Groundwater-Discharge Fens in the Tanana Lowlands, Interior Alaska, U.S.A.

Abstract

Large expanses of herbaceous floating mat wetlands (FMW) bordered by slightly higher uplands with forest or scrub occur in the northwest corner of the Tanana Flats between the Alaska Range and the Tanana River. Five major FMW systems, together with other outliers and extensions, are linear in shape and cover over 20 km2. Nutrient-rich and circumneutral water flows slowly through these areas toward the northwest and through outlets to the Tanana River. The floating mat vegetation consists of tall emergent macrophytes; mosses, in particular Sphagnum spp., are conspicuously absent and shrubs are infrequent. Although species dominance shifts over short distances on the mat, four community types can be recognized: (1) Menyanthes trifoliata, (2) Carex aquatilis, (3) Typha latifolia, and (4) Calla palustris. Below the water surface, the mat extends to a depth of 0.5 to 1.0 m and consists of rhizomes and roots in a matrix of well-decomposed peat and water. The mat then either directly overlies unfrozen gray silts at a depth of 1 m, or more commonly, floats on a clear-water or loose peat zone above more consolidated peat lying on unfrozen silt at a depth of 1.5 to 2.5 m. No permafrost or frozen ground was detected in late August or late winter below these floating mats but it is ubiquitous on the bordering uplands, 0.5 to 2 m above the FMW. The topographic location, apparent absence of permafrost, water chemistry, and vegetation composition suggest that these areas are fens fed by groundwater sources flowing out of the Alaska Range. Permafrost degradation and lateral expansion of these FMW is indicated by slumped blocks of forest peat, dead trees, and open water moats along the upland margin. Although floating mats are frequently described in the literature as occupying the edge of northern ponds and lakes, the FMW described here do not and they appear to be unique because of their large extent, absence of mosses, physiographic position, and presumed origin.