Coccolithophores in Water Samples and Fossil Assemblages in Sedimentary Archives of the Mediterranean Sea: A Review.

Coccolithophores are unicellular planktonic algae belonging to the phylum Haptophyta, and have been one of the most important contributors to calcium carbonate production in the oceans since the Middle-Late Mesozoic. They are attracting growing attention in the light of their potentiality in evaluating the response of marine organisms' calcification to ocean acidification. We present a review of living coccolithophores in the Mediterranean Sea and of the assemblages stored in the underlying surface sediments. Thus, we provide a case history from the Sicily Channel, a key area for Mediterranean oceanographic and palaeoceanographic studies. It deals with fossil coccolithophore (calcareous nannofossil) assemblages over the last 430,000 years, whose data were collected at a centennial-scale resolution. Water samples demonstrate that coccolithophore production in the Mediterranean Sea is seasonally controlled (Knappertsbusch, 1993). In winter, the nutrient uptake is rapidly exploited by r-strategist taxa such as placolith-bearing species. In summer, the productivity is at least one order of magnitude lower and the occurrence of a seasonal thermocline leads to the development of a vertical zonation. Lower photic zone species can still profit from a rising nutrient flux at the base of the thermocline, while a typical K-strategist community grows in the surface waters of the mixed layer. Such a seasonal configuration, with the development of a summer vertical zonation, is identical to what is observed in today's oceans at low-middle latitudes.