Abstract. The vegetation ecology of a paramo cushion mire was studied along four transects. Six local vegetation types were distinguished. Two types are called cushion bogs; one is dominated by Oreobolus cleefii and the other by Plantago rigida. Types 3 and 4 are mire vegetation dominated by bryophytes; one is characterised by Sphagnum sparsum, Breutelia sp. and Campylopus cucullatifolius, the other one is dominated by Lophozia subinflata with Cortaderia sericantha. Vegetation type 5 is dominated by submerged Sphagnum cyclophyllum; type 6 refers to aquatic vegetation with only Equisetum bogotense and algae. The relation between the first four vegetation types and edaphical and hydrological gradients is analysed by means of Canonical Correspondence Analysis (CCA). From the results of CCA it can be concluded that the occurrence of the Oreobolus cleefii type is restricted to areas with a thinner peat layer, a wider distance surface‐anaerobic zone and water table‐anaerobic zone, a higher NO2 concentration and a lower pH. The occurrence of the Plantago rigida type is restricted to a situation with a lower electrical conductivity of the ground water, a lower NO3 and PO4 concentration, and a higher Fe concentration. The occurrence of the Sphagnum type is restricted to places with a higher conductivity of the ground water, a higher NO3 and PO4 concentration, and a lower Fe concentration. The occurrence of the Lophozia subinflata with Cortaderia sericantha type is restricted to a situation with a lower K and Al concentration.

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Three sediment cores from two lakes, Fish Lake and Phalarope Lake, in Truelove Lowland, Devon Island, N.W.T. were analyzed for diatoms and chemical composition. Multivariate statistical techniques using a range of chemical variables successfully isolated three sediment groupings in the cores. Allochthonous and autochthonous chemical components in the sediments have been used to reconstruct paleoenvironmental conditions. The two lakes began approximately 10 600 years ago as shallow marine lagoons that were isolated from the sea as a result of glacio-isostatic rebound. Based on the presence of distinctive diatom assemblages, the three stratigraphic zones are identified as a basal marine zone, an intermediate and transitional brackish/marine zone and an upper freshwater zone. Following isolation from the sea, the lakes were flushed with freshwater produced by snow and ice melt. In Fish Lake, the period of transition from marine to freshwater, which began approximately 7000 years ago, lasted approximately 800 years. In Phalarope Lake, which was isolated from the sea approximately 5 000 years ago, flushing by fresh water was completed only within the last 300 years. Fe, Cr, and Mo in the sediments are associated with the isolation phase when lake sedimentation is sensitive to the presence of brackish water and erosion within the lake catchments. In particular, the precipitation of Mo as MoS 2 reflects the presence of hypolimnetic anoxia associated with lake isolation. During the early post-isolation phase the response of lake biota to an influx of nutrients is reflected in an increase in biological silica and organic carbon in the lake sediments. On the other hand, the generally low organic content of the sediments indicates that sedimentation in these lakes has been largely determined by variations in non-biogenic factors through time. During the mid Holocene the progressive stabilization of surface materials within the lake catchments is marked by decreasing Cr, As and Na in the sediments. At the same time, an increase in allochthonous Mn and Fe is attributed to progressive soil development. During the last 2500 years the catchments have experienced decreased erosion resulting in a decrease in both allochthonous clastic input and lake productivity.

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