The fingered tangle Laminaria digitata, a North Atlantic brown alga
The fingered tangle, Laminaria digitata, off the coast of Helgoland. (Photo: Markus Molis)
Laminaria species are several metres long brown algae with a leathery thallus structure that are commonly referred to as ‘kelps’. They form large underwater forests along light flooded rocky coastlines (kelp forests). They are especially important for coastal ecosystems as they provide protection, food and a habitat for many other marine organisms. The fingered tangle Laminaria digitata presented here is growing down to water depths of 20 m, depending on the locality, and has a life-span of 3 to 7 years. It is of great economic importance as it contains alginates, a polysaccharide which is used, for example, in food and cosmetic industries. In France, a biomass of approx. 75,000 t is harvested every year. Since kelp species preferably grow in cool waters not exceeding summer temperatures of 20°C, they are potentially endangered by global warming. Especially Laminaria species of mid and high latitudes are threatened by global climate changes, such as the greenhouse effect and stratospheric ozone depletion [1, 2]. They attach themselves to hard substrates by their holdfasts thereby being dependent on rocky shores. At the island of Helgoland (North Sea) there are three native kelp species: the fingered tangle Laminaria digitata (Fig. 1) presented here, the forest kelp (L. hyperborea) and the sugar kelp (Saccharina latissima).
The current distribution of the forest kelp (Laminaria hyperborea) (black lines along the coasts) and its forecasted change in distribution by the end of the 21st century (dark red and dark blue lines: summer and winter isotherms that currently determine the distribution of the species; orange and light blue line: projected shift of these isotherms for the period 2080-2100). Changed according to Müller et al. 2009
Since many years researchers of the department for seaweed biology at the Alfred Wegener Institute have been interested in the effect of the changing environmental stress factors temperature and ultraviolet (UV) radiation on seaweed ecosystems.
A new study demonstrates that the greenhouse effect will manifest itself especially in the Atlantic sector of the Arctic Ocean [2]. A sea-surface temperature increase of around 4°C is expected by the end of the 21st century. Studies on various kelp species demonstrate the possible impact. The forest kelp Laminaria hyperborea is an example which clearly shows the expected changes [2]. According to these investigations the temperature increase will induce a withdrawal of the species from Central and Western Europe (Fig. 2), with unforeseeable ecological consequences for the region there. In France (Brittany), where the forest kelp and fingered tangle are harvested, a decline in the landed biomass is already evident.
Freshly discharged healthy spores of a Laminaria sepcies from Spitzbergen after UV-B exposure that corresponds to the increased UV radiation measured in the Arctic [3, 4]. The spores are formed by the large kelps which represent the asexually reproducing sporophyte generation. (Photo: C. Wiencke)
Freshly discharged damaged spores of a Laminaria sepcies from Spitzbergen after UV-B exposure that corresponds to the increased UV radiation measured in the Arctic [3, 4]. The spores are formed by the large kelps which represent the asexually reproducing sporophyte generation. (Photo: C. Wiencke)
In contrast, an increase of UV radiation which is predicted as consequence of a further decrease in the stratospheric ozone concentration has a different impact. The affected kelp species will not change their biogeographic distribution, but advance their distribution to greater depths. In recent years, in situ studies in the Arctic investigating the fingered tangle L. digitata and other kelp species at different water depths showed that the germination rate of spores is inhibited by UV radiation in a species-specific manner (see Fig. 3). It became evident that this process determines the upper depth limit of adult individuals of the respective species [3]. Increased UV-B radiation (between 315 and 280 nm, the range that causes sunburn in humans) will therefore force algal species to greater depths but only to an extent where the light conditions at greater water depths are still sufficient for growth. Since kelp forests are populated by many species, are breeding grounds for many marine animals and, furthermore, influence currents, these changes will presumably cause unforeseeable ecological domino effects. In northern Spain the kelp forest has already deteriorated in recent years and is replaced by other algae.
[1] K. Bischof et al., Reviews in Environmental Science and Biotechnology 5, 141 (2006).
[2] R. Müller et al., Botanica Marina 52, 617 (2009).
[3] C. Wiencke et al., Journal of Ecology 94, 455 (2006).
[4] C. Wiencke, M. N. Clayton, M. E. A. Schoenwaelder, Marine Biology 145, 31 (2004).
Dr. Inka Bartsch, Prof. Dr. Christian Wiencke, Alfred Wegener Institute for Polar and Marine Research