Why the phosphorus retention of lakes does not necessarily depend on the oxygen supply to their sediment surface

Abstract

In order to improve the trophic state of Lake Sempach, a eutrophied lake in central Switzerland, its external phosphorus (P) load has been decreased and its hypolimnion has been artificially oxygenated to lower the lake‐internal P recycling. Based on more than 15 yr of experience, we conclude that the reduction of the external P load resulted in a corresponding decrease of the lake’s P concentration. However, contrary to initial expectations, increased hypolimnetic dissolved oxygen concentrations neither (1) reduced the P release from sediments during summer nor (2) resulted in anincreased permanent P retention. These observations warrant a reevaluation of the well‐accepted management strategy of decreasing the lake internal P cycling by maintaining an aerobic hypolimnion and sediment surface. We hypothesize that oxygenation only results in an increased permanent benthic P burial if, because of the depressed sulfide production, more ferrous phosphate (e.g., vivianite) and less FeS is deposited in the anoxic sediment. Hence, it is not the oxic sediment surface that directly affects the permanent redox‐dependent sediment P retention but the molar ratio of the available reactive Fe(II) : S2‐ : PO4 in the anoxic sediment. This ratio is driven by the settling rate and the nature of organic matter and particulate iron, as well as the supply of oxygen, nitrate, and sulfate to the sediment.

Groundwater-induced redox-gradients control soil properties and phosphorus availability across four headwater wetlands, New York, USA

“…The exact mechanism to explain these patterns, however, remains less certain. It is widely known that SO 4 2-reduction enhances iron mineral dissolution and consequently releases P (Caraco et al 1989;Roden and Edmonds 1997;Smolders and Roelofs 1993), but other processes secondary to iron and sulfur redox reactions also can increase P mobilization (Gachter and Muller 2003). In particular, alkalinity generation enhances internal eutrophication by neutralizing acidity generated by microbial respiration (Lamers et al 1998a;Smolders et al 2003) and facilitating microbial mineralization (Lamers et al 2001;Lucassen et al 2004a;Roelofs 1991;Smolders et al 1995), as well as by inhibiting P sorption onto iron oxides in sediments (Hawke et al 1989).…”

Eutrophication of aquatic ecosystems: Bistability and soil phosphorus

“…Rapid recycling rates are known from some enriched lakes (14,19). Various mechanisms for increased phosphorus release from enriched sediments of different lakes have been investigated (14,(19)(20)(21)(22). Some eutrophic lakes have failed to respond, or responded with long delays, to decreases in phosphorus input because of rapid recycling from sediments (12,17,(23)(24)(25), thereby demonstrating the effectiveness of recycling in maintaining a eutrophic state.…”

Tackling Hypoxia in the Baltic Sea: Is Engineering a Solution?

“…Although there are gaps in our understanding of hypoxia (21), remediation experience from lakes tells us that simply adding O 2 may not in fact help to mitigate eutrophication (26,27). Pilot experiments with other types of manipulations are underway, with further projects planned.…”