Impacto do uso do solo no crescimento, limitação nutricional, taxonomia e variação estequiométrica do perifíton em rios tropicais

Abstract

The rivers around the world are threatened by changes in land use. In this context, the periphyton is a great biological indicator due to its short life cycle, the autotrophic community and species richness. The aim of this study was to identify changes in structural and functional attributes of the periphytic assemblages in relation to the different land uses (pristine, mixed, and impacted); especially related to nutritional limitation, growth and accumulation, taxonomic composition and stoichiometry of the community. The study was conducted in three rivers (Santa Maria, Itaperiti, Anil) of Guapi-Macacu basin, RJ, Brazil. Nutritional limitation was accessed through Nutrient Diffusing Substrata allocated in situ; these substrata consist of agar solution enriched with nutrients (N, P, N+P) or not (control), which diffuse through the filter, which acts as a substrate for the periphyton growth. Growth was also assessed through colonization of tiles, sampled at 4 different dates; on the last date, samples were used to assess the taxonomic and stoichiometric composition of the periphyton. The results showed that periphyton community was limited primarily by light at pristine sites; the disturbed reaches were limited by nitrogen or phosphorus, depending on the prevailing land use agriculture with higher nitrogen supply, causing limitation by phosphorus, and livestock with higher phosphorus supply, causing nitrogen limitation. The disturbed site also showed higher periphyton biomass in terms of chlorophyll a. In addition, the highest community growth rates were found at the disturbed reaches. There were changes in the periphyton taxonomic composition along the impact gradient, with increasing pollution-tolerant genera and disappearence of non-tolerant genera. The diversity and evenness of algae taxa were higher in pristine sites. Canopy openess was an important fator determining presence/absence of taxa, while PO4 concentration was crucial for their abundances. Finally, the periphyton stoichiometric ratios measured as C:N, C:P and N:P - decreased according to the nutrient concentration in the water, indicating the potential for nutrient removal in polluted streams. This work was innovative in limnological area due to the stoichiometric approach and the large number of other biological variables. The difficulty of separating the environmental variables effects in this study in situ suggests the use of controlled experiments in mesocosms in latter studies