Long-Term Ecological Change in the Northern Wadden Sea

Abstract

The Wadden Sea is a shallow coastal region in the south eastern North Sea. Karl Mobius started ecological research in the northern Wadden Sea about 150 years ago studying the extensive oyster beds. With the foundation of a field station of the Biologische Anstalt Helgoland in List/Sylt in 1924 biological research in the Wadden Sea was continued to date. Several time series were initiated between the 1970s and the 1990s including a bi-weekly phytoplankton and zooplankton program and an observation program on macrobenthos. Three factors dominating the changes observed during the past decades are a rise in temperature, decreasing nutrients, and increasing invasions of non-native species. Phytoplankton blooms gradually decrease due to the combined effect of decreasing nutrient loads and increasing winter temperatures. Mean annual zooplankton abundance is stimulated by higher winter temperatures. Recently, invading species are increasingly dominating native mussel beds. For several invaders, a positive effect of temperature was shown. We expect that major pressures of change during the next years will be further species introductions, temperature increase, and reduced nutrient loads. On the long run (21st century), we expect sea level rise to be the key factor of coastal change through a loss of habitats with fine-grained sediments and intertidal sediments in general. A major challenge for coastal research will be to disentangle the interactive effects of these pressures on the long-term development of the Wadden Sea.