Oceanographic habitat of sponge reefs on the Western Canadian Continental Shelf

Abstract This paper describes oceanographic conditions in which siliceous (Hexactinellida, Hexactinosida) sponge reefs thrive near the heads of shelf canyons on the west coast of Canada. Unique in the present world ocean, these colonies consist of mounds (bioherms) up to 21 m in height and beds (biostromes), which extend for tens of kilometers at depths between 140 and 240 m. The relaxation of winter downwelling and occasional weak summer upwelling enhances northward up-canyon intrusions of nutrient rich bottom waters in Hecate Strait. As bottom waters cross the shelf, they are further enriched with nutrients and depleted in oxygen by remineralization processes. Silicate concentrations in waters around sponge reefs are thus enhanced during what is likely the sponge's main growing season. Bottom waters in the vicinity of the sponge reefs contain 43–75 μM silicate and 64–152 μM dissolved oxygen, and have a temperature range of 5.5–7.3 C and a salinity range of 33.2–34.2. Canyons harbouring the sponge reefs provide a means of concentrating particulate material that the sponges can use to enrich their habitat. Detrital rain supplies particulates to canyons in what appears to be tidally modulated pulses. Particulate material entrained in near bottom currents has a residence time of approximately 6 days in contact with the largest sponge reef and is effectively trapped by sponges, which enhances mound construction. A simple budget of Si uptake by the 425 km 2 area of the northern sponge reef shows that it has slight impact on the Si budget of this region. Scour by the ∼25 cm s −1 bottom tidal currents eliminates sediment accumulation on the reefs and the surrounding seabed.