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Sarah J. Davies, Sarah E. Metcalfe, Fernando Bernal-Brooks, Arturo Chaco n-Torres,John G. Farmer, A. B. MacKenzie and Anthony J. Newton

Lake Sediments Record Sensitivityof Two Hydrologically Closed Upland Lakesin Mexico to Human Impact

We provide evidence of rapid, recent environmentalchange in two lakes in the highlands of central Mexico.Multiple sediment cores were obtained from Lago deZirahuen (Michoacan) and Laguna de Juanacatlan (Jalis-co). Analysis of diatom assemblages, magnetic suscepti-bility, and metal concentrations was carried out, with thechronology provided by 210Pb dating, 14C dating, andtephrochronology. There is evidence of catchment distur-bance during the colonial period in both basins, but themost striking feature at both sites is the rapid change indiatom assemblages during the last 20 y, indicating the

onset of eutrophication. Limnological data from Lago deZirahuen support this interpretation, although none areavailable from Laguna de Juanacatlan. Paleolimnology isa powerful tool in tracking recent change, particularly in theabsence of regular limnological monitoring programs.These lakes appear to be highly sensitive to changes incatchment exploitation, which must be considered in futuredrainage basin management.

INTRODUCTION

The volcanic highlands of Central Mexico, or Trans-MexicanVolcanic Belt (TMVB), contain numerous lacustrine basins,many of which are hydrologically closed (Fig. 1). These have pro-

vided foci forhumansettlement forseveral thousand years and, asa result, many are experiencing considerable environmentaldegradation (1). Lake ecosystems are affected by increased soilerosion, nutrient enrichment, heavy metal pollution, andexcessive groundwater abstraction (2–4). In addition, a trend

toward drier climatic conditions since the 1970s has contributedto the lowering of some lake levels (4–6). Pioneering limnologicalwork by De Buen on the lakes of Michoaca ´ n during the early1940s (7, 8) provides a reference point for comparing results fromrecent investigations (9, 10), but such opportunities to examinelong-term trends in water quality in Mexico are very rare.Effective management of water resources requires a longertemporal perspective to ascertain the impacts of anthropogenicand climatic stressors and an alternative approach is required.

The application of paleolimnology as a tool for examiningrecent environmental change has become increasingly importantin Europe and North America (11). Diatoms are particularly

sensitive to changes in acidity and nutrient status. Diatom-basednumerical reconstructions have provided insights into surfacewater acidification (12) and cultural eutrophication (13). Amultiproxy approach is usually adopted, rather than relying onone line of evidence. For example, mineral magnetic propertiescan identify periods of enhanced soil erosion (14), whereasprofiles of heavy metals in lake sediments have been used toexamine the history of pollution from mining and smeltingactivity (15). The importance of paleolimnology in lake manage-ment is underlined by its inclusion in proposed methods fordetermining predisturbance reference conditions under theEuropean Union Water Framework Directive (16).

This study is part of a wider investigation into Late Holoceneclimate variability and human-environment interactions in

central Mexico. Here, we focus on the historical sediment record(ca. 500 y) from two lakes: Lago de Zirahue ´ n (Michoaca ´ n) andLaguna de Juanacatla ´ n (Jalisco) (Figs. 1 and 2). Of particularsignifcance is themost recent sedimentary record covering the lastc. 20 years. Diatom analysis combined with magnetic suscepti-

Figure 1. Map of central Mexico illustrating major closed basins in the TMVB and location of the two study sites.

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bility and heavy metal profiles help to elucidate the impacts of human activity on these catchments since at least the arrival of Spanish settlers in 1521. We demonstrate the value of paleo-

limnological records in providing an early warning signal of environmental degradation in Mexican lakes and suggest howthis approach may be developed.

Study Area

Zirahue n Basin. Lago de Zirahue ´ n (198260N, 1018440W) lieswithin a lava-dammed basin (Figs. 1 and 2) covering an area of 261 km2. The lake (approximately 10.5 km2) lies in the western-most corner of the basin at 2075 m above sea level (a.s.l.) and isencircled by mountains reaching 3100 m a.s.l. Average annualtemperature is 15.78C and average annual precipitation is1182 mm yrÀ1. Most precipitation (95%) falls in summer, betweenJune and September. The natural vegetation of the basin is mixedpine and oak forest, although this is now largely restricted to

steeper slopes due to clearance for settlement and agriculture.Lago de Zirahue ´ n has a maximum depth of 40 m (mean depth¼20.5 m). the lake has a pH of 8.4 and an electrical conductivity of 119 lS cmÀ1 (17). It is classified oligo-mesotrophic, although the

estimated phosphorus loading to the lake of 0.34 g mÀ2 yÀ1 (1996)suggests that lake waters of this closed basin should be richer innutrients than has been measured (18). Reasons suggested for thisdiscrepancy are: the sinking of cold, nutrient-rich inflowing waterbelow the photic zone undergoing an apparent incorporation of chemical content to the lake bottom under aerobic conditions;negligible amounts of sulfur thus preventing phosphorus releasefrom sediment; and that nitrogen and phosphorus may be co-limiting, so eutrophication will only occur with increases in bothnutrients (19). Despite the apparent lack of evidence for eutro-

phication, secchi disk transparency decreased by 2–3 m between1987 and 1996 based on sampling data (9, 18).

The two principal settlements in the basin, Santa Clara andOpopeo, both with populations greater than 10 000, are locatedon the main inflow to the lake, the Rı ´o de la Palma. On thelakeshore, the town of Zirahue ´ n has a population of ;2000, andthere are numerous small settlements. Santa Clara was animportant center for copper smelting during the eighteenthcentury. Around this time, colonial settlers established haciendas(large, private agricultural enterprises) within the basin (20).Much land has now been cleared for agriculture (maize andwheat). In the last decade, a commercial fruit farm has beenestablished on the south shore of the lake. Zirahue ´ n is heavilypromoted as a tourist attraction and accommodations have been

built, including numerous private luxury homes. An exit from therecently constructed motorway to Uruapan was provided toencourage more visitors to the lake.

Juanacatla n Basin. Laguna de Juanacatla ´ n (20837 0N,1048440W), also a lava-dammed lake, is situated in a remote lo-cation in the Sierra de Mascota (Figs. 1 and 2). This small basin(approximately 10 km2) is oriented in a northwest to southeastdirection. The lake occupies an area of approximately 0.5 km2 atthe northwest end of the basin and lies at ;2000 m a.s.l. It issurrounded by steep slopes that rise to a maximum of 2300 ma.s.l. The closest meteorological station is at Mascota, some 800m lower and 12 km away. Average annual temperature there is21.88C, although temperatures at Juanacatla ´ n are probablylower. Average annual precipitation at Mascota is 1026 mm yÀ1

(21). The basin slopes are dominated by dense pine and oakforest. During the 1950s, an artificial exit channel was dug at thewestern end of the lake in order to provide irrigation water. Incontrast to the Zirahue ´ n Basin, there is no limnological moni-toring program. Preliminary bathymetric measurements indicatea maximum depth of 25 m, but a detailed survey has not beenundertaken. Data obtained in 1998 indicate that the lake hasa pH of 8.8 and electrical conductivity of 148 lS cmÀ1. Its ioniccomposition, like Zirahue ´ n, is calcium-magnesium bicarbonate(17). In autumn 1998 and 2003, the lake was thermally andchemically stratified.

The region was important for colonial gold and silver mining,with mines being exploited from the seventeenth century onward(22); these are no longer in operation. It was not possible to estab-

lish from historical records whether mining actually took placewithin the basin. The principal activity today is agriculture (maizeand wheat cultivation and cattle farming). At the time of coring,settlement was restricted to small farming communities lying closeto, but not within, the basin. Human impacts therefore appearedto be minimal. In September 1998, work began on a luxury touristresort on the lakeshore aimed at attracting visitors and boostingthe local economy. This development was completed in mid-2003.

MATERIALS AND METHODS

During March 1998, sediment cores (60 and 80 cm long) wereobtained from both basins using a mini-Kullenberg corer. Thesediment-water interface was captured. Cores were sealed,

returned to the UK, and kept in cold storage at 48C. They wereX-rayed, opened, and the visual stratigraphy described beforesampling. Cores were analyzed at 4-cm intervals, with finerresolution sampling where major changes were identified. Mass

Figure 2. Photographs of a) Lago de Zirahue n and b) Laguna deJuanacatla n.

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specific magnetic susceptibility was measured. Metal concen-trations were determined on HNO3/HCl digests of core sectionsusing a Unicam SP9–800 flame atomic absorption spectrometer(23). Concentrations of silver in Core JL/98 were determined byinductively coupled plasma optical emission spectrometry (ICP-OES) using a Thermoelemental TJA IRIS instrument. Diatomsamples were prepared using standard techniques (24) and atleast 400 valves were counted in each sample. Species identifica-tion was carried out with reference to several published floras.

Chronological control on cores from Lago de Zirahue ´ n is

providedby 210Pb dating, radiocarbon dating, and the identificationof tephralayers from eruptions of Volca ´ n Paricutı ´n (A.D. 1943) andVolca ´ n Jorullo (A.D. 1759–1764). This is discussed in detailelsewhere (25). Lead-210 dating and AMS radiocarbon dating werealso used on cores from Laguna de Juanacatla ´ n. Unfortunately, itwas not possible to obtain a reliable 210Pb chronology for this site(discussed in more detail below). Results from the 1998 coringprogram at Juanacatla ´ n are compared with those from a sediment-water interface core (Core 1) obtained by R. Byrne (University of California at Berkeley) in 1990, on which diatom analysis wasundertaken by S. Metcalfe. The chronology of Core 1 was estab-lished by combining 210Pb dates with laminae counting (Byrne,unpubl. data), a summary of which is presented here.

RESULTS AND INTERPRETATIONLago de Zirahue ´ n

The four cores obtained from Lago de Zirahue ´ n revealeda spatially and temporally coherent record (25). Here, a summaryof data from Core AV/98, taken in 12 m of water in the subbasinof Agua Verde, is presented, providing an overview of changes inthe basin during the last ca. 500 y (Fig. 3). The core consists of grayish-brown and dark brown organic mud (gyttja), except forthe uppermost 5 cm, which are oxidized reddish silt. This layeroccurs across all the cores obtained and is likely the result of recent erosion of red soils around the lakeshore. An estimated age

of ca. A.D. 1550 was assigned to Core AV/98, based on extrapo-lation of 210Pb ages and comparison with historical tephra layers(25). Magnetic susceptibility is low between 58 and 50 cm, butrises steadily above this point, corresponding to the eighteenthcentury onward. The occurrence of the tephra from VolcanJorullo helps to date a noticeable peak in the lead and copperprofiles in the core. Although not highly pronounced, as thecoring site lies at the farthest point from the surface inflow, thesepeaks indicate contamination from colonial smelting activity atSanta Clara. According to historical records, smelting operations

peaked in 1789, diminishing greatly by the early twentiethcentury (26), although some coppersmiths continue to operate.Following the late-eighteenth century peak, copper concentra-tions remained slightly higher than those observed in the lowerportion of the core, suggesting continued contamination.

Prior to the mid-eighteenth century, the diatom flora of Lagode Zirahue ´ n is dominated by small species of the genus Fragilaria,which are common in circum-neutral to slightly alkaline environ-ments in central Mexico (27). However, the planktonic Cyclotellastelligera, which is found throughout the core in small amounts,increases in abundance through the latter half of the eighteenthcentury and is common until the 1970s. Diploneis elliptica, aspecies preferring slightly alkaline water (28), which is found inlakes, bogs, and springs, follows the same pattern. The ecology of Diploneis ellipticais notwell documented, butCyclotella stelligerais known to respond to increased phosphorus concentrations (29)and has been observed in horizons with increased mineral soilerosion (30). This distinctive change in diatom species composi-tion coincides with the rise in magnetic susceptibility. Archivalevidence indicates a marked increase in settlement and agricultu-ral activity in the basin from the eighteenth century onward (20).Such activities, combined with deforestation to provide charcoalfor copper smelting, would have enhanced soil erosion and in-creasednutrient inputs into the lake. Fine resolution diatom anal-ysis has shown that the deposition of the Jorullo tephra did not

Figure 3. Core AV/98, Lago de Zirahue n, illustrating magnetic susceptibility, copper and lead profiles, and a summary of diatom speciescomposition.




have a significant impact on the diatom flora (31), so changes inthe assemblage observed in the core cannot be attributed to this.

Since the 1970s, the diatomflora haschanged completely, withCyclotella ocellata at relative abundances reaching ;80%. Alsofound in small amounts for the first time in the 500-y record isFragilaria crotonensis. It is difficult to establish the ecology of Cyclotella ocellata from published literature and it has not beenfound in other lakes in central Mexico. Fragilaria crotonensis,however, is regarded as an indicator of eutrophication (32, 33). Itappears that the diatom flora has responded dramatically and

rapidly over a period of less than 10 y to recent land-use changes,including tourist development and commercial agriculture. Theinterpretation of a trend toward eutrophication is consistent withlimnological observations. Although classified as oligo-mesotro-phic, the phosphorus and inorganic nitrogen loading increasedby 36% and 45% respectively, between 1987 and 1996, along witha reduction in secchi depth (18).

Laguna de Juanacatla ´ n

Core 1, retrieved in 1990 in c. 10 m of water, yielded a finelylaminated sequence of millimeter-scale, alternating pale pink claylayers with greenish-brown organic layers (Fig. 4). It waspresumed that these layers represent annual couplets, with claylayers representing wet season deposition of allochthonous

material, but further monitoring is required to confirm this.Several thicker pink clay bands were identified, the mostprominent occurring at a depth of 36–39 cm. These have beeninterpreted as representing flood events resulting from tropicalstorms (Byrne pers. comm.). A chronological framework wasestablished by Byrne (unpubl. data), indicating a basal age for thecore of ca. A.D. 1520 and an age of A.D. 1820 for the clay layer.

The diatom record is dominated by the planktonic Cyclotellastelligera throughout, forming up to 80% of the total (Fig. 4).Synedra acus and Synedra nana are present in significant numbersduring the early part of the record until the late 1700s (52 cm).Also found in the lower portion of the core are Nitzschia palaeceaand Nitzschia gracilis. From the late eighteenth century, Synedra

species are replaced by Aulacoseira granulata and its variety,angustissima. Small Fragilaria species also occur for the first timeand the epiphyte Cocconeis placentula shows a slight increase inabundance. Previous research has shown that Synedra specieshave high silica and low phosphorus requirements and are themost effective competitors in systems with a limited phosphorussupply (34). Aulacoseira granulata and its varieties, however,have higher phosphorus requirements than Synedra. It istherefore likely that increased phosphorus availability from thelate 1700s onward led to the change in the diatom assemblage.

Three further cores were obtained in 1998 and showeda coherent pattern of change. Results from Core JL/98 obtainedin 11 m of water are presented here (Fig. 5). The sequence waslaminated except for two sections in the upper 15 cm. It was notpossible to obtain a reliable 210Pb profile due to small sample size.Cesium-137 was present in the top 3 cm of core, but was notdetectable in individual centimeter slices. This suggests that thetop3 cm represent the period after the 1963 weapons testing peak.Radiocarbon dates were obtained from a distinctive clay layer,thought to be correlative with that found in Core 1, and from 70-cm depth, where the laminae composition changed toward moreorganic-rich layers. Calibrated age ranges of A.D. 1445–1636 andA.D. 1338–1495 (2r) were obtained for the clay layer and thecompositional boundary, respectively (Table 1). These dates do

not correspond to those estimated for Core 1, which may be theresultof unreliable radiocarbon dates dueto inwashof oldcarbonfrom the catchment, or errors in the Core 1 chronology. We can,however, agree that the sequence represents at least the last 500 y.

Magnetic susceptibility is low throughout the core, but risesfrom 55 cm upward. A peak at 36–37 cm corresponds to the pinkclay layer. This layer also shows increased levels of silver, lead,and zinc. Below this layer, silver concentrations are below the de-tection limit (0.5 mg kgÀ1) of the ICP-OES, a value of 12 mg kgÀ1

being recorded within the layer itself. Above the layer, levelsdecline but remain higher than in the lower portion of core. Themagnetic susceptibility and metal profiles indicate a significantcatchment disturbance event, the date of which remains unclear.

Figure 4. Summary diatom diagram for Juanacatla n Core 1, retrieved in 1990. Dates are derived from a combination of laminae countingand 210Pb dating (Byrne, unpub. data).




The 14C date from JL/98 suggests that this may be the earlycolonial period, whereas Core 1 indicates a later age of 1820. Theincrease in silver from this point suggests that the initial distur-bance may have been related to exploration for mineralresources. Unfortunately, historical records do not specify thelocations of mines, but the town of Navidad, less than 10 kmdistant, was an important seventeenth century mining center.This time scale falls within the range of the calibratedradiocarbon date for the layer.

The diatom record from JL/98 follows the same pattern of Core 1, with domination of Cyclotella stelligera throughout, anda switch from Synedra species to Aulacoseira granulata var.angustissima occurring slightly before the deposition of the claylayer. The species composition of the uppermost sediments, how-ever, is completely different. Cyclotella stelligera is almost com-pletely replaced by Fragilaria crotonensis, which accounts formore than 90% of the assemblage. As discussed earlier, thisspecies is considered to be a eutrophication indicator. This dra-matic change must have occurred some time between 1990 and1998, indicating a very rapid response. In 1998, the catchmentwas not inhabited, but cultivation occurred along the flanks of the inflowing stream. It is not known whether any significantchanges in agricultural practices have occurred since 1990.

DISCUSSION AND CONCLUSIONS

Results from these two lakes show remarkably similar patterns.Both basins have experienced changes as a result of anthropo-

genic disturbance during the colonial period. In the Zirahue ´ nBasin, accelerated soil erosion and nutrient enrichment from themid-eighteenth century can be linked to increased anthropogenicactivity recorded in numerous historical documents. Whereassimilar events seem to occur in the record from Laguna deJuanacatla ´ n, these cannot be conclusively attributed to humanactivity due to a lack of archival documents for the area. Metalprofiles from both basins indicate contamination from colonialmineral exploitation. In Zirahue ´ n, copper smelting during the

eighteenth century resulted in increased copper and lead levels incore sediments. The timing and extent of colonial smeltingoperations at the town of Santa Clara are well documented fromarchival sources. It is likely that increased levels of silver inJuanacatla ´ n Core JL/98 also result from colonial mineralexploitation, but due to problems with the core chronology andlack of supporting historical data, this cannot be confirmed.

Diatom assemblages show a clear trend toward nutrient en-richment during the colonial period. The most dramatic featurethough is the rapid, recent change in diatom flora, occurring inZirahue ´ n during the early 1980s and in Juanacatla ´ n between1990and 1998. In both cases, composition changes completely, withspecies disappearing that had been present throughout the lastseveral hundred years. The almost total dominance of Cyclotella

ocellata in Zirahue ´ nand Fragilaria crotonensisin Juanacatla ´ n hasled to a significant decrease in the diversity of the diatom flora inthese lakes. Theuppermost samplesfrom thetwo lakes mayrepre-sent individual blooms (which would account for their almost

Figure 5. Core JL/98, Laguna de Juanacatla n, illustrating magnetic susceptibility, selected heavy metals, and a summary of diatom speciescomposition.

Table 1. AMS radiocarbon dates from Cores JL/98 and JD/98.

Core Depth (cm) Laboratory Code 14C Age (61r)* d13CPDB (60.1) Calibrated Age Range (A.D.: 2r)

Rel. Area underprobability distribution

JL/98 36–37 AA-32323 365 6 45 À30.1 1445–15341538–1636

0.4870.513

JL/98 69–70 AA-32324 4856

50À

31.4 1317–13531388–14951501–15071601–1613

0.0820.9030.0050.011

* 14C dates were produced by the University of Arizona AMS facility and calibrated using CALIB 4.1 (35).




monospecific nature), but plankton sampled at Juanacatla ´ n inMarch and September 1998 consisted almost entirely of Fragi-laria crotonensis, indicating that this species is dominant in bothwet and dry seasons, whereas Cyclotella ocellata [previously re-corded as C. kutzingiana (36)] has formed the dominant com-ponent of Zirahue ´ n phytoplankton samples since at least 1987.

Limnological records do not exist for Laguna de Juanacatlan,but instrumental data from Lago de Zirahue ´ n show a gradualtrend toward eutrophication. Compared with many lakes inMexico, Zirahue ´ n is regarded as being in a pristine state (18); its

clear, blue waters showing none of the telltale signs of nutrientenrichment. Subtle changes in nutrient concentrations in thewater column are now being observed. Alone, these may not givecause for concern, but the lake sediment records that the aquaticbiota has shown a major response. As tourist developmentincreases in the basin, andthe untreated sewage from the towns of Santa Clara and Opopeo continues to flow, the lake’s bufferingcapacity against excess nutrient inputs is likely to be exceeded.

It would be helpful to quantify the extent of nutrient enrich-ment in both Juanacatla ´ n and Zirahue ´ n. In Europe and NorthAmerica, successful reconstructions of total phosphorus havebeen produced using diatom-based transfer functions (13, 37). Acalibration dataset for central Mexico has been developed (17),but focused on a salinity and alkalinity. A nutrient training set is

now being developed by E. Hill (University of Nottingham). Aquantitative approach will help to establish baseline conditionsfor Mexican water bodies, many of which are suffering consider-able anthropogenic disturbance and will help assess the magni-tude of recent eutrophication.

Ourpaleolimnological study hasshown that Lago de Zirahue ´ nand Laguna de Juanacatla ´ n have experienced increased distur-bance during the colonial period, as a result of mineral exploi-tation and settlement. Historical records can contributesignificantly to their interpretation. The diatom records highlightthe sensitivity of lakes in the volcanic highlands of Mexico toanthropogenic activity. The recent, rapid response of the aquaticbiota to environmental change is cause for concern and mighthave gone undetected. Our results demonstrate a need for more

widespread monitoring of natural water bodies in Mexico,combined with analysis of recent lake sediment records.Together, these will help to inform decisions on drainage basinand water resources management in Mexico.

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36. Bernal-Brooks, F.W. 1988. Limnology of Lake Zirahuen Relative to Human Impacts. MScThesis, University of Guelph, Guelph, Ontario, Canada.

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38. Acknowledgments: S.J.D. was funded by a PhD studentship from the Department of Geography, University of Edinburgh, UK. Fieldwork was funded by the Dudley StampMemorial Trust, National Geographic, and the Carnegie Trust for the Universities of Scotland. We extend our thanksto Dr.Roger Byrne(Universityof California at Berkeley)for introducing us to Juanacatla ´ n and for providing access to core material andunpublished data. We acknowledge logistical support from Dra. Margarita Caballero(UNAM, Mexico City) and postgraduate students at INIRENA, UMSNH.

39. First submitted 11 May 2004. Revised manuscript received 22 Oct. 2004. Accepted forpublication 1 Nov. 2004.

Sarah Davies is a lecturer in the Quaternary EnvironmentalChange research group at the Institute of Geography and EarthSciences, University of Wales, Aberystwyth. Her address:IGES, University of Wales, Aberystwyth, SY23 3DB, [email protected]

Sarah Metcalfe is professor of Earth and EnvironmentalDynamics in the School of Geography, University of Notting-ham, University Park, Nottingham, NG7 2RD, [email protected]

Fernando Bernal-Brooks is a researcher in the Instituto deInvestigaciones Sobre Recursos Naturales at the UniversidadMichoacana de San Nicolas de Hidalgo, Av. San Juanito s/n,Col. San Juanito Itzıcuaro, Morelia, Michoacan. CP 58330,Morelia, Mexico.

[email protected] Chacon is a researcher in the Instituto de Investiga-ciones Sobre Recursos Naturales at the Universidad Michoa-cana de San Nicolas de Hidalgo, Av. San Juanito s/n, Col. SanJuanito Itzıcuaro, Morelia, Michoacan. CP 58330, Morelia,[email protected]

John Farmer is professor of Environmental Geochemistry in theSchool of Geosciences, University of Edinburgh, Joseph BlackBuilding, West Mains Road, Edinburgh, EH9 3JJ, [email protected]

Gus MacKenzie is a reader at the Scottish UniversitiesEnvironmental Research Centre, Scottish Enterprise Technol-ogy Park, East Kilbride, G75 OQF, [email protected]

Anthony Newton is a research fellow in the School ofGeosciences, University of Edinburgh, Drummond Street,Edinburgh, EH8 9XP, [email protected]


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