Seasonal phosphorus depletion and microbial responses to the change in phosphorus availability in a subarctic coastal environment

• Published in: Marine Chemistry
• Main Authors: Yoshimura, Takeshi, Kudo, Isao
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier B.V.
• Subjects: Phosphorus; Orthophosphate; Phosphatase; Phytoplankton; Bacteria; Coastal zone; 452; Subarctic
• Online Access: http://hdl.handle.net/2115/47275; https://doi.org/10.1016/j.marchem.2011.06.003

Temporal variations of phosphorus (P) pools (soluble reactive P (SRP), dissolved organic P (DOP), and particulate P (PP)), alkaline phosphatase activity (APA), and orthophosphate uptake were measured in Funka Bay to describe the annual P cycle in the bay and determine the phytoplankton and bacterial responses to the change in P availability. Observations over four years demonstrated that SRP concentrations in the surface waters decreased from the winter maximum of 1.0 μmol L^[-1] to 0.3 μmol L^[-1] at the end of the spring phytoplankton bloom, and then to under the detection limit by around August. The DOP concentration was relatively constant at 0.1 μmol L^[-1] throughout the year with higher concentrations up to 0.3 μmol L^[-1] just after the peak of the spring bloom. PP concentrations varied around 0.1 μmol L^[-1] with higher concentrations up to 0.6 μmol L^[-1] during the high chlorophyll a concentration period. SRP comprised 85% of the total P pool prior to the spring bloom, but the contribution decreased to less than 30% in the post-bloom period and DOP dominated the P pool in the surface. Most of the measured APA was less than 0.1 nmol L^[-1] min^[-1] in the whole water column, but exceptionally high values up to 1.4 nmol L^[-1] min^[-1] were observed in low SRP surface waters during summer. When SRP decreased from 0.3 μmol L^[-1] in spring to under the detection limit in summer, turnover time for orthophosphate at 10 m depth decreased from 61 days to 4.2 hours. The high APA and short turnover time for orthophosphate suggested that microbes in the surface mixed layer experienced severe P stress in summer. Under severely P stressed conditions, APA was detected mostly in the >10 μm fraction, suggesting that large phytoplankton would acquire P from DOP to overcome the P stress. This was supported by the result that large phytoplankton were outcompeted by the 0.2-1 μm sized microbes for orthophosphate uptake.