AIM: This work is aimed at extending the understanding of urea cycle in freshwater ecosystems. Its degradation rate concerning microorganisms activities was measured in the turbid waters of Pindaré and Turiaçu rivers located on the pre-Amazonian floodplain, Brazil; METHODS: The Pindaré and Turiaçu aquatic systems have distinct dry and rainy periods. The investigation was developed in the middle of the rainy period, and field activities were carried out in the extensive water bodies from the middle to lower reaches. The rate of degradation in the surface waters of the two aquatic systems was determined with in situ simulation technique using 14C-labelled urea. Photosynthetic rates were determined by the radiocarbon technique simultaneously with experimental measurements of urea degradation rate; RESULTS: Urea degradation rates (sum of carbon incorporation into particulate matter and CO2 liberation into water) were 2.0 mg urea C m-3 day-1 in the Pindaré and 17.1 mg urea C m-3 day-1 in the Turiaçu waters. Daylight values were obviously higher than those in the dark, and the urea degradation rates in Turiaçu showed much higher values than those in Pindaré. Most of the urea degradation occurred during the CO2 liberation phase. The average of chlorophyll a specific urea degradation rate was 0.13 and 0.83 mg urea C mg chl.a-1 day-1 in both river waters. The ratio of urea carbon degradation to photosynthetic carbon assimilation in both waters averaged 1.2 and 4.2%, respectively. The residence time of urea in the surface water was calculated as 2.3 to 4.5 days in the Pindaré and 0.21 to 0.50 days in the Turiaçu. Much shorter residence times were obtained in the Turiaçu due to the high degradation rate of urea; CONCLUSION: The correlation coefficient between the urea degradation rate and chlorophyll a or photosynthetic rate showed a statistically significant value in the Turiaçu. This indicates that in the Turiaçu system the urea degradation rate was proportional to the standing crop of phytoplankton and their photosynthetic rate. A strong relationship between the urea carbon incorporation rate and photosynthetic rate in the light bottles was observed, indicating that the carbon incorporation into the phytoplankton cells was also related to the photosynthetic rate. The present brief residence time indicates that the urea was being rapidly recycled in the euphotic zone of the investigated systems in the rainy period.
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