abundant at top of littoral zone covering chalk stone
No interactions present.
The British seaweeds that that were in genus Enteromorpha are now considered to be in the genus Ulva. Finding the currently accepted name is now made easy with the World Marine Register. http://www.marinespecies.org/index.php
This could be Ulva intestinalis, as that species is generally considered to be the commonest. However, identification of the "gut weeds" to species level in the field is difficult. The degree and pattern of branching is now considered to be environmentally, not genetically, determined. Microscopic characters which are useful are the arrangement of the cells, the chloroplast shape and the number of pyrenoids in the cells.
I do not know of an on-line key for these.
The definitive book is Green Seaweeds of Britain and Ireland, 2008. Edited by Juliet Brodie, Christine A Maggs and David M John. British Phycological Society. This has keys, colour photos, b&w photos, drawings.
Lat/Lng: 51.3827, 1.3276
OS grid ref: TR316701