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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°S, 51 to 57°W, in November 1995. Samples were collected by means of a Niskin bottle cast, and reverse-filtered through a 10 µm 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 µm mesh net paired samples indicated that absolute abundances of tintinnid species with diameters > 40 µm 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.

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Abstract. Proportions of foraminifers, tintinnids, polycystine radiolarians, pteropods and crustacean larval stages were estimated in a collection of 76 vertically stratified (0 – 100 m) 30 µm net microplankton samples from 16 stations along the Argentine shelf‐slope (around 200 m isobath – between 40 and 56° S), covered on 13 – 18 November 1996. Tintinnids were identified to species. Relative abundances of the microzooplankton assessed and chlorophyll a values allow to define two contrasting groups of stations: ‘deep’ and ‘shallow’. The former, located in pelagic, purely subantarctic Malvinas Current waters, hosted higher proportions of foraminifers and lower proportions of tintinnids, as well as less chlorophyll a (all differences were significant at the 0.1 % level). ‘Shallow’ stations were located in the area of the thermohaline front where the Patagonian Current comes in contact with the Malvinas Current, and were generally characterized by higher chlorophyll a levels (up to 3.7 µg Chl a · l–1). The distribution of tintinnid species, on the other hand, allowed no discrimination between these two areas, although some of the dominant forms showed much higher relative abundances in one of the two groups of stations. Twenty‐six tintinnid taxa were recorded, yet only 6 accounted for 95 % of the specimens identified. Tintinnid taxocoenoses were characterized by a few abundant species and many rare ones. Numbers of tintinnid species and specific diversity did not differ noticeably with depth and latitude. Cape Horn Current waters were detected in the area by the presence of expatriated organisms presumably originating at mid‐latitudes in the South Pacific Ocean.