Multi-indicator conductivity transfer functions for Quaternary palaeoclimate reconstruction

Diatoms (Bacillariophyceae; single-celled algae) and ostracods (Ostracoda; shelled microcrustacea) are known for their sensitivity to salinity. In palaeolimnology, the potential has yet to be tested for quantifying past salinity, lake level, and by inference, climate change, by application of multiple-indicator transfer functions. We used weighted averaging techniques to derive diatom (n = 91; r ²  = 0.92) and ostracod (n = 53; r ²  = 0.83) conductivity transfer functions from modern diatom, ostracod and water chemistry data collected in lakes of central, western and northern Turkey. Diatoms were better represented across the full gradient than ostracods, at intermediate levels of conductivity in particular, but both transfer functions were statistically robust. Because transfer functions are not infallible, we further tested the strength and simplicity of salinity response and the potential for identifying characteristic associations of diatom and ostracod taxa in different parts of the salinity gradient, to improve palaeoclimate reconstruction. We identified a subset of 51 samples that contained both diatoms and ostracods, collected at the same time from the same habitat. We used Two-Way Indicator Species Analysis of a combined diatom-ostracod data set, transformed to achieve numerical equivalence, to explore distributions in more detail. A clear ecological threshold was apparent at ~3 g l⁻¹ salinity, rather than at 5 g l⁻¹, the boundary used by some workers, equating to the oligosaline-mesosaline boundary. Other salinity boundaries were poorly defined, indicating lack of a simple, well-defined salinity response. We did, however, define characteristic associations of taxa, to facilitate the distinction to be drawn between a hydrologically open, fresh lake and an oligosaline lake, in palaeoenvironmental reconstruction. Over the rest of the salinity gradient, we highlighted the potential for the multi-proxy approach to strengthen ostracod-based reconstruction in particular, to overcome the problem of broad apparent tolerance ranges in common halophilic taxa such as Limnocythere inopinata, which often dominate in low-diversity fossil assemblages. The combination of multi-proxy quantitative reconstruction, complemented by qualitative understanding of ecological responses generated by the analysis, remains a powerful tool in Quaternary palaeoclimate research.     

Diatom-based total phosphorus (TP) and pH transfer functions for the Irish Ecoregion

A 72-lake diatom training set was developed for the Irish Ecoregion to examine the response of surface sediment diatom assemblages to measured environmental variables. A variety of multivariate data analyses was used to investigate environmental and biological data structure and their inter-relationships. Of the variables used in determining a typology for lakes in the Irish Ecoregion, alkalinity was the only one found to have a significant effect on diatom assemblages. A total of 602 diatom taxa were identified, with 233 recorded at three or more sites with abundances ≥1%. Generally diatom data displayed a high degree of heterogeneity at the species level and non-linear ecological responses. Both pH and total phosphorus (TP) (in the ranges of 5.1–8.5 and 4.0–142.3 μg l⁻¹ respectively) were shown to be the most significant variables in determining the surface sediment diatom assemblages. The calibration models for pH and TP were developed using the weighted averaging (WA) method; data manipulation showed strong influences on model performances. The optima WA models based on 70 lakes produced a jack-knifed coefficient of determination (r ² _jack) of 0.89 with a root mean squared error (RMSEP) of 0.32 for pH and r ² _jack of 0.74 and RMSEP of 0.21 (log₁₀ μg l⁻¹) for TP. Both models showed strong performances in comparison with existing models for Ireland and elsewhere. Application of the pH and TP transfer functions developed here will enable the generation of quantitative water quality data from the expanding number of palaeolimnological records available for the Irish Ecoregion, and thus facilitate the use of palaeolimnological approaches in the reconstruction of past lake water quality, ecological assessment and restoration.     

A history of aquatic plants in the Coorong,a Ramsar-listed coastal wetland,South Australia

The Coorong in South Australia is an internationally recognised ecologically significant coastal lagoon that extends 140 km south-east from the mouth of the River Murray. The Coorong has increasingly been impacted by a variety of human activities. Declining migratory bird abundance has been linked to the loss of Ruppia tuberosa, an aquatic plant that is the main feedstock for a wide variety of water birds. Analysis of Ruppia remains from a radiometrically dated core in the southern lagoon of The Coorong shows that the salt-tolerant annual Ruppia tuberosa has only been present at this site in recent times. By contrast, the perennial Ruppia megacarpa, which has limited tolerance to elevated salinity, appears to have been present at the site for several millennia, although it had never been observed in ecological surveys of this part of The Coorong. Diatom analysis from the same core reveals a shift from estuarine/marine assemblages to an assemblage reflective of elevated salinity levels. Charophyte, ostracod and foraminifera remains also indicate that the change in the aquatic plant community is associated with increased salinity at the study site since European settlement. Elevated salinity is the result of catchment modifications which have reduced freshwater inflows at the northern and southern extremities of The Coorong, and marine input via the Murray Mouth. This study demonstrates the utility of multiproxy palaeoecological data in addressing complex management questions. In the absence of such information, managers must ultimately rely on data sourced only from the historical record which, more often than not, is already skewed by the impact of European settlement.     

A caveat regarding diatom-inferred nitrogen concentrations in oligotrophic lakes

Atmospheric deposition of reactive nitrogen (Nr) has enriched oligotrophic lakes with nitrogen (N) in many regions of the world and elicited dramatic changes in diatom community structure. The lakewater concentrations of nitrate that cause these community changes remain unclear, raising interest in the development of diatom-based transfer functions to infer nitrate. We developed a diatom calibration set using surface sediment samples from 46 high-elevation lakes across the Rocky Mountains of the western US, a region spanning an N deposition gradient from very low to moderate levels (<1 to 3.2 kg Nr ha⁻¹ year⁻¹ in wet deposition). Out of the fourteen measured environmental variables for these 46 lakes, ordination analysis identified that nitrate, specific conductance, total phosphorus, and hypolimnetic water temperature were related to diatom distributions. A transfer function was developed for nitrate and applied to a sedimentary diatom profile from Heart Lake in the central Rockies. The model coefficient of determination (bootstrapping validation) of 0.61 suggested potential for diatom-inferred reconstructions of lakewater nitrate concentrations over time, but a comparison of observed versus diatom-inferred nitrate values revealed the poor performance of this model at low nitrate concentrations. Resource physiology experiments revealed that nitrogen requirements of two key taxa were opposite to nitrate optima defined in the transfer function. Our data set reveals two underlying ecological constraints that impede the development of nitrate transfer functions in oligotrophic lakes: (1) even in lakes with nitrate concentrations below quantification (<1 μg L⁻¹), diatom assemblages were already dominated by species indicative of moderate N enrichment; (2) N-limited oligotrophic lakes switch to P limitation after receiving only modest inputs of reactive N, shifting the controls on diatom species changes along the length of the nitrate gradient. These constraints suggest that quantitative inferences of nitrate from diatom assemblages will likely require experimental approaches.     

Twentieth century eutrophication of the St. Croix River (Minnesota–Wisconsin,USA) reconstructed from the sediments of its natural impoundment

Evaluation of land-use effects on coastal and marine ecosystems requires better understanding of the role of rivers in regulating mass transport from terrestrial to oceanic environments. Here we take advantage of the presence of a riverine lake to use paleoecological techniques to quantify impacts of logging, European-style agriculture, urbanization and continued terrestrial disturbance on mass transport and water quality in the northern drainage of the Mississippi River. Two 2-m sediment-cores recovered in 1999 from Lake St. Croix, a natural impoundment of the St. Croix River, were dated using ²¹⁰Pb and ¹³⁷Cs, and analyzed for historical changes (c. 1840–present) in sediment magnetic susceptibility, inorganic and organic matter content, biogenic silica, fossil pigments, and diatom microfossils. Inorganic sediment accumulation increased threefold between the mid-1800s and present, whereas clear signs of eutrophication were only evident after the mid-twentieth century when biogenic silica accumulation increased sixfold, diatom accumulation rates increased 20- to 50- fold, and the diatom community shifted from predominantly benthic species to assemblages composed mainly of planktonic taxa. Similarly, fossil pigment concentrations increased during the 1960s, and diatom-inferred total phosphorus (DI-TP) increased from ~30 μg TP l⁻¹ c. 1910 to ~60 μg l⁻¹ since 1990, similar to historical records since 1980. Together, these patterns demonstrate that initial land clearance did not result in substantive declines in water quality or nutrient mass transport, instead, substantial degradation of downstream environments was restricted to the latter half of the twentieth century. This is one of eight papers dedicated to the “Recent Environmental History of the Upper Mississippi River” published in this special issue of the Journal of Paleolimnology. D. R. Engstrom served as guest editor of the special issue.     

Use of sedimentary pigments to infer past phosphorus concentration in lakes

We propose a palaeolimnological method for inferring past total phosphorus (TP) concentrations in lake water from spectrophotometrically-measured sedimentary pigments, particularly total carotenoids (TC). Our approach is based on a highly significant statistical correlation (P < 0.0001) between pigment concentrations (total carotenoids) in the surface sediment of 28 Italian lakes (subalpine, large, deep, shallow, volcanic) and TP concentrations measured in these lakes at overturn when the core was collected. A transfer function was developed from this “training” set, and used to estimate past TP concentrations from pigment concentrations in sediment cores. The results generally agreed with TP values as measured by long-term water quality monitoring programs. Contrasting results were obtained by a comparison with diatom-inferred TP. While the diatom model showed a tendency to overestimate TP values higher than 100 μg l⁻¹, the pigment model correctly estimated TP in lakes when TP was <100 μg l⁻¹, but not when lakes were rich in macrophytes. In fact, lakes with extensive populations of aquatic submersed macrophytes and epiphytes are outliers in terms of the TC versus TP relationship. The root mean square error of prediction of the pigment model is lower than those derived from certain diatom—based inference models. The predicted and residual values are not related to the estimated values and their average is not statistically different from zero. Errors were estimated via a ‘leave-one-out’ re-sampling technique. The proposed method permits rapid and relatively inexpensive determination of reference trophic conditions.     

Calibration of Littoral Diatoms to Water Chemistry in Standing Fresh Waters (Flanders, Lower Belgium): Inference Models for Historical Sediment Assemblages

Relationships between littoral surface-sediment diatom assemblages and ambient limnological conditions were examined in 186 lentic fresh waters throughout lower Belgium (Flanders). Most of these waters were small, unstratified, alkaline and rich in nutrients. Using weighted-averaging techniques, robust and accurate transfer functions were developed for median pH-values ranging from 3.4 to 9.3 and dissolved inorganic carbon concentrations from <1.6 to 63 mg l⁻¹ (jackknifed r ²≈ 0.87, RMSEP <10% of the observed range), while a less precise model was obtained for sodium (2–571 mg l⁻¹; jackknifed r ² 0.69, RMSEP 9.9% of the range). Restricting the data set to circumneutral and alkaline sites (pH≥6.5) revealed the importance of additional variables, including calcium, silica, chemical oxygen demand and potential gross oxygen production (a proxy for metabolic activity and phytoplankton abundance). Calibration models for these variables were strong enough to be useful (jackknifed r ² 0.57–0.59, RMSEP 13.1–16.4% of the observed range), although estimations should not always be considered entirely independent. Except for the predominant pH gradient, removal of all taxa with a distribution unrelated to the variable of interest improved model performance. In general, such taxa were proportionally represented among taxa classified according to their principal habitat. Application of the present models to diatom assemblages of shallow-water sediments obtained from historical samples and, most importantly, herbarium-macrophyte specimens, will improve hindsight into regional freshwater conditions and add to base-line setting of ecological quality standards in a highly impacted region. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

The use of diatom records to establish reference conditions for UK lakes subject to eutrophication

A knowledge of pre-disturbance conditions is important for setting realistic restoration targets for lakes. For European waters this is now a requirement of the European Council Water Framework Directive where ecological status must be assessed based on the degree to which present day conditions deviate from reference conditions. Here, we employ palaeolimnological techniques, principally inferences of total phosphorus from diatom assemblages (DI-TP) and classification of diatom composition data from the time slice in sediment cores dated to ~1850 AD, to define chemical and ecological reference conditions, respectively, for a range of UK lake types. The DI-TP results from 169 sites indicate that reference TP values for low alkalinity lakes are typically <10 μg L⁻¹ and in many cases <5 μg L⁻¹, whilst those for medium and high alkalinity lakes are in the range 10–30 and 20–40 μg L⁻¹, respectively. Within the latter two alkalinity types, the deeper waters (>3 m mean depth) generally had lower reference TP concentrations than the shallow sites. A small group of shallow marl lakes had concentrations of ~30 μg L⁻¹. Cluster analysis of diatom composition data from 106 lakes where the key pressure of interest was eutrophication identified three clusters, each associated with particular lake types, suggesting that the typology has ecological relevance, although poor cross matching of the diatom groups and the lake typology at type boundaries highlights the value of a site-specific approach to defining reference conditions. Finally the floristic difference between the reference and present day (surface sample) diatom assemblages of each site was estimated using the squared chord distance dissimilarity coefficient. Only 25 of the 106 lakes experienced insignificant change and the findings indicate that eutrophication has impacted all lake types with >50% of sites exhibiting significant floristic change. The study illustrates the role of the sediment record in determining both chemical and ecological reference conditions, and assessing deviation from the latter. Whilst restoration targets may require modification in the future to account for climate induced alterations, the long temporal perspective offered by palaeolimnology ensures that such changes are assessed against a sound baseline.     

Cladocera as indicators of trophic state in Irish lakes

We examined the impact of lake trophic state on the taxonomic and functional structure of cladoceran communities and the role of nutrient loading in structuring both cladoceran and diatom communities. Surface sediment assemblages from 33 Irish lakes were analysed along a gradient of total phosphorus concentration (TP; 4.0–142.3 μg l⁻¹), using a variety of statistical approaches including ordination, calibration and variance partitioning. Ordination showed that the taxonomic structure of the cladoceran community displayed the strongest response to changes in lake trophic state, among 17 measured environmental variables. Trophic state variables chlorophyll-a and TP explained about 20% of the variance in both cladoceran and diatom assemblages from a set of 31 lakes. Procrustes analysis also showed significant concordance in the structure of cladoceran and diatom communities (P < 0.001). Thus, lake trophic state affects the taxonomic structure of both primary and secondary producers in our study lakes. We also found a significant decrease in relative abundance of taxa associated with both macrophytes and sediments, or sediments only, along the TP gradient (r = −0.49, P = 0.006, n = 30), as well as an increase in the proportion of the planktonic group (r = 0.43, P = 0.017, n = 30). This suggests that cladoceran community structure may also be shaped by lake trophic state indirectly, by affecting habitat properties. We found no relationship between lake trophic state and the relative abundance of each of three cladoceran groups that display different body size. We compared community structure between bottom and top sediment samples in cores from six Irish lakes. Results revealed similar trajectories of nutrient enrichment over time, as well as a strong shift in cladoceran functional structure in most systems. This study confirms that Cladocera remains in lake sediments are reliable indicators of lake trophic state. This study also highlights the fact that taxonomic and functional structure should both be considered to account for the multiple factors that shape cladoceran communities.     

Biogeochemical records of paleoenvironmental changes in Nainital Lake,Kumaun Himalayas,India

Rapid urbanization and increased tourism around Nainital Lake in the Kumaun Himalayan region in north India has raised concerns about sediment and water pollution. Lead-210 dated sediment cores from the lake represent ~95 years of accumulation and yield a mean sedimentation rate of ~4.7 mm year⁻¹. Total organic carbon (TOC), percent N and S and their atomic C/N and C/S ratios, stable isotopes (δ¹³C, δ¹⁵N, and δ³⁴S), and specific biomarkers (n-alkanes and pigments) were measured in the core. Organic matter is primarily derived from in-lake algal production and TOC flux varies from 1.0 to 3.5 g m⁻² year⁻¹. Sediments are anoxic (Eh −328 to −187 mV) and have low (0.10–0.30 g m⁻² year⁻¹) N, but high (0.37–1.0 g m⁻² year⁻¹) S flux. Shifts in δ¹³C, δ¹⁵N, and δ³⁴S suggest in-lake microbial processes dominated by denitrification and sulfate reduction. The sediments are dominated by short-chain hydrocarbons with low Carbon Preference Index values. The pigments indicate a gradual shift to cyanobacterial domination of the phytoplankton community in recent years. Despite an increase in external input of nutrients, the trophic state of the lake has remained largely unchanged, and the perceived human-induced impacts are limited.     

500 years of trophic-state history of a hypertrophic Dutch dike-breach lake

We present a palaeolimnological study encompassing five centuries of trophic-state change of the dike-breach lake De Waay located on the Rhine-Meuse delta (the Netherlands). Diatom-inferred total phosphorus (TP) concentrations indicate hypertrophic epilimnetic conditions (>300 μg l⁻¹ TP) since the formation of the lake in the fifteenth century until the end of the eighteenth century. Cladocera data support the reconstructed trophic state and indicate turbid conditions in lake De Waay during this period. High inferred TP concentrations as well as the amount of Ti in the sediment reflect numerous flooding events. From the nineteenth century onwards reconstructed TP concentrations decreased to 40–150 μg l⁻¹ due to improvements in sewage and dike systems that considerably diminished direct river flooding and seepage-derived nutrients. As a consequence, the increased stability of littoral habitats led to an increased diversity of the Cladocera assemblages. The most significant decrease in TP concentrations to ~40 μg l⁻¹ occurred between about 1900 and 1930. This mesotrophic phase was a consequence of the isolation of the lake from catchment drainage and the introduction of a highly elaborate flood control during this period. However, since the mid twentieth century a eutrophication trend is preserved in the record, likely related to increased agricultural activity in the vicinity of the lake. Our results emphasize that land-use and trophic-state history must be taken into account when evaluating the ecological status of lakes for water management and protection actions, especially for lakes in landscapes that are strongly modified by human action.     

Recent limnological changes in a saline lake of the Bolivian Altiplano,Lake Poopo

During the past twelve years, Lake Poopo, located on the Bolivian Altiplano, has had two main types of morphometry. Before 1985, its level was low and the depth shallow (maximum 3 m); there was no outlet and a strong salinity gradient existed from north to south. After 1985, the depth doubled, an outlet developed and the salinity became uniform throughout the lake (∼10 g L⁻¹). Before 1985, the phytoplankton was distinguished by a high number of diatom taxa and by the dominance of diatoms in the algal biomass. After 1985, while diatoms were still numerous in terms of species composition, Peridiniales or Chlorophyceae dominated the algal biomass. *** DIRECT SUPPORT *** A02GG003 00002     

Responses of Fragilarioid-dominated diatom assemblages in a small Arctic lake to Holocene climatic changes, Russell Island, Nunavut, Canada

We present a paleolimnological record spanning the Holocene from a small lake on Russell Island (Lake PW02), in the central Canadian Arctic Archipelago (74.07° N, 97.77° W, 182 m asl). Fragilarioid diatom types in the genera Pseudostaurosira, Staurosira and Staurosirella constitute >90% of valves in fossil samples. Using modern biogeographic data which specify the temperature optima of the Fragilarioid diatom taxa, we present new inferences about the timing of paleoclimatic changes in the central Arctic islands. The early Holocene was characterized by maximum values for sediment organic matter, and lower ratios of Staurosirella pinnata to Staurosira construens v. venter, suggesting warm summer air temperatures between about 9500−6500 cal year BP. Influxes of biogenic silica and diatom valves decreased following 4000 cal year BP, the sediment accumulation rate slowed and diatom taxa of the littoral zone diversified, suggesting cooler summers and more persistent lake ice. Variations in the species composition of the assemblages indicate paleoclimatic changes that are in broad agreement with other paleoenvironmental records from the Arctic including melt records from the Agassiz Ice Cap. Although autecological data remain incomplete for Fragilarioid taxa, our results indicate differences in these taxa in responses to paleoenvironmental change and underline the potential for the increased use of these taxa in paleoenvironmental reconstructions. The record from Lake PW02, as in other records from Arctic lakes with low algal diversity throughout the Holocene, shows a pronounced increase in diatom diversity since the 1920s, and diatom production since the 1970s far exceeds any recorded during the Holocene.     

Natural and anthropogenic aquatic environmental changes reconstructed by paleolimnological analyses in Lake Kitaura,central Japan

In order to assess the recent anthropogenic environmental changes in Lake Kitaura, central Japan, changes during the past few centuries were reconstructed from results of radiometric and tephrochlonological age determination, magnetic susceptibility measurements, total organic carbon analyses, total nitrogen analyses and fossil diatom analyses on a sediment core from the lake. A total of six major and sub-zones are recognized according to the diatom fossil assemblages, and we discuss aquatic environmental change in Lake Kitaura mainly based on these diatom assemblage change. Zone Ia and Zone Ib (older than AD 1707) are marine to brackish. In Zone IIa (AD␣1707–AD 1836), most of the brackish diatoms disappeared, and were replaced by freshwater species indicating a decrease in salinity. We interpret the salinity decrease in Zone I–IIa as a sea-level fall during the Little Ice Age. The salinity of the lake decreased to near freshwater conditions in Zone IIb (AD 1836–AD 1970), which could arise from alteration in River Tone or development of a sandspit in the mouth of River Tone in addition to sea-level change. In Zone IIIa (AD 1970–AD 1987), the diatom assemblage indicates a freshwater environment, and sedimentation rates increase rapidly. These changes reflect sedimentary environment change and an ecosystem transition due to the construction of the tide gate. In Zone IIIb (AD 1987–AD 2002), the diatom flux (valves cm⁻² y⁻¹) increased and species composition changed. The changes in Zone IIIb show a good agreement with limnological monitoring data gathered from the lake. These paleolimnological data suggest that the recent human-induced changes of the aquatic environment of the lake after the 1970s exceed rates during the period concerned in this study.     

A diatom dataset and diatom-salinity inference model for southeast Australian estuaries and coastal lakes

To quantify the relationship between diatom species assemblages and the water chemistry of southeast Australian estuaries and coastal lakes, a new dataset of 81 modern diatom samples and water chemistry data was created. Three hundred and ninety-nine species from 53 genera were identified in 36 samples from 32 coastal water bodies in eastern Tasmania and 45 samples from 13 coastal water bodies in southern Victoria. Multivariate statistical analyses revealed that the sampling sites were primarily distributed along salinity and nutrient gradients, and that salinity, nitrate + nitrite, phosphate and turbidity explained independent portions of variance in the diatom data. Species salinity optima and tolerances were determined and a diatom-salinity inference model (WAinv r ² = 0.72, r ²jack = 0.58, RMSEP = 0.09 log ppt) was developed. This new information on diatom species’ salinity preferences provides a useful tool for quantitatively reconstructing salinity changes over time from diatom microfossils preserved in the sediments of a range of estuaries and coastal lakes in southeast Australia. This is valuable for studies investigating long-term human impacts and climate change in the region.     

A 140-year record of recent changes in aquatic productivity in a remote,tropical alpine lake in the Rwenzori Mountain National Park,Uganda

Environmental change in many tropical, alpine habitats remains poorly resolved due to an absence of proximate and sustained observations. In the Rwenzori Mountains of East Africa, glaciers have receded rapidly over the last century, and here we assess the impact of this recession through palaeolimnological analyses of a 45 cm sediment core (Buju3) from Lake Bujuku which is closest to the ice-fields and partly supplied by melt water in-flows. ²¹⁰Pb and ¹³⁷Cs suggest that Buju3 has an average sedimentation rate of 2.9 mm year⁻¹ and the base of the core can be dated to 1864 ± 20 years. Contemporary diatom taxa found in the lake are dominated by Tabellaria flocculosa and Synedra spp., but also include Achnanthes minutissima and Fragilaria pinnata. However, the diatom flora for Buju3 is less diverse and dominated by small, tychoplanktonic species of Fragilaria. Over the period associated with glacial recession, organic carbon isotope analysis (δ¹³C) suggests a small but distinct increase in within-lake productivity, which increases in rate since the mid 1970s up to the present day, in line with a shift towards increased algal productivity (as highlighted by C/N ratios). However, the diatom and pollen records appear rather insensitive to changes in glacier recession since the late 19th century.     

Benthic foraminifera and their stable isotope composition in sediment cores from Lake Qarun,Egypt: changes in water salinity during the past ~500 years

We studied the sedimentology, benthic foraminifera, molluscs, and δ¹⁸O and δ¹³C of Ammonia tepida tests in two late Holocene sediment cores from Lake Qarun (Egypt). The cores, QARU2 (upper section, 8.2 m) and QARU4 (1.4 m), span approximately the past 500 years of sedimentation. Benthic foraminifera first appeared in the upper part of QARU2 at 314 cm depth, ca. AD 1550. This depth marks the beginning of colonization of the lake by foraminifera and indicates a change in lake water salinity, as foraminifera cannot tolerate fresh water. Initially, three species of benthic foraminifera colonized the lake, Ammonia tepida, Cribroelphidium excavatum and Cribrononion incertum. Relative abundance of these species fluctuated throughout cores QARU2 and QARU4 and highest overall faunal diversity occurred at the beginning of the twentieth century. High relative abundances of C. incertum and deformed tests are attributed to periods of greater lakewater salinity. Peaks in both δ¹⁸O and δ¹³C indicate times of higher evaporation and reduced fresh water inflow. Inferred salinity was high around AD 1700 and after AD 1990. Rapid response of climate proxy variables indicates the high sensitivity of Lake Qarun to environmental changes over the past several 100 years. Increases in lakewater Mg concentration during past evaporative events, associated with less fresh water inflow, probably provided conditions suitable for C. incertum to build its white or transparent tests. Gradual decrease of C. incertum, until its disappearance at 100 cm depth ca. AD 1890, indicates a more persistent trend in lake water chemistry. Higher concentrations of dissolved sulphates were the likely cause of this species disappearance. Recent, twentieth-century sediments were deposited under optimal salinity (37‰) for benthic fauna, but further environmental changes are indicated by the decrease or disappearance of several benthic foraminifera and mollusc species. Intermittent hypoxia in the lake’s bottom waters, caused by cultural eutrophication, may account for these most recent changes.     

Diatom-salinity relationships in 111 lakes from the Interior Plateau of British Columbia,Canada: the development of diatom-based models for paleosalinity reconstructions

Diatoms were identified and enumerated from the surface sediments of 111 lakes, 45 from the Kamloops region and 66 from the Cariboo/Chilcotin region, located on the southern Interior Plateau of British Columbia, Canada. This paper is an extension of another study which investigated the relationship of diatoms to salinity and ionic composition in 65 lakes from the Cariboo/Chilcotin region. The 111 lakes spanned a large gradient in salinity, ranging from fresh through hypersaline (late-summer salinity values ranged from 0.04 to 369 g l^–1), and included both carbonate- and sulphate-dominated lakes with sodium and magnesium as the dominant cations. The Kamloops region had more sulphate-dominated, hypersaline lakes and fewer carbonate-rich lakes than the Cariboo/Chilcotin region. Most lakes had higher salinities in the late-summer compared to the spring.Both salinity and brine-type were important variables that could explain the different diatom assemblages present in the lakes. The majority of diatom taxa had salinity optima in the freshwater to subsaline range (<3 g l^–1), and the taxa displayed a range of both narrow and broad tolerances along the salinity gradient. Weighted-averaging regression and calibration, and maximum likelihood techniques were used to develop salinity inference models from the diatom assemblages based on their relationship to the spring, late-summer and average lakewater salinity measurements. Simple weighted-averaging (WA) models generally produced the same or lower bootstrapped RMSEs of prediction than weighted-averaging with tolerance downweighting (WA(tol)) in the two regional and the combined datasets. Weighted averaging partial least squares (WA-PLS) showed little or no improvement in the predictive abilities of the datasets, as judged by the jackknifed RMSE of prediction. In all cases, the combined dataset of 102 lakes performed better than either of the smaller regional datasets, with relatively little difference between spring, average and late-summer salinity models. The maximum likelihood models gave lower apparent RMSEs of prediction in comparison to other methods; however, independent validation of this technique using methods such as bootstrapping were not undertaken because of the computer intensive nature of such analyses. These diatom-based salinity models are now available for reconstructing salinity and climatic trends from appropriately chosen closed-basin lakes in the Interior region of British Columbia.This is the second in a series of papers published in this issue on the paleolimnology of arid regions. These papers were presented at the Sixth International Palaeolimnology Symposium held 19–21 April, 1993 at the Australian National University, Canberra, Australia. Dr A. R. Chivas served as guest editor for these papers.     

Environmental variability in Lake Naivasha,Kenya, over the last two centuries

Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by ²¹⁰Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm⁻² year⁻¹ correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.     

New Zealand chironomids as proxies for human-induced and natural environmental change: Transfer functions for temperature and lake production (chlorophyll a)

The analysis of chironomid taxa and environmental datasets from 46 New Zealand lakes identified temperature (February mean air temperature) and lake production (chlorophyll a (Chl a)) as the main drivers of chironomid distribution. Temperature was the strongest driver of chironomid distribution and consequently produced the most robust inference models. We present two possible temperature transfer functions from this dataset. The most robust model (weighted averaging-partial least squares (WA-PLS), n = 36) was based on a dataset with the most productive (Chl a > 10 μg l⁻¹) lakes removed. This model produced a coefficient of determination () of 0.77, and a root mean squared error of prediction (RMSEP_jack) of 1.31°C. The Chl a transfer function (partial least squares (PLS), n = 37) was far less reliable, with an of 0.49 and an RMSEP_jack of 0.46 Log₁₀μg l⁻¹. Both of these transfer functions could be improved by a revision of the taxonomy for the New Zealand chironomid taxa, particularly the genus Chironomus. The Chironomus morphotype was common in high altitude, cool, oligotrophic lakes and lowland, warm, eutrophic lakes. This could reflect the widespread distribution of one eurythermic species, or the collective distribution of a number of different Chironomus species with more limited tolerances. The Chl a transfer function could also be improved by inputting mean Chl a values into the inference model rather than the spot measurements that were available for this study.