Assessing the extent and relative risk of aquatic stressors on stream macroinvertebrate assemblages in the neotropical savanna

• Published in: The Science of the total environment 2018-08, Vol.633, p.179-188
• Main Authors: Silva, Déborah R.O., Herlihy, Alan T., Hughes, Robert M., Macedo, Diego R., Callisto, Marcos
• Format: Article
• Language: English
• Subjects: Bioassessment; Cerrado; Land use; Physical habitat; Probabilistic survey design; Water quality
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2018.03.127

•Aquatic ecosystems are among the most threatened worldwide.•The extent and relative risks of stressors to biological condition were assessed.•We used a probabilistic survey design to obtain estimates of relative risk and extent.•Excess fine sediments posed the greatest risk to biological condition.•To improve biological condition, management practice should consider both RR and RE. [Display omitted] Freshwater ecosystems are among the most threatened by human activities, influencing losses of biodiversity. To efficiently address management practices to conserve and restore those ecosystems it is important to correctly identify and quantify the severity and magnitude of anthropogenic stressors degrading freshwater biota. In this study we assessed seven stressors describing poor water quality, physical habitat alteration, and land use by means of the relative risk (RR) and relative extent (RE) approach. The RR measures the co-occurrence probability of high stressor condition and poor biological condition. The RE measures the proportion of stream length in the region in high stressor condition. To obtain accurate estimations of RR and RE we used a probabilistic survey design to select a representative sample of perennial, wadeable and accessible streams within four hydrologic units in the neotropical savanna. Results were evaluated at two spatial scales: local – within each of the four hydrologic units, and regional – all four units combined. From 143 randomly selected sites we inferred our results to a target population of 9466 km of streams. Regionally, we found turbidity, % fine sediments and % agriculture as key stressors associated with poor biological condition. At the local scale, we also found that % pasture and total nitrogen were key stressors of biological condition, but their extent was relatively small. By evaluating both RR and RE we conclude that reducing excess sedimentation on streambeds should be the most effective means of improving biological condition over the region. That finding should guide decision makers and land managers to better focus their efforts and resources on improving biological condition of savanna streams.