Author Type

Graduate Student

Date of Award

Spring 5-1-2026

Document Type

Thesis

Publication Status

Version of Record

Submission Date

May 2026

Department

Marine Science and Oceanography

College Granting Degree

Charles E. Schmidt College of Science

Department Granting Degree

Marine Science and Oceanography

Degree Name

Master of Science (MS)

Thesis/Dissertation Advisor [Chair]

Mingshun Jiang

Abstract

The St. Lucie Estuary (SLE) on Florida’s east coast is vital for diverse aquatic life and provides essential services to local communities. However, the estuary’s health is threatened by water quality deterioration due to nutrient-driven phytoplankton blooms, particularly cyanobacteria, which have increased in frequency over recent decades. These blooms may worsen water quality by reducing dissolved oxygen, altering pH, and increasing turbidity. Moreover, some phytoplankton species, including Microcystis aeruginosa, a freshwater cyanobacteria species, produce harmful toxins, posing risks to both marine aquatic life and human health in this area. Primary nutrient sources in this estuary include watershed freshwater discharges, atmospheric deposition, sediment inputs, coastal ocean water exchanges, and Lake Okeechobee (LakeO) water via C-44 canal. It has been hypothesized that LakeO waters not only supply nutrients but also phytoplankton species to this estuary, and this freshwater with phytoplankton cells causes the bloom mainly. However, despite extensive field research and continuous monitoring efforts, the factors and processes driving cyanobacterial blooms in the SLE remain poorly understood and have yet to be effectively incorporated into a predictive model. Here, we hypothesize that 1) no nutrient limit cyanobacterial bloom in SLE; 2) during dry seasons (when estuary has high residence time), phytoplankton growth and losses are in a quasi-equilibrium state such that phytoplankton biomass is largely controlled by zooplankton

grazing; 3) freshwater flow from LakeO, is one of the major factor that drives cyanobacterial blooms in the estuary; and 4) diel vertical migration is important during wet season cyanobacterial bloom formation in surface water.

To better understand the cyanobacterial bloom dynamics, a recently developed coupled hydrodynamic-biogeochemical model has been adopted. The hydrodynamic model is based on the Regional Ocean Model System (ROMS). The biogeochemical model simulates nitrogen and phosphorus cycles, as well as key processes associated with phytoplankton blooms, including photosynthesis, nutrient uptake, zooplankton grazing, and microbial loop, among others. Initially, one year (2018) simulation has been completed and calibrated with available observational data. To test the proposed hypotheses, the simulation period was extended to multi-years (2015-2020). Several numerical experiments have been conducted to examine the importance of various factors that might drive cyanobacterial blooms, including the nutrient and cyanobacterial input from LakeO , zooplankton grazing, and residence time in the estuary. Results from these experiments was analyzed to understand the key drivers and processes influencing the cyanobacterial blooms in this estuary.

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