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.

光和养分是控制热带阴影化贫营养流中生物膜的自下而上因素。不管主要的资源来源（土生的还是异源的），光和养分的更多可用性都有利于生物膜的发育和放牧（自上而下的因素），并反映了这些生态系统的营养结构。抽水生态系统中初级生产力的提高可能反映了生物膜中所含藻类细胞对溶解的无机碳（DIC）的分离作用。一项全球荟萃分析表明，初级生产力的提高导致生物膜碳同位素比的增加。δ ¹³下，在巴西中部的源头。放牧大无脊椎动物的密度由电脉冲控制。在每条溪流的开放区域和阴影区域均实现了带电（不包括gra草机）和不带电网格。营养膜中的生物膜无灰干物质（AFDM）增加了一倍。放牧对生物膜生物量的影响对于营养丰富的流非常重要，AFDM减少了50％。此外，在开阔的树冠条件下，浓度甚至增加了约十倍，因此即使在原始流中，遮光也会影响叶绿素-a。有叶绿素a浓度之间的显著负相关关系δ ¹³C值。我们的研究提供了证据，表明自下而上的控件发生了变化，生物膜的生物量增加了。自上而下的控制大大减少了基于自身有机物的食物网中的生物膜。