Characteristic of Si:P:N ratio in bottom water in central Suo-Nada, western Seto Inland Sea

Abstract

The nutrient composition in seawater is considered to determine the regional phytoplankton species, its composition, and its productivity. In order to examine the nutrient composition ratio in Suo-Nada, western Seto Inland Sea, where the oligotrophication process is progressing, the relationship among silicic acid [Si(OH)4], phosphate (PO4),and total dissolved inorganic nitrogen (DIN; NO3 + NO2 + NH4) in bottom water was investigated. From April through July, as stratification progressed, a stagnant water mass appeared in the bottom layer of the central part of Suo-Nada. Nutrient and apparent oxygen utilization (AOU) concentrations in this bottom water increased in this period. A plot of nutrients against AOU concentrations showed good linearity for Si(OH)4, PO4, and DIN, which suggests that increase in nutrients was accompanied by dissolved oxygen consumption due to the degradation of organic matter. The Si:P:N ratio calculated based on the increased nutrient–AOU relationship in bottom water from April through October was determined to be 25:1:9. Meanwhile, the average Si:P:N ratio existing in bottom water during the same period was 28:1:8. The two ratios obtained showed good agreement in general, although the difference between them is probably attributed to the existence of preformed fractions of each nutrient. This finding suggests that the nutrient composition ratio in bottom water generally could be regulated by the biological uptake–regeneration process from April through October in this study area, and the obtained ratio (28:1:8) is a regional characteristic with large deviation from the Redfield–Brzezinski nutrient ratio of 15:1:16.