B34D-07:
Physical and biological controls on reach to catchment scale nutrient retention and streamwater composition
Wednesday, 17 December 2014: 5:30 PM
Tim P Covino, Colorado State University, Fort Collins, CO, United States, Brian L McGlynn, Duke University, Nicholas School of the Environment, Durham, NC, United States and Ellen Wohl, Colorado State University, Geosciences, Fort Collins, CO, United States
Abstract:
Physical and biological processes occurring within fluvial networks can have strong influence on catchment scale retention of water and nutrients. Quantifying the physical (i.e., hydrologic exchange) and biological (i.e., nutrient uptake) contributions to total retention and deciphering how they relate to catchment morphology remains a central challenge in the hydrologic and biogeo-sciences. Here we present examples from our research that highlight the interactions between biology, physical hydrology, and geomorphology and how they combine to influence nutrient retention and streamwater compositions. Biological nutrient uptake in streams can have substantial influence on downstream fluxes and induce nutrient transformation along stream networks. Additionally, hydrologic loss of water and associated nutrients from streams to surrounding groundwater systems can greatly elongate water and nutrient retention times. While in-stream nutrient uptake is often associated with hyporheic exchanges that occur at sub-meter scales, these are nested within a larger framework of fluvial exchanges (100s – 1000s of meters). Larger scale exchanges can lead to strong shifts in streamwater composition over relatively short spatial scales (~1km) and are often very pronounced along geomorphic transitions (e.g., mountain to valley) and/or catchment retention zones (e.g., alluvial aquifers, wetlands, lakes). In fact, 50 – 80% of the water in the channel can be exchanged and replaced by different water (i.e., groundwater) along geomorphic transitions/catchment retention zones that are ~1 km in scale. These features can enhance geochemical processing through extended interactions between water, sediment, and nutrients. Accordingly, we suggest that although catchment retention features may be limited in spatial extent (~1km) and frequency they have the capacity to play a disproportionately large role in controlling catchment retention dynamics and setting fluvial network streamwater compositions.
