An experiment in >1000 river and riparian sites found spatial patterns and controls of carbon processing at the global scale.

The River Continuum Concept (RCC) assumes that autochthonous primary production in forest streams is limited by light and is insufficient to sustain secondary production by consumers; they must therefore depend on allochthonous carbon from the surrounding forest. Recent studies have, however, questioned the importance of allochthonous carbon in stream food webs. There is a growing body of evidence using stable‐isotope techniques that demonstrate the importance of algae (autochthonous production) in the food webs of tropical streams. The actual contributions of autochthonous and allochthonous resources are rarely evaluated accurately because few studies consider the diet and the trophic efficiencies of the components of the food web or measure primary and secondary production to estimate the energy flow. We estimated the annual net primary productivity of periphytic microalgae (NPP) and the secondary production of macroinvertebrates (SP) from empirical models and we used stable isotopes of carbon and nitrogen to quantify the flow of material in food webs of five forest streams in the Guapi‐Macacu catchment, Rio de Janeiro, Brazil. NPP ranged from 46 g to 173 g dry mass (DM) m−2, whereas SP ranged from 0.90 g DM m−2 to 2.58 g DM m−2. The contribution of allochthonous carbon to the SP was more important than autochthonous sources and varied from 56% to 74% of all basal energy flow assimilated by primary consumers. The annual ingestion rate of basal sources varied from 8.08 g DM m−2 to 26.57 g DM m−2, with the allochthonous material contributing 76% and 87% of this. The annual ingestion rate of autochthonous material varied from 1.2% to 5.5% of the NPP. The present work suggests that the principal energy source for macroinvertebrates in streams of the Guapi‐Macacu catchment came from the riparian forest, as predicted by the RCC. However, this dependence appeared not to be driven by an absolute lack of autochthonous NPP, which seemed more than sufficient to sustain the entirety of macroinvertebrate SP.

According to the prevalent paradigm, the major source of carbon and energy for food webs of small forested streams in temperate regions is allochthonous material from the surrounding forest. Tropical streams have not been as well studied and there has been some speculation that their food webs are more aligned with in‐stream, algal production (autochthonous carbon). We studied food sources and consumers in four sites in a coastal tropical stream with gradients of 36–254 m elevation, 8–73% canopy cover and 11.9–7.1 km2 watershed area, with the expectation that the food web would incorporate proportionately more allochthonous material as shading increased with smaller stream size. We analysed stable isotope ratios of carbon and nitrogen in basal resources and fauna and compared the values of consumers to the potential sources using a mixing model to estimate the proportion of allochthonous and autochthonous material in their diets. The predominant source of carbon in the food web was from algal production at all sites. There was no distinct increase in the proportion of allochthonous contribution to the diets of primary consumers and predators with increasing shading, and they did not generally change their diet with shading. Thus the food web was based more on autochthonous resources than would be expected from the paradigm for temperate streams.

Summary Periphytic algae are an important source of energy fuelling stream food webs. Periphytic algal biomass can be controlled by bottom‐up and top‐down forces, but there are few studies that have investigated these effects simultaneously, especially in tropical streams, where periphyton can be a very important carbon source. Here, we investigated the bottom‐up effects of nutrients and the top‐down effects of grazers and predators in a coastal tropical stream in Brazil. We employed nutrient‐diffusing substrata to test for nutrient limitation, placed inside electric exclosures of different intensities, which prevented consumers from entering the electrified area. We used four types of substrata: one enriched with nitrogen alone (N), one with phosphorus alone (P), one with nitrogen + phosphorus (NP) and one control with no nutrients added (C). The electric exclosure had three treatments: high electricity that excluded both predators (Macrobrachium olfersi shrimps) and grazers (Baetidae mayflies), low electricity that excluded only shrimps, and a control treatment with no electricity where both shrimps and mayflies were allowed to enter. The limiting nutrient in the stream was nitrogen. Our results showed a greater magnitude of bottom‐up effects, with periphytic algae responding more strongly to nutrient addition than to grazers or predators. Top‐down control was not as strong, but periphytic algae responded negatively to grazers, especially in the absence of predators. This suggests that shrimps, either directly or indirectly, inhibit mayfly grazing activity. The response of periphytic algae to nutrients was stronger in the absence of grazers. Thus, top‐down effects, both directly and indirectly through a trophic cascade, can dampen the response to nitrogen addition, diminishing the degree of resource limitation.

Stream metabolism is affected by both natural and human-induced processes. While metabolism has multiple implications for ecological processes, relatively little is known about how metabolic rates are influenced by land use in tropical streams. In this study, we assessed the metabolic characteristics and related environmental factors of six streams located in a transition area from Cerrado to Atlantic Forest (São Carlos/Brazil). Three streams were relatively preserved, while three were flowing through more agriculturally and/or urban impacted watersheds. Surface water samples were analyzed for biological and physico-chemical parameters as well as discharge and percentage of canopy cover. Metabolism was determined through the single-station method to estimate gross primary production (GPP), ecosystem respiration (ER) and net ecosystem production (NEP) with BAyesian Single-station Estimation (BASE). Nutrient concentrations tended to be higher in impacted versus preserved streams (e.g., average total phosphorus between 0.028-0.042 mg L-1 and 0.009-0.038 mg L-1, respectively). Average canopy cover varied between 58 and 77%, with no significant spatial or seasonal variation. All streams were net heterotrophic (ER exceeded GPP) in all sampling periods. GPP rates were always lower than 0.7 gO2 m-2 d-1 in all streams and ER varied from 0.6 to 42.1 gO2 m-2 d-1.  Linear Mixed-Effect models showed that depth, discharge, velocity and total phosphorus are the most important predictors for GPP. For ER, depth, velocity and canopy cover are significant potential predictors. Canopy cover was the main light limiting factor and influenced stream metabolism. Our findings reinforced the concepts that shifts in the shading effect provided by vegetation (e.g., through deforestation) or changes in discharge (e.g., through land use conversion or water abstractions) can impact freshwater metabolism. Our study suggests that human activities in low latitude areas can alter tropical streams’ water quality, ecosystem function, and the degree of riparian influence. Our data showed that tropical streams can be especially responsive to increases of organic matter inputs leading to high respiration rates and net heterotrophy, and this should be considered to support management and restoration efforts.