Sediment phosphorus cycling in a nutrient-rich embayment in relation to sediment phosphorus pool and release

A semi-enclosed river-mouth bay (Kojima Bay) and the artificial lake within it (Kojima Lake) were studied to examine their nutrient storage and export processes and characteristics. The aim of this study was to perform mass balance analysis using long-term monitoring and historical accumulation data of phosphorus (P) via 210Pb measurements of accumulated sediment. The results are then compared with data based on analysis of P concentrations in core samples to delineate input and output nutrient fluxes in a semi-enclosed bay and an artificial lake, in order to evaluate if the lake sediment is a source or sink of P during the transportation process from the river to the bay. From 1980 to 2008, total P flux into the lake is 3520 tons. The lake sediment phosphorus retention estimated by the model (383 tons) was lower than that obtained by multiplying the average total P concentration in sediment samples by the sediment accumulation rate (1288 tons), possibly indicating that recycling from sediments deposited before 1980 contributed to the recent P cycle. The implication for these data is that the severe eutrophication of the lake in the 1970s led to greatly elevated P levels. The inner artificial lake had a significant amount of trapped P, and the production of diatoms increased in response to P enrichment in lake water, an initial sign of eutrophication. The modeled annual P flux into the lake sediments decreased from ~ 15 g m−2 yr−1 (a flux into the sediment) in 1980 to negative values (a flux out of sediment) by 2008. The decreasing trend of nutrients accumulating in lake sediments suggests the lake is slowly recovering from eutrophication, especially since the 1990s.