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Research Document 2021/002

Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2019

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

Abstract

An overview of chemical and biological oceanographic conditions in the Gulf of St. Lawrence (GSL) in 2019 is presented as part of the Atlantic Zone Monitoring Program (AZMP). Data from the AZMP regional monitoring program were analyzed and results were described in relation to long-term means (climatology) in the context of a strong warming event that began in 2010. During 2019, oxygen at 300 m depth reached the lowest concentration observed to date in the Estuary, while the second or third lowest concentrations of the time series were observed in other subregions. The nitrate inventories in the surface (0–50 m) layer were near or slightly above normal during wintertime and generally below normal during summer and fall, suggesting that a large nitrate drawdown occurred between March and June, particularly in the southern (sGSL) and eastern (eGSL) GSL. In 2019, the deep water (300 m) nitrate inventory was near normal in eGSL, a first since 2012 when intrusions of warm and salty waters started to be observed in association with above-normal nitrate inventories. However, deep phosphate and silicate inventories remained above normal in 2019 in the GSL. Despite an overall large spring nitrate drawdown, satellite imagery indicates that the spring bloom had below-normal magnitude and amplitude across the GSL. The annual anomalies of vertically integrated chlorophyll a (chl a; 0–100 m) were above normal in the western GSL (wGSL) and eGSL mostly because of high chl a concentrations during fall. Taxonomic analyses showed a large proportion of diatoms at Rimouski station in 2019, while most of the phytoplankton abundance consisted in small-sized cells at Shediac Valley station. Zooplankton biomass in 2019 was above normal at Rimouski station and in wGSL for the first time since 2013-2014, but remained below normal elsewhere in the GSL. The abundances of large calanoid were either near or slightly above normal in all subregions in 2019. It was mostly due to near-normal abundances of Calanus finmarchicus everywhere, high abundances of Calanus hyperboreus at Rimouski station and in wGSL (spring only), and to record high abundance of Calanus glacialis in eGSL. Small calanoid and warm-water-associated copepod abundances were generally near or above normal , in agreement with the trend observed in recent years. The abundances of cold-water-associated copepods were above normal in eGSL due to C. glacialis, and at a record high at Shediac Valley station due to both C. glacialis and Metridia longa. The phenology of C. finmarchicus at Rimouski station suggests a near-normal or slightly early timing of emergence from diapause and development into the adult stage. The peak of the early copepodite stages (CI–CIII) started early in May but was long-lasting and reached its maximum abundance in late June. The infrequent and irregular sampling at Shediac Valley limited our ability to describe seasonal patterns associated with nutrients or lower trophic levels at this station.

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