GEOECOLOGY OF LAGUNA-DE-BAY, PHILIPPINES .1. TECHNO-COMMERCIAL IMPACTON THE TROPHIC LEVEL STRUCTURE OF THE LAGUNA DE BAY AQUATIC ECOSYSTEM1968-1980

The human impact on quasi-natural systems was analyzed in constructing quantitative trophic models describing the changing situation of the Laguna de Bay ecosystem's aquatic compartment for two time frames. The trophic model developed for the late 1960s described the biological f ood chain during the pre-fishpen period. On the other hand, the model for the early 1980s emphasized the effect of introducing milkfish (Cha nos chanos) in fishpens, with special reference on its impact on the p rimary productivity of the lake's ecosystem. The sum of all flows to d etritus for the entire system was 3.2 times lower in 1980 than in 1968 . Due to the presence of milkfish in fishpens, the total net primary p roduction decreased twofold during the 1980s. The development of milkf ish as one central part of the lake's ecosystem was the result of the interplay of several factors related to commercial interests, politica l affairs, climatological changes and biological responses involved in the overproduction of milkfish during the 1980s. Preliminary analysis of the nitrate and phosphate loadings from 1973 to 1982 were consider ed as important indicators in analyzing pollution-related lake eutroph ication. Since the 1840s, the status of the Laguna de Bay ecosystem ha s demonstrated the steadily growing impact of human activities, especi ally on the dynamics of the lake's trophic structure.