Short-term and seasonal effects of water level variations on Eleocharis interstincta (VAHL) nutrient content in a tropical coastal lagoon
AMADO1, A.M., ESTEVES1, F.A., FIGUEIREDO–BARROS1, M.P. & SANTOS1, A.M.
1 Universidade Federal do Rio de Janeiro, Instituto de Biologia, Departamento de Ecologia, Laboratório de Limnologia, Ilha do Fundão, Rio de Janeiro, RJ Zip Code: 21941-540; P.O.BOX: 68020, Brazil.
ABSTRACT: We evaluated the influence of water level variation of a tropical coastal lagoon on the carbon, nitrogen, phosphorus and cell wall fraction contents in different structures of the aquatic macrophyte Eleocharis interstincta, in Cabiúnas lagoon (Parque Nacional da Restinga de Jurubatiba, State of Rio de Janeiro, Brazil). During the annual study, the water level decreased up to expose the sediment in the dry period and the maximum depth (141.0 cm) was registered in the rainy period. The sandbar, which separates the lagoon from the sea, was artificially breached, resulting in a sharp and rapid decrease in water level in the lagoon. After the breaching of the sandbar and the consequent water level lowering, the stems of E. interstincta died. This event strongly affected the nutrients contents, in almost all the structures of the macrophyte. In the E. interstincta studied stand all the stems died and later re-sprouted. Nitrogen and phosphorus stems contents increased significantly post-breaching (from 0.9 to 2.1 % D.W. and from 0.2 to 0.7 % D.W., respectively), while in the rhizomes accumulated nitrogen, but carbon and phosphorus did not. The N contents on detritus post-breaching increased indicating lack of leaching in the water abscence. We conclude that events capable of defining states of growth and death of stems of E. interstincta can regulate the N and P contents in the stems. The absence of water may reduce the process of leaching decreasing the C:N ratio, altering the nutritional quality of the detritus of E. interstincta and the decomposition processes. The rhizome was shown to be capable of supplying the large demand for N and P, after a stress event, reconstituting the population even in an oligotrophic, phosphorus-poor environment.
Key-Words: Aquatic Macrophytes, Water Level Changes, Nutrients Stoichiometry, Coastal Lagoon, Human Impacts.