Dorothee C. E. Bakker

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Southern Ocean deep-water carbon export enhanced by natural iron fertilization

The addition of iron to high-nutrient, low-chlorophyll regions induces phytoplankton blooms that take up carbon. Carbon export from the surface layer and, in particular, the ability of the ocean and sediments to sequester carbon for many years remains, however, poorly quantified. Here we report data from the CROZEX experiment in the Southern Ocean, which was conducted to test the hypothesis that the observed north–south gradient in phytoplankton concentrations in the vicinity of the Crozet Islands is induced by natural iron fertilization that results in enhanced organic carbon flux to the deep ocean. We report annual particulate carbon fluxes out of the surface layer, at three kilometres below the ocean surface and to the ocean floor. We find that carbon fluxes from a highly productive, naturally iron-fertilized region of the sub-Antarctic Southern Ocean are two to three times larger than the carbon fluxes from an adjacent high-nutrient, low-chlorophyll area not fertilized by iron. Our findings support the hypothesis that increased iron supply to the glacial sub-Antarctic may have directly enhanced carbon export to the deep ocean. The CROZEX sequestration efficiency (the amount of carbon sequestered below the depth of winter mixing for a given iron supply) of 8,600 mol mol-1 was 18 times greater than that of a phytoplankton bloom induced artificially by adding iron, but 77 times smaller than that of another bloom initiated, like CROZEX, by a natural supply of iron. Large losses of purposefully added iron can explain the lower efficiency of the induced bloom6. The discrepancy between the blooms naturally supplied with iron may result in part from an underestimate of horizontal iron supply.

The reinvigoration of the Southern Ocean carbon sink

Uptake uptick Has global warming slowed the uptake of atmospheric CO2 by the Southern Ocean? Landschützer et al. say no (see the Perspective by Fletcher). Previous work suggested that the strength of the Southern Ocean carbon sink fell during the 1990s. This raised concerns that such a decline would exacerbate the rise of atmospheric CO2 and thereby increase global surface air temperatures and ocean acidity. The newer data show that the Southern Ocean carbon sink strengthened again over the past decade, which illustrates the dynamic nature of the process and alleviates some of the anxiety about its earlier weakening trend. Science, this issue p. 1221; see also p. 1165 Carbon uptake by the Southern Ocean has increased again after its slowdown in the 1990s. [Also see Perspective by Fletcher] Several studies have suggested that the carbon sink in the Southern Ocean—the ocean’s strongest region for the uptake of anthropogenic CO2 —has weakened in recent decades. We demonstrated, on the basis of multidecadal analyses of surface ocean CO2 observations, that this weakening trend stopped around 2002, and by 2012, the Southern Ocean had regained its expected strength based on the growth of atmospheric CO2. All three Southern Ocean sectors have contributed to this reinvigoration of the carbon sink, yet differences in the processes between sectors exist, related to a tendency toward a zonally more asymmetric atmospheric circulation. The large decadal variations in the Southern Ocean carbon sink suggest a rather dynamic ocean carbon cycle that varies more in time than previously recognized.

Recent variability of the global ocean carbon sink

We present a new observation-based estimate of the global oceanic carbon dioxide (CO2) sink and its temporal variation on a monthly basis from 1998 through 2011 and at a spatial resolution of 1×1. This sink estimate rests upon a neural network-based mapping of global surface ocean observations of the partial pressure of CO2 (pCO2) from the Surface Ocean CO2 Atlas database. The resulting pCO2 has small biases when evaluated against independent observations in the different ocean basins, but larger randomly distributed differences exist particularly in high latitudes. The seasonal climatology of our neural network-based product agrees overall well with the Takahashi et al. (2009) climatology, although our product produces a stronger seasonal cycle at high latitudes. From our global pCO2 product, we compute a mean net global ocean (excluding the Arctic Ocean and coastal regions) CO2 uptake flux of −1.42 ± 0.53 Pg C yr−1, which is in good agreement with ocean inversion-based estimates. Our data indicate a moderate level of interannual variability in the ocean carbon sink (±0.12 Pg C yr−1, 1𝜎) from 1998 through 2011, mostly originating from the equatorial Pacific Ocean, and associated with the El Nino–Southern Oscillation. Accounting for steady state riverine and Arctic Ocean carbon fluxes our estimate further implies a mean anthropogenic CO2 uptake of −1.99 ± 0.59 Pg C yr−1 over the analysis period. From this estimate plus the most recent estimates for fossil fuel emissions and atmospheric CO2 accumulation, we infer a mean global land sink of −2.82 ± 0.85 Pg C yr−1 over the 1998 through 2011 period with strong interannual variation.

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.

Changes of carbon dioxide in surface waters during spring in the Southern Ocean

The fugacity of C02 (fCO2) and the content of chlorophyll a in surface-water were determined during consecutive sections between 47° and 60°S along 6°W in austral spring, October- November 1992. In the Polar Frontal region, the fCO2 of surface-water decreased from slightly below the atmospheric value to 50 μatm below it. This was accompanied by the development of diatom blooms. Seasonal warming of 1.2°C and air-sea exchange partly compensated the decrease of fCO2 by biological activity. Meanders of the Polar Frontal jet and a mesoscale eddy were reflected in spatial variability of fCO2 and chlorophyll a. Systematic observations indicated relationships between fCO2 and chlorophyll a, albeit changing with time. The combination of biological CO2- uptake with formation of Antarctic Intermediate Water (AAIW) makes the Polar Front a site of combined biological/physical CO2-drawdown from the atmosphere. In the southern part of the Antarctic Circumpolar Current (sACC) and the Southern Frontal region, fCO2 increased 7–8 μatm due to surface-water warming of 0.5°C. A sharp rise of surface water fCO2 of 13 μatm occurred south of the southern Frontal jet. As the ice-cover disappeared, the Boundary between the ACC and the Weddell Gyre released significant amounts of CO2. The Weddell Gyre would become a strong CO2-source after the imminent retreat of the ice. Clearly mechanisms behind changes of fCO2 in surface waters differ for the hydrographic regions. Interstitial brines of sea-ice had fCO2 as low as 100 μatm and had been depleted in nutrients. The summation of significant sources and sinks in the different regions indicates an overall minor oceanic CO2-sink of 0.3 mmol m−2 day−1 throughout the cruise, on the basis of the Wanninkhof relationship at in situ wind speed without skin effect. Uptake of C02 increased to 1.0 mmol m−2 day−1, when a uniform cold skin temperature difference of 0.2°C was assumed. The skin temperature difference derived from the physical model by Soloviev and Schl\ussel (1994a,b) had an average value of 0.2°C, leading to an uptake of CO2 of 1.2 mmol m−2 day−1. The measured skin temperature difference exceeded the calculated value. These assessments underline the uncertainty in the estimated air-sea exchange of C02 due to the thermal skin effect, the chosen parametrization of the gas transfer velocity, and the selected length of the wind speed interval. Limited understanding of the mechanistics of gas exchange, as well as large seasonal and spatial variability of the air-sea flux, still preclude a reliable estimate of the basin-wide annual flux for the Southern Ocean.

Ocean acidification and marine trace gas emissions

The oceanic uptake of man-made CO2 emissions is resulting in a measureable decrease in the pH of the surface oceans, a process which is predicted to have severe consequences for marine biological and biogeochemical processes [Caldeira K, Wickett ME (2003) Nature 425:365; The Royal Society (2005) Policy Document 12/05 (Royal Society, London)]. Here, we describe results showing how a doubling of current atmospheric CO2 affects the production of a suite of atmospherically important marine trace gases. Two CO2 treatments were used during a mesocosm CO2 perturbation experiment in a Norwegian fjord (present day: ∼380 ppmv and year 2100: ∼750 ppmv), and phytoplankton blooms were stimulated by the addition of nutrients. Seawater trace gas concentrations were monitored over the growth and decline of the blooms, revealing that concentrations of methyl iodide and dimethylsulfide were significantly reduced under high CO2. Additionally, large reductions in concentrations of other iodocarbons were observed. The response of bromocarbons to high CO2 was less clear cut. Further research is now required to understand how ocean acidification might impact on global marine trace gas fluxes and how these impacts might feed through to changes in the earths future climate and atmospheric chemistry.

Dissolved carbon dioxide in Dutch coastal waters

The role of shelf seas in global carbon cycling is poorly understood. The dissolved inorganic carbon system and air-sea exchange of carbon dioxide (CO2) are described for the Dutch coastal zone in September 1993. The inorganic carbon chemistry was affected by tidal mixing, wind speed, wind direction, freshwater input, stratification and coastal upwelling. Surface water had a variable fugacity of carbon dioxide (fCO2) between 300 and 800 μatm with short-term changes partly related to the tidal cycle. High contents of dissolved inorganic carbon (DIC) and CO2 in relatively saline water probably originated from mineralisation of accumulated organic matter in water and sediments farther out at sea and transport of water enriched in DIC into the coastal zone by upwelling. Air-sea exchange of CO2 ranged from —20 to 60 mmol m−2 day−1. These fluxes are critically discussed in the light of potential stratification. It is not possible to assess from this study whether the Dutch coastal zone is a net sink or source for atmospheric CO2.

δ13C of Southern Ocean suspended organic matter during spring and early summer: regional and temporal variability

Observations are presented for stable carbon isotope abundance (δ13C) and organic carbon and nitrogen content of suspended organic matter from the Southern Ocean (Circumpolar Current and Polar Front) during spring and early summer. The Polar Front Zone was characterized by elevated plankton biomasses and phytoplankton activity, which also increased significantly over the one-month investigation period. From the beginning of the phytoplankton bloom δ13C values of suspended organic matter in the Polar Front were high, exceeding values predicted from the relationship with CO2(aq) concentration observed in other areas of the Southern Ocean. Later in the season δ13C of suspended organic matter in the Polar Front became more negative despite continued high biomass and productivity. Ambient CO2 concentration and cell growth rate, therefore, are not the only factors controlling the δ13C of phytoplankton. The possible additional impact of shifts in nitrogen uptake regime is discussed.

Impact of the North Atlantic Oscillation on the trans‐Atlantic migrations of the European eel (Anguilla anguilla)

Glass eel catches and FAO (Food and Agricultural Organization) landings of the European freshwater eel (Anguilla anguilla) show a decrease over the past 20 years. The long-term trends in the time series mask an interannual fluctuation, which becomes apparent on the application of a high-pass filter and autocorrelation analysis. Both the FAO landings and the glass eel catches show interannual fluctuations with a repeat period of 6–8 years, similar to the period of the North Atlantic Oscillation (NAO). Most glass eel catch monitoring stations are in phase. The glass eel catches show a significant negative correlation with the NAO lagged by 0–2 years, consistent with the hypothesis that the positive NAO phase has an adverse impact on the larval survival in and migration from the Sargasso Sea spawning location, one year prior to the arrival of the glass eels in Europe and North Africa. The FAO landings can be divided into two groups of different phase that have an approximate correspondence to the NAO dipole in winter rainfall in Europe and North Africa. One group (P) comprises Denmark, Ireland, Morocco, Netherlands, Norway, Sweden, Tunisia, and the United Kingdom, and the other group (N) comprises France, Germany, Italy, Poland, Portugal, Spain, and Turkey. At least for the interannual fluctuations, the success of the glass eel fishery (and eel recruitment) may be coupled with the number of migrating silver eels from the N group of countries and uncoupled with the P group of countries.