Research Document 2022/021
Optical, Chemical, and Biological Oceanographic Conditions in the Labrador Sea between 2014 and 2018
By Ringuette, M. Devred, E., Azetsu-Scott, K., Head, E., Punshon, S., Casault, B. and Clay, S.
Abstract
The chemistry and biology of the Labrador Sea and adjacent shelves have undergone significant changes over the 2014–2018 period compared to previous years. The Atlantic Zonal Off-shelf Monitoring Program (AZOMP) revealed an increase in dissolved inorganic carbon and a decrease in pH, a trend that extends back to the beginning of the monitoring program in the mid-1990s. While the mean concentration of chlorofluorocarbon (CFC-12) over the water column has remained stable, the concentration of sulfur hexafluoride (SF6) has been increasing steadily since we began to measure it in 2011, reflecting the atmospheric history of these gasses. The mean temperature of the top layer (0–100 m) has been mainly below normal since 2011, except for the Labrador Shelf where two warmer-than-normal years were observed in 2015 and 2018. The lower-than-normal temperatures were associated with higher-than-normal nutrient concentrations in the same layer. The timing of the cruise, which tend to become earlier than usual because of constrained crew change dates, can partly explain these findings and point out the need for consistent dates in sea-going expeditions to remove uncertainties related to sampling time. Deep nutrients exhibited inter-annual variations up until 2014, and mostly remained above average today. Since 2014, both phytoplankton (as indicated by chlorophyll-a concentration) and mesozooplankton abundances have remained below the 1999–2010 average in all three regions of interest: the Labrador Shelf, the Labrador Central Basin, and the Greenland Shelf. Plankton monitoring by means of the Continuous Plankton Recorder (CPR) has been carried out in the northwest Atlantic since the early 1960s. In 2017 (the most recent data available), three indicators of phytoplankton biomass showed high values in the region adjacent to the Newfoundland Shelf (40°W to 45°W) compared with previous years/decades. This was also the case for the abundances of six out of seven mesozooplankton taxa. Further east, one of the phytoplankton color index and six of the seven mesozooplankton taxa showed positive anomalies in 2017. The abundance of one macrozooplankton taxon (Hyperiid amphipods) has increased throughout the northwest Atlantic over all sampling decades/years, while that of a second taxon (Euphausiids) has decreased. The abundances of three acid-sensitive taxa in 2017 were all higher than the 1990–2009 average throughout the sub-polar gyre and on the Newfoundland Shelf.
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