Structural characteristics of tropical headwater streams draining native vegetation and sugarcane cultivation

Abstract

There is a great relationship between riparian native vegetation and freshwater ecosystems, influencing the functioning and structure of streams and ensuring the maintenance of ecological integrity and functions, such as bank stability, structural stability, organic matter supply, sediment reduction, and thermal regulation, among others. However, several small streams are inserted in large-scale agricultural areas, suffering pressure due to the expansion of agriculture. One of the monocultures that causes stream degradation in São Paulo, Brazil, is sugarcane, causing changes in riparian vegetation characteristics, bank stability, channel geomorphology and hydraulics, and substrate cover, among others. The objective of this study was to evaluate which structural features are most affected in streams draining sugarcane crops and compare them with streams draining native vegetation, also verifying the seasonal influence on each land use. For this, we applied two protocols that characterize aquatic ecosystems, observing the distinction between land use and seasonality in the studied streams. Our results showed that the relevant and most affected parameters were vegetation cover, presence of riparian forest, bank stability, extent and frequency of rapids, velocity/depth regime, epifauna/substrate cover, alteration in the river/stream channel, and streamflow. PERMANOVA analysis comparing structural importance parameters, among different land uses showed significant difference (p < 0.01; F = 73.74) among land uses and showed no significant difference between seasonal periods for streams draining native vegetation (p > 0.05; F = 0.68) and draining sugarcane cultivation (p > 1; F = 0.82). Thus, our study demonstrated the importance of native vegetation in promoting better structural characteristics of headwater streams.