DIEBACK OF PHRAGMITES-AUSTRALIS - INFLUENCE ON THE DISTRIBUTION AND RATE OF SEDIMENT METHANOGENESIS

Methanogenesis was measured during the summer of 1994, in sediment cor es and bulk samples from a Phragmites australis wetland in northern Ju tland, Denmark. We compared sediment from healthy reed and dying-back reed, and an open lagoon resulting from die-back. Cores revealed varia bility 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. sedimen t) 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 suggeste d that both acetate-fermenting and CO2-reducing bacteria were present. Acetate additions suggested some co-limitation by substrate availabil ity, with acetate limitation occurring in the healthy site during July and in the die-back site during August. Lower rates during August, es pecially in the healthy area, were associated with low water levels wh ich resulted in more oxidized sediments. The data reveal highly variab le methanogenesis in the sediment which, when considered with sediment depths, indicates that sites of Phragmites die-back have significantl y greater rates of anaerobic mineralization than surrounding healthy w etland, and may be intense sources of methane.