Methylmercury Bioaccumulation in Stream Food Webs Declines with Increasing Primary Production.

Opposing hypotheses posit that increasing primary productivity should result in either greater or lesser contaminant accumulation in stream food webs. We conducted an experiment to evaluate primary productivity effects on MeHg accumulation in stream consumers. We varied light for 16 artificial streams creating a productivity gradient (oxygen production =0.048-0.71 mg O2 L(-1) d(-1)) among streams. Two-level food webs were established consisting of phytoplankton/filter feeding clam, periphyton/grazing snail, and leaves/shredding amphipod (Hyalella azteca). Phytoplankton and periphyton biomass, along with MeHg removal from the water column, increased significantly with productivity, but MeHg concentrations in these primary producers declined. Methylmercury concentrations in clams and snails also declined with productivity, and consumer concentrations were strongly correlated with MeHg concentrations in primary producers. Heterotroph biomass on leaves, MeHg in leaves, and MeHg in Hyalella were unrelated to stream productivity. Our results support the hypothesis that contaminant bioaccumulation declines with stream primary production via the mechanism of bloom dilution (MeHg burden per cell decreases in algal blooms), extending patterns of contaminant accumulation documented in lakes to lotic systems.

Effects of a chronic lower range of triclosan exposure on a stream mesocosm community.

Triclosan (5-chloro-2-(2,4-dichlorophenoxy)phenol) is an antimicrobial found in consumer soaps and toothpaste. It is in treated wastewater effluents at low parts-per-billion concentrations, representing a potentially chronic exposure condition for biota inhabiting receiving streams. For the present study, a naturally colonized benthos was created using flow-through indoor mesocosms; then, the benthic communities were dosed to achieve different in-stream triclosan concentrations (control, 0.1 µg/L, 0.5 µg/L, 1.0 µg/L, 5.0 µg/L, and 10 µg/L) for 56 d. Water quality parameters and endpoints from bacteria to macroinvertebrates, as well as interacting abiotic components, were measured. Effects of triclosan on specific microbial endpoints were observed at all doses, including an effect on litter decomposition dynamics at doses of 1.0 µg/L and higher. Resistance of periphytic bacteria to triclosan significantly increased at doses of 0.5 µg/L and above. By the end of dosing, the antimicrobial appeared to stimulate the stream periphyton at the 3 lowest doses, while the 2 highest doses exhibited decreased stocks of periphyton, including significantly lower bacteria cell densities and cyanobacteria abundance compared with the control. Other than an effect on benthic ostracods, the changes that occurred in the periphyton did not translate to significant change in the colonizing nematodes, the macroinvertebrate community as a whole, or other measurements of stream function. The results shed light on the role a low, chronic exposure to triclosan may play in effluent-dominated streams.

The distance that contaminated aquatic subsidies extend into lake riparian zones.

Consumption of emergent aquatic insects by terrestrial invertebrates is a poorly resolved, but potentially important, mechanism of contaminant flux across ecosystem borders leading to contaminant exposure in terrestrial invertivores. We characterized the spatial extent and magnitude of contaminant transfer from aquatic sediments to terrestrial invertebrate predators by examining riparian araneid spiders, terrestrial insects, and emergent aquatic insects for stable isotopes and polychlorinated biphenyls (PCBs, sum of 141 congeners) at Lake Hartwell, (Clemson, South Carolina, USA). PCB concentrations in aquatic insects were orders of magnitude higher than in terrestrial insects. Aquatic insect consumption by spiders (as indicated by delta13C and delta15N), PCB concentrations in spiders, and aquatic prey availability were greatest at the shoreline and declined inland, while terrestrial prey availability was invariant with distance. These patterns indicate PCB transfer to spiders through consumption of emergent aquatic insects extending to a distance of 5 m inland. Measurable, but much lower, PCBs were present in insect predators dominated by social wasps up to 30 m inland. These results illustrate the importance of emergent insects as vectors of contaminant transfer from lake sediments to riparian food webs, and that spiders are key predators in this process.

Temporal variation in spatial sources of discharge in a large watershed.

We examined how the spatial configuration of source areas for runoff varied over time in a large watershed, in order to understand processes governing material loading to rivers. Discharge source areas within the Fox River watershed (Wisconsin, US) were mapped for two individual discharge events. The spatial distribution of source areas varied between and over the duration of individual discharge events. Relative contribution to runoff by land cover types within source areas was quantified and compared to areal abundance of land covers in the watershed. Contributions of runoff by land cover types varied over time. Moreover, the degree to which different land cover types acted as source areas differed from their abundance in the watershed. Hence, areal quantifications of land cover within a watershed may not accurately represent what land covers are source areas over given time periods. Therefore, a source-area-based approach may yield more accurate spatial analysis of material loading patterns than a watershed-based approach.

Development of resource shed delineation in aquatic ecosystems.

We apply a concept derived from food web ecology to large-scale spatial patterns of material supply within and between watersheds and coasts by generalizing the definition "resource shed" to source areas for materials supplied to a receptor (e.g., a point location) over a specified time interval. Independent hydrologic and hydrodynamic models, coupled with a particle tracking model, were used to delimit resource shed total spatial extent and relative contributory importance for selected receptors in Lake Erie (North America) over varying time intervals. One resource shed was extended into the Maumee River watershed (OH) by integrating the lake and hydrologic models. Model validation was achieved through comparison with data from the 2005 International Field Years on Lake Erie (IFYLE) study. Resource shed size, orientation, and internal structure varied with receptor location, in-lake circulation, terrestrial precipitation, time interval, and season. River plume extent and interaction were illustrated, and model integration revealed the relative contributory importance of subwatershed catchments to an off-shore receptor.

Spider-mediated flux of PCBs from contaminated sediments to terrestrial ecosystems and potential risks to arachnivorous birds.

We investigated aquatic insect utilization and PCB exposure in riparian spiders at the Lake Hartwell Superfund site (Clemson, SC). We sampled sediments, adult chironomids, terrestrial insects, riparian spiders (Tetragnathidae, Araneidae, and Mecynogea lemniscata), and upland spiders (Araneidae) along a sediment contamination gradient. Stable isotopes (delta(13)C, delta(15)N) indicated that riparian spiders primarily consumed aquatic insects whereas upland spiders consumed terrestrial insects. PCBs in chironomids (mean 1240 ng/g among sites) were 2 orders of magnitude higher than terrestrial insects (15.2 ng/g), similar to differences between riparian (820-2012 ng/g) and upland spiders (30 ng/g). Riparian spider PCBs were positively correlated with sediment concentrations for all taxa (r(2) = 0.44-0.87). We calculated spider-based wildlife values (WVs, the minimum spider PCB concentrations causing physiologically significant doses in consumers) to assess exposure risks for arachnivorous birds. Spider concentrations exceeded WVs for most birds at heavily contaminated sites and were approximately 14-fold higher for the most sensitive species (chickadee nestlings, Poecile spp.). Spiders are abundant and ubiquitous in riparian habitats, where they depend on aquatic insect prey. These traits, along with the high degree of spatial correlation between spider and sediment concentrations we observed, suggest that they are model indicator species for monitoring contaminated sediment sites and assessing risks associated with contaminant flux into terrestrial ecosystems.

Aquatic invertebrate resting egg sensitivity to glutaraldehyde and sodium hypochlorite.

Ballast tank treatment technologies are currently in development to reduce the risk of acquiring or transporting viable aquatic organisms that could be introduced to ecosystems and become invasive. Aquatic invertebrate resting eggs represent a challenge to such technologies because of morphological and biochemical adaptations to stress that also protect eggs from artificial stressors. To evaluate the potential efficacy of chemical biocides for ballast tank treatment, the present study examined the acute toxicity of glutaraldehyde and sodium hypochlorite on resting eggs of the freshwater cladoceran Daphnia mendotae and marine brine shrimp (Artemia sp.). Glutaraldehyde was toxic to resting eggs of Artemia sp., as indicated by a lethal concentration to 90% of organisms (LC90) of 95% confidence interval (226 +/- 10 mg/L). Daphnia mendotae, in contrast, displayed erratic responses to glutaraldehyde. Sodium hypochlorite was similarly toxic to resting eggs of Artemia sp. and D. mendotae, which displayed LC90s of 86.5 +/- 3.0 and 78.3 +/- 1.6 mg/L, respectively. Burial in sediment protected resting eggs from toxicants. The present results corroborate those from previous investigations of resting egg sensitivity to artificial stressors, supporting the conclusions that resting eggs are less sensitive than other life stages to artificial stressors and that chemical biocide concentrations effective against other life stages may be ineffective against resting stages.

Effects of proposed physical ballast tank treatments on aquatic invertebrate resting eggs.

Adaptations in aquatic invertebrate resting eggs that confer protection from natural catastrophic events also could confer protection from treatments applied to ballast water for biological invasion vector management. To evaluate the potential efficacy of physical ballast water treatment methods, the present study examined the acute toxicity of heat (flash and holding methods), ultraviolet (UV) radiation (254 nm), and deoxygenation (acute and chronic) on resting eggs of the freshwater cladoceran Daphnia mendotae and the marine brine shrimp Artemia sp. Both D. mendotae and Artemia sp. were similarly sensitive to flash exposures of heat (100% mortality at 70 degrees C), but D. mendotae were much more sensitive to prolonged exposures. Exposure to 4,000 mJ/cm2 of UV radiation resulted in mortality rates of 59% in Artemia sp. and 91% in D. mendotae. Deoxygenation to an oxygen concentration of 1 mg/L was maximally toxic to both species. Deoxygenation suppressed hatching of D. mendotae resting eggs at oxygen concentrations of less than 5.5 mg/L and of Artemia sp. resting eggs at concentrations of less than 1 mg/L. Results suggest that UV radiation and deoxygenation are not viable treatment methods with respect to invertebrate resting eggs because of the impracticality of producing sufficient UV doses and the suppression of hatching at low oxygen concentrations. Results also suggest that the treatment temperatures required to kill resting eggs are much higher than those reported to be effective against other invertebrate life stages and species. The results, however, do not preclude the effectiveness of these treatments against other organisms or life stages. Nevertheless, if ballast tank treatment systems employing the tested methods are intended to include mitigation of viable resting eggs, then physical removal of large resting eggs and ephippia via filtration would be a necessary initial step.

Sensitivity of aquatic invertebrate resting eggs to SeaKleen (Menadione): a test of potential ballast tank treatment options.

The introduction of aquatic species in resting life stages by the release of ballast water is a less well-known but potentially important invasive species vector. Best-management practices designed to minimize transport of ballast water cannot eliminate this threat, because residual water and sediment are retained in ballast tanks after draining. To evaluate the potential efficacy of chemical treatment of residual material in ship ballast tanks, the present study examined the acute toxicity of the proposed biocide SeaKleen (menadione; Garnett, Watkinsville, GA, USA) on resting eggs of Brachionus plicatilis (a marine rotifer), a freshwater copepod, Daphnia mendotae (a freshwater cladoceran), and Artemia sp. (a marine brine shrimp). SeaKleen was toxic to resting eggs of all taxa. Daphnia mendotae resting eggs encased in protective ephippia were the least sensitive, as indicated by a 24-h lethal concentration of toxicant to 90% of organisms of 8.7 mg/L (95% confidence interval, +/- 0.1 mg/L). SeaKleen induced teratogenic effects in D. mendotae and Artemia sp. Exposure to sunlight quickly degraded SeaKleen, which lost all toxicity after 72 h outdoors. SeaKleen increased in toxicity slightly after 72 h in darkness. Burial of D. mendotae ephippia in natural lake sediment reduced SeaKleen toxicity by a factor of 20. Reduced toxicity in the presence of sediment raises serious doubts as to the potential for this, or any, chemical biocide to kill aquatic invertebrate resting stages buried in sediment retained in ship ballast tanks.