High-Frequency Observations of Phytoplankton Spring Bloom Dynamics in Baffin Bay Using Imaging Flow Cytometry

The FlowCytobot (IFCB) is a high-frequency submersible imaging flow cytometer that allows a detailed characterization of phytoplankton community composition. The IFCB was used to study the under-ice spring bloom dynamics at a fixed station (67˚28.774N, 63˚47.398W) in Baffin Bay from April 22^nd until July 10^th 2015. Seawater and sea-ice samples were collected every second day, at six different depths in water and at the bottom of ice cores. Preliminary analyses show an increase in algae abundance in sea-ice from the end of April to mid-June, reaching ca. 3000 cells mL^-1. As spring sets in, the abundance decreased rapidly to 250 cells mL^-1. Visual inspection of images showed the dominance of pennate diatoms such as Nitzschia frigida, Entomoneis spp. and Navicula spp. in the sea-ice biota from April to mid-June. Concurrently, we observed an abrupt increase in ice related algae abundance in the water column (ca. 25 to ca. 225 cells mL^-1). This suggests a “flushing” of sympagic algae from sea ice. Inspection of images from the seawater samples supports this idea by showing the same community, with a substantial proportion of pennate diatoms debris. Data also shows the onset of a phytoplankton bloom at the beginning of July, with a maximum abundance near surface deepening over time. The data suggest a shift towards a phytoplankton community, largely dominated by Thalassiosira spp. and Chaetoceros spp., with limited occurrences of <10µm flagellates and dinoflagellates. Results match commonly used algal biomass proxies like chl a concentration as shown by a strong correlation with cell abundance from the IFCB. Further comparisons with irradiance, water masses properties, sea-ice cover and algal pigments will improve our understanding of the under-ice spring bloom dynamics. Together with automated classification of images, this new method allows reduced sampling costs, time effective species identification and real-time visualisation of phytoplankton communities composition.