The fate of riverine nutrients on Arctic shelves

cess Biogeosciences Open Access Climate of the Past Open Access Biogeosciences, 10, 3661–3677, 2013 www.biogeosciences.net/10/3661/2013/ doi:10.5194/bg-10-3661-2013 © Author(s) 2013. CC Attribution 3.0 License. Techniques The fate of riverine nutrients on Arctic shelves Open Access Earth System 1 Laboratoire d’Océanographie de Villefranche, BP 8, UMR7093, CNRS & Univ. Pierre et MarieDynamics Curie (Paris VI), 06238 V. Le Fouest1 , M. Babin2 , and J.-É. Tremblay2 Correspondence to: V. Le Fouest () Received: 2 August 2012 – Published in Biogeosciences Discuss.: 2 October 2012 Revised: 19 February 2013 – Accepted: 7 May 2013 – Published: 4 June 2013 Open Access and nitrate supply are taken into account. This analysis unGeoscientific derscores the need to better contrast oceanic nutrient supply Development processes with Model the composition and fate of changing riverine nutrient deliveries in future scenarios of plankton community structure, function and production in the coastal AO. 1 Introduction Hydrology and Earth System Sciences Open Access Fifty years ago, the Arctic Ocean (AO) was perceived as a small contributor to the global carbon cycle because of its extensive sea-ice cover and the relatively low light levels experienced by phytoplankton (English, 1961). The AO is now Ocean Science thought to contribute ca. 14 % of the global uptake of atmospheric carbon dioxide (Bates and Mathis, 2009) and, as such, is an important actor in the global carbon cycle. As a consequence of warming, the AO tends to switch towards a more sub-Arctic state. The earlier and longer exposure of Solidearlier Earth surface waters to sunlight triggers vernal blooms in some parts of the Arctic Ocean (Kahru et al., 2011). Also, it has been suggested based on ocean colour remote sensing data that annual primary production (PP) is increasing (Arrigo et al., 2008). However, recent observations show that the density stratification (i.e. pycnocline) is persistent throughThe Cryosphere out the year (Tremblay et al., 2008) and strengthening as a result of increasing river discharge (Li et al., 2009). These conditions limit the vertical supply of nutrients offshore and favour small phytoplankton cells at the expense of large ones (Li et al., 2009). Present and future trends in Arctic PP will depend on nutrient inputs into the photic zone, driven either by ocean mixing, upwelling or external sources Open Access Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. Open Access Abstract. 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 horizontal river supply relative to oceanic processes is poorly constrained at the pan-Arctic scale. We compiled extensive historical (1954–2012) data on discharge and nutrient concentrations to estimate fluxes of nitrate, soluble reactive phosphate (SRP), silicate, dissolved organic carbon (DOC), dissolved organic nitrogen (DON), particulate organic nitrogen (PON) and particulate organic carbon (POC) from 9 large Arctic rivers and assess their potential impact on the biogeochemistry of shelf waters. Several key points can be emphasized from this analysis. The contribution of riverine nitrate to new PP (PPnew ) is very small at the regional scale (< 1 % to 6.7 %) and negligible at the pan-Arctic scale (< 0.83 %), in agreement 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 panArctic and regional scales, respectively. Corresponding figures for SRP are 28.9 % and 18.6–46 %. On the Beaufort and Bering 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 and not in summer when AO shelf waters are nitrogen-limited. Riverine DON is potentially an important nitrogen source for the planktonic ecosystem in summer, when ammonium supplied through the photoammonification of refractory DON (3.9 × 109 mol N) may exceed the combined riverine supply of nitrate and ammonium (3.4 × 109 mol N). Nevertheless, overall nitrogen limitation of AO phytoplankton is expected to persist even when projected increases of riverine DON Instrumentation Methods and Data Systems Open Access Villefranche-sur-Mer Cedex, France 2 Takuvik Joint International Laboratory, Université Laval (Canada) & Centre National de la Recherche Scientifique (France), Département de Biologie, 1045, Avenue de la Médecine, Québec (Québec), G1V 0A6, Canada Geoscientific M 3662 V. Le Fouest et al.: The fate of riverine nutrients on Arctic shelves (Tremblay and Gagnon, 2009). Mixing and upwelling replenish the photic zone with new nutrients transported upwards from below the pycnocline. These nutrients originate mostly from the local remineralization of settling organic matter and from the inflow of Atlantic and Pacific waters. Upward supply can result from tidal or wind-driven erosions of the pycnocline (Wassmann et al., 2006; Hannah et al., 2009; Le Fouest et al., 2011), upwelling when wind blows in a suitable direction along the shelf break (Tremblay et al., 2011) or the ice edge (Mundy et al., 2009) and eddy pumping in shallow anticyclonic eddies (Timmermans et al., 2008). The contribution of these oceanic processes relative to horizontal nutrient supply from rivers and adjacent seas to the Arctic PP regime is poorly constrained at the pan-Arctic scale (Tremblay and Gagnon, 2009). Continental rivers surrounding the AO are a potentially significant source of nutrients for circum-Arctic shelf seas. Arctic river discharge is high, representing 10 % of the global freshwater discharge pouring into only 1 % of the global ocean volume (Opshal et al., 1999). While the estimated input of allochthonous inorganic and organic compounds by rivers into the Arctic Ocean is not negligible (Holmes et al., 2000; Dittmar and Kattner, 2003), its biogeochemical significance in shelf waters remains unclear (McClelland et al., 2012). Riverine nitrate is derived from soil leaching (i.e. moved or dissolved and carried through soil by water) and terrestrial surface run-off (i.e. transported over land in the excess water when soil is infiltrated to full capacity). Soluble reactive phosphorus (SRP) originates from the weathering of crustal minerals (e.g. aluminium orthophosphate, apatite) and silicate from weathering of silicate and aluminosilicate minerals. Along the river path, the specificity of the lithological substrate and permafrost and the terrestrial vegetation are important factors governing the riverine nutrient flux. Glacial or thermokarst lakes also control the nutrient transport from the soil to the river. Around delta lakes, inorganic nutrients can be enhanced via processes involving flo (...truncated)