SEASONALITY OF METHANE EMISSIONS FROM FIVE LAKES AND ASSOCIATED WETLANDS OF THE COLORADO ROCKIES

Methane emissions were measured over a 15-month interval for five lakes and five associated wetland sites in the Rockies of Colorado at elevations between 2800 and 3600 m. Three of the five lakes accumulated dissolved methane under ice; accumulation was as high as 53-fold above ice-free water column concentrations in the shallowest lake. The combination of high dissolved oxygen and low dissolved inorganic nitrogen concentrations within the water column during ice breakup suggests that methane emissions rather than methane oxidation led to the substantial loss of dissolved methane from the shallowest lake at the time of spring thaw. A pulsed release of methane to the atmosphere at the time of ice breakup within lakes may be widespread at high latitudes and may play a role in the observed increase in tropospheric methane concentrations in the northern hemisphere. During the ice-free season, the mean emission rate was 1.6 mmol m(-2) d(-1) over open water for the five lakes, but the mean was much higher (13 mmol m(-2) d(-1)) over the Nuphar lutea beds located in one of the lakes. Open water emissions occurred primarily through diffusion rather than bubbling. For wetlands near the lakes, average lake emissions ranged from negligible to almost 6 mmol m(-2) d(-1); the average across all sites was 2.1 mmol m(-2) d(-1) during the warm season. There was no measurable emission during the winter months. Surface dissolved methane explains 40% of the variation in emissions from the open water sites; the combination of soil organic C content and soil temperature explains 40% of variation in emissions from the wetland sites. The data from the Southern Rockies and information that has accumulated on other lake types over the last 15 years indicate that lakes may be a larger source strength of methane than reported estimates.