Research Document 2019/009

Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2017

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 2017 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. Oxygen levels at 300 m reached their lowest concentration measured so far in several GSL regions during 2017. Negative anomalies in deep O₂ concentration were especially strong in central GSL and the Cabot Strait region. Nitrate inventories in the surface layer (0–50 m) were generally near normal everywhere in the GSL all year round. However, they were above normal in deep waters of eastern GSL (eGSL), which has been observed since 2012 and is associated with intrusions of warm and salty waters. Vertically integrated chlorophyll a (chl a; 0–100 m) was below normal in western GSL (wGSL) and above normal in southern GSL (sGSL) during summer and fall. In sGSL, the positive chl a anomaly measured in situ was actually the strongest recorded since 2002. However, according to satellite imagery, the spring bloom amplitude was generally below normal everywhere in the GSL, including sGSL. Satellite estimates show a near-normal surface chl a annual mean throughout the GSL despite low spring biomass. Dinoflagellate abundance reached a record low at Rimouski station, while it was above normal at Shediac Valley as were flagellates and ciliates at this station. In 2017, zooplankton biomass was below average everywhere in the GSL, but the main zooplankton functional groups and species abundances were all above normal in eGSL with the exception of Calanus hyperboreus. In wGSL and sGSL, C. finmarchicus, Pseudocalanus spp., and total copepod abundances showed negative anomalies. It is the first time since 2012–2013 that negative anomalies have been recorded for Pseudocalanus spp. Moreover, the widespread positive anomalies of small calanoid abundance that have been observed since 2008 were restricted to Rimouski station and eGSL in 2017. Higher-than-normal abundances of copepod species associated with warm water were again observed in 2017, continuing a trend observed since 2011. Development of C. finmarchicus was delayed at Rimouski station in 2017, unlike observations in recent years. Some of these phytoplankton and zooplankton dynamics could reflect the influence of a St. Lawrence freshwater runoff well above the normal in May and June 2017 and of year-round warmer-than-normal deep waters.

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