Aquatic macrophyte vegetation development in Krakenes Lake, western Norway, during the late-glacial and early-Holocene

Macrofossil analyses were carried out on the late-glacial and early-Holocene sediments of the radiocarbon-dated master core at Krakenes Lake, western Norway, to investigate the aquatic vegetation changes. Ranunculus sect. Batrachium and Nitella were the earliest pioneers after deglaciation ca. 12,300 C-14 yr BP. The Allerod vegetational succession was very slow during ca. 1000 C-14 yrs in a cool climate and conditions that were similar to those above tree-line in Norway today. The rapid cooling at the start of the Younger Dryas stadial caused extensive disturbance, and with the development of an active cirque glacier in the catchment, plants and animals were almost exterminated from the lake by inflow of permanently cold and turbid water. Rising temperatures caused the glacier to melt at the end of the Younger Dryas. The biotic response to the rapid warming was immediate, with pioneer Ranunculus sect. Batrachium and Nitella expanding within 1-3 decades, closely followed by other elodeids. The lake witnessed a remarkable isoetid succession, with phases dominated by Limosella aquatica, Subularia aquatica, Elatine hydropiper, Isoetes echinospora, and, later, I. lacustris. About 800 yrs into the Holocene most of the macrophytes declined. The short-lived isoetids became extinct, but other taxa probably survived vegetatively. The reasons for this decline are unknown, but are probably related to nutrient depletion in combination with other factors. About 550 yrs later, I. lacustris and Nymphaea colonised, and a stable flora and vegetation developed. This study illustrates the large and rapid changes that occurred over the first 1400 yrs of the Holocene in the macrophyte flora and vegetation in Krakenes Lake before stability was attained, pointing to the value of a palaeoecological study in tracing aquatic successions over time, and highlighting our lack of knowledge of the underlying ecological factors responsible for such rapid and marked changes.