A review of literature data on the degree of peat decomposition – an important parameter that yields data on environmental conditions during the peat-forming process, i.e., humidity of the mire surface, is presented. A decrease in the rate of peat decomposition indicates a rise of the ground water table. In the case of bogs, which receive exclusively atmospheric (meteoric) water, data on changes in the wetness of past mire surfaces could even be treated as data on past climates. Different factors shaping the process of peat decomposition are also discussed, such as humidity of the substratum and climatic conditions, as well as the chemical composition of peat-forming plants. Methods for the determination of the degree of peat decomposition are also outlined, maintaining the division into field and laboratory analyses. Among the latter are methods based on physical and chemical features of peat and microscopic methods. Comparisons of results obtained by different methods can occasionally be difficult, which may be ascribed to different experience of researchers or the chemically undefined nature of many analyses of humification.
Our study concerns the development of marginal zone of Lake Suchar Wielki (Wigry National Park). This humic lake is characterised by mire occurrence in its marginal zone, covered with a floating mat spreading on water surface. Biogenic sediments (peat archive) present in lakeside allow recognising the thousands years’ development pattern of lake surrounded by mire. The wetland records were analysed for plant macroremains, degree of peat decomposition, sediment geochemical features along with age assessment. Morphological characteristics of the catchment were integrated with the wetland records. Our research showed that after the lake origin in the Allerod, accumulation in its marginal zone was interrupted, which was probably connected with high precipitation in the past. Slope processes could be possible. Stabilisation of environmental conditions took place in the younger Holocene. Despite these perturbations marginal zone of the lake was permanently terrestrial in character, as indicated by values of atomic C:N ratio.