| Summary: | (1) I explore the hypothesis that the water regime is one of the major determinants of the difference between bog and fen in Sphagnum mires because of its effects on the decay loss and the mass accretion rate. This article deals with the recent mass balance and water regime and the variation in mass accretion rate during the last millennium, studied at two mires in southern Sweden.
(2) At one bog site and one fen site in the Åkhult mire the recent litter input to the acrotelm and mass accretion in the catotelm were 210 and 38 g m-2 a-1 at the bog site and 300 and 21 g m-2 a-1 at the fen site, respectively. The difference in productivity reflected the availability of plant nutrients and the pH. The smaller loss of mass at the bog site was a result of smaller water table amplitude and a higher median time water level (MTWL) than at the fen site. Through its effects on hummock formation, the water regime is also a major determinant of the surface structure.
(3) Around 1000 cal. BP the precipitation and the supply of mineral soil water to the two sites increased and the bog vegetation of that time was replaced by fen vegetation. Mass accretion first increased to 180 g m-2 a-1 but with the upward growth of the mire surface the rate decreased. The recent conditions at the fen site became established ~800 cal. BP, probably as a result of increased water table amplitudes. At the bog site the water regime changed from geogenous to ombrogenous ~600 cal. BP, contemporary with a decrease in precipitation. This change resulted in a rapid increase in mass accretion by 150 % followed by a slow decrease until recent times with low values. The same development was found to have occurred twice at the Store Mosse mire. Such an initial increase in the peat accretion rate must be important for the formation of permanently ombrotrophic conditions in raised bogs.
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