How river transport affects dissolved organic matter

Posted on December 13, 2016


In most rivers, the downstream transport of dissolved organic matter (DOM) makes large amounts of energy available to aquatic ecosystems. The River Continuum Concept (RCC) of Vannote et al. (1980) provided a useful framework for studying the influence of organic material from headwater streams on the structure and function of downstream ecosystems. The RCC hypothesized that the diversity of soluble organic carbon compounds would decline abruptly in headwater streams because easily altered dissolved DOM from the catchment would be rapidly removed while less easily altered DOM would be transported downstream. However, we now know that DOM is a complex mix of compounds that have a big impact on nutrient cycles, microbial food webs and the transport of metals and pollutants, and we have a better understanding of other complexities such as spatial patchiness and floodplain interactions. In light of these advances, the RCC treatment of DOM now seems oversimplified. A new model of DOM dynamics treats the river network as a series of chemical reactors. Each type of reactor (hillslopes, riparian wetlands, streams, rivers, lakes and reservoirs, groundwater) is described by a characteristic combination of values for land-water transport, water residence time, internal DOM production and DOM decay. The new model describes how the concentration and composition of dissolved organic carbon change as it moves downstream, and how these changes are controlled by hydrological and biogeochemical processes. It’s argued that the biogeochemical processes that produce and consume DOM (such as photosynthesis, respiration, photo-oxidation, and adsorption–desorption) combine to reduce downstream variation in DOM concentration and composition, even though hydrological processes (e.g., surface and groundwater flows, evapotranspiration) cause variability in discharge over time. This effect is driven by a shift from upstream hydrological dominance (by the drivers that introduce terrestrial DOM into streams), to downstream biogeochemical dominance. Hydrological mixing tends to reduce concentration variability, while biogeochemical processing also tends to reduce compositional variability. The new concept recognises that the influences of groundwater discharge and stored water (i.e., lakes, reservoirs) both increase downstream. Since these “reactors” are relatively stable in term of the concentration of dissolved organic carbon, they contribute to the convergence effect. The authors note that DOM-related processes are best studied by measuring concentration and composition at the same time, and that this can now be done with relatively simple and inexpensive optical instruments such as field-deployable fluorometers.

References:

Creed, I.F. et al. 2015. The river as a chemostat: fresh perspectives on dissolved
organic matter flowing down the river continuum. Canadian Journal of Fisheries and Aquatic Sciences 72, 1272–1285. http://www.uwo.ca/biology/faculty/creed/PDFs/Journal%20Articles/095%20Creed%20et%20al%20In%20Press%20-%20The%20river.pdf

Vannote, R.L. et al. 1980. The river continuum concept. Canadian Journal of Fisheries and Aquatic Sciences 37, 130–137. http://www.colorado.edu/geography/geomorph/envs_5810/vannote_1980.pdf

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