Coastal zones as important habitats of coccolithophores: A study of species diversity, succession, and life-cycle phases

Coccolithophores are unicellular calcifying eukaryotes with a complex life‐cycle; they are important primary producers, and major drivers of global biogeochemical cycles. The majority of research on coccolithophores has been focused on open ocean waters, while the knowledge of their roles in coastal ecosystems is limited. Early 20th century studies of the Adriatic coast produced major taxonomic papers describing half of the projected diversity of coccolithophores. Afterwards, there have been only scarce records of their diversity and ecology in the Adriatic, and coastal systems in general. We aimed to assess coccolithophore diversity and species succession, and closely examine their ecological preferences in the coastal area of the northern Adriatic. We recorded coccolithophores throughout the year at 100% frequency, documented 52 taxa, abundances as high as 2.4 × 105 cells L−1, and noted the winter domination over the phytoplankton community. Out of 52 observed coccolithophore taxa, 31 were heterococcolithophorids and 21 holococcolithophorids. Moreover, seven pairs of heterococcolith and holococcolith phases were noted, and two strategies of temporal separation of life‐cycle phases were observed. We report ecological preferences of those life‐cycle pairs and give in situ evidence that holo‐ and heterococcolith phases are widening the specie's ecological niches. This expansion allows the species to exploit a wider range of ecological conditions and enables phenotypic plasticity, important in response to changes in the environment. With this study we emphasize that coastal areas are highly productive for coccolithophore studies. Our results indicate that a shift in research effort on coccolithophores in the coastal waters is needed