In subarctic marine environments, nutrient stocks are replenished through physical and biogeochemical processes in winter, largely setting an upper limit on new primary production for the next growing season. In spring, marine nutrient stocks are modified by freshwater-associated additions, especially in coastal areas. Hydroelectric development of the La Grande River (LGR), in northern Québec, has shifted the timing of peak freshwater discharge from spring into winter, producing ten times the natural winter discharge. In this study, we used salinity, oxygen isotope ratio (δ18O), and nutrient (nitrate, phosphate) data collected from NEJB coastal waters in different seasons of 2016 and 2017. Here we quantify two main freshwater sources, established by freshwater tracers (LGR and sea-ice melt), and their influence on nutrient distributions along the coast. Our results show that LGR is the dominant source of freshwater to coastal waters throughout the year, especially during winter and serves as an important source of nitrate to nitrogen-limited coastal waters (winter concentrations of 4.53 μM and 3.18 μM respectively). Despite being a poor phosphate source, LGR (0.11 μM, compared to ambient seawater 0.66 μM), provides the largest portion of the phosphate stock in surface waters near the mouth of LGR. LGR regulation has changed the pattern of natural fluvial nitrate inputs, what was observed in spring (pre-development) is now observed in winter (post-development). Thus, high winter surface nitrate stocks (22.5 mmol m-2) are available to support primary production, but dispersed to offshore areas, prior to the onset of the growing season, which begins 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.