Assessment of water quality based on physico-chemical parameters and their relationships with the macro invertebrate communities of Mananga River in Cebu, Philippines

The Mananga River today is a source of potable water to meet the demands of a fast growing Cebu Metropolis. This study assessed the temporal and spatial changes in environmental quality of Mananga River based on selected physicochemical parameters in combination with the accompanying structural characteristics of its macroinvertebrate communities. Water and biological samples were collected from February to December 2006. Three sampling stations, each 100 m long, were selected to represent the upstream, midstream and downstream portions of the river. Study protocols were generally adopted from the United States Environmental Protection Agency (USEPA) Volunteer Stream Monitoring Manual (1997). The water quality of the Mananga River segments significantly varied with location, season and anthropogenic activities. Flow velocity and discharge, temperature, pH, nitrates, and total phosphates had significant temporal variations (P 0.05). Stream flow velocity was significantly correlated with pH, DO, nitrates, temperature, and TSS [total soluble solids] (P 0.01). Heavy rains were accompanied by increased amounts of nitrate-nitrogen and total phosphate, indicative of run-off from agricultural and domestic sources. Water temperature significantly correlated with DO [Dissolved Oxygen] pH and TSS (P 0.01), and the latter significantly correlated with BOD [Biological Oxygen Demand] (P 0.05). The BOD in the downstream exceeded the water quality standard (WQS) during low rainfall. Fecal coliform count (280-900,000 MPN/100mL) in all the sampling stations exceeded the DENR WQS for Class C waters, reflecting agricultural and domestic sources. Thirty-seven macroinvertebrate families representing 15 orders under 7 classes were recorded. The most abundant group was Class Insecta (25 families), followed by Class Gastropoda (5 families). Family Thearidae was the most persistent. High persistence was also observed for families Physidae, Caenidae, Elmidae, and Chironomidae. Plecoptera of the Ephemeroptera, Plecoptera and Trichoptera (EPT) groups was totally absent. Temperature had a negatively significant correlation with the most number of macroinvertebrate families (8 at P 0.05). Family Elmidae was negatively correlated with the most number of parameters (temperature at P 0.01; BOD and TSS at P 0.05). Family Neritidae positively correlated with BOD and fecal coliform (P 0.01 and P 0.05, respectively), indicating tolerance to organic pollution. Sampling stations S1 and S2 had more macroinvertebrate families (31 and 23, respectively) and higher overall H' or Shannon-Wiener diversity index (2.127 and 1.808, respectively) compared with sampling station S3 (10 families H'