PS 40-105 - Biogeochemical impacts of variable inundation on river sediments

Abstract: Currently, over half of the global river network undergoes wetting and drying cycles. Variable inundation in river systems has been shown to impact biogeochemical processes in the hyporheic zone, with past research finding an up to 66x increase in respiration upon rewetting dried sediments. Differences in respiration between rewetted and constantly inundated sediments, however, remains undetermined, as well as the factors influencing biogeochemical processing in wet and dry cycles. With projected increases in stream and river intermittency due to climate change, it is increasingly important to understand the biogeochemical impacts of these dynamic systems. To study the effects of variable inundation across the contiguous United States, we are using hyporheic zone sediments collected through crowdsourced sampling efforts. Sediments are either kept constantly inundated or allowed to air-dry over 21 days while being shaken to maintain aerobic conditions. After three weeks, aerated water is added to the sediments to remove headspace, and dissolved oxygen concentrations are measured every two minutes for two hours using a high-throughput method utilizing customized oxygen optodes. Results show that while some sites experience little difference in respiration between rewetted sediments and sediments kept inundated, others experience decreased respiration in sediments that were rewetted after drying compared to those kept constantly inundated. In comparison to previous work, which found an increase in respiration after rewetting dried sediments, this work shows that respiration in rewetted sediments may not be increased in comparison to constantly inundated sediments. Understanding the impacts of wetting and drying cycles on hyporheic zone respiration, along with factors governing the differences in response, provides necessary context for studying variably inundated systems.