[Michael Hoaster]

Thanks taricha. Yes, I would expect the mud to have abundant organic material, as well as all kinds of fauna, adding biodiversity. I have read that seagrass likes soft, muddy sand to root in. In the previous setup, I added some of this mud, then covered it with sand, so it's not exposed to light or the bulk water. The grasses responded well.

I got it from Florida Pets. It is actual, black, stinky mud. So you don't need a lot.

Rather than planting in clean sand, I want to give the grasses a fertile home right from the start, rather than waiting for nutrients to build up. And since they are true, rooted plants, they take up a lot of nutrients from the soil.

So hopefully, my dirty sand bed will provide a good home to both the grasses and micro fauna, adding nutrients and diversity to the foundation of the ecosystem.

While we're on the subject, I wanted to elaborate on the reasoning for using multiple sand grain sizes. Layers of different grain sizes are conducive to different pore water oxygen levels and thus different kinds of bacteria. By layering with coarse at the top, medium in the middle and fine on the bottom, I should get a gradient from aerobic to anaerobic to anoxic conditions. This should foster diversity in bacteria, which is a good thing. Also, the coarse top layer provides habitat (and refuge) for pods and other tiny creatures to feed on accumulating detritus.