An update on dissolved methane distribution in the North subtropical Atlantic Ocean

Abstract

Abstract. Methane (CH4) is the second most produced greenhouse gas after carbon dioxide, however the role of the open ocean in its natural cycle remains poorly constrained. Accumulating evidence indicates that a significant part of oceanic CH4 is produced in oxygenated surface waters as a by-product of phytoplanktonic activity. The subtropical North Atlantic Ocean between 26° N 80’W and 26° N 18’W was investigated for the distribution of dissolved CH4 concentrations and associated air-sea fluxes during winter 2020. Water samples from 64 stations were collected from the upper water column up to depths of 400 m. The upper oxic mixed-layer was oversaturated in dissolved CH4 with concentrations ranging between 3–7 nmol/l, with the highest values of 7–10 nmol/l found to the east of the transect, consistent with other subtropical regions of the world’s oceans. The high anomalies of dissolved CH4 appeared to be associated to phosphorus depleted waters and to a peak of regions of elevated phytoplankton abundance. Further investigations indicated a correlation between CH4 anomalies, phosphate depletion and the abundance of two ubiquitous pico-cyanobacteria, Synechococcus and Prochlorococcus, although other phytoplanktonic phyla cannot be excluded. The calculation of air-sea fluxes confirms the subtropical North Atlantic Ocean as a source of CH4, mainly produced by phytoplanktonic activity in surface waters.

High photoreactivity of chromophoric dissolved organic matter derived from Ulva prolifera and Sargassum

“…than the sea-air CH 4 efflux of 1.6-4.4 mmol m -2 d -1 in the upper Sargasso Sea (Holmes et al, 2000) or the average efflux of 1.9 mmol m -2 d -1 in the subtropical North Atlantic (Kolomijeca et al, 2022). The photoproduction rates of CO 2 , CO, and CH 4 in the Great Atlantic Sargassum Belt are also presented in Table 2.…”