Zhi-Ping Mei ^1* Diane Lavoie ^2* Nicolas Lambert ³ Michel Starr ² Joël Chassé ⁴ Will Perrie ¹ Zhenxia Long ¹

• ¹ Bedford Institute of Oceanography (BIO), Dartmouth, Canada
• ² Maurice Lamontagne Institute, Fisheries and Oceans Canada (DFO), Mont‐Joli, Quebec, Canada
• ³ Canadian Centre for Climate Modelling and Analysis (CCCma), Victoria, British Columbia, Canada
• ⁴ Gulf Fisheries Centre, Fisheries and Oceans Canada (DFO), Moncton, New Brunswick, Canada

The Gulf of St. Lawrence (GSL), along with the St. Lawrence Estuary, is the largest estuarine system in North America. It is a biologically productive sea and an important fishing ground in Canada. The objectives of this study areis to determine how climate changes will affect primary production (PP) in the GSL and on the eastern Scotian Shelf, and to determine the drivers of the changes. We use a regional ocean model forced with downscaled output of the Max-Planck Institute Earth System Model to study the changes in net primary productionPP under the climate change scenario of Representative Concentration Pathway 8.5 (RCP8.5). Results reveal a projected 13.4% decrease in annual primary productionPP across the GSL system over the next 680 years. This decline in primarily stems from reduced nutrient concentrations in the upper layer, despite the increase increased river runoff supplying nutrients supplied by rivers. Enhanced freshwater influx and ocean surface warming contribute to heightened stratification, that in turn reduce the vertical nutrient fluxes from deeper layers. This mechanism affects the upwelling of nutrient-rich nutrient supplywater at the head of the Laurentian Channel in the Lower St. Lawrence EstuaryLSLE, leading to a 22% reduction in the vertical nitrate flux which is reduced by about 22% in the future. Additionally, nutrient concentrations in the water masses entering the GSL at the Strait of Belle Isle and at Cabot Strait are also reduced. Due to declining sea-ice concentration, the phytoplankton bloom is expected to occur between 9 and 23 days earlier under a warmer future climate, and last for up to 24 days longer, depending on the GSL subregions, compared to the historical period than in history years (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020). Biological productivity at higher trophic levels, and thus fisheries productivity, could be negatively impacted under the RCP8.5 climate change scenario.