The effects of convective and wind-driven mixing on spring phytoplankton dynamics in the Southeastern Bering Sea middle shelf domain

Spring phytoplankton bloom conditions for the southeastern Bering Sea shelf were simulated with a coupled phytoplankton-nutrient-detritus model that received input from a physical mixed-layer model. The models captured the essential features of chlorophyll, dissolved inorganic nitrogen concentration, and temperature fields during the spring bloom onset and progression in 1980 and 1981. In contrast to critical depth theory, the occurrence of a shallow mixed-layer depth and a period of low wind speed were not sufficient to trigger the spring bloom. In both years, the spring bloom onset occurred in response to the cessation of convective mixing during a period of increasing atmospheric temperature and decreasing wind speed. Differences between 1980 and 1981 post-spring-bloom characteristics, however, resulted from differences in water column stability, and wind speed variability and magnitude through time. Those factors affected the vertical distributions of nitrogen and chlorophyll, and, therefore, phytoplankton growth rate. A high degree of model accuracy was indicated by low average RMSE values for euphotic zone model variable values compared to data. This was a consequence of the dominant role that meteorological forcing had on variable fields and processes during spring 1980 and 1981, and the application of a physical model that was specifically designed to model vertical mixing processes.