Nutrient inputs from subarctic rivers into Hudson Bay

Abstract

Hudson Bay (HB), a large subarctic inland sea, is impacted by rapid climate change and anthropogenic disturbance. HB plays crucial roles in supporting resident and migratory species of birds and marine mammals, providing subsistence to coastal communities, and exporting nutrients into the western Labrador Sea. To better constrain the impact of river nutrients on the HB ecosystem and to obtain a contemporary reference point by which future change can be evaluated, we estimated fluxes of nitrate plus nitrite (N), phosphate (P), and silicate using contemporary and historical nutrient data in conjunction with discharge estimates produced by three global climate models. Concentrations and molar ratios of the different nutrients exhibited large contrasts between different sectors of HB, which is attributed to the diversity of geological settings across distinct watersheds. With respect to the needs of primary producers, river waters were characterized by a shortage of P during winter and spring (N:P molar ratios in dissolved nutrients >16), nearly balanced N:P ratios in summer, and a shortage of N during fall (N:P < 16). Southwestern rivers made the largest regional contribution to the total annual delivery of all nutrients, followed by modest contributions from southern and eastern rivers, and minor ones from northwestern rivers. While the regulation of river flow in the Nelson and La Grande rivers had no discernible impact on nutrient concentrations and ratios, it clearly shifted nutrient transports toward the winter when biological activity in the estuaries is reduced. Finally, the potential amount of new production supported by riverine N inputs was nearly two orders of magnitude (1.8 × 1011 g C yr−1) lower than the new production supported by marine nutrients (7.3 × 1012 g C yr−1). Although the potential contribution of river nutrients to new primary production is small (2.4%) at the bay-wide scale, it can be significant locally.

Understanding the Physical Forcings Behind the Biogeochemical Productivity of the Hudson Bay Complex

“…Despite being barely non‐significant, correlation coefficients >0.5 suggest that the spring freshet in the system could partly be responsible for the lack of phytoplankton growth and accumulation in the coastal areas until July, potentially because of its large load of dissolved and particulate matter that hampers light penetration (McSweeney et al., 2017; Senneville et al., 2018) and the offshore transport of river freshwater (St‐Laurent et al., 2011). In late summer, nutrients brought by the rivers' runoff in the nutrient‐depleted surface layer may sustain higher chlorophyll a concentrations along the southern shores (Lee et al., 2023).…”