Agricultural influences on the magnitude of stream metabolism in humid tropical headwater streams

The production and respiration of organic carbon in streams (stream metabolism) is a fundamental ecosystem process. The extent to which the magnitude of stream metabolism changes with forest conversion to agriculture in humid tropical headwaters is poorly understood. We measured whole-stream metabolism in headwaters draining forest-agricultural boundaries to investigate metabolic rates in areas with abrupt land-use transitions and the role of remnant riparian vegetation. We used linear mixed models to test the hypotheses that gross primary production (GPP) and ecosystem respiration (ER) would be higher in agricultural areas due to higher light availability and nutrient concentrations, respectively. We found a 257% increase in GPP and 30% increase in ER in agricultural stream reaches. GPP was driven by light and ER was mainly controlled by GPP. These results highlight the overriding influence of light in agricultural streams with large fractions of upstream forest cover. Our findings suggest that high riparian canopy cover (similar to 90%) is necessary to support stream metabolic rates similar to forests in agricultural areas. This study adds to our understanding of the within-biome variation of metabolism resulting from agriculture, and the potential similarities between forested biomes and the role of tropical streams in the global carbon cycle.