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

Abstract

Abstract. In coastal regions, rivers and streams may be important\nsources of nutrients limiting to primary production in marine waters;\nhowever, sampling is still rarely conducted across the land-to-ocean aquatic\ncontinuum, precluding conclusions from being drawn about connectivity\nbetween freshwater and marine systems. Here we use a more-than-4-year\ndataset (2014–2018) of nutrients (nitrogen, phosphorus, silica, iron) and\ndissolved organic carbon spanning streams draining coastal watersheds and\nnearshore marine surface waters along the Central Coast of British Columbia,\nCanada, at the heart of the North Pacific coastal temperate rainforest\nregion. Mean freshwater and surface marine N : Si : P ratios were 5 : 20 : 1 ( P:Fe = 1:67 ) and 6 : 11 : 1 , respectively, showing relative consistency across the\nland–ocean interface but deviation from the extended Redfield ratio.\nInorganic nutrient concentrations ( NO 3 - + NO 2 - ,\n PO 4 3 - , Si(OH )4 ) in fresh waters were less than in the\nreceiving marine environment, indicating that freshwater nutrient inputs in\nthis region were of little importance to – or even diluted – the pool of\nreadily available inorganic nutrients in nearshore waters. Conversely,\nfresh waters increased the pool of organic-matter-associated nutrients,\nnamely dissolved organic nitrogen and iron. The organic-matter-rich\nlandscapes of the region yielded globally significant quantities of\ndissolved organic nitrogen (304–381\u2009kg\u2009km −2 \u2009yr −1 ) and iron\n(463–596\u2009kg\u2009km −2 \u2009yr −1 ), thus acting as important sources of\npotentially limiting nutrients to both nearshore and offshore waters. These\nexports may subsidise heterotrophic microbial communities capable of\ndirectly consuming and remineralising these nutrients, potentially\ncompensating for the dilution of inorganic nutrients by freshwater inputs.\nWe highlight the need to better understand nutrient limitation in coastal\nwaters and for concerted research efforts to study the spatial and temporal\ndynamism at the land–ocean interface along the northeast Pacific coast.