Temporal Phosphorus Dynamics in Shallow Eutrophic Lake Suwa, Japan

It can be difficult to decrease the water phosphorus (P) concentration in eutrophic shallow lakes, even if the external P loading is reduced, owing to a high level of internal P loading to surface water from sediment. However, in shallow Lake Suwa, Japan, lake water P concentration has largely decreased in recent years owing to low levels of internal P loading, as well as declining external P loading. We measured water/sediment P and iron (Fe) concentrations and the P release rate from sediment in Lake Suwa, and then compared it with data from the 1970s. In the 1970s, the P concentration throughout the lake water was high during the hypoxic period. Recently, however, the P concentration has increased only in the hypolimnion during the hypoxic period. This suggests that internal P loading from sediment to surface water has largely been suppressed during the hypoxic period in recent years. This may be due to (i) stronger water stratification from global warming, (ii) a greater decrease in the P release rate from the sediment owing to a decline in sediment P concentration from the 1970s to 2020, and (iii) stronger formation of the Fe–P cycle in Lake Suwa recently, compared with that in the 1970s. Our results indicated the need to reduce both external P loading, and internal P loading from sediment to water, for effective water quality improvement in shallow lakes.