Diatom community response to climate variability over the past 37,000 years in the sub-tropics of the Southern Hemisphere

Science of the Total Environment 468–469 (2014) 774–784

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Diatom community response to climate variability over the past 37,000 years in the sub-tropics of the Southern Hemisphere Sarah C. Hembrow a,⁎, Kathryn H. Taffs a, Pia Atahan b, Jeff Parr a, Atun Zawadzki b, Henk Heijnis b a b

School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia Institute for Environmental Research, Australian Nuclear Science and Technology Organisation, Kirrawee, NSW 2232, Australia

H I G H L I G H T S • • • • •

Lake McKenzie has provided an environmental record of the past ca 37,000 cal. yBP. 210 Pb dating results demonstrate a very slow sedimentation rate in Lake McKenzie. A hiatus between ca. 14,000 and 18,000 cal. yBP indicates a dry climate. Geochemical evidence indicates an increase of effective precipitation in the early Holocene. Lake McKenzie has revealed a long term drying trend from mid-Holocene to present.

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Article history: Received 7 May 2013 Received in revised form 2 September 2013 Accepted 2 September 2013 Available online 25 September 2013 Editor: C.E.W. Steinberg Keywords: Diatom Quaternary Lake McKenzie Biological proxies Climate change Isotopes

a b s t r a c t Climate change is impacting global surface water resources, increasing the need for a deeper understanding of the interaction between climate and biological diversity. This is particularly the case in the Southern Hemisphere sub-tropics, where little information exists on the aquatic biota response to climate variations. Palaeolimnological techniques, in particular the use of diatoms, are well established and can significantly contribute to the understanding of climatic variability and the impacts that change in climate have on aquatic ecosystems. A sediment core from Lake McKenzie, Fraser Island (Australia), was used to investigate interactions between climate influences and aquatic ecosystems. This study utilises a combination of proxies including biological (diatom), geochemical and chronological techniques to investigate long-term aquatic changes within the perched-dune lake. A combination of 210Pb and AMS 14C dates showed that the retrieved sediment represented a history of ca. 37,000 cal. yBP. The sedimentation rate in Lake McKenzie is very low, ranging on average from 0.11 mm to 0.26 mm per year. A sediment hiatus was observed between ca. 18,300 and 14,000 cal. yBP suggesting a period of dry conditions at the site. The diatom record shows little variability over the period of record, with benthic, freshwater acidic tolerant species dominating. Relative abundance of planktonic species and geochemical results indicates a period of increased water depth and lake productivity in the early Holocene and a gradual decrease in effective precipitation throughout the Holocene. Results from this study not only support earlier work conducted on Fraser Island using pollen reconstructions but also demonstrate that diatom community diversity has been relatively consistent throughout the Holocene and late Pleistocene with only minor cyclical fluctuation evident. This record is consistent with the few other aquatic palaeoecological records from the Southern Hemisphere sub-tropics. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Climate change and anthropogenic activities are threatening the quality and volume of surface water worldwide (Jun et al., 2011; Murray et al., 2012; Paerl and Paul, 2012), increasing the need for conservation and management of freshwater resources. It has been estimated that during ⁎ Corresponding author at: School of Environment, Science and Engineering, Southern Cross University, PO Box 157, Lismore, NSW 2480, Australia. Tel.: + 61 2 6626 9450; fax: + 61 2 6621 2669. E-mail address: [email protected] (S.C. Hembrow). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.09.003

the twentieth century more than half of the world's wetlands have been destroyed (World Resources Institute, 2005). Continued population growth, urban development and climate change have been identified as the key factors causing destruction of wetlands (World Resources Institute, 2005). Long term, high resolution records of aquatic biological response to climate fluctuations and anthropogenic impacts are needed to identify trajectories of environmental change to better understand ecological responses and improve management and conservation of wetlands. In particular, it is important to study near-pristine ecosystems to isolate and identify aquatic ecological responses to climate fluctuations (Thoms et al., 1999; Rasanen et al., 2006; Logan and Taffs, 2013).

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Few continuous long-term data sets are available to identify natural variability of physio-chemical conditions and the biological response to significant climatic events (Vandergoes et al., 2005; Fletcher and Moreno, 2012; Logan and Taffs, 2013). These data sets are rarely available as environmental monitoring can be expensive in terms of time and resources, and are rarely implemented over periods of more than one year (Smol, 2008). Palaeolimnology provides an alternative suite of approaches for clarifying limnological changes on time scales of decades or more (Logan et al., 2010; Logan and Taffs, 2013). There are discrepancies and knowledge gaps regarding wetland ecosystem response to climate variability, particularly in the Southern Hemisphere (Tibby, 2003; Saunders and Taffs, 2009; Meadows et al., 2010; Hobday and Lough, 2011; Tibby and Taffs, 2011). The lack of long term, continuous or near-continuous climate records of the Southern Hemisphere, particularly those spanning the last glacial maximum to present, limits detailed understanding and effective management of aquatic communities (Saunders and Taffs, 2009). The Southern Hemisphere climate is a result of the interaction of several regional circulation patterns including the El Niño–Southern Oscillation (ENSO), the Southern Annular Mode (SAM), the Intertropical Convergence Zone (ITCZ) and the Indian Ocean Dipole (IOD). Evidence for the changes in these climate mechanisms has recently been well reviewed in Gouramanis et al. (2013) and Neukom and Gergis (2011) and is an active area of research focus. Hence, a detailed understanding of climate variability and ecosystem response in the Southern Hemisphere is gradually improving with palaeoecological research enabling increased record length and resolution of some regional discrepancies (Sandiford et al., 2003; Kemp et al., 2012; Ohlendorf et al., 2012; McGlynn et al., 2013). For example, variations in the ENSO have been increasingly studied (Moy et al., 2002; Thompson et al., 2002; Braganza et al., 2009) with many continental temperature and rainfall variations now directly related to variations in ENSO activity (Gergis and Fowler, 2005; Brown et al., 2008; Fowler et al., 2012), including vegetation change (Donders et al., 2007) and fire intensity (Black et al., 2007). The varying influence of the westerly wind belt on Southern Hemisphere climate has identified movement in the location and strength of the westerly winds and related changes in rainfall and temperature in the high latitudes (Shulmeister et al., 2004; Fletcher and Moreno, 2012; Saunders et al., 2012, 2013). The ITCZ has been identified as a dominant climate mechanism in the Southern Hemisphere, increasing in strength from the mid-Holocene (Gagan et al., 2004; Donders et al., 2007; Abram et al., 2009; McGlynn et al., 2013). The influence of the Indonesian Ocean Dipole has only recently been identified and its influence on low latitude areas is being investigated (Abram et al., 2009; Gouramanis et al., 2013). Correlations between Southern Hemisphere climate records are of considerable interest to palaeoclimatologists, and while correlations are limited by accurately dated chronologies, they provide a deeper understanding of the climate mechanisms (Neukom and Gergis, 2011; Gouramanis et al., 2013; Mathiot et al., 2013). However, most studies of this nature focus on terrestrial ecosystem response. Few studies have focussed on the interaction between climate variability and aquatic ecology (Luoto and Nevalainen, 2012; Mackay et al., 2012; Morrongiello et al., 2012). Although some studies have good correlation, many long-term palaeoclimate data sets lack sufficient resolution and are regionally fragmented or discontinuous (Hesse et al., 2004; Kershaw et al., 2007; Neukom and Gergis, 2011; Saunders et al., 2012). In addition, many of these Southern Hemisphere studies have been conducted at high latitudes in temperate climates and understanding of mid-latitude sub-tropical climate remains neglected. This highlights the need for increased knowledge of environmental responses to changes in climate, in sub-tropical aquatic ecosystems in the Southern Hemisphere. The use of diatoms as aquatic biological indicators in aquatic palaeolimnological studies is well established (Dixit et al., 1999;

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Lim et al., 2001; Bradshaw et al., 2006), and has been used extensively for climate research (Taffs, 2001; Tibby and Reid, 2004; Gell et al., 2005; Haynes et al., 2007; Taffs et al., 2008; Logan and Taffs, 2013). Fossil diatoms enable inference of water quality conditions, and associated changes within the catchment area and climate that influences the biological functioning of a lake system. In combination with other proxies such as sediment geochemistry, diatoms can be used to infer aquatic and climatic variability (Valero-Garces et al., 1997; Raubitschek et al., 1999; Edlund and Stoermer, 2000; Smol and Stoermer, 2010; Logan and Taffs, 2011; Logan et al., 2011). The aim of this study is to infer diatom community responses to climate variability in sub-tropical eastern Australia using a combination of the diatom record, physical and geochemical proxies. This will aid understanding of the response of aquatic organisms to climate variability in the Southern Hemisphere aiding conservation efforts for freshwater resources. 2. Methods 2.1. Study site Fraser Island, (25°26′53.87″ S, 153°3′11.72″ E), contains numerous unique deflation lakes (Donders et al., 2006) that are formed through large scale wind erosion during cold, dry glacial conditions (Cohen, 2003) (Fig. 1). The environment is pristine and is recognised for its natural heritage value by UNESCO World Heritage listing (IUCN, 1992). The island is sub-tropical, located in the climatic transition zone between a tropical and temperate climate (Donders et al., 2006). This is a region expected to experience significant climatic changes in the next 100 to 200 years (Lindenmayer et al., 2011; Wernberg et al., 2011). Few studies providing long-term climate records exist for freshwater aquatic environments in the sub-tropics of the Southern Hemisphere (Newnham, 1999; Sandiford et al., 2003; Turney et al., 2006). Lake McKenzie is situated in the south west of Fraser Island (Fig. 1), and was selected for its relatively pristine catchment with minimal anthropogenic impacts. It is perched 100 m above sea level, covers an area of 130 ha and has a depth of 8.5 m (Sinclair, 2008). Several palynological studies have been conducted on the perched-dune lakes of Fraser Island. Analyses on vegetation dynamics (Longmore, 1997b; Donders et al., 2006), climate variability (Longmore and Heijnis, 1999), and geochemistry (Longmore et al., 1983; Torgersen and Longmore, 1984) have been conducted using pollen as the primary biological indicator with a small number of U/Th and 14C inferred ages supported by 137Cs analysis. These studies have focussed on Lake Allom, Hidden Lake and Old Lake Coomboo Depression in the northern section of the Island (Fig. 1). Further palaeoecological research on limnology, using different biological proxies and additional sites has the potential to contribute greatly to identifying climate variability and aquatic responses to climate (Gergis et al., 2009; Smol, 2010; Hembrow and Taffs, 2012). 2.2. Location and sample collection Two duplicate sediment cores (430 and 450 mm) were extracted in November 2010 from the deepest basin of Lake McKenzie at a depth of 8.3 m (Fig. 1). The cores were extracted using a Glew gravity corer (Glew et al., 2001) from an inflatable boat platform anchored during extraction. One core (core LM1) was sub-sampled at 2.5 mm using the Glew core extruder (Glew et al., 2001) for high resolution analysis, the other at 10 mm for chronological and other analysis (core LM2). Subsamples were placed into separate zip lock bags and kept below 4 °C. The samples from core LM1 were transported to Southern Cross University, and LM2 core samples to the Australian Nuclear Science and Technology Organisation (ANSTO) for laboratory analyses. The sediment cores were homogeneous and appeared dark-brown in colour and rich in organic matter. Visible signs of lateral banding or bioturbation were absent.

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Fig. 1. Location of Lake McKenzie, Fraser Island, Australia. a) the east coast of Australia, b) south eastern Queensland, and c) Fraser Island.

2.3. Chronology Thirteen samples were prepared for 210Pb dating and twelve samples for accelerator mass spectrometry (AMS) 14C dating. Subsamples from the upper 85 mm of core LM1 were selected for 210Pb dating. These subsamples were weighed and dried at 60 °C before being placed into plastic vials and sent to ANSTO for measurement. The total activity of 210Pb was established by measuring the grand-daughter isotope Polonium (210Po) which was presumed to be in secular equilibrium with 210Pb, the supported 210Pb was determined by measuring 226Ra. The unsupported 210Pb was calculated by subtracting 226Ra activity from total 210Pb activity (Harrison et al., 2002). As the unsupported 210Pb activity of the samples showed a monotonic decay profile the Constant Initial Concentration (CIC) model was used to calculate ages and mass accumulation rates (Appleby and Oldfield, 1983). AMS 14C dating was conducted on ten samples from core LM2 and two samples from core LM1 using the accelerator mass spectrometry laboratory located at ANSTO's Institute for Environmental Research. Due to a lack of terrestrial plant macro-fossils in the cores, pollen concentrates were prepared for AMS 14C dating. The preparation procedure involved sieving to collect a 10–150 μm size fraction, separation by heavy liquid flotation (LST; SG = 1.8) and treatment with NaOH (10%), HCl (10%) and H2SO4 (98%). A single large wood fragment was retrieved from core LM1 at 212.5 mm depth, and was prepared for AMS 14C dating using the standard method at ANSTO (Hua et al., 2001), which included an acid–alkali–acid (AAA) treatment. Dates were obtained using the intercepts method (Stuiver and Polach, 1977), and calibrated using the SHCal04 and IntCal09 curves (McCormac et al., 2004; Reimer et al., 2009) and the CALIB program (version 6.0) (Stuiver and Reimer, 1993). Single calibrated ages are reported here as the median age in the calibrated age range for the radiocarbon date and rounded to the nearest 100 years. The two sediment cores were correlated by a linear regression method using tie points identified on the C% curves of both cores (Oldfield et al., 1999; Hofmann et al., 2005).

Diatom residues were mounted onto microscope slides using the mounting medium Naphrax. Slides were then examined with an Olympus CX40 compound light microscope at 1000 × magnification with oil immersion, to identify species assemblages and abundance. A count of at least 300 frustules was attempted for each sample. Diatoms were identified using various taxonomic guides (Hendey, 1964; Foged, 1978, 1979; Krammer and Lange-Bertelot, 1986, 1988, 1991a,b; Vyverman et al., 1995; Hodgson et al., 1997; Gell et al., 1999; Sonneman et al., 2000). Photographs of the diatom species are archived with the first author. The relative abundance of each species is presented as a proportion of the total frustule count, and species representing less than 2% of the relative abundance were removed to increase the significance of the data. The species information was plotted using the “C2” program (Juggins, 2007). Diatoms were grouped according to their habitat and pH preferences (Hendey, 1964; Foged, 1978, 1979; Vyverman et al., 1995; Hodgson et al., 1997; Kelly et al., 2005; Royal Botanic Garden Edinburgh, 2007). 2.5. Sediment analysis Particle size analysis was conducted on untreated samples using a Mastersizer2000 particle analyser in conjunction with Hydro 2000 software. This analysis was undertaken to determine grain size and detect any anomalies occurring between sediment samples. Three measurements were taken on each sample and average values are reported. This process was repeated for all samples. The procedure consisted of three rinses between sample measurement (Cheetham et al., 2008). In addition, loss on ignition analysis (LOI) (550 °C) was conducted on samples at 10 mm resolution to determine the organic carbon content of sediments. The method of Bengtsson and Enell (1986) was followed with dry weight, bulk density and organic content being calculated during the LOI procedure (Bengtsson and Enell, 1986; Lotter and Sturm, 1994). 2.6. Stable isotope analysis

2.4. Diatoms The diatoms were extracted from 118 sediment samples using the method of Battarbee et al. (2001) using both HCl and H2O2 processing.

Bulk untreated sediment samples were dried and homogenised for stable isotope analyses of δ15N and δ13C, to infer changes in the depositional environment of the lake. Samples were processed

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using the conventional methods of Murase and Sugimoto (2001) and Papanicolaou et al. (2003) with acetanilide used as the working standard. Sediment samples were analysed using a DeltaV Isotope Ratio Mass Spectrometer (IRMS) at the Centre for Biogeochemistry Analytical Laboratory at Southern Cross University. δ13C values were obtained using a CO2 standard gas reference (99.996%, δ13CVPDB = −6.317), and all values stated are calibrated against the absolute standard NIST 8542. δ15N values were obtained using a N2 standard gas reference (99.99%, δ15NAIR = −1.706%), and all values stated are calibrated to the absolute standard NIST 8547. 3. Results

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other studies (Thinon, 1992; Hopkins et al., 1993; Carcaillet and Talon, 1996; Carcaillet, 2001a,b). Calibrated ages suggest a sedimentation rate of ca. 0.011 mm per year between 100 and 450 mm. A large difference in age is observed between samples on core LM1 at 150 mm depth (Table 2) suggesting that a hiatus occurs from ca. 14,000 to 18,300 yBP. A deposition model has been constructed using the P_sequence program in OxCal (v 4.1 Fig. 4) (Ramsey, 2008, 2009). Three dates were excluded from the deposition model as outliers (OZO411, OZN680 and OZN681), based on their poor linear fit with the other dates, the lower overall Agreement Index produced with their individual inclusion in P_sequence deposition models, and the results of outlier analyses showing their higher posterior probabilities of being outliers compared with the other dates.

3.1. Chronology 3.2. Diatoms The unsupported 210Pb profile (Fig. 2) shows a decreasing monotonic pattern of exponential decay in the surface of five samples, to 32.5 mm depth. Samples below 32.5 mm show activities very close to background levels and were thus excluded from the age calculations. The CIC model was applied to five measurements in the top 32.5 mm of the core and yielded a sedimentation rate of 0.22 mm per year (Table 1). Ten AMS 14C dates were obtained for core LM2 (Table 2). Uncalibrated dates ranged from 2395 ± 35 yBP at 110 mm depth to 31,870 ± 180 yBP at 450 mm and correspond to respective calibrated age ranges of 2342–2688 cal. yBP and 35,575–36,783 cal. yBP. Sample depths for core LM2 were correlated with sample depths for core LM1 using linear regression between six tie points identified on the C% curves (Fig. 3). Two AMS 14C dates were obtained from core LM1, one on a wood fragment collected at 212.5 mm depth and the other on a pollen residue prepared from a sediment attached to the wood fragment. The wood fragment returned at an age that was much younger than expected (11,470 ± 60 yBP) (Table 2). It is hypothesised that the wood fragment is out of sequence and may have moved downwards in the sediment since burial, potentially as a result soil–bioturbation processes, the occurrence of which has been documented in various

Diatoms were well preserved in the LM1 core until 220 mm depth after which diatoms showed signs of differing degrees of chemical dissolution. Consequently, a diatom count of greater than 300 frustules was achieved in the upper sediments to 220 mm depth. Slides between 227.5 and 290 mm depth recorded a total of less than 200 diatom frustules. Within the Lake McKenzie sediment core a total of 38 diatom species were recorded (Fig. 5). The diatom assemblage (N 2% relative abundance) remains predominantly benthic (e.g. Brachysira brebissonii (Kützing) Ross, Kobayasiella subtilissima (Cleve) Lange-Bertalot and Frustulia rhomboides (Ehrenberg) De Toni) (Fig. 5). Aulacoseira distans (Ehrenberg) Simonsen and Tabellaria flocculosa (Roth) Kützing are the only dominant planktonic species. Overall, the diatom assemblage is relatively stable with B. brebissonii dominant throughout the record. At the base of the core (below 227.5 mm) diatom preservation is poor making environmental inferences problematic. From 227.5 mm to 110 mm depth the relative abundance (RA) of B. brebissonii and K. subtilissima increases slightly, and F. rhomboides and Amphora arenaria (Donkin) decrease, a trend that continues upwards from 215 mm. In the upper core from 110 mm to

Unsupported 210Pb (Bq/kg) 0 0 2

100

200

300

400

500

600 2005 1999 1993

4

1987

6

1981 1975

Depth (mm)

10 12

1969 1963 1957 1951

14

1945

16

1939 1933

18 20 22

1927 1921 1915 1909

24

1903

26

1897 1891

28 30

1885 1879 Fig. 2. Unsupported 210Pb for Lake McKenzie core 1, plotted against depth and age in calendar years.

Calendar years

8

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Table 1 210 Pb results and age estimations for Lake McKenzie surficial sediments. CRS = constant rate of supply and CIC = constant initial concentration. ANSTO ID

Depth (mm)

Total 210Pb (Bq/kg)

Supported 210Pb (Bq/kg)

Unsupported 210Pb (Bq/kg)

Calculated CIC (years)

Calculated CRS (years)

Calculated age

M897 M898 M899 M900 M901

0–2.5 2.5–5 15–17.5 17.5–20 30–32.5

614.4 490.8 168.3 97.9 31.0

14.1 17.5 18.7 14.7 19.6

601.2 483.7 149.9 85.0 11.5

5 15 65 75.0 131

5 13 64 76 130

2005 1995 1945 1935 1879

± ± ± ± ±

12.0 23.7 5.7 4.1 0.8

± ± ± ± ±

1.8 1.6 2.1 1.6 1.6

the surface the dominant diatom assemblage is stable with minor fluctuations in abundance at 95 mm to 97.5 mm and 37.5 mm to 40 mm. A number of less dominant species peak in abundance in this section of the core. A summary of the diatom ecological preferences shows that aquatic conditions have favoured an acid tolerant diatom community throughout the record (Fig. 5), suggesting that the aquatic ecosystem has been acidic throughout the Holocene and late Pleistocene. The habitat preference summary also clearly demonstrates the dominance of benthic species during the last 37,000 years, with an increase of planktonic species in the early Holocene (~9,000 cal. yBP), which is sustained at high levels for approximately 6000 years. 3.3. Sediment analysis Particle size analysis of Lake McKenzie samples shows high silt composition in all samples, ranging from 42 to 88% of content (Figs. 6 and 7). The composition of sand particles also fluctuates down core, ranging from 12 to 56%. Clay particles were detected in negligible amounts with the highest, 3.62% at 252.5 mm, and the lowest 0.19% at 165 mm. The cumulative frequency distribution of all samples is relatively uniform with a range of particle sizes between φ 12.2 to φ −1.1 (0.2 μm to 2187.76 μm). 3.4. Stable isotopes Analysis of nitrogen and carbon isotopes suggests a number of significant sedimentary changes in the core (Fig. 7). Concentrations of δ13C fluctuated with a maximum of −22.6‰ at 170 mm and the minimum of −26.8‰ at 230 mm, indicating a range of 4.2‰. Total organic carbon content was at its lowest (8%) at a depth of 167 mm, with the highest values (58%) recorded at the base of the core. Results for δ15N show that concentrations fluctuate throughout the core with a maximum of 3.5‰ at 130 mm, and minimum of −0.5‰ at 60 mm, indicating a range of 4‰. Percent nitrogen was greatest in samples between 180 and 280 mm (1.1%), and the surface sediments (1.4%). Both total carbon and nitrogen show similar curves with a reduction of content at 180 mm (ca. 8000 cal. yBP) before gradually returning to similar high levels. The carbon to nitrogen ratio (C/N) varied throughout the

± ± ± ± ±

12.1 24.2 6.1 4.5 1.8

± ± ± ± ±

5.0 5 7.0 8 14

± ± ± ± ±

2 4 8 9 11

core with the maximum values (61) recorded at 314 and 331 mm (ca. 28,623 to 30,898 cal. yBP), and the lowest values (b20) recorded in the surface at 48 mm of sediment (ca. 1935 cal. yBP). The C/N shows a gradual decreasing trend up-core, but is interrupted at 263.6 mm depth (ca. 23,388 cal. yBP) with a value of 24 before increasing to 45 then continuing to decrease towards the surface. 4. Discussion 4.1. Chronology 210

Pb dating results demonstrate a very slow sedimentation rate in the near-surface sediment of Lake McKenzie. Only the surface at 35 mm of the sediment showed measureable levels of unsupported 210 Pb. Nevertheless, the results of the top five samples analysed provided a robust 210Pb chronology. The pattern of monotonic decay of unsupported 210Pb indicates that the CIC model is suitable for age estimations. An average sedimentation rate of 0.22 mm per year was calculated from the 210Pb dates. The AMS 14C dates also confirm the very slow sedimentation rate in Lake McKenzie with an average sediment accumulation rate of 0.011 mm per year below 35 mm depth. This is comparable to other studies on Fraser Island lakes: 0.01 mm per year at Old Lake Coomboo Depression (Longmore and Heijnis, 1999) and 0.06 mm per year at Lake Allom (Donders et al., 2006). A low sedimentation rate is to be expected on a sand island dominated by rainforest vegetation where sediment sources are few (Donders et al., 2006). A total of 12 AMS 14C dates were obtained for the Lake McKenzie cores. Tie points identified on C% curves were used to correlate the core depths and construct a chronology that includes dates from both cores LM1 and LM2. Key limitations associated with this tie point approach are that constant rates of deposition are assumed between tie points, and the selection of tie points was based on visual inspection of the C% curves only. Age reversals are apparent in the Lake McKenzie AMS 14C dates. Three anomalously young dates were produced and these were excluded from the deposition model. Contamination of lake sediments by young carbon can occur and be caused by plant roots penetrating the sediments, humic acids infiltrating into the sediments, or bioturbation (Olsson, 1991; Bjorck and Wohlfarth, 2001).

Table 2 Radiocarbon dates and calibrated ages for Lake McKenzie cores. Calibrated ages based on the IntCal09 and SHCal04 curves are provided (McCormac et al., 2004; Reimer et al., 2009). ANSTO code

Core

Original sampling depth (mm)

Corresponding depth on core LM1 (mm)a

Material type

14

C age (years BP)

Error (1σ)

IntCal09 (years BP, 2σ)

OZN680 OZN695 OZN683 OZN684 OZN685 OZO411 OZN686 OZO412 OZN687 OZN688 OZN689 OZN690

LM1 LM1 LM2 LM2 LM2 LM2 LM2 LM2 LM2 LM2 LM2 LM2

212.5–215 212.5–215 100–110 150–160 200–210 230–240 250–260 260–270 300–310 350–360 400–410 450–460

212.5–215 212.5–215 61–68 101–110 129–133 141–146 150–158 158–166 191–199 224–230 256–265 292–300

Pollen residue Wood fragment Pollen residue Pollen residue Pollen residue Pollen residue Pollen residue Pollen residue Pollen residue Pollen residue Pollen residue Pollen residue

19,150 11,470 2395 3785 6485 4515 12,110 15,100 18,670 23,270 30,940 31,870

±210 ±60 ±35 ±35 ±50 ±40 ±70 ±70 ±100 ±120 ±190 ±180

22,330–23,428 13,188–13,457 2342–2688 3999–4288 7279–7483 5044–5309 13786–14148 18026–18589 21872–22545 27785–28499 34924–36280 35575–36783

a

Corresponding depths are estimated using linear regression between tie points (Fig. 3).

SHCal04 (years BP, 2σ)

2183–2654 3931–4230 7260–7431 4885–5294

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Fig. 3. Tie points for cores LM1 and LM2 using C% curve.

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While the use of pollen concentrates for radiocarbon dating is designed to remove potential contamination sources, contaminants have been shown to occasionally persist with residual organic matter (Vandergoes and Prior, 2003; Newham et al., 2007). The low number of terrestrial plant macrofossils in the Lake McKenzie sediment cores resulted in the chronology relying on radiocarbon dated pollen concentrates. A single macrofossil encountered in the Lake McKenzie core (a wood fragment) yielded an age estimate that fitted poorly with the other radiocarbon dates, and was excluded from the deposition model. Potentially the anomalously young age of the wood fragment is a result of it being displaced in the sediment sequence, or having an origin as a plant root, occurring at the site during a period of lower water level. The deposition model shows that the age of the sediment core ranges from modern to ca. 37,000 cal. yBP, providing scope for a longterm analysis of aquatic responses to climate fluctuations. It is apparent from the age depth curve that sedimentation ceased in the period of 14,000–18,300 cal. yBP. This partly corresponds to a hiatus observed in the sediment record at Lake Allom between 12000 and 35000 cal. yBP (Donders et al., 2006). This was suggested to result from a reduction in moisture availability related to the last glacial maximum, a cold, dry and windy period during which both lakes may have been completely dried. With limited in-flow and out-flow, perched lakes such as these have water levels that are highly sensitive to change in precipitation or evaporation (Longmore and Heijnis, 1999). The Lake McKenzie hiatus is of much shorter duration than the Lake Allom hiatus, Lake McKenzie is currently about 5 m deeper than Lake Allom (Donders et al., 2006). Conversely, a second hiatus apparent at Lake Allom between 5500 and 8000 cal. yBP was not recorded in the Lake McKenzie sediments, possibly because it has deeper water and the dry period was insufficient to dry the lake. 4.2. Diatoms

Fig. 4. Age–depth curve for Lake McKenzie based on ten AMS 14C dates obtained from core 2.

Diatom preservation was excellent for the top 220 mm enabling confident inferences of environmental conditions. However, below 220 mm diatom preservation deteriorated. This can occur in lakes as a result of changes in physical and chemical conditions (Ryves et al., 2006) and lakes with low biogenic silica (Dong et al., 2008). Frustules were partially chemically dissolved, but showed little evidence of mechanical degradation. Given the age of the sediments at this depth (28,623 cal. yBP), this is a natural and expected phenomenon which could also indicate increased salinity or changes in pH levels (Ryves et al., 2006, 2009). The diatom data indicate only minor aquatic changes in Lake McKenzie over the last ca. 37,000 cal. yBP. Throughout this period B. brebissonii, a benthic freshwater acidic loving diatom (Wehr and Sheath, 2003), has been dominant although its relative abundance has not been constant and has increased over the last 5000 years. The other dominant benthic diatoms include K. subtilissima and F. rhomboides. These species are common in freshwaters, however, K. subtilissima has a preference for alkaline conditions while F. rhomboides is known to withstand waters of high acidity (Foged, 1979; Gell et al., 1999). In many occasions the abundance of F. rhomboides increases when K. subtilissima declines, suggesting alternating fluctuations of pH conditions in the water body. The planktonic species A. distans and T. flocculosa are a minor component of the assemblage. The under representation of planktonic species in the diatom assemblage may indicate environmental conditions not conducive to a diverse planktonic community such as poor light penetration due to turbidity or dissolved tannins (Smol et al., 2002; Smol and Stoermer, 2010). At the time of core retrieval water depth was ~8 m and transparency was low due to tannins in the water body sourced from surrounding vegetation. These tannins cause water to become acidic, with Lake McKenzie recording pH values around 4 (Sinclair, 2008; Water Quality and Ecosystem Health Unit, 2010). A. distans did have higher relative abundance following the sediment hiatus, from ca. 14,000 to

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Fig. 5. Dominant diatom species (N2% RA) plotted against depth and age (cal. 14C) in addition to summary diagrams of pH and habitat preferences for LM1. Ages are inferred from calibrated AMS 14C for Lake McKenzie LM2. Grey bar indicates hiatus.

5000 cal. yBP. The less prevalent species in the diatom community such as Pinnularia major (Kützing) Rabenhorst and T. flocculosa may be good indicators of changing water chemistry. Both species have a

tolerance to highly acidic waters (b5.0 pH), and prefer dystrophic systems (Werner, 1977; Gell et al., 1999). P. major occurs in fresh to brackish waters and often thrives in humic and shady habitats

100 90 80

Weight (%)

70 60 50 40 30 20 10 0 0.1

1

10

100

1000

Particle diameter (µm) Fig. 6. Cumulative frequency distribution by weight of Lake McKenzie sediment core.

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781

Fig. 7. Stable isotope, C%, N%, particle size analysis and loss on ignition results for Lake McKenzie. Grey bar indicates sediment hiatus.

(Kelly et al., 2005). T. flocculosa is a freshwater species that can occur in acidic peat swamps (Gell et al., 1999). Based on published literature describing ecological tolerances of diatom species, a summary of the pH optima and habitat preferences was constructed (Fig. 5). This shows that a majority of the diatom community prefers slightly acidic conditions with average pH optima of ~6.0. Species preferring alkaline conditions composed on average 23% of the overall community. The habitat summary diagram also shows minor fluctuating conditions inferred by the diatom community, in particular an increase in the planktonic A. distans between 14,000 and 5000 cal. yBP. Longmore (1997b) suggested a gradual drying throughout the Holocene based on pollen evidence from Old Lake Coomboo Depression and Hidden Lake. This may be supported by the diatom record showing a decrease in planktonic species over time. 4.3. Sediment analysis Sediment analysis demonstrates that the core is dominated by wellsorted silt particles, with sand sized particles forming only a minor component, on average 33%. The sediment particles are well sorted, generally indicating that energy within the depositional zone has remained low, and no major changes in sediment transportation have occurred (Lewis and McConchie, 1994a,b). Bulk density increases with depth as

the sediment becomes increasingly compacted (Bengtsson and Enell, 1986; Donders et al., 2006). Loss on ignition decreases with depth as a result of organic material breaking down (Longmore, 1997b). Naturally, there are minor fluctuations in both these parameters suggesting minor variation in environmental conditions and sources of organic matter. 4.4. Stable isotope ratios and elemental carbon and nitrogen content Total carbon was high compared to N%, reflecting the high organic content and the oligotrophic status of Lake McKenzie through time. The variation in δ13C values indicates there have been changes in the source of carbon entering the lake system over the study period. The δ13C values are within the range of terrestrial C3 plants (Rundel and Ehleringer, 1989; Logan et al., 2010), but are also within the range of other types of organisms, such as the green colonial alga Botryococcus (e.g. Huang et al., 1999; Fuhrmann et al., 2003). Botryococcus is known to have wide ranging δ13C compositions as a result of differences in photosynthetic conditions during periods of growth (Grice et al., 1998). The more positive δ13C between ca. 11,000 and 20,000 cal. yBP suggests a change in the major sources of organic matter over that time period. Variation in δ15N values is relatively small and therefore suggests that change to nitrogen consumption within the lake was minor. The increase in δ15N between ca. 4000 and 8000 cal. yBP infers an increase

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in primary productivity by algae and photosynthetic organisms (Smol and Cumming, 2000; Smol et al., 2002). This correlates with the increase in A. distans and Donders et al. (2006) findings of increased lake depth of Lake Allom during the early Holocene. The C/N ratios also suggest that there have been changes in the source of organic material entering the lake. C/N ratios are greater than 11 through the record (Fig. 7), and this suggests a dominant source of organic matter from terrestrial vegetation (Meyers, 2003). However, high C/N ratios are also reported in certain algae such as Botryococcus (Huang et al., 1999), which occur in Fraser Island lakes (Bayly et al., 1975). High C/N ratios in the lower section of the core (430 to 310 mm), and decreased C/N ratios occur in the upper sediments (from ca. 4000 cal. yBP to present) suggest variations in the input from terrestrial carbon sources, perhaps as a result of changes in effective precipitation (Drew et al., 2008), vegetation dynamics (Longmore and Heijnis, 1999) and climate mechanisms (Bickford and G., 2005). This is supported by Longmore's (1997a) findings of a trend towards increased aridity within the sediment record of Lake Allom and Hidden Lake, Fraser Island. This study provides a valuable record of diatom community related aquatic response to climate mechanisms within the sub-tropics of the Southern Hemisphere. The record shows Holocene variability that relates to other studies in the Southern Hemisphere (Longmore, 1997b; Longmore and Heijnis, 1999; Gagan et al., 2004; Shulmeister et al., 2004; Turney et al., 2006; Donders et al., 2007). At Lake McKenzie the diatom assemblage has not significantly changed in biodiversity or community composition. Given that few studies in the Southern Hemisphere have focused on freshwater aquatic biological community response to climate drivers, these findings provide significant insight. Increasing understanding of how freshwater ecosystems have responded to climate change over time will assist in the management and conservation of aquatic resources on a global scale. 5. Conclusions Lake McKenzie has provided a record of minor climate fluctuations ca. 37,000 cal. yBP, and the chronological record indicates a major hiatus between ca. 14,000 and 18,300 cal. yBP. The biological, sedimentological and geochemical records indicate only minor fluctuations within the lake over the past 37, 000 cal. yBP. The slight increase in the phytoplankton community in the early Holocene (~ 9,000 cal. yBP), and the increase in δ15N values between ca. 8000 and 4000 cal. yBP, suggest a wetter phase with increased lake depth and primary productivity. However, overall the trend has been for increasing abundance of benthic diatom species and a decreasing C/N ratio indicating shallower lake conditions and possibly a reduction of terrestrial plant material entering the lake due to declining effective precipitation. There are many minor fluctuations in the biological and geochemical records that are cyclical and may relate to climatic mechanisms such as the ENSO that merits further investigation in this region with higher resolution records. Further palaeoecological studies focusing on diatom communities and other aquatic biological communities within the Southern Hemisphere will provide greater understanding of the timing and nature of climatic processes on a regional scale and impacts on aquatic biota. Acknowledgements We would like to thank Kerry Wilsher and Linda Barry (ANSTO) for field work assistance, Mattheus de Carvellho de Carvellho (SCU) for isotope laboratory analysis and Mick Cheetham (Southern Cross Geoscience) for assistance with sediment analysis. The lead author is the recipient of an Australian Postgraduate Award (APA). Lastly, we would like to thank the reviewers of the manuscript for their valuable comments.

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