Research Document 2017/034

Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2015

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

Abstract

An overview of chemical and biological oceanographic conditions in the Gulf of St. Lawrence (GSL) in 2015 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. In 2015, sea-surface temperatures were near normal from May to November while deepwater temperatures reached an all-time high. Nitrate inventories in 2015 were near normal in the 0–50 m layer but strongly above normal in the deeper layer, the latter in association with high temperatures and salinities. Compared to the time series (1999–2010), the spring phytoplankton bloom began earlier, lasted longer, and had a greater magnitude in the NWGSL and NEGSL while the reverse was observed for the Magdalen Shallows and Cabot Strait. The shift away from a smaller-sized phytoplankton community at Rimouski station continued in 2015, with a second consecutive year of positive anomalies in the diatom/flagellate ratios, while increased relative abundances of the smaller-sized phytoplankton—dinoflagellates and ciliates—were observed at Shediac Valley. Zooplankton biomass was below normal throughout the GSL, with strong declines in the indices for Calanus finmarchicus, large calanoids, and cold-water copepods and strong increases for Pseudocalanus spp., small calanoids, warm-water copepods, and non-copepods. The overall high Gulf-wide temperatures (surface and deep layers) and salinities observed in 2015 likely led to the well-above-normal abundances of warm-water copepod species as well as the presence of specimens from the family Aetideidae, which appeared in the top 95% of the most abundant copepod taxa for the first time in 2014.

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