Explanations for enhanced growth of Odonthalia:
The enhanced growth of Odonthalia may be an artifact of laboratory conditions, and field tests are required for confirmation. No. The experiment is well designed to test the hypothesis. Not mentioned above is the fact that the mesocosms are flushed twice daily with natural seawater to simulate high- and low-tide conditions. The seawater is filtered to remove large particles but to allow phytoplankton to pass through for growth of the mussels.
Other chemicals essential for growth of algae are excreted by mussels. Yes, most notably small levels of phosphorus, but this does not diminish the impact that mussels have on algal growth. In this regard, the author notes that when nitrogen is seasonally depleted in Oregon coastal waters there is still a significant amount of phosphorus available.
Carbon dioxide is required for growth of plants and is released as a by-product of metabolism by mussels. Yes. This may also be a contributing factor in the results, but is probably minor owing to the daily flushing with natural carbon dioxide-containing seawater.
Other invertebrates growing on the mussel shells add to the ammonia available to the algae. Yes, but the mussels are cleaned off as much as possible before being used in the experiment. In any case, contribution of these epifauna in relation to the much larger mass of mussels would likely be small.