Looks like bivalves (specifically lorinid clams) are good for seagrasses, in combination with the sulfur bacteria that live in the clams' huge gills.  The clams provide a nice bed for the bacteria, which obtain their energy by oxidizing sulphides.  The bacteria pay rent to the clams.  In turn, the seagrasses are relieved of sulphide stress (think of stinky swamp muck) and grow vigorously, making lots of dead leaves and more sulphide.  Everybody wins.   Science.

Here's the UF press release.

Here's the short version from Science:

Submarine seagrass meadows are critical to fisheries and coastline protection and provide feeding grounds for many endangered species, including dugongs and turtles, and serve as a nursery for coral reef fish. The persistence and maintenance of seagrass ecosystems have been mysterious, because accumulation of organic matter in the beds should rapidly lead to toxic sulphide levels in the sediment. Using a meta-analysis and a field study, van der Heide et al. (p. 1432) atttribute the 100-million-year success of seagrasses to a three-stage symbiosis.

Seagrass beds worldwide contain high densities of small lucinid bivalves that have symbiotic sulphide-oxidizing bacteria in their gills. This association appears to relieve any sulphide stress for seagrasses, while the lucinids and their symbionts profit from the accumulation of degradable organic matter and oxygen release from seagrass roots.

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According to the report, Lucinid clams and their bacteria friends go back to the Silurian (when bony fish and a few land plants appeared), and seagrasses emerged in the Cretaceous (late dinosaur times).  They've been together for a long time.  I know hardly anything about seagrass beds, but this research seems to go a long way toward explaining how seagrass beds work.

There was also some clever experimentation that should be repeatable by people with modest facilities (maybe a back yard with some water containers) here in Florida.