Chlorophyta

The Chlorophyta (or green algae) occur in many different growth forms and sizes and as their name suggests, they are green in colour. As with terrestrial plants, the green colour comes from the pigment chlorophyll and this absorbs red light and reflects the green and blue end of the spectrum. Green algae thrive in shallow water where the long red wavelengths of light are most prevalent.

There are around 100 species of green algae in the British Isles in six orders. Most are marine although some orders such as the Chlorococcales, are mainly freshwater. The majority of marine green algal species occur around the entire coastline although a few species e.g. Codium fragile subsp. atlanticum and Cladophora pygmaea are northern species and Ulva olivascens and Codium fragile subsp. tomentosoides are more southern.

A great variation in life history is found in the Chlorophyta but one which occurs frequently is that involving a gametophyte, the gametes of which fuse to form a zygote which develops into unicellular phases. The unicellular phases form zoospores or aplanospores that appear to represent diploid sporophytes (although they can also be formed from zoospores or even gametes in some species).

A good text for identifying green seaweeds is Seaweeds of the British Isles, Volume 2 Clorophyta by Elsie M. Burrows (1987).