The fate of riverine nutrients on Arctic shelves

Abstract : Present and future levels of primary production(PP) in the Arctic Ocean (AO) depend on nutrient inputsto the photic zone via vertical mixing, upwelling and externalsources. In this regard, the importance of horizontalriver supply relative to oceanic processes is poorly constrainedat the pan-Arctic scale. We compiled extensive historical(1954–2012) data on discharge and nutrient concentrationsto estimate fluxes of nitrate, soluble reactive phosphate(SRP), silicate, dissolved organic carbon (DOC), dissolvedorganic nitrogen (DON), particulate organic nitrogen(PON) and particulate organic carbon (POC) from 9 largeArctic rivers and assess their potential impact on the biogeochemistryof shelf waters. Several key points can be emphasizedfrom this analysis. The contribution of riverine nitrateto new PP (PPnew) is very small at the regional scale (<1%to 6.7 %) and negligible at the pan-Arctic scale (<0.83 %), inagreement with recent studies. By consuming all this nitrate,oceanic phytoplankton would be able to use only 14.3%and 8.7–24.5% of the river supply of silicate at the pan-Arctic and regional scales, respectively. Corresponding figuresfor SRP are 28.9% and 18.6–46 %. On the Beaufort andBering shelves, riverine SRP cannot fulfil phytoplankton requirements.On a seasonal basis, the removal of riverine nitrate,silicate and SRP would be the highest in spring andnot in summer when AO shelf waters are nitrogen-limited.Riverine DON is potentially an important nitrogen source forthe planktonic ecosystem in summer, when ammonium suppliedthrough the photoammonification of refractory DON(3.9×109 mol N) may exceed the combined riverine supplyof nitrate and ammonium (3.4×109 mol N). Nevertheless,overall nitrogen limitation of AO phytoplankton is expectedto persist even when projected increases of riverine DONand nitrate supply are taken into account. This analysis underscoresthe need to better contrast oceanic nutrient supplyprocesses with the composition and fate of changing riverinenutrient deliveries in future scenarios of plankton communitystructure, function and production in the coastal AO.
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V. Le Fouest, M. Babin, J.-É. Tremblay. The fate of riverine nutrients on Arctic shelves. Biogeosciences, European Geosciences Union, 2013, 10 (6), pp.3661 - 3677. ⟨10.5194/bg-10-3661-2013⟩. ⟨hal-01089953v2⟩



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