engleski

Seasonal vegetation patterns of the introduced Caulerpa racemosa (Caulerpales, Chlorophyta) in the northern Adriatic (Vrsar, Croatia)

Background and purpose: Due to its high development capacity the invasive green alga Caulerpa racemosa is rapidly spreading in both the northwestern Mediterranean and the Adriatic Sea. In July 2004 it was signalized in the region of Vrsar (45°8'42''N ; 13°35'98''E), representing the northernmost settlements of this species in the Mediterranean. In order to assess seasonality in biomass and biometry in response to the cold seawater temperatures of the northern Adriatic the alga was studied in two contrasting habitats, where one had a sheltered sandy bottom while the other had a rocky bottom exposed to wave action. Materials and methods: C. racemosa was seasonally sampled by SCUBA diving from August 2004 to August 2005 in two habitats. Analysis of samples included the determination of the biomass dry weight, the number of erect axes, the length of erect axes and the length of stolons. Daily seawater temperature fluctuations were assessed by placing temperature Data Loggers in Caulerpa meadows. Results and conclusions: On the sheltered sandy bottom, inside the harbor of Vrsar, C. racemosa formed a dense and continuous meadow from 2 to 5 m depth which increased from approximately 500 m2 in August 2004 to 1500 - 2000 m2 in November 2004. Several years are necessary to achieve such extent of colonization. On the exposed rocky bottom approximately tens of colonies of 1 to 5 m2 were present during the August 2004 to February 2005 period suggesting a more recent colonization. While extremely low winter temperatures (8 - 9 °C) caused a drastic reduction of the covered area, the meadow recovered however, in August 2005 in the harbor, and in the exposed habitat the colonization also evolved into the meadow phase. In both habitats the alga showed similar general vegetative patterns, with maximum development in autumn and winter and minimum in spring. However, during the maximum development, biomass dry weight (1260  221 g m-2), number of erect axes (31956  8970 m-2) and stolon length (1445  470 m m-2) in the rocky habitat were higher than on the sandy bottom (biomass = 803  62 g m-2, number of erect axes = 15267  2748 m-2 and stolon length = 1180  230 m m-2).

colonization patterns; exposed habitat; introduced species; seasonal changes; sheltered habitat; winter temperatures; Caulerpa racemosa; northern Adriatic

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