Annual plankton dynamics in a coupled physical–biological model of the Strait of Georgia, British Columbia

• A coupled circulation-plankton model was developed for the Strait of Georgia.
• The model simulates a realistic estuarine circulation and plankton dynamics.
• Most of the nitrate entering the upper layer is exported out by horizontal advection.
• The model suggests that zooplankton grazing causes the decline of the spring bloom.
• Physical processes maintain the high spatio-temporal variability of plankton.

A three-dimensional coupled biophysical model was developed to study the dynamics of the plankton ecosystem in the Strait of Georgia (SoG) estuary. The ocean circulation component is an implementation of the Regional Ocean Modeling System (ROMS) and the lower trophic level ecosystem is a nine-compartment Nutrient–Phytoplankton–Zooplankton–Detritus (NPZD) model that includes two types of phytoplankton and of zooplankton. A three year hindcast (2007–2009) is used to examine the mean annual seasonal cycle of the local plankton dynamics. For realistic values of irradiance, wind forcing and fresh water fluxes, the model predicts a seasonal cycle of salinity, nutrients and plankton in reasonable agreement with observations. In particular, the model reproduces the main features of the estuarine circulation, the marked increase in phytoplankton biomass during spring followed by intermittent less intense blooms during summer and fall, as well as the seasonal pattern of zooplankton biomass. Model results show that primary production fluctuates between low values in January and high values in April, with an annual production of 270 ± 33 (gC m−2) in the SoG. Most primary production is fueled by nitrate (f-ratio of about 0.7). Upwelling is the main source of nitrate into the upper layer, while most of the nitrate entering the Strait is exported out by horizontal advection. Physical processes, such as freshwater inflow, wind events, tidal mixing, and horizontal transports are important in maintaining the high spatio-temporal variability of the local phytoplankton biomass and production. Horizontal variability is high during the growing season (March to September). In summer, phytoplankton concentrates near the surface, in the shallow mixed layer, resulting in sporadic nutrient limitation of phytoplankton growth. In the northern portion of the strait, weaker summer winds and tidal currents lead to increased near-surface stratification which restricts nutrient fluxes and limit phytoplankton growth. In the central region, nitrate and light limitation are equally important at limiting phytoplankton growth during the summer whereas, in the tidally mixed regions of the south, primary production is higher in summer than during the spring resulting in higher annual primary production in this region than elsewhere.