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

St. Pierre, Kyra A.; Hunt, Brian P. V.; Tank, Suzanne E.; Giesbrecht, Ian; Korver, Maartje C.; Floyd, William C.; Oliver, Allison A.; Lertzman, Kenneth P.

In coastal regions, rivers and streams may be important sources of nutrients limiting to primary production in marine waters; however, sampling is still 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-4-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 inline-formula M1inlinescrollmathml chem normal N : chem normal Si : normal P 43pt10ptsvg-formulamathimg67eccfb2859e42b6cfca5eac8d6e480e bg-18-3029-2021-ie00001.svg43pt10ptbg-18-3029-2021-ie00001.png ratios were inline-formula M2inlinescrollmathml normal 5 : normal 20 : normal 1 42pt10ptsvg-formulamathimg746e3598c4c2c0a4ebd59c05d9bed67b bg-18-3029-2021-ie00002.svg42pt10ptbg-18-3029-2021-ie00002.png (inline-formulaP:Feinline-formula=inline-formula1:67) and inline-formula M6inlinescrollmathml normal 6 : normal 11 : normal 1 42pt10ptsvg-formulamathimgc584a5ab53653f55a83b182a51e2e20e bg-18-3029-2021-ie00003.svg42pt10ptbg-18-3029-2021-ie00003.png , respectively, showing relative consistency across the land–ocean interface but deviation from the extended Redfield ratio. Inorganic nutrient concentrations (inline-formula M7inlinescrollmathml chem normal NO normal 3 - + normal NO normal 2 - 59pt16ptsvg-formulamathimgfc7350e160a73d22ed32c39ddc7cec46 bg-18-3029-2021-ie00004.svg59pt16ptbg-18-3029-2021-ie00004.png , inline-formula M8inlinescrollmathml chem normal PO normal 4 normal 3 - 29pt17ptsvg-formulamathimg903823d2610be1a1166032100b1ddcf0 bg-18-3029-2021-ie00005.svg29pt17ptbg-18-3029-2021-ie00005.png , Si(OHinline-formula)4) in fresh waters 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, fresh waters 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 kminline-formula−2 yrinline-formula−1) and iron (463–596 kg kminline-formula−2 yrinline-formula−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.

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St. Pierre, Kyra A. / Hunt, Brian P. V. / Tank, Suzanne E. / et al: Rain-fed streams dilute inorganic nutrients but subsidise organic-matter-associated nutrients in coastal waters of the northeast Pacific Ocean. 2021. Copernicus Publications.

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