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Research Document 2018/017

Optical, Chemical, and Biological Oceanographic Conditions on the Scotian Shelf and in the Eastern Gulf of Maine in 2016

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

Abstract

The condition of nutrients and plankton on the Scotian Shelf and in the Gulf of Maine were assessed in the context of continued warmer-than-normal surface and near bottom ocean temperatures in 2016, a pattern that started in 2008, and continued higher-than-normal stratification in summer and fall. Overall in 2016, deep nutrient inventories were lower than normal. While deep nitrate anomalies have been mixed in sign in recent years, inventories of deep silicate and phosphate have been mainly negative since 2013. Spring phytoplankton bloom initiation was early and bloom duration short in the east, and bloom magnitude was small and duration short in the central and western Scotian Shelf. The early spring bloom initiation timing in 2016 contrasts with the mainly later initiation timing observed on the Scotian Shelf in recent years. Observations in 2016 provide additional evidence for a persistent plankton community change in recent years.  The abundance of large phytoplankton, including diatoms, continued to be lower than normal. Zooplankton biomass and Calanus finmarchicus abundance also continued to be lower than normal, while non-copepod abundance was high. The abundance of arctic Calanus, an indicator of cold water on the Scotian Shelf, continued to be lower than normal. Higher than averageabundances of Oithona atlantica andwarm offshore species suggest greater influence of offshore waters in recent years, especially on the western Scotian Shelf. 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. In 2015, there was sampling in only 7 months between April and November on the Western Scotian Shelf (WSS) and in only 5 months between June and November on the Eastern Scotian Shelf (ESS). Due to the poor temporal coverage, annual average abundances and abundance anomalies could not be calculated for 2015.  Sampling on the WSS in April-May indicated (i) near normal levels for three phytoplankton groups and the dominant zooplankton taxa Calanus I-IV and C. finmarchicus V-VI, (ii) unusually low levels for two Arctic Calanus species and hyperiid amphipods, and (iii) low or near normal levels for seven other taxa. Sampling over the entire Scotian Shelf between June and November indicated (i) near normal levels for the three phytoplankton groups, (ii) lower than (June-September) or close to (October-November) normal abundances for Calanus I-IV, C. finmarchicus V-VI and most other taxa, but higher than normal abundances for coccolithophores and foraminifera on the ESS from September to November.

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