Nicole Cai

Sea-level Rise Impacts on Tidal Marshes and Estuarine Biogeochemical Processes
Cai et al. (2023)
We use a numerical model to investigate the impacts of Sea-level Rise (SLR) on the biogeochemical processes in the York River Estuary with extensive tidal marshes. The fully-coupled hydrodynamic-water quality-marsh model accounts for the spatial-temporal variations of the physical-biogeochemical interactions between tidal marshes and surrounding waters. This study focuses on the SLR scenario that the vertical accretion of the tidal marshes keeps up with the SLR. Results show that SLR increases the tidal range and flooding duration, which leads to enhanced porewater exchanges of materials between the tidal marshes and the surrounding waters. With an increase in shallow water habitats and light utilization in the shallow water under SLR, phytoplankton production increases in the shallow water of the York River. Consequently, the organic carbon in the open water is fueled by the shallow waters and the enhanced export of organic carbon from the marsh under SLR.
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The Roles of Tidal Marshes in the Estuarine Biogeochemical Processes
Cai et al. (2023)
A marsh model, which simulates the ecological functions of marshes at tidal, seasonal, and annual time-scales, is embedded inside SCHISM-ICM. This tidal marsh model simulates the growth and metabolism of the tidal marshes and links biological processes to nutrient dynamics in the water column and sediment. The entire coupled modeling system dynamically simulates nutrient recycling and physical transport of the materials between marshes and open water through wetting-drying processes. Model results suggest that tidal marshes influence the local diurnal DO cycle by exporting dissolved organic carbon and high sediment oxygen demand in the marsh system through the tidal exchange. The high deposition rates of organics and diurnal DO cycle enhance the sediment release of phosphorus. On the other hand, marshes tend to decrease dissolved inorganic nitrogen in the water column by settling particulate nutrients and enhancing the denitrification process. The developed tidal marsh model enhances eutrophication modeling and advances the understanding of the feedback effects between marsh biogeochemistry and estuarine eutrophication processes on a systemic scale.
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Bifurcate Responses of Tidal Range to Sea-level Rise in Estuaries with Marsh Evolution
Cai et al. (2022)
We investigated the responses of the tidal range to SLR in tidal marshes. We demonstrate the existence of bifurcate tidal responses: tidal range can either increase or decrease, depending critically on the marsh evolution. The result is then incorporated into the current framework of studying marsh resilience to SLR, indicating that the bifurcate tidal responses may help resilient marshes become more resilient while causing vulnerable marshes to become more vulnerable to SLR.
[paper] [supplementary]

Impacts of Sea Level Rise on Hypoxia and Phytoplankton Production in Chesapeake Bay
Cai et al. (2021)
With the projected SLR, enhanced gravitational circulation transports more oxygen-rich water in the bottom layer from the mouth. However, the pycnocline moves upwards along with increasing water depth, which largely prolongs the time for dissolved oxygen (DO) to be transported to the bottom. The altered physical processes contribute greatly to a larger HV bay-wide. Besides, SLR increases the whole Bay phytoplankton production, with a larger increase in shallow areas (e.g., 53% in areas with depth smaller than 1 m under SLR of 0.5 m). Enhanced light availability is suggested to be the major driver of blooming phytoplankton under SLR in shallow areas. While increased DO production over the euphotic zone is mostly released to the atmosphere and transported downstream, the increase in settled organic matter greatly promotes DO consumption in the water column.
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A Numerical Study of Hypoxia in Chesapeake Bay Using an Unstructured Grid Model with Non-smoothed Bathymetry
Cai et al. (2020)
A three-dimensional unstructured-grid hydrodynamic and water quality model (SCHISM-ICM) is applied successfully for Chesapeake Bay. Comparison with the model experiment results with bathymetry smoothing indicates that bathymetry smoothing, as commonly used for many systems, changes the stratification and lateral circulation pattern, resulting in more salt intrusion into shallow water regions, and an increase in the freshwater age. Consequently, a model with bathymetry smoothing can lead to an unrealistic prediction of the distribution of hypoxia and phytoplankton production. Local grid refinement shows significant improvement of model simulations on local stratification and water quality variables. Overall, the use of high-resolution unstructured grid model leads to a faithful representation of the complex geometry, and thus a seamless cross-scale capability for simulating water quality processes in the Bay including tributaries and tidal creeks.
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Impact of Submerged Aquatic Vegetation on Water Quality in Cache Slough Complex, Sacramento-San Joaquin Delta
Cai (2018)
A new SAV model was imbedded into the unstructured-grid SCHISM-ICM framework to study to impacts of SAV removal on the system. 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. In Cache Slough Complex, Sacramento-San Joaquin Delta, the feedback loop from SAV to the hydrodynamics plays the most important role in the water quality variables among all feedback loops.