Dead zone or oasis in the open ocean? Zooplankton distribution and migration in low-oxygen modewater eddies

Abstract

Abstract. The eastern tropical North Atlantic (ETNA) features a mesopelagic oxygen minimum zone (OMZ) at approximately 300–600 m depth. Here, oxygen concentrations rarely fall below 40 µmol O2 kg−1, but are expected to decline under future projections of global warming. The recent discovery of mesoscale eddies that harbour a shallow suboxic (< 5 µmol O2 kg−1) OMZ just below the mixed layer could serve to identify zooplankton groups that may be negatively or positively affected by ongoing ocean deoxygenation. In spring 2014, a detailed survey of a suboxic anticyclonic modewater eddy (ACME) was carried out near the Cape Verde Ocean Observatory (CVOO), combining acoustic and optical profiling methods with stratified multinet hauls and hydrography. The multinet data revealed that the eddy was characterized by an approximately 1.5-fold increase in total area-integrated zooplankton abundance. At nighttime, when a large proportion of acoustic scatterers is ascending into the upper 150 m, a drastic reduction in mean volume backscattering (Sv) at 75 kHz (shipboard acoustic Doppler current profiler, ADCP) within the shallow OMZ of the eddy was evident compared to the nighttime distribution outside the eddy. Acoustic scatterers avoided the depth range between approximately 85 to 120 m, where oxygen concentrations were lower than approximately 20 µmol O2 kg−1, indicating habitat compression to the oxygenated surface layer. This observation is confirmed by time series observations of a moored ADCP (upward looking, 300 kHz) during an ACME transit at the CVOO mooring in 2010. Nevertheless, part of the diurnal vertical migration (DVM) from the surface layer to the mesopelagic continued through the shallow OMZ. Based upon vertically stratified multinet hauls, Underwater Vision Profiler (UVP5) and ADCP data, four strategies followed by zooplankton in response to in response to the eddy OMZ have been identified: (i) shallow OMZ avoidance and compression at the surface (e.g. most calanoid copepods, euphausiids); (ii) migration to the shallow OMZ core during daytime, but paying O2 debt at the surface at nighttime (e.g. siphonophores, Oncaea spp., eucalanoid copepods); (iii) residing in the shallow OMZ day and night (e.g. ostracods, polychaetes); and (iv) DVM through the shallow OMZ from deeper oxygenated depths to the surface and back. For strategy (i), (ii) and (iv), compression of the habitable volume in the surface may increase prey–predator encounter rates, rendering zooplankton and micronekton more vulnerable to predation and potentially making the eddy surface a foraging hotspot for higher trophic levels. With respect to long-term effects of ocean deoxygenation, we expect avoidance of the mesopelagic OMZ to set in if oxygen levels decline below approximately 20 µmol O2 kg−1. This may result in a positive feedback on the OMZ oxygen consumption rates, since zooplankton and micronekton respiration within the OMZ as well as active flux of dissolved and particulate organic matter into the OMZ will decline.

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

“…This behavior is known as diel vertical migration (DVM) and also contributes to the activity of the biological pump as it enhances the export of organic matter from the photic zone by continued respiration, excretion and egestion in midwater layers (Burd et al, 2010;Hannides et al, 2009;Robinson et al, 2010;Steinberg et al, 2000). In addition to changes in temperature with depth, DVM organisms experience low O 2 concentrations during the daytime in OMZ regions (Brewer and Peltzer, 2009;Paulmier et al, 2011), and O 2 concentrations below a certain threshold level can restrict DVM of most zooplankton and nekton (e.g., Hauss et al, 2016). On a regional scale, the upper boundary of the oxycline is the single most critical factor structuring the habitat of most zooplankton organisms in the Peruvian upwelling system (Escribano et al, 2009).…”

Impact of mesoscale eddies on water mass and oxygen distribution in the eastern tropical South Pacific

“…contribute to cooling the surface layer in this region (Holte et al, 2013) For the Atlantic Ocean the low-oxygen eddy cores has been attributed to high productivity in the surface (Schütte et al, 2016b), enhanced respiration of sinking organic material at subsurface depth 10 (Fiedler et al, 2016) and a strong isolation of the eddy core (Karstensen et al, 2017). An anticyclonic mode water eddy observed at the Stratus mooring in February/March 2012 indicated high primary production just below the mixed layer (Stramma et al, 2014).…”

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

“…-reduced daily migrating zooplankton in low oxygen eddies (Karstensen et al, 2015;Hauss et al, 2016) might have resulted in less organic matter degradation due to missing "flux feeding". This could have caused less organic carbon flux attenuation and, thus, a higher bathypelagic organic carbon flux.…”

“…Acoustic backscatter data suggest that at least some zooplankters reduced their diel vertical migration behaviour in the low oxygen eddies (Karstensen et al, 2015;Hauss et al, 2016) but also in large-scale open ocean OMZs (e.g. Ayon et al, 2008).…”

“…Missing diel migration of a number of zooplankton groups due to the passage of the suboxic eddy (Karstensen et al, 2015;Hauss et al, 2016) could have resulted in less organic matter degradation of sinking particles due to reduced "flux feeding" within the suboxic/hypoxic zone (around 40-170 m). This depth range is the most active zone in terms of organic carbon turnover under normal conditions with sufficient oxygen (e.g.…”