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Saturday, Oct. 4, 2008 | When mysterious black blobs started washing up on Fiesta Island’s shorelines, the city of San Diego turned to one man to figure out what they were: Keith Merkel, an ecologist who works as a consultant for the city. He’s spent the last 20 years studying Mission Bay and the creatures that call the bay home. It didn’t take Merkel long to figure out the blobs were sea sponges, which live on the bay floor. But the city has not yet discovered what caused them to die and wash ashore.
We sat down to talk with Merkel, principal ecologist for Merkel and Associates, about the mystery, what could be responsible and just how lively Mission Bay is beneath the surface.
What could explain the die-off in these sponges?
It’s really unknown what’s going on. Sponges die like anything else. The fact that they’re ending up on the beach is not unique. But to be candid, we don’t know why they’re dying and whether or not this is a unique phenomenon or one to come in the future.
Is there a checklist of potential causes? Can you rule out a die-off of everything out there?
Not necessarily. If you look at the sponges as a community, there’s two very dense organisms in Mission Bay — there’s eelgrass beds, the sea grass that covers most of the bottom of the bay, and then there’s sponge communities. Beyond that, the organisms are fairly small, live in or on the sediments, they’re not dense in distribution: sea stars and worms and clams. Most would not float to the surface if they died. And even if they did float to the surface, finding small organisms like that up on the beach is probably unlikely.
There could be something that’s having an effect on the larger communities of the bay. My guess is that it’s unique to the sponges. It could be a variety of different things.
Let’s brainstorm some possibilities for what could be killing them: Some unique pollution runoff? A bottle of bleach or some kind of toxin?
It’s very unlikely it’s a runoff issue. The reason being that these sponges live on the bottom, offshore. The sponges are in depths of water that are seven to 12 feet deep. The bay is 2,000 acres. So a toxin discharge that would actually result in the level and extent of sponges washing up would be measured in thousands and thousands of gallons. It’d have to be very large. It’s more likely that it’s a naturally occurring-type thing.
You might pick up a phytoplankton toxin. It could be a disease, a fungus, something running through the population, or a parasite in the sponges. It could be silt. Sponges feed by bringing water through the sponge, so if you plug up all the pores, you effectively suffocate the sponge with fine sediment. The storm drain inputs right now are pretty low. You’d have to bring tank trucks in by the dozens to get this kind of an effect (from runoff).
The sponges don’t have many enemies down there?
They’re a positive thing for the bay. All they do is sit on the bottom and filter water all day long. They provide improved clarity. A lot of free swimming animals — planktonic animals — extrude mucus chains to feed. Those mucus strands are floating around in the water. In low circulation areas, you can accumulate just a string of particles on mucus strands floating around — it’s kind of an obnoxious look to it. Anything that sucks water through traps the particles and removes that. There’s thousands and thousands of sponges down there cleaning the water all the time. Losses of sponges are a public nuisance, because they stink and they smell and they make the beaches unattractive. But it also is a question of whether we’re losing something to the bay water quality — and what’s the magnitude of that.
Have you ever seen a die-off like this before?
I have not. You find dead or dying sponges in a sponge bed. Sponges are pretty hearty animals. I’ve never seen this magnitude.
That’s in the last 20 years?
I did a survey of Mission Bay in 1988. Since that time I’ve done a lot of work in the bay, but I’ve never seen anything like this before.
Any population estimate for sponges in the bay? And also, the percentage or number that have died in the last two weeks?
I’ve not seen numbers of what’s [died]. I would speculate that there are tens to hundreds of thousands of sponges on the bay floor. Many, many, many tons.
So we’re not seeing a majority dying?
No. Yesterday some of the divers from here went out and collected some sponges from the beds. The sponges I received are all healthy, viable sponges. None of the sponges picked up show any necrotic condition. My belief is that what we’re seeing is probably either from a localized area of the bed — where many sponges in close proximity are getting affected similar to what you’d see with a virus, fungus or pathogen going through the population. There’s nothing in the sponges collected that would suggest they’re unhealthy.
You talked about the sponges’ role in the food web. Are there things that rely on sponges for their existence as well?
There are very few things in the food web that aren’t eaten by something. Sponges are consumed by small organisms as well. There are nudibranchs that feed on the sponges. By and large, eating a sponge is not a great use of your time. They’re not particularly nutritious. Most of the sponge structure is made up of spicules — glass, little jacks made out of glass. To consume sponges by larger animals is kind of like eating the glass of a window. It’s real hard on the gut of an animal. Small animals can remove stuff around the spicules, but it’s not a huge food supply in Mission Bay.
How lively of a marine environment is Mission Bay under the surface? Is there a lot going on beneath the surface?
Oh yeah. Mission Bay is one of the most interesting bays I work in. I do a lot of work with eelgrasses, the principal seagrass within bays and estuaries in California. Mission Bay historically has run about 50 percent cover of eelgrass on the bottom, making it the third largest eelgrass bed population in California — behind San Diego and San Francisco bay. And it’s has the greatest coverage. San Francisco Bay has about 1 percent coverage. Mission Bay is actually hugely beneficial in terms of eelgrass. It has a diverse community of fish. You get more oceanic communities by Quivera Basin. And up the northern wildlife preserve you have marsh communities. In my mind, it’s one of the neatest bays around, because it has so many different characteristics to it.