Establishing the impact of acid mine drainage through metal bioaccumulation and taxa richness of benthic insects in a tropical Asian stream (The Philippines).

The Boac-Makulapnit River (The Philippines) is contaminated with heavy metals primarily due to acid mine drainage (AMD). Heavy-metal levels in river water, sediment, and algae suggest that Cu is the primary contaminant in this system. Copper concentrations decrease longitudinally, indicating a contamination gradient away from the mine. This is also reflected in the whole-body metal concentration of the caddisfly Hydropsyche, wherein higher Cu levels are observed in upstream stations (677-980 microg/g in 2001). In 2002, AMD formation was observed in a midstream station resulting from the oxidation of mine tailings in floodplains deposited during a 1996 tailings spill event. This consequently increased heavy-metal concentrations locally in environmental media and is likewise perceived to have caused the absence of aquatic insects in the midstream reach. Copper concentrations also increased in Hydropsyche found downstream of the new AMD site during this year. Furthermore, even the lower Cu concentrations in Hydropsyche observed in downstream stations are still at least six times higher than the established regional baseline value (29.9 microg/g), which is calculated from samples from clean tributary streams. These tributary streams also showed higher taxa richness for aquatic insect community in riffle zones. Both biomonitoring tools suggest that the whole length of the river is severely impacted. Rehabilitation projects will be underway next year; therefore, such monitoring programs will truly be beneficial when gauging river habitat recovery.