Decomposition of Egeria densa Planchon (Hydrocharitaceae) in a well oxygenated tropical aquatic ecosystem

The aims of this study were to evaluate the decomposition rate (KT) of the submerged aquatic macrophyte Egeria densa Planchon and measure the associated nutrients release, using both qualitative and quantitative methods. Additionally, we compared the effects of microorganisms alone versus microorganisms plus invertebrates on the decomposition process. The experiment was performed using two mesh litter bags (45 µm for microbial activity only and 2 mm for microbial and invertebrate activity) during two distinct periods: the dry season, September to November of 2003, and the rainy season, January to February of 2004 in the Campelo Lake (Northern of Rio de Janeiro State, Brazil). The KTs, calculated using a single exponential model, were higher in the 45 µm mesh bags in the dry season and in the 2 mm mesh in the rainy season (dry KT: 45 µm=0.0384, 2 mm=0.0355; rainy KT: 45 µm=0.0483, 2 mm=0.0666), when higher temperatures (rainy season ~30°C; dry season ~25°C) were observed. The half-life of the E. densa detritus ranged from 14 to 27 days. The action of microorganisms was the main process causing the decomposition of E. densa in the Campelo Lake, as shown by the decomposition rate of the 45 µm mesh bags. However, invertebrates were important for the fragmentation, catabolism and leaching processes within the detritus contained in the 2 mm litter bags, accelerating its degradation, especially during the rainy season. The accelerated loss of chemical compounds such as nitrogen, carbon and phosphorus in the remaining detritus throughout the decomposition process stresses the importance of the decomposition process for the rapid nutrient cycling in the Campelo Lake and for the maintenance of high primary productivity.