Impact of Submerged Aquatic Vegetation on Water Quality in Cache Slough Complex, Sacramento-San Joaquin Delta: a Numerical Modeling Study
Master of Science (M.Sc.)
Virginia Institute of Marine Science
Carlton H. Hershner
Submerged aquatic vegetation (SAV) plays a significant role in many aquatic systems, and impacts both physical and ecological quantities. It can baffle currents, attenuate waves, recycle nitrogen and phosphorus from the sediment bed, perform ecosystem function as a primary producer, and provide critical habitat for many aquatic species. Conversely, the invasive SAV, Egeria densa (Brazilian waterweed), in the San Francisco Bay & Delta has been a nuisance since its introduction into the system in the 1960s. It has displaced most of the native submersed aquatic plant species in the Delta and restructured the ecosystem, thus threatening the survival of several endangered native fishes such as Delta Smelt. Its impacts on the ecological system remain largely unknown and the need for assessment is growing. This multi-interdisciplinary study, incorporating biogeochemistry, hydrodynamics, and numerical computing and field survey data, accomplishes two main goals. The first goal is to develop a new SAV model imbedded into the unstructured-grid SCHISM-ICM framework. in addition to the advantages of directly simulating the SAV impact on hydrodynamics using high-resolution unstructured grids, this new SAV model can also simulate the competition between SAV and phytoplankton for light and nutrient supplies. The second goal is to apply the new model to Cache Slough Complex, Sacramento-San Joaquin Delta, to estimate the impact on the water quality from intervening SAV removal. Removal of SAV is already being studied in Little Hastings Tract and this study can serve to develop hypotheses for monitoring and ultimately guidance for managing SAV removal in the Bay-Delta region. We benchmark the new SAV model with the tests on the SAV biomass, growth and impacts on light supply and nutrient budget in the water column and sediment bed, respectively. Starting from a uniform biomass distribution, we simulate the evolution of biomass over seasonal scales and validate the calculated distribution with the observed distribution. The model is able to successfully simulate the SAV die-off process in areas where it is known to be unable to colonize. By applying the fully coupled SCHISM-ICM-SAV model in the Cache Slough Complex area, the changes of the water quality state variables due to SAV are estimated over spatial and seasonal scales. Generally, SAV increases the accumulation of phytoplankton by locally reducing flushing and thus increasing the residence time, but in the meantime, reduces its local growth rate due to light shading and nutrient competition. A combination of direct impact from SAV and indirect impact through changed phytoplankton results in changes in other water quality variables: dissolved oxygen and nutrients. SAV tends to increase oxygen and organic nutrients while decreasing inorganic nutrients. For this system, the feedback loop from SAV to the hydrodynamics plays the most important role in the water quality variables among all feedback loops.
© The Author
Cai, Xun, "Impact of Submerged Aquatic Vegetation on Water Quality in Cache Slough Complex, Sacramento-San Joaquin Delta: a Numerical Modeling Study" (2018). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1550153628.