Streams and rivers continually transport microscopic organic particles that originate from sources such as forests, upland soils, and streambed algae. These particles are important because they (1) carry food-energy from headwaters to larger streams and rivers, (2) transport pollutants through river systems, and (3) transfer organic carbon from terrestrial to marine ecosystems. This research asks three fundamental questions about particulate organic carbon in transport: is it biodegradable, how far downstream does it travel, and to what extent does it support life in larger streams and rivers? To answer these questions, we have assembled a research team with expertise in stream ecology, organic geochemistry, hydrologic engineering, and mathematical modeling that will perform chemical and biological characterizations of the particles, identify the important hydrodynamic processes controlling suspension and deposition, and construct a mathematical model that simulates the generation, transport, and decomposition of particles as they move downstream.
Stroud Water Research Center Education staff will develop curricula for middle- and high school classrooms which introduce modeling within the larger framework of aquatic sciences. The educators will work with researchers to develop the curricula and models, which range from conceptual diagrams to mathematical simulations. Because this research will emphasize the role of headwaters in subsidizing larger streams and rivers, the result should help guide policy regarding the protection of upstream ecosystems. Research results will be summarized for local watershed associations for dissemination to the public, and shared with national conservation organizations that provide technical information to legislative aides.
The upstream reaches of a river network supply downstream reaches with food energy in the form of microscopic organic particles. Some particles are consumed near their source, while others travel as far as the ocean. There are two reasons to know the fate of the organic particles that enter streams and rivers. First, it tells us the extent that downstream ecosystems depend on upstream systems and thus can guide policy for protecting headwater streams. Second, it can help us ascertain how much of the carbon that enters rivers is ultimately sequestered in river banks, reservoirs, estuaries, and oceans versus the amount that is returned to the atmosphere as the greenhouse gas, carbon dioxide. Our approach is based on quantifying "turnover length," which is the average distance an organic particle travels from its point of origin to the point of mineralization. The turnover length, in turn, can be represented as the product of a velocity and a time. The velocity is the "migration velocity" or rate at which a particle moves downstream through successive cycles of deposition and resuspension. The time is the "turnover time" or the time that the particle resides in the fluvial environment until mineralization. Seston is not uniform, but consists of particles with varying transport and degradation rates, so our task is to characterize seston as the ensemble of these characteristics. Given this framework, our work falls into three major areas: (1) characterization of seston as to source, lability (degradation rate), and factors such as size and density that affect transportability; (2) quantifying deposition and suspension rates and understanding the mechanisms of transport; and (3) synthesizing this information in a dynamic, hydrology-based, model of the river network. In the first area, we characterized stream seston more fully than previous work, showing that while most of the seston mass is from mineral-core particles, most of the seston carbon is in the form of algal and detrital, rather than mineral-core, particles. We quantified seston degradation rates and found that seston is substantially more labile than previously suspected. This result implies that the turnover length of seston in river networks is likely to be much shorter than previously inferred. This in turn points to tighter metabolic coupling of upstream to downstream reaches, with a higher proportion of fluvial carbon mineralized within the river network, and a lower proportion exported to estuaries and the ocean. In the second area, we confirmed that deposition of natural seston is rapid and similar to previous estimates of larger (more easily studied) fractions of the seston. We confirmed the hypothesis that seston deposition is regulated strongly by streambed biofilms. We found that seston concentrations vary on a daily cycle, increasing at night. This result, together with additional experiments, led us to conclude that seston resuspension is strongly regulated by bioturbation. In the third area, modeling seston transport, we found that the classical exponential model that has long been used to measure deposition rates is valid over long distances, but not over short distances. Methods for measuring particle deposition may need to be modified to account for deviations from the exponential model. Broader Impacts--With this project the Stroud Water Research Center initiated an innovative program to work with high school teachers through NSF’s RET (Research Experience for Teachers) program to develop curriculum materials that introduce mathematical modeling within the larger framework of aquatic sciences. Several teachers participated in both the research and education aspects of this project. Their curricular materials are posted on the Educational section of the Stroud Center’s website (www.stroudcenter.org/education/CurricularResources/index.htm). Of particular note is the Stella-based Land Management and Storm Flow Simulator, developed by science teacher William Richardson. This teaching tool inspired several other innovations that have incorporated into the Model My Watershed project (NSF 0929763). Model My Watershed interactively teaches fundamental concepts of hydrology by allowing students to delineate watersheds in their own neighborhood and investigate the implications of changing land use and management practices. Other results of this project are being disseminated through the education program of the Stroud Water Research Center, which provides teacher training workshops, hands-on programs for students (grades 4-12), and training for citizens monitoring groups.
