A study of the river basins and limnology of five humic ... · A study of the river basins and...

563RIVER BASINS AND LIMNOLOGICAL STUDY IN LAKES OF CHILOÉRevista Chilena de Historia Natural76: 563-590, 2003

A study of the river basins and limnology of five humic lakes onChiloé Island

Estudio de la cuenca y limnología en cinco lagos húmicos de la Isla Chiloé

L. VILLALOBOS1*, O. PARRA2, M. GRANDJEAN1, E. JAQUE3, S. WOELFL1 & H. CAMPOS1(†)

1Instituto de Zoología, Universidad Austral de Chile, Casilla 567, Valdivia, Chile;*(Corresponding autor) e-mail: [email protected]

2Centro EULA, Universidad de Concepción, Concepción, Chile3Departamento de Geografía, Universidad de Concepción, Concepción, Chile

ABSTRACT

From November 1996 to October 1997, the river basins of five humic lakes on Chiloé Island were studiedmonthly: Lakes Natri, Tepuhueico, Tarahuín, Huillinco and Cucao. The objective of this study was to know thecatchment area, river basin and the main physical, chemical and biological characteristics of these humic lakes.The trophic status, the actual loading, and the mass balances of phosphorus and nitrogen were determined inrelation to anthropogenic activities. Lakes Cucao and Huillinco were characterized by a marine influence. Allthe lakes had brown coloured waters, caused by humic substances, which limit their transparency. Lake Natriwas the deepest (58 m), whereas Lake Tepuhueico had the shallowest depth (25 m). Total phosphorus andnitrogen fluctuated between 23.5 and 35 µg L-1 and 197 and 380 µg L-1 (annual average) in lakes Natri,Tepuhueico and Tarahuín, respectively. Lakes Cucao and Huillinco showed extremely high concentrations oftotal nitrogen (annual average => 3,000 µg L-1) and total phosphorus (= 223 and 497 µg L-1), and were classifiedas hyper-eutrophic. Lake Tarahuín registered the greatest diversity of phytoplankton, with 55 species, includingCeratium hirundinella which also occurred in lakes Cucao and Tarahuín. The diversity of the zooplanktoncommunity varied across these lakes. The presence of Diaptomus diabolicus (Tumeodiaptomus d. Dussart 1979)(Cucao, Huillinco and Tepuhueico) is noteworthy since this extends its geographical distribution to the south.

Key words: river basin, limnological characterization, humic lakes, Chiloé, southern Chile.

RESUMEN

Durante noviembre de 1996 y octubre de 1997 mensualmente se estudió la cuenca de cinco lagos húmicoslocalizados en la Isla de Chiloé: Lagos Natri, Tepuhueico, Tarahuín, Huillinco y Cucao. El estudio tuvo comoobjetivo conocer el área de la cuenca, la cuenca hidrográfica y las principales características físicas, químicas ybiológicas de estos lagos húmicos. Se determinó el estado trófico, como también la carga actual y el balancemásico de fósforo y nitrógeno total, en relación a actividades antrópicas. Los lagos Cucao y Huillinco secaracterizaron por presentar influencia marina. Todos los lagos tienen aguas de color marrón, causadas por lassustancias húmicas, que limitan la transparencia. El lago Natri es el más profundo (58 m), mientras que elTepuhueico presentó la menor profundidad (25 m). Concentraciones de fósforo y nitrógeno total fluctuaron entre23,5 y 35 µg L-1 y 197 y 380 µg L-1 en los lagos Natri, Tepuhueico y Tarahuín. Mientras que los lagos Cucao yHuillinco mostraron para el nitrógeno y fósforo concentraciones extremadamente altas, para el nitrógeno total(promedio anual > 3.000 µg L-1) y para el fósforo (223 y 496 µg L-1) clasificándolos como hipereutróficos. Ellago Tarahuín registró la mayor diversidad fitoplanctónica con 55 especies. En los lagos Cucao y Tarahuín,destaca la presencia de Ceratium hirundinella. La comunidad zooplanctónica mostró diferencias en la diversida-des de especies en los lagos estudiados. En los lagos Cucao, Huillinco y Tepuhueico la presencia de Diaptomusdiabolicus (= Tumeodiaptomus d. Dussart 1979), amplía su distribución geográfica hacia el sur.

Palabras clave: cuenca hidrográfica, caracterización limnológica, lagos húmicos, Chiloé, Sur de Chile.

INTRODUCTION

The Great Island of Chiloé belongs to thearchipelago of Chiloé, which extends 180 kmfrom north to south in a straight line, and is

thus the second largest island of SouthAmerica, after Tierra del Fuego. This island iscrossed by a continuation of the continentalCoastal Range which is interrupted by theChacao channel separating the island from the

564 VILLALOBOS ET AL.

continent on the north. To the east the island isseparated from the continent by the Ancud andCorcovado gulfs.

Little information is available about thefreshwater bodies on Chiloé Island. Campos etal. (1984) studied the macrozoobenthos andfish communities of some rivers on this islandwhile Hedin & Campos (1991) compared thewatersheds of two contrasting environmentsincluding some streams on Chiloé Island.Campos et al . (1996) have studied thegeographic significance of the distribution ofthe native fish species, Cheirodon australe inLake Tarahuín. However, there is no detailedinformation on the limnology of the freshwaterbodies of this Island and only in recent yearshave various lakes and rivers been studied. Inaddition to the present work, the investigationsof Roberto Prado-Fiedler et al. and José Arenaset al. (unpublished results) should also bementioned.

The objective of the study reported here wasto investigate the catchment area of each riverbasin and the main physical, chemical andbiological characteristics, of five humic lakeslocated in Chiloé Island: Lakes Natri ,Tepuhueico, Tarahuín, Huillinco and Cucao.Based on the concentrations of phosphorus andnitrogen, and using the models of Ryding &Rast (1992) and Vollenweider (1976), theactual loadings and mass balances ofphosphorus and nitrogen in these lakes weremeasured. Due to the human influences onmany of the lakes of Chiloé Island duringrecent decades, the information given hererepresents a basis for future studies andplanning of sustainable management.

MATERIAL AND METHODS

Study site

The Chiloé Island (41o46’ and 46o59’ S, 72o30’to 75o26’ W) is located within an oceanicecological region with Mediterranean influence(di Castri 1968). The rainfall is 2.000-2.500mm, relative humidity averages 84 % (di Castri1968, Subiabre & Rojas 1994), with anhistorical annual average temperature of 10.5oC; minimum and maximum temperaturesaverage 6.9 oC and 14.2 oC, respectively (diCastri 1968).

The geological substrate generallycorresponds to precambrian metamorphics andrecent tertiary sediments located within thecoastal range. In the western part of ChiloéIsland brown podzolics soils predominate

(Brüggen 1950). Three overlapping drift layersare recognized (“Fuerte San Antonio,Intermedia y Llanquihue”), representing thelast glaciation and two or more previousglaciations (Subiabre & Rojas 1994).

The forests of Chiloé are dominated byNothofagus nitida, N. dombeyi with an almosttotal absence of N. obliqua . There is aformation of “Ñadis” shrub associations ofDrimys winteri and Embothrium coccineum,among other aquatic plants, partiallysubmerged (Hoffmann 1999), and formations ofN. obliqua and Laurelia sempervirens arefound within the central valley. At higheraltitudes, “alerces” (Fitzroya cupressoides) areprominent along with associations of N .antarctica and N. pumilio (Brüggen 1950).

The hydrographic basin of the River Cucao,at 840 km2 is the third largest on the island andcontains two elongated, interconnected lakes:Huillinco and Cucao, which drain into thePacific Ocean. Within this basin, the mostimportant tributary is the River Bravo (418 km2)which drains from Lake Tepuhueico into LakeHuillinco and, en route, receives water drainingLake Tarahuín (Subiabre & Rojas 1994).

Hydrographic river basins

To define the hydrographic river basins of lakesNatri, Tepuhueico, Tarahuín, Cucao andHuillinco (Fig. 1), topographic information wasobtained from maps 1:50.000 IGM (InstitutoGeográfico Militar) (Isla Lemuy, Chonchi,Rivers Anay and Cucao). The data base wasdigitalized in Autocad R12, and worked with theGeographic Information System (SIG) Idrisi, toobtain information on surface areas, length ofchannels and density of drainage in the basin.

Land use

The land uses of the five basins were identifiedthrough photo-interpretation of aerialphotographs 1:20.000 (S.A.F. 1995), analysisand interpretation of satellite images and landslopes. The cartographic information wasdigitalized in Autocad and input to the SIGIdrisi to obtain the surface areas occupied byeach land use in the basin.

Morphometry of slope

The slope is one of the physical characteristicsthat permit some processes such as soil erosionand land slips to become factors limiting theuse of land. The slope cartography wasanalysed according to the method of Brunett

565RIVER BASINS AND LIMNOLOGICAL STUDY IN LAKES OF CHILOÉ

Fig. 1: Location of the five lakes studied on Chiloé Island: Lakes Natri, Tepuhueico, Tarahuín,Cucao and Huillinco.Localización de los cinco lagos estudiados en la Isla de Chiloé: Lagos Natri, Tepuhueico, Tarahuín, Cucao y Huillinco.

(1963), which defines units that aremorphometrically homogenous between certainthresholds given by the author (Table 1). Theinformation on these maps was used tocalculate the degree of inclination.

Morphometry of lakes

To assess hydrological and morphometricparameters, various maps of 1:50.000 IGMwere used. Morphometric bathymetry wasdetermined using a Lowrence echosounder X-16 of 192 Khz frequency.

Field measurements

Routine physical, chemical and biologicalsampling was carried out monthly in all fivelakes from November 1996 to October 1997 ata central sampling station in each lake.

Samples were obtained using a Van Dornbott le (volume 5 L) at three depthsrepresenting the surface, middle and bottom ofthe lakes: Natri (0.1, 25, 50 m ), Tepuhueico(0.1, 8, 17 m), Tarahuín (0.1, 15, 30 m),Huillinco (0.1, 22, 45 m) and Cucao (0.1, 8,16 m). Additionally chemical samples wereobtained from the inflows to each lake.

Physical measurements

Temperature profiles were obtained using aKahlsico bathythermograph with an accuracy ofbetter than 0.2 °C. Water transparency wasmeasured at midday with a standard 20 cmdiameter Secchi disk, from the shady side ofthe boat. pH was measured with a pH-meter(EXTECH, USA). The colour and turbiditywere measured in the laboratory using aspectrophotometer at a wavelength of 440 nm.


Chemical measurements

During each monthly sampling, a house in VillaHuillinco was used as a laboratory and avehicle was adapted as a mobile laboratory.Thus, most of the chemical measurements weremade in situ when the samples were obtained,either in the boat or in the mobile laboratory.The analyses of nitrogen and phosphoruscompounds were made in the laboratory houseon the day of sampling, up to the point ofdigestion and evaporation, and were thenimmediately sent at an optimal temperature, bybus or car to the laboratory of the UniversidadAustral de Chile. Chemical analyses followedthe methods described by Eaton et al. (1995)and are described only briefly here. Dissolvedoxygen concentrations were determined byWinkler titration. For orthophosphate (PO4-P)and total phosphorus (TP) the molybdenum

blue method was used, but with a preliminaryacid-peroxide digestion for TP (Ammbühl &Schmidt 1965). The concentration of NO3-Nwas determined as the difference between theconcentrations of NO3-N + NO2-N and NO2-N.Analysis for NO2-N + NO3-N involvedreduction of NO3-N to NO2-N with sodiumsalicylate and salt of Seignette, prior to analysisas for NO2-N. The analysis of NO2-N involvedthe formation of a diazonium salt complexwhich reacted with chromotropic acid prior tocolourimetric analysis. The indophenol bluemethod was used for analysis of NH4-N, andtotal nitrogen concentrations were determinedas the sum of concentrations of Kjeldahlnitrogen + NO3-N + NO2-N.

Organic and inorganic seston (particulatematter) were quantified by filtering 1 L of lakewater through a previously tared fiber glassfilter (0.45 µm), dried at 60 oC for 24 h. The

TABLE 1

Surface, slopes and land use in the five Chiloé Island basinsSuperficie, pendiente y uso de suelo en cinco cuencas de la Isla de Chiloé

Natri Tepuhueico Tarahuin Huillinco Cucao

Subriver basic 14 8 15 15 11km2 km2 km2 km2 km2

Miraflores 10.9 Aguas Muertas 103.3 Pichihueico 13.1 Notué 220.8 Quilque 5.92 3.7 Tepuhueico 46.1 2 2.5 Bravo 167.7 Curahueldo 5.73 3.4 3 10.3 3 4.2 Trainel 59.5 Maquil 5.59 6.6 Others 5 22.0 4 2.4 Huillinco 14.0 Palpahuen 4.4

12 9.5 6 2.4 Cuduhue 12.7 6 2.5Others 9 12.4 10 3.4 Pinda 8.2 9 2.4

Others 9 10.2 Others 9 46.9 Others 5 5.7

Slope Area (km2) Area (km2) Area (km2) Area (km2) Area (km2)

0-2.9º 12.8 35.8 4.3 194.5 3.83-5.9º 5.9 31.3 1.3 30.1 2.66-8.9º 7.0 25.8 4.3 42.1 3.19-11.9º 4.2 33.4 7.9 64.4 4.212-16.9º 7.0 26.3 10.4 40.5 3.617-21.9º 4.3 11.5 6.2 29.7 4.222-26.9º 2.7 14.6 3.4 41.2 4.227-30.9º 1.7 2.1 0.0 21.4 3.831-35º 0.5 1.0 0.0 19.3 1.6> 35º 0.0 0.0 0.0 45.1 2.1

Land use Area (km2) Area (km2) Area (km2) Area (km2) Area (km2)

Mixed native forest 26.7 139.5 19.2 290.6 15.9Mixed native renewal 7.5 32.1 7.5 107.9 13.9Natural prairies 10.2 5.2 11.1 108.5 0.0Logging 1.8 4.3 4.2 1.9Wetland 0.3 1.0 0.3 13.5Agriculture 4.4Inhabitant 0.2Protected(National Park) 1.3


dry weight of organic particulate material wasobtained by weight difference. The filter wasincinerated for 6 h in a Muffle at 550 ºC untilonly ashes remained; the inorganic fraction ofthe seston was obtained by weight difference.Calcium, magnesium, sodium and potassiumwere measured using an atomic absorptionspectrophotometer, Perkin Elmer 2380.Chloride concentration was determined usingthe mercuric nitrate method. Sulphate wasmeasured by turbidimetry. Alkalinity wasdetermined by ti tration using 0.025 Nhydrochloric acid with bromocresol green-methyl red as indicator. Bicarbonate wascalculated from alkalinity.

To determine the nutrient concentrations inthe rainwater, a 1 m2 surface automaticcollector, connected to a Pyrex glass container,was set up in each river basin during the fieldtrip. Rainwater was cold preserved until itsanalysis, and total phosphorus and nitrogenconcentrations were determined according tothe methods described above.

Chlorophyll a was determined by filtrationof 3 L of lake water through Millipore glassfiber (GF/C) filters and extraction of pigmentfrom the filters with 90 % acetone. Absorbanceof the extract was measured with a ShimadzuUV-150-02 spectrophotometer and theequations of Scor-Unesco (1969) were thenused to estimate concentrations of chlorophylla after correction for accessory pigments(chlorophylls b and c).

Phytoplankton was collected with a VanDorn bottle at 0.1, 3, 5, 10 and 15 m, fixed withLugol’s iodine (1 % final concentration) in 100mL water samples, and counted using a ZeissInverted Microscope according to the standardUtermöhl technique (1958). Zooplankton wascollected using a Ruttner net with a 10.4 cmmouth opening and mesh size of 90 µm.Vertical hauls were carried out at differentdepths: Natri and Tarahuín (30-20, 20-15, 15-10, 10-6, 6-3, 3-0 m), Tepuhueico (20-15, 15-10, 10-6, 6-3, 3-0 m), Huillinco (50-30, 30-20,20-15, 15-10, 10-6, 6-3, 3-0 m) and Cucao (15-10, 10-6, 6-3, 3-0 m). The samples were fixedwith formaldehyde-sugar to a finalconcentration of 4 % (Haney & Hall 1973), andcounted in Bogorov chambers under 40-100 x.

The annual loads of phosphor and nitrogenwere calculated according to the equation ofRyding & Rast (1992, eq. 1). The mass balanceof phosphorous and nitrogen were calculatedaccording the equation of Vollenweider (1976,equation 2):

L = LT+ LDS+LDD+LA+LG+LS

Equation 1 (modified after Ryding & Rast1992): L = total load, LT = annual load of theinflows, LDS = direct and diffuse load of sewagewaters, LA= atmospheric contribution by rainwater, LG= load through the fish farmingentries, LS= liberation of nutrients from thesediment, and LT= annual load of the inflows.

Q1 [P]1 + Q2 [P]2 +......+ Qn [P]nL[p, n] =

Ao

where: L [p, n] is the load of phosphorus ornitrogen in µg m-2 year-1, Q1 the annual flow ofthe inflow, P the concentration of phosphorusor nitrogen from the corresponding inflow inthe flow, and Ao the area of the lake.

LDS = direct and diffuse load of sewage watersJA = K (kg per capita –1 year –1) N (1 – Rs) T year –1)

where: JA is the artificial phosphorus ornitrogen supply, K is a constant that in the caseof phosphorus K = 0.8 kg by person year –1, andfor nitrogen K = 3.8 kg year –1 (Dillon & Rigler1974). N is the number of inhabitants, Rs theretention coefficient of total phosphorus perseptic tank according to the filtration capacityof the soil (to filter bed) (Brandes et al. 1974).A Rs of 0.30 was calculated per septic tank byBrandes et al. (1974). T is the average numberof days of use by the inhabitants per year.

LA= atmospheric contribution by rain waterL[p, n ] = Pp, n ([p, n] /Ao)

where: L [p, n] is the load of phosphorus ornitrogen in µg m-2 year-1, Pp, n the annualprecipitation of phosphorus or nitrogen on thelake, [p, n] the average concentration ofphosphorus or nitrogen in the rain water. Ao thearea of the lake.

LG= load through the fish farming centresR = P + A - (S + M)

where: R is the supply of phosphorus ornitrogen to the lake, P the content ofphosphorus or nitrogen in the smoltsproduction, A is the content of phosphorus ornitrogen in the smolts leaving the fish farm,and M is the content of phosphorus or nitrogenin those which died. The nutrient loads fromthe fish farming were calculated according tothe amounts of food used in the production ofsmolts during the year. Chemical analyses wereconducted to quantify the amounts ofphosphorus and nitrogen present directly in the


food. The content of phosphorus and nitrogenin the fish was also considered, and such datawere taken from Rodriguez (1993). Thecalculations of the quantities of food used inthe production process were made using aconversion factor of 1.5.

LS= liberation of nutrients from the sediment

where Ls is considered as internal load ofphosphorus or nitrogen (Lint) and wascalculated according to the mass balancedescribed in the equation 2.

Lint = Lout - Lext ± TPEquation 2 (Vollenweider 1976)

where Lint = internal load of total phosphorus ornitrogen during a period of study, Lout = loss oftotal phosphorus or nitrogen in the water

column during a period of study, Lext = externalload of total phosphorus or nitrogen in thewater column during a period, and ± TP =difference positive or negative in the totalphosphorus or nitrogen content in the watercolumn during the annual period of study.

RESULTS

Hydrographic system and land uses in the diffe-rent River Basin: Lake Natri

The catchment area of Lake Natri drains a totalarea of 46.5 km2 on the central eastern slope ofChiloé Island, and empties through the RiverNatri which ends in the inner sea. Fourteen sub-basins were recognized (Fig. 2) which, accordingto the classification of Strahler (1987), comprise anetwork of 25 first order and six second order

Fig. 2: Hydrology of the catchment area of the five lakes studied on Chiloé Island: (A) Natri, (B)Tepuhueico, (C) Tarahuín, (D) Cucao and (E) Huillinco.Hidrología de las cuencas hidrográficas los cinco lagos estudiados en la Isla de Chiloé: (A) Natri, (B) Tepuhueico, (C)Tarahuín, (D) Cucao y (E) Huillinco.


rivers. The Miraflores stream, located in thenorth-west, constitutes the largest sub-river basin(Table 1). The Natri basin in general hasmoderate slopes (0.5- 8.9o) (Table 1). Someerosive processes were observed in areas withslopes greater than 17o (southern slopes) causedby change in the land use (grazing of cattle)

Lake Tepuhueico

The basin of this lake comprises part of thehydrographic system of Lake Cucao and drainsthe south-western slope to 42o47’ S. The basincovers a surface of 182.1 km2. LakeTepuhueico receives two important inflows(Fig. 2): Aguas Muertas and Tepuhueico, anditself empties towards the north through theRiver Bravo which flows into Lake Huillinco.In the basin of Lake Tepuhueico eight sub-riverbasins were recognized (Fig. 2) and thishydrographic network consists of 32 first order,10 second order and 1 third order river, theAguas Muertas, which is exceptional for its103.3 km2. The sub-river basins of the RiverTepuhueico are the second most extensive(Table 1). The basin of Lake Tepuhueico has amorphometry of predominantly moderateslopes with 52 % of the territory less than 12o.

Lake Tarahuín

This lake receives drainage from the easternslope of the Huillinco-Cucao hydrographicsystem to 42o43' S. With a catchment area of38.2 km2 the lake is orientated in a north-southdirection and drains through the River Tarahuínto the River Bravo. Fifteen smaller sub-riverbasins (Fig. 2), corresponding to rivers andstreams have been recognised in this basin.Twenty-two first order and 4 second orderstreams make up the hydrographic network. Themost important of these is the River Pichihueico(Table 1), draining the south-eastern slope. Thesecond most important are those numbered 3(north slope) and 10, the latter receiving itswater from a small lagoon. None of the twelveremaining sub-basins exceed 2.5 km2 in surfacearea and are primarily seasonal run-off waters.The Tarahuín basin has predominantly moderateslopes (< 12o, 50 % of the surface); areas withvery strong slopes (22-26.9o) were found only inthe 8.9 % of the area (Table 1).

Lake Huillinco

Drains the north-eastern slope of the Huillinco-Cucao hydrographic system to 42o30' S. In thisbasin there are 15 sub-basins (Fig. 2), of rivers

and streams forming a particularly densenetwork which consists of 110 first order and 50second order water courses. The most importantof these is the River Notué, which drains 43 %of the territory on the north slope. The riversBravo and Trainel are also significant sub-basins(Table 1). The basin of Lake Huillinco shows apredominance of gentle slopes (0.5-2.9o) (Table1) but in the north of the area there are steepslopes associated with the mountains of theCoastal range. In the south and east, gentle tomoderate slopes predominate, associated withplatforms and fluvio-glacial terraces.

Lake Cucao

The basin of Lake Cucao drains the westernslope of its hydrographic network to 42o S;with a basin covering 33.2 km2. It l iesperpendicular to the coastline and drainsthrough the River Cucao to the Pacific Ocean,which exerts a strong marine influence on lakesCucao and Huillinco (Fig. 1). Eleven sub-basins were recognised in the basin of LakeCucao (Fig. 2), comprising 18 first, six secondand one third order streams, the most importantof which is the River Quilque, draining thesouth slope. The rivers Curahueldo, Maquil andPalpahuén are the next most important sub-basins (Table 1). The Cucao basin ischaracterised by slopes over 12o (50 %) (Table1) whose maximum altitudes reaches 800m.a.s.l. at the north- western end and theminimum (5 m of altitude) is at its inflow.

Land use

Most of river basins (nearly 70 %) are coveredwith mature or regenerating native forestconsisting of species such as: “canelo” (Drimyswinteri), “Tepú” (Tepualia stipularis), “Lumablanca” (Myrceugenia chrysocarpa), “Olivillo”(Aextoxicon punctatum), “Avellano” (Gevuinaavellana), “Mañío” (Podocarpus nubigenana(male) and Saxe-Gothea conspicua (female))and “Coigüe” (Nothofagus nitida, N. dombeyi,N. antarctica and N. pumilio). This type offorest cover reaches around 94 % in the basinof Lake Tepuhueico. On the other hand, in thebasins of lakes Natri, Tarahuín and Huillincoabout 22 % is natural prairie. Associated withLakes Natri , Tepuhueico, Tarahuín andHuillinco there are small areas of wetlandwhich are of importance (Table 1). In the basinof Lake Cucao 4.1 % of the outstandinglandscape is destined to be a National Park.Only in the basin of Lake Huillinco, is there avery small percentage (0.03 %) covered by


human habitation (Table 1). Huillincorecognized as a Village by the NationalInstitute of Statistics, is located on the easternshore of the lake with 88 houses inhabited by apopulation of 336.

Limnological characterization

The results presented in this section as annualaverages or means correspond to the arithmeticmean calculated for the study period fromNovember 1996 to October 1997. The trophicstatus mentioned by the authors in differentsections of the text was based on data given byWetzel (1983).

Morphometric features

Table 2 and Fig. 3A-E show the morphologicalfeatures of all the lakes. Lake Natri has anelongated form, extending from east to west,and drains through the River Natri. It ispredominantly shallow (< 20 m) around themargins. Lake Tepuhueico extends in a north-west to south-eastern orientation and drains viathe River Bravo. It has an irregular roundedshape with a constriction in the middle where itreaches its maximum depth. Most of this lake ismore than 15 m deep (Fig. 3B). Lake Tarahuínlies in an east - west direction, and drains to thewest via the River Tarahuín, which joins the

TABLE 2

Locations and morphometric parameters of Lakes Natri,Tepuhueico, Tarahuín, Cucao and Huillinco

Localización y parámetros morfométricos de los lagos Natri, Tepuhueico, Tarahuín, Cucao y Huillinco

Natri Tepuhueico Tarahuín Huillinco Cucao

Latitude 42o 47’ S 42o 47’ S 42o 43’ S 42o 40’ S 42o 38’ S

Longitude 73o50’ W 73o 58’ W 73o 45’ W 73o 57’ W 74o 40’ W

Altitude (m) 39.0 25.0 66.0 13.0 10.0

Volume (km3) 0.273 0.128 0.170 0.395 0.127

Surface area (Ao) (km2) 7.8 14.3 7.7 19.1 10.6

Catchment area (Ad) (km2) 46.5 182.1 38.1 529.8 33.2

Maximum length (lm) (km) 6.3 5.2 6.4 8.1 7.9

Maximum breadth (bm) (km) 1.3 3.1 1.5 3.2 1.8

Minimum breadth (km) 0.6 1.9 0.5 0.8 0.7

Mean breadth (b) (km) 1.2 2.5 0.9 1.9 1.1

Maximum depth (Zm) (m) 58.0 25.0 33.0 47.0 25.0

Mean depth (Z) (m) 35.0 9.0 22.2 20.7 12.0

Relative depth (Zr) (%) 1.8 0.6 1.1 1.0 0.7

Shore line (L) (km) 16.4 24.7 20.0 25.9 22.9

Development of shore line (DL) 1.7 1.8 2.0 1.7 2.0

Ad/Ao 6.0 12.7 4.9 27.7 3.1

Ratio of mean to maximum depth Z:Zm 0.6 0.4 0.7 0.4 0.5

Development of volume 1.8 1.1 2.0 1.3 1.4


Fig. 3: Bathymetric maps of the lakes: (A) Natri, (B) Tepuhueico, (C) Tarahuín, (D) Cucao y (E)Huillinco.Mapas batimétricos de los lagos: (A) Natri, (B) Tepuhueico, (C) Tarahuín, (D) Cucao y (E) Huillinco.

River Bravo. This lake is also elongated with asmall constriction around the middle, and ispredominantly less than 15 m deep (Fig. 3C).Lake Huillinco extends from east to west, withan irregular shape of an almost roundedreceptacle with depth diminishing regularlytowards the coast where it drains to LakeCucao through the channel Caldera (Fig. 3E).Lake Cucao extends in a north-west to south-eastern direction and empties into the PacificOcean through the River Cucao. The lake hasan elongated shape with a predominance of

zones less than 15 m but the distributionof depths was irregular on the middle of thelake (Fig. 3D).

Physical characterization

The seasonal cycles of water temperature fromthe surface (0.1 m) to the bottom are shown inFig. 4. The lakes showed similar temperaturesranging from 8.6 oC (Huillinco, July, 0.1 m) to20.5 oC (Cucao, January, 0.1 m). Of thefreshwater lakes Lake Tarahuín and especially


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C)

Ta r

a huí

n, (

D)

Hui

llin

c o y

(E

) C

uca o

.


Lake Natri, can be classified as monomictictemperate, since they exhibited a wintercirculation, and thermal stratificationthroughout the summer (Fig. 4A, 4C). Duringwinter, Lake Natri was isothermal throughoutthe water column between 11.6 and 10.0 oC(Fig. 4A), very similar to Lake Tarahuín.Thermal stratification began in the middle ofspring (October) and lasted until autumn. InLake Natri the depth of the thermocline variedbetween 5 and 25 m while the hypolimniumtemperature remained between 9.5-12.0 oC. Thesummer stratification in Lake Tarahuín wassimilar to that in Lake Natri (Fig. 4C). LakeTepuhueico was different in that there was notypical thermal stratification during summer(Fig. 4B). The maximal temperature differencebetween the surface and the bottom of the lakeduring summer was only 3 oC. Throughout thewinter (June-August) the lake was isothermalfrom 9.4 to 11.3 oC.

The thermal regimes of lakes Cucao andHuillinco showed two patterns (Fig. 4D, 4E).The first was observed from the end ofNovember to the end of March (spring-summer) when the surface temperature variedbetween 16.5 and 20.0 oC. From 9 (10) m thetemperature decreased abruptly by around 5 to7 oC but below 10 m rose again by 2 to 3 ºCwithin two or three meters (Fig. 4D, 4E).Below 20 m, in the case of Lake Huillinco, thetemperature was stable at approximately 14.5oC. In this profile the thermocline was recordedat between 5 and 10 m. In the second pattern aninverted thermocline was observed fromautumn to the onset of spring (April-October).The surface temperature (8.6-16 oC) down to 5to 8 (10) m deep was colder than water in themiddle of the column while below this depththe temperature increased again by 2 to 7 oC,reaching maximum differences in July (Fig.4D, 4E). Below 20 m in Lake Huillinco thetemperature was stable down to the bottom.

In all the lakes the transparency measuredwith the Secchi disk was very low (Fig. 5),with a minimum of 2.5 m in the freshwaterLake Tepuhueico (July, Fig. 5), and maximumin lakes Natri and Tarahuín with ca. 6.4 m. Inlakes Cucao and Huillinco the transparency waseven lower and ranged between 1.9 and 3.6 m(Fig. 5D, 5E).

The water in all the lakes was brown incolour indicating dissolved organic acid mattersuch as humic acid of vegetable origin. Thecolour of lakes Natri, Tepuhueico and Tarahuínfluctuated between 8.3 Unid Pt.-Co. (Tarahuín)to 170 Unid Pt.-Co. (Tepuhueico) (Table 3).Lakes Natri and Tarahuín showed similar

annual averages compared to Lake Tepuhueico,whereas lakes Cucao and Huillinco exhibitedslightly more densely coloured waters than thefreshwater lakes.

Chemical characterization

The pH values indicated acidity in all thelakes. The lowest annual average pH wasmeasured in Lake Tepuhueico (Table 3). Ingeneral, the range of pH in the freshwaterlakes remained below neutrality, while thevalues in lakes Cucao and Huillinco werenearer to neutrality. The highest conductivitieswere measured in lakes Cucao and Huillinco,resulting from the marine influence (Table 3).In both these lakes the conductivity showed astrong vertical gradient. In Lake Cucao theannual average conductivity at the surface was2.461 µScm-1; whereas at the bottom, thecorresponding value was 27.684 µScm-1. InLake Huil l inco the annual averageconductivity was higher, at 3.512 µScm-1 and40.658 µScm-1 at the surface and bottom,respectively. The salinity at the bottom oflakes Cucao and Huillinco had an annualaverage of 19 and 29 0/00, respectively.

In all the lakes there was a decrease in theoxygen concentration from the surface to thebottom. In lakes Natri and Tarahuín themaximum monthly average oxygenconcentration (11 mg L-1) was measuredduring September (Natri) and December(Tarahuín) . The minimum concentrat ionamong the freshwater lakes was measured inLake Tarahuín where, from March to May, theconcentration at the bottom was 0.2 mg L-1.Lakes Cucao and Huillinco showed largedifferences in their oxygen concentrationswithin a vertical profile: at the surface (0-3 m)values of 10-11 mg L-1 were found, whereasthe bottom was anoxic. An extreme conditionwas observed in Lake Huillinco, where therewas no oxygen below 18 m. In Lake Cucaobelow 14 m an annual average of only 1 mg L-

1 was measured.The nitrate concentrations were very high.

Among the freshwater lakes, Lake Natrishowed the highest concentration (Table 3) andthe lowest was found in Lake Tarahuín with aminimum in April of 65 µg L-1. In lakes Cucaoand Huillinco nitrate concentrations (Table 3)were extremely high with an exceptional 7.658µg L-1 in Lake Huillinco (June). There was avertical gradient throughout the year, with thehigher concentrations in the bottom of the lake.Nitrite concentrations were low in lakes Natriand Tarahuín and highest in Lake Cucao (Table


Fig

. 5:

Tra

nspa

renc

y m

easu

red

wit

h a

Sec

chi

disk

in

the

five

lak

es s

tudi

ed o

n C

hilo

é Is

land

: (A

) N

atri

, (B

) T

epuh

uei-

co, (

C)

Tar

ahuí

n, (

D)

Hui

llin

co y

(E

) C

ucao

.T

rans

pare

ncia

med

ida

c on

disc

o S

e cc h

i e n

los

cin

c o l

a gos

est

udia

dos

e n l

a Is

la d

e C

hilo

é : (

A)

Na t

ri,

(B)

Te p

uhue

ico,

(C

) T

a ra h

uín,

(D

) H

uill

inc o

y (E

) C

uca o

.


TA

BL

E 3

Sum

mar

y (a

nnua

l m

ean

and

rang

e) o

f th

e pr

inci

pal

phys

ical

and

che

mic

al c

hara

cter

isti

cs o

f fi

ve C

hilo

é la

kes

Res

umen

(m

edia

anu

al y

ran

go)

de l

as p

rinc

ipal

es c

arac

terí

stic

as f

ísic

as y

quí

mic

as d

e ci

nco

lago

s de

Chi

loé

Nat

riT

epuh

ueic

oT

arah

uin

Hui

llin

coC

ucao

Col

or (

Uni

d .P

t-C

o)41

.4 (

22.2

-5 8

.3)

131.

4 (7

2.5-

170.

0)40

.0 (

8.3-

61.7

)92

.8 (

49.2

-134

.2)

81.6

(55

.8-1

16.7

)

Tur

bidi

ty (

mg

L-1

)2.

8 (2

.5-2

.3)

5.3

(4.0

-6.5

)2.

8 (2

.5-3

.7)

4.3

(2.8

-5.3

)3.

9 (3

.2-5

.0)

pH6.

4 (6

.2-6

.7)

5.8

(5.6

-6.1

)6.

7 (6

.3-7

.0)

6.9

(6.6

-7.0

)6.

8 (6

.4-7

.2)

Con

duct

ivit

y (m

S c

m-1

)41

.4 (

37.3

-44)

30.2

(25

.9-3

3.5)

50.9

(44

.4-5

7.4)

27,3

26 (

18,6

50–3

7,40

0)15

6,85

8.0

(9,7

57-1

9,01

0)

Tot

al d

isso

lved

sol

ids

(mg

L-1

)43

.4 (

42.7

-44.

7)31

.6 (

30.0

-34.

0)52

.9 (

48.7

-52.

9)94

2.9

(700

.0–1

,087

)73

2.8

(400

.0–1

,332

.0)1

Sal

init

y (‰

)18

.9 (

17.6

-28.

0)10

.4 (

7.8-

12.0

)

Oxy

gen

(mg

L-1

)8.

5 (6

.7-1

1.7)

9.5

(8.1

-11.

2)8.

4 (5

.1-1

1.9)

3.5

(3.0

-4.2

)6.

0 (4

.7-7

.2)

NO

3-N

(µg

L-1

)12

7.8

(44.

7-18

2.3)

110.

4 (1

00.4

-128

.1)

34.9

(14

.6-8

1.5)

796.

3 (5

1.5–

2,03

8.8)

486.

3 (4

0.3-

961.

3)

Nit

rate

(N

O3,

µg

L-1

)56

5.9

(197

.8-8

08.7

)44

5.0

(247

.3-4

45.0

)15

4.4

(64.

6-36

1.3)

3,52

7.5

(228

.0–9

,031

.7)

2,15

4.2

(178

.6–4

,258

.3)

Nit

rite

(N

O2,

µg

L-1

)2.

1 (0

.1-4

.3)

6.3

(4.0

-9.2

)1.

9 (0

.7-3

.6)

8.0

(3.5

-31.

0)24

.8 (

4.8-

121.

1)

Am

mon

ium

(N

H4,

µg

L-1

)9.

3 (0

.7-3

7.4)

13.9

(n.

d.5.

6-27

.3)

9.3

(n.d

. 0.

7-46

.1)

1,47

0.0

(113

.8-2

782.

8)12

3.5

(10.

5-30

3.5)

Org

anic

nit

roge

n (µ

g L

-1)

165

(46.

2-39

9.1)

144.

0 (7

2.8-

198.

9)15

0.7

(21.

0-49

8.6)

5,13

8.5

(490

.2-9

453.

8)24

48.9

(78

.4–6

,652

.7)

Tot

a l n

itro

gen

(µg

L-1

)30

2.7

(142

.7-4

49.9

)26

0.3

(214

.1-2

60.3

)19

6.9

(71.

3-51

4.3)

8,95

1.8

(829

.0–2

0,44

5.1)

3,06

6.2

(189

.7–8

,343

.6)

Ort

hoph

osph

a te

(PO

4, µ

g L

-1)

5.7

(1.3

-9.2

)2.

8 (1

.8-4

.4)

3.4

(0.8

-10.

7)88

.3 (

45.6

-107

.7)

16.9

(8.

4-49

.9)

Tot

a l p

hosp

horu

s (µ

g L

-1)

23.6

(13

.0-5

1.6)

15.9

(7.

3-33

.7)

23.5

(9.

9-52

.4)

496.

9 (8

1.4–

2,60

7.4)

223

(41.

5-44

3.0

)

Org

a nic

se s

ton

(mg

L-1

)1.

1 (0

.6-1

.9)

1.5

(0.9

-2.2

)1.

7 (1

.0-4

.9)

2.2

(1.3

-1.7

)1.

8 (1

.0-3

.1)

Inor

gani

c se

ston

(m

g L

-1)

0.5

(0.1

-1.8

)0.

4 (0

.1-1

.3)

1.0

(0.3

-3.9

)5.

6 (4

.2-8

.7)

3.8

(1.5

-6.9

)

Ca l

c ium

(C

a , m

g L

-1)

1.7

(1.3

-3.1

)1.

4 (0

.8-2

.6)

3.3

(2.5

-4.8

)18

1.5

(91.

8-25

3.0)

111.

9 (6

7.4-

157.

3)

Mag

nesi

um (

Mg,

mg

L-1

)1.

2 (1

.0-1

.6)

0.9

(0.7

-1.7

)1.

6 (1

.4-1

.9)

562.

2 (1

0.1-

777.

6)30

1.5

(98.

1-48

9.0)

Sod

ium

(N

a , m

g L

-1)

4.7

(3.7

-6.8

)4.

4 (3

.1-5

.3)

4.8

(3.4

-6.3

)1,

386.

6 (5

7.7–

4,90

5.2)

1300

.0 (

111.

5–2,

186.

0)

Pot

a ssi

um (

K,

mg

L-1

)0.

7 (0

.3-1

.9)

0.6

(0.1

-1.8

)0.

9 (0

.5-1

.8)

211.

0 (1

50.3

-308

.3)

115.

7 (7

7.6-

183.

1)

Clo

ruro

(C

l, m

g L

-1)

8.8

(2.6

-13.

6)8

(2.3

-10.

3)8.

4 (2

.1-1

4.1)

11,4

73 (

1504

.0–1

5,43

2.0)

7,05

5.0

(1,1

78.4

–9,3

00.0

)

Sul

pha t

e (S

O4,

mg

L-1

)28

.0 (

20.1

-34.

5)62

.2 (

53.2

-72.

3)23

.7 (

7.7-

30.6

)19

,241

.1 (

6,73

9.5–

31,5

11.5

)12

,839

.0 (

7147

.0-3

8,01

0.5)

Bic

a rbo

nate

(H

CO

3, m

g L

-1)

12.9

(10

.2-1

6.9)

9.8

(6.9

-11.

4)22

.5 (

20.9

-24.

0)18

3.0

(150

.0-1

99.9

)72

.1 (

58.6

-84.

4)

Alk

a lin

ity

(mva

l L

-1)

0.2

(0.2

-0.3

)0.

2 (0

.1-0

.2)

0.4

(0.3

-0.4

)3.

0 (2

.5-3

.3)

1.2

(1.0

-1.4

)


3), with a maximum of 44.7 µg L-1 (January).Among the freshwater lakes, ammoniumconcentrations were similar and not very high(Table 3). Lakes Cucao and Huillinco showedhigh ammonium concentrations and differenceswere observed between the surface (33.0 and19.6 µg L-1) and bottom (279 and 2,798 µg L-

1), indicating a denitrification process. Amongthe freshwater lakes, the maximumconcentration of organic nitrogen was found inlake Natri with an annual average of 165 µg L-

1, and the lowest in Lake Tepuhueico (Table 3,Fig. 6). Lakes Cucao and Huillinco hadconcentrations corresponding to hyper-eutrophic lakes and, in general, the organicnitrogen constituted a considerable portion ofthe total nitrogen (Fig. 6).

The highest annual average oforthophosphate among the freshwater lakes wasobserved in Lake Natri, with 5.7 µg L-1 (Table 3,Fig. 7). Lake Cucao had moderate to highconcentrations of orthophosphate (Fig. 7), whilea high annual concentration was measured inLake Huillinco (Table 3). However, in thesurface waters of both lakes (0.1-3 m) very lowconcentrations of orthophosphate were measured(annual average Huillinco = of 3.1 µgL-1) and inLake Huillinco there was a strong verticalgradient, reaching an annual average of 164 µgL-1 at the bottom.

Among the studied freshwater lakes, totalphosphorus exceeded the limit of mesotrophy,reaching similar concentrations in lakes Natriand Tarahuín (Table 3, Fig. 7). On the otherhand, in Lake Cucao the total phosphorusexhibited extraordinarily high values even in thesurface water, with an annual average of 169mgL-1, as also in the deep water (297 µg L-1). Asimilar situation was observed in the surfacewater of lake Huillinco, but the concentrationswere still higher in the middle and bottom of thislake, reaching annual averages of 606.4 and731.0 µg L-1, respectively.

Their ionic compositions clearlydifferentiate lakes Natri , Tarahuín andTepuhueico from the other two lakes. They hadlow concentrations of cations (calcium,magnesium, sodium and potassium) and anions(bicarbonate and chloride) (Table 3).Nevertheless, high sulphate concentrationswere recorded in the lakes and in the rivers(Table 3). On the other hand, in lakes Huillincoand Cucao, very high concentrations of anionsand cations were observed.

The concentrations of suspended sediment(particles smaller of 70 µm), or seston, were as lowin the lakes as in the rivers and consisted mainly oforganic matter. The alkalinity was also low.

Characterization of the lakes inflows andoutflows

The main inflows and outflow of each lakewere also studied (L. Villalobos unpublishedresults). Those inflows were chosen whichcontributed a permanent water supply to thelake and some of the characteristics of theserivers are summarized here. In the basin ofLake Natri the largest drainage area was that ofriver number 2 (Miraflores), and for LakeTepuhueico the River Aguas Muertas standsout as having the largest drainage area andlength (Fig. 2). In the basins of lakes Tarahuínand Cucao the river drainages were of moderatesurface area while the largest drainage area wasrepresented by the River Bravo with 167.7 km2

and, its tributary the outflow from LakeTarahuín (River Tarahuín).

The inflows, in general, showed similarlimnological characteristics. In the majority,the minimum temperature (4.9 oC in the RiverTepuhueico) was recorded in August (winter)and the maximum (17. 5 oC) in January(summer) in the River Bravo. Most of the riverscontained strongly coloured water with valuesup to 80 Unit. Pt. Co. and pH with a tendencyto acidity (5.9-7.0). Their conductivities variedbetween 24.4 µS cm-1 (Notué, Lake Huillinco)and 464.0 µS cm-1 (Curahueldo, Lake Cucao).Most inflows showed high concentrations ofnitrate (> 200 µL-1), but the total nitrogen laywithin the limit of oligotrophy (200-400 µL-1).Orthophosphate too showed concentrationsbelow the limit of mesotrophy (< 15 µL-1),while the total phosphorus normally registeredhigher concentrations (16-27 µL-1). It must bepointed out that the major contribution ofnutrients to these lakes came from the riverswith the highest discharge. The outflowsshowed similar physical and chemicalcharacteristics to those observed for the surfacewater mass of the corresponding lakes.

Annual load of lakes

The annual loads of phosphorus and nitrogen ina lake result from the cumulated contributionsof all river inflows, plus localized and diffusesources in the drainage area (equation 1). Thesesources are summarised in Table 4.Considering the annual load entering throughthe inflows from the basin of the River Cucao,the contributions can be arranged in ahierarchy. Lakes Tarahuín and Tepuhueicodrain into Lake Huillinco, which finally drainsto Lake Cucao. It was evident that this is thereason for the high load of nutrients, especially


Fig

. 6:

Con

cent

rati

ons

of t

otal

and

org

anic

nit

roge

n m

easu

red

mon

thly

in

five

lak

es o

n C

hilo

é Is

land

: (A

) N

atri

, (B

)T

epuh

ueic

o, (

C)

Tar

ahuí

n, (

D)

Hui

llin

co y

(E

) C

ucao

.C

onc e

ntra

c ion

e s d

e ni

tróg

eno

orgá

nic o

y t

ota l

med

ida s

men

sua l

men

te e

n c i

nco

lago

s de

la

Isla

de

Chi

loé :

(A

) N

a tri

, (B

) T

e puh

ueic

o, (

C)

Ta r

a huí

n, (

D)

Hui

llin

c o y

(E

) C

uca o

.


Fig

. 7:

Con

cent

rati

ons

of t

otal

pho

spho

rus

and

orth

opho

spha

te m

easu

red

mon

thly

in

five

lak

es o

n C

hilo

é Is

land

:(A

) N

atri

, (B

) T

epuh

ueic

o, (

C)

Tar

ahuí

n, (

D)

Hui

llin

co y

(E

) C

ucao

.C

onc e

ntra

c ion

e s d

e or

tofo

sfa t

o y

fósf

oro

tota

l m

edid

a s m

ensu

a lm

ente

en

c inc

o la

gos

de l

a Is

la d

e C

hilo

é : (

A)

Na t

ri,

(B)

Te p

uhue

ico,

(C

)T

a ra h

uín,

(D

) H

uill

inc o

y (

E)

Cuc

a o.


phosphorus, entering Lake Cucao (Table 4). Inthe case of Lake Natri the small contributionsof phosphorus and nitrogen were explained byits independence from other river basins andthe direct relationship between the size of thedrainage area and the surface of the lake(relation Ad:Ao, Table 2). Interpretation of thenitrogen loadings are more difficult, given thecomplex cycles of this nutrient. The direct anddiffuse loads of residual waters were related tothe locations of human density and agriculturalactivities, which were particularly developed inthe drainage areas of the lakes Huillinco, Cucaoand Natri. Low loadings from all these sourceswere recorded for lakes Tarahuín andTepuhueico, which have the lowest populationdensities in their drainage areas. Thecontribution of nutrients from rain water was,

in general, in direct relationship to the surfacearea of each lake (Table 2 and 4). The load ofnutrients from fish farming centres dependedon the number of fish farms in each waterbody: extreme values were registered in lakesTepuhueico (lowest) and Natri (highest).Nevertheless, in terms of percentage, thesmallest contribution of nutrients from fishfarming was found in Lake Cucao (2.7 % ofphosphorus and 8 % of nitrogen), whereas, thehighest was measured in Lake Natri with 80 %of both phosphorus and nitrogen derived fromfish farming (Table 4).

Mass balance of lakes

The mass balance establishes the amount oftotal phosphorus and nitrogen persisting in a

TABLE 4

Load and mass balance of total phosphorus and nitrogen in five lakes of the Chiloé IslandCarga y balance de masa del fósforo y nitrógeno total en cinco lagos de la Isla de Chiloé

Load of lake LT LDS LA LG LS L

Phosphorus (mg m-2 y-1)

Natri 95 38 25 763 0 921

Tepuhueico 948 4.7 60 204 0 1,216

Tarahuín 202 14 52 590 0 858

Huillinco 1,603 34 65 571 1,976 4,249

Cucao 7,617 44 26 217 0 7,904

Nitrogen (mg m-2 y -1)

Natri 843 346 131 5,326 0 6,646

Tepuhueico 8,455 43 593 1,196 766 11,053

Tarahuín 1,442 127 132 3169 0 4,870

Cucao 13,906 402 486 1311 0 16,105

Huillinco 19,633 312 181 3,780 52,696 76,602

Mass balance

Phosphorus (Tons) Lint Lout Lext ± D TP

Natri -5.0 2.2 7.2 +0.02

Tepuhueico -8.6 8.8 17.5 +0.03

Tarahuín -4.9 1.7 6.6 +0.01

Huillinco 38 80 43 +1.2

Cucao -55 28 84 +0.52

Nitrogen (Tons) Lint Lout Lext ± D TP

Natri -32 19 52 +0.3

Tepuhueico +11 158 147 +0.6

Tarahuín -24 13 37.5 +0.13

Huillinco +1,006 1,447 457 +16.1

Cucao -748 735 1491 +7.5


lake according to the income and egress ofthese nutrients. Indirectly the mass balancemeasures the nutrients incorporated from thesediments into the water column, or those thatare integrated into the sediments from the waterbody (internal loading). The mass balance thusbecomes a useful tool with which to obtain anestimation of the magnitude of the internal loadand/or sedimentation of nutrients (phosphorusand nitrogen), within the water mass. Thepresence of internal loading occurs when apositive value is obtained in the mass balanceequation given by Vollenweider (1976,equation 2), whereas a negative value meansthat there are processes of nutrientsedimentation. However, for nitrogen, theinterpretation of flows is more complex due tothe gaseous phase within its cycle so theappearance of negative flows does notnecessarily mean sedimentation. For nitrogen itcan be due to the balance between nitrificationand denitrification.

According to the mass balance (Table 4) forLake Huillinco, internal loading correspondedto 38 tons of phosphorus and 1,006 tons ofnitrogen, which is evident since the mass ofnutrients in its outflow was higher than in itsinflows. The presence of internal loading inLake Huillinco was confirmed by the anoxicconditions in most of the water column. Incontrast, Lake Tepuhueico registered internalloading of 11 tons of nitrogen (Table 4); but, aswas mentioned previously, due to the lowoxygen concentration in the bottom of this lake,the internal load would be related to thebalance between denitrification andnitrification. In the others lakes, sedimentationof phosphorus and nitrogen was observed.

Biological characterization: Chlorophyll a

The concentrations of chlorophyll a in the fivelakes ranged between 0.56 µg L-1 (Huillinco,Julio) and 70.5 µg L-1 (Natri, March). In all thelakes the maximum was registered in the periodMarch-April. The lowest levels of chlorophylla were observed in lakes Huillinco and Cucao(annual average < 3.4 µg L-1). Lake Tepuhueicohad concentrations lower than 13 µg L-1, withsome lower than 1 µg L-1, specially at the endof winter and during spring. The concentrationsin Lake Tarahuín hardly varied, with amaximum of 14 µg L-1 (April). In contrast,Lake Natri showed two extraordinarily highconcentrations during May and April (autumn);but in the remaining period the concentrationsobserved were below 10 µg L-1.

Phytoplankton

The main characteristics of the phytoplanktoncommunity are summarized in Table 5. In all 5lakes, 87 species were determined most ofwhich were diatoms (Bacillariophyceae, 32species) or green algae (Chlorophyceae, 33species). Lakes Tepuhueico, Huillinco andCucao showed similarities in species richness,abundance and seasonal behavior withmaximum biomass and density during summer.These three lakes also presented a remarkablepredominance of the species Ankistrodesmusmirabilis (green algae) and Chroomonas sp.(Cryptophyceae). These lakes had low algaldensities and biomasses (Fig. 8 and 9),especially Cucao and Huillinco, in spite of theirhigh concentrations of nutrients (Fig. 6 and 7).The opposite situation was observed in lakesNatri and Tarahuín. Lake Natri had itsmaximum algal biomass and density duringspring, Lake Tarahuín during winter (Fig. 8 and9). In lakes Cucao and Huillinco there was astrong gradient in the vertical distribution ofthe species, related to the salinity gradientwhereas, in the fresh water lakes the verticaldistribution of the phytoplankton was morehomogenous. In lakes Cucao and Huillincospecies of diatoms characteristic of mixohalineenvironments were recorded. In lakes Tarahuínand Cucao the presence of Ceratiumhirundinella, a species apparently introduced toChilean freshwater systems through fishfarming activities, is noteworthy.

Zooplankton

The most important characteristics of thezooplankton communities are shown in Table 6.In the freshwater lakes, rotifers constituted thepredominant group in terms of density, Keratellacochlearis and its varieties (K. cochlearis var.teca and K. c. var. tropica and K. c. var.quadrata) being the most important species,followed by Polyarthra vulgaris (Fig. 10). Itmust be pointed out that Keratella cochlearisvarieties were recorded for the first time inSouth Chilean lakes. In Lake Tarahuín importantspecies such as Trichocerca similis andChonochilus unicornis were also found amongthe rotifers. Among the copepods, the cyclopoidTropocyclops prasinus was of importance,especially during summer. In general, speciessuch as Boeckella gracilipes and Mesocyclopslongisetus were present in all the lakes, but atlow densities. In Lake Tepuhueico the presenceof Diaptomus diabolicus was of importance.Among the cladocerans, the species Bosmina


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Fig. 8: Phytoplankton density in the five lakes on Chiloé Island: (A) Natri, (B) Tepuhueico, (C)Tarahuín, (D) Huillinco y (E) Cucao.Densidad del fitoplancton en cinco lagos de la Isla de Chiloé: (A) Natri, (B) Tepuhueico, (C) Tarahuín, (D) Huillinco y (E)Cucao.


Fig. 9: Phytoplankton biomass in the five lakes on Chiloé Island: (A) Natri, (B) Tepuhueico, (C)Tarahuín, (D) Huillinco y (E) Cucao.Biomasa del fitoplancton en cinco lagos de la Isla de Chiloé: (A) Natri, (B) Tepuhueico, (C) Tarahuín, (D) Huillinco y (E)Cucao.


chilensis appeared in all the lakes, reaching itsmaximal density during February in Lake Natri(Fig. 10). Considering the five lakes, thepresence in Lake Tarahuín of Daphnia pulexstands out in spite of its low densities.

In lakes Cucao and Huillinco species werefound which represent both the freshwater andmarine environments (Fig. 10). Lake Cucaowas found to have a high zooplankton density,while in Lake Huillinco density was low, butwith greater species richness. In Lake Cucaothe Rotifers (Fig. 10) predominated, registeringtheir maximum density in March with K.cochlearis, and its varieties, and Hexarthrafennica, the latter also being found duringFebruary. A second peak was observed inDecember with a predominance of Synchaetasp.. During winter, in general, the zooplanktondensity declined, but was maintained at a lowlevel by the marine plankton. Of the freshwaterspecies, D. diabolicus should be noted,especially in May, although at low densities.

In Lake Huillinco more freshwater specieswere found with one peak during March and asecond in February and October. During MarchK. cochlearis and its varieties predominated withFilinia longiseta, crustacean larvae and marineplankton. The copepod D. diabolicus appearedduring the spring and summer, reaching itsmaximum in March. In both lakes Tropocyclopsprasinus and M. longisetus were foundsporadically, the remaining crustaceans speciesbeing recorded at low densities. As in LakeCucao, during winter the marine planktonprevailed. Three taxa of marine origin wererepresented in the plankton of Lake Huillinco:larvae and adults of Poliqueta andAppendicularia. Their high densities were foundduring June, July and August, coincident with adecreasing freshwater plankton. In Lake Cucao,as at L. Huillinco, the same species of adult andlarval stages of marine plankton appeared, andoccasionally a species of Decapoda. Within theMarine Plankton the Poliqueta larvae appearedin greatest numbers (90 %).

In their vertical distributions, the freshwaterspecies were concentrated mainly between 10and 6 m deep, while the species of marineorigin located around 10 m, since the salinity atthis depth was higher than at the surface.

DISCUSSION

Five hydrographic river basins on Chiloé Islandwere studied for the first time. They containthree freshwater lakes (Natri, Tepuhueico andTarahuín) and two (Huillinco and Cucao) which

are strongly influenced by the sea. The lakes aremostly of recent origin, mainly associated withthe last glaciation, but lakes Huillinco andCucao have also been influenced by tectonicevents (present study and Laugenie, 1982).Lakes Tarahuín, Tepuhueico, Huillinco andCucao belong to the extensive hydrographicsystem of the River Cucao; whereas the basin ofLake Natri is isolated. Lake Huillinco has thelargest catchment area and, based on therelationship of Ad:Ao (Table 2), is the basin mostsusceptible to anthropogenic influences(deforestation, agriculture etc). Similarsituations have been discussed with regard to thelarge lakes located in South Chilean Patagonia(Soto & Campos 1996).

Their physical, chemical and biologicalcharacteristics suggest several differencesamong these lakes and based on their thermalprofiles we differentiated: (1) Lake Natri as atypical monomictic temperate lake; (2) LakeTarahuín which does not develop a completethermocline during summer; (3) LakeTepuhueico which occupies an intermediateposition and, (4) lakes Huillinco and Cucaowhich both have an inverted stratification, withthree differentiated water masses as a result ofthe marine influence, and can therefore bedescribed as meromictic lakes influenced bythe external supply of salt (Wetzel 1983). Thestrong fluctuations in the depth of thethermocline (5-25 m) in Lake Natri and theabsence of a thermocline in Lake Tarahuín areexplained by the shallow depth of the latter andthe strong winds in this area, which preventstable stratification. The winds are, however,not strong enough to destroy the markedchemoclines in lakes Huillinco and Cucao. Inaddition, all these lakes can be classified asdystrophic due to their brown coloured waters,which can be explained by the input ofallochthonous organic matter with a highcontent of humic substances. The browncolouration is associated with remarkably lowtransparencies which prevent higherchlorophyll concentrations in all the lakes.

In all five lakes total phosphorusconcentrations were high compared to those oforthophosphate (Fig. 7). Brown waters rich inorganic matter have been known to exhibithigher total phosphorus concentrations whenthey are compared with clear water lakes(Hutchinson 1957, Wetzel 1983, Jones 1992).There is empirical evidence relating the strongassociation of phosphate and iron withdissolved material of high molecular weight inwaters rich in humic substances (Shaw et al.2000). The ionic composition and pH of such


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Fig. 10: Zooplankton density in the five lakes on Chiloé Island: (A) Natri, (B) Tepuhueico, (C)Tarahuín, (D) Huillinco y (E) Cucao.Densidad del zooplankton en cinco lagos de la Isla de Chiloé: (A) Natri, (B) Tepuhueico, (C) Tarahuín, (D) Huillinco y (E)Cucao.


waters also influence the speciation of iron andphosphate (Shaw et al. 1996), making thechemical reactions more complex than thosepostulated for clear lakes. Thus, under certainconditions, humic substances can decrease thephosphate available to algae and in that way wecould, in part, explain the low orthophosphateconcentrations found in the lakes studied here.

In lakes Cucao and Huillinco the organicmatter from both internal and external sourcessettles towards the bottom, having originated bydecomposition in anoxic conditions. Due to theirvery strong chemoclines, the passage of oxygento the bottom of these lakes is very improbable,and the consequences of this are unpredictablebecause in Lake Huillinco a strong odour ofsulphuric acid was detected when we extractedsamples from the deep zones.

In lakes Huillinco and Cucao phytoplanktonbiomass and zooplankton density were very lowwhich could be explained, on the one hand, by theanoxic environment from 7-10 m down to thebottom of these lakes, making only a thin layer ofwater available for productive processes, and onthe other, by the constant marine influence onthese lakes which prevents the establishment of aplankton community under conditions of variablesalinity. Nevertheless, the highest diversity ofspecies was found in the plankton community ofLake Huillinco. In the zooplankton communitiesof all the lakes the richness of rotifer and copepodspecies stands out, but most of these speciesappeared sporadically and in low densities. Intheir vertical distribution, the species are, to alarge extent, distributed between 3 and 10 m, withthe rotifers and cladocerans predominating in thesuperficial water. The small sizes of theorganisms in the zooplankton communities ofthese lakes also deserve attention and couldindicate a displacement of the trophic chain asproposed by Brooks & Dodson (1965). Anotherinteresting point is the presence of severalvarieties of the rotifer K. cochlearis, whichappear to be associated with the strong winds andturbulence (Schmidt-Araya, personalcommunication). The presence of D. pulex(Tarahuín) in the zooplankton community is alsoworthy of remark due to the scarcity of Daphniaspecies recorded at this latitude, in both large anddeep North Patagonian lakes and lakes ofmoderate depth (> 50 m), the factors determiningits presence being unknown. Also remarkable isthe extension in its distribution to the South ofDiaptomus diabolicus (= Tumeodiaptomus d.).

With regard to the nutrients, the freshwaterlakes showed an annual average of around 300µg L-1 total nitrogen (Fig. 6), whereas in lakesCucao and Huillinco, the annual mean

exceeded 3,000 µg L-1. Hedin & Campos(1991) found low nutrient concentrations inChiloé stream waters suggesting highlyweathered, nutrient poor soils. Oyarzún et al.(1997) investigated the watershed of LakeHuillinco and found that the contributions oftotal nitrogen and phosphorus in the streams,especially nitrogen in organic form, wereexported mainly from native forests. In thelarge Lake Rupanco (Chilean North Patagonia)Oyarzún (2000) found nutrients to be near tothe level of mesotrophy, with high nitrogeninflow through the streams, due principally tochanges in land use (deforestation, agriculture),which accelerate the eutrophication process.Similar changes in the land use of river basinson Chiloé Island could have immeasurableconsequences, because the natural contributionof organic humic substances from the forestmaintains an equilibrium among the physico-chemical and biological compounds. At the timeof our investigation, a high percentage (70-80%) of mature native and regenerating forest stillcovers these hydrographic basins on Chiloé.

Phosphorus concentration has long beenrelated to the trophic status of a lake, especiallyas indicated by chlorophyll concentration(Vollenweider 1969, Dillon & Rigler 1974,Carlson 1977, Schindler 1978, among others).Both phosphorus and nitrogen have been putforward as limiting factors for phytoplanktongrowth (Schindler 1978, Burns 1991, Soto &Campos 1995, among others). Nutrient input toa lake depends on external contributions fromits drainage basin through rain water and landuse (agriculture, forestry, urban, industrial etc.)and, according to Ryding & Rast (1992), massbalances are the only reliable method forclarifying the origins of various nutrients withrespect to the nutrient flow dynamics, betweenthe sediments and the water mass.Eutrophication models have been used onlyrecently in South Chile (Campos et al. 2001),due to the fact that in recent decades a largenumber of lakes, including those on ChiloéIsland, have been used for fish farmingactivities such as smolt growth centres. It isassumed that these activities result in unnaturaladditional inputs of nutrients to the lakes. Withthe aim of controlling the actual loads of thelakes, and their mass balances, we applied themodels of Ryding & Rast (1992) andVollenweider (1976). As shown in the results,Lake Natri demonstrated the major impact offish farming activities and this situation hasbecome serious, since the renewal time of thewater in Lake Natri, the largest of all the lakesin this study, is in the order of 3.5 years (H.


Campos et al. unpublished results, calculatedafter Larsen & Mercier 1976).

The actual nutrient loads and mass balancesof lakes Huillinco and Cucao constitute quite acomplex system in relation to the dynamics oftheir water masses, due to the strong marineinfluence, especially during periods of hightides (Fig. 11). The two lakes are joined by theCaldera Channel, are of different depths andalso have water columns of different salinitiesand chemoclines of different strengths, andtherefore different nutrient cycles. Theimpossibility of complete circulation in thesewater bodies allows an accumulation ofphosphorus and nitrogen, among othercompounds, making even more complex thecalculation of actual loads and critical loads ofthese lakes.

To explain the high salinity found at thebottom of Lake Huillinco we hypothesize thatduring the 1960’s earthquake, a large wavecould have brought a considerable mass of saltwater into this system. Because Lake Huillincois deeper than both its outflow channel andLake Cucao, part of this salt water was retainedwithin the lake without possibili ty ofcirculation and evacuation. In addition, salt isprobably carried into the lake throughatmospheric transport.

Chile is characterised by its long latitudinalextent and diverse geography, encompassing avariety of lake systems, all of them differentfrom the Chiloé lakes. In northern Chile, highmountain lakes (Altiplano) dominate (Doradoret al. 2003), plus small saline ponds. Numerousmesotrophic or eutrophic reservoirscharacterise the freshwater bodies of centralChile, the largest being Lake Rapel (Vila et al.

Fig. 11: Model proposed for the dynamic processes of the water in the Lake Huillinco-Cucaosystem.Modelo propuesto para los procesos de la dinámica del agua en el sistema de los lagos Cucao-Huillinco.

2000). In this region there are also some deepoligotrophic, high Andean lakes (Geller 1992,Tartarottii et al. 1999). In the northern part(37o55’ S, 73o18’ W) of the southern lakedistrict are the lakes which 1 Parra et al (2001)called Nahuelbutanos, characterized by andtheir coastal or mountain influence anddifferences in their trophic status. The lakes ofChiloe Island differ from all of these lakesystems, principally because of their differentwater chemistry, origins, hydrological regimes,and the influence of their catchment areas.

CONCLUSIONS

Within southern Chile, the Chiloé lakes can becompared to those of north Patagonia (38-26o

S, 71-19o W, Thomasson 1963) both systemshaving glacial origins and the latter beingstrongly influenced by volcanic activity(Campos 1984). However, in terms of theirmorphometric characteristics, the Chiloé lakesare shallower and much smaller than those ofnorth Patagonia. Their temperature regimesvary through warm-monomictic to polymicticor meromictic, whereas the north Patagonianlakes are all warm monomictic (Campos 1984,Soto 2002). A unique feature of the Chiloélakes is the presence of humic substanceswhich presumably limits their productivitythrough high light attenuation and the reducedavailability of nutrients, due to the formation ofcomplexes with phosphorus. Finally,concerning the phyto- and zoo-planktoncommunities, the North Patagonian lakes showlower species diversity with a dominance ofdiatoms and calanoid species in comparison to


the humic Chiloé lakes (Soto & Zuñiga 1991,Villalobos 1994, this study).

To conclude, this investigation is one of thefirst multidisciplinary studies looking at thedrainage basin inflows, land uses andcharacteristics of the catchment areas,influencing the limnological characteristics,nutrient loadings and mass balances of thesefive lakes on Chiloé Island. The resultsconstitute a basis for future studies aimed atevaluating anthropogenic influences and theirconsequences for the streams and lacustrinesystems of the Island.

ACKNOWLEDGEMENTS

This study was financed by a Grant from theFondo de Investigación Pesquera, Project FIP96-54 and a grant from the Universidad Australde Chile. The investigation was initiated by thesenior author Hugo Campos, who died in 1998during a field trip. We dedicate this work to hismemory in recognition of his worthylimnological work in Chile. The authors thankDr. M. Burgis for helpful comments on themanuscript and correction of the English. Wealso thank and appreciate the comments of twoanonymous reviewers. L. Villalobos givesspecial thanks to César Cuevas for his helpfulcomments and assistance. We acknowledge thetechnical assistance of Gloria Agüero, RaúlArriagada and Rosa Cárcamo.

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Associate Editor: Vivian MontecinoReceived April 17, 2001; accepted June 27, 2003

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