Development of an acute sediment toxicity test using an endemic benthic macroinvertebrate, Chironomus species to assess the toxicity of Philippines' Pasig River sediments

In the absence of a sediment quality guideline, mere characterization regarding the identity and levels of the constituents present in sediments, will not contribute much to an effort of establishing the impact of sediment quality to the aquatic environment. Available standard methods for sediment toxicity tests require the use of species that may not be accustomed to a tropical climate such as in the Philippines. In this regard, area of this study was to assess toxicity of the sediment from Pasig River through the use of an endemic midge (order Diptera, family Chironomidae) collected from Los Banos, Laguna, Philippines as test species. Pasig River is one of the most important waterways in the Philippines, traversing six major cities of Metro Manila Such strategic location of the River, also explains why it is heavily impacted by both point and nonpoint sources of toxicants. Sediment samples were collected in April 2007, month of dry season and November 2007, month of wet season at five sampling stations along the Pasig River-Delpan, Nagtahan, Pandacan, Guadalupe, and Bambang. A modified 96-hour acute toxicity test was adopted in this study in which sediment samples were mixed with "control" sediment, also obtained locally and subsequently and exposed to the test species. Results suggested that the most polluted and most toxic was the Delpan station which yielded the lowest LC50. The LC50 was experimentally determined via probit analysis. Moreover, it was determined that sediment samples collected during the dry season exhibited lower toxicity towards these organisms than those samples collected during the wet season possibly due the effect of runoff. In view of the results obtained, the locally developed ecotoxicological assay performed on sediments of Pasig River can be used to assess and to distinguish sites along Pasig River that could be potentially toxic based upon its response to the locally grown Chironomids species.