The fate of riverine nutrients on Arctic shelves

[摘要] Present and future levels of primary production (PP) in the Arctic Ocean(AO) depend on nutrient inputs to the photic zone via vertical mixing,upwelling and external sources. In this regard, the importance of horizontalriver supply relative to oceanic processes is poorly constrained at thepan-Arctic scale. We compiled extensive historical (1954–2012) data ondischarge and nutrient concentrations to estimate fluxes of nitrate, solublereactive phosphate (SRP), silicate, dissolved organic carbon (DOC),dissolved organic nitrogen (DON), particulate organic nitrogen (PON) and particulate organic carbon (POC) from 9 largeArctic rivers and assess their potential impact on the biogeochemistry ofshelf waters. Several key points can be emphasized from this analysis. Thecontribution of riverine nitrate to new PP (PP_new) is very small at theregional scale (< 1% to 6.7%) and negligible at the pan-Arcticscale (< 0.83%), in agreement with recent studies. By consumingall 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 andregional scales, respectively. Corresponding figures for SRP are 28.9%and 18.6–46%. On the Beaufort and Bering shelves, riverine SRP cannotfulfil phytoplankton requirements. On a seasonal basis, the removal ofriverine nitrate, silicate and SRP would be the highest in spring and not insummer when AO shelf waters are nitrogen-limited. Riverine DON ispotentially an important nitrogen source for the planktonic ecosystem insummer, when ammonium supplied through the photoammonification of refractoryDON (3.9 × 10⁹ mol N) may exceed the combined riverine supply ofnitrate and ammonium (3.4 × 10⁹ mol N). Nevertheless, overall nitrogenlimitation of AO phytoplankton is expected to persist even when projectedincreases of riverine DON and nitrate supply are taken into account. Thisanalysis underscores the need to better contrast oceanic nutrient supplyprocesses with the composition and fate of changing riverine nutrientdeliveries in future scenarios of plankton community structure, function andproduction in the coastal AO.

[学科分类] 地球化学与岩石