Sparking and sustaining blooms
While predicting red tide blooms
remains tricky, it’s clear that widespread
and multifaceted factors drive their ini-
tiation each year. For decades, research-
ers have been amassing evidence that
K. brevis blooms start near the seafloor
about 10 to 40 miles offshore in the Gulf.
This makes the species somewhat unique
among bloom-causing algae, many of
which begin to expand inshore or near-
shore, flourishing in the high nutrient
concentrations, warm temperatures,
and sunlight that most phytoplankton
require to survive and reproduce.
What may set K. brevis apart is its
ability to thrive on a variety of nutrients
and to get those nutrients in a number
of ways. In several papers published in a
2014 special issue of the journal Harm-
ful Algae, researchers including Heil
documented that K. brevis can subsist
on nutrients from undersea sediments,
decaying fish, atmospheric deposits,
and estuarine water. They can consume
other plankton species or use nutrients
those species produce, and can absorb
nutrients from the water following the
decay of a filamentous cyanobacterial
genus called Trichodesmium.^4 “Karenia
brevis is super flexible,” Hubbard says.
The observation that the dinofla-
gellate can dine on rotting Trichodes-
mium, which occasionally also blooms
in the Gulf, links red tides in the area
to events that extend far beyond the
state of Florida and the Gulf of Mexico.
In 2001, scientists published evidence
that dust storms in the Sahara Desert in
early 1999 supplied nutrients that sup-
ported subsequent blooms of Trichodes-
mium, which requires high iron levels
to thrive. Levels of dissolved phospho-
rus plummeted as it was consumed by
the cyanobacteria, and dissolved nitro-
gen increased, potentially fueling a
nitrogen-hungry K. brevis bloom that
reddened Gulf waters in the fall of the
same year.^5While it’s not known whether large,
sustained red tide events are caused by
one or multiple dinoflagellate popula-
tions, once K. brevis gets blooming, the
phytoplankton are sometimes swept
toward shore by upwelling and prevail-
ing currents into surface waters, where
they continue to proliferate. This pro-
cess involves yet another suite of drivers
that include nutrient inputs from fertil-
izers in groundwater runoff, fish killed
by the bloom, faulty wastewater sys-
tems, even air pollution—and, of course,
local currents.
A recent study tracked the extensive
movement of Florida’s exceptional 2017–
2019 red tide event and suggested that its
movement into nearshore waters skirting
the state’s panhandle may have resulted
from the passage of Tropical Storm Gor-
don across the Gulf from the southwest
coast of Florida to Texas and Louisiana
in early September 2018. The storm, the
study reported, may have disrupted the
upwelling that had brought the bloom
into nearshore waters on the state’s west
coast, allowing the red tide to move northA RISING TIDE:AK. brevis bloom dyed coastal
waters off of Sarasota, Florida, in August 2018,
during the 2017-2019 red tide event.
VINCE LOVKO/MOTE MARINE LABORATORY