2024 Abstracts

Drying and Rewetting of Riverbed Sediments Leads to Biogeochemical Cold Moments and Shifts Dissolved Organic Matter Thermodynamics

Authors

James Stegen* (james.stegen@pnnl.gov), Vanessa Garayburu-Caruso, Kenton Rod, Maggi Laan, Dillman Delgado, Lupita Renteria, Sophia McKever, Amy Goldman, Brieanne Forbes

Institutions

Abstract

Currently, over 60% of the global river network undergoes annual wetting and drying cycles. Variable inundation has been shown to impact biogeochemical processes and dissolved organic matter (DOM) composition in riverbed sediments. Differences in respiration between rewetted and constantly inundated sediments, however, remains undetermined, as well as the factors influencing biogeochemical mechanisms and organic matter transformations in wet and dry cycles. With projected increases in stream and river intermittency due to climate change, it is increasingly important to understand effects of drying on key biogeochemical processes like sediment respiration and DOM composition across diverse river ecosystems. To do this, the project studies the effects of variable inundation across 56 sites in the contiguous United States. Researchers conducted laboratory manipulative experiments with sediments from each site where they were either kept constantly inundated or allowed to air-dry over 21 days while being shaken to maintain aerobic conditions. Both treatments were then fully inundated with saturated riverine water and aerobic respiration rates were measured noninvasively using customized oxygen optodes. Samples were collected for ultra-high resolution mass spectrometry analysis post respiration measurements. The team investigated the effect of drying on DOM chemistry (e.g., DOM properties, elemental composition, and chemical classes) and thermodynamics (e.g., Gibbs free energy and lambda) and the effects of these changes on respiration rates. Results show that while some sites experience little difference in respiration between rewetted and inundated sediments, others experience decreased respiration, or cold moments, in rewetted sediments compared to those kept constantly inundated. Additionally, drying sediments resulted in a broad range of influences on DOM thermodynamics, respiration rates, and the link between them.

Understanding the impacts of wetting and drying cycles on sediment respiration and how drying affects DOM chemistry and thermodynamics is essential to generate mechanistic inferences regarding the effects of stream intermittency in global biogeochemical cycles.