Amazing Arctic Moss

Arctic lakes are inhospitable places for plants. Darkness reigns for at least ten months of the year as sunlight is intercepted by a thick layer of ice and snow. The annual average water temperature hovers just above 0°C, reaching a maximum of only 4–6°C at the height of summer. Nutrient levels are also extremely low, particularly nitrogen. With such environmental challenges, it is no wonder that there is a dearth of plant species. And yet, in some polar lakes, a green carpet covers the lake bottom, extending to great depths (Fig. 1). These hardy aquatic plants are mosses, ancient plants with small leaves, non-woody stems, and lacking true roots. In Arctic and Antarctic lakes, mosses are typically the only multicellular plants present.

Figure 1: A blanket of moss covering the bottom of Char Lake, Cornwallis Island.

In recent years, a group of Danish scientists1 have studied two mosses, Drepanocladus revolvens and Calliergon giganteum, from lakes in the Canadian High Arctic, trying to decipher the secrets to their perseverance. By examining the shoots of these mosses, which have distinct annual segments, the researchers were able to determine rates of growth, and longevity. As it turns out, Arctic mosses are the turtles of the plant world; growing slowly as little as 1 cm per year per shoot. The leaves remained green and alive for at least four years, and shoots live for 7 to 10 years. These statistics give these Arctic mosses pride of place among the biology books as the slowest-growing, longest-lived freshwater macrophytes ever recorded.

Their longevity and slow growth are likely a result of the short growing season, the cold temperatures, and the lack of essential nutrients. The long-lived shoots store nutrients, so that new leaves can be made as soon as possible each spring; more leaves translate into more tissue to photosynthesize and hence, more energy. The low temperatures constrain the rate of enzymatic activity, and hence the rate that nutrients can be used, resulting in low growth rates. The lack of nutrients in Arctic freshwaters is extreme; the concentrations of nitrogen found in the tissues of the mosses in this study were much less than what was previously considered the critical concentration for growth!

In Arctic seas, there is a much greater diversity of macrophytes, algae (seaweed) (Fig. 2) mostly. These plant species must contend with the same cold temperatures, and low levels of sunlight; but marine arctic waters are comparatively rich in nutrients. It is only the nitrogen-starved mosses of the fresh waters that exist in a sleepy hollow state, vegetating on great green carpets at the bottom of Arctic lakes.

Figure 2: Algae like this are a part of the plant community in Arctic seas.

1 Sand-Jensen K, Riis T, Markager S, Vincent WF. 1999. Slow growth and decomposition of mosses in Arctic lakes. Can. J. Fish. Aquat. Sci. 56: 388–393.