An Assessment of the Benthic Community Structure: Implications on the Water Quality of Lake Palacpaquen, San Pablo City, Philippines Perez, Terresita., Caasi, Hazel. de la Torre, Benjamin. Department of Environmental Science, Ateneo de Manila University, Loyola Heights, Quezon City, Philippines hectares (25.22%) covered with fish cages and fish pens -(City Government of San Pablo, 2000) RA 8550 limit: 10-12% of the total lake area To investigate the macroinvertebrates diversity and relate this to the water quality and physical features of Lake Palacpaquen. General Objective Preliminary Field Work Initial Survey of Sampling Site Identification of Sampling Points Sampling Points: Inlet, Pool, Periphery, Outlet Benthic macroinvertebrates sampling Physical characterization In-situ water quality check Water depth Sediment/ Substrate Other site features Substrate type % particle size weight Grain/particle size Statistical Analysis Application of BBI, FBI, and other indices and metrics Sorting Measurement of taxa richness and diversity Laboratory processing Invertebrate identification Methodology Abstract Water Quality pH levels are within the standard for Class C waters, which is (DAO No. 34, 1990). Minimum DO conc. is 5.0 mg/L for Class C (DAO No. 34, 1990). Turbidity levels were highest in January for all sites. Conductivity was lowest in December and highest in January across all sites. Substrate Characterization November January Macroinvertebrate Diversity AngulyagraBellamyaPomaceaPhysellaGyraulus TarebiaPhysastraThiaraMelanoides Corbicula Chironomidae (midge larvae)Tipulidae (cranefly) Ceratopogonidae (biting midge) Stratiomyidae Culicidae (mosquito) Dixidae (dixid midge) Adephaga (water beetle) Caenis (mayfly) Cloeon (mayfly) Sericostomatidae (caddisfly) Helicopsychidae (caddisfly) Lumbricidae (worm) Octogomphus (dragonfly) Glossiphoniidae (leech) Macroinvertebrate Diversity Lake is possibly affected by organic pollution. In situ water quality parameters had low correlations with biotic indices but they contribute to physical characteristics of habitat together with the substrata. Conclusion Substrate composition at different points in the lake varied in type and sizes, from large organic debris to finer particles. BBI: Lake is moderately polluted to lightly polluted. FBI: substantial pollution is likely; water quality is fairly poor. Evenness is negatively correlated with % dominance (r = ). Taxa richness is positively correlated with H’ (r = 0.791). Indices and MetricsNovemberDecemberJanuary Shannon-Wiener Diversity Index (H') Taxa Richness (Family) Evenness Index (E) % Dominant H’, TR and E were lowest in November and highest in January. Lake Palacpaquen is the shallowest and second largest lake in the Seven Lakes of San Pablo City, Laguna, Philippines. The lake is threatened by water quality deterioration caused by anthropogenic activities. The benthic macroinvertebrate community of the lake was investigated. The impact of water quality deterioration on the benthic community was evaluated using the Belgian Biotic Index (BBI) and the Family-Level Biotic Index (FBI). Samples were obtained from four stations in the lake: inlet, pool, periphery and outlet and in-situ water quality parameters were measured. Substrate varied from organic litter (leaves, twigs, roots) to gravel and fine sediments. DO concentrations and turbidity were generally lower in November while pH level, temperature and conductivity were lower in December, following a series of heavy rainfall. Dominance of mollusca genera Melanoides, Tarebia, Bellamya and Angulyagra were observed to dominate. Six other genera of mollusks were found, including the bivalve Corbicula and the foreign species, Pomacea canaliculata. Larvae of the Family Chironomidae were abundant in the pool. Taxa diversity was highest in the periphery and the lake outlet in December and January. Diversity of the entire lake obtained from composite samples was highest in January (H’= 0.834). BBI scores were lowest in November and highest in January for all sites. Based on the BBI, Lake Palacpaquen was moderately polluted in November and December, and was lightly polluted to unpolluted in January. FBI scores were lowest in January (6 = fairly poor), but this rating is still indicative of the possibility of substantial pollution.