Diatom responses to late-glacial and early-Holocene environmental changes at Krakenes, western Norway

A stratigraphic diatom sequence is presented for the period from 13,870-9,1 70 cal BP from Krakenes Lake, western Norway. Changes in species assemblage s are discussed with reference to the changing environmental conditions in the Allerod, Younger Dryas, and the early Holocene and to the development o f the aquatic ecosystem. The site is sensitive to acidification, and diatom -based transfer functions are applied to estimate the past pH status. The c ombination of rapid sediment accumulation together with an excellent calibr ated radiocarbon chronology means that the rate of inferred pH change and a ssociated increase in [H+] can be assessed and compared with recent, anthro pogenically acidified situations. The Allerod diatom assemblages are dominated by benthic taxa particularly F ragilaria species, indicating an unproductive, alkaline, turbid, and immatu re system. Diatoms are absent in the early part of the Younger Dryas, but s ubsequently a sparse planktonic flora develops reflecting decreased turbidi ty and/or increased nutrient supply. A clear sequence of diatom assemblages is seen in the early Holocene. A short-lived peak of Stephanodiscus specie s indicating a period of increased nutrient availability occurred at ca. 11 ,500 cal BP. Throughout the early Holocene, acid-tolerant species increasin gly replaced less acidophilous, circumneutral taxa. The lake became slightly more acid during the Allerod, but this was statist ically insignificant in a trend test involving regression of pH or [H+] in relation to age. Diatom-inferred pH declined rapidly during the early Holoc ene period investigated (9,175-11,525 cal BP) with a statistically signific ant overall rate of 0.024 pH units per 100 yrs. This consisted of an older (ca. 11,525-10,255 cal BP) phase, where pH fell more rapidly at up to 0.095 pH units per 100 yrs; and a younger phase after ca. 10,500 cal BP where th e pH fall was extremely slow (0.008 pH units per 100 yrs) and was not stati stically significant. In the Allerod a combination of low catchment productivity together with di sturbance, weathering, and minerogenic inwash ensured that the base-cation status remained relatively high. In the Holocene the catchment soils stabil ised and base cations were sequestered by terrestrial vegetation and soil. This resulted in reduced base-cation leaching and this, together with the p roduction of organic acids caused the lake to acidify, reaching an equilibr ium by ca. 10,000 cal BP.