A PALEOLIMNOLOGICAL STUDY: ESTABLISHING THE IMPACTS OF HUMAN ACTIVITIES ON TWO PHILIPPINE FRESHWATER CRATER LAKES

Abstract

Most Philippine freshwater lakes are threatened by eutrophication arising from proliferation of unsustainable aquaculture activities and atmospheric deposition. Small freshwater lakes remain understudied in the Philippines, yet are crucial in the provisioning of ecosystem services and livelihoods of lake communities. Lake monitoring programmes are hampered by deficient data for past limnological lake conditions, and typically occur only after significant environmental degradation. This study utilised paleolimnological techniques to reconstruct the timing, magnitude and cause of past environmental changes by anthropogenic activities on two small Philippine freshwater crater lakes, Lake Mohicap and Lake Yambo. One sediment core was retrieved from each lake for analysis in this study. Chronology of sediment cores was established using a series of radiocarbon and radiometric dating techniques. Past limnological conditions were then inferred from changes in diatom assemblages with time, with understanding of their ecology used in wider literature. SCPs and trace metals further complemented diatoms as indicators to trace impacts from atmospheric deposition. Findings revealed that major shifts in the productivity levels of the lakes were related to multiple stressors from early human disturbances (~AD 1895), atmospheric deposition (~AD 1960), onset of aquaculture (~AD 1970) and possibly climate warming (~AD 1990). At present, both lakes have significantly higher productivity levels due to combined effects of all the above stressors. Future development and management of the seven crater lakes in San Pablo City has to account for these multiple stressors for effective long-term management.