Bioavailability of dissolved organic phosphorus in the Baltic Sea

We determined bioavailable, dissolved organic phosphorus (BAP) by measuring changes in dissolved organic phosphorus (DOP) in experiments using 0.8 mu m filtered seawater, representing the free bacteria fraction, with the addition of carbon and nitrogen. Experiments were conducted at 3 stations in the central Baltic Sea from May to July 2004, a period characterized by a decrease in phosphate concentrations, and the development of diazotrophic cyanobacteria blooms. DOP concentrations ranged between 0.32 and 0.52 mu M in May, and declined to 0.20 mu M in July, Changes in DOP were caused by fluctuations in the BAP decreasing from 0.34 mu M in May to 0.03 mu M in July. The refractory DOP pool ranged from 0.14 to 0.21 mu M, and did not vary seasonally, The DOP turnover and P release from BAP was calculated using 2 different approaches: (1) [gamma-P-33]ATP hydrolysis time, and (2) the time needed for DOP degradation in the experiments. Based on [y-P-33]ATP hydrolysis times, BAP is recycled several times per day, except at 2 stations in May when the time is longer. According to the second approach, BAP is recycled within 3 or 4 d without seasonal differences. The fast [gamma-P-33]ATP turnover indicates that BAP comprises a fraction of compounds (such as ATP) that are degraded very rapidly. Another pool of BAP compounds exists which are turned over at a slower rate. Thus, the [gamma-(33)p]ATP does not represent the total BAP turnover.