Slow growth and decomposition of mosses in Arctic lakes

Aquatic mosses are often the exclusive form of macrophytic vegetation in Ar ctic lakes. Despite the cold nutrient-poor water and the short ice-free sum mer, the mosses form dense stands on the lake bottom down to great depths. The environmental conditions suggest that moss growth and decomposition are extremely slow, but logistical and methodological difficulties have so far precluded direct measurements of the processes. Here. we use temporal chan ges in the size and density of leaves along the axis of moss shoots collect ed from different depths in Char Lake and North Lake in the Canadian High A rctic to reconstruct the annual growth and decomposition of the mosses duri ng the past 10-17 years. Our results show low but remarkably constant annua l elongation rates (about 10 mm shoot(-1)) in the long-lived shoots that ca rry green leaves for several years and decompose slowly. Cold temperatures and low nutrient supply in combination with the short Arctic growing season can account for the low growth rate, the low decomposition rates, and the unprecedented longevity of these moss communities relative to other submerg ed macrophytes.