The main aim of this PhD thesis was to investigate regulating factors of the following processes from the nitrogen cycle in aquatic ecosystems: biologic nitrogen fixation, nitrification and denitrification. Although these processes have been investigated in sediment, periphyton and rizosphere of an aquatic macrophyte in some ecosystems, most of the research has been performed in an Amazonian lake (Lake Batata -PA) and a coastal lagoon (Lagoon Imboassica - RJ). Biological nitrogen fixation has been measured with the acetylene reduction method and it has been shown that the availability of organic carbon can regulate the occurrence of this process. Denitrification has been measured with the acetylene blockage and isotope pairing methods and it has been shown that under anoxic conditions, first nitrate and then organic carbon are the main regulating factors to this process and that phosphate availability can also limit this process. Nitrification has been measured with the isotope dilution technique and it has been shown that phosphate can limit this process. This was the first record of phosphate limitation on nitrification in an aquatic ecosystem. The process of bioturbation can enhance nitrification and denitrification and promote an increase in consumption or production of ammonia from the sediment to the water column.

email: aeprast@biologia.ufrj.br