Si(OH)4- AOU characteristics in bottom water of Seto Inland Sea, Japan, in summer

The regeneration of a nutrient is accompanied by the consumption of dissolved oxygen (DO) and could regulate the productivity of an aquatic system. The relationship between regenerated silicic acid [Si(OH)4] and Apparent Oxygen Utilization (AOU) in bottom water was investigated at four sites of the Seto Inland Sea, western Japan in summer. In the study period, a stratified condition developed, and the bottom water seemed to be isolated from the overlying water at all four sites. A similar horizontal distribution pattern of Si(OH)4 and AOU was observed in the bottom water at the four study sites. A plot of Si(OH)4 against AOU in the bottom water showed good linearity, which suggested that the Si(OH)4 increase (dissolution) was accompanied by DO consumption due to the degradation of organic matter. Different ratios of regenerated Si to AOU [slope in a Si(OH)4-AOU plot] of 0.17 μM/μM, 0.36 μM/μM, 0.31 μM/μM, and 0.15 μM/μM for the four study sites, respectively were obtained, and these ratios seemed to deviate from the ratio based on the traditional Redfield-Brzezinski proportions (0.11). These results are considered to reflect the distinctive Si(OH)4-AOU characteristics of the Seto Inland Sea.