Synergistic impacts of sediment contamination and dam presence on river functioning

1. Dam presence is commonly associated with strong accumulation of polluted sediments. In spite of this context of multiple stressors, physical effects are often solely considered in the ecological assessment of the dam impacts. 2. We studied four ‘reservoir/downstream reach’ systems differing in levels of sediment contamination in reservoirs. Using assemblages and biotrait (i.e. ecological or biological attribute) responses of macroinvertebrate communities and leaf litter breakdown, we examined the individual effects and potential interactions between sediment contamination and dam presence along the gradient of ecotoxic pressure. 3. Leaf breakdown rates ranged from 0.0044° per day in the most contaminated reservoir to 0.0120° per day in the reference reservoir. Comparisons of community trait profiles among reservoirs highlighted a gradient of trait responses to sediment contamination. 4. In the absence of toxic contamination, the dam‐induced modifications in biotraits of invertebrate assemblages were not related to a reduction of leaf litter breakdown. Conversely, contaminated sediment in reservoir induced strong functional disturbances (i.e. bioecological shifts and reduction of leaf litter breakdown) downstream of dams. 5. Key biotrait categories positively related to leaf litter breakdown rate have been identified. They corresponded mainly to shredders and/or small‐sized (<0.5 cm) insects, using aquatic (e.g. crawlers) or aerial (e.g. fliers) active dispersal strategies. In addition, trait categories positively correlated to contamination level have been considered as ‘response’ traits. They corresponded to large‐sized (>4 cm) species, having several generations per year (polyvoltin), using asexual reproduction and/or disseminating by drift (aquatic, passive). 6. In the current context of ecological continuity restoration, this study has identified the risks associated with the presence of historical contamination in the run‐of‐river reservoirs for downstream ecosystem health.     

Multiple stressors and multifunctionality: limited effects on an illuminated benthic system

The bulk of experiments that study stressor effects on ecosystem functioning consider only individual functions one at a time, and such narrow focus may well bias our understanding of the overall impact on ecosystem functioning. We used data from six published experiments in which marine illuminated sediment systems were exposed to nutrient enrichment, toxicants, sedimentation and warming, either alone or in combination. Measured functions were primary production, community respiration, inorganic nitrogen and phosphorus fluxes, and autotrophic biomass. We calculated two indices of multifunctionality that simultaneously considered all six functions: (i) a weighted average level of the functions and (ii) the number of functions that simultaneously exceed a critical threshold level. Stressors affected individual functions both positively and negatively, but multifunctionality was generally unaffected by both single and joint stressors. The filtering capacity of coastal illuminated sediment systems thus appears resilient to exposure to moderate levels of multiple stressors, most probably due to the robustness of the benthic microalgal community. We recommend using a multifunctionality approach in future studies on cumulative stressor effects on ecosystem functioning, particularly when considering functions related to ecosystem services.     

Determining the influence of habitat and chemical factors on instream biotic integrity for a Southern Ohio watershed

The relative influences of water quality and in-streamhabitat on receiving water biology were investigatedin the Little Miami River, Ohio, a national and statescenic river located in the southwest corner of thestate. Monitored sample data for water chemistry,habitat, invertebrates and fish were obtained fromfederal and state sources and integrated via ageographical information system (GIS), using a uniquestream segmenting system. Locations and flowinformation for municipal wastewater treatment plants(WWTPs) were also integrated into the system. Therelative importance of water chemistry and habitat onbiological sample information was determined usingmultivariate forward stepwise regression. Theprincipal environmental factors that influencedbiological responses were in-stream habitat andpercent cumulative WWTP effluent at mean flow. Cumulative WWTP effluent at low flow dilution,nutrients and total toxic units from metals andammonia did not correlate with biological responses(e.g. number of mayfly species, invertebrate communityindex (ICI), number of fish species and index ofbiotic integrity (IBI)). We found the GIS-based datamethods to be highly useful for understanding theprincipal processes that guide receiving water health.     

The Anna Karenina Principle Applied to Ecological Risk Assessments of Multiple Stressors

Successful ecological risk assessments are all alike; every unsuccessful ecological risk assessment fails in its own way. Tolstoy posited a similar analogy in his novel Anna Karenina: “Happy families are all alike; every unhappy family is unhappy in its own way.” By that, Tolstoy meant that for a marriage to be happy, it had to succeed in several key aspects. Failure on even one of these aspects, and the marriage is doomed. In this paper, I argue that the Anna Karenina principle also applies to ecological risk assessments involving multiple stressors. In particular, I argue that multiple stressors assessments and environmental decision making will not have a happy marriage unless the following can be achieved: (1) there must be societal and political buy-in to the assessment and decision-making process; (2) the assessment must have the latitude to consider a wide range of stressors and potential risk management options; (3) there must be a commitment to following a rigorous focusing of the assessment and to expending resources for model development and data collection; and (4) an adaptive management strategy must be adopted wherein risk management actions are undertaken, system response intensively observed and assessed, and revised management actions taken as appropriate. Failure to meet any of the above criteria for success will doom a multiple stressors assessment and prevent its use in effective decision-making.