Phytoplankton (specifically algae and photosynthetic bacteria) are responsible for the production of about 50% of the Earth's O2. That is a fairly large role in a natural ecosystem.
I don't know exactly how much O2 is produced by macrophytes (higher plants) in a typical aquatic ecosystem, but O2 is required for the respiration of plants, fish, and other organisms. Algae take care of that need pretty well.
I would venture a guess that any "healthy" aquatic environment would include algae. I also don't believe its presence would necessarily signify an "unhealthy" aquatic environment although I imagine an excess thereof applies implications of what one might call an imbalance.
I don't know exactly how much O2 is produced by macrophytes (higher plants) in a typical aquatic ecosystem, but O2 is required for the respiration of plants, fish, and other organisms. Algae take care of that need pretty well.
I would venture a guess that any "healthy" aquatic environment would include algae. I also don't believe its presence would necessarily signify an "unhealthy" aquatic environment although I imagine an excess thereof applies implications of what one might call an imbalance.