Assessing the Ecological Quality Status in tropical Indian estuaries: testing the applicability of benthic foraminiferal indices

The ecological quality status (EcoQS) of Vellar and Uppanar estuaries (Southeast coast of India) has been monitored monthly, using a combination of foraminiferal (Foram Stress Index: FSI and exp(H'_bc) indices and abiotic (Pollution Load Index: PLI, Dissolved Oxygen: DO, and Total Organic Carbon: TOC) parameters. The Uppanar Estuary shows relatively higher values of PLI and TOC and lower DO values than Vellar Estuary. The highest value of TOC and PLI are recorded during the monsoon season. These variations are well mirrored by the change in exp(H'_bc) and FSI. The lowest values of exp(H'_bc) are observed with the monsoon season and could be ascribed by an overall reduction of salinity, and to the highest level of TOC and PLI in response to enhanced river discharge. The FSI also exhibits great variability with significant higher values in the Vellar Estuary than in the Uppanar Estuary. The EcoQS evaluated by a combination of pollution- (i.e., PLI, TOC and DO) and foraminiferal-based [i.e., FSI and exp(H'bc)] indices are highly consistent (73.4%). The most frequent disagreement among indices is mostly associated to Uppanar Estuary and, particularly, in the inner stations. This difference might be related to a time-lag response of benthic foraminifera in terms of diversity and assemblages' compositions as well as of the pollution indicators in response to enhanced riverine input. This study further supports the application of foraminiferal-based indices in EcoQS assessment in transitional environments including tropical Indian estuaries. It also fills the gap of knowledge by providing a seasonal perspective on the variation of EcoQS based on a monthly-scale sampling.