Direct and indirect effects of an antifouling biocide on benthic microalgae and meiofauna

Abstract

When multitrophic, natural communities are exposed to a contaminant, indirect, food-web mediated effects may become as important as direct toxic effects. In this study the antifouling agent copper pyrithione (CPT) was used as a model toxicant to study toxic effects on microphytobenthos (MPB) and meiofauna, as well as on the interaction between them. The hypothesis was that exposure to CPT will decrease meiofaunal grazing pressure and that this effect would result in increased primary production and biomass of MPB. Two laboratory experiments (16 and 29days long) were performed, using natural sediment that was exposed to both acute and repeated exposure of CPT (final dose 5 nmol CPT per g dry weight sediment). Variables measured included light utilization efficiency of MPB (PAM fluorometry), primary production ( 14C-uptake), chl a content of the sediment, algal composition, meiofauna biomass and meiofaunal grazing pressure on MPB ( 14C-incubations). Although CPT, added as a single dose or repeatedly, affected both MPB and meiofauna, no strong mechanistic evidence for a top-down control was found. The clearest direct CPT effects were seen for MPB function (light utilization efficiency and primary production), while algal biomass and composition were only marginally affected. MPB recovered rapidly from the toxicant exposure, a finding that verified the conclusion from previous experiments that the MPB community is highly resilient. The magnitude of the direct response of the microalgae depended on whether the total dose of CPT was added as one initial dose or divided between several additions; effects were larger after one initial dose. Although meiofauna biomass decreased at CPT exposure in both experiments, this decline was not reflected in decreased grazing pressure on MPB. Meiofauna grazed between 1 and 12% of the algal biomass per day. The lack of a clear top-down effect from CPT on MPB could have several both biological and methodological reasons, such as low sensitivity of the meiofaunal species to CPT, and a combination of simultaneously operating bottom-up and top-down effects from CPT.