Riparian deforestation may strongly affect stream functioning, with consequences for biodiversity and ecosystem services. These effects can be assessed using bioindicators relating to biotic community structure and ecosystem functioning. We evaluated the effects of riparian deforestation on 1. measures of community structure using aquatic benthic invertebrates, and 2. an aspect of ecosystem functioning, aquatic leaf processing. We selected sites along gradients of riparian land use in four Atlantic rainforest streams and measured physical and chemical properties for their association with riparian deforestation. We sampled benthic invertebrates and calculated metrics of community structure at each site. We measured rates of leaf processing using leaves of a common riparian tree, Guarea guidonia. Riparian deforestation was accompanied by increasing concentration of ammonia, water current and temperature and decreasing nightly oxygen saturation. Invertebrate diversity decreased and community metrics changed with deforestation as expected of negative impacts. Leaf processing decreased with deforestation. Although there were significant differences in physical and chemical measurements among streams, the gradients in community and ecosystem responses were similar, thus suggesting that both types of bioindicators were useful for monitoring changes and relating them to loss of biodiversity and ecosystem function.

The conversion of forests to agriculture in tropical areas profoundly changes adjacent streams by modifying hydrological conditions, altering light regimes, and increasing nutrient concentrations. In this study, we used an integrative approach to examine how transformations of intact forests affected the physical, chemical, and biological properties of periphyton, in three Brazilian Atlantic rainforest streams. We found that riparian land use change affected the stream periphyton in a variety of ways that were linked to the availability of light and nutrients. Periphyton standing stocks and accrual rates of new periphyton biomass on tiles were higher in deforested reaches than forested reaches. Linear mixed-model analyses showed that the increase of chlorophyll-a in the periphyton was explained by the increase in deforestation and soluble reactive phosphorus concentration. Deforestation also altered periphyton stoichiometry as deforested streams exhibited lower C:P, whereas C:N ratios decreased with increasing NH4+ concentration that was higher in some deforested reaches. Periphyton productivity appeared to be limited by light in forested reaches and by nutrients in deforested reaches. There was differential availability of nitrogen and phosphorus in the deforested reaches, depending on land use type, and this resulted in different nutrient limitation. Periphyton community structure shifted from taxa less tolerant to high nutrients and light found in forested sites, to species tolerant to these conditions dominating periphyton assemblages in deforested sites. The loss of canopy cover was the strongest predictor of community composition for all sites, whereas phosphorus concentration was the best predictor of algal abundance in deforested reaches. This study highlights the complex effects of forest clearing on stream periphyton, ranging from changes to biomass accrual, nutrient limitation, stoichiometry, and community structure. We show the importance of using a comprehensive approach to help determine and predict how deforestation impacts stream ecosystems.

Consumer mineralisation of nutrients can affect ecosystem processes, such as primary productivity, community respiration and biogeochemical cycling. Researchers have not tested, however, the influence of standard incubation methods on mineralisation rates. Therefore, the influence of varied incubation methods on consumer mineralisation rates is poorly understood. Here, we investigated how incubation conditions affected the mineralisation rates of ammonium and phosphate of two species of fish and two species of shrimp. For each species, we measured mineralisation rates during the day and at night in two incubation environments (standard and modified to reduce stress). We also estimated the effect of conspecific density (1, 3 or 5 individuals) and the duration of incubation (after 15, 30, 45, 60 and 75min) on mineralisation for a species of shrimp. Our results indicated mass-specific N mineralisation by three species was influenced by the time of day, and for two of the species, mass-specific N mineralisation was also influenced by incubation conditions. Mass-specific mineralisation rates for both N and P by a species of shrimp decreased significantly with increasing incubation time, but we did not detect significant differences in mineralisation among incubation durations or densities for either element. Mass-specific mineralisation rates for P were negatively correlated with body size for all studied species, as previously reported. Only the two species of shrimp showed the expected negative relationship between mass-specific N mineralisation rates and body size. Our results suggest that the mechanisms by which organisms excrete elements are differentially influenced by incubation conditions. Our study highlights the need to develop methods specific to the organisms of interest, especially the duration and the time of day of the incubation. Additionally, our attempt to influence mineralisation rates by modifying chamber design was mostly ineffective, indicating that this measure may not be a priority for future studies.

Nutrient limitation assessment is important to understand stream ecosystem functioning. Aquatic primary producers are often limited by nitrogen, phosphorus, or both, as assessed by nutrient diffusing substrata (NDS), a common method for assessing nutrient limitation in streams. But little is known regarding how this method relates to patterns of nutrient uptake at the whole‐stream scale. We combined two techniques to examine nutrient limitation in a tropical stream. First, we conducted two NDS experiments using ammonium, nitrate, and phosphate alone and combined, to determine nutrient limitation at substrata scale over several weeks. Second, we conducted whole‐stream nutrient addition experiments over the course of a year, using nutrients alone and in combination, to test theoretical predictions about uptake characteristics of limiting and nonlimiting nutrients. NDS results consistently indicated N limitation. Ambient uptake length (SW‐amb) suggested either P limitation (shorter SW‐amb for P than N) or colimitation (similar SW‐amb for both nutrients). The relationship between N uptake and concentration when added alone or with P suggested P limitation, colimitation, or neither, depending on the date. We speculate that the different conclusions arise from differences in the spatial and temporal scale assessed by these techniques and the microbial processes involved, and the potential for physical processes influencing whole‐stream uptake estimates. We conclude that nutrient limitation is not as categorical as NDS results often imply, rather habitat, resource, and biotic diversity result in nutrient uptake rates that do not necessarily conform to predictions drawn from fine scale, process‐specific bioassays such as chlorophyll‐a accrual on NDS.

We estimated the net primary productivity of periphyton algae and rate of macroinvertebrate grazing in a third-order stream in coastal Atlantic forest, Brazil. Net primary productivity was estimated from the growth curve of periphyton chlorophyll, based on the relationship between algal density and algal production, with baetid ephemeropteran grazers excluded by highintensity electric shocks. We fit a logistic model and cite the maximum productivity 44.7mgCm 2 d 1 . We manipulated grazing pressure by excluding the shrimp Macrobrachium olfersi by low-intensity electrical exclusion—the shrimp was known to have a negative effect on ephemeropteran grazing and to cause a trophic cascade on periphyton. Thus, rate of grazing was estimated at the normal level of grazing and with grazing enhanced by the exclusion of shrimps. Normal level of grazing reduced periphyton algal stock (6mg chlorophyll m 2 ) by slightly less than 50%, enhanced grazing by slightly more than 50%. This implied that grazing removed algae at approximately the maximum net productivity—that is 19%d 1 and24%d 1 ofalgalstockinnormalandenhancedgrazingrespectively,equivalentto 44.1mgC m 2 d 1 in both cases. The observed net primary productivity obtained by direct measurementmatchesthatexpectedfrompredictiveequationsbasedonchlorophylllevels,but isatthelowerpartoftherangeobservedinworldwideandtropicalsurveys.Itmatchesresultsof recentstudiesinthetropicswhichusedopen-channeldielchangesinoxygentocalculategross primary productivity. The method appears reliable and is potentially applicable to situations in which other methods are constrained by low productivity and other technical difficulties.

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.