Abstract.Since 2003 the macroalgal flora of the rocky intertidal zone at the marine nature conservation island of Helgoland (North Sea) has been subject of quantitative percentage cover estimations within an area of 160 x 230 m using approx. 150 regularly spaced, georeferenced sampling points covering the major intertidal biotopes. For the first time seasonal changes of all seaweed species were quantified and progressive dispersion of foreign species was visualized in a geographic information system (GIS). It became apparent that not only annual species but also perennial and pseudo-perennial species may exhibit a strong seasonality in total coverage. Continuous strong winds in winter 2006/2007 led to a drastic decrease of formerly dense Fucus serratus beds while the co-dominant flora and diversity of this biotope was hardly influenced. It became evident that the repeated spatial mapping is a valuable tool to document variability present in the intertidal zone. Furthermore spatial data may also be used as parameter for macrophytobenthos classification required in the water framework directive.