Status and drivers of dissolved oxygen in regulated rivers

Sammanfattning

Oxygen is critical for life and plays a pivotal role in ecosystem functioning supporting both respiration and biogeochemical cycles. In natural aquatic environments its availability is primarily regulated by photosynthesis and aeration. Water flow and its natural regime influences community composition both directly and indirectly. While Hydropower provides a renewable energy source, water regulation poses a significant threat to the ecological integrity of regulated rivers. In response European Water Framework Directive (WDF) emphasizes the importance of conserving and restoring the ecological potential of heavily modified water bodies (HMWBs). In this study I investigate the temporal and spatial variation of Dissolved oxygen (DO) at an hourly resolution in three regulated rivers (Luleälven, Umeälven, Ljungan) and two free-flowing rivers (Vindelälven, Piteälven) in central and northern Sweden over summer months using Generalized Additive Model (GAM). I found significantly lower levels of mean DO in regulated rivers with a higher variability, and occurrence of severe hypoxia (< 3mg/l) and anoxia (0mg/l) at times. Furthermore, DO showed strong correlation with time of the day, hourly discharge, and duration of zero flow in regulated river. Thus, I emphasize the need for a high spatial and temporal monitoring of DO in regulated rivers to enable accurate and early detection of harmful hypoxic events in HMWBs, Additionally, I argue the importance of including ecological limits and requirements in hydropeaking schemes, along with the strict enforcement of environmental flows (e-Flow) improving sustainability of hydroelectric energy production.