Effects of CO.sub.2 perturbation on phosphorus pool sizes and uptake in a mesocosm experiment during a low productive summer season in the northern Baltic Sea

Studies investigating the effect of increasing CO.sub.2 levels on the phosphorus cycle in natural waters are lacking although phosphorus often controls phytoplankton development in many aquatic systems. The aim of our study was to analyse effects of elevated CO.sub.2 levels on phosphorus pool sizes and uptake. The phosphorus dynamic was followed in a CO.sub.2 -manipulation mesocosm experiment in the Storfjärden (western Gulf of Finland, Baltic Sea) in summer 2012 and was also studied in the surrounding fjord water. In all mesocosms as well as in surface waters of Storfjärden, dissolved organic phosphorus (DOP) concentrations of 0.26 ± 0.03 and 0.23 ± 0.04 µmol L.sup.-1, respectively, formed the main fraction of the total P-pool (TP), whereas phosphate (PO.sub.4) constituted the lowest fraction with mean concentration of 0.15 ± 0.02 in the mesocosms and 0.17 ± 0.07 µmol L.sup.-1 in the fjord. Transformation of PO.sub.4 into DOP appeared to be the main pathway of PO.sub.4 turnover. About 82 % of PO.sub.4 was converted into DOP whereby only 18 % of PO.sub.4 was transformed into particulate phosphorus (PP). PO.sub.4 uptake rates measured in the mesocosms ranged between 0.6 and 3.9 nmol L.sup.-1  h.sup.-1 . About 86 % of them was realized by the size fraction  1000 µatm during periods when phytoplankton biomass increased. In addition, we found significant relationships (e.g., between PP and Chl a) in the untreated mesocosms which were not observed under high fCO.sub.2 conditions. Consequently, it can be hypothesized that the relationship between PP formation and phytoplankton growth changed with CO.sub.2 elevation. It can be deduced from the results, that visible effects of CO.sub.2 on P pools are coupled to phytoplankton growth when the transformation of PO.sub.4 into POP was stimulated. The transformation of PO.sub.4 into DOP on the other hand does not seem to be affected. Additionally, there were some indications that cellular mechanisms of P regulation might be modified under CO.sub.2 elevation changing the relationship between cellular constituents.