Phosphorus recycling in sediments of the central Baltic Sea

Abstract. Benthic fluxes of phosphorus (P) were measured in situ in the Eastern Gotland Basin (EGB), central Baltic Sea, using benthic landers. A total of 40 flux measurements of dissolved inorganic P (DIP) on 13 stations at water depths ranging 30–210 m and under different oxygen regimes were carried out on three cruises during three consecutive years (2008–2010) in August–September. Our study is the first to report in situ DIP fluxes in the Baltic proper, and it provides the most comprehensive dataset of benthic fluxes of DIP and dissolved organic P (DOP) in the Baltic proper existing to date. DIP fluxes increased with increasing water depth and with decreasing bottom water oxygen concentration. Average DIP fluxes were calculated for oxic bottom water conditions (− 0.003 ± 0.040 mmol m⁻² d⁻¹), hypoxic conditions (0.027± 0.067 mmol m⁻² d⁻¹) and anoxic conditions (0.376 ± 0.214 mmol m⁻² d⁻¹). The mean DIP flux at anoxic bottoms was higher than previous estimates based on ex situ measurements of pore water gradients. The DIP flux was positively correlated with the organic carbon inventory of sediment, and the benthic flux of dissolved inorganic carbon (DIC) at anoxic stations, but these variables were uncorrelated at oxic stations. The positive correlation between DIP and DIC fluxes suggests that the benthic DIP efflux from anoxic bottoms in the Baltic Proper is mainly controlled by rates of deposition and degradation of organic matter. The flux from anoxic sediment was very P rich in relation to both C and nitrogen (N). The average C : P ratio in fluxes at anoxic accumulation bottoms was 69 ± 15, which is well below the Redfield C : P ratio of 106 : 1. At oxic stations, however, the C : P flux ratio was much higher than the Redfield ratio, consistent with well-known P retention mechanisms associated with iron and bacteria in oxidised sediment. Using a benthic mass balance approach, a burial efficiency estimate of 0.2–12% was calculated for the anoxic part of the EGB, which suggests that anoxic Baltic sediments are very efficient in recycling deposited P. Based on the measured fluxes and the average areal extent of anoxic bottoms during years 1999–2006, an internal DIP load of 152 kton yr⁻¹ was calculated. This is almost 9 times higher than the average external total phosphorus (TP) supply to the Baltic proper during the same period. This comparison clearly highlights the dominance of internally regenerated P as a DIP source in the Baltic Sea.