Morphology and distribution of the marine diatom Azpeitia africana (Janisch ex A. Schmidt) G. Fryxell & T. P. Watkins in the South China Sea

The purpose of the present paper is to study the morphological structure and variability of Azpeitia africana and to determine its geographical distribution in the surface sediments of the South China Sea (SCS). Sediment samples were collected with grabs or box corers in one cruise in 2001 and two cruises in 2007. The sampling stations were located between 3°56.61′–20°59.37′N and 108°30.68′–116°46.70′E, where the water depth ranged from 72 m to 4 238 m. The diatom was observed by phase contrast microscopy and scanning electron microscopy. Microscopical observation showed that A. africana had circular valves with the areolar lines radiating from the eccentric ring. The central rimoportula had an external tube recessed on the edge of a central ring. The marginal rimoportulae were not evenly spaced, and they were positioned more closely together in one quadrant than in the others. Azpeitia africana is the most abundant diatom species in the southern region of the SCS, and accounted for 0.9%–5.6% of all diatom species in the Xisha Islands area. Average cell density of A. africana was 1.1×10 5 valves/g. The percentage abundance of A. africana was low (0%–2.5%) in the northern regions of the SCS and the Sunda Shelf, and it was not detected in the northwestern continental shelf (shallow water area) and northern Kalimantan Island shelf. Our results suggested that A. africana is a typical warm water species and that it could be used as an indicator of the warm Pacific Ocean water, including the Kuroshio Current, flowing into the SCS.