Pore water phosphate and ammonia below the permanent halocline in the south-eastern Baltic Sea and their benthic fluxes under anoxic conditions

In this paper the results of a study on the distribution of pore water phosphates and ammonia, and their fluxes under anoxic condition in a deep (> 70 m) accumulation-type bottom of the south-eastern Baltic Sea, namely in the Gdańsk Deep and the adjacent areas, are presented. All measurements were taken during the growth period, i.e. in September 2000, April 2001 and June 2002. Benthic phosphate and ammonia fluxes were estimated using Fickʹs First Law. Phosphate and ammonia concentrations ranged from 7.5 to 266.3 μmol dm− 3 and from 53.6 to 1248.3 μmol dm− 3, respectively. The values recorded in the central part of the Gdańsk Deep were lower than those found both on its slopes and on the SW slope of the Gotland Deep. The lowest phosphate contents were typical of the Oblique Sill which separates the Gdańsk and Gotland Deeps. In 1993–2002, as a result of anoxia the sediments in the Gdańsk Deep released about 5.1 × 103 t P and 22.8 × 103 t N. These loads supplied on average 1.5% and 0.9% of phytoplanktonʹs demand for P and N, respectively. In comparison to the total external load of nutrients discharged to the Gulf of Gdańsk (i.e. 8.79 × 103 t year− 1 Ptot and 130.79 × 103 t year− 1 Ntot; [Witek, Z., Humborg, Ch., Savchuk, O., Grelowski, A. and Łysiak-Pastuszak, E., 2003. Nitrogen and phosphorus budgets of the Gulf of Gdańsk (Baltic Sea). Est. Coast. Shelf Sci., 57:239–248.]), the return flux of P and N from the anoxic sediments to the water column in the Gdańsk Deep was a minor source of these elements.