Patterns in tintinnid species composition and abundance in relation to hydrological conditions of the southwestern Atlantic during austral spring

The tintinnid community was assessed on a collection of 42 samples taken at depths of 5, 10, 25 and 50 m from 17 oceanographic stations located along 34 to 62 degree S, 51 to 57 degree W, in November 1995. Samples were collected by means of a Niskin bottle cast, and reverse-filtered through a 10 mu m mesh filter. A total of 60 tintinnid taxa were recorded, most of which with a distribution circumscribed to Antarctic, Subantarctic or Brazil-Malvinas Confluence waters, thus allowing the distinction of Antarctic, Subantarctic and Transitional biogeographic zones, respectively. A comparison based on the analysis of Niskin bottle versus flowmetered 35 mu m mesh net paired samples indicated that absolute abundances of tintinnid species with diameters >40 mu m almost did not differ between collecting methods. Tintinnid abundances and species structure reflected the degree of variability in the hydrological conditions of the southwestern Atlantic between 1994 and 1995, Sea surface temperature fluctuations within Brazil-Malvinas Confluence waters involved pronounced changes in the tintinnid communities of the Transition Zone: the increase of southward warm water transport during 1994 favoured the dominance of subtropical species along the ecotone, while its decrease during 1995 favoured the contribution of subantarctic species. Low hydrological variability in the Subantarctic and Antarctic Zones was reflected in a quite similar tintinnid community structure during these 2 years. Acanthostomella norvegica forma typica and Cymatocylis antarctica forma typica in the Subantarctic Zone, and Codonellopsis gaussi and Cymatocylis convallaria in the Antarctic Zone, might be regarded as key tintinnid species, based on their persistence and dominance.