Bioretention cell age and construction style influence stormwater pollutant dynamics.

In urbanized landscapes, green infrastructure is proposed as a method for using relatively small plots of land to manage stormwater and protect receiving ecosystems from pollutants. Bioretention cells can infiltrate stormwater from roads and parking lots, and as stormwater passed through the soils, metals can be removed. Metal removal and storage has been demonstrated in laboratory media columns and field-scale test cells, but we have an incomplete understanding of metal removal and accumulation in aging bioretention cells in the field. We surveyed 25 bioretention cells (0-7 years of service) for soil physicochemistry to determine which characteristics related to soil metal (Cu, Pb, and Zn) concentrations. We collected soil cores and treated them with simulated stormwater to measure potential rates of metal removal under different metal and salt concentrations. Older bioretention cells had higher Cu, Pb, and Zn concentrations in soil, which indicates accumulation and storage over time. The oldest cells had metal concentrations that were not a risk to human health but which approached screening thresholds for potential impairment of plants. Soil organic matter content (OM) was positively associated with metal concentrations which highlights the importance of OM in the functioning of cells. Retrofit bioretention cells were younger with less OM and more clay than cells built concurrently with new parking lot construction (i.e., de novo), but after 2.7 years of service, soil OM was similar between retrofit and de novo builds. Soil cores from de novo bioretention cells retained more stormwater metals than soil cores from retrofit cells, and this was likely due to differences in soil OM. Elevated road salt in stormwater was associated with less effective metal removal and leaching of Zn from soils. Overall, these data suggest that management (e.g., salting) and design (e.g., construction) decisions can greatly influence metal removal function of bioretention cells.

Hitting Reset on Sediment Toxicity: Sediment Homogenization Alters the Toxicity of Metal-Amended Sediments.

Laboratory testing of sediments frequently involves manipulation by amendment with contaminants and homogenization, which changes the physicochemical structure of sediments. These changes can influence the bioavailability of divalent metals, and field and mesocosm experiments have shown that laboratory-derived thresholds are often overly conservative. We assessed the mechanisms that lead to divergence between laboratory- and field-derived thresholds; specifically, we assessed the importance of slow equilibration to solid-phase ligands and vertical stratification. To mimic natural physicochemical conditions, we uniquely aged sediment with a flow-through exposure system. These sediments were then homogenized and compared, toxicologically, with freshly metal-amended sediments in a 28-d chronic toxicity bioassay with the amphipod Hyalella azteca. We assessed concentration-response relationships for 3 metals (copper, nickel, and zinc) and 5 geochemically distinct sediments. We observed minimal differences in growth and survival of H. azteca between aged and freshly spiked sediments across all sediments and metals. These trends suggest that a loss of toxicity observed during long-term sediment aging is reversed after sediment homogenization. By comparison with mesocosm experiments, we demonstrate that homogenizing sediment immediately before toxicity assays may produce artificially high toxicity thresholds. We suggest that toxicity assays with sediments that maintain vertical redox gradients are needed to generate field-relevant sediment metal toxicity thresholds. Environ Toxicol Chem 2019;38:1995-2007. © 2019 SETAC.

Global patterns and drivers of ecosystem functioning in rivers and riparian zones.

River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth's biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented "next-generation biomonitoring" by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale.

Ranking stressor impacts on periphyton structure and function with mesocosm experiments and environmental-change forecasts.

Streams are being subjected to physical, chemical, and biological stresses stemming from both natural and anthropogenic changes to the planet. In the face of limited time and resources, scientists, resource managers, and policy makers need ways to rank stressors and their impacts so that we can prioritize them from the most to least important (i.e., perform 'ecological triage'). We report results from an experiment in which we established a periphyton community from the Huron River (Michigan, USA) in 84 experimental 'flumes' (stream mesocosms). We then dosed the flumes with gradients of six common stressors (increased temperature, taxa extinctions, sedimentation, nitrogen, phosphorus, and road salt) and monitored periphyton structure and function. A set of a priori deterministic functions were fit to each stressor-endpoint response and model averaging based on AICc weights was used to develop concentration-response best-fit predictions. Model predictions from different stressors were then compared to forecasts of future environmental change to rank stressors according to the potential magnitude of impacts. All of the stressors studied altered at least one characteristic of the periphyton; however, the extent (i.e., structural and functional changes) and magnitude of effects expected under future forecasts differed significantly among stressors. Elevated nitrogen concentrations are projected to have the greatest combined effect on stream periphyton structure and function. Extinction, sediment, and phosphorus all had similar but less substantial impact on the periphyton (e.g., affected only structure not function, smaller magnitude change). Elevated temperature and salt both had measurable effects on periphyton, but their overall impacts were much lower than any of the other stressors. For periphyton in the Huron River, our results suggest that, among the stressors examined, increased N pollution may have the greatest potential to alter the structure and function of the periphyton community, and managers should prioritize reducing anthropogenic sources of nitrogen. Our study demonstrates an experimental approach to ecological triage that can be used as an additional line of evidence to prioritize management decisions for specific ecosystems in the face of ecological change.

Metal Oxides in Surface Sediment Control Nickel Bioavailability to Benthic Macroinvertebrates.

In aquatic ecosystems, the cycling and toxicity of nickel (Ni) are coupled to other elemental cycles that can limit its bioavailability. Current sediment risk assessment approaches consider acid-volatile sulfide (AVS) as the major binding phase for Ni, but have not yet incorporated ligands that are present in oxic sediments. Our study aimed to assess how metal oxides play a role in Ni bioavailability in surficial sediments exposed to effluent from two mine sites. We coupled spatially explicit sediment geochemistry (i.e., separate oxic and suboxic) to the indigenous macroinvertebrate community structure. Effluent-exposed sites contained high concentrations of sediment Ni and AVS, though roughly 80% less AVS was observed in surface sediments. Iron (Fe) oxide mineral concentrations were elevated in surface sediments and bound a substantial proportion of Ni. Redundancy analysis of the invertebrate community showed surface sediment geochemistry significantly explained shifts in community abundances. Relative abundance of the dominant mayfly (Ephemeridae) was reduced in sites with greater bioavailable Ni, but accounting for Fe oxide-bound Ni greatly decreased variation in effect thresholds between the two mine sites. Our results provide field-based evidence that solid-phase ligands in oxic sediment, most notably Fe oxides, may have a critical role in controlling nickel bioavailability.

Nickel Partitioning and Toxicity in Sediment during Aging: Variation in Toxicity Related to Stability of Metal Partitioning.

Metals in sediment can be complexed by minerals, partition between solid and aqueous phases, and cause toxicity at high concentrations. We studied how the oxidation of surface sediment that occurs during aging alters the partitioning and toxicity of Ni. Two sediments (Burntwood and Raisin) were amended with Ni, equilibrated, incubated in a flow-through flume, and examined for sediment physicochemistry and toxicity to Hyalella azteca (7 day growth). Through time, the sediment surface (5 mm) was oxidized, acid-volatile sulfide concentrations declined in Raisin sediment, and amorphous Fe oxides increased. Porewater Ni concentrations declined through time but total Ni concentrations in sediment were unchanged, suggesting changes in Ni partitioning through time. Both sediments elicited a toxic dose-response by H. azteca early in the aging process; but only Burntwood, for which Ni was primarily partitioned to Fe oxide minerals, exhibited a consistent dose-response during aging. Low total Ni concentrations (20 mg kg-1) in Raisin sediment reduced H. azteca growth at initiation, but all Ni treatments (up to 3000 mg kg-1) exhibited similar growth after 12 days of aging. The dynamic toxicity observed in Raisin sediment was likely due to the instability of NiS in surface sediments early in the aging process. These data suggest that short-term toxicity assays with homogenized Ni-amended sediment (i.e., standard sediment toxicity tests) may be accurate for sediments where Ni speciation is dominated by oxidized ligands; however, under high-AVS and high-Fe conditions, calculated toxicity thresholds may be overly conservative (here by >100-fold) with respect to natural sediment conditions.

Net methylmercury production in 2 contrasting stream sediments and associated accumulation and toxicity to periphyton.

Periphyton uptake of bioaccumulative methylmercury (MeHg) may be an important entryway into the food web of many stream ecosystems where periphyton can be dominant primary producers. The net production of MeHg in stream sediment, its bioaccumulation in periphyton, and the potential toxicity of divalent Hg (Hg[II]) and MeHg in sediment to periphyton were investigated with a 67-d in situ incubation experiment using chemical exposure substrates containing either a fine-grained, organic-rich or a sandy, low-organic sediment, each amended with varying concentrations of mercuric chloride. Methylmercury was produced in sediment, and concentrations increased with greater amounts of added Hg(II); however, the net production of MeHg was inhibited in the highest Hg(II) treatments of both sediments. The range of total Hg concentrations that inhibited MeHg production was between approximately 80 000 ng Hg and 350 000 ng Hg per gram of organic matter for both sediments. Periphyton colonizing substrates accumulated MeHg in proportion to the concentration in sediment, but periphyton exposed to the sandy sediment accumulated approximately 20-fold more than those exposed to the organic-rich sediment relative to sediment MeHg concentrations. Toxicity of either Hg(II) or MeHg to periphyton was not observed with either periphyton organic content, net primary production, or respiration as endpoints. These results suggest that in situ production and bioaccumulation of MeHg in stream ecosystems can vary as a function of sediment characteristics and Hg(II) loadings to the sediment. Environ Toxicol Chem 2016;35:1759-1765. © 2015 SETAC.

Toxicological effects of short-term resuspension of metal-contaminated freshwater and marine sediments.

Sediments in navigation-dominated waterways frequently are contaminated with a variety of particle-associated pollutants and are subject to frequent short-term resuspension events. There is little information documenting whether resuspension of metal-contaminated sediments has adverse ecological effects on resident aquatic organisms. Using a novel laboratory approach, the authors examined the mobilization of Zn, Cu, Cd, Pb, Ni, and Cr during resuspension of 1 freshwater and 2 coastal marine sediments and whether resuspension and redeposition resulted in toxicity to model organisms. Sediment flux exposure chambers were used to resuspend metal-contaminated sediments from 1 site in Lake DePue, Illinois (USA), and 2 sites in Portsmouth Naval Shipyard, Maine (USA). Short-term (4-h) resuspension of sediment at environmentally relevant suspended particulate matter concentrations (<1 g/L) resulted in metal mobilization to water that was sediment and metal specific. Overall, the net release of metals from suspended particles was limited, likely because of scavenging by organic matter and Fe oxides that formed during sediment interaction with oxic water. Minimal toxicity to organisms (survival of Hyalella azteca and Daphnia magna; survival, growth, and tissue metal concentration of Neanthes arenaceodentata; bioluminescence of Pyrocystis lunula) was observed during 4-h exposure to resuspended sediments and during 4-d to 10-d post-exposure recovery periods in uncontaminated water. Redeposited suspended particles exhibited increased metal bioavailability and toxicity to H. azteca, highlighting the potential for adverse ecological impacts because of changes in metal speciation. It is important to consider interactions between organisms' life histories and sediment disturbance regimes when assessing risks to ecosystems.

Deforestation and cultivation mobilize mercury from topsoil.

Terrestrial biomass and soils are a primary global reservoir of mercury (Hg) derived from natural and anthropogenic sources; however, relatively little is known about the fate and stability of Hg in the surface soil reservoir and its susceptibility to change as a result of deforestation and cultivation. In southwest Ohio, we measured Hg concentrations in soils of deciduous old- and new-growth forests, as well as fallow grassland and agricultural soils that had once been forested to examine how, over decadal to century time scales, man-made deforestation and cultivation influence Hg mobility from temperate surface soils. Mercury concentrations in surficial soils were significantly greater in the old-growth than new-growth forest, and both forest soils had greater Hg concentrations than cultivated and fallow fields. Differences in Hg:lead ratios between old-growth forest and agricultural topsoils suggest that about half of the Hg lost from deforested and cultivated Ohio soils may have been volatilized and the other half eroded. The estimated mobilization potential of Hg as a result of deforestation was 4.1 mg m(-2), which was proportional to mobilization potentials measured at multiple locations in the Amazon relative to concentrations in forested surface soils. Based on this relationship and an estimate of the global average of Hg concentrations in forested soils, we approximate that about 550 M mol of Hg has been mobilized globally from soil as a result of deforestation during the past two centuries. This estimate is comparable to, if not greater than, the amount of anthropogenic Hg hypothesized by others to have been sequestered by the soil reservoir since Industrialization. Our results suggest that deforestation and soil cultivation are significant anthropogenic processes that exacerbate Hg mobilization from soil and its cycling in the environment.

Copper Sediment Toxicity and Partitioning during Oxidation in a Flow-Through Flume.

The bioavailability of transition metals in sediments often depends on redox conditions in the sediment. We explored how the physicochemistry and toxicity of anoxic Cu-amended sediments changed as they aged (i.e., naturally oxidized) in a flow-through flume. We amended two sediments (Dow and Ocoee) with Cu, incubated the sediments in a flow-through flume, and measured sediment physicochemistry and toxicity over 213 days. As sediments aged, oxygen penetrated sediment to a greater depth, the relative abundance of Fe oxides increased in surface and deep sediments, and the concentration of acid volatile sulfide declined in Ocoee surface sediments. The total pool of Cu in sediments did not change during aging, but porewater Cu, and Cu bound to amorphous Fe oxides decreased while Cu associated with crystalline Fe oxides increased. The dose-response of the epibenthic amphipod Hyalella azteca to sediment total Cu changed over time, with older sediments being less toxic than freshly spiked sediments. We observed a strong dose-response relationship between porewater Cu and H. azteca growth across all sampling periods, and measurable declines in relative growth rates were observed at concentrations below interstitial water criteria established by the U.S. EPA. Further, solid-phase bioavailability models based on AVS and organic carbon were overprotective and poorly predicted toxicity in aged sediments. We suggest that sediment quality criteria for Cu is best established from measurement of Cu in pore water rather than estimating bioavailable Cu from the various solid-phase ligands, which vary temporally and spatially.

Copper and nickel partitioning with nanoscale goethite under variable aquatic conditions.

Metal contaminated sediments can be toxic to aquatic organisms and are common in human-dominated ecosystems, which results in metals being a leading cause of ecosystem impairment. Bioavailability of metals is influenced by their affinity for dissolved and solid-phase ligands, including iron (Fe) oxyhydroxides, which have been hypothesized to reduce metal toxicity in sediments. The authors examined the adsorption kinetics of copper (Cu) and nickel (Ni) with goethite (α-FeOOH) and characterized the influences of solute metal concentration, pH, ionic strength, and humate concentration on steady-state partitioning of the metals with goethite under conditions representative of natural aquatic environments. Copper and Ni readily adsorbed to goethite, and steady-state partitioning was achieved within 2 h. Although ionic strength had no effect on metal partitioning, adsorption of Cu and Ni to goethite was enhanced by alkaline pH and reduced by competition with humate. Because distribution coefficient (KD ) values for Cu and Ni from the present study are comparable to values measured in natural systems, the authors hypothesize that goethite may contribute significantly to the adsorption of both Ni and Cu to particles in the environment. The authors suggest that incorporating binding by Fe oxides in metal bioavailability models should be a priority for improving risk assessment of metal-contaminated oxic sediments.

Predator-induced defenses in tadpoles confound body stoichiometry predictions of the general stress paradigm.

Predation is known to have both direct and indirect effects on nutrient cycling in terrestrial and aquatic ecosystems, and the general stress paradigm (GSP) has been promoted as a theory for describing predator-mediated indirect effects on nutrient cycling. The GSP predicts that prey exposed to predators will produce glucocorticosteroids, which have a host of physiological effects including gluconeogenesis, increased respiration, excretion of N and P, and increases in body C:N. We tested the nutrient predictions of the GSP using anuran larvae, which exhibit morphological defenses in addition to behavioral defenses for which the GSP was conceived. Genetically similar Hyla versicolor tadpoles were placed in mesocosms either in the presence or absence of a fed predator (Dytiscus verticalis), and after two weeks, tadpoles exposed to predators exhibited strong induced defenses with large, tubular bodies, larger tails, and reduced activity. Tadpole body %C and N:P increased with no change in C:N, which is contrary to expectations from the GSP. Statistical models suggested that changes in body morphology (e.g., tail muscle width) rather than behavioral defenses (i.e., reduced activity) were most likely responsible for predator-mediated differences in body stoichiometry. This study suggests that strong morphological defenses may overwhelm or counteract the nutrient predictions of the GSP.

Evaluating the performance of diffusive gradients in thin films for predicting Ni sediment toxicity.

Diffusive gradients in thin films (DGTs) rapidly measure labile fractions of metal and are promoted as an assessment tool for bioavailability. Using macroinvertebrate community composition as a response, this study compared the predictive ability of DGT-measured Ni with acid volatile sulfide (AVS) and organic carbon (OC) corrected Ni [(SEM(Ni)-AVS)/f(OC)] and total Ni concentrations. In two experiments, sediments were amended with Ni and placed within either a streamside mesocosm or deployed in situ. DGT-measured Ni concentrations (C(DGT)) increased with increasing total Ni, were greater at depth, and decreased over time. Relationships between Ni C(DGT) and sediment geochemistry indicated a shift in Ni partitioning from AVS-bound to Fe- and Mn-associated Ni. In both experiments, DGT-measured Ni poorly predicted the invertebrate response to metal, whereas models that included total Ni or (SEM(Ni)-AVS)/f(OC) effectively predicted the invertebrate response for the streamside mesocosm and in situ experiments, respectively. C(DGT) overestimated the available Ni fraction, possibly due to sampling either nonbioavailable solid-phase Ni or Ni irrespective of cations competing at the biotic ligand. We suggest that C(DGT) cannot replace (SEM(Ni)-AVS)/f(OC) for predicting invertebrate response to sediment Ni, and greater understanding of metal species lability to DGTs is needed before assuming equivalence between bioavailable and DGT-labile metals in sediments.

Interactive effects of phosphorus and copper on Hyalella azteca via periphyton in aquatic ecosystems.

This research examined the interaction between dissolved copper and phosphorus, with respect to their effects on the freshwater amphipod Hyalella azteca feeding on periphyton. Field-collected periphyton communities were exposed to different nutrient and metal conditions in indoor recirculating streams. H. azteca were then exposed to water and periphyton from these streams. There was rapid Cu accumulation by periphyton but the total Cu concentration of periphyton was not directly related to dissolved P. In terms of H. azteca growth, an interactive effect was found between Cu and P as growth was reduced more than expected in the low Cu-high P treatment. Our data suggest that eutrophic conditions result in greater Cu toxicity to benthic macroinvertebrates at lower metal concentrations, likely due to higher assimilation efficiency of dietary Cu from periphyton incubated under eutrophic conditions. These results imply that non-additive interactions between multiple stressors may cause ecosystem effects as detected in standard laboratory bioassays conducted under controlled conditions.

Nickel phase partitioning and toxicity in field-deployed sediments.

The pool of bioavailable metal in sediments can be much smaller than total metal concentration due to complexation and precipitation with ligands. Metal bioavailability and toxicity in sediment is often predicted from models of simultaneous extracted metal and acid volatile sulfide (SEM-AVS); however, studies of the applicability of these models for Ni-contaminated sediments have been conducted primarily in laboratory settings. We investigated the utility of the SEM-AVS models under field conditions: Five lotic sediments with a range of sulfide and organic carbon contents were amended with four concentrations of Ni, deployed in streams for eight weeks, and examined for colonizing macroinvertebrates. After four weeks, colonizing macroinvertebrates showed a strong negative response to the Ni-treated sediments and SEM-AVS models of bioavailability differentiated between toxic and nontoxic conditions. By Week 8, relationships deteriorated between colonizing macroinvertebrates and SEM-AVS model predictions. Total Ni in the sediment did not change through time; however, Ni partitioning shifted from being dominated by organic cabon at deployment to associations with Fe and Mn. Combined geochemical and toxicity results suggest that Fe and Mn oxides in surface sediments resulted in Ni being less available to biota. This implies that current SEM-AVS models may overestimate bioavailable Ni in sediments with oxic surface layers and sufficient Fe and Mn.

Non-native earthworms in riparian soils increase nitrogen flux into adjacent aquatic ecosystems.

Riparian zones are an important transition between terrestrial and aquatic ecosystems, and they function in nutrient cycling and removal. Non-native earthworms invading earthworm-free areas of North America can affect nutrient cycling in upland soils and have the potential to affect it in riparian soils. We examined how the presence of earthworms can affect riparian nutrient cycling and nutrient delivery to streams. Two mesocosm experiments were conducted to determine how (1) the biomass of earthworms and (2) earthworm species can affect nutrient flux from riparian zones to nearby streams and how this flux can affect streamwater nutrients and periphyton growth. In separate experiments, riparian soil cores were amended with one of four mixed earthworm biomasses (0, 4, 10, or 23 g m(-2) ash-free dry mass) or with one of three earthworm species (Aporrectodea caliginosa, Lumbricus terrestris, L. rubellus) or no earthworm species. Riparian soil cores were coupled to artificial streams, and over a 36-day period, we measured nutrient leaching rates, in-stream nutrient concentrations, and periphyton growth. Ammonium leaching increased with increasing biomass and was greatest from the A. caliginosa treatments. Nitrate leaching increased through time and increased at a greater rate with higher biomass and from cores containing A. caliginosa. We suggest that the overall response of increased nitrate leaching [90% of total nitrogen (N)] was due to a combination of ammonium excretion and burrowing by earthworms, which increased nitrification rates. During both experiments, periphyton biomass increased through time but did not differ across treatments despite high in-stream inorganic N. Through time, in-stream phosphorus (P) concentration declined to <5 microg l(-1), and periphyton growth was likely P-limited. We conclude that activities of non-native earthworms (particularly A. caliginosa) can alter biogeochemical cycling in riparian zones, potentially reducing the N-buffering capacity of riparian zones and altering stoichiometric relationships in adjacent aquatic ecosystems.