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 saltmarsh sediments suggest that microalgal blooms are a response to reduced grazing pressure by crustaceans. Nevertheless, this and alternative hypotheses for microalgal blooms in contaminated sediments have not been explicitly examined. Here, we used microcosms of saltmarsh sediment to examine influences of diesel fuel on benthic microalgae as they relate to (i) direct effects associated with reduced grazing and (ii) indirect effects associated with enhanced nitrogen availability. In both diesel fuel-contaminated sediment and in sediment where grazing was experimentally reduced (by microwaving the sediment fraction >125 mu m), microalgal biomass more than doubled after 5 days; while biomass in control microcosms did not change. NH4+ efflux in diesel fuel-contaminated sediment was significantly higher than in uncontaminated sediment after 14 days. Microalgae in uncontaminated sediments were not nitrogen limited (NH4+ additions did not stimulate growth). In diesel fuel-contaminated sediments, however, microalgae were nitrogen limited, and all ambient NH4+ was consumed. We conclude that, in diesel fuel-contaminated sediments, grazer mortality leads to increased growth of microalgae and ultimately to nitrogen limitation; longer-term microalgal growth is supported by the enhanced flux of NH4+ that occurs in contaminated sediments. The enhanced NH4+ flux is likely a consequence of an altered microbial community and could have long-term biogeochemical consequences for the ecological health of contaminated coastal communities.