Chironomids are important laboratory model organisms used to assess toxicity in freshwater environments. Cell and tissue features are not commonly used as chironomid markers to detect toxicity, but they could be extremely helpful in identifying acute and chronic effects of pollutants. The nervous system is an excellent cellular candidate since it is reactive to toxic substances. However, a detailed description of the chironomid nervous system is required prior to considering it as a candidate for a cellular toxicity marker. The present study describes the central ganglia, nerves, axons, and the neuromuscular system of Chironomus vitellinus (Freeman, 1961) to facilitate its use as a model organism in environmental studies. We find that the structure of the C. vitellinus central nervous system is identical to that observed in other Chironomus larvae. We then focused our study on the first abdominal segment and labeled the 31 hemi-segmental muscles according to a nomenclature based on their position and orientation. We also characterized their innervation and assigned the nerves a nomenclature based on their terminals' location in the muscle tissue. Finally, we investigated the neuromuscular junctions (NMJs) throughout this segment and defined four types of NMJs illustrating their great variability in size and shape. We selected a model NMJ, VEL 2, and quantified its mean bouton number and muscle size. Together with documenting a neurobiological system that could be informative to insects' comparative biology, these results could help establish the Chironomus NMJ as an aquatic toxicity marker.

Hurricanes are major natural disturbances that significantly influence tropical ecosystems. While most research focuses on large-bodied organisms, understanding the impact of hurricanes on small-bodied biota, such as meiofauna, is crucial, especially as climate change models predict more frequent and intense storms. Puerto Rico offers a unique setting to study these effects, as hurricanes and tropical storms are frequent. This research examined the short- (Post-Hurricane A) and medium-term (Post-Hurricane B) impacts of Hurricanes Irma and María (September 2017) on stream meiofaunal communities in a tropical stream in the Luquillo Experimental Forest. Twelve samples were collected monthly from pools across two stream reaches for eight months before and after the hurricanes. Environmental variables, such as discharge, sediment composition, and biotic data, were recorded. Meiofauna were identified to the lowest possible taxonomic level and categorized by phyla and feeding groups. Results showed a significant increase in both richness and abundance of meiofauna following the hurricanes. Richness and abundance peaked during Post-Hurricane A and declined slightly during Post-Hurricane B. This trend was linked to an increase in coarse particulate organic matter, potentially the result of defoliation and debris from hurricane damage of riparian forest. Community structure analyses revealed significant differences between pre- and post-hurricane periods. Variables such as stream discharge, macroinvertebrate presence, sediment size, and shrimp abundance were all influenced by hurricane impacts and correlated with changes in meiofaunal communities. During Post-Hurricane A, models explaining meiofaunal variation involved variables associated with ecosystem disturbance. In contrast, Post-Hurricane B models were simpler, suggesting a level of ecological stabilization. These findings suggest that hurricanes influence meiofaunal communities, but that these organisms are likely benefiting from hurricane disturbance. Given expected increases in hurricane activity due to climate change, hurricanes may play a long-term role in shaping the structure and function of tropical stream communities, in particular for small-body size organisms.

Leaf litter in headwater streams serves as an important energy source for the aquatic food web. However, the timing, magnitude, and source dynamics of leaf litter inputs into tropical montane streams have received less attention. Here we present an 8-year study of leaf litter inputs from two tropical montane first-order headwater streams in the Luquillo Experimental Forest, Puerto Rico that include a prolonged drought period and major hurricanes. Leaf litter was collected across different landscape sources such as canopy and riparian inputs, and litter transported from upstream. The different sources of leaf litter were compared to environmental variables (precipitation, wind, solar radiation, and temperature) to identify potential drivers of inputs to streams and exports. Leaf litter from upstream was the greatest contributor (50-95%) and can vary with storms and droughts. Transitions between dry and wet periods led to pulses of leaf litter in both streams. Increasing precipitation, solar radiation, and wind led to increased leaf litter inputs but varied across sources. The variable responses across sources to climatic events suggest that the timing and magnitude of litter inputs to headwater streams may be specific by source. Moreover, the results underscore the value in considering the forest cover of the watershed landscape in its entirety when quantifying organic matter inputs as they depend on upstream conditions that go beyond the immediate reach-adjacent riparian zone.

[This corrects the article DOI: 10.1371/journal.pone.0247382.].

Understanding how natural ecosystems respond to disturbance events is a major goal in ecology, especially as extreme climatic events become more frequent with climate change. We evaluated the effects of hurricanes Irma and María on multiple aspects of stream ecosystem stability (i.e. resistance, engineering resilience, recovery and temporal stability) over 5 years post-disturbance. We fitted eight models to analyse recovery trajectories and used a Bayesian structural equation model to examine interactions among variables during recovery. Most variables exhibited low resistance, with recovery remaining incomplete and high variability during the return to pre-hurricane conditions. Canopy openness increased sevenfold, and although the initial disturbance caused a fivefold rise in leaf litter fall, subsequent sampling showed a significant drop in organic matter input, while epilithic algae slightly decreased. Decapod and macroinvertebrate responses varied across streams. Recovery trajectories differed by variable, including exponential (epilithic algae), logarithmic (leaf litter), inverted logistic (decapods), linear (canopy openness) and humped curves (macroinvertebrate density). Analyses of ecological interactions revealed differing associations and dynamic feedback among variables. This study underscores the profound, lasting effects of hurricanes on tropical streams and emphasizes the need for long-term monitoring to understand ecosystem responses to increasingly intense and frequent disturbances.

Forested headwater streams disproportionately rely on inputs of organic matter to fuel their food webs, and characterizing the breakdown of organic matter offers insights into ecosystem function. Organic matter breakdown rates can be influenced by the availability of limiting nutrients, and describing patterns of breakdown rates across nutrient gradients is increasingly relevant as inland waters undergo eutrophication. Here, we determined the breakdown rates of coarse woody debris (kwood) across 5 streams located at La Selva Biological Station, Costa Rica, that receive a gradient of interbasin modified groundwater inputs, creating a gradient in P concentration (6-134 mu g/L soluble reactive P [SRP]). The fastest breakdown rate (kwood = 0.77/y) occurred in the stream with the highest SRP, and kwood was positively correlated with SRP across the 5 streams. Further, we characterized the assemblage of macroinvertebrates from wood packs. Macroinvertebrate assemblages were different between the 5 streams, with more dense and diverse assemblages in streams with higher SRP and faster breakdown rates. Our results contribute to a growing field of study on the effect of nutrients on organic matter dynamics in inland waters by characterizing the effect of SRP on breakdown rates of wood in tropical streams.

La teoría de representaciones lineales consiste en estudiar grupos abstractos a través de sus representaciones como grupos de matrices. En este artículo, presentamos una aplicación de esta teoría en química, específicamente en la aproximación de las frecuencias vibracionales de una molécula usando las representaciones del grupo de simetrías de dicha molécula. Para explicar el procedimiento, primero presentamos la definición del grupo de simetrías de una molécula, describimos cómo obtener la aproximación de las frecuencias vibracionales mediante un sistema lineal de ecuaciones diferenciales, y revisamos los conceptos básicos de la teoría de representaciones lineales. Esperamos que este texto sea útil tanto para químicos interesados en los fundamentos matemáticos del campo de la simetría molecular, como para matemáticos interesados conocer aplicaciones las matemáticas en otras ciencias.

AimInsects are one of the least studied taxa, with most species lacking basic ecological and biogeographical information. This problem is particularly acute in the tropics, where low sampling effort hampers accurate estimates of species richness at scale and potentially confounds efforts to identify the drivers of biogeographical gradients. Here, we evaluate the quality of the data on the distribution and diversity of odonate species in the Neotropics, while also examining the influence of sampling completeness on climate-richness relationships using a comprehensive database of odonates.LocationThe Neotropics.TaxonOdonata.MethodsUsing 56,535 records collected from 1970 to 2021, we assess whether climate-species richness models vary under different scenarios of survey completeness.ResultsOur survey compilation revealed that most Neotropical diversity of Odonata likely remains unknown. Only 1% of the one-degree cells covering the Neotropics held reliable information on odonate species richness, with particularly severe gaps in the Caribbean, Central America, northeastern Brazil and northern Chile. Temperature, precipitation and potential evapotranspiration exert consistent effects on Odonata richness across the entire Neotropics, regardless the level of survey completeness. Whereas seasonality-related variables are less important predictors of species richness at the biogeographical scale.Main ConclusionsBy highlighting areas where inventories are more reliable and identifying regions that require increased data collection efforts and mobilization, our assessment offers a roadmap for improving the reliability of odonate inventories in the Neotropics. Furthermore, our findings underscore the importance of accounting for varying levels of survey completeness in macroecological models to reveal robust climate-species richness relationships. Simultaneously, they highlight strong climatic predictors of species richness, irrespective of survey effort intensity. These predictors provide a solid foundation for modelling and predicting odonate species richness in the Neotropics.

Globally, freshwater fishes are among the taxa most vulnerable to climate change but are generally understudied in tropical island ecosystems where climate change is predicted to alter the intensity, frequency and duration of extreme flow events. These changes may impact stream ecosystems and native and non-native biota in complex ways. We compiled an extensive dataset of fish assemblages collected at 119 sites across the Caribbean island of Puerto Rico from 2005 to 2015. We coupled these data with stream flow indices and dam height to understand associations between flow and fish assemblage structure. Sixteen percent of sites contained exclusively non-native species, 34% contained exclusively native species, and 50% contained native and non-native species. We built generalised linear models and conducted all subsets model selection to identify extreme flow variables explaining variation in native and non-native species richness and biomass. We also built models to determine the combined effects of extreme flows and the presence of non-native species on native species richness and biomass. Extreme flows and dam height were important in explaining variations in native and non-native species richness and biomass. Model averages showed native biomass decreased by 0.42 kg/ha with a 1-m increase in dam height, by 0.05 kg/ha with 1 cm/s increase in maximum mean daily high flow and by 3.45 kg/ha with each additional day increase in maximum high flow duration, and increased by 2.06 kg/ha with each additional day increase in mean high flow duration. Model averages predicted that non-native biomass increased by 1.32 kg/ha with a 1-m increase in dam height and by 0.01 kg/ha with each additional day increase in mean high flow duration, and decreased by 0.36 kg/ha with each additional day increase in maximum high flow duration. Model averages also predicted an increase in native and non-native biomass of 0.71 gage and 0.06 kg/ha, respectively, with each additional day increase in maximum low flow duration. The combined effects of non-native species presence and extreme flows changed the relationship between maximum high and low flow durations and native biomass. Model averages showed that native biomass increased by 1.83 kg/ha with each additional day increase in maximum high flow duration and decreased by 2.52 kg/ha with each additional day increase in maximum low flow duration when non-native species were present. Native fishes may be able to better cope with longer maximum durations of low flows than expected when non-native fishes are absent. In mixed fish assemblages, extended maximum durations of high flows may act as a control of non-native species and dampen their negative effect on native species, but longer maximum durations of low flows may heighten the negative effects of non-native fishes. Our results are informative for tropical island ecosystems globally and can guide the management and conservation of native fishes, particularly when faced with the dual threats of climate change and non-native species. Managers may consider increasing efforts to conserve native fishes in Caribbean rivers by maintaining connectivity and habitat complexity while preventing non-native species introductions.

Hurricanes are major disturbances with important consequences to stream ecosystems as they create major floods and remove riparian vegetation. Understanding their impacts is a priority, as hurricane intensity is expected to increase due to global climate change. Mayfly assemblages in streams fill a diversity of ecological roles and functions. They are important consumers of algae by scraping benthic biofilms and detritivores associated with fine particles and leaf litter. Other taxa are filterers and even predators. Mayflies are also important prey items in aquatic and terrestrial food webs. Here, we assessed the effects of two consecutive hurricanes that impacted Puerto Rico in 2017 to understand how hurricane-induced changes in the environment alter mayfly composition, secondary production and emergence. The study was conducted in the Luquillo Experimental Forest, Puerto Rico. Mayflies were sampled as nymphs and emerging adults for 6 months before and 17 months after hurricanes Irma and Mar & iacute;a hit the island in September 2017. Leaf litter inputs, canopy cover and chlorophyll a concentrations were monitored along with mayflies. Mayfly assemblages were dominated by two genera of Leptophlebiidae before the hurricane, Neohagenulus (two species: N. julio Traver, 1938, N. luteolus Traver, 1938) and Borinquena (one species: B. carmencita Traver, 1938). Both genera decreased in density after the hurricanes and were replaced with the Baetidae Cloeodes maculipes Traver, 1938 as the dominant taxon. This pattern was observed in both nymph and emerging adult densities. The secondary production of Leptophlebiidae species was highest before hurricane disturbance, with the Baetidae C. maculipes showing the opposite pattern. Neohagenulus had an annual production of 445 mg m-2 year-1, C. maculipes of 153 mg m-2 year-1 and B. carmencita of 68 mg m-2 year-1. Overall, the mayfly assemblages in our studied stream are vulnerable to hurricane disturbances. Expected increases in hurricane impacts might result in assemblage shifts that could change assemblage composition and alter energy flows within the ecosystem. Los huracanes son disturbios mayores con consecuencias importantes sobre los ecosistemas de quebradas ya que crean crecidas y remueven la vegetaci & oacute;n ribere & ntilde;a. Entender los impactos de los mismos es una prioridad, ya que la intensidad de los huracanes se espera aumente a ra & iacute;z del cambio clim & aacute;tico global. Los Ephemeroptera llenan una diversidad de funciones en las quebradas. Son importantes consumidores de algas que raspan de la biopel & iacute;cula b & eacute;ntica y son detrit & iacute;voros que consumen part & iacute;culas finas y hojarasca. Otros taxa son filtradores y algunos hasta depredadores. Los Ephemeroptera son presas importantes en las redes alimentarias acu & aacute;ticas y terrestres. Estudiamos los efectos de dos huracanes consecutivos que impactaron Puerto Rico en 2017 con el fin de entender c & oacute;mo los huracanes inducen cambios en el ambiente y alteran la composici & oacute;n, producci & oacute;n secundaria, y emergencia de Ephemeroptera. El estudio se realiz & oacute; en el Bosque Experimental de Luquillo, Puerto Rico. Los Ephemeroptera se muestrearon como ninfas y adultos emergentes por seis meses antes y 17 meses despu & eacute;s de los huracanes. Tambi & eacute;n se monitorearon las entradas de hojarasca, cobertura de dosel, y la concentraci & oacute;n de clorofila. Los ensamblajes fueron dominados por dos g & eacute;neros de Leptophlebiidae antes de los huracanes, Neohagenulus (dos especies: N. julio Traver, 1938, N. luteolus Traver, 1938), y Borinquena (una especie: B. carmencita Traver, 1938). Ambos disminuyeron en densidad luego de los huracanes y fueron reemplazados por el Baetidae Cloeodes maculipes Traver, 1938 como tax & oacute;n dominante. Este patr & oacute;n fue observado en las ninfas y los adultos emergentes. La productividad secundaria de Leptophlebiidae fue m & aacute;s alta antes de los huracanes, y los Baetidae C. maculipes presentaron el patr & oacute;n opuesto. Neohagenulus tuvo una producci & oacute;n anual de 445 mg m-2 a & ntilde;o-1, C. maculipes de 153 mg m-2 a & ntilde;o-1, y B. carmencita de 68 mg m-2 a & ntilde;o-1. En general, los ensamblajes de Ephemeroptera en nuestra quebrada de estudio son vulnerables a los disturbios creados por huracanes. El esperado aumento en los impactos de los huracanes puede resultar en cambios en la composici & oacute;n de los ensamblajes y alterar el flujo de energ & iacute;a dentro del ecosistema.

Leaf litter decomposition is a key ecosystem process in headwater streams, influenced by physical fragmentation, microbial degradation and feeding activity by stream biota. In some tropical streams, feeding by freshwater shrimps can exert strong top-down control on leaf litter decomposition, however, variation in shrimp macroconsumer effects across small spatial scales or among years is not well-known. We ran 50-day macroconsumer exclusion experiments to measure shrimp effects on leaf decomposition in two adjacent headwater streams in Puerto Rico, in 2017 (immediately prior to two Category 4 and 5 hurricanes) and again in 2018 and 2019, to assess shrimp effects in the context of post-hurricane conditions that included reduced canopy cover and higher shrimp (Atya and Xiphocaris) counts. Leaf decomposition was faster when shrimp had access to leaf packs, but only in the study stream with larger pools, which also had higher overall shrimp counts. However, increased shrimp abundances following the hurricanes did not result in faster decomposition, potentially because shrimp diets shifted toward algae post-hurricanes when canopies were more open. We conclude that shrimp effects on leaf litter breakdown may vary between adjacent streams that differ in habitat conditions and that increasing local shrimp abundances may fail to accelerate decomposition.

High abundance of trees capable of biological N-fixation (henceforth “N-fixers”) in tropical forests has been hypothesized to drive higher stream nitrate (NO3) concentrations compared to temperate counterparts. However, to date there have been no empirical linkages of stream NO3 concentrations with the productivity of tropical forests. Here, we combined three unique long-term datasets from La Selva Biological Station, Costa Rica: 21 years of (1) mean annual stream NO3-N concentrations in six stream sites within the same watershed, (2) annual growth of trees, and (3) annual leaf litterfall. We hypothesized that years of greater growth of N-fixer tree species and of greater leaf litterfall would be correlated with higher stream water NO3-N concentrations. We also hypothesized that landscape position mediates these relationships, with growth of N-fixer trees on adjacent slopes being more strongly correlated to stream NO3-N than the growth of such trees on upland plateau sites. We found that mean annual stream NO3-N concentrations were consistently high (160–260 µg L−1). There was substantial interannual variation in leaf litterfall (inter-year range: 5.4 to 8.1 Mg ha−1 year−1), growth of N-fixers (inter-year range: 1.2 to 2.2 Mg ha−1 year−1), and growth of all other tree species (inter-year range: 2.1 to 3.2 Mg ha−1 year−1). To assess stream NO3-N relationships with forest productivity, we used water conductivity to account for dilution resulting from variable discharge. We found that NO3-N concentrations were positively related to the annual growth of the N-fixers on nearby slopes, and were negatively correlated with annual leaf litterfall. Stream NO3-N concentrations were not related to the growth of N-fixers or other tree species in the more removed plateau areas. Using a mass balance, we estimated that symbiotic N fixation can account for 7–29

Rapid urbanization will increase the number of novel stream ecosystems in the Southeastern United States. The green sunfish (Lepomis cyanellus) is a globally widespread, invasive species that is particularly well-adapted to urban stream conditions. The trophic ecology of green sunfish is understudied, especially in the novel ecosystems where they appear to thrive. We assessed predation by green sunfish in the food web of a heavily engineered and restored urban stream in Raleigh, North Carolina. We sampled fish species composition, size structure, abundance, the benthic macroinvertebrate assemblage, and fish diets during two seasons. Green sunfish was the sole species inhabiting the study stream, with the exception of a single goldfish. The population size structure indicated potential overcrowding in our study ecosystem. Chironomidae was the most common taxa in both the benthic macroinvertebrate assemblage and in fish diets during the summer. We observed a seasonal shift in diets to lesser prey-specific abundance of Chironomidae and a greater overall abundance of terrestrial prey from summer to winter. Green sunfish can persist in small restored urban streams of the Southeast US where virtually no other fish occur, and they utilize benthic invertebrates and terrestrial prey as resources.

Meiofauna is a group of heterotrophic organisms smaller than macroinvertebrates but larger than microfauna and characterized by groups such as testate amoebae, ciliates, and nematodes. They are a link between bacteria and resources and macroinvertebrates. However, tropical meiofauna is poorly studied; thus, our goal was to characterize meiofaunal community composition and abundance and assess potential environmental variables controlling these community dynamics. Monthly samplings of meiofauna were conducted for eight months in Quebrada Prieta, El Yunque National Forest, Puerto Rico. Sampling was made in 12 pools, and data on discharge, sediment characteristics, and biotic variables were also collected. A total of 62 meiofaunal morphospecies were identified, with nematodes dominating the community, followed by testate amoebae. Bacterivores and detritivores taxa dominated the community. Meiofaunal abundance was negatively related to discharge and positively to the percentage of coarse sand, nitrate, and macroinvertebrate abundance. The composition of meiofauna in Quebrada Prieta is like the composition reported for temperate streams, at least in major meiofaunal groups present. However, the community in Quebrada Prieta was dominated by testate amoebae. In contrast, temperate streams are often dominated by rotifers and nematodes. Both abiotic and biotic variables are important for meiofaunal communities in the headwater streams in Puerto Rico.

The relative importance of allochthonous and autochthonous carbon (C) as sources of energy for tropical stream food webs remains an open question. Allochthonous C might be the main energy source for small and shaded forest streams, while autochthonous C is more likely to fuel food webs draining land uses with less dense vegetation. We studied food webs in cloud forest streams draining watersheds with forests, coffee plantations, and pastures. Our goal was to assess the effects of those land uses on the C source and structure of stream food webs. The study took place in tropical montane streams in La Antigua Watershed, in eastern Mexico. We selected three streams per land use and sampled biofilm and leaf litter as the main food resources, and macroinvertebrates and aquatic vertebrates from different trophic guilds. Samples were analyzed for δ13C and δ15N isotopes. Using a Bayesian mixing model, we estimated the proportional assimilation of autochthonous and allochthonous carbon by each guild. We found that consumers were mostly using allochthonous C in all streams, regardless of watershed land use. Our findings indicate that montane cloud forest streams are dominated by allochthony even in watersheds dominated by pastures. Abundant precipitation in this life zone might facilitate the movement of allochthonous C into streams. While food webs of streams from coffee plantations and pastures also rely on allochthonous resources, other impacts do result in important changes in stream functioning.

Food webs are complex ecological networks that reveal species interactions and energy flow in ecosystems. Prevailing ecological knowledge on forested streams suggests that their food webs are based on allochthonous carbon, driven by a constant supply of organic matter from adjacent vegetation and limited primary production due to low light conditions. Extreme climatic disturbances can disrupt these natural ecosystem dynamics by altering resource availability, which leads to changes in food web structure and functioning. Here, we quantify the response of stream food webs to two major hurricanes (Irma and María, Category 5 and 4, respectively) that struck Puerto Rico in September 2017. Within two tropical forested streams (first and second order), we collected ecosystem and food web data 6 months prior to the hurricanes and 2, 9, and 18 months afterward. We assessed the structural (e.g., canopy) and hydrological (e.g., discharge) characteristics of the ecosystem and monitored changes in basal resources (i.e., algae, biofilm, and leaf litter), consumers (e.g., aquatic invertebrates, riparian consumers), and applied Layman's community-wide metrics using the isotopic composition of 13 C and 15 N. Continuous stream discharge measurements indicated that the hurricanes did not cause an extreme hydrological event. However, the sixfold increase in canopy openness and associated changes in litter input appeared to trigger an increase in primary production. These food webs were primarily based on terrestrially derived carbon before the hurricanes, but most taxa (including Atya and Xiphocaris shrimp, the consumers with highest biomass) shifted their food source to autochthonous carbon within 2 months of the hurricanes. We also found evidence that the hurricanes dramatically altered the structure of the food web, resulting in shorter (i.e., smaller food-chain length), narrower (i.e., lower diversity of carbon sources) food webs, as well as increased trophic species packing. This study demonstrates how hurricane disturbance can alter stream food webs, changing the trophic base from allochthonous to autochthonous resources via changes in the physical environment (i.e., canopy defoliation). As hurricanes become more frequent and severe due to climate change, our findings greatly contribute to our understanding of the mechanisms that maintain forested stream trophic interactions amidst global change.

Background Land use is a major factor determining stream water physicochemistry. However, most streams move from one land use type to another as they drain their watersheds. Here, we studied three land use scenarios in a tropical cloud forest zone in Mexico. We addressed three main goals, to: (1) assess how land use scenarios generate different patterns in stream physicochemical characteristics; (2) explore how seasonality (i.e., dry, dry-to-wet transition, and wet seasons) might result in changes to those patterns over the year; and (3) explore whether physicochemical patterns in different scenarios resulted in effects on biotic components (e.g., algal biomass). Methods We studied Tropical Mountain Cloud Forest streams in La Antigua watershed, Mexico. Streams drained different three scenarios, streams with (1) an upstream section draining forest followed by a pasture section (F-P), (2) an upstream section in pasture followed by a forest section (P-F), and (3) an upstream forest section followed by coffee plantation (F-C). Physicochemistry was determined at the upstream and downstream sections, and at the boundary between land uses. Measurements were seasonal, including temperature, dissolved oxygen, conductivity, and pH. Water was analyzed for suspended solids, alkalinity, silica, chloride, sulfate, magnesium, sodium, and potassium. Nutrients included ammonium, nitrate, and phosphorus. We measured benthic and suspended organic matter and chlorophyll. Results Streams presented strong seasonality, with the highest discharge and suspended solids during the wet season. Scenarios and streams within each scenario had distinct physicochemical signatures. All three streams within each scenario clustered together in ordination space and remained close to each other during all seasons. There were significant scenario-season interactions on conductivity (F = 9.5, P < 0.001), discharge (F = 56.7, P < 0.001), pH (F = 4.5, P = 0.011), Cl− (F = 12.2, P < 0.001), SO42− (F = 8.8, P < 0.001) and NH4+ (F = 5.4, P = 0.005). Patterns within individual scenarios were associated with stream identity instead of land use. Both P-F and F-C scenarios had significantly different physicochemical patterns from those in F-P in all seasons (Procrustes analysis, m12 = 0.05–0.25; R = 0.86–0.97; P < 0.05). Chlorophyll was significantly different among scenarios and seasons (F = 5.36, P = 0.015, F = 3.81, P = 0.42, respectively). Concentrations were related to physicochemical variables more strongly during the transition season. Conclusion Overall, land use scenarios resulted in distinctive water physicochemical signatures highlighting the complex effects that anthropogenic activities have on tropical cloud forest streams. Studies assessing the effect of land use on tropical streams will benefit from assessing scenarios, rather than focusing on individual land use types. We also found evidence of the importance that forest fragments play in maintaining or restoring stream water physicochemistry.

Disturbances can alter the structure and function of ecosystems. In stream ecosystems, changes in discharge and physicochemistry at short, intermediate, and long recurrence intervals can affect food webs and ecosystem processes. In this paper, we compare pH regimes in streams at La Selva Biological Station, Costa Rica, where episodic acidification frequency across the stream network varies widely due to buffering from inputs of bicarbonate-rich interbasin groundwater. To examine the effects of acidification on ecosystem structure and function, we experimentally increased the buffering capacity of a headwater stream reach and compared it to an unbuffered upstream reach. We compared these reaches to a naturally buffered and unbuffered reaches of a second headwater stream. We quantified ecosystem structural (macroinvertebrate assemblages on leaf litter and coarse woody debris) and functional responses (leaf litter and coarse woody debris decomposition rates, and growth rates of a focal insect taxon [Diptera: Chironomidae]). Non-metric multidimensional scaling and analysis of similarity revealed that macroinvertebrate assemblages were relatively homogenous across the four study reaches, although the naturally buffered reach was the most dissimilar. Ecosystem function, as measured by chironomid growth rates, was greater in the naturally buffered reach, while decomposition rates did not differ across the four reaches. Our results indicate that biological assemblages are adapted to pH regimes of frequently acidified stream reaches. Our experiment informs the effects on structure and function at short time scales in streams that experience moderate acidification, but larger magnitude acidification events in response to hydroclimatic change, as projected under climate change scenarios, may induce stronger responses in streams.

The role of streams and rivers in the global carbon (C) cycle remains unconstrained, especially in headwater streams where CO2 evasion (FCO2) to the atmosphere is high. Stream C cycling is understudied in the tropics compared to temperate streams, and tropical streams may have among the highest FCO2 due to higher temperatures, continuous organic matter inputs, and high respiration rates both in-stream and in surrounding soils. In this paper, we present paired in-stream O2 and CO2 sensor data from a headwater stream in a lowland rainforest in Costa Rica to explore temporal variability in gas concentrations and ecosystem processes. Further, we estimate groundwater CO2 inputs (GWCO2) from riparian well CO2 measurements. Paired O2–CO2 data reveal stream CO2 supersaturation driven by groundwater CO2 inputs and large in-stream production of CO2. At short time scales, CO2 was diluted during storm events, but increased at longer seasonal scales. Areal fluxes in our study reach show that FCO2 is supported by greater in-stream metabolism compared to GWCO2. Our results underscore the importance of tropical headwater streams as large contributors of carbon dioxide to the atmosphere and show evaded C can be derived from both in-stream and terrestrial sources.