Cycling of Urea Associated with Photosynthetic Activity of Phytoplankton in the Euphotic Zone of Tropical Lakes, Brazil.

Abstract

The in situ decomposition rate of urea was measured in the euphotic zone of three tropical lakes, Dom Helvécio, Jacaré and Carioca, located in the Rio Doce Valley Lake System, Brazil, during rainy and dry seasons. The decomposition rate of urea (as the sum of urea carbon incorporation into particulate matter and CO2 liberation into water from urea) in surface waters was 15 to 41 μmol urea⋅m-3⋅d-1, except for low rates observed in Lake Jacaré in the rainy season. High decomposition rates were observed in Lake Carioca which is eutrophic in character. These rates decreased with depth. The greater part of urea decomposition took place during the phase of CO2 liberation. The urea turnover time at surface waters in Lakes Dom Helvecio, Jacaré and Carioca was calculated, respectively, as 6, 280 and 4 days in the rainy season and 7, 8 and 2 days in the dry season; the values increased with depth. The present results indicate that urea in waters is decomposed by phytoplankton rather than bacteria and that it makes a significant contribution as a nitrogen source for phytoplankton. In tropical lakes, urea is rapidly recycled in the upper euphotic zone during both rainy and dry seasons.