Research Document - 1999/192
Hexactinellid Sponge Reefs on the British Columbia Continental Shelf: Geological and Biological Structure with a Perspective on their Role in the Shelf Ecosystem.
By K.W. Conway
Abstract
Globally unique sponge reefs dominated by species of hexactinellid sponges occur in the deep shelf troughs on the western Canadian continental shelf. Submersible dives reveal these reefs to consist of dense populations of hexactinosan sponges that cover bioconstructions that are up to 18 m high and many kilometres wide. The non-living portion of the reef, in the subsurface, is composed of a framework of sponge skeletons encased in a matrix of modern clay trapped by the sponges. Three species of hexactinosan sponge form this skeletal framework through the biological fixing and deposition of opaline silica, which fuses the spicules of the skeleton of the individual sponges. This structure also allows inter- and con-specific attachment of young sponges onto skeletons of deceased hexactinosan sponges, which permits the multi-generational habitation of reef sites. The mounds (bioherms) and sheet-like accumulations (biostromes) cover a low angle, non-depositional, iceberg scoured seafloor, relict since the deglaciation of the region, about 13 thousand years ago. The base of the oldest sponge reefs date from approximately 9000 years BP. The reefs discontinuously cover about 700 km2 of seafloor in Queen Charlotte Sound and Hecate Strait at depths between 165 and 230 metres.
The sponge reefs have been subject to damage by seafloor trawling in the past decade. Sidescan sonar data indicate that intensive trawling has impacted one of the four hexactinellid sponge reef complexes. Repeat sidescan sonar surveys accomplished in 1988 and 1999, indicate that seafloor scouring by trawling has occurred at a sponge reef complex in southern Queen Charlotte Sound. Trawl marks identified with sidescan sonar record the passage of trawl doors across many sponge biohermal structures in water depths of 210 to 220 metres. The importance of the sponge reefs to the ecology of the continental shelf is largely unknown. Qualitative submersible observations suggest that species of crab, shrimp, prawns and rockfish utilize interstices within and between the sponges as refugia. A cyclicity of habitation by sponges followed by sediment coverage of mound surfaces is inferred from core and photographic data.
The sponge reefs, as geological features, are most closely related to Upper Jurassic siliceous sponge reefs, which stretched in a belt 7000 kilometres wide across the northern Tethys and Atlantic Ocean margins. The analogue, which the modern sponge reefs provide for the extinct reef belt, represents a unique opportunity to gain insight into what was the largest bioconstruction in Earth History. In view of the globally unique and fragile nature of the sponge reefs, and the unknown contribution that these reefs make to the shelf ecosystem, recommendations for habitat management include (1) restriction of mobile fishing gear deployment and other types of seafloor dragging in the sponge reef complexes; (2) further biological, biophysical and environmental studies of the sponge reefs to define ecological relationships and the critical physical environmental conditions of reef formation and growth; and (3) further surveys to allow assessment of reef "health" and the nature and extent of impacts sustained to date.
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