Non-redfieldian mesopelagic nutrient remineralization in the eastern North Atlantic subtropical gyre

Abstract

The eastern boundary of the North Atlantic subtropical gyre (NASTG) receives organic materials produced in the nearby Canary upwelling system (CanUS), which has important implications for the metabolic balance of the region. Here we computed absolute geostrophic, Ekman and eddy diffusive transports and net budgets of oxygen, nitrate, and phosphate, with an inverse model applied to annual climatologies from the World Ocean Atlas 2013, in a large box covering the eastern boundary of the NASTG. The surface waters (<100 m) of the box exported 4.2 ± 1.1 mol O 2 m −2 yr −1 to the adjacent ocean and atmosphere, and imported 0.40 ± 0.17 mol N m −2 yr −1 and 22.2 ± 9.5 mmol P m −2 yr −1 of nitrate and phosphate, respectively, indicating that net autotrophy prevailed in the euphotic waters of the eastern NASTG. The central, intermediate and deep waters (100 m-bottom) imported 6.7 ± 3.3 mol O 2 m −2 yr −1 , and exported 1.05 ± 0.35 mol N m −2 yr −1 and 50.2 ± 21.8 mmol P m −2 yr −1 of nitrate and phosphate, respectively, indicative of net remineralization. Mesopelagic remineralization rates exceeded epipelagic net community production, implying a net deficit of carbon and nutrients in the region. Remineralization occurred at high N:P molar ratios (∼28) in central waters, and thus, an excess nitrate (DINxs = NO 3-16•PO 4) was produced at a rate of 2.2 ± 1.6 × 10 11 mol N yr −1. This excess nitrate was exported to the gyre interior, influencing the biogeochemistry of the whole NASTG and leaving the eastern margin with a strong nitrogen deficit, relative to carbon and phosphorus. According to the available literature, the organic carbon export from the CanUS could be sufficient to account for the carbon deficit, but the source of nitrogen remains elusive.

Lateral transports of mass, inorganic nutrients and dissolved oxygen in the Cape Verde Frontal Zone in summer 2017

“…In the same way, inorganic nutrients entered with −58 ± 54 /−3.5 ± 3.3 /−34 ± 32 and −65 ± 39 /−4.2 ± 2.5 /−42 ± 26 kmol s −1 of NO 3 / PO 4 / SiO 4 H 4 from east and south and left with 35 ± 46 / 2.40 ± 3.13 / 28 ± 37 and 57 ± 53 / 3.9 ± 3.6 / 40 ± 37 kmol s −1 of NO 3 / PO 4 / SiO 4 H 4 northward and westward. These transports do not match with the lateral advection ranges of Burgoa et al (2020) north of our domain where inorganic nutrients transports are smaller than in the CVB but they are in agreement with the climatological ranges reported by Fernández-Castro et al (2018).…”

“…In the third CW layer, there was also important northward transport of inorganic nutrients through transect N. In IW and DW, these transports came from the south and east and they left also mainly westward as with mass transports. The most intense and important transports of O 2 and inorganic nutrients occur in the deepest CW layer and the shallowest two IW layers (Fernández-Castro et al, 2018;Burgoa et al, 2020). In these three layers, O 2 enters with −280±262 and −340±206 kmol s −1 from east and south and it leaves with 95±124 and 255±238 kmol s −1 northward and westward.…”

Seasonal variability in mass, nutrients and DOC lateral transports off Northwest African Upwelling System

“…This is quite in agreement with the local maximum of remineralization found for all tracers in the upper intermediate layerFernández-Castro et al (2018).CW levels are featured by a relatively high biological production and therefore a nutrient deficit, and also by large geostrophic velocities. During fall the amount of IN that enters the box through N and S transects is larger than the IN quantity that leaves the box through the W transect.…”

“…The relevance of lateral advective transports on the spatio-temporal distribution of biogeochemical variables in the ocean has historically received little attention (Ganachaud, 1999;Ganachaud and Wunsch, 2002b). Specifically in the EBUS off Northwest Africa, some recent manuscripts related to lateral advective transports of biogeochemical variables have shed light on this topic (Álvarez and Álvarez-Salgado, 2009;Alonso-González et al, 2009;Santana-Falcón et al, 2017;Fernández-Castro et al, 2018).…”

Cape Verde Frontal Zone in summer 2017: lateral transports of mass, dissolved oxygen and inorganic nutrients

“…This number indicates that the total O 2 consumption within the box is 163.8 kmol s −1 , as the lateral transport integrated for the whole sampled water column was −277.34 kmol s −1 . On the other hand, the inorganic nutrient positive balances indicate that the domain is producing inorganic nutrients, likely as a consequence of remineralization below the photic layer; the nutrient import from the atmosphere is considered negligible compared with lateral transports, according to climatological values reported by Fernández-Castro et al (2019). Hence, this domain would be acting as an heterotrophic box, as revealed by the net oxygen consumption, with remineralization of N, P and Si below the photic layer.…”