Note
An
n
-alkane proxy for the sedimentary input of submerged/
¯oating freshwater aquatic macrophytes
K.J. Ficken
a,b,*, B. Li
a,1, D.L. Swain
b, G. Eglinton
aaBiogeochemistry Research Centre, Department of Earth Sciences, Wills Memorial Building, Queens Road, Bristol BS8 1RJ, UK
bDepartment of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK
Received 31 May 2000; accepted 12 June 2000
Abstract
Lipid analysis of aquatic plants from lakes on Mt. Kenya, E. Africa, revealed that non-emergent (submerged and ¯oating-leaved) species displayed enhanced abundances of mid-chain length, C23and C25n-alkanes. In contrast, the emergent aquatic plants hadn-alkane distributions similar to those of the terrestrial vegetation, typically dominated by the long-chain length homologues (>C29). A proxy ratio,Paq, has been formulated to re¯ect the non-emergent aquatic macrophyte input to lake sediments relative to that from the emergent aquatic and terrestrial plants.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Lakes; Lipids;n-Alkanes; Aquatic macrophytes;d13C
1. Introduction
Aquatic macrophytes are very common in fresh to oligosaline lakes, especially in the tropics (Lind and Morrison, 1974). Together with epiphytic algae on their stems and leaves, aquatic macrophytes probably con-tribute a signi®cant proportion of the sedimenting par-ticulate organic carbon in these small, shallow lakes (Wetzel, 1983). Interpreting pollen percentage data to provide an estimate of macrophyte abundance is di-cult and does not readily yield a measure of the carbon ®xed by aquatic higher plants. The identi®cation of biomarkers and biomarker proxies for aquatic plants should assist in the interpretation of the lipid and car-bon isotope (d13C) signatures obtained from lacustrine sediments.
Few published data exist on the lipid composition of submerged and emergent aquatic macrophytes (Barnes and Barnes, 1978; Cranwell, 1984; Ogura et al., 1990). Previous studies have indicated that emergent macro-phytes such as Juncus eusus, Typha latifolia and
Phragmites communis display n-alkane and n-alkanol
distributions characteristic of terrestrial higher plants i.e. >C27 (Barnes and Barnes, 1978; Cranwell, 1984), whereas studies of submerged and ¯oating plants such
asNajas marina, Potamogetonspp.,Vallisneria gigantea,
Elodea nuttaliandPosidonia Oceanicaindicate that the
n-alkane distributions of these plants maximise at C21, C23or C25 (Barnes and Barnes, 1978; Cranwell, 1984; Ogura et al., 1990; Viso et al., 1993).
Plant communities surrounding the freshwater lakes on Mt. Kenya vary greatly as a function of altitude: terrestrial vegetation around the lakes ranges from montane rainforest to sparse, Afroalpine communities. There is also considerable variation in the aquatic plant communities. Aquatic macrophytes are very abundant in L. Nkunga (1820 m a.s.l.) and Sacred Lake (2350 m a.s.l.), both of which have very limited open water areas. They are less common in two lakes above the treeline (L. Rutundu, 3078 m a.s.l. and L. Ellis, 3500 m a.s.l.).
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* Corresponding author.
E-mail address:gg®[email protected] (K.J. Ficken).
1 Present Address: College of Chemical and Light Industrial
2. Experimental
2.1. Sample description
Plants were collected in September 1995 and January/ February 1996 from L. Nkunga, Sacred Lake, L. Rutundu and L. Ellis. From these sites, 17 samples of aquatic macrophytes (6 submerged, 2 ¯oating and 9 emergent) and 3 common tree and shrub species were selected for lipid extraction. The plants were dried immediately after collection by pressing between sheets of newspaper over a bank of light bulbs. The paper was changed daily until the plants were dry, usually after 2±3 days. Since hydrocarbons in printers' ink may con-taminate pressed plants, a piece of the newspaper was extracted in exactly the same way as for the plants, the hydrocarbons giving a smooth distribution (CPI=0.98) ofn-alkanes (n-C23ÿ34), centred aroundn-C26.
2.2. Lipid extraction and instrumental analysis
The dried plants (leaves, stems and ¯owers) were ®nely sliced into small pieces with a clean scalpel and extracted using a solvent system of sequentially decreasing polarity (Ficken et al., 1998). The free fatty acids were separated from the neutral lipids by solid phase extraction. Neutral lipids were separated by TLC on silica gel 60G pre-eluted with ethyl acetate. Devel-opment with 1% acetic acid in hexane:ethyl acetate (7:2 v:v) gave bands corresponding in mobility to hydro-carbons, alcohols and polar compounds, respectively. The hydrocarbons were further fractionated into adduct and non-adduct fractions by urea adduction. Gas chro-matography (GC), GC/mass spectrometry (GC/MS) and compound-speci®c carbon-isotope ratio mass spec-trometry (GCirms) were performed as described by Ficken et al. (1998).
3. Results and discussion
The terrestrial, emergent and non-emergent (sub-merged/¯oating) species all contained high concentra-tions ofn-alkyl lipids (up to 4190, 2378 and 2897mg/g, respectively; Table 1) suggesting that aquatic macro-phytes are an important source of lipids in freshwater lake sediments. The n-alkanoic acids were generally most abundant, with the exception of Podocarpus
lati-folius andNymphaea nouchali (3607 and 2085 mg/g n
-alkanes, respectively), with then-alkanols being the least abundant (Table 1). However, when all three classes of lipid are considered (Fig. 1), then-alkanes exhibited the most signi®cant dierences in distribution between the three plant types. The terrestrial and emergent plants showed similar n-alkane distributions, maximising at C29or C31 (Fig. 1, Table 1). However, the submerged/
¯oating plants maximised at C23or C25 n-alkane, with the exception ofNymphaea nouchaliandOttelia ulvifolia
which maximise at C21and C29, respectively.
We, therefore, propose a proxy, Paq, for submerged/ ¯oating aquatic macrophyte input versus emergent and terrestrial plant input to lake sediments, based onn-alkane data. It expresses the relative proportion of mid-chain length (C23, C25) to long-chain length (C29, C31) homo-logues:
Paq C23C25= C23C25C29C31
For the modern plants (Table 1), this proxy gives average values of 0.09 for terrestrial (range 0.01±0.23), 0.25 for emergent (range 0.07±0.61) and 0.69 for sub-merged/¯oating species (range 0.48±0.94). At the 95% con®dence level, the probability that the terrestrial and emergent plants are the same is 7%, the probability that the terrestrial and submerged/¯oating plants are the same is 0.09% and the probability that the emergent and submerged/¯oating plants are the same is 0.001%. Therefore, based on this pilot study, Paq<0.1 corre-sponds to terrestrial plants, 0.1±0.4 to emergent macro-phytes and 0.4±1 to submerged/¯oating macromacro-phytes. However, when applied to the study of sediment extracts, a given value of Paq will re¯ect a particular mixture of inputs from two or more of these sources. For example, an intermediate value of 0.1±0.4 could indicate a mixture of inputs from terrestrial, emergent and submerged/¯oating aquatic macrophytes.
We have tested this index using the surface sediments of four lakes on Mt. Kenya for which lipid concentra-tions and modern plant data are available (Table 1). Both Lake Nkunga and Sacred Lake give high Paq values (0.82 and 0.5, respectively), in accord with the observed high density of submerged/¯oating macro-phytes (Table 1). Futhermore, in cores from these lakes, high relative concentrations of the C23 and C25 n -alkanes (Ficken et al., 1998; Huang et al., 1999) corre-spond to high counts of aquatic pollen taxa, notably
Nymphaea(Coetzee, 1967; Swain, 1999) con®rming that
Paq re¯ects the nature of the plant input to the late Quaternary sedimentary record. The other two lakes, Lake Rutundu and Small Hall Tarn, both give inter-mediate Paq values, corresponding to a mixed input from submerged/¯oating, emergent and terrestrial plants. Lake Rutundu (Paq=0.37), which has abundant submerged/¯oating Potamogetonin its shallower parts, is bordered by a fringe of emergent macrophytes and surrounded by ericaceous shrubland. Small Hall Tarn, (Paq=0.26), has far fewer submerged/¯oating and emergent macrophytes than Lake Rutundu and a rela-tively lowPaqvalue.
The d13C values of bulk leaf tissue from most of the plants in this study (Table 1) range from ÿ30.9 to
ÿ22.8%, with a mean value ofÿ26.3%, indicating that
n-Alkyl lipid distributions, carbon isotope and proxy (Paq) values of (A), plant species and (B), surface sediments from four lakes on the slopes of Mt Kenya
Lipid distributions Carbon isotope values Proxy
n-Alkanes n-Alkanols n-Alkanoic acids n-Alkanes n-Alkanoic acids
(A) Plant Pamg/g
Juniperus procera 337 23-35 33 167 22-30 30 533 14-34 32 ÿ26.0 n.m. n.m. n.m. n.m. n.m. n.m. 0.23
Podocarpus latifolius 3607 21-35 29 92 24-32 30 491 14-34 30 ÿ27.5 n.m. n.m. n.m. n.m. n.m. n.m. 0.01
Emergent/sub-aerial
Ottelia ulvifolia 295 19-35 29 65 20-32 30 910 14-32 24 ÿ24.6 n.m. n.m. n.m. n.d. n.m. n.m. 0.48
Potamogeton
Utricularia re¯exa 467 19-35 25 127 20-32 24 1938 14-30 20 ÿ24.7 n.m. n.m. n.m. n.m. n.m. n.m. 0.64 Unknown sp. 433 20-35 25 88 20-30 30 2025 14-34 20 n.m. n.m. n.m. n.m. n.m. n.m. n.m. 0.56
n-alkanoic acids. All values are in%relative to PDB. n.m.=not measured.
they use the C3photosynthetic pathway. Exceptions are the emergent sedgeCyperus immensus(ÿ9.7%) and the submerged Hydrillasp. (ÿ14.9%), which utilise the C4 pathway, and the emergent Crassula sp. (ÿ22.8%) which utilises the CAM pathway (Keeley et al., 1986; Bowes and Salvucci, 1989). The compound-speci®cd13C values of then-alkanes andn-alkanoic acids (Table 1) in general parallel those of bulk leaf tissue, though there are signi®cant deviations. The weighted carbon-isotope averages of the mid-chain (C23, C25) n-alkanes can be expected to re¯ect the availability of dissolved CO2 and HCO3ÿto the submerged leaves of the non-emergent macrophytes (Smith and Walker, 1980; Keeley and Sandquist, 1992).
4. Conclusions
A proxy ratio,Paq, has been formulated on the basis of a limited survey of then-alkyl lipids extracted from aquatic macrophytes from four lakes on the slopes of Mt. Kenya. It expresses the abundance of the C23and C25n-alkanes relative to the C29and C31homologues. It provides an approximate measure of the sedimentary input from submerged/¯oating aquatic macrophytes relative to that from emergent and terrestrial species.
More extensive surveys of lipids from modern plants and sediments will be required to test the validity of this approach.
Acknowledgements
We thank the Oce of the President, Nairobi, for research permission. Financial support was provided by the NERC (GR3/9523). We are indebted to the Gov-ernment of Yunnan Province, People's Republic of China for their support of B. Li. Dr. A. Agnew and Mr. S. Mathai are thanked for plant identi®cations. Dr. R. Evershed, Mr. J. Carter and Mr. A. Gledhill are grate-fully acknowledged for access to the NERC GC±MS and GC±IRMS facilities (GR3/2951, GR3/3758, GR3/ 7731) at the University of Bristol.
Associate EditorÐA.G. Douglas
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