Fine threads of lime green seaweed coil over the beach stones by the river that crosses the shore at Waulkmill Bay on Mainland, Orkney. Larger stones provide a secure attachment for the clumps of Horned Wrack that also favours the lowered salinity occurring with freshwater input at that location. At first I thought that these bright threads were a filamentous algae akin to Cladophora – but now I think it is a type of tubular seaweed like Gutweed (Ulva spp.) in which the narrow tubes have become flattened. Or maybe it is a combination of both.
Marine Life Information Network Filamentous green seaweeds on low salinity infralittoral mixed sediment or rock.
Bunker, F. StP. D., Maggs, C. A., Brodie, J. A., Bunker, A. R. 2017 Seaweeds of Britain and Ireland. Second Edition, Wild Nature Press, Plymouth, UK