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Morphological and ecological investigation of rare planktonic dinoflagellate Prorocentrum arcuatum Issel in the Adriatic Sea

Morphology and seasonal distribution of the rare planktonic bloom forming dinoflagellate Prorocentrum arcuatum in Medulin Bay are described in this study. It demonstrates first record of high abundance of this species in the coastal waters of the Adriatic Sea. Samples were collected fortnightly in the period from July 2013 to October 2014 for analysis of phytoplankton community structure and for environmental parameters. Using both light and scanning electron microscopy, a morphometric study of P. arcuatum was performed according to cell shape, size, ornamentation of thecal plates and apical spine. Cells of P. arcuatum are asymmetrical, medium to large. At the anterior side it is broadest at the middle part of the cell and attenuated at the posterior end. This species posses marked torsion of the cells, which is distinctive feature in relation to other Prorocentrum species. The cell surface is covered with shallow depressions and scattered pores. The anterior apical spine is long, sharp and broadest at the base. P. arcuatum was almost continuously occurring in Medulin Bay throughout 15 months of the research period. The cell density of P. arcuatum varied from 80 cells L-1 to 13.200 cells L-1. Regarding vertical distribution, abundances of this species between surface and bottom layer were highly correlated. Variation in length of P arcuatum cells was noticeable having the smallest cells in winter months ; from December till March, while the largest cells occurred in May, June and July. In the period of the highest proliferation of P. arcuatum high temperatures were accompanied by high salinity and therefore both parameters should be accomplished. Also, negative correlation of P. arcuatum with all forms of inorganic nitrogen (NO3-, NO2-, TIN, NTOT) was found.

dinoflagellatae; Prorocentrum arcuatum; Adriatic Sea

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