Functional threshold responses of benthic macroinvertebrates to environmental stressors in reservoirs.

Reservoirs are aquatic ecosystems created by humans to supply water needs. They can impair aquatic diversity due to the lack of connectivity, reduced water volume, and pressures exerted by surrounding human activities. These changes are expected to produce abrupt fluctuations in the reservoirs' environment, thus influencing the structure and functioning of aquatic communities. Therefore, this study aimed to understand the impact of a range of environmental stressors in reservoirs on benthic macroinvertebrates by analyzing their functional threshold response. Biological data were collected in six reservoirs from the semi-arid region of Northeast Brazil, as case study. A total of 37.874 benthic macroinvertebrates belonging to 35 taxa were collected. Nevertheless, almost 90% of this abundance belonged to three species alone, considered generalists, with multivoltine reproduction and from the gatherer-collectors feeding group. Increases in environmental stressors such as salinity, nitrate, ammonia, and dissolved solids led to the selection of macroinvertebrates with specific traits (e.g., protected body, gill respiration, and large body size). These functional traits showed differences in their threshold response depending on the stressors and are indicators of the effects of these stressors on the reservoirs. Some of the potential sensitive traits (with a negative threshold response to the stressor) could also associate with other stressors, demonstrating that tolerance of benthic macroinvertebrates is defined by a set of functional characteristics. Overall, the increase in stressor' gradients selected functionally tolerant organisms with high resistance capacity, but these were represented by dominant species. This resulted in low diversity in the reservoirs, which may compromise ecosystem functioning, and raises concerns about adequate management of the systems.

Extreme drought scenario shapes different patterns of Chironomid coexistence in reservoirs in a semi-arid region.

Semi-arid regions are particularly prone to extreme climate events such as droughts, which result in drastic fluctuations in the water volume of aquatic ecosystems, including artificial ones. As these climate extremes intensify, species must adapt, however, not all species can persist under new climate regimes in such a short period of time. In this study, we evaluated how fluctuations in the water levels of reservoirs, caused by drought, affect Chironomidae diversity patterns in a semi-arid region. We studied six reservoirs (256 sites) in two basins in Northeastern Brazil, exposed to different levels of anthropic impact. Sampling was carried out in 2014, 2015 (both extremely dry years) and 2019. A dead water volume was attained during the extreme drought in 2015, consequently affecting the reservoir and resulting in a low diversity, abundance, and functional redundancy of the Chironomidae assemblages. Despite precipitation increases in 2019, some reservoirs continued to be water deficient. These drastic water fluctuations led to different patterns in Chironomidae taxonomic and functional diversity, which were also influenced by anthropic stressors. Thus, the most impacted basin presented lower diversity, with some species and trait turnover between reservoirs. The opposite trend was observed in the least impacted basin. Overall, taxonomic and functional diversity decreased with decreasing water volume, resulting in a community dominated by small-medium sized individuals with multivoltine cycles and hemoglobin and diapause resistant forms, conferring higher tolerance to water stress. The drought and consequent water volume fluctuations throughout the years seemed to exacerbate the water quality due to pre-existing exposure to anthropic impacts (e.g., domestic discharge, fishing activity, agriculture, livestock). This resulted in biotic homogenization, with an observed loss of taxa and traits. This study reinforced the need to implement habitat conservation and water quality improvement strategies to prevent further ecosystem damage in the face of climate change uncertainty.

Performance of richness estimators for invertebrate inventories in reservoirs.

Biological inventories combined with the estimation of species richness represent a useful tool for the analysis of the pattern of species distribution in different regions. This study aimed to (i) comparatively evaluate the performance of non-parametric richness estimators for invertebrate inventories in reservoirs between ecoregions and (ii) to assess whether the efficiency (bias, precision and accuracy indices) of the estimators is altered when applied to sites from different ecoregions. The study was conducted in the ecoregions Central Pediplano of the Borborema Plateau (Paraíba River basin) and Northern Sertaneja Depression (Piranhas-Assu River basin), semiarid region of Brazil. Six reservoirs were selected and benthic macroinvertebrates were sampled at 141 sites distributed along the littoral zone, in 4 periods (June, September, December 2014 and March 2015). The organisms were identified to the family level, except for Chironomidae, identified to the genus level. We comparatively analyzed six non-parametric richness estimators: Jackknife 1, Jackknife 2, Chao1, Chao 2, ICE, and Bootstrap, and three performance indicators: bias, precision, and accuracy. ICE and Jackknife 2 had more stable results for total species richness, but with different performance between ecoregions for bias, precision, and accuracy. Variation in performance of the estimators may be associated with differences in species richness and frequency between ecoregions. ICE and Jackknife 2 proved to be the best estimators for biological inventories of aquatic invertebrates in reservoirs in studies comparing data from different ecoregions, due to accuracy and precision, while Bootstrap is the least indicated, given greater bias and less accuracy and precision.