Modelagem da teia trófica de um reservatório no semiárido em cenários de alterações de nutrientes e esforço pesqueiro em período de seca prolongada

Abstract

Understanding the trophic webs of aquatic environments is a crucial step in guiding the decision making process aimed at improving the water quality of continental aquatic ecosystems. Man-made lakes are aquatic ecosystems that play important roles for human populations due to their multiple uses, such as supplying cities, animal consumption, fishing, and energy production. In semi-arid tropical regions, where water scarcity is a constant, these ecosystems can be used even more. In addition, natural and artificial changes associated with biota, multiple uses and the region's climate (such as rising temperatures and long periods of drought) favor resistant species. These changes in the structure and functioning of these environments cause environmental degradation and deteriorate water quality. The aim of this study was to quantify the trophic web of a man-made lake in the Brazilian semiarid region, in order to describe the main mechanisms (drivers) that act in the structure and dynamics of this ecosystem. Thus, we used the Ecopath with Ecosim software to represent a man-made lake located at the Seridó Ecological Station (ESEC) in the semi-arid region of the brazilian Northeast. We made three models with the same data set (May 2012 to April 2014): an annual model, a monthly model with the same characteristics as the annual and a monthly one with nutrients as part of the trophic web, simulating variations related to the dynamics of nutrients and plankton. The simulations were tested under the influence of two forcing functions (phosphorus and radiation FF) and under different impact scenarios (changes in nutrient loading and fish removal). The model that best represented the ecosystem was the monthly model, which showed results more consistent with the reality of the man-made lake, which was a eutrophic and was facing severe drought during the data monitoring period. The modeling results indicated that the ESEC man-made lake is an ecosystem whose most of the energy comes from the detritus chain, with high primary productivity, with low resilience and that although the key species are top predators, they do not seem to influence the dynamics of plankton, showing a dynamic of bottom-up control and corroborating with the results of previous studies, in tropical climate, and of a biomanipulation experiment carried out in the man-mande lake at the same time as the data collection.