Transit times and governing processes in high-mountain wetlands

High-mountain wetlands have the potential to store and release large volumes of water, providing crucial supplies to highland communities and receiving lowlands, especially in seasonally dry climates due to their flow regulation capacity and extended residence times. Despite their significance to mountain hydrology and water resources, gaps remain in understanding the connection between wetlands and streams. This study uses a combination of fluorescent tracing and high frequency monitoring of rainfall-runoff and wetland water levels to assess the movement and timing of flow through wetlands. The experiments were conducted during the wet or dry season in 5 representative study catchments (0.38 km² – 12.58 km²) with varying wetland coverage, from Northern Ecuador to Southern Peru. Fluorescein was introduced into wetlands and monitored downstream with activated carbon samplers for 5-12 months. Our results suggest transit times from less than 1 week to upwards of 3 ½ months, with one experiment seeing little to no response. Results indicate that wetlands are likely far more hydrologically connected to streams in the wet season than in the dry season, where in some cases they may not be connected at all. Several peaks in fluorescein concentration during the wet season may suggest that the wetlands contribute to streamflow via multiple pathways. The potential lack of fluorescein response at one site could indicate a very high transit time or that the wetlands did not feed the stream at the monitored locations during the monitoring period. The results demonstrate a complex connection between wetlands and streams depending on location and season, amongst other factors. However, persistent contributions from wetlands to streams observed several months after dye introduction support their significance to downstream, year-round water supply. We discuss potential hypotheses for divergent wetland behaviours and provide a baseline for further investigation.