\u3ci\u3eAulacoseira stevensiae\u3c/i\u3e sp. nov. (Coscinodiscophyceae, Bacillariophyta), a new diatom from Ho Ba Bê, Bac Kan Province, Northern Viêt Nam

A new species of Aulacoseira Thwaites is described from piston core samples from Ho Ba Bê in the karst region of northern Viêt Nam. Although it closely resembles Aulacoaseira subborealis (Nygaard) Denys, Muylaert & Krammer, A. stevensiae Weide sp. nov. is designated a new species based mainly on morphological differences in the spines, including invariably inclined spines that are rounded, differences in the Ringleiste, areola pattern and overall size. Aulacoseira stevensiae is present throughout a core that spans the last 500 years. It was a major component of the diatom community, but the populations have recently decreased, possibly being outcompeted by Discostella Houk & Klee and Cyclotella (Kützing) Brébisson species