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Trophic Relations and Nutrient Recycling in a Tropical Floodplain Lake

Danny REJAS ALURRALDE researcher laureate

°1968 Bolivia
Licenciate in biology, Universidad Mayor de San Simón, Cochabamba, Bolivia, 1997

Trophic Relations and Nutrient Recycling in a Tropical Floodplain Lake

A great deal is already known about the ecological function of lakes, rivers and alluvial tributaries in our moderate climatic zone. However, when it comes to such systems in tropical environments, our knowledge is far more limited and occasionally even non-existent. Nevertheless it is very important for us to have a better understanding of how the food web works in tropical aquatic systems in order to organize fishing in a sustainable way. Fish are an essential part of the daily nutrients required in tropical regions.
This is why Danny Rejas Alurralde decided to study the ecosystem and in particular the food pyramid in Laguna Bufeos in the alluvial plains of the River Ichilo in Bolivia for his Ph.D. Laguna Bufeos is an oxbow lake created when a bend in the river was cut off following a natural or anthropogenic straightening of the river bed. The food pyramid in a small lake like Laguna Bufeos comprises the classical transfer of nutrients of phytoplankton (algae) to zooplankton, to herbivorous fish (plant-eaters) and predatory fish (fish-eaters) on the one hand, and on the other the microbiological loop that works in the opposite way: instead of converting organic matter into a higher level organism, dead plankton and possibly organic material from outside are transformed into a solution of inorganic nutrient substances.
Since there is a high level of fish production in Laguna Bufeos (about 11 g carbon per sq. m. per year) this must be based on a high level of plankton and/or microbial production. However, neither contribute very much towards the production of fish (only 0.82 g carbon per sq. m. per year).
Measurements and experiments led Danny Rejas Alurralde to conclude that most organic substances are transferred from the bottom to the top of the food web by way of short chains. About 90% of the transfer fluxes in the food web at Laguna Bufeos appear to move directly from the detritus to the fish. This detritus is a mixture of coarse organic waste that includes dead phytoplankton, bacteria and plants.

report: Prof. W. Baeyens, Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Belgium