Internal phosphorus loading due to sediment anoxia in shallow areas: implications for lake aeration treatments

Shallow lake sediments may be anoxic despite overlying aerated water. In the current study, we aimed to ascertain the contribution of shallow areas to internal phosphorus (P) loading due to sediment anoxia in stratifying lakes. Moreover, we analyzed relationships of the key water quality variables with internal P loading due to sediment anoxia originating solely from stratifying areas (IPₒbₛ) and that accounting also for the shallow areas (IPₚᵣₑd) for a set of Finnish lakes, including intentionally aerated and non-aerated lakes. Finally, using a broader set of lakes worldwide, we established a specific combination of lake characteristics that predict sediment P release due to sediment anoxia and linked it to the practices of aeration. Our results showed that shallow lake areas (a difference between IPₚᵣₑd and IPₒbₛ) contributed about half of the total P flux due to sediment anoxia. While all of the studied water quality variables related significantly to IPₚᵣₑd, only the concentration of total phosphorus (TP) in the near-bottom water layer related significantly to IPₒbₛ. This indicates the key importance of P release of shallow areas for water quality. The concentrations of TP in the surface water layer and chlorophyll a were significantly dependent on IPₚᵣₑd irrespectively of the treatment (aerated lakes or not). P supply from shallow areas may affect aeration effectiveness in stratifying lakes. IPₚᵣₑd was found to be dependent on the specific combination of lake characteristics (including mean and maximum depth, lake and catchment area, external P loading) PC3, driven mainly by external P loading. Hence, external load reduction should be considered as the first priority in lake water quality management. By linking the dependence of IPₚᵣₑd on PC3 to aeration practices, we determined the conditions that promise increased effectiveness of aeration treatments.