Bioavailability of dissolved organic phosphorus in the Baltic Sea

Abstract

We determined bioavailable, dissolved organic phosphorus (BAP) by measuring changes in dissolved organic phosphorus (DOP) in experiments using 0.8 μm 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 μM in May, and declined to 0.20 μM in July. Changes in DOP were caused by fluctuations in the BAP decreasing from 0.34 μM in May to 0.03 μM in July. The refractory DOP pool ranged from 0.14 to 0.21 μM, and did not vary seasonally. The DOP turnover and P release from BAP was calculated using 2 different approaches: (1) [γ-33P]ATP hydrolysis time, and (2) the time needed for DOP degradation in the experiments. Based on [γ-33P]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 [γ-33P]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 [γ-33P]ATP does not represent the total BAP turnover. © Inter-Research 2006.