Fate of Seasonal, Terrestrial Nutrient Inputs to a Shallow Seagrass Dominated Embayment

A highly seasonal input of water-borne nutrients flows into Geographe Bay from a network of agricultural drainage systems, from July to September (winter) when the drains flow. Near-shore nutrient concentrations increased from 15 to 150 μg l−1for total phosphorus and from 200 to 2000 μg l−1for total nitrogen. However, greater than 100 m offshore, these nutrients were no longer detectable. The yearly input from terrestrial sources (255 tonnes of nitrogen) accounts for two-thirds of the nitrogen required to produce maximum biomass of the seagrassPosidonia sinuosa. Other sources of nutrients are likely to be important in maintaining seagrass biomass and productivity. These sources include seagrass wrack decomposition, sediment nutrient release and groundwater inflow to the embayment. Nitrogen appeared to be the limiting nutrient for seagrass growth in Geographe Bay in summer when nutrients were not in excess.