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Assessment of Disturbance at Three Spatial Scales in Two Large Tropical Reservoirs
Citation:
Morais, L., B. de Oliveira Sanches, G. Santos, Phil Kaufmann, R. Hughes, J. Molozzi, AND M. Callisto. Assessment of Disturbance at Three Spatial Scales in Two Large Tropical Reservoirs. JOURNAL OF LIMNOLOGY. MPS Multimedia Print Service, Domodossola, Italy, 76(2):240-252, (2017).
Impact/Purpose:
The manuscript entitled “Assessment of disturbance at three spatial scales in two large tropical reservoirs” by Leticia Morais, Barbara Sanches, Philip Kaufmann, Robert Hughes, Gilmar Santos, Joseline Molozzi, and Marcos Callisto, is intended for submission to the Journal of Limnology. This work is a demonstration of the application of the USEPA’s National Aquatic Resource Survey (NARS) methods in two hydropower reservoirs in Brazil. The researchers adapted the randomized systematic sample site selection design of NARS rivers to the complex shoreline littoral area of these two large reservoirs, and successfully applied EPA’s National Lakes Assessment field methods to those 80 near-shore reservoir sites. Their application of this survey approach demonstrates that these methods could be used routinely to assess reservoir condition in Brazilian and other South American regions with similar challenges. Interestingly, even though the reservoirs are very large, and there was considerable variation in local disturbance and stressors, benthic macroinvertebrate assemblages were relatively homogeneous among the 40 sites within each reservoir. Within each reservoir, macroinvertebrate indices did not show strong association with disturbance. By contrast, the reservoir average macroinvertebrate assemblage metrics strongly reflected the difference in the mean level of disturbance between the two reservoirs. For EPA, this is an important result, because it suggests that reservoir-wide average condition assessments based on biotic assemblages and physical-chemical stressors in very large reservoirs may be as representative as those used by EPA’s National Lakes Assessment for smaller lakes and reservoirs. This study examines two of a total of four large Brazilian reservoirs that will be examined in other articles in preparation. The results of these surveys will aid the growing effort to establish standardized approaches to quantify ecological condition globally.
Description:
Large reservoirs vary from lentic to lotic systems in time and space. Therefore our objective was to assess disturbance gradients for two large tropical reservoirs and their influences on benthic macroinvertebrates. We tested three hypothesis: 1) a disturbance gradient of environmental conditions can be observed in the reservoirs; 2) each reservoir would exhibit a different gradient of disturbance; and 3) the disturbance gradient would influence the structure and composition of benthic assemblages. For each reservoir, we assessed land use (macroscale), physical habitat structure (mesoscale), and water and sediment quality (microscale). We sampled 40 sites in the littoral zones of both Três Marias and São Simão Reservoirs (Minas Gerais, Brazil). At the macroscale, we measured cover percentages of land use categories in 500 m diameter buffer areas at each site. At the mesoscale, we assessed the presence of human disturbances and the complexity of cover in the riparian and drawdown zones. At the microscale, we assessed water quality and sediment at each macroinvertebrate sampling station. To evaluate anthropogenic disturbance of each site, we calculated an integrated disturbance index (IDI) from a buffer disturbance index (BDI), a local disturbance index (LDI), and a micro disturbance index (MDI). For each site, we calculated richness and abundance of benthic macroinvertebrates, richness of Chironomidae, abundance and percent Chironomidae individuals, abundance and percent EPT individuals, richness and percent EPT taxa, abundance and percent resistant individuals, and abundance and percent non-native individuals. We also evaluated the influence of disturbance on benthic macroinvertebrate assemblages at the entire-reservoir scale. The BDI, LDI and IDI had significantly greater average scores at São Simão than at Três Marias Reservoir. Percent sensitive taxa and percent EPT individuals were greater, and percent chironomids were less, in the latter. We also observed clear disturbance gradients within both reservoirs at macro (BDI) and mesoscales (LDI), but an insignificant range in MDI results. However, we found no significant relationship between the benthic macroinvertebrate metrics and the BDI, LDI, and IDI within a single reservoir. Hence, in those reservoirs we believe that the distribution of the benthic macroinvertebrates was influenced by other factors or that reservoir macroinvertebrates (dominated by chironomids) were poor indicators of disturbance at the site scale.
