Seasonal patterns of grazing and nutrient limitation of phytoplankton in a eutrophic lake

Abstract

Observations and experiments on zooplankton grazing and nutrient (N and P) limitation of phytoplankton of Lake Mendota, Wisconsin, revealed that the relative strength of the two factors varied markedly during the seasonal succession of phytoplankton. Furthermore, seasonal changes in grazing and nutrient limitation were caused not only by seasonal changes in zooplankton community structure and nutrient availability but also by changes in phytoplankton community structure, which led to changes in edibility to zooplankton. The spring phytoplankton bloom of diatoms and small flagellates was highly vulnerable to grazing by both cyclopoid copepods and Duphnia, although Daphnia had a much stronger effect. Increased Duphnia grazing in late spring resulted in low phytoplankton biomass (the clear-water period) and a phytoplankton community dominated by colonial green algae-taxa vulnerable to grazing by Daphnia but not other zooplankton. Nutrients (N and P) did not limit phytoplankton growth during the spring bloom or the clear-water period. After the clear-water period, the summer phytoplankton community was dominated by blue-greens and Ceratium. Grazing effects by both Daphnia and copepods were low in summer, while nutrient limitation (both N and P) became severe. Reduced grazing impacts in summer may have resulted from prior intense grazing impacts, which led to dominance of grazing-resistant taxa. These results suggest that seasonal variation in the strength of grazing and nutrient controls in eutrophic lakes results from changes in zooplankton biomass and community structure, nutrient availability, and phytoplankton community structure interacting to determine phytoplankton seasonal succession.Freshwater phytoplankton communities often undergo pronounced seasonal succession (Reynolds 1984). The successional pattern in a lake is fairly repeatable among years, and patterns among lakes are somewhat predictable according to trophic status (e.g. Reynolds 1984;Sommer et al. 1986). Understanding the mechanisms causing seasonal succession and its consequences for other trophic levels, however, is still limited ' Present address:

Pattern recognition of grazing dynamics in response to fish removal (Lake Wolderwijd, Netherlands) using non-supervised artificial neural networks

“…In spring during the clear-water periods, grazing overwhelms nutrients but in summer, nutrient may limit the phytoplankton communities, similar to the results of a study on Lake Mendota (Vanni and Tempte [18]). According to Reynolds [19], phytoplankton is grazed by zooplankton.…”

Fluxes and transformations of aquatic pigments in Lake Mendota, Wisconsin

“…5) suggests that these groups were not grazed extensively. In situ zooplankton grazing experiments confirm the greater grazing of spring than summer phytoplankton (Vanni and Temte 1990).…”

Seasonal phytoplankton nutrient limitation patterns as revealed by bioassays over Baltic Sea gradients of salinity and eutrophication

“…Seasonality in nutrient limitation of the planktonic community has been previously observed in several environments, mainly in lakes (Pollingher et al 1988, Vanni & Temte 1990, Elser et al 1990, Sterner 1994 but also in a variety of coastal environments (D'Elia et al 1986, Paasche & Erga 1988, Fisher et al 1999). Our results demonstrate that the seasonal patterns can be highly reproducible, and they can vary distinctly even between closely related environments (as in the northern basins Bay of Bothnia and Bothnian Sea; Figs.…”