Remarks on the morphology and distribution of some rare centric diatoms in southern Brazilian continental shelf and slope waters

This paper provides information on the morphology and occurrence of extant diatoms of the family Asterolampraceae and Azpeitia species of the southernmost Brazilian continental shelf and slope waters in the Southwestern Atlantic Ocean (28 - 34°S), under the influence of Tropical, Subtropical, Subantarctic Waters, terrestrial discharges from La Plata River and Lagoa dos Patos lagoon. Plankton was sampled (20 µm net; vertical hauls) at the same 27 stations in winter 2005 and summer 2007. Among Asterolampraceae, Asteromphalus flabellatus was the most frequent species (57% of samples) observed mainly in winter samples (92%) in Subtropical Shelf Water but also under the influence of La Plata River plume (salinity <35). Lower frequencies were observed for Asterolampra marylandica, Asteromphalus elegans, Asteromphalus heptactis and Spatangidium arachne. Four species of Azpeitia were observed: A. barronii and A. neocrenulata for the first time in the South Atlantic Ocean while A. africana and A. nodulifer had already been registered in equatorial and southern areas of Brazil. All Azpeitia species were rare (19%) in offshore samples (100-200 m depth. Remarks on the morphology based on light and scanning electron microcopy observations are provided along with their distribution in the study area.