Boilers on the ocean surface

Image: "Southwest Breaker" in Bermuda - A Boiler Reef that we surveyed


Today was a fine day to be surveying the coral reefs around Bermuda; calm seas, blue skies and plenty of healthy reefs to see. In a narrow band, along the south side of the island are algae cup reefs or ‘boiler reefs’ as they are locally known. The top of these reefs grow very close to the surface (and are often exposed) so water churning over them looks like it is boiling; consequently, these reefs are quite hazardous to boats and can only be studied on calm sea days. Bermuda is well known for these boiler reefs, the most well known being Northeast Breaker and Southwest Breaker (pictured above). Both of these reefs provide habitat for a huge range of marine life and some of the areas best scuba dive locations.

Algae cup reefs are shaped like giant goblets and are unique to high wave action areas. With a slender base, rising up to a full head, these reefs often succumb to bioerosion and sand scouring and topple over at their narrow base. The head can vary in diameter from 1 to 40 metres across and they can rise 12 metres from the sea floor, often joining together to make a complex group.

Corals struggle to grow on the surface of these reefs - probably due to the intensity of the wave action - but can grow along the walls. Algae cup reefs are made from hard crustose coralline algae, a sheet like seaweed that has heavily calcified tissues, and Corroding Worm Shells (Dendropoma annulatus), vermetid gastropods that embed their tiny shells, with an opening of only 1mm wide, in the algae mass and together form a hard limestone structure.

Studying algae cup reefs

Algae cup reefs are often bored and drilled into by echinoderms and bivalves. These voids collect sediments and intertidal species, washed into the spaces by the frequent and intense wave action. Layers are created, which essentially, record the changes in the ocean over time.