A new proposal for zonation of the midlittoral in the Bay of Koper (Gulf of Trieste, northern Adriatic) based on macroalgal communities

The general scheme of vertical zonation of organisms of marine hard substrate in the uppermost zones, as suggested by many authors (STEPHENSON & STEPHENSON, 1949, 1972; LEWIS, 1964, 1972; BALLESTEROS & ROMERO, 1988), in principle, corresponds to the scheme of the Mediterranean (PÉRÈS & PICARD, 1964; PÉRÈS, 1967; COPPEJANS, 1974; BELLAN-SANTINI et al., 1994, 2002) and the Adriatic Sea (PÉRÈS & GAMULIN-BRIDA,1973; BAKRAN-PETRICIOLI, 2007, 2011). They comprehend supralittoral and midlittoral zones affected by waves, submitted to sea level variations caused by wind, atmospheric pressure and tides. Slightly different is the division of the midlittoral into smaller parts (e.g. horizons). In the Mediterranean (including the middle and southern Adriatic Sea), the midlittoral was divided into the upper midlittoral that is moisturized by waves and splashing, and the lower midlittoral, which is under direct influence of waves and fluctuations in sea level (PÉRÈS & PICARD, 1964; PÉRÈS & GAMULIN-BRIDA, 1973; COPPEJANS, 1974; BELLAN-SANTINI et al., 2002). This scheme was accepted also in Slovenia (LIPEJ et al., 2000; LIPEJ et al., 2004), although the vertical distribution of organisms, especially algae, of the midlittoral zone of the Gulf of Trieste received comparatively little attention. The zonation of midlittoral mainly based on macroalgal communities was studied at six sites of the Bay of Koper (Slovenian south-east part of the Gulf of Trieste) in spring (April) and autumn (November) 2010. The study, using the random selection method, involved different types of substrate (limestone, marl and sandstone). It was surveyed from the upper midlittoral to the infralittoral fringe (from the mean higher high water to the mean lower low water), the upper limit of the occurrence of the brown macroalgae Cystoseira compressa. The zonation of the midlittoral zone was divided into three horizons (upper, middle and lower) by biotic (leader species) and abiotic (sea level) criteria. By using multivariate technique the main dissimilarity was found between the macroalgal communities of the upper and lower horizon. The species that most contributed to this difference was Blidingia minima. The main differences in species composition in different substrates were found between limestone and marl. The species Chaetomorpha linum mainly characterized such differences, while the species Fucus virsoides was the main contributor to dissimilarity among the middle horizons on limestone and sandstone.