Mechanistic Links Between Climatic Forcing and Model-Based Plankton Dynamics in the Strait of Georgia, Canada

Large scale climate indices such as the North Pacific Gyre Oscillation (NPGO) have been shown to influence the physiology, ecology, and phenology of phytoplankton and zooplankton, yet the mechanisms by which they are linked are not well-defined. We used a three-dimensional coupled biophysical model, SalishSeaCast, to determine the mechanistic links between the NPGO and plankton dynamics in the Central Strait of Georgia, Canada. First, we compared bottom-up processes during NPGO positive (cold-phase) and negative (warm-phase) years. Then, we conducted a series of model experiments to determine the effects of the NPGO on local physical drivers by switching individual parameters between a typical warm and cold year. The model showed that thermal forcing had the strongest influence on spring bloom timing resulting in an earlier increase in spring diatom biomass during warm-phase years. Due to the conditions set up during the spring, warm-phase years exhibited lower overall summer diatom biomass and an earlier shift to nanoflagellate-dominance compared to cold-phase years. Our systematic model experiments revealed that variability in wind-driven resupply of nutrients to the surface waters during the summer had the most significant impact on diatom biomass, and ultimately on the food available to zooplankton. The zooplankton model classes grazed on a higher proportion of nanoflagellates during the summer of warm-phase years, suggesting a poorer quality diet. Results from this study are relevant in the context of other climate signals (e.g., El Niño) favoring weaker winds or increased stratification, which limit the amount of nutrients being replenished to the surface waters.