Mechanisms affecting macroalgal zonation in the northern Baltic Sea

The effects of exposure-related factors (ice scraping, waves and water clarity) on macroalgal zonation in the tideless and brackish northern Baltic Sea were analysed. The effects of these mutually correlated factors were separated on the basis of their ability to circumvent obstacles-in this case islands. Ice scraping is restricted to the exposed shore of an island because it is caused by movements of large ice pieces. Waves are able to bend around an island and a sheltered shore is always affected to some extent by waves. Clarity is a property of a water mass and on average the same water mass surrounds a small island without consistent differences between its exposed and sheltered shores. Three modified exposure indices, based on Effective Fetch, were tentatively used to model the behaviour of these three factors in a labyrinth-like archipelago. The upper and lower depth limits of the most common belt-forming macroalgal species were recorded at 79 study sites on the southern coast of Finland. Regressions between each exposure index and nine specific depth limits were calculated. The best-fitting exposure indices divided the depth limits into three groups. The four uppermost (upper or lower) limits were best explained by the wave index, the two intermediate ones by the ice index and the three lowermost limits by the clarity index. The r(2) values for the regressions varied between 20 and 85%. The precise processes potentially responsible for individual depth limits are discussed.