Limnetica 39
Determination of the Trophic State Index (TSI) using remote sensing, bathymetric survey and empirical data in a tropical reservoir
The reservoirs are important freshwater reserves that suffer with domestic and industrial wastes, causing qualitative and quantitative changes in their physicochemical composition, as well as causing fauna and flora impacts. One of the consequences is eutrophication, a process characterized by increased concentration of nutrients in aquatic ecosystems, mainly phosphorus and nitrogen. In order to classify waterbodies according to their trophic degrees, chlorophyll-a and phosphorus concentration data are usually obtained in water samples, feeding an equation to obtain the Trophic State Index (TSI). The method used at the present work aim to determine the TSI based on remote sensing (RS), by using the ratio between the near-infrared (NIR) and red (RED) images to infer the chlorophyll-a concentration in wet and dry seasons. This method allows a synoptic vision of the reservoir, providing support for decision-making process at low cost, especially when a large amount of data is available and/or generated, increasing quality in consonance with technological development. However, we need to consider the influence of the bottom of shallow areas when using orbital images to evaluate the waterbodies conditions, since the emerging radiation of the euphotic zone is responsible for the results reliability. Thus, it is necessary the exclusion of shallow areas, which may compromise the spectral response of the satellite images due to the bottom effects, before the development of the TSI RS Map. We created the TSI RS Maps (observed) and then we validated it with the Empirical TSI Maps (expected), obtained in 28 geo-coded points along the riverine-transition-lacustrine gradient, both for wet and dry season. We performed the chi-square test that shows how much difference exists between the observed and the expected. Both seasons presented x2 = 1, df = 27, with critical value (0.005) of 49.64. Therefore, we found no differences between the observed and the expected. Our results reinforce the potentiality of the RS to perform the TSI favoring the identification of eutrophic areas in inland water systems with low turbidity.
