Alchichica is a deep (maximum depth 62 m) crater lake located in the state of Puebla (19° 24' N and 97° 24' W), Central Mexico (Figure 1). The lake is warm monomictic 15. Mixing takes place from the end of December or beginning of January until the onset of the stratification period by the end of April or beginning of May. A well-developed thermocline is present from June-July up to October-November. After November, the thermocline becomes deeper and weaker until its breakup in late December or early January.

Alchichica is a unique Mexican hyposaline (8.3–9 g l^-1; Na-Mg and Cl-HCO₃) and alkaline (pH = 8.8–10.0) aquatic system characterized by endemic biota and distinctive features such a tufa towers ring. Among the endemic biota there have been described the atherinid fish Poblana alchichica 16, the ambystomatid salamander Ambystoma taylorii 17, the isopod Caecidotea williamsi 18, and more recently the centric diatom Cyclotella alchichicana 19.

Sampling took place at mid-day monthly at the central and deepest part of the lake during 2001. In situ profiles of temperature, dissolved oxygen, pH and conductivity (K₂₅) were obtained with a calibrated Hydrolab^® DS3/SVR3 multiparameter water-quality data logger and logging system (discrete readings every meter). Ten water samples (depth 2, 5, 10, 15, 20, 25, 30, 40, 50 and 60 m deep) for phytoplankton analysis were obtained with a 6-liter Niskin-type water sampler. Two 500 ml sub-samples from each sampling depth were fixed, one with 4% formaldehyde and the other with Lugol's solution (1%). Phytoplankton were counted in 50 ml settling chamber with a Zeiss inverted microscope D following the Utermöhl method 2021. Valves of C. choctawhatcheeana were counted at a magnification of 806×. Additional material was cleaned through acid oxidation. Aliquots were dried onto cover slips and mounted in Naphrax 22. Slides were examined by phase-contrast microscopy. Microphotographs were taken with a Nikon Lobophot-2 photomicroscope. For scanning electron microscopy (SEM), cover slips with the dried material were mounted on aluminum stubs and coated with pure silver. We used a JEOL JSM-5200 microscope (working distance 10 mm, accelerating voltage 25 kV). For the description of the valve morphology we followed the terminology in 32324.

Morphological traits included for comparison were valve diameter, number and arrangement of the marginal and central fultoportulae (strutted processes), presence of the marginal rimoportula (labiate process), presence of marginal spines, presence of granules, density of striae, and the structure of the central area of the valves.

Frustules drum-shaped in girdle view, seldom forming short chains (i.e. 2–3 cells). In LM the specimens showed an indistinct structure. Valves are circular, 5–12 μm (mean 8.6 μm, N = 100) in diameter (Figure 2a). In the SEM, the external view of the valve shows marginal striae of equal length, radiating from the center of the valve, and extending to the mantle edge; striae 12–14 in 5 μm (Figure 2b, d). The striae start at the transition of the central to marginal area with two rows of areolae becoming three towards the valve face/mantle junction (Figure 2d–e). The central area is colliculate 38 showing a conspicuous tangential undulation and the openings of the central fultoportulae (Figure 2b, d). Small silica granules around the margin irregularly distributed were presented (Figure 2c, f). No spines on the marginal area of the valve were observed. Externally the rimoportula is visible on one of the interstria as a slit-like opening (Figure 2c). The internal view of the valve shows 7–14 marginal fultoportulae per valve, on every second, third or fourth costa, each having two satellite pores (Figure 3a–c). These fultoportulae open to the exterior as circular openings on every second, third or fourth interstria (Figure 2c, e). The single marginal rimoportula is placed on one costa radially oriented between the fultoportulae (Figure 3a–b). Central area is smooth, usually with two central fultoportulae, occasionally four. Each fultoportula is surrounded by three satellite pores (Figure 3a, d). We found some girdle views in which we observed the ligular area and open band (Figure 2f), and they were similar to those showed in 1.

Distribution of C. choctawhatcheeana is wide both latitudinal as well as longitudinal 12. It is a cosmopolitan species inhabitant of coastal brackish waters and saline lakes. It was first described from brackish-water estuary of the Choctawhatchee Bay, Florida 1. and after that, from the large estuary of Chesapeake Bay, Maryland an Virginia 1025, from the Baltic Sea 2627, from the Apalachee Bay, Florida 12 and recently from a Croatian estuary 28 and from Brazilian tropical waters 2930.

In spite of the species was first discovered inhabiting estuarine waters, there are numerous reports of C. choctawhatcheeana from inland saline lakes of Saskatchewan, Canada (Waldsea Lake, Basin Lake and Deadmoose Lake) 8, Nevada, USA (Walker Lake and Pyramid Lake) 313233, California, USA (Salton Sea) 25, La Pampa, Argentina (Laguna La Amarga) 9.

There are also reports of fossil material of C. choctawhatcheeana from the North America (Devil's Lake, Medicine Lake, Moon Lake) and North Africa (Adrar Bous, Nigeria) 8 and San Luis, Argentina (Salinas del Bebedero basin) 11.

C. choctawhatcheeana is able to tolerate water temperatures in the range of 10° to 30°C and wide ranges of salinities 1. The presence in the Baltic Sea, between 3 and 11 ‰ 2627, and the Salton Sea, with a salinity of 40 ‰ 25. demonstrates that C. choctawhatcheeana is tolerant to wide ranges of salinity fluctuation. Wilson et al. 34 in an examination of diatom assemblages from 219 saline and freshwater lakes, found a range of salinity tolerance from 5.14 ‰ to 79.80 ‰ for C. choctawhatcheeana. Prasad & Nienow 12 suggested that salinity in excess of 20 ‰ coupled with temperatures in excess of 25°C might be detrimental to its growth. Recently this species has been found in Apalachee Bay, an oligotrophic bay system in the northeastern Gulf of Mexico 12, in a karstic estuary of the Zrmanja River, Croatia 28 and in a tropical coastal lagoon, southeast Brazil 2930.

Lake Alchichica environmental characteristics such as its alkaline and saline waters rich in sodium chloride, large amounts of carbonate-bicarbonates, magnesium and sulphates, correspond to the type of habitat described previously for the species 17.

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°C. Dissolved oxygen ranged from anoxic (non detectable) up to saturation (7 mg l^-1). Alchichica is an oligotrophic lake 735 with low nutrient (N-NH₃ between non detectable (n.d.) and 0.98 mg l^-1, N-NO₂ n.d.-0.007 mg l^-1, N-NO₃ 0.1–1.0 mg l^-1, P-PO₄ n.d.-0.54 mg l^-1) and chlorophyll "a" concentrations (mean < 5 μg l^-1).

In 2001 year C. choctawhatcheeana showed low densities (0–48 cells ml^-1) along the time when the water column of the lake is mix (January and February) and nutrients are available for phytoplankton growth (Figure 4). From March to May, when the lake begins the stratification process, an increase in density (0–139 cells ml^-1) was observed. Maximum densities (7–3877 cells ml^-1) were found from June to October, along the stratificated period of the lake. During the first months of this period, the higher densities were observed near the surface (between 2 and 15 m depth), but at the end of the stratification (September and October) the maximum density values were at 40 m depth, maybe due to the sedimentation process of the cells. In this year C. choctawhatcheeana seemed to have an important role in the development of the deep chlorophyll maximum observed in the lake at the end of the stratification. The stratification season in Lake Alchichica showed a phosphorous limitation at the epilimnion, nonetheless C. choctawhatcheeana developed high densities, specially at the level of the metalimnion (20–40 m).

C. choctawhatcheeana described by Prasad 1 is often misidentified as C. caspia Grunow 36373839. Furthermore, Maidana & Romero 9 stated that C. choctawhatcheeana is related to a group of species widely distributed in saline continental and marine waters named by Hakanson et al. 27 as the "C. striata complex". Carvalho et al. 8 found that the species occurring in saline lakes (recent and sub-fossil North American material) were quite different from C. caspia. They studied material closely resembling C. choctawhatcheeana already described 1. This species had a colliculate external central area and one to several fultoportulae in the central area, whereas C. caspia has a smooth external central area with numerous (13–40) valve-face fultoportulae 278.

C. choctawhatcheeana and C. hakanssoniae are validly described species, however, considered as synonym 32627. The only difference between both taxa is that C. choctawhatcheeana forms chain-like colonies meanwhile C. hakanssoniae is single celled 1327.

Morphological features of the Alchichica population examined are similar to those considered in descriptions 18912 (i.e. size, distribution of marginal fultoportulae, lower number of central fultoportulae, number of satellite pores of the marginal and central fultoportulae, position and morphology of the rimoportula, central area of the external view, and presence of small siliceous granules) (Table 1).

The Alchichica material differed from C. choctawhatcheeana of Prasad's original diagnosis in that in the Lake Alchichica the valves exceeded the diameter (5–12 μm) given for the type material of C. choctawhatcheeana (3.0–9.5 μm), and it does not forms or seldom forms short chains (2–3 cells) in contrast of up to 20 cell chains. The presence of the small silica granules around the margin of the external view of the valve irregularly distributed meanwhile in Prasad's diagnosis a ring of siliceous granules is present near the valve margin; it has been suggested that this differences could be attributed to early stages of speciation, as this widely distributed species could be adapted to local conditions 12. All other features were within the range of variation described by the authors previously mentioned. The correct identity of this small species is essential, because it could affect the results and conclusions of present and future studies, since it is apparently a very widespread species 10.

C. choctawhatcheeana inhabits several similar North American water bodies. It has been found in Pyramid and Walker Lakes in Nevada. The ecological traits of both lakes are similar than those of Alchichica: they are hyposaline, alkaline and deep lakes. They are also monomictic lakes where C. choctawhatcheena and the filamentous diatom Chaetoceros elmorei are found together. Remarkably, in the three lakes the filamentous cyanobateria Nodularia spumigena is also an important phytoplankton species, developing blooms along the summer season 3132.

In Alchichica, during the 2001 year, the higher densities were observed from September to November, when a thermal stratification was present in the lake and nutrient concentration at the epilimnion was very low. In contrast, Oliva et al. 7 found the higher C. choctawhatcheeana densities in Alchichica Lake along the 1998 year from January to March, during the mixing season. In other saline lakes, for example in the Walker 40, diatoms usually are dominant along fall and winter, as was observed in Alchichica. It can use nutrients along the water column during the mixing period, but when nutrients in the upper layer are scarce, C. choctawhatcheeana can be located in very high densities into a well defined depth, the metalimnion, where light intensity is low but nutrient concentrations are high. Due to it small size, the contribution of C. choctawhatcheeana to phytoplankton biomass in lake Alchichica is low, but it could be an important food resource for the lake's zooplankton.