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 Sept ember (winter) when the drains flow. Near-shore nutrient concentration s increased from 15 to 150 mu g l(-1) for total phosphorus and from 20 0 to 2000 mu g l(-1) for total nitrogen. However, greater than 100 m o ffshore, these nutrients were no longer detectable. The yearly input f rom terrestrial sources (255 tomes of nitrogen) accounts for two-third s of the nitrogen required to produce maximum biomass of the seagrass Posidonia sinuosa. Other sources of nutrients are likely to be importa nt in maintaining seagrass biomass and productivity. These sources inc lude seagrass wrack decomposition, sediment nutrient release and groun dwater inflow to the embayment. Nitrogen appeared to be the limiting n utrient for seagrass growth in Geographe Bay in summer when nutrients were not in excess. (C) 1998 Academic Press Limited.