Does historical exposure to hydrocarbon contamination alter the response of benthic communities to diesel contamination?

A microcosm experiment was used to compare the influence of diesel contamin ation on two benthic salt-marsh communities, one chronically exposed to pet roleum hydrocarbons for decades (Louisiana [LA]) and the other relatively u ncontaminated (Mississippi [MS]). Initial meiofaunal community composition of the two sites was similar. Higher organic content of MS sediments should have reduced bioavailability, and thus the toxicity of hydrocarbons relati ve to the LA site. Nevertheless, although responses to diesel contamination at the two sites were generally qualitatively similar, a species-specific and several community-response variables were influenced to a much greater degree in the MS community. In particular, the abundance of total nauplii, ostracods, and copepods were negatively impacted to a greater extent in MS than in LA, as was grazing by ostracods on benthic microalgae. Nematode:cop epod ratios in contaminated sediments were much higher in MS than in LA sed iments. Pseudostenhelia wellsi (a benthic copepod) nauplii suffered greater adverse effects of diesel in MS than in LA. We conclude that the MS commun ity was more sensitive to diesel contamination than was the LA community. T he differential sensitivity is presumably a manifestation of different tole rances to hydrocarbon contaminants, mediated by a higher proportion of more tolerant species and/or increased tolerance among individual species in LA . Although the MS site was more sensitive to diesel contamination, qualitat ive responses of the LA and MS communities were similar, and comparable to previous studies of diesel contamination. The spatial and temporal consiste ncy of diesel impacts on salt-marsh communities suggests that hydrocarbon c ontamination results in predictable community responses. Specifically, crus tacean (e.g. copepods, ostracods, and nauplii) benthos are most sensitive t o hydrocarbons. Reductions in abundance and grazing activity of crustaceans leads to enhanced algal biomass, reduced copepod diversity, and alters com petitive interactions among meiofauna. (C) 2000 Elsevier Science Ltd. All r ights reserved.