The principal aim of this study was to evaluate how nutrient regime and TN:TP ratios regulate algal chlorophyll in Korean agricultural reservoirs. We developed empirical models of log-transformed CHL-TN:TP ratio based on seasonality. Winter algal chlorophyll was highly influenced by the ratio of TN:TP (R² = 0.78, p<0.01) compared to chlorophyll-a in the spring (R² = 0.38, p<0.01), summer (R² = 0.50, p<0.01), and autumn (R² = 0.44, p<0.01), respectively. Regression analysis of log-transformed TN:TP ratios-TP and TN:TP ratios-TN on CHL-a showed that the ratio of TN:TP are highly influenced by the concentration of TP rather than TN. The ambient nutrient concentrations of the agricultural reservoirs showed positive linear relationships with water quality parameters of water temperature, pH, BOD, COD, DO, and TSS. Summer monsoon is an important factor that determines the nutrient regime, algal chlorophyll, and suspended solids in the reservoirs. Nutrient enrichment bioassays (NEBs) suggest that phosphorus was the primary limiting nutrient for algae production in the agricultural reservoirs. Analysis of trophic state index deviation (TSID) showed that P-limitation and biogenic turbidity (non-algal light limitation) were key regulating factors for algal growth in the reservoirs. Overall, our study suggested that the TN:TP ratio was the key regulating factor for algal biomass in the reservoirs.