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  1. Beisner, Beatrix E (Ed.)
    Abstract Consumer nutrient recycling influences aquatic ecosystem functioning by altering the movement and transformation of nutrients. In hypereutrophic reservoirs, zooplankton nutrient recycling has been considered negligible due to high concentrations of available nutrients. A comparative analysis ( Moody and Wilkinson, 2019) found that zooplankton communities in hypereutrophic lakes are dominated by nitrogen (N)-rich species, which the authors hypothesized would increase phosphorus (P) availability through excretion. However, zooplankton nutrient recycling likely varies over the course of a growing season due to changes in biomass, community composition and grazing pressure on phytoplankton. We quantified zooplankton, phytoplankton and nutrient concentration dynamics during the summer of 2019 in a temperate, hypereutrophic reservoir. We found that the estimated contribution of zooplankton excretion to the dissolved nutrient pool on a given day was equivalent to a substantial proportion (21–39%) of the dissolved inorganic P standing stock in early summer when P concentrations were low and limiting phytoplankton growth. Further, we found evidence that zooplankton affected phytoplankton size distributions through selective grazing of smaller phytoplankton cells likely affecting nutrient uptake and storage by phytoplankton. Overall, our results demonstrate zooplankton excretion in hypereutrophic reservoirs likely helped drive springtime phytoplankton dynamics through nutrient recycling while grazing influenced phytoplankton size distributions. 
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  2. Abstract Rivers that do not flow year-round are the predominant type of running waters on Earth. Despite a burgeoning literature on natural flow intermittence (NFI), knowledge about the hydrological causes and ecological effects of human-induced, anthropogenic flow intermittence (AFI) remains limited. NFI and AFI could generate contrasting hydrological and biological responses in rivers because of distinct underlying causes of drying and evolutionary adaptations of their biota. We first review the causes of AFI and show how different anthropogenic drivers alter the timing, frequency and duration of drying, compared with NFI. Second, we evaluate the possible differences in biodiversity responses, ecological functions, and ecosystem services between NFI and AFI. Last, we outline knowledge gaps and management needs related to AFI. Because of the distinct hydrologic characteristics and ecological impacts of AFI, ignoring the distinction between NFI and AFI could undermine management of intermittent rivers and ephemeral streams and exacerbate risks to the ecosystems and societies downstream. 
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  3. null (Ed.)
  4. Abstract

    Microevolution can have consequences at higher levels of ecological organization. Although divergence among populations can be rapid and driven by anthropogenic changes to the environment, the ecological relevance of evolution induced by human activities remains poorly understood.

    A frequent way in which human activities drive microevolution is the increase in supply of nutrients such as phosphorus (P) that are required for fitness‐relevant traits such as growth and reproduction. Because higher P concentrations decrease P‐use efficiency and feeding rate in heterotrophic consumers such asDaphnia, we hypothesized that such adjustments should alter consumer–resource dynamics.

    We examined how cultural eutrophication in temperate lakes causes trait variation in the grazerDaphnia pulicaria. We tested for variation inDaphniatraits and genetic variation in the metabolic enzyme phosphoglucose isomerase (Pgi) which are each known to respond to eutrophication. We then examined the impact of this variation on consumer–resource dynamics using a combination of experiments and a multi‐lake survey.

    We found thatDaphniafrom hypereutrophic lakes responded to experimental hypereutrophic conditions with increased growth rates and fecundity when raised on P‐fertilized seston, but had reduced performance on P‐poor seston relative to eutrophic sourceDaphnia. These results suggest thatDaphniamay face a trade‐off in performance at low versus excess P that may be mediated in part by genetic variation at thePgilocus.

    The variation observed in laboratory growth experiments scaled up toDaphniapopulations in both mesocosm experiments and among lakes. In both the mesocosm experiment and in the lake survey,Daphniafrom hypereutrophic source lakes reached high biomass while phytoplankton biomass also remained high.

    Given the prevalence and rapid eutrophication of freshwater ecosystems worldwide, these results indicate that considering the potential effects of evolutionary change in ecosystem models could be useful in forecasting the effects of anthropogenic environmental change on pivotal ecosystem services.

     
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  5. Abstract

    Herbivorous ectothermic vertebrates are more diverse and abundant at lower latitudes. While thermal constraints may drive this pattern, its underlying cause remains unclear. We hypothesized that this constraint stems from an inability to meet the elevated phosphorus demands of bony vertebrates feeding on P‐poor plant material at cooler temperatures because low gross growth efficiency at warmer temperatures facilitates higher P ingestion rates. We predicted that dietary carbon:phosphorus (C:P) should exceed the threshold elemental ratio between carbon and P‐limited growth (TERC:P) for herbivores feeding at cooler temperatures, thereby limiting the range of herbivorous ectothermic vertebrates facing P‐limited growth.

    We tested this hypothesis using the Andean suckermouth catfishesAstroblepusandChaetostoma.Astroblepusare invertivores that inhabit relatively cool, high‐elevation streams whileChaetostomaare grazers that inhabit relatively warm, low‐elevation streams. We calculated TERC:Pfor each genus across its elevational range and compared these values to measured values of food quality over an elevational gradient in the Andes. We also broadly summarized measurements of TERC:Pacross diverse groups of fishes.

    Supporting our hypothesis, we found that dietary C:P was predicted to exceed the TERC:Pfor the grazerChaetostomanear the highest elevation where this genus has been recorded. Conversely, the TERC:Pfor the invertivoreAstroblepuswas consistently higher than that ofChaetostomaand thus its dietary C:P never approached the TERC:P. We found that, among all fishes, omnivores had higher average TERC:Pthan invertivores, and TERC:Pdid not vary with temperature.

    Our results suggest that, at least for Andean suckermouth catfishes, cool temperatures constrain herbivory at higher elevations. Increased gross growth efficiency at cooler temperatures evidently restricts the ability of P‐limited consumers to meet P demand. However, our survey of fish TERC:Pestimates suggests that some fishes are able to circumvent this constraint through behavioural and life‐history adaptations that reduce P demand or increase P use efficiency.

    The physiological trade‐offs underlying these functional shifts reveal that geographic dietary patterns can be predicted by stoichiometric theory, but variation in food quality and consumer traits that reduce P demand and/or increase P efficiency can create exceptions to these patterns.

    Aplain language summaryis available for this article.

     
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