Effects of recent increases in salinity and nutrient ... - Springer Link

Hydrobiologia (2010) 649:207–216 DOI 10.1007/s10750-010-0246-3

PRIMARY RESEARCH PAPER

Effects of recent increases in salinity and nutrient concentrations on the microbialite community of Lake Clifton (Western Australia): are the thrombolites at risk? Michael D. Smith • Sarah E. Goater • Elke S. Reichwaldt • Brenton Knott • Anas Ghadouani

Received: 28 October 2009 / Revised: 4 March 2010 / Accepted: 23 March 2010 / Published online: 3 April 2010 Ó Springer Science+Business Media B.V. 2010

Abstract The Yalgorup lakes, a groundwater-fed system in south-western Australia recognized as a Ramsar wetland, hold significant scientific and conservation value due to the presence of a unique range of lake systems, resident waterfowl and, on the eastern shore of Lake Clifton, the presence of the only thrombolite reef in the southern hemisphere. Recent concern over changing physico-chemical conditions in the lakes, particularly an increase in salinity, prompted this study: the current status of the inherent thrombolite community is unknown. Salinity, total phosphorous (TP), phosphate, total nitrogen (TN), nitrate, chlorophyll-a and relative abundance of the thrombolite microflora were measured in Lake Clifton to analyse changing conditions in this lake and to determine the effect of these water parameters on the thrombolite community. Comparisons with historical data revealed a significant increase in

Handling editor: David Hamilton M. D. Smith E. S. Reichwaldt A. Ghadouani (&) Aquatic Ecology and Ecosystem Studies, School of Environmental Systems Engineering, M01, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia e-mail: [email protected] S. E. Goater B. Knott School of Animal Biology, M092, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia

salinity since 1985 and a possible increase in phosphorus concentrations in the lake in the recent decade, although historical nutrient data are rather sparse. The increased salinity may be due to concentration of lake water through a combination of high evaporation, long-term reduction in rainfall and increased groundwater abstraction. Comparison of the composition of the thrombolite community with historical data indicates a large reduction in relative abundance of Scytonema sp. and other filamentous cyanobacterial species, which are believed to be fundamental for the thrombolite structure. It is concluded the changing physico-chemical environment of the Yalgorup Lakes may have led to the decline in important genera in the thrombolite community; however, the mechanisms underlying this change remain unknown. Keywords Thrombolites Microbialites Ramsar wetland Salt lake Randomized intervention analysis Salinity

Introduction Salt lakes within Yalgorup National Park, Western Australia, are listed under the Ramsar convention as wetlands of international importance due to their resident waterfowl population, the importance to inhabitant fauna and the presence of a reef of thrombolites on the eastern shore of Lake Clifton

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(Ramsar, 2008). The thrombolite community of Lake Clifton was assessed as critically endangered in 2000 due to its highly restricted distribution, making it susceptible to extinction by processes such as increased nutrient concentrations and change in salinity and/or water level (Luu et al., 2004). Relatively small changes in climate are likely to cause large changes in the functioning of salt lakes (Williams, 1993). This sensitivity gives them scientific significance, especially in the context of global changes that the biosphere has been experiencing in recent decades (Williams, 2002; Knott et al., 2003; Zheng et al., 2004). Thrombolites, together with other microbialitic communities such as stromatolites and benthic microbial communities (BMCs) have great scientific value providing the oldest evidence of life on Earth (age c. 3.5 billion years) and the ancient ecosystems probably signalled the first appearance of cellular organization and photosynthesis (Carr & Whitton, 1973). Today, the occurrence of modern examples of these structures is restricted worldwide to few areas only, including the Bahamas, Mexico, Bermuda and Western Australia (Moore et al., 1984). Lake Clifton supports the largest living, nonmarine microbialite community in the southern hemisphere (Moore & Burne, 1994); consequently, with the limnological processes of the lake threatened by recent human interventions, it becomes critical to increase efforts to elucidate the functioning of this ecosystem. The thrombolite reef in Lake Clifton, part seasonally, part permanently inundated, covers a total area c. 6 km2 (Luu et al., 2004). Thrombolites are discrete organo-sedimentary structures produced by some combination of sediment trapping, sediment binding and precipitation resulting from the metabolic activity of communities of photosynthetic prokaryotes (cyanobacteria and purple sulphur bacteria), eukaryotic microalgae (e.g., diatoms) and chemoautotrophic and chemoheterotrophic microbes (Moore, 1991). Thrombolites display an internal clotted structure (Moore & Burne, 1994) and, although the ability to form it is not limited to one group or morphological form of cyanobacteria (Golubic, 1976), sheathed motile filamentous cyanobacteria are more efficient mat constructors than unicellular unsheathed organisms given that sheaths promote trapping and binding of microorganisms and sediment particles (Pentecost & Riding, 1986; Burne & Moore, 1987). While the geological interpretation of these structures as fossils

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Hydrobiologia (2010) 649:207–216

has been a subject of long debate, recent investigations particularly of modern analogues with live photosynthetic organisms, mainly cyanobacteria, provide strong evidence supporting the notion that these structures are the product of microbe–sediment interactions (Allwood et al., 2006). The elements of the Lake Clifton thrombolite community as originally described by Neil (1984) included filamentous (Scytonema, Oscillatoria, Anabaena) and unicellular cyanobacteria (Aphanocapsa, Chroococcus, Aphanothece). Diatoms and other eukaryotic algae (e.g. Chlorococcus, Gloecystis, Dichotomosiphon) were also reported in significant numbers. Later, Moore & Burne (1994) noted that the most abundant cyanobacterium was Scytonema sp. Concurrent stable isotope (d13C) analyses of the thrombolites and their environmental carbon sources in Lake Clifton suggested that the main process responsible for the formation of the thrombolites in Lake Clifton was the photosynthetically influenced precipitation of aragonite, a polymorph of calcium carbonate, predominantly by Scytonema but also by other members of the BMC (Moore & Burne, 1994). Scytonema is a well known, though not obligate, calcifier (Pentecost & Riding, 1986) requiring lownutrient conditions and fresh to brackish waters (Black, 1933). This accords with the presence in Lake Clifton of the thrombolite reef within the zone of continuous discharge of fresh groundwater along the eastern shore (Moore et al., 1984; Moore, 1987). The low salinity, together with high alkalinity (higher concentration of HCO3- and CO32-) of the calciumrich groundwater, supplies the ions necessary for calcification and thus provides a favourable environment for the construction of thrombolites (Moore, 1987). Although microbialite communities occur in other coastal lakes in Western Australian along a gradient of salinity levels, ranging from as low as 0.14 g/l (Lake Richmond: Kenneally et al., 1987) up to 53 g/l (Lake Thetis: Grey et al., 1990), the composition of each community is unique. Furthermore, it is likely that changes in salinity in lakes lead to a shift in the microbes that dominate the communities, possibly replacing these with a different microbial community that may not facilitate carbonate deposition and thrombolite formation. Concerns about changing conditions in Lake Clifton were first raised by Knott et al. (2003)


who suggested a dramatic increase in the salinity of Lake Clifton, probably starting from 1992. However, no published work has ever followed up on the issue of increasing salinity within Lake Clifton and the possible ecological effect on the thrombolites. Given that the thrombolite reefs are thought to require low salinity and nutrient conditions for survival (Black, 1933; Rosen et al., 1996), it is likely that the community will be significantly affected by any permanent increases in salinity or nutrient concentrations. The objectives of the present study were to (1) identify the climatic and hydrological drivers of the changes in salinity levels observed in the last 20 years and (2) to determine the response of the microbialitic communities to these changes.

Materials and methods Study site Lake Clifton is a shallow coastal salt lake situated in the Yalgorup National Park, c. 100 km south of Perth, Western Australia (Fig. 1) (Moore et al., 1984; Knott et al., 2003). The regional climate is Mediterranean, with hot, dry summers and cool, wet winters. The lake is approximately 21.5 km long by 1.5 km wide, covering an area of roughly 17.8 km2 (Commander, 1988), although the lake’s area becomes much smaller in summer when the southern section of the lake dries out completely (Rosen et al., 1996). The northern basin is the deepest part of the lake, with a maximum depth of 3 m (Moore, 1987). Lake Clifton is a groundwater sink with no surface outflow (Commander, 1988), leading to seasonally changing properties of the lake with variable groundwater and rainfall inputs (Moore, 1987; Commander, 1988; Moore, 1993). Moore (1987) observed that the peak lake water level occurred 4 months after the rainfall maximum, thus highlighting the significance of groundwater to the lake system. The local aquifer consists of a thin freshwater lens overlying a lower, hypersaline layer (Commander, 1988). Overall, hydraulic gradients in the catchment are small, and groundwater flow is relatively slow (Commander, 1988). Surface water inflows to the lake occur only after large rainfall events (Davies & Lane, 1996).

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Thrombolite sampling and analyses We sampled the thrombolites on three separate occasions during daylight hours in September and October 2006 at the Lake Clifton Boardwalk (Coordinates 32°44.719S, 115°39.229E) (Fig. 1). On two occasions, we took two samples from randomly chosen thrombolite structures, whereas we took only one sample on the third sampling date. For each sample, we took approximately 30 g of material from the top 10 mm of the thrombolite and preserved it in 70% ethanol. All specimens were examined microscopically using a Leica DME light microscope (Leica, USA). We identified organisms according to the descriptions and images in Neil (1984). Features used for identification included size, cell shape, the presence of heterocysts and branching. As samples were preserved we could not use the factor motility for identification. With the exception of diatoms, which we simply identified as diatoms, all organisms observed were identified to the level of genus and then classified into five groups of organisms: Scytonema sp., filamentous cyanobacterial species other than Scytonema sp., unicellular cyanobacteria, diatoms and other eukaryotic algae. We adopted this classification from Neil (1984) to allow a direct comparison with her results. We estimated the relative abundance of each group in each sample by homogenizing 0.5 g of sample in 15 ml of water each. A small part of this suspension was then placed on a Helber counting chamber and the number of organisms intersecting the lines of the grids of the large square was counted. Three slides were made per sample, which were averaged to give the overall count number. We counted between 290 and 650 organisms per slide. The relative abundance of each group was then calculated and expressed as a percentage of total number of organisms. Water quality sampling and analyses We collected water samples at the Lake Clifton Boardwalk on the same days on which we sampled thrombolites. These samples were analysed for nitrate, filterable reactive phosphate, total nitrogen (TN) and total phosphorus (TP) concentrations. We determined TN and TP after the samples were digested to convert all nitrogen forms to nitrate, and all phosphorus forms to phosphate, respectively.

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Fig. 1 Location of Lake Clifton. a Shows the locality of Lake Clifton and the main sampling site at the boardwalk off Mt. John Road in the northern part of the lake marked with 1; image taken from Rosen et al. (1996), b photograph of the thrombolites taken from the boardwalk on 31 January 2007,

looking south-west, c photograph of the Lake Clifton Boardwalk taken looking west on 31 January 2007. The water level is shown at an unusually low level that occurred after the study period

Digestion was achieved after autoclaving 5 ml samples at 120°C for 30 min in a 1:1 mix of sample and 4% alkaline persulphate solution (Ebina et al., 1983).

Nitrate-N and phosphate-P analyses were determined by standard colorimetric methods (APHA, 1995) on a segmented flow auto-analyser (Alpkem, Wilsonville,

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OR, USA). All laboratory analyses were performed by the South Coast Nutrient Analysis Laboratory (Albany, Western Australia). We determined chlorophyll-a on the first two sampling dates (three replicates each). For this, we filtered 400 ml of water sample over glass fibre filters which were then extracted with 90% acetone. Chlorophyll-a was then determined fluorometrically after acidification. Historic salinity data for 1985–2005 were provided by the Department of Environment and Conservation (DEC, J. Lane). As salinity changes seasonally in Lake Clifton, we only used salinity values from September and November of each year to make them comparable with our study. Historic salinity values were determined using four separate measurements: direct conductivity reading from the site, calculation from conductivity performed by a government laboratory, calculation from conductivity performed by the Western Australian Chemistry Centre (WACC) and determination of total dissolved solids. The salinity values we used in this study represent the average of these four measurements. Salinity measurements in the present study were taken with a Hydrolab probe (DS5, Hydrolab, Hach Environmental, USA). All salinity data were collected at the Lake Clifton boardwalk. Annual rainfall data were provided by the Australian Bureau of Meteorology. Data for the period 1984–2006 represent the annual rainfall average between the closest weather stations at Waroona, Clifton Lodge and Mandurah. Long-term rainfall data were available only for Mandurah. Statistical analyses We performed randomized intervention analysis (RIA) on the salinity to determine whether the change was statistically significant. RIA uses a bootstrapping technique to compute the probability that an observed impact is due to chance in the absence of a control group. It constitutes an advancement of the Monte Carlo simulation, developed by Carpenter et al. (1989) and modified more recently by Zhang et al. (2001). We chose arbitrary cutoff dates to determine when the change, if any, occurred. Each run consisted of 10,000 iterations and employed a 5% level of statistical significance. The analyses were run using a routine written in Matlab (Version 6.5).

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We used linear regression analyses to identify whether the decrease in rainfall over time was significant and to detect possible relationships between salinity and either lake level or rainfall (SPSS 17.0). We compared relative abundances of the five organism groups in the thrombolite community from the earlier study by Neil (1984) with relative abundances data obtained in this study and calculated differences. As no raw data were available from Neil’s (1984) work, we extracted the mean and SE values from Fig. 11 of her thesis with WinDig 2.5 (freeware; http://www.unige.ch/sciences/chifi/cpb/windig.html).

Results Salinity increased significantly in Lake Clifton over the period 1985–2006 (Fig. 2). Randomized intervention analysis of salinity was conducted using salinity data from 1985 to 2005 only, as these data were collected by DEC using a consistent method but one different to that we used in 2006. RIA helped identify three significantly different phases (P \ 0.01) in the dataset, as illustrated in Fig. 3. In 1985–1993, salinity remained relatively constant with an average value of 18.2 g/l (SD 1.65). In 1994, salinity increased significantly from the previous period to an average level of 24.1 g/l (SD 2.0) for the period 1994–2001. From 2001 to 2005 salinity increased significantly again to an average of 32.2 g/l (SD 4.21), with an increase in yearto-year variability as previously recorded. A linear regression of rainfall over time revealed a significant decrease in rainfall since 1981 (Fig. 4; P \ 0.01). Linear regression analyses between salinity and lake level and also rainfall were used to estimate possible correlations between the respective variables. All regression coefficients were statistically significant (P \ 0.001, respectively). Correlation of salinity with water level of Lake Clifton returned an r2 = 0.39 (Fig. 5), whereas the correlation with annual rainfall returned an r2 = 0.38 (Fig. 6). Chemical water analysis data and chlorophyll-a data for our project are summarized in Table 1 together with TN and TP values from two earlier studies (Moore, 1993; Rosen et al., 1996). Based on our data as well as previous analyses of historical data (Rosen et al., 1996), there seems to be an increase in phosphorus concentration in the lake;

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Fig. 2 Depth and salinity data from Lake Clifton for the period 1985–2005 and 1985–2006, respectively. Recordings were taken at the same location close to Lake Clifton boardwalk during the months of September and November of each year. Depth measurements are expressed in depth gauge local datum (m)

50

5

40

4

30

3

20

2

Lake level (m)

Salinity (g/L)

Ocean's average salinity

Salinity Depth 1

10 1984

1988

1992

1996

2000

2004

Year 40 35

1200 1100

Ocean's average salinity

Rainfall (mm)

Salinity (g/L)

1000 30 25 20

900 800 700 600

15

500

10 1985 - 1993

1994 - 2000

2001 - 2005

Time period

Fig. 3 Box plot of Lake Clifton salinity for the time period 1985–2005 from salinity data from the DEC. Bars represent the 5/95th percentiles. Salinity measurements were taken next to the Lake Clifton Boardwalk. Randomised intervention analysis shows that there is a statistically significant difference (P \ 0.01) between the three time periods. N = 18 for 1985– 1993, N = 14 for 1994–2000, N = 10 for 2001–2005

however, the nutrient data are rather sparse and make it difficult for a formal comparison. The results of thrombolite sampling are shown in Fig. 7. Relative abundance analysis of the thrombolite microflora suggests that unicellular cyanobacteria and diatoms are the dominant groups in 2006. Comparison of our data with historical data from Neil (1984) shows that the relative abundance of the

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400 1980

1985

1990

1995

2000

2005

Year

Fig. 4 Linear regression of rainfall (mm) over time (r2 = 0.26; P \ 0.01; y = -9.704 * x ? 20194). Broken lines indicate the 95% confidence band. Rainfall data were taken as the annual average between recorded measurements at Mandurah, Clifton Lodge and Waroona and covers the period 1985–2006

five groups has changed since 1984. In particular, there has been a large relative decrease in both filamentous cyanobacteria and Scytonema sp. On the other hand, the relative numbers of diatoms and unicellular cyanobacteria increased between the two studies, whereas the relative percentage of eukaryotic species, including Chlorococcus sp., Ulothrix sp., Mougetia sp., Listeria sp. and Cosmarium sp., remained stable.


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4.8

Lake level (m)

4.6

4.4

4.2

4.0 10

15

20

25

30

35

40

Salinity (g/L)

Fig. 5 Linear regression between lake water level measured as depth gauge local datum (m) and salinity (g/l) in Lake Clifton (r2 = 0.39; P \ 0.001; y = -0.02 * x ? 4.68). Broken lines indicate the 95% confidence band. Both measurements were taken from the Lake Clifton Boardwalk in September and November for the time period 1985–2005 50 45

Salinity (g/L)

40 35 30 25 20 15 10 500

600

700

800

900

1000

1100

Rainfall (mm)

Fig. 6 Linear regression between rainfall (mm) and salinity (g/l) for Lake Clifton (r2 = 0.38; P \ 0.001; y = -0.032 * x ? 51.0). Broken lines indicate the 95% confidence band. Salinity measurements were taken in September and November at the Lake Clifton Boardwalk over the time period 1985–2006. Rainfall data were taken as the annual average between recorded measurements at Mandurah, Clifton Lodge and Waroona and covers the period 1985–2006

Discussion Until the late 1980s, Lake Clifton was considered the only permanently hyposaline lake in Yalgorup National Park and the only lake that contained a thrombolite community (Moore, 1991). The data presented in this study show an increase in the salinity of Lake Clifton (Figs. 2, 3), confirming concerns for

the lake voiced by Knott et al. (2003). The salinity increase in Lake Clifton has been significant and salinity measured in this study (September and October 2006) was higher than salinity in the ocean. Taking into account that salinity in this lake is usually lowest in the months August through October it can be concluded that Lake Clifton can no longer be considered hyposaline at any stage of the year. The physico-chemical changes in Lake Clifton appear to have affected the thrombolite community. In earlier studies, the thrombolites were dominated by Scytonema sp. and other filamentous cyanobacteria (Neil, 1984; Moore & Burne, 1994). Our study indicates that thrombolites are now dominated by diatoms and unicellular cyanobacteria. Based on the findings, it can be concluded that the microbialite community structure has changed significantly since 1984. The decline of Scytonema sp. in the thrombolite community is particularly concerning, given that it has been suggested to be responsible for the biologically influenced precipitation of aragonite in this lake (Moore, 1991; Moore & Burne, 1994) which, in turn, is vital to thrombolite formation. Moore (1991) and Moore & Burne (1994) claimed that precipitation was found only around Scytonema filaments, not around unicellular cyanobacteria. However, Thompson et al. (1990) showed, that Synechococcus, a unicellular cyanobacteria is able to precipitate calcium carbonate in thrombolites in Green Lake, New York. Putting together this information we might argue that precipitation due to Scytonema has certainly decreased in Lake Clifton, however, such a change does not necessarily mean that the thrombolites have stopped growing. It might rather be that unicellular cyanobacteria, which are now relatively more abundant in the thrombolytic community in Lake Clifton, will continue to precipitate and thus keep the thrombolite alive. However, if future studies show that Lake Clifton’s unicellular cyanobacteria are not able to maintain thrombolite growth and if the community change is permanent, then the Lake Clifton thrombolite community has lost much of its scientific value as a ‘natural laboratory of international significance’ (Knott et al., 2003). The thrombolite forming organisms may then be replaced in a succession under conditions that favour more halophilic organisms. Given that the thrombolite community has been subjected to such a significant change in salinity, the

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Table 1 Water chemistry data from the thrombolite sampling site near the Clifton Boardwalk off Mount John Road on the three sampling dates of the present study and from earlier studies by Moore (1993) and Rosen et al. (1996) Date

Lake depth (m)

pH

TN (mg/l)

NO3-N (mg/l)

TP (mg/l)

PO4-P (mg/l)

Chl-a (lg/l)

Salinity (g/l)

Present study 15 September 2006

1.1

8.1

3.97

\0.005

0.13

0.04

2.53

42.9

21 September 2006

1.2

8.3

2.99

\0.005

0.12

0.02

2.71

47.2

16 October 2006

–

–

3.55

\0.005

0.07

0.02

–

47.9

October 1979

–

–

3.05

–

0.017

0.014

–

–

November 1979

–

–

1.89

–

0.225

0.005

–

–

October 1991

–

–

2.08

–

0.013

0.013

–

–

September 1992

–

–

1.72

–

0.005

0.005

–

–

Moore (1993)

Rosen et al. (1996)

Rosen et al. (1996) stated that phosphate was the only abundant form of P in Lake Clifton and therefore used this value as the TP value

(a)

1984 2006

50 40 30 20 10 0 40

(b)

20 0 -20

ga e

o.

a

al ry ot ic ka Eu

ni ce l

.c ya n

m U

en t la m Fi

to ne

.c

ia to D

Sc y

ya no

s

.

-40 m

Deviation from 1984 data (%)

Relative abundance (%)

60

Fig. 7 a Relative abundance (mean, SE) of the five organism groups in the thrombolite community in samples from 1984 (Neil, 1984) (N = not stated) and from the present study (N = 5); values for mean and SE from Neil’s data (1984) were extracted from hand-drawn figures from her work with WinDIG (http://www.unige.ch/sciences/chifi/cpb/windig.html; freeware, version 2.5; b difference between the data from the two studies

changes in the community structure are not surprising. Unicellular cyanobacteria and diatoms are believed to be more resilient in extreme conditions

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than filamentous bacteria. Also, Scytonema sp. is believed to require conditions of low nutrients (Rosen et al., 1996), although this has never been directly confirmed. High nutrient levels would encourage an increased phytoplankton and macrophyte (e.g., Cladophora) growth which would lead to light competition and subsequent shading of thrombolites, thus reducing photosynthesis rate of the organisms inside the thrombolites (Rosen et al., 1996). In fact, previous studies (e.g., Rosen et al., 1996) reported an increased Cladophora growth in Lake Clifton attributed to the export of readily available phosphorus via groundwater. Although Scytonema is thought to prefer a low salinity environment (Black, 1933), it tolerates salinities higher than 65 g/l in the intertidal pools of Hamelin Pool, Western Australia (Moore, 1987). Still, it might be that the Scytonema sp. in Lake Clifton’s thrombolites have adapted to the local environment and is not able to tolerate high salinity. At this stage we can only speculate whether the increase in salinity is directly responsible for the decline of Scytonema sp. and other filamentous cyanobacteria. It is difficult to determine the cause of the increase in salinity in Lake Clifton. The concentration of lake solutes is due to variability in rainfall, evaporation and groundwater levels in the catchment. Almost 40% of the change in salinity is attributable by regression analysis to lake water level, which likely represents the concentration of ions pre-existing in the lake. Despite the generally decreasing trend in


rainfall (Fig. 4), only 38% of the change in salinity is attributable to rainfall (Fig. 6). No long-term data exists for evaporation or groundwater level, so the relative importance of this factor in determining salinity cannot be assessed. Other factors, such as a suspected biological disturbance associated with the introduction of black bream (Acanthopagrus butcheri) to the lake in the 1990s (Chaplin et al., 1998; Norriss et al., 2002), may have also contributed to the decline of filamentous cyanobacteria in the thrombolite community. Although black bream are omnivorous there is no evidence that they feed directly on the microbialite community. However, they are known to feed on fauna associated with the thrombolites (Sarre et al., 2000) which might increase the top–down pressure on this community. The physico-chemical changes in the Yalgorup Lakes will not affect just the thrombolites, but the entire lake ecology. The thrombolite structures have been reported to provide habitat for a range of organisms (e.g., Annelida, Insecta, Arthropoda) which may depend on them for food or shelter from predation (Konishi et al., 2001). Disruption of the thrombolite community will transcend through to these organisms. The future prognosis for the currently existing Lake Clifton thrombolite community is not good. Global climate models predict decreased rainfall for southwest Australia over the next few decades (IPCC, 2007). This will further decrease freshwater flows to the lake, leading to additional increases in salinity. Also, with the expansion of the southern suburbs of the city of Mandurah, the nearest urban concentration (Fig. 1) likely to continue, land directly to the east of Lake Clifton will be subjected to pressures associated with urban development. Higher nutrient loads to the lakes and increased rates of groundwater extraction are likely to result, placing further stress on the Yalgorup Lake System. Large experimental studies in other systems made a clear link between land clearing and changes in the functioning of lakes especially in relation to increased algal blooms (Ghadouani et al., 2006). Appropriate management strategies are required to mitigate these threats. Whether the Lake Clifton thrombolites will cease to grow cannot be predicted at this stage but the data presented here all indicate the strong need for a substantive and consistent research focus for this lake of international limnological significance.

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Conclusion In summary, this study shows that the biological characteristics of the Lake Clifton thrombolite community have changed significantly since the community was initially studied in 1984. In particular, the decline of the filamentous cyanobacterium Scytonema sp. raises serious concern as this genus previously represented the foundation for formation of the thrombolites. The observed changes within the community are co-occurring with physico-chemical changes in Lake Clifton, namely an increase in salinity, as well as additional changes such as a suspected increase in phosphorus export to the lake along with complex changes to the hydrology of the region and a decrease in rainfall. Ultimately, more research is needed to better understand the changes taking place in Lake Clifton, and their ecological effect on the lake’s thrombolites, fauna and the limnology of the lake. Acknowledgements Support was provided for this project by the School of Environmental Systems Engineering at the University of Western Australia. Rainfall data is courtesy of the Western Australian Bureau of Meteorology. The authors would like to thank the thrombolite recovery team, the City of Mandurah, the Department of Environment and Conservation of Western Australia especially J. Lane and S. Dutton for providing support to this study and by providing historical salinity data and lake water levels. Many thanks also to D. Krikke for field and laboratory assistance during the study.

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