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Research Document 2021/060

Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2020

By Blais, M., Galbraith, P.S., Plourde, S., Devred, E., Clay, S., Lehoux, C. and Devine, L

Abstract

An overview of chemical and biological oceanographic conditions in the Gulf of St. Lawrence in 2020 is presented as part of the Atlantic Zone Monitoring Program (AZMP). AZMP data as well as data from regional monitoring programs are analyzed and presented in relation to long-term means in the context of a strong warming event that began in 2010. These long-term means are now calculated on data from 1999–2020 (1999–2015 in earlier reports). Oxygen levels at 300 m reached a record low concentration in the Estuary and at Rimouski station. Nitrate inventories in the surface layer (0–50 m) of the Gulf were either near or slightly above normal. Nitrate mid-layer inventories (50–150 m) were above normal in the northern and central Gulf, while they were close to normal in Magdalen Shallows and Cabot Strait. In the bottom layer (150 m–bottom), positive nitrate anomalies were found in all Gulf regions. Positive deep nitrate anomalies have been observed regularly since 2012 in Cabot Strait and the central Gulf in association with intrusions of warm and salty waters, but they have been rare in the Estuary over the last decade. The recent increase in the nitrate inventory of the bottom layer is mostly associated with negative anomalies of the N:P ratio and positive anomalies of the Si:N ratio. There were strong positive anomalies of vertically integrated chlorophyll a (chl a; 0–100 m) during late summer in the northeast Gulf, during fall in the Estuary, and, to a lesser extent, in the central Gulf during both seasons. Elsewhere, vertically integrated phytoplankton biomass was close to normal except in Cabot Strait, where it was below normal. Most regions have shown either near-normal or above-normal chl a inventories during fall since about 2014. In contrast, phytoplankton biomass derived from satellite data showed negative annual and fall anomalies in most of the ocean colour averaging areas over the last three years. Spring bloom metrics were mostly near normal, except for high bloom amplitudes and magnitudes in the Magdalen Shallows and Cabot Strait boxes. Zooplankton biomass was close to normal almost everywhere in the Gulf; it was only below normal in the central Gulf/Cabot Strait region. Calanus finmarchicus, C. hyperboreus, and large calanoid abundances were also generally close to normal, except that C. hyperboreus abundance was above normal in the northwest Gulf and below normal on the Magdalen Shallows. Small calanoid abundances were near normal everywhere despite record low abundance of Pseudocalanus spp. on the Magdalen Shallows and low abundances in the northwest Gulf and at Rimouski station. Abundances of warm-water-associated copepods were slightly above normal in most regions and at a record high on the Magdalen Shallows. The lack of sampling during spring precluded characterization of Calanus finmarchicus phenology at Rimouski station this year.

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