Understanding the Factors Controlling Biofilm As an Autochthonous Resource in Shaded Oligotrophic Neotropical Streams

Light and nutrients are bottom-up factors in controlling biofilm in tropical shaded oligotrophic streams. Regardless of the predominant resource source (autochthonous or allochthonous), greater availability of light and nutrients favors the development and, later, grazing (top-down factor) of the biofilm, with reflections on the trophic structure of these ecosystems. The increase in primary productivity in lotic ecosystems may reflect the fractionation of dissolved inorganic carbon (DIC) by the algal cells contained in the biofilm. A global meta-analysis showed that an increase in primary productivity leads to enrichment of the carbon isotope ratios of biofilm. The objective of this study was to evaluate how top-down (grazing) and bottom-up (light and nutrient) factors act on the biofilm and whether algal biomass determines the isotopic fractionation of δ13C in headwaters of Central Brazil. The density of grazing macroinvertebrates was controlled with electric pulses. Electrified (excluding grazers) and non-electrified grids were implemented in open and shaded areas in each stream. The biofilm ash-free dry mass (AFDM) doubled in nutrient-enriched streams. The effect of grazing on the biomass of the biofilm was significant for the nutrient-enriched streams, with a 50% reduction in the AFDM. Additionally, shading influenced chlorophyll-a even in pristine streams, as the concentration increased approximately ten-fold under open canopy conditions. There was a significant negative relationship between chlorophyll-a concentrations and δ13C values. Our study provides evidence that biofilm biomass increases as a result of changes in bottom-up controls. Top-down control significantly reduced biofilm in the food web based on autochthonous organic matter.