Water salinity, oxygen isotope, and nutrient data from southeast Hudson Bay and James Bay

The purpose of this research was to study the influence of altered freshwater discharge on the seasonality of nutrient distributions near La Grande River, northeastern James Bay. In subarctic marine environments, winter is a time when nutrient stocks are replenished through physical and biogeochemical processes, largely setting an upper limit on new primary production for the next growing season. In spring, freshwater-associated nutrient additions from sea ice melt and river inflow modify marine nutrient stocks and influence primary production, especially in coastal areas. In northeastern James Bay (NEJB), Qubec, hydroelectric development of the La Grande River (LGR) has shifted the timing of peak freshwater discharge from spring into winter, producing ten times the natural winter flows. In situ observations of coastal water properties in this area are limited to a handful of studies, none of which simultaneously included nutrients and freshwater tracers to distinguish the influence of river water from sea-ice melt. In this study, we used salinity, oxygen isotope ratio (18O), and nutrient (nitrate, phosphate) data collected from NEJB coastal waters during six campaigns spanning 2016 and 2017 to quantify sea-ice melt and river water content and their influence on nutrient distributions within the LGR plume. Our results show that the LGR is the dominant source of freshwater to coastal waters in both winter and summer and an important source of nitrogen to nitrogen-limited coastal waters. Regulation of the LGR has shifted fluvial nitrate inputs from spring (pre-development) to winter (post-development) producing a mismatch between high surface nitrate stocks available to support primary production now occurring in winter and the growing season, which can begin only after the return of light. In NEJB, the timing and magnitude of primary production, dependent on nutrients in the water column, is expected to have been impacted by altered freshwater input, reducing overall production in local areas and potentially increasing production further downstream with cascading effects on the marine ecosystem.