The Physical Context of Seasonal and Inter-Annual Variability in Phytoplankton across the Scotian Shelf: Insights from Profiling Gliders

Understanding how phytoplankton respond to their physical environment is key to predicting how bloom dynamics might change under future climate change scenarios. Phytoplankton are at the base of most marine food webs and play an important role in drawing CO₂ out of the atmosphere. We use 5 years of simultaneous CTD, irradiance, fluorescence by chlorophyll a and optical backscattering observations obtained from Slocum glider missions across the Scotian Shelf, near Halifax, Nova Scotia, Canada, to examine the seasonal and inter-annual variability in proxies of phytoplankton abundance and type, and their physical context. Consistent with previous studies, we find that the main spring bloom occurs before the onset of the seasonal stratified layer. In fact, the largest chlorophyll observations consistently occurred while there was very little stratification in the upper 60 meters. This first bloom type, which was diatom dominated, was followed by a phytoplankton assemblage dominated by pico- and nano-phytoplankton and dinoflagellates associated with the nutrient poor, more stable summertime conditions. We examine the conditions throughout the entire growing season and compare the conditions during the initiation of the spring diatom bloom between years.