A significant number of manatees have died this year as a result of red tide blooms off the southwest coast of Florida, more than 180 so far. These red tide blooms are largely caused, and made more severe, by agricultural and urban runoff that the alga Karenia brevis then uses for food. And now in addition to these deaths on the west coast, a significant number of manatees having begun dying on the east coast. The current state-wide population of manatees is estimated to be around 3,300 individuals, minus these recent deaths.
The red tide alga Karenia brevis is naturally occurring, but blooms have been increasing in frequency and severity as a result of increasing agricultural and urban runoff, and also as a result of increasing water temperatures. The alga produces a variety of potent neurotoxins that are deadly to fish, turtles, birds, and mammals. People are susceptible, but because of limited exposure their symptoms are often mild. Breathing in the aerosolized particles can cause respiratory problems in susceptible individuals though.
The east coast deaths, unrelated to Karenia brevis, remain as something of a mystery though. No sick manatees have been found, so biologists don’t have much to work with yet, simply dead and living individuals. There is speculation that the deaths may be related to the recent blooms of a few different species of algae, which caused the death of 31,000 acres of sea grass in the area. Manatees use sea grass as one of their primary food sources, it’s possible that they were forced to less nutritious sources of food as result of the recent loss. These recent east coast blooms were largely caused by agricultural and urban runoff.
The area where many of the dead manatees have been found is also the location where hundreds of dead pelicans have been found so far this year. The dead pelicans were all “emaciated and full of parasites.”
The pollution that caused the eastern bloom isn’t entirely from agriculture though, as Mother Jones described it, it’s a “Miracle-Gro of fertilizers, sewage, manure, and pet wastes that fuels algae blooms.”
With regards to the western red tide bloom, it has been argued that since red tides predate European settlement, and develop 10-40 miles offshore, that it means they are a “natural” occurrence and not caused/worsened by human pollution. But there is considerable debate in the regard, with some occurrences being easily linked to pollution and some showing no direct link. There is no doubt though that when exposed to nutrient pollution that it fuels their development and growth. And they seemingly have been increasing in recent years. Higher water temperatures have been associated with higher bloom rates, so they are very likely to increase in the coming years as a result of climate change.
It’s an open question whether manatees will be able to survive into the near future, or whether they will soon become a part of the 6th great mass extinction event, as most of the other megafauna animals have.
“NOAA reports that red tides off southwest Florida caused mass die-offs of endangered manatees in 1963, 1982, 1996, 2002, and 2003. So these episodes seem to be increasing in frequency. Florida’s manatee population is estimated at 4,000 to 5,000 (some estimates say as low as 3,000) individuals—about half the total world population of the species, according to the IUCN Red List.”