Nitrogen regeneration by the surf zone penaeid prawn Macropetasma africanus

Nitrogen excretion of individual Macropetasma africanus (Balss) from an exposed beach/surf zone in Algoa Bay, South Africa was monitored under laboratory and field conditions in relation to body mass, temperature and feeding during 1985. Excretion rate experiments were performed on starved prawns at 15°, 18°, 20° and 25°C, as well as on individuals fed on four different diets (mussel, fish, shrimp and natural diet) at 15° and 20°C. The ratios of the excreted compounds to total nitrogen excreted were similar for the four diets despite differences in their nitrogen content and in the amount of food consumed. At 15° and 20°C, ammonia excretion rates of fed individuals were four to seven times higher than in starved prawns. the excretion rates were not correlated with nitrogen content of diets. M. africanus recycles 1 557 g NH₄–N per metre strip per year or 1 832 g total nitrogen m^-1 yr^-1, which constitute 12 and 14%, respectively, of total phytoplankton requirements of the surf zone. This study indicates that large motile crustaceans, when abundant, can play an important role in nutrient recycling in turbulent marine environments.