Research Document 2019/059

Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2018

By Blais, M., Galbraith, P.S., Plourde, S., Scarratt, M., Devine, L. and Lehoux, C.

Abstract

An overview of chemical and biological oceanographic conditions in the Gulf of St. Lawrence (GSL) in 2018 is presented as part of the Atlantic Zone Monitoring Program (AZMP). Data from the AZMP regional monitoring program were analyzed and described in relation to long-term means in the context of a strong warming event that began in 2010. During 2018, oxygen at 300 m reached the lowest concentration observed so far in the GSL. The deep oxygen anomalies were particularly strong from Cabot Strait to the northwest GSL, while the anomaly stayed relatively stable in the Estuary compared to 2017. Nitrate inventories in the surface (0– 50 m) and mid-water column (50–150 m) layers were generally below normal everywhere in the GSL during summer and fall, but were near normal in the surface layer during wintertime, suggesting that a large nitrate drawdown occurred between March and June in all regions. As well in 2018, above-normal nitrate inventories were observed in deep waters (150 m–bottom) of the eastern GSL (eGSL), a pattern associated with intrusions of warm and salty waters occurring since 2012. The annual anomalies of vertically integrated chlorophyll a (chl a; 0–100 m) were above normal in all regions mostly because of high chl a concentrations during fall. More specifically, phytoplankton biomass reached record highs during summer in the western GSL (wGSL) and during fall in eGSL. In accordance with the large spring nitrate drawdown, satellite observations show that the spring bloom started earlier, lasted longer, and showed an above-normal magnitude than normal in most regions, the main exception being the northeast GSL. Zooplankton biomass increased in 2018 compared to 2016 and 2017 but remained below normal almost everywhere in the GSL. In most regions, large calanoid abundance was also below normal in 2018, mostly related to the decline in Calanus hyperboreus abundance at Rimouski station and of Calanus finmarchicus in eGSL and southern GSL (sGSL). Small calanoid abundances were generally above normal in wGSL and sGSL and near normal in eGSL, in agreement with the trend observed since 2014. Abundances of warm-water- associated copepods were also above normal in wGSL and sGSL but near normal in eGSL. In the latter region, cold-water-associated copepod abundance was higher than the long-term mean for the fourth consecutive year. Phenology of C. finmarchicus at Rimouski station suggests a near-normal timing of emergence from diapause and development into the adult stage. However, the peak of the early copepodite stages (CI–CIII) was long-lasting and reached its maximum abundance only in July. The infrequent and irregular sampling at Shediac Valley ur ability to describe seasonal patterns associated with nutrients or lower trophic levels at this station.

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