Termination of the 2017-2018 Florida Red Tide – A Tracer Model Perspective

Abstract:

The 2017-2018 Karenia brevis harmful algal bloom on the west coast of Florida was the worst red tide occurrence since 2005. It peaked in fall, abated in winter 2018, and disappeared in early 2019. Here we examine the termination process of this red tide event based on a passive tracer simulation using the West Florida Coastal Ocean Model (WFCOM). To consider the role of the coastal ocean circulation in terminating the red tide event under a protracted upwelling scenario, as observed, a tracer was introduced uniformly throughout the water column over the region were the nearshore K. brevis concentration was observed to be large. Independent of biological growth or decay processes, the upwelling circulation resulted in a substantial reduction in tracer concentration over the three-month simulation. The tracer initialized along the central West Florida Shelf coastal area was transported southward and offshore, ridding most of the formerly high concentration region while sustaining relatively high concentrations from the region of Sanibel Island to Naples, FL. Whereas nutrient flux from the Caloosahatchee River is often argued as being related to K. brevis red tide, the longer residence time in this area may also have important implications for K. brevis red tide in this region. This simplistic experiment demonstrates that in the same way that K. brevis blooms initiated offshore, and transported to the nearshore along the bottom, can manifest as high concentration blooms along the coastline, if the offshore source abates, then the nearshore bloom can quickly abate as well.