Report Number: 160
Year: 2016

Nutrient Assessment of Togcha River, Estuary & Bay: Use of Dominant Sedimentary Phosphorus (P) Fractions to Identify Anthropogenic P Contributions & Potential Impacts

The Togcha River is the smallest of several rivers found in southern Guam. It is ~6 km long with a catchment area of ~18 sq km and drains into sea at Togcha Bay on the eastern side of the island. The river receives stormwater runoff from a nearby housing estate and effluent from a sewage treatment plant (STP) in the same area. It is also receives surface and subsurface drainage from two golf courses. To gain insight into the nutrient status of the waterway, phosphorous levels were examined in surface sediments from 9 sites in the river above the estuary, 14 sites in the estuary itself, and 15 sites in the bay. Sediment collections were made in 1999: at the end of the dry season in May and towards the end of the wet season in October. A sequential extraction technique was employed to isolate the following five P fractions from the sediments: loosely sorbed P, ferric-iron bound P, authigenic apatitic P (including calcium carbonate associated P), detrital apatitic P, and organic P. All samples were also analyzed for total organic carbon and reactive iron. Average total P concentrations in river sediments ranged from 518-1030 µg/g (dry weight) over the study period. Mean levels in the estuary and bay ranged from 621-705 µg/g and 245-269 µg/g respectively. Overall, organic P was the dominant P fraction identified in the river and estuary and accounted for ~30-40% of total P. Ferric-iron bound P and detrital apatitic P ranked next in order of abundance followed by loosely sorbed P and authigenic apatitic P. In bay sediments, ferric-iron bound P was the dominant fraction in all samples with average levels representing close to 50% of the total P pool. Loosely sorbed P ranked second in order of abundance and averaged ~25% of the pool, followed by organic P and then the apatitic P fractions. Significant temporal differences between overall means were identified for each P fraction in at least one of the three locations studied. Several P fractions were also significantly correlated with one another. These findings are addressed and explained where possible in the text. The P status of the region was evaluated by reference to reported P levels in aquatic sediments from elsewhere on Guam and other parts of the world. Comparative assessments based on total P, total inorganic P and organic P were found to be of limited use because of the inability to differentiate between biologically available and non-biologically available forms of P. Since ferric-iron bound P and loosely sorbed P are potentially available to the biota they were considered to be of much greater value as an assessment tool. Collectively these fractions are referred to as non-apatitic P and together represent less than 30% of total P in clean coastal sediments and upwards of 75% of total P in sediments from grossly polluted waters. Based on these figures it was concluded that the Togcha River and estuary are mildly to moderately enriched with P, depending on the season, while the immediately adjacent portions of Togcha Bay are grossly enriched year round. The implications of the data are discussed with respect to upstream discharges from the STP, dominant estuarine processes that liberate sediment bound P into the water column, and the periodic algal blooms that occur in the bay.

M. Thomas Nadcau
Gary R.W. Denton