It is known that phytoplankton biomass or turbidity are lower in waters with submerged macrophytes than those without submerged plants at a given nutrient level. We hypothesize that presence of submerged macrophytes would lower phytoplankton biomass below levels expected by total phosphorus levels through various mechanisms and that phytoplankton biomass would decrease more as the biomass increase of the submerged macrophytes. To find submerged macrophytes effectively lowering phytoplankton growth, we conducted spatial field surveys at 21 water bodies and a temporal monitoring at Seung-un 1 Reservoir, Anmyyeondo Island. We measured chlorophyll ${alpha}$ concentrations and total phosphorus (TP) concentrations from waters in patches of submerged macrophytes with measurements of submerged plant biomass. Majority of our sites with submerged macrophytes showed much less chlorophyll a concentrations than the predicted ones from literature. Among submerged macrophytes studied, Myriophyllum spicatum and Hydrilla verticillata showed patterns of lowering chlorophyll ${alpha}/TP$ ratios with increase of their biomass in both spatial and temporal surveys.