There aren’t really that many things in the ocean that are red. So it’s kind of funny that unpleasant ocean things that are red sometimes beleaguer us. A part of the conundrum is because of unfortunate naming.
In SW Florida, we have red tide and red drift algae. Biologically they are not related. What our red scourges share is the color red, their co-occurrence in SW Florida and that their growth is enhanced by increased nutrient levels in the water.
Most planktonic organisms are microscopic, and either free-floating, or swimming in the water column. Plankton can be animals, called zooplankton or plants, phytoplankton. Phytoplankton have high reproductive rates, and populations can increase significantly over short time periods; such rapidly growing plankton populations are called ‘blooms’.
Red tide is a phenomenon caused by a single-celled, phytoplankton classified as a Dinoflagellate. Although there are about five different species in our region, the organism most commonly linked to fish kills and red tide occurrences is Karenia brevis. They manufacture and contain a suite of toxic chemical compounds known as brevitoxins. When present in seawater, brevitoxins affect the nervous systems of vertebrates, and may ultimately lead to their death. Each K. brevis cell has reddish pigments in it, and water with high densities of this plankton can take on a reddish tint – hence the name red tide.
The tide portion of that name is a nebulous left over from early (perhaps 18th century) descriptions of such events – tides have nothing to do with red tide outbreaks. A new and preferred name for a ‘red tide event’ is an ‘harmful algal bloom’, more conveniently called “HAB”.
Now to our other challenge, red drift algae.
Marine algae – those plants that normally grow in the ocean, commonly called seaweed, come in three basic varieties; green, brown and red. Most of these algae are benthic – that is they are attached to the ocean floor. Red algae are a native, natural component of the sea floor in our region. Under normal conditions, it is a subtle component of benthic communities – whose populations are kept in check by grazing organisms and the amount of dissolved nutrients available for growth. Although attached to the sea floor, the plants of many red algae are slightly positively buoyant – this buoyancy keeps them erect under water.
As the nutrient levels in the waters of SW Florida increase, we have witnessed an increase of red benthic algae growing offshore on the sea floor. When this algae dies, or becomes dislodged, it floats to the surface (recall its positive buoyancy) and is collected into mats by wind and currents. Then, when currents and tides are favorable, these mats of dying and decomposing algae are blown ashore. Left ashore by the receding tide, we know these mats of algae as ‘red drift algae’. It is not toxic, but its decay usually produces a foul odor.
We had massive freshwater discharges from the Caloosahatchee River this summer and fall. That water was very nutrient rich. In other words, we spent months adding fertilizer to our ocean, and now it is not surprising if we see an increase in the amount of red drift algae on our shores.
The presence of large amounts of red drift algae on our beaches is another gentle calling card reminding us that all is not how it should be, there under the sea.
To help restore our oceans, please email the Army Corp of Engineers (firstname.lastname@example.org), and the South Florida Water Management District (email@example.com). Just ask that they fix our water problems. Won’t take more than ten minutes of your time; and the number of emails they get counts.