Effect of salinity induced pH changes on benthic foraminifera: a laboratory culture experiment

Abstract. The coastal water pH varies with salinity. Therefore, to study the effect of salinity induced pH variations on benthic foraminifera, live specimens of Rosalina globularis were subjected to different salinities (10, 15, 20, 25, 30, 35 and 40 ‰) with pH varying from 7.2 to 8.2. A total of 210 specimens were used and the experiment was conducted in replicates. It was observed that the salinity induced pH changes affect the calcification of foraminifera. However the response is not linear. The maximum growth is reported in the specimens kept at 35 ‰ salinity (pH 8.0) while the rest of the specimens maintained at salinity higher or lower than 35 ‰, showed comparatively lesser growth. A significant drop in pH severely hampers the calcification capability of benthic foraminifera. Specimens kept at 10 and 15 ‰ (pH 7.2 and 7.5, respectively) became opaque within two days of lowering the salinity and later on their tests dissolved within 24 and 43 days, respectively. Besides calcification capability, pH also affects reproduction. No specimen reproduced at 10 and 15 ‰ salinity while only a few specimens (3 %) reproduced at 20 ‰. As compared to 10–20 ‰ salinity, ∼60 % reproduction was observed in specimens subjected to 25–40 ‰ salinity. The drop in pH also decreased the calcification rate as specimens at 20 ‰ salinity took twice the time to reach maturity than normal range (25–40 ‰). We conclude that salinity induced drop in pH adversely affects the calcification capability and reproduction in benthic foraminifera. It is inferred that the time required to reach reproductive maturity increases at the extreme salinity tolerance limits. However, beyond a certain limit, a further increase in pH does not affect benthic foraminifera; rather they respond to salinity as per their salinity tolerance range.