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Identifying hydropower dam operations that lessen detrimental effects on downstream fauna could inform conservation strategies for native fishes. We compared occurrence of native fishes in 20 shoal habitats downstream from two differently operated hydropower dams in the Coosa River system, Georgia, USA. Species richness averaged 7 and 11, respectively, in surveys downstream from (1) a hydropeaking dam and (2) a dam with a re-regulation structure that stabilized downstream flows. In contrast, surveys in two nearby reference communities averaged 19 and 24 species. Species persisting downstream from the dams tended toward water-column orientation, larger body size, longer life span, and greater prevalence in tributary stream collections, compared with missing or rarely captured species. We observed no evidence of recovery toward reference conditions when operations were paused for 28 months at the hydropeaking dam. Our observations suggest that (1) strongly contrasting dam operations can result in similar alterations to native fish assemblages, potentially reflecting effects of thermal alteration by hypolimnetic water release, and (2) periodic dispersal from tributary streams may enhance fish persistence in flow-altered rivers. Key words: freshwater fishes, hydrologic alteration, reference communities, species traits, dispersal, Introduction Ecologists have documented widespread alterations of river communities imposed by hydropower dams. Large dams fragment river systems, imperiling populations of migratory fishes blocked from spawning, nursery, and foraging habitats [...]

Increased temperatures are altering rates of organic matter (OM) breakdown in stream ecosystems with implications for carbon (C) cycling in the face of global change. The metabolic theory of ecology (MTE) provides a framework for predicting temperature effects on OM breakdown, but differences in the temperature dependence of breakdown driven by different organismal groups (i.e., microorganisms vs. invertebrate detritivores) and litter species remain unresolved. Over two years, we conducted 12 60‐day leaf litterbag incubations in 20 headwater streams in the southern Appalachian Mountains (USA). We compared temperature dependence (as activation energy, Ea) between microbial and detritivore‐mediated breakdown, and between a highly recalcitrant (Rhododendron maximum) and a relatively labile (Acer rubrum) leaf species. Detritivore‐mediated breakdown had a higher Ea than microbial breakdown for both leaf species (Rhododendron: 1.48 > 0.56 eV; Acer: 0.97 > 0.29 eV), and Rhododendron breakdown had a higher Ea than Acer breakdown for both organismal groups. Similarly, the Ea of total (coarse‐mesh) Rhododendron breakdown was higher than the Ea of total Acer breakdown (0.89 > 0.52 eV). These effects for total breakdown were large, implying that the number of days to 95% mass loss would decline by 40% for Rhododendron and 26% for Acer between 12°C (our mean temperature value) and 16°C (+4°C, reflecting projected increases in global surface temperature due to climate change). Despite patterns in Ea, overall breakdown rates were higher for microbes than detritivores, and for Acer than Rhododendron over most of our temperature gradient. Additionally, the Ea for a subset of the microbial breakdown data declined from 0.40 to 0.22 eV when fungal biomass was included as a model predictor, highlighting the key role of fungi in determining the temperature dependence of litter breakdown. Our results imply that, as streams warm, routing of leaf litter C to detritivore‐mediated fates will increase faster than predicted by previous studies and MTE, especially for labile litter. As temperatures rise, earlier depletion of autumn‐shed, labile leaf litter combined with rapid breakdown rates of recalcitrant litter could exacerbate seasonal resource limitation and alter carbon storage and transport dynamics in temperate headwater stream networks. [ABSTRACT FROM AUTHOR]

Leaf litter dominates particulate organic carbon inputs to forest streams. Using data‐informed simulations, we explored how litter type (slow‐ vs. fast‐decomposing species), pulsed autumn litter inputs, groundwater‐mediated temperature regimes, and climate warming affect litter breakdown in a 3rd‐order stream network. We found that the time‐dependent interactions of these variables govern network‐scale litter breakdown phenology, with greater thermal sensitivity of slow‐decomposing litter for both current and future scenarios. Groundwater thermal inputs modified litter breakdown phenology by reducing spring and summer and elevating winter litter breakdown fluxes. Under future warming scenarios, the source depth of contributing groundwater influenced summer detrital resources; shallow groundwater‐fed streams had reduced summer resources compared to deep groundwater‐fed streams. Our results demonstrate that predicting in‐stream carbon cycling requires explicit consideration of the phenology of resource inputs and the seasonal timing of environmental factors, notably stream thermal regimes. [ABSTRACT FROM AUTHOR]

Anthropogenic impacts on the landscape can drive biotic homogenization whereby distinct biological communities become more similar to one another over time. Land-use change in the Southern Appalachian region of the United States is expected to result in homogenization of the highly diverse freshwater fish communities as in-stream habitat alterations favor widespread cosmopolitan species at the expense of more narrowly distributed highland endemic species. We compiled four datasets spanning 25 years to (1) evaluate the effects of environmental factors on relative abundance and richness of highland endemic vs. cosmopolitan species in this region and (2) test for taxonomic homogenization, measured as a change in beta diversity over time. We found that several environmental factors differentially affected highland endemic and cosmopolitan species, with the proportion of forested land cover in a watershed most strongly predicting higher relative abundance and richness of highland endemic species. Our analysis of beta diversity change, however, shows mixed evidence of taxonomic homogenization, depending on how common species are weighted. Shifts in community composition, with or without homogenization, may warrant attention in biodiversity conservation planning. Les impacts de l'activite humaine sur le paysage peuvent entrainer une homogeneisation biotique qui fait en sorte que des communautes biologiques distinctes deviennent plus semblables les unes aux autres au fil du temps. Il est prevu que les changements d'utilisations du sol dans la region du sud des Appalaches des Etats-Unis se traduiront par l'homogeneisation de communautes de poissons d'eau douce de grande diversite etant donne que les modifications des habitats de cours d'eau favorisent les especes cosmopolites repandues au detriment d'especes endemiques de hautes terres de repartition plus restreinte. Nous avons compile quatre ensembles de donnees couvrant 25 ans afin (1) d'evaluer les effets de facteurs environnementaux sur l'abondance et la richesse relatives des especes endemiques de hautes terres et des especes cosmopolites dans cette region et (2) verifier s'il y a homogeneisation taxonomique en mesurant d'eventuelles variations de la diversite beta au fil du temps. Nous constatons que plusieurs facteurs environnementaux ont des effets differentiels sur les especes endemiques de hautes terres et les especes cosmopolites, la proportion de couverture forestiere dans le bassin versant etant le predicteur le plus fort de plus grandes abondances et richesses relatives d'especes endemiques de hautes terres. Notre analyse des variations de la diversite beta fait toutefois ressortir des preuves mitigees d'homogeneisation taxonomique, dependamment de la methode de ponderation des especes repandues. Il pourrait etre utile de tenir compte des changements de composition des communautes, avec ou sans homogeneisation, dans la planification de la conservation de la biodiversite. [Traduit par la Redaction], Introduction A key consequence of the global decline in biodiversity is the increasing homogenization of biotic communities, the result of a loss of endemic species and the expansion of non-native, [...]

The conversion of forestlands to alternative land uses is a growing worldwide concern, given the wide range of provisioning and regulating ecosystem services (ES) provided by forests. We applied a scenario-based land-use/land-cover (LULC) projection technique integrating societal preferences, conservation policies, and socio-economic factors to the Upper Flint River Watershed in the Atlanta, Georgia (USA) metropolitan area. We employed the InVEST modeling toolset to assess the impact of anticipated LULC changes on ES under each development scenario. Our simulations projected a consistent conversion from Deciduous/Mixed Forests to either Urban or Evergreen forests across all scenarios, leading to a significant decline in ES. We quantified the economic impacts of this ES loss, conservatively estimated as representing millions of dollars per year under a Business as Usual scenario in just carbon and water services alone. Integrating social and policy drivers into our projection approach yielded policy-relevant results and identified the need for conservation policy instruments to protect forested ecosystems with higher conservation values. Existing conservation policies are unlikely to stem the loss of important ES, and there may be a need to consider more aggressive policies to prevent further degradation of watersheds, such as the one analyzed here. [ABSTRACT FROM AUTHOR]

Understanding species' associations with physical habitat conditions is a fundamental goal of ecology. For organisms that occupy lotic ecosystems, relationships to streamflow are of particular importance, but these associations are unstudied for most species. We tested the predictability of fish–microhabitat relationships in river shoals (shallow, rocky areas with relatively swift water flow) using a large data set from the Conasauga River in Georgia, USA. Our objective was to assess the consistency of species‐specific relationships with flow‐dependent variables (depth, velocity, Reynolds number and Froude number) while accounting for other microhabitat variables (e.g. vegetation). We used data from 8285 seine‐sets collected during late summer or autumn at 26 sites over 12 years to relate occurrence and counts of 22 fish species to habitat variables using generalised linear multiple regression models. Results showed that microhabitat models explained a substantial amount of the variation in counts for some species, although other species were poorly predicted. We classified 16 species as velocity specialists and nine species as depth specialists, with six species specialised for depth and velocity and three species classified as depth and velocity generalists. The variability in habitat associations that we observed suggests that species will be unevenly affected by anthropogenic activities that alter flows. [ABSTRACT FROM AUTHOR]