Dissolved phosphorus in the Baltic Sea — Occurrence and relevance

The spatial variability of dissolved organic phosphorus (DOP) concentrations throughout the whole Baltic Sea was investigated during two cruises in summer 2008 and in late winter/early spring 2009. In addition, bioavailable phosphorus was detected in the central Baltic and in the Gulfs of Bothnia and Finland during the summer cruise.Highest summer DOP concentrations of 0.32 ± 0.05 μM have been estimated in the Baltic Proper, followed by the Gulf of Finland to be having a mean concentration of 0.25 ± 0.01 μM. DOP decreased down to 0.12 μM in the Bothnian Bay. Similar low concentrations were found in the Skagerrak. The same spatial DOP pattern but at a lower concentration level like in summer 2008 was estimated in the central and northern regions during the cruise in late winter/early spring 2009. The bioavailable and refractory DOP fractions (B-DOP and R-DOP, respectively) were determined in time course experiments excluding C- and N-limitation. In all basins, the R-DOP constituted a higher proportion of DOP compared to B-DOP. A gradient has been observed from the northernmost station of the Bothnian Bay to the southern Gotland Basin. In the Bothnian Bay, B-DOP exhibited the lowest amounts of only 0.01 μM comprising 8% of DOP, highest concentrations occurred in the Gotland Basin constituting a proportion of 25–29%. In the Gulf of Finland, B-DOP comprised a proportion up to 46% of DOP at lower concentrations compared to the Gotland Basin. Thus the Bothnian Bay was depleted in total bioavailable phosphorus (DIP + B-DOP) opposite to the southern Baltic Proper where the sum of B-DOP + DIP was 40 times higher.

Research Highlights
► Distribution pattern of DOP was studied in surface water of the Baltic Sea.
► High concentrations occurred in the Baltic Proper.
► Low concentrations were found in the Bothnian Bay and in the Kattegat /Skagerrak.
► DOP in the Bothian Bay was only refractory and not bioavailable.
► Bioavailable DOP was high in the southern Gotland Basin and the Gulf of Finland.