The benthic foraminiferal community in a naturally CO<sub>2</sub>-rich coastal habitat of the southwestern Baltic Sea

Abstract

Abstract. It is expected that the calcification of foraminifera will be negatively affected by the ongoing acidification of the oceans. Compared to the open oceans, these organisms are subjected to much more adverse carbonate system conditions in coastal and estuarine environments such as the southwestern Baltic Sea, where benthic foraminifera are abundant. This study documents the seasonal changes of carbonate chemistry and the ensuing response of the foraminiferal community with bi-monthly resolution in Flensburg Fjord. In comparison to the surface pCO 2 , which is close to equilibrium with the atmosphere, we observed large seasonal fluctuations of pCO 2 in the bottom and sediment pore waters. The sediment pore water pCO 2 was constantly high during the entire year ranging from 1244 to 3324 µatm. Nevertheless, in contrast to the bottom water, sediment pore water was slightly supersaturated with respect to calcite as a consequence of higher alkalinity (A T ) for most of the year. Foraminiferal assemblages were dominated by two calcareous species, Ammonia aomoriensis and Elphidium incertum, and the agglutinated Ammotium cassis. The one-year cycle was characterised by seasonal community shifts. Our results revealed that there is no dynamic response of foraminiferal population density and diversity to elevated sediment pore water pCO 2 . Surprisingly, the fluctuations of sediment pore water undersaturation ( calc ) co-vary with the population densities of living Ammonia aomoriensis. Further, we observed that most of the tests of living calcifying foraminifera were intact. Only Ammonia aomorienis showed dissolution and recalcification structures on the tests, especially at undersaturated conditions. Therefore, the benthic community is subjected to high pCO 2 and tolerates elevated levels as long as sediment pore water remains supersaturated. Model calculations inferred that increasing atmospheric CO 2 concentrations will finally lead to a perennial undersaturation in sediment pore waters. Whereas benthic foraminifera indeed may cope with a high sediment pore water pCO 2 , the steady undersaturation of sediment pore waters would likely cause a significant higher mortality of the dominating Ammonia aomoriensis. This shift may eventually lead to changes in the benthic foraminiferal communities in Flensburg Fjord, as well as in other regions experiencing naturally undersaturated calc levels.

Food availability outweighs ocean acidification effects in juvenile Mytilus edulis: laboratory and field experiments

“…perturbations that are expected through air-sea CO 2 uptake in the average surface ocean in the next 100 years (Cao & Caldeira, 2008;Borges & Gypens, 2010;Haynert et al, 2012). The observed fluctuations in seawater pCO 2 over the seasonal cycle directly translate into changes in mussel haemolymph pCO 2 .…”

“…In contrast to the open ocean, where seawater pCO 2 is close to atmospheric values, pCO 2 in coastal habitats is much more variable (Feely et al, 2010;Cai et al, 2011;Melzner et al, 2012;Hofmann et al, 2011). Shallow temperate estuaries such as the Western Baltic Sea, with a strong seasonal vertical stratification due to salinity and temperature gradients are characterized by higher and more fluctuating seawater pCO 2 Melzner et al, 2012, Haynert et al, 2012. In these areas, community respiration leads to decreases in pO 2 and increases in pCO 2 below the thermocline.…”

Atmospheric CO2availability does not equate to increased nodularin production in diazotrophic cyanobacteria, but does induce varying responses in net photosynthesis and N2fixation rates

“…The latter displayed the weakest response towards eCO 2 in the long term, but the highest increase in short-term response of NP at eCO 2 in the gas exchange cuvette, suggesting an overall low expression of HCO 3 receptors. In their natural benthic environment, elevated CO 2 of up to 3000 µatm can occur due to high organic matter decomposition and remineralisation (Haynert et al, 2012). The capability to swiftly downregulate the CCM is an important prerequisite for attaining high CO 2 tolerance in this environment.…”

Response of benthic foraminifera to ocean acidification in their natural sediment environment: a long-term culturing experiment

“…To date, no ocean acidification studies were reported to pursue culturing experiments with benthic foraminifera in their natural sedimentary environment. The field study of Haynert et al (2012) in Flensburg Fjord exhibited that the carbonate chemistry of sediment pore water differed strongly from the conditions in the overlying near-bottom water. Sediment pore water pCO 2 was constantly high, ranging from 1244 to 3324 µatm during the entire year.…”