Relationship between diatoms and water quality (TN, TP) in sub-tropical east Australian estuaries

Nutrient over-enrichment of estuarine environments is increasing globally. However, it is difficult to determine the eutrophication trend in estuaries over long periods of time because long-term monitoring records are scarce and do not permit the identification of baseline environmental conditions. In this study, preliminary diatom based transfer functions for the inference of total phosphorus (TP) and total nitrogen (TN) in east-Australian sub-tropical estuaries were developed to address the deficiency in knowledge relating to historical estuary water quality trends. The transfer functions were created from a calibration set consisting of water quality and associated surface sediment diatom assemblage data from fifty-two sub-tropical estuaries in New South Wales and Queensland, Australia. Following data screening processes, Canonical Correspondence Analysis confirmed that TP and TN both explained significant, independent variation in the diatom assemblages. Variance partitioning, however, indicated that the TP was confounded with and may receive some strength from TN. WA and WA-PLS 2 component models for TP that included all calibration set sites yielded statistically weak results based on the jack-knifed r ² scores $ \left( {r_{\text{jack}}^{{^{ 2} }} \, = 0.22\;{\text{and}}\;0. 2 2 {\text{ respectively}}} \right) $ . Removal from the calibration set of 12 sites that had all PO₄, NH₄, NO₂, and NO_x concentrations below detection limit resulted in a substantial improvement in WA-PLS 2 component TP model scores $ \left( {r_{\text{jack}}^{{^{ 2} }} \; = \;\,0.69} \right) $ , indicating that this model is statistically robust, and thus suitable for down core nutrient reconstructions. Caution, however, is required when developing diatom based inference models in Australian estuaries as nutrient cycling processes may have the potential to influence diatom based transfer functions. The model reported on here provides a foundation for reconstructing nutrient histories in eastern Australian sub-tropical estuaries in the absence of monitoring data.     

Depth distribution of chironomids and an evaluation of site-specific and regional lake-depth inference models: a good model gone bad?

We used 39 surface samples from Marcella Lake, Yukon Territory, to examine the distribution of chironomid head capsules in relation to depth and to develop a site-specific (intralake) inference model for reconstructing past lake levels. Ten of the 34 most-frequently occurring taxa encountered in the surface-sediment samples are significantly related to depth. We then applied the site-specific inference model and a previously developed regional model to samples from deep- and shallow-water cores from Marcella Lake. The inferences were compared to an independent Digerfeldt-type reconstruction of lake level history and to moisture inferences drawn from pollen data. Although the site-specific model was good in having better performance statistics than the regional model, it was bad at producing depth reconstructions because most samples from the long cores lacked suitable analogues in the site-specific training set. None of the chironomid-based reconstructions was a good match to the Digerfeldt-type reconstruction. Inconsistencies remain between the paleohydrological inferences derived from the chironomid depth models, the Digerfeldt-type reconstruction and pollen-inferred reconstructions of past moisture regimes.     

Modern influences on chironomid distribution in western Ireland: potential for palaeoenvironmental reconstruction

Ireland provides a unique setting for the study of past climates, as its climate is dominated by westerly airflow from the North Atlantic and readily responsive to changes in North Atlantic circulation patterns. Although there has been substantial research on Ireland’s past environments, quantitative palaeolimnological research, especially chironomid-based research, has been lacking. In order to further develop chironomid-based palaeolimnological investigations, a calibration set was constructed to determine the dominant environmental controls on modern chironomids in western Ireland. Chironomid subfossils were collected from surface sediments of 50 lakes. The lakes were characterised with 36 environmental variables, including physical attributes, lake water characteristics, lake sediment characteristics and land cover within each catchment. In this exploratory study, no specific environmental variable was targeted and lakes were chosen to span gradients of latitude, elevation, depth and trophic status. Redundancy analysis showed that six environmental variables—mean July air temperature, lake depth, dissolved organic carbon, and percentage catchment land cover of agriculture, peat bog and scrubland—captured a large and statistically significant portion of the variance in the chironomid data. July temperature and agricultural land cover were the most dominant environmental variables, with July temperature proving the most suitable for inference model development. A classical weighted-averaging model was developed to estimate July air temperature, with a coefficient of determination (r _jack ² ) of 0.60 and root mean square error of prediction (RMSEP) of 0.57 °C. Results suggest that summer temperature is the dominant influence on chironomid distribution across a wide variety of lake types, and the relatively small RMSEP should allow for more accurate reconstructions of Ireland’s relatively subdued Holocene temperature fluctuations.     

Diatom–environment relationships and a transfer function for conductivity in lakes of the Badain Jaran Desert, Inner Mongolia, China

We describe a dataset of 26 modern diatom samples and associated environmental variables from the Badain Jaran Desert, northwest China. The influence of electrical conductivity (EC) and other variables on diatom distribution was explored using multivariate analyses and generalized additive modeling of species response curves. A transfer function was derived for EC, the variable with the largest unique effect on diatom variance, as shown by partial canonical correspondence analysis. Weighted-averaging partial least squares regression and calibration provided the best model, with a high coefficient of determination ( $ {\text{r}}_{\text{boot}}^{2} $  = 0.91) and low prediction error (RMSEP_boot = 0.136 log₁₀ μS cm^?1). To assess its potential for palaeosalinity and palaeoclimate reconstructions, the EC transfer function was applied to fossil diatom assemblages from ²¹⁰Pb-dated short sediment cores collected from two subsaline lakes of the Badain Jaran Desert. The diatom-inferred (DI) EC reconstructions were compared with meteorological data for the past 50 years and with remote sensing data for the period AD 1990–2012. Changes in DI–EC were small and their relationship with climate was weak. Moreover, remote sensing data indicate that the surface areas and water depths of these lakes did not change, which suggests that water loss by evaporation is compensated by groundwater inflow. These results suggest that the response of these lakes to climate change is mediated by non-climatic factors such as the hydrogeological setting, which control recharge from groundwater, and may be non-linear and non-stationary.     

Paleotemperature reconstruction in tropical Africa using fossil Chironomidae (Insecta: Diptera)

Fossil assemblages of chironomid larvae (non-biting midges) preserved in lake sediments are well-established paleothermometers in north-temperate and boreal regions, but their potential for temperature reconstruction in tropical regions has never before been assessed. In this study, we surveyed sub-fossil chironomid assemblages in the surface sediments of 65 lakes and permanent pools in southwestern Uganda (including the Rwenzori Mountains) and central and southern Kenya (including Mount Kenya) to document the modern distribution of African chironomid communities along the regional temperature gradient covered by lakes situated between 489 and 4,575 m above sea level (a.s.l). We then combined these faunal data with linked Surface-Water Temperature (SWTemp: range 2.1–28.1°C) and Mean Annual Air Temperature (MATemp: range 1.1–24.9°C) data to develop inference models for quantitative paleotemperature reconstruction. Here we compare and discuss the performance of models based on different numerical techniques [weighted-averaging (WA), weighted-averaging partial-least-squares (WA-PLS) and a weighted modern analogue technique (WMAT)], and on subsets of lakes with varying gradient lengths of temperature and other environmental variables. All inference models calibrated against MATemp have a high coefficient of determination ( r_\textjack² r_{\text{jack}}^{2}  = 0.81–0.97), low maximum bias (0.84–2.59°C), and low root-mean-squared error of prediction (RMSEP = 0.61–1.50°C). The statistical power of SWTemp models is generally weaker ( r_\textjack² r_{\text{jack}}^{2}  = 0.77–0.95; maximum bias 1.55–3.73°C; RMSEP = 1.39–1.98°C), likely because the surface-water temperature data are spot measurements failing to catch significant daily and seasonal variation. Models based on calibration over the full temperature gradient suffer slightly from the limited number of study sites at intermediate elevation (2,000–3,000 m), and from the presence of morphologically indistinguishable but ecologically distinct taxa. Calibration confined to high-elevation sites (>3,000 m) has poorer error statistics, but is less susceptible to biogeographical and taxonomic complexities. Our results compare favourably with chironomid-based temperature inferences in temperate regions, indicating that chironomid-based temperature reconstruction in tropical Africa can be achieved.     

Independent measurement of biogenic silica in sediments by FTIR spectroscopy and PLS regression

We present an independent calibration model for the determination of biogenic silica (BSi) in sediments, developed from analysis of synthetic sediment mixtures and application of Fourier transform infrared spectroscopy (FTIRS) and partial least squares regression (PLSR) modeling. In contrast to current FTIRS applications for quantifying BSi, this new calibration is independent from conventional wet-chemical techniques and their associated measurement uncertainties. This approach also removes the need for developing internal calibrations between the two methods for individual sediments records. For the independent calibration, we produced six series of different synthetic sediment mixtures using two purified diatom extracts, with one extract mixed with quartz sand, calcite, 60/40 quartz/calcite and two different natural sediments, and a second extract mixed with one of the natural sediments. A total of 306 samples—51 samples per series—yielded BSi contents ranging from 0 to 100 %. The resulting PLSR calibration model between the FTIR spectral information and the defined BSi concentration of the synthetic sediment mixtures exhibits a strong cross-validated correlation ( \( {\text{R}}^{ 2}_{\text{cv}} \)  = 0.97) and a low root-mean square error of cross-validation (RMSECV = 4.7 %). Application of the independent calibration to natural lacustrine and marine sediments yields robust BSi reconstructions. At present, the synthetic mixtures do not include the variation in organic matter that occurs in natural samples, which may explain the somewhat lower prediction accuracy of the calibration model for organic-rich samples.     

A midge-salinity transfer function for inferring sea level change and landscape evolution in the Hudson Bay Lowlands,Manitoba, Canada

We compared water chemistry and environmental data with midge assemblage data, using multivariate analysis to assess the environmental gradients that limit midge (Chironomidae, Chaoboridae and Ceratopogonidae) distributions in the Hudson Bay Lowlands, northeastern Manitoba, Canada. Midge remains, comprising 62 taxa, were obtained from surficial sediments of 63 ponds. Ponds were sampled to maximize the salinity gradient. Specific conductance ranged from 46 to 29,000 μS cm^?1. Proximity to the coast was a principal determinant of pond salinity, with ponds closer to Hudson Bay shoreline more saline that those farther away. Multivariate analysis indicated that midge distributions have a significant relationship ( $ {\text{r}}_{\text{boot}}^{2} = 0.68 $ ) with salinity in the data set. This work will allow paleolimnological inferences of midge community responses to changing sea level (i.e. salinity) via isostatic rebound within the Hudson Bay Lowlands, and will provide essential limnological information to scientists and managers in a region where understanding of aquatic ecosystems is limited. One undescribed midge taxon was dominant in ponds with the highest salinities and may be a key indicator for inferring highly saline environments.     

A Comparative Analysis of Favorability Mappings by Weights of Evidence,Probabilistic Neural Networks,Discriminant Analysis,and Logistic Regression

This study compares the performance of favorability mappings by weights of evidence (WOE), probabilistic neural networks (PNN), logistic regression (LR), and discriminant analysis (DA). Comparisons are made by an objective measure of performance that is based on statistical decision theory. The study further emphasizes out-of-sample inference, and quantifies the extent to which outcome is influenced by optimum variable discretization with classification and regression trees (CARTS).Favorability mapping methodologies are evaluated systematically across three case studies with contrasting scale and geologic information:

Stable isotopes and trace elements in modern ostracod shells: implications for reconstructing past environments on the Tibetan Plateau,China

Stable isotopes and trace elements in ostracod shells have been used widely in paleolimnological investigations of past lake hydrochemistry and climate because they provide insights into past water balance and solute evolution of lakes. Regional differences in lake characteristics and species-specific element fractionation, however, do not permit generalization of results from other regions or ostracod species to the southern Tibetan Plateau, in part because most common taxa from the southern Tibetan Plateau are endemic to the area. This study evaluated relations between present-day environmental conditions and the geochemical composition of modern ostracod shells from the southern Tibetan Plateau, to assess the suitability of using shell chemistry to infer hydrological conditions. We studied nine lakes and their catchments, located along a west–east transect in the south-central part of the Tibetan Plateau. Stable oxygen and carbon isotope values and trace element concentrations in recent shells from the four most abundant ostracod species (Leucocytherella sinensis, ?Leucocythere dorsotuberosa, Limnocythere inopinata, Tonnacypris gyirongensis) were measured, together with hydrochemical properties of host waters at the time of sampling. Results revealed significant between-species differences in stable isotope fractionation and trace element incorporation into shell calcite. Stable oxygen and carbon isotope values of ostracod shells were correlated significantly with the stable isotope composition of the respective water body \( \left( {\updelta^{18} {\text{O}}_{{{\text{H}}_{ 2} {\text{O}}}} \,{\text{and }}\updelta^{13} {\text{C}}_{{{\text{H}}_{ 2} {\text{O}}}} } \right) \), reflecting salinity and productivity, respectively. Offsets between δ¹⁸O_shell and δ¹³C_shell and inorganic calcite, the latter representing isotopic equilibrium, suggest shell formation of T. gyirongensis during spring melt. L. sinensis reproduces throughout the monsoon season until September and shows several consecutive generations, and L. inopinata molts to the adult stage after the monsoon season in August/September. The influence of pore water δ¹³C was displayed by L. inopinata, suggesting shell calcification within the sediment. Mg/Ca_shell is primarily influenced by water Mg/Ca ratios and salinity and confirms the use of this shell ratio as a proxy for precipitation-evaporation balance and lake level. In addition, Sr/Ca and Ba/Ca can be used to infer changes in salinity, at least in closed-basin lakes with calcite saturation. Observed effects of water Sr/Ca and salinity on Sr/Ca incorporation are biased by the presence of aragonite precipitation in the lakes, which removes bioavailable Sr from the host water, resulting in low Sr/Ca_shell values. Changes in carbonate mineralogy affect the bioavailability of trace elements, a process that should be considered in paleoclimate reconstructions. Oxygen isotopes and Mg/Ca_shell ratios were unaffected by water temperature. Positive correlations among Fe/Ca, Mn/Ca and U/Ca in ostracod shells, and their negative correlation with δ¹³C, which reflects organic matter decay, show the potential to infer changes in redox conditions that can be used to reconstruct past oxygen supply to bottom waters and thus past water-circulation changes within lakes. The intensity of microbial activity, associated with organic matter decomposition, can be inferred from U/Ca ratios in ostracod shells. These findings highlight the value of fossil ostracod records in lake deposits for inferring paleoenvironmental conditions on the southern Tibetan Plateau.     

Oxygen isotope fractionation in the ostracod <Emphasis Type="Italic">Eucypris mareotica</Emphasis>: results from a culture experiment and implications for paleoclimate reconstruction

Oxygen isotope analysis of the adult ostracod Eucypris mareotica cultured at controlled temperatures (10, 15, and 19°C) was used to measure isotopic fractionation during shell calcification. The ostracod shells that precipitated at experimental temperatures are almost in isotopic equilibrium with the culture water as compared to the oxygen isotope fractionation of inorganic carbonates. Moreover, they had almost constant offsets from equilibrium based on the oxygen isotope fractionation of inorganic carbonates. The δ¹⁸O values of ostracod shells from the 10°C cultures were higher than those of the 15 and 19°C cultures by 1.6 and 2.7‰, respectively. The observed fractionations are shown by the regression equations: *20c 10^° \textC:d ^{1 8} \textO_{\textostracod} = 1. 1 7+ 0. 5 7d ^{1 8} \textO_\textwater 1 5^° \textC:d ^{1 8} \textO_{\textostracod} = - 0. 4 8+ 0. 6d ^{1 8} \textO_\textwater 1 9^° \textC:d ^{1 8} \textO_{\textostracod} = - 1. 6+ 0. 6d ^{1 8} \textO_\textwater \begin{array}{*{20}c} { 10^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = 1. 1 7+ 0. 5 7\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 5^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 0. 4 8+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ { 1 9^\circ {\text{C}}:\delta^{ 1 8} {\text{O}}_{\text{ostracod}} = - 1. 6+ 0. 6\delta^{ 1 8} {\text{O}}_{\text{water}} } \\ \end{array} The fractionation factors (α) are slightly lower for the 15 and 19°C cultures, but slightly higher for the 10°C culture, as compared to inorganic carbonates (O’Neil et al. in J Chem Phys 51:5547–5558, 1969). The oxygen fractionation factors of E. mareotica are very close to those of synthetic calcite formed in isotopic equilibrium. The ‘vital offsets’ of valve-δ¹⁸O for E. mareotica is so small that we can neglect its effect when using the δ¹⁸O of E. mareotica living in lake waters with high pH and salinity to reconstruct the paleoenvironment. The paleotemperature or paleoisotopic composition of lake water interpreted from a core of lacustrine sediment may be closer to the true values when the δ¹⁸O data for E. mareotica are used.     

Cladoceran and chironomid assemblages as qualitative indicators of water depth in subarctic Fennoscandian lakes

The relationship between surface-sediment cladoceran and chironomid communities to lake depth was analysed in 53 lakes distributed across timberline in northern Fennoscandia using multivariate statistical approaches. The study sites are small and bathymerically simple, with water depth ranging from 0.85-27.0 m (mean 6.36 m). Maximum lake depth was the most important factor in explaining the cladoceran distributions and the second most important factor in explaining the chironomid distributions in these subarctic lakes, as assessed on the basis of a series of constrained RDAs, Monte Carlo permutation tests, and variance partitioning. Quantitative inference models for maximum lake depth were created for both groups of animals. Well-performing calibration functions for predicting lake depth were obtained in each case using linear partial least squares (PLS) regression and calibration, weighted averaging (WA) with an 'inverse' deshrinking regression, and weighted averaging partial least squares (WA-PLS). Quantitative reconstructions of lake level fluctuations should be possible from cladoceran and chironomid core data with a root mean squared error of prediction (RMSEP), as estimated by jack-knifing, of about 1.6-3.0 m.     

Defining drivers of nitrogen stable isotopes (δ15N) of surface sediments in temperate lakes

The nitrogen stable isotopic signature (δ¹⁵N) of sediment is a powerful tool to understand eutrophication history, but its interpretation remains a challenge. In a large-scale comparative approach, we identified the most important drivers influencing surface sediments δ¹⁵N of 65 lakes from two regions of Canada using proxies that reflect watershed nitrogen (N) sources, internal lake microbial cycling and productivity. Across regions, we found that water column total nitrogen (TN),  %N in the sediments and lake morphometric variables were the best predictors of sedimentary δ¹⁵N, explaining 66 % of its variation. Significant relationships were also found between sediment δ¹⁵N and human-derived N load ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.23, p < 0.001), the latter being a strong predictor of TN ( \( R_{{{\text{adj}} .}}^{2} \)  = 0.68, p < 0.001). Despite a relatively strong overall relationship, variation partitioning revealed an interesting difference in the dominant variable that influenced regional δ¹⁵N. Alberta lake sedimentary δ¹⁵N signature was dominated by human derived N load. In contrast, internal processing appeared to be more important in Quebec lakes, where sediment δ¹⁵N was best explained by  %N in the sediments and lake volume. Overall, our findings support the use of δ¹⁵N in paleolimnological investigations to reconstruct changing N sources to lakes but also highlight that regions may have distinctive drivers. Interpretations of sediment δ¹⁵N are likely to be strongest when multiple lines of evidence are employed and when placed in a regional context.     

Diatom habitats,species diversity and water-depth inference models across surface-sediment transects in Worth Lake,northwest Ontario,Canada

We analyzed surface-sediment samples collected along transects from three sub-basins of a relatively large (~115 ha), bathymetrically complex lake, in northwest Ontario, Canada, to assess the reproducibility of diatom species habitats and diversity along a water-depth gradient. Transects displayed different orientations with respect to prevailing wind direction and varied in complexity and degree of slope along the lake bottom. Each transect consisted of three replicate samples at a resolution of ~1 m water depth from ~1 to 30 m for the two deep-basin transects and from ~1 to 18 m in the shallower basin. Distinct diatom assemblages were identified in all transects: (1) a near-shore community composed largely of attached life-forms and some motile benthic taxa, (2) a mid-depth community composed largely of motile life-forms and other benthic taxa that are adapted to lower light conditions (e.g. Staurosirella pinnata), and (3) a deep-water community dominated by planktonic taxa. Species richness was highest in the benthic zones (<9 m), with greatest species evenness in the mid-depth zone (~3–9 m). Species richness and evenness were highly correlated across the three transects (r = 0.89–0.93, p < 0.01). Diatom-inferred depth models were developed from the individual transects to assess reproducibility and applicability for down-core analyses using modern analog (MAT) and weighted-averaging (WA-PLS) approaches. Coefficients of determination (r ²) for these models ranged from 0.80 to 0.98, and RMSEP ranged from 1.2 to 4.2 m. The models developed from the transect with the highest resolution sampling, gentlest non-complex slope and shallowest maximum depth were the strongest ( r_\textMAT² = 0.97 r_{\text{MAT}}^{2} = 0.97 ; r_{\textWA - PLS}² = 0.98 r_{\text{WA - PLS}}^{2} = 0.98 ) and had the lowest RMSEP (MAT = 1.2 m, WA-PLS = 1.3 m). These inference models can be used to infer past fluctuations in the depth of the benthic/planktonic boundary from cores retrieved near this ecotone and provide a sensitive record of the past change in location of the benthic zone. These types of data can be used to assess past variability in droughts and lake levels to better plan for potential future extremes. Such records incorporate more realistic estimates of natural variability than the ~100-year instrumental records currently used by water resource managers.     

Carbon cycling in Lake Erie during cultural eutrophication over the last century inferred from the stable carbon isotope composition of sediments

A ~106-cm sediment core from the eastern basin of Lake Erie was examined to investigate biogeochemical processes in this large lake during its cultural eutrophication over the last century. We measured stable carbon isotopes of total organic carbon and calcium carbonate (δ¹³C_TOC and d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} ) as well as the concentrations of total organic carbon (TOC) and calcium carbonate (CaCO₃). δ¹³C_TOC and TOC show a strong positive correlation throughout the core and record changes in phytoplankton productivity and nutrient loading. CaCO₃ and TOC concentrations display a negative correlation throughout the core, suggesting that CaCO₃ concentrations are controlled primarily by decomposition of TOC in the hypolimnion and the sediments, although temperature and invasive mussels are also potential controlling factors. d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values show a positive correlation with δ¹³C_TOC between 1909 and 1969, indicating phytoplankton productivity was the primary control for d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} values during eutrophication. However, a negative correlation between d¹³ \textC_{\textCaCO 3} \delta^{13} {\text{C}}_{{{\text{CaCO}}_{ 3} }} and δ¹³C_TOC from 1970 to 2002 suggests that these two proxies tracked different aspects of the carbon cycle in the lake in more recent times. The cause for the negative correlation is not yet known, but it is perhaps associated with temperature variations and seasonal differences in productivity.     

The use of high-resolution gridded climate data in the development of chironomid-based inference models from remote areas

The development of chironomid-based air temperature inference models in high latitude regions often relies on limited spatial coverage of meteorological data and/or on punctual measurements of water temperature at the time of sampling. The use of simple linear regression to relate air temperature and latitude was until recently the best method to characterize the air temperature gradient along a latitudinal gradient. However, recent studies have used high-resolution gridded climate data to develop new chironomid-based air temperature inference models. This innovative approach has, however, never been further analyzed to test its reliability. This study presents a method using ArcGIS^® to extract air temperatures from a high-resolution global gridded climate data set (New et al. 2002) and to incorporate these new data in a variety of chironomid-based air temperature inference models to test their performance. Results suggest that this method is reliable and produces better estimates of air temperature and will be helpful in the development of further quantitative air temperature inference models in remote areas.     

Surface sediment Chironomidae (Insecta: Diptera) distributions along an ecotonal transect in subarctic Fennoscandia: developing a tool for palaeotemperature reconstructions

We used multivariate statistical techniques to analyse the distributions of surface sediment chironomid assemblages with respect to surface-water temperature, and an additional set of 27 environmental variables, in 30 freshwater lakes of northern Fennoscandia. Our study transect spans boreal coniferous forest to subarctic tundra and includes a steep temperature gradient. Canonical correspondence analysis (CCA) with forward selection and associated Monte Carlo permutation tests revealed that there were statistically significant (P<0.05) relationships between chironomid distributions and two environmental variables, namely lakewater temperature and maximum lake depth. A constrained CCA with temperature as the only predictor variable suggested that the relationship between lakewater temperature and chironomid composition was sufficiently robust for developing a weighted-averaging (WA) based quantitative inference model that will allow palaeotemperature reconstructions using subfossil chironomid remains preserved in lake sediments.     

Sieve mesh size and quantitative chironomid paleoclimatology

The minimum sieve mesh size for fossil chironomid analysis is usually set at 100 μm, to ensure adequate recovery of small species. Yet taking into account the labor intensity of sorting and identifying fossil chironomid remains, the large numbers of samples requiring processing in paleoclimate studies with high temporal resolution, and the increasing need to engage non-specialist analysts in this work, it seems appealing to sieve samples through a larger mesh size that would mainly retain easier-to-see, easier-to-extract and easier-to-identify fossil specimens. In this study we evaluated the influence of sieve mesh size on chironomid-based quantitative paleoenvironmental reconstructions in African lakes. We developed two chironomid-salinity calibration data sets based on either the >100 μm or >150 μm size fractions of surface-sediment fossil assemblages, and compared the performance of salinity-inference models derived from them. We find that, despite ∼35% additional fossil loss, restriction to >150 μm data did not appreciably affect individual taxon optima or tolerances, and resulted in only a modest reduction of mean taxon richness per lake. Parameters of statistical model performance were as good, or better than those of models based on the >100 μm data, albeit only after excluding two atypically dilute (<100 μS/cm) lakes from the calibration. Application of inference models based on >150 μm or >100 μm data to a 200-year fossil record from Lake Abiyata, Ethiopia, produced very similar trends, amplitudes and uncertainty ranges of inferred past salinity change. Restriction to >150 μm data reduced the mean fossil yield of core samples by ∼20% on average, i.e. fossil loss was markedly lower than in surface-sediment samples. Using the larger mesh size reduced sample processing time by up to 50%, partly by removing a significant proportion of visually obstructive organic debris, and partly by allowing a greater fraction of chironomid fossils to be identified directly in the sorting tray. The fraction of 1st instar group taxa in surface-sediment samples was reduced from 13% to 3%, increasing the mean taxonomic resolution of fossil assemblages, and thus their ecological specificity.     

Geochemical Characteristics and Hydrocarbon Expulsion of Lacustrine Marlstones in the Shulu Sag,Bohai Bay Basin,Eastern China: Assessment of Tight Oil Resources

Natural Resources Research - In recent years, tight oil exploration has made significant progress in the lower part of Shahejie Formation ( $$ {\text{Es}}_{3} ^{{\text{L}}} $$ ) in the Shulu Sag,...     

The potential of site-specific and local chironomid-based inference models for reconstructing past lake levels

We examined the relationship between three key environmental variables (water depth, loss-on-ignition, and bottom-water temperature) and fossil chironomid distributions sampled from within-lake gradients in three small, moderately deep (18–35 m), maar lakes on St Michael Island, western Alaska. Site-specific (one lake, 29 samples) and local (three lakes, 87 samples) inference models for reconstructing water depth were developed using partial least squares regression and calibration. These models and a previously published regional model (136 lakes, one central-lake sample from each) are used to infer water depths from 78 fossil samples spanning the last ~30,000 ¹⁴C years B.P. at Zagoskin Lake. Although the site-specific [r ² _boot = 0.90, root mean square error of prediction (RMSEP) = 1.76] and local (r _boot² = 0.68, RMSEP = 4.36) inference models have better performance statistics than the regional model, few clear trends among all three models exist in the lake-level reconstruction. We propose that multiple, within-lake sampling of gradients can be used to improve the performance statistics of water-depth transfer functions and ultimately reconstruct paleohydrology in regions known to exhibit large fluctuations in moisture balance through time given that: (1) adequate analogs are established and (2) taphonomic processes important to benthic invertebrate remains are more fully understood.     

Eutrophication of a small, deep lake in southern New Zealand: the effects of twentieth-century forest clearance, changing nutrient influx, light penetration and bird behaviour

This study provides a high resolution multi-proxy record of the response of an aquatic ecosystem (Alexander Lake) to forest clearance in New Zealand in the late twentieth century (ca. 1950–2006 AD). New chironomid-based transfer functions for lake water total nitrogen (TN) concentration were applied to the Alexander Lake chironomid record. A test of the significance of reconstructions based on multiple model types indicates that a model with the highest r² and lowest root mean squared error of prediction may not necessarily perform the best when applied to a particular site. The chironomid-based TN reconstruction and other proxies suggest a complex response by a stained water (dystrophic) lake in a forested catchment to deforestation. Minor perturbations and nutrient influx may favour increased phytoplankton production, but continued light attenuation by dissolved organic carbon and humic compounds prevents proliferation of submerged macrophytes. Complete mechanical forest clearance resulted in a short term pulse of nutrients and eutrophication. The long term effect of deforestation was to increase light penetration and favour the growth of submerged macrophytes. Continued eutrophication of Alexander Lake could be due to a contribution of bird-derived nutrients. Deforestation around Alexander Lake has created a perfect moulting site for Paradise Shelducks (Tadorna variegata Gmelin). The input of total phosphorus from T. variegata could be enough to trigger blooms of Microcystis that currently occur in the lake. Changes in bird behaviour in response to changes in vegetation should therefore be considered a possible result of past (including prehistoric) and future deforestation in New Zealand.