Benthic foraminifera under elevated CO2 conditions
Zusammenfassung der Projektergebnisse
The project focused on living benthic foraminifera at elevated atmospheric CO2 concentrations. We investigated foraminifera from shallow waters of marginal seas where the impact of ocean acidification will be felt first. The objectives of our study were the individual responses of benthic foraminifera to high CO2 concentrations, the ensuing impact on the composition and population density of shallow-water foraminiferal assemblages, and the expected changes of benthic foraminiferal carbonate production. Culturing experiments were performed with single specimens of the boreal species Ammonia aomoriensis from the western Baltic Sea and living assemblages from Kiel Fjord, Baltic Sea, and Caroni River estuary, Gulf of Paria, Trinidad. The calcification rates of Ammonia at different partial pressures of CO2, temperatures and salinities were determined and changes of the associated fauna w a s monitored over a period of six months. The experiments were accompanied by a one-year field study in Flensburg Fjord, Baltic Sea, where living foraminiferal assemblages and hydrochemistry of bottom- and sediment pore waters was investigated every other month. The culturing experiments with single specimens revealed that growth rates of Ammonia aomoriensis differed significantly between pCO2 treatments. The specimens growed by 19 % during six weeks at atmospheric and slightly higher CO2 levels, and they were found intact after the experiment. At partial pressures >1200 µatm, the test diameter decreased by up to 22 % and the tests were severely damaged by corrosion. A larger test diameter and less corrosion were observed at higher temperatures, whereas no relationship could be found between salinity and test growth. During the field study at Flensburg Fjord, we observed very high pCO2 values in the bottom and pore water of near-surface sediments. The pore water was calcite supersaturated by a high alkalinity, which prevailed during most time of the year. An elevated pCO2 had no substantial effects on the foraminiferal preservation. The long-term culturing experiment showed a very similar pattern as the field study. A higher pore water alkalinity and calcite supersaturation facilitated growth, an increase of the benthic foraminiferal population densities and even consecutive reproduction at high pCO2 levels in the supernatant water. Field observations and experimental studies revealed a clear relationship between calcification of Ammonia aomoriensis and saturation state Ωcalc. The species growed at supersaturation (Ωcalc >1), whereas growth ceased and the tests dissolved at undersaturation (Ωcalc <1). Complete decalcification was effected at Ωcalc <0.5 where the majority of the individuals died. Consequently, it is not pCO2, as we initially thought, but rather Ω calc affecting the calcification. Calcareous benthic foraminifera in shallow-water environments will therefore be able to stand increasing atmospheric CO2 concentrations as long as the seawater in their immediate microhabitat is calcite supersaturated by a mediating substrate sustaining a high alkalinity. Furthermore, the long-term experiments revealed foraminiferal production rates of 0.47 g carbonate m-2 yr-1, of which 0.27 g m-2 yr-1 was accumulated in the sediment of the culturing vessels under high pCO2 levels in the supernatant water. These rates are one magnitude higher than previously reported from Kiel Bight at the same water depth from where our culturing samples were taken. Benthic foraminifera produce 0.1 Gt carbonate yr-1 in shallow-water environments on a global scale today. Planktonic foraminifera export 2.9 Gt yr-1 to the deep ocean. Progressing ocean acidification will cause a reduction in planktonic foraminiferal carbonate production by as they will be directly exposed to undersaturated surface waters. With reference to the results of our study, we may expect that benthic foraminiferal carbonate precipitation in marginal seas will sustain and even gain a higher importance for the global carbonate budget in the future.
Projektbezogene Publikationen (Auswahl)
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2011. Biometry and dissolution features of the benthic foraminiferal species Ammonia aomoriensis at high pCO2. Marine Ecology Progress Series, 432: 53-67
Haynert, K., Schönfeld, J. Riebesell, U., Polovodova, I.
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2012: The benthic foraminiferal community in a naturally CO2-rich coastal habitat of the southwestern Baltic Sea. Biogeosciences, 9: 4421-4440
Haynert, K., Schönfeld, J., Polovodova-Asteman, I., Thomsen, J.
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2013: Response of benthic foraminifera to ocean acidification in their natural sediment environment: a long-term culturing experiment. Biogeosciences Discussions, 10: 9523-9572
Haynert, K., Schönfeld, J., Schiebel, R., Wilson, B., Thomsen, J.
