Basic Limnological Survey of Twenty-One Northern and Central Mongolian Lakes
Kevin D. Robinson1, Michael F. Rosenmeier2 and Nergui Soninkhishig3
1Department of Geosciences, Paul C. Rizzo Associates, 500 Penn Center Blvd, Pittsburgh, PA 15235
2Department of Geology and Planetary Science, University of Pittsburgh, 4107 O’Hara St., Pittsburgh, PA 15260, USA
3Department of Botany, School of Biology and Biotechnology, National University of Mongolia, Ulaanbaatar 210646, Mongolia
This survey report presents basin morphology, water quality, and sedimentological data from twenty-one Mongolian lakes, and is meant to be used as a resource for future geological and biological investigations. The lakes are organized in three separate groups based on geographic location, and the survey results from each lake are described in detail. A short discussion of local and regional factors infl uencing solute concentrations and pH levels of the lakes’ waters is presented. The lakes present at latitudes lower than N 50°00’ (i.e. central and north/central lakes) are distributed across an area of approximately 150,000 km2 and vary considerably with respect to their water quality characteristics. Regional precipitation patterns as a function of geographic location and/or catchment specifi c processes are indentifi ed as the driving mechanisms for variations in solute concentrations and pH levels in these lakes. The surveyed lakes present at latitudes higher than N50°00’ (i.e. northern lakes) are distributed across a smaller area (approximately 180 km2) and have relatively little variation in water quality characteristics. These lakes straddle the local taiga/alpine tundra transition zone, and elevation (i.e. valley placement) is identifi ed as the driving mechanism for inter-lake variations in solute concentrations and pH levels.
Keyword: limnology,lake,water quality,survey,sediment cores
The diverse ecology/geology of Mongolian lakes has long been recognized (Berkey & Morris, 1927) and a history of limnological monitoring within Mongolia exists (Kondratiev, 1929; Dulmaa, 1979; Kozhov et al., 1965; Kuznetsov, 1968; Tserensodnom, 1971; Sevastyanov & Dorofeyk, 2005), yet a great number of Mongolian lakes remain unstudied. Of the 3,500 lakes distributed throughout Mongolia’s rugged and sparsely populated landscape, most have remained isolated from human activities. Such isolation has preserved a great number of natural, undisturbed lake systems, offering biologists and geologists the unique opportunity to study such systems in their pristine state. Here, we report observations on the physical and chemical properties of twenty-one northern and central Mongolian lake systems, the majority of which were previously unstudied (Table 1; Fig. 1). The simple objective of this study is to provide a basic understanding of the nature of each lake system. Furthermore, this paper aims to document the baseline conditions of each ecosystem in order to better assess any future alterations.
Material and Methods
Physiolimnological data were collected during June, July, and August of 2005 (Table 2). Lake names were assigned as labeled on existing topographical maps, or, when unlisted, informal local names were designated. Elevations and locations were recorded using a Garmin GPS 76 hand-held unit. Transparency measurements via Secchi disk were taken during the late morning to early afternoon hours, and were measured off the shaded side of the boat.
Water temperature, dissolved oxygen (DO), specifi c conductivity (SpC), and pH measurements were taken using a Hydrolab Quanta G water quality monitoring system at a 0.5 m interval until a depth of 10.0 m, after which a 1.0 m interval was deemed appropriate. Exceptions to this method occurred when clear metalimnetic characteristics were visible above or below 10.0 m and, thereby, the measurement routine was altered to best resolve stratifi cation. Water depth measurements were routinely recorded using a Garmin GPS 176 sonar unit. Lake water total dissolved solids (TDS) concentrations were calculated from average water SpC and temperature values (Fofonoff, 1985). Short sediment cores were collected with a percussion corer designed to retrieve an undisturbed sediment water interface. The uppermost, unconsolidated sediments of each core were extruded in the fi eld by upward extrusion into a sampling tray fi tted to the top of the core barrel. Deeper core sections were stored in polycarbonate tubes, and either transported intact to the University of Pittsburgh or to the National University of Mongolia for sampling, and ultimately stored in cold room facilities at 4°C. All sediment cores were split, photographed, and Munsell color, texture, and sedimentary structures noted. Diatom samples were counted on sediment samples from one northern Mongolian lake (Sanjin Nuur) using untreated smear slides due to the high abundance of unaltered diatoms present in the sediment (Table 3). A minimum of 300 diatom valves were identifi ed and cataloged using a compound microscope with oil immersion optics at 1000´ magnifi cation. Taxonomy of diatoms was based on Krammer & Lange-Bertalot (1986; 1991) and Flower et al. (1998).
Radiocarbon ages were determined on sediment samples from six lakes by accelerator mass spectrometry (AMS) at the University of California - Irvine Keck Carbon Cycle AMS Facility (UCI; Table 4). All samples consisted of aquatic plant macrofossils and were pretreated with standard acid-baseacid techniques (Abbott & Stafford 1996). Calibrated ages were determined with the CALIB REV4.4.2 Radiocarbon Calibration Program (Stuiver & Reimer 1993; Stuiver et al., 2005) and presented in cal yr B.P. (1950 = 0 cal yr B.P.).
All surveyed lakes were located between 46-52°N and 95-102°E (Fig. 1). For ease of discussion we group lakes by their latitudinal location. Central Mongolian lakes are considered those present between N46°00’ and N48°00’ (n = 3). North-central Mongolian lakes are those present between N48°00’ and N50°00’ (n = 12). Northern Mongolian lakes are those present at latitudes higher than N50°00’ (n = 6). Central Mongolian Lakes All central and north-central Mongolian surveyed lakes are of defl ation basin origin. They primarily reside in granodiorite bedrock and have large surface area/depth ratios. They are all located in steppe, forest steppe, or dessert-steppe ecosystems. When visible inputs or outputs were seen, the lakes were given an open basin status, although the large surface area of some lakes prohibited proper investigation of basin hydrology. The assignment of an open basin status to the central and north-central lakes is therefore largely arbitrary, and the assumption that lakes are not open systems should be taken with caution. Shireet Nuur (Table 1) is located within the Naiman Nuur protected area. The northeastern basin was explored for this study and yielded a maximum depth measurement of 6.0 m. The water temperature profi le spanned 14.0°C at the surface water interface to bottom water values of 9.1°C and lacked a clear thermocline (Table 2). DO values spanned 9.84 – 8.83 mg L-1 and similarly exhibit a generally homogeneous profi le. Water conditions were basic (pH 8.32 - 9.65), fresh (SpC 0.068 mS cm-1, TDS 44.1 mg L-1), and moderately clear (Zsecchi = 3.5 m). Sediment core SHR-A-25-VI-05 (Table 1) consists of homogenous clay with no visible stratigraphic horizons. Hцh Nuur (Table 1) is an open system consisting of a western and southern basin, of which only the southern basin was investigated due to inclement weather. The southern basin is entirely surrounded by raised grasslands, while the western basin is surrounded by steep-sided hills. The lake has a surface area of approximately 13.0 km2. Depths of up to 12.0 m were observed, but local residents claim the western basin is deeper with submerged trees present. Local residents also claim shell materials are often found along the shoreline, although no shell materials were observed during the time of sampling. The temperature profi le exhibited an isothermal profi le with values ranging from 10.2 – 9.4°C (Table 2). DO had its highest concentration (9.84 mg L-1) at the air-water interface. Water conditions were slightly basic (pH 8.14 - 8.90), fresh (SpC 0.059 mS cm-1, TDS 37.7 mg L-1) and extremely clear (Zsecchi = 8.0 m).
Sediment core HOH-A-27-VI-05 (Table 1) sediments consist of dark brown homogenous clay with no visible stratigraphic horizons. Tsagaan Nuur (Table 1) is an open system located within the Otgon Tenger Strictly Protected Area. A large central basin and a smaller western basin are present, with only the central basin being studied (Fig. 2a). The lake has a surface area of approximately 3.5 km2, and a maximum observed depth of 28.0 m. Water temperatures ranged from 14.5 – 4.2°C with a clear thermocline present at a depth of 6.0 m (Fig. 2a, Table 2). DO values ranged from 8.60 – 4.77 mg L-1 and exhibited a slight metalimnetic oxygen maximum, likely attributed to phytoplankton growth. The water was slightly basic (pH 8.54 – 7.87), fresh (SpC 0.22 mS cm- 1, TDS 158 mg L-1), and moderately clear (Zsecchi = 5.2 m). Sediment core TSA-A-3-VI-05 (Table 1) consists of organic rich clay with thick beds (>10 cm) to diffuse sub-millimeter laminations. Sediment core TSA-B-3-VI-05 (Table 1) consists of massive gravel material with no visible stratigraphic features. North-Central Mongolian Lakes Terkhiin Tsagaan Nuur (Table 1) is a relatively large open system with two east-west basins and a surface area of approximately 60 km2. A maximum depth of 14.0 m was found in the central area of the western basin. Water temperatures ranged from 13.6 – 9.6°C with a thermocline present from 7.0 – 9.0 m (Table 2). DO values ranged from 8.46 – 5.77 mg L-1 and exhibited a clinograde profi le. The water was basic (pH 8.33 – 8.01), fresh (SpC 0.0149 mS cm-1, TDS 96 mg L-1), and relatively clear (Zsecchi = 2.4 m). Sediment core TTN-A-29-VI-05 (Table 1) consists of bioturbated organic clay with no visible stratigraphic horizons. Khar Nuur (Table 1) is present in a desert/ steppe ecosystem surrounded by grasslands to the south and sand dune fi elds to the north (Fig. 3). Abundant shell material was present along the shoreline. A maximum depth of 12.8 m was observed in the central region of the lake. Water temperatures ranged from 13.9 – 9.7°C with a slight thermocline occurring from 4.0 - 6.0 m (Table 2). Dissolved oxygen values ranged from 8.96 – 7.81 mg L-1 with the highest values occurring in the bottom waters. The water was basic (pH 8.9 – 9.6), slightly brackish (SpC 0.545 mS cm-1, TDS 355 mg L-1), and clear (Zsecchi = 4.0 m). Sediment core HAR-A-04-VII-05 (Table 1) consists of organic rich clay with trace shell fragments and visible stratigraphic horizons. Thin beds (<10 cm) to laminations (<1 cm) are present in the sediment core, including thin (<10 cm) sand deposits (Fig. 4b). Baga Nuur (Table 1) is located just northwest of Khar Nuur with a sand dune fi eld separating the lakes (Fig. 3). A maximum depth of 15.0 m was observed in the central region of the lake. Water temperatures ranged from 16.6 – 8.7°C with a slight thermocline occurring from 6.0 – 8.0 m (Table 2; Fig. 2b). DO values ranged from 8.38 mg L-1 at the surface-water interface to 2.15 mg L-1 at bottom depths. The water was basic (pH 9.07 – 8.00) with the highest observed pH values occurring in the depths directly overlying the thermocline. The water was fresh (SpC 0.345 mS cm-1, TDS 218 mg L-1) and relatively turbid (Zsecchi = 1.3 m). Abundant macrophyte communities surrounded the shoreline to a water depth of 4.0 m. Sediment cores BAG-A-05- VII-05 and BAG-B-05-VII-05 (Table 1) consist of organic rich clay with shell fragments and sand horizons (Fig. 4a). Tsegeen Nuur (Table 1; Fig. 3) has a maximum observed depth of 8.5 m in the central region of the lake (Table 2). Water temperatures ranged 17.66°C to 1.71°C with a clear thermocline present at 5.0 - 6.5 m (Fig. 2c). Tsegeen Nuur water was mesosaline (SpC 12.8 mS cm-1, TDS 10,691 mg L-1) and basic (average pH 8.9). DO values showed a strong clinograde profi le with values ranging from 7.52- 1.71 mg L-1. Water clarity was low (Zsecchi = 0.5 m). Sediment core TGN-A-06-VII-05 (Table 1) consists of grey clay with thin-to-medium beds of highly organic black clay. Diffuse laminations are present in the organic horizons (Fig. 4d). Kholboo Nuur (Table 1) consists of two large basins connected by a small intermittent strait (Fig. 5). Only the northeastern basin was explored for this study. Maximum observed depth of 8.5 m occurred along the southwestern shore (Table 2). Water temperatures were relatively consistent (15.5 – 15.0°C), and DO displayed a homogeneous profi le (8.00 – 7.16 mg L-1). The water was basic (9.38 – 9.24 pH), oligosaline (SpC 2.67 mS cm-1, TDS 1726 mg L-1), and relatively turbid (Zsecchi = 1.3 m).
Sediment core HOL-A-01-VII-05 (Table 1) consists of clay with interspaced diffuse submillimeter dark and light brown laminations and limited bioturbation. Oigon Nuur (Table 1) is a large (surface area approximately 73.5 km2), shallow (Zmax = 3.5 m) (Table 2), mesosaline lake located northwest of Kholboo Nuur (Fig. 5). Due to extremely inclement weather, neither water quality data nor sediment cores were acquired. Bust Nuur (Table 1) is a relatively large lake northwest of Kholboo Nuur (Fig. 5). The lake is circular with a small island located in the center. Only the lake area northwest of the island was surveyed for this investigation. A maximum depth of 12.0 m was found (Table 2). Water temperatures ranged from 14.6°C to 5.5°C with a clear thermocline at 7.0 - 9.0 m depth. The DO values ranged from 9.1 – 3.8 mg L-1 and the DO profi le was consistent with the temperature profi le. The water was basic (pH 9.2) and oligosaline (SpC 3.19 mS cm-1, TDS 2388 mg L-1). Sediment core BST-A-30-VI-03 (Table 1) consists of organic-rich clay with visible stratigraphic horizons. Multiple sections of diffuse sub-millimeter laminations exist within the core with massive intervals of organics and clay (Fig. 4c). Zuun Nuur (Table 1) is relatively large lake (surface are approximately 17.5 km2) located southeast of Sangiin Dalai Nuur (Fig. 6). Only the western basin was investigated in this study. A maximum observed depth of 12.7 m occurred just east of the large southwestern peninsula (Table 2). Water temperatures were relatively consistent (14.0 - 13.4°C), and DO displayed a homogeneous profi le (6.18 – 5.00 mg L-1). The water was basic (pH 8.99 – 9.04), oligosaline (SpC 4.670 mS cm-1, TDS 3254 mg L-1), and turbid (Zsecchi = 1.0 m). Sediment core ZUN-A- 16-VIII-05 (Table 1) consists of grey fi ne clay with visible stratigraphic horizons.
Multiple sets of sub-millimeter laminations, silt layers, sand layers, and organic horizons are present. Gandan Nuur (Table 1) is located southwest of Sangiin Dalai Nuur (Fig. 6) and has maximum observed depth of 5.2 m located in the center region of the lake (Table 2). Abundant macrophytes were present throughout the lake. Water temperatures were consistent to a depth of 4.5 m (14.3°C) and DO displayed a homogeneous profi le (6.18 – 6.11 mg L-1). The water was slightly basic (pH 8.71) and fresh (SpC 0.398 mS cm-1, TDS 243 mg L-1). Secchi depth measurements were not taken and a sediment core was not retrieved due to inclement weather. Sangiin Dalai Nuur (Table 1) is the largest lake (surface area approximately 181.9 km2) in this study and is comprised of two large basins (Fig. 6). Only the southeastern basin was surveyed due to the extreme size of this lake. A maximum depth of 6.5 m was found 3.5 km offshore (N49°11.034, E099°01.990). All physical characteristics measured throughout the water column displayed a consistent profi le. The water was warm throughout (15.5°C), basic (pH 9.2), oligosaline (SpC 5.06 mS cm-1, TDS 3411 mg L-1), and DO values were high (5.06 mg L-1). No sediment cores were retrieved due to high wave action and inclement weather. Tunamal Nuur (Table 1; Fig. 6) has a maximum depth of 9.7 m observed in the central region of the lake (Table 2). Water temperatures were consistent throughout the water column (16.3 – 15.8°C) and DO displayed a homogeneous profi le (9.07 - 9.10 mg L-1). The water was basic (9.07 pH), oligosaline (SpC 5.65 mS cm-1, TDS 3724 mg L-1), and turbid (Zsecchi = 1.0 m). Sediment core TUN-A-14-VIII-05 (Table 1) consists of highly organic dark grey clay. Two distinct clay horizons make up the majority of the core with sub-millimeter diffuse laminations present in the basal sediments. Tsavdan Nuur (Table 1) is a hypersaline lake with a consistent depth of 1.0 m. A thick salt crust was present at the sediment water interface and active salt precipitation was occurring in the water column. Sediment core TVD-A-06-VII-05 (Table 1) was retrieved through a perforation in the salt crust and consists of course grained dark grey clay with dipping sand and gravel deposits. Northern Mongolian Lakes All northern lakes associated with this study are open systems located within the Baruun Taiga mountain complex west of the Darkhad Valley, Hцvsgцl Aimag (Fig. 7).
The lakes all straddle the local tree line and the taiga/ alpine tundra transition zone. Evidence for late Pleistocene glaciation in the watershed is provided by the presence of cirque and moraine complexes and surveyed lakes likely originate from pro-glacial processes. The region is characterized by continuous permafrost. Sanjin Nuur (Table 1) is a paternoster lake positioned at the headwall of the lower valley and drains northeastward (Fig. 7). Hydrologic inputs are limited to overfl ow from a small basin located to the south and snowmelt from the limited catchment (~1.0 km2). A maximum depth of 17.4 m was observed in the southern regions of the lake with an average depth of 3.1 m (Table 2; Fig. 8a). The lake has a surface area of approximately 0.09 km2. The lake was thermally stratifi ed with water temperatures varying from an air-water interface value of 11.9°C to a water-sediment interface value of 4.6°C. A clear thermocline was visible at 6.0 - 8.0 m. The DO profi le displayed a slight positive heterograde profi le with a metalimnetic oxygen maxima occurring at 6.5 m (7.99 mg L-1). The pH of the water was circumneutral (7.75 – 6.87 pH) with a maximum value occurring at the metalimnion. The lake water was extremely fresh (SpC 0.01 mS cm-1, TDS 9.00 mg L-1) and transparent (Zsecchi = 4.8 m). Four sediment cores (Table 1) ranging in length from 0.83 – 1.10 m consist of homogeneous diatomaceous clay with limited visible stratigraphic horizons. A basal age of approximately 2350 cal yr B.P. was determined on one Sanjin Nuur sediment core (SAN-A-6- VIII-05; Table 4). The sediments of Sanjin Nuur core SN-B-03 contain 54 diatom species (Table 3) of 25 genera. The dominant species present in the sediments is an unidentifi ed Cyclotella that is closely allied to C. rossii (Grunow) Hеkansson. Other species that are abundant (>5% in any one sample) include Pliocaenicus costatus var sibiricus, Fragilaria tenera, Achnanthidium minutissimum, Aulacoseira alpigena, Aulacoseira lirata, Aulacoseira ambigua, and Pseudostaurosira brevistriata. Mustei Nuur (Table 1) is located in a cirque formation above local tree-line (Fig 7). The only hydrologic input is surface runoff from its highly limited catchment.
A maximum depth of 28.0 m was observed along the western shoreline (Table 2; Fig. 8b). The lake was thermally stratifi ed with water temperatures varying from 14.0°C at the surface waters to a bottom water value of 5.1°. A clear thermocline was present at 6.0 - 8.0 m water depth. The lake is ultra-oligotrophic with an average specifi c conductivity value of 0.007 mS cm-1. The water was circumneutral (pH 7.08), extremely fresh (SpC 0.007 mS cm-1, TDS 7.00 mg L-1) and exceptionally transparent (Zsecchi = 11.2 m). Sediment core MUS-A-7-VIII-05 (Table 1) consists of diatomaceous clay with visible stratigraphic horizons and has a basal age of approximately 9370 cal yr B.P. (Table 4). Organics, silt and sand layers, bedding, and laminations are present throughout the sediment core. Ganbold Nuur (Table 1) is located at the headwall of a valley with hydrologic inputs from several small basins located in the surrounding cirque complexes (Fig. 7). A maximum depth of 21.9 m was observed in along the southern shoreline (Table 2; Fig. 8c). The lake was thermally stratifi ed with water temperatures varying from 12.7°C at the surface waters to 3.7°C at the bottom waters; the thermocline occurred at 1.5 - 3.0 m water depth. The dissolved oxygen values vary from a surface water value of 7.44 mg L–1 to a bottom water value of 3.32 mg L-1 and showed a positive heterograde profi le with a metalimnetic oxygen maxima occurring at 3.5 m (8.38 mg L-1). The water was circumneutral (pH 7.01), extremely fresh (SpC 0.027 mS cm-1, TDS 20.00 mg L-1), and highly transparent (Zsecchi = 5.6 m). Sediment cores GAN-A-10-VIII-05 and GANB- 10-VIII-05 (Table 1) consist of diatomaceous clay with thick bedding and thin organic and silt laminations (Fig. 4e). A basal age of approximately 1330 cal yr B.P. was determined in core GAN-B-10-VIII-05 (Table 4). Tsogtoo Nuur (Table 1) is located downvalley of Ganbold Nuur (Fig. 7). Its main hydrologic inputs are overfl ow from Ganbold Nuur and Batbold Nuur. A maximum depth of 4.0 m was observed in the central region of the lake (Table 2). Due to the shallow nature and the assumed homogeneity of the lake, only surface waters were measured for physical characteristics. Tsogtoo Nuur surface water had a temperature of 16.6°C, DO concentration of 6.3 mg L-1, a pH of 7.62, and was fresh (SpC 0.017 mS cm-1, TDS 15.0 mg L-1). Light penetrated throughout the entire water column (Zsecchi = Zmax). Sediment core TSO-A-9-VIII-05 (Table 1) consists of homogenous clay with no visible stratigraphic horizons and has a basal age of approximately 9650 cal yr B.P. (Table 4). Batbold Nuur (Table 1) is present above tree line in a cirque formation located directly west of Tsogtoo Nuur (Fig. 7). Its only hydrologic input is surface fl ow from its limited catchment. A maximum depth of 15.5 m was observed in the central regions of the lake (Fig. 8d). The lake is thermally stratifi ed with water temperatures varying from surface water values of 15.8°C to bottom water values of 5.3°C, and has a thermocline occurring from 5.0 - 7.0 m (Table 2). The DO profi le exhibits an orthograde profi le with maximum values occurring at 7.5 m. The water had pH values ranging from 8.4 – 6.6 with a maximum value at 5.0 m, and was fresh (SpC 0.017 mS cm-1, TDS 14.0 mg L-1). Sediment core BAT-A-8-VIII-05 (Table 1) consists of clay with diffuse massive transitions and has a basal age of approximately 8080 cal yr B.P. (Table 4). Mandakh Nuur (Table 1) is the lowest and largest (surface area approximately 1.4 km2) surveyed lake within the Baruun Taiga Mountains (Fig. 7). A maximum depth of 6.0 m was observed in the central region of the lake (Table 2). Water temperatures ranged from 18.4°C at surface waters to 10.4°C at bottom waters. Dissolved oxygen increased with depth from 6.16 – 7.11 mg L-1. The water pH had an average value of 7.0 and was fresh (SpC 0.019 mS cm-1, TDS 16 mg L-1). Sediment core MAN-A-8-VIII-05 consists of organic-rich homogeneous clay with no visible stratigraphic horizons and has a basal age of approximately 760 cal yr B.P. (Table 4).
In this study, the northern surveyed lakes are considered to be unique with respect to geological setting, physiolimnological and morphological characteristics, and origin, and are thereby discussed separately from the central and north-central lakes. The climate characteristics of the northern region are considered uniform across the northern lake systems as the area encompassing the northern surveyed lakes is relatively small (~180 km2). Climatic variation is therefore not discussed as a mechanism for interlake physical and physiolimnological variations in the northern surveyed lakes. This assumption, coupled with the smaller surface area of the northern lake systems, allows for a more concise discussion of the mechanisms driving any interlake variability.
The overall large surface area of the central and north-central surveyed lakes resulted in a lack of specifi c data for some lakes (i.e. maximum depth, surface area). This lack of information, coupled with the signifi cant spatial distribution of the lake systems, limits any discussion of inter-lake variability to general, large-scale infl uences (i.e. regional climate variations).
Factors Controlling Solute Concentrations in Mongolian Lake Water The northern lakes varied little in their physical and physiolimnological characteristics, although a general negative correlation between elevation (i.e. valley placement) and SpC values exists (Fig. 9a). Such a correlation is expected, as a lower valley placement amplifi es catchment size, increasing runoff and infl ow sources and leading to a higher concentration of chemical solutes. Because the northern lakes are all open systems, elevation should be the dominant infl uence on solute concentration. This inference is supported by the lack of correlation between surface area and SpC concentrations (Fig. 9b). Central and north-central surveyed lakes varied considerably in their surface area (1.45 to 181.9 km2), elevation (1667 to 2625 m.a.s.l.), latitude (N46°31 to N49°25), longitude (E095°39 to E101°50) and specifi c conductivity (0.059 to 12.8 mS cm-1). Based on TDS concentrations, the lakes can be characterized as fresh, oligosaline, or mesosaline. Similar to the northern lakes, elevation has a negative correlation to solute concentration (Fig. 10a) and no correlation to surface area (Fig. 10b). Precipitation rates rather than catchment placement are likely responsible for solute concentration in the central and north-central lakes. However, annual average precipitation totals for surrounding WMO weather stations (Table 5) show an increasing trend in precipitation with decreased elevation (Fig. 11a), suggesting that precipitation as a function of elevation does not play a major role in solute concentration. A pattern of increased specifi c conductivity with decreasing longitude is observed (Fig. 10c) and is in agreement with local precipitation patterns (Fig. 11b). Yet, specifi c conductivity concentrations increase with latitude (Fig. 10d), opposite to the precipitation trends (Fig. 11c).
Factors Controlling pH of Mongolian Lake Water All northern surveyed lakes had circumneutral waters.
Any variation in pH between such lake systems is likely to be a function of biological activity. A negative correlation between elevation and pH is found in these lake systems (Fig. 12a). These lakes straddle the local tree-line elevation. The catchments surrounding the upper lakes have thin and poorly developed soils with an exposed rocky substrate (metamorphosed granite) and sparse vegetation. The lower elevation lakes have soil development and vegetation within their broader catchment areas, likely resulting in an enhanced delivery of nutrients to the water column and, thereby, a high promotion of internal biological activity. The positive relation between pH and surface area (Fig. 12b) and the negative relation between pH and maximum depth (Fig. 12c) similarly suggest a biological component as the driving infl uence on water pH levels. The large shallow lakes provide a greater habitat for benthic species likely enhancing both biological diversity and production. The positive relation between pH and lake turbidity (Fig. 12d) support such inferences toward a biologically mediated mechanism. The pH levels of the central and northcentral lake systems are largely controlled by catchment specifi c processes, such as edaphic and/or hydrologic processes. If pH levels of such lake systems were determined by a climatic mechanism (i.e. precipitation), a positive relation between pH and elevation and longitude with a negative relation between pH and latitude would be expected, in accordance with the regional precipitation patterns (Fig. 11a-c). Actual fi ndings (Fig. 13a-c) are in opposition to all of the above assumptions, supporting the inference of catchment specifi c pH determinants.
This survey provides basic but valuable information for twenty-one lake systems throughout central and northern Mongolia. The surveyed infl ation depression lakes in central and north-central Mongolia exhibit a wide range of physical variation in their basin morphology, waters, and sediments, refl ecting the complex geological, edaphic, and hydrologic conditions of the survey area. Using the data collected in this study, the lakes can be classifi ed into three groups based upon chemical enrichment: fresh, oligosaline, and mesosaline. Evidence suggests catchment specifi c processes as determinates for inter-lake variability. The lakes located in the Baruun Taiga Mountains of northern Mongolia exhibit less physical variability, with any inter-lake variability likely attributed to valley placement (i.e. elevation). Further investigation into the chemical make-up of each lake system is required before any accurate characterization can be completed. Major ion concentrations should be measured to examine the internal nutrient and chemical dynamics of each individual system. Furthermore, +18O and +D ratios of water samples should be measured to properly assign each lake system an open or closed basin status. The lake sediment cores also stand to provide a wealth of information about the lakes’ ages, biological communities, chemistries, and the histories of regional climatology and ecology both within lakes and their surrounding watersheds. Therefore, with more intensive analysis of the materials collected as part of this survey, the basic nature of each lake system as presented by this investigation may be expanded to a complex and holistic study of the limnological and paleolimnological dynamics of each lake system, providing a more thorough understanding of the mechanisms determining inter-lake variability on a both spatial and temporal scales.
This work was funded, in part, by the Institute of International Education United States Fulbright Scholar Research Program. Special thanks to the Department of Botany, National University of Mongolia, for facilitating the research and to Dr. Michael Walther of the Department of Geography, National University of Mongolia for providing intellectual support and laboratory facilities. Enkhbaatar Demchig and the American Center for Mongolian Studies provided invaluable logistical support and assistance throughout this project. Other support was provided by Steve Saunders of the North American Mongolian Business Council and the late Dr. G. Alec Stewart of the University Honors College, University of Pittsburgh, to whom this paper is dedicated in honor of his unwavering support and promotion of Mongolian-American academic collaboration and scientifi c exploration.
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