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Water quality characteristics of a high altitude oligotrophic Mexican lake
Aquatic Ecosystem Health and Management 1 (1998) 237–243
Water quality characteristics of a high altitude oligotrophic Mexican lake A. Chacon-Torres*, C. Rosas-Monge Instituto de Investigaciones sobre los Recursos Naturales, Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoaca´n Mexico
Abstract Lake Zirahuen, a small (10.48 km 2) tropical high-mountain lake located in the Mexican Neovolcanic Axis, is under severe anthropogenic pressure. Its maximum depth is 43 m. The annual water balance is controlled by differences between rainfall, plus seepage, watershed runoff, and the main stream ‘La Palma’ plus evaporation. Tropical latitudinal conditions and high altitude produce thermal characteristics intermediate between temperate and tropical lakes. Transparency is good (up to 7.0 m). The nutrient relationship for N:P suggests that phosphorus remains limiting to primary productivity. Maximum phytoplankton chlorophyll a occurs in September at the end of the rainy season. Trophic state models suggest that Lake Zirahuen is an oligotrophic system. However, increasing erosion loads and untreated sewage input indicate the need for compatible watershed and lake management strategies which will ensure the conservation of the system and its sustainable development. 䉷 1999 Elsevier Science Ltd and AEHMS. All rights reserved. Keywords: Tropical lakes; High-altitude lakes; Lake Zirahuen; Trophic state; Water quality; Oligotrophic
1. Introduction The design of compatible and efficient management strategies for inland waters require a sound knowledge of structure and function of the aquatic ecosystem and the watershed area. Moreover, these strategies must be designed from regional baseline environmental information that can be used to increase the efficiency of management practices. High-altitude lakes located in tropical areas represent systems with ecological, cultural and economic importance. In Latin America, these lakes are usually inhabited by native fauna which are exploited by local people for a subsistence economy. However, the increase of the * Corresponding author. Privada de Juan Escutia No. 100, Colonia Chapultepec Norte, Morelia, Michoaca´n, CP 58260, Mexico.
human population and the adoption of noncompatible economic systems have often caused over-exploitation of natural resources, a decrease in biodiversity and environmental deterioration (Chacon-Torres, 1993). Lake Zirahue´n, located at 2075 m above sea level in the Mexican Neovolcanic Axis (Fig. 1), is considered to be one of the most important lakes in Mexico (Chaco´n-Torres and Mu´zquiz-Iribe, 1991). Its high transparency is unique among natural Mexican lakes located in volcanic environments. At present, Lake Zirahuen supports an increasing tourist industry, aquaculture, and is undergoing cultural eutrophication. There is insufficient information about its ecology and potential productivity to establish a sustainable program for development and conservation. De Buen (1943) described the lake as the youngest in Michoaca´n and compared it to Lakes Cuitzeo and Patzcuaro. At that time, De Buen
1463-4988/99/$19.00 䉷 1999 Elsevier Science Ltd and AEHMS. All rights reserved. PII: S1463-498 8(98)00013-X
238
A. Chacon-Torres, C. Rosas-Monge / Aquatic Ecosystem Health and Management 1 (1998) 237–243
Fig. 1. Geographical location of Lake Zirahuen, Mexico.
mentioned the risk of ageing and deterioration because of inadequate forestry practices. Bernal (1988) described the lake as a tropical monomictic system with the thermocline located at 15 m depth by the end of the summer. The light attenuation coefficient was reported to be very low (0.2 m ⫺1) supporting high Secchi disc transparency values. During the last 30 years, increased erosion and P loading have led Lake Zirahuen to evolve into an oligo-mesotrophic system. Chacon-Torres and Muzquiz-Iribe (1991) found increases of suspended solids (SS), chlorophyll a (CHL) and P concentration. They also reported the depletion of hypolimnetic dissolved oxygen (DO) and the possible release of dissolved reactive P. The deterioration in lake quality is faster than expected for other regional lakes. Recognising its small surface area (10.48 km 2) in relation to its drainage area (260 km 2), it is clear that watershed management could play a key role in the conservation of lake quality. This paper presents results of annual limnological monitoring in Lake Zirahuen focusing on water quality characteristics and lake trophic status to encourage the development of a lake management program for sustainable use and conservation of the Lake Zirahuen ecosystem.
380 km northwest of Mexico City, and lies at 19⬚ 26’ 18’’ N and 101⬚ 44’ 25’’ W (Fig. 1). The drainage basin is 260.81 km 2 with a total lake surface area of 10.48 km 2; the watershed area ratio is about 25:1. The lake has one permanent tributary stream known as ‘La Palma’. However, during the rainfall season (July–September), the lake is fed by intermittent streams. Although major water inputs are derived from surface streams, variations in lake level are observed during the year. The average annual range of level is 0.35 m.
2. Study area
Bedrock of the drainage area consists of Tertiary and Quaternary volcanic rocks. Volcanic rocks are mostly basalts and andesites. Volcanic activity is extremely high in the region. There are 40 extinct volcanoes. The lake basin itself originated as an obstruction of lava flows during the Pleistocene. Soil
2.1. Geographic location Lake Zirahuen is at 2075 m above sea level, near the southern edge of the Mexican Altiplano, about
2.2. Geology Lake Zirahuen is surrounded by volcanic mountains and steep-sloped terrain. The geomorphology of the watershed corresponds to five topographic environments: 1. open water represented by the lake basin; 2. lakeshore islets and alluvial deposits affected by seasonal fluctuations in lake level; 3. the lower slopes of the Sierra, located from 2100 to 2300 msl including lower slopes of volcanic hills, mountains, and lava flows; 4. the upper slopes of the Sierra, located between 2300 and 2800 msl; and 5. alpine regions, including the upper peaks from 2500 to 3300 msl.
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239
Table 1 Fish fauna of Lake Zirahuen, Michoacan, Mexico. Indigenous species are denoted by *; introduced species by ⫹ Family and species Atherinidae Chirostoma estor copandaro* Chirostoma attenuatum zirahuen* GoodeidaE Alloophorus robustus* Goodea atripinnis* Neoophorus diazi* Centrarchidae Micropterus salmoides ⫹ Salmonidae Oncorhynchus mykiss ⫹ Cyprinidae Cyprinus carpio ⫹
Local name
Food habits
Kurucha Kuerepo
Carnivorous (fish) Carnivorus (zooplankton)
Chehua Tirhu´ Choromu
Carnivorous (fish & insects) Herbivorous (phytoplankton) Carnivorous (zooplankton)
Trucha
Carnivorous (fish)
Trucha arcoiris
Carnivorous (fish & zooplankton)
Carpa
Omnivorous (benthos)
types associated with the volcanic origins of the region are represented by Andosols covering up to 75% of the drainage area. Secondary soils located in the upper region are basically lithosols, luvisols and feozems.
2.3. Vegetation Despite strong anthropogenic influences during last century because of the introduction of inappropriate agricultural and forestry methods, 40% of total surface of the watershed of Lake Zirahuen is covered with vegetation. Maximum elevations are covered by fir forest (Abies religiosa) associated with pine (Pinus pseudostrobus) and alpine grass (Muhlenbergia macroura and Agrostis tolucensis). The upper slopes of the Sierra are dominated by pine (P. leiophylla and P. michoacana) and oak (Quercus laurina, Q. rugosa). The lower slopes of the Sierra are vegetated by mountain deciduous forest, dominated by Alnus acuminata arguta, Carpinus caroliniana, Dendropanax arboreus, and Salix bonplandiana. Deforested areas have been replaced by herbaceous vegetation including Baccharis heterophylla, Brickellia pendula, and Eupatorium areolare. Aquatic vegetation is represented primarily by emergent and submerged macrophytes including Potamogeton illinoensis, Scirpus californius and
Sagittaria latifolia (Chacon-Torres and MuzquizIribe, 1991). 2.4. Fish fauna Fish fauna consists of five native and three introduced species (Table 1). The average total annual catch between 1984 and 1993 was 12.53 metric tons (12.5 kg ha ⫺1). The composition of the fishery included largemouth bass Micropterus salmoides (42.3%), charal (Chirostoma attenuatum zirahuen) (33.7%), pez blanco (Chirostoma estor copandaro) (18.1%), chehua (Alloophorus robustus, Goodea atripinnis, and Neoophorus diazi) (5.0%), carp (Cyprinus carpio) (0.5%) and rainbow trout (Onchorhyncus mykiss) (0.4%). 3. Methods Water sampling was conducted at monthly intervals from 1989 to 1994, at 15 stations using a Van Dorn bottle. Chemical variables measured were DO, pH, total dissolved solids (SS), chlorophyll a (CHL) conductivity, hardness, alkalinity, N as ammonia (NH4⫹), nitrite (N02⫺), nitrate (NO3⫺) and organic (total, dissolved and particulate) and P (orthophosphate and total inorganic) (APHA, 1989). Water temperature was measured with a thermistor at various depths throughout the water column, water
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Table 2 Optical values of Lake Zirahuen, Mexico for 1987 and 1994. kd vertical attenuation coefficient; c beam attenuation coefficient; Eu euphotic depth; Zsd Secchi disc depth; CHL clorophyll a concentration; SS suspended solids concentration Optical parameter
1987
1994
Mean
kd (m ⫺1) c (m) Eu (m) Zsd (m) CHL (mg l ⫺1) SS (mg l ⫺1)
0.23 1.20 19.80 7.00 3.77 0.80
0.45 3.36 10.20 5.00 3.92 33.00
0.34 2.28 15.00 6.00 3.84 16.90
visibility was measured with Secchi disc according to Preisendorfer (1986), and surface and subsurface light was measured at various depths using an underwater irradiometer equipped with an ambient cell (Kahlsico underwater irradiometer No. 268WA310). Transmittance readings were taken at various depths using a 1 m pathlength transmissometer standardized in clear water at 100% transmittance (Kahlsico digital in situ transmittance meter No. 269WA170). Vertical attenuation coefficient (kd), beam attenuaton coefficient (c) and euphotic depth according to Jerlov (1976) and Kirk (1983). Primary production (PP) of phytoplankton was estimated by measuring DO concentration changes using the dark and light bottle technique (Vollenweider, 1969). Fifteen incubations were made monthly at two sites located at opposite sites along the maximum lake length, respectively. Two replicates per location were incubated at surface, 2.0 and 5.0 m depth from 9:00 am to 5:00 pm. Primary production in grams C m ⫺2 were estimated according to Vollenweider (1969) and Cole (1983).
4. Results and discussion 4.1. Thermal regime In January, the mean temperature at the lake surface is about 14⬚C with a maximum difference of 2⬚C (i.e. 12⬚C) at the bottom (43 m). In February, the lake surface temperature starts to rise (17⬚C), increasing the difference from the bottom temperature of 14⬚C. As the warm season continues from March to October,
Table 3 Water quality and primary productivity for Lake Zirahuen, Michoacan, Mexico (1989 – 1994). Mean value of 500 water samples Parameter
Mean value
Physical Water temperature (⬚C) Secchi disc transparency (m) Euphotic depth (m) Electric conductivity (mSem ⫺1) Chemical
16.5 6.0 15.0 75.0
pH Total alkalinity (mg l ⫺1) Carbonates (mgl ⫺1) Bicarbonates (mgl ⫺1) Total hardness (mg l ⫺1 CaCO3) Dissolved oxygen (mg l ⫺1) Oxygen saturation (%) Dissolved reactive phosphorus (mg l ⫺1) Total phosphorus (mg l ⫺1) Chlorophyll a (mg l ⫺1) Total solids (mg l ⫺1) Suspended solids (mg l ⫺1) Total dissolved solids (mg l ⫺1) Biochemical oxygen demand (mg l ⫺1) NO 3 (mg l ⫺1) NO2 (mg l ⫺1) NH4 (mg l ⫺1) Total organic nitrogen (mg l ⫺1) Total dissolved N (mg l ⫺1) Total particulated N (mg l ⫺1) Na (mg l ⫺1) K (mg l ⫺1) Fe (mg l ⫺1) Ca (mg l ⫺1) Mg (mg l ⫺1) Biological
8.1 70.0 0.0 63.8 24.18 7.30 93.00 5.39 8.69 3.77 14.00 2.00 12.00 1.55 0.015 0.006 0.28 0.158 0.035 0.123 2.38 1.84 0.74 1.77 4.80
Primary production (g Cm ⫺2y ⫺1) Phytoplankton
15.75
maximum water temperatures of 23⬚C at the surface and 17⬚C at the bottom occur. This temperature range persists throughout the summer and part of the autumn. By the end of the rainy season and the beginning of winter the difference between surface and bottom water temperature decreases to 2⬚C, with a surface water temperature of 19⬚C and a bottom water temperature of 17⬚C. Intermittent surfacelayer stratification occurs in well-sheltered areas on sunny, calm afternoons; when the temperature of the
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near-surface layer can rise to 3⬚C higher than the water below. 4.2. Optical properties The optical environment of Lake Zirahuen in 1987 and 1994 is summarized in Table 2. Vertical attenuation coefficients are relatively low with a mean value of 0.34 m ⫺1. Similarly, beam attenuation coefficients are relatively low, with a mean value of 2.28 m. The mean value of Secchi disc readings is 6.0 m. Minimum Secchi disc values occur during autumn and winter following runoff from the watershed. The center of the lake is generally more transparent than the rest of the lake with a maximum value of 7.00 m. De Buen (1943) reported a maximum Secchi transparency of 7.50 m and a minimum value of 6.40. Values of optical parameters reflect an increase of turbidity through time. Euphotic depth has been reduced almost 10 m from 1987 to 1994. Increase of turbidity in Lake Zirahuen results from silt and other inorganic materials introduced to the lake by hydrological and a eolic erosion. This is a consequence of intensive deforestation in the catchment. 4.3. Water quality Chemical characteristics of Lake Zirahuen reflect the parent geology of the basin (Table 3). Soils located around the catchment area are derived from volcanic ash, magma and basic lavas. These materials are rich in olivine, andesite and pyroxenes which mostly alkaline ferromagnesian minerals and susceptible to weathering (Mohr et al., 1972). Higher concentrations of Na and Mg relative to K and Ca are attributed to the olivine basalts in the basin. Sodium is the dominant cation, followed by Mg ⬎ K ⬎ Ca ⬎ Fe. Most of the hardness is attributed to Mg. Values of conductivity and total dissolved solids are relatively low considering the closed nature of Lake Zirahuen. Relatively high pH and alkalinity categorize Lake Zirahuen as a slightly alkaline lake with its inorganic carbon components dominated by bicarbonates (91%) and free CO2 (9%). Although maximum values of pH are similar to the lake mean value (8.1), lowest pH values down to 7.5 are observed as a possible result of nocturnal respiratory processes. Dissolved oxygen concentrations in Lake Zirahuen
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are usually near saturation. However, major vertical differences are detected in the water column during the spring and summer seasons. Oxygen concentrations at the bottom near depletion (0.5 mg l ⫺1), probably due to thermal differences and organic matter decomposition. High levels of oxygen saturation at the surface are the result of 1. strong wind action and surface drift generated from early afternoon to midnight, and 2. density currents generated from the tributary stream ‘La Palma’. Assessment of CHL concentrations indicate an even distribution in the lake with an average value of 3.8 mg l ⫺1. However, the small bay located at the extreme southwest and known as ‘Rinco´n de Agua Verde’ (the Green Water Corner), always has higher concentrations of CHL than the rest of the lake with values up to 5.2 mg l ⫺1. Environmental conditions in this area such as wind shelter, organic matter inputs from the deciduous forest and shallow profile (12 m) favour phytoplankton activity. 4.4. Primary productivity Phytoplankton productivity in the lake was estimated to be 15.7 g C m ⫺2 y ⫺1. There is no significant spatial difference within the lake except in the area of Rincon de Agua Verde where higher PP values, up to 27.3 g C m ⫺2y ⫺1, have been obtained. Nutrient levels are likely to be increased by rainfall and erosion loads and by concentration during the dry season. Temperature does follow a seasonal pattern which probably affects species composition and biomass during the spring and summer. 4.5. Phosphorus loading Estimated P loadings from the watershed to Lake Zirahuen suggest that the major contribution of total P loads is from sources represented by agricultural and urban activities (58.54 and 22.90%). Secondary contributions of P inputs are rural sources derived from an approximate 15 000 human population distributed in 26 towns and villages without sewer systems. It is estimated that a total of 54 574 kg P y ⫺1 are generated within the watershed area. However, based on the P concentration, Lake Zirahuen may still be categorized as predominantly
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oligotrophic, with its major P contributions coming from anthropogenic activities.
4.6. Trophic state Based on a year of water quality data (1989–1990), the trophic state index proposed by Shannon and Brezonik (1972) applied to water quality variables in Lake Zirahuen suggest its oligotrophic condition. Similarly, indices proposed by Carlson (1977) indicate oligotrophic conditions in Lake Zirahuen. Lake Zirahuen is a tropical, high-altitude lake with temperature discontinuity during the spring and summer. This thermal difference could be the result of density differences caused by the hydrodynamic pattern under the influence of the ‘La Palma’ stream. The process of thermal stratification was detected on only one occasion during the summer of 1991, when an unusually hot summer season occurred in Mexico. However, thermal stratification has not been detected since then. A more intensive study is needed to define thermal characteristics in Lake Zirahuen. The lake is increasing in turbidity due to the suspended volcanic silt, heavy erosion loads and untreated sewage inputs. The lake is moderate in hardness and is a bicarbonate–alkaline water body with predominance of Na and K. Oxygen levels are frequently near saturation at the surface as a result of wind action and hydrodinamics. The lake is receiving untreated wastewater from municipal and agricultural sources. Thus, the water has high levels of suspended matter and P compounds. However, concentrations of inorganic N are very low, suggesting N limitation. Primary productivity data indicate that Lake Zirahuen is not a productive system. These values are exclusively related to phytoplankton activity. Although various aquatic ecosystems depend mostly upon phytoplankton productivity, others receive considerable contribution from macrophytes, periphyton and microphytobenthos. In Lake Zirahuen, macrophyte communities are restricted to narrow peripheral areas located in littoral zones with smooth slope profile. Thus, macrophyte productivity is very low and phytoplankton is the only source of organic carbon.
4.7. Ecosystem management In order to achieve regional strategies for an efficient lake management in Zirahuen, it is important to consider the following. 1. An intensive and compatible program for forest management should be introduced, including reforestation with native and regional conifers which can also be used to support artesanal activities. 2. Simultaneously with forest management, a permanent program for soil erosion control should be incorporated. Most of the drainage area is represented by andosol soils distributed in a high slope terrain. Thus, water seepage and local humidity have to be conserved using appropriate vegetation cover in creeks and seasonal streams. 3. An efficient sewage treatment system has to be designed for the ‘La Palma’ stream to reduce its load of SS, organic matter, bacterial activity and nutrients. 4. Simultaneously, a program to reduce the indiscriminate use of pesticides and fertilizers in the basin has to be made, encouraging some concepts of organic agriculture. 5. Finally, a permanent water quality program should be established to detect changes in some key water variables such as water transparency, SS, organic matter, DO, total P, and CHL. This monitoring program would be used to interpret the effects of watershed management and lake quality characteristics.
References APHA (American Public Health Association), 1989. Standard Methods for Examination of Water and Wastewater, 17th ed. Washington, DC: APHA, 2134 pp. Bernal, B.F., 1988. Limnology of Lake Zirahuen relative to cultural impacts, MSc. Thesis. University of Guelph, Canada, 132 pp. Carlson, R.E. (1977). A trophic state index for lakes. Limnol. Oceanog., 22(2), 361–369. Chacon-Torres, A., 1993. Lake Patzcuaro, Mexico: watershed and water quality deterioration in a tropical high-altitude Latin-American lake. Lake and Reservoir Management, 8(1), 37–47. Chacon-Torres. A., Mu´zquiz-Iribe, L.E., 1991. El lago de Zirahue´n, Michoaca´n, Me´xico. Reconocimiento ambiental de una cuenca
A. Chacon-Torres, C. Rosas-Monge / Aquatic Ecosystem Health and Management 1 (1998) 237–243 michoacana. Monografı´as Biologı´a Acua´tica No. 2. Secretarı´a De Difusio´n Cultural. Editorial Universitaria, U.M.S.N.H. Morelia, Mich. Me´xico. 30 pp. Cole, G.A., 1983. Textbook on Limnology, 3rd ed. The C.V. Mosby Company, St. Louis, MO. De Buen, F., 1943. Los lagos michoacanos. I. Caracteres generales. El lago de Zirahuen. Rev. Soc. Mex. Hist. Nat., 4, 211–232. Jerlov, N.J., 1976. Marine Optics. Elsevier, Amsterdam, 310 pp. Kirk, J.T.O., 1983. Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press, Cambride, 401 pp. Mohr, E.C.J., Van Baren, F.A., Van Schuyleborgh, J., 1972.
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Tropical Soils. A Comprenhensive Study of their Genesis, 3rd. ed. Mouton-Ichtiar Baru-Van Hoeve, The Hague, 300 pp. Preisendorfer, R.W., 1986. Secchi disc science: visual optics of natural waters. Limnol. Oceanogr., 31, 909–926. Shannon, E.E., Brezonik, P.L., 1972. Eutrophication Analysis. A Multivariate Approach. J. San. Eng. Div. ASCE. SA1, 37–58. Vollenweider, R.A. (Ed.), 1969. A Manual on Methods for Measuring Primary Production in Aquatic Environments. IBP Handbook no. 12. Blackwell, Oxford.
Water quality characteristics of a high altitude oligotrophic Mexican lake A. Chacon-Torres*, C. Rosas-Monge Instituto de Investigaciones sobre los Recursos Naturales, Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Michoaca´n Mexico
Abstract Lake Zirahuen, a small (10.48 km 2) tropical high-mountain lake located in the Mexican Neovolcanic Axis, is under severe anthropogenic pressure. Its maximum depth is 43 m. The annual water balance is controlled by differences between rainfall, plus seepage, watershed runoff, and the main stream ‘La Palma’ plus evaporation. Tropical latitudinal conditions and high altitude produce thermal characteristics intermediate between temperate and tropical lakes. Transparency is good (up to 7.0 m). The nutrient relationship for N:P suggests that phosphorus remains limiting to primary productivity. Maximum phytoplankton chlorophyll a occurs in September at the end of the rainy season. Trophic state models suggest that Lake Zirahuen is an oligotrophic system. However, increasing erosion loads and untreated sewage input indicate the need for compatible watershed and lake management strategies which will ensure the conservation of the system and its sustainable development. 䉷 1999 Elsevier Science Ltd and AEHMS. All rights reserved. Keywords: Tropical lakes; High-altitude lakes; Lake Zirahuen; Trophic state; Water quality; Oligotrophic
1. Introduction The design of compatible and efficient management strategies for inland waters require a sound knowledge of structure and function of the aquatic ecosystem and the watershed area. Moreover, these strategies must be designed from regional baseline environmental information that can be used to increase the efficiency of management practices. High-altitude lakes located in tropical areas represent systems with ecological, cultural and economic importance. In Latin America, these lakes are usually inhabited by native fauna which are exploited by local people for a subsistence economy. However, the increase of the * Corresponding author. Privada de Juan Escutia No. 100, Colonia Chapultepec Norte, Morelia, Michoaca´n, CP 58260, Mexico.
human population and the adoption of noncompatible economic systems have often caused over-exploitation of natural resources, a decrease in biodiversity and environmental deterioration (Chacon-Torres, 1993). Lake Zirahue´n, located at 2075 m above sea level in the Mexican Neovolcanic Axis (Fig. 1), is considered to be one of the most important lakes in Mexico (Chaco´n-Torres and Mu´zquiz-Iribe, 1991). Its high transparency is unique among natural Mexican lakes located in volcanic environments. At present, Lake Zirahuen supports an increasing tourist industry, aquaculture, and is undergoing cultural eutrophication. There is insufficient information about its ecology and potential productivity to establish a sustainable program for development and conservation. De Buen (1943) described the lake as the youngest in Michoaca´n and compared it to Lakes Cuitzeo and Patzcuaro. At that time, De Buen
1463-4988/99/$19.00 䉷 1999 Elsevier Science Ltd and AEHMS. All rights reserved. PII: S1463-498 8(98)00013-X
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A. Chacon-Torres, C. Rosas-Monge / Aquatic Ecosystem Health and Management 1 (1998) 237–243
Fig. 1. Geographical location of Lake Zirahuen, Mexico.
mentioned the risk of ageing and deterioration because of inadequate forestry practices. Bernal (1988) described the lake as a tropical monomictic system with the thermocline located at 15 m depth by the end of the summer. The light attenuation coefficient was reported to be very low (0.2 m ⫺1) supporting high Secchi disc transparency values. During the last 30 years, increased erosion and P loading have led Lake Zirahuen to evolve into an oligo-mesotrophic system. Chacon-Torres and Muzquiz-Iribe (1991) found increases of suspended solids (SS), chlorophyll a (CHL) and P concentration. They also reported the depletion of hypolimnetic dissolved oxygen (DO) and the possible release of dissolved reactive P. The deterioration in lake quality is faster than expected for other regional lakes. Recognising its small surface area (10.48 km 2) in relation to its drainage area (260 km 2), it is clear that watershed management could play a key role in the conservation of lake quality. This paper presents results of annual limnological monitoring in Lake Zirahuen focusing on water quality characteristics and lake trophic status to encourage the development of a lake management program for sustainable use and conservation of the Lake Zirahuen ecosystem.
380 km northwest of Mexico City, and lies at 19⬚ 26’ 18’’ N and 101⬚ 44’ 25’’ W (Fig. 1). The drainage basin is 260.81 km 2 with a total lake surface area of 10.48 km 2; the watershed area ratio is about 25:1. The lake has one permanent tributary stream known as ‘La Palma’. However, during the rainfall season (July–September), the lake is fed by intermittent streams. Although major water inputs are derived from surface streams, variations in lake level are observed during the year. The average annual range of level is 0.35 m.
2. Study area
Bedrock of the drainage area consists of Tertiary and Quaternary volcanic rocks. Volcanic rocks are mostly basalts and andesites. Volcanic activity is extremely high in the region. There are 40 extinct volcanoes. The lake basin itself originated as an obstruction of lava flows during the Pleistocene. Soil
2.1. Geographic location Lake Zirahuen is at 2075 m above sea level, near the southern edge of the Mexican Altiplano, about
2.2. Geology Lake Zirahuen is surrounded by volcanic mountains and steep-sloped terrain. The geomorphology of the watershed corresponds to five topographic environments: 1. open water represented by the lake basin; 2. lakeshore islets and alluvial deposits affected by seasonal fluctuations in lake level; 3. the lower slopes of the Sierra, located from 2100 to 2300 msl including lower slopes of volcanic hills, mountains, and lava flows; 4. the upper slopes of the Sierra, located between 2300 and 2800 msl; and 5. alpine regions, including the upper peaks from 2500 to 3300 msl.
A. Chacon-Torres, C. Rosas-Monge / Aquatic Ecosystem Health and Management 1 (1998) 237–243
239
Table 1 Fish fauna of Lake Zirahuen, Michoacan, Mexico. Indigenous species are denoted by *; introduced species by ⫹ Family and species Atherinidae Chirostoma estor copandaro* Chirostoma attenuatum zirahuen* GoodeidaE Alloophorus robustus* Goodea atripinnis* Neoophorus diazi* Centrarchidae Micropterus salmoides ⫹ Salmonidae Oncorhynchus mykiss ⫹ Cyprinidae Cyprinus carpio ⫹
Local name
Food habits
Kurucha Kuerepo
Carnivorous (fish) Carnivorus (zooplankton)
Chehua Tirhu´ Choromu
Carnivorous (fish & insects) Herbivorous (phytoplankton) Carnivorous (zooplankton)
Trucha
Carnivorous (fish)
Trucha arcoiris
Carnivorous (fish & zooplankton)
Carpa
Omnivorous (benthos)
types associated with the volcanic origins of the region are represented by Andosols covering up to 75% of the drainage area. Secondary soils located in the upper region are basically lithosols, luvisols and feozems.
2.3. Vegetation Despite strong anthropogenic influences during last century because of the introduction of inappropriate agricultural and forestry methods, 40% of total surface of the watershed of Lake Zirahuen is covered with vegetation. Maximum elevations are covered by fir forest (Abies religiosa) associated with pine (Pinus pseudostrobus) and alpine grass (Muhlenbergia macroura and Agrostis tolucensis). The upper slopes of the Sierra are dominated by pine (P. leiophylla and P. michoacana) and oak (Quercus laurina, Q. rugosa). The lower slopes of the Sierra are vegetated by mountain deciduous forest, dominated by Alnus acuminata arguta, Carpinus caroliniana, Dendropanax arboreus, and Salix bonplandiana. Deforested areas have been replaced by herbaceous vegetation including Baccharis heterophylla, Brickellia pendula, and Eupatorium areolare. Aquatic vegetation is represented primarily by emergent and submerged macrophytes including Potamogeton illinoensis, Scirpus californius and
Sagittaria latifolia (Chacon-Torres and MuzquizIribe, 1991). 2.4. Fish fauna Fish fauna consists of five native and three introduced species (Table 1). The average total annual catch between 1984 and 1993 was 12.53 metric tons (12.5 kg ha ⫺1). The composition of the fishery included largemouth bass Micropterus salmoides (42.3%), charal (Chirostoma attenuatum zirahuen) (33.7%), pez blanco (Chirostoma estor copandaro) (18.1%), chehua (Alloophorus robustus, Goodea atripinnis, and Neoophorus diazi) (5.0%), carp (Cyprinus carpio) (0.5%) and rainbow trout (Onchorhyncus mykiss) (0.4%). 3. Methods Water sampling was conducted at monthly intervals from 1989 to 1994, at 15 stations using a Van Dorn bottle. Chemical variables measured were DO, pH, total dissolved solids (SS), chlorophyll a (CHL) conductivity, hardness, alkalinity, N as ammonia (NH4⫹), nitrite (N02⫺), nitrate (NO3⫺) and organic (total, dissolved and particulate) and P (orthophosphate and total inorganic) (APHA, 1989). Water temperature was measured with a thermistor at various depths throughout the water column, water
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Table 2 Optical values of Lake Zirahuen, Mexico for 1987 and 1994. kd vertical attenuation coefficient; c beam attenuation coefficient; Eu euphotic depth; Zsd Secchi disc depth; CHL clorophyll a concentration; SS suspended solids concentration Optical parameter
1987
1994
Mean
kd (m ⫺1) c (m) Eu (m) Zsd (m) CHL (mg l ⫺1) SS (mg l ⫺1)
0.23 1.20 19.80 7.00 3.77 0.80
0.45 3.36 10.20 5.00 3.92 33.00
0.34 2.28 15.00 6.00 3.84 16.90
visibility was measured with Secchi disc according to Preisendorfer (1986), and surface and subsurface light was measured at various depths using an underwater irradiometer equipped with an ambient cell (Kahlsico underwater irradiometer No. 268WA310). Transmittance readings were taken at various depths using a 1 m pathlength transmissometer standardized in clear water at 100% transmittance (Kahlsico digital in situ transmittance meter No. 269WA170). Vertical attenuation coefficient (kd), beam attenuaton coefficient (c) and euphotic depth according to Jerlov (1976) and Kirk (1983). Primary production (PP) of phytoplankton was estimated by measuring DO concentration changes using the dark and light bottle technique (Vollenweider, 1969). Fifteen incubations were made monthly at two sites located at opposite sites along the maximum lake length, respectively. Two replicates per location were incubated at surface, 2.0 and 5.0 m depth from 9:00 am to 5:00 pm. Primary production in grams C m ⫺2 were estimated according to Vollenweider (1969) and Cole (1983).
4. Results and discussion 4.1. Thermal regime In January, the mean temperature at the lake surface is about 14⬚C with a maximum difference of 2⬚C (i.e. 12⬚C) at the bottom (43 m). In February, the lake surface temperature starts to rise (17⬚C), increasing the difference from the bottom temperature of 14⬚C. As the warm season continues from March to October,
Table 3 Water quality and primary productivity for Lake Zirahuen, Michoacan, Mexico (1989 – 1994). Mean value of 500 water samples Parameter
Mean value
Physical Water temperature (⬚C) Secchi disc transparency (m) Euphotic depth (m) Electric conductivity (mSem ⫺1) Chemical
16.5 6.0 15.0 75.0
pH Total alkalinity (mg l ⫺1) Carbonates (mgl ⫺1) Bicarbonates (mgl ⫺1) Total hardness (mg l ⫺1 CaCO3) Dissolved oxygen (mg l ⫺1) Oxygen saturation (%) Dissolved reactive phosphorus (mg l ⫺1) Total phosphorus (mg l ⫺1) Chlorophyll a (mg l ⫺1) Total solids (mg l ⫺1) Suspended solids (mg l ⫺1) Total dissolved solids (mg l ⫺1) Biochemical oxygen demand (mg l ⫺1) NO 3 (mg l ⫺1) NO2 (mg l ⫺1) NH4 (mg l ⫺1) Total organic nitrogen (mg l ⫺1) Total dissolved N (mg l ⫺1) Total particulated N (mg l ⫺1) Na (mg l ⫺1) K (mg l ⫺1) Fe (mg l ⫺1) Ca (mg l ⫺1) Mg (mg l ⫺1) Biological
8.1 70.0 0.0 63.8 24.18 7.30 93.00 5.39 8.69 3.77 14.00 2.00 12.00 1.55 0.015 0.006 0.28 0.158 0.035 0.123 2.38 1.84 0.74 1.77 4.80
Primary production (g Cm ⫺2y ⫺1) Phytoplankton
15.75
maximum water temperatures of 23⬚C at the surface and 17⬚C at the bottom occur. This temperature range persists throughout the summer and part of the autumn. By the end of the rainy season and the beginning of winter the difference between surface and bottom water temperature decreases to 2⬚C, with a surface water temperature of 19⬚C and a bottom water temperature of 17⬚C. Intermittent surfacelayer stratification occurs in well-sheltered areas on sunny, calm afternoons; when the temperature of the
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near-surface layer can rise to 3⬚C higher than the water below. 4.2. Optical properties The optical environment of Lake Zirahuen in 1987 and 1994 is summarized in Table 2. Vertical attenuation coefficients are relatively low with a mean value of 0.34 m ⫺1. Similarly, beam attenuation coefficients are relatively low, with a mean value of 2.28 m. The mean value of Secchi disc readings is 6.0 m. Minimum Secchi disc values occur during autumn and winter following runoff from the watershed. The center of the lake is generally more transparent than the rest of the lake with a maximum value of 7.00 m. De Buen (1943) reported a maximum Secchi transparency of 7.50 m and a minimum value of 6.40. Values of optical parameters reflect an increase of turbidity through time. Euphotic depth has been reduced almost 10 m from 1987 to 1994. Increase of turbidity in Lake Zirahuen results from silt and other inorganic materials introduced to the lake by hydrological and a eolic erosion. This is a consequence of intensive deforestation in the catchment. 4.3. Water quality Chemical characteristics of Lake Zirahuen reflect the parent geology of the basin (Table 3). Soils located around the catchment area are derived from volcanic ash, magma and basic lavas. These materials are rich in olivine, andesite and pyroxenes which mostly alkaline ferromagnesian minerals and susceptible to weathering (Mohr et al., 1972). Higher concentrations of Na and Mg relative to K and Ca are attributed to the olivine basalts in the basin. Sodium is the dominant cation, followed by Mg ⬎ K ⬎ Ca ⬎ Fe. Most of the hardness is attributed to Mg. Values of conductivity and total dissolved solids are relatively low considering the closed nature of Lake Zirahuen. Relatively high pH and alkalinity categorize Lake Zirahuen as a slightly alkaline lake with its inorganic carbon components dominated by bicarbonates (91%) and free CO2 (9%). Although maximum values of pH are similar to the lake mean value (8.1), lowest pH values down to 7.5 are observed as a possible result of nocturnal respiratory processes. Dissolved oxygen concentrations in Lake Zirahuen
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are usually near saturation. However, major vertical differences are detected in the water column during the spring and summer seasons. Oxygen concentrations at the bottom near depletion (0.5 mg l ⫺1), probably due to thermal differences and organic matter decomposition. High levels of oxygen saturation at the surface are the result of 1. strong wind action and surface drift generated from early afternoon to midnight, and 2. density currents generated from the tributary stream ‘La Palma’. Assessment of CHL concentrations indicate an even distribution in the lake with an average value of 3.8 mg l ⫺1. However, the small bay located at the extreme southwest and known as ‘Rinco´n de Agua Verde’ (the Green Water Corner), always has higher concentrations of CHL than the rest of the lake with values up to 5.2 mg l ⫺1. Environmental conditions in this area such as wind shelter, organic matter inputs from the deciduous forest and shallow profile (12 m) favour phytoplankton activity. 4.4. Primary productivity Phytoplankton productivity in the lake was estimated to be 15.7 g C m ⫺2 y ⫺1. There is no significant spatial difference within the lake except in the area of Rincon de Agua Verde where higher PP values, up to 27.3 g C m ⫺2y ⫺1, have been obtained. Nutrient levels are likely to be increased by rainfall and erosion loads and by concentration during the dry season. Temperature does follow a seasonal pattern which probably affects species composition and biomass during the spring and summer. 4.5. Phosphorus loading Estimated P loadings from the watershed to Lake Zirahuen suggest that the major contribution of total P loads is from sources represented by agricultural and urban activities (58.54 and 22.90%). Secondary contributions of P inputs are rural sources derived from an approximate 15 000 human population distributed in 26 towns and villages without sewer systems. It is estimated that a total of 54 574 kg P y ⫺1 are generated within the watershed area. However, based on the P concentration, Lake Zirahuen may still be categorized as predominantly
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oligotrophic, with its major P contributions coming from anthropogenic activities.
4.6. Trophic state Based on a year of water quality data (1989–1990), the trophic state index proposed by Shannon and Brezonik (1972) applied to water quality variables in Lake Zirahuen suggest its oligotrophic condition. Similarly, indices proposed by Carlson (1977) indicate oligotrophic conditions in Lake Zirahuen. Lake Zirahuen is a tropical, high-altitude lake with temperature discontinuity during the spring and summer. This thermal difference could be the result of density differences caused by the hydrodynamic pattern under the influence of the ‘La Palma’ stream. The process of thermal stratification was detected on only one occasion during the summer of 1991, when an unusually hot summer season occurred in Mexico. However, thermal stratification has not been detected since then. A more intensive study is needed to define thermal characteristics in Lake Zirahuen. The lake is increasing in turbidity due to the suspended volcanic silt, heavy erosion loads and untreated sewage inputs. The lake is moderate in hardness and is a bicarbonate–alkaline water body with predominance of Na and K. Oxygen levels are frequently near saturation at the surface as a result of wind action and hydrodinamics. The lake is receiving untreated wastewater from municipal and agricultural sources. Thus, the water has high levels of suspended matter and P compounds. However, concentrations of inorganic N are very low, suggesting N limitation. Primary productivity data indicate that Lake Zirahuen is not a productive system. These values are exclusively related to phytoplankton activity. Although various aquatic ecosystems depend mostly upon phytoplankton productivity, others receive considerable contribution from macrophytes, periphyton and microphytobenthos. In Lake Zirahuen, macrophyte communities are restricted to narrow peripheral areas located in littoral zones with smooth slope profile. Thus, macrophyte productivity is very low and phytoplankton is the only source of organic carbon.
4.7. Ecosystem management In order to achieve regional strategies for an efficient lake management in Zirahuen, it is important to consider the following. 1. An intensive and compatible program for forest management should be introduced, including reforestation with native and regional conifers which can also be used to support artesanal activities. 2. Simultaneously with forest management, a permanent program for soil erosion control should be incorporated. Most of the drainage area is represented by andosol soils distributed in a high slope terrain. Thus, water seepage and local humidity have to be conserved using appropriate vegetation cover in creeks and seasonal streams. 3. An efficient sewage treatment system has to be designed for the ‘La Palma’ stream to reduce its load of SS, organic matter, bacterial activity and nutrients. 4. Simultaneously, a program to reduce the indiscriminate use of pesticides and fertilizers in the basin has to be made, encouraging some concepts of organic agriculture. 5. Finally, a permanent water quality program should be established to detect changes in some key water variables such as water transparency, SS, organic matter, DO, total P, and CHL. This monitoring program would be used to interpret the effects of watershed management and lake quality characteristics.
References APHA (American Public Health Association), 1989. Standard Methods for Examination of Water and Wastewater, 17th ed. Washington, DC: APHA, 2134 pp. Bernal, B.F., 1988. Limnology of Lake Zirahuen relative to cultural impacts, MSc. Thesis. University of Guelph, Canada, 132 pp. Carlson, R.E. (1977). A trophic state index for lakes. Limnol. Oceanog., 22(2), 361–369. Chacon-Torres, A., 1993. Lake Patzcuaro, Mexico: watershed and water quality deterioration in a tropical high-altitude Latin-American lake. Lake and Reservoir Management, 8(1), 37–47. Chacon-Torres. A., Mu´zquiz-Iribe, L.E., 1991. El lago de Zirahue´n, Michoaca´n, Me´xico. Reconocimiento ambiental de una cuenca
A. Chacon-Torres, C. Rosas-Monge / Aquatic Ecosystem Health and Management 1 (1998) 237–243 michoacana. Monografı´as Biologı´a Acua´tica No. 2. Secretarı´a De Difusio´n Cultural. Editorial Universitaria, U.M.S.N.H. Morelia, Mich. Me´xico. 30 pp. Cole, G.A., 1983. Textbook on Limnology, 3rd ed. The C.V. Mosby Company, St. Louis, MO. De Buen, F., 1943. Los lagos michoacanos. I. Caracteres generales. El lago de Zirahuen. Rev. Soc. Mex. Hist. Nat., 4, 211–232. Jerlov, N.J., 1976. Marine Optics. Elsevier, Amsterdam, 310 pp. Kirk, J.T.O., 1983. Light and Photosynthesis in Aquatic Ecosystems. Cambridge University Press, Cambride, 401 pp. Mohr, E.C.J., Van Baren, F.A., Van Schuyleborgh, J., 1972.
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Tropical Soils. A Comprenhensive Study of their Genesis, 3rd. ed. Mouton-Ichtiar Baru-Van Hoeve, The Hague, 300 pp. Preisendorfer, R.W., 1986. Secchi disc science: visual optics of natural waters. Limnol. Oceanogr., 31, 909–926. Shannon, E.E., Brezonik, P.L., 1972. Eutrophication Analysis. A Multivariate Approach. J. San. Eng. Div. ASCE. SA1, 37–58. Vollenweider, R.A. (Ed.), 1969. A Manual on Methods for Measuring Primary Production in Aquatic Environments. IBP Handbook no. 12. Blackwell, Oxford.