Gairsay Ballast Mounds – Flora and Fauna

Volunteer divers joined a team of archaeologists from the Orkney Research Centre for Archaeology last month to start the second phase of an archaeological project to explore Orkney’s early maritime heritage.

The fieldwork concentrated on Milburn Bay on the small Orcadian island of  Gairsay. The volunteer divers not only discovered ballast mounds but in the clear water also filmed an astonishing array of flora and fauna.

The ballast mounds are colonised by a distinct assemblage of species that sets them apart from the surrounding seabed.  The most obvious constituent is the green alga Codium fragile, which grows abundantly on the mounds and less so in the surrounding area.  Its bright green, branched structure forms a dense canopy that adds to the sheltered habitat already provided among the ballast stones.  Sea squirts are also abundant on the mounds, particularly the large pink species

Its bright green, branched structure forms a dense canopy that adds to the sheltered habitat already provided among the ballast stones.  Sea squirts are also abundant on the mounds, particularly the large pink species Ascidia mentula, distinguished by the white spots around the lip of its inlet siphon. Numerous other species were present around and among the ballast stones, including the sea urchin (Echinus esculentus), green crabs (Carcinus maenus) and a variety of small juvenile fishes.

Thanks to Sula Diving. ORCA staff, Paul Sharman, Senior Projects Manager and Sandra Henry, Marine Archaeologist, are leading the project.


ORCA-704