Influence of grazing and nitrogen on benthic algal blooms in diesel fuel-contaminated saltmarsh sediments

Previous observations of the co-occurrence of high mortality among benthic crustaceans and blooms of benthic microalgae in diesel fuel-contaminated sa ltmarsh sediments suggest that microalgal blooms are a response to reduced grazing pressure by crustaceans. Nevertheless, this and alternative hypothe ses for microalgal blooms in contaminated sediments have not been explicitl y examined. Here, we used microcosms of saltmarsh sediment to examine influ ences of diesel fuel on benthic microalgae as they relate to (i) direct eff ects associated with reduced grazing and (ii) indirect effects associated w ith enhanced nitrogen availability. In both diesel fuel-contaminated sedime nt and in sediment where grazing was experimentally reduced (by microwaving the sediment fraction >125 mu m), microalgal biomass more than doubled aft er 5 days; while biomass in control microcosms did not change. NH4+ efflux in diesel fuel-contaminated sediment was significantly higher than in uncon taminated sediment after 14 days. Microalgae in uncontaminated sediments we re not nitrogen limited (NH4+ additions did not stimulate growth). In diese l fuel-contaminated sediments, however, microalgae were nitrogen limited, a nd all ambient NH4+ was consumed. We conclude that, in diesel fuel-contamin ated sediments, grazer mortality leads to increased growth of microalgae an d ultimately to nitrogen limitation; longer-term microalgal growth is suppo rted by the enhanced flux of NH4+ that occurs in contaminated sediments. Th e enhanced NH4+ flux is likely a consequence of an altered microbial commun ity and could have long-term biogeochemical consequences for the ecological health of contaminated coastal communities.