Spring phytoplankton bloom and associated lower trophic level food web dynamics on Georges Bank: 1-D and 2-D model studies

A coupled biological–physical model was developed and tested in one-dimensional (1-D, vertical) and two-dimensional (2-D, cross-sectional) domains to examine the spring phytoplankton bloom and associated lower trophic level food web dynamics on Georges Bank (GB). The biological model consists of nine compartments: dissolved inorganic nutrients (nitrate, ammonium and silicate), phytoplankton (large and small size classes), zooplankton (large and small size classes), and detrital organic nitrogen and biogenic silica. The 1-D model results showed that in the shallow central bank, the timing and duration of spring blooms are closely linked to the light intensity and its downward penetration, while the intensity of blooms is regulated by initial nutrient concentrations and zooplankton grazing pressure. In the deeper flank area, the bloom dynamics is directly controlled by the seasonal development of stratification. The interactions between the shallow and deep regions of the bank were examined by a 2-D model, which showed that the cross-sectional gradients of biological quantities were caused mainly by the shallow-deep topographic transition and tidal mixing. Between the shallow and deep regions, a possible phytoplankton maximum concentration area was seen in the model at the time before the formation of the tidal-mixing front. Once the tidal-mixing front was established during late spring, the model showed a relatively high concentration of phytoplankton near the front as the result of the tidally driven up-front nutrient flux. Both the 1-D and 2-D models captured the basic seasonal cycles of the nutrients and phytoplankton in the central bank, but failed to reproduce those patterns in the deep flank regions, where horizontal advection might play a significant role.