Biogeochemistry and Ecology of Oxygen Depleted Eddies in the Eastern Tropical North Atlantic

State of research field

The recent discovery of isolated low oxygen (O2) water masses in the generally well ventilated open ocean region near the Cape Verde Archipelago changed our understanding of oceanic processes in this area. The eastern tropical North Atlantic (ETNA) is characterized by a highly productive coastal upwelling system off northwest Africa, enhanced Saharan dust deposition, and a moderate O2 minimum zone (OMZ) with lowest O2 concentrations just under 40 µmol kg-1. Current understanding is that the ETNA OMZ has been expanding over the past decades both in terms of vertical extent and intensity. Nevertheless, the recently observed exceptionally low O2 concentrations just below the mixed layer ranging from hypoxic (<20 µmol kg-1) to even anoxic (<1 µmol kg-1) conditions have never been reported before for the ETNA (Fig. 1).

Figure 1
Left: Time-series record of sensor-based subsurface O2 measurements conducted continuously from 2006 on at the CV Ocean Observatory (CVOO) long-term mooring site. Shaded areas denote hypoxic and anoxic anomalies below the 40 µmol kg-1 threshold (dashed line). Right: A hydrographic section through the outer rim of the O2 anomaly in 2010, as recorded accidently by a glider.

These O2 depleted isolated water masses were attributed to mesoscale eddies which originated in the highly productive coastal Mauritanian upwelling and propagated westwards. Acoustic Doppler Current Profiler observations at the Cape Verde Ocean Observatory (CVOO) suggest that zooplankton and nekton diurnal vertical migration (DVM) was inhibited for 3 weeks due to the presence of subsurface anoxia in the core of the eddy. Mesoscale eddies are being recognized as biogeochemical hot-spots of up to basin-wide relevance for the world’s oceans.

Scientific rationale

Autonomous observations of O2 depleted eddies in the ETNA suggest that their subsurface layers were subject to intensified respiration of organic matter. Resulting hypoxic to anoxic conditions would entail a local shift both in (i) biogeochemical cycling of nitrogen (N) and carbon (C) and in (ii) zooplankton and nekton community behavior. In particular, the N-cycle is strongly impacted by changing O2 conditions as the microbial communities react by differential expression of key functional genes, and O2 begins to limit oxidative pathways and reductive pathways are turned on. Resulting N-loss processes such as denitrification and anammox, which are generally assumed to be absent in open water of the ETNA, might be triggered by low O2 conditions in the eddy. Moreover, microbial production of N2O along with elevated levels of N2 and CO2 at shallow depths are likely to result in a rapid atmospheric release of N and C across the sea-air interface.

Identifying these key biogeochemical processes and their magnitudes will provide invaluable insights into these poorly described anomalies.

Zooplankton play a pivotal role in export of particulate and dissolved matter from the surface. Besides their important biogeochemical role, mesozooplankton are an integral part of the pelagic food web, forming the main prey items of small pelagic fishes and their larval stages. Hindrance of their DVM as recently observed at CVOO would make these organisms particularly vulnerable to predation, impacting both on the zooplankton and nekton community, but could have positive effects on fish feeding ecology.

How does a low-O2 eddy impact vertical and horizontal distribution of zooplankton, and how would this affect the ecological community and the organic matter flux within and outside the eddy?

So far, only autonomous observations are available but no direct biogeochemical and ecological measurements of these eddies. Dedicated surveys of open-ocean eddies are still rare due to technical and logistical limitations and the high risk of not finding an anomaly. Recent effort within the framework of a larger ship-based field expedition failed to find one of these eddies because the sub-optimal timing, limited ship-time and difficulties with the sensitivity of satellite-based.

Thus, underlying biogeochemical processes as well as the actual ecosystem response still remain unclear.

Methods and concepts

The observational concept is based on two pillars:
(i)Glider-based exploratory surveys (remote survey)
(ii)Ship-based hydrographical survey (site survey)

Figure 2
Left: Proposed remote survey (red line) in the vicinity of CVOO (black cross). The light gray line and arrow denote the propagation of the anoxic anticyclonic modewater-type eddy (ACME) which hit CVOO in Feb. 2010. Right: Composite map of sea level anomaly for the time of the eddy passage at CVOO in 2010, superimposed by the intended sampling scheme for the site survey (white dots).

The glider equipped with biogeochemical sensors will be used to explore the area east of the CVOO 1-2 months ahead of the season with enhanced eddy activity (Fig 2). This survey will be flanked by making use of optimized satellite data (sea level anomaly). The ship-based survey will be carried out shortly after successful detection of a low O2 eddy (within 2 weeks), covering two hydrographic sections (0 – 500 m) throughout the eddy (Fig. 2, right). The following objectives will be addressed:

Biogeochemical processes: Performing extensive sampling for various parameters of the N- and C-cycle (e.g., dissolved gases, gene abundance and expression, and isotopic and isotopomeric signatures).

Ecological response: Performing vertically stratified zooplankton sampling (day/night), determine particulate organic matter distribution.

Capacity building: Extending the existing bilateral cooperation by involving a student of the Universidade de Cabo Verde (UNI-CV) in field work and post-cruise analysis in Kiel.



Funded by: Cluster of Excellence "The Future Ocean" / DFG

Contact: Prof. Dr. Arne Körtzinger (GEOMAR)