Predicting Transport of Nutrients from Three Tributary Rivers of Taal Lake, Philippines
Inflows from the Balete, Wawa and Laurel Rivers and from fish cages within Taal Lake, Batangas, Philippines were studied to assess the input of nitrogen (N) and phosphorus (P) into the lake. Physical parameters such as temperature, total dissolved solids (TDS), pH, conductivity and dissolved oxygen (DO) were recorded in situ. Nutrient concentration and wind data collected were used as inputs in a modified nutrient transport model simulating advection and dispersion of nutrient concentrations in Taal Lake. Results showed significant concentrations of P (P < 0.01) and N (P < 0.05) in Laurel River, with mean discharges of 5 and 4 kg d-1, respectively, while Balete and Wawa Rivers contributed high mean discharges of 85 kg d-1 nitrate-nitrogen (NO3-N). Measurements of TDS and conductivity were similarly significant (P < 0.01) in the fish cages. Simulated distribution patterns of nutrients revealed that during the Northeast (NE) monsoon, nutrient discharges from the Laurel and Balete Rivers were transported southward while nutrients coming from the Wawa River moved in the northwest and southwest directions. During the Southwest (SW) monsoon, nutrients from Laurel were transported northwest while nutrients from Wawa and Balete moved in the southwest direction. High N and P concentrations in Laurel could be attributed to the neutral pH measured throughout the study period since P is soluble and N-fixers grow best at circum- neutral pH. NH4-N and NO3-N are soluble ions and are easily leached, so that losses of these ions from soils to freshwaters reflect the amount of fixation in the soils. A nutrient transport model of Taal Lake was formulated to quantitatively analyze the transport and fate of P and N nutrients that enter the lake via tributaries. Nutrient discharges into the lake were transported by water movements which had three main influences: height differences of surface level with gravitational flow, density with buoyancy or sinking and surface wind stress with transfer of momentum as well as energy. Wind stress was responsible for the slow nutrient transport in the lake.
Key Words: advection and dispersion model, nitrogen, nutrient transport, phosphorus, Taal Lake, water quality sampling
Abbreviations: DO – dissolved oxygen, NAMRIA – National Mapping and Resource Information Authority, PHIVOLCS – Philippine Institute of Volcanology and Seismology, SP – soluble phosphorus, TDS – total dissolved solids, TP – total phosphorus