Field observations were made during two years on the seasonal variation and the balance of three forms of phosphorus (particulate phosphorus, PP; dissolved inorganic phosphorus, DIP; and dissolved organic phosphorus, DOP) in the water column at two stations situated in the eutrophicated coastal region of the Seto Inland Sea, Japan.
In the high temperature season concentration of PP was definitely higher than in the low temperature season. High correlation between the concentrations of PP and chlorophyll a suggested that PP was mostly composed of the phosphorus contained in the phytoplankton, especially at high PP levels. Maximal PP values were observed in dense phytoplankton blooms, and were as high as 0.91－1.06µg-at/l.
Proportion of DOP in the total phosphorus of seawater was considerably high throughout the year, averaging 38.7 and 41.7% at each station. An increase of DOP in the water column coincided with, or followed with a short delay, an increase of PP.
DIP was consumed in the course of the growth of phytoplankton in upper layers during stratification period. There was indication that DIP was supplied to the water column from the outside during September through October. The influx of land water and the liberation of dissolved phosphorus from the bottom sediments were suggested as two main routes of phosphorus supply to the water column.
In conclusion, then, it appears that in this sea region the production and decomposition of phytoplankton played an important role in the annual cycle of phosphorus.