Ten Years of Artificial Mixing and Oxygenation:  No Effect on the Internal Phosphorus Loading of Two Eutrophic Lakes

Hypolimnetic aeration was used in conjunction with reductions in phosphorus (P) inputs in order to lower the P concentrations in two eutrophic lakes. Based on more than 10 years of experience with artificial mixing and hypolimnetic oxygenation we conclude that the lakes' internal P cycling was not affected by increased hypolimnetic dissolved oxygen (D.O.) concentrations. We show that irrespective of oxic conditions in the hypolimnion the sediment/water interface remained anoxic due to unchanged high sedimentation rates of organic matter. This may explain why oxygenation did not increase the P retention capacity of the sediment. However, the presented information indicates that, contrary to general assumption, anoxic sediment surface and high P release rates from lake sediments may not be cause-effect related but simply two parallel symptoms of one common cause: excessive organic matter and P sedimentation exhausting the stock of hypolimnetic D.O. and exceeding the P retention capacity of the sediment after diagenesis. Therefore, it is not surprising that fighting one symptom (an anoxic hypolimnion and sediment surface) does not solve the other one (a high P release rate). These findings and considerations based on more than 10 years of experimental experience warrant a reevaluation of the well accepted theoretical management strategy of limiting lake internal P cycling by maintaining an aerobic hypolimnion and sediment surface.