Water and plant samples were collected from Princess Royal Harbour, Oyster Harbour and King George Sound between December 1987 and February 1989. Mean nutrient and chlorophyll a concentrations in the waters of Oyster Harbour were higher than either Princess Royal Harbour or King George Sound. In contrast, water clarity was lower.
The water quality of Princess Royal Harbour in 1988/89 had improved significantly since the survey conducted in 1978n9, and was similar to the water quality in King George Sound.
A high proportion of phosphorus entering Oyster Harbour is in a dissolved inorganic form. During floods a buoyant, nutrient-rich layer of freshwater flows over the top of the denser marine water of Oyster Harbour and out into King George Sound.
Seagrass leaf biomass reached a seasonal maximum in spring/summer at all sites. Seagrass biomass and shoot density was lower in the two harbours than in King George Sound. Stands of P. sinuosa in Princess Royal Harbour were particularly sparse. Nutrient concentrations in seagrasses, epiphytes and periphyton indicated that Oyster Harbour was more nutrient enriched than Princess Royal Harbour and King George Sound.
Light was found to be the dominant factor affecting seagrass leaf growth in all three waterbodies. Maximum leaf production rates per shoot were highest in spring, but maximum rates per unit area of meadow occurred in summer. Production rates were less consistent in the harbours than in King George Sound indicating a reduced capacity to lay down below-ground storage reserves, creating an increased vulnerability to unfavourable conditions such as prolonged periods of low light levels.
Macroalgal smothering appears to be the major cause of seagrass decline in Princess Royal Harbour. In contrast, epiphytes are implicated as the main cause of seagrass decline in Oyster Harbour, apart from the south-east comer of the harbour where dense accumulations of macroalgae occur. This difference may be due to the better water clarity in Princess Royal Harbour favouring the proliferation of macroalgae, while the higher nutrient loading and relatively poor light conditions in Oyster Harbour may favour the growth of epiphytes
