The effects of PAH contamination and grazing on the abundance and composition of microphytobenthos in salt marsh sediments (Pass Fourchon, LA, USA): II: The use of plant pigments as biomarkers

A field study was conducted in 1996 and 1997 that examined effects of tempo ral and spatial variability of polycyclic aromatic hydrocarbons (PAHs), mac ro-epifaunal abundance, and dissolved inorganic nutrients on the abundance and composition of microphytobenthos in salt marsh sediments in Pass Fourch on, LA (U.S.A.). Plans pigment biomarkers were used to monitor seasonal pat terns in benthic microalgal abundance and composition across a PAH contamin ation gradient created by past produced-wafer pumping. The microphytobenthi c community at Pass Fourchon was primarily composed of diatoms and cyanobac teria, 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 general ly ranged from 0.2 to 15.9 mu g g dry sediment(-1). Significantly higher co ncentrations of dissolved inorganic nitrogen (DIN) were observed in porewat ers at the highly contaminated (high PAHs) station; ammonium had the highes t range (160 to 8500 mu M at Station la). Higher DIN concentrations were li kely due to rapid remineralization of microphytobenthic detritus at the hig hly contaminated station. Microphytobenthic response to enhanced DIN availa bility was likely hindered by PAH toxicity and/or macro- and meiobenthic gr azing effects. In September of each study year, significant seasonal increa ses in total PAH concentration in surface sediments were observed at the co ntaminated sire (Station 1a). Examination of PAH isomer ratios indicates th is seasonal pattern was likely due to physical mixing events (i.e. storms) and not to changes in PAH inputs or preferential remineralization of certai n PAHs. (C) 2000 Academic Press.