High resolution mapping of community structure in three glass sponge reefs (Porifera, Hexactinellida)

Glass sponge reefs (Porifera, Hexactinellida) are unique to the Pacific coast of Canada. To date, the locations and extents of reefs have only been assessed by multibeam echosounders, a method that does not resolve where live, dead and buried sponges are within a reef. We performed fine-scale (25 and 12.5 m grids) photographic surveys using remote operated vehicles and carried out GIS and semivariogram analysis to produce high resolution maps of the spatial distribution and abundance of glass sponges among 3 reefs in the Strait of Georgia and also to determine the community structure of animals associated with the reefs. Within a reef, glass sponges are clustered in spatially dependent patches 35 to 72 m in diameter, which we interpret to result from growth and reproduction processes. Reefs occur within a narrow range of depths (30 to 50 m), which may be explained by suspension feeding requirements for flow, which is accelerated over the underlying elevated bedrock mounds and ridges. The local abundance of fish and crustaceans was significantly higher in the presence of glass sponges while significantly lower abundances of molluscs and other sponges were found within the reefs. Also, we determined that a dense population of reef sponges can process water at 83 000 l s–1, removing carbon at a rate of 0.96 g C m–2d–1. Our high resolution mapping establishes a biological baseline for 3 glass sponge reefs in the Strait of Georgia.