Have We Missed the Message of the Manatees?

by Dr. John C. Ogden

Originally published in The Tampa Tribune on 1/27/97.

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The most recent statewide survey shows that a record number of 415 manatees died in Florida waters in 1996. While an estimated 70 were killed by collisions with boats and other human-related activities, biologists have now determined that the greatest single cause -- killing more than 150 manatees over a three-month period -- was a red tide that lingered off Florida's south coast early last year. This phenomenon has been reported again and again as "naturally-occurring."

Given our inability to stop running over these slow swimming, nationally-beloved mammals with motorboat propellers, there is something soothing in the knowledge that the cause of a tragic 15 to 20 percent decline in the remaining Florida population of manatees is not our fault. Or is there?

Was the red tide that killed the manatees simply an unavoidable stroke of nature? Unfortunately, there are reasons to believe that we humans are not blameless.

Red tides are an annoying but familiar fact of life to Floridians. Late last winter, for reasons not fully understood, a microscopic, single-celled plant which causes toxic red tides (Gymnodinium breve) bloomed far out to sea near the middle of the vast, shallow west Florida shelf. Red tide blooms are most likely initiated by currents which concentrate these cells -- combined with an upwelling of deep ocean water rich in the natural fertilizers, or nutrients -- which sustain their growth.

After it started, the red tide swept inshore via strong northwest winds and currents, eventually concentrating between Charlotte Harbor and Naples. In contrast to normal years, this red tide occurred earlier in the year and ultimately moved farther than usual into manatee feeding areas in inner harbors and river mouths.

At the same time, manatees -- which seek warmth in spring-fed rivers -- were driven out of these sanctuaries by abnormally cold weather. They headed seaward into waters contaminated by the red tide, where wave action ruptured the red tide cells -- thereby releasing their potent toxin into water droplets which were inhaled by the air-breathing manatees. Manatees also ingested toxin-laden organisms as they fed on bottom vegetation. This combination proved fatal even to healthy adult manatees.

While red tides have been reported for centuries, they are now increasing in frequency, duration and size all over the world. In many cases, red tides are driven by pollution of near shore waters caused by excessive nutrients from sewage, agriculture and runoff from streets, parking lots, lawns and golf courses.

For example the "cell from hell," an aggressive red tide organism (Pfisteria piscida) which kills fish with its toxins and then, changing form, consumes them, killed millions of fish near the Neuse River in North Carolina in the summer of 1995. The bloom of this organism -- so toxic that it causes a variety of human health problems and is hazardous to handle even in the laboratory -- has been attributed to the enormous proliferation of hog farms in the state. Unusually heavy spring rains that year caused runoff of polluted waters from fertilized farm fields and triggered a heavy growth of the organism.

But what does all this have to do with us Floridians?

First, we live in one of the most rapidly growing regions in the nation. The concentration of so many people in any coastal area inevitably leads to increased runoff of nutrients into near shore waters. While the initiation of the red tide this winter may have been a natural event, its size, duration and virulence once it entered shallow waters could be attributable to runoff of nutrients, as have red tides elsewhere in the world.

Second, communities all over Florida are depleting their already limited fresh water supplies. Could it be that the unusually high salinity in river mouths and inner harbors of southwest Florida this winter was the result of reduced river flows due to development and excessive use of fresh water?

Answers to these questions are not yet known. And they certainly will not be found by uncritical acceptance of red tides as "natural" -- especially those of the virulence experienced this winter. A red tide is currently lingering off Texas and we are once again approaching red tide season in Florida.

We must develop more comprehensive programs to monitor our rivers, harbors and ocean so that we can do more than react in panic when an event of this magnitude occurs again.

Nature has provided the "wake-up call." Manatees are a symbol both of the fragile nature of our coastal environment and of our responsibility to sustain the health of the ocean and its resources for future generations. Who is watching the ocean? Have we missed the message of the manatees?

Dr. John C. Ogden is Director of the Florida Institute of Oceanography, Professor of Biology at the University of South Florida.