Land uses modify isotopic composition in aquatic macroinvertebrates from semideciduous Atlantic Forest stream sites

Human land uses modify energy flux and nutrient cycles between terrestrial and freshwater ecosystems. Because aquatic macroinvertebrates are key components in nutrient cycling and food webs, their isotopic compositions can provide clues about the energy and matter flows between riparian-aquatic ecosystems through their producer–consumer relationships. Therefore, our objective was to investigate how land use affects food assimilation by aquatic macroinvertebrates. We measured the δ13C and δ15N values of aquatic macroinvertebrates and potential food sources at 12 stream sites under different riparian land uses: secondary riparian forests and agricultural (pasture and sugarcane). We had two hypotheses: (1) the δ15N of aquatic macroinvertebrates would be higher in sites with more agricultural areas and (2) the overall contribution of C4 plants to aquatic macroinvertebrates would be low in all sites, but higher in sites with more agricultural areas. We found higher values of δ15N in sites with forested riparian zones than those with agricultural riparian zones, rejecting our first hypothesis. The disturbances in riparian zones are indicated in δ13C and δ15N aquatic macroinvertebrate signatures. The second hypothesis was partially supported: i.e., the C4 plants represented < 12% of aquatic macroinvertebrate diet composition, the lowest assimilation rates regardless of the riparian land use, but it was not necessarily higher at sites with more agricultural areas. Our results reinforce the greater importance of C3 plants and autochthonous material contribution to aquatic macroinvertebrate diets and the importance of protecting riparian vegetation, which plays a fundamental role in aquatic ecosystem functioning in tropical headwater streams.