Bioavailable dissolved organic phosphorus and phosphorus use by heterotrophic bacteria

Dissolved organic phosphorus (DOP) concentrations were measured in surface water at 3 stations in the central Baltic Sea from May to July 2005. The amount of bioavailable DOP (BAP) was estimated in time course experiments using 0.8 µm filtered seawater and carbon plus nitrogen addi- tions with the aim of verifying results obtained in 2004. The consumption of dissolved inorganic phos- phorus (DIP) and DOP by heterotrophic bacteria under natural water conditions was determined in parallel experiments without any substrate amendment. In both years, most DOP and BAP concentra- tions were in the same range of between 0.20 and 0.31 µM, and 0.04 and 0.12 µM, respectively, with a tendency to decline from spring to summer. BAP constituted a proportion of 33.2 to 60.0% of DOP. Bacterial phosphorus use differed between the Gotland and Bornholm Basins. In the eastern and south-eastern Gotland Basin, DIP was taken up completely, independently from initial concentrations in substrate addition experiments. Without any substrate addition, less DIP was used in May. In sub- sequent investigations in June and July, the DIP was taken up in full. Under natural conditions, the uptake of BAP ranged between 18 and 78% and reached 100% in summer when heterocystous cyanobacteria occurred in surface water. In the Bornholm Basin, all the BAP was used in July despite the availability of DIP. These results indicate that the phosphorus uptake in both basins may be car- bon and/or nitrogen limited in May. The limitation was repressed in summer in the Gotland Basin, but was sustained in the Bornholm Basin. Thus, the BAP use could satisfy the carbon demand in the Bornholm Basin, whereas it fulfils the phosphorus demand in the Gotland Basin.

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