Non‐native Chinese mystery snail ( Bellamya chinensis ) supports consumers in urban lake food webs

Valuing the contributions of non-native species to people and nature

While decision-making can benefit from considering positive and negative outcomes of change, over the past half-century, research on non-native species has focused predominately on their negative impacts. Here we provide a framework for considering the positive consequences of non-native species relative to relational, instrumental, and intrinsic values. We demonstrate that their beneficial outcomes are common and profoundly important for human well-being. Identified benefits include social cohesion, cultural identity, mental health, food and fuel production, regulation of clean waters, and attenuation of climate change. We argue that long-standing biases against non-native species within the literature have clouded the scientific process and hampered policy advances and sound public understanding. Future research should consider both costs and benefits of non-native species.

Human health risk from consumption of aquatic species in arsenic-contaminated shallow urban lakes

Arsenic (As) causes cancer and non-cancer health effects in humans. Previous research revealed As concentrations over 200 μg g-1 in lake sediments in the south-central Puget Sound region affected by the former ASARCO copper smelter in Ruston, WA, and significant bioaccumulation of As in plankton in shallow lakes. Enhanced uptake occurs during summertime stratification and near-bottom anoxia when As is mobilized from sediments. Periodic mixing events in shallow lakes allow dissolved As to mix into oxygenated waters and littoral zones where biota reside. We quantify As concentrations and associated health risks in human-consumed tissues of sunfish [pumpkinseed (Lepomis gibbosus) and bluegill (Lepomis macrochirus)], crayfish [signal (Pacifastacus leniusculus) and red swamp (Procambarus clarkii)], and snails [Chinese mystery (Bellamya chinensis)] from lakes representing a gradient of As contamination and differing mixing regimes. In three shallow lakes with a range of arsenic in profundal sediments (20 to 206 μg As g-1), mean arsenic concentrations ranged from 2.9 to 46.4 μg g-1 in snails, 2.6 to 13.9 μg g-1 in crayfish, and 0.07 to 0.61 μg g-1 in sunfish. Comparatively, organisms in the deep, contaminated lake (208 μg g-1 in profundal sediments) averaged 11.8 μg g-1 in snails and 0.06 μg g-1 in sunfish. Using inorganic As concentrations, we calculated that consuming aquatic species from the most As-contaminated shallow lake resulted in 4-10 times greater health risks compared to the deep lake with the same arsenic concentrations in profundal sediments. We show that dynamics in shallow, polymictic lakes can result in greater As bioavailability compared to deeper, seasonally stratified lakes. Arsenic in oxygenated waters and littoral sediments was more indicative of exposure to aquatic species than profundal sediments, and therefore we recommend that sampling methods focus on these shallow zones to better indicate the potential for uptake into organisms and human health risk.

An assessment of assumptions and uncertainty in deuterium-based estimates of terrestrial subsidies to aquatic consumers

The deuterium ratio (² H/¹ H) in tissue is often used to estimate terrestrial subsidies to aquatic consumers because of strongly differentiated values between terrestrial and aquatic primary producers. However, quantitative deuterium-based analyses of terrestrial resource assimilation are highly dependent on several poorly defined assumptions. We explored the sensitivity of these estimates to assumptions regarding environmental water contributions to consumer deuterium content (ω) and algal photosynthetic hydrogen discrimination (ε_H ). We also tested whether ¹³ C/¹² C and ² H/¹ H-based estimates of terrestrial resource assimilation give similar outcomes. The average of the 12 experiments that have directly estimated proportional contributions of environmental water to consumer tissue ² H/¹ H was 0.27 ± 0.11 (mean ± SD), with similar values for invertebrates and fish. Conversely, of the 28 field studies that have used ² H/¹ H to characterize aquatic food webs, all but one assume a value that is less than our current best estimate, usually substantially less. A reanalysis of the raw data from four recent case studies indicates the calculated terrestrial contribution to aquatic consumers is extremely sensitive to this assumption. When the authors' original assumptions were used (i.e., ω = 0.16 ± 0.05), the estimated proportional contribution of terrestrial resources to aquatic consumers (θ_T ) averaged 29 ± 17%, and when ω = 0.27 was used the average estimated assimilation of allochthonous resources was ≈0.00. A compilation of published photosynthetic hydrogen discrimination values for microalgae averaged ε_H  = -150 ± 27‰ (SD, n = 99), and a sensitivity analysis showed the outcomes of these calculations were also strongly influenced by uncertainty in ε_H . There was no statistical association between ¹³ C/¹² C and ² H/¹ H-based estimates of terrestrial subsidies (r = -0.12, n = 274). This analysis indicates that the assumptions in deuterium-based estimates of terrestrial resource assimilation are highly influential but poorly constrained; therefore, the impact of these assumptions on calculated outputs must be carefully assessed and thoroughly reported. Due to the highly uncertain assumptions inherent in deuterium-based analyses, we urge much more caution when using this approach to estimate terrestrial subsidies to consumers in aquatic ecosystems.

Emerging threats and persistent conservation challenges for freshwater biodiversity

In the 12 years since Dudgeon et al. (2006) reviewed major pressures on freshwater ecosystems, the biodiversity crisis in the world's lakes, reservoirs, rivers, streams and wetlands has deepened. While lakes, reservoirs and rivers cover only 2.3% of the Earth's surface, these ecosystems host at least 9.5% of the Earth's described animal species. Furthermore, using the World Wide Fund for Nature's Living Planet Index, freshwater population declines (83% between 1970 and 2014) continue to outpace contemporaneous declines in marine or terrestrial systems. The Anthropocene has brought multiple new and varied threats that disproportionately impact freshwater systems. We document 12 emerging threats to freshwater biodiversity that are either entirely new since 2006 or have since intensified: (i) changing climates; (ii) e-commerce and invasions; (iii) infectious diseases; (iv) harmful algal blooms; (v) expanding hydropower; (vi) emerging contaminants; (vii) engineered nanomaterials; (viii) microplastic pollution; (ix) light and noise; (x) freshwater salinisation; (xi) declining calcium; and (xii) cumulative stressors. Effects are evidenced for amphibians, fishes, invertebrates, microbes, plants, turtles and waterbirds, with potential for ecosystem-level changes through bottom-up and top-down processes. In our highly uncertain future, the net effects of these threats raise serious concerns for freshwater ecosystems. However, we also highlight opportunities for conservation gains as a result of novel management tools (e.g. environmental flows, environmental DNA) and specific conservation-oriented actions (e.g. dam removal, habitat protection policies, managed relocation of species) that have been met with varying levels of success. Moving forward, we advocate hybrid approaches that manage fresh waters as crucial ecosystems for human life support as well as essential hotspots of biodiversity and ecological function. Efforts to reverse global trends in freshwater degradation now depend on bridging an immense gap between the aspirations of conservation biologists and the accelerating rate of species endangerment.

Response diversity, nonnative species, and disassembly rules buffer freshwater ecosystem processes from anthropogenic change

Integrating knowledge of environmental degradation, biodiversity change, and ecosystem processes across large spatial scales remains a key challenge to illuminating the resilience of earth's systems. There is now a growing realization that the manner in which communities will respond to anthropogenic impacts will ultimately control the ecosystem consequences. Here, we examine the response of freshwater fishes and their nutrient excretion - a key ecosystem process that can control aquatic productivity - to human land development across the contiguous United States. By linking a continental-scale dataset of 533 fish species from 8100 stream locations with species functional traits, nutrient excretion, and land remote sensing, we present four key findings. First, we provide the first geographic footprint of nutrient excretion by freshwater fishes across the United States and reveal distinct local- and continental-scale heterogeneity in community excretion rates. Second, fish species exhibited substantial response diversity in their sensitivity to land development; for native species, the more tolerant species were also the species contributing greater ecosystem function in terms of nutrient excretion. Third, by modeling increased land-use change and resultant shifts in fish community composition, land development is estimated to decrease fish nutrient excretion in the majority (63%) of ecoregions. Fourth, the loss of nutrient excretion would be 28% greater if biodiversity loss was random or 84% greater if there were no nonnative species. Thus, ecosystem processes are sensitive to increased anthropogenic degradation but biotic communities provide multiple pathways for resistance and this resistance varies across space.