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 hypoli mnetic oxygenation we conclude that the lakes internal P cycling was not af fected 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 indi cates that, contrary to general assumption, anoxic sediment surface and hig h P release rates from lake sediments may not be cause-effect related but s imply two parallel symptoms of one common cause: excessive organic matter a nd 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 tan anoxic hypolimnion and sedime nt surface) does not solve the other one (a high P release rate). These fin dings and considerations based on more than 10 years of experimental experi ence warrant a reevaluation of the well accepted theoretical management str ategy of limiting lake internal P cycling by maintaining an aerobic hypolim nion and sediment surface.