O efeito da cobertura vegetal na contribuição de um consumidor aquático na ciclagem de nutrientes

Abstract

Due to the irrigation demand of plantations and cattle keeping in rural areas, rivers and streams suffer many damages by the expansion of these activities, like the removal of riparian vegetation, silting and leaching of nutrients. The nutrient inputs into aquatic environment changes physical and chemical processes. The aquatic animals are involved in many ecosystemic processes, one of them being the nutrient cycling, in which consumers supply nutrients to the environment where they live. The consumers excretion rates importance is influenced by organisms specific factors like biomass, and by ecosystem features, like primary production demand for nutrients. The aim of this study was to evaluate the functional importance as nutrient suppliers in streams with different human impacts. The sites were classified into three levels of impact (forested, medium-impact, impacted) according to their vegetation cover. To calculate the areal excretion rate, the fish density was estimated by the method of "Catch-Effort". Afterwards the fish were taken to incubation in order to estimate the ammonium and phosphate excretion rates for each impact level. Each subject was allocated in a plastic bag with a known quantity of filtered water for 60 minutes. Water samples were collected from the river for control. To estimate the environmental demand for nutrients was used TASCC method (Tracer Additions for Spiraling Curve Characterization), which consists of instant injection of nutrients along with a conservative tracer. Consumer contribution was calculated from the percentage of areal excretion rate relative to demand. No relationship was observed between excretion rates and the canopy cover gradient nor the concentrations of dissolved nutrients. The results did not corroborate the hypothesis that sites with more impact would present lower contribution of the excretion to the nutrient demand, since no relation was found between the contribution and the canopy cover gradient and nor between the dissolved nutrients concentrations