Rain-fed streams dilute inorganic nutrients but subsidise organic matter-associated nutrients in coastal waters of the northeast Pacific Ocean

Abstract. In coastal regions, rivers and streams may be important sources of nutrients limiting to primary production in marine waters; however, sampling is still only rarely conducted across the land-to-ocean aquatic continuum, precluding conclusions from being drawn about connectivity between freshwater and marine systems. Here we use a more than four year dataset (2014–2018) of nutrients (nitrogen, phosphorus, silica, iron) and dissolved organic carbon spanning streams draining coastal watersheds and nearshore marine surface waters along the Central Coast of British Columbia Canada, at the heart of the North Pacific coastal temperate rainforest region. Mean freshwater and surface marine N : Si : P ratios were 4 : 18 : 1 (P : Fe = 1 : 58) and 6 : 11 : 1, respectively, showing relative consistency across the land-ocean interface but deviation from the extended Redfield ratio. Inorganic nutrient concentrations (NO₃⁻+NO₂⁻, PO₄³⁻, Si(OH)₄) in freshwaters were less than in the receiving marine environment, indicating that freshwater nutrient inputs in this region were of little importance to – or even diluted – the pool of readily available inorganic nutrients in nearshore waters. Conversely, freshwaters increased the pool of organic matter-associated nutrients, namely dissolved organic nitrogen and iron. The organic matter-rich landscapes of the region yielded globally significant quantities of dissolved organic nitrogen (304–381 kg km⁻² y⁻¹) and iron (463–596 kg km⁻² y⁻¹); thus acting as important sources of potentially limiting nutrients to both nearshore and offshore waters. These exports may subsidise heterotrophic microbial communities capable of directly consuming and remineralising these nutrients, potentially compensating for the dilution of inorganic nutrients by freshwater inputs. We highlight the need to better understand nutrient limitation in coastal waters and for concerted research efforts to study the spatial and temporal dynamism at the land-ocean interface along the northeast Pacific coast.