Vertical mixing effects on the phytoplankton bloom in the southeastern Bering Sea midshelf

Abstract

A vertically one‐dimensional ecosystem model was developed and applied to the southeastern Bering Sea middle shelf. The physical model includes a 2.5‐level turbulence model. Tidal forcing was introduced to improve representation of tidal mixing in addition to wind stirring and thermal stratification. The simulated currents, thermocline and mixed‐layer depth (MLD) agree well with observations. The biological model was adapted from Eslinger et al. (2001) with nine compartments. The onset, magnitude and duration of the spring phytoplankton blooms were realistically reproduced at 12 m, 24 m, and 44 m in the standard run. The spring phytoplankton bloom was dominated by diatoms, and the post blooms by flagellates, which agree with previous studies in the region. The peak phytoplankton biomass reached 8 mmol N m−3, or approximately 16 mg Chl m−3, comparable to that observed in the PROBES program in 1980 and 1981 (Eslinger and Iverson, 2001). The simulated primary productions were within the range of 60 to 200 g C m−2/yr estimated in previous studies. Sensitivity studies revealed distinct effects of tidal mixing, wind stirring, thermal stratification and their impacts on the timing and magnitude of the phytoplankton bloom and the gross and net primary production. Links of MLD with primary production and species were found. If a constant MLD is used in the model, there exists a maximum gross primary production (GPP) at MLD = 24 m. Model results reveals that the predominant phytoplankton species changes from flagellates when MLD < 15 m to diatoms when MLD > 15 m.