49 Use of biomarkers and core incubations to investigate how changes in benthic ecosystems influence sediment n fluxes in western australia

Abstract

The west coast of Australia is dominated by warm, oligotrophic waters, shallow carbonate‐rich sediments and little terrigeneous nutrient input. Irradiance and light penetration are generally high so that benthic primary production is substantial and can equal that in the water column. Microphytobenthos has been shown to have a major influence on nutrient cycling in northern hemisphere waters, but little work has been published on this subject describing the Western Australian marine environment. This presentation focuses upon anthropogenic impacts on microphytobenthos and sediment‐water column nitrogen cycling. The effects of artificial enclosures (harbours) on microphytobenthic communities and nitrogen cycling were examined over an annual cycle at 2 locations in Cockburn Sound, Western Australia. Core and bulk sediment samples were taken from bare sandy areas in water 6–7 m deep at paired sites inside and outside sea walls. Ex‐situ core incubations were used to determine rates of N2 fixation, denitrification, fluxes of macronutrients and oxygen. Bulk sediment samples were analysed for pigments, fatty acids, sterols and grain size. Significant differences were found in both flux and biomarker data between the 2 areas, inside and outside the harbours and between summer and winter. Biomarker data gave information about the algal, bacterial and faunal composition of the sediments and how it changed across the same temporal and spatial scales as the fluxes. The combination of process studies and biomarker information promises to be a powerful tool for resolving a range of questions on the magnitude and mechanisms of nitrogen inputs and exports from these shallow ecosystems.