Tracing the early development of harmful algal blooms with the aid of Lagrangian coherent structures

TL;DR

This study uses simulated Lagrangian coherent structures and a population dynamics model to trace and understand the early development of harmful algal blooms, linking nutrient sources to environmental and human factors.

Contribution

It introduces a novel approach combining LCSs and population dynamics modeling to identify early HAB development stages and nutrient sources.

Findings

LCSs effectively trace HAB origins before satellite detection.

Two nutrient sources linked to HAB: coastal upwelling and river runoff.

Human activities contribute significantly to nutrient loading and HAB development.

Abstract

Several theories have been proposed to explain the development of harmful algal blooms (HABs) produced by the toxic dinoflagellate \emph{Karenia brevis} on the West Florida Shelf. However, because the early stages of HAB development are usually not detected, these theories have been so far very difficult to verify. In this paper we employ simulated \emph{Lagrangian coherent structures} (LCSs) to trace the early location of a HAB in late 2004 before it was transported to an area where it could be detected by satellite imagery, and then we make use of a population dynamics model to infer the factors that may have led to its development. The LCSs, which are computed based on a surface flow description provided by an ocean circulation model, delineate past and future histories of boundaries of passively advected fluid domains. The population dynamics model determines nitrogen in two components, nutrients and phytoplankton, which are assumed to be passively advected by the simulated surface currents. Two nearshore nutrient sources are identified for the HAB whose evolution is found to be strongly tied to the simulated LCSs. While one nutrient source can be associated with a coastal upwelling event, the other is seen to be produced by river runoff, which provides support to a theory of HAB development that considers nutrient loading into coastal waters produced by human activities as a critical element. Our results show that the use of simulated LCSs and a population dynamics model can greatly enhance our understanding of the early stages of the development of HABs.