Benthic recruitment of zooplankton in an acidic lake

Abstract

In recent years, most studies of the benthic microbial food web have either been descriptive or were restricted to the measurement of within sediment process rates. Little is known about benthic–pelagic coupling processes such as recruitment. We, therefore, developed an ex situ core incubation procedure to quantify the potential for microbial recruitment from the benthos to the pelagic in an acidic mining lake, Mining Lake 111 (ML 111; pH 2.6), in eastern Germany. Our data suggest that considerable zooplankton recruitment from the benthos takes place. Heliozoan and rhizopod recruitment in both summer and winter sediment cores was highest when they were incubated at 20 °C. Maximum heliozoan recruitment was 23 (±9 S.E.) individuals cm −2 day −1 (40% initial standing stock daily) in the winter 20 °C incubation. Maximum rhizopod recruitment was 6 (±2 S.E.) individuals cm −2 day −1 in the summer 20 °C incubation. Little or no recruitment was apparent for either taxa when winter cores were incubated at 5 °C, implying a temperature cue. Conversely, the rotifer, Cephalodella hoodi, exhibited a maximum recruitment of 6 (±2 S.E.) individuals cm −2 day −1 during the winter 5 °C incubation, representing 30% of initial standing stock daily, but little recruitment when incubated at 20 °C. Cephalodella may have responded to an increased winter benthic food supply; in situ winter Chl a concentrations in the benthos were 3.4 times higher than those in the summer. The importance of this was reinforced by the poor pelagic food supply available in ML 111. In situ, Heliozoa, rhizopods and Cephalodella were first observed in the epilimnion of ML 111 in spring or early summer, suggesting active or passive recruitment following lateral transport from littoral sediments. Benthic–pelagic coupling via recruitment is potentially important in understanding the pelagic food web in ML 111 and warrants further investigation in this and other aquatic environments.