Diatom and Macroinvertebrate assemblages to inform management of Brazilian savanna's watersheds

Human activities are increasingly affecting freshwater ecosystems and biodiversity, especially in neotropical regions like the Brazilian savanna. Limited research and data availability have inhibited the development and implementation of systematic bioassessment programs and management guidelines. Identifying drivers of biological assemblages' composition and ecological thresholds along human disturbance gradients is an important step to protect and recover freshwater ecosystems while avoiding threats to biodiversity, goods and services of value to humans. The objectives of this study were to: 1) assess changes in the composition and density of periphytic diatom and macroinvertebrate assemblages relative to natural and human disturbance gradients in Brazilian savanna's streams, and 2) identify ecological thresholds for direct and indirect human disturbances and potential indicator taxa that could inform the development of biomonitoring programs in Brazil and other neotropical countries. Samplings were carried out in 52 stretches of streams located in central Brazil during two campaigns throughout 2018. Ordination analyses (NMDS) were applied to identify the main drivers of biological assemblages' composition and analyses of taxa distribution in disturbance gradients (TITAN) were carried out to detect possible thresholds and potential bioindicator taxa. Our results pointed out that the scale of land use in the catchment, treated sewage input, and water quality variables (nitrate, phosphate, and conductivity) were the main drivers of diatom and macroinvertebrate assemblages. Taxa such as Eunotia (diatoms) and some families of the orders Ephemeroptera, Plecoptera, and Trichoptera (EPT, macroinvertebrates) were associated with natural conditions, while Nitzschia palea and Gomphonema (diatoms) and Oligochaeta and Hirudinea (macroinvertebrates) were related to environments with a higher degree of disturbance. Although ecological thresholds along disturbance gradients varied among taxa and biotic groups, our results revealed that relatively minor increases in land use in the riparian zone (>0%) and in the upstream catchment (0-33%) were sufficient to trigger significant changes in macroinvertebrate and diatom assemblages. The limits of tolerance for conductivity, nitrate and phosphate were also low, varying between 7-163 mu S.cm(-1), 0.3-1.0 mg.L-1 and 0.03-1.5 mg.L-1, respectively. In general, the values we found were more restrictive than those provided by Brazilian government guidelines, suggesting that the latter would be insufficient to maintain the integrity of biological assemblages in Brazilian savanna's watersheds. We provided valuable knowledge about the sensitivities and tolerances of diatom and macroinvertebrate's taxa/assemblages that can be especially useful for the proper freshwater and watersheds management.

Automated summary

This study found that land use scale, treated sewage input, and water quality variables were the main drivers of biological assemblages in Brazilian savanna streams. Ecological thresholds along disturbance gradients varied among taxa and biotic groups, with relatively minor increases in land use or water quality parameters triggering significant changes in macroinvertebrate and diatom assemblages. The limits of tolerance for conductivity, nitrate, and phosphate were lower than those provided by Brazilian government guidelines, underscoring the need for more stringent measures to maintain the integrity of biological assemblages in these watersheds. Valuable knowledge about the sensitivities and tolerances of diatom and macroinvertebrate taxa can inform better freshwater and watershed management practices.