Die-back of Phragmites australis: Influence on the distribution and rate of sediment methanogenesis

Methanogenesis was measured during the summer of 1994, in sediment cores and bulk samples from a Phragmites australis wetland in northern Jutland, Denmark. We compared sediment from healthy reed and dying-back reed, and an open lagoon resulting from die-back. Cores revealed variability with depth and between sites, with the highest rates coinciding with layers of organic gyttja, and negligible methane production from the underlying sand base. Methanogenesis rates in the lagoon and die back sites were higher (up to 100-150 nmol h(-1) g(-1) dry wt. sediment) than in the healthy reed (50-80 nmol h(-1) g(-1)), with the highest rates being recorded from May to July. At these times, methanogenesis was markedly temperature limited; samples incubated at 30 degrees C a non-limiting temperature, gave rates as high as 200-400 nmol h(-1) g(-1) for the lagoon and die-back areas and 150 nmol h(-1) g(-1) for the healthy area. Addition of 8 mM acetate and H-2/CO2 headspace suggested that both acetate-fermenting and CO2-reducing bacteria were present. Acetate additions suggested some co-limitation by substrate availability, with acetate limitation occurring in the healthy site during July and in the die-back site during August. Lower rates during August, especially in the healthy area, were associated with low water levels which resulted in more oxidized sediments. The data reveal highly variable methanogenesis in the sediment which, when considered with sediment depths, indicates that sites of Phragmites die-back have significantly greater rates of anaerobic mineralization than surrounding healthy wetland, and may be intense sources of methane.