Long-term variations of three types of phosphorus concentrations in highly eutrophic shallow Lake Kasumigaura, with special reference to dissolved organic phosphorus.

Abstract

Seasonal variations of particulate-phosphorus (PP), dissolved organic phosphorus (DOP), and dissolved reactive phosphorus (RP) concentrations at ten stations in highly eutrophic shallow Lake Kasumigaura were monthly or biweekly monitored during May 1977 to March 1987. Marked increases in total phosphorus concentration at all stations were directly related to the increases in the standing crop of phytoplankton by the blooming of blue- green algae in summer. Under the heavy blooming of the blue-green algae, a clear peak in RP concentration appeared at all stations every summer after 1981. The RP concentrations were 10-20μg·l-1 at lake center and 20-120μg·l-1at the innermost parts of Takahamairi and Tsuchiurairi Bays. The DOP concentrations were nearly constant in the range of 8-25μg·l-1 throughout the years. The linear relationship with a constant between DTP and RP concentrations suggested that DOP might be composed of considerably refractory components. The characterization of the DOP obtained by reverse osmosis, ultra filtration, and freeze-drying and the DOP production experiments using sample incubations with 32P-phosphate suggested that the DOP in the lake water might mainly originate from phytoplankton and cannot be the substrate for at least bacterial alkaline phosphatase.