Peaks and transient dynamics of ecological and biogeochemical variables following impoundment in boreal reservoirs.

Across the globe, reservoirs represent nearly 10 % of the world's freshwater. River impoundment strongly alters the hydrological regime of aquatic ecosystems which subsequently affect the ecological (e.g., primary production, fish biomass) and biogeochemical variables (e.g., nutrient, mercury, and carbon cycles which includes Green House Gas emissions; GHG). We examined the transient dynamics and co-variation of biogeochemical and ecological variables from unique long-term time series (40 years of data) from Hydro-Québec boreal reservoirs, with data before and after impoundment. To do so, we applied curve fitting analysis on the data from eight plausible scenarios and model selection. Following impoundment, most variables increased, peaked, and then decreased over time (clear hump-shaped patterns; six over eight variables). Model predictions peaked between three- and 11-years post-impoundment and returned to pre-impoundment levels after about nine- to 40-years. Variables also followed a clear sequence where GHG emissions (CO2, CH4) peaked first, immediately after impoundment, followed by an increase in phosphorus and Chl-a. Total mercury in fish peaked a few years later for non-piscivorous fish and was followed closely by piscivorous fish. This work provides the first comprehensive and holistic description of the transitory nature and co-variation of ecological and biogeochemical variables following reservoir impoundment.

The impacts of hydropower on freshwater macroinvertebrate richness: A global meta-analysis.

Hydroelectric dams and their reservoirs have been suggested to affect freshwater biodiversity. Nevertheless, studies investigating the consequences of hydroelectric dams and reservoirs on macroinvertebrate richness have reached opposite conclusions. We performed a meta-analysis devised to elucidate the effects of hydropower, dams and reservoirs on macroinvertebrate richness while accounting for the potential role played by moderators such as biomes, impact types, study designs, sampling seasons and gears. We used a random/mixed-effects model, combined with robust variance estimation, to conduct the meta-analysis on 107 pairs of observations (i.e., impacted versus reference) extracted from 24 studies (more than one observation per study). Hydropower, dams and reservoirs did significantly impact (P = 0.04) macroinvertebrate richness in a clear, directional and statistically significant way, where macroinvertebrate richness in hydropower, dams and reservoirs impacted environments were significantly lower than in unimpacted environments. We also observed a large range of effect sizes, from very negative to very positive impacts of hydropower. We tried to account for the large variability in effect sizes using moderators, but none of the moderators included in the meta-analysis had statistically significant effects. This suggests that some other moderators (unavailable for the 24 studies) might be important (e.g., temperature, granulometry, wave disturbance and macrophytes) and that macroinvertebrate richness may be driven by local, smaller scale processes. As new studies become available, it would be interesting to keep enriching this meta-analysis, as well as collecting local habitat variables, to see if we could statistically strengthen and deepen the conclusions of this meta-analysis.