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Research Document 2022/018

Optical, Chemical, and Biological Oceanographic Conditions on the Scotian Shelf and in the eastern Gulf of Maine during 2020

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

Abstract

Ocean physical conditions in the Maritimes Region in 2020 were characterized by generally warm surface and bottom temperatures. The year 2020 was the third warmest of the 51-year time series with 2012 having been the warmest on record. Surface and deep nutrients inventories remained mainly near or below normal on the Scotian Shelf (SS) following the trend of the last 6–7 years. Higher-than-normal deep nitrate concentrations associated with warm salty water were observed in the fall on the eastern side of the Cabot Strait section and at the offshore stations of the Halifax section. The spring phytoplankton bloom on the central and the western SS was later than normal and shorter in duration with either near- or lower-than-normal magnitude. On the eastern SS, bloom conditions developed near the shelf break with normal onset timing and duration. Observations at the Halifax-2 (HL2) station indicated a continued pattern of mainly lower abundance of diatoms and higher abundance of ciliates and flagellates. Zooplankton indices (i.e., Calanus finmarchicus, Pseudocalanus spp., total copepods and non-copepods abundance, and mesozooplankton biomass) were mainly near or lower than average across most of the region but slightly above normal on the Browns Bank section. The abundance of copepod indicator species (i.e., Arctic Calanus, warm-offshore species and warm-shelf species) was generally near or slightly lower than normal, although spatial variability was observed. Observations at HL2 also indicate a continued pattern of generally higher-than-normal abundance of small cyclopoid copepods (e.g., Oithona atlantica).

Surface temperatures in Bedford Basin were, on average, slightly above normal in 2020, consistent with general trends observed across the SS. From January to September, bottom temperatures continued the trend of primarily negative anomalies observed in 2019. In October, both bottom temperature and salinity reverted to above-normal conditions, suggesting the occurrence of an offshore shelf-water intrusion event. Bottom nitrate, silicate, particulate organic carbon and nitrogen all reverted to below-normal conditions in October, coinciding with the timing of the suspected shelf-water intrusion event. Bottom chlorophyll and phytoplankton conditions were all below normal in 2020, marking a continuation of the primarily negative anomalies observed in these parameters since 2010.

The 2019 Continuous Plankton Recorder data indicated lower-than-normal annual abundances of diatoms and dinoflagellates on the Eastern Scotian Shelf (ESS) and Western Scotian Shelf (WSS), while the values of the phytoplankton colour index, a proxy for the phytoplankton biomass, were near (WSS) or above (ESS) normal. The annual abundances of Calanus CI–IV and C. finmarchicus CV–VI were above or near normal on the ESS and below normal on the WSS. Abundances of the other taxa (two arctic Calanus species, three small copepod taxa, two macrozooplankton taxa, and three acid-sensitive taxa) were near or lower than normal on the ESS and WSS, except for hyperiid amphipods, which were more abundant than normal on the ESS.

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