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Seasonal Distribution and Morphological Changes of Ceratium hirundinella in Two Mediterranean Shallow Lakes

The paper presents different morphological changes between two forms of C. hirundinella in shallow karstic Mediterranean lakes, Ponikve and Njivice. Both lakes are situated on the island of Krk, Northern Adriatic See. Trophic state parameters (chlorophyll a, phytoplankton biomass, species composition) as physical and chemical features (temperature, Secchi depth, pH, conductivity, oxygen, nitrates, nitrites, ammonium, phosphates) were measured during 1997/98. and 2000/01. Net samples were taken at one vertical profile at each lake. C. hirundinella f. gracile was presented during the whole investigated period in Lake Njivice. The same species was dominant in Ponikve Lake during 1997/98., while in 2000/01. strong develop of C. hirundinella f. austriacum was occurred. C. hirundinella f. austriacum was restricted to spring-summer time. Spring populations of Ceratium hirundinella f. austriacum showed larger variations in number of horns. It occurred as a formtype with five horns as well as a formtype without an antapical horn. C. hirundinella f. gracile showed a slight reduction and variation of the fourth horn through the changing seasons. An analysis of morphometric measurements between the samples showed the significant differences in length, width and V3 angle.

Ceratium formtype; size variation; morphological changes; seasonal distribution

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