The dependence on temperature and salinity of dissolved

Recurring latitudinal patterns of the dissolved inorganic carbon (DIC) content and the fugacity of CO2 (fCO2) were observed in East Atlantic surface waters with strong gradients at hydrographic fronts. The dissolved inorganic carbon chemistry clearly displayed the effects of oceanic circulation and of persistent surface water processes. In two cases inorganic carbon components could be used as an indicator of the origin of hydrographic features. Surface water fCO2 below the atmospheric value, low DIC and low salinity north of the equator were ascribed to a combination of high rainfall and low wind speed in the Intertropical Convergence Zone and of biological uptake of CO2. Low surface water DIC and salinity delineated the Congo outflow. Along the cruise tracks calculated titration alkalinity (TA) had an almost linear relationship with salinity, while DIC had an apparent dependence on temperature and salinity. The latter dependence was tested by comparing observed DIC to DIC estimated from fCO2 and a reference value of TA normalised to salinity. Different scenarios of temperature, salinity, fCO2 and nutrient contents were applied. Changes of DIC were found to be indeed related to both temperature and salinity. The latitudinal distribution of DIC could be inferred with an accuracy of 17 μmol kg−1 and a standard deviation of 13 μmol kg−1 from in situ salinity, in situ temperature and the reference values of TA and nutrient contents normalised to in situ salinity (scenario D). The applied technique of estimating DIC from temperature and salinity is a powerful diagnostic tool to evaluate the spatial distribution of DIC.

Calcium carbonate saturation states along the West Antarctic Peninsula

The waters along the West Antarctic Peninsula (WAP) have experienced warming and increased freshwater inputs from melting sea ice and glaciers in recent decades. Challenges exist in understanding the consequences of these changes on the inorganic carbon system in this ecologically important and highly productive ecosystem. Distributions of dissolved inorganic carbon (CT), total alkalinity (AT) and nutrients revealed key physical, biological and biogeochemical controls of the calcium carbonate saturation state (Ωaragonite) in different water masses across the WAP shelf during the summer. Biological production in spring and summer dominated changes in surface water Ωaragonite (ΔΩaragonite up to +1.39; ∼90%) relative to underlying Winter Water. Sea-ice and glacial meltwater constituted a minor source of AT that increased surface water Ωaragonite (ΔΩaragonite up to +0.07; ∼13%). Remineralization of organic matter and an influx of carbon-rich brines led to cross-shelf decreases in Ωaragonite in Winter Water and Circumpolar Deep Water. A strong biological carbon pump over the shelf created Ωaragonite oversaturation in surface waters and suppression of Ωaragonite in subsurface waters. Undersaturation of aragonite occurred at < ∼1000 m. Ongoing changes along the WAP will impact the biologically driven and meltwater-driven processes that influence the vulnerability of shelf waters to calcium carbonate undersaturation in the future

Winter-summer differences of carbon dioxide and oxygen in the Weddell Sea surface layer

Mid-winter total inorganic carbon (TCO2) and oxygen measurements are presented for the central fully ice-covered Weddell Sea. Lateral variations of these properties in the surface layer of the central Weddell Sea were small, but significant. These variations were caused by vertical transport of Warm Deep Water into the surface layer and air-sea exchange before the ice cover. Oxygen saturation in the surface layer of the central Weddell Sea was near 82%, whereas in the eastern shelf area this was 89%. Surprisingly, pCO2, as calculated under the assumption of (reported) conservativeness of alkalinity, was also found to be below saturation (86-93%). This was not expected since ongoing Warm Deep Water entrainment into the surface layer tends to increase the pCO2. Rapid cooling and subsequent ice formation during the previous autumn, however, might have brought about a sufficiently low undersaturation of CO2, that as to the point of sampling had not yet been replenished through Warm Deep Water entrainment.In the ensuing early summer the measurements were repeated. In the shelf area and the central Weddell Sea, where the ice-cover had almost disappeared, photosynthesis had caused a decrease of pCO2 and an increase of oxygen compared to the previous winter. Inbetween these two regions there was an area with significant ice-cover where essentially winter conditions prevailed.Based on the summer-winter difference a (late-winter) entrainment rate of Warm Deep Water into the surface layer of 4-5 m/month was calculated. A complete surface water balance, including entrainment, biological activity and air-sea exchange, showed that between the winter and summer cruises CO2 and oxygen had both been absorbed from the atmosphere. The TCO2 increase due to entrainment of Warm Deep Water was partly countered by (autumn) cooling, and partly through biological drawdown. Part of the CO2 removed through biological activity sinks down the water column as organic material and is remineralised at depth. It is well-known that bottom water formation constitutes a sink for atmospheric CO2. However, whether the Weddell Sea as a whole is a sink for CO2 depends on the ratio of two counteracting processes, i.e. entrainment, which increases CO2 in the surface and the biological pump, which decreases it. As deep water is not only entrained into the surface, but also conveyed out of the Weddell Sea, the relative importances of these (CO2-enriched) deep water transports are important as well

Mechanisms controlling the air-sea CO2 flux in the North Sea

The mechanisms driving the air–sea exchange of carbon dioxide (CO2CO2) in the North Sea are investigated using the three-dimensional coupled physical–biogeochemical model ECOHAM (ECOlogical-model, HAMburg). We validate our simulations using field data for the years 2001–2002 and identify the controls of the air–sea CO2CO2 flux for two locations representative for the North Sea's biogeochemical provinces. In the seasonally stratified northern region, net CO2CO2 uptake is high (View the MathML source2.06molm-2a-1) due to high net community production (NCP) in the surface water. Overflow production releasing semi-labile dissolved organic carbon needs to be considered for a realistic simulation of the low dissolved inorganic carbon (DIC) concentrations observed during summer. This biologically driven carbon drawdown outcompetes the temperature-driven rise in CO2CO2 partial pressure (pCO2pCO2) during the productive season. In contrast, the permanently mixed southern region is a weak net CO2CO2 source (View the MathML source0.78molm-2a-1). NCP is generally low except for the spring bloom because remineralization parallels primary production. Here, the pCO2pCO2 appears to be controlled by temperature

Ocean acidification and calcium carbonate saturation states in the coastal zone of the West Antarctic Peninsula

The polar oceans are particularly vulnerable to ocean acidification; the lowering of seawater pH and carbonate mineral saturation states due to uptake of atmospheric carbon dioxide (CO2). High spatial variability in surface water pH and saturation states (Ω) for two biologically-important calcium carbonate minerals calcite and aragonite was observed in Ryder Bay, in the coastal sea-ice zone of the West Antarctic Peninsula. Glacial meltwater and melting sea ice stratified the water column and facilitated the development of large phytoplankton blooms and subsequent strong uptake of atmospheric CO2 of up to 55 mmol m-2 day-1 during austral summer. Concurrent high pH (8.48) and calcium carbonate mineral supersaturation (Ωaragonite ~3.1) occurred in the meltwater-influenced surface ocean. Biologically-induced increases in calcium carbonate mineral saturation states counteracted any effects of carbonate ion dilution. Accumulation of CO2 through remineralisation of additional organic matter from productive coastal waters lowered the pH (7.84) and caused deep-water corrosivity (Ωaragonite ~0.9) in regions impacted by Circumpolar Deep Water. Episodic mixing events enabled CO2-rich subsurface water to become entrained into the surface and eroded seasonal stratification to lower surface water pH (8.21) and saturation states (Ωaragonite ~1.8) relative to all surface waters across Ryder Bay. Uptake of atmospheric CO2 of 28 mmol m-2 day-1 in regions of vertical mixing may enhance the susceptibility of the surface layer to future ocean acidification in dynamic coastal environments. Spatially-resolved studies are essential to elucidate the natural variability in carbonate chemistry in order to better understand and predict carbon cycling and the response of marine organisms to future ocean acidification in the Antarctic coastal zone

Impact of ocean acidification and high solar radiation on productivity and species composition of a late summer phytoplankton community of the coastal Western Antarctic Peninsula

The Western Antarctic Peninsula (WAP), one of the most productive regions of the Southern Ocean, is currently undergoing rapid environmental changes such as ocean acidification (OA) and increased daily irradiances from enhanced surface‐water stratification. To assess the potential for future biological CO2 sequestration of this region, we incubated a natural phytoplankton assemblage from Ryder Bay, WAP, under a range of pCO2 levels (180 μatm, 450 μatm, and 1000 μatm) combined with either moderate or high natural solar radiation (MSR: 124 μmol photons m−2 s−1 and HSR: 435 μmol photons m−2 s−1, respectively). The initial and final phytoplankton communities were numerically dominated by the prymnesiophyte Phaeocystis antarctica, with the single cells initially being predominant and solitary and colonial cells reaching similar high abundances by the end. Only when communities were grown under ambient pCO2 in conjunction with HSR did the small diatom Fragilariopsis pseudonana outcompete P. antarctica at the end of the experiment. Such positive light‐dependent growth response of the diatom was, however, dampened by OA. These changes in community composition were caused by an enhanced photosensitivity of diatoms, especially F. pseudonana, under OA and HSR, reducing thereby their competitiveness toward P. antarctica. Moreover, community primary production (PP) of all treatments yielded similar high rates at the start and the end of the experiment, but with the main contributors shifting from initially large to small cells toward the end. Even though community PP of Ryder Bay phytoplankton was insensitive to the changes in light and CO2 availability, the observed size‐dependent shift in productivity could, however, weaken the biological CO2 sequestration potential of this region in the future