Seasonality in pH and temperature for shallow zones of the Bering Sea from measurements of coral Clathromorphum nereostratum

Increasing atmospheric CO2 concentrations are potentially affecting marine ecosystems twofold, by warming and acidification. The rising amount of CO2 taken up by the ocean lowers the saturation state of calcium carbonate, complicating the formation of this key biomineral used by many marine organisms to build hard parts like skeletons or shells. Reliable time-series data of seawater pH are needed to evaluate the ongoing change and compare long-term trends and natural variability. For the high-latitude ocean, the region facing the strongest CO2 uptake, such time-series data are so far entirely lacking. Our study provides, to our knowledge, the first reconstruction of seasonal cycle and long-term trend in pH for a high-latitude ocean obtained from 2D images of stable boron isotopes from a coralline alga.

Annual response of two Mediterranean azooxanthellate temperate corals to low-pH and high-temperature conditions

Ocean acidification (OA) and warming related to the anthropogenic increase in atmospheric CO2 have been shown to have detrimental effects on several marine organisms, especially those with calcium carbonate structures such as corals. In this study, we evaluate the response of two Mediterranean shallow-water azooxanthellate corals to the projected pH and seawater temperature (ST) scenarios for the end of this century. The colonial coral Astroides calycularis and the solitary Leptopsammia pruvoti were grown in aquaria over a year under two fixed pH conditions, control (8.05 pHT units) and low (7.72 pHT units), and simulating two annual ST cycles, natural and high (+3 °C). The organic matter (OM), lipid and protein content of the tissue and the skeletal microdensity of A. calycularis were not affected by the stress conditions (low pH, high ST), but the species exhibited a mean 25 % decrease in calcification rate at high-ST conditions at the end of the warm period and a mean 10 % increase in skeletal porosity under the acidified treatment after a full year cycle. Conversely, an absence of effects on calcification and skeletal microdensity of L. pruvoti exposed to low-pH and high-ST treatments contrasted with a significant decrease in the OM, lipid and protein content of the tissue at high-ST conditions and a 13 % mean increase in the skeletal porosity under low-pH conditions following a full year of exposure. This species-specific response suggests that different internal self-regulation strategies for energy reallocation may allow certain shallow-water azooxanthellate corals to cope more successfully than others with global environmental changes.

Age models and summer sea surface temperature and winter sea ice concentration for the EPILOG-LGM time slice in the Pacific Southern Ocean

Sea surface temperatures and sea-ice extent are the most critical variables to evaluate the Southern Ocean paleoceanographic evolution in relation to the development of the global carbon cycle, atmospheric CO2 variability and ocean-atmosphere circulation. In contrast to the Atlantic and the Indian sectors, the Pacific sector of the Southern Ocean has been insufficiently investigated so far. To cover this gap of information we present diatom-based estimates of summer sea surface temperature (SSST) and winter sea-ice concentration (WSI) from 17 sites in the polar South Pacific to study the Last Glacial Maximum (LGM) at the EPILOG time slice (19,000-23,000 cal. years BP). Applied statistical methods are the Imbrie and Kipp Method (IKM) and the Modern Analog Technique (MAT) to estimate temperature and sea-ice concentration, respectively. Our data display a distinct LGM east-west differentiation in SSST and WSI with steeper latitudinal temperature gradients and a winter sea-ice edge located consistently north of the Pacific-Antarctic Ridge in the Ross sea sector. In the eastern sector of our study area, which is governed by the Amundsen Abyssal Plain, the estimates yield weaker latitudinal SSST gradients together with a variable extended winter sea-ice field. In this sector, sea-ice extent may have reached sporadically the area of the present Subantarctic Front at its maximum LGM expansion. This pattern points to topographic forcing as major controller of the frontal system location and sea-ice extent in the western Pacific sector whereas atmospheric conditions like the Southern Annular Mode and the ENSO affected the oceanographic conditions in the eastern Pacific sector. Although it is difficult to depict the location and the physical nature of frontal systems separating the glacial Southern Ocean water masses into different zones, we found a distinct temperature gradient in latitudes straddled by the modern Southern Subtropical Front. Considering that the glacial temperatures north of this zone are similar to the modern, we suggest that this represents the Glacial Southern Subtropical Front (GSSTF), which delimits the zone of strongest glacial SSST cooling (>4K) to its North. The southern boundary of the zone of maximum cooling is close to the glacial 4°C isotherm. This isotherm, which is in the range of SSST at the modern Antarctic Polar Front (APF), represents a circum-Antarctic feature and marks the northern edge of the glacial Antarctic Circumpolar Current (ACC). We also assume that a glacial front was established at the northern average winter sea ice edge, comparable with the modern Southern Antarctic Circumpolar Current Front (SACCF). During the glacial, this front would be located in the area of the modern APF. The northward deflection of colder than modern surface waters along the South American continent leads to a significant cooling of the glacial Humboldt Current surface waters (4-8K), which affects the temperature regimes as far north as into tropical latitudes. The glacial reduction of ACC temperatures may also result in the significant cooling in the Atlantic and Indian Southern Ocean, thus may enhance thermal differentiation of the Southern Ocean and Antarctic continental cooling. Comparison with temperature and sea ice simulations for the last glacial based on numerical simulations show that the majority of modern models overestimate summer and winter sea ice cover and that there exists few models that reproduce our temperature data rather well.

Effects of Ocean Acidification and Warming on Sperm Activity and Early Life Stages of the Mediterranean Mussel (Mytilus galloprovincialis)

Larval stages are among those most vulnerable to ocean acidification (OA). Projected atmospheric CO2 levels for the end of this century may lead to negative impacts on communities dominated by calcifying taxa with planktonic life stages. We exposed Mediterranean mussel (Mytilus galloprovincialis) sperm and early life stages to pHT levels of 8.0 (current pH) and 7.6 (2100 level) by manipulating pCO2 level (380 and 1000 ppm). Sperm activity was examined at ambient temperatures (16-17 °C) using individual males as replicates. We also assessed the effects of temperature (ambient and = 20 °C) and pH on larval size, survival, respiration and calcification of late trochophore/early D-veliger stages using a cross-factorial design. Increased pCO2 had a negative effect on the percentage of motile sperm (mean response ratio R= 71%) and sperm swimming speed (R= 74%), possibly indicating reduced fertilization capacity of sperm in low concentrations. Increased temperature had a more prominent effect on larval stages than pCO2, reducing performance (RSize = 90% and RSurvival = 70%) and increasing energy demand (RRespiration = 429%). We observed no significant interactions between pCO2 and temperature. Our results suggest that increasing temperature might have a larger impact on very early larval stages of M. galloprovincialis than OA at levels predicted for the end of the century.