Sea bottom anoxia in the Archipelago Sea, northern Baltic Sea—Implications for phosphorus remineralization at the sediment surface

The effects of the hydrophysical environment on oxygen conditions and P remineralization at sediment surface were studied in the Archipelago Sea, northern Baltic Sea. At 94 sites, the water column was profiled for conductivity, temperature and oxygen, and the topmost sediment was collected for P and C fractions and (oxy)hydroxide Fe analysis. The near-bottom water and sediment parameters as well as visual observations on the vertical colour distribution in the sediment cores were used to statistically classify the sediment surfaces into oxic, anoxic and suboxic (fluctuating oxygen conditions) bottoms. The anoxic bottoms occurred in the basins with water depths less than 47 m, and the suboxic bottoms were most common in the basins with depths between 20–60 m, while the oxic bottoms dominated in the depths over 60 m. It was concluded that the preferential development of oxygen deficiency in the shallow basins results from the combined effects of complex topography and seasonal temperature stratification on both vertical and lateral bottom water exchange. In the anoxic bottoms, C / P ratios suggest enhanced release of P compared to the oxic and suboxic bottoms. In the suboxic bottoms, high inorganic P concentrations and the strong coupling between P and Fe suggest active transformation of organic P to the inorganic P phases, probably due to the fluctuating oxygen conditions. Differences in the spatial distribution of the P, C and Fe were insignificant except for detrital P, which decreased seawards.