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THE DIATOM FLORA OF TWO ACID LAKESIN CENTRAL MÉXICOMargarita Caballero-Miranda aa School of Geography and Earth Resources, The University of Hull,Hull, HU6 7RX, United Kingdom

To cite this article: Margarita Caballero-Miranda (1996): THE DIATOM FLORA OF TWO ACID LAKES INCENTRAL MÉXICO, Diatom Research, 11:2, 227-240

To link to this article: http://dx.doi.org/10.1080/0269249X.1996.9705381

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Diatom Research (1996) Volume 1 1 (2), 227-240

THE DIATOM FLORA OF TWO ACID LAKES IN CENTRAL MEXICO

Margarita Caballero-Miranda*

School of Geography and Earth Resources, The University of Hull, Hull, HU6 7RX, United Kingdom

The diatom flora from two high altitude, tropical lakes is reported in this paper. These lakes (“Lago de la Luna” and “Lago del Sol”) lie within the crater of one of the major volcanoes in Central MCxico: the Nevado de Toluca (4,620 rn ad). Both had diluted, acidic water (pH < 6, electrical conductivity (E.C.) < 25 pS/crn and total dissolved solids (T.D.S.) < 22 mg/l) with pH, E.C. and T.D.S. being lower in the “Lago de la Luna” than in the “Lago del Sol”. The diatom flora from the more acidic “Lago de la Luna” was dominated by Achnanthes helvetica, Cyrnbella perpusilla and Navicula NTA and that from the less acidic “Lago del Sol” was dominated by Navicula pseudoscutiformis and Aulacoseira distans. Only one diatom species: Achnanthes levanderi, was abundant in both lakes. Two of the reported species: Navicula NTA and N. NTB, had an uncertain taxonomic identification and a description of these diatoms is included.

La flora de diatomeas de dos lagos tropicales de altura es reportada en este trabajo. Estos lagos (Lago de la Luna y Lago del Sol) se localizan dentro del crater de uno de 10s principales volcanes del centro de MCxico: el Nevado de Toluca (4,630 m snm). Ambos lagos tienen aguas icidas y diluidas (pH < 6, conductividad elCctrica (C.E.) < 25 pS/crn y solidos disueltos toales (S.D.T.) < 22 mg/l), con el pH, la C.E. y 10s S.D.T. siendo mas bajos en el Lago de la Luna que en el Lago del Sol. La flora de diatomeas del lago con aguas mas k idas (Lago de la Luna) estuvo dominada por Achnanthes helvetica, Cymbella perpusilla y Navicula NTA mientras que la del menos acido (“Lago del Sol”) estuvo dominada por Navicula pseudoscutiformis y Aulacoseira distans. Solo una especie de diatomeas fue abundante en ambos lagos: Achnanthes levanderi. Dos de las especies reportadas tienen una posici6n taxon6mica dudosa: Navicula NTA y N. NTB por lo que se incluye una descripci6n de estas especies.

INTRODUCTION

The study of the Mexican diatom flora has, in general, received little attention and at present there is only one published survey on diatom distributions in Central Mexico (Metcalfe 1988). To contribute to the knowledge of the geographical and ecological distribution of the diatom species in this area, several lakes in Central MCxico were selected for diatom studies (Caballero, in preparation). This survey is part of a wider research project on the late Quaternary environmental evolution of the Basin of MCxico (Lozano Garcia et. al. 1993) and was undertaken because a better knowledge of the distribution of the local diatom flora is considered to be necessary for more

* Present address: Laboratorio de Paleomagnetismo, Instituto de Geofisica, UNAM, Ciudad Universitaria, MCxico D.F. 04510, MCxico.

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228 M. CABALLERO-MIRANDA

accurate diatom based palaeolimnological reconstructions. Amongst the lakes included in that survey, only two had acidic waters: “Lago de la Luna” and “Lago del Sol”. These lakes, both located within the crater of one of the major volcanoes in Central Mexico (the “Nevado de Toluca”), had a characteristic diatom flora that included two Nuviculu species of unclear taxonomic identification. It was considered relevant to report the diatom flora of these acidic, high altitude, tropical lakes given the restricted knowledge of the diatom species distribution in the area and the general interest in the ecology and distribution of diatoms in acidic environments that has developed since the 1960s in relation to the problem of lake acidification (e.g. Flower 1986, Smol et. al. 1986, Charles et. ul. 1986, Dixit & Smol 1995).

The Study Area

The “Nevado de Toluca” (4,620 m asl) is a stratovolcano built up mainly by dacitic lava flows, located about 80 km southwest of Mexico City (19”lO’N and 99’45’W: Fig. 1). Its summit is characterized by an explosion crater that contains the two studied lakes: “Lago de la Luna” (Lake of the Moon) and “Lago del Sol” (Lake of the Sun), and a central plug called “El Ombligo” (The Navel). The crater was formed about 11,600 years ago when the volcano was last active (Bloomfield & Valastro 1977) and water accumulation probably led to the formation of these lakes soon after this event.

The climate at the summit of the Nevado is cold, with a summer rainy season (UNAM- DETENAL 1970). Mean annual precipitation near the summit is 1,243 mm and mean annual temperature is 4 “C. The vegetation surrounding the lakes is sparse and is strongly influenced by the cold climate and the high altitude of the site. It has been classified by Rzedowski (1981) as “zacatonal alpino” (alpine meadow) and is mainly formed by a few species of “fascicle” growing grasses. Lower on the slopes of the volcano, below the tree line (ca. 4,000 m asl), the vegetation consists of Pinus or Abies forests. ’

METHODS

Field measurements, water and diatom samples were taken at each of the lakes. Field measurements included: water depth (m), Secchi Disk visibility (m), surface water temperature (“C) and water pH and electrical conductivity (E.C., in yS / cm).

Water samples were collected in air-tight bottles that were labelled and transported to the laboratory in a cool container. Determinations of major ions were carried out using standard techniques (Armienta et ul. 1987) at the Analytical Chemistry Laboratory of the Institute of Geophysics, National University of Mkxico (UNAM). Concentrations of Ca”, Mg”, Na’, K’, HC03-, C1-, NO3-, SiO2 and total dissolved solids (T.D.S.) were measured and are reported as mg / 1.

Diatom samples were collected at each lake from all the microhabitats in which diatoms were likely to be found: mid-lake bottom sediments, littoral sediments, rocks and plants. Samples from bottom sediments were collected using a scoop with a plastic lid that was dragged for a about 0.5 m over the bottom of the lake. Other samples were collected directly by hand. All samples were kept in plastic bags that were sealed, labelled and stored in a cool container until they were processed in the laboratory. Rock and plant samples were boiled and scraped with a small brush to detach diatoms from their surface and the remaining sediment was then cleaned using the method described below. Sediment samples were dried overnight (1 6 to 18 hr) at 80 “C and 0.5 g of dry sediment were cleaned using the following method:

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DIATOM FLORA OF TWO ACID LAKES IN CENTRAL MEXICO 229

M E X l C

. ..

10000’ 99’20’

Fig. 1 . Location map of the “Nevado de Toluca” lakes: “Lago del Sol” and “Lago de la Luna”, Central Mexico.

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230 M. CABALLERO-MIRANDA

a)

b)

samples were gently boiled with 10% HC1 and concentrated (30 vol.) H202 until all carbonates and organic matter were eliminated; the remaining sediment was rinsed with distilled water and centrifuged at 1,500 rpm for 15 min, or left to settle for 24 hr. This was repeated twice more to eliminate all remaining acid or peroxide;

c) the residue was diluted to 30 ml and kept in plastic vials. Slides for microscope analysis were prepared with 200 pl of cleaned diatom suspensions and

using Naphrax as the mounting medium. Further dilutions were made if necessary to achieve a clear view of individual valves. The microscope analysis was carried out using a Zeiss Photomicroscope at a magnification of 1 ,250~. At least 400 diatom valves were counted for each sample except for one in which diatom abundance was very low; in this case only 100 valves were counted. Species abundances are reported as a percentage of the total count (relative abundance). Standard floras were used for species identification (Hustedt 1927-66, Patrick & Reimer 1966-75, Schoeman & Archibald 1976-80, Gasse 1986a, Germain 1981, Krammer & Lange-Bertalot 1986-91). Estimates of diatom total abundance were also made for the mid-lake sediment samples following the aliquot method described by Battarbee (1986) and are reported as number of valves per gram of dry sediment (vlvs / g.d.s.). The presence of Chrysophyte statospores was also recorded and is reported as number of cysts per 100 diatom valves. Some diatom species were observed under the scanning electron microscope (Cambridge Instruments Stereoscan 360 microscope). For this purpose aliquots of the diatom samples were dried on a small coverslip, mounted on an aluminum stub and coated with carbon.

For a more objective description of the diatom species distribution in the two lakes studied, diatom relative abundance data were analysed using the classification program TWINSPAN (Hill 1979). The analysis was carried out using all the default settings defined by the program. This program performs a divisive, hierarchical classification for both samples and species, and identifies groups of species that have a similar distribution within the samples.

RESULTS

The lakes:

The “Lago de la Luna” is a small lake of about 25,000 m2 (0.025 km’). At the time of sampling (January, 1991) the maximum depth was 8 m, the pH was 4.87, and the electrical conductivity was 17.3 pS / cm (Table la). Its water was cold (~10°C) and very clear, with a Secchi Disk visibility that nearly reached the bottom of the lake. Macroscopic vegetation was not evident. The “Lago del Sol” is a bigger and deeper lake, with an area of about 175,000 m2 (0.175 km2). The maximum recorded depth at the time of sampling (January, 1991) was 12 m; pH was 5.9 and electrical conductivity 24.6 yS / cm (Table la). It also had cold (<lO°C) and clear water with a Secchi Disk visibility of 8 m. The macroscopic vegetation of this lake was dominated by filamentous green algae.

The water ionic composition of both lakes was similar, with very low content of dissolved ions. Chlorides, sodium, potassium, calcium and silica were below detectionable limits and only magnesium and carbonates could be quantified (Table lb). Even though sulphate was not measured directly it is probably important in both lakes, particularly at “Lago de la Luna” where, given the expected balance between the major anions and cations, it might be the dominant anion.

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DIATOM FLORA OF TWO ACID LAKES IN CENTRAL MEXICO 231

Table 1. Physical and chemical characteristics of the Nevado de Toluca lakes, in Central MCxico. la. Field measurements, lb. Water chemical analyses BDL = Below Detection Limits (DL in mg/l are: Na= 1.0, K= 0.5, Ca= 0.8, C1= 1 .O, Si02= 2.5) E.C. = Electrical Conductivity, T.D.S. = Total Dissolved Solids

a. Area pH E.C. Depth Visibility Temperature

(m2) (pS/cm) (m) (m) (“C) LAKE

“Lago de la Luna” 25,000 4.87 17.3 8 8 9 “Lago del Sol” 175,000 5.90 24.6 10-12 8 7

“LagodelaLuna” BDL. BDL BDL 10.90 BDL 6.10 1.15 BDL 19.2 “Lagodel Sol” BDL BDL BDL 3.92 BDL 12.20 0.26 BDL 21.5

The diatom flora:

A total of eleven diatom samples was collected and analysed, three from “Lago de la Luna” and eight from “Lago del Sol” (Table 2). Full diatom counts (400 valves) were possible for only ten of these samples. A reduced count of only 100 valves was made in sample 1. A total of twenty-one diatom taxa were identified and the presence of plain, round Chrysophyte statospores, 5-1 0 pm diameter was also recorded (Table 3). Amongst the diatom taxa there were two species which could not be clearly identified. They are reported as Navicula NTA (Figs 23-29) and Navicula NTB (Figs 30-36). A description of these forms is included at the end of this paper.

In the “Lago de la Luna” nine diatom species were present. Achnanthes helvetica (Figs 5-10), A. levanderi (Figs 15-22), Navicula NTA (Figs 23-29) and Cymbella perpusillu (Figs 37, 38) had the highest overall relative abundances (Table 3). Plain, round Chrysophyte statospores were also observed in the samples from this lake. Chrysophyte statospores were apparently more abundant in sample 1 (littoral coarse sediment) than in the rest of the samples, but this might only be a reflection of the low diatom numbers in this sample, as the relative abundance of these cysts is reported as numbers / 100 diatoms. Diatoms were abundant in all the samples except for number 1 (littoral coarse sediment). Total diatom concentration in the mid-lake sediment sample (number 3) was 146 x lo6 v1vdg.d.s.

In the “Lago del Sol” nineteen diatom taxa were identified. Navicula pseudoscutiformis (Figs 11-14) was the most abundant species in these samples, but Aulacoseiru distans (Figs 2 4 ) and Achnanthes levunderi (Figs 15-22) were also common (Table 3). Plain, round Chrysophyte statospores were also observed in these samples. Diatoms were abundant in all the samples. Total diatom concentration in the mid-lake sediment samples (8 and 9) had an average value of 360 x lo6 vlvs/g.d.s.

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232 M. CABALLERO-MIRANDA

Table 2. Diatom samples collected from the Nevado de Toluca lakes: “Lago del Sol” and “Lago de la Luna”, in Central MCxico.

I “LAGO DE LA LUNA” “LAGO DEL SOL” I I I NUMBER KIND OF SAMPLE I NUMBER KIND OF SAMPLE 1

1 littoral coarse sediment 2 rocks 3 mid-lake bottom sediment

4 5 6 7 8 9 10 11

littoral coarse sediment littoral sediment rocks filamentous algae mid-lake bottom sediment mid-lake bottom sediment littoral sediment rocks

From a first inspection of the diatom species relative abundance data (Table 3) it seems clear that several species were more common in one of the “Nevado de Toluca” lakes or the other. The result of the TWINSPAN (Hill 1979) analysis of the diatom relative abundance data (Table 4) supports this approach as the “Nevado de Toluca” samples were clearly divided into two groups with different species composition: those from the “Lago de la Luna” (sample group “1, including samples 1, 2 and 3) and those from the “Lago del Sol” (sample groups “0, including samples 9, 10,6, 8, 11, 4, 5 and 7). The TWINSPAN species classification suggests that the diatom taxa present in these two lakes can be divided into four groups and one outlier species (a species with a distinctive distribution in the samples): 1) a group of twelve species that were present only in the less acidic “Lago del Sol” (“00 codes in Table 4, including among others Aulacoseira distans, Cymbella gracilis, Navicula minima and N. pseudoscutiformis); 2) a group of two diatom taxa that were present in both lakes at generally low relative abundances (“01 codes in Table 4, including Pinnularia spp. and Surirella linearis); 3) the outlier species (Achnanthes levanderi), which was present in both lakes at generally high relative abundances (code “10 in Table 4); 4) a group of five species that, even though present in both lakes, had a higher abundance in the more acidic “Lago de la Luna” (“110 codes in Table 4, including amongst others Achnanthes helvetica, Cymbella perpusilla and Navicula NTA) and 5 ) a group formed by two diatom taxa (Eunotiu spp and Hantzschia amphioxys) with very low relative abundance and which were found only in the “Lago de la Luna” (“111 codes in Table 4).

DISCUSSION

The chemical characteristics of the Nevado de Toluca lakes: “Lago de la Luna” and “Lago del Sol” are very similar, both having acidic waters with low electrical conductivity and total dissolved solids. In both, the dominant cation was magnesium. Carbonates, and probably sulphates, were the dominant anions. The “Lago de la Luna” had, however, lower pH, electrical conductivity and total dissolved solids than the “Lago del Sol” (Table la). The diatom flora of both lakes was, nevertheless different. The “Lago de la Luna” had a lower number of diatom species present and a slightly lower diatom total abundance in the bottom sediment samples than the “Lago del Sol” but, most importantly, the most common diatom species at each lake were different. In the “Lago de la Luna”

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DIATOM FLORA OF TWO ACID LAKES IN CENTRAL MEXICO 233

Figs 2-22. Most common diatom species in the “Nevado de Toluca” lakes. Scale bar in light microscope (LM) photographs = 10 pm, scale bar in scanning electron microscope (SEM) micrographs = 5 pm. Figs 2 4 . Aulacoseira distuns (Ehr.) Sim., LM; Figs 5-7. Achnunthes helvetica Hust., LM; Figs 8-10. Achnanthes helvetica Hust., SEM; Figs 11-14. Navicula pseudoscutiformis Hust., LM; Figs 15-19. Achnanthes levanderi Hust., SEM; Figs 2&22. Achnunthes levanderi Hust, LM.

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DIATOM FXORA OF TWO ACID LAKES IN CENTRAL MEXICO 235

Achnanthes helvetica, A. levanderi, Cymbella perpusilla and Navicula NTA were the most abundant species while in the “Lago del Sol” Navicula pseudoscutiformis, in addition to Aulacoseira distans and Achnanthes levanderi were the most common. Only this last species (A. levanderi) was relatively abundant in both lakes; the rest were clearly more common in one lake than the other. The most common species in the “Lago del Sol” (in the “00 groups in Table 4) were completely absent in the “Lago de la Luna”. On the other hand, the most common species in the “Lago de la Luna” (in group *110 in Table 4) were also present, even though at lower relative abundances, in the “Lago del Sol”. Given the very low abundances of the two taxa that were found only in the “Lago de la Luna” (Eunotia spp. and Hantszchia amphioxys, *11 in Table 4) this distribution is not considered to be representative of these forms and is, therefore, not discussed further.

Table 4. TWINSPAN classification of the diatom species present in the Nevado de Toluca lakes: “Lago del Sol” and “Lago de la Luna”, in Central MCxico. 1 = 0-2%; 2 = 2-5%; 3 = 5-10%; 4 = 10-20%; 5 = >20%.

Samples:

Species classification

Fragilaria cons. venter Fragilaria pinn. lancett. Navicula minima Nitzschia gracilis Fragilaria brevistriata Fragilaria cons. exigua Frustulia rhomboides Gomphonema parvulum Aulacoseira distans Navicula NTB Navicula pseudoscutifor. Cymbella gracilis Pinnularia spp. Surirella linearis Achnanthes levanderi Achnanthes helvetica Cymbella perpusilla Navicula NTA Stenopter. delicatissima Chrysophyte statocysts Eunotia spp. Hantzschia amphioxys

Samples classification

“Sol” 9 1 0 6 8 1 1 4 5 7

- 1 1 1 1 1 1 1 _ _ _ 1 1 1 1 1 3 1 3 3 3 4 3 4 - 1 2 2 1 2 2 2 - 1 1 1 1 - 1 1 1 - 1 1 - - 1 - 1 1 1 1 1 1 1 1 1 1 1 1 1 - 1 1 3 5 4 4 5 3 4 4 2 4 3 3 4 3 3 3 5 5 5 5 5 5 5 5 4 4 3 2 3 2 2 2 1 3 2 2 2’1 1 1 2 1 - 2 - - - - 5 2 3 3 3 4 4 3 - - 2 2 2 2 2 2 1 3 1 3 3 2 2 2 1 1 1 2 1 1 1 1 1 - - 1 1 - - 1 1 - 1 1 1 1 1 1 - - - - - - - -

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1

0 0 0 1 1 1

“Luna” 1 2 3

0 0 0 0 0 0 0 0 0 0 0 0

_ _ _ _ _ _ _ _ _ _ _ _

_ - - 0 0 1 0 - - _ 0 0 1 1 0 0 _ _ - 0 0 1 1 0 1 _ _ _ 0 0 1 1 0 1 _ _ _ 0 0 1 1 0 1 _ _ _ 0 0 1 1 0 1 _ _ _ 0 0 1 1 0 1 _ _ _ 0 0 1 1 1

O 1 + I- 1 1 1 - 1 1 0 1 1 ; : ; : - J I 4 4 2 1 1 0 - 3 2 1 1 0 5 2 2 1 1 0 - 1 - 1 1 1 1 1 - 1 1 1

1 1 1

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Taking into account the geographical proximity of both lakes, the differences in the diatom species distribution between them should probably be attributed to their slightly different water chemistry. It is considered possible that species such as Navicula pseudoscutiformis, N. minima, N. NTB, Aulacoseira distans and Cymbella gracilis (in TWINSPAN group *OO, see Table 4) are excluded from the “Lago de la Luna” because of its lower pH and, in general, water mineral content. This interpretation is in agreement with the reported ecological preferences for N. pseudoscutifomis and N. minima, which are considered to range from indifferent to alkaliphilous (Patrick & Reimer 1966-75, Lowe 1974, Krammer & Lange-Bertalot 1986-91). The dominant presence of N. pseudoscutiformis in the “Lago del Sol” diatom assemblage suggests that the optimal conditions for the development of this species are cool, moderately acidic waters.

Species such as Achnanthes helvetica, Cymbella perpusilla, Navicula NTA and the Chrysophyte statospores (in TWINSPAN group “110, see Table 4), on the other hand, seem to have a greater tolerance or preference for the more acidic and diluted conditions of the “Lago de la Luna”. Only the distribution of Achnanthes levanderi, a species commonly present in circumnetural, low nutrients, alpine lakes (Hustedt 1927-66, Arzet et al. 1986, Krammer & Lange-Bertalot 1986-91), appears to be unaffected by the pH and water mineral content differences between both lakes.

The diatom flora from the “Nevado de Toluca” differs markedly from that reported for other acidic lakes. It is different from the diatom assemblages found in the tropical, high altitude, acid systems studied by F. Gasse in Africa (Gasse 1986a, 1986b), where Eunotia and Pinnularia spp. are the dominant taxa. It is also different from the diatom flora found in other acidic lakes in North America (Canada and USA) or in Northern Europe where diatoms such as Brachysira brebissonii (= Anomoeoneis serians var. brachysira), Asterionella ralfsii var. americana, Aulacoseira distans (= Melosira distans), Eunotia spp., Fragilaria acidobiontica, Frustulia rhomboides, Pinnularia spp. and TabellariafZocculosa are some of the most common taxa (Dam et al. 1981, DeNicola 1986, Flower 1986, Ford 1986, Leclerq & Depiereux 1987, Whitmore 1989, Sweets et al. 1990, Hamilton et al. 1992, Stabell 1993, Dixit & Smol 1995). Except for Aulacoseira distans, these species are absent in the “Nevado de Toluca” lakes or are present only in low relative abundances (< 5%) while the abundant Achnanthes helvetica, A. levanderi, Navicula pseudoscutiformis and Cymbella perpussilla, are uncommon in those systems.

This work is only a first attempt to widen the knowledge on the diatom species distribution in Central MCxico. Its main aim was to report the water chemistry and diatom flora of two acidic, high altitude lakes in this area, including the two Navicula species of uncertain taxonomic identification that were present in these lakes. A description of these forms will close this paper:

Naviculu NTA (Figs 23-29):

This diatom has a narrow elliptical valve with short rostrate to capitate ends, a narrow axial area and an undifferentiated central area. Size is variable, with length between 19-25 pm (most common 20 pm) and width between 4.5-5.6 pm. The raphe is straight, filiform, with straight central terminals and polar terminals bent towards the same side. Striae can be slightly radiate in the central area, but most commonly they are parallel, becoming slightly convergent towards the ends. They are clearly distinguishable under the light microscope, with density varying between 16-19 / 10 pm, becoming more closely spaced towards the poles. Under the scanning electron microscope (SEM) the punctae are equally spaced and have a slot shape, aligned lengthwise with the apical axis. Density of punctae is 3 5 4 1 / 10 pm.

Under the light microscope this diatom appears similar to N. submolesta Hust. even though N. NTA has bigger valves and somewhat coarser striae than reported for N. submolesta (Krammer & Lange-Bertalot 1986-9 1; Schoeman & Archibald 1976-80). In the TEM (transmission electron

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DIATOM FLORA OF TWO ACID LAKES IN CENTRAL MEXICO 237

Figs 23-38. Most common diatom species in the “Nevado de Toluca” lakes (cont.). Scale bar in light microscope (LM) photographs = 10 pm, scale bar in scanning electron microscope (SEM) micrographs = 5 pm. Figs 23-25. Navicula NTA, LM; Figs 26-29. Navicula NTA, SEM.; Figs 30, 31. Navicula NTB, LM; Figs 32-36. Navicula NTB, SEM; Fig. 37. Cymbella perpusilla Cleve-Euler, SEM; Fig. 38. Cymbella perpusilla Cleve- Euler, LM.

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microscope) photographs in Schoeman and Archibald (1976-SO), however, N. submolestu has long narrow rectangular puncta rather the slot shaped ones of this variety.

Nuvicula NTB (Figs 30-36):

This diatom has a lightly silicified elliptical valve with broadly rounded ends, a clear axial area and a circular central area. Length ranges between 13.3-19.5 pm; width between 4.2-6.6 pm. The raphe is filiform, with slightly unilaterally bent central terminals and abruptly bent (towards opposite side as central terminals) polar terminals. Striae are slightly radial near the central nodule, becoming parallel towards the polar nodules. The central one to four striae are shorter and more widely spaced than the rest. Striae appear to be very fine under the light microscope, with density varying between 28 and 32 / 10 pm. Under the SEM the punctae appear to be equally spaced, round to slightly elongated along the transapical axis with a density of 35-41 / 10 pm.

Although it is convenient to refer to it as a Naviculu, it is clear from its ultrastructure that it is not a species of Naviculu in its narrow sense (Naviculae Lineolatae sensu Krammer & Langte- Bertalot 1986-91). When observed under the light microscope this species was considered to be a thinly silicified form related to Bruchysiru brebissonii but the SEM micrographs showed that it does not belong to this genus as it does not have a marginal ridge or a clear zone running around the whole valve. A more likely candidate is Selluphora Mereschkowsky, since it has simple round poroids, and central raphe terminals bent slightly the opposite way to the polar terminals (see Mann 1989).

ACKNOWLEDGEMENTS

The present research was possible thanks to the support of the Direction of Academic Staff Affairs (DGAPA) at UNAM (project IN-103589 and a research grant to the author). The water chemical analyses were carried out in the Analytical Chemistry Laboratory at the Institute of Geophysics, UNAM, by Dr Maria Aurora Armienta, Nora E. Cenizeros, Alejandra Aguayo and MSc Tino Juirez. The help of Mr Antonio Altamira was invaluable for the preparation of the diatom plates. The author would also like to express her gratitude to Dr Cecilia Caballero for her help in the preparation of the manuscript, to Dr Sarah Metcalfe for her support during the realisation of this research, to Dr Jaime Urrutia for his initial encouragement to undertake it and to the anonymous reviewers of this paper for their valuable comments.

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