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Research Document 2020/002

Optical, Chemical, and Biological Oceanographic Conditions on the Scotian Shelf and in the Eastern Gulf of Maine During 2017

By Johnson, C., Devred, E., Casault, B., Head, E., Cogswell, A., and Spry, J.

Abstract

Ocean nutrient and plankton conditions on the Scotian Shelf and in the eastern Gulf of Maine were assessed in the context of continued warmer than normal surface and near bottom ocean temperatures in 2017, a pattern that started in 2008, and continued higher than normal stratification in summer and fall. Overall in 2017, deep nutrient inventories were lower than normal over the entire region of interest, which is the first time this pattern has been observed since the time series began in 1999. Anomalies of surface nitrate and silicate were positive on the Eastern Scotian Shelf (ESS) and negative or near normal elsewhere, while surface phosphate anomalies were negative in the entire region. The amplitude and magnitude of the phytoplankton bloom were below normal for the second year in a row. Observations in 2017 provide additional evidence for a persistent plankton community change in recent years. Anomalies of water-column integrated chlorophyll-a, a proxy for phytoplankton biomass, were negative, as in 2016. The abundance of large phytoplankton, including diatoms, continued to be lower than normal, especially in summer. Zooplankton biomass and Calanus finmarchicus abundance also continued to be lower than normal, while non-copepod abundance was high. The abundance of arctic Calanus, a cold water zooplankton indicator, continued to be lower than normal on the Scotian Shelf, a trend that started in 2013. Higher than averageabundances of Oithona atlantica andwarm offshore copepods suggest a greater influence of offshore waters in recent years. Changes in phytoplankton and zooplankton communities observed in recent years suggest changes in prey fields for planktivorous fish, birds, and mammals and could be associated with changes in the fate of primary production in the ecosystem.

Continuous Plankton Recorder (CPR) data become available one year later than data collected by the Atlantic Zone Monitoring Program (AZMP). In 2016, annual averages for two CPR phytoplankton indices (PCI – phytoplankton colour index – and dinoflagellate abundance) were near the 1992–2015 average values on both the Eastern and Western Scotian Shelf (WSS), while those of a third (diatom abundance) were below average in both areas. Monthly diatom abundances and PCI values indicated an early, short spring bloom on the ESS and a low intensity, short bloom on the WSS, consistent with 2016 satellite observations. The Calanus copepodite I–IV (CI–IV) and C. finmarchicus CV–VI annual average abundances were near the 1992–2015 average values in both regions. In situ sampling at Halifax-2 has shown low levels of C. finmarchicus since 2011 (compared with 1999–2010), with the decreases occurring mainly in the levels of CVs in summer. The CPR observations suggest that this decrease is in the sub-surface portion of the CV population. Among the other taxa, C. glacialis CV-VI (on the ESS) and copepod nauplii and Oithona spp. (both on the WSS) were unusually low in abundance, while hyperiid amphipods and three acid-sensitive taxa (coccolithophores, foraminifera, pteropods) were unusually abundant on the ESS.

Bedford Basin Compass station surface conditions in the fall of 2017 (Oct–Dec) were the warmest on record for the time series. The Compass station phosphate to nitrate ratio continued to match a new regime that has emerged since 2011, likely in response to declining soluble phosphate inputs associated with sewage treatment advancements and Federal laws controlling acceptable phosphate concentrations in detergents.

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