Dissolved organic phosphorus production and decomposition during open ocean diatom blooms in the subarctic Pacific

• DOP dynamics during two open ocean diatom blooms were observed.
• DOP concentration significantly increased with the buildup of phytoplankton biomass.
• Net DOP production corresponded to 4–5% of the consumed SRP.
• 5–13% of newly accumulated organic P was partitioned into DOP.
• Newly produced DOP was decomposed by bacteria within 3 weeks of incubation.

A significant part of phosphorus (P) in seawater is found in the dissolved organic matter (DOM) fraction as DOP, which plays a key role in the marine biogeochemical cycle of P. The DOM pool size changes with biological activity, but DOP production and decomposition processes, unlike carbon (C) and nitrogen, have been only infrequently studied during phytoplankton blooms when rapid production and accumulation of organic matter occurs. We observed the DOP dynamics during two phytoplankton blooms dominated by centric diatoms, the first induced by an in situ mesoscale iron enrichment experiment in the western subarctic Pacific in summer 2001 (SEEDS) and the second that occurred naturally in spring 2003 in the Oyashio region. DOP concentration increased with the buildup of phytoplankton biomass with DOP/chlorophyll-a production ratios (mol/g) of 0.0027 ± 0.0004 and 0.0044 ± 0.0010 for the SEEDS and Oyashio blooms, respectively. During the SEEDS and Oyashio blooms the amount of net DOP production corresponded to (4.9 ± 0.7) % and (4.5 ± 0.6) % of the consumed soluble reactive P, and (5.5 ± 0.8) % and (13 ± 3) % of the newly accumulated organic P was partitioned into DOP, respectively. Seawater culture bottle experiments showed that newly produced DOP during the bloom development was decomposed by free living bacteria over a time scale of a month even under soluble reactive P available conditions. C:P for the decomposed DOM (molar ratio of 78–88) showed a similar value to in situ net produced DOM (66) and POM (83) but much lower than that for the bulk DOM (395–706), suggesting that the composition of the freshly produced DOM with high lability differs significantly from the bulk DOM.