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Stream carbon movement in a changing climate

One of the most defining characteristics of the Pacific Coastal Temperate Rainforest is the flow of water. From land to sea, stream systems facilitate a massive transfer of organic matter into the ocean. As water moves through plants and soil, it carries dissolved organic carbon, often from decomposing plant and soil materials it comes in contact with. The transport of materials in streams has tremendous impacts for this ecoregion as well as the global carbon budget: acre for acre, the watersheds of the Gulf of Alaska transport 36 times as much dissolved organic carbon as the world average.

This process is an important source of nutrients in coastal oceans, where marine organisms from microbes to whales rely on dissolved organic carbon as the base of a complex food web. With support from the ACRC, graduate student Megan Behnke (Florida State University) is studying how water biogeochemistry changes as it travels through non-glaciated watersheds in southeast Alaska. The chemical processes that alter dissolved organic carbon during its flow to the ocean are little understood. By determining the chemical “fingerprints” that aspects of terrestrial ecosystems (such as different types of trees, or different soil types) impart on rain and groundwater, we can better understand what factors have the greatest influence on water chemistry.

The geography of southeast Alaska contains a patchwork of peatlands, forested wetlands, and forested uplands. By conducting repeated sampling in these ecosystems, Behnke can determine how landscape elements affect the biogeochemistry of the watershed. Using an ultra high-resolution mass spectrometer, Behnke will identify the chemical compounds in samples and match the complex chemical fingerprints to their origins. In addition to this focus, Behnke also studies how microbes eat away at organic matter over the course of stream flow. While distinct chemical fingerprints may be apparent at the stream source, microbial activity could simplify water chemistry into one unified signature as it travels to the ocean.

This research will address other unknowns, such as measuring the contribution of organic matter from trees carried by rainfall into the soil, and the rate of movement of old carbon stored in peatlands back into the ecosystem. Behnke will also examine trends related to high water flow following storm events, and low water flow at dry intervals, as well as how seasonality affects the movement of organic material.

The transport of organic matter through coastal watersheds has implications for ecosystems and communities, as well as billion-dollar fishing and tourism industries in the PCTR. In a warming climate, retreating glaciers will initiate a number of changes to southeast Alaska landscapes. As a result of deglaciation in watersheds, a greater portion of the landscape will be dominated by forest. This research will inform our understanding of the mechanisms of dissolved organic carbon transport in a changing climate, and the bioavailability of these materials to marine life.


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