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Despite decades of research and management efforts, eutrophication remains a persistent threat to inland waters. As nutrient pollution intensifies in the coming decades, the implications for aquatic greenhouse gas (GHG) emissions are poorly defined, particularly the responses of individual GHGs: carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O). The biogeochemical controls of each gas can differ, making it difficult to predict the overall effect of nutrient pollution on the net radiative forcing of aquatic ecosystems. Here, we induced eutrophication of small nitrogen (N)-limited agricultural reservoirs and measured changes in diffusive GHG emissions within a before-after-control-impact (BACI) study design during June to September 2021. Each gas exhibited a unique response to 300% increases in primary production, with a shift from an overall CO 2 source to a sink, a modest increase in N 2 O flux, and, unexpectedly, no significant change in CH 4 emissions. The lack of net directional change in CO 2 -equivalent GHG emissions in fertilized reservoirs during the summer contrasts findings from empirical studies of eutrophic lakes. Our findings illustrate the difficulty in extrapolating among different sized ecosystems and suggest that forecast 2-fold increases in agricultural N fertilization by 2050 may not result in consistently elevated GHG emissions during summer, at least from small reservoirs in continental grassland regions.

Hatchery-reared salmon smolt used for supplementary stocking often display poor migration behavior compared to wild smolt, which reduces the success of this management action. Oxazepam, an anxiolytic drug, has been shown to intensify salmon smolt migration in mesocosm experiments, and treatment with this drug has, therefore, been suggested as a management option to improve downstream smolt migration. In this study, we tested this by assessing migration performance of hatchery-reared Atlantic salmon (Salmo salar) smolt along a 21-km long natural river-to-sea migration route in a boreal river in Northern Sweden. Using acoustic telemetry, the migration rate and survival of smolt that had been exposed to oxazepam (200 μg L−1, N = 20) was monitored and compared with a control group (N = 20) of unexposed smolt. Exposed smolt took significantly longer time to initiate migration after release compared to the control fish, but after that we observed no significant difference in downstream migration speed. However, exposed smolt had considerably higher probability of being predated on compared to control smolt. We attribute these results to increased risk-taking and higher activity in oxazepam-exposed smolt, which in turn increased initial non-directional exploratory behavior and decreased predator vigilance. These results are discussed based on current concerns for ecological implications of behavioral modifications induced by pharmaceutical pollution and climate change. We conclude that exposure to oxazepam is an unsuitable management option to prime migration of reared salmon in natural systems. Unlabelled Image • Bold an less anxious smolt becomes more easy prey. • Oxazepam exposed initiated migration at slow rate. • Downstream migration success is negatively affected by oxazepam exposure. [ABSTRACT FROM AUTHOR]

Adaptive management is a powerful means of learning about complex ecosystems, but is rarely used for recovering endangered species. Here, we demonstrate how it can benefit woodland caribou, which became the first large mammal extirpated from the contiguous United States in recent history. The continental scale of forest alteration and extended time needed for forest recovery means that relying only on habitat protection and restoration will likely fail. Therefore, population management is also needed as an emergency measure to avoid further extirpation. Reductions of predators and overabundant prey, translocations, and creating safe havens have been applied in a design covering >90,000 km². Combinations of treatments that increased multiple vital rates produced the highest population growth. Moreover, the degree of ecosystem alteration did not influence this pattern. By coordinating recovery involving scientists, governments, and First Nations, treatments were applied across vast scales to benefit this iconic species. [ABSTRACT FROM AUTHOR]

Rapid changes in state have been documented for many of Earth's ecosystems. Despite a growing toolbox of methods for detecting declining resilience or early warning indicators (EWIs) of ecosystem transitions, these methods have rarely been evaluated in whole-ecosystem trials using reference ecosystems. In this study, we experimentally tested EWIs of cyanobacteria blooms based on changes in the spatial structure of a lake. We induced a cyanobacteria bloom by adding nutrients to an experimental lake and mapped fine-resolution spatial patterning of cyanobacteria using a mobile sensor platform. Prior to the bloom, we detected theoretically predicted spatial EWIs based on variance and spatial autocorrelation, as well as a new index based on the extreme values. Changes in EWIs were not discernible in an unenriched reference lake. Despite the fluid environment of a lake where spatial heterogeneity driven by biological processes may be overwhelmed by physical mixing, spatial EWIs detected an approaching bloom suggesting the utility of spatial metrics for signaling ecological thresholds. [ABSTRACT FROM AUTHOR]

Ecosystems sometimes shift between different states or dynamic regimes. Theory attributes these shifts to multiple ecosystem attractors. However, documenting multiple ecosystem attractors is difficult, particularly at spatial and temporal scales relevant to ecosystem management. We manipulated the fish community of a lake with the goal of causing trophic cascades and shifting the food web from a planktivore-dominated state to an alternate piscivore-dominated state. We evaluated evidence that the shifts in the fish community comprise alternate attractors using two complementary approaches. First, we calculated phase space trajectories to visualize the shift between attractors. Second, we computed generalized autoregressive conditional heteroskedasticity (GARCH) models and the Brock–Dechert–Scheinkman (BDS) test for linearity. The reconstructed phase space trajectories show the system departing a point attractor, entering a limit cycle, and then shifting to a new point attractor. The GARCH and BDS results indicate that linear explanations are not sufficient to explain the observed patterns. The results provide evidence for alternate attractors based on high-frequency time series of field measurements. [ABSTRACT FROM AUTHOR]

Regime shifts in stochastic ecosystem models are often preceded by early warning signals such as increased variance and increased autocorrelation in time series. There is considerable theoretical support for early warning signals, but there is a critical lack of field observations to test the efficacy of early warning signals at spatial and temporal scales relevant for ecosystem management. Conditional heteroskedasticity is persistent periods of high and low variance that may be a powerful leading indicator of regime shift. We evaluated conditional heteroskedasticity as an early warning indicator by applying moving window conditional heteroskedasticity tests to time series of chlorophyll- a and fish catches derived from a whole-lake experiment designed to create a regime shift. There was significant conditional heteroskedasticity at least a year prior to the regime shift in the manipulated lake but there was no significant conditional heteroskedasticity in an adjacent reference lake. Conditional heteroskedasticity was an effective leading indicator of regime shift for the ecosystem manipulation. [ABSTRACT FROM AUTHOR]

The succession of vegetation, soil development, water quality changes, and carbon and nitrogen dynamics are summarized in this article for a pair of 1-hectare flow-through-created riverine wetlands for their first 15 years. Wetland plant richness increased from 13 originally planted species to 116 species overall after 15 years, with most of the increase occurring in the first 5 years. The planted wetland had a higher plant community diversity index for 15 years, whereas the implanted wetland was more productive. Wetland soils turned hydric within a few years; soil organic carbon doubled in 10 years and almost tripled in 15 years. Nutrient removal was similar in the two wetlands in most years, with a trend of decreased removal over 15 years for phosphorus. Devitrification accounted for a small percentage of the nitrogen reduction in the wetlands. The wetlands were effective carbon sinks with retention rates of 1800-2700 kilograms of carbon per hectare per year, higher than in comparable reference wetlands and more commonly studied boreal peatlands. Methane emission rates are low enough to create little concern that the wetlands are net sources of climate change radiative forcing. Planting appears to have influenced carbon accumulation, methane emissions, and macrophyte community diversity. [ABSTRACT FROM AUTHOR]

Lakeshore residential development (LRD) reduces coarse woody habitat (CWH) in lakes resulting in negative effects on fishes. We tested whether the addition of CWH could reverse those effects. We added CWH to Camp Lake, a lake with CWH abundances similar to developed lakes, following 2 years of study of the fish populations in the reference and treatment basins. Both basins were monitored for 4 years following the manipulation. Specifically, we tested for changes in the population dynamics (densities, size-structure, growth), diet, and behavior (habitat use) of bluegill ( Lepomis macrochirus) and largemouth bass ( Micropterus salmoides). CWH addition had no discernible effect on fish population dynamics. Diet and behavioral responses were more pronounced in the treatment basin. Prey diversity and availability increased. Piscivory increased, with decreased reliance upon terrestrial prey, for largemouth bass. Habitat use was positively correlated with CWH branching complexity and abundance. Our study suggests that negative effects observed in fish populations through CWH reductions cannot be reversed in the short-term by adding CWH. We recommend that regulations governing the LRD process be protective of CWH. [ABSTRACT FROM AUTHOR]

Recent research suggests that secondary production in aquatic systems can be driven by inputs of energy from terrestrial sources. Temporary forest ponds appear to be unproductive ecosystems that are reliant upon allochthonous inputs of energy to support secondary production, but the functioning of these systems has not been well quantified. To assess the metabolic state of this type of ecosystem as well as to quantify the importance of terrestrial subsidies of carbon to ecosystem function, we conducted an experiment in which we manipulated the amount of leaf litter in ponds. Litter was either removed or removed and replaced (that is, control) from the dry basins of ponds immediately after leaf abscission. Once the ponds filled, we monitored net ecosystem production (NEP) on a biweekly basis from 9 April to 27 May 2002. All ponds were consistently net heterotrophic; however, NEP was significantly less negative in removal ponds. Furthermore, removal ponds also had lower levels of respiration (R) and higher dissolved oxygen levels than control ponds. The removal of litter had no effect on gross primary production, indicating that the difference in NEP between treatments was driven by the change in R. Therefore, it appears that terrestrial inputs of organic carbon support heterotrophic respiration in these ponds, and that the endogenous production of carbon is insufficient to support secondary production. [ABSTRACT FROM AUTHOR]

The impact of climate warming on the littoral zone of a boreal lake ecosystem was studied experimentally for three growing seasons in two artificial ponds (10×27 m) and in replicated chamber experiments. One pond was enclosed in a plastic greenhouse and another untreated pond served as a reference system. During the growing seasons temperature in the greenhouse was maintained at levels 2–3 °C higher than ambient with a computer-controlled ventilation system. One growing season prior to initiation of the experiment, a vegetated littoral zone with equal densities of water horsetail ( Equisetum fluviatile) was established in both ponds. Although changes occurred in the species dominance ( E. fluviatile - Alisma plantago-aquatica - Sparganium erectum spp. microcarpum - Elodea canadensis) within the three years of the study, the emergent macrophytes emerged earlier and grew better in the warmer conditions of the greenhouse pond compared with those in the reference pond. The difference in above-ground biomass throughout the growing seasons was >2 fold and after three experimental growing seasons the difference in below-ground biomass of macrophytes was 2.5-fold between the ponds. In replicated chamber experiments the biomass growth of E. fluviatile was also significantly higher in a 2–3 °C higher temperature than under ambient conditions. An ecosystem-scale induced change, characterized by a heavy growth of filamentous algae (mainly chlorophytes) was evident in the vegetated littoral zone of the greenhouse pond. A hypothesis that macrophyte rhizomes function as `phosphorus pumps' from the sediment and thus accelerate eutrophication in a warmer climate should be further studied. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) is an abundant form of organic matter in stream ecosystems. Most research has focused on the watershed as the source of DOC in streams, but DOC also comes from leaching of organic matter stored in the stream channel. We used a whole-ecosystem experimental approach to assess the significance of leaching of organic matter in the channel as a source of DOC in a headwater stream. Inputs of leaf litter were excluded from a forested Appalachian headwater stream for 3 years. Stream-water concentration, export, and instream generation of DOC were reduced in the litter-excluded stream as compared with a nearby untreated reference stream. The proportion of high molecular weight (HMW) DOC (more than 10,000 daltons) in stream water was not altered by litter exclusion. Mean DOC concentration in stream water was directly related to benthic leaf-litter standing stock. Instream generation of DOC from leaf litter stored in the stream channel contributes approximately 30% of daily DOC exports in this forested headwater stream. This source of DOC is greatest during autumn and winter and least during spring and summer. It is higher during increasing discharge than during base flow. We conclude that elimination of litter inputs from a forested headwater stream has altered the biogeochemistry of DOC in this ecosystem. [ABSTRACT FROM AUTHOR]

The direct effects of large-scale disturbances are readily studied because their effects are often apparent and result in large changes to ecosystems. Direct effects can cascade through the ecosystem, leading to indirect effects that are often subtle and difficult to detect. Managing anthropogenic disturbances, such as chemical contamination, requires an understanding of both direct and indirect effects to predict, measure, and characterize the impact. Using a replicated whole-ecosystem experiment and path analyses (assesses the effects of a set of variables on a specified outcome, similar to multiple regression), we examined the direct and indirect effects of a glyphosate-based herbicide and nutrient enrichment on wetland communities. The latter did not impact any measured endpoints. The strongest drivers of macrophyte, benthic invertebrate, and amphibian assemblages were the ephemerality and the size of wetlands, factors which were not altered by herbicide applications. The herbicide had a direct negative effect on macrophyte cover, amphibian larval abundance, and the proportion of predatory benthic invertebrates. However, both amphibians and invertebrates were positively affected by the reduction in the macrophyte cover caused by the herbicide applications. The opposing directions of the direct and indirect effects lead to no net change in either group. The compensatory dynamics observed herein highlight the need for a better understanding of indirect effects pathways to determine whether common anthropogenic disturbances alter the ecological communities in small wetland ecosystems. Unlabelled Image • Glyphosate application has a direct negative effect on wetland macrophyte cover. • The indirect effect of macrophyte reduction on the amphibian and benthic invertebrate communities opposed the direct effect. • Compensatory dynamics make it difficult to determine the overall effects of chemical contamination on wetland communities. • Natural variation in abiotic environmental factors has a greater effect on wetland communities than herbicide application. [ABSTRACT FROM AUTHOR]