The effects of polycyclic aromatic hydrocarbon contamination and grazing on the abundance and composition of microphytobenthos in salt marsh sediments (Pass Fourchon, LA) - I. A microcosm experiment

A microcosm experiment was conducted to examine the effects of polycyclic aromatic hydrocarbon (PAHs) contamination on the abundance and composition of microphytobenthos in surface (0-10 cm) sediments collected from a salt marsh in Pass Fourchon, LA. Plant pigment biomarkers were used to monitor changes in the microphytobenthos in sediments (over 60 days) with treatments containing high and low concentrations of PAMs. Grazing pressure on microphytobenthos by the periwinkle snail Littorina irrorata as well as its responses to PAH contamination were also examined. Concentrations of total PAHs declined in sediments over the duration of the experiment in all microcosms. There was a pattern of increasing relative abundance of high-molecular-weight PAHs in High-PAM sediments (5.5-16 ppm). Microphytobenthic biomass, estimated by chlorophyll-a concentrations, did not significantly change in the treatments with Low-PAM sediments (0.04-0.33 ppm); however, biomass decreased significantly in all treatments after 4 days from 64 mu g to 2-8 mu g g dry sed.(-1). Concentrations of the carotenoids fucoxanthin (a biomarker for diatoms) and zeaxanthin (a biomarker for cyanobacteria) were consistent with chlorophyll-a, showing little change in Low-PAM treatments; sediments from the High-PAM treatments showed significantly higher concentrations of fucoxanthin and zeaxanthin (1.8 and 1.2 mu g g dry sed(-1), respectively) than the Low-PAM treatments during the initial phase of the microcosm experiment. However, concentrations of fucoxanthin and zeaxanthin decreased dramatically after 4 days to 0.1-0.3 mu g g dry sed-L and 0.01-0.15 mu g g dry sed(-1): respectively. Increased total phaeophorbide concentrations (a biomarker for metazoan grazing) over the first 12 days of the experiment in treatments with and without L. irrorata additions indicated that meiobenthos may have been important in grazing-down microphytobenthos. High-Exposure snails (from High-PAM field sites) gained more weight during the initial phase of the experiment when microphytobenthic abundance was decreasing but eventually lost more weight than did Low-Exposure snails (from Low-PAM field sites) by the end of the experiment. Although there was considerable variability in the snail growth values, we speculate that the High-Exposure snails were more active in feeding during this experiment and, after the initial decline in microphytobenthos, became resource-limited throughout the remainder of the experiment. (C) 1999 Elsevier Science B.V. All rights reserved.