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Macroalgae in Estuaries

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).

What’s good about them?

Green macroalgae

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.

What’s bad about them?

In many coastal waters, macroalgae abundance has increased in recent decades due to nutrient enrichment, also known as eutrophication.

Red drift algae

As nutrient levels in coastal environments change so do the plants species. All plants require light, water and nutrients to grow. Studies have shown that seagrasses generally dominate in waters with low nutrients and high light availability. In contrast, macroalgae (and microscopic microalgae) tend to dominate where there is less light and more nutrients.

Because macroalgae (and microalgae) are better able to make use of low light levels than are seagrasses they have competitive edge when nutrient levels increase. This is because seagrasses are rooted to the substrate and as such are subject to shading effects of unattached macroalgae covering them. When seagrasses are shaded their ability to photosynthesize is reduced and ultimately their growing season is shortened.

So, one could expect as nutrient levels increase, seagrasses which are slow growing, to be replaced by macroalgae, which are fast growing. If nutrients continue to increase macroalgae will be replace by even faster growing microalgae — microscopic algae, also called phytoplankton (as seen in graphic below). From an ecosystem perspective, the shift from seagrass to microalgae could have cascading impacts as animals dependent upon seagrass for food and shelter, spotted seatrout for instance, are replaced by less desirable species such as jellyfish. When macroalgae have negative effects on ecosystems, they can be considered harmful algal blooms, even though they do not produce toxins.

This graphic shows the ecosystem effects of nutrient enrichment in Mar Menor coastal lagoon. Gamito et al. (2005).

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 may be important. To complicate this, different species of macroalgae can be limited by nitrogen or by phosphorus in the same estuary.

One of the biggest challenges when nutrient levels increase is identifying where the increase is coming from. Regular water monitoring programs cannot get to that detail without including some sophisticated and often costly sampling. Determining what nutrient sources are contributing to macroalgae growth will enable resource managers to implement corrective action hopefully before major shifts in plant species occur.

Of course, in the end it’s important to note that simply seeing macroalgae is not cause for concern. Recall, as primary producers (photosynthesizers) they do serve an important role, particularly in winter months when seagrass growth is naturally slowed. The key is keeping our bays and estuaries healthy to ensure a healthy balance in macroalgae and seagrass abundance.

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