Hydrobiologia – Supplementary Information (Gell et al.)
Methods
Sample collection
Sampling of Lake Colac was undertaken in November 2008 and The Curdies Inlet was sampled in
March 2009. At both lakes sediments were collected using a piston corer a Russian ‘D’ section coring
device; an 80 cm long core was retrieved from Colac and almost 6 metres of sediment was taken from
the Curdies estuary.
Chronological analyses
Lead-210, Caesium-137 and Radon-226
Sediment samples from the upper 20 cm of Lake Colac were analysed for 210Pb, 226Ra and
137
Cs by direct gamma assay at Liverpool University’s Environmental Radioactivity Laboratory, UK.
Radiometric dates for the core were calculated using the constant rate of supply dating model
(Appleby, 2001). The atmospheric fallout of 210Pb is known to be much lower in Australia and as a
result, the dating was restricted to the upper 8 cm of the core.
Radiocarbon (Carbon-14)
The deeper core sections of Lake Colac were dated using AMS 14C dating of bulk sediment samples;
dates were obtained in order to establish a full chronology for the sediment sequence.
All of the samples for radiocarbon dating, were prepared using an Acid-Alkali-Acid (AAA)
Treatment. The results were reported as conventional radiocarbon years before present (BP, relative to
AD 1950). Calibrated ages were derived from 14C dates using the OxCal program (v4.1.7;
Bronk Ramsey, 2009) using the southern hemisphere calibration curve. The radiocarbon dating was
undertaken at the University of Waikato’s Radiocarbon Laboratory, New Zealand.
Diatom analysis
The standard technique of diatom analysis follows the method as set out by Battarbee (1986).
Approximately 0.5 g of wet weight material per sample was used for the analysis, to which 30%
hydrogen peroxide (H2O2) was added to oxidise the organic material present and heated on a hotplate
to 90 °C for 4 hours, topping up the H2O2 when necessary to prevent the samples from boiling dry. In
the case of extremely organic-rich sediment, the samples were left in cold H2O2 for 24 hours to
prevent the samples from boiling over when heated. Following the oxidisation process, 10%
hydrochloric acid (HCl) was added to the samples to dissolve any carbonates present and to neutralize
the sample. Samples were washed four times after the chemical treatments to remove any salts that
had formed during the preparation process. The samples were allowed to settle for 24 hours between
each wash. After washing the samples were diluted and placed on coverslips and allowed to dry.
These strewn slides were mounted in Naphrax and at least 300 valves per sample were counted in
parallel transects under oil-immersion phase-contrast light microscopy (LM) at x1000 magnification
on a Zeiss research microscope.
A variety of general (Krammer and Lange-Bertalot, 1986-1991) and region-specific floras (e.g.
Sonneman et al., 2000) were consulted, and valves identified to species level where possible. The
dissolution of the diatom valves was assessed using a four-scale system (pristine and dissolved; Ryves
et al., 2001). This ratio varies from 0 (all valves partly dissolved) to 1 (perfect preservation). All data
was recorded manually on count sheets and then transferred into an electronic database to allow
further analyses.
Pollen analysis
1 cm3 sub-samples were taken from 2 mm slices at regular intervals where possible from each core for
pollen and charcoal analysis. Preparation involved sediment dispersal in 10% sodium pyrophosphate,
treatment with potassium hydroxide to remove organic matter, sieving through mesh sizes of 180 μm
and 7μm to remove unwanted large and small fragments respectively, treatment with hydrochloric
acid to remove carbonates, acetolysis (a mixture of acetic anhydride and sulphuric acid) to reduce
cellulose matter and darken grains to make them easier to identify, treatment with hydrofluoric acid to
dissolve silicate material and heavy liquid (specific gravity 2.0) separation of any remaining mineral
matter from the pollen residue using sodium polytungstate. A known amount of exotic Lycopodium
spores was added to each sample at the beginning of the preparation process, to allow for pollen and
charcoal concentrations to be calculated. Prepared samples were mounted on microscope slides for
identification and counting of pollen grains and charcoal.
Pollen and charcoal counts were undertaken using either an Olympus model BHB or a Zeiss Axiorod
compound light microscope at x 600 magnification. Sample counts were continued until a total of 100
grains of major regional south-east Australian dryland taxa had been recorded. All pollen taxon
percentages were calculated in relation to this major pollen taxon sum to prevent taxa such as
Chenopodiaceae that can be derived from locally growing as well as regional plants, and local
aquatics, distorting regional vegetation reconstructions. Microcharcoal particles, greater than 5 μm
maximum diameter, were counted with Lycopodium spores along selected transects and expressed as
particles per cm3 or as charcoal/pollen sum ratios.
Statistical analysis
TILIA [Pollen data]
The results of pollen analysis on each lake are presented as two pollen diagrams prepared using
TILIA (Grimm, 2004); one for predominantly aquatic taxa and one for mainly dryland pollen taxa and
charcoal. A moisture index, based on percentages of predominantly freshwater taxa (Myriophyllum,
Pediastrum and Botryococcus) and saline tolerant taxa (Chenopodiaceae, Myriophyllum muelleri and
Ruppia), is included on the aquatic pollen diagram. Each diagram was zoned independently, on either
all aquatic taxa or pollen sum taxa, using a stratigraphically constrained hierarchical classification
technique, CONISS, contained within TILIA (Grimm, 2004).
C2 [Diatom data]
For all stratigraphic diatoms, all diatom specis >10% in any sample are shown as percentage
abundance and ordered according their down-core weighted averaging abundance (species ordered in
terms of their occurrence in the core). The cores are plotted against depth and dates (where available)
are presented alongside the y-axis. The diatom zones presented are based on the output of ZONE.
Zone [Diatom data]
The stratigraphical diatom data from each core were divided into assemblage zones using optimal sum
of squares partitioning (Birks and Gordon, 1985) by the program ZONE (version 1.2; Juggins, 2002).
Ordination: Detrended Correspondence Analysis (DCA) [Diatom data]
Indirect ordination analyses were carried out using CANOCO 4.5 (ter Braak and Šmilauer, 2002) to
identify the dominant trends within the data. Initially a Detrended Correspondence Analysis (DCA;
Hill and Gauch, 1980) with detrending by segments, and down-weighting of rare species, was used to
explore the main patterns of taxonomic variation among sites and to estimate the compositional
gradient lengths of the first few DCA axes. The diatom percentage data were transformed using log
transformation in an attempt to reduce clustering of abundant or common taxa at the centre of origin
(Leps and Šmilauer, 2003). The gradient lengths allow the determination of the most appropriate
response model for further analysis. If the gradients were sufficiently long (>1.5 s.d.), it indicated that
numerical methods based on a unimodal response model were most appropriate (e.g. [Detrended]
Correspondence Analysis; CA or DCA).
Results: Curdies Inlet
The diatom stratigraphy from the Curdies Inlet, Peterborough spans the last c. 6000 years. Eightythree species were identified across 75 samples (selected taxa >10% are shown in Figure 1), and three
statistically significant zones were identified. Dating of this core is being undertaken and the dates,
where presented, are from a parallel dated sequence that has been correlated on the basis of the major
changes in the diatom stratigraphy.
CUR-1 (550-50 cm) is dominated by three main species, Pinnularia yarrensis (river derived?) and the
marine/brackish flora Grammatophora oceanica and Paralia sulcata. These species are consistently
present in the lower record in abundaces greater than 20%. Pinnularia yarrensis is present in
generally lower abundance (from 3-45%) compared to Grammatophora oceanica (8-52%) and Paralia
sulcata (3-54%). Whilst there are small fluctuations in the presence of these taxa in the lower
stratigraphy (notably around 500 cm for all 3 taxa; and 150 cm and 70 cm for P. sulcata which is
temporarily replaced by the saline/nutrient tolerant Cyclotella meneghiniana), there is little in the way
of variation. Whilst many of the inland ‘rain-gauge’ lakes were demonstrating sensitivity to shifts in
moisture balance over the last 6000 years, the Curdies Inlet appears to be relatively stable over the
long-term, showing influences of both the freshwater river and tidal/marine realms.
The first statistically significant shift in the diatom assemblage data is noted at c. 50 cm (CUR-2; 50-7
cm), where there is a decrease in the presence of Pinnularia yarrensis, which may be an indicator of
declining brackish influence on the Curdies estuary. Grammatophora oceanica still has a strong
presence in this zone (with abundances between 20 and 40%) and Paralia sulcata increases in
abundance, to percentages higher than those recorded in the lower stratigraphy (10-80%). In addition
to this, the diversity of diatom taxa increases dramatically in this zone, with the appearance of 7 other
taxa with abundances >10% in any one sample. CUR-2 has taxa indicative of fresh, oligotrophic
conditions (Discostella stelligera) interspersed with periods of higher salinity (Amphora
coffeaeformis) and higher salinity/nutrients (Cyclotella meneghiniana); there is also an increase in
species indicative of well formed littoral areas and the establishment of submerged aquatic vegetation
coincident with the less saline conditions (Sellaphora pupula, Planothdium delicatulum and
Cocconeis krammeri). Towards the top of the zone the assemblage sees the appearance of taxa
indicative of more brackish water (Cocconeis scutellum and Cocconeis placentula). This precedes a
major switch in the diatom assemblage data at 7 cm (CUR-3; 7-0 cm). The uppermost zone sees the
disappearance of the Pinnularia yarrensis following its decline through CUR-2 and the decline and
disappearance of the former dominant taxa Grammatophora oceanica and Paralia sulcata, perhaps as
a result of the estuary becoming increasingly isolated from the marine environment (the development
of sand bars across the estuary mouth are frequently observed at the Curdies Inlet). Whilst a number
of species from CUR-2 are still present in CUR-3, they tend to occur in lower abundances (D.
stelligera, A. coffeaeformis, P. delicatulum and C. krammeri) although S. pupula does increase in
abundance. The dominant species in the upper sediments are Cocconeis placentula, C. scutellum and
C. stauroneiformis and suggest a well-established submerged macrophyte community in the estuary,
likely as a result of relatively shallow water. Cocconeis placentula and C. scutellum are present in
abundnces >40%; Cocconeis stauroneiformis peaks in abundance at 7 cm (40%) and then declines
towards the surface (10%). These species are tolerant of brackish conditions and can withstand
fluctuating water chemistry having broad tolerances to a range of chemical parameters.
Results: Lake Colac
Lead-210 Chronology
Lead-210 dates were calculated using the Constant Rate of Supply (CRS) model, placing 1964, the
year of peak nuclear weapons fallout in the southern hemisphere, at a depth of about 6 cm. The 210Pb
data below 6 cm was not reliable. The deepest sample, had significant unsupported 210Pb
concentrations (9-9.4 cm) and is dated to the early part of 20th century. These results are reasonably
consistent with the 14C dates. In the absence of a good 137Cs record the 210Pb chronology is treated
with some caution.
Radiocarbon Dating
Seven dates were obtained from the Lake Colac sediment core, and were all taken below the
appearance of Pinus in the pollen record to ensure that the samples submitted would be old enough for
the use of this dating technique. Five of the seven dates obtained from Lake Colac fall in stratigraphic
order (Gell et al., 2012), suggesting that whilst there is likely to be limited disturbance in the core
sequence (Supplementary Table 2).
Terrestrial and aquatic pollen
There are extreme variations in taxon representation below about 35 cm in Lake Colac
(Supplementary Figures 2 and 3), a number of which may be the result of differential preservation
rather than vegetation change. Consequently, the diagram is not presently zoned and is little
considered below this depth. Perhaps the most significant change in the diagram occurs around 25 cm,
the zone LCP-3/4a boundary, with a switch from a macrophyte-dominated to an algal-dominated
aquatic system. The dominant macrophyte is the submerged plant Myriophyllum, identified to the
species M. salsugineum from recorded seeds, although values are exceeded by those of Rumex
between 35 and 40 cm depth. Myriophyllum is replaced largely by the alga Botryococcus. The change
may represent an increase in water depth although, considering that the maximum attainable depth of
the lake is only about 2.5 m, it may be more related to a loss of visibility due perhaps to increased
turbidity. This event is dated to about 500 years ago, long before the arrival of Europeans, whose
activities may have been considered a cause of the change in state of Lake Colac. However, it is a
common phenomenon for radiocarbon ages to appear several hundred years too old in recent
sedimentary records, possibly due to reworking and redeposition of carbon with intensification of
human impact on the landscape, and this event may mark the earliest possible time of European
arrival. There is, though, little indication of influence on dryland vegetation and unless evidence
emerges for special, early attention to the lake, then a more natural cause, such as an increase in
effective rainfall, seems most likely.
The initial impact of Europeans, or perhaps more specifically their agriculture, is at the subzone LCP4b boundary, marked by a sustained increase in spores of the moist ground liverwort Anthoceros and
dryland understorey Pteridium (bracken). The former may also indicate a fall in lake level, as also
possibly suggested by the reduction in Botryococcus values. This period also witnesses the beginning
of a sustained decline in pollen of Casuarinaceae, a common feature of early European impact due to
the selection of its timber for fuel and construction. There is no doubt that agriculture was well
established by the subzone LCP-4c/5 boundary as exotics, in the form of shelter trees Pinus and
Cypress Pine (Cupressaceae), and the pastoral weed Plantago, had arrived and begun flowering.
Earlier occurrences of pine pollen are no doubt contaminant resulting from the method of sediment
sampling and perhaps also pollen in the atmosphere, as pine pollen is a notorious contaminant.
The zone 4c/5 boundary indicates the establishment of a landscape similar to that of today with
extensive pine plantations and a more open tree cover demonstrated by high percentages of Poaceae
as well as reduced eucalypt and Casuarina percentages. Through the period though there are general
reductions in disturbance taxa such as Plantago, Pteridium and Anthoceros that may reflect increasing
conservation values. However, Asteraceae Liguliflorae percentages have increased and stay high in
this period. Perhaps they indicate weeds of a more arable agricultural system this century. It is
interesting that M. salsugineum has not re emerged considering that conditions have been frequently
dry during the last 80 years, yet Myriophyllum muelleri, an indicator of more saline aquatic
environments, is conspicuous through zone 1. Its only previous representation of note is a peak within
zone 3 and could indicate a dry period prior to European presence. However, the very different
assemblages associated with the early and later phases of M. muelleri demonstrate the importance of
factors other than water availability on aquatic succession.
Macrofossils
Samples for macrofossil analysis were much smaller than desired, due to high-resolution sampling of
the sediment cores; however they were sufficient to provide an idea of the types of macro remains that
were present. They were also sufficient to provide some taxonomic resolution to the pollen data.
The macrofossil record contained six identified vascular plant taxa, one freshwater sponge species,
and between 3 and 5 charophyte species, were present. The record also contained a number of
invertebrate macrofossils including 5 or 6 cladoceran species, 1 bryozoan, at least 4 trichopteran
(caddis-flies) taxa and a range of other beetles, true bugs, ostracod and mites. A summary diagram is
presented with selected macrofossil taxa (Supplementary Figure 4).
The presence of macrofossils in the lower part of the record adds some taxonomic refinement to the
pollen data; species were assigned to the aquatic pollen genera Myriophyllum salsugineum
(Myriophyllum), Lepilaena cf. bilocularis (Lepilaena) and Azolla filiculoides (Azolla), and to the
families cf. Chenopodium glaucum (Chenopodiaceae) and cf. Bolboschoenus medianus (Cyperaceae).
The macrofossils also add ecological depth to the interpretation of the record, reflecting both the
submerged lake vegetation (Myriophyllum salsugineum, Lepilaena cf. bilocularis and Azolla
filiculoides) and those species growing at the lake edge (Juncus cf. pallidus, cf. Bolboschoenus
medianus and cf. Chenopodium glaucum). Freshwater to brackish conditions are suggested by the
plant species and their modern habitat preferences.
The presence of Daphnia sp. throughout the lower section of the record supports moderately fresh to
brackish water and indicates that salinity levels were no greater than 5.8‰ during this period. In
agreement, the charophytes present in the lower sediments Chara globularis/vulgaris type and Nitella
sp. suggest freshwater fluctuating to brackish water with salinity no greater than 5‰. The
disappearance of these taxa, along with all Trichopteran types and Cladoceran type 4, around 25 cm
could support the proposed drought and increase in salinity shown by the diatom record at this point
(or may simply be an artefact of the small sample size).
The second main change occurs at 47.6-47.8 cm where there is a loss of three cladoceran types
coincident with the appearance of Myriophyllum salsugineum, cf. Chenopodium glaucum, Azolla
filiculoides and Lepilaena cf. bilocularis. This coincides with the inferred AD 1000 drought in the
diatom record.
Diatoms
The diatom stratigraphy shows much change in the condition of Lake Colac over the last 5550 years
(Supplementary Figure 5). Seven zones have been statistically identified in the core.
The broadly tolerant Cyclotella meneghiniana is usually common. The base of the core (zone LCD-1)
supports an unusual population of aerophilous taxa suggesting a shallow, muddy substrate of
inwashed littoral sediments. This scenario is supported by the testate amoebae: diatom ratio. Testate
amoeba are small soil dwelling creatures and are often washed into lake sediments from the littoral
areas. In this research testate amoeba scale are used to infer in-wash from the littoral areas, while they
may also be used to infer the extent of littoral areas around the lake (which may occur during periods
of lower lake levels). The halophilous taxon Amphora veneta is also present revealing a naturally
brackish condition for Lake Colac. Throughout LCD-2 (44 cm) the diatom flora is represented by taxa
with varying salinity tolerance, but including some preferring fresh or oligosaline conditions e.g.
Gomphonema affine, Nitzschia lacuum. The appearance of Amphora coffeaeformis and Navicula
tenelloides in LCD-3 marks an increase in salinity. From here progressive changes occur with small
peaks in the eutraphentic Nitzschia palea, and increase in the nutrient-salinity planktonic form
Actinocyclus normanii (LCD-4). This latter change reveals an increase in depth, and salinity, possibly
suggesting increased catchment contribution of salt, independent of drought. The section from 30-25
cm hosts the greatest representation of saline taxa, and particular those capable of withstanding
hypersaline conditions (Amphora coffeaeformis, Gyrosigma spp. (particularly G. attenuatum) and
Navicula tenelloides, as well as Amphora veneta, and a peak in the Chaetoceros: diatom ratio). The
period represented in LCD-4 (c. 27 cm) marks the best evidence for a major drying event, which is
currently dated to AD 1500, prior to the arrival of Europeans and concomitant with the onset of the
northern hemisphere’s climate phenomenon the “Little Ice Age”. Other, lesser saline events appear in
zone LCD-5 (21 and 11 cm). An increase in sediment flux and lake water turbidity may be reflected
by the increase in Fragilaria spp. which peak after this AD 1500 salinity event.
The Chaetoceros: diatom ratio, and abundance of Actinocyclus normanii, increases through LCD-6
suggesting increasing lake salinity. The abundance and diversity of salinity indicators increases in
LCD-7 (from 3 cm) and reaches a peak for the entire record in the middle of this zone. It includes the
presence of the halobiontic Staurophora wislouchii. The plankter Cyclotella meneghiniana and
tychoplanktonic Fragilaria spp. are largely lost.
The ordination analyses (DCA) particularly DCA axis 2 suggest unprecedented conditions in the Lake
Colac system in the last 30 years than has been observed for the entire 6000 year record. This change
in the statistical analyses is supported by a 3-fold increase in the diatom-inferred conductivity (a
proxy for salinity).
Acknowledgements
Pollen analysis and the interpretation of the pollen data were undertaken by Prof. Peter Kershaw and
Dr Merna McKenzie (Monash University). Macrofossil analysis was undertaken by Tara Lewis (PhD
candidate at Monash University).
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