Diel variation in methane emissions from stands of Phragmites australis (Cav.) Trin. ex Steud. and Typha latifolia L. in a boreal lake

Methane emission from Phragmites australis and Typha latifolia stands was quantified in Lake Vesijarvi, southern Finland. There were differences in emission rates at different times of the day in all the vegetation zones studies, but a clear pattern was found only occasionally. In the inner reed zone with shallow water (<2 cm) the lowest emissions were usually recorded at night when the rates varied from 0.5 to 11.8 mg CH4 m(-2) h(-1). The pattern in diel emissions became more obvious towards the end of the growing season and the difference between the night and day emissions was highest in August, when the rates were 3.3 and 85.9 mg CH4 m(-2) h(-1) at 00.00 and 12.00 h, respectively. Significant differences in emission rates between night and day were only recorded in August and September. In the mixed cattail-reed zone the pattern in emission rates was more obscure than in pure stands of reed. The ratios between the lowest and highest emissions were similar to those determined in the inner reed zone; in August, when the pattern was clearest, the ratio was 0.18. No patterns of emissions were observed in the outer reed zone, where the water depth was > 10 cm and the number of shoots low. The emissions were highest in this zone, but most of them took place through ebullition. In July, when the emissions were maximal, the plant-mediated rates of emissions varied from 15.5 to 80.0 mg CH4 m(-2) h(-1) and ebullition from 29.5 to 55.8 mg CH4 m(-2) h(-1). In the inner reed zone the diel changes could be related to irradiance, whereas in the mixed cattail-reed zone this relation was not clear. The role of temperature in diel variations was even more modest than that of irradiance. Neither of the species closed stomata at night, but still showed diel patterns in lacunal methane concentrations. Methane probably escaped through the lower part (< 10 cm above the sediment surface) of the stems as the lacunal methane concentrations were always highest here. (C) 2001 Elsevier Science B.V. All rights reserved.