Transition from calcareous to agglutinated shelled benthic foraminifera at near neutral pH in a tropical estuary

Among the most important environments of the coastal zone are the estuaries where fresh water from land mixes with the sea water creating one of the most biologically productive regions on the earth. A wide range of human activities affect the estuaries and endanger their biodiversity due to unchecked development in many coastal areas across the world. A large fluctuation is observed in the pH in different parts of the estuaries. In view of the ocean acidification, it is important to delineate the pH tolerance range of the organisms thriving in the estuaries. In this study, we assess the effect of pH on the distribution and diversity of benthic foraminifera in a tropical estuary. The abundance and shell composition of both the living and recently dead benthic foraminifera was studied in the surface sediments collected from coarse sand rich pockets as well as the fine-grained organic matter rich sediments on the margins of the Terekhol estuary in the Goa state of India. The salinity, pH of the ambient water was noted and the sediment characteristics were analysed. The salinity ranged from 0.04 to 22.43 from the upstream station to the river mouth whereas pH varied from 6.99 to 7.84. The highest salinity and pH were recorded at the river mouth stations. Total Suspended Material (TSM) decreased from river mouth to upstream, with the highest TSM at the river mouth station (9.5 mg/l). The highest sand (%) was in the river mouth region and a higher percentage of sand was invariably observed in the middle part of the estuary. The sand and silt dominated the sediments throughout the estuary and the clay content was <24 % on all the stations. As compared to the middle channel stations, the sediments collected from the adjacent banks had higher organic carbon (C_org) content throughout the estuary. The highest C_org (4.52%) was observed at the upstream station. Except two stations in the river mouth, all of the stations in the Terekhol estuary showed higher C_org/N ratio (>10) indicating terrestrial input. A relatively higher CaCO₃ (%) was observed in the river mouth region as compared to the upstream region. Benthic foraminiferal abundance was much higher in fine grained and C_org rich sediments on the banks as compared to sandy sediments devoid of the organic matter in the middle of the estuary. A distinct transition from the abundance of calcareous benthic foraminifera to agglutinated benthic foraminifera was observed at nearly neutral (7.14) pH. Ammonia dominated the calcareous benthic foraminifera at low pH, suggesting its wide pH tolerance range. This lower pH tolerance range is much lower than previously reported. Only, agglutinated benthic foraminifera were found at station with pH lower than 7.14. We report a much lower pH tolerance range of calcareous benthic foraminifera.