Riverine Influence on Coastal Biogeochemistry Along a 400-km Latitudinal Gradient in Eastern James Bay

Rivers integrate climate signals, landscape gradients, and environmental disturbances at the watershed scale, strongly influencing downstream ecosystems and ultimately coastal waters. Watershed environmental gradients therefore exert a strong local influence on coastal river plumes, yet it is unclear how regional-scale gradients involving multiple watersheds are coupled to broad patterns of coastal marine biogeochemistry and productivity. Here, we aimed to establish connections between the physicochemical properties of rivers draining into the highly riverine-influenced James Bay (JB) and of the properties of coastal waters along its entire eastern shore. We clustered 17 river outlets and over 140 coastal sites along a 400-km latitudinal gradient of the eastern JB, sampled during two consecutive summers, according to trends in nutrients, suspended particulate matter, colored dissolved organic matter, freshwater discharge, and salinity. Our findings reveal notable latitudinal changes in the physicochemical properties of both rivers and coastal waters along JB, which were spatially coherent. Whereas river discharge exerts a significant influence—higher discharge amplifies the riverine impact on coastal waters—this alone does not account for the observed variability along the coast. The riverine influence differs among areas and depends on variables considered. In this study, we identified biogeochemical transition zones and assessed the impact of river exports on coastal waters along JB, and this integrative approach could be applied to disentangle river-coast interactions in other regions.