An environmental record of changes in sedimentary organic matter from Lake Sattal in Kumaun Himalayas, India

S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 7 ( 2 0 0 9 ) 2 7 8 3 – 2 7 9 5

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An environmental record of changes in sedimentary organicmatter from Lake Sattal in Kumaun Himalayas, India

Preetam Choudharya, Joyanto Routhb,⁎, Govind J. Chakrapania

aDepartment of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee 247667, IndiabDepartment of Geology and Geochemistry, Stockholm University, 10691 Stockholm, Sweden

A R T I C L E D A T A

⁎ Corresponding author. Tel.: +46 80 674 7824E-mail address: [email protected] (J.

0048-9697/$ – see front matter © 2008 Elsevidoi:10.1016/j.scitotenv.2008.12.020

A B S T R A C T

Article history:Received 19 April 2008Received in revised form10 December 2008Accepted 11 December 2008Available online 20 January 2009

Sattal a small mountainous lake in the Kumaun Himalayas has been impacted by variouscultural activities in recent years.We explored the effects of human-induced changes in thislake by using various geochemical proxies. Shifts in TOC and N flux, C/N ratio, stableisotopes (δ13C and δ15N), n-alkane, and pigment concentrations in sediments indicate asteady increase in primary productivity over the last few decades. The trophic status of thelake has changed from mesotrophic to eutrophic condition. The C/N, CPI, and TAR basedratios in sediments indicate accumulation of algal matter derived primarily from in situproduction, with limited input of terrestrial organic matter from the watershed. The low(between 0.1 and 1‰) δ15N values imply N2-fixation by cyanobacteria, and the decrease inδ13C values up-core represent the effect of sewage input and land based runoff, or possiblecontribution from microbial biomass. The pigments change from non-N2 fixingcyanobacterial species to the N2-fixing community, and are consistent with the proxy-based productivity shifts inferred in the lake. The deeper sediments are affected by post-diagenetic changes causing an increase in δ13C (and possibly δ15N) due to mineralization oforganic C and N.

© 2008 Elsevier B.V. All rights reserved.

Keywords:ProductivityStable isotopesHydrocarbonsPigmentsCyanobacteria

1. Introduction

Organic matter (OM) in lake sediments consist of a complexmixture of lipids, carbohydrates, proteins and other organiccompounds. Terrigenous (allochthonous) OM in lakes origi-nates from the catchment and consists mainly of degradedleaves, grass and soil OM; additional contributions from moredistant sources are delivered by precipitation and wind. Onthe other hand, biota (animals, plants, and bacteria) within thewater column and sediments contribute towards the aquatic(autochthonous) OM. Useful information concerning thegeochemical characteristics of sedimentary OM deposited inlakes have been widely studied to interpret both contempor-ary and paleoenvironmental histories (Bernasconi et al., 1997;Meyers, 2003; Das et al., 2007; Routh et al., 2007, 2008). Forexample, elemental ratios and stable isotope composition of

.Routh).

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sedimentary OM in lakes help in distinguishing the differentOM sources, primary productivity, and prevalent nutrientstatus (Meyers, 1997; Hodell and Schelske, 1998; Routh et al.,2004). Similarly, specific biomarkers e.g. n-alkanes and pig-ments are unambiguous tools because of their low suscept-ibility to degradation, which make them useful to trace thedifferent OM sources, address diagenetic alteration, andpreservation issues in lake sediments (Hodgson et al., 1998;Meyers and Lallier-Verges, 1999; Routh et al., 2007, 2008; Daset al., 2008).

Lakes in mountainous terrain are considered an extremeenvironment because they are small and sensitive ecosystemswith rapid flushing rates (Vreca and Muri, 2006). Mountainouslakes respond quickly to any direct anthropogenic influence,or human-induced changes in the catchment. These effectsand changes are potentially recorded in the sediments, which

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2784 S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 7 ( 2 0 0 9 ) 2 7 8 3 – 2 7 9 5

can be used as means of reconstructing past conditions overdifferent timescales (Battarbee et al., 2002; Vreca and Muri,2006; Choudhary et al., in press).

Lake Sattal is an important group of small interconnectedwater bodies in the Kumaun Himalayas situated in Nainitaldistrict, Uttarakhand state (northwest India). The lake is animportant source of drinking water as well as a popular venuefor recreational activities. Sattal is ecologically fragile, and dueto rapid urbanization in its catchment, the lake is experiencingan increase innutrient loadingand rapiddeteriorationofwaterquality (Chakrapani, 2002; Das, 2005). Previous studies onSattal primarily focused on the regional tectonics and asso-ciated structural features (Valdiya, 1988), sediment accumula-tion (Das et al., 1994), and heavy metal pollution (Chakrapani,2002; Das, 2005). These studies suggest that the lake water isinfluenced by the surrounding lithology, and possibly fromvarious agricultural and anthropogenic activities in the catch-ment. However, these studies do not elucidate the actualsource(s) of OM in the lake, their effect on nutrient dynamics,or in productivity shifts affecting the lake's trophic status.

In the present study, OM extracted from the sediment coresfrom Sattal Lake was characterized to determine their domi-nant source(s) and fate. Secondly, variations in primaryproductivity in response to nutrient inputs were correlatedwith recent increases in various human-induced activities inthe catchment. To the best of our knowledge, the present studyis one of the few paleolimnological investigations in theHimalayan lake systems in India. Hence, useful generaliza-tions can be drawn based on the use of different geochemicalproxies to assess the environmental impact of human-inducedchanges in these fragilemountainous lakes. Further, the studyillustrates the potential of this type of proxy based analyses to

Fig. 1 –Bathymetric map of Lake Sattal showing the sample loca

reconstruct trophic changes in lakes in areaswhere hardly anywater column data are available. The information could beused for environmental assessment and suitablemanagementof similar lakes in this region and/ other areas.

2. Study area

Sattal lake is located near Bhimtal town in Nainital district(29°20′15″N latitude and 79°32′00″E longitude; Fig. 1). ‘Sat’means seven and ‘tal’ is lake in native language; Sattal is agroup of seven interconnected water bodies in the KumaunHimalayas. The lake is situated at an altitude of 1286 m abovemean sea level below the lush and fertile fruit orchards of theMehragaon Valley. Sattal is surrounded by dense forests setagainst the backdrop of oak and pine trees. Sattal is 1.3 km inlength, 0.2 km inwidth, andhasamaximumdepthof 20m.Thesurface area is 0.04 km2 and the catchment area is 4.4 km2; thewater volume is 2,598,500 m3 (Sharma et al., 1982; Pant andJoshi, 1987). The annual rainfall in the area is 1741 mm, whichis mostly caused by the northwest monsoons. Rainfall, localstreams, and springs are the primary sources of water into thelake. The main outflow from the lake is near the Gaula River(Fig. 1).

Sattal formed during the Holocene as a result of tectonicactivity and uplift of sediments in the Tibetan and Indo-Gangetic plains (Das, 2005). The catchment lithology ispredominantly composed of quartzite and phyllite (Pant andJoshi, 1987). Das et al. (1994) indicated that thewater chemistryand sedimentation in Sattal is mainly influenced by erosionalprocesses in the catchment in response to tectonic, climatic,and anthropogenic disturbances. Themain sources of external

tion (modified from Nainital Development Authority, India).

2785S C I E N C E O F T H E T O T A L E N V I R O N M E N T 4 0 7 ( 2 0 0 9 ) 2 7 8 3 – 2 7 9 5

input of nutrients into the lake are increase of anthropogenicand agricultural activities in the catchment, which result inlowdissolvedoxygen andhighnitrogenandphosphorus levels(Chakrapani, 2002; Das, 2005). The lake water is alkaline(pH 9.7) due to high photosynthesis, which makes it under-saturated in CO2 (Das, 2005). Pant and Joshi (1987) identified 54different phytoplankton species in Sattal.

3. Materials and methods

Sediment cores were collected during December 2004 from thedeepest part of the lake at 20 m. A gravity corer was used toobtain two relatively undisturbed sediment cores of 55 mmdiameter and 35–43 cm length (Fig. 1). The sampling pointswere located b1 m from each other. The cores consist ofgrayish black-colored fine-grained non-bioturbated sedi-ments. Little variation of grain-size occurred between thecores. Each core was sliced into 2 cm layers in the field and Ehwasmeasured immediately; the sampleswere packed into air-tight zip lock bags and refrigerated until further analysiscommenced. Because of the high water content, there was notenough sample material left after freeze-drying to measure allthe geochemical proxies in the same sediment interval.Hence, the longest core (SL 1, 43 cm) was used for establishingthe sediment chronology, whereas the other core (SL 2, 35 cm)was used for various geochemical analyses. However, certainmeasurements were done in both cores in order to correlatethe sediment intervals. These included measuring the bulk

Fig. 2 –Distribution of 210Pbex, total activity, age–depth relation anfrom Lake Sattal. The dates are calculated by the CRS model.

density followed by estimating the water content andporosity. In addition, loss-on-ignition (LOI) was measured inall samples as described in Heiri et al. (2001). Pre-weighed sub-samples were oven-dried at 105 °C for 8 h, reweighed and thenignited in a muffle furnace at 550 °C for 5 h. Samplereproducibility of duplicate runs was ca. ±3%.

The sediment chronology was established based on the210Pb method (Appleby and Oldfield 1978). The measurementof 210Pb was based on the α-measurement of 210Po, which wasassumed to be in secular equilibrium with its parent. Theprocedure involved adding 209Po (from NIST, USA; activity0.8815 Bq g−1) as a tracer, and leaching the sediment samplewith aqua regia (Kumar et al., 2007). The residual solid wasfiltered and treated with HCl. The Polonium nuclides (210Poand 209Po) were deposited on copper disks by adding ascorbicacid in HCl solution prior to alpha counting. The standardcounting error was b10% in the upper section of the core andslightly higher in the deeper sections. The supported activityfor 210Pb was estimated from the asymptote, which wassubtracted from the total 210Pb activity to get the unsupported210Pb activity i.e., excess 210Pb (210Pbex). In order to correct theeffect of sediment compaction, 210Pbex at each depth wasmultiplied by the dry density. The CRS (Constant Rate ofSupply) method of 210Pb dating was used to derive thesediment accumulation rate (Fig. 2). The model assumed aconstant 210Pb flux, but variable sedimentation rate (Applebyand Oldfield, 1978).

In SL 2 core, the sediments were used for characterizing theOM. The C and N isotopic compositions of acid-treated

d mass accumulation rate in a sediment core (SL 1) collected


samples were analyzed using a continuous flow systemconsisting of a Carlo Erba elemental analyzer coupled to aFinnigan MAT Delta Plus mass spectrometer. Data arereported in the conventional delta (δ) notation vs. ViennaPeeDee Belemnite (V-PDB) for C and atmospheric N2 for totalN. The precision for C and N isotope analyses were ±0.18‰and ±0.06‰, respectively. Reproducibility of duplicate ana-lyses was ±0.1‰.

The TOC content and sedimentation rate was used toestimate the paleoproductivity (PP) in sediments according toIshiwatari et al. (2005):

PP = kTOC � Dð Þ= 0:0030� S0:3� � ð1Þ

where D was the dry bulk density (mg cm−3) and S was thesedimentation rate (cm kyr−1). PPwas also calculated based onthe distribution of hydrocarbons (n-ΣC15,17,19) representing in-lake algal production.

Approximately 1–2 g of freeze-dried sediment wasextracted with a mixture of CH2Cl2 and MeOH (9:1 v/v) for thesolvent extractable hydrocarbons. A Dionex Automated Sol-vent Extractor 300 was used (programmed for three extractioncycles at 1000 psi and 100 °C). The extracts were reduced usinga Büchli Rotovapor and injected in pulsed splitless mode intoan Agilent 6890 gas chromatograph equipped with a CIS-4Gerstel inlet and DB5-MS column (30m×0.25mm i.d.×0.25 µmfilm); the oven temperature was held at 35 °C for 6 min,increased to 300 °C at 5 °Cmin−1 and held there for 20min. Thechromatograph was interfaced with an Agilent 5973 massspectrometer operated at 70 eV in full-scan mode (m/z 50–500 amu). External and internal standards (S-4066 fromCHIRON, Norway and deuterated perylene from CambridgeLaboratory, USA, respectively) were used for quantification.The hydrocarbons were used to calculate various hydrocarbonratios in order to characterize the sedimentary OM. Theseratios were calculated as indicated below:

1) Carbon preference index (CPI; Allan and Douglas, 1977)representing the predominance of odd over even n-alkanes:

CPI =P

C23 � C31ð Þodd +P

C25 � C33ð Þodd2P

C24 � C32ð Þevenð2Þ

2) Terrigenous aquatic ratio (TAR; Bourbonniere and Meyers,1996) quantifying the in situ algal vs. terrestrial OM:

TAR =C27 + C29 + C31ð ÞC15 + C17 + C19ð Þ ð3Þ

3) Paq ratio (Ficken et al., 2000) quantifying the different planttypes in the lake (e.g. submerged vs. emergent planttypes):

Paq =C23 + C25ð Þ

C23 + C25 + C29 + C31ð Þ ð4Þ

Pigments were extracted for 2 min by ultra-sonication inHPLC-grade acetone (2 ml g−1 sediment) and stored overnightat 3 °C (Bianchi et al., 2000, 2002). After filtration (with 0.02 µmfilters; Gelman GHP Acrodisc 13), the samples were injected

into an HPLC system, consisting of a Waters 2690 separationmodule coupled to a Waters 996 photodiode array UV/VISdetector (at 450 nm). The injector was connected via a guardcolumn to a RP-18 LiChroCART column (5 µm particle size,250mm×4.6mm internal diameter). The gradient (1mlmin−1)program started with 100% mobile phase A (80:20 methanol:0.5 M ammonium acetate) after injection (50 µl; Westmanet al., 2003). This was followed by 100% mobile phase B (90:10acetonitrile: water) for 4 min, and to 25% B and 75% C (100%ethyl acetate) for 28 min. The programwas changed to 100% Bfor 5 min with a final ramping to 100% A for 4 min. Thepigment standards were purchased from DHI, Denmark.Chlorophyll was analyzed colorimetrically on a Hitachi U-1100 spectrometer following the method of Dere et al. (1998;absorbance measured at 470, 645 and 662 nm). The concentra-tions were calculated according to Lichtentaler et al. (1985):

Ca = 11:75A662 � 2:350A645 ð5Þ

Cb = 18:61A645 � 3:960A662 ð6Þ

Total Carotene = 1000A470 � 2:270Ca � 81:4Cb=227 ð7Þ

where Ca is Chl a and Cb is Chl b. Typical precision of duplicateruns was ≤3%.

4. Results

The210Pbex activity curve in the core is shown in Fig. 2. Presenceof 210Pbex was noted until 35 cm. The 210Pb activity after 35 cmwas constant and hence, it was assumed to representsupported activity. 210Pbex declined from the maximum valueof 985 mBq g−1 near the surface to 62 mBq g−1 at 35 cm depth.Average sedimentation rate based on the CRS method wasestimated as 4.3 mm yr−1, and the mass accumulation rate ofsediments varied between 96 and 160 g m−2 yr−1. Based onthese values, thedeepest part of the corewasdated to be∼1922AD.

The TOC flux ranged between 2.7 g m−2 yr−1 to 12.7 g m−2

yr−1; TOCvaluesweremoreor less the same (~3.8 gm−2 yr−1) upto 27 cm, but above this depth, TOC increased gradually (Fig. 4).The N flux increased up-core from 0.3–1.2 g m−2 yr−1. Inaddition, most of the OM in sediments (N60%) was algalderived. The paleoproductivity estimates based on sedimenta-tion rate and TOC indicated a steady increase in PP up-core.The minimum PP estimate (320 g C m−2 yr−1) was observed at29 cm,whereas themaximumvalue (589 g Cm−2 yr−1) occurredat 3 cm (Fig. 4). Paleoproductivity calculated based on theamount of algal derived n-ΣC15,17,19 hydrocarbons (represent-ing autochthonous production of sedimentary OM) was 1.8–3.6 g Cm−2 yr−1; the low valueswere present towards the lowerhalf of the core.

Eh measured in the sediments were between −110 and−171 mV suggesting an increase of anoxic conditions up-core(Fig. 4). The C/N ratio decreased up-core from 14.6 to 11.5. δ13Cof OM in sediments ranged from −26.7 to −30.2‰; the valuesgradually decreased in young sediments (Fig. 4). The δ15N

Fig. 3 –LOI and porosity measurements in Lake Sattal (coresSL 1 and SL 2).


values fluctuated between 1.3 and 0.1‰. The highest (1.4‰)δ15N value occurred at 33 cm, whereas the lowest (0.1‰) valuewas recorded at 1 cm.

The biomarker concentrations in sediments were normal-ized with TOC to compensate for depositional variations, andto express the enrichment or depletion of these compoundsrelative to TOC. The n-alkane data showed noticeable varia-tions with depth (Fig. 5). The odd numbered short chained n-alkanes (C15, C17, and C19) indicated low concentration inbottom sediments. The minimum (0.16 µg mg−1 of TOC)hydrocarbon concentration occurred at 29 cm, whereas themaximum value (4.4 µg mg−1 of TOC) occurred between 1 and11 cm. CPI values ranged between 0.2 and 1.6; higher (1.6) CPIvalues occurred at 15–23 cm depth. The TAR values rangedbetween 1.4 and 2.7. The maximum (N2.2) TAR value occurredin the 19–23 cm interval, whereas the minimum (1.4) valueoccurred around 15–35 cm interval. Paq values varied between0.67 and 0.09, and indicated frequent variations with depth.

Specific pigments identified in Sattal were zeaxanthin,echinenone, myxoxanthophyll, β,β-carotene, peredinin, andlutein (Fig. 6). Zeaxanthinwas found throughout the core, withthe maximum concentration of 3.1 mmol g−1 of TOC at 25 cm;the average value was 2.0 mmol g−1 of TOC. Althoughechinenone indicated values between 1.7 and 4.8 mmol g−1

of TOC, the pigment was completely absent below 27 cm.Myxoxanthophyll had low (between 0.30 and 2.0 mmol g−1 ofTOC) concentration, andwas present only in few samples. β,β-carotene first appeared around 15 cm and the concentrationwas 2.2 mmol g−1 of TOC. The highest (28 mmol g−1 of TOC)concentration of β,β-carotene was around 11 cm. Lutein andperedinin occurred after 11 cm; their concentration ranged from0.41–3.6 mmol g−1 of TOC and 1.1–1.5 mmol g−1 of TOC,respectively. Chlorophyll (Chl) a ranged from 0.05–1.7 mmol g−1

of TOC, and Chl b ranged from 0.03–0.30 mmol g−1 of TOC.The value for zeaxanthin/β,β-carotene ratio was close to

zero up to 15 cm, but above this depth, the ratio varied between0.09 and 1.7. Likewise, the value for Chl a/β,β-carotene wasnegligible until 15 cm, but above this depth, the valuesfluctuated between 0.02 and 1.2. The ratio of cyanobacterialpigments e.g. echinenone/zeaxanthin showed an increasingtrend in younger sediments; the lowest (0.56) value occurred at27 cm and the highest (2.5) value occurred at 5 cm.

5. Discussion

5.1. Sediment chronology

There were no discernible changes in color, texture, and grainsize in the cores (data not shown). Moreover, porosity and LOImeasurements in sediments from both cores in Lake Sattalwere comparable (Fig. 3). Because the two cores were locatedb1 m from one another, were retrieved at the same time, theyshould hence display similar sediment accumulation rates.Extrapolating ages from the long core (SL 1) to the short core(SL 2) in which all the paleolimnological proxies have beenmeasured, while not ideal, is therefore probably acceptable.

The interpretation of 210Pb profile and the model used incalculating the age and sedimentation rate depends on theassumptions concerning the supply of 210Pb and its behavior.

Researchers justify using the CIC model based on the semi-logarithmic trend of total 210Pb concentration per unit weightof sediments vs. depth, which is characterized by amonotonicdecline until a region of unchanging 210Pb concentration isreached (Krishnaswamy et al., 1971; Koide and Bruland, 1975).In the present study, the plot between 210Pb(ex) and cumulativedrymassdoesnot showanexponential decrease in activity i.e.,the plot is non-monotonic representing fluctuations in 210Pbactivity (Fig. 2). Because the lake is located in a tectonicallyactive area it is probably incorrect to assume a constantsedimentation rate over an extended time period. Moreover,the region is highly prone to landslides (Kumar and Nachiap-pan, 1999), and hence, the CRS method is reasonable forestimating the mass accumulation rate (e.g. Edgington et al.,1991; Benoit and Rozan, 2001; Kumar et al., 2007).

The average sedimentation rate based on the 210Pb methodin this study is ca. 4.3 mm yr−1 (Fig. 2). Earlier, Das et al. (1994)reported an average sedimentation rate of 2.6–3.3 mm yr−1,whereas in a recent study by Kumar et al. (2007) sedimentationrate has been estimated between 5.4±0.03 and 8.1±0.05 mmyr−1. The large variability in sedimentation rates between thesestudiesprobably results fromdifferent sampling locations in thelake. The lake is shaped as two outstretched arms attached to atorso; these arms have steep slopeswith accumulation bottomsoccurring at different depths (Fig. 1). Moreover, depending onthe topography of the lake-bottom, sediment accumulation canvary between different parts of the lake. Finally, the age-depthrelationship in a core is influenced by the sampling resolutione.g. 210Pb measurement by Das et al. (1994) was done at 6 cminterval, which is coarse in a rapidly sedimenting basin.

The sedimentmass accumulation range is low (96 and 160 gm−2 yr−1) despite the fact that Sattal is eutrophic. This impliesthat autochthonous production of OM does not translate torapid accumulation of sedimentary OM in the lake bottom dueto microbial degradation in the water column. Because lessthan one third of the total primary productivity is actuallyexported from the photic zone in lacustrine systems (Bernas-coni et al., 1997 and references therein), the low massaccumulation rate in Sattal seems acceptable. Moreover, alake with high productivity and eutrophic planktonic assem-blage could annually turn over vigorously to poorly preserve


OM in its sediment record. In fact there are examples of manyeutrophic lakes, whereby ongoing processes in the watercolumn impact mass accumulation rates, and their overallvalues are low e.g. Lugano (Bernasconi et al., 1997), Norrviken(Routh et al., 2008), Nylandssjön (Gälman et al., 2008), and Erie(Philip Meyers, personal communication). Notably, two factorswhichplay a crucial role in impactingmass accumulation ratesare P level (Bernasconi et al., 1997) and water depth (Meyers,1997). High (30–40 mg/m3) P levels as in case of Lake Luganoresults in low (~100 g m−2 yr−1) mass accumulation rate due torapid degradation in the photic zone. Likewise, longer sinkingtimes in deeper lakes imply greater chance for degradation oflabile components in the water column, and hence, massaccumulation rate is low. Themass accumulation rate in Sattalis slightly higher in younger sediments (Fig. 2). This is mostlikely due to changes in land use pattern in the catchment —urbanization, construction of new settlements, and horticul-ture that have all increased in recent years.

5.2. Total organic carbon

TOC is mostly composed of algal derived OM in Sattalsediments. The TOC flux in Sattal sediments indicates that

Fig. 4 –Distribution of Eh, total organic carbon (TOC) and nitrogen(Cal%), and paleoproductivity in Lake Sattal (core SL 2).

within the last eight decades, the values have significantlyincreased from 2.7 g m−2 yr−1 to 12.7 g m−2 yr−1 (Fig. 4). It isunlikely that variation inTOCcan be due to differences in grainsize since the sediments are characterized by silt-to-clay sizefractions throughout the core. Instead variation in TOC ismostlikely related to in-lake productivity and OM preservation. TheTOC flux is low (between 2.7 and 3.9 g m−2 yr−1) in bottomsediments (27–35 cm). Likewise, PP estimates based on TOCcontent (320–411 g C m−2 yr−1) and algal derived n-ΣC15,17,19

hydrocarbons (1.7–3.6 g C m−2 yr−1; Fig. 4) are low in bottomsediments. However, the TOC flux coincides with the high (upto 589 g C m−2 yr−1) PP estimate in sediments deposited above27 cm. Because hydrocarbons constitute a very small fractionof TOC, the measured PP values based on TOC are higher thanPP values calculated based on the amount of algal-derived n-ΣC15,17,19 hydrocarbons. The elevated TOC values up-core canresult from better preservation of OM under anoxic conditionsbecause higher biological oxygen demand results in reducedmicrobial activity (Pedersen and Calvert, 1990; Hodell andSchelske, 1998). Consistent with this idea, a steady decline inEh occurs from −110 to −179 mV up-core (Fig. 4).

Meyers (1997) indicated that primary production in meso-trophic freshwater bodies is from 100–310 g Cm−2 yr−1, whereas

flux, atomic Corganic/Ntotal, δ13Corganic, δ15Ntotal, algal matter


in eutrophic systems it is from370–640 g Cm−2 yr−1. Baseduponthese rough estimates, the trophic status in Sattal has changedfrom mesotrophic to eutrophic condition. This change thatstarted around the 1960s is evidently related to the culturaleutrophication of Sattal, and is associated with increase inurbanization and higher inputs of nutrients and sewage(Chakrapani, 2002; Das, 2005). Consistent with this idea, weobserve an increase in N flux (0.3–1.2 g m−2 yr−1) in sedimentsduring the 1960s, which coincides with urban development andincrease in agricultural activities in the catchment. Urbandevelopment leads to deforestation and higher inputs of nitratebearing domestic waste into the lake. The rapid shift in trophicstatus highlights the fragile state of these mountainous lakes,which are extremely vulnerable to human-induced changes inthe catchment. Moreover, this change poses a mitigationchallenge and threat to the lake's ecological diversity.

5.3. Corganic/Ntotal ratio

The C/N ratio is widely used to infer OM sources in lacustrinesediments (Meyers, 1994; Routh et al., 2004, 2007; Gälmanet al., 2008). Terrestrial plants consist of cellulose and hence,C/N ratio is high (20 or greater). In contrast, algae are rich inproteins and have low (8 to 10) C/N ratio (Meyers andIshiwatari, 1993; Talbot, 2001). In Sattal, the Corganic/Ntotal

ratio is between 11 and 14; these values imply that OM ispredominantly derived from in-lake phytoplankton produc-tion. Moreover, OM that reaches the lake bottom has alreadybeenworked over such that only themore recalcitrant fractionsurvives. Therefore, it is justifiable to use bulk parameters asCorganic/Ntotal to interpret the OM sources.

The inorganic N fraction in sediments can sometimes alterthe Corganic/Ntotal ratio (Talbot, 2001). The intercept for theregression line of the scatter plot for Ntotal vs. Corganic is belowzero (−0.05), and implies that the inorganic N fraction in Sattalsediments is negligible. Based on the facts that inorganic N isvery low in these sediments, and C/N (wt.%) of algal matter is~8 and that of terrigenous OM is ~25 (Meyers, 2003), we canestimate the percentage of algal derived OM (Cal%). Thecalculated Cal% values range from 63–81%. This confirmsthat OM in Sattal sediments is essentially autochthonous(algal derived). Moreover, the interpretation about OM sourcesin Sattal based on Corganic/Ntotal ratio also matches with thebiomarker data (see below).

Diagenetic processes can alter the C/N ratio of sedimentingOM with burial time/depth (Meyers, 1994, 1997; Brenner et al.,1999), and mislead determination of OM sources in lakesediments. The increase in Corganic/Ntotal values with age isrelated to microbial degradation of nitrogenous proteins insedimentary OM. In addition, newly deposited sedimentscontain easily degradable material from algae. In a recentstudy by Gälman et al. (2008) on varved sediments fromNylandssjön, the authors indicate that a major part of the Cand N loss from sediment occurs within 5 years afterdeposition, and thereafter, the rate of loss is low. By assumingthe Corganic/Ntotal value as 12 at steady state (a temporallyconstant value), 14 for the burial ratio, and 8 for algal-derivedOM, we estimated the amount of N and C that is lost in Sattalbottomsediments during diagenesis.WhileN loss ranges from9–18%, the loss for TOC is higher in deeper sediments i.e., 19–

28%. The loss in C and N due to post-depositional changes inSattal is roughly similar to those in Lake Ontario (20–30% C;Hodell and Schelske, 1998), Lake Lugano (15–20% C; Lehmannet al., 2002), and Nylandssjön (20% C and 30% N; Gälman et al.,2008). These studies demonstrate that 1) diagenetic processesassociated with preferential remineralization of N can changetheC/N ratiowith depth/timeparticularly in small lakes, and 2)increase in C/N ratio with depth is most likely process relatedrather than indicating input of different proportions of OMfrom variable sources (algal vs. terrestrial OM).

5.4. Stable carbon isotope

Stable carbon isotopes in aquatic plants reflect kinetic isotopefractionation that results from slow biological assimilation ofthe heavier isotope δ13C (Fogel and Cifuentes, 1993; Herczeget al., 2001; Kendall et al., 2001). Phytoplankton use 12C inpreference over 13C and results in OM depleted in 12C. Asproductivity increases, the dissolved inorganic carbon (DIC)pool gets depleted in 12C, whereas OM gets enriched in 13C(Meyers, 1997; Teranes and Bernasconi, 2005). Thus, in lakesystems where primary productivity plays a dominant role oncycling of carbon, OM becomes progressively enriched in 13Cwith increased productivity— both seasonally and in terms oflong-term eutrophication (Teranes and Bernasconi, 2005).

Sattal sediments shows decrease in δ13C values up-core; thebottom sediments record −26.7‰, whereas towards the top ofthe core, the δ13C value is −30.1‰ (Fig. 4). Unfortunately, δ13Cvalues of OM produced by lacustrine algae and vascular plantsare often indistinguishable because both type of plants useisotopically identical sources of inorganic carbon i.e., CO2

dissolved in water or in the atmosphere. Input from macro-phyte biomass can however affect the δ13C values; macro-phytes display awide range of δ13C values from −30‰ to −12‰in sedimentary OM (Boutton, 1991). Consistent with this,macrophytes are present in Sattal and their input to the OMpool has increased (based on hydrocarbon distribution — Paqratio, Fig. 5, see below). In particular, carbon isotopevalues thatare significantly depleted in 13C suggest that bacteriallyassimilated C contribute to the sedimentary OM pool, eitheras coupled or uncoupled to surface water phytoplanktonproduction (Hollander and Smith, 2001; Teranes and Bernas-coni, 2005). Distribution of biomarkers in Sattal confirmsmicrobial biomass based on the presence of algal-derivedhydrocarbons and cyanobacterial input (see below) similar toLake Baldeggersee (Teranes and Bernasconi, 2005). Thesestudies by Hollander and Smith, (2001) and Teranes andBernasconi (2005) indicate that the low δ13C trend is linked toP input, and development of anoxic conditions associatedwitheutrophication. Likewise, the steady depletion in δ13C valuescan be explained by recycling of lighter isotope due tomethanogenesis. This phenomenon is common when lakesbecome eutrophic and anoxic conditions develop (Gu andSchelske, 1996; Brenner et al., 1999; Teranes and Bernasconi,2005; Vreca and Muri, 2006). However, we have not measuredmethane in Sattal sediments, or in the water-column andhence, the role of methanogenesis in affecting δ13C ofsedimentary OM is probably speculative.

The δ13C values of OM in Sattal are in fact similar to manyeutrophic water bodies affected by external input of nutrients

Fig. 5 –Total hydrocarbon concentration and ratios (CPI, TAR, Paq) in Lake Sattal sediments (core SL 2).


(Schelske and Hodell, 1995; Brenner et al., 1999; Routh et al.,2004; Teranes and Bernasconi, 2005; Vreca and Muri, 2006).Delivery of isotopically light DIC e.g. from land based run-offcan result in production of isotopically light algal OM (Meyersand Teranes, 2001; Meyers, 2003). Moreover, deforestation inthe catchment can lead to such changes in bulkC andN isotopesignatures (Routh et al., 2007). In particular, input of δ13Cdepleted sewageeffluents can result in low δ13C in sedimentaryOM. For example, in Lake Petén Itzá (Guatemala) input ofsewage results in OM that is depleted in δ13C although it is ahighly productive water body (Rosenmeier et al., 2004).Consistent with this idea, Chakrapani (2002) reported a sharpincrease in sewage effluents in recent years in Sattal and alsoother lakes in the Kumaun region. Thus, the most plausiblereasons of 13C-depleted sedimentary OM record in Sattal inmore recent years are input of external nutrients, or contribu-tion frommicrobial biomass— all pointing towards the effectsof increasing cultural eutrophication in thismountainous lake.

5.5. Stable nitrogen isotope

The use of 14N in preference over 15N by phytoplanktonmakesthem useful to determine primary productivity in lakes(Brenner et al., 1999; Tenzer et al., 1999; Teranes andBernasconi 2000; Talbot, 2001). N isotopes are also useful fordetermining OM sources (Meyers, 1997; Talbot, 2001). Aquaticplants use dissolved inorganic nitrogen (DIN), which isisotopically enriched in 15N by 7‰ to 10‰ relative to atmo-spheric N (0‰). Hence, OM produced by phytoplankton hasδ15N values of 8‰ (Meyers, 1997, 2003). Because δ15N in DINvaries depending upon the source inputs (see Kendall, 1998),phytoplankton can in fact show a broad range of δ15N valuesranging from −3 to 18‰ (Sharp, 2007). Land plants useatmospheric N2 to produce OM, and have δ15N values between0 and 2‰ (Meyers, 2003). Likewise, OM produced during N2-fixation by cyanobacteria under eutrophic conditions, indicateδ15N values between −1.0 and 3.0‰ (Fogel and Cifuentes, 1993).

The δ15N trend in Sattal sediments shows a gradual up-coredecline (Fig. 4), the values change from 1.4‰ to 0.1‰. Lowerδ15N values result from greater availability of DIN for biologicalutilization, which typically stimulate primary productivity in

lakes (Hodell and Schelske, 1998; Meyers and Teranes, 2001).The low δ15N values correspond with low C/N, which isdiagnostic of algal derived OM and elevated PP estimates.This is consistent with studies in Florida lakes, which indicateconcurrent shifts to lower δ15N and C/N values with increasedprimary productivity and/or trophic state (e.g. Gu et al., 1996;Brenner et al., 1999).

The δ15N values of OM in Sattal sediments fall within therange of isotopically light OM produced by N2-fixing cyano-bacteria (Fogel and Cifuentes, 1993; Talbot, 2001). Consistentwith this, Sattal sediments indicate the dominance of cyano-bacterial pigments (Fig. 6), which covarieswith δ15N values (seebelow). Eutrophic conditions arising as a result of high nutrientavailability and productivity shifts coupled with low dissolvedoxygen levels encourage cyanobacteria to thrive in thesewaters. Cyanobacterial blooms are common in eutrophiclakes, which have low dissolved oxygen and N content, buthigh P levels (Brenner et al., 1999; Talbot and Laerdal, 2001;Wasmund et al., 2001). Under such conditions, the N2-fixingcyanobacteria use atmospheric N2 dissolved in lake water, andin the process, decrease the δ15N values to −1 to +3‰ (Brenneret al., 1999).

5.6. Diagenetic impact on stable isotopes

Unraveling changes based on the alteration of primary isotopicsignal bymicrobial degradation, variable source inputs, or post-diagenetic changes are crucial for understanding OM preserva-tion and its characteristics. While carbohydrates and proteinsare generally enriched, lipids are depleted in 13C (Degens, 1969;Deines, 1980); selective loss of the former components couldlead to the decrease of δ13C values in residual OM. Therefore,decline in δ13C values with time can either be explained by thepreferential removal of OM compounds enriched in 13C, orrepresentinghigher input of organic compoundsdepleted in 13C(e.g. Lehmann et al., 2002). Previous studies indicate that δ13C ofOM in lake sediments is generally resistant to isotopic alterationduring water-column, or post-burial diagenesis particularly, inorganic-rich sediments under anoxic conditions (Meyers andEadie, 1993; Schelske and Hodell, 1991, 1995; Meyers, 1994, 1997;Routh et al., 2007; Das et al., 2008). In contrast, other studies

Fig. 6 –Distribution of phytoplankton pigments (total carotene, Chl a, Chl b, β,β-carotene, zeaxanthin, echinenone,myxoxanthophyll, lutein, and peredinin) and their ratios (zeaxanthin/β,β-carotene, Chl a/β,β-carotene, echinenone/zeaxanthin) in Lake Sattal (core SL 2).


indicate that post-diagenetic processes can affect the bulk OM,and shift δ13C values (e.g. Benner et al., 1987; Lehmann et al.,2002; Gälman et al., 2008). However, these changes are morenoticeable towards the top of the core, and occur immediatelyafter deposition.

In Sattal, the upper part of the core, which is affected byhuman activities in the catchment has δ13C values around−30.1‰; the mid-section, which is most likely unimpacted byanthropogenic activities or diagenetic changes shows δ13Cvalues around −29.0‰; towards the bottom of the core δ13Cvalues increase to −26.7‰ (Fig. 4). Because stable C and Nisotope values increase with depth systematically (i.e. beyond27 cm), this can be an effect of post-diagenetic changes. In orderto place limits on the possible role of diagenesis, we estimatedthe weighted average of δ13C in sedimentary OM based on asimple binarymixingmodel (Schultz andCalder, 1976) involvingvariable inputs of terrestrial and algal-derived OM:

d13C = fterr OMTd13C + falgal OMTd13C� � ð8Þ

where fterr OM and falgal OM represent the fraction of terrestrialand algal-derived inputs, respectively. Although algal input in

the sediment intervals increases from 63% to 76% up-core(Fig. 4), the weighted average of δ13C value is not expected tochange beyond −29‰ because terrestrial and algal-derived OMdonot have unique δ13C signatures (Meyers, 2003). However, theδ13C values in Sattal increase by ~3.5‰ in bottom sediments(Fig. 4). This shift in δ13C values implies post-diagenetic alte-ration due to selective remineralization of C. Consistent withthis idea, we note a decrease of up to 28% organic C in bottomsediments.

While N loss in bottom sediments ranges from 9–18%, theδ15N values indicate a narrow range (0.1 to 1.4‰; Fig. 4). Theseδ15N values coincide with N-derived OM associated withcyanobacteria and/ terrestrial and aquatic plants. Estimatingthe effect(s) of post-diagenetic change on N isotopes based ona binary mixing model is contentious due to the absence ofunique δ15N signature for different OM sources, and the smallshift in δ15N values (~1.0‰) observed in Sattal sediments.Talbot (2001) concludes that while diagenesis can impact theδ15N values, available limnological data are inconclusivebecause of lack of: 1) significant change in the N-isotopecomposition, and 2) a clear pattern of change in lacustrinesediments investigated in various studies.


5.7. Distribution of hydrocarbons

Due to their low resistance to decomposition hydrocarbonsserve as promising tools to determineOMsources in lacustrineenvironments (Meyers, 1997; Meyers and Lallier-Verges, 1999;Routh et al., 2007, 2008; Das et al., 2008). There are two distinctsources of biotic hydrocarbons, and their molecular character-istics, serve as proxies for changes in OM sources and delivery.For example, hydrocarbon composition of most aquatic algaeand photosynthetic bacteria are dominated by n-C17 alkane(Cranwell et al., 1987, Tenzer et al., 1999); abundance of n-C17

alkane reflects enhanced primary productivity. In contrast,vascular plants contain large proportions of n-C27, C29, and C31

alkanes in their epicuticular waxy coatings (Cranwell et al.,1987; Rieley et al., 1991; Tenzer et al., 1999). Abundance of theselong-chained hydrocarbons reflects larger input of land-derived OM that is transported into a lake from the watershed.

The concentration of total hydrocarbons in Sattal sedi-ments shows an increase up-core (Fig. 5). The values are low(between 0.26 and 0.44 µg mg−1 of TOC) between 27 and 35 cm,but start to increase upwards, and represent a maximum of2.23 µgmg−1 of TOC in surface sediments. The lowhydrocarbonconcentration in bottom sediments compared to the overlyinglayers is most likely related to the low PP. The steady increasein n-ΣC15,17,19 alkanes from 0.23 to 0.44 µg mg−1 of TOC isconsistent with evidences from other paleolimnologicalproxies confirming the increase of in-lake productivity up-core.

CPI values representing strong odd/even predominanceand high CPI values (N5) correspond to n-alkanes fromepicuticular waxes of higher plants (Peters et al., 2005). Incontrast, n-alkanes from bacteria and algae show a weak odd-over-even predominance and low CPI values (≈1; Cranwellet al., 1987). The CPI values in Sattal sediments are between 0.2and 1.6 (Fig. 5) suggesting the dominance of algal derived OM.The results also imply that surface runoff does not transportsignificant amounts of terrestrial OM from the hill slopes. Thisinterpretation is consistent with the TAR results. The low TARvalues (between 1.4 and 2.7) favor algal dominated OM. The Paqvalues are between 0.09 and 0.6 suggesting a mixture ofemergent and submerged fresh water vegetation (Mead et al.,2005) in the lake. Consistentwith this, the lake is dominated byPolygonum, Hydrilla, Potamogeton, Myriophyllum, and Vallisneria(Raina and Petr, 1999).

5.8. Pigments

Pigments in lake sediments are mainly derived from plank-tonic and benthic algal communities, phototrophic bacteria,aquatic macrophytes, and minor contributions of undegradedpigments from terrestrial plants (Leavitt, 1993). Because oftheir specificity, pigments are often used to study paleoenvir-onmental conditions (Sanger, 1988; Leavitt, 1993). For example,ubiquitous pigments such as β,β-carotene, Chl a, and pheo-phytin are valuable indicators of algal abundance and primaryproductivity. Likewise, taxon-specific carotenoids allow dis-tinction between different algal groups such as: siliceous algaeand dinoflagellates (fucoxanthin), cryptophytes (alloxanthin),chlorophytes (lutein, Chl b, pheophytin b), diatoms (diatox-anthin), and cyanobacteria (echinenone, zeaxanthin).

The total carotene content in Sattal sediments is between 1.3and10mmol g−1 of TOC in bottomsedimentswhich reaches themaximum value of 34 mmol g−1 of TOC around 11 cm. Increasein pigment concentrations ismore prominent after 1970 (Fig. 6),indicating enhanced anthropogenic input, which affects pri-mary productivity. This is consistent with the other paleolim-nological proxies discussed above. Moreover, the low redoxpotential in the upper layers may further enhance thepreservation of algal pigments (Leavitt, 1993; Bianchi et al.,2000). Notably, concentrations of specific carotenoids varyindicating differences in individual degradation rates, contribu-tion by specific phytoplankton communities, or a combinationof both factors. The distribution of different pigments in Sattalmatches the description of phytoplankton study. The studycarried over a period of 18months in 1979 (Pant and Joshi, 1987),indicate the distribution of different phytoplankton species asChlorophyceaeNDinophyceaeNCyanophyceae. In terms of ver-tical distribution, phytoplankton indicate the abundance ofdiatoms in surface water, green algae and dinoflagellate in themiddle strata, and cyanobacteria in deeper waters. Moreover,phytoplankton distribution in the water-column indicated astrong seasonal influence.

Chl a and b indicate high concentrations up-core, and implyelevated algal productivity (Fig. 6). Chl a is a non-specificmarker because it can be produced by all types of plants. Incontrast, Chl b typically produced by Chlorophytes, Eugleno-phytes, and Tracheophytes (Leavitt, 1993), indicates a steadyincrease in more recent sediments. Similarly β,β-carotenerepresents a maximum of 28 mmol g−1 of TOC at 11 cmcoincidingwith higher PP (Figs. 4 and 6). Althoughβ,β-carotenemay persist under oxygenated conditions (Bianchi et al., 2000),it is absent in bottom sediments. The absence of β,β-caroteneis perhaps not related to degradation, but more likely, it isrelated to the low productivity at these depths. A similar trendfor Chl a and b supports the idea (Fig. 6). Researchers have usedthe ratio of Chl a/β,β-carotene to indicate productivity relatedchanges in aquatic environments (e.g. Fogg and Belcher, 1961;Griffiths, 1978). Consistent with this idea, the low (between 0.1and 1.8) value for Chl a/β,β-carotene ratio in Sattal sedimentsimplies an increase in primary productivity in recent years.

Concentration of lutein increases after 15 cm (Fig. 6). Thepresence of lutein alongwith Chl b represents the abundance ofChlorophytes in surface sediments. However, the total concen-tration of lutein is low compared to the cyanobacteria specificpigments i.e., echinenone and zeaxanthin. This implies thatlutein is most likely not well preserved in these sediments. Theabsence of myxoxanthophyll in bottom sediments suggestsrapid degradation due to the presence of sugar containinggroupswithin thepigment. Fucoxanthinproducedbydiatoms ischaracteristically absent throughout the core. However, theabsence of fucoxanthin does not mean diatoms are absent inthis lake because fucoxanthin is very unstable (Leavitt, 1993),and extensive degradation of this compound occurs duringdiatom grazing by zooplankton (Pandolfini et al., 2002). Finally,the appearance of peredinin in surface sediments representsthe presence of dinoflagellates, which can bloom under P-richconditions (Wu and Chou, 1998; Findlay et al., 1999). Consistentwith this, Pant and Joshi (1987) indicate that dinoflagellates(Peridinium willei, P. cinctum, Gymnodinium fuscum) contribute~90% of the total phytoplankton standing stock (from January–


March). Peredinin is however amongst themost labile pigments(Leavitt, 1993), and its absence in bottom sediments is mostlikely related to its poor stability and preservation.

The distribution of two specific cyanobacterial pigmentsnamely echinenone and zeaxanthin in Sattal sediments isparticularly interesting. Zeaxanthin concentration in bottomsediments is low (1.0 to 2.1 mmol g−1 of TOC), but increasessharply to 3.1 mmol g−1 at 25–27 cm depth; above this interval,zeaxanthin indicates a decrease in concentration up-core(Fig. 6). The presence of zeaxanthin indicates cyanobacteria,which are specific for non-N2-fixing species e.g. Microcystis andOscillatoria (Paerl, 1988; Leavitt, 1993; Bianchi et al., 2000).Zeaxanthin is very stable (Bianchi et al., 2000, 2002), andhence, the observed variability is not the result of degradation,but it is the response of phytoplankton communities to nutrientinflux. Echinenone considered a specific marker for thefilamentous N2-fixing cyanobacteria e.g. Anabaena, Nostoc, andAanbaenopsis (Bianchi et al., 2002) indicates a steady increase inconcentration up-core (Fig. 6). These cyanobacteria thrive wellinN2-limitedconditionsbecauseof their symbiotic relationwithother algae, and the capability to fix atmospheric N2 (Ashton,1981; De Nobel, 1995, 1997; Beaumont et al., 2000). During therainy season,Nostoc takes over after the blooming of Volvocaleswhichdeplete thedissolvednitrate level inSattalwater— this isbelieved to be an adaptive character of these cyanobacteria(Pant and Joshi, 1987). Echinenone is however absent in bottomsedimentsmost likely due to its lower stability then zeaxanthin(Bianchi et al., 2002). Alternatively, it represents the absence ofN2-fixing cyanobacteria in bottom sediments. The change inphytoplankton communities in the lake is further supported bythe zeaxanthin/β,β-carotene ratio. The ratio increases from0.67to 1.7 (Fig. 6), and represents a shift in phytoplankton speciesfrom green algae to cyanobacteria. Likewise, the ratio betweenechinenone/zeaxanthin represents the dominance of N2-fixingcyanobacteria in surface sediments (Fig. 6).

The distribution of specific cyanobacterial pigments inSattal matches well with the light δ15N OM (0.75 to 0.12‰), andn-alkane inferred changes in phytoplankton communities/productivity. This change in phytoplankton communities inSattal can be due to the high (7.1 µg l−1) P input in recent years(Das, 2005), which prompt the N2-fixing cyanobacteria toproliferate. Studies in the Anabaena dominated LakeBosumtwi, Ghana (Talbot and Johannessen 1992) and theBaltic Sea (Bianchi et al., 2000; Borgendahl andWestman 2007),indicate a strong correlation between the distribution ofcarotenoids and N isotopes. Likewise, recent studies in LakesZeekoevlei (Das et al., 2008), Norrviken (Routh et al., 2008), andNainital (Choudhary et al., in press) indicate a strong correla-tion between δ15N, n-alkane, and cyanobacterial pigments. Allthese lakes have been impacted by human-induced changesand pigments provide detailed information about in-lakeprocesses, trophic shifts, and change in phytoplankton com-munities, which is not readily available from other proxies.

6. Conclusions

The results of the study indicate that anthropogenic activitiesin thewatershed have impacted Lake Sattal. TOC values and%N show increase in concentrations core-up. The TOC inferred

PP estimate nearly doubled in the last eight decades, which isrelated to increase in N and P input to the lake. Moreover, thetrophic status has changed to eutrophic condition in Sattal.The lake is primarily dominated by algal-rich OM; this issupported by the C/N and hydrocarbon (CPI, TAR) data. Theδ13C value of sedimentary OM implies input of externalnutrients or contribution from microbial biomass pointingtowards the effects of increasing cultural eutrophication inrecent years. In addition, post-diagenetic changes have alsoaffected the δ13C values in deeper sediments. The low δ15Nvalues in these sediments are related to N2-fixation by cya-nobacteria. The pigment data conform to productivity relatedchanges, in particular, the shift in phytoplankton species inrecent years. The lake is presently dominated by N2-fixingcyanobacteria over other phytoplankton e.g. diatoms, dino-flagellates, and non-N2-fixing species in younger sediments.

Acknowledgements

P. Parthasarathy and R. Saini helped in sampling the lake.Supriyo Das helped with the pigment analysis. Klara Hajnaland Heike Seigmund are acknowledged for their help in thelaboratory. Dr. BhismKumar is acknowledged for providing thelab facilities for lead dating. Discussionwith PhilipMeyers andVolker Bruechert was helpful. We thank the anonymousreviewers for their careful review and suggestions. TheSwedish Research Link-Asia program funded the study. PCwas supported on a CSIR fellowship.

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An environmental record of changes in sedimentary organic matter from Lake Sattal in Kumaun Himalayas, India