Glass sponges are conspicuous members of the deep-sea fauna, but in the northeastern Pacific they form unusual reefs covering kilometers of seafloor. Individual sponges in fjords can process up to 10 m 3 water d 21 osculum 21 ; sponge reefs must therefore process considerable volumes and could significantly affect local water properties. We measured, in situ, the flux of carbon and nitrogen through Aphrocallistes vastus, the dominant reef-building species on Fraser Ridge reef, and calculated the energetics of feeding for all reefs in the Strait of Georgia, British Columbia. Sponges removed up to 90% of bacteria from the water and released ammonium. Because of the high density of sponges, high volumetric flow rates (up to 210 6 35 m 3 m 22 d 21 , mean 6 standard error, 95% confidence interval (CI) 132-288 m 3 m 22 d 21 ), and the efficient extraction of bacteria, we calculate a grazing rate of 165 6 29 m 3 m 22 d 21 (95% CI 102-228 m 3 m 22 d 21 ) for sponge reefs, the highest benthic grazing rate of any suspension-feeding community measured to date. Reefs of A. vastus extract seven times more carbon (3.4 6 1.4 g C m 22 d 21 ) than can be supported by vertical flux of total carbon alone and therefore require productive waters and steady currents to sustain their strong grazing. We calculate that modern sponge reefs in the northeastern Pacific remove 2.27 3 10 5 6 0.91 3 10 5 kg of bacterial carbon daily, nearly an order of magnitude less than the 1.38 3 10 6 6 0.55 3 10 6 kg removed by past sponge reefs estimated to have covered the continental shelf.

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-2 d-1. Our high resolution mapping establishes a biological baseline for 3 glass sponge reefs in the Strait of Georgia.

Deoxygenation in the global ocean is predicted to induce ecosystem-wide changes. Analysis of multidecadal oxygen time-series projects the northeast Pacific to be a current and future hot spot of oxygen loss. However, the response of marine communities to deoxygenation is unresolved due to the lack of applicable data on component species. We repeated the same benthic transect (n = 10, between 45 and 190 m depths) over 8 years in a seasonally hypoxic fjord using remotely operated vehicles equipped with oxygen sensors to establish the lower oxygen levels at which 26 common epibenthic species can occur in the wild. By timing our surveys to shoaling hypoxia events, we show that fish and crustacean populations persist even in severe hypoxia (<0.5 mL L(-1) ) with no mortality effects but that migration of mobile species occurs. Consequently, the immediate response to hypoxia expansion is the collapse of community structure; normally partitioned distributions of resident species coalesced and localized densities increased. After oxygen renewal and formation of steep oxygen gradients, former ranges re-established. High frequency data from the nearby VENUS subsea observatory show the average oxygen level at our site declined by ~0.05 mL L(-1) year(-1) over the period of our study. The increased annual duration of the hypoxic (<1.4 mL L(-1) ) and severely hypoxic periods appears to reflect the oxygen dynamics demonstrated in offshore source waters and the adjacent Strait of Georgia. Should the current trajectory of oxygen loss continue, community homogenization and reduced suitable habitat may become the dominant state of epibenthic systems in the northeast Pacific. In situ oxygen occurrences were not congruent with lethal and sublethal hypoxia thresholds calculated across the literature for major taxonomic groups indicating that research biases toward laboratory studies on Atlantic species are not globally applicable. Region-specific hypoxia thresholds are necessary to predict future impacts of deoxygenation on marine biodiversity.

Resale or republication not permitted without written consent of the publisher Aphrocallistes vastus and Heterochone calyx are reef-building glass sponges, whose heavily silicified skeletons create a large reservoir of silicon. Photo: S. P. Leys, V. Tunnicliffe, and ROPOS

Sponges link the microbial loop with benthic communities by feeding on bacteria. Glass sponge reefs on the continental shelf of western Canada have extremely high grazing rates, consuming seven times more particulate carbon than can be supplied by vertical flux alone. Unlike many sponges, the reef building species Aphrocallistes vastus has no microbial symbionts and removes little dissolved organic carbon. To determine how reef sponges therefore get enough food to sustain such substantial grazing we measured stable carbon and nitrogen isotope signatures of water, sediment and sponge tissues. To ensure samples were temporally associated, we also studied the duration particles were retained in tissues in controlled feeding studies using microscopic beads and 13C-labeled bacteria. Although fecal pellets were expelled from sponges within 24 hours of feeding, intact bacteria were still found in tissues and sponge tissues retained elevated 13C levels for at least 14 days. These independent lines of evidence suggest that carbon in reef sponge tissues may reflect food consumed from days to weeks earlier. Stable isotope analysis suggests that heterotrophic bacteria ingested by the sponges comes from a confluence of trophic subsidies: from terrestrial and oceanic sources, and also potentially on sediment-borne bacteria resuspended by tidal currents.