Research Document 2023/045
Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2021
By Blais, M., Galbraith, P.S., Plourde, S., Lehoux, C. and Devine, L.
Abstract
An overview of chemical and biological oceanographic conditions in the Gulf of St. Lawrence in 2021 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 warming trend that began in 2010. These long-term means are typically calculated using 1999–2020 as a reference period. Oxygen levels at 300 m reached a record-low concentration in the northwest and central Gulf and in Cabot Strait. Nitrate inventories in the surface layer (0–50 m) of the Gulf were either near or below normal, with record-low levels in the Estuary and northwest Gulf. Mid-layer nitrate inventories (50–150 m) were also below normal in these two subregions, but near or above normal in the other regions. In the bottom layer (150 m–bottom), positive anomalies were found in all Gulf regions with record-high inventories everywhere but in the Estuary. Positive deep nitrate anomalies have been observed regularly since 2012 in the Cabot Strait and the central Gulf and are associated with intrusions of warm and salty waters, but they have been less common 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. Annual inventories of vertically integrated chlorophyll a (chl a; 0–100 m) were near normal in all regions except for the Estuary and northwest Gulf, where they were above normal. In these two subregions, the second highest inventories of chl a were observed during fall. Most regions have regularly shown above-normal chl a inventories during fall since about 2014. In contrast, phytoplankton biomass derived from satellite data mostly showed negative annual and fall anomalies in most of the ocean colour polygons over the last four years, including several record-low values in 2021. Spring bloom metric anomalies indicated that the bloom was generally on time but was slightly less intense than normal. Zooplankton biomass was either near or below normal everywhere in the Gulf, with a record-low level in the Estuary/northwest Gulf region. Calanus finmarchicus annual abundance was near normal except at Rimouski station, where it was slightly above normal. C. hyperboreus abundance showed widespread negative anomalies, including a record-low level in the northeast Gulf. Small calanoids were at record-high abundances at Rimouski station and in the Estuary/northwest Gulf region, and their abundances were also above normal in the Magdalen Shallows, and at the Shediac Valley station. Abundances of warm-water copepods were also at record-high levels in the Estuary/northwest Gulf region as well as in northeast Gulf and Magdalen Shallows, and abundances were also above normal in other regions. Cold-water copepod abundances were either near normal or above normal, and record-high values were observed at both high-frequency monitoring stations. The characterization of C. finmarchicus phenology at Rimouski station indicated that the peak of early copepodite stages (CI–CIII) was the earliest and longest-lasting peak ever recorded.
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