Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific Oceans

Abstract

Abstract. Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs). There are numerous feedbacks between oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes. We investigated the pelagic biogeochemistry of OMZs in the eastern tropical North Atlantic and eastern tropical South Pacific during a series of cruise expeditions and mesocosm studies. The following summarizes the current state of research on the influence of low environmental oxygen conditions on marine biota, viruses, organic matter formation and remineralization with a particular focus on the nitrogen cycle in OMZ regions. The impact of sulfidic events on water column biogeochemistry, originating from a specific microbial community capable of highly efficient carbon fixation, nitrogen turnover and N2O production is further discussed. Based on our findings, an important role of sinking particulate organic matter in controlling the nutrient stochiometry of the water column is suggested. These particles can enhance degradation processes in OMZ waters by acting as microniches, with sharp gradients enabling different processes to happen in close vicinity, thus altering the interpretation of oxic and anoxic environments.

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

“…On the one hand, nitrogen (nitrate) limitation in the surface water north of the Cabo Verde islands combined with low growth rates of the primary producers (both diatoms and coccolithophores) would explain the elevated C : N ratios of organic matter (e.g. Laws and Bannister, 1980;Martiny et al, 2013;Löscher et al, 2015a). However, since oxygen : nitrate ratios are about twice as high in the eddy compared to the surrounding waters, enhanced nitrogen recycling could explain the extraordinarily high C : N ratios as well .…”

“…Koopmans et al, 1996). Thus, evidence of bacterial communities detected in oxygen minimum zones (OMZs), including green sulfur bacteria and anammox bacteria (see summary in Löscher et al, 2015a), could not be found with our methods. However, detection of these compounds requires the respective bacterial stocks to be present in concentrations above a certain detection threshold and/or an effective export mechanism for them leading to incorporation into sinking particles.…”

“…Oxygen levels in the future oceans might decrease significantly and develop into OMZs due to increasing global warming and water column stratification (e.g. Stramma et al, 2008Stramma et al, , 2010Codispoti, 2010;Löscher et al, 2015a). These potential changes may in turn influence the nitrogen cycle and the operation of the biological pump, e.g.…”

“…Moreover, local denitrification in the low oxygen eddy core (e.g. Löscher et al, 2015a) could contribute to increasing C : N ratios;…”

“…However, detection of these compounds requires the respective bacterial stocks to be present in concentrations above a certain detection threshold and/or an effective export mechanism for them leading to incorporation into sinking particles. Although one might expect these compounds to be present during the ACME passage when low oxygen conditions prevailed in the subsurface waters (Löscher et al, 2015a), it is likely the populations have not reached significant levels, because suboxic conditions just recently developed within the eddy. Another possible explanation is that the bacteria were present in the suboxic zone of the eddy, but did not interact and were not attached to the fast-sinking organic-rich particles, which originated in the surface layer and later constituted the mass flux.…”