Spatial and temporal variations in nutrients in water and riverbed sediments at the mouths of rivers that enter Lake Hachiro, a shallow eutrophic lake in Japan

Abstract We evaluated the spatial and temporal variations in dissolved inorganic nitrogen (DIN = NO3− + NO2− + NH4+) and soluble reactive phosphorus (SRP) concentrations and the processes that control them in water and riverbed sediments at the mouths of five main rivers (MTN, BBM, IKW, TYK, and BFM) that enter Lake Hachiro, a shallow eutrophic lake in Japan. At the river mouths, we sampled water monthly and analyzed the nutrient concentrations and dissolved N2O and CH4 concentrations. We also sampled riverbed sediments to determine the SRP release potential, inorganic P fractions, and denitrification potential (dNp). Riverine SRP concentrations showed large spatial and temporal variations, with values increasing from summer to autumn in the three southernmost rivers (IKW, TYK, and BFM). High dissolved CH4 concentrations were observed in association with high SRP concentrations, indicating that SRP was released from the sediments under hypoxic conditions. In contrast, NO3− concentrations did not differ among the rivers and decreased during summer, likely owing to denitrification, thereby decreasing DIN/SRP ratios to a level suitable for the formation of summer algal or cyanobacterial blooms in the three southernmost rivers. Sediment incubation demonstrated high SRP release rates and high dNp values in the sediments of these rivers due to their higher iron (Fe)-bound P and organic matter contents. The riverine SRP concentration was significantly correlated with dissolved CH4 and sediment dNp in positive, and DIN/SRP in negative. In conclusion, differences in Fe-bound P and sediment organic matter content suggested to cause spatial and temporal variations in water quality, with high SRP release and denitrification under hypoxic summer conditions, resulting in lower DIN/SRP ratios that may trigger algal or cyanobacterial blooms in this shallow eutrophic lake.