The Effects of PAH Contamination and Grazing on the Abundance and Composition of Microphytobenthos in Salt Marsh Sediments (Pass Fourchon, LA, U.S.A.): II: The Use of Plant Pigments as Biomarkers

Abstract

A field study was conducted in 1996 and 1997 that examined effects of temporal and spatial variability of polycyclic aromatic hydrocarbons (PAHs), macro-epifaunal abundance, and dissolved inorganic nutrients on the abundance and composition of microphytobenthos in salt marsh sediments in Pass Fourchon, LA (U.S.A.). Plant pigment biomarkers were used to monitor seasonal patterns in benthic microalgal abundance and composition across a PAH contamination gradient created by past produced-water pumping. The microphytobenthic community at Pass Fourchon was primarily composed of diatoms and cyanobacteria, as indicated by the carotenoid biomarkers fucoxanthin and zeaxanthin, respectively. Microphytobenthic abundance showed no consistent pattern of variation across stations or seasons; chlorophyll a concentrations generally ranged from 0·2 to 15·9μgg dry sediment−1. Significantly higher concentrations of dissolved inorganic nitrogen (DIN) were observed in porewaters at the highly contaminated (high PAHs) station; ammonium had the highest range (160 to 8500μM at Station 1a). Higher DIN concentrations were likely due to rapid remineralization of microphytobenthic detritus at the highly contaminated station. Microphytobenthic response to enhanced DIN availability was likely hindered by PAH toxicity and/or macro- and meiobenthic grazing effects. In September of each study year, significant seasonal increases in total PAH concentration in surface sediments were observed at the contaminated site (Station 1a). Examination of PAH isomer ratios indicates this seasonal pattern was likely due to physical mixing events (i.e. storms) and not to changes in PAH inputs or preferential remineralization of certain PAHs.