Research Document - 2015/071

Chemical and Biological Oceanographic Conditions in the Estuary and Gulf of St. Lawrence during 2014

By L. Devine, S. Plourde, M. Starr, J.-F. St-Pierre, L. St-Amand, P. Joly and P. S. Galbraith

Abstract

An overview of chemical and biological oceanographic conditions in the Gulf of St. Lawrence (GSL) in 2014 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 that was somewhat attenuated in 2014. Phytoplankton and zooplankton abundance indices and nutrient inventories were relatively coherent through the time series (1999–2014) between the high-frequency monitoring sites and among sections and subregions. Late winter nutrient inventories were near or above normal in 2014, continuing the overall positive tendency that began in 2013. The spring bloom began later than had been observed in recent years, which coincided with the colder overall conditions in fall 2013/winter 2013–2014 (delayed ice retreat). In addition, the spring bloom magnitude was below normal and of shorter duration across the region. The differences between winter (maximum) and late spring (minimum) nitrate inventories were below normal in many regions of the GSL, confirming that primary production was lower than normal during spring 2014. In fall, chlorophyll a levels were nevertheless above normal in many regions of the GSL. For a third consecutive year, highly positive deep-water (> 200 m) nitrates were associated with high temperature and salinity. Conditions in the GSL were different compared to the St. Lawrence Estuary, where chlorophyll a was above normal during spring, summer, and fall. A striking increase in the relative abundance of diatoms was also seen at Rimouski station but not at Shediac Valley. The strong spring freshet affected the zooplankton community (lower abundances of Calanus finmarchicus and modified phenology), and overall higher temperatures and salinities likely resulted in increased abundances of warm-water copepod species. In contrast, water temperatures in the southern Gulf were near normal and phytoplankton abundance was high, both of which appeared to influence the zooplankton community in a coherent way. Our results and independent evidence from ecosystem surveys indicate that modifications to the abundance of large zooplankton might be due to a combination of environmental conditions (bottom-up processes) and an increase in predation pressure (top-down processes).

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