Biogeochemical response of the coupled river-hyporheic zone system to organic matter inputs and unexpected sensitivity to temperature

Abstract:

Rivers play an important role in nutrient and carbon cycles. Many biogeochemical reactions responsible for nutrient uptake and carbon cycling are occurring in the sediment at the groundwater-surface water mixing interface known as the hyporheic zone (HZ). However, estimates of the contribution from the HZ to whole ecosystem carbon cycling and respiration vary drastically, demonstrating that the mechanisms of carbon cycling in rivers are not fully understood. Additionally, given dynamic environmental conditions that cause fluctuations in nutrient and carbon content, various flow regimes and temperature, investigations of the HZ separately from the river domain limit mechanistic understanding of river corridor function as a whole. Using a flume, we investigate the effects of addition of organic matter to the flume channel on aerobic respiration across the surface channel and the HZ domains under changing environmental temperature conditions. Measurements were focused on the spatial and temporal distribution of CO₂ and oxygen through a combination of continuous water quality monitoring with in-stream sondes, two-dimensional dissolved oxygen planar optode that imaged the hyporheic zone, and pore-water sampling. Our observations indicate distinct CO₂ and oxygen patterns in the surface channel and in the HZ in response to the addition of various amounts of organic matter. We observed an immediate and linear response in channel CO₂ and oxygen levels from the addition of organic matter, and a more complex non-linear response in the HZ. Our observations suggest that temperature is an important control on oxygen fluctuations in the HZ. Oxygen in the HZ is very sensitive to environmental temperature under low organic matter additions and less sensitive to temperature conditions with higher organic matter additions. These observations suggest that to understand carbon cycling in river corridors, particularly through experiments, an integrated approach is necessary where physical, chemical, and biological interactions between the channel and the HZ are fully coupled.