Impact of sediment type, light and nutrient availability on benthic diatom communities of a large estuarine bay: Moreton Bay, Australia

Shifts in diatom species composition may be used to infer past changes in environmental conditions in fresh, estuarine and marine systems. Establishing the primary drivers of present day diatom community composition is a vital step in their use as a proxy for past conditions. Moreton Bay, Australia, has experienced extensive modification of its catchments and western and southern shorelines. Regional weather patterns and terrestrial runoff have created a gradient in water quality from relatively degraded western and southern areas to relatively pristine northern and eastern areas. The aim of this study was to examine the relative impact of short term changes to light and/or nutrient availability and long term changes in sediment type, light and nutrient availability to subtidal benthic community composition. Short term changes were imposed using a manipulative field experiment whilst long term changes were obtained from a field survey of sites across the gradient of water quality. Diatoms were found to be the dominant microalgal group at all studied sites. The diatom communities were comprised primarily of small benthic epipsammic species and community composition was primarily driven by changes in sediment silt content. Short term changes in light and/or nutrient availability had little impact on community composition. In this open estuarine system the use of diatom indices to infer past water quality must take into account the sediment silt content in their interpretations