From the UNIVERSITY OF SOUTH FLORIDA and the Gilda Radner School of Universal Climate Blame comes this “never mind” moment. You may recall that last year, blaming Florida’s red tide event was the low-hanging fruit for journos, like Angela Fritz of the Washington Post “Capital Weather Gang” who, without any direct evidence, speculated breathlessly: How climate change is making ‘red tide’ algal blooms even worse

Except when you wait for the science to catch up to headlines, Angela gets to have a Gilda Radner moment.

Ocean circulation likely to blame for severity of 2018 red tide

ST. PETERSBURG, Fla. (April 18, 2019)- The harmful algae that causes red tide is currently at near undetectable levels in Florida waters compared with the much higher concentrations at this time last year. The red tide algae, Karenia brevis, causes respiratory issues, is responsible for massive fish kills and is often blamed for damaging tourism.

While traces of the bloom are always present offshore in the Gulf of Mexico, a new study published in the Journal of Geophysical Research-Oceans finds ocean circulation made 2018 the worst year for red tide in more than a decade.

By affecting the nutrient levels offshore, marine scientists at the University of South Florida (USF) showed that the ocean circulation played a controlling role. If nutrient levels offshore are high in spring due to the upwelling of deeper ocean waters, then there tends not to be major red tide blooms along the shoreline in fall. Such upwelling did not occur in winter and spring of 2018, allowing a new bloom to form offshore in spring and summer 2018. An upwelling circulation then set in toward the end of July, ensuring that the newly formed bloom would be carried to the coastline along the bottom where it reinforced what had already been in place from 2017.

Tropical Storm Gordon temporarily disrupted the upwelling circulation, allowing some of the new bloom to be carried to the Florida Panhandle. After the passage of Gordon, the upwelling circulation then allowed the bloom to be transported offshore at the surface to eventually be carried to the Florida’s east coast by the Gulf Stream. Thus, the rare occurrence of Karenia brevis at three different locations (Florida’s west, Panhandle and east coasts) may be attributed to the ocean circulation.

“This further demonstrates that the ocean circulation is the major determinant of Florida’s, Karenia brevis harmful algae blooms, dispelling the myth that land-based fertilizers are to blame,” said Robert Weisberg, PhD, Distinguished University Professor of Physical Oceanography. “While pollutants can exasperate an existing red tide, they are not the root cause.”

In addition to ocean circulation models, the team at USF and collaborators with the Florida Fish and Wildlife Conservation Commission (FWC) deployed an autonomous underwater glider for a near month-long mission. Its sensors detected relatively high chlorophyll and low oxygen levels near the sea floor, along with upwelling circulation. On-site sampling also helped pinpoint the initiation zone for all three regions to be the middle shelf some 30 to 50 miles off the coast from north of Tampa Bay to Sarasota Bay.

Weisberg and his colleagues have accounted for the occurrence or lack of occurrence of major red tide blooms in 20 of the past 25 years based on the ocean circulation conditions. While recent sampling shows very low concentrations of Karenia brevis offshore, which is not a cause for immediate concern, it is too early to speculate on what future conditions may be. Weisberg expects to have a better idea of the possible severity of 2019’s red tide season in mid-June.

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The paper:

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JC014887

The Coastal Ocean Circulation Influence on the 2018 West Florida Shelf K. brevis Red Tide Bloom

Abstract

Blooms of the harmful alga, Karenia brevis on the west Florida continental shelf are thought to initiate offshore before manifesting as a nuisance along the coastline. Contributing to such blooms are a complex sequence of events occurring within oligotrophic waters, which in any given year may or may not be facilitated by the ocean circulation. Once initiation occurs, the delivery from the region of offshore origination to the region of coastline manifestation requires an upwelling circulation, whereby K. breviscells are advected shoreward along the bottom. The 2018 K. brevis bloom was particularly intense owing to cells from the preceding 2017 bloom being reinforced by a newly formed bloom in 2018, a year when the offshore conditions in spring through early summer were again favorable for bloom development. As an event response to determine the potential for new cells to be delivered to the shore, a glider was deployed from 24 August 2018 to 17 September 2018 with a track line designed to map water properties over the hypothesized initiation region. The coastal ocean circulation during the deployment interval was generally upwelling favorable, but the passage of Tropical Storm Gordon temporarily disrupted this flow, after which K. brevis appeared along the Florida Panhandle coast. Strong upwelling then reestablished and K. brevis was subsequently observed along Florida’s east coast. We describe the glider deployment, the K. brevis observations, and we use a numerical circulation model to account for the K. brevis manifestation as occurred along Florida’s west, Panhandle, and east coasts.

Plain Language Summary

We account for the intensity and location of the 2018 Karenia brevis red tide outbreak on the west Florida continental shelf by a combination of water property observations and numerical circulation model simulations. These confirm the initiation region being offshore, the manifestation region being along the shoreline and the delivery mechanism (from initiation to manifestation) being an upwelling favorable coastal ocean circulation. The intensity is attributed to the cells remaining in the manifestation region from the prior 2017 bloom being reinforced by cells newly formed offshore in 2018.