The Bare Zone of Swedish Rocky Shores: Why Is It There?

Rocky shores world-wide are characterized by two factors; low species richness and zonation of dominant species, both related to changing water levels. In Swedish waters, sea level fluctuates by up to 2 m due to changes in atmospheric pressure, onand off-shore winds, and to a lesser extent, due to small tides. As pressure varies irregularly over time, sea level fluctuations are unpredictable. Yet, Swedish rocky shores have a zonation pattern which largely corresponds to that of tidal shores. However, two differences appear: firstly, littoral organisms of Swedish shores require more submersion time over the year than species of tidal shores, and secondly, a bare azoic zone separates the marine and terrestrial species. Non-tidally induced fluctuations of sea level increase the variance of submersion period length by several orders of magnitude, and the prolonged periods of either submersion or emersion act as disturbances in an already stressed environment. Grime's plant strategy hypothesis predicts that no plants withstand a habitat both under stress and disturbances. The bare zone of atidal shores seems to support this.