www.nature.com/scientificreports OPEN Relationships Between Environmental Conditions And Fish Assemblages In Tropical Savanna Headwater Streams Thiago Bernardi Vieira 1,2* & Francisco Leonardo Tejerina-Garro 3,4 Riparian vegetation plays an important role in providing energy to small watercourses and maintaining ecological processes through organic matter input and together with hydrological and geomorphological watercourse characteristics infuence on fsh assemblages. The goal of this paper was partitioning and quantifying the infuence of riparian zone (type of riverbank substrate, bank slope, type of riparian vegetation cover and percentage of riparian vegetation cover on the main channel), physical habitat (stream channel width and depth, type of substrate and aquatic habitat in channel, water velocity and organic matter), water quality (turbidity, temperature, conductivity, pH, dissolved oxygen and chlorophyll concentration) and spatial variables (linear distances between sampled points) on fsh assemblages (richness and abundance per species) in headwater streams of the Upper Paraná River basin, Central Brazil. For this purpose, it was performed a variation partitioning analysis between riparian, physical habitat, water and spatial variables sets and a Redundancy Analysis to quantify the infuence of variables on the fsh assemblages. Only the physical habitat and water quality variables infuenced the fsh assemblages (richness and abundance per species). Freshwater fsh assemblages are structured by variables related to both water quality and riparian vegetation1–6. In this sense, warmer waters exhibit higher fsh abundance and biomass while highly oxygenated waters may lead to greater species diversity7–9. Riparian vegetation is a transitional semiterrestrial system10 that provides energy in watercourses through the input of organic matter2. Leaves deposited on the watercourse bed contribute indirectly to fsh food because they act as a substrate for numerous microorganisms11 and insects12,13. In addition, riparian trees and roots restrict channel widening, cause channel deepening and add coarse woody debris favoring fsh concealment and channel complexity. Te infuences of water and riparian vegetation on fsh assemblages are not independent2,10,14,15; that is, ripar- ian vegetation may directly or indirectly infuence water variables16. For example, water temperature is directly infuenced by riparian vegetation, which regulates the watercourse insolation level17,18 and infuences primary production19. Conversely, channel depth and substrate heterogeneity are indirectly infuenced by riparian vege- tation because the riparian zone regulates the entry of sediment that can be deposited into the watercourse10,20,21. Another factor that should not be neglected is the spatial factor (e.g., the river network), which includes geographical barriers that hamper or prevent species migration between locations. Abundance and richness are diversity metrics that are spatially structured22–28. Spatial factors are a consequence of the geological and local climatic infuence on the streams in a river network29–31 and the position of the watercourse along a longitudinal gradient (upstream-downstream32) for the 1st-3rd 33 and 4th-7th order34 streams. A spatial model coupled with a 1Laboratório de Ictiologia de Altamira – LIA, Universidade Federal do Pará (UFPA), Campus Altamira. Rua Coronel José Porfírio 2515, São Sebastião, Altamira, PA, CEP 68372-040, Brazil. 2Programa de Pós-Graduação em Biodiversidade e Conservação – PPGBC, Universidade Federal do Pará (UFPA), Campus Altamira. Rua Coronel José Porfírio 2515, São Sebastião, Altamira, PA, CEP 68372-040, Brazil. 3Laboratório de Biodiversidade, Programa de Mestrado em Sociedade, Tecnologia e Meio Ambiente, UniEVANGÉLICA, Av. Universitária km. 3,5, Cidade Universitária, Anápolis, GO, CEP 75083-515, Brazil. 4Centro de Biologia Aquática, Escola de Ciências Agrárias e Biológicas, Pontifícia Universidade Católica de Goiás - Campus II, Av. Engler s/n, Jardim Mariliza, Goiânia, GO, CEP 74605-010, Brazil. *email: [email protected] SCIENTIFIC REPORTS | (2020) 10:2174 | https://doi.org/10.1038/s41598-020-59207-9 1 www.nature.com/scientificreports/ www.nature.com/scientificreports river network accurately explains fsh richness patterns35,36. Additionally, the river network acts as a corridor37, facilitating fsh species dispersion38 or acting as a flter39. Furthermore, the individual infuence of water, riparian or spatial processes on the structure of fsh assem- blages is not necessarily consistent40. Instead, the infuence of these processes results more ofen from their inter- action41. Terefore, physical habitat variables infuence fsh assemblages either alone27 or in combination with water quality variables42. Te aim of this paper was to partition and quantify the infuence of riparian, physical habitat, water quality and spatial variables on fsh assemblages in headwater streams located in the Upper Paraná River basin, Central Brazil. Results A total of 4879 specimens belonging to 59 species and 19 families were collected (Table 1). Infuence of the environmental conditions on fsh assemblages. Te variation partitioning analysis indicated that fsh abundance variation is explained by water quality (18.7% of variation), physical habitat (8.4%), spatial (6.2%) and riparian zone variables (5.1%; Fig. 1). Te interactions among the spatial variables, water qual- ity and physical habitat explain 16.7% of the variation, those among the physical habitat, riparian zone and water quality explain 8.9%, those between the physical habitat and water quality explain 8.2%, and the other interactions represent ≤3.4% (Fig. 2). Te Procrustes analyses indicated a signifcant correlation between the fsh abundance and the physical habitat (M2 = 0.295; p < 0.001) and water quality variables (M2 = 0.565; p < 0.001) and no signif- cance for the riparian zone (M2 = 0.200; p = 0.526) or spatial variables (M2 = 0.150; p = 0.744). All the non-metric multidimensional scaling NDMS analyses performed had a good ft (stress < 0.02). Fish assemblages-environmental conditions relationships. According to the broken stick criteria, there were two signifcant axes for the PCAs performed separately on the water quality (77% of the variation) and physical habitat (81% of the variation), eight axes for the PCA performed on the riparian zone (87% of the varia- tion) and three signifcant MENs (Moran’s Index = 0.01 for each one) for the spatial variables. Te multiple linear regression showed no signifcant relation between fsh abundance and the variables of the 2 four groups considered (r = 0.566; F (12, 27) = 2.536; p = 0.128). In contrast, a signifcant relation was observed between fsh richness (r² = 0.784; F (12, 27) = 5.865; p = 0.001) and the PCA-1 of the physical habitat variables (p = 0.005; Table 2). All of the other compartments did not display signifcant relationship (Table 2). Te relationship between assemblage and physical habitat variables is detailed by the RDA (total variance explained by the two axes = 53.4%; F (10, 17) = 3.543; p = 0.003). Te frst axis (35.17%) was positively correlated with conductivity, dissolved oxygen and chlorophyll concentration and negatively correlated with water temper- ature, whereas the second axis (18.23%) was positively correlated with organic matter, channel depth, pH and channel width and negatively correlated with turbidity (Fig. 2). Te characins Piabina argentea and Astyanax altiparanae and the scrapetooths Parodon nasus were related to high values of water conductivity, dissolved oxy- gen and chlorophyll concentration, whereas the characins Astyanax fasciatus and Astyanax scabripinnis and the poeciliid Poecilia reticulata were associated with elevated water temperature values. Te scrapetooths Apareiodon ibitiensis, the headstander Leporinus microphthalmus, and the toothless Steindachnerina insculpta were associated with elevated organic matter and pH values and a large and deep channel stream. Te characin Bryconamericus stramineus, the callichthyid armored catfsh Aspidoras fuscoguttatus and the South American darter Characidium zebra were correlated with high values of turbidity (Fig. 2). Discussion Te riparian zone does not display any signifcant infuences on fsh abundance or richness in the headwater streams sampled. Similar results using a diferent methodology were obtained for fsh diversity43 in 1st to 3rd order headwaters streams in the Amazon region. Tis result suggests a low infuence of riparian vegetation removal, assessed indirectly in this paper by the variables of the riparian zone group (type and percentage of the vegetation cover), on fsh assemblages. However, studies focused on this subject have stressed the infuence of the riparian zone on fsh assemblages in the Amazon (channel fragmentation, deforestation44; mechanized agriculture43), São Francisco (deforestation42,45) and Paraná River basin (deforestation45), the last two of which contain the same vegetation cover of the area sampled in this paper (i.e., Cerrado). Te spatial component also showed no signifcant infuence on fsh assemblages. Te abundance and rich- ness of plants and animals, including stream organisms, are spatially structured45,46 because of the infuence of geology, the local climate30 and the watercourse position along a longitudinal gradient32, especially for 1st to 3rd order streams33. However, if the 1st and 2nd order streams sampled in this study were in the same geologic (a com- bination of Precambrian metamorphic rocks,