In-situ, high-resolution evidence from water-sediment interface for significant role of iron bound phosphorus in...

Potential release of phosphorus (P) bound to iron (Fe) is critical because of the aggravating effects on P load in aquatic ecosystems. However, the process is largely unknown due to the absence of in-situ high-resolution evidence. Dissolved oxygen (DO), ferrous ion (Fe²⁺), and dissolved reactive phosphate (DRP) in interstitial water of sediment columns from a eutrophic shallow lake were measured using the novel colorimetric planar optode imaging method and ZrO-Chelex DGT technology during controlled experimental episodes. The solid Fe and P fractions in sediments were also simultaneously evaluated by employing sequential extraction procedure and spectra scanning analysis including SEM-EDS and ⁵⁷Fe-Mössbauer spectroscopy. The results demonstrated that the DO penetration depths were accordingly regulated with time, the depths depended on the oxygen supply patterns, and oxygen depletion occurred at anaerobic intervals. Considerable increases of concentrations and diffusion of Fe²⁺ and DRP in interstitial water upward from the deep layer into the overlying water were mirrored by decreased concentrations of solid Fe bound P and mineral phase Fe(II) during an anaerobic episode. This confirmed that the re-dissolution of solid Fe bound P pools is the most important source of labile P, and aggravates the P budget in lake water via anaerobic intervals. The reduction-precipitation mechanism of Fe bound P during different oxidation scenarios indicated that the Fe bound P in sediments can act as intermediates between Po and Ca bound P, and result in the permanent burying of authigenic Ca bound P. Significantly positive correlations (R² ≥ 0.7783, n = 74) between labile Fe²⁺ and DRP on both redox conditions also provided explicit evidence for the critical role of redox controlling Fe in labile P cycling at the lacustrine sediment-water interface. These findings provide improved insight for potential controlling effort of Fe coupled P to labile P depending on the oxygen supply in shallow-water hypereutrophic lakes.