Microscopic view of Karenia brevis. Courtesy of Mote Marine Laboratory

Sanibel and Captiva have recently experienced the first red tide event in several years. Red tide affects all of us in this region in many ways, and the effects of red tide can be very severe. Fortunately the majority of this bloom has remained offshore and had little or no impact on the shorelines of our islands.

The very premature (and perhaps overly optimistic) prediction on the future of this particular algal bloom is that it appears to be diminishing. For biologists, algal blooms like this are an opportunity to investigate and perhaps better understand the interesting dynamics of this complex marine phenomenon.

Let’s start with a brief refresher about Karenia brevis, the organism that causes red tide. These single-celled organisms are classified as protists. The protists are a difficult group to comprehend; however, a defining character is that they are single-celled organisms. Some protists are plant-like and undergo photosynthesis; some are animal-like and obtain their nutrition by preying on other organisms.  Two groups of plant-like protists are common in the marine waters of SW Florida: diatoms and dinoflagellates. Karenia brevis is a dinoflagellate.

Many plant-like protists are found in ocean habitats and can be a dominant component of marine phytoplankton communities. Oceanic phytoplankton communities are vital to life as we know it on planet Earth. Although difficult to calculate, estimates of global atmospheric oxygen content indicate that over 50% of the oxygen in the atmosphere is created by photosynthesis from marine phytoplankton communities. These phytoplankton communities link nearly every living thing on this planet through the cycling of oxygen.

The Gulf of Mexico and most oceanic water bodies are not big homogeneous bodies of water. It would be convenient if they were, but they are not. They are composed of fairly distinct, adjacent groups of water arranged in complex three-dimensional patterns. Each of these small groups of water has fairly unique combinations of physical and chemical properties. These unique combinations allow different phytoplankton communities to flourish at any given time in any given place in the ocean. When the chemical and physical water properties are favorable, some phytoplankton communities can reproduce very rapidly and achieve very high population densities – this growth pattern is known as a plankton bloom. Plankton blooms of K. brevis cause red tide.

Nature is very complex. Natural communities are typically composed of many, many species that occupy a variety of niches and trophic levels. We really do not understand the processes at play in phytoplankton communities very well. We don’t know what causes the favorable conditions that lead to most blooms, but we do know that in a red tide bloom, there are many other species of phytoplankton present.

It appears that the red tide bloom of October 2011 is fragmenting and diminishing. We don’t really know why, but here is a plausible explanation. The recent wind associated with our big storm fronts has stirred the oceans and suspended particles from the bottom. In the resuspension of these sediments, nutrients have been added to the water where the red tide bloom is – this is really a change in the physical and chemical properties of that patch of water. Recent samples indicate that diatoms are growing very well, and diatom populations are blooming. It appears that non-harmful diatoms are now outcompeting K. brevis in that patch of water K. brevis so previously dominated.

So, although far too early to be entirely accurate, it appears that our islands might just be spared the ill effects of red tide – because of a storm and some really interesting interplay between the physical and biological components in a patch of phytoplankton-laden water.

One aspect of this particular bloom is clear. We need a better understanding of phytoplankton community dynamics. They are crucial to our lives in southwest Florida and to the lives of nearly all living things on Earth. Sanibel Sea School is a volunteer partner of the Florida Wildlife Research Institute; we go to these blooms to collect samples for analysis. We spend our time and money to help the State of Florida gather data to help advance the understanding of the rich marine community that supports us.