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The Boundary Reefs: Glass sponge (Porifera: Hexactinellida) reefs on the international border between Canada and the United States


Hexactinellid sponge reefs have been discovered in shallow-water areas in Portland Canal on the international boundary between Southeast Alaska and British Columbia. The reefs were first observed on multibeam imagery data collected in 2008 and were examined in detail with a survey undertaken with a remotely operated vehicle in April 2010. The Boundary Reefs consist of three distinct reef areas at depths between 53 and 107 m.

Framework constructing, dictyonine sponges (Aphrocallistes vastus, Heterochone calyx, and Farrea occa) form bioherms up to 200 m in diameter and 21 m in height on glacial sediments, and extensive beds on moraines and glacial promontories. Variations in the morphology and structure of some of the Portland Canal reefs are attributed to variations in sedimentation compared to other shelf and fjord reef sites in British Columbia.

Development of extensive oxide crusts, pervasive colonization by zooanthid corals on reef surfaces, and the largely skeletal nature of the reefs suggest very low sedimentation rates as a result of low overall riverine inflow to the surface waters of the fjord and possibly reduced glacial meltwater input in recent years. A second complex of small reefs was discovered near Benjamin Island north of Juneau, Alaska. These reefs are the northernmost documented in the world and are very shallow, occurring at depths between 22 and 56 m. The two newly discovered reef sites are separated by almost 500 km indicating that the Inside Passage of Southeast Alaska may harbour additional undiscovered sponge reefs.

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