Nitrogen Dynamics Along the Sacramento River and Links to Phytoplankton Dynamics: Resolving Spatial and Temporal Variability Using In-Situ, High-Frequency Measurements and Other Tools

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The overall project objective is to further our understanding of the link between nitrogen and phytoplankton dynamics in the Sacramento River and to elucidate effects of wastewater treatment plant effluent on food web dynamics.

Sacramento River reach

Map of the Sacramento River reach that will be monitored. Flow along this stretch of river is generally downstream-I80 to Isleton except during low flow periods when tidal forcings can cause flow to reverse.

(Public domain.)

Declines in phytoplankton blooms and shifts in species composition have been documented in the Bay-Delta. Recent work demonstrates that elevated concentrations of ammonium (NH4) in the Sacramento River attributable to Sacramento Regional Wastewater Treatment Plant (SRWWTP) effluent can lead to precipitous drops in phytoplankton nitrate uptake, which is associated with decreases in primary production. Because declines in phytoplankton abundance are linked to the pelagic organism decline, the addition of NH4 is widely believed to be an important "bottom-up effect" on Delta fish populations. However, declines in chlorophyll-a concentrations have also been documented in water moving down the Sacramento River above the SRWWTP. These observations highlight the need to better understand nitrogen cycling in the Sacramento River and its links to primary productivity, phytoplankton abundance, and community composition. We hypothesize that one reason nitrogen and phytoplankton dynamics are not yet fully understood is that data generated to date are largely limited to "snapshots" from discrete samples. Due to complex hydrodynamics, variable effluent loads, and diel biological processing, this approach makes it difficult to compare changes in water quality between two sites, because each sample can represent a significantly different parcel of river water, both of which may contain a different fraction of effluent and have experienced significantly different biogeochemical processing.

We propose to significantly improve our understanding of nutrient and phytoplankton dynamics by incorporating the use of in-situ, high-frequency sensors to continuously track changes in key water quality parameters during longitudinal, Lagrangian-based, transects of the Sacramento River. As part of this study, in collaboration with the Sacramento Regional County Sanitation District (SRCSD), wastewater outflows from the treatment plant will be halted for approximately 12 hour periods on two occasions (spring and summer/fall) to investigate nutrient and phytoplankton dynamics in the absence of wastewater contributions.

The overall project objective is to further our understanding of the link between nitrogen and phytoplankton dynamics in the Sacramento River and to elucidate effects of wastewater treatment plant effluent on food web dynamics. Central to this approach is (a) the use of high-frequency, in-situ measurements and (b) coordination with the SRWWTP to halt discharge twice during monitoring.

Longitudinal transects of the 67 km (42 mile) reach of the Sacramento River between the I-80 Bridge and Isleton will be conducted during two seasons, in the spring and summer/fall of 2013. During each season, a pair of transects will be conducted; once when wastewater effluent is being discharged from the Sacramento Regional WWTP into the river, and another immediately preceding or following that when wastewater outflows are halted for up to 12 hours. The river will be navigated by a boat equipped with a suite of in-situ sensors designed to measure and record continuous water-quality data. In addition, discrete samples will be made at a minimum of 5 specified stations and analyzed to measure constituent concentrations, identify algal species composition, determine phytoplankton primary production, estimate NH4-N and NO3-N uptake rates, and to obtain a full suite of isotopic measurements. Differences in all of these measurements due to the presence/absence of wastewater effluent will allow us to determine if changes in algal dynamics can be directly linked to wastewater inputs.

Relevance and Benefits: This project relates explicitly to the mission of the U.S. Geological Survey to provide scientific information that will be utilized by decision-makers to effectively manage the landscape and water resources to mitigate water-related natural hazards, and protect recreational and ecological use aquatic health, and environmental quality (USGS, 2007).