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Research Document 2020/037

Optical, Chemical, and Biological Oceanographic Conditions on the Scotian Shelf and in the Eastern Gulf of Maine during 2018

By Casault, B., Johnson, C., Devred, E., 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 mainly warmer than normal surface and near bottom ocean temperatures in 2018, a pattern that started in 2010, and in light of increasing stratification. Overall in 2018, deep nutrient inventories were lower than normal over the entire region of interest, continuing a pattern that started three (for nitrate) to five (for phosphate and silicate) years ago. Anomalies of surface nitrate were negative at the high frequency sampling stations (Halifax-2 and Prince-5) and in the eastern part of the region (Cabot Strait and Louisbourg sections) and positive in the central (Halifax section) and western (Browns Bank section) part. Surface phosphate and silicate anomalies were negative over the entire region. Anomalies of the amplitude and magnitude of the spring phytoplankton bloom, and of the water-column integrated chlorophyll a, a proxy for phytoplankton biomass, were mostly positive or near zero in the eastern Scotian Shelf and Cabot Strait, and negative in the central and western Scotian Shelf. Observations in 2018 provide additional evidence for a persistent plankton community change in recent years with lower abundance of large phytoplankton (diatoms), especially in summer, as observed at Halifax-2. Zooplankton biomass and Calanus finmarchicus abundance also continued to be mainly lower than normal, while non-copepod abundance was mainly 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. Above normalabundances of offshore copepods on the Scotian Shelf, particularly Oithona atlantica as observed at Halifax-2, 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 and secondary production in the ecosystem.

The Bedford Basin surface temperatures were warmer than normal for 8 of 12 months in 2018, with the warmest September of the time series. Conversely, November and December were the coolest since 2003 and 2007, respectively. 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.

The 2017 Continuous Plankton Recorder (CPR) data, as reported here, indicate that the annual averages for the Phytoplankton Colour Index (PCI) were close to normal for the Eastern Scotian Shelf (ESS) and Western Scotian Shelf (WSS), while diatom and dinoflagellate abundances were lower, as seen at Halifax-2. Monthly values indicated a late, intense short spring bloom (WSS), as also observed at Halifax-2 in 2017. Calanus I–IV (mostly C. finmarchicus CI–IV) levels were higher than (ESS) or near (WSS) normal, while C. finmarchicus CV–VI levels were near normal (ESS, WSS). Annual abundances of C. finmarchicus have been low since 2011 at Halifax-2. The decreases are in CVs in summer/fall, with CPR observations suggesting they are in the sub-surface population. Among the other taxa, most 2017 annual mean abundances were near the 1992–2015 averages, except for C. hyperboreus CIII–VI and hyperiid amphipods (higher on ESS) and euphausiids (lower on WSS).

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