Population ecology and mass development of Enteromorpha spp. (Chlorophyta) in the Wadden Sea intertidal at the island of Sylt (North Sea)

Since the 1950s, nutrient concentrations of the North Sea and the adjacent Wadden Sea waters have increased through anthropogenic inputs. Eutrophication in estuaries and coastal waters may explain the extensive growth of several green algal genera on mudflats in this area. The formation of dense mats of Enteromorpha over large areas of the North Frisian Wadden Sea has been well documented. Apparently, Enteromorpha grows from the sediment in spring and develops into thick mats within a few weeks. Decay processes in the algal mats are associated with decreased oxygen exchange between sediment and the overlying water column. As a consequence many endo- and epibenthic organisms suffocate. This study, the practical part of which was carried out between 1992 and 1994, is concerned with the development of Enteromorpha in sandy and muddy intertidal areas of Konigshafen Bay (island of Sylt, North Sea). A total of 13 Enteromorph species was found to occur in Koenigshafen Bay. Three of them were regularly represented in the algal mats (E. flexsuosa, E. prolifera, and E. radiata). The species E. ralfsii was of local importance. Also, E. clathrata and E. torta were found commonly, though often with low biomass. The presence and development of spores, germlings and adult plants were studied for at least 15 months by regular sampling in the field. Additionally, laboratory investigations were carried out. Spores, germlings and adult plants were shown to survive for at least 10 months at both 5 degree C and at 15 degree C. Germlings with a maximum thallus length of 5 mm continued growth after 8 months in the dark. In intertidal areas were adult plants were present throughout the year, they did not form dense aggregations during winter months but, nevertheless, produced spores in that period.