THE ROLE OF PICOPLANKTON (CYANOBACTERIA AND PLASTIDIC PICOFLAGELLATES) IN THE DIET OF TINTINNIDS

Food vacuole contents in naturally occurring populations of tintinnids in surface waters at the mouth of the Bay of Villefranche were examined from November 1988 to December 1989. Three types of prey were considered: cyanobacteria (Synechococcus spp.), plastidic pico- and nanoflagellates. The relative frequency (%) of these were observed in the diet of 18 tintinnid species. Results confirm that nanoflagellates constitute the dominant fraction in the diet of most of the tintinnids examined. They are preferentially ingested by some species, such as Amphorides quadrilineata and Favella sp. Nevertheless, picoplankton is ingested throughout the year by all the tintinnids, and can reach up to 100% of the ration mainly during winter and summer. Cyanobacteria are the prey most ingested by Rhabdonella spiralis and the only prey observed in food vacuoles of Salpingella acuminata. As indicated by the Strauss linearity index values, picoflagellates are actively grazed by some species (e.g. Dictyocysta elegans, Eutintinnus spp.). The implications of the ingestion of picoplankton by tintinnids in the food webs are discussed.