Permeability of riparian forest strips in agricultural, small subtropical watersheds in south-eastern Brazil

Abstract

If riparian buffer zones are ineffective in preventing C4 plant carbon from upland areas reaching the stream sediment, the composition of stream fauna can be significantly altered. The permeability of riparian forest strips in agricultural, small subtropical watersheds in south-eastern Brazil was measured in nine watersheds categorised according to the predominant land cover of the legally required 30-m buffer riparian zone. Four watersheds with well preserved riparian forest along the 30-m buffer zone were designated as FOREST watersheds; three watersheds, with a predominance of C4 grasses from sugarcane to pasture, mixed with preserved riparian forests, were designated MIXED watersheds; and two watersheds were termed PASTURE-SUGAR because their entire 30-m buffer zone was covered by C4 plants. Stable carbon (δ13C) isotopes were used as tracers of upland C4 carbon in sediments, suspended particulate organic carbon, terrestrial and aquatic invertebrates and two species of neotropical fish. Although the intact 30-m buffer zone of riparian forests did not entirely prevent the input of C4 to the river environment and food web, there was a significant increase in C4 carbon in those watersheds where the buffer zone was not covered by riparian forests. These findings emphasise the importance of riparian forests in mitigating disturbance in streams and support efforts to preserve such riparian corridors.