A dinâmica da produtividade autotrófica e do metabolismo perifítico em córregos tropicais: o papel regulador de herbívoros e predadores

Abstract

Animals can affect stream ecosystems by promoting physical changes to the environment and playing important roles in modulating resource availability and ecosystem-level processes. When considering the role of aquatic consumers, we need to distinguish their bioturbation and grazing actions from their final trophic relations. Chapter 1 examines the relative importance of consumption and bioturbation of atyid shrimps and baetid mayflies in the redistribution and processing of periphyton. Our results indicate that only a small part of the removal (~20%) is due to direct ingestion, with the remainder of the material (~80%) being removed by bioturbation. Periphyton, as a metabolic compartment of streams, is subject to food webs and can be altered due to grazing by consumers or trophic relationships that regulate the activity of those grazers. Chapter 2 aims to understand how these cascading trophic relationships regulate the grazing effect of Potimirim brasiliana on algal stock and periphytic metabolism. The presence of predatory fish reduces shrimp activity, generating substrates with greater accumulation of material. Our results exibits shows different effects of P. brasiliana grazing depending on the characteristics of the periphyton. The grazing of these shrimp was positively related to primary productivity and respiration in substrates with a greater amount of accumulated material. In substrates with less accumulation of material, herbivory pressure nullified any beneficial effect of shrimp grazing on algae. Chapter 3 investigates how riparian forest shading and aquatic invertebrate grazing can influence periphyton in terms of measures of metabolism, total biomass and algal stock. We investigated the effects of grazing on periphyton through electrical exclusion experiments in a stretch of river with low canopy cover and another with high canopy cover. Our results show that light and grazing are strongly related, as the effects of light availability on algal productivity, total biomass and chlorophyll stock occur mainly in the absence of consumers. The effects of consumers are more pronounced with the increase in light availability. Finally, Chapter 4 presents a meta-analysis of a series of experiments with freshwater decapods. Our goals with the meta-analysis were analyze how different portions of the biomass respond to decapods, to test whether different groups of decapods promote different effects on periphyton biomass and to investigate how the duration of the experiments affects the effects of decapods on periphyton biomass. The results showed a strong effect of freshwater decapods on periphyton biomass, revealing an overall average effect size capable of reduce the standing stock of periphyton biomass by half in the presence of these decapods. It was also revealed that the type of biomass measurement, the identity of the decapods and the duration of the experiments influenced the average size of periphytic biomass responses on the presence of decapods.