The paradox of internal phosphorus loading from oxic areas of iron-rich eutrophic boreal lakes: insights from porewater geochemistry

Internal phosphorus (P) loading from sediments is an important component of total P supply to the water column of many eutrophic lakes.  A long-standing paradigm states that the magnitude of internal loading through diffusion of P is limited in the presence of iron (Fe) oxides at the sediment-water-interface, due to efficient sorption and co-precipitation of P with oxide minerals. Iron-rich sediments underlying oxic water columns in shallow lake areas are thus expected to retain, rather than release P. However, recent statistical investigations have suggested that oxic epilimnetic areas of stratifying lakes may be responsible for a significant fraction of the internal P loading in these systems [1], implying a "leaky" seal of Fe oxides even under oxic conditions. Here we study the mechanisms of internal P loading in two Fe-rich eutrophic lakes in southern Finland through geochemical analysis of porewaters, over one annual cycle at five study sites per lake. Diffusive flux calculations using Fick's Law, and upscaling to whole-lake areal estimates, confirm that shallow (approx. <10 m) areas dominate internal P loading even during stratified conditions in summer. Furthermore, the highest instantaneous fluxes of the study were observed in shallow sites in late summer. The results suggest in shallow eutrophic settings with a high organic matter flux to sediments and elevated summer temperatures, remineralization reactions at the sediment-water interface regenerate P efficiently enough to escape capture by Fe oxides, even under sediment molar Fe/P ratios >20.     

 

[1] Tammeorg, O., Möls, T., Niemistö, J., Holmroos, H., & Horppila, J. (2017). The actual role of oxygen deficit in the linkage of the water quality and benthic phosphorus release: potential implications for lake restoration. Science of the Total Environment, 599, 732-738.