As I write, the sun is finally out and it’s hot! Many terrestrial plants have been waiting for a day like this to open their buds and put on a growth spurt. On the other hand, my little pond has been green for several weeks now, supporting luxuriant growth of filamentous green algae. You’ve probably been seeing them in your waters too.
Green algae are a very diverse group. We are most likely to notice the filamentous green species like Spirogyra and Mougeotia because they form large masses of bright green in quieter water and tangled in with aquatic plants, but green algae come in many shapes and sizes. There are single celled and colonial species as well as filaments. Green algae grow in both fresh and salt water. In freshwater, they are found in a wide range of conditions from low to high nutrients, from acidic bogs to lakes, and from seeps to fast flowing rivers.
Green algae as a group have chlorophyll a and b as their main photosynthetic pigments. These pigments are housed within distinct structures called chloroplasts, with chloroplast shape being a key taxonomic feature. They store food as starch and have cellulose cell walls. Some green algae are mobile, using flagella, others aren’t able to move. Some are planktonic, floating free in the water. Others grow firmly attached and still others only loosely interact with the sediment. With more than 7000 known species, they come in an amazing array of sizes, shapes, and shades of green.
Green algae can reproduce vegetatively or asexually, meaning the mature cell divides in some way to produce additional cells with identical genes and appearance. Desmids are a beautiful example of vegetative division. Others like Ulothrix may turn all of their cellular material into asexual spores and release them to grow in a new place. Still others, like Volvox, form identical miniature colonies that are freed when the parent colony breaks open. At certain periods in their life cycle, green algae also undergo sexual reproduction, pairing with another individual to form new genetic combinations. You might enjoy an older but interesting time-lapse video of Volvox that shows both sexual and asexual reproduction.
Like other algae and most plants, many green algae have specific growth requirements and are found in only at certain times of the year. Water temperature, available light, and available nutrients all play a role in this seasonal succession of species. Green algae are often pretty tasty so grazing by zooplankton, insects, and young fish also determines how long a particular species may be present.
Under the right environmental conditions, green algae can become very abundant and rise to the level of nuisance growth. They can then have significant impacts on lakes, ponds, rivers, and streams – decreased water clarity, masses of decaying biomass on shorelines, and decreased oxygen levels in lakes and ponds as masses of green algae decay. Watershed nutrient management is often key to reducing nuisance levels of green algae growth.
It’s likely that the filamentous green algae you see along your shoreline in April is not the same species you see in August. If you’re interested in finding out, there are inexpensive magnifiers and microscopes that allow you to look more closely. You can also look online for images. INaturalist has a wonderful array of green algae photos. You can also browse photos in open access online taxonomic keys to see the amazing diversity in shape, size, and structure of freshwater green algae. Some good keys are: