Changing structure of benthic foraminiferal communities due to declining pH: Results from laboratory culture experiments

Abstract

The ocean absorbs large amounts of CO2 emitted from human activities, which results in a decrease in seawater pH. Marine calcifying organisms such as foraminifera, are most likely to be affected by this declining pH. In this study, we collected sediments from five stations of different depths (34–73 m) in a continental shelf of the Yellow Sea. The entire benthic foraminiferal communities together with sea sediments were cultured under three constant pHs (8.3, 7.8, and 7.3) for 6 and 12 weeks in the laboratory to study their responses to pH or incubation time. The microcosm’s experimental results obtained showed that most of the foraminiferal community parameters (abundance, species richness, Margalef index, and Shannon-Wiener diversity) decreased significantly (p<0.05) with the decline in pH in all the tested stations. The responses of foraminifera to the decline in pH were species-specific, for instance, Protelphidium tuberculatum and Cribroelphidiumfrigidum were highly sensitive to declining pH and were finally eliminated at low pH, while some species (e.g., Lagenammina atlantica, Verneuilinulla advena, V. propinqua, Haplophragmoides applanata, and H. canariensis) could tolerate low pH and acted as pH-tolerant species. In addition, the proportion of hyaline taxa showed a significant (p<0.05) positive correlation with pH, while agglutinated type showed a negative response. Furthermore, different incubation times (6 and 12 weeks) showed significant effects on the nearshore communities other than the offshore treatments, which were, however, entirely declined after 6 weeks’ incubation under low pH manipulation. Our results indicated that nearshore foraminiferal communities showed rather a resilience to the declining pH and the offshore foraminifera, especially those in the central area of the Yellow Sea Cold Water Mass were found to be more sensitive to the decline in pH in the continental shelf sediments of the Yellow Sea.