Annual export production in the interior Weddell Gyre estimated from a chemical mass balance of nutrients

Abstract

Nitrate, phosphate and silicate data are presented from 1992 austral winter and 1998 austral autumn cruises with ''FS Polarstern'' in the Weddell Gyre. Because in the Weddell Gyre, away from the boundary current, the surface layer is eventually formed from upwelled deep water, the difference in nutrient concentrations between these layers can be used to compute net nutrient consumptions (identical with the export production). This method renders a value for the export production that is based on observed annual changes. The results are consistent for two years and two regions within the central gyre. The calculated net nitrate and phosphate consumptions were scaled to net carbon consumptions using canonical Redfield ratios, yielding 16-17 mmol C kg À1 yr À1 . This equals 2174 g C m À2 yr À1 as a robust estimate for the marginal ice zone. The net annual silicate consumption in the surface layer, which equals the export of biogenic silica, amounts to 15-18 mmol kg À1 yr À1. There is a tendency for higher values in the eastern Weddell Gyre. The estimated silicate consumption of about 1.8 mol Si m À2 yr À1 is relatively high compared to earlier estimations of biogenic silica export. The silicate to carbon consumption ratio of about 1 is very high, and documents the dominance of diatoms in the export of organic material. r 2002 Elsevier Science Ltd. All rights reserved.Re´sumeŚ ont pr! esent! ees les distributions verticales de nitrate, de phosphate et de silicate en Mer de Weddell, pour les p! eriodes de l'hiver austral 1992 et de l'automne austral 1998. Les eaux de surface du tourbillon " a grande ! echelle de la Mer de Weddell (temps de r! esidence ! egal " a 2.9 ans) sont form! ees par l'upwelling des eaux profondes. La diff! erence de concentrations des sels nutritifs entre les couches profondes et de surface permettent de calculer la consommation annuelle, ! equivalente " a la production export! ee de l'! el! ement nutritif consid! er! e vers les couches profondes. Les r! esultats sont comparables pour les deux sc! enarios annuels ! etudi! es. La production export! ee de carbone pour les eaux de surface de la zone marginale de la glace, calcul! ee " a partir des consommations annuelles en nitrates et phosphates apr" es transformation gr# ace aux rapports de Redfield, est estim! ee " a 16-17 mmol C kg À1 yr À1 soit en moyenne 2174 g C m À2 yr À1 . La consommation annuelle de silicate est estim! ee " a 1.8 mol Si m À2 yr À1 , relativement ! elev! ee en *Corresponding author. Tel.: +49-421-2182931; fax: +49-421-2187018. E-mail address: hoppema@physik.uni-bremen.de (M. Hoppema).0967-0645/02/$ -see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 -0 6 4 5 ( 0 2 ) 0 0 0 0 6 -1 comparaison des estimations ant! erieures. Le rapport molaire Si/C, voisin de 1 dans le mat! eriel export! e, traduit la dominance des diatom! ees dans l'export de mati" eres organiques.

Whole season net community production in the Weddell Sea

“…Since the NCP represents the amount of net carbon production in the surface layer that is available for downward transfer, its value may be compared with the export production, the more so as we claim to have captured almost the entire annual NCP of the vegetative season. The annual export production in the central Weddell Sea, determined from a nutrient balance of the surface and subsurface layers, was reported to be 21 § 4 g C m ¡2 year ¡1 (= 1.7 mol m ¡2 year ¡1 ) (Hoppema et al 2002) which is insigniWcantly diVerent to our estimate of NCP. However, this may be fortuitous.…”

“…Fortuitous or not, this value is not very diVerent from the reported export production for the central Weddell Sea (Hoppema et al 2002). In austral autumn and winter, remineralization of organic matter produced in the preceding period will occur in the surface layer, which will reduce the NCP to its real annual value, which in turn should be equal to the annual export production.…”

Anthropogenic CO2 in the Southern Ocean: Distribution and inventory at the Indian‐Atlantic boundary (World Ocean Circulation Experiment line I6)

“…When differentiating between the two formation regions (results not shown), we found The eastern North Atlantic Deep Water (NADW-E) and North Indian Deep Water (NIDW) are the deep outflow waters defined by Broecker et al [1985], and Indian Water is the oxygen minimum/nutrients maximum layer described by Donohue and Toole [2003]. The southern source waters are Weddell/Circumpolar Deep Water (WDW/CDW), Ice Shelf Water (ISW), and Winter Water (WW) originating from either the Weddell Sea [Lindegren and Josefson, 1998;Hoppema et al, 1995Hoppema et al, , 1999Hoppema et al, , 2002 or Prydz Bay [Middleton and Humphries, 1989;Wong et al, 1998;Frew et al, 1995;Gibson and Trull, 1999]. Figure A1.…”

Distributions, trends and inter-annual variability of nutrients along a repeat section through the Weddell Sea (1996–2011)

“…However, nitrite also has inherent relevance within the nitrogen cycle of the Weddell Sea but only very few publications treat this subject. Only few nitrite data for the Weddell region have been previously published (Nöthig et al, 1991;Cota et al, 1992 andHoppema et al, 2002a). Nitrite is a dynamic, short-lived intermediate product of nitrification (nitrate formation from ammonium) and is excreted by phytoplankton (Lomas and Lipschultz, 2006); it is generally found in significantly non-zero concentrations only in the oceanic surface layer where often a primary nitrite maximum is found at the base of the euphotic zone (Lomas and Lipschultz, 2006).…”