Community structure of diatoms present in the surface sediments of eight Andean lakes of central Chile

In this work, the community structure of diatoms deposited in the surface sediments of eight Chilean Andean lakes located between 32 ° 49 ‘and 38 ° 48’ S was analyzed. A total of 99 diatom taxa were found, distributed in 48 genera. Of the total taxa, 74 were identified at the species level, all of them cosmopolitan except for Eunotia andinofrequens, Gomphonema pupae, Pinnularia araucanensis, and Pinnularia acidicola. among others, which are described only for the Southern Hemisphere. On the other hand, the analyzed samples presented different floristic compositions of diatoms. 

In this way, in the victimological assemblages deposited in the sediments of the lagoons of height Ocho, Haifa, Ensueño and Negra located above 2,860 m of sea level, benthic species abounded, typical of oligotrophic and acidic waters such as Achnanthidium exiguum, Achnanthidium minutissimum.

Encyonema minutum, Pinnularia acidicola, and Planothidium lanceolatum. Planktonic diatoms, alkaline, and mesotrophic water characteristics such as Asterionella Formosa, Aulacoseira distans, Aulacoseira granulata, Cyclotella stelligera and Rhopalodia gibba abounded in the victimological assemblages of lakes Galletué, Icalma and Laja, located below 1,360 m above sea level.

Keywords: species composition, Bacillariophyceae, surface sediment, Andean lakes, Chile.

ABSTRACT

In this research the taxonomic structure of diatoms in sediments of high mountain lakes was studied. These lakes are located in Chile between 32 ° 49 ‘and 38 ° 48’ S in the Andean Cordillera. A total of 99 diatom taxa distributed in 48 genera were identified and all this taxa are cosmopolitan excepting a Eunotia andinofrequens, Gomphonema punae, Pinnularia araucanensis and Pinnularia acidicola, which are know only for the Southern Hemisphere.

The assemblages of diatoms were different in the studied lakes. So the high mountain lakes Ocho, Huifa, Ensueño and Negra, dominated benthic diatoms which are typical of oligotrophic and acid waters asAchnanthidium exiguum, Achnanthidium minutissimum, Encyonema minutum, Pinnularia acidicola and Planothidium lanceolatum. 

In the assemblages from lakes Galletué, Icalma and Laja planktonic diatoms were more abundant, which are common in alkaline and mesotrophic waters, eg, Asterionella formosa, Aulacoseira distans, Aulacoseira granulata, Cyclotella stelligera and Rhopalodia gibba.

Keywords: species composition, Bacillariophyceae, superficial sediment, Andean lake, Chile.

Diatoms (Bacillariophyceae) constitute an important component in phytoplankton communities, but most of their species are sessile and are associated with benthic and littoral communities (Round et al. 1990). However, most of the ecological and biogeographic work on these microalgae is carried out on the basis of the communities that inhabit the phytoplankton.

On the other hand, in lake sediments it is possible to find both benthic forms and forms from the water column (Reynolds et al. 1982) for this reason, sediments provide a temporal and spatial record of what inhabits the lake system ( Anderson & Battarbee 1994).

Due to the above, diatoms deposited in lake sediments can provide environmental information on the processes that take place in the aquatic ecosystem (Brugam 1983). This is possible, because between the sedimentary layers that are deposited slowly and with little disturbance, the siliceous remains of the diatoms are archived, which can be used as indicators of the environmental conditions of the lake at the time of deposit (Smol & Cumming 2000). As noted by Charles et al. (1994), many anthropic and natural disturbances that have affected lakes can be evaluated by using the remains of diatoms stored in the sediments.

One of the difficulties in conducting diatom-based studies is the lack of information about the composition and distribution of species in a geographic region of interest (Smol & Cumming 2000).

In relation to the diatom work carried out in our country, it is necessary to mention that the knowledge of the diatomological flora of continental waters is scarce, being limited to certain geographical areas of the country, namely, the North zone and the Center-South zone (Urrutia et al. 2000, Cruces et al. 2001, Cruces et al. 2006, Rivera 2006).

An important geographic area for the study of diatoms is the Andes mountain range. Its great longitudinal extension that crosses very different climatic zones and its development parallel to the coast create the ideal conditions for endemic species to develop (Rumrich et al. 2000). This geographic domain is little studied, and it is possible to mention in the Andes of northern Chile the works of Tapia et al. (2004), Díaz & Maidana (2005) and Rivera & Cruces (2005); in the Andes of central Chile, the studies by Rivera (1970), Parra et al. (1993) and Cruces et al. (2006) and throughout its north-south extension, the work of Rumrich et al. (2000).

Currently, a project under development is carrying out studies in a group of mountain lakes in central-northern Chile, of which five are included in this study. These systems are far from being intervened by human activities given their difficult access, which maintains their pristine conditions and makes them interesting study units to learn about the diversity and floristic composition of diatoms.

The composition of benthic assemblages of these five systems was compared to the benthic flora of three cordilleran systems located in south-central Chile. According to the above, The objective of this work was to study and compare the floristic composition and diversity of diatom assemblages present in the surface sediments of eight lake bodies located in the Andes of north-central and south-central Chile. This work constitutes a first approach to the knowledge of the diatom flora that inhabits high Andean Chilean lakes.

MATERIALS AND METHODS

Study area

The study area covered two sectors of the Andes mountain range: the first located between 32 ° 49 ‘S and 34 ° 06’ S and the second between 37 ° 26 ‘S and 38 ° 48’ S ( Fig. 1). The first sector included the Inca, Ocho, Ensueño, Haifa, and Negra lagoons, located above 2,860 m above sea level.

The climate in this part of the Andes is cold temperate in height, with abundant snow precipitations (0.6 to 6.4 m) and rainfall (28.5 to 471.7 mm) in the winter months (June -September) and a dry summer season (December-March) that reaches 16 ° C (CONAF 2004). Vila & Mühlhauser (1987) mention that in high-altitude lakes the thermal regime depends on latitude and altitude, most of them being polymictic, with frequent periods of total mixing; or amictic, permanently stratified. 

The limnological data published in Table 1were taken by Grosjean et al. (unpublished results) between the months of November and March of the years 2004 and 2005. The nutrient contents measured in the sampling period reveal an oligotrophic condition for these lakes. The pH, conductivity, and temperature in these lagoons were measured directly by a Hanna brand conductivity meter and the data of the main ions were analyzed in the Environmental Chemistry Laboratory of the EULA Environmental Sciences Center of the University of Concepción.

The second sector under study included lakes Laja, Icalma and Galletué, located in the foothills of the Andes, below 1,360 m of altitude and in the areas of birth of the river system of the Biobío river (Parra et al. 1993). According to this author, the Galletué and Icalma lakes are subject to strong climatic variations between winter and summer, presenting stratification in summer and marked circulation in winter, which classifies them as temperate monomictic lakes. Furthermore, this author classifies these lakes according to his observations as oligotrophic. The climate of this sector is temperate humid with abundant rainfall between the months of May to August, also accompanied by snowfalls.

The environmental and limnological characterization of the water quality data from lakes Icalma and Galletué was obtained from the work carried out by Parra et al. (1993). For its part, the limnological information on Lake Laja was obtained by Urrutia et al. (unpublished results) in February 1999. The data from lakes Icalma and Galletué presented in Table 1 correspond to the average of values ​​measured between the months of January and March of the year 1991.

Diatom assemblages contained in the surface sediments of sedimentary columns collected in the deepest part of each lake were analyzed. For this, a Uwitec brand “gravity corer” was used. The sedimentary columns of the Inca, Huifa, Ocho, Negra and Ensueño lagoons were obtained between the months of November and March of the years 2004 and 2005. For their part, the sedimentary columns belonging to the Laja, Icalma and Galletué lakes were obtained between the months of January and February of the year 2000.

From each one of the samples, 0.1 g of dry sediment was taken, which was oxidized with hydrogen peroxide and / or sulfuric acid, following the conventional methodologies of Hasle & Fryxell (1970) and Battarbee (1986). Subsequently, permanent preparations were made with Hyrax resin (IR = 1.7), which were analyzed qualitatively for taxonomic purposes and quantitatively to determine the relative abundances of the species, counting a minimum of 500 diatom valves for each preparation in duplicate. The identification of diatom frustules was carried out using a Cari Zeiss photon microscope with 1000x magnification and a JEOL / DEO scanning electron microscope and this identification followed Krammer & Lange-Bertalot (1991, 2000), Barber & Carter (1996) and Rumrich et al. (2000).

Data treatment

Specific diversity was determined by calculating the Shannon-Wiener diversity index (H ‘), richness (H max = log2S, where S is the number of taxa) and equity (H7H max ) in diatom associations ( Krebs 1989). Using a bootstrap protocol, 95% confidence intervals were calculated to compare the Shannon-Wiener diversity index values ​​in the samples (Hammer et al. 2001).

Dendrograms were generated using the Unweighted Arithmetic Average Clustering (UPGMA) method. Because the samples presented a high number of dominant and rare species, the similarity was measured by the Morisita index (Wolda 1981). For this, the relative abundance data transformed to log (logo) were used. The robustness of the dendrograms was determined by bootstrap values ​​(Manly 1997). All calculations were performed in the PAST-Palaeontological Statistics program, see. 1.50 (Hammer et al. 2001).

A total of 99 diatom taxa distributed in 48 genera were found in the surface strata of the eight lakes studied ( Table 2 ). Of the total taxa, 74 were identified at the species level, all of the cosmopolitan with the exception of Eunotia andinofrequens (Lange-Bertalot) Lange-Bertalot, Gomphonema araucana Lange-Bertalot, Pinnularia araucanensis Lange-Bertalot, Rumrich & Krammer and Pinnularia acidicolaVan der Vijver & Le Cohu, which are described only for the Southern Hemisphere. 

Of the identified species, only 12% presented abundances greater than 10% in at least one of the samples and were defined as “dominant”, 45% presented abundances between 1 and 10% and the rest presented abundances less than 1% and they were classified as “rare”.

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