Global transition towards renewable energy production has increased the demand for new and more flexible hydropower operations. Although commonly considered as green energy, hydropower operations can cause severe environmental problems upstream and downstream of the power plant, including decreased habitat quality and quantity. Before management and stakeholders can make informed choices on potential mitigations, it is essential to understand how the hydropower reservoir ecosystems respond to water level regulation (WLR) impacts and how these are likely modified by the reservoirs’ abiotic and biotic characteristics. Yet, most reservoir studies have been case-specific, which hampers large-scale planning and evaluation of mitigation actions across various reservoir ecosystems.
Methodology
Here, we investigated how the effect of the magnitude, frequency and duration of WLR on fish populations varies along environmental gradients. We used biomass, density, size, condition and maturation of brown trout (Salmo trutta L.) in Norwegian hydropower reservoirs as a measure of ecosystem response, and tested for interacting effects of WLR and lake morphometry, climatic conditions and fish community structure. Norway is the sixth largest hydropower producer in the world. Its large number of reservoirs with variable environmental characteristics and operational water level regimes, but species-poor communities, provided an under-utilized opportunity to evaluate hydropower impacts on reservoir fish populations and ecosystems. Brown trout was chosen as the focal study species, because it is the dominant fish species in many Norwegian reservoirs and because generalist salmonids are known to reflect the overall productivity and changes in physical and biological status of lakes.
As response variables for our analyses, we used data derived from fish surveys conducted in 102 Norwegian hydropower reservoirs between 1973 and 2009. As predictor variables, we included measures of lake morphometry, productivity, climate, and fish community composition. Water level data for the selected reservoirs were obtained from a database managed by the Norwegian Water Resources and Energy Directorate.
Outcomes
Brown trout populations showed different responses to the magnitude, frequency and duration aspects of WLR. While the WLR magnitude and WLR frequency had notable impacts, WLR duration had no significant effects on brown trout. The WLR impacts were most evident when using brown trout density and condition (a measure of weight relative to length) as measures of population status and occasionally when using biomass. In contrast, we found no clear effects of WLR on brown trout mean weight or female maturity length, although brown trout tended to become smaller with increasing WLR frequency.
We found support for our hypothesis that WLR affects brown trout populations, with some WLR effects modified by local conditions. The effects of WLR magnitude on brown trout biomass, density and condition were modified by the reservoir morphometry such as area size and shoreline development, and fish community composition. Overall, reservoir morphometry interacted with WLR magnitude when using biomass, density, or mean condition as a measure of brown trout population status, although morphometric characteristics that modified the measured biological response varied.
The main results are shown in Fig. 1. The lines present predicted regression values for the significant main effects. All modelled and presented data are standardized to have a mean of zero and a standard deviation of one. Brown trout biomass and density increased with WLR magnitude, particularly in large and complex-shaped reservoirs, but the positive relationships were only evident in reservoirs with no other fish species. Moreover, increasing WLR frequency was associated with increased brown trout density, but decreased condition of individuals within the populations.
Figure 1: The responses of brown trout (a) density and (b) condition to increasing water level regulation (WLR) frequency.
Conclusions
Our study demonstrates that local environmental characteristics and the biotic community strongly modify the hydropower-induced WLR impacts on reservoir fishes and ecosystems, and that there are no one-size-fits-all solutions to mitigate environmental impacts. This knowledge is vital for sustainable planning, management and mitigation of hydropower operations that need to meet the increasing worldwide demand for both renewable energy and ecosystem services delivered by fresh waters.
To increase renewable energy capacity, and at the same time reduce the overall negative impacts of WLR on ecosystems and their related services, it is essential to identify waterbodies in which new or altered hydropower operations should be either avoided or conducted. This study provides important insights into the factors that need to be considered in sustainable planning, management and mitigation of hydropower development. These include variation in the reservoirs’ abiotic and biotic characteristics as well as their operational water level regimes. Hence, environmental drivers modified the responses of brown trout populations to different WLR patterns.
For reservoirs formed by damming lakes, our results suggest that those with restricted littoral zones (that is, having small areas and/or steep slopes) and sympatric fish communities are most vulnerable to WLR impacts. However, it is important to note that conclusions drawn regarding WLR impacts depend on the complicated interactions among environmental variables that can, in some instances, produce unexpected effects, such as the positive correlation between brown trout biomass and WLR magnitude in reservoirs with large surface areas.
Applying a more holistic reservoir management strategy that considers local reservoir conditions, hydrological alterations and possible habitat restorations for resident fish and overall ecosystem status is a prerequisite for the sustainable development of future hydropower production.
Eloranta, A. P., Finstad, A. G., Helland, I. P., Ugedal, O., Power, M. (2018). Hydropower impacts on reservoir fish populations are modified by environmental variation. Science of the Total Environment, 618, 313-322.
Contact: Michael Power, Department of Biology
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