Natural, internal dynamics are coupled to physical forcing and occur at a range of temporal scales. The tides, for example, cause substantial, predictable changes in environmental conditions over ~6 – 24 hours, i.e. within a day, but also longer periods occur. Next, all across Europe distinct seasonality is apparent in coastal waters and on land. Seasonality, from an ecological perspective, is fairly well predictable with day length, light availability and/or temperature (e.g. Vermaat & Verhagen, 1996, Marba et al., 1996). Climate change, however, may lead to an uncoupling of physical forcing and biotic responses (Phillipart et al,. 2003). Thirdly, at larger temporal scales, weather patterns change between years and generate highly variable sequences of comparatively dry, wet, warm or cold years, the North Atlantic Oscillation being an important underlying forcing factor. Variation at all these three scales has profound, nested, impacts on the functioning of coastal ecosystems, and hence on the communities of living biota populating coastal habitats. A substantial European research effort has been invested in better understanding these natural dynamics of coastal ecosystems.

Transitional dynamics, or a change into another type of habitat, is probably the result of a substantial change in forcing factors, whether abiotic or biotic. The growing body of literature on resilience of ecosystems coupled to positive feedbacks and alternative stable states (e.g. Van de Koppel et al., 2001), suggests that also the accumulation of individually small changes may lead to a drastic change in ecosystem state, or, a transition from one habitat type to another. Examples of habitat transitions are the drastic declines in seagrass beds across Europe (e.g. Giesen et al., 1990, Frederiksen et al., 2004a) We will summarise both types of dynamics briefly here (Table 3) and have merged several habitats that appear to co-occur. Mobile sediment may generate apparently cyclic transitions between different habitats, both on land and in the sea. Rare, extreme events, such as heavy storms and seismic movements of the earth crust may cause considerable change in coastal habitats due to massive, incidental relocations of sediment. Particularly in the Mediterranean, seismic incidents are sufficiently frequent over longer time scales to be taken into consideration. Because of the erratic, unpredictable outcome in terms of habitat dynamics, seismic events have not been incorporated into Table 3. Isostatic sea level rise as well as coastal subsidence are additional natural processes leading to gradual or abrupt transitions in coastal habitats. Succession in shore meadows of the Northeastern Baltic are a good example. Climate change effects will be discussed later.