Posts Tagged 'Antarctic'



Combined effects of ocean acidification and increased light intensity on natural phytoplankton communities from two Southern Ocean water masses

The composition of phytoplankton communities plays a major role in the efficiency of the biological carbon pump and energy transfer to higher trophic levels. Phytoplankton community composition can be significantly affected by changes in environmental conditions. We investigated the effect of increased pCO2 and light intensity on natural communities from two Southern Ocean water masses, the Subtropical Frontal Zone (STFZ) and Subantarctic Surface Waters (SASW). The community in both experiments shifted to predominately dinoflagellates under high pCO2 and high light and the community composition was significantly different between both treatments at the end of the incubation. In the STFZ assemblage, the combination of increased light and pCO2 had a small positive effect on diatom, coccolithophores and picoeukaryote abundance at the beginning of the experiment while higher pCO2 alone had no or a negative effect. In the SASW assemblage, the combination of increased light and pCO2 had a negative effect on diatom abundance while lower pH/higher pCO2 alone resulted in an increase in diatom counts compared to the control. Coccolithophores grew only in the control treatment. Our results show that there are taxon-specific and locality specific differences in natural phytoplankton community responses to increased light and pCO2 within low nutrient regions.

Continue reading ‘Combined effects of ocean acidification and increased light intensity on natural phytoplankton communities from two Southern Ocean water masses’

Decadal‐scale increases of anthropogenic CO2 in Antarctic Bottom Water in the Indian and western Pacific sectors of the Southern Ocean

We determined decadal‐scale increases of anthropogenic CO2 in the water column using data sets collected 17 years apart (1994‐1996 and 2012‐2013) along a transect at nominal 62°S in the Indian and western Pacific sectors of the Southern Ocean. Large increases of anthropogenic CO2 (up to 9.1 ± 1.5 μmol kg–1), closely following atmospheric CO2 increases, were found in Antarctic Bottom Water (AABW), previously considered a small sink of anthropogenic CO2. Vertical distributions of anthropogenic CO2 increases showed significant positive correlations with those of changes in CFC‐12 and SF6, implying that the distributions were mainly controlled by physical processes such as ventilation and circulation. Calculated uptake rates of anthropogenic CO2 by AABW were between 0.29 and 0.39 mol m–2 yr–1 in five longitudinal segments of the transect. In accounting for the large increase of anthropogenic CO2 in AABW, sea surface conditions in the formation region of AABW are important.

Continue reading ‘Decadal‐scale increases of anthropogenic CO2 in Antarctic Bottom Water in the Indian and western Pacific sectors of the Southern Ocean’

Adult Antarctic krill proves resilient in a simulated high CO2 ocean

Antarctic krill (Euphausia superba) have a keystone role in the Southern Ocean, as the primary prey of Antarctic predators. Decreases in krill abundance could result in a major ecological regime shift, but there is limited information on how climate change may affect krill. Increasing anthropogenic carbon dioxide (CO2) emissions are causing ocean acidification, as absorption of atmospheric CO2 in seawater alters ocean chemistry. Ocean acidification increases mortality and negatively affects physiological functioning in some marine invertebrates, and is predicted to occur most rapidly at high latitudes. Here we show that, in the laboratory, adult krill are able to survive, grow, store fat, mature, and maintain respiration rates when exposed to near-future ocean acidification (1000–2000 μatm pCO2) for one year. Despite differences in seawater pCO2 incubation conditions, adult krill are able to actively maintain the acid-base balance of their body fluids in near-future pCO2, which enhances their resilience to ocean acidification.

Continue reading ‘Adult Antarctic krill proves resilient in a simulated high CO2 ocean’

CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales

The cause of changes in atmospheric carbon dioxide (CO2) during the recent ice ages is yet to be fully explained. Most mechanisms for glacial–interglacial CO2 change have centred on carbon exchange with the deep ocean, owing to its large size and relatively rapid exchange with the atmosphere1. The Southern Ocean is thought to have a key role in this exchange, as much of the deep ocean is ventilated to the atmosphere in this region2. However, it is difficult to reconstruct changes in deep Southern Ocean carbon storage, so few direct tests of this hypothesis have been carried out. Here we present deep-sea coral boron isotope data that track the pH—and thus the CO2 chemistry—of the deep Southern Ocean over the past forty thousand years. At sites closest to the Antarctic continental margin, and most influenced by the deep southern waters that form the ocean’s lower overturning cell, we find a close relationship between ocean pH and atmospheric CO2: during intervals of low CO2, ocean pH is low, reflecting enhanced ocean carbon storage; and during intervals of rising CO2, ocean pH rises, reflecting loss of carbon from the ocean to the atmosphere. Correspondingly, at shallower sites we find rapid (millennial- to centennial-scale) decreases in pH during abrupt increases in CO2, reflecting the rapid transfer of carbon from the deep ocean to the upper ocean and atmosphere. Our findings confirm the importance of the deep Southern Ocean in ice-age CO2 change, and show that deep-ocean CO2 release can occur as a dynamic feedback to rapid climate change on centennial timescales.

Continue reading ‘CO2 storage and release in the deep Southern Ocean on millennial to centennial timescales’

Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high‐latitude Southern Ocean

Although the Southern Ocean is thought to account for a significant portion of the contemporary oceanic uptake of carbon dioxide (CO2), flux estimates in this region are based on sparse observations that are strongly biased towards summer. Here we present new estimates of Southern Ocean air‐sea CO2 fluxes calculated with measurements from biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project during 2014‐2017. Compared to ship‐based CO2 flux estimates, the float‐based fluxes find significantly stronger outgassing in the zone around Antarctica where carbon‐rich deep waters upwell to the surface ocean. Although interannual variability contributes, this difference principally stems from the lack of autumn and winter ship‐based observations in this high‐latitude region. These results suggest that our current understanding of the distribution of oceanic CO2 sources and sinks may need revision and underscore the need for sustained year‐round biogeochemical observations in the Southern Ocean.

Continue reading ‘Autonomous biogeochemical floats detect significant carbon dioxide outgassing in the high‐latitude Southern Ocean’

Climatic modulation of surface acidification rates through summertime wind forcing in the Southern Ocean

While the effects of the Southern Annular Mode (SAM), a dominant climate variability mode in the Southern Ocean, on ocean acidification have been examined using models, no consensus has been reached. Using observational data from south of Tasmania, we show that during a period with positive SAM trends, surface water pH and aragonite saturation state at 60°–55° S (Antarctic Zone) decrease in austral summer at rates faster than those predicted from atmospheric CO2 increase alone, whereas an opposite pattern is observed at 50°–45° S (Subantarctic Zone). Together with other processes, the enhanced acidification at 60°–55° S may be attributed to increased westerly winds that bring in more “acidified” waters from the higher latitudes via enhanced meridional Ekman transport and from the subsurface via increased vertical mixing. Our observations support climatic modulation of ocean acidification superimposed on the effect of increasing atmospheric CO2.

Continue reading ‘Climatic modulation of surface acidification rates through summertime wind forcing in the Southern Ocean’

Ocean acidification studies and the uncertainties relevance on measurements of marine carbonate system properties

The global ocean has a key role on the Earth’s climate system. It possesses a direct connection with the atmospheric gases, including the greenhouses, allowing exchanges between those compartments and oceanic storage of carbon. Through the years, this exchange of gases occurred based on gas equilibrium between ocean and atmosphere. After the Industrial Revolution, human activities have increased the emissions of greenhouse gases, mainly carbon dioxide (CO2), which changed the atmospheric concentration from ~280 ppm of CO2 to values as high as 391 ppm between c.a. 1750 and 2011 (Ciais et al., 2013). Recently, the measured CO2 atmospheric values are ranging near or above 400 ppm, as recorded by the Mauna Loa observatory, in Hawaii (daily CO2 measurements information available on www.scripps.ucsd.edu). A regional study in the south-southeast Brazilian continental shelf agrees with this value, which has measured an average of 396.7±2.5 ppm in the atmosphere during the spring of October 2014 (Kerr et al., 2016). This enhancement is reflected in the ocean, which has absorbed about 25% to 30% of the anthropogenic atmospheric CO2 emissions (Sabine and Tanhua, 2010); Le Quére et al., 2016). The CO2 uptake by the oceans directly affects the seawater chemistry and marine biogeochemical processes, impacting both the ecosystems and their respective biota (Doney et al., 2009).

Continue reading ‘Ocean acidification studies and the uncertainties relevance on measurements of marine carbonate system properties’


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