Abstract
Seven cruises during June 2005–November 2006 were conducted to study the variations in apparent oxygen utilization in Hong Kong coastal waters, and PO4 addition experiments were conducted in the summer to study bacterial responses to PO4 additions. Apparent oxygen utilization (AOU) was significantly correlated with PO4 concentrations. AOU was more responsive to PO4 when DIN/PO4 ratio was 50:1 in the summer wet season, and PO4 was more responsive to AOU in the dry season when N/P ratio was close to or less than 16:1, which suggested less regenerated PO4 during oxygen consumption in the former case than in the latter case. The PO4 addition experiment showed that 0.1 μM PO4 addition increased bacterial respiration (BR) by 20 ± 4%, but a 0.2-μM PO4 addition saturated BR even when DIN/PO4 was 80:1, suggesting that there was a limit for PO4 stimulation of bacterial respiration. This finding of a 0.2-µM PO4 saturation concentration for stimulating bacterial respiration is significant, and it also indicates that the release of PO4 during regeneration might be reduced by the same concentration.
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