Primary grazers

Primary grazers among the zooplankton constitute the next stage of the trophic web. We picture the primary grazers as feeding on algae, but many of them are also grazing on bacteria: as ecologists say, they're mixotrophic. In the biofilm, grazing protozoans help regulate bacterial colonies. Grazing pressures never entirely eliminate any strain of bacteria, but by keeping some living spaces and ecological niches less than completely filled, grazing keeps the diversity of strains high and in constantly self-renewing phases. Wastewater engineers have found that grazing pressures from protists actually enhance the rate at which decomposers reduce organics. In wastewater treatment plants where ciliates have experimentally been eliminated and amoebas and flagellates reduced to small populations, the effluent water became turbid and was found to contain higher levels of bacteria and suspended solids. Large healthy ciliate communities graze voraciously on dispersed bacteria and have the ability to flocculate suspended matter and its associated bacteria. In the 1980s T. Fenchel found that swimming nanoflagellates in experimentally enriched seawater cleared 10⁵ times their own cell volumes of water in an hour: that's hundreds of thousands times their cell volume, a metabolism that makes a hummingbird laid-back by comparison!
 
The result of a healthy plankton population  is clearer water with fewer bacteria in it; so, a well-balanced and diverse community of microscopic plankton is the surest cure for green water, which is simply a bloom of phytoplankton.