A mechanism for onset of diatom blooms in a fjord with persistent salinity stratification

We demonstrate that episodic high freshwater influx from the Manapouri hydroelectric power station and storms promote diatom blooms (15 mg chlorophyll m−3) during spring, summer and autumn in Doubtful Sound, New Zealand, a fjord with semi-permanent salinity stratification. Diatom blooms form when mixing of the surface low salinity layer (LSL – high in silicic acid and low in nitrate and phosphate) and the underlying saline layer (SL – low in silicic acid but relatively high in nitrate and phosphate) reduce nutrient limitations in the thin photic zone of the fjord. We used time series of salinity, macronutrient concentrations, irradiance, and chlorophyll a (February 1997 to January 1999) from stations along the main axis of the fjord to investigate this proposed mechanism. A multiple regression analysis reveals that increases in wind velocity and silicic acid and decrease in phosphate explained 27% of the variance in phytoplankton stocks, indicating that natural and anthropogenic increases in freshwater and silicic acid input and mixing of the LSL and SL play an important role in supporting a predominant diatom community in this system.