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Limnology of an Equatorial High Mountain Lake in Ecuador, Lago San Pablo*

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Limnology of an Equatorial High Mountain Lake in Ecuador, Lago San Pablo* Limnologica 30 (2000) 113-120 LIMNOLOGICA http://www.urbanfischer.de/j ournals/limno © by Urban & Fischer Verlag Technical University of Berlin, Institute of Technical Environmental Protection, Department Water Quality Control, Berlin, Germany Limnology of an Equatorial High Mountain Lake in Ecuador, Lago San Pablo* GUNTER GUNKEL With 7 Figures and 4 Tables Key words: Limnology, tropical lakes, high mountain lakes, Ecuador, Lake San Pablo, South America Abstract Tropical high mountain lakes in the Andean area of Ecuador are a A special type of tropical lake are the mountain lakes in special type of lake, that had not been studied. They are cold water the equatorial zone. These lakes are situated directly on the lakes situated 3,000 to 4,000 m above sea level, directly on the equa- equator where they receive intensive illumination without tor. Limnological investigations were undertaken to determine the seasonal variation. Such lakes are found in the Andes at succession of phyto- und zooplankton and to elucidate basic limno- about 3,000 to 4,000 m above sea level. Because of their logical processes, such as thermal stratification and mixing process- es. high elevations these lakes contain cold water at tempera- Today Lake San Pablo, the largest lake in Ecuador, is a eutrophic tures less than 20 °C. There are only a few lakes of this type, lake due to the input of sewage and other nutrients from the catch- all situated in the Andes of Ecuador, Columbia and northern ment area, which originate from intensive agriculture and land ero- Peru. The well-known Lake Titicaca, also in the high Andean sion. Some data on water chemistry of the lake are presented. Lake region, has been intensively studied for many years (LAz- San Pablo is a monomictic lake with a short mixing period during ZARO 1981; DEJOUX 1992). This lake is situated at 15 ° south July to September. Stratification of the lake and mixing processes latitude, where a significant seasonal variation of the climate caused by nocturnal cooling are of great significance for the eu- occurs, and cannot be considered an example of an equatori- trophication, which occurred mainly during the last decade. al high mountain lake. Initial data on the phytoplankton, zooplankton and macrophytes Before this study, no continuous limnological research are provided, which show that the biocoenosis has low diversity. Considering the high nutrient level, phytoplankton biomass is low. had been performed in Ecuador, and the knowledge of the lake and river systems is very poor. Only a few studies had been completed on water chemistry and lake classification 1. Introduction (LOFFLER 1959; STEINITZ-KANNAN et al. 1983; BAIGUN & MARINONE 1995), in addition, some taxonomic studies had The limnology of tropical lakes is of great interest and has been carried out on the phytoplankton (ROTT 1981), become a main field of contemporary limnological research Characeae (GUERLEQUIN 1981), Rotatoria (KosTE & to overcome the lack of knowledge. Tropical lakes are warm BOrrOER 1989, 1992) and the Ostracoda and Copepoda water lakes situated in the tropical and subtropical parts of (LOFFLER 1963). Asia, Africa, Central and South America. The chemical and The radiation input to tropical lakes is very high and the physical properties as well as the biological processes of water body heats up during the daytime and cools at night. tropical lakes differ significantly to those of temperate lakes Heat transfer between the water surface and the atmosphere (LEW~S 1987), due to temperature and thermal stratification, produces convective nocturnal mixing. Tropical lakes, situ- radiation and primary production, diversity of fauna and ated in the lowland, are polymictic at a high temperature flora and metabolic processes in the water body. level, and water exchange occurs every night due to diver- gence and convergence processes. This intensive water ex- change, the high radiation and the temperature level are pa- * This paper is dedicated to Prof. Dr. HARTMUTKAUSCH on the occa- rameters, not adequately accounted for in the nutrient load- sion of his 60 th birthday. ing concept of VOLLENWEIDER (VOLLENWEIDER1968; OECD 0075-9511/00/30/02-113 $12.00/0 113 1982; TUNDISI 1990). Therefore a modified nutrient loading dard limnological equipment is used and analyses are completed fol- concept was developed for tropical South American lakes by lowing the US Standard Methods for the Examination of Water and the Centro Panamerican de Ingenieria Sanitaria y Ciencias Wastewater and the German Standard Rules for Water Analysis del Ambiente (CEPIS 1990). However the application of this (DEV). Chemical analyses are performed by the Subsecretaria Saneamiento de Quito and the Escuela Politecnica Nacional de model to tropical high mountain lakes is questionable, and Quito (1998-2000), phyto- and zooplankton analyses are completed nothing has been learned about eutrophication processes in by the Technical University of Berlin. this type of lake. The surroundings, especially the catchment area of the main Investigations are carried out at Lake San Pablo, Ecuador, water source, the Rio Itambi, was studied by the author, using aerial in the high Andes. The aim of this study is to describe the photographs taken in 1994. limnological processes of this type of lakes under considera- tion of the eutrophication processes. This includes the turn over of the nutrientes, the limitation of primary production as 3. Study Area well as the succession of phyto- and zooplankton. Lake San Pablo is a high mountain lake, 2,660 m above sea level (ASL), in the Andes of Ecuador, South America, 2. Material and Methods situated near Otavalo at 0°12 ' north latitude and 78°13 ' east longitude. It is a natural lake, the largest one of Ecuador. The first investigations on the water quality in Lake San Pablo, Lake San Pablo is a nearly circular lake with a shoreline de- Ecuador, were realised in 1993/1994 by the Subsecretaria velopment factor of 1.21 and a steep slope of the shore. The Saneamiento de Quito which sampled monthly inflow, outflow and maximum depth is 35.2 m, and the mean depth is 26.0 m the water at three sites in the lake for one year. Their data are being (Fig. 1). The lake surface area is 583 hectares, and its volume evaluated by the author. In 1996 investigations had been performed is 140.106 m 3. about the occurrence of macrophytes, and since 1997, limnological investigations are carried out in Lake San Pablo by GUNKELet al. The main water source is the Rio Itambi, which con- These include studies of the stratification, its stability, mixing pro- tributes 90% of total input, a small creek with a 20-years cesses and the annual succession of phyto- and zooplankton by sam- mean flow of 1.4 m 3 sec 1. In addition there are some period- pling every two weeks, a mapping of the sediments and an evalua- ically inflowing brooks and a few springs. The source of the tion of the eutrophication processes. In these investigations, stan- Rio Itambi is 3,600 m ASL, and its catchment area extends to 4,000 m ASL. The outlet is the so called Desaguadera. The water N residence time is about 3.2 years (ZEVALLOS 1992). The catchment T area is 14,790 hectares, and the ratio of the catchment to the lake surface amounts to 1:26. The catchment area of the Rio Itambi had been mapped (Table Pucara i 1), and it shows a very high risk of nutrient input from intensive agri- culture due to the slope of the area. Agriculture is practiced up to 3,600 m ASL, and up to this el- evation, crop growing is the main land use with four to six harvests San Pablo per year. The intensive cultiva- i1##tj tion, steep slope of the fields and high precipitation rate results in / much erosion as well as a high input of nutrients into the lake. In- tensive land use for the past few Totora . .- "" decades led to the destruction of the ancient wet high mountain forests, and we must assume that 1km San Rafael this is one significant factor for eutrophication processes occur- Fig. 1. Map of Lake San Pablo, Ecuador. ring today in Lake San Pablo. 114 Limnologica 30 (2000) 2 Table 1. The catchment area of the Rio Itambi, the main source of hypolimnic temperature is rather high. Consequently, the water for Lake San Pablo with a catchment area of 12,133 hectares thermal stratification of the lake is weak. (basis: aerial photographs from 1994). Mixing processes may be caused by nocturnal cooling and the development of convergence water currents, which Flowing water system leads to nocturnal mixing. Besides the evaporation rate is Rios Itambi and La Compafiia 8.1 km Axillary creeks 11.6 km high, and this may lead to a cooling of the surface water. An- Temporary axillary creeks 80.7 km other important factor is the lack of coriolis forces in the Irrigation ditches 1.6 km equatorial. No deflection of currents produced by the winds occurs under these conditions. Consequently, the effects of Slope of the catchment area wind are greater than in temperate areas. The stability of the 0-5% slope 7.2% thermal stratification is very significant for the distribution 5-12% slope 5.8% 12-25% slope 14.8% of oxygen and nutrients. Investigations to quantify the mix- 25-50% slope 21.6% ing processes using oxygen isotope concentrations, drifting 50-70% slope 18.7% bodies and continuous registration of the thermal stratifica- > 70% slope 31.9% tion are planned. The biology of cold high mountain lakes is also of interest Soils Volcanic ashes (Duripan or Cangahua) 42.7% because these are isolated lakes in the tropics.
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