An ecological engineering for treating eutrophication is studied. Loop analysis is used for analysing the combination of food chains in the ecological engineering. In the engineering, shellfishes(SF) are used for cleaning excessive algae(AL). For changing the parghway of nutrients transfering,aquatic plants(AP) are cultured and are consumed by the farmed fishes (FS). By catching shellfish and fish , organic matter (OM) and inorganic salts (NP) in the water body can be exported along the pathways of algae(AL...