Morphological study of Cyclotella choctawhatcheeana Prasad (Stephanodiscaceae) from a saline Mexican lake

BACKGROUND Cyclotella choctawhatcheeana Prasad 1990 is a small centric diatom found in the plankton of water bodies with a wide range of salt concentrations. This paper describes the morphological features of the valve of C. choctawhatcheeana, from Alchichica lake, a hyposaline lake located in Central Mexico, and provides information about their ecology with respect to water chemistry and distribution in the water column along the annual cycle. Alchichica, and their neighbor lake Atexcac, are the only Mexican water bodies where C. choctawhatcheeana has been registered. RESULTS Morphological differences were found with respect to the original description. The valves of C. choctawhatcheeana from Alchichica exceeded the diameter (5-12 microm) given for the type material (3.0-9.5 microm), and it does not forms or seldom forms short chains (2-3 cells) in contrast of up to 20 cell chains. Other difference was the presence of irregularly distributed small silica granules around the margin of the external view of the valve, meanwhile in Prasad's diagnosis a ring of siliceous granules is present near the valve margin; all other features were within the range of variation of the species. Maximum densities (up to 3877 cells ml(-1)) of C. choctawhatcheeana were found in Alchichica lake from June to October, along the stratificated period of the lake. Low densities (48 cells ml(-1)) when the water column was mixed, in January and February. C. choctawhatcheeana of Lake Alchichica was found in an ample depth range from 20 m down to 50 m. Conductivity (K25) ranged between 13.3 and 14.5 mS cm(-1) and the pH between 8.8 and 10.0. Water temperature fluctuated between 14.5 and 20 degrees C. Dissolved oxygen ranged from anoxic (non detectable) up to saturation (7 mg l(-1)). CONCLUSION The morphology of C. choctawhatcheeana from Alchichica corresponded to the original description, with exception of some secondary traits. C. choctawhatcheeana can grow in several different environmental conditions. It can use nutrients along the water column during the mixing period in the lake. But when nutrients are scarce, C. choctawhatcheeana, can be located in very high densities, into a well defined depth layer of the lake, being an important contributor to the depth chlorophyll maximum (DCM). The species seems to be a small size but significant component of the phytoplankton in the saline Mexican lake Alchichica.