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Using Remote Sensing to Predict How Landscape Factors Influence Ecosystem Processes
Each year in the United States, approximately $1 billion is spent to restore degraded streams. Unfortunately, the scientific understanding necessary for linking restoration actions to identified objectives is often lacking, a problem that can result in missed goals at best and environmental destruction at worst. In light of this critical knowledge gap, researchers with the National Center for Earth-surface Dynamics (NCED) are working to understand the factors that drive stream ecosystem processes in order to generate a sound base of knowledge to enable better prediction of stream response to perturbations such as restoration management, as well as changes in biota, land-use and climate.
NCED Principal Investigators are currently at work in the Angelo Coast Range Reserve—an 8000 acre tract of old growth forest in the northern California Coast Range that surrounds 5 km of the South Fork Eel River and watersheds of several of its tributaries—attempting to build that base of knowledge. Here, NCED PIs Mary Power, Miki Hondzo and Jacques Finlay are investigating linkages of ecosystem processes like algal production, aquatic insect emergence, carbon and nitrogen cycling, and changes in mercury bioavailability.
NCED, an NSF Science and Technology Center, is at the forefront of a new approach to such investigations. Feeding off of its center mode of operation, scientists from previously disparate fields such as geomorphology, engineering, and ecology are working together to understand how biota and ecosystems are linked to landscapes and hydrology, providing predictive insights into how important ecosystem services such as salmon spawning and water quality are linked to each other and to the landscape. 
A recent fruit of this multidisciplinary labor has been the identification of consistent thresholds in biological activity in stream networks. Power, Finlay, and colleagues have found that large macroalgal blooms accrue in stream reaches with drainage areas above 100 km2, a threshold likely driven by the increased light availability that accompanies larger stream size. Over the course of the summer, the blooms change color—from green to yellow to rusty brown as the algal filaments become covered first with non-nitrogen fixing, then with thick layers of nitrogen fixing epiphytic diatoms. The seasonal and spatial variation in the color of the blooms, which can be monitored with cameras located throughout the reserve, may reveal rates of stream nitrogen fixation. In this nitrogen-limited river ecosystem, nitrogen loading is linked to biotic growth—for example, rates of insect emergence are 25x higher from rusty algal mats.
Surprisingly, the presence of the macroalgal blooms in the South Fork Eel River may also be linked to mercury cycling, even in this relatively pristine watershed. Recent fieldwork of NCED graduate student Martin Tsui and his advisor Jacques Finlay suggests that the blooms may be in-stream hot spots for the methylation of mercury, a transformation that greatly increases mercury bioavailability and rate of bioaccumulation in aquatic food webs. If this is the case, understanding what drives mercury methylation in the macroalgal blooms will be essential to understanding the drivers behind stream water quality and biotic contamination.
In light of the importance of these macroalgal assemblages, NCED researchers are using LiDAR (Light Detection and Ranging) Digital Elevation Models (generated by the NSF funded National Center for Airborne Laser Mapping) of bare earth and vegetation canopy to model light radiation regimes striking the ground and/or streambed channels. Analysis is now underway to determine how well the light models predict the location of large algal blooms in a 5 km reach of the South Fork Eel River.
If they are successful in creating models that predict how landscape factors such as light availability influence algal productivity, and therefore nutrient cycling, aquatic insect emergence, and mercury methylation, the NCED researchers will be well on their way to linking landscapes with ecosystems and understanding the factors that drive ecosystem processes, information that is vital for planning restoration efforts or understanding environmental response to management decisions or land-use and climate change.
Image: Scanning electron micrograph by Rex Lowe showing epiphytic diatoms piled thickly upon one another atop Cladophora algal strands.
Figure: A 100 km2 light threshold controlling the growth of Cladophora, a branched macroalga, may account for a threshold in total dissolved nitrogen in the South Fork Eel River in the Angelo Coast Range Reserve in California. From Power, Finlay, et al. 2009 Freshwater Biology
