Can the ecological status of three differentially impacted monsoonal tropical estuaries in NW India, be adequately assessed by a common estuarine benthic index?

Abstract

Effective management measures for improving rapidly deteriorating estuaries around the world are hugely dependent on accurate evaluation of the ecological status, which is a challenge in itself considering the Estuarine Quality Paradox. Several Benthic Biotic Indices (BBIs) have been tested in estuaries in different bio-geographical zones with no concurrence whatsoever about the best approach in evaluating the ecological status. An often suggested approach is to apply several indices concurrently to gauge the overall status. However, this method typically results in more complexities due to the multiple contradicting conclusions derived by different indices besides increasing the computational load and the requirement of data matrices. Therefore, it would be reasonable to validate available BBIs in various estuaries of a biogeographical zone and select a common estuarine benthic index that can respond effectively to diverse stressors, considering the natural estuarine gradients. Towards this goal, five most frequently employed BBIs viz. AZTI's Marine Biotic Index (AMBI), Multivariate AZTI's Marine Biotic Index (M-AMBI), BENTIX, Benthic Opportunistic Polychaete Amphipoda index (BOPA) and Benthic Opportunistic Oligochaete Amphipoda index (BO2A) were applied in the three differently stressed estuaries (Ulhas, Amba and Savitri) of NW India during the three major seasons (premonsoon, monsoon and postmonsoon). It was aimed to test if any of the applied BBIs could appropriately determine the ecological status across salinity zones of all the estuaries and thus can act as a common estuarine index. While the indices generally concurred in high salinity zones of the heavily polluted Ulhas, discrepancies in ecological status were observed in other zones and estuaries, mostly erroneous when compared to the pre-determined status. As monsoonal index values appeared to have been influenced by the disturbance due to fresh water inflow during this season, the average of the two non-monsoonal seasons was considered for deriving the final status by all indices. Among the BBIs, AMBI exhibited stronger response to pressure gradients and was most successful in discriminating the correct ecological status of most of the salinity zones across the three estuaries. Thus, this index could reasonably be considered for ecological status evaluation of estuaries of NW India irrespective of the diverse anthropogenic stressors and macrobenthic composition. It is recommended that this conclusion be reaffirmed by repeating similar investigations in other monsoonal tropical estuaries of this zone.