Control of periphyton standing crop in an Atlantic Forest stream: the relative roles of nutrients, grazers and predators

1. 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. 2. 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. 3. 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.