Limnological characteristics of an oxbow lake in a lower reach of the Shibetsu River in Hokkaido Island, Japan

Limnological characteristics of a shallow oxbow lake (maximum depth about 2 meters) were investigated in a lower reach of the Shibetsu River in Hokkaido Island, Japan. Water temperatures, under-water light attenuation coefficients, and concentrations of dissolved oxygen, nitrogen, phosphorus and chlorophyll-a were measured on July 21 and November 17 in 2001 and July 30 in 2002. Water temperature varied among sampling stations and depths, ranging from 10°C to 24°C in July, while all measurements were constant at about 5°C in November. Dissolved oxygen concentrations at all stations on all sampling dates were 10 mg L-1 or more. Its maximum concentration reached 25 mg L-1 with 250% saturation at the lake bottom on July 21 in 2001. The depths of sampling stations that obtained high concentrations of dissolved oxygen were between 50 cm and 60 cm. Water temperatures at these depths ranged from 10°C to 15°C, which were lower than surface water by 5°C to 10°C. A large filamentous green alga, Spirogyra sp., propagated in these depths. Concentrations of dissolved nitrogen in lake water varied from 11 μg L-1 to 250 μg L-1, which declined in July at all stations and depths. Concentrations of PO43--P showed a range from 7 μg L-1 to 14 μg L-1, not so different among stations, depths and sampling dates. Particulate phosphorus and chlorophyll-a concentrations were approximately constant between 33 μg L-1 and 35 μg L-1 and 10 μg L-1 and 13 μg L-1, respectively. The concentrations of total phosphorus and chlorophyll-a on 21 July 2001 indicate that trophic status of the oxbow lake is in meso-eutrophic conditions. Under-water light attenuation coefficients (k) ranged from 1 m-1 to 2 m-1 at all stations and sampling dates. These values are comparable to the maximum value for eutrophic lakes with water-bloom of phytoplankton. However, because chlorophyll-a concentrations in this oxbow lake were lower than those in the eutrophic lakes, under-water light seemed to be attenuated by non-living particles and dissolved organic matter. Therefore, there is a possibility that the oxbow lake is in dystrophic conditions. These results suggested that the limnological characteristics of the oxbow lake were greatly different from those of the main channel of the Shibetsu River. It is believed that the oxbow lake maintains the characteristics of a low-land marsh, which was former landscape of the lower region of the Shibetsu River watershed