The influence of lateral hydrologic connectivity on fluvial flux and ecosystem metabolism in a river-floodplain system
Date
2016
Authors
Wegener, Pamela, author
Covino, Tim, advisor
Kampf, Stephanie, committee member
Wohl, Ellen, committee member
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Abstract
Within mountainous watersheds and river networks, low-gradient valley bottoms can function as locations of high retention and biogeochemical processing. We evaluated hydrologic dynamics, nutrient flux, and aquatic ecosystem metabolism across the snowmelt hydrograph from May – October 2015 along two river segments of the North Saint Vrain Creek, Rocky Mountain National Park, Colorado: a valley-confined channel (confined segment) and an unconfined wet valley (unconfined segment) directly downstream. We observed significant differences in water, carbon, and nutrient flux, ecosystem metabolism, and lateral hydrologic connectivity dynamics among these contrasting segments. The confined segment was a consistent source of water, carbon, and nutrients and exported 14.4 mm of water, 26 g NO3-N ha-1, 41 g dissolved organic nitrogen (DON) ha-1, and 721 g dissolved organic carbon (DOC) ha-1 per 100 m of river length over the study period. In contrast, the unconfined segment exhibited variable source-sink dynamics and stored 1.2 g NO3-N ha-1 and 1.8 g DON ha-1, and exported only 1.1 mm of water and 8 g DOC ha-1 per 100 m. The retention of water, DOC and N related to the strength of lateral connectivity between the river and the floodplain, which in turn, influenced fluvial ecosystem metabolism rates. Gross primary productivity (GPP) and ecosystem respiration (ER) rates were higher and more variable in the unconfined versus confined river segment; average GPP was +1.01 ± 0.76 g O2 m-2 d-1 and ER was -1.77 ± 1.10 g O2 m-2 d-1 in the unconfined segment, whereas average GPP was +0.09 ± 0.14 g O2 m-2 d-1 and ER was -0.72 ± 0.50 g O2 m-2 d-1 in the confined segment. We found that along the unconfined segment, metabolism rates generally increased from high to low flows and that the greatest increases occurred in a floodplain side-channel with intermittent surface water connections with the main channel. Combined, our data suggest a conceptual model where DOC and N are delivered to floodplain water-bodies via lateral hydrologic connections during high flows, and are subsequently utilized when velocities decline and processing rates are maximized.
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Subject
carbon
fluvial flux
nitrogen
fluvial ecosystem metabolism
beaver meadow
hydrologic connectivity