Mapping declining oxygen in an aquaculture area (Pangasinan, Philippines) and in a bay impacted with untreated domestic waste (Manila Bay, Philippines)

Eutrophic conditions have often characterized the waters of Bolinao and Anda, Pangasinan due to extensive milkfish farming in the area. Increased nutrients and organic matter input from feed wastage have been observed in the channel as well as depleted oxygen levels which often lead to fish kills resulting to millions of losses. Manila Bay, on the other hand, drains a densely populated watershed with virtually no sewage treatment. It has evidence of hypoxia, eutrophication, and algal blooms. Surface, bottom, and water column average dissolved oxygen (DO) values according to the following criteria were compiled for the area: DO < 5.0 mg/l, DO < 2.0 mg/l, and DO <0.5 mg/l representing low, hypoxic, and near anoxic dissolved oxygen conditions, respectively. Data used were from the different field surveys for the two sites. The frequency of DO levels meeting these criteria per station over time for the two areas was then mapped out to show the stations which often have low, hypoxic, or near anoxic dissolved oxygen conditions representing mid to high-risk areas for oxygen depletion, respectively. There are 3 stations along Guiguiwanen Channel in Pangasinan that have low dissolved oxygen at least 75% of the time in the whole water column and near bottom hypoxic conditions at least 20% of the time. These are deeper stations that have less aeration. In the aquaculture area in Pangasinan, the areas that are often at risk for oxygen depletion are along channels that have restricted flow and increased organic matter input due to the different marine aquaculture activities. In Manila Bay, 7 (out of 35) stations had low dissolved oxygen 40% of the time. These were located near the coast and rivers which carries untreated domestic waste into the bay. However, deeper stations in Manila Bay also experience low oxygen due to poor ventilation.