2009 gunkel-beulker cuicocha irh

2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1434-2944/09/102-0103

Internat. Rev. Hydrobiol. 94 2009 1 103125

DOI: 10.1002/iroh.200811071

GNTER GUNKEL* and CAMILLA BEULKER

Berlin University of Technology, Dept. of Water Quality Control, Strae des 17 Juni 135, Sekr. KF 4, D-10623 Berlin, Germany; e-mail:[email protected]

Limnology of the Crater Lake Cuicocha, Ecuador,a Cold Water Tropical Lake

key words: Andes, atelomixis, caldera lake, Ecuador, Lake Cuicocha, travertine

Abstract

Cuicocha (3380 m a.s.l.) is a young, a few hundred years old volcanic lake in the western cordilleras of the Ecuadorian Andes with some post-volcanic activities, such as emission of volcanic gases and input of hydrothermal water. Water chemistry is influenced by the emission of CO2 and weathering of the young andesitic rocks in the water shed. A calcium cycle exists in the lake with intensive biological Ca precipitation at the flanks and formation of travertine crusts, while in the hypolimnion dissolution of Ca carbonate occurs. The crater lake is oligotrophic, biodiversity is low; the littoral flora and fauna is more important than the pelagic species. In the littoral zone, a small Totora zone occurs, followed by submerged macrophytes down to 35 m water depth. Phyto- and zooplankton occur down into the hypolimnion. Phytoplankton is strongly influenced by down-welling of water (atelomixis) and by copre-cipitation with detritial flocs.

1. Introduction

Volcanic lakes, built up in a caldera, are strongly influenced by volcanic activities such as gas emissions and hydrothermal water springs. Thus, different lake types are formed, and have been classified according to their physical water constraints by PASTERNACK and VAREKAMP (1997). These authors distinguished volcanic lakes with different levels of activ-ity, namely cool to hot acid-brine lakes, reduced to oxidized, acid-saline lakes, acid-sulphate lakes and bursting to buoyant plume bicarbonate lakes; only neutral dilute volcanic lakes do not show any activity. Besides this heterogeneity in water chemistry, a wide range of mor-phometric characters occur as well as different lake genesis which influence the limnology of the lakes (LARSON 1989; VAREKAMP et al., 2000; ARMIENTA et al., 2000; AGUILERA et al., 2006). Some basic knowledge exists regarding the limnology of crater lakes (VZQUEZ et al., 2004; SCHABETSBERGER et al., 2004) especially from the long research program on the Crater Lake in Oregon, which was mainly concerned with water chemistry, stratification processes and water currents (LARSON, 1996; LARSON et al., 1996; NELSON et al., 1996; CRAWFORD and COLLIER, 1997).

Limnology of crater lakes is of high interest, because they are young lakes, situated in an area with soils in a state of development. Many of the calderas are deep and have formed lakes with a depth of a few hundred meters, mostly situated in high mountain regions. Some

* Corresponding author

104 G. GUNKEL and C. BEULKER

2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.revhydro.com

of these represent a scarce type of cold tropical lake (LFFLER, 1964; CASALLAS and GUNKEL, 2001; ROLDAN and RUIZ, 2001; SCHABETSBERGER et al., 2004), and a few investigations on this type of lake have been carried out mainly concerning water mixing processes (TALLING, 1969; GUNKEL and CASALLAS, 2002, 2002a), primary production (KINZIE et al., 1998) and occurrence of zooplankton (GREEN, 1995). The morphometry of caldera lakes is frequently determined by the steep slopes of the flanks and non-eroded lake shores. Water chemistry is strongly influenced by the source of water input (rainwater or hydrothermal water) and by the weathering of volcanic deposits in the watershed. In some cases, the continuous input of volcanic gases and/or hydrothermal water leads to ionic rich water (OHBA et al., 1994; NELSON et al. 1996; RONDE et al., 2002), and a chemocline can build up (WOOD et al., 1984). Inflow of hydrothermal water and the energy flux from the volcano strongly influences the stratification processes (CAMERON and LARSON, 1993; CHRISTENSON, 1994; PASTERNACK and VAREKAMP, 1997). This can lead to an irregular overturn due to inverse thermal stratifica-tion, and convergent currents are observed (GOECKE, 1997; CRAWFORD and COLLIER, 1997). Fauna and flora are determined by the young age of the lakes, the high mountain position, and the mixing processes, and it must be assumed that the complexity of the biocoenosis is reduced.

Crater lakes are poorly investigated worldwide. Since the CO2 eruption of Lake Nyos, Cameroon, followed by intense international research to analyse the phenomenon of limnic eruptions (LE GUERN and SIGVALDASON, 1989, 1990; KUSAKABE, 1994; MARTINI et al., 1994), there have been concentrated efforts to obtain detailed information on other volcanic lakes with potentially dangerous CO2 accumulation. However, there is still a deficit of knowledge on volcanic lakes, especially in the Andes of South America, even though this is a region with a great number of active volcanoes. In Ecuador, two lakes are known to be active volcanic lakes, the Quilotoa (AGUILERA et al., 2000; GUNKEL et al., 2008) and the Cuicocha (von HILLEBRANDT and HALL, 1988; GUNKEL et al., pers. com.), located in the high Andine region > 3000 m above sea level (a.s.l.). Both the Quilotoa and Cuicocha volcanoes, have formed large and deep caldera lakes and currently show post-volcanic activities in form of volcanic gas emissions and input of geothermal water. The volcanic gases, mainly CO2, are accumu-lated in the lake water but without the risk of a limnic eruption (GUNKEL et al., pers. com.).

Investigations on the Cotacachi-Cuicocha complex carried out by SAUER (1971), MOTHES and HALL (1991) and GRUPE et al. (2008) provided a clear picture of the history of the volcano and the petrogenesis of the erupted lavas, and VON HILLEBRANDT and HALL (1988) developed a volcanic hazard map of the surrounding areas.

Lake Cuicocha was the focus of a detailed limnological study with the objective of a comprehensive characterization of the volcanic lake and its watershed in respect to petrol-ogy, morphometry, hydrophysics, water chemistry and limnology. Several scientific field campaigns were carried out between 2003 and 2006; primary results have been published regarding macrophytes (KIERSCH et al., 2004), Al polymerisation (GUNKEL et al., pers. com.) and the risk assessment of a limnic eruption (GUNKEL et al., pers. com.).

2. Methods

2.1. Studied Site

Cuicocha is a parasitic volcano of the Cotacachi volcano, which was active in the Pleistocene period, and is located in the western cordilleras of the Ecuadorian Andes, situated about 100 km north of Quito. Lake Cuicocha was formed after the collapse of the Cuicocha domo and is a young caldera lake (Fig. 1). Today, the caldera has a diameter of 3.2 km and a maximum depth of 450 m, filled by a lake to a water depth of 148 m. Recent volcanic activities of the Cuicocha volcano include emission of volcanic gases and some hydrothermal water inflow at the lake bottom as well as at the shore line.

Limnology of a Crater Lake 105


The water input into the lake is of low quantity, due to the small catchment area, and to the moderate annual precipitation of 1320 mm (20042006). The caldera has no continuous inflow and water inflow occurs via a periodic waterfall, two cascades with low flow rates and the mentioned hydrothermal entries; the lake has no superficial outflow. The soils consist of volcanic deposits, mainly andesite with a high SiO2 and Al2O3 content (5761 % SiO2, 1718% Al2O3; GRUPE et al., 2008) and its weathering products. The soils are in an early stage of development with a low clay content classified as andisols covered by paramo vegetation (ZEHETNER, 2003).

2.2. Sampling

Lake Cuicocha was investigated using a regular monitoring program and sampling was carried out twice a year for four years (20032006). The twice yearly investigation periods were February to April (rainy period) and July to October (dry period), from 08/2003 to 04/2006. The data base for the water chemistry included 6 vertical profiles (in 08/2003, 09/2003, 03/2003, 08/2004, 03/2005, 08/2005), and lake profiler diagrams (T, pH, cond., E7) were based on 10 profiles (in 08/2003, 09/2003, 03/2004, 08/2004, 03/2005, 08/2005, 04/2006 with 2275 data points). Investigations on further parameters (water physics and stratification stability at the crater rim and near the islands) were based on 29 lake profiles, determined during the regular investigation periods.

For depth determination and detailed bathymetric mapping of the lake, a double frequency sonar (50 and 200 kHz; Garmin Fishfinder 250 C) in combination with GPS (Garmin 60CS) was used; the bathymetric map was developed by the sonar GPS data and digitalised topographic maps (1 : 15 000, Souris, IRD). The use of sonar allowed detection of the lake floor as well as recognition of volcanic gas emissions and resuspended sediments as a consequence of gas eruptions. Furthermore, the use of sonar facilitated the positioning of the equipment in a specific water depth and thus allowed high precision for data registration near the sediment.

Temperature, pH, conductivity, CO2 and redox potential were determined by a lake profiler (Ocean Seven 316, Idronaut, Italy) with extremely high accuracy of the probes (with values for temperature + 0.003 C, for pH 0.01 pH units and for redox potential 1 mV). The CO2, probe was calibrated immediately after utilization by chemical determinations of CO2/HCO3 and CO32 according to the German Standard Methods for pKa and pKb analysis (DEV 2005). The density of the water was cal-culated under consideration of salinity, local pressure and temperature using the formula of CHEN and MILLERO (1986) for natural waters and is expressed as (S, T, Psurface) (= density 1000 kg/m3); salinity was calculated by the total ionic content. Oxygen concentration within the carbonate precipitates was deter-mined using optical oxygen sensor of 2 mm with a Fibox 3 oxygen meter (PreSens, Germany). Water samples were taken using a Ruttner water sampler (Hydrobios, Germany) at 78 m (9 samples in 178 m; N 01811.4 W 782210.9) and 148 m depth (13 sample in 1148 m; N 01742.3 W 782126.7; Fig. 2), portions of the water samples were filtered immediately, using 0.45 m cellulose-acetate filters for cation and anion analyses. Water samples for chemical analyses were preserved by HNO3 as well as HCl (both at pH ~ 1), using HDPE bottles.

Gas sampling was done directly with GC vials at the lake shore and on Island Yerovi.Samples of suspended material for Scanning Electron Microscopy was obtained by filtration of

250 mL lake water from the water sampler, immediately after sampling in the boat, using 0.4 m poly-carbonate filters (Nuclepore). The wet polycarbonate filters were stored in plastic bags with taps soaked with formaldehyde (37%) for preservation.

Sediment samples were collected by a sediment gravity corer, 5 cm with a sediment capturer. Sedi-ment sampling was recognized to be very difficult due to only a thin sediment layer on the stony floor and due to sediment oversaturation by gases, which led to the loss of sediments while degassing during lifting of the equipment. Sediment samples were prepared using an HCl/HNO3 acid digestion method (VDLUFA 1991) and chemically analysed using the analytical methods mentioned below. Sedimentation behaviour of seston was studied using 8 sediment traps, 8 cm , 50 cm length, which were exposed in the western bay of the lake for two periods, each one for 2 weeks at depths of 15, 30, 50 and 70 m (N 01815.3 W 782214.9). Samples were preserved with 4% glutaraldehyde solution for Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS) analyses.

A SONY HCR-HC16E digital video camera was modified as an underwater camera and was pro-tected by a purpose-built aluminium housing and equipped with 4 underwater lamps (NEMO 8C Xenon, 14 watt). With this camera, sediment type and gas emissions with sediment resuspension were regis-



tered as well as the penetration of the sediment core sampler into the sediment. During two campaigns, divers were used for sampling and underwater filming, and the experiences of the divers were helpful for further investigations.

Phytoplankton samples were collected from the water sampler (11 depths in 0145 m), preserved with Lugols solution. Zooplankton was collected using a plankton net (55 m) with trapping mechanism (a profile was made by 8 sections from 0145 m), and preserved with HANEY and HALL solution. Plants were collected using a macrophyte anchor (KIERSCH et al., 2004) in 1998 within 6 profiles from the lake surface down to 40 m. Sampling of the invertebrates was done by a fine sieve and a plankton net at 6 different positions in the littoral zone, surficial and down to a few meter water depth, preserved with 4% formaldehyde. Fish control was done by using a sonar (Garmin Fishfinder 250 C).

2.3. Chemical Analyses

Nitrite was determined using the Merck test (Aquaquant, 0.005 0.1 mg L1 NO2) immediately after the sample was collected. Chemical analyses for CO2, HCO3 and CO32 were undertaken on the sam-pling day applying the German Standard Methods for pKa and pKb determination (DEV 2005). Oxygen concentration was analysed using the WINKLER method, soluble reactive phosphorus (SRP), ammonia and nitrate were analysed according to US APHA Methods (1998) in the Laboratory of Chemistry, University Central, Quito, Ecuador. Water chemistry of the non-reactive cations and anions were carried out in the laboratory of the Berlin University of Technology, Dept. of Water Quality Control, Germany, using the acid-preserved water samples. Water samples for the determination of total amounts of ions were digested in an autoclave under acidic conditions using K2S2O8 (121 C, 1.3 bars for 2 h). Total phosphorus and total nitrogen were analysed photometrically by flow injection analysis in accordance with EN ISO 15681-1 (2004) and 11905-1 (1998), respectively (Foss Tecator FIAstar 5000) with a detection limit of 0.005 g L1.

The cations Ca2+, Na+, Mg+, K+, As2+ and Fe3+ were analysed by flame AAS (GBC Scientific Equip-ment, Pty. Ltd. Victoria, Australia), lower concentrations of Li+, Fe3+ and Mn2+ (< 0.1 mg L1) were ana-lysed by graphite furnace ASS (Varian Spectra A-400). The anions Cl, SO42 and NO3 were analysed using an ionic chromatograph (AS 50 Dionex) with CD 20 detector, GD 50 gradient pump and an AS 11 column for separation. Boron was determined photometrically in accordance with DIN 38405-D17 (DEV 1981; Dr. LANGE LCK 307). Analyses of the gases CO2, CO, O2, N2, N2O, and CH4 were carried out using a GC with FID and TCD detectors at the Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Berlin, Germany. 14C-analyses of soils were carried out by the Leibnitz Institute for Applied Geosciences, Hannover, Germany.

2.4. Biological and Microscopic Analyses

The determination of seston (bacteria, precipitations, detrital flocs) was done by Scanning Electron Microscopy as with Energy Dispersive Spectroscopy. For SEM-EDS analyses the wet polycarbonate filters were fixed on a bracket, air dried and then sputtered with gold or carbon. A SEM-EDS Hitachi S 2700 electron microscope was used with an acceleration voltage of 20 kV and an IDFix hardware and software from SAMx for analysis.

Phytoplankton was determined qualitatively using a Zeiss Laboval 4 and quantitatively according to the Utermhl method (DEV 2007) using a Olympus microscope CK30, about up to 5 transects of the Utermhl chamber (2 to 50 ml) were counted for quantitative determination, for the calculation of the biovolume 20-100 cells were measured in the Zeiss using an ocular with a reticle. The biovolume was calculated using related geometrical bodies (PADISK and ADRIAN, 1999). Diversity, abundance and biovolume were determined for each of the 11 sampling depths.

Determination of macroinvertebrates was carried out by Dr. CORREOSO, Catholic University of Quito.



3. Results

3.1. Lake Formation and Morphometry

The strato volcano Cotacachi was active during the middle Pleistocene period up to 630 000 years ago (INECEL 1983), which documents the end of volcanic activities. Cui-cocha, a parasitic volcano of the Cotacachi, began its activity with a series of eruptions 45003000 years BP followed by the collapse of the dome and a caldera was formed. Final eruptions took place more or less simultaneously about 14501330 BP and led to the formation of four domes within the caldera, nowadays the islands of Yerovi and Wolf (GRUPE et al., 2008; Fig. 1), and the differentiation of the caldera into two main basins of different depth. Following the extinction of the Cuicocha volcano, the caldera was partly filled with sediment from the catchment area; fine grained and clay material sealed existing fissures and fractures within the caldera and after the end of the colmation process, lake water accumulation began, probably a few hundred years ago.

Nowadays Lake Cuicocha is situated about 3072 m a.s.l. and has a maximum water depth of 148 m, a mean depth of 72 m; the volume amounts to 0.28 km3 and the lake surface 3.78 km2 (Table 1). The lake morphometry is determined by the caldera flanks with steep inclinations, and in some parts of the crater rim, a water depth of 100 m is reached only 20 m from the shore line. The bathymetric map (Fig. 2) was developed on the basis of 1250 data points and shows two lake basins, one with a maximum depth of 148 m extending east of the islands, and the other with a depth of 78 m, situated in the western area of the lake.

From the catchment area, water inflow into the caldera occurred until recently, however, the watershed is small (18.2 km2) with a surrounding factor of only 5.9. Additional hydro-thermal water has been entering into the caldera and mainly influences the water chemistry. The lakes water conductivity clearly points to a significantly increased ionic content com-pared with the inflow water of the catchment, and a rough estimate shows in total about 30% hydrothermal water, based on rain water inflow and recent hydrothermal water quality (see below).

3.2. Lake Stratification and Mixing Processes

Lake Cuicocha is a weakly thermal stratified lake with a monomictic cycle, and overturn occurs during the windy dry period from July to October (Fig. 3). The epilimnion stretches down to 40 m. Temperature differences between surface waters (approx. 1618 C) and the deep water body were small and amounted to about 2 C during stratified periods. The stabil-ity of the stratification in Lake Cuicocha was further reduced due to nocturnal cooling and deep lake mixing, the so called atelomixis, which was analysed in detail in the nearby Lake San Pablo (GUNKEL and CASALLAS, 2002, 2002a). During a 24 hour period in March during stratification period (Fig. 4), heating occurred in the upper water body of 50 m, mainly as a consequence of daily radiation input; and in the deeper water layer of 70 to 100 m depth a decrease in temperature was registered due to convection currents which occurred during the night after cooling of the surface layer.

An input of hydrothermal water was detected in the 78 m lake basin, and near the lake bottom both temperature and conductivity increased significantly (T = about 0.04 C; Cond = about 25 S cm1; Fig. 5), resulting in a decreasing density of (S,T) = 0.00915 kg m3 (P < 0.0001) in the 70 m water depth compared to the 55 m depth. In the whole lake density of the water increased from the surface ( = 999.0191) to the ground ( = 999.2731 in 78 m depth, = 999.2986 in 148 m depth), but no chemocline was build up and the hydrothermal water was quickly mixed with the lake water. The inflow of hydrothermal water at the lake floor could not be quantified. Well known is the hydrothermal water inflow at the shore line



of Island Yerovi, which was used for water chemical analyses, in 2005 an additional inflow of hydrothermal water was observed near the Island Yerovi in 510 m water depth.

The conductivity profile in the 148 m lake basin showed no increase, and an inflow of hydrothermal water must be excluded.

In the western basin of Lake Cuicocha, at 78 m depth, a permanent emission of CO2 took place detected by sonar and few gas bubbles reached the lake surface. The composition of the gas (see below) clearly pointed to the volcanic origin. The emission of the gas led to a resuspension of sediments proved by sonar and underwater filming and to a partial mixing of the water body by billows during upraising of gas bubbles.

Figure 1. Lake Cuicocha with domos Yerovi (left) and Wolf (right).

Table 1. Morphometric data of Lake Cuicocha.

Parameter Lake Cuicocha

Lake coordinates NorthSouth N0 18 49.9 W78 21 40.1 N0 17 32.6 W78 21 28.8WestEast N0 18 22.1 W78 22 33.1 N0 18 03.9 W78 20 52.8Watershed (km2) 18.2Surrounding factor 5.9Lake water level (m a.s.l.) 3,072Length (m) 3,238Width (m) 2,232Shore line without islands (km) 10.14 Shore line with islands (km) 14.43Shore line development without islands 2.9Shore line development with islands 4.2Surficial area (m2) 3,781,012Maximum depth (m) 148Mean depth (m) 72Relative depth (%) 3.3Volume (m3) 282,053,575Volume of the islands underwater (m3) 103,648,907Volume of Jerovi above water line (m3) 9,011,699Volume of Wolf (m3) above water line (m3) 29,381,720



3.3. Water Chemistry and Post-Volcanic Activities

Gas analyses showed that the main composition was CO2 (51%) and N2 (23%) with smaller amounts of O2 (3.0%) CH4 (1.7%) and CO (0.3%). Gaseous emissions of boron compounds led to increased BO43 concentrations in the lake water, a typical component of volcanic gases.

The pH of Lake Cuicocha in the epilimnic water (040 m) was weakly alkaline, the pH was raised to 8.3 as a consequence of loss of CO2 from the oversaturated water to the atmos-phere and primary production. The hypolimnion (> 40 m) exhibited neutral conditions with a pH down to 7.0 (Fig. 6), caused by CO2 input from volcanic gases from the lake floor in the western basin (GUNKEL et al., pers. com.).

The carbon dioxide concentrations were elevated in the whole water column with the highest concentrations in the deep water layers > 60 m, a consequence of the hypolimnic gas input in the shallow basin, combined with high wind-induced horizontal water cycling between the different parts of the lake. In the deep water, an oversaturation of CO2 of up to 78 times, compared with the atmosphere and the local pressure was observed, this cor-responded to a CO2 concentration of 40 mg L1 or 31 mL CO2 per litre water. Nevertheless, the in situ CO2 saturation of the hypolimnic water was low, due to the high pressure of the water column in the deep water, where great quantities of CO2 are soluble. Taking into consideration the partial pressure of the gas, leaving the volcano (xi = 0.39%), the in situ saturation of CO2 in the bottom-near water layers amounted to only 0.08%.

Figure 2. Bathymetric map of Lake Cuicocha, = sampling points, 1 = cascades, 2 waterfall.



Figure 3. Temperature isopleths of Lake Cuicocha, based on 10 profiles.

Figure 4. Temperature profiles and temperature differences over a 1 day period during lake stratifica-tion period, Lake Cuicocha, 148 m depth position, 30/31.3.2004.



The dissolved ionic concentration of Lake Cuicocha resulted in a conductivity of about 810 S cm1; this was elevated compared with the water inflow of the cascades ( 260 S cm-1) and a nearby rain water-filled caldera lake, Lake Mojanda (mean 43 S cm1). Thus the Cuicocha lake water must be influenced significantly by the input of hydrothermal water.

Lake Cuicocha is a sodium hydrogen carbonate water (60.6 mg L1 Na+, 189 mg L1 HCO3) with significant amounts of magnesium and calcium as cations (29.8 mg L1 Mg+, 46.8 mg L1 Ca2+) and chloride as anion (70.5 mg L1 Cl, Table 2). Ions of minor concen-trations were the cations K+, Li+, Fe3+, Mn2+, Al3+ and the anions SO42, SiO32, B(OH)4, PO43.

In Lake Cuicocha the trophic level is very low, and a median phosphorus concentration of 12 g L1 Ptotal was observed. The nitrogen concentrations were also shown to be low (mean 96 g L1 Ntotal). The N/P ratio (median = 10.0, mass basis) for most data showed P limita-tion. Due to the volcanic origin of the lake, the siliceous concentration (median = 21 mg L1 SiO2) was very high and exceeded the normal range in natural lakes.

Pelagic calcium concentration varied between 39.856.1 mg L1 (5- and 95-percentile), and the epilimnic Ca saturation index of + 0.75 (030 m) showed a slight oversaturation. Nevertheless, precipitation of calcium carbonate was registered at the shore line of Lake Cuicocha. These precipitations were continuously built up by epilithic algae and cyanobac-teria (see below), leading to CaCO3 crusts of about 1 cm in thickness, which reached down deep into the epilimnion. Wave action partly destroyed these crusts, forming fine carbonate debris, which sank to the lake bottom, where dissolution of the CaCO3 precipitates occurred under conditions of negative Ca saturation index (S = 20.2 at 45135 m, see Fig. 8). An up-welling of hypolimnic water by the lake floor heating (see 3.2.) led to an internal calcium cycle.

Figure 5. Increases in temperature and conductivity with decrease of the density near the lake bot-tom in the 78 m lake basin; (n = 107; variances R2 for temperature = 0.8298, conductivity = 0.8913,

density = 0.8297).



The inflow of hydrothermal water was analysed at the shore of Island Yerovi. The water was of relatively low temperature (2326 C), rich in sodium (247 mg L1 Na+), calcium (120 mg L1 Ca2+) and iron (7.4 mg L1 Fe3+) as cations and sulphate (193 mg L1 SO42), chloride (171 mg L1 Cl) and silicate (110 mg L1 SiO2) as anions.

3.4. Littoral Zone and Travertine Formation

At the littoral zone of Lake Cuicocha the reed plant Totora (Schoenoplectus totora) built up a dense but narrow vegetation zone, followed by a few submerged plants including Myriophyllum quitense (from 02.3 m depth), Potamogeton illinoensis (28 m), P. pectina-tus (214 m), Chara rusbyabana (2729 m), Ch. globularis (830 m) and Nitella acuminata (2035 m). As a new proof Drepanocladus capillifolius, a Bryophyta, Fam. Amblystegiace-ae, was found at 2028 m depth (KIERSCH et al., 2004). The biomass of the submerged macrophytes was very high and they built up dense stands with very long plants with several meters lengths on the high flanks.

The invertebrates of the littoral zone were dominated by Hyalella cf. dentata, Fam. Malacostraca; GONZALEZ and WATLING (2002). Other species with regular mass develop-ment were Dugesia sp. and some Gastropoda species, Biomphalaria sp. and Potamopyrgus antipurvarum (Table 3). The biodiversity was very low and the few species occurred in high abundance.

Only one fish species was observed and filmed by divers, the Andean catfish Astroblepus ubidiai (Siluriform), living in the upper part of the littoral zone was a new proof for this lake. This fish is a resident species of the high Andes, with a few remnant populations in this area (VLEZ-ESPINO and FOX, 2005).

The colonization of littoral interfaces by benthic algae and bacteria was of high ecological importance. A dense biofilm with intensive photosynthetic activity induced the precipita-tion of travertine on the caldera flanks and consisted mainly of motile pennate diatoms (Epithemia argus, Mastogloia smithii, Cymbella pusilla and Rhoicosphenia abbreviata as dominant species) and one frequent cyanobacteria species, Calothrix parietina, which lived

Figure 6. pH isopleths of Lake Cuicocha, based on 10 profiles.



Table 2. Water chemistry of Lake Cuicocha, median and percentiles of 20032005; n = 84.

Parameter 5 % Median 95 %

pHepilimnion 7.65 8.14 8.58pHhypolimnion 7.18 7.37 7.86Conductivity (S cm1) 797.2 812.3 818.2Natotal (mg L1) 55.9 60.6 69.0Catotal (mg L1) 39.8 46.8 56.1Ktotal (mg L1) 4.7 5.5 6.4Litotal (mg L1) 0.100 0.110 0.135Mgtotal (mg L1) 25.3 29.8 41.2Fe dissolved (mg L1) 0.000 0.006 0.022Fetotal (mg L1) 0.004 0.0120 0.073Mntotal (mg L1) 0.001 0.005 0.083Altotal (mg L1) 0.003 0.009 0.139B+ (mg L1) 2.4 3.9 5.8HCO3 (mg L1) 138 189 351Cl (mg L1) 64.0 70.5 74.4SO42 (mg L1) 9.2 13.4 18.3Si dissolved (mg L1) 17.1 20.0 23.3Sitotal (mg L1) 17.0 21.0 23.4Ntotal (mg L1) 0.029 0.096 0.296Ptotal (mg L1) 0.004 0.011 0.027SRP (mg L1 PO4-P) 0.002 0.005 0.013N/P (weight based) 2.5 10.0 28.9O2epilimnion (mg L1) 5.9 6.7 7.0O2hypolimnion (mg L1) 0.8 2.0 3.2

Table 3. Fauna in Lake Cuicocha (rare to mass describes the relative abundance in a seven grade scale).

Class/Subclass/Family Taxon Abundance

Littoral fauna:Turbellaria Dugesia sp. frequentGastropoda, Planorbidae Biomphalaria sp. frequent

Gyraulus sp. less frequent Physidae Physa sp. less frequent Hydrobidae Potamopyrgus antipurvarum massBivalvia, Spaeridae Sphaerium forbesi less frequentCrustacea, Ostracoda Cypris sp. frequent Malacostraca Hyalella cf. dentata mass

Insecta, Zygoptera Oxyallagma dissidens less frequentFishes: Osteichthyes, Siluridae Astroblepus ubidiai rare

Zooplankton:Crustacea, Phyllopoda Daphnia pulex pulex 0.23 n L1Crustacea, Copepoda Metacyclops mendocinus WIERZEJSKI 0.117 n L1 Malacostraca Hyalella cf. dentata 0.0010.06 n L1



at the surface and inside the pore system of the CaCO3 crusts (Fig. 7). The intensive primary production in this biofilm was proved using oxygen measurements, which showed a mean O2 oversaturation of 199% (min. = 152%, max. = 216%, n = 13) 5 mm inside the CaCO3 crusts, resulting in a microenvironment with increased pH and favoured carbonate precipitation; the CaCO3 crusts were composed of pure calcium and contained only traces of magnesium.

3.5. Plankton Community

The phytoplankton community was characterized by Diatomeae (Aulacoseira granulata, Fragilaria ulva var. acus, Fragilaria sp.), Crytophyceae (Rhodomonas and Cryptomonas), Euglenophyceae (Trachelomonas volvocina) and some Chlorococcales species (Oocystis lacustris, O. naegli, O. parva and Scenedesmus linearis, S. quadricauda, S. acuminatus,

Figure 7. Calcium carbonate precipitations within the biofilm at the lake shore, REM picture of the calcium crusts with some diatoms embedded in CaCO3.



Table 4. Occurrence of phytoplankton in Lake Cuicocha, abundance of algae (n mL1), n.d. = not determined. The abundance is given as the maximum cell number in one of the 11 sampling depths (sampling depths: 0, 5, 15, 30, 45, 60, 75, 90, 105, 120, 135, 145m), *) biomass without Mougeotia sp.

and Spirogyra sp.

03/2004 08/2004 03/2005 08/2005

CyanobacteriaeCyanobacteria n.d. 3.306DiatomeaePennalesAsterionella formosa < 4Fragilaria sp. < 4 72F. ulva var. acus 18 22Navicula sp < 4 < 4Nitzschia sigmoides 4Pennales n.d. < 4 < 4 91 4CentralesMelosira sp. < 4Aulacoseira granulata 117 49 108Aulacoseira sp. 108DinophyceaePeridinium sp. < 4 4Gymnodinium sp. 9 < 4Cryptophyceae 4Rhodomonas minuta 9Rhodomonas sp. 90Cryptomonas sp. 9 4 13 31EuglenophyceaeTrachelomonas volvocina 1.054 22 40 9Trachelomonas verrucosa 31 13 4ChlorophyceaeVolvocalesChlamydomonas sp. 81 40 439ChlorococcalesMonoraphidium komarkovae 251 58Oocystis lacustris 157 175 27Oocystis naegeli 9 4 22 9Oocystis parva 117Chlorella sp. < 4Lagerheimia wratislaviensis 4Planktosphaeria gelatinosa < 4Pediastrum boryanum 144Scenedesmus linearis 36Scenedesmus quadricauda 332Scenedesmus acuminatus 72Tetraedron minimum 4Botryococcus braunii < 4Dictyosphaerium ehrenbergianum < 4Lagerheimia longiseta 18Neglectella sp. 13Schroederia setigera < 4CladophoralesCladophora sp. 583ZygnematalesMougeotia sp. 193Spirogyra sp. 108DesmidialesCosmarium sp. < 4 4Abundance (n mL1) 1094 193 3485 363Biomass (m3 mL1) 784,000 218,000 3,808,000 118,000

247,900*)Depth of abundance maxima 075 m 075 m 130140 m 4575 m



Table 4). In general, the abundance (< 1100 cells mL1) and biomass (< 400 000 m3 mL1) were small, and corresponded with the oligotrophic character and the Secchi depth of 13.0 14.9 m. On one occasion the occurrence of Mougeotia and Spirogyra at 140 m depth led to a maximum biomass of 3 800 000 m3 mL1.

The vertical algae distribution showed continuously occurrence of algae in the whole water column down to 140 m, and cell number as well as biovolume did not had any significant maximum in the epilimnic zone, while other parameters such as E7, conductivity, pH, CO2 and the Ca saturation index showed an analogous stratification to temperature (Fig. 8).

Algal species were regularly found in deep water and maximum abundance occurred down to 75 m, and an extreme maximum abundance of Mougeotia and Spirogyra occurred once at 130140 m (Fig. 9). The depth distribution during stratification periods point out clearly that most species occurred in the whole water column (Chlamydononas, Scenedesmus linearis, Oocystis lacustris, Trachelonomas volvocina, T. verrucosa, Aulacoseira sp.), but a few species were found only in the upper water body down to 80 m such as Scenedesmus acuminatus, Lagerheimia longiseta and Monoraphidium komarkovae. Other species, mostly filamentosus forms occurred only in depth >> 80 m such as Aulacoseira granulata, Pedias-trum boryanum, Mougeotia sp. and Spirogyra sp. (Fig. 9).

In the plankton some iron-oxidising bacteria such as Siderocapsa occurred frequently, and Metallogenium-like structures were also found.

The zooplankton biocoenosis of Lake Cuicocha was formed by two species, the filter feeder Daphnia pulex pulex and the carnivorous Metacyclops mendocinus (Table 3). Zoo-

Figure 8. Vertical stratification of physical-chemical water parameters (redox potential, conductivity, temperature, pH, CO2, calcite saturation index) and depth distribution of plankton (phytoplankton cell number and biomass, abundance of Daphnia pulex, Metacyclops mendocinus and Hyalella cf. dentata)

in the 148 m depth position of Lake Cuicocha, 18.3.2004.



plankton abundance was low and the maximum number of animals amounted to 7 n L1, the mean abundance was 1.8 n L1 (Metacyclops mendocinus) and 0.7 n L1 (Daphnia pulex pulex), respectively. The vertical distribution of the zooplankton stretched down to a depth of 140 m (Fig. 8).

In the pelagic water body, the invertebrate found in the littoral zone, Hyalella cf. dentata was found regularly. Hyalella occurred in the whole water column with a few animals per litre (Fig. 8), but was frequent at the lake bottom at 78 m depth, proved by under water filming; Hyalella was also found at 148 m depth in the plankton net samples, however, at this depth no intensive filming was performed.

In the pelagial of Lake Cuicocha no fish were found by use of sonar, Astroblepus ubidiai occurred only in the littoral zone. A few decades before, the trout (Oncorhynchus mykiss) was introduced into Lake Cuicocha as well as into the nearby Lake San Pablo and Lake Mojanda, however, in Lake Cuicocha the population has broken down, while in the other lakes, the fish populations have increased.

3.6. Sediments and Sediment Resuspension

The sediments in Lake Cuicocha formed a very thin layer of up to 1015 cm on volcanic rocks a few decimetres in diameter, proved by underwater photography (see GUNKEL et al., 2008). The sediments consisted mainly of mineral compounds and sedimented diatom frus-tules. These sediments were poor in Ca (1020 g kg1 ds, = dry substance) but rich in P (2446 g kg1 ds). The sediments were anaerobic with sulphide formation in the form of

Figure 9. Vertical distribution of some dominate algae species in the 148 m depth position of Lake Cuicocha (17.03.2005).



pyrites, only the surface layer was oxidized down to a few millimetres. In particular, the sur-face layer in the 78 m deep area consisted of brown flocs a few millimetres in diameter.

The sediment traps showed extraordinary high sedimentation rates with the higher rates in deeper exposed traps (13 mm fresh material within 2 weeks), in contrast to the low abundance of plankton in the pelagial. The trapped material consisted of the same type of flocs as were found at the lake bottom, recognizable by a similar chemical composition. The settled material of both origins was poor in Ca (12 g kg1 ds) but rich in Fe (74 g kg1 ds) and consisted of algae, detritus and masses of iron oxidising bacteria (Fig. 10). Observations with

Figure 10. Sediment flocs in Lake Cuicocha, detritus, diatom cell is embedded in flocs with iron- oxidising bacteria, SiO2(H2O)n precipitations and filamentosus fungi structures, free water, 40 m depth,

18.08.2005, in situ filtration.

Figure 11. Formation of amorphous SiO2(H2O)n in Lake Cuicocha sediment on Nuclepore filters, 148 m depth, 31.08.2004; EDS analyses verified the Si composition with traces of Al and Fe.



sonar and the underwater camera registered resuspension of sediment and sediment flocs by gas bubbles rising up to 20 m.

Sediment forming processes were supported by two linked water chemistry processes, the precipitation of polymeric Al(OH3)n and the formation of amorphous SiO2(H2O)n; due to the atomic characteristics mixed precipitations are common. In the water as well as in the sedi-ment, Si precipitation occurred and formed amorphous structures (Fig. 11). Al precipitation occurred in the water column due to a decrease in pH to about 7, which meant that down-welling of epilimnic water leading to oversaturation of the Al polymer, and gelatinous Al polymers as well as Al microcrystals like gibbsite were observed (GUNKEL et al., 2008b). Both processes were frequent and promoted the coagulation of flocs in the water, formed by detritus, bacteria and living algal cells.

4. Discussion

4.1. Characteristics of Caldera Lakes

Crater lakes are a special type of lake, which are classified by the lake formation pro-cess (caldera as an intrusion of the magma chamber of a volcanic dome, maar as explosive craters), without consideration of any geological or climatic settings. Thus caldera lakes pos-sess a wide scattering of ecological characteristics. The physical and chemical parameters of caldera lakes were summarized by PASTERNAK and VAREKAMP (1997) and VAREKAMP et al. (2000). Nevertheless caldera lakes are characterized by properties of ecological interest: caldera lakes are young lakes in a state of development, mostly located at higher altitudes; only maar lakes and some marine atoll lakes are found in the plain. The watershed is nor-mally covered by young volcanic deposits, with weathering processes and the beginning of soil formation. The watershed is small, the lakes do not have a regular outflow; caldera lakes have no connection with the ground water table. Especially in young caldera lakes the accumulated sediment layer is very small.

Lake Cuicocha is characterized by the young age of about 500 years with weathering of the rocks in the water shed and the lake succession processes such as rock falls and land slides from the crater rim, deposition of sediments, colmation and nutrient accumulation, and water chemistry is strongly influenced by these processes as well as by post volcanic activities such as gas emission and hydrothermal water inflow. One effect is the resuspension of sediments is a process which determines the water chemistry as well as the development of the phytoplankton (WEYHENMEYER, 1998).

The colmation of the lake basin is unstable or insufficient due to the small sediment layer and the post-volcanic activities. Triggered by an earthquake in 1987, the lake colmation layer was destroyed and from this time a decrease in the water level of about 30 cm per year occurred. Water percolation leads to a higher risk of phreatic-magmatic eruption, meaning an explosive eruption, when the infiltrating water contacts the magma of hot rocks (MASTIN and WITTNER, 2000; GUNKEL et al., 2008).

Post-volcanic emissions including gases or hydrothermal springs are frequent in volcanic lakes and affect lake stratification and water chemistry. In Lake Cuicocha the ionic content of the water was increased, compared with a nearby inactive caldera lake, Lake Mojanda, which clearly points to the high significance of the hydrothermal inflow.

4.2. Physical Characteristics of Lake Cuicocha: Mixing Processes

Besides wind-induced water cycling, lake mixing processes of Lake Cuicocha are deter-mined by post-volcanic activities like the emission of volcanic gases, hydrothermal underwa-



ter springs and a heat flux via the lake floor. Gas emissions and hydrothermal water springs at the lake floor lead to a mixing of the lake due to billows with upraising gas bubbles or divergent currents of heated water. These processes can be detected only by the application of special monitoring (use of divers, sonar, underwater camera and a lake profiler).

Thus the stability of the thermal stratification of Lake Cuicocha is diminished, and atelo-mixis occurs, that means daily or frequent periodic mixing down into the hypolimnion e.g. by nocturnal cooling in periods of low density gradients (TALLING, 1969). Increased losses of phytoplankton by sedimentation take place, and the atelomixis leads to a more oligotrophic status of the lake (TAVERA and MARTNEZ-ALMEIDA, 2005), who postulated that atelomixis was the driving force in determining phytoplankton composition. In the nearby Lake San Pablo, atelomixis was proved and led to hypolimnic phytoplankton maxima (GUNKEL and CASALLAS, 2002, 2002a).

4.3. Water Chemical Characteristics of Lake Cuicocha

Water chemistry of the caldera lake is determined by rain water interacting with vol-canic deposits in the catchment area and represents a new (young) terrestrial-aquatic linked ecosystem. The export of nutrients and other ions from the watershed, being composed of volcanic deposits, lava, volcanic rocks or ashes, which undergo an intensive mineralization and soil forming process determine the ionic composition. Typical for the Andean zone is the high export of Al and Si as characteristic compounds of the andesites, while the content of Ca must be regarded as low. Lake Cuicocha, only a few hundred years old, reflects these conditions: Al and Si are increased, Al polymerisation and microcrystal formation (GUN-KEL et al., pers. com.), amorphous SiO2(H2O)n precipitation as a process of sediment build-ing. AlSi polymerisation processes promote the formation of flocs and it must be assumed that the stability of flocs is also increased.

Lake Cuicocha is an oligotrophic lake, due to low P and N input from the watershed with young soils, the andisols (ZEHETNER et al., 2003) and due to no accumulation of nutrients in the young lake. These nutrients are further reduced by intensive precipitation with iron oxides being a consequence of hydrothermal water inflow.

In Lake Cuicocha CO2 accumulation with pH change and changes in the lime carbonic acid equilibrium occur.

The water of Lake Cuicocha has a low Ca and Mg concentration, however, in the epilim-nion, the calcite saturation index regularly shows a positive value, meaning oversaturation, whereas the hypolimnion values decrease to levels < 0. Travertine formation occurs as a biological CaCO3 precipitation in an extended biofilm of a few millimetres thickness with high abundance of diatoms and median abundance of cyanobacteria. It is of interest that only traces of magnesium occurs in the precipitates. The productivity of the biofilm, which stretched down to more than 15 m is very high (sampling at deeper areas of the crater flanks was not possible due to diving limitations). Part of the travertine crusts are eroded by wave action and form a fine carbonate sand in the littoral zone, which sink to the lake floor, where it is dissolved. This Ca cycle Ca precipitation in the epilimnic zone, sedimentation of Ca sands, re-dissolution in the hypolimnic zone and introduction of dissolved Ca in the epilimnion by regular partial lake mixing is the basis of the intensive development of the biofilm with travertine formation.

Similar travertine formation is a frequent process at the crop-out of hydrothermal springs, where the water flow is a source of Ca, Mg and corresponding anions. In lakes it has also been observed at a small rate, induced by photosynthetic pH changes (GOLUBI, 1969). In crater lakes intensive travertine formation is known e.g. from Lake Quilotoa, Ecuador (up to 50 cm thickness, own observations) and the Ries crater, Germany (RIDING, 1979), but in Lake Quilotoa an abiotic formation process with extreme Ca over-saturation and aragonite



precipitates can also become of high significance, and under these conditions CaMg-car-bonate precipitates occur.

In the hypolimnion the Ca concentration is in the range of undersaturation, due to decreas-ing pH caused by increased CO2 concentrations. This Ca undersaturation in the hypolimnion leads to re-dissolving of biogenic precipitated Ca, resulting in a very small amount of Ca in the lake sediments of < 1%. Consequently the sediment building rate is reduced and the lake colmation process is hindered and slowed, and the stability of the colmating layer must be regarded as low.

4.4. Biocoenosis of Lake Cuicocha

The phytoplankton community is characterized by a low diversity, which corresponds to the nearby glacial Lake San Pablo (Lake San Pablo: 31 species, Cuicocha: 37 species), but only 8 species occur in both lakes (GUNKEL and CASALLAS, 2002). Most of the species in Lake Cuicocha are wide spread, endemic species of Latin America were not found, espe-cially of the group of diatoms (LANGE-BERTALOT, pers. com. after sample revision).

The dominant species in 2004/2005 are Aulacoseira granulata, Fragilaria ulva var. acus, Trachelomonas volvocina, Oocystis lacustris, O. parva and Clamydomonas sp., which have to be classified as meso- to eutraphent species (trophic value 33.5) with low to median saprobe character (saprobic value 2), these species are cosmopolite with a wide ecological tolerance (LUB 2005). Only Monoraphidium komarkovae is a more oligotraphent species (trophic value 1.5). The more eutrophic species in Lake Cuicocha do not correspond to the high water transparency and the low phosphorous concentrations, but it must be considered, that the algae biomass is low and the continuity is less.

Of high interest is the vertical distribution of the algae with a significant distribution below the epilimnic zone, the calculated euphotic depth is 1820 m (TILZER, 1988), but most of the algae we found in depth > 20 m; this is a phenomenon already investigated in the nearby Lake San Pablo, and atelomixis as deep nocturnal mixing caused the temporary residence in the aphotic zone (GUNKEL and CASALLAS, 2002, 2002a). A significant part of the population is even in more than 80 m depth, some species such as Aulacoseira granu-lata, Pediastrum boryanum, Mougeotia sp. and Spirogyra sp. had a maximum distribution in 130148 m. This paradox seems to be a consequence of the intensive flocculation processes due to Al and Si precipitation, the flocs trap algae cells and promote sedimentation processes and lead to an accumulation of algae near the sediment.

The biocoenosis of the zooplankton is characterized by some ubiquists which are highly abundant, however, the biodiversity is strictly reduced. Only one filter-feeding Phyllopoda, one carnivorous Copepoda and one Malacostraca form a pelagic food web with low phyto-plankton and bacterioplankton at the primary trophic level. Top predation by fish is missing, a catfish being the only inhabitant of the littoral zone. A very restricted food web was also observed in the nearby Lake San Pablo, a glacial lake of 38 m depth (CASALLAS and GUNKEL, 2001). Without doubt high mountain lakes in this tropical area show infrequent colonization by aquatic organisms, and the fauna and flora are characterized by introduced species (direct introduction of fish, and by ballast water from water planes). Tropical Middle America is a barrier for migration to cold water species from North to South America, and inversion of species occurred only from the south of South America via the Andes mountain range.

Little is known about the occurrence of Hyalella cf. dentata, a species of the new world, and even the description of some Hyalella species must be revised (GONZALEZ and WATLING, 2002). Hyalella cf. dentata found in many South American lakes, even in salt lakes, and in some North American lakes (GONZALEZ and WATLING, 2002). However, information on the habitat and ecology of this organism is insufficient. Although Hyalella cf. dentata is a her-bivorous littoral species, its occurrence in the pelagial and at the lake bottom was regularly



registered at 148 m, and at the 78 m depth a high abundance was observed. However, due to the extension of the lake, no leaves or similar particulate organic material can accumulate in the pelagial and on the lake floor. It can be assumed, that filamentous algae such as the desmids Mougeotia and Spirogyra, observed in deep water in high biomass (Table 4), play a role in the feeding behaviour of these organisms.

This investigation did not include any microbiological studies, however, some interesting effects were observed. Iron-oxidizing bacteria were frequent, mostly of the genus Side-rocapsa as well as Metallogenium-like structures (KLAVENESS, 1977), which were classified by EMERSON et al. (1989) as manganese-oxidizing fungi. These bacteria, mainly the filamen-tous forms of Metallogenium-like structures promote the build-up of flocs and increase their stability. Flocs are cycled in the water body by sediment resuspension and in turn strongly influence water quality. The role of flocs in chemical and biological processes in lakes must be regarded as very important, however, available knowledge is still insufficient (WEYHEN-MEYER, 1998). In Lake Cuicocha flocs played a significant role in the occurrence of algae, serving as a substrate for adhesion and as a nutrient source in the oligotrophic lake.

The lack of fish within the pelagial may have been caused by the high CO2 concentration, however, it should be noted that in the nearby high Andean lakes only a few species such as a hybrid (Carassius carassius, C. auratus) called carp, trout (Oncorhynchuss mykiss) and pike (Micropterus salmoides) occur. Fish biodiversity is extremely low in these lakes and little is known about the native biocoenosis (e.g., Atroblepus ubidiai), because trout, carp and pike were introduced into all these lakes.

The littoral macrophytes had a high biomass, and invertebrates in the littoral zone were frequent, whereas the planktic community was of low abundance and biomass. This increased importance of the littoral biocoenosis has also been observed in other tropical high moun-tain lakes, and one reason for this seems to be the increased losses of pelagic species due to deep diurnal mixing processes (GUNKEL and CASALLAS, 2002, 2002a). In Lake Cuicocha this process is forced by the formation of flocs and the coagulating effect of polymeric Al precipitation, by the formation of amorphous SiO2(H2O)n and filamentous Metallogenium like structures, and by the stabilization of flocs by the abundant occurrence of Siderocapsa colonies.

5. Acknowledgements

This study is part of a cooperation project of the Berlin University of Technology, Germany, Dept. Water Quality Control with support from the Dept. of Environment and Marine Science and Technology (VWS), Dipl. Geol. B. GRUPE, and the Central University of Quito, Facultad de Ingeniera en Geologa, Minas, Petrleos y Ambiental (FIGEMPA), Ecuador, Ing. F. VITERI. It was financed by the Deutsche Forschungsgemeinschaft, Germany (DFG, German Scientific Society) and the Ministry for Economic Development and Cooperation, Germany (BMZ). Logistic support was provided by the National Park of Cotacachi, Ibarra, Ecuador. Dr. P. CASPER, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Germany, carried out gas analyses. Dr. GABRIELE GUNKEL, Berlin, Germany, carried out the phytoplank-ton analyses. Dr. CORREOSO, Catholic University of Quito carried out the systematic identification of the invertebrates. 14C-analyses of soils were carried out by the Leibnitz Institute for Applied Geosciences, Hannover, Germany.

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Manuscript received March 6th, 2008; revised July 11th, 2008; accepted August 19th, 2008

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