Diel patterns of dissolved organic carbon and nitrate in streams: The influence of moisture conditions and watershed scale

Abstract:

Diel variations of solute concentrations in aquatic ecosystems reflect coupled influences of abiotic and biotic processes. Driven by solar radiation and temperature, processes such as autotrophic production, heterotrophic consumption and photodegradation will exhibit diel, or 24 hour, cycles. Stream size and moisture conditions will also influence these processes. Hydrologic flow paths will influence solute concentrations. While much research has focused on diel patterns of dissolved oxygen (DO), little is known about the response of nutrients and dissolved organic matter (DOM) to these processes. Understanding diel cycles of DOM and nutrients will help scientists better predict how disturbances in the terrestrial environment and watershed processes could modify solute concentrations and thus effect water quality and ecosystem function. This research sought to ascertain: What are the coupling patterns among the diel cycles of DOC, nitrate-N, and other solutes and what are the mechanisms controlling that coupling (hydrology, biology, spatial scale)? What insights do high-frequency sensors provide on diel patterns?

We studied diel patterns across stream orders in a forested watershed in the Piedmont region of Maryland. Sensors were installed and grab samples were collected from three stream locations within a 79 ha catchment to capture the spatial effect on the diel patterns. UV and fluorescence sensors were used to capture the daily cycles of DOC, nitrate-N, turbidity, temperature, DO, specific conductance, and fluorescent DOM. Groundwater elevation was measured every 30 minutes and stream discharge was measured every 20 minutes. Hourly grab samples were collected over three days for lab measurements of the diel periodicity in fluorescence excitation-emission matrix—an indicator of DOM composition—and major cations and anions. Preliminary results suggest diel patterns of DOC and nitrate-N were more evident during a long dry period in the forested watershed.