Occurrence of the rare microflagellates Prorocentrum arcuatum Issel and Hermesinum adriaticum Zacharias in the marine Lake Rogoznica (eastern Adriatic coast)

Abstract

         The marine Lake Rogoznica is a small, karstic habitat with potential anthropogenic influence, situated in the eastern-central Adriatic coast. In this naturally eutrophic, hypoxic and periodically anoxic lake, an investigation of the microplankton community was performed during the 1995- 1998 period when the ecology of the lake was greatly influenced by strong stratification and the appearance of anoxic conditions in the entire water column. The microplankton community (maximum abundance 1.06 x 107 cells L^-1) was composed of 40 taxa, mainly diatoms (62.5%) and dinoflagellates (29%). The research provided evidence of exceptionally dense populations and the seasonally-recurrent appearance of two rare microflagellates: Prorocentrum arcuatum Issel (dinoflagellate) and Hermesinum adriaticum Zacharias (heterotrophic microflagellate). The development of H. adriaticum and P. arcuatum mostly increased during the summer under conditions of nitrate deficiency, while the co-dominant diatom Chaetoceros curvisetus Cleve appeared during the spring when higher nitrate concentrations were present. Canonical Correspondence Analysis (CCA) indicated the importance of temperature, oxygen, silicate and nitrate on the incidence of these dominant species. The investigation of these microflagellates has allowed for the gaining of insight into their ecophysiological characteristics as well as the monitoring of their distribution in the Mediterranean, which has possibly expanded due to climate changes.