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 a romatic hydrocarbon (PAHs) contamination on the abundance and composition o f microphytobenthos in surface (0-10 cm) sediments collected from a salt ma rsh in Pass Fourchon, LA. Plant pigment biomarkers were used to monitor cha nges in the microphytobenthos in sediments (over 60 days) with treatments c ontaining high and low concentrations of PAMs. Grazing pressure on microphy tobenthos by the periwinkle snail Littorina irrorata as well as its respons es 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 a bundance of high-molecular-weight PAHs in High-PAM sediments (5.5-16 ppm). Microphytobenthic biomass, estimated by chlorophyll-a concentrations, did n ot significantly change in the treatments with Low-PAM sediments (0.04-0.33 ppm); however, biomass decreased significantly in all treatments after 4 d ays from 64 mu g to 2-8 mu g g dry sed.(-1). Concentrations of the caroteno ids fucoxanthin (a biomarker for diatoms) and zeaxanthin (a biomarker for c yanobacteria) were consistent with chlorophyll-a, showing little change in Low-PAM treatments; sediments from the High-PAM treatments showed significa ntly 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 se d-L and 0.01-0.15 mu g g dry sed(-1): respectively. Increased total phaeoph orbide 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 microph ytobenthos. High-Exposure snails (from High-PAM field sites) gained more we ight during the initial phase of the experiment when microphytobenthic abun dance was decreasing but eventually lost more weight than did Low-Exposure snails (from Low-PAM field sites) by the end of the experiment. Although th ere was considerable variability in the snail growth values, we speculate t hat the High-Exposure snails were more active in feeding during this experi ment and, after the initial decline in microphytobenthos, became resource-l imited throughout the remainder of the experiment. (C) 1999 Elsevier Scienc e B.V. All rights reserved.