Development of a novel in-sediment passive sampler for profiling orthophosphate and internal phosphorus release near the sediment–water interface in a eutrophic lake

Abstract

Internal phosphorus (P) release from lake sediments is now recognized as an important P supply that maintains eutrophication, especially in lakes where stratification induces hypoxic conditions in the bottom waters. Freshwater lakes are increasingly threatened by eutrophication and harmful algal blooms. Therefore, to manage lakes, it is important to quantify the internal P release. The internal flux of P (i.e., the orthophosphate (PO₄) released from the sediments) may be miscalculated by the methods used to date, such as sediment core samples because the concentrations may be affected when the sediment is disturbed, and the spatial resolution of the sampling may be low. In this study, we developed a novel in-sediment passive sampler to determine the PO₄ flux from sediment and deployed it in a eutrophic lake, Lake Barato in Sapporo, Japan. We also deployed Chemcatcher passive samplers for PO₄ at the same time to investigate the change in the PO₄ concentrations in the water column. With these methods, we obtained the vertical and horizontal distributions of the PO₄ concentrations in the sediment porewater across approximately 10 × 20 cm close to the sediment–water interface (SWI) and in the water column. We observed relatively large centimeter-scale PO₄ hotspots within the shallow sediment layers (−1 to −5 cm below the SWI). These PO₄ hotspots were significantly larger during the summer season than in the other seasons, when thermal stratification and hypoxia influenced the P release. The PO₄ fluxes calculated with data from the in-sediment passive samplers ranged from 0.05 to 0.37 mg-P/m²/d, and were considerably lower than the estimates from the conventional sediment core sampling methods. In addition, the data from the Chemcatcher passive samplers showed that the temporal patterns in the time-weighted average PO₄ concentrations (around 10 µg-P/L) in the water column were consistent with the patterns from the in-sediment sampler. The results suggest that the in-sediment sampler provided a high-resolution vertical profile of the PO₄ concentrations near the SWI with minimal sediment disturbance, and that passive sampling techniques could be used to monitor the fluxes of PO₄ released from sediments and the PO₄ concentrations in the water column.