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Distribution and composition of Cystoseira stands along the west Istrian coast (northern Adriatic, Croatia) and comparison with historical data

The northern Adriatic represents the northernmost biogeographic sector of the Mediterranean. Additionally, due to north Italian rivers runoff, it represents one of the most eutrophicated regions of the Mediterranean. Differently to other north Adriatic areas, the sea bottom along the west Istrian coast is mainly rocky, i.e. particularly adapted to harbor Cystoseira (Phaeophyceae, Fucales)-dominated assemblages. The west Istrian coast spans approximately 100 km straight from north to south. This particular orientation produces large scale gradients of oceanographic variables and wave action intensities. A series of Cystoseira demography surveys were conducted during the last 10 years at different depths. From 0 to 1 m depth Cystoseira stands were mainly composed of Cystoseira amentacea, which attained maximum abundances along southern sectors of the coast. Around 1 m depth stands were dominated by C. crinita or C. barbata. The composition of these stands likely depends on local gradients of wave action and anthropogenic pressure. From 4 m depth to the rocky bottom edge (usually between 10 and 20 m) mixed stands were dominated by C. foeniculacea or C. corniculata. In some sites C. spinosa was a major component of these mixed stands. C. compressa and C. humilis were found along the whole coast at all depths as components of mixed stands. Monospecific stands of C. compressa were noticed only in moderately polluted sites. Comparison with historical data revealed long term changes of Cystoseira spp. abundance. A phase of regression, occurring during the last decades of the past century, was followed by a phase of recovery. At present, the composition of stands in sites not directly subjected to urban pollution might be considered similar to that assessed during the 1950s.

Cystoseira; mixed stands; long-term changes; eutrophication; rocky bottoms; northern Adriatic

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