Enhanced Bioaccumulation and Transfer of Monomethylmercury through Periphytic Biofilms in Benthic Food Webs of a River Affected by Run-of-River Dams

Ruin-of-the-rivers? A global review of run-of-the-river dams

The classification of a hydropower scheme as run-of-the-river (or run-of-river; ROR) evokes an image of a low-impact installation; however, examination of eight case studies worldwide shows that substantial negative societal and ecological impacts are tied to them, albeit in somewhat different ways. We conclude that ROR dams not only potentially displace communities, disrupt livelihoods, and degrade environments in surrounding areas, but they also divert water from areas of need, impact aquatic ecology through habitat destruction and disruption of fish migrations, emit non-trivial amounts of greenhouse gases over the lifespan of the project, and disrupt streamflow in downstream river sections. While these negative impacts vary on a case-by-case basis, medium and large ROR dams consistently have multiple and cumulative impacts, even when not having appreciable reservoirs. We contend that many impactful dams do not qualify as low-impact ROR projects, despite being defined as such. Such mislabeling is facilitated in part by the ambiguous definition of the term, which risks the ROR concept being used by proponents of impactful structures to downplay their negative effects and thus mislead the public or gain status, including within the Clean Development Mechanism in relation to mitigating climate change.

Biofilm-mediated heavy metal bioaccumulation and trophic transfer in a mining-contaminated river

Biofilms, essential for material circulation and energy flow in aquatic ecosystems, markedly enrich heavy metals in water environments. However, the impact of these accumulated metals on organisms feeding on biofilms remains poorly unknown. This study involved a year-long seasonal survey along the Bijiang River, located next to Asia's largest lead (Pb)-zinc (Zn) mine, conducted to investigate the role of biofilms in nutrient and metal transfer in food webs. In total, 355 biotic and abiotic samples, including water, biofilms, and aquatic biota, were analyzed for the presence of eight heavy metals (arsenic [As], cadmium, chromium, copper, Pb, nickel, Zn, and iron) as well as stable carbon (δ13C) and nitrogen (δ15N) isotopes. Wide ranges of δ13C and δ15N values indicated diverse dietary carbon sources and trophic positions in the Bijiang River (maximum trophic level: 4.28). A Bayesian mixing model revealed that periphytic biofilms were the dominant basal carbon source, especially in spring, whereas in summer, consumers exploited more diverse food sources, possibly because feeding on spring biofilms enhanced predator feeding efficiency. Metals tended to be biodiluted along food chains owing to their higher concentrations in biofilms and benthic organisms as well as their chemical forms. Although diet did not significantly affect heavy metal accumulation in fish, those relying on biofilms as the main carbon source showed significantly higher As (p = 0.048) and Pb (p = 0.007) levels compared with those relying on C4 plants. Overall, this study highlights the critical role of periphytic biofilms in nutrient and metal dynamics in aquatic food webs.