH 4 + -N and 93 μg L -1 SRP). Simocephalus vetulus had a maximum grazing rate of 8-10% d -1 in both years. Although Daphnia maintain clear water during most of the growing season, plant-associated microcrustaceans such as Sida crystallina and Simocephalus vetulus were directly responsible for clearing the water during the intense filtration episodes. Plant-associated crustacean grazers can be an important link helping sustain plant beds in the face of eutrophication and therefore perpetuate the plant-dominated state in shallow lakes.