*Dirección para correspondencia

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Abstract
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High-altitude mountain lakes remain understudied, mostly because of their relative inaccessibility. Laguna de Guatavita, a small, equatorial, high-altitude crater lake in the Eastern Range of the Colombian Andes, was once of high cultural importance to pre-Columban inhabitants, the original location of the legendary El Dorado. We investigated the factors regulating the primary production in Laguna de Guatavita (4°58’50” N - 73°46’43” W, alt. 2 935m.a.s.l., area: 0.11km², maximum depth: 30m), ]]> 4⁺, NO₃^- , NO₂^-; soluble reactive phosphorus). Primary production was also determined, by oxygen generation, on each day of the campaign. Our results showed that the ]]> 2.h) but that many of the regulating factors were not those anticipated intuitively. The lake is demonstrably meromictic, reminiscent of karstic dolines in higher latitudes, its stratification being maintained by solute- concentration gradients. Light penetration is poor, attributable to the turbidity owing to fine calcite and other particulates in suspension. Net primary production in the mixolimnion of Laguna de Guavita is sensitive to day-to-day variations in solar irradiance at the surface. However, deficiencies in ]]>

Key words: chlorophyll a, photosynthetic rate, stratification, meromictic structure, sacred lakes.

Resumen

Los factores que regulan la ]]> N - 73°46’43” W, altura 2 935m.s.n.m., área 0.11km² y profundidad máxima 30m), que también tuvo importancia cultural para los indígenas precolombinos y los orígenes de la leyenda de El Dorado, fueron investigados. La relativa lejanía del lugar requirió una ]]> 2.h, pero ]]>

Palabras clave: clorofila a, tasa fotosintética, estratificación, estructura meromíctica, lagos sagrados.

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High-altitude mountain lakes remain relatively under-researched, mostly because of their remoteness and relative inaccessibility render them logistically difficult locations ]]>
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Whereas the photosynthetic responses of phytoplankton to short-term variations in light intensity are now well documented (Marra 1978, Yoder & Bishop 1985, Kroon et al. 1992, Mallin & Paerl 1992), longer-term adaptive adjustments –in swimming behaviour, pigmentation, and growth rates– eventually affecting changes in species composition - are still being resolved ]]> prima-facie sites for investigation of the influences of low latitude and high altitude on fundamental hydrobiological processes. Even in this particular region, there are inevitable logistic difficulties in organizing and sustaining relevant systematic field studies. In the end, the results of an earlier survey of one of these lakes (Zapata 2001, Rivera et al. 2005, Donato 2010) were decisive in selecting Laguna Guatavita as the site for the present study.

The first purpose of this paper is to convey the results of this investigation. It was necessarily undertaken in a series of intense 16-18 day field campaigns that were designed to obviate a large number of distant sampling expeditions. However, as we shall demonstrate, almost every aspect of the limnology of Laguna de Guatavita was substantially outside our expectations. Our findings showed that the environmental factors that substantially ]]>

It is relevant to mention that there was another factor influencing our selection of Laguna de Guatavita as a study site and it has less to do with limnology and rather more to its significance in the long history of the pre-Columbian settlement of the central Andes by indigenous people. At ]]>

Materials and methods

Laguna de Guatavita is one of a series of heterogeneous lakes located in the Eastern Range of the Colombian Andes, supposed to have been formed towards the end of the Pleistocene glaciation, 10 000-12 000 years before present (Psenner & Catalan 1994). The lakes vary in their morphometry, hydrology, chemical composition and stratification, as well as the nature and ]]> et al. 2005). Its area (0.114km²) and current altitude of its surface (2 935m.a.s.l) are regulated by an artificial channel, which was cut in the year 1 580 as part of an attempt (largely successful) to recover Muiscan treasures from the lake and which dropped the water level by about 20m. The maximum depth of the present lake is 30m (mean depth: 13.1m, relative depth: 7.9%) and it has a ]]> 3. It has a small, wooded topographic catchment (area: 0.67km²). Rivera et al. (2005) also revealed the presence of a steep chemical gradient in the lake, separating the upper and lower water masses and confining photosynthetic production to the insolated part of the water column (Zapata 2001); carbon fixation rates in the order 24-165mgC/m².d were consistent with those said to typify oligo productive lakes.
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Our field studies were necessarily carried out in a series of three intensive campaigns, as the relative remoteness of Laguna de Guatavita from Bogotá made frequent, routine visits quite impractical. The campaigns, each lasting 16-18 days, were initiated during April 2003, August 2003 and April 2004. On each day of each campaign, net primary production (NPP) was estimated from the change in oxygen concentration in isolates of plankton exposed in light- and darkened- bottles, suspended in situ in the water column (basically the method of Gaarder & Gran (1927), as later modified by Vollenweider (1974). All oxygen concentration measurements were determined titrometrically by the standard Winkler method. Exposures commenced between 10.00-11.00 on each occasion and were run for 4-6 hours, i.e., until 14.00-16.00.

Vertical profiles of dissolved ]]> PAR, 400-700nm: Wetzel & Likens 2001). Profiles of light intensity were also measured every 15min throughout each ]]> PAR readings, using the Microcall Origin 3.5 program; from mean irradiance measurements at each depth (I_z) and supposing conformity to the typical exponential series I_z=I_o e^-kz (where I₀ is the intensity of light immediately beneath the water surface); the coefficient of vertical light extinction k_z=(ln I_z- ln I₀)/z was solved ]]>

Water samples were collected in the field for immediate measurement of nutrient and chlorophyll-a content. The concentrations of combined inorganic nitrogen (NH₄⁺, NO₃^- , NO₂^-) were determined separately, following standard APHA (1998) methods. ]]>

Chlorophyll-a concentrations were determined in samples collected at selected depths (0.1, 2, 4 and 8m). Hot extracts in 90% acetone were cooled and read in a Hewlett-Packard model 8453E; the formula of Jeffrey & Humphrey (1975), in APHA ]]> 3.h) and the corresponding chlorophyll concentration (mgchla/m³).

The program Winsurfer 6.0 (Goleen Software 1995) was used to generate the graphical representations of ]]>

Results

Stratification: The vertical distribution of oxygen isotherms and temperature through the period of ]]> 1A, 2A, 3A) reveals an ongoing stratification lake stably stratified throughout the year, with the 2mg/L isopleth consistently located at a depth close to 10m below the surface. At the same time, the corresponding distribution ]]> 1B, 2B, 3B) is only sometimes well matched to the oxygen. Moreover, the temperature differences between the superficial layers and the deep water, perhaps as little as 0.5^oC, do seem unlikely to be solely responsible for maintaining the evidently strong stratification. In figure 4, we summarize the daily information related ]]>
In contrast, the vertical gradient of specific electrical conductance was pronounced and recurrent throughout the sampling campaigns, separating the remarkably solute-dilute waters (~10μS/cm) of the upper part of the water column (≤11m depth) from deeper waters of ≥70μS/cm at depths of >20m. We deduce the observed density stratification in the Guatavita Lake and its apparent year-long persistence is a quasi-permanent feature. Without further evidence to the contrary, it is reasonable to regard the lake as being conventionally meromictic. We may thus justify reference henceforth to the upper mixolimnion and lower monimolimnion of Laguna de Guatavita.

Physico-chemical factors of biological relevance: Light income to the lake was found to be variable from day to day (Table 1). However, there is no evidence of significant seasonality in the light incomes recorded (fluctuations in duration and maximum intensity are not expected at this latitude!): the variations likely owe principally to day-to-day differences in the extent and duration of cloud cover. On the other hand, gradients of underwater light penetration have been found to be typically steep, with coefficients of extinction consistently falling between 1.0/m (in August 2003) and reaching values of 1.48/m in April, 2003 and 1.6/m in April, 2004 (Table 1). The implied levels of turbidity, largely attributable to suspended fine particles of clay and calcite, seem to be a persistent feature of the underwater light climate. Using conventional approximations (from Reynolds 2006), the depth of the euphotic zone (wherein net photosynthesis is supportable) is estimated rarely to have exceeded 3.5-4.5m; the Secchi disk measurements recorded during the three campaigns (Table 1) point to a similar extrapolation.

Investigations of nutrient availability failed to detect nitrate in any of the samples for all campaigns, while nitrite was found only once (April, 2004). Inorganic combined nitrogen was represented by ammonium concentrations between 3.17-5.55 μMN/L. Average values of SRP were measured in April up to a maximum of 0.95μMP/L in April 2003; in April 2004, values ≤0.54 μMP/L were observed. The modest concentrations of both nitrogen and phosphorus are not atypical of oligotrophic lakes; we deduce that availability of nitrogen is more likely than that of phosphorus to regulate phytoplankton production and the biomass supportive capacity of Laguna Guatavita.

Distribution of chlorophyll and biomass:Isopleths of chlorophyll-a concentration in the vertical direction during each of the three campaigns are respectively illustrated in figures 5 A, B, C. Concentrations in the upper mixolimnion were generally lower than 12mg chla/m3 but, to a greater or lesser extent, were steeply graded to maxima of 20-28mg chla/m³ located in the pycnocline region. In each case, the main photosynthetic algae comprised mainly Cryptophytes, whose dominance of the phytoplankton of Laguna de Guatavita had been noted previously by Orjuela (2004). To the extent that the chlorophyll is healthy and functional, the algae are required to be substantially photoadapted to be capable of sustaining net photosynthesis at depths where residual light levels are usually less than <10mM/m.

Received 29-VII-2011. Corrected 30-I-2012. Accepted 29-II-2012.