Zooplankton Community as Indicator of Trophic Status of Lake Tadlac in Los Baños, Laguna, Philippines

Tadlac Lake was heavily used for aquaculture until the late 1990s which resulted in a deterioration of its water quality. Aquaculture operations were banned to rehabilitate the lake after a massive fish kill in 1999. However, no thorough assessment of the rehabilitation's effectiveness has been conducted. In order to assess the success of the rehabilitation efforts, this study evaluated the lake's trophic status through study of its zooplankton community and physical-chemical parameters. Zooplankton samples and physical-chemical parameters were collected once a month from October 2017 to March 2018. Overall, 25 zooplankton species were documented during the sampling period. Rotifera dominated the zooplankton community with a relative density of 46.19%, followed by Copepoda (36.70%) and Cladocera (17.11%). Eutrophic indicator species Brachionus forficula had the highest density among the zooplankton taxa, followed by Keratella tropica and Brachionus havanaensis. Significant variation in zooplankton density was documented over the sampling period (p<0.05). Cluster analysis and analysis of similarities (ANOSIM) showed variation in the zooplankton density across the sampling months. Similarity percentage (SIMPER) revealed that the differences in plankton density were due to the densities of the most abundant taxa. Canonical correspondence analysis (CCA) showed that the zooplankton community was highly influenced by dissolved oxygen, conductivity, pH, biological oxygen demand, and temperature. The high density of the eutrophic indicator zooplankton species and the nutrient concentration of the lake indicated that Tadlac lake was still under eutrophic conditions. Biotic indices further confirmed that the lake was experiencing eutrophic conditions with moderate organic pollution. Therefore, additional approaches are necessary to effectively control the nutrient enrichment in the lake to improve its condition.