What Determines Seasonal and Interannual Variability of Phytoplankton and Zooplankton in Strongly Estuarine Systems? Application to the semi-enclosed estuary of Strait of Georgia and Juan de Fuca Strait

A coupled biological–physical box model is developed to investigate the seasonal and interannual variability of marine plankton in strongly estuarine systems such as the semi-enclosed estuary of the Strait of Georgia and Juan de Fuca Strait on the west coast of Canada. The estuarine circulation not only supplies nutrients to the euphotic layer but also transports plankton between the straits, causing an asymmetrical distribution of plankton biomass in the estuary. A specific set of biological parameters can be chosen so that the model predicts a large spring bloom and nutrient limitation in the Strait of Georgia but high nutrient levels and no spring bloom in the Juan de Fuca Strait, in agreement with observations. However, as the plankton growth and mortality rate parameters are varied over a realistic range, the plankton also exhibit two other ecosystem behaviours: one with a large spring bloom in the Juan de Fuca Strait and one with a low zooplankton stock in the Strait of Georgia. To determine possible causes for observed interannual variability of the planktonic ecosystem, we have run the coupled biophysical model with stochastic variation of the Fraser River runoff and the shelf salinity. The plankton populations are found to be insensitive to the interannual variability in the estuarine circulation. It is suggested that marine phytoplankton and zooplankton might respond more significantly to climate variability (or change) through changes in their biological rate parameters. 2000 Academic Press