TRACKING LONG-TERM CHANGES IN CLIMATE USING ALGAL INDICATORS IN LAKE SEDIMENTS

Interest in climate change research has taken on new relevance with the realization that human activities, such as the accelerated release of the so‐called greenhouse gases, may be altering the thermal properties of our atmosphere. Important social, economic, and scientific questions include the following. Is climate changing? If so, can these changes be related to human activities? Are episodes of extreme weather, such as droughts or hurricanes, increasing in frequency? Long‐term meteorological data, on broad spatial and temporal scales, are needed to answer these questions. Unfortunately, such data were never gathered; therefore, indirect proxy methods must be used to infer past climatic trends. A relatively untapped source of paleoclimate data is based on hindcasting past climatic trends using the environmental optima and tolerances of algae (especially diatoms) preserved in lake sediment profiles. Paleophycologists have used two approaches. Although still controversial, attempts have been made to directly infer climatic variables, such as temperature, from past algal assemblages. The main assumption with these types of analyses is that species composition is either directly related to temperature or that algal assemblages are related to some variable linearly related to temperature. The second more commonly used approach is to infer a limnological variable (e.g. water chemistry, lake ice cover, etc.) that is related to climate. Although paleolimnological approaches are broadly similar across climatic regions, the environmental gradients that paleophycologists track can be very different. For example, climatic inferences in polar regions have focused on past lake ice conditions, whereas in lakes near arctic treeline ecotones, paleophycologists have developed methods to infer past lakewater‐dissolved organic carbon, because this variable has been linked to the density of coniferous trees in a drainage basin. In closed‐basin lakes in arid and semiarid regions, past lakewater salinity, which can be robustly reconstructed from fossil algal assemblages, is closely tied to the balance of evaporation and precipitation (i.e. drought frequency). Some recent examples of paleophycolgical work include the documentation of striking environmental changes in high arctic environments in the 19th century believed to be related to climate warming. Meanwhile, diatom‐based reconstructions of salinity (e.g. the Great Plains of North America and Africa) have revealed prolonged periods of droughts over the last few millennia that have greatly exceeded those recorded during recent times. Marked climatic variability that is outside the range captured by the instrumental record has a strong bearing on sustainability of human societies. Only with a long‐term perspective can we understand natural climatic variability and the potential influences of human activities on climate and thereby increase our ability to understand future climate.     

Do warmer winters change variability patterns of physical and chemical lake conditions in Sweden?

In this study, the effect of a warmer winter climate on variability patterns of physical and chemical lake conditions was examined by using monthly air temperature data from 72 meteorological Swedish sites, ice breakup data from 77 Swedish lakes and monthly data of 17 water chemical variables from 11 nutrient-poor Swedish reference lakes during 1988–2005. The results showed significantly increasing variations of lake ice breakup dates and nitrate concentrations over Sweden along with increasing winter air temperatures. Variability patterns of other water chemical variables were not affected by warmer winters. Nitrate concentrations increased their variability in spring and early summer not only between lakes but also within lakes, which was attributed to a climate-induced increase in spring nitrate concentrations in particular in southern Sweden, while summer nitrate concentrations remained rather constant and low all over Sweden (median 10 μg l⁻¹). Since nitrate concentrations play an important role for primary production, highly varying concentrations will be a challenge for biota to adapt.     

Elemental variability in the coralline alga Lithophyllum yemenense as an archive of past climate in the Gulf of Aden (NW Indian Ocean)

This study presents the first algal thallus (skeleton) archive of Asian monsoon strength and Red Sea influence in the Gulf of Aden. Mg/Ca, Li/Ca, and Ba/Ca were measured in Lithophyllum yemenense from Balhaf (Gulf of Aden) using laser ablation inductively coupled plasma mass spectrometry, and Mg/Ca ratio oscillation was used to reconstruct the chronology (34 y). Oscillations of element rates corresponding to the algal growth between 1974 and 2008 were compared with recorded climate and oceanographic variability. During this period, sea surface temperatures (SST) in Balhaf recorded a warming trend of 0.55°C, corresponding to an increase in Mg and Li content in the algal thallus of 2.1 mol‐% and 1.87 μmol‐%, respectively. Lithophyllum yemenense recorded decadal SST variability by Li/Ca, and the influence of the Pacific El‐Niño Southern Oscillation on the NW Indian Ocean climate system by Ba/Ca. Additionally, algal Mg/Ca, Li/Ca, and Ba/Ca showed strong and significant correlations with All Indian Rainfall in the decadal range indicating that these proxies can be useful for tracking variability in the Indian monsoon system, possibly due to changes of the surface wind system, with deep water upwelling in summer, and a distinct seasonality.     

Late Holocene palaeoecology of Lago Verde: evidence of human impact and climate change in the northern limit of the neotropics during the late formative and classic periods

Multiproxy analysis (pollen, diatom, charcoal) on a 6 m core from Lago Verde (Sierra de Los Tuxtlas), shows evidences of environmental changes and human impact on the evergreen rainforest on the tropical lowlands of eastern Mexico during the last ca. 2,800 years. Prehistoric human occupation is recorded since the late Formative throughout the middle Classic (250 b.c.–ca. a.d. 800) by the presence of maize pollen, a low abundance of tropical arboreal taxa and a high abundance of herbaceous pollen and charcoal particles. This occurred under a scenario of very low lake levels from which dry conditions are inferred based on the dominance of aerophilous and periphytic diatom taxa. After ca. a.d. 800 the site was abandoned and forest regeneration started, at the same time higher lake levels, an indication of more humid conditions, were established. In the absence of human disturbance, tropical forest regeneration was rapid (ca. 200 years). Fluctuations in pollen composition during times of low human population at the site are related to climate variability, with the highest tropical forest diversity and lake levels recorded during the Little Ice Age. Modern human impact is also recorded and compared with the prehistoric deforestation event. Comparison with palaeoecological records from Yucatan and the highlands of central Mexico offers a Mesoamerican perspective of climatic variability giving evidence that the late Formative and early to middle Classic demographic expansion occurred under a scenario of climates dryer than present, with the Postclassic characterized by moister conditions. The end of the Classic (ca. a.d. 800–1000) is identified as a period of rapid climate change which marks one of the most important cultural transitions in Mesoamerica.     

Ostracod distribution in Ulungur Lake (Xinjiang,China) and a reassessed Holocene record

Ostracod shells in surface sediments from Ulungur Lake (Xinjiang, China) belong mainly to Limnocythere inopinata as the dominant species, and Candona neglecta and Darwinula stevensoni as accompanying, less abundant taxa. Shells of an additional nine species were recorded only sporadically. The three most abundant ostracods have wide tolerance ranges in terms of salinity, substrate and water depth. The similarly recorded bivalve Pisidium subtruncatum, and the gastropods Gyraulus chinensis and Radix auricularia belong to the most tolerant representatives of the genera. The bivalve and gastropods, in addition to the ostracod assemblage, reflect the fact that Ulungur Lake has experienced strong lake level and salinity variations due to water withdrawal in the catchment and the counteracting diversion of river waters to the lake in recent decades. The substrate in Ulungur Lake is typically fine-grained, apart from the delta region of the Ulungur River channel, which is marked by relatively coarse-grained detrital sediments barren of ostracod shells. This channel was created 40 years ago to divert water to Ulungur Lake and support its local fisheries and recreational facilities. A reassessed Holocene ostracod record from the lake shows that a significantly higher salinity and lower lake level existed in the early Holocene before 6.0 ka in response to the regional climate. In contrast, a higher lake level and lowest salinity is inferred for the late Holocene period between ca. 3.6 and 1.3 ka before present. Afterwards, the lake level declined and salinity increased in response to regional moisture reduction, although conditions similar to the early Holocene lake status were not re-established. Our surface-sediment-derived data provide a baseline for analysis of future environmental variations due to global climate change and regional water management.     

Impact of chronic and pulse dilution disturbances on metabolism and trophic structure in a saline Mediterranean stream

Non-diatom benthic algae from 104 streams in southern California were studied. We present a novel method for quantification of non-diatom algae that seeks to improve upon two important aspects of existing methods: separate processing of macroalgae and microalgae to avoid sample blending and consequent loss of macroalgal integrity, and for better viewing, counting a well-mixed microalgal subsample on a standard microscope slide instead of using a counting chamber. Our method provided high-quality taxonomic and quantitative data with low uncertainty. A total of 260 algal taxa were recorded, 180 of which were identified to species level. The median total algal biovolume per site was 22.7 mm³ cm⁻² (range: <0.001–836.9 mm³ cm⁻²), the median species number was 11 (range: 2–43). Total algal biovolume and species number correlated with canopy cover (negative) and water temperature (positive), but not with measured water chemistry constituents. The proportion of heterocystous cyanobacteria and Zygnemataceae were strongly negatively correlated with nitrate concentrations and TN. The proportion of red algae was negatively correlated with TP. Species optima calculations combined with indicator species analysis identified >40 algal species as potential indicators of nutrient conditions. Proposed here is a practical tool for non-diatom algal quantification that enhances its application to stream bioassessment.     

Plankton composition and water chemistry in the mixing zone of the Selenga River with Lake Baikal

Chemical and biological components of the Selenga River waters, the largest tributary of Lake Baikal, differ significantly from the lake waters. Active transformation processes of river waters into the lake ones occur in the vast barrier-like zone in the river-sea boundary areas. This study presents results on the spatial distribution and dynamics of water chemistry as well as the quantity and diversity of phyto- and bacterioplankton at a distance of 14 km off the Selenga River mouth. The most representative tracers of river and lake waters are total amount of ions and sulphates. Principal changes of chemical and biological parameters were fixed at 1–3 km off the Selenga River mouth that was determined as a mixing zone. Intense development of phytoplankton and eukaryotic picoplankton causing the decrease of nitrate and phosphate concentrations and organic matter rise were registered in this area. Gradual replacement of river phytoplankton by the lake one, abundance reduction of microorganisms and organotrophic bacteria and percentage increase of oligotrophic and psychrotolerant bacteria occurred in the mixing zone. Replacement of PC-rich picocyanobacteria by PE-rich ones was also recorded here. At a distance of 5–7 km off the shore, nutrient concentration and plankton composition were similar to those of Lake Baikal.     

Ostracod-based environmental reconstruction over the last 8,400 years of Nam Co Lake on the Tibetan plateau

From a 332-cm long lacustrine core taken at 60 m water depth and 25 surface sediment samples taken at different water depth sites in Nam Co lake (4,718 m a.s.l.) in the middle-south part of the Tibetan Plateau, we identified nine species of ostracods (Crustacea: Ostracoda) belonging to six genera. Using lithological data, auto-ecological information of the recovered taxa and an ostracod-based transfer function for water depth reconstruction, we distinguished three main environmental stages over the past 8,400 years: during Stage I (8,400–6,800 yr BP), the climate changed from warm-humid to cold-humid, and eventually to cold-arid. The water depth of the site was much lower than today and changed from an estimated 50 to 20–30 m. During Stage II (6,800–2,900 yr BP), environmental conditions were again warm-humid, turning into a cold-arid episode. The lake water depth initially stayed much shallower than today, but then gradually deepened to around 50 m. At the earlier period of Stage III (2,900 yr BP–present), the climate became again warm-humid from cold-arid status. There was a cold-dry event between 1,700 and 1,500 yr BP, which intensified afterward while the surface run off weakened. Early in this stage, lake depth decreased slightly, but then it continuously deepened to 60 m. Our results revealed that central Tibet experienced wavily warm toward tendency in early Holocene, a shift from warm-humid to cold-dry conditions in the middle Holocene, and from warm-humid to cold-dry conditions in the late Holocene. They also show that ostracod assemblages are not only indicative of cold-warm conditions, but are also usable to imply the dry–wet status of a lake area by the inferring water depth variations. Finally, this study provides baseline data on (natural) climate change in this mountain region against which to compare global change impacts.     

Precipitation patterns, dissolved organic matter and changes in the plankton assemblage in Lake Escondido (Patagonia, Argentina)

Global warming affects the hydrological cycle by increasing the frequency and intensity of extreme rainfall events and dry spells. These changes potentially affect the quantity and quality of dissolved organic matter (DOM) input into lakes. In this study, we investigated if changes in precipitation over a 3-year period correspond to changes in DOM and whether these changes affect light attenuation and plankton community composition. We sampled Lake Escondido, a shallow, oligotrophic Andean lake, nine times, analyzing coloured DOM and plankton community composition. During the study period, we observed that variations in the precipitation regime correlated with DOM parameters (water colour and molecular weight), and this, in turn, affected the plankton composition. Chlorophyll a concentrations of both phytoplanktonic fractions (less than and greater than 2 μm) were related to water colour and TDP. We observed in the small fraction (<2 μm) an increase in phycocyanin-rich cells during periods of high water colour. Larger phytoplanktonic cells (>2 μm) presented two biomass peaks corresponding to increases of the cyanophyte Chroococcus planctonicus and of the haptophyte Chrysochromulina parva. As precipitation decreased, the lake became more transparent, favouring C. planctonicus and mixotrophic oligotrich ciliates with endosymbiotic Chlorella. In the context of global climate change, our results highlight the potential impact of changes in precipitation patterns and, consequently, in DOM quality on the plankton community.     

Dynamic of phytoplankton size-class and photosynthetic activity in a tropical hypereutrophic lake: the Yaounde municipal lake (Cameroon)

This study carried out within the framework of a multi-disciplinary project, aimed at highlighting the fundamental processes governing the functioning of the hypertrophic Yaoundé Municipal Lake. It was based on the hypothesis that, even within a small range of lake trophic status, important variations can occur in the species composition, biomass and photosynthetic activity of the phytoplankton size-fractions. For this purpose, samples were taken at weekly intervals from November 1996 to December 1997 at a fixed set of depths in the water column. Spatial-temporal fluctuations of some physical–chemical variables, associated with biological variables such as phytoplankton size-class species composition, phytoplankton size-class biomass, phytoplankton primary production and chlorophyll-a were analysed. The water transparency was low and rarely exceeded 100 cm. Conductivity values relatively higher increased generally from the top to the lake bottom. The oxygen deficiency, and sometimes anoxia, recorded from 2.5 m depth resulted in high quantities of ammonium nitrogen. Total phosphorus and total Kjeldahl nitrogen concentrations were characteristics of hypertrophic lakes. The fertility of this biotope favoured the development of a high phytoplanktonic community with remarkable physiological adaptations to the variations of the nutritive potentials of the lake, characterized by the size-structure of these organisms. Cells of small size (<12 μm) contributed up to 11.42% of the total phytoplanktonic biomass. Species with average size (12–45 μm), dominated by Chlamydomonas spp., represented a more significant contribution reaching up to 69.85%, whereas the cells of big size (>45 μm), mainly Euglenophyta, maintained the relatively most important biomass, accounting for up to 89.85% of the total phytoplanktonic biomass. Chlorophyll-a concentrations are among the highest reported for both fresh water and sea water, being a consequence of high proportions of Chlorophyta and Euglenophyta. This led to intense phytoplanktonic photosynthetic activity which continued throughout the year, even though it was confined to the upmost first meter of the water column. Analyses pointed out the allogenic nature of the functioning of this urban lake ecosystem, due to a poor waste management on the surrounding landscape. Handling editor: J. Padisak     

Microbiology and geochemistry of great boiling and mud hot springs in the United States Great Basin

A coordinated study of water chemistry, sediment mineralogy, and sediment microbial community was conducted on four >73°C springs in the northwestern Great Basin. Despite generally similar chemistry and mineralogy, springs with short residence time (~5–20 min) were rich in reduced chemistry, whereas springs with long residence time (>1 day) accumulated oxygen and oxidized nitrogen species. The presence of oxygen suggested that aerobic metabolisms prevail in the water and surface sediment. However, Gibbs free energy calculations using empirical chemistry data suggested that several inorganic electron donors were similarly favorable. Analysis of 298 bacterial 16S rDNAs identified 36 species-level phylotypes, 14 of which failed to affiliate with cultivated phyla. Highly represented phylotypes included Thermus, Thermotoga, a member of candidate phylum OP1, and two deeply branching Chloroflexi. The 276 archaeal 16S rDNAs represented 28 phylotypes, most of which were Crenarchaeota unrelated to the Thermoprotei. The most abundant archaeal phylotype was closely related to “Candidatus Nitrosocaldus yellowstonii”, suggesting a role for ammonia oxidation in primary production; however, few other phylotypes could be linked with energy calculations because phylotypes were either related to chemoorganotrophs or were unrelated to known organisms. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Developing biomonitoring protocols for shallow Arctic lakes using diatoms and artificial substrate samplers

Growing concerns over effects of climate warming and other stressors on shallow Arctic lakes and ponds stimulate the need to develop and implement effective protocols to track changes in ecological integrity. This study assesses seasonal and spatial variability of periphytic diatom communities in a shallow Arctic lake in northern Yukon Territory to establish biomonitoring protocols. Artificial substrate samplers, which mimic macrophytes, allow direct measurement of biotic responses to shifting environmental conditions and control for possible confounding factors (e.g., accrual time and microhabitat type). Artificial substrate samplers were deployed at three locations and retrieved at three times (early, mid, and late) during the ice-free season. Analyses identified that diatom abundance increased exponentially and community composition changed significantly over the ice-free season, despite little variability in water chemistry, but did not differ among the three sampling locations within the lake. Patterns of seasonal succession in diatom community composition were characterized by first arrival of well-dispersed taxa, which included several planktonic taxa, followed by a transitional phase composed of planktonic and periphytic taxa, and culminated with dominance by periphytic species, mainly Achnanthes minutissima (Kützing). Results highlight the role of seasonal succession on artificial substrate colonization and the need to deploy artificial substrate samplers for the duration of the ice-free season to capture peak periphytic algal abundance. Low spatial variability of shallow Arctic lakes allows for samplers to be deployed at one single location to characterize diatom community composition.     

Climate drivers of diatom distribution in shallow subarctic lakes

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Primary production of epipsammic algal communities in Lake Balaton (Hungary)

Benthic algal communities can play an important role in matter and energy flux of shallow lakes. Their contribution to total primary production of lakes has been largely unexplored. The aim of this study was to estimate the primary production of the epipsammic algal communities at different water depths in Lake Balaton (Hungary) with photosynthetic measurements performed in laboratory. The photosynthesis of the benthic algae of different origin was studied at nine different irradiance levels, in three replicates. The maximum photosynthetic rate (P _max) was always higher in samples from the shallow parts than those from the deeper regions of the lake. Along the west–east longitudinal axis of the lake P _max decreased in the southern part and increased in the middle of the lake as a consequence of differences in the chlorophyll-a concentrations. Knowing P _max, I _k, global radiation and extinction coefficient, the primary production (mg C m⁻² day⁻¹) of the epipsammic algal community was calculated at different water depths. In the shallow regions at 0.5 and 1 m water depth 75–95% and 60–85% of the production was attributable to the epipsammon. The percentage contribution of epipsammon was at 2 m water depth 20–65%. In the deeper pelagic region (>3 m) more than 85% of the primary production originated from the phytoplankton.     

Sustainability assessment and comparison of efficacy of four P-inactivation agents for managing internal phosphorus loads in lakes: sediment incubations

A novel application of a continuous flow incubation system (CFIS) was used to assess four phosphorus (P) inactivation agents—alum, Phoslock™, a new modified zeolite (Z2G1 or Aqual-P™), and allophone—when used as sediment capping agents to manage internal P loads in lakes. The CFIS technique allowed combined efficacy and sustainability assessment, including: (1) flux measurements during simulation of stratified (anoxic) and mixed (aerobic) conditions on the same sediment through multiple cycles to assess the longevity of a range of product doses; (2) simulation of a summer algal bloom collapse and subsequent burial of the products; and (3) investigation of non-target effects on nitrification and denitrification processes at the sediment–water interface. Minimum P-removal dose rates were found to differ substantially at 80 g m⁻² for alum, 190 g m⁻² for Z2G1, 220 g m⁻² for allophane and 280 g m⁻² for Phoslock™, for similar capping layer thickness of about 2 mm, and would be effective for at least 4 years. All products temporarily suppressed nitrification and denitrification under aerobic conditions, and it may be important to minimise product application to any permanently aerobic zones, such as the littoral areas of a lake. While the aluminium (Al)-based products did not enhance Al fluxes in the CFIS, lanthanum (La) was released at a near constant rate of around 2 mg La m⁻² day⁻¹ from the Phoslock™ treatments over a period of at least 14 days. Spatial variability of sediment P, bioturbation, and burial are factors that will affect up-scaling these results to a whole lake.     

Geochemical evidence for the formation of a large miocene “travertine” mound at a sublacustrine spring in a soda lake (Wallerstein castle rock, nördlinger ries, Germany)

Summary “Travertines” (tufa pinnacles) of the Miocene Riescrater basin have been investigated to test whether carbon, oxygen and strontium isotopes can be used for the recognition of fossil subaquatic spring deposits in high-alkalinity settings. The Ries basin “travertines” have so far been interpreted as a product of subaerial to sublacustrine artesian springs discharging calcareous groundwater into a freshwater or slightly saline lake. However, recent studies on microfacies and fabric development propose a formation at Ca²⁺-supplying sublacustrine springs of a soda lake. Geochemical analysis of “travertines” of the castle rock Wallerstein, including “sickle-cell” limestones, thrombolites, non-skeletal stromatolites, and speleothems, now support the latter interpretation. High Sr contents surpassing that of the contemporaneously formed dolomitic algal biocherms of the lake shore point to an aragonitic composition of primary precipitates. the δ¹³C values of diagenetically moderately to weakly altered “travertine” facies types are in the same range of the impact-brecciated Upper Jurassic limestones, thus, are inconsistent with a mixture of soil-derived CO₂ and CO₃ ²⁻ from the Jurassic limestones. In addition, the δ¹⁸O values are too high to support a significant contribution of CO₃ ²⁻ by meteoric waters seeping through marine Jurassic limestones. Instead the δ¹³C and δ¹⁸O values indicate an origin of the CO₃ ²⁻ from a lake water body characterized by evaporation. This is consistent with a sodium-rich lake water as indicated by high sodium contents of aragonitic algal bioherms of the lake shore. The⁸⁷Sr/⁸⁶Sr isotope ratio of the “travertine” mound carbonates are consistent with calculated mixing of spring waters discharging from the crystalline basement and lake water high in dissolved inorganic carbon. This points to an origin of the divalent cations from sublacustrine spring waters. In turn,⁸⁷Sr/⁸⁶Sr isotope ratios of green algal reef carbonates of the lake shore are closer to that of the Upper Jurassic carbonates, due to surface run-off from surrounding limestone uplands.     

Spatial and seasonal variations in benthic algal assemblages in streams in monsoonal Hong Kong

Samples from stone surfaces were collected in pools within four unpolluted hillstreams (two shaded and two unshaded) in monsoonal Hong Kong (lat. 23°N) to elucidate the extent of spatial (within and among streams) and temporal (seasonal) variations in algal biomass and assemblage composition. Sampling continued for over 12 months, incorporating the dry season when streams were at baseflow, and the wet season when spates were frequent. We anticipated that algal biomass would be lower in shaded streams and during the wet season, with associated seasonal differences in assemblage composition or relative abundance of different growth forms (e.g. erect versus prostrate). Benthic chlorophyll a (a proxy for algal biomass) varied among streams from an annual mean of 11.0–22.3 mg m⁻². Dry-season standing stocks were 18% higher than during the wet season when spate-induced disturbance reduced algal standing stocks. Algal biomass varied significantly at the stream scale, but not at the pool scale, and was lower in unshaded streams, where standing stocks may have been limited by high densities of algivorous balitorid loaches (mainly Pseudogastromyzon myersi). An overriding effect of grazers on algal biomass could also have reduced variations resulting from spate-induced disturbance. Significant differences in assemblage composition among streams, which were dominated by diatoms and cyanobacteria (totally 82 taxa) were not systematically related to shading conditions. Seasonal variations in algal assemblages were statistically significant but rather minor, and did not involve major shifts in composition or growth form caused by spate-induced disturbance. The abundance of filamentous cyanobacteria in all the streams may have been due to ‘gardening’ by balitorid loaches that removed erect or stalked diatoms and favoured cyanobacteria that persist through basal regeneration of filaments. This explanation requires validation through manipulative experiments. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Luis Mauricio Bini     

Combining monitoring,models and palaeolimnology to assess ecosystem response to environmental change at monthly to millennial timescales: the stability of Blue Lake,North Stradbroke Island,Australia

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Investigation of bathymetry and water quality of Lake Nam Co, the largest lake on the central Tibetan Plateau, China

Comprehensive field investigations have been conducted four times on Nam Co, central Tibet, from September 2005 to September 2008. Here, we present the preliminary results focusing on the bathymetric survey and water quality measurements. The isobathic map shows that Nam Co is a high-altitude, deep lake where a flat and large basin lies in the central part with a water depth of more than 90 m. Water depth data from the northwestern bank areas of Nam Co provide unquestionable evidence of rising water levels in the last 3 decades because of the formation of two small islands that were still peninsulas in the 1970s. Water quality measurements taken at 19 stations during three summer field campaigns (2006, 2007 and 2008) covering almost all of the lake areas showed that the temperature, pH, dissolved oxygen and electric conductivity of surface water are on average 11.43°C, 9.21, 8.90 mg l⁻¹ and 1,851 μS cm⁻¹, respectively. The surface water shows no obvious spatial variability among all the stations. Vertical fluctuations of profiles, however, display some differences in thermocline and related parameters, such as pH and dissolved oxygen. According to the vertical variations of water quality parameters, the water column in relatively deep lake areas of Nam Co could be divided into three layers with distinctly various features: the epilimnion is from the surface to about 18–20 m depth in which the parameters are homogeneous with higher temperature and abundant sunlight; the metalimnion ranged from 20–60 m where a thermocline develops; the deepest layer forms a cold and dark hypolimnion.