Reservatórios no semiárido e sua avaliação ecológica: dos padrões ambientais e biológicos à elaboração de ferramenta de suporte à reabilitação

Abstract

The reservoirs represent transitional waters between rivers and lakes and have as main goal the water storage to supply the human demands (e.g., electric power generation, cities supply, irrigation or recreation). Although artificial, the reservoirs are important elements on landscape, being ecosystems that support biota conservation, especially in regions where drought events are frequent (as arid and semi-arid regions) and its hydrographic networks are predominantly represented by intermittent rivers. The reservoirs may be freshwater ecosystems even more important in future, as according the climate changes predictions, watersheds will increase their intermittency. In fact, the number of dams is growing in the entire world, at a rate of 700 dams/year. This data is worrying and challenging at the same time, as the development of several human activities associated to these ecosystems, requires the monitoring their health status, or ecological status, and the elaboration of rehabilitation measures to improve their degradation status. Thus, the main aim this thesis was analyze ecological characteristics of reservoirs (environmental, structural and functional patterns) to classified their ecological status, as well as to elaborate a multivariate tool to simulate the effect of rehabilitation measures in macroinvertebrate communities. Our dataset was obtained from six reservoirs on Brazilian Northeast, semi-arid region. These reservoirs belong to two watersheds: Piranhas-Assu river (Rio Grande do Norte state) and Paraíba river (Paraíba state). The samplings were performed during four periods: June, September and December (2014) and March (2015). This period was classified as atypical, with an intense and prolonged drought, which led to the analysis of the effects of extreme events. Our results had demonstrated the high degradation level of reservoirs, including the in the increase of densities of an invader mollusk (Melanoides tuberculata). We found that the diversity among sites at watersheds is mainly maintained by random substitution (turnover), however in the driest period, when the environmental conditions are most limiting, the species loss (nestedness) is a factor with similar importance for the distribution pattern of diversity, resulting in poorer communities dominated by tolerant species (as Melanoides tuberculata). The diversity variation over time is reflected on changes in communities functional patterns. Indeed, along the drought period there was a loss in functional richness and increasing of functional evenness, indicating biotic homogenization. Additionally, analyzed the variation of trophic gradients we observed high trophic levels and also of dissolved organic carbon (DOC). Sites with highest trophic level (during the drier period) were those with lower DOC values. Under these conditions, the autochthonous DOC indicates be an important source to DOC in ecosystem and favor the eutrophication process. All results obtained show that the poor ecological status is a reality and therefore our largest challenge is to propose useful measures to reverse the low ecological condition. Therefore, we elaborated a predictive model to simulate the effect of rehabilitation measures in macroinvertebrate communities and tested its applicability the study sites. The results are promising and demonstrate that a change in variables associated to eutrophication alone (e.g., phosphorus, transparency and chlorophyll-a) results in changes in the ecological status (environmental and biotic improvement) of reservoirs, including expansion of sensible taxa. The obtained results from of this thesis gathers information on the fluctuation of structural and functional patterns of the communities under intense perturbation condition and provides important insights to biomonitoring of reservoirs and its watersheds. The low ecological quality in semi-arid reservoirs is worrisome and actions to the reversal from degradation status are need. Here, we given a promising step in direction to these actions, and thus future studies may be develop with this method, considering different levels of improvement from others impact factors.