Influence of Drying on Riverine Sediment Biogeochemistry Across the Contiguous United States

Authors

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

Institutions

Pacific Northwest National Laboratory, Richland, WA

Abstract

Global change is altering where and when there is enough water for streams to flow. This leads to changes in where and when riverine sediments are inundated with an overlying water column, and it affects the associated wet and dry dynamics those sediments experience. Previous work has shown that these changes in inundation and wet and dry dynamics have strong influences over biogeochemical rates, organic matter chemistry, and organismal ecology. For example, a global study of riverbed sediments showed that respiration rates increased up to 66-fold upon rewetting. There is a knowledge gap regarding how respiration rates in rewetted sediments compare to sediments that are consistently inundated, and also regarding what factors govern the effect size of drying on respiration.

To help address this gap, researchers are conducting a manipulative laboratory experiment using sediments from the shallow hyporheic zone (~5 cm into the riverbed). The sediments are being crowdsourced from across the contiguous U.S. to span a broad range of environmental conditions. The experiment has two treatments: (1) sediments are allowed to air dry; and (2) sediments are kept inundated. In both cases sediments are constantly shaken to encourage aerobic conditions. After three weeks of these conditions, aerated water is added to both treatments to remove all headspace and oxygen (O) consumption is measured over two hours using custom built O optodes that provide data every two minutes. Results show that in some sites drying and rewetting has almost no influence over respiration rates. Yet in other sites, drying and rewetting leads to dramatically lower respiration rates in air dried sediments compared to rates in consistently inundated sediments. These initial outcomes complement previous work showing that while respiration rates following rewetting may be elevated compared to dry conditions, rates following rewetting may not be elevated compared to rates in consistently inundated sediments. Combining insights from previous work and the current experiment can, therefore, provide an increasingly holistic understanding of the biogeochemical impacts of changes in where and when streams flow.