Impact of lateral circulation on transport and fate of buoyant particles in an estuary
Date
2025
Authors
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Publisher
University of Delaware
Abstract
Estuaries are a crucial interface between land and the open ocean and are the site of many physical, chemical, biological, and geological processes. For example, they distribute buoyant particles that can be natural (e.g., seafoam, plankton, organic matter) or pollutants (e.g., plastics, oil). While plastics and other buoyant particles are receiving substantial attention in the literature, there are few systematic studies that examine their distribution in an estuary. The Delaware Bay is an ideal system to develop an understanding of particle distributions in an estuary. The Delaware Bay exhibits gravitational circulation and has a deep central channel with shallow flanks, allowing for understandings from theoretical and idealized models to be applied to the bay. Additionally, the Delaware Bay has been the subject of many observational and numerical modeling studies. Here we will use a combination of a realistic hydrodynamic model, an idealized model, and observational data to study the movement of particles in an estuary. ☐ The goal of this dissertation is to systematically study the distribution of particles in the Delaware Bay. We perform this study in three parts. In the first part, we use a hydrodynamic model and particle tracking to study organismal exposure to plastics in the Delaware Bay. We find that exposure to plastics is not constant, but can vary throughout the bay based on initial location and plastics buoyancy effects. Additionally, organismal behavior, such as diel vertical migration, can alter exposure due to slower currents at depth not transporting the organism as far as the faster moving currents transporting plastics at the surface. In part two, we use an idealized estuarine model, a realistic hydrodynamic model, and observational data, to understand the patchy, inhomogeneous distribution of particles in the Delaware Bay as well as the along-channel transport. We find that when vertical mixing is high, a tidally-averaged up-estuary current region forms in the center of the bay and traps particles. Conversely, when vertical mixing is low, the entire surface of water column has down-estuary currents and particles are flushed. In the third and final part, we explore the combined effects and competition of tidally-averaged and time-dependent convergence in a tidally-resolved idealized estuary model. We find that Lagrangian motion can greatly impact the rate and location of aggregation and must be considered to fully understand particle trajectory. We also find the direction of Lagrangian convergence or Lagrangian divergence to be related to the level of stratification in the estuary. We conclude that particle motion in an estuary is controlled by a combination of tidally-averaged and time-dependent motion that aggregates particles, debris, and organisms, and that the underlying tidally-averaged flow is critical in determining if particles are trapped or released from the estuary.
Description
Keywords
Aggregation, Buoyant particles, Estuaries, Lateral circulation, Particle transport, Marine studies