Most Japanese people eat seaweed daily. Seaweed farms are common. L. littorea feeds on sea lettuce. Sea lettuce is a good indicator of pollution. If there is a lot of sea lettuce then there must be a good source of nitrogen near by.
Sea lettuce loves nitrogen but nitrogen is a pollutant in high doses.
It is in the oxygen picture above. The white tips of the algae is calcium carbonate. That is the same stuff sea shells are made of. A close view of Ulva. You will find that each of these algae are very particular about where they live.