DOI: 10.5061/dryad.w3r2280ws
title: Slow drawdown, fast recovery: Stream macroinvertebrate communities improve quickly after large dam decommissioning

description: Dam removal is increasingly considered as a river restoration tool for
 impoundments that harm the environment or have exceeded their lifespan.
 However, few studies report the ecological consequences of large dam
 removal. We performed a multiple Before-After/Control-Impact
 (mBACI) study to investigate the consequences of the decommissioning of a
 large dam (42 m high) on instream habitat and invertebrate communities in
 a temperate, forested catchment of northern Spain. Before
 decommissioning, a lack of fine sediments and high concentrations of
 manganese and iron occurred below the dam but decreased downstream.
 Invertebrate taxa richness and diversity were reduced, and
 pollution-sensitive taxa were missing just below the dam. The
 drawdown of the reservoir, the first step towards its decommissioning,
 mobilized stored sediments causing frequent turbidity peaks downstream,
 which nevertheless, caused no detrimental effects on macroinvertebrate
 communities. One year after the drawdown, the communities downstream from
 the dam, as well as those in the newly formed stream in the area formerly
 impounded by the reservoir, became very similar to those in control
 reaches, showing a successful restoration project. Synthesis and
 applications: Dam decommissioning helps restore instream habitats and
 facilitates the recovery of invertebrate communities in a very short time
 frame if there are nearby sources of potential colonizers. Slow drawdown
 reduces the transport of the sediments accumulated in the reservoir and
 their potential downstream impacts, even more if before drawdown the
 reservoir is kept full for years to promote the retention of sediments in
 marginal areas that will later be readily colonized by trees.
description: <em>Experimental design:</em> Our study followed a
 multiple before-after / control-impact (mBACI) design (Underwood, 1994).
 We defined four control sites, one (C1) upstream from the dam and three
 (C2 to C4) in free-flowing tributaries, as well as four impact sites (I1
 to I4) at increasing distances downstream from the dam. All eight reaches
 were sampled before (before the period), during (drawdown period), and
 after (after the period) the drawdown. Additionally, three sites located
 within the reservoir (R1 to R3) were also sampled in the after period
 (Fig. 1). These sites could not be sampled either before the period when
 the reservoir was full or during the drawdown, when safe wading in the
 newly carved stream channel was still impossible.
 <em>Habitat characteristics:</em> We characterized
 benthic substrate composition following Wolman (1954), with 100 substrate
 particles collected per site and period while zig-zagging along 100 m-long
 reaches. <em>Benthic invertebrates:</em>
 Invertebrates were sampled in November 2017 (before), October 2019
 (drawdown), and November 2020 (after), one year after the end of the
 drawdown. On each occasion, we took 5 random samples with a Surber net
 (0.09 m<sup>2</sup>, 500 µm-mesh) and preserved them in 70%
 ethanol. Invertebrates were sorted in the laboratory, counted, and
 identified under a binocular microscope (Tachet et al., 2010). Most
 invertebrates were identified to genus level (74.6% of the taxa), but some
 Amphipoda, Coleoptera, Ephemeroptera, Trichoptera, Mollusca, and Diptera
 were only identified to family level (13.6%). Acari and Oligochaeta were
 left at these taxonomic levels (Table S4). We estimated taxa richness (S),
 Shannon-Wiener diversity index (H´), and total density (TD, individuals
 m<sup>-2</sup>) for each sample. We also estimated the IASPT
 (Iberian Average Score Per Taxon) index, which is widely used in Spanish
 biomonitoring programs to represent the average sensitivity to pollution
 of the taxa found (Guareschi et al., 2017). IASPT is calculated as the
 IBMWP value (Alba-Tercedor, 2002) divided by the number of scoring
 families present.
description: # Slow drawdown, fast recovery: Stream macroinvertebrate communities
 improve quickly after large dam decommissioning
 [https://doi.org/10.5061/dryad.w3r2280ws](https://doi.org/10.5061/dryad.w3r2280ws) ## Description of the data and file structure We followed a before-after/control-impact design to investigate the consequences of the decommissioning of a large dam (Enobieta Reservoir, North Spain) on invertebrate communities. We sampled substrate grain size and invertebrate communities before (Before, B), during (Drawdown, D), and after (After, A) the drawdown of the reservoir at 4 sites located below the dam (Impact reaches, I) and in 4 nearby undammed reaches (Control reaches, C). Additionally, three sites located within the reservoir (Reservoir reaches, R) were also sampled in the after period. On each sampling date and site, we characterized benthic substrate composition following Wolman (1954) with 100 substrate particles collected per site and period. Substrate class assessment was based on the Wentworth scale. Also, we took 5 random samples with a Surber net (0.09 m2, 500 µm-mesh) and preserved them in 70% ethanol. In the laboratory, invertebrates were identified under a binocular microscope (Tachet et al., 2010). The dataset provided comprises two sheets entitled “Grain Sizes” and “Invertebrates”, respectively. The Grain Size sheet is organized into 5 columns. The site indicates the location where the data was collected, while the CI column refers to the category to which each study site belongs, either control (C) if the study site is located in reaches without dam effects, impact (I) in sites located below the dam, or reservoir sites (R). The latter refers to the study sites located in the newly created stream channel within the reservoir area. The reach column gives more detailed information on each study site, indicating, by means of I1 to I4, the position in which the study sites are located with respect to the dam, with I1 being the site closest to the dam and I4 the furthest from the dam. Reservoir sites are also categorized from R1 to R3, with R1 being the study site that first emerged during the reservoir drawdown and R3 the last one. The period column categorizes the time during which the data was collected within the restoration project, referring to the pre-drawdown period (B), the drawdown period (D), and the after-drawdown period (A). The last column, named Substrate sizes (mm), contains granulometric data collected from the study during each sampling period). Similar to the Grain Sizes sheet, the Invertebrates sheet provides contextual information about each data entry's location and timing. It also contains a detailed taxonomic list of invertebrates identified within the study sites and represents abundance data for each taxon within a given sample. Also, invertebrate community descriptors, such as Species richness (S), Shannon diversity index (H´), total density (TD, m-2), and IASPT biomonitoring index are provided per sampling site and period.