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River food webs and salmon productivity in Southeast Alaska

a glacial and clear water river convergeRivers and streams in Southeast Alaska provide essential rearing habitat for some of the most productive salmon fisheries on the planet. Within these rivers salmon rely on a complex and interconnected tapestry of feeding relationships known as a food webs for growth and survival. The diversity of the Southeast Alaska landscape allows vastly different river systems, and potentially very different food webs, to coexist within a relatively small area. A single watershed can contain a mosaic of silt-heavy glacial rivers, snowmelt creeks, and wetland-dominated rainfall streams that have distinct temperature, hydrology, and nutrient characteristics. Because each link in the food web is influenced by the distinct characteristics of these stream types, they likely provide a template for unique food webs to exist across the landscape. For instance, food resources could peak at different times in glacier, snowmelt, and rainfall streams, creating a more stable food base throughout the year for juvenile salmon that move in search of more abundant food sources before migrating out to the ocean.

 
Variation in watershed characteristics may drive distinct food web structures across the landscape. The way energy makes it from biofilms and aquatic invertebrates to juvenile salmon may be different depending on where within the watershed they are rearing. Illustration by Matt Dunkle.

In a warming climate, one of the unforeseen consequences of diminished glacier and snowmelt contribution to watersheds could be the loss of unique food webs that provide important feeding opportunities for rearing salmon. As glaciers recede and disappear from some watersheds and precipitation shifts from snow to rain, the physical and chemical characteristics that make a glacial or snowmelt stream different from a rainwater stream may fade. Watershed deglaciation and snow-to-rain shifts could simplify the hydrology of Southeast Alaska, driving the loss of the unique food webs that each stream type supports. As food webs change, distinct cycles of food availability may be lost, resulting in watersheds that no longer maintain a stable food base for fish populations year round. To understand how important landscape diversity is to sustaining salmon productivity, researchers Matt Dunkle (PhD student, University of Idaho) and Ryan Bellmore (USFS, Pacific Northwest Research Station) are studying the food web pathways that support salmon production in glacial, snowmelt and rainfall rivers with support from ACRC and University of Alaska Southeast (Jason Fellman and Eran Hood).

researchers analyze stream samples on a river bank

Throughout the year, Dunkle and Bellmore will be conducting monthly surveys of abundance and composition of biofilm (microbial life found on rock surfaces), aquatic invertebrates, and fishes. Salmon species, along with trout, sculpin, and others are measured and weighed, marked with scannable tags, checked for stomach contents and released. In all, these observations will help illuminate the seasonal and spatial variety of food webs that fish populations rely on. Initial field observations and model results suggest that different aquatic invertebrate communities occupy each stream type, and the peak invertebrate biomasses occur at different times of year. This is the first study of this magnitude and focus for southeast Alaska, where one of the largest fishing industries in the world operates. Understanding food web dynamics of southeast Alaska watersheds can inform freshwater fisheries management in the face of changing climate and landscape dynamics.

 

Stream carbon movement in a changing climate

sampling equipment in a wetlands areaOne 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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