Macroalgae are large species of algae often referred to as seaweeds. When not attached to bottom sediments, they are commonly called drift algae. Macroalgae come in colors of red, green, brown and black, and they randomly wash up on beaches and shorelines just about everywhere.
Macroalgae are different from seagrasses and other common plants such as grasses, shrubs and trees. These more complex plants have vascular tissues to transport water and nutrients throughout the plant (much like veins and arteries transport blood in humans). Macroalgae lack these vascular tissues. Instead water, and nutrients are absorbed directly across their surfaces. Vascular plants also have true roots, stems and leaves, which macroalgae do not have (although some have evolved structures that are similar).
Macroalgae, like all plants, photosynthesize. That is, they convert the energy in sunlight into usable chemical energy, specifically carbohydrates such as sugars. The carbohydrates are stored in or used by the plant, and eventually transferred to other organisms when the plant is consumed or decays.
Chemically reactive pigments are the key to how plants capture the energy of sunlight for photosynthesis. However, since each pigment reacts with only a narrow range of the spectrum, there is usually a need for plants to produce several kinds of pigments, each of a different color. This allows them to capture more of the sun’s energy.
Macroalgae are typically classified into three broad groupings — green, red, and brown algae.. Brown algae gets its color from the brown to olive-colored pigment fucoxanthin (FEW-koh-zan-thin). Red algae get their color from phycoerythrin (FIE-koh-era-thrin). Bright green is from chlorophyll. But to keep things confusing — because all things in nature are confusing — red algae can appear light green to brown, green algae can appear yellow, brown algae can appear … well, you get the picture! You can’t judge an alga by its color alone, even though the major groups are named for the colors we expect them to be.
The macroalgae we experience most frequently in Charlotte Harbor are red algae. Their phycoerythrins absorb blue light and reflect red light. The blue wavelengths of sunlight penetrate deeper into the water, making it possible for red algae to carry out photosynthesis in deep or dark waters. Common red algae types found in the Harbor include Gracilaria, Hypnea and Acanthophora. All of these are similar in appearance.
Less common in the Harbor, but most abundant worldwide, are green algae. In Charlotte Harbor the green algae typically seen include Ulva and Caulerpa. Ulva, also known as sea lettuce, contains many species. Some look much like pieces of plastic wrap. Others are stringy and super slimy when they begin to decay. Caulerpa is an attached alga that is either feathery or straplike and looks like it could be a seagrass.
The brown algae species I see most frequently is Sargassum. Most Sargassum species are planktonic, meaning they drift in the water column, but the one I frequently see in the Harbor is attached. At times, Sargassum is locally abundant south of Alligator Creek and near the mouth of the Caloosahatchee River. Sargassum can be identified by small berry-like bladders that are filled with gas and help with buoyancy. Floating Sargassum can become a real nuisance when it washes up on beaches. This problem is particularly bad seasonally on the east coast of Florida, where large rafts pile up, decay and smell really bad. Of course, this stinky mess is also full of life, so scraping it off leads to an overall less healthy beach.
In many coastal waters, macroalgae abundance has increased in recent decades due to nutrient enrichment. Studies indicate that nitrogen is generally the nutrient that seems to control macroalgae growth rates throughout the year. However, phosphorus alone — or, at certain times of the year, both nitrogen and phosphorus together — can also have an effect. To complicate this, different species of macroalgae can be limited by nitrogen or by phosphorus in the same estuary.
I periodically receive phone calls and emails from boaters or anglers who are concerned that macroalgae abundance is on the rise in Charlotte Harbor. There are certainly seasonal patterns that give rise to more macroalgae, particularly in the winter and early spring. Whether we are experiencing an increasing trend or seasonal patterns is unclear, but it’s something that will need to be studied and possibly addressed in the future.