Cyclotella (Bacillariophyceae) species present in sediments dating to Marine Isotope Stage 5 from Lake Chalco, central Mexico, with special reference to two new species: Cyclotella poyeka and Cyclotella tlalocii

Lake Chalco is one of the few lakes in Mexico that has a long, continuous sedimentary sequence covering the Upper Pleistocene (> 150 000 yrs). The diatom content of its lacustrine sediments includes a variety of centric species. In particular, the sediments dating to Marine Isotope Stage 5 (MIS5) were alternately dominated by four Cyclotella spp: C. meneghiniana Kützing, C. quillensis Bailey, and two new species which are described here: C. poyeka and C. tlalocii. These two new Cyclotella spp. resemble C. stylorum Brightwell, because of the presence of marginal chambers. Nevertheless, the new species from Lake Chalco have a different structure of the marginal chambers and marginal fultoportulae with three satellite pores. Cyclotella poyeka and C. tlalocii differ from each other by the relative proportion of the central area to valve face diameter, the central fultoportula arrangement, and the striation pattern. Ecological affinities of the four Cyclotella species in the MIS5 sediments from Lake Chalco are discussed based on their modern distribution (for C. meneghiniana and C. quillensis) and their fossil assemblages (for C. quillensis, C. tlalocii and C. poyeka), leading to the conclusion that C. tlalocii was a freshwater species while C. meneghiniana, C. quillensis, and C. poyeka thrived in saline conditions.