Posts Tagged 'Antarctic'

Iron sources alter the response of Southern Ocean phytoplankton to ocean acidification

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.

Continue reading ‘Iron sources alter the response of Southern Ocean phytoplankton to ocean acidification’

Carbonate system properties in the Gerlache Strait, Northern Antarctic Peninsula (February 2015): II. Anthropogenic CO2 and seawater acidification

Highlights

  • Anthropogenic carbon has reached the deep waters (≥ 100 m) of Gerlache Strait, Antarctica.
  • The main intrusion of anthropogenic carbon in the deep basin of the Gerlache Strait is through the advection of HSSW-derived water into the region.
  • A small additional input of anthropogenic carbon at depth can trigger the dissolution of aragonite.


Abstract

During the NAUTILUS I cruise in February 2015, the carbonate and associated physical properties were measured throughout the water column in the Gerlache Strait—off the southern coast of the Northern Antarctic Peninsula (NAP). Part I of this study (Kerr et al., 2017, this issue) focused on the net sea–air carbon dioxide (CO2) flux, whereas the same dataset was used here to estimate the extent of anthropogenic carbon (Cant) storage in the Gerlache Strait deep basin. In addition, the impact of Cant increases on pH and calcium carbonate saturation states for calcite and aragonite (ΩCa and ΩAr, respectively) were evaluated. The results indicate that, up to the present, 21.2 ± 16.7 μmol kg−1 of Cant have been injected to the deep and bottom layers of the Gerlache Strait. Two mechanisms may have contributed to that fact: (i) the pathway of the Cant follows that of the high salinity shelf waters inflow coming from both the Bransfield Strait and the Northwestern Weddell Sea shelf, and (ii) the Gerlache Strait is absorbing a significant amount of Cant from the atmosphere. Therefore, pH, ΩAr and ΩCa are decreasing in the deep waters of the Gerlache Strait. Since ΩAr is already very close to 1 at depth, any small additional input of Cant will trigger the dissolution of aragonite.

Continue reading ‘Carbonate system properties in the Gerlache Strait, Northern Antarctic Peninsula (February 2015): II. Anthropogenic CO2 and seawater acidification’

Potential impact of carbonate chemistry change (pCO2) on krill and krill-based food chain in the Southern Ocean with emphasis on embryogenesis of Antarctic krill

In the Southern Ocean, it is still not certain that overall krill biomass may decline because of drastic increase in pCO2, and consequent decline in pH. However, there is evidence that ecological vacuums created by krill population collapses caused by ecosystem shifts in Western Antarctic Peninsula (WAP) region led to replacement of Antarctic krill Euphausia superba by soft-bodied salps Salpa thomsoni. There is yet another questionable hypothesis that by the end of 21st century, ocean acidification stress, coupled with thermal increase, may synergistically induce physiologically critical stress to Antarctic krill in some areas of the Southern Ocean, egg development of krill may drastically decrease and in the 23rd century krill may even become extinct. I have earlier reported on normal krill egg development in relation to thermal change and high pressure (George and Stromberg, 1985). Recent experiments on krill development under different pCO2 conditions by Kawaguchi et al. (2011, 2013) suggest that we may witness 20 to 70 % reduction in Antarctic Krill by 2100 as direct consequence of pH decline. Such a scenario may lead to demise of krill-eating top-predators like baleen whales, seals and different species of Antarctic penguin populations. We now know that Adelaide penguins are decreasing in Bransfield Strait region off of the Western Antarctic Peninsula but increasing in Ross Sea region. Such a shift in breeding colonies moving from northern to southern WAP region and Ross Sea areas is not attributed to any decline in krill biomass but recent decadal melting of sea-ice as documented by remote sensing (George and Hayden, 2017). In this paper the main focus revolves around implications of changing chemistry of the Southern Ocean caused by absorption of anthropogenic carbon dioxide.
Continue reading ‘Potential impact of carbonate chemistry change (pCO2) on krill and krill-based food chain in the Southern Ocean with emphasis on embryogenesis of Antarctic krill’

Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO2 tolerance in phytoplankton productivity

High-latitude oceans are anticipated to be some of the first regions affected by ocean acidification. Despite this, the effect of ocean acidification on natural communities of Antarctic marine microbes is still not well understood. In this study we exposed an early spring, coastal marine microbial community in Prydz Bay to CO2 levels ranging from ambient (343 μatm) to 1641 μatm in six 650 l minicosms. Productivity assays were performed to identify whether a CO2 threshold existed that led to a decline in primary productivity, bacterial productivity, and the accumulation of Chlorophyll a (Chl a) and particulate organic matter (POM) in the minicosms. In addition, photophysiological measurements were performed to identify possible mechanisms driving changes in the phytoplankton community. A critical threshold for tolerance to ocean acidification was identified in the phytoplankton community between 953 and 1140 μatm. CO2 levels ≥ 1140 μatm negatively affected photosynthetic performance and Chl a-normalised primary productivity (csPP14C), causing significant reductions in gross primary production (GPP14C), Chl a accumulation, nutrient uptake, and POM production. However, there was no effect of CO2 on C : N ratios. Over time, the phytoplankton community acclimated to high CO2 conditions, showing a down-regulation of carbon concentrating mechanisms (CCMs) and likely adjusting other intracellular processes. Bacterial abundance initially increased in CO2 treatments ≥ 953 μatm (days 3–5), yet gross bacterial production (GBP14C) remained unchanged and cell-specific bacterial productivity (csBP14C) was reduced. Towards the end of experiment, GBP14C and csBP14C markedly increased across all treatments regardless of CO2 availability. This coincided with increased organic matter availability (POC and PON) combined with improved efficiency of carbon uptake. Such changes in phytoplankton community production could have negative effects on the Antarctic food web and the biological pump, resulting in negative feedbacks on anthropogenic CO2 uptake. Increases in bacterial abundance under high CO2 conditions may also increase the efficiency of the microbial loop, resulting in increased organic matter remineralisation and further declines in carbon sequestration.

Continue reading ‘Ocean acidification of a coastal Antarctic marine microbial community reveals a critical threshold for CO2 tolerance in phytoplankton productivity’

Autonomous observing platform CO2 data shed new light on the Southern Ocean carbon cycle.

While the number of surface ocean CO2 partial pressure (pCO2) measurements has soared the recent decades, the Southern Ocean remains undersampled. Williams et al. [2017] now present pCO2 estimates based on data from pH-sensor equipped Bio-Argo floats, which have been measuring in the Southern Ocean since 2014. The authors demonstrate the utility of these data for understanding the carbon cycle in this region, which has a large influence on the distribution of CO2 between the ocean and atmosphere. Biogeochemical sensors deployed on autonomous platforms hold the potential to shape our view of the ocean carbon cycle in the coming decades.

Continue reading ‘Autonomous observing platform CO2 data shed new light on the Southern Ocean carbon cycle.’

Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment

The Southern Ocean provides a vital service by absorbing about one sixth of humankind’s annual emissions of CO2. This comes with a cost – an increase in ocean acidity that is expected to have negative impacts on ocean ecosystems. The reduced ability of phytoplankton and zooplankton to precipitate carbonate shells is a clearly identified risk. The impact depends on the significance of these organisms in Southern Ocean ecosystems, but there is very little information on their abundance or distribution. To quantify their presence, we used coulometric measurement of particulate inorganic carbonate (PIC) on particles filtered from surface seawater into two size fractions: 50–1000 μm to capture foraminifera (the most important biogenic carbonate forming zooplankton) and 1–50 μm to capture coccolithophores (the most important biogenic carbonate forming phytoplankton). Ancillary measurements of biogenic silica (BSi) and particulate organic carbon (POC) provided context, as estimates of the abundance of diatoms (the most abundant phytoplankton in polar waters), and total microbial biomass, respectively. Results for 9 transects from Australia to Antarctica in 2008–2015 showed low levels of PIC compared to northern hemisphere polar waters. Coccolithophores slightly exceeded the biomass of diatoms in Subantarctic waters, but their abundance decreased more than 30-fold poleward, while diatom abundances increased, so that on a molar basis PIC was only 1 % of BSi in Antarctic waters. This limited importance of coccolithophores in the Southern Ocean is further emphasized in terms of their associated POC, representing less than 1 % of total POC in Antarctic waters and less than 10 % in Subantarctic waters. NASA satellite ocean colour based PIC estimates were in reasonable agreement with (though somewhat higher than) the shipboard results in Subantarctic waters, but greatly over-estimated PIC in Antarctic waters. Contrastingly, the NASA Ocean Biogeochemical Model (NOBM) shows coccolithophores as overly restricted to Subtropical and northern Subantarctic waters. The cause of the strong southward decrease in PIC abundance in the Southern Ocean is not yet clear. Poleward decrease in pH is small and while calcite saturation decreases strongly southward it remains well above saturation (> 2). Nitrate and phosphate variations would predict a poleward increase. Temperature and competition with diatoms for limiting iron appear likely to be important. While the future trajectory of coccolithophore distributions remains uncertain, their current low abundances suggest small impacts on overall Southern Ocean pelagic ecology.

Continue reading ‘Distribution of planktonic biogenic carbonate organisms in the Southern Ocean south of Australia: a baseline for ocean acidification impact assessment’

Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania

Biogeochemical change in the water masses of the Southern Ocean, south of Tasmania, was assessed for the 16-year period between 1995 and 2011 using data from 4 summer repeats of the WOCE/JGOFS/CLIVAR/GO-SHIP SR03 hydrographic section (at ~ 140° E). Changes in temperature, salinity, oxygen, and nutrients were used to disentangle the effect of solubility, biology, circulation and anthropogenic carbon (CANT) uptake on the variability of dissolved inorganic carbon (DIC) for 8 water mass layers defined by neutral surfaces (ϒn). CANT was estimated using an improved back-calculation method. Warming (~ 0.0352 ± 0.0170 °C yr−1) of Subtropical Central Water (STCW) and Antarctic Surface Water (AASW) layers decreased their gas solubility, and accordingly DIC concentrations increased less rapidly than expected from equilibration with rising atmospheric CO2 (~ 0.86 ± 0.16 μmol kg−1 yr−1 versus ~ 1 ± 0.12 μmol kg−1 yr−1). An increase in apparent oxygen utilisation (AOU) occurred in these layers due to either remineralization of organic matter or intensification of upwelling. The range of estimates for the increases of CANT were 0.71 ± 0.08 to 0.93 ± 0.08 μmol kg−1 yr−1 for STCW and 0.35 ± 0.14 to 0.65 ± 0.21 μmol kg−1 yr−1 for AASW, with the lower values in each water mass obtained by assigning all the AOU change to remineralization. DIC increases in the Sub-Antarctic Mode Water (SAMW, 1.10 ± 0.14 μmol kg−1 yr−1) and Antarctic Intermediate Water (AAIW, 0.40 ± 0.15 μmol kg−1 yr−1) layers were similar to the calculated CANT trends. For SAMW, the CANT increase tracked rising atmospheric CO2. As a consequence of the general DIC increase, decreases in total pH (pHT) and aragonite saturation (ΩAr) were found in most water masses, with the upper ocean and the SAMW layer presenting the largest trends for pHT decrease (~ −0.0031 ± 0.0004 yr−1). DIC increases in deep and bottom layers (~ 0.24 ± 0.04 μmol kg−1 yr−1) resulted from the advection of old deep waters to resupply increased upwelling, as corroborated by increasing silicate (~ 0.21 ± 0.07 μmol kg−1 yr−1), which also reached the upper layers near the Antarctic Divergence (~ 0.36 ± 0.06 μmol kg−1 yr−1) and was accompanied by an increase in salinity. The observed changes in DIC over the 16-year span caused a shoaling (~ 340 m) of the aragonite saturation depth (ASD, ΩAr = 1) within Upper Circumpolar Deep Water that followed the upwelling path of this layer. From all our results, we conclude a scenario of increased transport of deep waters into the section and enhanced upwelling at high latitudes for the period between 1995 and 2011, probably linked to a positive trend in the Southern Annular Mode. Although enhanced upwelling lowered the capacity of the AASW layer to uptake atmospheric CO2, it did not limit that of the newly forming SAMW and AAIW, which exhibited CANT storage rates (~ 0.41 ± 0.20 mol m−2 yr−1) twice that of the upper layers.

Continue reading ‘Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania’


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Ocean acidification in the IPCC AR5 WG II

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