Trophic Development of Taal Lake [Philippines]

anglais. An assessment of the current trophic stae of Taal Lake was done by reviewing post limnological data and integrating these with the initial results of an on-going study on the lake. A sufficient number of sampling points in 1989 gave rise to a data set that may be considered representative of the baseline conditions in the lake before and in the early beginnings of the floating cage aquaculture technology in the lake. The turning point in the lakes' trophic development seems to have occurred between the early 1990s and 1998 when the Ceratium blooms first appeared. Today, microcystis along with Ceratium, is progressively building up its populations in the lake. The results of the assessment point to the following: a) Taal Lake has advanced tremendously in its trophic state from mesotrphic-oligotrophic state in 1989 to its eutrophic state. b) Physically, the lake is moderately eutrophic. c) based on alkalinity, the lake is highly productive, but s ome areas seem to manifest diminished alkalinity. d) based on total inorganic nitrogen, total phosphorus, and total organic carbon, the lake is eutrophic. e) Based on chl a, it is oligotrophicthe trophic states of the five sampling stations are: a) Balete-slightly eutrophic in 1989, now eutrophic, b) Mataas-na-Kahoy-oligotrophic in 1989, now eutrphic. c) Sta. Teresita- no baseline evaluation, presumed oligotrophic in the paststill oligotrophic. d) Alas-as-oligotrophic in 1989now still oligotrophic e) Bonintiang Malaki- no baseline evaluation in the pastnow slightly eutrophic. Trophic build-up in a body of water is a natural phenomenon. A eutrophic body of water is beneficial to its fisheries because this ensures a good source of natural food supply for the fish, at the same time it lowers the cost of fish production in aquaculture. However, extreme eutrofication is not desirable because it brings about aesthetic problems and undermines aquatic health. The need for Taal Lake is a rational management that takes cues from the results of scientific findings and makes use of available technology. The present results tend to show that aquaculture and river inflows are the potential sources of nutrient enrichment in the lake over the past years. This enrichment has been supporting the lakes' high fishery productivity and is responsible for its good fish (tilapia) quality. Lake management should strictly regulate the further growth of the lake's use for aquaculture. The heyday of aquaculture continues to this day. To ensure that this will extend to the future, municipalities should now divert the existing lakeward flow of leachates away from their open garbage dumps. Taal Lake's shoreline communities should revive their traditional respect for their Lake (which the authors witnessed in the beginning of their Taal Lake studies in 1989), particularly on distancing their latrines adequately from the waters edge and avoiding the lake's pollution with solid wastes including raw animal excreta. Taal Lake today needs vigilance more than ever to prevent its present highly acceptable share from burning into a deplorable condition.

anglais. CF: Nishibori professorial Chair Lecture (on the Environment in the field of Environmental Science and Management) 30 Jun 2005 College, Laguna (Philippines)