Colleen Nagel Aquatic Ecology of the Tropics North-Central Venezuela Surface Area: 350 km^2 Metamorphic rocks from marine sediments (bedrock) Aragua Valley : Cordillera de las Costas to the north and the Serrania del Interior to the south Northern Hemisphere ITCZ Depth~40 meters Warm, monomictic lake, hyper-eutrophic Turmero, Venezuelan city, in catchment of Lake Valencia Precipitation: 1,000- 1,500mm Loss of water: Evaporation and subsurface out-seepage During high lake level periods water is lost from outflow over sill at 427 m in SW part of lake Fig. 1. Geographic location of Lake Valencia, Venezuela. X denotes the sampling site. Average net photosynthesis rate: 7.5 gCm^-2d^-1 High primary productivity lowers O2 levels in deeper water Cyanobacteria (90% of plankton biomass) dominate over diatoms and green algae Emerged macrophytes Floating macrophytes Submerged macrophytes Phytoplankton Microcystis Zooplankton Benthos Fish: Sarotherodon mossambicus (tilapia), Petenia kraussii, Symbranchus marmoratus (freshwater eel), and Rhamdia quelen (catfish) Petenia kraussii Population of Aragua Valley: 2 million people Main crops: Sugar cane, vegetables, fruit trees (banana, mango, citrus), tobacco, cotton, and maize Industrialized Lake salinity has increased over the past several decades, due to input waters being diverted and groundwaters were pumped for agricultural irrigation. Raw sewage discharge, high nutrient concentration and high rates of primary production Financed by Interamerican Bank for Development Laws: Organic Law for Environment (1976) Ordinance for Classification of Waterbodies According to Its Use (1978) Control natural resources, waterbodies water treatment, and industrial development