Macroinvertebrate trophic responses to nutrient addition in a temperate stream in South America

The present continuous anthropogenic pressure has resulted in an enhancement of nutrient inputs into rivers and streams. This situation has worsened, mainly in agricultural areas, causing an accelerated eutrophication. Macroinvertebrate feeding strategies reflect the species' adaptations to environments. For a characterization of the macroinvertebrate trophic response to eutrophication, we added nitrogen and phosphorus into the La Choza stream and examined the gut contents of those taxa in order to analyze dietary alterations and assign each macroinvertebrate to a functional feeding group (FFG). Complementary C and N stable isotopes analysis was carried out. The gut contents of all taxa studied contained principally detritus, but statistical analyses indicated significant differences before and after fertilization in some taxa. At the control site, and the treatment site before fertilization, the FFGs maintained constant proportions, the gathering-collector species being dominant in all samples. After fertilization the composition of the taxa at the treatment site varied. The 13 C isotopes values showed that most taxa used detritus as a basal resource. 15 N values were generally coincident with gut content analyses. These results provide a prediction of the functional responses of macroinvertebrates to the environmental consequences expected from accelerated land use. The functional responses constitute a powerful tool to assess eutrophication and its consequences in temperate plain streams.