Glass sponge reefs form an alien world that is right at our doorstep and unique to the BC coast. During my masters research (I’ve since moved on to a PhD project) I had the opportunity to look at how tidal waters flow over the sponges.
One of these sponge reefs has made Fraser Ridge its home. In the shadow of the Vancouver International Airport, Fraser Ridge sits in about 200 m of water a few kilometres off shore. Scoured down to the rock by tides, the ridge is surrounded by muddy goo dropped by the Fraser River. If you have ever flown from Victoria to Vancouver, you’ve passed over Fraser Ridge.
I was lucky enough to watch in real time on an high-definition screen while ROPOS (a robotic submersible) explored the sponge reef at Fraser Ridge. A ‘snow storm’ of settling sediment obscured the view as ROPOS ‘flew’ towards the reef. As the reef finally became visible, my first thought was that I was watching a scene from a science fiction movie where a space ship is approaching a city on another planet.
Glass sponges form elaborate constructions of fiberglass. Touching them feels like pink insulation – smooth and prickly at the same time. Their colour palate is pale – I only saw yellow, peach and white sponges (other colours can be seen elsewhere). Some look like a crime scene for a mass mannequin massacre, hollow limbs sticking up at odd angles, while others look like a discombobulated jumble of bowling pins of different sizes. Nothing like them exist on land.
To form a reef, live sponges build on the scaffolding left by their ancestors. When sponges die, their structure remains which fills with sediment forming a solid base for new sponges to build on. At Fraser Ridge the reef reaches a height of about 14 m.
Glass sponge reefs are unique to BC, but that wasn’t always the case. When dinosaurs lumbered on land, glass sponge reefs grew over large swaths of the shallow oceans. Studying modern glass sponge reefs gives us a window into a past ecosystem. Today, these reefs host a diverse ecosystem by providing shelter and food. Juvenile fish spend their youth darting about spongy spines and crevices while shrimp use their delicate claws to clean alga off the sponges.
Since sponges have chosen to stay put and build, their food must come to them. They filter feed by pumping water through elaborated internal canals (there are some great videos of dye experiments that show this). A sponge’s sustenance comes by water currents and that’s where my work comes in as I’m interested in how water moves.
We used data from an ADCP (acoustic doppler current profiler) to look at the structure of the flow over the ridge. An ADCP works by sending out an acoustic pulse that reflects off the little particles found throughout the water column. The instrument then calculates the water’s velocity at different depths. For my project, this data was collected from a ship and from a mooring we put on the bottom next to the sponge reef.
Local topography, perhaps even augmented by the sponge reef’s own structures, can change how the tides flow – it turns out to be a balance between the water velocity and density. At Fraser Ridge, we found a pocket of intermittently faster flow lasting only a few hours in the region of the sponge reef. This period of high-speed flow flushed away the fine sediment from the river, preventing the sponges from being buried and brought in food. When the tides wane, a cloud of tasty delights is left for the sponges to filter through.
–Jeannette Bedard, OSS Member
Photo credit: Sponge Reef on Fraser Ridge, VENUS, Ocean Networks Canada, University of Victoria