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  • 1.
    Lejon, Anna G.C.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Ecosystem response to dam removal2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis aims to improve our understanding of how riverine ecosystems respond to dam removal. Riverine and particularly riparian ecosystems are among the most variable and important features of all landscapes. They connect landscape elements both longitudinally and laterally, and are governed by processes such as flooding, erosion and deposition that create dynamic, diverse and heterogeneous habitats. In fact, riparian zones are among the world’s most species-rich habitats. Worldwide there are millions of dams that fragment stream and river systems, regulate flows and degrade ecosystems. Dams impact freshwater, marine and terrestrial ecosystems and threaten biodiversity by disrupting organism movements and energy flows in the landscape. An important upstream effect of dams is inundation of habitats and development of new shorelines around impounded areas. Effects downstream of dams are mainly caused by changed hydrological regimes and retention of organic and inorganic materials in reservoirs, leading to reduced transport and dispersal of for example seeds to reaches downstream. The removal of dams create expectations that biota will eventually recover. We have studied a number of dam removal projects in Sweden. Our experimental results showed that following dam removal, newly exposed soils in former impoundments were rapidly colonized by pre-removal species. Their species richness increased slightly with time and their species composition indicated a slow change towards that in the reference site. In addition, the vegetation in formerly impounded areas showed a direction of change from lentic riparian plants (high proportion of aquatics) towards lotic ones, consisting of native perennials typical of free-flowing streams. We also found that the apprehensions that former impoundments would turn into pools of mud did not come true; in fact, a process towards more pristine channel morphology was observed. After removal there was erosion and downstream transport of sediment. We found only minor effects on macroinvertebrate communities. For example, a few species decreased over the years, suggesting that dam removal in itself might cause a temporary disturbance. This highlights the importance of long-term studies after dam removal, and also the importance of comparisons with pre-removal conditions and stretches unaffected by dams. Thorough documentation of executed dam removal projects and distribution of the results and experiences are tremendously important in the planning process of future decommissioning projects. Also, our experiences have taught us that in order to attain a successful dam removal it is important to involve stakeholders such as non-governmental organizations and local inhabitants in the process.

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    Ecosystem response to dam removal
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    Omslag
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    Spikblad
  • 2.
    Lejon, Anna G.C.
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Malm Renöfält, Birgitta
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Conflicts associated with dam removal in Sweden2009In: Ecology and Society, ISSN 1708-3087, Vol. 14, no 2, p. 4-Article in journal (Refereed)
    Abstract [en]

    The increasing number of deteriorating old dams that need renovation or have lost their function make dam removal a viable management option. There are at least four major reasons for dam removal: safety, law and policy, economy, and ecology. Here we discuss 17 Swedish dams that were recently considered for removal. Because dam removal usually causes controversy, dam removal initiatives may succeed, fail, or result in a compromise such as a bypass channel for migrating fish. We identify and discuss three major obstructions to dam removal: funding, cultural-historical values, and threatened species. To facilitate dam removal, the reasons for, and the effects of, dam removal must be carefully explained, and the public and stakeholders must be kept informed. In complicated cases in which compromise solutions may be the most feasible outcome, the integration of the knowledge of different stakeholders is crucial. The involvement of diverse stakeholders increases their willingness to find compromises, thus avoiding conflicts and failures.

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    Lejon et al_EcolSoc2009
  • 3.
    Lejon, Anna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Malm Renöfält, Birgitta
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Dam removal effects on riparian vegetationManuscript (preprint) (Other academic)
    Abstract [en]

    Dams cause substantial damage to stream and river landscapes, especially because of flow regulation and channel fragmentation that constrain environmental structures and processes. Dam removal, on the other hand, initiates succession of plant communities as a response to new morphological and hydrological conditions in the channel. We studied the vegetation in riparian reaches upstream and downstream of a dam construction in the Nissan stream in southern Sweden before and after its removal, using a Before-After-Control-Impact (BACI) design. We monitored the vegetation and different environmental variables at three different water levels (summer low, middle and spring high) in the impoundment, downstream of the dam, as well as in an unimpacted reach located within the same river system upstream of the area affected by the dam. Following dam removal, plant colonisation was fast on newly exposed soils in the former impoundment and species richness increased slightly without major changes of the dominant species. The reach downstream of the dam exhibited minor changes after dam removal, comparable to those in the reference reach. The vegetation response implies that the post-removal vegetation in the impoundment area was more similar to that of the previous impoundment than to that of the reference reach, suggesting low seed rain and local recruitment.

  • 4.
    Lejon, Anna
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Malm Renöfält, Birgitta
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Succession of riparian plants following dam removal in a boreal stream in central SwedenManuscript (preprint) (Other academic)
    Abstract [en]

    Worldwide there are numerous aging and non-performing dams that may face removal, for economic or other reasons.  Dam removal initiates succession of plant and animal communities as a response to new morphological and hydrological conditions in the channel. We studied the succession of plant communities in riparian reaches upstream and downstream of a recently removed dam in central Sweden over 3 years. We monitored the vegetation development at each site and compared it with a reference site in an unimpacted upstream reach. The two reaches located in the former reservoir developed new riparian zones following dam removal. Plant colonisation in the new riparian zone was fast and species composition became increasingly similar to that of the reference reach. Dam removal largely restored species composition in the riparian zones that were formed in the previous reservoir, indicating that an appropriate species pool was available and that conditions for natural regeneration of riparian vegetation were sufficient.  However, a significant decline in species richness in the downstream reach following dam removal may imply that the upstream and downstream effects of removal may differ and that the removal itself may have disturbed the reach downstream of the former dam. Although remaining timber floating structures and four hydroelectric dams upstream may hamper a more complete vegetation recovery we foresee many years of riparian vegetation development before the successional processes slow down.

  • 5.
    Malm Renöfält, Birgitta
    et al.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Lejon, Anna G.C.
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Jonsson, Micael
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Nilsson, Christer
    Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
    Long-term taxon-specific responses of macroinvertebrates to dam removal in a mid-sized Swedish stream2013In: Rivers Research and Applications: an international journal devoted to river research and management, ISSN 1535-1459, E-ISSN 1535-1467, Vol. 29, no 9, p. 1082-1089Article in journal (Other academic)
    Abstract [en]

    Dam removal to restore ecologically impaired rivers is becoming increasingly common. While the target often is to facilitate fish migration, dam removal has also been assumed to benefit other types of organisms. Because few studies thus far deal with effects of dam removal on stream macroinvertebrates, and because results have been equivocal, we investigated both short- and longer-term dam-removal effects on downstream macroinvertebrate communities. We did this in a before-and-after study of the removal of a dam located in a south Swedish stream. We sampled the benthic fauna 6 months prior to dam removal and both 6 months and 3.5 years after the dam was removed. We compared species composition, taxonomic richness, total densities, and densities of macroinvertebrate groups before and after dam removal and between downstream and reference sites. We found that dam removal reduced some macroinvertebrate taxa at the downstream site, but we found no effect on community composition. While this corroborates results from previous short-term studies, we also found a reduction of taxonomic richness and that some dam-removal effects persisted or even increased over time. The most likely explanation for the suppression of benthic macroinvertebrate richness following dam removal is a significantly increased sediment transport from the former reservoir, and a subsequent loss of preferred substrates. Our results indicate that adverse dam-removal effects may be long-lasting, but taxon-specific. We therefore call for longer-term studies on a variety of organisms to better understand how dam removal may influence downstream macroinvertebrate communities.

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