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Response of a benthic food web to hydrocarbon contamination

Direct and indirect effects of diesel‐contaminated sediment on microalgae, meiofauna, and meiofauna‐microalgae trophic interactions were examined in a microcosm study of the sediment community from a Spartina alterniflora salt marsh. Microcosms of natural sediment were given small daily doses of contaminated sediment over a 28‐d period, creating low‐, medium‐, and high‐diesel treatment concentrations of ∼0.5, 5.5, and 55 ppm PAH, respectively. Diesel caused initial (within 7 h) reductions in microalgal grazing by meiobenthic harpacticoid copepods. Over longer periods of exposure (7–28 d), grazing on microalgae by copepods as a group was reduced in high‐diesel treatments, primarily because of high copepod mortality. In contrast, grazing by and abundance of Cletocamptus deitersi (a copepod) was significantly enhanced in high‐diesel treatments. Concurrent with reduced grazing by copepods, nematode grazing rates increased significantly in high‐diesel treatments, indicating possible competition for microalgae between copepods and nematodes. In spite of transiently enhanced grazing by nematodes and C. deitersi, total meiofaunal grazing on microalgae was reduced in high‐diesel treatments. Increased Chl a : pheopigment ratios in contaminated sediments were also indicative of reduced grazing pressure. A large (10×) increase in microalgal biomass was observed in high‐diesel treatments and was likely a consequence of reduced meiofaunal grazing. The general responses observed in microcosms were also observed in a field study of polycyclic aromatic hydrocarbon contamination. Collectively, our data indicate that benthic microalgal biomass is controlled by meiofaunal grazing and that meiofauna may compete for limited algal resources. Furthermore, consideration of multiple trophic levels and their interactions allows a more complete and ecologically meaningful understanding of the mechanisms by which contaminants induce changes in natural communities.