Oxidation of dissolved methane in a eutrophic, shallow lake: Lake Kasumigaura, Japan

Long‐term seasonal variations in water‐column methane oxidation were studied in the eutrophic, shallow Lake Kasumigaura, Japan. The entire water column was oxic throughout the year. Measurements were taken monthly from July 1991 to March 1996. The oxidation rates were derived from time‐course dissolved methane concentration measurements during the bottle incubation at in situ temperature. The existence of methane oxidation activity in the oxic lake water was confirmed. Methane oxidation 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 dissolved methane was usually shorter than half a day. Methane oxidation activity does not depend on water temperature, dissolved methane concentration, dissolved oxygen, or dissolved inorganic nitrogen. The annual average methane consumption rate was 12.3 ± 15.5 mg CH4 m−2 d−1. Methane oxidation was the dominant methane sink. consuming an annual average of 74% of dissolved methane in the water column of Lake Kasumigaura.