COMPOSITIONAL SHIFTS IN SHALLOW-WATER FORAMINIFERAL ASSEMBLAGES IN RESPONSE TO PH VARIATIONS: INSIGHTS FROM A CULTURE EXPERIMENT WITH PROPAGULES

Variations in ocean pH in shallow-water environments have implications on calcifying organisms. These include increased mortality, decalcification, or shifts in community composition. Previous works included field and laboratory studies on single species or entire assemblages. Here, we applied pH variations on a foraminiferal propagule bank to gather insights into the effect of changing pH on the composition of experimentally grown assemblages.

We isolated a local propagule bank from a mudflat in Corfu (Greece) by separating the fine fraction of the sediment (<53 µm) through sieving. The resulting material contained juveniles and propagules of benthic foraminifera, which were set-up for growth experiments in the laboratory. The experimental design included four pH treatments of 6.5, 7.2, 7.8 and 8.5 at constant temperature and salinity (22°C and 38 ppt) for 5 weeks. At the termination of the experiment, the material was washed over 63 µm before analyzing the grown assemblages.

We found high numbers of grown individuals (825–1564 per replicate) and a high survivability rate throughout the experiment (78–87%). The treatments with higher pH (7.8 and 8.5) revealed assemblages that were dominated by monothalamous and porcelaneous species, whereas the treatments with lower pH (6.5 and 7.2) showed a reduction in porcelaneous and an increase in agglutinated species. Several taxa showed significant positive or negative correlations with decreasing pH values.

Our results are congruent with previous findings that foraminifera in their natural sediment exhibit some resilience towards changing pH values. Other studies also reported compositional shifts towards a dominance of agglutinated taxa with decreasing pH (culture and field observations). Our study on propagules adds to the insights on the mechanisms behind these shifts and offers a glimpse into possible effects of decreasing ocean pH in the future.