Does large dam removal restore downstream riparian vegetation diversity? Testing predictions on the Elwha River, Washington, USA.
Large dams and their removal can profoundly affect riparian ecosystems by altering flow and sediment regimes, hydrochory, and landform dynamics, yet few studies have documented these effects on downstream plant communities. Ecological theory and empirical results suggest that by altering disturbance regimes, reducing hydrochory, and shifting communities to later successional stages, dams reduce downstream plant diversity. Dam removal could reverse these processes, but the release of large volumes of sediment could have unexpected, transient effects. Two large dams were removed on the Elwha River in Washington State, USA, from 2011 to 2014, representing an unprecedented opportunity to study large dam removal effects on riparian plant communities. Our research objectives were to determine: (1) whether the Elwha River dams were associated with lower downstream plant diversity and altered species composition across riparian landforms pre-dam removal, and (2) whether dam removal has begun to restore downstream diversity and composition. To address these objectives, we compared plant species richness and community composition in river segments above, below, and between the two dams. Plant communities were sampled twice before (2005 and 2010) and four times after (2013, 2014, 2016, and 2017) the start of dam removal, with 2013 and 2014 sampled while the upstream dam removal was ongoing. Prior to dam removal, native species richness was 41% lower below dams compared with the upstream segment; 6 years after dam removal began, it increased ~31% between the dams, whereas nonnative species richness and cover were not apparently affected by dams or their removal. Deposition caused by large volumes of released reservoir sediment had mixed effects on native species richness (increased on floodplains, decreased elsewhere) in the lowest river segment. Plant community composition was also different downstream from dams compared with the upstream reference, and has changed in downstream floodplains and bars since dam removal. In the long term, we expect that diversity will continue to increase in downstream river segments. Our results provide evidence that (1) large dams reduce downstream native plant diversity, (2) dam removal may restore it, and (3) given the natural dynamics of riparian vegetation, long-term, multiyear before-and-after monitoring is essential for understanding dam removal effects.
Brown RL
，Thomas CC
，Cubley ES
，Clausen AJ
，Shafroth PB
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《ECOLOGICAL APPLICATIONS》
Dams in the Cadillac Desert: downstream effects in a geomorphic context.
This paper was motivated by the 25th anniversary of the publication of Marc Reisner's book, Cadillac Desert: The American West and its Disappearing Water. Dams are ubiquitous on rivers in the United States, and large dams and storage reservoirs are the hallmark of western U.S. riverscapes. The effects of dams on downstream river ecosystems have attracted much attention and are encapsulated in the serial discontinuity concept (SDC). In the SDC, dams create abrupt shifts in continua of downstream changes in physical and biotic properties. In this paper, we develop a framework for understanding how channel geometry and network structure influence how the physical components of habitat and the biota rebound from discontinuities set up by large dams. We apply this framework to data describing the flow regime, temperature, sediment flux, and fish community composition below Garrison Dam on the Missouri River, Glen Canyon Dam on the Colorado River, and Flaming Gorge Dam on the Green River. Sediment flux in dam tailwaters is under strong control by channel geometry. By contrast, dam-related changes in temperature and flow variation are not significantly modulated by channel geometry or tributary inputs if flow volumes are small (Missouri and Colorado River tributaries). Instead, small tributaries provide near-native conditions (flow and temperature variation) and, as such, provide key refuges for biota from novel habitats in mainstem rivers below large dams. Unregulated tributaries that are large relative to their respective mainstem (e.g., Yampa River) provide refuges as well as significant amelioration of flow and temperature effects from upstream dams. Finally, the proportion of native fish increases with distance from dam and exhibits sharp increases near tributary junctions. These results suggest that tributaries-even minor ones in terms of relative discharge-act as key refugia for native species in regulated river networks. Moreover, large, unregulated tributaries are key to restoring continuity in physical habitat and the biota in large regulated rivers.
Sabo JL
，Bestgen K
，Graf W
，Sinha T
，Wohl EE
... -
《-》
Multiple stressors shape invertebrate assemblages and reduce their trophic niche: A case study in a regulated stream.
Few studies have addressed how the diversity of basal resources change with stream regulation and the potential consequences on river biota. We sampled invertebrates above and below a series of dams, over two years, at both downwelling and upwelling zones. In each zone, we recorded the daily temperature and flow variations, estimated the algal development, measured the available resources, and analysed carbon and nitrogen stable isotope compositions of the invertebrate community. The number of hydrological pulses were typically higher below the dams than above the dams especially during high-flow periods whereas the groundwater outlets had minor effects on invertebrate assemblages. Invertebrate abundance, richness and diversity tended to decrease below the dams. Co-inertia analysis showed that flow and temperature variations, and eutrophication explained most of the variance in the invertebrate assemblages, which comprised a higher number of resilient taxa below than above the dams. The proportions of pesticide-sensitive invertebrates were lower below the dams and ovoviviparous and more generalist taxa were prominent. We did not observe the expected CPOM decrease and FPOM increase downstream. Accordingly, the proportions of each functional feeding group were remarkably similar above and below the dams despite the long distance between the sectors (>100 kms). The diversity of basal resources used within assemblages progressively increased downstream above dams. In contrast, the diversity of resources used by organisms below the dams decreased from upstream to downstream suggesting a significant influence of flow regulation on aquatic food webs. Finally, the shorter trophic chains for the invertebrate assemblages below the dams suggests that the effects of stream regulation and eutrophication induced a simplification of food webs. To our knowledge, this study is the first to connect taxonomic and functional trait changes in response to multiple stressors with the associated modifications in isotopic niches within aquatic invertebrate assemblages.
Understanding how stream regulation and associated anthropogenic pressures act on aquatic assemblages and trophic niches is necessary to guide management actions.
We aimed to investigate the functional responses (traits and trophic niches) of aquatic invertebrate assemblages to stream regulation and eutrophication.
We used univariate and multivariate analyses to compare the invertebrate assemblages above and below the dams and to assess the contributions of hydrology (including groundwater supplies to the river), temperature and eutrophication to the variability in the composition of invertebrate assemblages. We also considered the relative utilization of a selected set of traits describing invertebrate resilience, resistance and specialization to address the potential functional effects of stream regulation on invertebrate assemblages. Finally, carbon and nitrogen isotope analyses allowed us to characterize the length and width of invertebrate assemblage food webs as related to the availability and diversity of basal resources.
Invertebrate abundance and richness generally decreased below the dams, with the highest impacts on insect taxa. Co-inertia analysis showed that stream regulation and eutrophication were main drivers of the aquatic invertebrate assemblages. The analysis separated the sites above and below the dams according to flow and temperature variation, whereas eutrophication appeared as a secondary stressor that separated the sites within each sector. Furthermore, the series of dams resulted in (i) a higher proportion of resilient (e.g., multivoltine) and resistant (ovoviviparous) taxa and a majority of generalists in assemblages below dams, (ii) an impact on the classical dynamics of CPOM (decrease) and FPOM (increase) sources from upstream to downstream, and (iii) a reduction in the diversity of resource use and in the trophic chain length of invertebrate assemblages below dams. The cooler and less oxygenated upwelling zones had lower invertebrate abundance; however, contrary to our expectation, the variation in the groundwater supply did not affect the composition of epigean invertebrate assemblages.
This study provides insights about the impacts of flow regime alteration and eutrophication on food webs that may have been caused by regulation of permanent streams. To our knowledge, this is the first to connect taxonomic and functional trait changes in response to multiple stressors with the associated modifications in energy fluxes in aquatic invertebrate assemblages. This study suggests that bed stability, which is associated with a reduction in channel mobility below the dams and with moderate eutrophication, may provide the shelter and resources that can locally favour invertebrate assemblage dynamics and lessen the effects of flow regulation. In addition, the study suggests that the biological trait-based approach and isotope analysis are complementary approaches for addressing ecosystem functioning. The relative utilization of traits indicates the functional potential of aquatic invertebrate assemblages to face multiple stressors whereas isotope analysis is an expression of the actual effect of the stressors on the trophic structure of aquatic invertebrate assemblages.
Dolédec S
，Simon L
，Blemus J
，Rigal A
，Robin J
，Mermillod-Blondin F
... -
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