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

Abstract

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.

Fish introductions and light modulate food web fluxes in tropical streams: a whole‐ecosystem experimental approach

“…), but stronger effects of light or nutrients in others (e.g., Lourenço‐Amorim et al. ). The relative importance of top‐down and bottom‐up drivers is likely to depend on the response variable.…”

“…, Lourenço‐Amorim et al. ). Our manipulation of light, rather than nutrients, contrasts with these other previous studies, which focused on increased nutrient availability as a bottom‐up driver, perhaps because the two fold increase in light in this study is not as great as the increase in many nutrient addition studies.…”

“…, Lourenço‐Amorim et al. ) and lake (e.g., Hillebrand and Kahlert ) ecosystems, but effects of consumers and nutrients are often antagonistic rather than synergistic. In Neotropical streams in both Brazil and Venezuela, algal biomass and chlorophyll a increase strongly in response to the addition of nutrients, but decline in response to top‐down effects of consumers (Flecker et al.…”

Dynamics of algal production and ephemeropteran grazing of periphyton in a tropical stream

“…Ambient mean nutrient concentration values, as measured in June 2012, were 127 μg L −1 NO 3 + NO 2 ‐N, 2.1 μg L −1 NH 4 ‐N and 6.8 μg L −1 PO 4 ‐P. The periphyton algae at this site were shown to be limited by nitrogen (Lourenço‐Amorim et al, ).…”

Do changes in riparian zones affect periphyton growth and invertebrate colonization on rocky substrates in Atlantic Forest streams?

“…Predation, for example, might have been an important structuring factor for the assemblages (McIntosh & Townsend, 1996;Fairchild & Holomuzki, 2005;Holomuzki et al, 2010). The studied streams contain macroconsumers (crabs and shrimps) that may control invertebrate communities, as observed by Lourenço-Amorim et al (2014) and Costa et al (2016) in Atlantic Forest streams located in the state of Rio de Janeiro and Espírito Santo, Brazil.…”