Are streams pipes or processors of organic carbon from their watersheds? Reflections from an LTER fellow

Each year the KBS LTER program awards two graduate students with summer research fellowships. Here Sydney Ruhala describes the research her 2016 summer fellowship supported. Sydney is an M.S. student in Jay Zarnetske’s lab at Michigan State University. She recently was awarded best poster presentation in basic research at the 2016 Society for Freshwater Science meeting in Sacramento, CA – congrats, Sydney!

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As a graduate student in the Department of Earth and Environmental Sciences at Michigan State University, I have spent my summers wading through the rivers of southwestern Michigan in order to study dissolved organic carbon dynamics at the stream-groundwater interface. My research focuses on understanding if these streams are pipes or processors of organic carbon from their watersheds.

Dissolved organic carbon (DOC) is a master water quality parameter in aquatic ecosystems. It fuels stream metabolism, regulates biogeochemical cycles, complexes with trace metals, and produces carbon dioxide (CO₂) when mineralized. Over the last several decades, stream DOC concentrations have been increasing across the Northern Hemisphere. Despite its critical roles and increasing concentrations, little is known about DOC processing in streams. My goal is to address the following questions: 1) where does DOC processing occur within rivers? 2) What are the dominant biogeochemical transformation processes? And 3) how do those processes affect the fate and function of DOC that is transported downstream.

The hyporheic zone (HZ) is the zone in the stream bed where groundwater interacts with the stream water. This area may be a significant processor of stream DOC due to its rich biological, chemical, and hydrological diversity. To assess the role of the HZ on DOC processing, I am using optical techniques, such as absorbance and fluorescence spectroscopy, as a diagnostic tool to quantify how DOC quantity (that is, DOC concentration) and quality (that is, the chemical structure of DOC such as molecular structure and molecular weight) change across the stream-groundwater interface. It is crucial to understand the impact that DOC processing in the HZ has on DOC quantity and quality because those properties may control the bioavailability of DOC transported to downstream ecosystems.

Last summer, my co-advisor Dr. Jay Zarnetske, fellow lab members, and I sampled the watershed of Augusta Creek in southwestern Michigan, which includes the KBS LTER. We collected hyporheic porewater samples, along with surface and groundwater samples, at 16 sites varying in stream size and land cover throughout the Augusta Creek watershed. We sampled water within the stream bed sediments using a unique sampler, called a MINIPOINT sampler that draws water up through narrow stainless steel tubes that extend to various depths and can be inserted with minimal disturbance.

My results indicate that the HZ is a significant processor of DOC when compared to surface water and groundwater and suggest that microbial activity drives these transformations. Detailed results from the August 2015 sampling were presented at the 2016 Society for Freshwater Science Annual Meeting in Sacramento, California.

This summer, with the support of a 2016 KBS LTER summer research fellowship, I am able to conduct a second sampling campaign in Augusta Creek to further assess DOC dynamics at the stream-groundwater interface. Specifically, I am interested in observing temporal patterns in DOC quantity and quality within the HZ. For example, are the observed trends consistent between years at an individual site? What about across a stream order or across the entire watershed? Results from this sampling event will be presented at the 2017 Society for Freshwater Science Annual Meeting in Raleigh, North Carolina.