Monitoring of key water quality parameters in Laguna de Bay was carried out from 1986 to 1996 to determine trends in water quality and the trophic state of the lake. Monthly sampling and field analyses were made at randomly selected sites of the lake. Three-year trend analysis of the annual mean values of some key water quality parameters showed the worsening condition of the lake. Ammonia-nitrogen, nitrate-nitrogen, ortho-phosphate, total phosphorus, chemical oxygen demand, turbidity, conductivity, and chlorides increased while alkalinity, clarity, and dissolved oxygen decreased. The lake is judged to be extremely trophic or dystrophic. Uses of the lake as a fishery resource and as a source of domestic water supply are briefly discussed.

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