Abstract
Abstract. The Beta Triangle, a region of the oligotrophic subtropical eastern North Atlantic Ocean, is notorious for its enigmatic oxygen, carbon, and nitrogen balances, in which nutrient supply is said to explain only a fraction of production necessary for estimated carbon export. Rates of dissolved organic carbon accumulation and dissolved organic nitrogen utilization in surface water and an assessment of oxygen utilized, organic matter consumed, and nitrate and phosphate regenerated in subsurface water, show that conventional production estimates miss substantial shares of biotic production. The shallow export of total organic carbon, predominantly dissolved (DOC), by subduction is responsible for about 50–70% of apparent oxygen utilization in subsurface water between the base of the surface layer at ca. 140 m and ca. 195 m depth, but it is insignificant below. Additionally, there is an estimated accumulation of 1.0 to 1.75 mol DOC m−2 a−1 in surface water. Including DOC dynamics in its carbon balance reveals the surface of this ultra-oligotrophic part of the ocean to be net autotrophic. Increasing subsurface values of excess nitrogen (DINxs) imply the export of nitrogen from surface water stemming from production not exclusively fuelled by new nitrate supplied from below. Total organic nitrogen (almost exclusively dissolved, DON) is consumed in the surface layer at a rate estimated at 0.13 to 0.23 mol m−2 a−1. There is no variation in dissolved organic phosphorus (DOP) in the same direction. DON utilization thus contributes to the pronounced subsurface DINxs signature. DOC export and accumulation are important in the carbon balance in surface and near-surface water. DON utilization and, probably, N2 fixation contribute significant amounts to the nitrogen supply of surface water. These processes can close part of the enigmatic carbon and nitrogen balances in the Beta Triangle. There are, however, no comparable processes which can explain the equally enigmatic situation concerning phosphorus supply in this area.
Highlights
1.1 The problemThe biotic contribution to the air-sea flux of carbon dioxide in the oligotrophic subtropical gyres of the oceans has been discussed controversially regarding its magnitude and even its direction (del Giorgio et al, 1997; Williams, 1998; Serret et al, 2001, 2009; del Giorgio and Duarte, 2002; LopezUrrutia et al, 2006; Riser and Johnson, 2008; a short history of the ensuing controversy is given by Williams et al, 2004)
Focusing on water masses of northern origin, isopycnal gradients are determined only for the region north of this boundary, i.e. the southernmost data point we use is at 22.9◦ N. This is essentially that part of our transect which is within the Beta Triangle (Fig. 1a)
The top panel gives a contour plot of the concentration of the respective substance or property
Summary
The biotic contribution to the air-sea flux of carbon dioxide in the oligotrophic subtropical gyres of the oceans has been discussed controversially regarding its magnitude and even its direction (del Giorgio et al, 1997; Williams, 1998; Serret et al, 2001, 2009; del Giorgio and Duarte, 2002; LopezUrrutia et al, 2006; Riser and Johnson, 2008; a short history of the ensuing controversy is given by Williams et al, 2004) These regions have been described to be heterotrophic, i.e. more organic carbon is thought to be consumed there than produced – even within the productive surface layer, the euphotic zone. Experimental and in-situ geochemical evidence can often not be reconciled
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