Influence of Grazing and Nitrogen on Benthic Algal Blooms in Diesel Fuel-Contaminated Saltmarsh Sediments

Abstract

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 µm), microalgal biomass more than doubled after 5 days; while biomass in control microcosms did not change. NH 4 + efflux in diesel fuel-contaminated sediment was significantly higher than in uncontaminated sediment after 14 days. Microalgae in uncontaminated sediments were not nitrogen limited (NH 4 + additions did not stimulate growth). In diesel fuel-contaminated sediments, however, microalgae were nitrogen limited, and all ambient NH 4 + was consumed. We conclude that, in diesel fuel-contaminated sediments, grazer mortality leads to increased growth of microalgae and ultimately to nitrogen limitation; longerterm microalgal growth is supported by the enhanced flux of NH 4 + that occurs in contaminated sediments. The enhanced NH 4 + 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.

Impact of a simulated oil spill on benthic phototrophs and nitrogen‐fixing bacteria in mudflat mesocosms

“…Hydrobia ulvae is equally plastic in its feeding habits (Riera, ), with the microphytobenthos forming a major component of its diet (Fenchel et al ., ). Removal, either manually or by addition of diesel oil, of other grazers not investigated here, such as harpacticoid copepods, has also been shown to result in an increase in diatom abundance (Carman et al ., ). Thus, the oil‐induced increase in 16S rRNA sequences from diatom chloroplasts (Fig.…”

“…However, as argued in the previous section, the dominance of cyanobacteria at day 21 is most likely a consequence of their ability to fix dinitrogen. It is notable that Carman and colleagues () found an increase in diatom abundance, whereas cyanobacterial abundance remained low and constant, after addition of diesel oil to mudflat mesocosms. However, unlike in our study, the mesocosms of Carman and colleagues () did not become nitrogen‐limited, suggesting that nitrogen limitation in our mesocosms was the key factor driving the cyanobacterial bloom.…”

“…It is notable that Carman and colleagues () found an increase in diatom abundance, whereas cyanobacterial abundance remained low and constant, after addition of diesel oil to mudflat mesocosms. However, unlike in our study, the mesocosms of Carman and colleagues () did not become nitrogen‐limited, suggesting that nitrogen limitation in our mesocosms was the key factor driving the cyanobacterial bloom. The enhanced diatom growth, coupled with activity of hydrocarbon‐degrading bacteria, would further increase nitrogen depletion.…”

EFfects of Diesel and Interactions with Copper and Other Metals in an Estuarine Sediment Microbial Community

“…Also, the response of a microbial community can range from no effect to an increase in biomass caused by use of diesel as a carbon source [37] to a decrease in biomass caused by the toxicity of diesel [38], depending on the amount and timing of the diesel addition. Furthermore, the oxidation of diesel can exhaust oxygen, driving the system anaerobic and shifting the community composition [37], or nitrogen and phosphorous can limit the increase in microbial biomass because of diesel addition [2,38,39]. Hence, PLFA analysis can be used as a sensitive indicator of petroleum contamination, but the nature of the shift in the PLFA will depend on the experimental details.…”

“…A series of previous microcosm studies (see [2][3][4][5][6][7] and references therein) have examined the effects of diesel and metals on estuarine benthic invertebrates. In the present work, these studies are extended to effects on the estuarine sediment microbial community as determined by polar lipid fatty acid (PLFA) analysis [8].…”