A New Trophic State Index for Lagoons

This paper proposes a new nitrogen-based trophic state index (TSI) for the estimation of status of eutrophication in a lagoon system. Nitrite-nitrogen (NO2-N) is preferable because of its greater abundance in Chilika lagoon and its relation to other criteria of trophic state, for example, chlorophyll-a (Chl-a) and Secchi disk depth (SDD). Nitrite is preferable over nitrate because the former decreases the fluorescence and affects photosynthesis, thereby controlling primary production. This paper also computes TSI using Chl-a and SDD. The three parameters account for the biological, chemical, and physical characteristics of the lagoon. It will be possible to estimate the TSI of freshwater and brackish water lagoons and other water bodies using the new expressions taking into consideration the spatial and temporal variability in the dataset. Depending on the data availability, alternative TSI (Chl-a) and TSI (SDD) can account for the biological and physical contributions to eutrophication. The estimated TSI can account for Chl-a and NO2-N up to 322.18？mg？m？3 and 61.99？μg L？1, respectively. The TSI based on these three parameters can serve as a complimentary and predictive tool for lagoon management and field programs to monitor the health of a lagoon. 1. Introduction It is much easier to convey to the public the status and nature of a lagoon through an index, which provides the scientific accord of eutrophication and character of the lagoon. The monitoring of water quality, especially in the inland water bodies, aquifers, lakes, reservoirs, and lagoons, is important as it helps with the management of the eutrophication and productivity of the water body. Carlson [1] made early attempts to define an index, termed trophic state index (TSI), which could tell about the trophic status and nature of the lakes. Based on this trophic index, further classification of eutrophication of a lake into oligotrophic, mesotrophic, and eutrophic was possible. Because of water pollution and contamination of inland and coastal water bodies, it has become important to develop methods to determine such index of eutrophication of coastal lagoons that provides better spatial and temporal coverage. Carlson [1] presented the trophic condition on a 0–100 scale; however, the author reported three different TSI values, that is, total phosphorus (TP), chlorophyll-a (Chl-a), and Secchi disk transparency depth (SDD) [2]. To eliminate this confusion, Osgood [3] suggested using differences in the indices to assess the water quality in lakes. The basis of the development of an index of