First Measurements of Spring Primary Production in central Hudson Bay

Lisa Matthes1, Laura A Dalman1, Jens K Ehn1, Jean-Eric Tremblay2, Janghan Lee3, Simon Belanger4, Ilka Peeken5 and C. J. Mundy1, (1)University of Manitoba, Centre for Earth Observation Science, Winnipeg, MB, Canada, (2)Laval University, Biology, Quebec City, QC, Canada, (3)Laval University, Quebec City, QC, Canada, (4)Université du Québec à Rimouski, Biology, Chemistry and Geography, Rimouski, QC, Canada, (5)Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung, Polar Biological Oceanography, Bremerhaven, Germany
The central Hudson Bay is considered a region of relatively low annual primary production due to the strong surface stratification and a seasonal sea ice cover lasting for up to nine months, and was corroborated by so far predominately post-bloom autumnal observations in this region. However, remote sensing of surface chlorophyll a concentration suggest a strong pulse of phytoplankton production in early summer while the ice cover rapidly ablates. So far, no studies have directly observed primary production for either ice algae or phytoplankton in central Hudson Bay during this time period. A six-week scientific expedition on board the CCGS Amundsen in June-July 2018 as part of the BaySys project, was used to fill this knowledge gap by studying algal biomass, species composition, and primary production in the pack-ice and the water column accompanied by simultaneous light and nutrient measurements from the bottom sea ice and along vertical profiles. Sampling was carried out along transects across the bay and thus contrasting ice-covered to ice-free conditions.

Results show an unexpectedly large contribution of sub-ice algal communities to spring primary production in north-eastern Hudson Bay in contrast to low ice algal production at the bottom of thick (> 2m) sediment-laden ice in southern Hudson Bay. Under-ice phytoplankton production was consistently low throughout the bay. However, a large phytoplankton bloom with a pronounced subsurface chlorophyll maximum (SCM) was observed in the north-western polynya, which opened up in May, highlighting the importance of polynyas in this region. The location of the SCM in the water column, which was shallower nearshore and at the ice edge compared to the centre of the ice-free region, overlapped with an observed nutrient depletion in the surface water layer, suggesting that a surface bloom had occurred prior to the observation. Here, we present a summary of the biological results during the BaySys project, including the first estimate of primary production and its partitioning between sea ice and phytoplankton, for the central Hudson Bay during the previously unexplored early summer period.