During the Eyes on Seagrass Blitz, one of the questions we ask is “Is there macroalgae present?” But what is macroalgae, and how can I tell it apart from seagrass?
What is the difference between seagrass and algae?
Seagrass is a true plant with vascular tissues and roots that attach the seagrass to the seafloor and take up nutrients. Here in Florida, we have seven species of seagrass, and the most common species growing in the Indian River Lagoon (IRL) is shoal grass (Halodule wrightii).
There are also many species of macroalgae that you can find in the lagoon. Macroalgae are photosynthetic, like plants, but they do not have vascular tissues. Some are attached to the seafloor with a holdfast, but that does not take up nutrients like true roots.
You will often see drift algae in the lagoon. The term drift algae refers to many different species of algae that grow unattached to the seafloor and can therefore drift around in the water. They do often get caught up on seagrass or other obstacles in the water and so may stay put in one place for a while but are not growing out of the seafloor from a holdfast. The drift algae in our area can be a wide range of colors, including red, brown, tan, or green.
There are some species of macroalgae that grow attached to the seafloor, including Caulerpa prolifera. Caulerpa prolifera is a bright green alga that grows up from the seafloor, similar to seagrass, but does not have true vascular tissues so appears as a flat color instead of seeing veins up and down the blades. I have not seen this species up in the far northern IRL, but I know there are some areas a little south of me where there are dense beds of it! While Caulerpa does provide some habitat benefits for small organisms, unfortunately, it does not provide the same nutrition benefits to grazers like manatees as seagrasses do. You can read more about the increase Caulerpa in the lagoon and see more photos in this article about research taking place at FAU Harbor Branch.
What role does algae have in the environment?
Macroalgae are important primary producers that are food for some organisms and take up nutrients in the environment as they grow. They can also provide habitat to small invertebrates and fish, and some attached species may be able to help with sediment stability. However, if there is too much macroalgae, it can also have a negative impact on the ecosystem. A large quantity of drift macroalgae may cover the seagrass bed and prevent it from getting sufficient sunlight to photosynthesize. There are also many types of smaller algae, including single-celled phytoplankton that float in the water and epiphytic algae that grows attached to the seagrass blade itself. These algae can grow rapidly if there are too many nutrients in the lagoon, which then prevents the seagrass from getting the sunlight they need to photosynthesize and survive. A healthy estuary will have some macroalgae, but in an estuary with poor water quality, we will see more algae than seagrass.
When you submit your sightings to the Eyes on Seagrass Blitz in the Indian River Lagoon this month, remember to look out for drift algae and Caulerpa and note if you see either around the seagrass!
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