Optical, Chemical, and Biological Oceanographic Conditions on the Newfoundland and Labrador Shelf during 2018

Ocean nutrients and plankton conditions on the Grand Bank and Newfoundland and Labrador shelves in 2018 were assessed and compared to long-term average conditions in the region to highlight relationships between biogeochemical oceanographic conditions and marine primary (phytoplankton) and secondary (zooplankton) production. Overall, optical and chlorophyll a indices indicated above normal phytoplankton biomass in April-May 2018 at the high frequency sampling station (S27). Phytoplankton production indices during the remainder of the year were consistent with normal conditions. Silicate and nitrate profiles in spring indicated uptake to depths nearly two-fold deeper compared to the climatology and implies that a portion of primary production occurred deeper within the euphotic layer. The annual chlorophyll anomalies along the cross-shelf sections also indicated higher phytoplankton production in 2018 which contrasts with lower levels observed back to the early 2010’s. Although broad-scale ocean colour imagery during spring 2018 was consistent with Atlantic Zone Monitoring Program (AZMP) in situ observations, bloom indices at smaller regional scales indicated limited bloom amplitude and magnitude. The general trend of increasing zooplankton abundance observed on the Grand Bank since the start of the monitoring program continued in 2018. The abundance of large, energy-rich Calanus finmarchicus copepods was mainly near normal on the Grand Bank and the NL shelves in 2018 after 4 years of low abundance. The abundance of the small copepod taxa Pseudocalanus spp. and Oithona spp. was back to near normal levels after 3–4 years of high abundances with the exception of the southeastern Grand Bank where abundances remained above normal. The abundance of non-copepod zooplankton continued to be above normal across the region for a 6th consecutive year, reaching either highest or second highest levels of the time series at S27 and on all oceanographic sections. The production cycles of C. finmarchicus and Pseudocalanus spp. at S27 were delayed ~1 month in 2018 with evidence of the production of a second generation. Large (>1 mm) planktonic organisms were mainly responsible for the increased zooplankton biomass in all oceanographic cross-shelf sections. The biomass of small zooplankton (<1 mm) showed negative anomalies across the region, continuing a trend that started more than a decade ago. The relative abundance of near-surface phyto- and zooplankton taxa based on the Continuous Plankton Recorder (CPR) survey indicated enhanced levels of small copepods and acid-sensitive plankton on the northern Grand Bank. Changes in ocean acidification and carbonate chemistry indicate seasonal variations in pH and in saturation horizons of aragonite and calcite. Ocean carbon measurements generally indicate that water masses are saturated in regard to carbonate ions but observations identified under-saturation of aragonite at intermediate and deeper bottom waters at certain stations. The overall pattern of variation among nutrients, phytoplankton biomass and zooplankton abundance highlights the relationship between the biogeochemical environment and the primary and secondary production. There are persistent signs of a shift in copepod community size structure characterized by a decline of large energy-rich calanoid copepods in favor of smaller copepod taxa. More research is needed to understand the underpinnings of these important changes in zooplankton community structure and their potential impact on the transfer of energy to higher trophic levels.

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