Amazonian run-of-river dam reservoir impacts underestimated: Evidence from a Before-After Control-Impact study of freshwater turtle nesting areas

Abstract

1. Although the construction of hydropower dams is widely assumed to generate myriad negative impacts on biodiversity there remains a lack of robust scientific evidence documenting the magnitude of these impacts particularly across highly biodiverse tropical waterways. Hydropower expansion is an increasing threat to the Endangered yellow-spotted river turtle (Podocnemis unifilis) across its tropical South American range. 2. Turtle nesting-areas were monitored as an indicator of dry-season river level changes following run-of-river dam reservoir filling. A Before-After-Control-Impact (BACI) study design was established with multi-year field campaigns monitoring turtle nesting-areas along 66 km of river upstream of the dam, separated into 33 km control and impact zones. 3. The cause and extent of changes in nesting-areas was established using Generalized Additive Models. Nesting-area density was examined in relation to four variables: Before-After, Control-Impact, distance to the dam and precipitation. The extent of changes was examined by comparing the proportion of nesting-areas remaining in subzones during the four years after reservoir filling. 4. Comparison of the proportion of nesting-areas remaining after dam construction showed that a permanent dry season river level rise extended more than 20 km beyond impact assessment limits. On average the density of nesting-areas declined 69% (from 0.48 to 0.15 per km) across 33 km of river directly impacted by the dam. This loss was reflected in a significant BACI interaction. The variation in nesting-areas was not explained by seasonal precipitation, whilst nesting-area density increased significantly further away from the dam. 5. Standardized monitoring of freshwater turtle nesting-areas provided an effective means to quantify impacts of hydropower developments across biodiverse yet rapidly changing waterways. The negative impacts should be preventable by mitigation actions including habitat restoration and dry season flow regulation. Such measures are also likely to benefit multiple species in highly diverse Amazonian rivers increasingly impacted by run-of-river dams.

Impacts of Dams on Freshwater Turtles: A Global Review to Identify Conservation Solutions

“…Nests of turtle species that use terrestrial nesting habitats (e.g., river banks) may be at greater risk from flooding (Bodie, 2001), as is the case of species of the South American genus Podocnemis (Eisemberg et al, 2016; Gallego-García & Castaño-Mora, 2008; Norris et al, 2018a; Norris et al, 2020), genus Elseya in Australia (Espinoza et al, 2022), and of the North American Emydidae and Trionychidae (Pitt et al, 2021; Tornabene et al, 2018). Changes to a river's annual discharge cycle caused by dams can therefore reduce the availability of nesting areas (Bárcenas-García et al, 2022; Norris et al, 2018a; Tornabene et al, 2018), as well as the duration of low water levels, which may affect the behavior and nesting success of these species (Eisemberg et al, 2016; Espinoza et al, 2022; Tornabene et al, 2018).…”

“…In Brazil, the filling of a hydropower dam reservoir resulted in the flooding and loss of 3.9 hectares of nesting habitats and areas used by the yellow-spotted river turtle Podocnemis unifilis (Norris et al, 2018a). Besides Norris et al (2018a) and (Bárcenas-García et al, 2022) who applied a before-after control-impact study design, no other study evaluated freshwater turtle nesting patterns with baseline monitoring previous to dam installation.…”

“…In addition, to mitigate dam impacts and prevent the loss of species and ecological functions, environmental authorities must conduct more robust and rigorous Environmental Impact Assessments (Bárcenas-García et al, 2022; Norris et al, 2018a), as well as provide support to supervise compliance with mitigation actions and monitoring effectiveness (Guo et al, 2021; Valiente-Banuet et al, 2015).…”

Impacts of dams on freshwater turtles: a global review to identify conservation solutions

“…river banks) may be at greater risk from flooding (Bodie, 2001), as is the case of species of the South American genus Podocnemis (Eisemberg et al, 2016; Gallego-García & Castaño-Mora, 2008; Norris et al, 2018a; Norris et al, 2020), genus Elseya in Austrailia (Espinoza et al, 2022), and of the North American Emydidae and Trionychidae (Pitt et al, 2021; Tornabene et al, 2018). Changes to a river’s annual discharge cycle caused by dams can therefore reduce the availability of nesting areas (Bárcenas-García et al, 2022; Norris et al, 2018a; Tornabene et al, 2018), as well as the duration of low water levels, which may affect the behavior and nesting success of these species (Eisemberg et al, 2016; Espinoza et al, 2022; Tornabene et al, 2018).…”

“…In Brazil, the filling of a hydropower dam reservoir resulted in the flooding and loss of 3.9 hectares of nesting habitats and areas used by the yellow-spotted river turtle Podocnemis unifilis (Norris et al, 2018a). Besides Norris et al (2018a) and (Bárcenas-García et al, 2022) who applied before-after control-impact study design, no other study evaluated freshwater turtle nesting patterns with baseline monitoring previous to dam installation.…”