Drivers of Ecosystem Respiration in the Middle Rio Grande Floodplain Aquifer

Little is known about carbon flux and ecosystem metabolism in river floodplains, despite their potentially large and dynamic contributions to carbon storage and loss. We hypothesized that the region of intermittent saturation (ROIS), which is the subsurface interface between terrestrial and aquatic ecosystems, is a biogeochemical ‘hotspot’ for ecosystem respiration and carbon transformation. Groundwater depth and variability in the ROIS facilitate interactions between aquatic and terrestrial processes, but their role in carbon fluxes is poorly understood. We developed a novel method utilizing high-frequency sensors in groundwater wells to capture biogeochemical ‘hot moments’ of ecosystem respiration in the ROIS of the riparian floodplain of the Rio Grande in New Mexico. We deployed four EXO1 sondes in monitoring wells across two floodplain sites for one year to capture biogeochemical activity and groundwater variability in the ROIS. Sensors recorded temperature, conductivity, turbidity, fluorescent dissolved organic matter, dissolved oxygen, and groundwater depth in 15-minute intervals. We observed patterns of ecosystem respiration, quantified the size of each respiration event, and estimated annual rates of carbon loss from the ROIS. We found that higher variation of groundwater depth in the two days preceding a respiration event significantly correlated with larger respiration events across wells and sites. Our estimates suggest that floodplains respire large amounts of carbon in the ROIS, possibly exceeding rates typically observed in rivers and terrestrial ecosystems. Our work demonstrates the effectiveness of collecting continuous in situ data from the ROIS and suggests that groundwater variation is a key driver of floodplain ecosystem respiration.