OXIDATION OF DISSOLVED METHANE IN A EUTROPHIC, SHALLOW LAKE - LAKE-KASUMIGAURA, JAPAN

Long-term seasonal variations in water-column methane oxidation were s tudied in the eutrophic, shallow Lake Kasumigaura, Japan. The entire w ater column was oxic throughout the year. Measurements were taken mont hly from July 1991 to March 1996. The oxidation rates were derived fro m time-course dissolved methane concentration measurements during the bottle incubation at in situ temperature. The existence of methane oxi dation activity in the oxic lake water was confirmed. Methane oxidatio n was distinctly seasonal, with low activity from January to April and high activity from August to November. Maximum methane oxidation was observed in late summer or early autumn, when the turnover time of dis solved methane was usually shorter than half a day. Methane oxidation activity does not depend on water temperature, dissolved methane conce ntration, dissolved oxygen, or dissolved inorganic nitrogen. The annua l average methane consumption rate was 12.3 +/- 15.5 mg CH4 m(-2) d(-1 ). Methane oxidation was the dominant methane sink, consuming an annua l average of 74% of dissolved methane in the water column of Lake Kasu migaura.