Hidden biosphere in an oxygen-deficient Atlantic open-ocean eddy: future implications of ocean deoxygenation on primary production in the eastern tropical North Atlantic

Abstract

The eastern tropical North Atlantic (ETNA) is characterized by a highly productive coastal upwelling system and a moderate oxygen minimum zone with lowest open-ocean oxygen (O2) concentrations of approximately 40 μmol kg−1. The recent discovery of re-occurring mesoscale eddies with close to anoxic O2 concentrations (< 1 μmol kg−1) located just below the mixed layer has challenged our understanding of O2 distribution and biogeochemical processes in this area. Here, we present the first microbial community study from a deoxygenated anticyclonic modewater eddy in the open waters of the ETNA. In the eddy, we observed significantly lower bacterial diversity compared to surrounding waters, along with a significant community shift. We detected enhanced primary productivity in the surface layer of the eddy indicated by elevated chlorophyll concentrations and carbon uptake rates of up to three times as high as in surrounding waters. Carbon uptake rates below the euphotic zone correlated to the presence of a specific high-light ecotype of Prochlorococcus, which is usually underrepresented in the ETNA. Our data indicate that high primary production in the eddy fuels export production and supports enhanced respiration in a specific microbial community at shallow depths, below the mixed-layer base. The transcription of the key functional marker gene for dentrification, nirS, further indicated a potential for nitrogen loss processes in O2-depleted core waters of the eddy. Dentrification is usually absent from the open ETNA waters. In light of future projected ocean deoxygenation, our results show that even distinct events of anoxia have the potential to alter microbial community structure with critical impacts on primary productivity and biogeochemical processes of oceanic water bodies

Bathypelagic particle flux signatures from a suboxic eddy in the oligotrophic tropical North Atlantic: production, sedimentation and preservation

“…5a). These estimates match rather well with data determined in the low oxygen ACME in 2014 (Löscher et al, 2015b). The latter authors obtained chlorophyll concentrations and carbon uptake rates within the eddy of up to 3 times as high as in the surrounding waters.…”

“…This oxygen depletion may depend on the amount and type of settling organic-rich marine snow particles being remineralised in the subsurface waters (e.g. Löscher et al, 2015b;Karstensen et al, 2015) and on specific eddy physics as well.…”

“…The single depth oxygen observations show minimal values below 2 µmol kg −1 in the eddy (Karstensen et al, 2015) and it is not unlikely that denitrification in the eddy will also alter the δ 15 N to more positive values in the nitrate source waters (Liu and Kaplan, 1989). Löscher et al (2015b) found a transcription of the key gene for denitrification in a low oxygen ACME in the Cabo Verde region studied in 2014.…”

“…Our δ 15 N record in winter-spring 2009-2010 may reflect episodic nutrient injection into the euphotic zone of the ACME (Karstensen et al, 2015b), leading to increased particle formation and fluxes documented in February-March 2010 in the lower trap (Fig. 5c, Fig.…”

Oxygen utilization and downward carbon flux in an oxygen-depleted eddy in the eastern tropical North Atlantic

“…Based on the SLA data the "outside stations" during ISL and M105 were located 43 and 54 km away from the supposed eddy center, respectively. However, shipborne acoustic Doppler current profiler data (ADCP; see Hauss et al, 2016) as well as SLA data (Löscher et al, 2015) suggest a radius of this eddy of approx. 50-55 km.…”