Carlson's index, a poor cursor of trophic status assessment of Pulicat Lagoon, southeast coast of India

Pulicat Lagoon is a productive coastal lagoon on the southeast coast of India. Several studies of its physico-chemical and biological characteristics have been undertaken. This project was the first to attempt to represent the lagoon's ecological status in the form of a trophic state index. The investigation applied Carlson's Trophic State Index (TSI) to determine the trophic status of Pulicat lagoon over the years 2005 and 2006. The observations corresponding to the TSI scores for the transparency of the water column (represented by Secchi Disk Depth, an important parameter for calculating TSI) were compared with observations obtained during field sampling. Discrepancies were observed in the representation of the transparency of the water column as well as the trophic condition and those predicted by the TSI. These discrepancies suggested that TSI was not an effective index of the trophic status of Pulicat lagoon. The study thus proves the fallibility of classical indices for a case-specific scenario. The investigation points out that there is a need to develop newer and more accurate indices to represent water quality in a lagoon.