Search

In this article, we analyze the impacts of climate change on Antarctic marine ecosystems. Observations demonstrate large-scale changes in the physical variables and circulation of the Southern Ocean driven by warming, stratospheric ozone depletion, and a positive Southern Annular Mode. Alterations in the physical environment are driving change through all levels of Antarctic marine food webs, which differ regionally. The distributions of key species, such as Antarctic krill, are also changing. Differential responses among predators reflect differences in species ecology. The impacts of climate change on Antarctic biodiversity will likely vary for different communities and depend on species range. Coastal communities and those of sub-Antarctic islands, especially range-restricted endemic communities, will likely suffer the greatest negative consequences of climate change. Simultaneously, ecosystem services in the Southern Ocean will likely increase. Such decoupling of ecosystem services and endemic species will require consideration in the management of human activities such as fishing in Antarctic marine ecosystems. Expected final online publication date for the Annual Review of Marine Science Volume 12 is January 3, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Macrozooplankton and its grazing pressure shape ecosystem structures and carbon pathways in the Southern Ocean. Here, we present the implementation of “polar macrozooplankton” as a plankton functional type and a related fast-sinking detritus class (fecal pellets) into the biogeochemical model REcoM-2. We use the model to assess major carbon pathways and ecosystem structure in the Southern Ocean south of 50°S. The model represents zooplankton biomass and its spatial distribution in the Southern Ocean reasonably well in comparison to available biomass data. A distinct difference of our model from previous versions is the seasonal pattern of particle formation processes and ecosystem structures in the Southern Ocean. REcoM-2 now captures high zooplankton biomass and a typical shift from a dominance of phytodetrital aggregates in spring to zooplankton fecal pellets later in the year. At sites with high biomass of macrozooplankton, the transfer efficiency of particulate organic carbon can be as high as 50%, and the carbon content of the exported material increases. In our simulations, macrozooplankton is an important component of the Southern Ocean plankton community, contributing up to 0.12 Pg C per year (14%) to total modeled carbon export across 100 m depth. Macrozooplankton changes the phytoplankton composition and supports the recycling of macronutrients. These results support the important role of macrozooplankton such as krill in the Southern Ocean and have implications for the representation of Southern Ocean biogeochemical cycles in models.

A quantitative analysis of any environment older than the instrumental record relies on proxies. Uncertainties associated with proxy reconstructions are often underestimated, which can lead to artificial conflict between different proxies, and between data and models. In this paper, using ordinary least squares linear regression as a common example, we describe a simple, robust and generalizable method for quantifying uncertainty in proxy reconstructions. We highlight the primary controls on the magnitude of uncertainty, and compare this simple estimate to equivalent estimates from Bayesian, nonparametric and fiducial statistical frameworks. We discuss when it may be possible to reduce uncertainties, and conclude that the unexplained variance in the calibration must always feature in the uncertainty in the reconstruction. This directs future research toward explaining as much of the variance in the calibration data as possible. We also advocate for a “data-forward” approach, that clearly decouples the presentation of proxy data from plausible environmental inferences.

Cell division of the coccolithophore Emiliania huxleyi and other phytoplankton typically becomes entrained to diel light/dark cycles under laboratory conditions, with division occurring primarily during dark phases and production occurring during light phases. Under these conditions, increases in cell and biomass concentrations deviate from exponential functions on time scales < 24 h. These deviations lead to significant diel variations in common measurements of phytoplankton physiology such as cellular quotas of particulate organic and inorganic carbon (POC, PIC) and their production rates. Being time-dependent, only the temporal mean of the various values during the day are comparable between experiments. Deviations from exponential growth furthermore imply that increases in cell and biomass concentrations cannot be expressed by the daily growth rate μ_{24 h} (typically determined from daily increments in cell concentrations). Consequently, conventional calculations of production as the product of a cellular quota (e.g., POC quota) and μ_{24 h} are mathematically incorrect. To account for this, we here describe short-term changes in cell and biomass concentrations of fastdividing, dilute-batch cultures of E. huxleyi grown under a diel light/dark cycle using linear regression. Based on the derived models, we present calculations for daily means of cellular quotas and production rates. Conventional (time-specific) measurements of cellular quotas and production differ from daily means by up to 65% in our example and, under some circumstances, cause false “effects” of treatments. Intending to reduce errors in ecophysiological studies, we recommend determining daily means—mathematically or by adjusting the experimental setup or sampling times appropriately.

The genesis of phytoplankton blooms and the fate of their biomass in iron-limited, high-nutrient−low-chlorophyll regions can be studied under natural conditions with ocean iron fertilization (OIF) experiments. The Indo-German OIF experiment LOHAFEX was carried out over 40 d in late summer 2009 within the cold core of a mesoscale eddy in the productive southwest Atlantic sector of the Southern Ocean. Silicate concentrations were very low, and phytoplankton biomass was dominated by autotrophic nanoflagellates (ANF) in the size range 3−10 μm. As in all previous OIF experiments, the phytoplankton responded to iron fertilization by increasing the maximum quantum yield (Fv/Fm) and cellular chlorophyll levels. Within 3 wk, chlorophyll levels tripled and ANF biomass doubled. With the exception of some diatoms and dinoflagellates, the biomass levels of all other groups of the phyto- and protozooplankton (heterotrophic nanoflagellates, dinoflagellates and ciliates) remained remarkably stable throughout the experiment both inside and outside the fertilized patch. We attribute the unusually high biomass attained and maintained by ANF to the absence of their grazers, the salps, and to constraints on protozooplankton grazers by heavy predation exerted by the large copepod stock. The resistance to change of the ecosystem structure over 38 d after fertilization, indicated by homogeneity at regional and temporal scales, suggests that it was locked into a stable, mature state that had evolved in the course of the seasonal cycle. The LOHAFEX bloom provides a case study of a resistant/robust dynamic equilibrium between auto- and heterotrophic ecosystem components resulting in low vertical flux both inside and outside the patch despite high biomass levels.

The LOHAFEX iron fertilization experiment was conducted for 39 days in the closed core of a cyclonic mesoscale eddy located along the Antarctic Polar Front in the Atlantic sector of the Southern Ocean. Mixed layer (ML) waters were characterized by high nitrate (~ 20 μM), low dissolved iron (DFe ~ 0.2 nM) and low silicate concentrations (below 1 μM) restricting diatom growth. Upon initial fertilization, chlorophyll-a doubled during the first two weeks and stabilized thereafter, despite a second fertilization on day 21, due to an increase in grazing pressure. Biomass at the different trophic levels was mostly comprised of small autotrophic flagellates, the large copepod Calanus simillimus and the amphipod Themisto gaudichaudii. The downward flux of particulate material comprised mainly copepod fecal pellets that were remineralized in the upper 150 m of the water column with no significant deeper export. DFe concentrations in the upper 200 m were not significantly affected by the two fertilizations but after day 14 showed a greater variability (ranging from 0.3 to 1.3 nM) without a clear vertical pattern. Particulate iron concentrations (measured after 2 months at pH 1.4) decreased with time and showed a vertical pattern that indicated an important non-biogenic component at the bottom of the mixed layer. In order to assess the contribution of copepod grazing to iron cycling we used two different approaches: first, we measured for the first time in a field experiment copepod fecal pellet concentrations in the water column together with the iron content per pellet, and second, we devised a novel analytical scheme based on a two-step leaching protocol to estimate the contribution of copepod fecal pellets to particulate iron in the water column. Analysis of the iron content of isolated fecal pellets from C. simillimus showed that after the second fertilization, the iron content per fecal pellet was ~ 5 fold higher if the copepod had been captured in fertilized waters. We defined a new fraction termed leachable iron (pH 2.0) in 48 h (LFe48h) that, for the conditions during LOHAFEX, was shown to be an excellent proxy for the concentration of iron contained in copepod fecal pellets. We observed that, as a result of the second fertilization, iron accumulated in copepod fecal pellets and remained high at one third of the total iron stock in the upper 80 m. We hypothesize that our observations are due to a combination of two biological processes. First, phagotrophy of iron colloids freshly formed after the second fertilization by the predominant flagellate community resulted in higher Fe:C ratios per cell that, via grazing, lead to iron enrichment in copepod fecal pellets in fertilized waters. Second, copepod coprophagy could explain the rapid recycling of particulate iron in the upper 100–150 m, the accumulation of LFe48h in the upper 80 m after the second fertilization and provided the iron required for the maintenance of the LOHAFEX bloom for many weeks. Our results provide the first quantitative evidence of the major ecological relevance of copepods and their fecal products in the cycling of iron in silicate depleted areas of the Southern Ocean., This work was supported by the Government of Spain (MINECO CTM2008-01864-E/ANT and CTM2014-59244-C3-3-R). LML was supported by the Campus de Excelencia Internacional Program (Ministerio de Educación, Cultura y Deportes, España).

A recent numerical cell model, which explains observed light and carbonate system effects on particulate organic and inorganic carbon (POC and PIC) production rates under the assumption of internal pH homeostasis, is extended for stable carbon isotopes (12C, 13C). Aim of the present study is to mechanistically understand the stable carbon isotopic fractionation signal (e) in POC and PIC and furthermore the vital effect(s) included in measured ePIC values. The virtual cell is divided into four compartments, for each of which the 12C as well as the 13C carbonate system kinetics are implemented. The compartments are connected to each other via trans-membrane fluxes. In contrast to existing carbon fractionation models, the presented model calculates the disequilibrium state for both carbonate systems and for each compartment. It furthermore calculates POC and PIC production rates as well as ePOC and ePIC as a function of given light conditions and the compositions of the external carbonate system. Measured POC and PIC production rates as well as ePIC values are reproduced well by the model (comparison with literature data). The observed light effect on ePOC (increase of ePOC with increasing light intensities), however, is not reproduced by the basic model set-up, which is solely based on RubisCO fractionation. When extending the latter set-up by assuming that biological fractionation includes further carbon fractionation steps besides the one of RubisCO, the observed light effect on ePOC is also reproduced. By means of the extended model version, four different vital effects that superimpose each other in a real cell can be detected. Finally, we discuss potential limitations of the ePIC proxy.

Measuring temperature and salinity profiles in the world's oceans is crucial to understanding ocean dynamics and its influence on the heat budget, the water cycle, the marine environment and on our climate. Since 1983 the German research vessel and icebreaker Polarstern has been the platform of numerous CTD (conductivity, temperature, depth instrument) deployments in the Arctic and the Antarctic. We report on a unique data collection spanning 33 years of polar CTD data. In total 131 data sets (1 data set per cruise leg) containing data from 10 063 CTD casts are now freely available at doi:10.1594/PANGAEA.860066. During this long period five CTD types with different characteristics and accuracies have been used. Therefore the instruments and processing procedures (sensor calibration, data validation, etc.) are described in detail. This compilation is special not only with regard to the quantity but also the quality of the data – the latter indicated for each data set using defined quality codes. The complete data collection includes a number of repeated sections for which the quality code can be used to investigate and evaluate long-term changes. Beginning with 2010, the salinity measurements presented here are of the highest quality possible in this field owing to the introduction of the OPTIMARE Precision Salinometer.

Antarctic Intermediate Water (AAIW) formation constitutes an important mechanism for the export of macronutrients out of the Southern Ocean that fuels primary production in low latitudes. We used quality-controlled gridded data from five hydrographic cruises between 1990 and 2014 to examine decadal variability in nutrients and dissolved inorganic carbon (DIC) in the AAIW (neutral density range 27 < γ n

In the rapidly changing Arctic Ocean, marine primary productivity is tightly linked to the balance between light and nutrient limitation. To capture this balance in ocean general circulation biogeochemical models (OGCBMs), a good representation of the physics is important due to the tight bio-physical coupling in the Arctic. Using a horizontal model resolution of a few kilometers makes it possible to resolve an increasing number of small scale processes, that otherwise need to be parameterized in OGCBMs. Such high resolution is, however, commonly not possible due to computational constrains. Utilizing an unstructured mesh approach, we have run the finite element sea-ice ocean model (FESOM 1.4) coupled to the biogeochemical model REcoM2 in a global configuration with an Arctic-wide resolution of 4.5 km. This resolution is so far unprecedented for a global biogeochemical setup, and here we present an analysis of the mean state of the model. FESOM-REcoM2’s integrated Arctic net primary production (NPP) averages 445 Tg C yr−1 for the years 2011 to 2015, a value that is in the middle of the range compared to estimates from the literature. Most production takes place in the inflow regions of the Nordic and Chukchi Seas, and 32% is associated with the sea ice zone, the latter including the marginal ice zone and below-ice productivity. Light limits production to some degree at all latitudes north of 60°N, with growth becoming nutrient limited following the initial spring bloom in most places. The model reproduces the relatively low surface concentration of dissolved inorganic nitrogen (DIN) that has been observed in the central Arctic Ocean, as well as the low surface DIN concentration towards the end of the growth season further to the south, thereby also capturing widespread subsurface chlorophyll maxima (SCM). The SCMs are found in the whole Arctic except for the areas where sea ice concentration is high the whole year. They have a duration of two weeks to five months. The balance between nutrient and light limitations, both in the vertical and horizontal direction, highlights that decreased light limitation in a future ice free Arctic Ocean will not necessarily induce an increase in NPP due to increasing nutrient limitation, and that further studies of the role of the SCMs are required.

Coccolithophores play an important role in the marine carbon cycle. Variations in light intensity and external carbonate system composition alter intracellular carbon fluxes and therewith the production rates of particulate organic and inorganic carbon. Aiming to find a mechanistic explanation for the interrelation between dissolved inorganic carbon fluxes and particulate carbon production rates, we develop a numerical cell model for Emiliania huxleyi, one of the most abundant coccolithophore species. The model consists of four cellular compartments, for each of which the carbonate system is resolved dynamically. The compartments are connected to each other and to the external medium via substrate fluxes across the compartment-confining membranes. By means of the model we are able to explain several pattern observed in particulate organic and inorganic carbon production rates for different strains and under different acclimation conditions. Particulate organic and inorganic carbon production rates for instance decrease at very low external CO2 concentrations. Our model suggests that this effect is caused mainly by reduced HCO3− uptake rates, not by CO2 limitation. The often observed decrease in particulate inorganic carbon production rates under Ocean Acidification is explained by a downregulation of cellular HCO3− uptake.

The rise in anthropogenic CO2 and the associated ocean acidification (OA) will change trace metal solubility and speciation, potentially altering Southern Ocean (SO) phytoplankton productivity and species composition. As iron (Fe) sources are important determinants of Fe bioavailability, we assessed the effect of Fe-laden dust versus inorganic Fe (FeCl3) enrichment under ambient and high pCO2 levels (390 and 900 μatm) in a naturally Fe-limited SO phytoplankton community. Despite similar Fe chemical speciation and net particulate organic carbon (POC) production rates, CO2-dependent species shifts were controlled by Fe sources. Final phytoplankton communities of both control and dust treatments were dominated by the same species, with an OA-dependent shift from the diatom Pseudo nitzschia prolongatoides towards the prymnesiophyte Phaeocystis antarctica. Addition of FeCl3 resulted in high abundances of Nitzschia lecointei and Chaetoceros neogracilis under ambient and high pCO2, respectively. These findings reveal that both the characterization of the phytoplankton community at the species level and the use of natural Fe sources are essential for a realistic projection of the biological carbon pump in the Fe-limited pelagic SO under OA. As dust deposition represents a more realistic scenario for the Fe-limited pelagic SO under OA, unaffected net POC production and dominance of P. antarctica can potentially weaken the export of carbon and silica in the future.

A dominant Antarctic ecological paradigm suggests that winter sea ice is generally the main feeding ground for krill larvae. Observations from our winter cruise to the southwest Atlantic sector of the Southern Ocean contradict this view and present the first evidence that the pack-ice zone is a food-poor habitat for larval development. In contrast, the more open marginal ice zone provides a more favourable food environment for high larval krill growth rates. We found that complex under-ice habitats are, however, vital for larval krill when water column productivity is limited by light, by providing structures that offer protec- tion from predators and to collect organic material released from the ice. The larvae feed on this sparse ice-associated food during the day. After sunset, they migrate into the water below the ice (upper 20 m) and drift away from the ice areas where they have previously fed. Model analyses indicate that this behaviour increases both food uptake in a patchy food environment and the likelihood of overwinter transport to areas where feeding conditions are more favourable in spring.

In coupled biogeochmical–ocean models, the choice of numerical schemes in the ocean circulation component can have a large influence on the distribution of the biological tracers. Biogeochemical models are traditionally coupled to ocean general circulation models (OGCMs), which are based on dynamical cores employing quasi-regular meshes, and therefore utilize limited spatial resolution in a global setting. An alternative approach is to use an unstructured-mesh ocean model, which allows variable mesh resolution. Here, we present initial results of a coupling between the Finite Element Sea Ice–Ocean Model (FESOM) and the biogeochemical model REcoM2 (Regulated Ecosystem Model 2), with special focus on the Southern Ocean. Surface fields of nutrients, chlorophyll a and net primary production (NPP) were compared to available data sets with a focus on spatial distribution and seasonal cycle. The model produces realistic spatial distributions, especially regarding NPP and chlorophyll a, whereas the iron concentration becomes too low in the Pacific Ocean. The modelled NPP is 32.5 Pg C yr−1 and the export production 6.1 Pg C yr−1, which is lower than satellite-based estimates, mainly due to excessive iron limitation in the Pacific along with too little coastal production. The model performs well in the Southern Ocean, though the assessment here is hindered by the lower availability of observations. The modelled NPP is 3.1 Pg C yr−1 in the Southern Ocean and the export production 1.1 Pg C yr−1. All in all, the combination of a circulation model on an unstructured grid with a biogeochemical–ocean model shows similar performance to other models at non-eddy-permitting resolution. It is well suited for studies of the Southern Ocean, but on the global scale deficiencies in the Pacific Ocean would have to be taken into account.

The δ13C value measured on benthic foraminiferal tests is widely used by palaeoceanographers to reconstruct the distribution of past water masses. The biogeochemical processes involved in forming the benthic foraminiferal δ13C signal (δ13Cforam), however, are not fully understood and a sound mechanistic description is still lacking. We use a reaction–diffusion model for calcification developed by Wolf-Gladrow et al. (1999) and Zeebe et al. (1999) in order to quantify the effects of different physical, chemical, and biological processes on δ13Cforam of an idealised benthic foraminiferal shell. Changes in the δ13C value of dissolved inorganic carbon (δ13CDIC) cause equal changes in δ13Cforam in the model. The results further indicate that temperature, respiration rate, and pH have a significant impact on δ13Cforam. In contrast, salinity, pressure, the δ13C value of particulate organic carbon (δ13CPOC), total alkalinity, and calcification rate show only a limited influence. In sensitivity experiments we assess how combining these effects can influence δ13Cforam. We can potentially explain 33 to 47% of the interglacial-to-glacial decrease in δ13Cforam by changes in temperature and pH, without invoking changes in δ13CDIC. Furthermore, about a quarter of the − 0.4‰ change in δ13Cforam observed in phytodetritus layers can be accounted for by an increase in respiration rate and a reduction in pH.

Ikaite (CaCO3·6H2O) has only recently been discovered in sea ice, in a study that also provided first direct evidence of CaCO3 precipitation in sea ice. However, little is as yet known about the impact of physico-chemical processes on ikaite precipitation in sea ice. Our study focused on how the changes in pH, salinity, temperature and phosphate (PO4) concentration affect the precipitation of ikaite. Experiments were set up at pH from 8.5 to 10.0, salinities from 0 to 105 (in both artificial seawater (ASW) and NaCl medium), temperatures from 0 to −4 °C andPO4 concentrations from0 to 50 μmol kg−1. The results show that in ASW, calcium carbonate was precipitated as ikaite under all conditions. In the NaCl medium, the precipitates were ikaite in the presence of PO4 and vaterite in the absence of PO4. The onset time (τ) at which ikaite precipitation started, decreased nonlinearly with increasing pH. In ASW, τ increased with salinity. In the NaCl medium, τ first increased with salinity up to salinity 70 and subsequently decreased with a further increase in salinity; it was longer in ASW than in the NaCl medium under the same salinity. τ did not vary with temperature or PO4 concentration. These results indicate that ikaite is very probably the only phase of calcium carbonate formed in sea ice. PO4 is not, as previously postulated, crucial for ikaite formation in sea ice. The change in pH and salinity is the controlling factor for ikaite precipitation in sea ice. Within the ranges investigated in this study, temperature and PO4 concentration do not have a significant impact on ikaite precipitation.

Stratospheric ozone depletion and emission of greenhouse gases lead to a trend of the Southern Annular Mode (SAM) towards its high-index polarity. The positive phase of the SAM is characterised by stronger than usual westerly winds that induce changes in the physical carbon transport. Changes in the natural carbon budget of the upper 100 m of the Southern Ocean in response to a positive SAM phase are explored with a coupled ecosystem-general circulation model and regression analysis. Previously overlooked processes that are important for the upper ocean carbon budget during a positive SAM period are identified, namely export production and downward transport of carbon north of the Polar Front (PF) as large as the upwelling in the south. The limiting micronutrient iron is brought into the surface layer by upwelling and stimulates phytoplankton growth and export production, but only in summer. This leads to a drawdown of carbon and less summertime outgassing (or more uptake) of natural CO2. In winter, biological mechanisms are inactive and the surface ocean equilibrates with the atmosphere by releasing CO2. In the annual mean, the upper ocean region south of the PF loses more carbon by additional export production than by the release of CO2 into the atmosphere, highlighting the role of the biological carbon pump in response to a positive SAM event.

Chemical weathering is an integral part of both the rock and carbon cycles and is being affected by changes in land use, particularly as a result of agricultural practices such as tilling, mineral fertilization, or liming to adjust soil pH. These human activities have already altered the terrestrial chemical cycles and land-ocean flux of major elements, although the extent remains difficult to quantify. When deployed on a grand scale, Enhanced Weathering (a form of mineral fertilization), the application of finely ground minerals over the land surface, could be used to remove CO2 from the atmosphere. The release of cations during the dissolution of such silicate minerals would convert dissolved CO2 to bicarbonate, increasing the alkalinity and pH of natural waters. Some products of mineral dissolution would precipitate in soils or be taken up by ecosystems, but a significant portion would be transported to the coastal zone and the open ocean, where the increase in alkalinity would partially counteract "ocean acidification" associated with the current marked increase in atmospheric CO2. Other elements released during this mineral dissolution, like Si, P, or K, could stimulate biological productivity, further helping to remove CO2 from the atmosphere. On land, the terrestrial carbon pool would likely increase in response to Enhanced Weathering in areas where ecosystem growth rates are currently limited by one of the nutrients that would be released during mineral dissolution. In the ocean, the biological carbon pumps (which export organic matter and CaCO3 to the deep ocean)may be altered by the resulting influx of nutrients and alkalinity to the ocean. This review merges current interdisciplinary knowledge about Enhanced Weathering, the processes involved, and the applicability aswell as some of the consequences and risks of applying the method.

Hunt J.R., George L. ... et al.-- 42 pages, 11 figures, We compare and contrast the ecological impacts of atmospheric and oceanic circulation patterns on polar and sub-polar marine ecosystems. Circulation patterns differ strikingly between the north and south. Meridional circulation in the north provides connections between the sub-Arctic and Arctic despite the presence of encircling continental landmasses, whereas annular circulation patterns in the south tend to isolate Antarctic surface waters from those in the north. These differences influence fundamental aspects of the polar ecosystems from the amount, thickness and duration of sea ice, to the types of organisms, and the ecology of zooplankton, fish, seabirds and marine mammals. Meridional flows in both the North Pacific and the North Atlantic oceans transport heat, nutrients, and plankton northward into the Chukchi Sea, the Barents Sea, and the seas off the west coast of Greenland. In the North Atlantic, the advected heat warms the waters of the southern Barents Sea and, with advected nutrients and plankton, supports immense biomasses of fish, seabirds and marine mammals. On the Pacific side of the Arctic, cold waters flowing northward across the northern Bering and Chukchi seas during winter and spring limit the ability of boreal fish species to take advantage of high seasonal production there. Southward flow of cold Arctic waters into sub-Arctic regions of the North Atlantic occurs mainly through Fram Strait with less through the Barents Sea and the Canadian Archipelago. In the Pacific, the transport of Arctic waters and plankton southward through Bering Strait is minimal. In the Southern Ocean, the Antarctic Circumpolar Current and its associated fronts are barriers to the southward dispersal of plankton and pelagic fishes from sub-Antarctic waters, with the consequent evolution of Antarctic zooplankton and fish species largely occurring in isolation from those to the north. The Antarctic Circumpolar Current also disperses biota throughout the Southern Ocean, and as a result, the biota tends to be similar within a given broad latitudinal band. South of the Southern Boundary of the ACC, there is a large-scale divergence that brings nutrient-rich water to the surface. This divergence, along with more localized upwelling regions and deep vertical convection in winter, generates elevated nutrient levels throughout the Antarctic at the end of austral winter. However, such elevated nutrient levels do not support elevated phytoplankton productivity through the entire Southern Ocean, as iron concentrations are rapidly removed to limiting levels by spring blooms in deep waters. However, coastal regions, with the upward mixing of iron, maintain greatly enhanced rates of production, especially in coastal polynyas. In these coastal areas, elevated primary production supports large biomasses of zooplankton, fish, seabirds, and mammals. As climate warming affects these advective processes and their heat content, there will likely be major changes in the distribution and abundance of polar biota, in particular the biota dependent on sea ice.

Carbon dioxide removal (CDR) approaches are efforts to reduce the atmospheric CO2 concentration. Here we use a marine carbon cycle model to investigate the effects of one CDR technique: the open ocean dissolution of the iron-containing mineral olivine. We analyse the maximum CDR potential of an annual dissolution of 3 Pg olivine during the 21st century and focus on the role of the micro-nutrient iron for the biological carbon pump. Distributing the products of olivine dissolution (bicarbonate, silicic acid, iron) uniformly in the global surface ocean has a maximum CDR potential of 0.57 gC/g-olivine mainly due to the alkalinisation of the ocean, with a significant contribution from the fertilisation of phytoplankton with silicic acid and iron. The part of the CDR caused by ocean fertilisation is not permanent, while the CO2 sequestered by alkalinisation would be stored in the ocean as long as alkalinity is not removed from the system. For high CO2 emission scenarios the CDR potential due to the alkalinity input becomes more efficient over time with increasing ocean acidification. The alkalinity-induced CDR potential scales linearly with the amount of olivine, while the iron-induced CDR saturates at 113 PgC per century (on average ~1.1 PgC yr−1) for an iron input rate of 2.3 Tg Fe yr−1 (1% of the iron contained in 3 Pg olivine). The additional iron-related CO2 uptake occurs in the Southern Ocean and in the iron-limited regions of the Pacific. Effects of this approach on surface ocean pH are small (< 0.01).

The Southern Ocean supports diverse and unique ecosystems that have been impacted by more than two centuries of exploitation and are now experiencing rapid changes in ocean temperature and seasonal ice cover due to climate warming. Understanding and projecting responses of Southern Ocean marine ecosystems to changing climate conditions and direct human impacts, such as fisheries, requires integrated ecosystem analyses at scales previously unexplored. Here we consider the main ecological and modelling challenges in predicting the responses of Southern Ocean ecosystems to change, and propose three interlinked focus areas that will advance the development of integrated models for Southern Ocean ecosystems. The first focus area is development of fundamental understanding of the factors that determine the structure and function of the food webs at multiple scales. Ecological research in the Southern Ocean is often centred on key species or localised systems, a tendency which is reflected in existing food web and ecosystem models. To build on this, a systematic analysis of regional food web structure and function is required. The second focus area is development of a range of mechanistic models that vary in their resolution of ecological processes, and consider links across physical scales, biogeochemical cycles and feedbacks, and the central role of zooplankton. These two focus areas underlie the third, which is development of methodologies for scenario testing across a range of trophic levels of the effects of past and future changes, which will facilitate consideration of the underlying complexity of interactions and the associated uncertainty. The complex nature of interactions determining Southern Ocean ecosystem structure and function will require new approaches, which we propose should be developed within a scale-based framework that emphasises both physical and ecological aspects.

The growing field of ocean acidification research is concerned with the investigation of organism responses to increasing pCO2 values. One important approach in this context is culture work using seawater with adjusted CO2 levels. As aqueous pCO2 is difficult to measure directly in small-scale experiments, it is generally calculated from two other measured parameters of the carbonate system (often AT, CT or pH). Unfortunately, the overall uncertainties of measured and subsequently calculated values are often unknown. Especially under high pCO2, this can become a severe problem with respect to the interpretation of physiological and ecological data. In the few datasets from ocean acidification research where all three of these parameters were measured, pCO2 values calculated from AT and CT are typically about 30% lower (i.e. ~300 μatm at a target pCO2 of 1000 μatm) than those calculated from AT and pH or CT and pH. This study presents and discusses these discrepancies as well as likely consequences for the ocean acidification community. Until this problem is solved, one has to consider that calculated parameters of the carbonate system (e.g. pCO2, calcite saturation state) may not be comparable between studies, and that this may have important implications for the interpretation of CO2 perturbation experiments.

We analyzed 214 new core-top samples for their CaCO3 content from shelves all around Antarctica in order to understand their distribution and contribution to the marine carbon cycle. The distribution of sedimentary CaCO3 on the Antarctic shelves is connected to environmental parameters where we considered water depth, width of the shelf, sea-ice coverage and primary production. While CaCO3 contents of surface sediments are usually low, high (> 15%) CaCO3 contents occur at shallow water depths (150–200 m) on the narrow shelves of the eastern Weddell Sea and at a depth range of 600–900 m on the broader and deeper shelves of the Amundsen, Bellingshausen and western Weddell Seas. Regions with high primary production, such as the Ross Sea and the western Antarctic Peninsula region, have generally low CaCO3 contents in the surface sediments. The predominant mineral phase of CaCO3 on the Antarctic shelves is low-magnesium calcite. With respect to ocean acidification, our findings suggest that dissolution of carbonates in Antarctic shelf sediments may be an important negative feedback only after the onset of calcite undersaturation on the Antarctic shelves. Macrozoobenthic CaCO3 standing stocks do not increase the CaCO3 budget significantly as they are two orders of magnitude lower than the budget of the sediments. This first circumpolar compilation of Antarctic shelf carbonate data does not claim to be complete. Future studies are encouraged and needed to fill data gaps especially in the under-sampled southwest Pacific and Indian Ocean sectors of the Southern Ocean.

We introduce a novel, simple method to measure sinking velocity of particles and aggregates in roller tanks. Using this noninvasive method, it is possible to follow changes in sinking velocities on the same aggregates during time and to make paired measurements of aggregate sinking velocity and composition. Particles and aggregates are video recorded in roller tanks, and their sinking velocity is derived from the orbital trajectories. This new method is compared with three other methods (using roller tanks, a vertical flow system, and a sedimentation column), which have not previously been inter-calibrated. Agar spheres and diatom aggregates were used as model particles in all experimental systems. No method showed significantly different sinking velocities of agar spheres compared with those calculated by theory. Paired measurements showed that sinking velocities from 70 to 700 m d(-1) were linearly correlated between different methods. Highest sinking velocities were measured in a sedimentation column followed by those measured in roller tanks and in the vertical flow system, respectively. The average difference of sinking velocity measured with the different methods ranged from 8% to 11% for agar spheres, and up to 20% for diatom aggregates.

article i nfo C fractionation CA C4 photosynthesis PEPC RubisCO The effect of pCO2 on carbon acquisition and intracellular assimilation was investigated in the three bloom- forming diatom species, Eucampia zodiacus (Ehrenberg), Skeletonema costatum (Greville) Cleve, Thalassio- nema nitzschioides (Grunow) Mereschkowsky and the non-bloom-forming Thalassiosira pseudonana (Hust.) Hasle and Heimdal. In vivo activities of carbonic anhydrase (CA), photosynthetic O2 evolution, CO2 and HCO3 − uptake rates were measured by membrane-inlet mass spectrometry (MIMS) in cells acclimated to pCO2 levels of 370 and 800 μatm. To investigate whether the cells operate a C4-like pathway, activities of ribulose- 1,5-bisphosphate carboxylase (RubisCO) and phosphoenolpyruvate carboxylase (PEPC) were measured at the mentioned pCO2 levels and a lower pCO2 level of 50 μatm. In the bloom-forming species, extracellular CA activities strongly increased with decreasing CO2 supply while constantly low activities were obtained for T. pseudonana. Half-saturation concentrations (K1/2) for photosynthetic O2 evolution decreased with decreasing CO2 supply in the two bloom-forming species S. costatum and T. nitzschioides, but not in T. pseudonana and E. zodiacus. With the exception of S. costatum, maximum rates (Vmax) of photosynthesis remained constant in all investigated diatom species. Independent of the pCO2 level, PEPC activities were significantly lower than those for RubisCO, averaging generally less than 3%. All examined diatom species operate highly efficient CCMs under ambient and high pCO2, but differ strongly in the degree of regulation of individual components of the CCM such as Ci uptake kinetics and extracellular CA activities. The present data do not suggest C4 metabolism in the investigated species.

Despite their microscopic size, marine phytoplankton are responsible for about half of the global primary production and represent the basis of the marine food web. This diverse group of organisms drives important biogeochemical cycles, exporting massive amounts of carbon to deep waters and sediments, and strongly influencing ocean–atmosphere gas exchanges. Anthropogenic climate change will result in significant alterations in the marine environment over the next 100 yr and beyond. The increase in atmospheric CO2 has already caused significantly higher aquatic CO2 concentrations and lower pH values (‘ocean acidification’) than in pre-industrial times. Rising temperatures will also impact surface ocean stratification, which in turn will affect the surface-water light regime and nutrient input from deeper layers. Phytoplankton will be affected by these environmental changes in many ways. In this article we assess the possible responses of different phytoplankton groups with regard to the expected physico-chemical changes. In addition to summarizing laboratory and field studies, we outline the current understanding of the underlying mechanisms that cause processes such as photosynthesis, calcification, and nitrogen fixation to be sensitive to ocean acidification. We describe different approaches to manipulate carbonate chemistry (e.g. acid/base or CO2 addition), discuss their potential to simulate future ocean acidification, and allude to common problems in experiments caused, for instance, by high biomass or the use of buffers. In addition to guidelines for CO2 perturbation experiments, we argue that it is essential to look at multiple environmental factors in combination with CO2, to aim for process-understanding rather than correlation, and to assess a wider diversity of phytoplankton species both in laboratory and field studies.

A biochemical model of Antarctic krill Euphausia superba was developed to investigate the physiological mechanisms which enable krill to survive winter, when food is scarce. In this mod- eling approach data sets on the biochemical composition of krill and its food sources are combined into a model that takes food quality into account rather than just food availability during different seasons. Krill is defined in terms of protein, neutral lipid, polar lipid, carbohydrate, chitin, and ash content, and the model tracks krill neutral lipid content separately from weight. The model includes parameterizations of filtration, ingestion, and metabolic processes which determine krill growth rate. Initial ratios of protein, neutral lipid, polar lipid, carbohydrate and ash change in chitin response to the biochemical composition of food as krill grows. Model results show that a diet of phytoplankton food alone may be sufficient for krill to grow to observed sizes but may not be sufficient to provide the summer lipid resources that are observed in the field and that are necessary for krill to reproduce and survive winter. The inclusion of sea ice algae as an additional food is beneficial for krill at the end of winter but does not significantly change summer krill lipid content. However, the amount of lipids accumulated within krill increases significantly when krill feeds on lipid-rich heterotrophic food, which points to the importance of carnivory, even in times when phytoplankton food is available. The strategy of combusting body components to produce energy (shrinking) is found to provide the greatest source of energy for krill of all sizes during times of prolonged starvation.

This paper primarily reviews our recent work on the biology and geochemistry of foraminifera with respect to their use as paleoceanographic proxies. Our approach to proxy validation and development is described, primarily from a modeler’s point of view. The approach is based on complementary steps in understanding the inorganic chemistry, inorganic isotope fractionation, and biological controls that determine paleo-tracer signals in organisms used in climate reconstructions. Integration of laboratory experiments, field and culture studies, theoretical considerations, and numerical modeling holds the key to the method’s success. We describe effects of life-processes in foraminifera on stable carbon, oxygen, and boron isotopes as well as Mg incorporation into foraminiferal calcite shells. Stable boron isotopes will be used to illustrate our approach. We show that a mechanism-based understanding is often required before primary climate signals can be extracted from the geologic record because the signals can be heavily overprinted by secondary, non-climate related phenomena. Moreover, for some of the proxies, fundamental knowledge on the thermodynamic, inorganic basis is still lacking. One example is stable boron isotopes, a paleo-pH proxy, for which the boron isotope fractionation between the dissolved boron compounds in seawater was not precisely known until recently. Attempts to overcome such hurdles are described and implications of our work for paleoceanographic reconstructions are discussed.

Coccolithophores play a crucial role in the marine carbon cycle and thus it is interesting to know how they will respond to climate change. After several decades of research the interplay between intracellular processes and the marine carbonate system is still not well understood. On the basis of experimental findings given in literature, a numerical cell model is developed that describes inorganic carbon fluxes between seawater and the intracellular sites of calcite precipitation and photosynthetic carbon fixation. The implemented cell model consists of four compartments, for each of which the carbonate system is resolved individually. The four compartments are connected to each other via H+, CO2, and View the MathML sourceHCO3− fluxes across the compartment-confining membranes. For CO2 accumulation around RubisCO, an energy-efficient carbon concentrating mechanism is proposed that relies on diffusive CO2 uptake. At low external CO2 concentrations and high light intensities, CO2 diffusion does not suffice to cover the carbon demand of photosynthesis and an additional uptake of external View the MathML sourceHCO3− becomes essential. The model is constrained by data of Emiliania huxleyi, the numerically most abundant coccolithophore species in the present-day ocean.

Phytoplankton blooms in surface waters of the oceans are known to influence the food web and impact microbial as well as zooplankton communities. Numerous studies have investigated the fate of phytoplankton-derived organic matter in surface waters and shelf sediments, however, little is known about the effect of sinking algal biomass on microbial communities in deep-sea sediments. Here, we analyzed sediments of four regions in the Southern Atlantic Ocean along the Antarctic Polar Front that had different exposures to phytoplankton bloom derived organic matter. We investigated the microbial communities in these sediments using high-throughput sequencing of 16S rRNA molecules to determine microorganisms that were active and catalyzed reporter deposition fluorescence in situ hybridization to infer their abundance and distribution. The sediments along the Antarctic Polar Front harbored microbial communities that were highly diverse and contained microbial clades that seem to preferably occur in regions of high primary productivity. We showed that organisms affiliated with the gammaproteobacterial clade NOR5/OM60, which is known from surface waters and coastal sediments, thrive in the deep-sea. Benthic deep-sea NOR5 were abundant, diverse, distinct from pelagic NOR5 and likely specialized on the degradation of phytoplankton-derived organic matter, occupying a similar niche as their pelagic relatives. Algal detritus seemed to not only fuel the benthic microbial communities of large areas in the deep-sea, but also to influence communities locally, as we found a peak in Flavobacteriaceae-related clades that also include degraders of algal biomass. The results strongly suggest that phytoplankton-derived organic matter was rapidly exported to the deep-sea, nourished distinct benthic microbial communities and seemed to be the main energy source for microbial life in the seafloor of vast abyssal regions along the Antarctic Polar Front.

Ikaite (CaCO3�6H2O) has recently been discovered in sea ice, providing first direct evidence of CaCO3 precipitation in sea ice. However, the impact of ikaite precipitation on phosphate (PO4) concentration has not been considered so far. Experiments were set up at pH from 8.5 to 10.0, salinities from 0 to 105, temperatures from 24°C to 0°C, and PO4 concentrations from 5 to 50 mmol kg-1 in artificial sea ice brine so as to understand how ikaite precipitation affects the PO4 concentration in sea ice under different conditions. Our results show that PO4 is coprecipitated with ikaite under all experimental conditions. The amount of PO4 removed by ikaite precipitation increases with increasing pH. Changes in salinity (S >=35) as well as temperature have little impact on PO4 removal by ikaite precipitation. The initial PO4 concentration affects the PO4 coprecipitation. These findings may shed some light on the observed variability of PO4 concentration in sea ice.

The results of an existing one-dimensional diagnostic model that calculates the fugacity of CO 2 (fCO 2 ) in the surface layer of the southern ocean were compared with in situ observations from different ocean sectors and seasons. Our model is based on the translation of monthly variations of constraints fields into surface water fCO 2 variations, and was used to assess the CO 2 uptake of the southern ocean. In situ observations are useful to verify the model results and were here applied to improve the estimation of the CO 2 uptake of the southern ocean south of 50 S. The model reproduces the fCO 2 distribution in both Pacific and Indian sectors of the southern ocean satisfactorily, the mean deviation being only 5 μatm. This diserepancy requires only a minor modification of the CO 2 uptake calculated by the model for that area. By contrast, the model strongly underestimates the fCO 2 levels in carly spring and early winter in the Weddell gyre. This indicates that the CO 2 uptake by the Atlantic sector of the southern ocean as calculated by the model, amounting to 0.47 GtC yr -1 . should be reduced, possibly by about half of this value. The reason for this mismatch lies in the use of climatological physical constraints by the model, that do not sufficiently well describe reality. Partly, the mismatch is also caused by a difference of seasonal stage between the model which reflects climatological conditions and the real ocean which is affected by interannual variability. Based on this study it is concluded that the CO 2 uptake of the southern ocean south of 50 S is likely to lie somewhere between 0.6 and 0.7 GtC yr -1 for the 1990s, which is a high value compared to estimates from other investigations.

Foraminifera are the most important source of information for oceanographic and climatic reconstruction on glacial–interglacial as well as on much longer time-scales. The information is contained in the chemical composition, especially the isotopic ratios, of the calcitic shells (e.g., δ 11 B , δ 13 C , δ 18 O ). Based on the assumption that our understanding of the major parameters controlling stable isotope incorporation is complete, these geochemical proxies have been used to reconstruct glacial ice volumes, sea surface and deep water temperatures, ocean circulation changes and shifts between carbon reservoirs. However, recent laboratory experiments have demonstrated that the δ 13 C and δ 18 O are not only strongly dependent on the carbonate chemistry of the culture medium but that the so-called `vital-effects' are probably mediated via perturbations of the local carbonate system. These findings have an important impact on the interpretation of isotope data. For instance, the carbonate system of the glacial ocean was quite different from that of the Holocene and since the onset of the industrial revolution the carbonate chemistry of the surface water must have changed drastically. As a first step towards a better understanding of the isotopic fractionation processes we present results of a diffusion-reaction model of the carbonate system (CO2, HCO3−, CO32−, H+, OH−, B(OH)3, B(OH)4−) in the microenvironment (the diffusive boundary layer) of living planktic foraminifera. The carbon fluxes associated with the main life processes (calcification, respiration and symbiont photosynthesis) lead to substantial perturbations in pH and significant shifts in the concentrations of CO2, CO32− and other components in the vicinity of the foraminifer. Consequently, the carbonate chemistry of the ambient environment is quite different from that of the bulk sea water. Comparison with pH-microelectrode measurements confirm our numerical results. Our results further demonstrate that the symbionts must use bicarbonate as an additional carbon source for photosynthesis as the calculated CO2 fluxes are not sufficient to support measured rates of oxygen evolution. The simulations also show that for the fast calcification of Globigerinoides sacculifer the supply of carbonate ions is insufficient and therefore use of bicarbonate or an internal pool for carbon is required, whereas no such pool is necessary for the much slower calcification in Orbulina universa.

International audience; Antarctic and Southern Ocean (ASO) marine ecosystems have been changing for at least the last 30 years, including in response to increasing ocean temperatures and changes in the extent and seasonality of sea-ice; the magnitude and direction of these changes differ between regions around Antarctica that could see populations of the same species changing differently in different regions. This paper reviews current and expected changes in ASO physical habitats in response to climate change. It then reviews how these changes may impact the autecology of marine biota of this polar region: microbes, zooplankton, salps, Antarctic krill, fish, cephalopods, marine mammals, seabirds, and benthos. The general prognosis for ASO marine habitats is for an overall warming and freshening, strengthening of westerly winds, with a potential pole-ward movement of those winds and the frontal systems, and an increase in ocean eddy activity. Many habitat parameters will have regionally specific changes, particularly relating to sea-ice characteristics and seasonal dynamics. Lower trophic levels are expected to move south as the ocean conditions in which they are currently found move pole-ward. For Antarctic krill and finfish, the latitudinal breadth of their range will depend on their tolerance of warming oceans and changes to productivity. Ocean acidification is a concern not only for calcifying organisms, but also for crustaceans such as Antarctic krill; it is also likely to be the most important change in benthic habitats over the coming century. For marine mammals and birds, the expected changes primarily relate to their flexibility in moving to alternative locations for food and the energetic cost of longer or more complex foraging trips for those that are bound to breeding colonies. Few species are sufficiently well studied to make comprehensive species-specific vulnerability assessments possible. Priorities for future work are discussed. This article is protected by copyright. All rights reserved.

Over the last four decades, different hypotheses of Ca(2+) and dissolved inorganic carbon transport to the intracellular site of calcite precipitation have been put forth for Emiliania huxleyi (Lohmann) HayMohler. The objective of this study was to assess these hypotheses by means of mathematical models. It is shown that a vesicle-based Ca(2+) transport would require very high intravesicular Ca(2+) concentrations, high vesicle fusion frequencies as well as a fast membrane recycling inside the cell. Furthermore, a kinetic model for the calcification compartment is presented that describes the internal chemical environment in terms of carbonate chemistry including calcite precipitation. Substrates for calcite precipitation are transported with different stoichiometries across the compartment membrane. As a result, the carbonate chemistry inside the compartment changes and hence influences the calcification rate. Moreover, the effect of carbonic anhydrase (CA) activity within the compartment is analyzed. One very promising model version is based on a Ca(2+) /H(+) antiport, CO2 diffusion, and a CA inside the calcification compartment. Another promising model version is based on an import of Ca(2+) and HCO3 (-) and an export of H(+) .

[1] We combined data sets of measured sedimentary calcium carbonate (CaCO3) and satellite-derived pelagic primary production to parameterize the relation between CaCO3 content on the Antarctic shelves and primary production in the overlying water column. CaCO3 content predicted in this way was in good agreement with the measured data. The parameterization was then used to chart CaCO3 content on the Antarctic shelves all around the Antarctic, using the satellite-derived primary production. The total inventory of CaCO3 in the bioturbated layer of Antarctic shelf sediments was estimated to be 0.5 Pg C. This quantity is comparable to the total CO2 uptake by the Southern Ocean in only one to a few years (dependent on the uptake estimate and area considered), indicating that the dissolution of these carbonates will neither delay ocean acidification in this area nor augment the Southern Ocean CO2 uptake capacity.

Assmy, Philipp ... et. al.-- 6 pages, 4 figures, Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon- sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux, P.A. was supported through Deutsche Forschungsgemeinschaft–Cluster of Excellence “The Ocean in the Earth System” and the Centre for Ice, Climate and Ecosystems at the Norwegian Polar Institute

A predictive model of carbon isotope fractionation (sigma p) and abundance (delta13C phyto) is presented under circumstances where photosynthesis is strictly based on CO2(aq) that passively diffuses into marine phytoplankton cells. Similar to other recent models, the one presented here is based on a formulation where the expression of intracellular enzymatic isotope fractionation relative to that imposed by CO2(aq) transport is scaled by the ratio of intracellular to external [CO2(aq)], ci/ce. Unlike previous models, an explicit calculation of ci is made that is dependent on ce as well as cell radius, cell growth rate, cell membrane permeability to CO2(aq), temperature, and, to a limited extent, pH and salinity. This allows direct scaling of ci/ce to each of these factors, and thus a direct prediction of sigma p and delta13C phyto responses to changes in each of these variables. These responses are described, and, where possible, compared to recent experimental and previous modeling results.

Schuiling et al. (1) question our conclusion (2) that the annual dissolution rate of olivine is limited by the saturation of waters with silicic acid (H4SiO4), which is one product of the dissolution reaction of olivine. In support of this point they discuss findings of CO2 sequestration in a mine in Yukon, Canada, claiming that a minimum of 1,700 g C m−2 y−1 was sequestered between 1978 and 2004 by silicate weathering and precipitation of (mainly) magnesium carbonates (3). This value is approximately …

We studied the evolution of the isotopic composition of dissolved Si (d30SiDSi) and biogenic Si (d30SiBSi) during a diatom bloom induced by an iron fertilization experiment in a Southern Ocean (SO) eddy. Dissolved Si (DSi) consumption was clearly boosted as a result of iron addition. We estimate an apparent Si isotopic fractionation factor of -1.36 parts per thousand +/- 0.11 parts per thousand, not significantly different from estimates based on in vitro incubations under Fe-replete conditions and in agreement with previous SO studies under Fe-depleted conditions. The temporal variations of the d30Si are best reconstructed considering the fertilized surface waters as an open system, i.e., DSi is continuously supplied from the winter water. Using vertical diffusivity, we estimate a supply of 6.7 +/- 1.2 mmol Si m-2 d-1 representing 30% +/- 5% of total DSi demand. We estimate biogenic silica (BSi) production at 22.1 +/- 1.8 mmol Si m-2 d-1, similar to BSi production under natural conditions in the SO. The DSi use preceding the start of the fertilization was conservatively estimated at 49% +/- 4%, and as a result of the Fe-fertilization, a further 31% +/- 4% of the initial DSi reservoir was consumed. The BSi export was estimated to be 17.4 +/- 1.7 mmol Si m-2 d-1. Our results suggest that for this study three main processes were acting significantly on d30Si signatures, biological uptake, Si supply from subsurface, and export of BSi, and that the magnitude of production and export in such a Fe-fertilized experiment are similar to those for natural diatom blooms occurring in the Southern Ocean.

The numerical modelling community is an important user group of ocean observations requiring data of global coverage for model parameterisation and evaluation. Dynamic Green Ocean Models (DGOMs) are a class of ocean biogeochemistry models that represent various types of plankton with distinct functions in food webs and biogeochemical cycles. DGOMs are used to study the feedbacks between climate and ocean biogeochemistry, particularly those mediated by ecosystem dynamics that influence CO2, DMS, and N2O fluxes to and from the atmosphere. DGOMs require experimental data for the parameterization of plankton growth and loss rates and of ecological interactions, and a range of observations for their evaluation. The most urgent data needs are: (1) decadal trends in surface ocean pCO2 and subsurface O2, (2) biomass (in carbon concentration) and (3) growth rates as a function of temperature for the important plankton types, and (4) sinking flux of particulate organic carbon. A global coverage is essential to evaluate the model mean state. Repeated measurements for all seasons are most useful to evaluate the model response to environmental change. These data can be obtained by a combination of platforms, including remote sensing, repeat sections and gliders, and oceanic and atmospheric time-series stations.

Seawater carbonate chemistry is typically calculated from two measured parameters. Depending on the choice of these input parameters, discrepancies in calculated pCO2 have been recognized by marine chemists, but the significance of this phenomenon for CO2 perturbation experiments has so far not been determined. To mimic different pCO2 scenarios, two common perturbation methods for seawater carbonate chemistry (changing either DIC or TA) were applied using state-of-the-art protocols and equipment. The carbonate system was over-constrained by measuring DIC, TA, pH, and pCO2. Calculated pCO2 matched measured pCO2 if pH and TA or pH and DIC were chosen as input parameters, whereas pCO2 calculated from TA and DIC was considerably lower than measured values. This has important implications for CO2 perturbation experiments. First, calculated pCO2 values may not be comparable if different input parameters were used. Second, responses of organisms to acidification may be overestimated when using TA and DIC for calculations. This is especially troublesome for experiments with calcifiers, as carbonate ion concentration and thus calcite or aragonite saturation state are overestimated. We suggest refraining from measuring TA and DIC only and rather include pH as input parameter for carbonate chemistry calculations.

In this laboratory study, we monitored the buildup of biomass and concomitant shift in seawater carbonate chemistry over the course of a Trichodesmium bloom under different phosphorus (P) availability. During exponential growth, dissolved inorganic carbon (DIC) decreased, while pH increased until maximum cell densities were reached. Once P became depleted, DIC decreased even further and total alkalinity (TA) dropped, accompanied by precipitation of aragonite. Under P-replete conditions, DIC increased and TA remained constant in the postbloom phase. A diffusion-reaction model was employed to estimate changes in carbonate chemistry of the diffusive boundary layer. This study demonstrates that Trichodesmium can induce precipitation of aragonite from seawater and further provides possible explanations about underlying mechanisms.

Geoengineering is a proposed action to manipulate Earth’s climate in order to counteract global warming from anthropogenic greenhouse gas emissions. We investigate the potential of a specific geoengineering technique, carbon sequestration by artificially enhanced silicate weathering via the dissolution of olivine. This approach would not only operate against rising temperatures but would also oppose ocean acidification, because it influences the global climate via the carbon cycle. If important details of the marine chemistry are taken into consideration, a new mass ratio of CO 2 sequestration per olivine dissolution of about 1 is achieved, 20% smaller than previously assumed. We calculate that this approach has the potential to sequestrate up to 1 Pg of C per year directly, if olivine is distributed as fine powder over land areas of the humid tropics, but this rate is limited by the saturation concentration of silicic acid. In our calculations for the Amazon and Congo river catchments, a maximum annual dissolution of 1.8 and 0.4 Pg of olivine seems possible, corresponding to the sequestration of 0.5 and 0.1 Pg of C per year, but these upper limit sequestration rates come at the environmental cost of p H values in the rivers rising to 8.2. Open water dissolution of fine-grained olivine and an enhancement of the biological pump by the rising riverine input of silicic acid might increase our estimate of the carbon sequestration, but additional research is needed here. We finally calculate with a carbon cycle model the consequences of sequestration rates of 1–5 Pg of C per year for the 21st century by this technique.

A one-dimensional model of Fe speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-Series Observatory (TENATSO) site. Among diverse processes affecting Fe speciation, this study is focusing on investigating the role of dust particles in removing dissolved iron (DFe) by a more complex description of particle aggregation and sinking, and explaining the abundance of organic Fe-binding ligands by modelling their origin and fate. The vertical distribution of different particle classes in the model shows high sensitivity to changing aggregation rates. Using the aggregation rates from the sensitivity study in this work, modelled particle fluxes are close to observations, with dust particles dominating near the surface and aggregates deeper in the water column. POC export at 1000 m is a little higher than regional sediment trap measurements, suggesting further improvement of modelling particle aggregation, sinking or remineralisation. Modelled strong ligands have a high abundance near the surface and decline rapidly below the deep chlorophyll maximum, showing qualitative similarity to observations. Without production of strong ligands, phytoplankton concentration falls to 0 within the first 2 years in the model integration, caused by strong Fe-limitation. A nudging of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profiles, assuming a decay time of 7 years for weak ligands. This indicates that weak ligands have a longer decay time and therefore cannot be modelled adequately in a one-dimensional model. The modelled DFe profile is strongly influenced by particle concentration and vertical distribution, because the most important removal of DFe in deeper waters is colloid formation and aggregation. Redissolution of particulate iron is required to reproduce an observed DFe profile at TENATSO site. Assuming colloidal iron is mainly composed of inorganic colloids, the modelled colloidal to soluble iron ratio is lower that observations, indicating the importance of organic colloids.

[1] We report on the discovery of the mineral ikaite (CaCO3·6H2O) in sea-ice from the Southern Ocean. The precipitation of CaCO3 during the freezing of seawater has previously been predicted from thermodynamic modelling, indirect measurements, and has been documented in artificial sea ice during laboratory experiments but has not been reported for natural sea-ice. It is assumed that CaCO3 formation in sea ice may be important for a sea ice-driven carbon pump in ice-covered oceanic waters. Without direct evidence of CaCO3 precipitation in sea ice, its role in this and other processes has remained speculative. The discovery of CaCO3·6H2O crystals in natural sea ice provides the necessary evidence for the evaluation of previous assumptions and lays the foundation for further studies to help elucidate the role of ikaite in the carbon cycle of the seasonally sea ice-covered regions

During phytoplankton growth a fraction of dissolved inorganic carbon (DIC) assimilated by phytoplankton is exuded in the form of dissolved organic carbon (DOC), which can be transformed into extracellular particulate organic carbon (POC). A major fraction of extracellular POC is associated with carbon of transparent exopolymer particles (TEP; carbon content = TEPC) that form from dissolved polysaccharides (PCHO). The exudation of PCHO is linked to an excessive uptake of DIC that is not directly quantifiable from utilisation of dissolved inorganic nitrogen (DIN), called carbon overconsumption. Given these conditions, the concept of assuming a constant stoichiometric carbon-to-nitrogen (C:N) ratio for estimating new production of POC from DIN uptake becomes inappropriate. Here, a model of carbon overconsumption is analysed, combining phytoplankton growth with TEPC formation. The model describes two modes of carbon overconsumption. The first mode is associated with DOC exudation during phytoplankton biomass accumulation. The second mode is decoupled from algal growth, but leads to a continuous rise in POC while particulate organic nitrogen (PON) remains constant. While including PCHO coagulation, the model goes beyond a purely physiological explanation of building up carbon rich particulate organic matter (POM). The model is validated against observations from a mesocosm study. Maximum likelihood estimates of model parameters, such as nitrogen- and carbon loss rates of phytoplankton, are determined. The optimisation yields results with higher rates for carbon exudation than for the loss of organic nitrogen. It also suggests that the PCHO fraction of exuded DOC was 63±20% during the mesocosm experiment. Optimal estimates are obtained for coagulation kernels for PCHO transformation into TEPC. Model state estimates are consistent with observations, where 30% of the POC increase was attributed to TEPC formation. The proposed model is of low complexity and is applicable for large-scale biogeochemical simulations.