Internal phosphorus cycling in water quality in restored lakes and the effectiveness of lake restoration
Agricultural environments are known for their large phosphorus (P) footprints. Given that enriched P negatively affects water quality in downstream ecosystems, current freshwater restoration and water quality management is aimed at reducing P concentrations in both runoff and aquatic ecosystems. However, little is known about the role of internal loadings on water quality and complex biogeochemical processes and subsequent internal P release can impede the effectiveness of restoration and management measures. Phosphorus availability and rate of internal P release (i.e. internal loading) from sediment are highly influenced by biological activity and chemical balance between precipitation and dissolution of P. Moreover, seasonal oxygen depletion in lakes can accelerate internal P release, which can act as an internal driver of persistent eutrophication. In this study, we aimed to understand how internal P cycling varies in response to time since restoration, trophic status and other environmental factors in lakes. We set up a flow-through cores experiment with 6-replicated sediment collected from 5 restored lakes. Two separate core experiments were conducted under aerobic and anaerobic conditions. Inflow and outflowing water samples were collected for 20 days to quantify internal P flux from sediment. Sediment total P contents and phosphatase activity were measured at the end of the each experiment.
Dissolved inorganic P (DIP) concentrations in inflow water, which were collected from the center of each lake, varied between lakes, ranging 50-502 µg/L. We found a clear P retention in sediment in high P loading condition while more P was released from the sediment of lakes with low P concentrations in overlying water. DIP concentrations in outflow water samples were fluctuated throughout the experiment periods. However, cumulative P in the outflow water appeared to be higher under anaerobic condition. Our results can be explained by facilitated dissolution and subsequent diffusion of sediment P into the water column under anaerobic condition and with low P concentrations in the overlying water. This study indicates that sediment P flux behaves differently depending on environmental conditions in lakes, such as sediment characteristics, stratification and trophic status. Given the importance of P flux and the contents in water-sediment interface, understanding the internal P dynamics and the relationship with environmental factors can help in determining the effectiveness of restoration and water quality management strategies.