Phosphorus release from coastal Baltic Sea sediments as estimated from sediment profiles

We used the decline in total phosphorus (P) concentration with depth in sediment profiles from the North-western Baltic Proper coastal zone to calculate the site-specific amount of sediment P eventually to be released to the water column: The potentially mobile P. P fractionation revealed that iron bound P dominated the potentially mobile P at sites with oxic surface sediment layers. Organic P forms were also a major constituent of the potentially mobile P pool. We determined that 1–7 g P/m2 were potentially mobile at our sites, and the turnover time of this P pool was considered short, i.e., less than a decade. To determine long-term average P fluxes to and from the surface sediment layer, we first multiplied the constant and relatively low P concentration in deeper sediment layers with the sediment accumulation rate to gain the P burial rate. Then the average total P concentration in settling matter was multiplied with the sediment accumulation rate to estimate the depositional P flux at each site. The difference between the depositional and burial rates represents the long-term average release rate of sediment P and varied between 1.0 and 2.7 g P/m2 yr among our sites. These rates are at the same order of magnitude as values reported from other areas of the Baltic Sea, and constitute a major source of P to the water column.

Research highlights
► P concentration declined with sediment depth until it stabilized at 10 cm in the profile.
► The sediment cannot bind and permanently bury all deposited P.
► The elevated P concentration in surface sediment represents P to be released.