Carbon export production in the subantarctic zone and polar front zone south of Tasmania

We studied the water column distribution of total 234Th in subantarctic (SAZ) and polar front (PFZ) zone systems south of Tasmania during mid-austral summer 2007. The objective was to assess whether the observed zonal differences in biomass in this sector translated into variability of the carbon export and sequestration potential, and to identify possible causes inducing this variability. This study is part of a broader investigation focusing on macro- and micronutrient availability controlling ecosystem functioning in this area. Surface deficits in 234Th activities were observed at every station. 234Th export fluxes calculated from the 234Th activity deficits assuming steady state conditions showed higher 234Th export fluxes in the western than eastern SAZ, both of which were higher than those in the PFZ. 234Th fluxes sampled by free-drifting IRS and PPS-3 sediment traps at 150 and 170 m during short-term deployments ( similar to 6 days) at the three process stations were significantly lower than those obtained by the 234Th-deficit method. Possible reasons for this discrepancy are discussed. Carbon export fluxes were calculated based on the total 234Th fluxes (234Th deficit method) and the C:Th ratio for the > 54 pm particle size fraction from the appropriate export depth. The > 54 pm C:Th ratio was significantly lower in the eastern SAZ than the western SAZ or PFZ, resulting in carbon export fluxes that were lowest in the eastern SAZ. Overall, export fluxes range from 3.6 +/- 1.5 to 13.2 +/- 3.1 mmol C m-2 d-1. Carbon export fluxes are compared with gross primary production, new production and mesopelagic remineralization fluxes obtained by others during the same cruise. Contrary to expectations, we found higher export production in the PFZ and the western SAZ where biomass and dissolved Fe were lower than in the eastern SAZ. Significant differences in community structure of both primary producers and consumers likely contributed to this difference between the three regions. This pattern of higher shallow export continued into the mesopelagic, with lower remineralization efficiency in PFZ and SAZ-West compared to SAZ-East. Within the SAZ, east and west sites thus differed in their efficiency of carbon sequestration into the deep ( > 600 m) water column, with SAZ-West exceeding the sequestration capacity of SAZ-East. If the SAZ-East region can be used as an analog for a future climate warming scenario, our results suggest that export production at high latitudes could decrease, despite increased primary production

²³⁴Th-based export fluxes during a natural iron fertilization experiment in the Southern Ocean (KEOPS)

Five iron-fertilization experiments in the Southern Ocean have clearly demonstrated that adding iron increases primary production, but the implications for carbon export to the ocean interior have been less clear. This reflects both observational limitations of short-term experiments and their uncertain relevance to quantifying ecosystem level processes that are likely to be structured differently under conditions of punctual versus persistent stimulation. To avoid these biases, KEOPS (KErguelen Ocean and Plateau compared Study) investigated the naturally iron-fertilized Kerguelen Plateau region in the Indian Sector of the Southern Ocean that exhibits an annual phytoplankton bloom. Here, we report particulate organic carbon (POC) and nitrogen export from this system based on the Th-234 approach. Results indicate that the export fluxes were variable both on and off the Kerguelen Plateau (9.0-38.4 mmol C m-2 d-1 and 1.6-4.8 mmol N m-2 d-1) and were in the range of values reported for natural Southern Ocean ecosystems. Export fluxes were compared at two reference stations, one above and one outside the Plateau. The station above the plateau was characterized by higher iron supply and export fluxes compared to the station outside the plateau. The difference in the export flux between these two reference stations defines the export excess induced by iron fertilization. It was 10.8+/-4.9 mmol C m-2 d-1 and 0.9+/-0.7 mmol N m-2 d-1 at 100 m, and 14.2 +/- 7.7 mmol C m-2 d-1 and 2.0 +/- 1.3 mmol N m-2 d-1 at 200 m. This POC export excess was similar to those found during other studies of artificial (SOFeX) and natural (CROZEX) iron fertilization in the Southern Ocean. The examination of the export efficiency (defined as the ratio of export to primary production) revealed significant variability over the plateau related to the temporal decoupling of production and export during the demise of the bloom. On average, the export efficiency was lower over the plateau than in surrounding waters, suggesting that increased iron supply may increase total export but lower export efficiency. Our findings are very important for evaluating present and past carbon cycling in the Southern and global oceans and for assessing predictive scenarios of carbon cycling and budget

Barium cycling along WOCE SR3 line in the Southern Ocean

Spring and summer profiles of dissolved and particulate barium (Ba) from WOCE SR3 line (145°E) in the Southern Ocean are compared and seasonal evolutions discussed. Fluxes are estimated from mass conservation equations and from differences in reservoir contents between seasons. Subtraction of barium is observed at mesopelagic depths (upper 600 m) and appears to exceed up to tenfold the combined local build-up and the deep-ocean fluxes of particulate Ba, pointing towards significant dissolution to take place in intermediate and deep waters. Although regression analyses identify silicate as the major predictor of dissolved Ba, Ba and silicate are clearly uncoupled in surface waters where Ba behaves more similar to nitrate, excluding diatoms as users of Ba. Moreover, we observe a southward decrease in the Ba vs. silicate regression slopes driven by the conditions in intermediate and deep waters and mainly marked by the location of the Polar Front. These findings corroborate existing knowledge about the predominant control by barite formation and dissolution on the oceanic Ba cycle. It suggests a decreased dissolution of barite south of the Polar Front as compared to the situation in the Polar Front Zone and the Subantarctic Zone. This is in agreement with the fact that, except for deep waters, the Antarctic Circumpolar Current water column is oversaturated with respect to barite, in contrast to the situation north of the Polar Front, where the whole water column is undersaturated [Jeandel C., B. Dupre, G. Lebaron, C. Monnin and J.F. Minster, 1996. Longitudinal distributions of dissolved barium, silica and alkalinity in the western and southern Indian Ocean. Deep-Sea Res. 1, 43 (1), 1-31; Monnin, C., C. Jeandel, T. Cattaldo and F. Dehairs, 1999. The marine barite saturation state of the world's ocean. Mar. Chem., 65, 253-261]

Contrasting regimes of production and potential for carbon export in the Sub-Antarctic and Polar Frontal Zones south of Tasmania

We report on mid-summer gross primary production and new production in the Sub-Antarctic Zone (SAZ) and the Polar Frontal Zone (PFZ) south of Tasmania during the SAZ-Sense expedition (January-February 2007). The aim of our study was to assess how well documented regional variability in surface Chl-a biomass translates into variability of primary production and potential export in the area. The selected sites for process studies contrasted in terms of euphotic and mixed layer depths, macro nutrient concentrations and primary production with short term (days) temporal variability of production. Daily euphotic layer integrated gross primary production (from short term 13C incubations) was higher in the SAZ than in the FEZ by about an order of magnitude. Within the SAZ highest production was reached south-west of Tasmania, contrasting with surface ocean Chl-a biomass which was highest in the eastern SAZ. In most cases regenerated production (from 15N-ammonium uptake experiments) was significantly larger than new production (from 15N-nitrate uptake experiments) with f-ratios mostly <= 0.3. Mixed layer and euphotic layer depths, relative availability of nitrate and ammonium, level of Fe sufficiency, grazing pressure, and opposing effect of Fe and ammonium on nitrate uptake, appear to control these regional differences in SAZ production. Overall low new production values reflected a low relative potential for carbon export in the area, confirming the low export ratios reported by others. These conditions prevail throughout the study area but, unexpectedly, are most marked in the eastern SAZ where Fe was reported not to be limiting. For the eastern SAZ we speculate that the availability of ammonium sustained by grazing pressure inhibited nitrate uptake and primary production by counteracting the effects of Fe sufficiency and mixed layer shallowness, which are factors potentially conducive to enhanced production. This lower production combined with decreased f-ratios led to the observed poor potential for carbon export and poor carbon sequestration in the SAZ-East

Marine ecosystems'responses to climatic and anthropogenic forcings in the Mediterranean

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Marine ecosystems' responses to climatic and athropogenic forcings in the Mediterranean

The semi-enclosed nature of the Mediterranean Sea, together with its smaller inertia due to the relative short residence time of its water masses, make it highly reactive to external forcings, in particular variations of water, energy and matter fluxes at the interfaces. This region, which has been identified as a 'hotspot' for climate change, is therefore expected to experience environmental impacts that are considerably greater than those in many other places around the world. These natural pressures interact with the increasing demographic and economic developments occurring heterogeneously in the coastal zone, making the Mediterranean even more sensitive. This review paper aims to provide a review of the state of current functioning and responses of Mediterranean marine biogeochemical cycles and ecosystems with respect to key natural and anthropogenic drivers and to consider the ecosystems¿ responses to likely changes in physical, chemical and socio-economical forcings induced by global change and by growing anthropogenic pressure at the regional scale. The current knowledge on and expected changes due to single forcing (hydrodynamics, solar radiation, temperature and acidification, chemical contaminants) and combined forcing (nutrient sources and stoichiometry, extreme events) affecting the biogeochemical fluxes and ecosystem functioning are explored. Expected changes in biodiversity resulting from the combined action of the different forcings are proposed. Finally, modeling capabilities and necessity for modeling are presented. Modeling acts as an integrative tool to investigate the question of how climate change and anthropogenic activities impact the cycle of biogenic elements and marine ecosystems. A synthesis of our current knowledge of expected changes is proposed, highlighting relevant questions for the future of the Mediterranean ecosystems that are current research priorities for the scientific community. Finally, we discuss how these priorities can be approached by national and international multi-disciplinary research, which should be implemented on several levels, including observational studies and modeling at different temporal and spatial scales.JRC.H.5-Land Resources Managemen

Marine ecosystems’ responses to climatic and anthropogenic forcings in the Mediterranean

Corrigendum : https://doi.org/10.1016/j.pocean.2011.08.003International audienceThe semi-enclosed nature of the Mediterranean Sea, together with its smaller inertia due to the relative short residence time of its water masses, make it highly reactive to external forcings, in particular variations of water, energy and matter fluxes at the interfaces. This region, which has been identified as a “hotspot” for climate change, is therefore expected to experience environmental impacts that are considerably greater than those in many other places around the world. These natural pressures interact with the increasing demographic and economic developments occurring heterogeneously in the coastal zone, making the Mediterranean even more sensitive. This review paper aims to provide a review of the state of current functioning and responses of Mediterranean marine biogeochemical cycles and ecosystems with respect to key natural and anthropogenic drivers and to consider the ecosystems’ responses to likely changes in physical, chemical and socio-economical forcings induced by global change and by growing anthropogenic pressure at the regional scale. The current knowledge on and expected changes due to single forcing (hydrodynamics, solar radiation, temperature and acidification, chemical contaminants) and combined forcing (nutrient sources and stoichiometry, extreme events) affecting the biogeochemical fluxes and ecosystem functioning are explored. Expected changes in biodiversity resulting from the combined action of the different forcings are proposed. Finally, modeling capabilities and necessity for modeling are presented. A synthesis of our current knowledge of expected changes is proposed, highlighting relevant questions for the future of the Mediterranean ecosystems that are current research priorities for the scientific community. Finally, we discuss how these priorities can be approached by national and international multi-disciplinary research, which should be implemented on several levels, including observational studies and modeling at different temporal and spatial scales

Marine ecosystems’ responses to climatic and anthropogenic forcings in the Mediterranean

Corrigendum : https://doi.org/10.1016/j.pocean.2011.08.003International audienceThe semi-enclosed nature of the Mediterranean Sea, together with its smaller inertia due to the relative short residence time of its water masses, make it highly reactive to external forcings, in particular variations of water, energy and matter fluxes at the interfaces. This region, which has been identified as a “hotspot” for climate change, is therefore expected to experience environmental impacts that are considerably greater than those in many other places around the world. These natural pressures interact with the increasing demographic and economic developments occurring heterogeneously in the coastal zone, making the Mediterranean even more sensitive. This review paper aims to provide a review of the state of current functioning and responses of Mediterranean marine biogeochemical cycles and ecosystems with respect to key natural and anthropogenic drivers and to consider the ecosystems’ responses to likely changes in physical, chemical and socio-economical forcings induced by global change and by growing anthropogenic pressure at the regional scale. The current knowledge on and expected changes due to single forcing (hydrodynamics, solar radiation, temperature and acidification, chemical contaminants) and combined forcing (nutrient sources and stoichiometry, extreme events) affecting the biogeochemical fluxes and ecosystem functioning are explored. Expected changes in biodiversity resulting from the combined action of the different forcings are proposed. Finally, modeling capabilities and necessity for modeling are presented. A synthesis of our current knowledge of expected changes is proposed, highlighting relevant questions for the future of the Mediterranean ecosystems that are current research priorities for the scientific community. Finally, we discuss how these priorities can be approached by national and international multi-disciplinary research, which should be implemented on several levels, including observational studies and modeling at different temporal and spatial scales