Nutrient and chlorophyll a variability at six stations associated with mussel farming in Pelorus Sound, 1984–85

Abstract

Abstract The large nutrient and chlorophyll a concentration variability in Pelorus Sound resulted from the complex hydrodynamic regime operating on the three major nutrient sources—sediment remineralisation, advection from Cook Strait, and the river inflows. Concentrations changed over time periods which ranged from short (minutes to hours), associated with the passage of tidal fronts and isolated patches of water, to long (days to months) associated with floodwater, retention times, and oceanic exchange. Spatial variability indicated that high nitrogen (N) concentrations and phytoplankton biomass in the inner sounds were associated with rainfall whereas the highest levels in the outer sounds were associated with sediment remineralisation, oceanic exchange, and the seasonal pattern in Cook Strait. Exceptions occurred when high phytoplankton biomass produced in the inner sounds was flushed out of the sounds following a storm event. Low N concentrations and phytoplankton biomass in summer throughout Pelorous Sound coincided with low rainfall and low concentrations in Cook Strait, and may have resulted in a food shortage for the cultivated mussels within Pelorous Sound at that time. Accumulation of N in the bottom waters of poorly flushed embayments resulted in deep chlorophyll a maxima developing below the level of the mussel farms. Internal waves on the halocline provided vertical displacement of both nutrients and phytoplankton biomass by up to 6 m independent of the tidal cycle, periodically moving this food source within the depth range of the cultivated mussels in those embayments.