Modeling Nitrogen Reduction Benefit to Invasive Aquatic Vegetation vs. Native Phytoplankton

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Phytoplankton comprise the bottom of the aquatic food web and the abundance of phytoplankton serves as an indicator of healthy aquatic habitats. In the Sacramento-San Joaquin River Delta (Delta), competing with phytoplankton for required nitrogen, invasive aquatic vegetation (IAV) has increased exponentially in recent years. Once established, IAV can negatively impact local ecosystems and infrastructure. In this research, scientists will determine how reductions dissolved inorganic nitrogen levels (specifically loads of ammonium), due to an upgrade to Sacramento’s Regional Wastewater Treatment Plant, will affect phytoplankton vs. IAV production. Using a synthesis of existing data, scientists will contribute to the ongoing Delta SCHISM modeling effort, the only modeling effort in the region that accounts for the growth and decay of IAV. 

Both phytoplankton and aquatic vegetation require nutrients for growth. Understanding linkages between nutrients and these "primary producers" of the ecosystem is critical for understanding the environmental impacts of management actions that change nitrogen inputs to the Delta. Specifically, such information is key to understanding the potential effects that upgrades to the Sacramento Regional Wastewater Treatment Plant (Regional San) will have on the levels and types of nutrients being introduced into the Delta via the plant's discharge.

Map of the Cache Slough Complex in the Sacramento-San Joaquin River Delta.

Map of the Cache Slough Complex in the Sacramento-San Joaquin River Delta. Specific wetlands recently studied by groups at the USGS include Wildlands, Little Holland Tract, Little Hastings Tract, and French Island.​​​​​​​

Science Plan

An effective tool in this research will be the study of hydrodynamic-biogeochemical models that use existing observational data. These models will allow for forecasting and management scenario testing. The previously mentioned SCHISM modeling effort will then help scientists understand the competition between unfavorable and favorable primary producers (i.e., IAV vs. phytoplankton). The SCHISM modeling effort, specifically designed to evaluate prospective and ongoing policy and management interventions, has been calibrated for hydrodynamics throughout the Delta. 

We expect the validation and calibration of the SCHISM model in the Cache Slough Complex may identify reaches where more data will be useful. Therefore the study has built in additional fieldwork that can gap fill pre-existing data. Additional data collection will be targeted toward data gaps identified with model validation and may include short-term water quality and flow collection or high-speed water-quality mapping.  

The area of focus in this study will be the Cache Slough Complex (CSC) where, over the last decade, there has been an increased growth of IAV and where the USGS has collected an abundance of data. The CSC also supports native fish habitat and has been shown to be a refuge for the endangered Delta Smelt. Additionally, the CSC is downstream of Regional San and the treatment plant provides the largest contribution of dissolved inorganic nitrogen to the region - relative to local agricultural inputs and smaller WWTP’s. Given the CSC is within the zone of influence for Regional San’s forthcoming upgrade and the subregion has shown to have gradient of water-residence times that include backwater reaches where nutrient limitation has been observed, there is management interest as to whether DIN reduction may limit primary production or create habitat for less favorable producers. The CSC has potential importance as a phytoplankton production hotspot for a larger domain of the Bay-Delta. IAV in this region affects hydrodynamics and impairs nutrient cycling modeling efforts because it takes up and releases nutrients and supports a community of organisms less favorable to native fish, possibly impacting primary production and food web dynamics. 

Relevance and Benefits

There are several reasons that current modeling efforts do not meet the needs of forecasting the reduction of wastewater-derived nutrients and the subsequent food web response. This proposed work will advance region-wide modeling efforts as they relate to future science investigation in the San Francisco Bay and Delta system. The work also supports the monitoring networks and techniques for determining water quality and the comprehensive understanding of the interactions among aquatic ecosystems, hydrology and biogeochemistry. The project's data synthesis, analysis, and modeling efforts support the Water Quality Processes Portfolio of the Water Mission Area (WMA); the effort will assist with defining ‘the relationships between human activities and changes in surface water. Additionally, the data synthesis and subsequent ‘gap-filling’ with future data collection is in parallel with the Next Generation Water Observing Systems portfolio of the WMA.