Xuemei Zhai Thesis Defense (Paul Moorcroft, Advisor)


Wednesday, August 25, 2021, 3:30pm

Title: What makes Stellwagen Bank a Productive Coastal Region?

Abstract: Stellwagen Bank lies at Massachusetts Bay’s eastern boundary with the Gulf of Maine and is encompassed by the Gerry E. Studds Stellwagen Bank National Marine Sanctuary.  The bank is a shoal with primarily sandy bottom and with water depth ranging from 20 meters to 40 meters. The waters across the bank are highly productive and support large and diverse planktonic and benthic communities, resident and migratory large fish diversity and fish biomass; it is also one of the most active whale feeding grounds in coastal waters of the United States.  In this study models of increasing complexity to investigate the factors responsible for the high biomass and production of this narrow, shallow off-shore bank.

In Chapter 1, after reviewing the physical and biological environment of Stellwagen Bank, I proposed the potential candidates of physical and biological factors that might contribute to this rich ecosystem: zooplankton diel migration, water depth, vertical mixing, tidal currents and topography.  Chapter 2 introduces and describes the methods and mathematical models which deal with five biological state variables: Phytoplankton (P), Zooplankton (Z), Ammonia (A), Nitrate (N) and Detritus (D).  Their equations are explained and discussed, and a zero-dimensional solution is analyzed.

Chapter 3 presents and analyzes the dynamics of the ocean ecosystem model when implemented in s single vertical dimension. The analysis reveals that zooplankton diel migration is a major factor affecting the biomass concentrations and spatial distribution of the model’s five biological state variables within the vertical water column.  Variation in water depths and diffusion also affect stability of the system.  With either shallower water depth or lower diffusivity, the model is less stable and even if it reaches stabilization eventually, it takes longer time than deeper water or higher diffusivity.

The one-dimensional model analyzed in Chapter 3 omits the effects of movement on or off the bank.  In Chapter 4, a two-dimensional (2D) cross-bank profile model that includes tidal flow is applied to investigate the biological and physical influencing biomass patterns and accumulations across Stellwagen Bank. In line with the over-arching goals of the thesis the 2D model focuses on determining the relative contributions of different factors and processes that are responsible for sustaining high biomass in the larger scale Stellwagen Bank ecosystem.  The results of the 2D model show that zooplankton diel migration, tidal currents, and topography are key interacting factors responsible for the high biomass and distribution pattern of phytoplankton, zooplankton and dissolved inorganic nitrogen (DIN) concentration over the Bank. In conjunction with zooplankton diel migration, tidal currents independently contribute to these biological effects; however, when zooplankton diel migration and tidal currents combine their effects becomes stronger and the spatial distribution of zooplankton biomass changes, becoming more concentrated on the bank’s flanks rather than the top of the bank under the influence.

In Chapter 5, I summarize the effects of the main physical and biological factors that contribute to the high productivity, discuss the implications derived from the model, and suggest future work.

Committee: James J. McCarthy (Advisor, posthumous), Paul R. Moorcroft (Advisor), Glenn R. Flierl (Co-Advisor, MIT), Robert M. Woollacott