Phosphorus cycling in a coastal marine sediment, Aarhus Bay, Denmark

Seasonal variation in P sedimentation, sediment P release, and sediment P pools was studied in a coastal marine sediment at 16-m water depth. Net sedimentation of P amounted to 51–63 mmol m−2 yr−1, compared to a sediment P release of 34 mmol m−2 yr− 1. The resulting deficit corresponded with a burial flux of 18 mmol P m−2 yr−1. Iron-bound P was the most dynamic P pool in the mixed surface sediment, where it made up 175 mmol m−2 of a total of 530 mmol P m−2. Iron-bound P decreased rapidly with depth and contributed only 3.5% to the burial flux. P sedimented in spring was retained in the pool of iron-bound P until September–October, when half of the annual P release occurred. The autumn maximum in P release seemed to be linked to sulfate reduction by two mechanisms: sulfide was primarily responsible for the reduction of ferric oxyhydroxides (FeOOH) and hence the release of iron-bound P, and precipitation of FeS and FeS2 restricted the upward migration of Fe2+ but not of dissolved reactive phosphate (DRP) and resulted in a saturation of sorption sites for DRP on FeOOH in the sediment surface layer. The saturation of sorption sites was reflected by a minimum ratio of FeOOH to iron-bound P in surface sediment measured in October. Seasonal changes in this ratio provided the best correlation with the DRP efflux (R = −0.74), indicating that adsorption onto FeOOH is probably the most important factor controlling sediment P release.