Effects of the decommissioning of the Enobieta Dam (Navarre, North Iberian Peninsula) on stream ecosystem structure and functioning

130 p.Debido al envejecimiento de las presas y el mayor interés por restaurar los ecosistemas fluviales, el número de proyectos de desmantelamiento de presas ha aumentado en las últimas décadas. Aunque el objetivo del desmantelamiento de presas es mejorar el hábitat, la biodiversidad y el funcionamiento de los ecosistemas fluviales alargo plazo, también puede causar impactos ecológicos a corto plazo, principalmente debido a la movilización del sedimento acumulado en el embalse. En todo el mundo ya se han eliminado más de 2.000 presas, pero los impactos y beneficios de esta importante intervención han sido poco estudiados, especialmente en el caso de las grandes presas.Además, hay aún menos información sobre los efectos de la demolición de presas en el funcionamiento de ecosistemas que en otras variables. No obstante, los impactos son probables ya que la hidrología, la morfología del cauce y la biodiversidad están estrechamente vinculados a los procesos ecosistémicos. Esta tesis explora los efectos del desmantelamiento de Enobieta, una presa de gran tamaño localizada en el norte de la Península Ibérica, en la estructura y el funcionamiento de los ecosistemas fluviales. Para ello se aplicó un diseño experimental denominado multipe before-after/control-impact (mBACI), que permite controlar la variabilidad espacial y temporal. Antes de que comenzara el desmantelamiento de la presa, observamos la falta de sedimento aguas abajo de la presa. También se midieron altas concentraciones de manganeso y hierro, causadas por la hipoxia creada en el hipolimnion durante la estratificación. Sin embargo, estas concentraciones disminuyeron conforme aumentaba la distancia desde la presa,llegando en el punto más lejano a la presa a valores similares a los medidos en los tramos control. La concentración de amonio siguió un patrón similar al observado para los metales. Como cabía esperar, la presa causó grandes efectos sobre la estructura de las comunidades de macroinvertebrados, incluyendo su densidad, su riqueza taxonómica y su índice de diversidad de Shannon. Por el contrario, la concentración de clorofila y biomasa del biofilm, así como la captación de nutrientes y el metabolismo, pero no la descomposición de materia orgánica, fueron similares en los tramos control e impacto. El vaciado de la presa de Enobieta provocó la erosión y el transporte aguas abajo de los sedimentos acumulados en el embalse, especialmente cuando se retiró el azud que emergió en el vaso del embalse. Eltransporte de sedimentos resultó en picos de turbidez aguas abajo, principalmente durante los eventos de lluvia. Sin embargo, esos picos no fueron superiores a los observados en las cuencas cercanas. Durante la fase de vaciado, las concentraciones de manganeso, hierro y amonio aumentaron. Con excepción de la biomasa y la captación de nutrientes, el resto de las variables relacionadas con el biofilm disminuyeron, probablemente debido al aumento en el transporte de sedimento. Sorprendentemente, las diferencias observadas en las comunidades de macroinvertebrados disminuyeron durante la fase de vaciado. Un año después, todas las variables excepto la descomposición de la materia orgánica, fueron similares en los tramos control e impacto, lo que demuestra el éxito de este proyecto de restauración. Este éxito también se evidenció con la rápida recuperación de las comunidades acuáticas y el bosque en la zona anteriormente anegada por el embalse.En general, la presa de Enobieta constituía un punto negro en el valle de Artikutza, fragmentando el río y alterando su geomorfología, la calidad del agua y la estructura y el funcionamiento de las comunidades biológicas. Este estudio demuestra un efecto positivo del vaciado de la presa en la estructura y el funcionamiento del río sin apenas efectos negativos apreciables. No obstante, los impactos y beneficios del desmantelamiento de presas podrían depender en gran medida de las condiciones específicas de la zona afectada

Short-term effects of a large dam decommissioning on biofilm structure and functioning

Aging dams and the rising efforts to restore stream ecosystems are increasing the number of dam decommissioning programs. Although dam decommissioning aims at improving in-stream habitat, biodiversity, and ecosystem functioning in the long term, it might also cause ecological impacts in the short term due to the mobilization of the sediment accumulated in the reservoir. Benthic biofilm in particular can be impaired by episodes of high turbidity and scouring. We conducted a multiple before-after/control-impact experiment to assess the effects of the drawdown of a large dam (42 m tall), a first step to its decommissioning, on biofilm structure (biomass and chlorophyll-a) and functioning (metabolism, nutrient uptake, and organic matter breakdown). Our results show that the reservoir drawdown reduced the autotrophic biofilm biomass (chlorophyll-a) downstream from the dam, which in turn lowered metabolism. However, nitrogen and phosphorus uptake by the biofilm was not affected. Organic matter breakdown was slower below the dam than in nearby undammed reaches before and during drawdown. All drawdown effects quickly disappeared and reaches downstream from the dam approached values found in nearby undammed reaches. Thus, our results indicate that the effects of reservoir drawdown on stream biofilms exist but may be small and disappear rapidly.This research was supported by the Fundación BBVA (064-17). The authors also acknowledge the financial support from the Basque Government (Consolidated Research Group: Stream Ecology 7-CA-18/10) predoctoral fellowships from the University of the Basque Country UPV/EHU (M.A.). The authors are especially grateful to the municipality of Donostia-San Sebastian and the staff of Artikutza for their continuous support during the experiment

The drawdown phase of dam decommissioning is a hot moment of gaseous carbon emissions from a temperate reservoir

Dam decommissioning (DD) is a viable management option for thousands of ageing dams. Reservoirs are large carbon sinks, and reservoir drawdown results in important carbon dioxide (CO2) and methane (CH4) emissions. We studied the effects of DD on CO2 and CH4 fluxes from impounded water, exposed sediment, and lotic water before, during, and 3-10 months after drawdown of the Enobieta Reservoir, north Iberian Peninsula. During the study period, impounded water covered 0-100%, exposed sediment 0-96%, and lotic water 0-4% of the total reservoir area (0.14 km(2)). Areal CO2 fluxes in exposed sediment (mean [SE]: 295.65 [74.90] mmol m(-2) d(-1)) and lotic water (188.11 [86.09] mmol m(-2) d(-1)) decreased over time but remained higher than in impounded water (-36.65 [83.40] mmol m(-2) d(-1)). Areal CH4 fluxes did not change over time and were noteworthy only in impounded water (1.82 [1.11] mmol m(-2) d(-1)). Total ecosystem carbon (CO2 + CH4) fluxes (kg CO2-eq d(-1)) were higher during and after than before reservoir drawdown because of higher CO2 fluxes from exposed sediment. The reservoir was a net sink of carbon before reservoir drawdown and became an important emitter of carbon during the first 10 months after reservoir drawdown. Future studies should examine mid- and long-term effects of DD on carbon fluxes, identify the drivers of areal CO2 fluxes from exposed sediment, and incorporate DD in the carbon footprint of reservoirs.This study was funded by the projects Alteration of carbon sinks and sources in shrinking inland waters (Alter-C), the Spanish Ministry of Science, Innovation and Universities (refs: PID2020-114024GB-C31 funded by MCIN/AEI/10.13039/501100011033/) and Effects of the drawdown of Enobieta Reservoir (Navarre) on the biodiversity and functioning of river ecosystems (DESEMBALSE), Foundation BBVA (ref: PI064-17). AM was supported by an FI grant from the Agencia de Gestio d'Ajuts Universitaris i de Recerca (AGAUR) of the Generalitat de Catalunya. DvS and BO acknowledge support through the Consolidated Research Group 2017SGR0976. RM acknowledges support by the Generalitat de Catalunya through the Consolidated Research Group 2017SGR1124, and by the CERCA program. AE and MA support of the Basque Government through the Consolidated Research Group IT951-16. AM got a predoctoral grant by the University of the Basque Country (UPV/EHU). DvS is a Serra Hunter Fellow

Short-term effects of a large dam decommissioning on biofilm structure and functioning

Aging dams and the rising efforts to restore stream ecosystems are increasing the number of dam decommissioning programs. Although dam decommissioning aims at improving in-stream habitat, biodiversity, and ecosystem functioning in the long term, it might also cause ecological impacts in the short term due to the mobilization of the sediment accumulated in the reservoir. Benthic biofilm in particular can be impaired by episodes of high turbidity and scouring. We conducted a multiple before-after/control-impact experiment to assess the effects of the drawdown of a large dam (42 m tall), a first step to its decommissioning, on biofilm structure (biomass and chlorophyll-a) and functioning (metabolism, nutrient uptake, and organic matter breakdown). Our results show that the reservoir drawdown reduced the autotrophic biofilm biomass (chlorophyll-a) downstream from the dam, which in turn lowered metabolism. However, nitrogen and phosphorus uptake by the biofilm was not affected. Organic matter breakdown was slower below the dam than in nearby undammed reaches before and during drawdown. All drawdown effects quickly disappeared and reaches downstream from the dam approached values found in nearby undammed reaches. Thus, our results indicate that the effects of reservoir drawdown on stream biofilms exist but may be small and disappear rapidly

Impacts of diffuse urban stressors on stream benthic communities and ecosystem functioning: A review

Catchment urbanisation results in urban streams being exposed to a multitude of stressors. Notably, stressors originating from diffuse sources have received less attention than stressors originating from point sources. Here, advances related to diffuse urban stressors and their consequences for stream benthic communities are summarised by reviewing 92 articles. Based on the search criteria, the number of articles dealing with diffuse urban stressors in streams has been increasing, and most of them focused on North America, Europe, and China. Land use was the most common measure used to characterize diffuse stressor sources in urban streams (70.7 % of the articles characterised land use), and chemical stressors (inorganic nutrients, xenobiotics, metals, and water properties, including pH and conductivity) were more frequently reported than physical or biological stressors. A total of 53.3 % of the articles addressed the impact of urban stressors on macroinvertebrates, while 35.9 % focused on bacteria, 9.8 % on fungi, and 8.7 % on algae. Regarding ecosystem functions, almost half of the articles (43.5 %) addressed changes in community dynamics, 40.3 % addressed organic matter decomposition, and 33.9 % addressed nutrient cycling. When comparing urban and non-urban streams, the reviewed studies suggest that urbanisation negatively impacts the diversity of benthic organisms, leading to shifts in community composition. These changes imply functional degradation of streams. The results of the present review summarise the knowledge gained to date and identify its main gaps to help improve our understanding of urban streams.This study has received funding from the Iberian Association of Limnology (AIL) through the project URBIFUN (Urbanization effects on the relationship between microbial biodiversity and ecosystem functioning), awarded to Míriam Colls and Ferran Romero. Authors thank as well the Basque Government (Consolidated Research Group IT951-16) and the MERLIN project 101036337 – H2020-LC-GD-2020/H2020-LC-GD-2020-3.info:eu-repo/semantics/publishedVersio