Remote and local drivers of oxygen and nitrate variability in the shallow oxygen minimum zone off Mauritania in June 2014

Abstract

Abstract. Upwelling systems play a key role in the global carbon and nitrogen cycles and are also of local relevance due to their high productivity and fish resources. To capture and understand the high spatial and temporal variability in physical and biogeochemical parameters found in these regions, novel measurement techniques have to be combined in an interdisciplinary manner. Here we use high-resolution glider-based physical–biogeochemical observations in combination with ship-based underwater vision profiler, sensor and bottle data to investigate the drivers of oxygen and nitrate variability across the shelf break off Mauritania in June 2014. Distinct oxygen and nitrate variability shows up in our glider data. High-oxygen and low-nitrate anomalies were clearly related to water mass variability and probably linked to ocean transport. Low-oxygen and high-nitrate patches co-occurred with enhanced turbidity signals close to the seabed, which suggests locally high microbial respiration rates of resuspended organic matter near the sea floor. This interpretation is supported by high particle abundance observed by the underwater vision profiler and enhanced particle-based respiration rate estimates close to the seabed. Discrete in situ measurements of dissolved organic carbon and amino acids suggest the formation of dissolved organic carbon due to particle dissolution near the seabed fueling additional microbial respiration. During June an increase in the oxygen concentration on the shelf break of about 15 µmol kg−1 was observed. These changes go along with meridional circulation changes but cannot be explained by typical water mass property changes. Thus our high-resolution interdisciplinary observations highlight the complex interplay of remote and local physical–biogeochemical drivers of oxygen and nitrate variability off Mauritania, which cannot be captured by classical shipboard observations alone.

Influence of intraseasonal eastern boundary circulation variability on hydrography and biogeochemistry off Peru

“…SUNA measurements are based on the absorbance 130 spectra of ultraviolet light (Sakamoto et al, 2009). Data post-processing followed Karstensen et al (2017) and Thomsen et al (2019). Finally, the SUNA nitrate concentrations were calibrated against the nitrate measurements by the autoanalyzer using a linear fit.…”

Controls on redox-sensitive trace metals in the Mauritanian oxygen minimum zone

“…At deeper levels (300-500 m depth), flow was found to be equatorward during both seasons. The shallow (< 300 m depth) boundary circulations turn offshore at the southern flank of the Cape Verde frontal zone (CVFZ) (e.g., Tomczak, 1981;Zenk et al, 1991) at about 20 • N, separating SACW from more saline and O 2rich central waters formed in the North Atlantic (NACW). The circulation in June 2014 was typical for a relaxation period characterized by strong poleward flow over the shelf and the upper continental slope between the surface and 250 m depth (Klenz et al, 2018;Thomsen et al, 2019).…”

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

“…The CVF also influenced its adjacent waters, as observed, for example, in the minima of O 2 and the maxima of NO 3 and PO 4 sampled just below 150 m in the tropical side of the front, which might be indicating a local remineralization (Fig. 6) (Thomsen et al, 2019).…”

“…At IW levels, the variability is mainly related to the AAIW flowing northward to the Canary Islands basin (Machín and Pelegrí, 2009;Machín et al, 2010). The shadow zone documented by Kawase and Sarmiento (1985) and the development of an oxygen minimum zone (OMZ) within the CW and IW levels was centered in transect S between 100 and 800 m depth with its core around 400 m between isoneutrals 27.1 and 27.3 kg m −3 (Karstensen et al, 2008;Brandt et al, 2015;Thomsen et al, 2019). That distribution matches well with the one provided by Peña-Izquierdo et al (2015), with high concentrations of NO 3 and PO 4 .…”