Reportedly, Posidonia oceanica has a tremendous capability 
Someone ought to have a look at it. Some references:
- Inversion of acoustic waveguide propagation features to measure oxygen synthesis by Posidonia oceanica
- High Net Primary Production of Mediterranean Seagrass (Posidonia oceanica) meadows determined with aquatic eddy covariance (about aquatic eddy covariance)
- The value of the seagrass Posidonia oceanica: A natural capital assessment
- The photosynthetic capacity of the seagrass Posidonia oceanica: influence of nitrogen and light
ecoquant
