Nutrient and sediment concentrations in the Pagsanjan–Lumban catchment of Laguna de Bay, Philippines

The off-site transport of nutrients and sediments can have detrimental effects on water quality and ecological health of waterways. This study was conducted to determine the concentrations of nutrients and sediments in surface waters contributing to Laguna de Bay, Philippines. Four sites representing various land uses and non-point and point sources of contamination (rice, vegetable and coconut production systems, and piggery) were selected and instrumented with automated water samplers for time weighted collection of water samples. Total suspended solids (TSS), total Kjeldahl nitrogen (TKN), total phosphorus (TP) and total carbon (TC) were determined in unfiltered samples. The effluent from piggery farms (a point source) had the highest TSS, TKN, TP and TC concentrations. For the other three sites, the dominant land use upstream of the sampling sites greatly influenced the concentrations of nutrients and sediment. Among the croplands, the site associated with rice as a dominant land use in the Pagsanjan–Lumban catchment contributed more suspended sediments and higher concentrations of TKN and TP as compared to the vegetable (Lucban) and coconut dominated (Cavinti) sites. In the majority of cases at all sites the TKN and the TP concentrations exceeded the Australian trigger values for freshwater quality for total nitrogen of 0.35 mg/L and TP of 0.10 mg/L. In addition to land use, TKN was correlated with TSS particularly in the Majayjay (R2 = 0.28), Pagsanjan (R2 = 0.51) and Lucban (R2 = 0.36) sampling sites. The relationship between TP and TSS was also significant at Lucban (R2 = 0.29) but not at the other sites. Rice paddies are subjected to wet cultivation (puddling) and have loose topsoil and are kept flooded most of the time which allows larger amounts of suspended sediments and nutrients to leave with the drainage water.

► First extensive monitoring of nutrient and sediment concentrations in major rivers in the Pagsanjan–Lumban catchment.
► Current land management practices greatly influenced the concentration of nutrients and sediments in rivers in the study area.
► A nonpoint source of contamination (piggery effluent) contributed highest concentrations of TKN, TP, TC and TSS.
► Off-site movement of total Kjeldahl nitrogen was strongly associated with the suspended sediments.