Threshold Responses in the Taxonomic and Functional Structure of Fish Assemblages to Land Use and Water Quality: A Case Study from the Taizi River

Biological functional traits help to understand specific stressors that are ignored intaxonomic data analysis. A combination of biological functional traits and taxonomic data ishelpful in determining specific stressors which are of significance for fish conservation and riverbasin management. In the current study, the Taizi River was used as a case study to understand therelationships between the taxonomic and functional structure of fish and land use and waterquality, in addition to determining the thresholds of these stressors. The results showed thattaxonomic structure was significantly affected by the proportion of urban land and specificconductivity levels, while functional metrics were influenced by the proportions of farmland andforest. Threshold indicator taxa analysis found that Phoxinus lagowskii, Barbatula barbatula nuda,Odontobutis obscura, and Cobitis granoei had negative threshold responses along the gradients ofurban developments and specific conductivity. There was a significant change in fish taxonomiccomposition when the proportion of urban land exceeded a threshold of 2.6–3.1%, or specificconductivity exceeded a threshold of 369.5–484.5 μS/cm. Three functional features—habitatpreference, tolerance to disturbances, and spawning traits—showed threshold responses to theproportion of farmland and forest. The abundance of sensitive species should be monitored as partof watershed management, as sensitive species exhibit an earlier and stronger response to stressorsthan other functional metrics. Sensitive species had a positive threshold response to the proportionof forest at 80.1%. These species exhibited a negative threshold response to the proportion offarmland at 13.3%. The results of the current study suggest that the taxonomic and functionalstructure of fish assemblages are affected by land use and water quality. These parameters shouldbe integrated into routine monitoring for fish conservation and river basin management in the TaiziRiver. In addition, corresponding measures for improving river habitat and water quality shouldbe implemented according to the thresholds of these parameters.