Macroalgae are large varieties of algae often referred to as seaweeds. When not attached to the bottom, they are commonly called drift algae. I’ve also heard them called gorilla snot, which at times is quite fitting. Macroalgae come in colors of red, green, brown and black, and 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 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.
Macroalgae, like all plants, photosynthesize — that is, they convert sunlight energy into usable chemical energy, specifically carbohydrates such as sugars. The carbohydrates are stored in or used by the plant, and then transferred to other organisms when the plant is consumed.
To capture the energy of sunlight, plants use special pigments. 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 — to capture more of the sun's energy.
Macroalgae are typically classified into three broad groupings — green, red and brown algae — based on the different types of photosynthetic pigments found in their cells. Brown algae gets its color from the pigment fucoxanthin (few-ko-ZAN-thin), red algae get their color from phycoerythrin (fie-ko-ERA-thrin), and 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!
The macroalgae we experience most frequently in Charlotte Harbor are red algae. The phycoerythrins in red algae absorb blue light and reflect red light. The blue wavelengths of sunlight penetrate further down in 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.
Green algae are less common in the Harbor, but most abundant worldwide. 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 bright green 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 almost 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 have small berry-like bladders that are gas filled 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 produce a pungent low-tide smell.
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 may be important. When large amounts of phosphorus and nitrogen occur together, each can magnify the effects of the other. To complicate this (as if it needed complication), different species of macroalgae can be limited by nitrogen or by phosphorus in the same estuary.
Periodically, I receive phone calls and emails from boaters and anglers who are concerned that macroalgae abundance is on the rise in Charlotte Harbor too. There are certainly seasonal patterns that give rise to more macroalgae, notably in 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 addressed in the future.
To get the ball rolling, this April I am debuting a new citizen science monitoring program. Hopefully you’ll want to participate. We’ll be collecting species and abundance information on local seagrass and macroalgae, so that we can establish baseline conditions and trends over time. You'll hear more on this in the coming months, or you can contact me if you just can't wait.