Sediments play an important role in dissolved inorganic phosphorus (DIP) recycling, which needs to be precisely quantified in eutrophic estuaries. A coupled field data and diagenetic modelling approach was used to study P dynamics in two estuaries (Elorn and Aulne, Brittany, France).

An existing model (OMEXDIA) was extended with phosphorus (P) benthic cycle by adding dissolved inorganic phosphorus (DIP) and particulate P fractions (organic P, Fe-bound P and Ca-bound P). The model was fitted to pore water oxygen, nitrate, ammonium, oxygen demand units (reduced substances that were produced by the anoxic mineralization; Fe2+, Mn2+ and H2S; ODU), DIP, sediment organic P and C, Fe-bound P and Ca-bound P data from four seasons (February, May, July and October 2009) in two eutrophic estuaries (Elorn and Aulne). These two systems were investigated along the salinity gradient with 3 stations per estuary.

The model shows that high organic C fluxes deposited in the sediments (23–98 mmol m-2 d-1) caused high organic P mineralization rates, which is the modeled main benthic source of DIP (77%). The DIP recycling fluxes were calculated in Elorn (463 ± 122 μmol m-2 d-1) and in Aulne (320 ± 137 μmol m-2 d-1) estuaries, and overall 85% of DIP produced in the sediment was recycled to the overlying water. A limited but substantial proportion (15%) was precipitated as Ca-bound P except in the upstream reach of the Aulne in February, where the integrated rate of Ca-bound P precipitation constitutes a major sink of DIP (83%).

The high DIP recycling fluxes in Elorn and Aulne estuaries, integrated over the entire area, were 3–20 times larger than the river DIP input. This comparison of external and internal DIP loads shows the major role of these sediments as sources of DIP to the estuary and the potential storage of formerly discharged nutrients.