Conversion of tropical forests to agriculture alters the accrual, stoichiometry, nutrient limitation, and taxonomic composition of stream periphyton

The conversion of forests to agriculture in tropical areas profoundly changes adjacent streams by modifying hydrological conditions, altering light regimes, and increasing nutrient concentrations. In this study, we used an integrative approach to examine how transformations of intact forests affected the physical, chemical, and biological properties of periphyton, in three Brazilian Atlantic rainforest streams. We found that riparian land use change affected the stream periphyton in a variety of ways that were linked to the availability of light and nutrients. Periphyton standing stocks and accrual rates of new periphyton biomass on tiles were higher in deforested reaches than forested reaches. Linear mixed-model analyses showed that the increase of chlorophyll-a in the periphyton was explained by the increase in deforestation and soluble reactive phosphorus concentration. Deforestation also altered periphyton stoichiometry as deforested streams exhibited lower C:P, whereas C:N ratios decreased with increasing NH₄⁺ concentration that was higher in some deforested reaches. Periphyton productivity appeared to be limited by light in forested reaches and by nutrients in deforested reaches. There was differential availability of nitrogen and phosphorus in the deforested reaches, depending on land use type, and this resulted in different nutrient limitation. Periphyton community structure shifted from taxa less tolerant to high nutrients and light found in forested sites, to species tolerant to these conditions dominating periphyton assemblages in deforested sites. The loss of canopy cover was the strongest predictor of community composition for all sites, whereas phosphorus concentration was the best predictor of algal abundance in deforested reaches. This study highlights the complex effects of forest clearing on stream periphyton, ranging from changes to biomass accrual, nutrient limitation, stoichiometry, and community structure. We show the importance of using a comprehensive approach to help determine and predict how deforestation impacts stream ecosystems.