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

International audienceCalcifying foraminifera are expected to be endangered by ocean acidification, However, the response of a complete community kept in natural sediment and over multiple generations under controlled laboratory conditions has not been constrained to date. During six month incubation, foraminiferal assemblages were treated with pCO2 enriched seawater of 430, 907, 1865 and 3247 muatm pCO2. The fauna was dominated by Ammonia aomoriensis and Elphidium species, whereas agglutinated species were rare. After 6 months incubation, pore water alkalinity was much higher in comparison to the overlying seawater. Consequently, the saturation state of Omegacalc was much higher in the sediment than in the water column in all pCO2 treatments and remained close to saturation. As a result, the life cycle of living assemblages was largely unaffected by the tested pCO2 treatments. Growth rates, reproduction and mortality, and therefore population densities and size-frequency distribution of Ammonia aomoriensis varied markedly during the experimental period. Growth rates varied between 25 and 50 mum per month, which corresponds to an addition of 1 or 2 new chambers per month. According to the size-frequency distribution, foraminifera start reproduction at a diameter of 250 mum. Mortality of large foraminifera was recognized, commencing at a test size of 285 mum at a pCO2 ranging from 430 to 1865 muatm, and of 258 mum at 3247 muatm. The total organic content of living Ammonia aomoriensis has been determined to be 4.3% of dry weight. Living individuals had a calcium carbonate production rate of 0.47 g m-2 yr-1, whereas dead empty tests accumulated at a rate of 0.27 g m-2 a-1. Although Omegacalc was close to 1, some empty tests of Ammonia aomoriensis showed dissolution features at the end of incubation. In contrast, tests of the subdominant species, Elphidium incertum, stayed intact. This species specific response could be explained by differences in the elemental test composition, in particular the higher Mg-concentrations in Ammonia aomoriensis tests. Our results emphasize that the sensitivity to ocean acidification of endobenthic foraminifera in their natural sediment habitat is much lower compared to the experimental response of specimens isolated from the sediment.