Prediction of Trophic State of San Marcos Lagoon Based on AQUATOX Eutrophication Model

Eutrophication of lakes is related to the decrease in water quality. It generates problems that are particularly important if the water is used for irrigation and human consumption. This research assesses and predicts the trophic state of San Marcos Lagoon using the AQUATOX model. Six sampling campaigns were carried out, for three months and at intervals of two weeks. Simple and compound water samples were taken at three different points in the lagoon to measure physicochemical and biological parameters such as chlorophyll-a, phosphates, nitrates, cyanobacteria, dissolved oxygen, among others. Additionally, a sensitivity analysis was performed to determine the critical parameters of the model, which will be used to improve the model fit. The comparison between the measured and simulated data indicates that the current trophic state of the San Marcos Lagoon varies between eutrophic and hypereutrophic. Three error indexes used at calibration and validation stages to calculate the goodness of fit (R2, RMSE, and ER). The obtained R2 for chlorophyll-a, total phosphorus and Secchi disc depth validation were 0.89, 0.97 and 0.93, respectively. Once the model is validated, the trophic state prediction is made for the next six months, obtaining 1.56 ug/L for chlorophyll-a, 114 ug/L for total phosphorus and 1.21 m for Secchi disc depth. These values correspond to a hypereutrophic state. Also, the simulation of a mitigation scenario is presented, where a mesotrophic state is obtained through the renewal of 25% of water volume and detritus removal.