Colleen NagelAquatic 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 HemisphereITCZDepth~40 metersWarm, 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 macrophytesFloating macrophytesSubmerged macrophytesPhytoplanktonMicrocystisZooplanktonBenthosFish: Sarotherodon

mossambicus (tilapia), Petenia kraussii, Symbranchus marmoratus (freshwater eel), and Rhamdia quelen (catfish) Petenia kraussii

Population of Aragua Valley: 2 million peopleMain crops: Sugar cane, vegetables, fruit trees

(banana, mango, citrus), tobacco, cotton, and maize

IndustrializedLake 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