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Understanding the interactions among anthropogenic stressors is critical for effective conservation and management of ecosystems. Freshwater scientists have invested considerable resources in conducting factorial experiments to disentangle stressor interactions by testing their individual and combined effects. However, the diversity of stressors and systems studied has hindered previous syntheses of this body of research. To overcome this challenge, we used a novel machine learning framework to identify relevant studies from over 235,000 publications. Our synthesis resulted in a new dataset of 2396 multiple‐stressor experiments in freshwater systems. By summarizing the methods used in these studies, quantifying trends in the popularity of the investigated stressors, and performing co‐occurrence analysis, we produce the most comprehensive overview of this diverse field of research to date. We provide both a taxonomy grouping the 909 investigated stressors into 31 classes and an open‐source and interactive version of the dataset (https://jamesaorr.shinyapps.io/freshwater‐multiple‐stressors/). Inspired by our results, we provide a framework to help clarify whether statistical interactions detected by factorial experiments align with stressor interactions of interest, and we outline general guidelines for the design of multiple‐stressor experiments relevant to any system. We conclude by highlighting the research directions required to better understand freshwater ecosystems facing multiple stressors. [ABSTRACT FROM AUTHOR]

Cyanobacterial blooms in lakes of low nutrient status are recent ecological surprises. Culling of planktivorous fish may help suppress phytoplankton blooms via a trophic cascade effect. To test this hypothesis, we conducted a 90-day experiment adjacent to a shallow oligomesotrophic lake increasingly beset by midsummer cyanobacterial blooms in the presence of high abundances of minnows and sparse herbivorous zooplankton. The single-factor ([+ or -] three spottail shiners, Notropis hudsonius) experimental design was replicated 10 times for a total of twenty 1200 L capacity mesocosms. Contrary to the trophic cascade hypothesis, minnow removal decreased the abundance of bosminids capable of grazing cyanobacteria. Nevertheless, removal of the minnows significantly both suppressed phytoplankton biomass and offset the development of cyanobacteria, such as Gloeotrichia echinulata. Lower concentrations of phosphorus and nitrogen in the fishless relative to stocked mesocosms best explained these differences in the phytoplankton community. Our findings highlight how fisheries management practices that enhance minnow populations in lakes of low productivity may inadvertently contribute to cyanobacterial blooms through increased nutrient cycling. Les proliferations de cyanobacteries dans des lacs pauvres en nutriments sont des evenements ecologiques recents surprenants. L'elimination de poissons planctonivores pourrait aider a reduire les proliferations de phytoplancton par un effet de cascade trophique. Pour verifier cette hypothese, nous avons mene une experience de 90 jours attenante a un lac oligomesotrophe peu profond faisant de plus en plus l'objet de proliferations estivales de cyanobacteries en presence d'une grande abondance de menes et de zooplancton herbivore peu abondant. Le plan experimental monofactoriel ([+ or -] trois queues a tache noire, Notropis hudsonius) a ete reproduit 10 fois pour un total de vingt mesocosmes d'une capacite de 1200 L. Contrairement aux predictions decoulant de l'hypothese de la cascade trophique, le retrait de menes a entraine une baisse de l'abondance de bosminides capables de se nourrir de cyanobacteries. Le retrait des menes a entraine toutefois une baisse significative de la biomasse de phytoplancton et limite le developpement de cyanobacteries comme Gloeotrichia echinulata. De plus faibles concentrations de phosphore et d'azote dans les mesocosmes exempts de poissons comparativement aux mesocosmes empoissonnes expliquent le mieux ces differences dans la communaute de phytoplancton. Nos constatations soulignent le fait que des pratiques de gestion des peches qui accroissent les populations de menes dans les lacs de faible productivite, en intensifiant le cycle des elements nutritifs, pourraient participer a des proliferations de cyanobacteries. [Traduit par la Redaction], Introduction Cyanobacterial blooms in freshwater ecosystems have long been associated with eutrophic conditions (Smith and Schindler 2009), but cyanobacteria are frequently also found in less productive lakes (Carey et al. [...]

Catastrophic state changes often occur in ecosystems as consequences of shifts in the dominant life forms, such as in the case of sportfish introduction into naturally fishless mountain lakes. While the pronounced ecological displacement of native communities by these novel predators (i.e., weak resistance) is well documented, few studies have also examined the dynamics of their recovery rates (i.e., "engineering resilience") following stressor removal. We addressed this knowledge gap by quantifying the taxonomic and species trait‐based ordination trajectories of crustacean zooplankton communities in stocked alpine lakes, which differed in how they were restored to a natural fishless state over a period of six decades. Resilience and resistance trajectories differed in each lake as the ecological importance of trait‐based species recolonization potential (e.g., asexual vs. sexual reproduction) replaced that of tolerance of size‐selective predation by fish (e.g., body size, pigmentation, motility). As a result, re‐establishment of large‐bodied zooplankton following the elimination of fish restored annual secondary production. In contrast, a more gradual unidirectional ordination trajectory toward smaller zooplankton body size and a decline in total zooplankton biomass occurred in a nearby alpine lake containing a protected native bull trout population. Although recovery trajectories returned via routes that differed from those of earlier resistance trajectories (i.e., hysteresis), we discovered that full taxonomic and functional recovery was realized in all the restored fishless lakes within three decades, regardless of the approach used to achieve ecosystem restoration. [ABSTRACT FROM AUTHOR]

Deposition of nutrients over mountainous regions in many parts of the world is increasing due to human activities. To assess the level of agreement between chemically-inferred and experimental evidence of the potential sensitivity of alpine lake ecosystems along the Eastern Front range of the Canadian Rockies to increased deposition of nitrogen (N) and phosphorus (P), nutrient limitation was investigated in 29 sites using water chemistry and nutrient amendment bioassays of phytoplankton. The two lines of evidence agreed in 55% of all cases with discrepancies mainly involving instances of co-limitation. Dissolved inorganic N:total P (DIN:TP) mass ratios frequently suggested P limitation while the corresponding bioassays supported meta-analytical evidence of the prevalence of co-limitation of freshwater primary producers. Based on previous evidence of low DIN:TP mass ratios and experimental N amendments stimulating algal communities in shallower alpine sites, we hypothesized that nutrient limitation of phytoplankton shifts from being P- to N-driven with decreasing lake depth. However, we found little evidence of N limitation from bioassays of phytoplankton from the shallower (< 2-m deep) lakes. Most phytoplankton were co-limited by P plus N amendments (42%), while 38% responded to only P, 10% to only N, and 10% to neither nutrient. Overall, DIN:TP mass ratios and bioassay results showed that chromophytes and chlorophytes drove the greater responsiveness of whole communities to P relative to N. Since the short-term bioassay experiments often demonstrated the alpine phytoplankton to be NP-co-limited, our findings highlight a need for further research into potential synergistic fertilizing effects of anthropogenic N emissions and P deposition from forest fires on lake ecosystems at high elevations. [ABSTRACT FROM AUTHOR]