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Pool of mobile phosphorus 

Estimating the pool of mobile phosphorus in offshore soft sediments of the Baltic Proper

Twelve sediment cores were collected in the Baltic Proper between 61 m and 175 m water depth and a number of phosphorus fractions were analyzed. Integrating the concentrations over the depth profiles, the amounts of mobile phosphorus were estimated in each core. The results were presented in the scientific journal Air, Soil and Water Research (2012:5 1–13).

It was found that sediments below the redox cline in the Baltic Proper contained small amounts of mobile phosphorus. The total amount of mobile phosphorus in the entire Baltic Proper sediments below 65 m water depth was estimated to between 55,000 tonnes and 156,000 tonnes or between less than 10% to around 25% of the phosphate in the system (water plus sediments). This represents the maximum amount of phosphorus that could possibly be released to the water column from these areas. We argue that the most reasonable estimate of the pool of mobile phosphorus in the sediments is the lower number.

The amounts of mobile phosphorus in sediment cores with oxidized surface layers were higher compared with sediment cores with reduced surfaces, indicating that there is a potential phosphorus-binding capacity in sediments below the redox cline if oxic conditions improved. Oxygenation of the Baltic Proper bottom water between 65 m and 100 m could probably remove around 100,000 tonnes of phosphorus from the water column and reduce phosphorus concentrations in the deep water by on average 30 mg/m3, which would possibly be felt also in the surface water.

Comparing mobile phosphorus content in soft sediments from two sub-systems of the Baltic Sea 

The role of sediments in the phosphorus (P) dynamics of the Baltic Sea is often highlighted. The impact of the sediments on the water column is strictly limited by the amount of potentially mobile P in the sediments. During 2012 we performed detailed measurements of different P forms with different mobility in ten sediment cores from two Baltic Sea sub-systems. The results were presented at the ASLO Aquatic Sciences Meeting, New Orleans 17-23 February, 2013. A scientific manuscript has also been submitted to the journal Plos One.  

One sub-system is characterized by anoxic conditions in the bottom waters overlying the accumulation sediments, whereas the bottom waters of the other sub-system are generally oxidized which is also reflected in the sediments. Integrating the sediment concentrations over the depth profiles, amounts of mobile phosphorus were estimated in each core.

Average mobile P content in the sediments of the oxidized basin was found to be an order of magnitude higher than in the anoxic basin. By contrast, the bottom waters of the anoxic basin contained dramatically higher P concentrations indicating that the available mobile P was largely present above the sediments. Organic P was also much more abundant in the oxidized sediments challenging the hypothesis that biodegradation is slower under anoxic conditions.

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