Even a cubic foot of water would be well under 100lbs. I doubt a cubic foot of mussels + water to fill the rest could weigh a ton.
It may be possible for (especially stationary structures) to accumulate enormous columns of biofouling materials, but for such a layer to reach > 20 feet thick (possibly closer to 50 feet without counting the interstitial water) sounds unreasonable. But I'm not knowledgable in this area.
It may be possible for (especially stationary structures) to accumulate enormous columns of biofouling materials, but for such a layer to reach > 20 feet thick (possibly closer to 50 feet without counting the interstitial water) sounds unreasonable. But I'm not knowledgable in this area.