The effect of seasonality in phytoplankton community composition on CO2 uptake on the Scotian Shelf

We characterise seasonal patterns in phytoplankton community composition on the Scotian Shelf, northwest Atlantic Ocean, through a study of the numerical abundance of different cell sizes — pico-, nano- and microphytoplankton. Cell abundances of each size class were converted to cellular carbon and their seasonal patterns compared with the partial pressure of carbon dioxide (pCO₂) also measured at the study site. We observed a persistent drawdown of CO₂ throughout the summer months, despite nutrient depleted conditions and apparent low biomass suggested by the chlorophyll record. This drawdown was associated with a summertime phytoplankton assemblage numerically dominated by small phytoplankton that reach their peak abundance during this period. It was found that phytoplankton carbon during this period accounted for approximately 10% of spring bloom phytoplankton carbon and pointed to the importance role that small cells play in annual CO₂ uptake.