Green algae love well-lit areas

Green algae grow along Finland’s rocky and stony shores in well-lit zones, which are almost at the water’s surface.

Mermaid’s hair is the most common green alga

The most common species of green algae in Finland is the well-known Mermaid's hair alga, i.e. Cladophora glomerata. It is found on almost every shoreline as a bushy, bright green growth that conquers the water's edge in early summer. This green alga grows as single filaments up to 30 centimetres in length. In ideal conditions, this species can form a dense zone of vegetation which drifts back and forth in the waves.

Fun fact: in Finnish, the name of this green alga species is quite masculine! It is called "viherahdinparta", which describes the green beard of the sea god Ahti!

On rocky seabeds which are deeper or more fragmented, the green Mermaid's hair alga only occurs here and there in individual small tufts. In Finland, the deepest observations of this alga are from depths of about seven metres, where the rocky surfaces are usually covered by red algae.

In addition to Mermaid's hair alga, there are other species of Cladophora along the coastline. For example, the common green-branched weed (Cladophora rupestris) is usually found in deeper and more saline water, thriving below the bladder wrack zone. Also, in more brackish water grows the fragile and cloud-like growth of C. fracta.

 Spotted black and white isopod munch on green algae
Green algae makes a tasty snack for grazers like herbivorous isopods..

Gutweeds thrive on nutrient-rich shores

Other green algae, such as the so-called gutweeds or grass kelp, also thrive in the same habitats as the Mermaid’s hair alga. Gutweeds are usually brighter green in colour than Mermaid’s hair seaweed and have broader fronds than filamentous algae. These fronds often contain gas bubbles left behind after photosynthesis. By squeezing these smooth and shiny algae, the bubbles can be popped out into the surrounding water.

Like the Mermaid’s hair alga and its cousins, gutweed algae are fast-growing and conquer hard underwater surfaces in a very short time. These intestine-like algae thrive in eutrophicated waters can be found equally well along the nutrient-rich shores of bird colony islands and estuaries alike.

There are three gutweed species found in Finland; i.e. large gutweed, slender sea-lettuce, and winding nori. To separate one from another requires a microscope and a practised eye.

 Green algae species Ulva grows light green in clear water
As its name suggests, gutweed has a tubular structure.

Green algae:

  • Mermaid’s hair (Cladophora glomerata)
  • Common green branched weed (Cladophora rupestris)
  • Fin. Hentoahdinparta, “the fragile beard of the sea god Ahti” (Cladophora fracta)
  • Gutweeds (Ulva spp.)
  • Gutweed or grass kelp (Ulva intestinalis)
  • Slender sea-lettuce (Ulva linza)
  • Winding nori (Ulva flexuosa)
  • Lake ball, moss ball or marimo ball (Aegogrophila linnaei)