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

Abstract. Calcifying 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 6 months of incubation, foraminiferal assemblages were kept and treated in natural sediment with pCO₂-enriched seawater of 430, 907, 1865 and 3247 μatm pCO₂. The fauna was dominated by Ammonia aomoriensis and Elphidium species, whereas agglutinated species were rare. After 6 months of incubation, pore water alkalinity was much higher in comparison to the overlying seawater. Consequently, the saturation state of Ω_calc was much higher in the sediment than in the water column in nearly all pCO₂ treatments and remained close to saturation. As a result, the life cycle (population density, growth and reproduction) of living assemblages varied markedly during the experimental period, but was largely unaffected by the pCO₂ treatments applied. According to the size–frequency distribution, we conclude that foraminifera start reproduction at a diameter of 250 μm. Mortality of living Ammonia aomoriensis was unaffected, whereas size of large and dead tests decreased with elevated pCO₂ from 285 μm (pCO₂ from 430 to 1865 μatm) to 258 μm (pCO₂ 3247 μatm). The total organic content of living Ammonia aomoriensis has been determined to be 4.3% of CaCO₃ weight. Living individuals had a calcium carbonate production rate of 0.47 g m⁻² a⁻¹, whereas dead empty tests accumulated a rate of 0.27 g m⁻² a⁻¹. Although Ω_calc was close to 1, approximately 30% of the empty tests of Ammonia aomoriensis showed dissolution features at high pCO₂ of 3247 μatm during the last 2 months of incubation. In contrast, tests of the subdominant species, Elphidium incertum, stayed intact. Our results emphasize that the sensitivity to ocean acidification of the endobenthic foraminifera Ammonia aomoriensis in their natural sediment habitat is much lower compared to the experimental response of specimens isolated from the sediment.