Processamento foliar em um córrego de mata atlântica: o papel dos invertbrados aquáticos e o uso de diferentes metodologias

Abstract

The role of macroconsumers and aquatic insects play in the leaf processing in tropical rivers has been discussed. In some tropical streams macroconsumers can directly increase the leaf processing rate (k) through consumption and bioturbation. However, they can also reduce this rate indirectly by decreasing the colonization of leaves by insects aquatics (trophic cascade). Some methodologies of fauna exclusion have been used to evaluate the role of these animals in the leaf processing. To test the role of fauna in the leaf processing and the effect of different methodologies, we set up two experiments between the months of April and August 2013 on Córrego da Andorinha, a third order stream located at Ilha Grande, Rio de Janeiro. We used leaves of two species of common plants in our study area and with different degrees of toughness: Erythroxylum pulchrum and Miconia prasina. In the first experiment, the presence of fauna was manipulated by electric exclusion. In the control treatment (without electrification) shrimp and insects had access to the leaves, the treatment with low shock intensity only the shrimps were excluded and with high intensity both shrimp and insects with size greater than 5 mm were excluded. In the second experiment, the presence of fauna was manipulated by electric exclusion or liter bags with different mesh sizes. The treatments of electric exclusion were the same of first experiment. Since the mesh treatments were: 12 mm - without fauna exclusion; 2 mm - excluding shrimp; and 0.2 mm - excluding shrimp and insects water. In the electric fence, Erythroxylum showed a significantly higher processing rate in the treatment with low shock, where only the shrimps were excluded (k = 0,016 ± 0,001 day-1), since the litter bags methodology its higher processing rate was found in bag with mesh 12mm (0,008 ± 0,001 day-1), where we find a high abundance of shrimp Potimirim brasiliana, which may have influenced positively the processing rate due to bioturbation. Regarding the leaves of Miconia, processing rates did not differ between treatments in terms of methodology using electric fences (F2,43= 1,69, p= 1,95) and with bags (F2,44= 2,18, p= 0,124). However, both species had lower processing rates on the bags, showing that the results of leaf processing of fauna exclusion may depend on the methodology. Regarding the fauna, in the experiment with leaves of Erythroxylum the electric fence, it was possible to identify a trophic cascade effect, where the presence of shrimp Macrobrachium olfersi influence negatively the abundance and activity of aquatic insects, causing also smaller leaf processing rate