Distribution and biomass of seagrasses and algae, and nutrient pools in water, sediments and plants in Princess RoyalHarbour and Oyster Harbour

Sediment nutrient concentrations were generally highest in the deepest regions of both harbours due to sediment focusing. The sediments in the two harbours were the largest nutrient pool. Macroalgae were the largest plant nutrient pool, accounting for over 80% of plant biomass in Princess Royal Harbour and over 50% in Oyster Harbour. Macroalgae contained about 90% of the nitrogen and 70% of the phosphorus pool associated with aquatic plants in Princess Royal Harbour, and about 60% of nitrogen and 34% of the aquatic plant phosphorus pool in Oyster Harbour. The high mean areal macroalgal biomass (406 g m•2) in Princess Royal Harbour was indicative of a highly eutrophic system. This was also supported by the characteristics of biomass, cover and carbonate content of algal epiphytes on seagrasses. The results of this study indicated that Princess Royal Harbour was more eutrophic than Oyster Harbour. Dense macroalgal beds were found in the south-eastern comer of both harbours. Prevailing wind directions are such that these areas are the most sheltered and therefore are suitable for macroalgal growth and accumulation. Circulation patterns are also compatible with the accumulation of drift algae in this area of the harbours (Mills, 1987). The rate of loss of seagrass area in the two harbours since 1984 was lower than that estimated between 1981 and 1984 by Bastyan (1986). Considerable losses of biomass are, however, continuing to occur, particularly in Princess Royal Harbour where 60% of above-ground seagrass biomass present in 1984 was lost by 1988. At present, major losses are occurring in the shallow dense seagrass beds, presumably due to macroalgal smothering. Present epiphyte loads were not considered to be high enough to cause significant losses of seagrass. An increase in light supply is probably responsible for the seagrass regeneration recorded in some deeper areas of Oyster Harbour. This trend is expected to reverse if water quality deteriorates again. Macroalgal biomass in both harbours must be reduced to halt the loss of seagrass beds in shallow waters. The role of algal epiphytes in the decline of seagrass in both harbours may be more significant than implied from the results of this study due to the large interannual variation in nutrient loading to the harbours.