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Atmospheric nitrous oxide (N2O) is a potent greenhouse gas thought to be mainly derived from microbial metabolism as part of the denitrification pathway. Here, we report that in unexplored peat soils of Central and South America, N2O production can be driven by abiotic reactions (≤ 98 %) highly competitive to their enzymatic counterparts. Extracted soil iron positively correlated with in-situ abiotic N2O production determined by isotopic tracers. Moreover, we found that microbial N2O reduction accompanied abiotic production, essentially closing a coupled abiotic-biotic N2O cycle. Anaerobic N2O consumption occurred ubiquitously (pH 6.4-3.7), with proportions of diverse clade II N2O-reducers increasing with consumption rates. Our findings show denitrification in tropical peat soils is not a purely biological process, but rather a “mosaic” of abiotic and biotic reduction reactions. We predict hydrological and temperature fluctuations differentially affect abiotic and biotic drivers and further contribute to the high N2O flux variation in the region.

Atmospheric nitrous oxide (N

Human impacts, particularly nutrient pollution and land-use change, have caused significant declines in the quality and quantity of freshwater resources. Most global assessments have concentrated on species diversity and composition, but effects on the multifunctionality of streams and rivers remain unclear. Here, we analyse the most comprehensive compilation of stream ecosystem functions to date to provide an overview of the responses of nutrient uptake, leaf litter decomposition, ecosystem productivity, and food web complexity to six globally pervasive human stressors. We show that human stressors inhibited ecosystem functioning for most stressor-function pairs. Nitrate uptake efficiency was most affected and was inhibited by 347% due to agriculture. However, concomitant negative and positive effects were common even within a given stressor-function pair. Some part of this variability in effect direction could be explained by the structural heterogeneity of the landscape and latitudinal position of the streams. Ranking human stressors by their absolute effects on ecosystem multifunctionality revealed significant effects for all studied stressors, with wastewater effluents (194%), agriculture (148%), and urban land use (137%) having the strongest effects. Our results demonstrate that we are at risk of losing the functional backbone of streams and rivers if human stressors persist in contemporary intensity, and that freshwaters are losing critical ecosystem services that humans rely on. We advocate for more studies on the effects of multiple stressors on ecosystem multifunctionality to improve the functional understanding of human impacts. Finally, freshwater management must shift its focus toward an ecological function-based approach and needs to develop strategies for maintaining or restoring ecosystem functioning of streams and rivers. Generalitat de Catalunya, Grant/Award Number: 2017SGR0976; Portuguese Foundation for Science and Technology, Grant/Award Number: CEEIND/02484/2018 and UIDP/04292/2020

Glyphosate-based herbicides can be harmful to the environment and human health. Especially in developing countries, these herbicides are often used indiscriminately in agricultural and urban areas. Here, we optimized a simple and efficient flow injection-based spectrophotometric method to monitor environmentally relevant glyphosate concentrations in surface waters. The method was then used to assess the environmental mobility of glyphosate in Southeast Brazil by monitoring surface runoff from experimental agricultural soil plots that received glyphosate applications in 2015. Further, water samples from low-order streams were collected in five agricultural, urban, and natural areas, as well as from the 5th-order Rio das Mortes during the rainy season. Finally, 20 drinking water sources were sampled in urban, rural, and agricultural areas. Runoff from reference plots without glyphosate application showed concentrations below the method's detection limit of 0.49 mg.L

Riparian areas are recognized for their buffering capacity regarding phosphorus and nitrogen from agricultural and urban runoff. However, their role in attenuating nutrient loads of rivers receiving point source nutrient inputs (e.g., from wastewater treatment plants, WWTPs) is still little understood. Here, we investigated whether ammonium (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP) retention were influenced by the riparian land use in three Brazilian rivers receiving WWTP effluents. We hypothesized that nutrient attenuation would be potentially influenced by the hydrological connectivity between the main channel and riparian areas with native vegetation. We estimated retention from longitudinal patterns of dilution-corrected nutrient concentrations below the WWTPs. We assessed nutrient retention during periods with high (i.e., the wet) and low connectivity (i.e., the dry season). Relationships between non-conservative (nutrients) and conservative (chloride) solutes in both seasons were used to identify potential changes in the river chemistry due to the hydrological connectivity with the riparian areas. We also evaluated the relationship between net uptake velocities (Vf-net) and the accumulated percent native vegetation cover in the 100-m buffer using linear regressions, comparing the response for each nutrient between seasons with Analysis of Covariance. Slopes of regressions between nutrients and chloride significantly differed between seasons for NO3-N and SRP but not for NH4-N. The relationships between Vf-net and accumulated native vegetation in the riparian buffer presented steeper slopes for SRP in the wet than in the dry season. No significant relationships between NO3-N Vf-net and native vegetation cover were observed in either season. In contrast, increases in Vf-net with increasing vegetation cover were observed for NH4-N in the dry season. In periods with expected higher connectivity, NO3-N and SRP concentrations tended to be lower relative to chloride concentrations, with a potential effect of native vegetation in the riparian area on SRP retention. Our results suggest that seasonal connectivity between nutrient-rich river water and riparian areas is likely to induce changes in the predominant nutrient transformation processes, thereby favoring either nutrient retention or export in such rivers.

Made available in DSpace on 2021-06-25T10:29:12Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-08-10 Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) It has been postulated that eutrophication causes replacement of n3 highly unsaturated fatty acids (n3 HUFA) rich taxa, such as Bacillariophyta, Cryptophyta and Dinophyta, with taxa poor in these fatty acids (FA), such as Chlorophyta and Cyanobacteria. Such a change in community composition at the basis of the food web may alter the FA composition of consumer tissues. Here, we investigated the effects of eutrophication on phytoplankton composition and FA profiles of seston and muscle of two omnivorous fish species (Astyanax fasciatus and Astyanax altiparanae) from reservoirs of different trophic status in Southeast Brazil. The phytoplankton composition and seston FA profiles reflected the degree of eutrophication at most of the studied sites. Three of the five most eutrophic sites were dominated by cyanobacteria and had the highest saturated fatty acid (SFA) and lowest polyunsaturated fatty acid (PUFA) relative contents among all sites. In contrast, the remaining two sites presented a higher phytoplankton diversity and higher relative contribution of sestonic PUFAs with 18 carbons (C18) and HUFAs than less eutrophic systems. However, there were no clear effects of sestonic FA profiles on the FA profiles of muscle of both fish species. A higher percentage of n3 HUFAs was found in the fish samples from a hypereutrophic and cyanobacteria dominated reservoir than in those from sites with a more diverse phytoplankton community in which fish mainly showed higher percentages of C18 PUFA. These results suggest a lack of a direct relationship between the degree of eutrophication and the percentage of n3 HUFAs in both fish species, which can be caused by specific characteristics of the reservoirs that may modulate eutrophication effects. Therefore, consumer FA biochemistry seemed to be dictated by their ability to select, accumulate, and modify dietary FAs, rather than by the eutrophication degree of the studied tropical reservoirs. Departamento de Fisiologia Instituto de Biociências Universidade de São Paulo, Rua do Matão, 101 Departamento de Geociências Universidade Federal São João del-Rei Universidade Anhembi Morumbi, Campus Mooca Departamento de Botânica Universidade Federal de Minas Gerais Centro de Pesquisa de Recursos Hídricos Instituto de Pesca Departamento de Ecologia Instituto de Biociências Universidade de São Paulo Universidade Estadual Paulista (Unesp) Instituto de Ciência e Tecnologia Universidade Estadual Paulista (Unesp) Instituto de Ciência e Tecnologia FAPESP: 2014/16320-7 FAPESP: 2016/07363-0 FAPESP: 2017/11530-1

Aquaculture impacts on aquatic organic matter and ecosystem integrity are poorly understood, especially in tropical regions. Here, we investigated the impacts of Nile tilapia net cage farming on the elemental stoichiometry, fluorescence components, and stable isotopes of dissolved organic matter (DOM) of the large, tropical Furnas Reservoir (SE Brazil). Early-stage fish farming, i.e., relatively small and recently implemented farms, had detectable incipient effects on DOM characteristics, and these effects differed between reservoir branches. In the less eutrophic Rio Grande branch of the reservoir, we found a reduction in natural humic-like DOM components and an increase in a protein-like DOM component as far as 100 m away from fish farms. Further, we observed a decrease in δ15N-TDN due to fish farming. In the more eutrophic Rio Sapucai branch, there were only local decreases in C:N ratios, as well as rises in C:P and N:P of DOM due to fish farming. These results suggest that early-stage fish farming had local but detectable effects on aquatic DOM that depended on previous eutrophication levels and highlight the need to assess the early impacts of fish farming on tropical reservoirs by combining different monitoring strategies.

Fatty acids (FAs) and their metrics have been used to detect and assess the impacts of urbanization and agriculture on aquatic ecosystems. Here, we investigated whether seston FAs are also useful to characterize and understand early-stage aquaculture impacts in a large tropical reservoir (Furnas Reservoir, SE Brazil). We tested the hypothesis that single FAs, as well as selected FA metrics in the seston fraction, are efficient markers of net-cage fish farming effects. In general, fish farming had only minor effects on standard water chemical variables, mainly small increases in ammonium, nitrate, and dissolved organic nitrogen concentrations. By increasing concentrations of several polyunsaturated FAs, early-stage fish farming improved sestonic food quality in the more oligotrophic branch of the reservoir under drought conditions. However, in general, increases in concentrations of bacterial FAs, due to fish farming, suggested organic matter (OM) subsidies from non-ingested and non-assimilated fish feed. In the more eutrophic reservoir branch, seston FA profiles suggested that fish farming caused an increase of low-quality food resources, such as cyanobacteria. Thus, background impact levels may determine the biochemical responses of tropical reservoirs to fish farming. Higher contributions of potentially sewage-derived and bacterial FAs during drought conditions, especially at reference sites of the more oligotrophic branch, suggested that drought shifted OM inputs towards anthropogenic sources, thereby overwriting land-use related differences between reservoir branches and homogenizing their environmental conditions. In conclusion, FA variables were useful to evaluate and understand environmental conditions, as well as the effects of early-stage fish farming and drought, and should be considered in impact assessments in tropical lentic ecosystems.

High Rate Algal Ponds (HRAP) are a sustainable alternative for wastewater treatment. However, HRAPs must be shallow (h = 0.2–0.3 m) for adequate sunlight penetration and require relatively long hydraulic retention times (HRT, >5 d) for effective pollutant removal, which results in high area footprints. In this context, a deep HRAP (h = 1.2 m) was equipped with red light-emitting diodes (LED, 660 nm) to provide photosynthetic active radiation in the aphotic zone. A conventional HRAP (HRAP1) and the LED-enhanced HRAP (HRAP2) were operated with identical working volumes (1.25 m3) and HRT (10 d), treating digestate dilutions of 5% (Stages S-0, S-I), 25% (S-II, S-III) and 50% (v/v) (S-IV to S-VI). The HRAP2 used only 25% of the area needed by the HRAP1 but LEDs used 1.4 (S-0 to S-II), 2.3 (S-III, S-IV) and 3.4 (S-V) times the energy consumed in HRAP1. HRAP2 presented similar performance to HRAP1 during the treatment of 5% and 25%-digestate, achieving removals above 75% for TSS, COD, TAN, and TKN. However, HRAP1 showed better treatment performance at 50%-digestate, likely due to more intense light radiation induced by its shallow depth, resulting in higher microalgal activity. Biomass productivity decreased with increasing digestate concentration, showing averages in HRAP1 and HRAP2 of 9 and 3 gVSS·m−2d−1, respectively, during feeding with 5%-digestate. With 25%, average productivities ranged from 2 to 5 and 2 to 6 gVSS·m−2d−1, and from 0 to 6 and 0 to 4 gVSS·m−2d−1 with 50%-digestate, respectively. Both systems showed reasonable nitrogen and phosphorus recoveries in the biomass (TN: 8–17% and 6–34%; TP: 10–45% and 5–48% for HRAP1 and HRAP2, respectively). Nitrogen volatilization, likely as NH3, N2O, and N2, was caused by stripping and nitrification-denitrification in both systems. Our data suggest that using LED-lighting to complement sunlight in deeper HRAPs is a promising alternative for enhancing microalgae-based reactors for digestate treatment.

Os herbicidas à base de glifosato têm amplo espectro de ação e estão associados a uma série de impactos ambientais. O glifosato é amplamente utilizado em áreas agrícolas e urbanas e é frequentemente encontrado em águas superficiais e no solo. Apresenta efeitos nocivos para o funcionamento ecossistêmico, devido à sua toxicidade para organismos de todos os níveis tróficos, além de casos de bioacumulação relatados. Além disso, a contaminação de recursos hídricos com glifosato põe em perigo a saúde humana. Medidas de mitigação são necessárias para reduzir os impactos do glifosato na saúde ecossistêmica e humana, com controles mais rígidos necessários para prevenir a sua crescente contaminação em águas superficiais e subterrâneas no território brasileiro.

Nesse caderno especial a crise hídrica no Brasil se revela como uma crise complexa, multi-facetada, intrinsicamente associada ao mau gerenciamento político e econômico e a um presente marcado por políticas públicas voltadas para a produção e o crescimento econômico, a despeito de questões ambientais, sociais e da saúde humana. Os diversos trabalhos aqui apresentados são fruto do Simpósio Internacional “Crise Hídrica: desafios e soluções”, realizado na Universidade de Federal de São João del-Rei em abril de 2019, e abordam o mosaico de desdobramentos causados não apenas pela escassez de água para abastecimento, sobretudo a luz das mudanças climáticas, mas também pelo mau gerenciamento dos recursos hídricos e de suas bacias de drenagem. São abordados ainda a falta de valoração ambiental e, em seu aspecto social e humano mais relevantes, a falta de valoração da condição humana como parte integrante de um cenário, onde a exploração dos recursos hídricos e do meio-ambiente se tornam justificativa para perdas ambientais e humanas, em seus mais variados aspectos. Torna-se evidente que a crise hídrica no Brasil, assim como no resto do mundo, é uma crise ambiental, política, econômica, social e de saúde pública. Apenas após compreendermos essa natureza transdisciplinar da crise hídrica, em seus mais diversos graus de complexidade, seremos capazes de propor soluções verdadeiramente efetivas frente à expansão de um problema, que não é novo, e certamente não se solucionará a curto e médio prazos.

Dissolved organic matter (DOM) can be an important source of energy and nutrients in aquatic ecosystems and play important roles in carbon and nutrient dynamics of natural and impacted environments. In order to assess the effects of early stage fish farming on dissolved organic carbon (DOC) concentrations as well as DOM fluorescence indices, we took water samples along transects from six fish farms towards reference sites in a large tropical reservoir (Furnas Reservoir, Southeast Brazil) between April 2013 and December 2016. While DOC concentrations did no change along transects, small but significant changes in fluorescence indices were detectable in up to 100 m from fish farms. Higher fluorescence index (FI) values near fish farms pointed to small increases in microbial production due to fish farming. Only in the more pristine of the two main reservoir branches, small differences in the freshness index (β:α) and the peak T/peak C ratio along transects indicated that the relative contribution of recently produced DOM and the biochemical oxygen demand of DOM increased due to aquaculture. The humification index did not respond to fish farming. In summary, current early-stage fish farming did not appear to cause major impacts on DOM quantity and quality, but future increases in net cage and fish farm densities, as well as longer operation times may change this assessment. The fluorescence indices FI, β:α and peak T/peak C ratio may be useful early warning signals for monitoring fish farm impacts in tropical reservoirs.

The dataset (Data File S1; DOI: 10.6084/m9.figshare.8863721) and the R code used to generate the results (Code S1; DOI: 10.6084/m9.figshare.8863655), including step by step explanations of the statistical tests, have been deposited in Figshare Digital Repository (https://figshare.com/projects/Sediment_Respiration_Pulses_in_Intermittent_Rivers_and_Ephemeral_Streams/66104)., Intermittent rivers and ephemeral streams (IRES) may represent over half the global stream network, but their contribution to respiration and carbon dioxide (CO2) emissions is largely undetermined. In particular, little is known about the variability and drivers of respiration in IRES sediments upon rewetting, which could result in large pulses of CO2. We present a global study examining sediments from 200 dry IRES reaches spanning multiple biomes. Results from standardized assays show that mean respiration increased 32–66-fold upon sediment rewetting. Structural equation modelling indicates that this response was driven by sediment texture and organic matter quantity and quality, which, in turn, were influenced by climate, land use and riparian plant cover. Our estimates suggest that respiration pulses resulting from rewetting of IRES sediments could contribute significantly to annual CO2 emissions from the global stream network, with a single respiration pulse potentially increasing emission by 0.2–0.7%. As the spatial and temporal extent of IRES increases globally, our results highlight the importance of recognizing the influence of wetting-drying cycles on respiration and CO2 emissions in stream networks., We thank Y. Etxeberria, L. Sánchez, C. Gutiérrez, G. LeGoff and B. Launay for laboratory support. DvS was supported by a Short-Term Scientific Mission of the COST Action CA15113 (SMIRES, Science and Management of Intermittent Rivers and Ephemeral Streams, www.smires.eu), supported by COST (European Cooperation in Science and Technology) and received additional funding from the EU’s 7th Framework Programme for research, technological development and demonstration under grant agreement No. 603629 (GLOBAQUA) and a Grant for Research Groups of the Basque University System (IT-951-16) funded by the Basque Government. RM and BO were supported by the Spanish Ministry of Science, Innovation and Universities through project C-HYDROCHANGE (CGL2017-86788-C3-2-P and CGL2017-86788-C3-3-P). FA was funded by the Swiss National Science Foundation grants No. PP00P3_150698 and PP00P3_179089. NC was supported by the EU project LIFE+ TRivers (LIFE13 ENV/ES/000341). SDL received funding from the EU’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 748625. PP and MP were supported by INTER-COST project LTC17017. The authors declare that they have no competing interests.

Climate change and human pressures are changing the global distribution and the extent of intermittent rivers and ephemeral streams (IRES), which comprise half of the global river network area. IRES are characterized by periods of flow cessation, during which channel substrates accumulate and undergo physico‐chemical changes (preconditioning), and periods of flow resumption, when these substrates are rewetted and release pulses of dissolved nutrients and organic matter (OM). However, there are no estimates of the amounts and quality of leached substances, nor is there information on the underlying environmental constraints operating at the global scale. We experimentally simulated, under standard laboratory conditions, rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase across 205 IRES from five major climate zones. We determined the amounts and qualitative characteristics of the leached nutrients and OM, and estimated their areal fluxes from riverbeds. In addition, we evaluated the variance in leachate characteristics in relation to selected environmental variables and substrate characteristics. We found that sediments, due to their large quantities within riverbeds, contribute most to the overall flux of dissolved substances during rewetting events (56%–98%), and that flux rates distinctly differ among climate zones. Dissolved organic carbon, phenolics, and nitrate contributed most to the areal fluxes. The largest amounts of leached substances were found in the continental climate zone, coinciding with the lowest potential bioavailability of the leached OM. The opposite pattern was found in the arid zone. Environmental variables expected to be modified under climate change (i.e. potential evapotranspiration, aridity, dry period duration, land use) were correlated with the amount of leached substances, with the strongest relationship found for sediments. These results show that the role of IRES should be accounted for in global biogeochemical cycles, especially because prevalence of IRES will increase due to increasing severity of drying events., In this study we experimentally simulated under laboratory conditions rewetting of leaves, riverbed sediments, and epilithic biofilms collected during the dry phase from intermittent rivers and ephemeral streams covering global spatial scale. We determined the amounts and quality of the leached nutrients and dissolved organic matter, assessed their inter‐substrate and cross‐climate differences, and estimated areal fluxes from 1 m2 of riverbeds. In addition, we evaluated the variance in leachate characteristics related to selected environmental variables and substrate characteristics.

Agricultural activities can affect the delivery of nutrients to streams, riparian canopy cover, and the capacity of aquatic systems to process nutrients and sediments. There are few measures of nutrient uptake and metabolism from tropical or subtropical streams in general, and even fewer from tropical regions of South America. We examined ammonium (NH₄ ⁺) and soluble reactive phosphorus (SRP) retention in streams in Brazil and Argentina. We selected 12 streams with relatively little or extensive agricultural activity and conducted whole-stream nutrient additions and measurements of gross primary production and ecosystem respiration. We used multiple linear regression to determine potential drivers of nutrient uptake metrics across the streams. Nutrient concentrations and retention differed significantly between land use categories. Both NH₄ ⁺ and SRP concentrations were higher in the agricultural sites (means of 161 and 495 μg l⁻¹, respectively), whereas metabolic rates were slower and transient storage was smaller. Our analysis indicated that agriculture increased ambient uptake lengths and decreased uptake velocities. The regression models revealed that ambient SRP had a positive effect on NH₄ ⁺ uptake and vice versa, suggesting strong stoichiometric controls. Drivers for nutrient uptake in streams with low-intensity agriculture also included canopy cover, temperature, and ecosystem respiration rates. Nutrient assimilation in agricultural sites was influenced by a higher number of variables (gross primary production for SRP, discharge, and transient storage for both nutrients). Our results indicate agricultural activity changes both the magnitude of in-stream nutrient uptake and the mechanisms that control its variation, with important implications for South American streams under agricultural intensification., Instituto de Limnología "Dr. Raúl A. Ringuelet"

Aim Neotropical biomes are highly threatened by land-use changes, but the catchment-wide biogeochemical effects are poorly understood. Here, we aim to compare exports of dissolved nitrogen (N) and phosphorus (P) from natural and human-impacted catchments in the Neotropics. Location Neotropics. Methods We measured streamwater nutrient concentrations and exports in 20 south-eastern Brazilian catchments with different land uses (natural Cerrado/semi-deciduous forest, pasture, intensive agriculture and urban areas) and conducted a meta-analysis on nutrient exports from Neotropical catchments, both natural and human-impacted. Results Organic forms dominated dissolved nutrient exports in central/south-east Brazil in both natural and human-dominated catchments. Our meta-analysis suggests that there is wide geographic variability in the natural dominance of organic versus inorganic nutrient exports across the Neotropics, and for N a tendency for inorganic and organic forms to vary inversely across sites. We found strong land-use effects, especially in urban areas. In areas naturally dominated by organic N exports, land use did not overturn that dominance, but rather increased the concentration of both inorganic and organic N. In catchments dominated by inorganic N exports or showing equivalent organic and inorganic exports, land use also caused organic exports to be dominant. Land-use effects on P were unclear, probably owing to a paucity of available data, but our results from south-east Brazil suggested that land use also increases dissolved P exports. Main conclusions We show that dominance of inorganic versus organic nutrient exports is highly variable across natural Neotropical catchments, a pattern contrasting with the general tendency for dominance of organic N in natural temperate catchments. Further, we found a general dominance of organic N in most human-impacted catchments, which is in contrast to strong dominance of inorganic N in most human-impacted temperate catchments. Finally, we find that urbanization exerts the strongest impacts on nutrient exports, thus underscoring wastewater treatment as a critical management priority under future land-use change.

International audience; Perennial rivers and streams make a disproportionate contribution to global carbon (C) cycling. However, the contribution of intermittent rivers and ephemeral streams (IRES), which sometimes cease to flow and can dry completely, is largely ignored although they represent over half the global river network. Substantial amounts of terrestrial plant litter (TPL) accumulate in dry riverbeds and, upon rewetting, this material can undergo rapid microbial processing. We present the results of a global research collaboration that collected and analysed TPL from 212 dry riverbeds across major environmental gradients and climate zones. We assessed litter decomposability by quantifying the litter carbon-to-nitrogen ratio and oxygen (O2) consumption in standardized assays and estimated the potential short-term CO2 emissions during rewetting events. Aridity, cover of riparian vegetation, channel width and dry-phase duration explained most variability in the quantity and decomposability of plant litter in IRES. Our estimates indicate that a single pulse of CO2 emission upon litter rewetting contributes up to 10% of the daily CO2 emission from perennial rivers and stream, particularly in temperate climates. This indicates that the contributions of IRES should be included in global C-cycling assessments.

Stable isotope techniques are widely applied in aquatic food web research to infer trophic position or to estimate the relative contributions of different dietary resources. The accurate consideration of carbon (C) and nitrogen (N) discrimination in organisms is a critical prerequisite for such studies. Isotopic discrimination of invertebrates may differ systematically between temperate and tropical freshwater environments, but there are little data available on discrimination factors of tropical invertebrates. Here, we analyzed the C (Δ13C) and N discrimination (Δ15N) of eight taxa of benthic freshwater invertebrates from a Southeastern Brazilian tropical catchment, six predator and two shredder species. Predators showed a high variability in Δ13C (−1.5 to 1.3; min–max), but shredders exhibited a lower variability and had negative Δ13C values (−2.1 to −1.8). Values of Δ15N were also highly variable among both predators and shredders, but shredders had negative and lower values (−0.9 to −0.1) than predators (0.0 to 9.1). Tissue turnover rates were equal to or higher than 0.02 d−1 for all invertebrates, suggesting that experiment durations of 50 days may be sufficient for future isotopic discrimination experiments with tropical freshwater invertebrates. Our results suggest that Δ13C enrichment, and to a minor degree also Δ15N enrichment, may not always occur in tropical freshwater invertebrates.

Here, we compared sensor precision and drift among four commonly used dissolved oxygen (DO) sensor types under laboratory conditions, and evaluated their accuracy of measurement of stream ecosystem metabolism (EM) and subsequent assessment of stream ecosystem health in the field. Of all tested sensor types, optical sensors had the highest precision, followed by miniaturized polarographic sensors. Traditional polarographic and pulsed polarographic sensors had a lower precision. The traditional polarographic sensors were the only ones exhibiting detectable sensor drift that had to be corrected for, prior to calculations of EM. In parallel EM measurements in two rural and two pristine tropical streams, the four compared DO sensor types estimated relatively similar gross primary production (GPP), ecosystem respiration (R) and GPP:R ratios in each stream, and classified all streams as highly heterotrophic. Nonetheless, parallel GPP and R estimates with different sensors had relative standard deviations of the mean between 6.2% and 34.9%, and between 7.9% and 16.3%, respectively. Thus, sensor accuracy can be a major source of variability in EM estimates. There were only minor differences in stream ecosystem health assessments among different DO sensors, compared to differences between different assessment methods. While all tested sensor types yielded useful ecosystem health classifications, the optical sensor type was the most accurate, and thus, the most suitable for functional impact assessment and monitoring.

Stream nutrient uptake is an important ecosystem service, because it supplies human needs such as water purification. Uncertainty in nutrient uptake measurement determines, in combination with effect magnitude, whether effects of land use, stream restoration or climate change can be detected by this method. However, measurement uncertainty for nutrient uptake metrics is rarely reported. Here, we compared whole-stream phosphate and nitrate uptake among three pristine and three agricultural tropical Cerrado streams. We quantified uncertainty in uptake metrics estimated by kinetic nutrient-addition experiments and evaluated their potential to detect land-use effects. Ambient phosphate and nitrate uptake lengths (SW) ranged from 11 to 106 m and from 29 to 357 m, respectively, in pristine streams and from 41 to 586 m and from 76 to 1372 m in agricultural streams. Moderate measurement uncertainty in SW allowed for the detection of land-use impacts with this metric. However, derived nutrient uptake metrics, such as ambient uptake rates and uptake velocities, as well as parameters of whole-stream saturation kinetics, were associated with high measurement uncertainties that prevented the detection of potential land-use effects. Sensitivity analyses suggested that avoiding high enrichment levels in kinetic addition experiments and increasing sampling effort for plateau nutrient and tracer concentration at the first and last sampling stations of investigated stream reaches are the most promising strategies to reduce measurement uncertainty. Analyzing nutrient uptake without considering measurement uncertainty might lead to poor interpretation in case studies and meta-analyses, such as incorrect evaluations of human impacts or comparisons among systems.

Organic matter decomposition and ecosystem metabolism have been suggested as promising functional indicators of stream ecosystem integrity, but tests of their applicability to tropical systems are rare. Here, we tested the suitability of the ecosystem processes leaf decomposition (LD), community respiration (CR) and gross primary production (GPP) as functional indicators of catchment and riparian corridor land use impacts on 16 rural, tropical headwater streams. Agricultural and urban land cover contributed 0.0–78.3% and 0.0–8.3%, respectively, to total catchment land cover, and 0.0–61.2% and 0.0–7.8%, respectively, to land cover in a riparian corridor of 20 m width of the investigated streams. Median rates of GPP and CR in the studied streams were 0.4 g O2 m−2 d−1 (0.0–1.6, min–max) and 3.3 g O2 m−2 d−1 (1.8–15), respectively. According to the double exponential model, median leaf litter decay coefficients of the recalcitrant and the labile litter fraction were 0.006 d−1 (−0.017 to 0.016) and 0.15 d−1 (0.06–0.86), respectively. While land use in the riparian corridor was correlated to the studied ecosystem processes, whole catchment land use did not exhibit such relations. Nitrogen enrichment and increased CR, both related to urban land use, were the main impacts, indicating that already low levels of urban land cover can have considerable effects on stream integrity in the investigated rural tropical landscape. Although spanning a much wider gradient, agricultural land cover was a less important stressor, and stimulated GPP was related to stream morphology, but not to nutrient enrichment. Leaf decomposition was negatively correlated with both the agricultural and the urban impacts, suggesting that high rates of leaf decomposition are an indicator of pristine conditions in the studied streams. In conclusion, measurements of LD, CR and GPP appear to be suitable, and complementary functional indicators of stream integrity and provided insights into mechanisms of land use impacts on streams in tropical rural catchments.

We isolated phytoplankton assemblages from five tropical lakes differing in environmental conditions and incubated them under different extreme growth conditions. Conditions were: under light with nutrient addition (L þ N), under light with nutrient and organic carbon addition (L þ N þ OC), and in the dark with nutrient and organic carbon addition (N þ OC). These were intended to simulate phytoplankton succession during early, intermediate and late eutrophication stages, respectively. Decreases in species diversity and richness in N þ OC assays from all lakes, except the most eutrophic one, suggested consistent diversity and richness declines during late eutrophication stages. Declines in species diversity were generally caused by a stronger reduction in species richness than evenness, indicating that species exclusion is a more probable response to eutrophication than co-existence and dominance of strong competitors. Under N þ OC conditions, the osmotrophic chlorophyte Chlamydomonas was generally dominant, indicating that this flagellate may prevail in late eutrophic communities in the tropical lakes studied. The L þ N and L þ N þ OC assays did not differ from each other in species diversity, but exhibited differences in species composition and richness. This suggests that the changing role of the phytoplankton community in whole-lake matter fluxes during early eutrophication stages could be a more important research question, than predicting the community’s diversity response in tropical lakes.

This study examined patterns of stream sediment granulometry, organic matter (OM) and metal concentration, and surface water characteristics in a catchment in the Brazilian Iron Quadrangle that is highly impacted by surface iron mining and gully erosion. Sediment granulometry indicated fine sediment deposition at impacted stream sites, i.e., tendencies towards bimodal particle size distributions with an additional peak in the sand fraction at impacted stream sites that did not occur at pristine reference sites, as well as towards smaller mean sediment particle sizes at impacted sites than at reference sites. Impacted sites also had significantly lower sediment OM contents than reference sites. Sediment heavy metal and arsenic concentrations did not differ between impacted and reference sites and were generally below published target or threshold effect concentrations. Impacts on surface water characteristics occurred only locally at a site that received tailings pond effluent from an iron mine and had very low pH and conductivity values. Sediment characteristics exhibited substantial spatial variability in the studied tropical catchment, showing that land use impacts can hardly be detected in routine monitoring and impact assessment studies that adopt a before–after control-impact approach and do not consider pristine reference streams. These results underline the importance of high-resolution and long-term sediment monitoring as well as integrated basin-scale sediment management programs.

We investigated how the lipid composition (fatty acids and sterols) of benthic microbial mats, which represent an important basal food resource for stream food webs, differs between tropical streams located in protected pristine and agricultural Cerrado savannah areas. The total microbial biomass and lipid composition differed significantly between pristine and agricultural streams in parallel with differences in water quality and hydrodynamic characteristics. Agricultural streams exhibited lower total biomass of benthic microbial mats than pristine streams. However, the higher concentrations of essential polyunsaturated fatty acids, such as linoleic acid (LIN, 18:2ω6), α-linolenic acid (ALA, 18:3ω3), and eicosapentaenoic acid (EPA, 20:5ω3), that were observed in agricultural streams suggest enhanced lipid complexity and a higher nutritional quality of the microbial community relative to pristine streams. Meanwhile, pristine stream microbial communities had higher total concentrations of saturated fatty acids and cholesterol than those of agricultural streams, reflecting their heterotrophic microbial communities. Moreover, stream morphotype and associated differences in the hydrodynamic characteristics affected the community composition and thereby also the lipid composition of microbial mats. Land-use-induced changes in the total biomass and lipid composition of microbial communities may affect the trophic transfer of energy in stream food webs, leading to changes in the composition and productivity of primary consumers and their predators, and thereby affecting stream ecosystem functioning.

Pollution abatement through phosphorus and nitrogen retention is a key ecosystem service provided by streams. Human activities have been changing in-stream nutrient concentrations, thereby altering lotic ecosystem functioning, especially in developing countries. We estimated nutrient uptake metrics (ambient uptake length, areal uptake rate, and uptake velocity) for nitrate (NO3&ndash, N), ammonium (NH4&ndash, N), and soluble reactive phosphorus (SRP) in four tropical Cerrado headwater streams during 2017, through whole-stream nutrient addition experiments. According to multiple regression models, ambient SRP concentration was an important explanatory variable of nutrient uptake. Further, best models included ambient NO3&ndash, N and water velocity (for NO3&ndash, N uptake metrics), dissolved oxygen (DO) and canopy cover (for NH4&ndash, N), and DO, discharge, water velocity, and temperature (for SRP). The best kinetic models describing nutrient uptake were efficiency-loss (R2 from 0.47&ndash, 0.88) and first-order models (R2 from 0.60&ndash, 0.85). NO3&ndash, N, NH4&ndash, N, and SRP uptake in these streams seemed coupled as a result of complex interactions of biotic P limitation, abiotic P cycling processes, and the preferential uptake of NH4&ndash, N among N-forms. Global change effects on these tropical streams, such as temperature increase and nutrient enrichment due to urban and agricultural expansion, may have adverse and partially unpredictable impacts on whole-stream nutrient processing.

Correction to: Nature Geoscience https://doi.org/10.1038/s41561-018-0134-4, published online 21 May 2018. In the version of this Article originally published, the affiliation for M. I. Arce was incorrect; it should have been: 5Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB), Berlin, Germany. This has now been corrected in the online versions of the Article.

Summary 1. The tropical Cerrado savannah, the second-largest South American biome, is undergoing the most rapid land-use change in South America, including a wholesale shift from native vegetation to pasture and cropland. However, the consequences of this development for aquatic ecosystem functioning remain unclear. In this study, we investigated how agriculture affects chemical, physical and biological characteristics of headwater streams in the Brazilian Cerrado, and how these changes relate to an important ecosystem function, i.e. ecosystem metabolism. 2. Three paired samplings of pristine and agricultural streams were conducted considering the most abundant morphotypes of Cerrado streams. Nutrient concentrations, stream morphometry and hydrodynamics, sediment transport, as well as benthic microbial biomass (BMB), and its chlorophyll a (Chl-a) content were measured. In parallel, whole-stream gross primary production (GPP) and community respiration (CR) were estimated using a diel open-channel dissolved O2 change technique. 3. Agricultural streams had consistently higher nutrient concentrations, less variability in channel morphology, smaller channel cross-sectional areas, smaller transient storage zones, higher current velocities and higher boundary shear stresses than pristine streams. At base flow, agricultural streams exhibited a midstream band of shifting sediments, while pristine streams had stable sediments. Both agricultural and pristine streams were dominated by thick microbial mats. Due to differences in hydrodynamics and sediment stability, these microbial mats covered the entire stream bed in pristine streams, but were restricted to the stream margins in agricultural streams. 4. As a result, BMB of agricultural streams was diminished by a factor of 7.5 compared to pristine streams. Interestingly, CR of agricultural streams was diminished only by a factor of 2.0, because higher BMB-specific respiration efficiencies, potentially due to increased nutrient availability, compensated for lower BMB due to physical stress. Similarly, the 3.3-fold elevated GPP in agricultural compared to pristine streams was only to a minor degree due to increased benthic Chl-a, but mainly a result of 2.4-fold higher Chl-a-specific primary production efficiencies, potentially due to increased nutrient availability. 5. In conclusion, agriculture impacted the studied Cerrado streams through two antagonistic mechanisms: physical stress resulted in decreased BMB and whole-stream CR by preventing BMB accrual in the central streambed, but higher nutrient availability led to increased primary production and respiration efficiencies in marginal zones, as well as higher whole-stream GPP. Finally, measures of whole-stream metabolism were useful indicators of stream ecosystem health and allowed for the differential assessment of the effects of physical stress and eutrophication.

We analysed diel cycles of total protein, lipid and carbohydrate content of seston in four seasonal sampling campaigns in a tropical hypereutrophic reservoir. Physical and water chemical variables and taxonomic composition of phytoplankton were measured in parallel. Seston lipid and carbohydrate contents exhibited highest values during the day, while protein content was highest at night. Carbohydrate content was negatively correlated with nitrate and nitrite concentrations. Lipid content was negatively correlated with temperature and positively with soluble reactive phosphorus (SRP) concentrations. Protein content was positively correlated with temperature and negatively with SRP concentrations. In all sampling campaigns, the biochemical composition was correlated to the biovolume of phytoplankton. However, relationships differed between seasons. In the dry season, the relative biovolume of non-dominant groups such as cyanobacteria and bacillariophyceae was strongly correlated with all biochemical compounds of seston. During the rainy season, significant correlations were detected between biochemical compounds of seston and biovolumes of the dominant groups, chlorophyceae and euglenophyceae. We conclude that diel fluctuations in environmental conditions as well as changes in the community composition of phytoplankton affected seston biochemical composition during the diel cycles investigated. However, the type of interaction was clearly influenced by seasonality in this tropical reservoir.

Eutrophication results in a deficiency of n-3 LC-PUFA (long-chain polyunsaturated fatty acids) in aquatic food chains, affecting fish nutrition and physiology. The trophic transfer of FA (fatty acids) to fish species of different feeding habits was investigated in two reservoirs in southeast Brazil-the mesotrophic Ponte Nova Reservoir (PN) and the hypereutrophic Billings Reservoir (Bil). Total FA profile of stomach contents and adipose tissue, triacylglycerols (TAG), and phospholipids (PL) from liver and muscle of the omnivorous Astyanax fasciatus and the carnivorous Hoplias malabaricus were analyzed by gas chromatography. A prevalence of n-6PUFA, as 18:2n-6 (linoleic acid) and 20:4n-6 (arachidonic acid, ARA) was observed in the stomach contents and in the tissues of A. fasciatus from the PN reservoir. In contrast, n-3 LC-PUFA, as 20:5n-3 (eicosapentaenoic acid, EPA) was accumulated in fish tissues from Bil, resulting in higher n3/n6 and EPA/ARA ratios, compared to fish from PN. This differential FA accumulation was also observed for H. malabaricus, but differences were slightly minor, and no changes were observed in the EPA/ARA ratios between fish from both reservoirs. Regardless reservoir, FA profiles of TAG resembled that of their diet, whereas FA profiles of PL were more conservative and mainly comprised by LC-PUFA. We conclude that reservoir trophic status affected the FA composition of food resources available to these fish species, resulting in differential allocation of n-3 and n-6 FA. As expected, FA profile of the investigated fish species also reflected their feeding habit and physiological demands.

We report the influence of different nutritional modes—autotrophy, mixotrophy, and heterotrophy—on the fatty acid and sterol composition of the freshwater flagellate Ochromonas sp. and discuss the ecological significance of our results with respect to the resource competition theory (rct). Polyunsaturated fatty acids (PUFAs) are the most efficient biochemical variable distinguishing between nutritional modes of Ochromonas sp. Decreasing concentrations of PUFAs were observed in the order autotrophs, mixotrophs, heterotrophs. In mixotrophs and heterotrophs, concentrations of saturated fatty acids were higher than those of monounsaturated fatty acids and PUFAs as a result of bacterivory. Stigmasterol was the main sterol in Ochromonas sp., regardless of nutritional mode. Mixotrophs showed higher growth rates than heterotrophs, which could not be explained by rct. Heterotrophs, in turn, exhibited higher growth rates than autotrophs, which were cultured under the same light conditions as mixotrophs. Mixotrophs can synthesize PUFAs, which are important for many physiological functions such as membrane permeability and growth. Thus, mixotrophy facilitated efficient growth as well as the ability to synthesize complex and essential biomolecules. These strong synergetic effects are due to the combination of biochemical benefits of heterotrophic and autotrophic metabolic pathways and cannot be predicted by rct.

Many tropical biomes are threatened by rapid land-use change, but its catchment-wide biogeochemical effects are poorly understood. The few previous studies on DOM in tropical catchments suggest that deforestation and subsequent land use increase stream water dissolved organic carbon (DOC) concentrations, but consistent effects on DOM elemental stoichiometry have not yet been reported. Here, we studied stream water DOC concentrations, catchment DOC exports, and DOM elemental stoichiometry in 20 tropical catchments at the Cerrado–Atlantic rainforest transition, dominated by natural vegetation, pasture, intensive agriculture, and urban land cover. Streams draining pasture could be distinguished from those draining natural catchments by their lower DOC concentrations, with lower DOM C:N and C:P ratios. Catchments with intensive agriculture had higher DOC exports and lower DOM C:P ratios than natural catchments. Finally, with the highest DOC concentrations and exports, as well as the highest DOM C:P and N:P ratios, but the lowest C:N ratios among all land-use types, urbanized catchments had the strongest effects on catchment DOM. Thus, urbanization may have alleviated N limitation of heterotrophic DOM decomposition, but increased P limitation. Land use—especially urbanization—also affected the seasonality of catchment biogeochemistry. While natural catchments exhibited high DOC exports and concentrations, with high DOM C:P ratios in the rainy season only, urbanized catchments had high values in these variables throughout the year. Our results suggest that urbanization and pastoral land use exerted the strongest impacts on DOM biogeochemistry in the investigated tropical catchments and should thus be important targets for management and mitigation efforts.

The biochemical factors that determine the food quality of protists for rotifers are poorly understood. We evaluated population growth rates and egg production of the rotifer Keratella quadrata fed with four protist species growing on either an algal or a bacterial diet. The cryptomonad Cryptomonas phaseolus, considered as a good quality prey, and assays without prey served as controls. Population growth rates and egg numbers of K. quadrata were correlated with single biochemical compounds (fatty acids, amino acids and sterols) of the protists. Feeding on the alga C. phaseolus or the algivorous ciliates resulted in enhanced population growth rates and high egg production by K. quadrata, whereas feeding on bacterivores supported only moderate egg production but no population growth. The rotifers' egg production was correlated with the protist biochemical composition, including polyunsaturated fatty acids (PUFAs), the sterol desmosterol, ergosterol, stigmastanol and the amino acid leucine. No significant relationships were observed between population growth rates of the rotifers and the protists' biochemistry, suggesting that population growth and reproduction of K. quadrata may have different nutritional requirements. To our knowledge, this is one of the first studies to analyze a large variety of biochemical compounds to determine the food quality of protists for a zooplankton predator and the first study to analyze the biochemical quality of protists to a rotifer species.

Ammonium retention was studied in 12 tropical headwater streams differing in channel morphology and hydraulic characteristics. Results from NaCl and NH4Cl injections combining a one-dimensional transport model with the nutrient spiraling concept were evaluated. Transient solute storage was large in swamp reaches, with ratios of storage zone and main channel cross-sectional area (AS/A) of up to 18, and fractions of residence time due to transient storage (F 200med) of up to 94%. Moreover, transient storage was at an intermediate level in step-pool and meandering reaches and low in run reaches. Maximum ammonium uptake potentials (UMAX) varied by a factor of 15 due to differing stream morphologies, and ammonium retention was high in swamp and step-pool reaches, with UMAX values as high as 1.8 mg N·m−2·min−1. Between 52% and 85% of ecosystem retention occurred in storage zones. Additionally, differences in ammonium uptake efficiency in the storage zones of contrasting stream morphotypes were negligible; hence...

The aim of this study was to assess land use effects on the density, biomass, and instantaneous secondary production (IP) of benthic invertebrates in a fifth-order tropical river. Invertebrates were sampled at 11 stations along the Rio das Mortes (upper Rio Grande, Southeast Brazil) in the dry and the rainy season 2010/2011. Invertebrates were counted, determined, and measured to estimate their density, biomass, and IP. Water chemical characteristics, sediment heterogeneity, and habitat structural integrity were assessed in parallel. Total invertebrate density, biomass, and IP were higher in the dry season than those in the rainy season, but did not differ significantly among sampling stations along the river. However, taxon-specific density, biomass, and IP differed similarly among sampling stations along the river and between seasons, suggesting that these metrics had the same bioindication potential. Variability in density, biomass, and IP was mainly explained by seasonality and the percentage of sandy sediment in the riverbed, and not directly by urban or agricultural land use. Our results suggest that the consistently high degradation status of the river, observed from its headwaters to mouth, weakened the response of the invertebrate community to specific land use impacts, so that only local habitat characteristics and seasonality exerted effects.

Sestonic biochemical composition (lipids, proteins and carbohydrates) may change with varying environmental fluctuations. These changes and the effects in the nutritional status of algae consists of an actual increasing source of interest. The aims of this work were to establish the relationship between biochemical composition of seston and 1) a range of physical, chemical and climatological factors, and 2) the natural fluctuation in the species composition in phytoplankton assemblages of the Pampulha Reservoir, an urban eutrophic lake located in Belo Horizonte, MG. Seasonal changes in the biochemical composition were observed in this study. None of the considered abiotic factors alone seem to affect the biochemical composition. So the effects could only be understood by interactions among different environmental factors. On the other hand, the dominant algal groups probably have some influence in the observed changes in biochemical composition of seston.

Land-use change, such as agricultural expansion and urbanization, can affect riverine biological diversity and ecosystem functioning. Identifying the major stressors associated with catchment land-use change is a prerequisite for devising successful river conservation and restoration strategies. Here, we analyzed land-use effects on the fatty acid (FA) composition and concentrations in suspended particulate organic matter (SPOM) along a fourth-order tropical river, the Rio das Mortes. Thereby, we aimed at testing the potential of fatty acids in riverine suspended particulate organic matter (SPOM-FAs) as indicators of land-use change in tropical catchments, and at identifying major human impacts on the biochemical composition of SPOM, which represents an important basal energy and organic matter resource for aquatic consumers. River water SPOM and total FA concentrations ranged between 2.8 and 10.2 mg dry weight (DW) L− 1 and between 130.6 and 268.2 μg DW L− 1, respectively, in our study. Urbanization was the only land-use category correlating with both FA composition and concentrations, despite its low contribution to whole catchment (1.5–5.6%) and riparian buffer land cover (1.7–6.6%). Higher concentrations of saturated FAs, especially C16:0 and C18:0, which are the main components of domestic sewage, were observed at sampling stations downstream of urban centers, and were highly correlated to urbanization, especially within the 60 m riparian buffer zone. Compared to water chemical characteristics (inorganic nutrients, dissolved oxygen, pH, and specific conductance) and river habitat structural integrity, FA variables exhibited a higher variability along the investigated river and were more strongly correlated to urban land use, suggesting that SPOM-FA profiles may be an efficient indicator of urban land-use impacts on larger tropical rivers. High total FA concentrations in the SPOM of urbanized tropical rivers may represent high-energy biochemical subsidies to food webs, potentially leading to changes in functional ecosystem characteristics, such as bacterial and suspension-feeder production.

Land-use change, i.e., the transformation of natural land into agricultural and urban area is among the most important human impacts on habitat quality and species diversity. In this study, we investigated the effects of land use on river habitat quality and sediment granulometry in a larger tropical river, affected by urbanization and agricultural land use. We selected 15 representative sampling reaches in the Rio das Mortes basin, 12 of them along the main river from its headwater to its mouth, and 3 in major tributaries. A habitat survey was conducted in these reaches in the dry season 2010 and sediment samples were taken for granulometry analyses. Sub-basin land cover of reaches was dominated by natural vegetation (41.6 to 60.2% of total land cover), followed by agricultural land cover (38.4% to 56.9%) and urban land cover (1.4% to 5.6%). Sediments were dominated by poorly to moderately sorted silts to sands, little conducive to diverse biological communities. According to the river habitat survey, all investigated river reaches exhibited moderately to totally disturbed habitat integrity, due to diverse and often co-occurring human impacts, such as riparian deforestation, water abstraction, sand and gravel extraction, and margin erosion. Only one of the investigated sampling reaches exhibited the minimum riparian forest corridor width demanded by the Brazilian Forest Code. Our results indicated that river habitat and sediment quality mainly depended on conditions in the direct vicinity of river reaches. Accordingly, initial cost-effective restoration of aquatic habitats could be achieved by relatively simple channel restoration measures and the protection of the riparian corridor in the investigated tropical river. Mudanças no uso e ocupação do solo representam um importante impacto antrópico sobre a qualidade de habitats e a diversidade biológica. Os efeitos do uso do solo sobre a qualidade de habitats e a granulometria do sedimento em um rio tropical impactado por urbanização e agricultura fora m investigados. Foram selecionados 15 trechos de amostragem representativos da bacia do Rio das Mortes, sendo 12 localizados no leito principal, desde a cabeceira até a foz, e os restantes em três dos principais tributários. Um levantamento de habitat foi realizado nestes trechos durante a estação seca de 2010 e amostras de sedimento foram coletadas para análise granulométrica. A cobertura da bacia nos trechos estudados foi dominada por vegetação natural (41.6 a 60.2%), seguida por agricultura (38.4 a 56.9%) e urbanização (1.4 a 5.6%). Os sedimentos foram dominados por silte e areia, pouco favoráveis à diversidade biológica. De acordo com o levantamento de habitat, todos os trechos apresentaram integridade moderada ou totalmente perturbada, devido a diversos impactos frequentes, tais como remoção de mata ciliar, captação de água, extração de areia e cascalho e erosão. Apenas um dos trechos estudados apresentou largura de corredor ripário em concordância com o exigido pelo Código Florestal Brasileiro. Os resultados indicam que a integridade de habitat e a qualidade do sedimento riverinos dependem principalmente das condições presentes na vizinhança imediata dos trechos estudados. Assim, a restauração custo-efetiva dos habitats aquáticos poderia ser alcançada por medidas simples de restauração do canal e a proteção da vegetação ripária neste rio tropical.

We investigated the impacts of effluent discharge from small flow-through fish farms on stream water characteristics, the benthic invertebrate community, whole-system nitrate uptake, and ecosystem metabolism of three tropical headwater streams in southeastern Brazil. Effluents were moderately, i.e. up to 20-fold enriched in particulate organic matter (POM) and inorganic nutrients in comparison to stream water at reference sites. Due to high dilution with stream water, effluent discharge resulted in up to 2.0-fold increases in stream water POM and up to 1.8-fold increases in inorganic nutrients only. Moderate impacts on the benthic invertebrate community were detected at one stream only. There was no consistent pattern of effluent impact on whole-stream nitrate uptake. Ecosystem metabolism, however, was clearly affected by effluent discharge. Stream reaches impacted by effluents exhibited significantly increased community respiration and primary productivity, stressing the importance of ecologically sound best management practices for small fish farms in the tropics.

Algivorous ciliates represent an important link in freshwater food webs, as they transfer energy and biochemical matter from their algal prey to mesozooplankton predators. However, it is still unknown how dietary composition influences the biochemical composition of ciliates. We analyzed the sterol composition of the algivorous ciliates Balanion planctonicum and Urotricha farcta and compared it to the sterol composition of their diet--the cryptomonad Cryptomonas phaseolus. The sterol composition of the ciliates did not resemble that of their algal diet. Ergosterol [(22E)-ergosta-5,7,22-trien-3beta-ol] was the sterol in highest concentration in C. phaseolus, whereas stigmasterol [(24S)-24-ethylcholesta-5,22E-dien-3beta-ol] was dominant in both B. planctonicum and U. farcta. Moreover, the occasionally higher sterol concentrations in the ciliates than in their algal diet suggest sterol accumulation by the ciliates. We conclude that dietary sterol composition influences the composition of the two algivorous ciliates studied, but species-specific differences in metabolism probably determine the ultimate sterol composition of the ciliates.

The focus of our study was to determine whether the biochemical composition of two algivorous ciliates, both fed the same alga, resembles that of their diet. By comparing both ciliated protozoa we intended to identify species-specific differences in the metabolic features of these ciliates. Carbon- and cell-specific concentrations of fatty acids and essential amino acids were investigated for the ciliates Balanion planctonicum and Urotricha farcta grown on the cryptomonad Cryptomonas phaseolus. Stepwise discriminant analyses (SDA) indicated differences in the biochemical composition between ciliates and their diet and between the two ciliated protozoa. Carbon-specific fatty acid concentrations were usually higher in the ciliates than in their diet, especially concentrations of monounsaturated and some polyunsaturated fatty acids. Except for tryptophan, valine, and lysine, amino acid concentrations were higher in the ciliates than in C. phaseolus. Furthermore, differences in the polyunsaturated fatty acids accounted for the largest discrepancies between the two ciliated protozoa. The higher concentrations in the ciliates compared to their diet suggest that these species are capable of efficiently ingesting, assimilating or possibly synthesizing some fatty acids and amino acids. We conclude that dietary fatty acid and amino acid composition influences the composition of the two ciliated protozoa to a minor extent, and that species-specific differences in fatty acid and amino acid metabolism may be more important determinants of the biochemical composition of the studied ciliates. Moreover, the metabolism of polyunsaturated fatty acids seems to differ more profoundly between the two ciliated protozoa than the metabolism of other fatty acid classes or amino acids.

Agricultural land covers approximately 40% of Earth’s land surface and affects hydromorphological, biogeochemical and ecological characteristics of fluvial networks. In the northern temperate region, agriculture also strongly affects the amount and molecular composition of dissolved organic matter (DOM), which constitutes the main vector of carbon transport from soils to fluvial networks and to the sea and is involved in a large variety of biogeochemical processes. Here, we provide first evidence about the wider occurrence of agricultural impacts on the concentration and composition of fluvial DOM across climate zones of the northern and southern hemispheres. Both extensive and intensive farming altered fluvial DOM towards a more microbial and less plant-derived composition. Moreover, intensive farming significantly increased dissolved organic nitrogen (DON) concentrations. The DOM composition change and DON concentration increase differed among climate zones and could be related to the intensity of current and historical nitrogen fertilizer use. As a result of agriculture intensification, increased DON concentrations and a more microbial-like DOM composition likely will enhance the reactivity of catchment DOM emissions, thereby fuelling the biogeochemical processing in fluvial networks and resulting in higher ecosystem productivity and CO2 outgassing.