Arne Körtzinger

Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes

Climate model predictions1, 2 and observations3, 4 reveal regional declines in oceanic dissolved oxygen, which are probably influenced by global warming5. Studies indicate ongoing dissolved oxygen depletion and vertical expansion of the oxygen minimum zone (OMZ) in the tropical northeast Atlantic Ocean6, 7. OMZ shoaling may restrict the usable habitat of billfishes and tunas to a narrow surface layer8, 9. We report a decrease in the upper ocean layer exceeding 3.5 ml l−1 dissolved oxygen at a rate of ≤1 m yr−1 in the tropical northeast Atlantic (0–25° N, 12–30° W), amounting to an annual habitat loss of ~5.95×1013 m3, or 15% for the period 1960–2010. Habitat compression and associated potential habitat loss was validated using electronic tagging data from 47 blue marlin. This phenomenon increases vulnerability to surface fishing gear for billfishes and tunas8, 9, and may be associated with a 10–50% worldwide decline of pelagic predator diversity10. Further expansion of the Atlantic OMZ along with overfishing may threaten the sustainability of these valuable pelagic fisheries and marine ecosystems.

Sensitivities of marine carbon fluxes to ocean change

Throughout Earths history, the oceans have played a dominant role in the climate system through the storage and transport of heat and the exchange of water and climate-relevant gases with the atmosphere. The oceans heat capacity is ≈1,000 times larger than that of the atmosphere, its content of reactive carbon more than 60 times larger. Through a variety of physical, chemical, and biological processes, the ocean acts as a driver of climate variability on time scales ranging from seasonal to interannual to decadal to glacial–interglacial. The same processes will also be involved in future responses of the ocean to global change. Here we assess the responses of the seawater carbonate system and of the oceans physical and biological carbon pumps to (i) ocean warming and the associated changes in vertical mixing and overturning circulation, and (ii) ocean acidification and carbonation. Our analysis underscores that many of these responses have the potential for significant feedback to the climate system. Because several of the underlying processes are interlinked and nonlinear, the sign and magnitude of the oceans carbon cycle feedback to climate change is yet unknown. Understanding these processes and their sensitivities to global change will be crucial to our ability to project future climate change.

Tracking the variable North Atlantic sink for atmospheric CO2

A Happy Marriage The fluxes of CO2 between the atmosphere and ocean are large and variable, and understanding why the concentration of atmospheric CO2 changes as it does, depends on accurately determining the details of those fluxes. One of the major obstacles in the way of quantifying this exchange is that there are too few measurements available, both temporally and geographically. Watson et al. (p. 1391) report results from a happy marriage of science and commerce—data collected by instruments fitted onto commercial ships plying the waters of the North Atlantic Ocean—that has generated the largest and most comprehensive set of measurements of ocean pCO2 ever collected. These data allow the oceanic CO2 sink to be monitored with unprecedented accuracy and will help researchers precisely map regional interannual air-sea fluxes. Data from instrumented commercial ships reveal substantial interannual variations of carbon dioxide flux between the ocean and the air. The oceans are a major sink for atmospheric carbon dioxide (CO2). Historically, observations have been too sparse to allow accurate tracking of changes in rates of CO2 uptake over ocean basins, so little is known about how these vary. Here, we show observations indicating substantial variability in the CO2 uptake by the North Atlantic on time scales of a few years. Further, we use measurements from a coordinated network of instrumented commercial ships to define the annual flux into the North Atlantic, for the year 2005, to a precision of about 10%. This approach offers the prospect of accurately monitoring the changing ocean CO2 sink for those ocean basins that are well covered by shipping routes.

Trends in marine dissolved oxygen: Implications for ocean circulation changes and the carbon budget

Recent measurements and model studies have consistently identified a decreasing trend in the concentration of dissolved O2 in the ocean over the last several decades. This trend has important implications for our understanding of anthropogenic climate change. First, the observed oceanic oxygen changes may be a signal of the beginning of a reorganization of large-scale ocean circulation in response to anthropogenic radiative forcing. Second, the repartitioning of oxygen between the ocean and the atmosphere requires a revision of the current atmospheric carbon budget and the estimates of the terrestrial and oceanic carbon sinks as calculated by the Intergovernmental Panel on Climate Change (IPCC) from measurements of atmospheric O2/N2.

An estimate of anthropogenic CO2 inventory from decadal changes in oceanic carbon content

Increased knowledge of the present global carbon cycle is important for our ability to understand and to predict the future carbon cycle and global climate. Approximately half of the anthropogenic carbon released to the atmosphere from fossil fuel burning is stored in the ocean, although distribution and regional fluxes of the ocean sink are debated. Estimates of anthropogenic carbon (Cant) in the oceans remain prone to error arising from (i) a need to estimate preindustrial reference concentrations of carbon for different oceanic regions, and (ii) differing behavior of transient ocean tracers used to infer Cant. We introduce an empirical approach to estimate Cant that circumvents both problems by using measurement of the decadal change of ocean carbon concentrations and the exponential nature of the atmospheric Cant increase. In contrast to prior approaches, the results are independent of tracer data but are shown to be qualitatively and quantitatively consistent with tracer-derived estimates. The approach reveals more Cant in the deep ocean than prior studies; with possible implications for future carbon uptake and deep ocean carbonate dissolution. Our results suggest that this approachs applied on the unprecedented global data archive provides a means of estimating the Cant for large parts of the worlds ocean.

The seasonal pCO2 cycle at 49°N/16.5°W in the northeastern Atlantic Ocean and what it tells us about biological productivity

A 2-year record of mixed layer measurements of CO2 partial pressure (pCO2), nitrate, and other physical, chemical, and biological parameters at a time series site in the northeast Atlantic Ocean (49N/16.5W) is presented. The data show average undersaturation of surface waters with respect to atmospheric CO2 levels by about 40 ± 15 matm, which gives rise to a perennial CO2 sink of 3.2 ± 1.3 mol m2 a1. The seasonal pCO2 cycle is characterized by a summer minimum (winter maximum), which is due to the dominance of biological forcing over physical forcing. Our data document a rapid transition from deep mixing to shallow summer stratification. At the onset of shallow stratification, up to one third of the mixed layer net community production during the productive season had already been accomplished. The combination of high prestratification productivity and rapid onset of tratification appears to have caused the observed particle flux peak early in the season. Mixed layer deepening during fall and winter reventilated CO2 from subsurface respiration of newly exported organic matter, thereby negating more than one third of the carbon drawdown by net community production in the mixed layer. Chemical signatures of both net community production and respiration are indicative of carbon overconsumption, the effects of which may be restricted, though, to the upper ocean. A comparison of the estimated net community production with satellite-based estimates of net primary production shows fundamental discrepancies in the timing of ocean productivity.

At-sea intercomparison of two newly designed underway pCO2 systems — encouraging results

Two newly designed underway systems for the measurement of CO2 partial pressure (pCO2) in seawater and the atmosphere are described. Results of an intercomparison experiment carried out in the North Sea are presented. A remarkable agreement between the two simultaneously measured (pCO2) data sets was observed even though the spatial variability in surface pCO2 was high. The average difference of all l -min averages of the seawater pCO2 was as low as 0.15 μatm with a standard deviation of 1.2 μatm indicating that no systematic difference is present. A closer examination of the profiles shows that differences tend to be highest during maxima of the pCO2 gradient (up to 14 μatm/min). The time constants of both systems were estimated from laboratory experiments to 45 s, respectively, 75 s thus quantitatively underlining their capability of a fast response to pCO2 changes

A significant CO2 sink in the tropical Atlantic Ocean associated with the Amazon River plume

] Rivers can have a significant impact on the hydrog-raphy and biogeochemistry of large ocean areas. This isespecially true for the Amazon River, by a considerablemargin the largest river in the world. Like in most aquaticecosystems, respiration exceeds autochthonous gross pri-mary production in the Amazon. The resulting negative netecosystem production is fueled by (particulate and dis-solved) organic carbon leaking from the terrestrial systemand causing the Amazon, like most rivers, to be highlysupersaturated in CO

High Quality Oxygen Measurements from Profiling Floats: A Promising New Technique

Abstract Two state-of-the-art profiling floats were equipped with novel optode-based oceanographic oxygen sensors. Both floats were simultaneously deployed in the central Labrador Sea gyre on 7 September 2003. They drift at a depth of 800 db and perform weekly profiles of temperature, salinity, and oxygen in the upper 2000 m of the water column. The initial results from the first 6 months of operation are presented. Data are compared with a small hydrographic oxygen survey of the deployment site. They are further examined for measurement quality, including precision, accuracy, and drift aspects. The first 28 profiles obtained are of high quality and show no detectable sensor drift. A method of long-term drift control is described and a few suggestions for the operation protocol are provided.

Oxygen minimum zone in the North Atlantic south and east of the Cape Verde Islands

The open-ocean oxygen minimum zone (OMZ) south and east of the Cape Verde Islands is studied from CTD hydrography, ADCP velocities, Argo float trajectories, and historical data, with a focus on the zonal supply and drainage paths. The strongest oxygen minimum is located north of the North Equatorial Countercurrent (NECC) at about 400 to 500-m depth just above the boundary between Central Water and Antarctic Intermediate Water (AAIW). It is shown that the NECC, the North Equatorial Undercurrent at 4 to 6°N, and a northern branch of the NECC at 8 to 10°N are the sources for oxygen-rich water supplied to the OMZ in summer and fall. A weak eastward NECC at 200-m depth also exists in winter and spring as derived from Argo floats drifting at shallow levels. Historical oxygen data from 200-m depth confirm this seasonality showing high (low) oxygen content in summer and fall (spring) within the supply paths. Compared to the strong oxygen supply at 150 to 300-m depth, the ventilation of the OMZ at 300 to 600-m depth is weaker. Westward drainage of oxygen-poor water takes place north of the Guinea Dome, i.e., north of 10°N, most pronounced at 400 to 600-m depth. In July 2006 the total eastward transport of both NECC bands above σ θ = 27.1 kg m−3 at 23°W was about 13 Sv (1 Sv = 106 m3 s−1). About half of this water volume circulates within the Guinea Dome or recirculates westward north of the Guinea Dome.