Shallow-lake sediments release nutrients by complete destratification events.

The development of phytoplankton communities in hypereutrophic shallow lakes, often used for aquaculture, is not fully understood and can sometimes be unpredictable. Focusing on the abiotic factors that regulate their succession, we recorded short-term mixing events in a shallow lake and examined their relationship with nutrient release from sediments in the lab. In situ measurements reveal a dynamic cycle of mixing and stratification during summer, when the lake mostly stratifies during the day and mixes at night, depending on wind conditions. The studied lake was stratified 45% of the time and, on average, mixed every 1.5 days. In terms of hydrodynamics, the velocities of surface and bottom waters are similar in magnitude, regardless of whether conditions are calm or windy. Stirred-core experiments recreated both lake hydrodynamic regimes and the observed patterns of destratification at the study site. Temporal destratification experiments show that the sediment releases more solutes during complete mixing than during partial destratification, due to an increase in sediment/water concentration gradients. This results in more phosphorus and ammonia being released, and more nitrate being consumed by sediments, when the water column is fully mixed compared to when a bottom layer remains unmixed. The effect of dissolved oxygen did not directly influence nutrient release by Fe-P compounds dissolution, as oxygen above the sediment did not fall below 50% saturation, but mixing enhanced the transport of electron acceptors to the sediment. The cycle of stratification and mixing appears to be a key factor in internal loading under oxic conditions.