Untitled DocumentThe influence of episodic flow events on nutrient and sediment loads in semiarid streams
Macro Theme Area:
River Systems [Project ID: R09]
PI:
Tom Meixner
CO-PI(s):
Paul Brooks, David Goodrich, Roger Bales
Basin focus:
San Pedro
Specific area in
basin /
field sites:
500 m reach near Boquillas; Walnut Gulch
Summary/Goals: To maintain a functioning riparian system, one must understand the linkages between water, vegetation, animals, and water quality (nutrients). This project provides a process-level understanding of nutrient fluxes and respiration reponses during storm events. Specifically, we are investigating inputs of sediments and nutrients during storm events due to high intensity rainfall and overland flow. Determining the importance of scour and fill on both microbial function and nutrient levels are our goals. To reach that aim, we monitored respiration changes on a point bar during the monsoon season. Changes in nutrient concentrations, particle size range and composition were also monitored in the river as well as the pore water in the point bar.
Activities and outcomes during past year:
Based on our results from the previous year, the work was concentrated at the interface at the head of the point bar. The interface was chosen, as this is a place where stream water flows into the sediments. During this monsoon, three flow events (>2 m3 s-1), in July and August, were monitored for nitrate, nitrite, sulfate, chloride, bromide, fluoride, TOC and DON. In situ respiration measurements were made by measuring the change in dissolved oxygen (DO) before and after flow events, above and below the water table near the stream interface. Methane, N2O and CO2 flux measurements were also made above the water table at these times. During flow events dissolved levels of TOC, TDN, nitrite, and nitrate all increased, while sulfate, chloride and bromide all decreased, furthermore Cl:Br concentration ratios changed during events from over 300 for the first event to 250 for later events, baseflow levels changed from about 175 at the beginning of the monsoon season to about 125 in August indicating changing water sources (e.g. soil water, runoff and rainfall). After a flow event CH4 and CO2 fluxes more than tripled from 0.6 mmol/hr to 21 mmol/hr and 6.6 mol/hr to 232 mol/hr respectively, which signifies that methanogenisis increases in the sediments at this time. Immediately after the floods, microbial respiration also increased (~600 mg O2/(kg sed*hr) with the input of nutrients and sediment to the system. This increase below the water table seems to be linked to DOC and DON concentration, the source of the water and organic matter as well as the influx of smaller particles. Above the water table the main factor controlling microbial respiration seems to be the input of water to the system, as the highest rates are right after a flood event. Once the floodwater recedes (<0.3 m3 s-1), respiration rates decreased (~90 mg O2 /(kg sed*hr). Flow events appear to trigger both aerobic and anaerobic processes in the sediment. The results for this season were considerably better than for the previous year due to better particle size analysis, conducting respiration experiments in situ at the point bar and cooperation with Dean Martens in measuring methane fluxes and soil analyses. Although this study was conducted on a small reach (50 m) in the San Pedro River, they can provide some speculation about the importance of these processes in the whole river basin. Respiration rates increased in response to inputs of water, dissolved organic carbon and nitrogen species, inorganic nitrogen species and particulate organic matter. Sediments that were typically dry responded less to the addition of nutrients and water than saturated sediment did to nutrients. This suggests as the San Pedro dries out after the rainy season, the zones where there will be active respiration will be saturated sediments that have active exchange with the river: hyporheic zone, parafluvial zones, and gravel bars. Microbial processes in these zones will continue to use the nutrients that have entered the river system at during the monsoon season until a specific nutrient becomes limiting or the next rainy system occurs.
Plans for the upcoming year:
The second manuscript from A.K. Huth's dissertation is being reworked linking hydrograph separation and nutrient inputs to the San Pedro.
A manuscript is being prepared on the respiration response in a point bar to storm events over the monsoon season.
To continue this work, our emphasis will be shifted from the main stem of the river to a tributory. Sediment samples will be collected on a hillslope and from the Walnut Gulch tributary, the San Pedro River (both upstream and downstream of the tributary) to test the hypothesis that during a monsoon storm, the main source of nutrients is from the upland terraces.
