A new trophic state index for assessing eutrophication of Laguna de Bay, Philippines
Eric Jan Escober, Maria Pythias B. Espino
Abstract
For decades, Laguna de Bay has been affected by urban, industrial, and agricultural wastes resulting in nutrient-enrichment, decline in productivity and biodiversity loss, among other environmental problems. Eutrophication is a perennial problem of the lake that remains unabated. In this study, a calibration of Carlson's original trophic state index (TSI) was made to develop a new TSILdB model using recent monitoring data on chlorophyll-a, nutrients (nitrate, nitrite, ammonium, phosphate) and Secchi depth and the Philippine water quality guidelines. Nutrient concentrations were determined using ion chromatography whereas chlorophyll-a and Secchi depth were measured using field devices. TSILdB and four comparable models classify Laguna de Bay as generally hypereutrophic. A TSI assessment provides an alternative to existing water quality guidelines and is practical in communicating policies for the protection and management of the lake. Based on the hypereutrophic thresholds of TSILdB, adjustments in the current guidelines to abate further eutrophication and attain a sustainable trophic state of the lake are proposed as follows: 5.3568 µg/L for chlorophyll-a; 0.295 m for transparency; 25 µg/L for total phosphorus; and 2.72 mg/L for total nitrogen.