Effects of dam decommissioning on carbon cycling in reservoirs. Case study of the Enobieta Reservoir, N Iberian Peninsula

Abstract

[eng] The overall objective of this thesis is to explore the effects of DD on C cycling in reservoirs because there are many dams under removal and many more predicted in the near future. To answer this question, we conducted this study in the Enobieta Reservoir (Navarre, N Iberian Peninsula) that underwent drawdown between 2017 and 2019. The central objectives are addressed in the main three chapters (chapters 3, 4, and 5), each corresponding to a separate article publication that addresses several specific objectives.

Overall question: This thesis will answer the overarching question of how DD affects C dynamics in reservoirs by altering water depth, impoundment status, and vegetation cover through terrestrialization.

Chapter 3: This chapter aimed to assess the effects of the drawdown phase of DD on CO2 and CH4 emissions in the Enobieta Reservoir. The specific objectives of this chapter were (1) to compare CO2 and CH4 emissions among impounded water, exposed sediment, and lotic water, (2) to examine the influence of water depth on CO2 and CH4 emissions in reservoirs, (3) to constrain spatial and temporal trends of CO2 and CH4 emissions in exposed sediment after reservoir drawdown, and (4) to assess how reservoir drawdown affects CO2 and CH4 emissions in the river reach downstream of reservoirs.

Hypotheses for chapter 3: We expected that (1) CO2 emissions will be highest in lotic water, followed by exposed sediment, and lowest in impounded water, while CH4 emissions will be highest in impounded water due to higher anoxia, followed by lotic water, and lowest in exposed sediment, (2) shallow water will exhibit higher CO2 and CH4 emissions than deep impounded water, (3) CO2 and CH4 emissions in exposed sediment will vary over space and time due to spatial and temporal changes in the content and reactivity of sediment OM, and (4) reservoir drawdown will increase CO2 and CH4 emissions in the river reach downstream of the reservoir due to enhanced transport of OM and those gases downstream from the reservoir.

Chapter 4: This chapter sought to estimate the content and reactivity of sediment OM in the study reservoir. The specific objectives of this chapter were (1) to understand the temporal changes in the content and reactivity of sediment OM in exposed sediments, (2) to compare the decomposition efficiency of bulk sediment OM and sediment water-extractable organic matter (WEOM), and (3) to compare the content and reactivity of sediment OM in reservoir sediments with the content and reactivity of sediment OM in other inland waters and soils.

Hypotheses for chapter 4: We predicted that (1) the content and reactivity of sediment OM will change with sediment exposure time due to the consumptive loss of sediment OM, (2) the decomposition efficiency will be lower for bulk sediment OM compared to WEOM because WEOM is the most bioavailable fraction of OM, and (3) the content and reactivity

of sediment OM in the Enobieta Reservoir and other lentic inland waters will be comparable, but higher than in lotic inland waters.

Chapter 5: This chapter aimed to understand how vegetation biomass affects CO2 fluxes in the exposed sediment of the study reservoir. The specific objectives were to (1) assess spatial and temporal dynamics of terrestrialization of exposed sediment after reservoir drawdown, (2) compare CO2 fluxes between bare sediment and sediment with vegetation, and (3) assess how the incorporation of C into vegetation biomass affects C fluxes in exposed sediment.

Hypotheses for chapter 5: We expected that (1) stable (e.g., flat) and old (i.e., with long sediment exposure time) environments will exhibit greater terrestrialization than steep and young environments, (2) CO2 fluxes from sediment with vegetation will be higher than from bare sediment due to the supply of labile OM by the growing vegetation and its contribution to respiration, and (3) the incorporation of C into vegetation biomass will reduce C fluxes in exposed sediment.