We propose that phosphorus and nitrogen can be removed from polluted water using an ecological water treatment system consisting of periphyton and fish. In the proposed system, polluted water flows through a series of vessels, and the nutrients are taken up by periphyton growing on porous screens. Algal-grazing fish feed on the periphyton and either assimilate or egest the nutrients in mucus-bound feces that settle from the water into a sediment trap. Both the fish and their feces can be harvested as nutrient sinks. In this study we examined the effects of an algal-grazing cichlid (Tilapia mossambica) and a stoneroller minnow (Campostoma anomalum) on nutrient removal by the proposed water treatment system. In two experiments using tank-mesocosms, we found that the algalgrazing fish consumed the periphyton and transferred nutrients into the fish’s body tissues or the sediments. The mass of phosphorus sequestered in the nutrient sink in the body tissues of the fish was approximately equivalent to the phosphorus sink in the sediments. High removal rates for phosphorus and nitrogen suggest that the proposed ecological water treatment system may be an effective ecotechnology for removal of nutrients from polluted water.
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