Biogeochemical budgets in the eastern boundary current system of the North Atlantic: Evidence of net heterotrophy and nitrogen fixation

Abstract

Thermohaline and chemical data from three WOCE (World Ocean Circulation Experiment) cruises conducted in 1997 and 1998 define the MedBox region, bounded by the Strait of Gibraltar, 24uN, 41uN, and 22uW. The carbon budget indicates that the MedBox is a heterotrophic region, where carbon is mineralized at a net rate of 17 6 11 g C m 22 yr 21 , supported by the input of allochthonous organic matter, mainly in the dissolved form, from the adjacent ocean. Dissolved organic carbon accounts for 90% of the organic carbon demand. In vitro measurements to estimate the net community production of the study area differ largely from our geochemical budget estimation, likely reflecting different spatial and temporal scales and/or terms or processes not taken into account by both methods. The nitrogen budget of the MedBox pointed to a significant atmospheric input via N 2 fixation (3.4 6 3.1 g N m 22 yr 21 ).

Low contribution of N2 fixation to new production and excess nitrogen in the subtropical northeast Atlantic margin

“…As previously mentioned, Álvarez and Álvarez-Salgado (2007) estimated a net nitrogen excess accumulation rate for the area bounded between 25º 42ºN and 20ºW of 40 ± 22 × 10 10 mol N y 1 or 658 ± 354 µmol N m 2 d 1 with a geochemical box model, which they hypothesized should be due to N 2 fixation after discounting other possible sources. This contrasts with the low average N 2 fixation rate measured in this study.…”

“…In the eastern subtropical North Atlantic, Álvarez and Álvarez-Salgado (2007) calculated an excess of 40 ± 22 × 10 10 mol N y 1 for the region bounded by 25º 42ºN and 20ºW. These authors tentatively attributed this excess to N 2 fixation, but the lack of in situ measurements in the area could not confirm it by that time.…”

Quasi-synoptic transport, budgets and water mass transformation in the Azores–Gibraltar Strait region during summer 2009

“…The subtropical eastern North Atlantic is a region of eastern boundary ventilation (Barton, 1998;Arístegui et al, 2004;Álvarez and Álvarez-Salgado, 2007) due in part to entrainment of central waters into Mediterranean Outflow Water (MOW) (van Aken, 2000;Álvarez et al, 2005;Fusco et al, 2008) and also to meridional spread and mixing of low-oxygen high-nutrient subtropical and recently ventilated subpolar central water modes (Ríos et al, 1992;Pérez et al, 2001). The former leads to an overturning circulation, whereby central waters enter the region and saltier, intermediate water flows out below, spreading to the Atlantic Ocean.…”

In vivo electron transport system activity: a method to estimate respiration in natural marine microbial planktonic communities

“…A long-lasting scientific controversy exists in oligotrophic subtropical waters between data extrapolated from direct respiration measurements (by Winkler method and bottle incubations or by in vitro ETS estimations), that suggest net heterotrophy in the euphotic zone (Duarte et al 1999;Serret et al 2001), and geochemical evidences (based on inorganic carbon, nutrient, and dissolved oxygen concentration data in the euphotic layer or integrated sediment trap data), which support that the upper ocean is a net source of oxygen to the atmosphere (Karl et al 2003;Álvarez and Álvarez-Salgado 2007;Riser and Johnson 2008). In addition, Marañón et al (2007) reported that microbial plankton community respiration in the North Atlantic Subtropical Gyre determined with the Winkler method did not reconcile with respiration rates estimated for the different microplankton groups.…”