Peter Gell

The Development of a Diatom Database for Inferring Lake Salinity, Western Victoria, Australia: Towards a Quantitative Approach for Reconstructing Past Climates

The development of a modern data set of 156 diatom samples from salt lakes has provided evidence of the tolerance of a large number of taxa to the salinity of lake waters. Thirty taxa have been recorded from 30 or more samples and so have been well characterised. A further 42 taxa have been recorded from 10 or more samples. The lakes sampled range in salinity from the freshwater–oligosaline boundary to well into the hypersaline range, so the upper and lower salinity tolerance limits of many species were investigated. Canonical correspondence analysis of the data set showed that salinity was the most important of the tested parameters influencing the diatom assemblages in the samples. Randomisation tests have provided correlation values between measured and predicted salinity comparable with those gained from other major salt lake diatom data sets, suggesting that this set is a good predictor of lake salinity.

Tareena Billabong – a palaeolimnological history of an ever-changing wetland, Chowilla Floodplain, lower Murray–Darling Basin, Australia

A 427-cm sediment core was extracted from Tareena Billabong, a Murray River floodplain wetland in the extreme south-west of New South Wales, Australia. Analysis of fossil diatoms and pollen, sediment 210Pb and 137Cs profiles and radiocarbon and luminescence dating reveal that Tareena Billabong has undergone substantial environmental change in its ~5000-year history. Shortly after its formation, the billabong was a freshwater lagoon with a diatom flora dominated by Synedra ulna and Planothidium lanceolatum. An increase in Aulacoseira granulata, a river plankton dominant today, reflects two phases of increased connectivity with the Murray River in the mid to late Holocene. A shift to lagoonal taxa after ~3000 years BP is attributed to water balance and river-flow changes, possibly associated with regional climate change. Importantly, it appears to have undergone an extended phase of increasing turbidity, and possibly wetland salinity, commencing ~3000 years BP. Sedimentation increased at least 15-fold in the European phase. Billabong salinity increased markedly soon after European settlement, reaching a peak in the late 1800s AD. While regulation then increased the degree of connection between the billabong with the River in the 1920s AD, salinity levels remained high. Increased salinity is revealed by increases in the diatom taxa Amphora spp., Cyclotella meneghiniana, Gyrosigma acuminatum, Planothidium delicatulum and Tryblionella hungarica and by declines in Casuarinaceae, Eucalyptus, Myriophyllum and Cyperaceae pollen. Tareena Billabong was subjected to considerable environmental pressures from the early stages of European settlement in terms of sediment load, hydrological change and salinity.

A diatom species index for bioassessment of Australian rivers

The Diatom Index for Australian Rivers (DIAR), originally developed at the genus level, was reformulated at the species level with data from diatom sampling of rivers in the Australian Capital Territory, New South Wales, Queens- land, South Australia and Victoria. The resulting Diatom Species Index for Australian Rivers (DSIAR) was significantly correlated with the ARCE (Assessment of River Condition, Environment) index developed in the Australian National Land and Water Resources Audit (NLWRA), and with nine of the ARCEs constituent indices and sub-indices, across 395 river reaches in south-eastern Australia. These correlations were generally stronger than those shown by the biological index that was used to assess river condition in the NLWRA, the ARCB (Assessment of River Condition, Biota) index based on macroinvertebrates and the Australian River Assessment System (AUSRIVAS). At a finer spatial scale, DSIAR was strongly and significantly correlated with measures of catchment urbanisation for streams in the eastern suburbs of Melbourne, Victoria. DSIAR scores across south-eastern Australia bore little relationship to the latitude, longitude or alti- tude of sampling sites, suggesting that DSIAR is not greatly affected by macro-geographical position. In addition, DSIAR scores did not vary greatly among small-scale hydraulic environments within a site. DSIAR appears to have potential as a broad-scale indicator of human influences on Australian rivers, especially the effects of agricultural and urban land use, and also for impact studies at a local scale. Further evaluation is warranted to test the sensitivity of the index to natural variables such as catchment geology, and to assess its performance in northern, western and inland Australia.

Development of diatom-based tools for assessing stream water quality in south-eastern Australia: assessment of environmental transfer functions

Models were developed of relationships among water quality, geospatial and species-level diatom data for 465 samples collected from stream sites across south-eastern Australia. Transfer functions were derived from weighted averaging and artificial neural network approaches. Analysis of spatial variations in species assemblages was used to divide the sites into two groups according to site elevation. The strongest predictive models for the upland group associated diatom assemblages with conductivity, longitude, altitude, and to a lesser extent pH, NOx and TKN. The strongest predictors for the lowland group were longitude and conductivity, but artificial neural network models performed well for NOx and temperature. The importance of the geospatial variables suggests that there may be a capacity to develop diatom sub-regions within which robust models for other water quality variables important to management can be generated.

Holocene vegetation change, Aboriginal wetland use and the impact of European settlement on the Fleurieu Peninsula, South Australia

A fossil pollen and charcoal record from a cyperaceous swamp on the Fleurieu Peninsula, South Australia, was investigated to provide a history of pre and post-European settlement vegetation change. It was found that the swamp initiated sometime before 8220 calibrated years BP. High rates of peat development and the expansion of swamp species between 7000 BP and 4500 cal. BP indicated wet conditions at that time. The swamp became drier in the late Holocene and some peat may have been lost through deflation. Macroscopic charcoal and Typha pollen suggested that Aborigines deliberately burned the upland wetlands during the mid to late Holocene. Prior to European settlement climate changes generated community shifts in the terrestrial vegetation. The record revealed a transition from an early Holocene Eucalyptus woodland to an Allocasuarina wet-heath in the humid mid-Holocene, a community type with no modern analogue in the region. In the drier late Holocene, a Eucalyptus-dominated woodland returned. The impacts of European settlement were clearly seen in changes in sedimentation rates and in both terrestrial and wetland flora. Allocasuarina verticillata declined early in the European period and fire tolerant species were promoted, before the almost complete removal of native vegetation through broad scale land clearance and its replacement with nonnative pasture species. Compositional changes to the swamp flora were marked through the European phase with Acacia expanding early in settlement and later being replaced by Leptospermum, in response to changed fire and regional hydrological regimes. The impact of European land use is discussed in relation to Holocene climate-driven vegetation changes and aspects of Aboriginal land use.

Diatom-salinity relationships in wetlands: assessing the influence of salinity variability on the development of inference models

Diatoms are among the most widely used indicators of human and climate induced wetland salinity history in the world. This is particularly as a result of the development of diatom-based models for inferring past salinity. These models have primarily been developed from relationships between diatoms and salinity measured at the time of sampling or during the preceding year. Although within site variation in salinity has the potential to reduce the efficacy of such models, its influence has been rarely considered. Hence, diatom–conductivity relationships in eight seasonally monitored wetlands have been investigated. In developing a diatom–conductivity transfer function from these sites, we sought to assess the influence of conductivity variation on diatom inference model performance. Our sites were characterised by variability in conductivity that was not correlated to its range and thus were well suited to an investigation of this type. We found, contrary to expectations, that short-term (seasonal) changes in conductivity which were often dramatic did not result in unduly reduced transfer function performance. By contrast, sites that were more variable in the medium term (5–6 years) tended to have larger model errors. In addition, we identified a secondary ecological gradient in the diatom data which could not be related to any measured variable (including pH, turbidity or nutrient concentrations).

LIMPACS––Human and Climate Interactions with Lake Ecosystems: setting research priorities in the study of the impact of salinisation and climate change on lakes, 2005–2010

Is part of a special issue with the theme: Resilience and Restoration of Soft-Bottom Near-Shore Ecosystems; Salt Lakes: Salinity, Climate Change and Salinisation

Against the tide: the freshening of naturally saline coastal lakes, southeastern South Australia

Diatom analyses of sediment cores extracted from three lakes in coastal southeastern South Australia reveal that, for most of the mid-late Holocene, they were shallow, brackish to saline systems with limited flow of water from continental sources. The construction of a substantial network of drains in the early years of settlement, to maximise transportation and agricultural production through wet winters, lead to abrupt freshening of the lakes. Interestingly, despite substantial nutrient loads to Lake Bonney SE (there are two Lakes Bonney and Frome in South Australia, which is why the lakes in the southeast of the state are differentiated with ‘SE’) associated with the commissioning and expansion of pulp and paper mills, a wastewater treatment plant discharge and agricultural runoff, there is only moderate evidence of nutrient enrichment in the lake, possibly because the post-impact assemblages are dominated by taxa with broad ecological preferences. Despite being preserved within a conservation park, eutrophication associated with agriculture is evident in the diatom assemblages of Lake Frome SE, which has a catchment more than twice that of Lake Bonney SE. Mullins Swamp, on the other hand, supports few indicators of eutrophic conditions. The freshening of these lakes is against the tide of salinisation from rising saline groundwaters in most wetlands across southeastern Australia.

Wetland and terrestrial vegetation change since European settlement on the Fleurieu Peninsula, South Australia

Microfossil, sediment and documentary records provide a history of European land use and its impact on the vegetation of the Fleurieu Peninsula, South Australia. Two sedimentary cores were analysed for their fossil pollen and charcoal composition. Chronologies were established using a combination of 210Pb, 14C and microfossil markers. Primary and secondary evidence for the spatial expansion of land uses in the region were compiled providing local-, bioregional- and regional-scaled European settlement histories. The settlement and land-use histories of the major vegetation types in the region were different and were closely determined by the nature of the vegetation itself. The sedimentary and microfossil records indicate that wetland and terrestrial vegetation have undergone sequential changes of composition. There is evidence of a decline in fire-sensitive understorey species and the decline is likely due to intensive firing and grazing of scleropyllous woodlands and forests early in European settlement. Early-settlement native forestry practices were intensive, however they did not alter overstorey tree composition. Mid-twentieth-century wholesale vegetation clearance is clearly marked in the pollen record by a decline in Eucalyptus and increase in herbaceous species. Wetland vegetation was highly impacted by European land practices through changes to sediment inputs and hydrological conditions that began prior to catchment clearance, during the phase of intensive firing and grazing. Through the integration of multiscaled, ecosystem-specific historical settlement histories and palaeoecological analysis, correlations between past land uses and biotic responses can be confidently demonstrated.

Diatoms and ostracods as mid-Holocene palaeoenvironmental indicators, North Stromatolite Lake, Coorong National Park, South Australia ∗

The Holocene carbonate sequence of perennial North Stromatolite Lake, located adjacent to the Coorong Lagoon near Salt Creek, South Australia, includes a prominent sapropelic unit (7 – 12% total organic carbon), in places more than 2 m thick, that was sampled for the purpose of radiocarbon dating and documenting its diatom and ostracod biostratigraphy. The recovered ostracods were also subjected to carbon and oxygen isotopic analysis. The bulk organic matter at the base of the sapropel yielded an uncalibrated 14C age of 6080 ± 60 y BP. Diatoms, where preserved, are almost exclusively benthic. Stratigraphic variation of the proportions of key indicator species in diatom assemblages records a marked oscillation between oligosaline and eusaline conditions in the hypolimnion during deposition of the sapropel. Ostracod carbon isotope data indicate that the lake at this time was eutrophic, thereby enriching the dissolved inorganic carbon of the hypolimnion in 13C. However, the observed secular variation in δ13C implies a mid-sapropel drop in productivity, caused by a freshening of the lake. Ostracod δ18O values display an overall increase through the sapropel consistent with the rising salinity of the hypolimnion. The existence of a flourishing benthic ostracod community, together with the valve ornamentation of Osticythere baragwanathi, indicates that the bottom waters were well oxygenated. Thus, anoxia was not a prerequisite for sapropel accumulation. The biostratigraphy and chemostratigraphy of the sapropel concur in suggesting a lack of climatic uniformity during its deposition, a period of ∼1200 years. This study therefore highlights the potential of diatoms and ostracods in shallow perennial alkaline lakes along the Coorong coastal plain as proxies for short-term (102 – 103 years) Holocene palaeoenvironmental change in southeastern Australia.