Effect of salinity induced pH/alkalinity changes on benthic foraminifera: A laboratory culture experiment

The salinity of coastal waters in the vicinity of seasonally fresh water fed estuaries changes tremendously and reportedly affects the living calcite secreting organisms like foraminifera, as well as their dead remains. The precise mechanism of adverse effect of such seasonal salinity changes on calcite secreting organisms is, however not clear. The seasonal fresh water influx from the estuaries also affects the pH and alkalinity of the coastal seawater. Therefore, to understand the effect of salinity induced pH/alkalinity variations on benthic foraminifera, living specimens of Rosalina globularis were subjected to different salinity. Additionally, water samples were collected from an estuary during both monsoon and post monsoon season to understand the relationship between salinity, pH and total alkalinity (TA). The pH decreased with decreasing salinity during both the seasons. A similar decrease in TA with decreasing salinity was also observed but only till 20 psu salinity, below which the TA increased with decreasing salinity. Even though the maximum growth was reported in specimens kept at 35 psu salinity, growth of specimens maintained at >25 psu salinity, was same. Specimens kept at 10 psu and 15 psu salinity, however were much smaller and turned opaque within two days of lowering the salinity and later on their tests dissolved within 24 and 43 days, respectively. No specimen reproduced at 10 psu and 15 psu salinity, while only a few specimens (3%) reproduced at 20 psu salinity. As compared to 10–20 psu salinity, ∼60% reproduction was observed in specimens subjected to 25–40 salinity. The specimens maintained at 20 psu salinity took twice the time to reach maturity than those subjected to 25–40 psu salinity. Since a big drop in pH was observed at 10–15 psu salinity (pH 7.2 and 7.5, respectively), while the alkalinity was still higher, we suggest that fresh water influx induced drop in pH adversely affects calcification and reproduction in benthic foraminifera. The response is, however not linear as 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. It is further inferred that the time required to reach reproductive maturity increases at the extreme salinity tolerance limits. Dissolution of calcareous foraminifera below 20 psu salinity, suggests that salinity induced changes control the carbonate inventory in the coastal regions subjected to seasonal fresh water influx.

Saraswat R., Kouthanker M., Kurtarkar S. R., Nigam R., Naqvi S. W. A. & Linshy V. N., 2015. Effect of salinity induced pH/alkalinity changes on benthic foraminifera: A laboratory culture experiment. Estuarine, Coastal and Shelf Science 153:96–107. Article (subscription required).

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