Sediment Water Interactions in the Swan River Estuary: Findings and Management Implications from Benthic Nutrient Flux Surveys, 2000 - 2006

Geoscience Australia has conducted four surveys in the Swan River Estuary to investigate benthic nutrient fluxes and their impact on water quality. Surveys were undertaken in March 2000, March 2001, September 2001 and October 2006, and both the upper and lower sections of the estuary were sampled. This report details the findings of the most recent benthic nutrient survey (October 2006) and compares benthic fluxes at selected sites during all four surveys.

During the October 2006 survey, very high nutrient fluxes were recorded in the upper estuary muddy sites. Combined with very low denitrification efficiencies, large sediment nutrient pool sizes and hypoxic bottom waters, these muds are a significant source of bioavailable nutrients to the water column.

Between 2000 and 2006 there has been a significant increase in the amount of organic matter decomposition and nutrient release from the muddy sediments in the upper estuary. A similar pattern is observed in the central basin, however, the change is not as severe. The shallow sandy margins of the lower estuary are sites of photosynthetic production, however, these differ between benthic and pelagic production depending on the light attenuation. When light is available at the sediment surface benthic production is evident, when light penetration is insufficient to reach the sediment surface pelagic production is more evident.