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

Abstract

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.

Land use differentially alters microbial interactions and detritivore feeding during leaf decomposition in headwater streams

“…Regardless of some of its limitations, this study effectively uses an NDS manipulation in novel ways to test how aquatic microbes and detritivores respond to labile C and nutrient availability in catchments of differing land‐use types. Relative to other NDS studies, we measured a wide variety of inter‐related response variables (i.e., decomposition and detritivore feeding) that would not have been possible using other NDS substrates such as wood veneers or glass fibre filters (Ren et al, 2019; Tromboni et al, 2019; Wyatt et al, 2019). Microbial interactions, decomposition and detritivore feeding are all connected in natural settings, but most NDS studies cannot address these relationships.…”

“…We predicted that because streams in agricultural areas have higher amounts of nutrient pollution, nutrient NDS treatments in agriculture streams will not change decomposition rates relative to control treatments. Forested streams, however, typically are nutrient-limited (Tromboni et al, 2019). We anticipated that adding nutrients will alleviate limitations, resulting in increased decomposition rates.…”

Substratum‐associated microbiota

“…For littoral benthic diatoms, however, their responses to nutrient enrichment may be limited in lakes with short water residence times when water turbulence is an overriding stressor (Rimet, Feret, Bouchez, Dorioz, & Dambrine, 2019). As part of research to understand how the conversion of tropical forests to agriculture affects stream periphyton, Tromboni et al (2019) found periphyton assemblage structure shifted from taxa less tolerant of the high nutrients and light of forested sites to those tolerant of these conditions in deforested sites. Overall, the loss of canopy cover was the strongest predictor of community composition for all sites, whereas P concentration was the best predictor of algal abundance in deforested reaches.…”