Biophysical Survey of the Hinatuan Enchanted River Underwater Cave System (HERUCS) and its Connecting River, Surigao del Sur, Southern Philippines

A rapid survey inside the Hinatuan Enchanted River Underwater Cave System (HERUCS) and its connecting river was done to gather baseline data on the biophysical aspects of the cave and river ecosystem. It involved collection of water samples, sediments and macrofauna. Water samples were analyzed for the following parameters, namely: nitrates, phosphates, turbidity, BOD, pH, alkalinity, conductivity and total coliform. Sediment samples were analyzed for total organic matter and grain size. A listing of macrofauna found within the length of the established sampling station was also collected, photo-documented and identified. Water from inside the cave have a pH range of 7.35– 7.39, a BOD of 1.67– 4.00 mg/L, a 16 ppt salinity (at cave entrance), and, nitrate and phosphate values of 0.32 –0.44 mg/L and 0.10–0.52 mg/L, respectively. The mean total coliform count of the water sample from the upstream sampling site was 1,700 MPN/100ml. There were 5 organisms belonging to 4 families recorded from inside the cave while 43 organisms belonging to 9 families were collected from its connecting river. Majority of the macrofauna samples collected were marine to brackish-water species and can be considered accidentals. Although there were no stygobites caught, the possibility of finding new species is high considering the depth of the cave and its many unexplored chambers. Further explorations are highly recommended with the use of mixed gas (i.e., Nitrox) for longer bottom time and more thorough sampling.