Tracking deepwater oxygen recovery using sedimentary chironomid assemblages: Peninsula Lake revisited

Journal of Paleolimnology - Peninsula Lake, Ontario, Canada, is a Precambrian Shield lake that has experienced many environmental stressors since European settlement of the watershed in the...     

Climate-mediated changes in small lakes inferred from midge assemblages: the influence of thermal regime and lake depth

Subfossil midge (Chironomidae and Chaoboridae) assemblages were examined in the surficial sediments (0?C1?cm) from small inland lakes in the Experimental Lakes Area (ELA) of northwestern Ontario, Canada. In these boreal lakes, maximum depth (Z_max), alkalinity, Secchi depth and chlorophyll-a concentrations explained significant amounts of variation in the subfossil assemblages. Constrained ordinations (redundancy analysis) indicated that the relationship between Z_max (as sqrt Z_max) and assemblage composition was strong enough to develop a paleolimnological inference model. Model statistics suggested that a robust model was generated (r ²?=?0.78, RMSEP?=?0.533, max bias?=?0.674); however, when the model was applied to a subfossil stratigraphy from an ELA lake sediment core, results suggested that the inference model had produced an unrealistically shallow Z_max inference. Further analyses indicated that thermal regime had a strong influence on assemblage composition; when the influence of thermal regime was partialled out, there was a much weaker relationship between Z_max and assemblage composition, particularly for stratified lakes. A thermal regime inference model was subsequently developed, which, when applied to the lake sediment core stratigraphy, indicated that the shallow Z_max inference may have been the result of a period of increased mixing or polymixis in this stratified lake. Water column mixing may increase due to hypolimnetic warming and increased water clarity resulting from declines in dissolved organic carbon. In a training set where there are strong correlations between lake depth and assemblage composition, this relationship is not necessarily a strict function of lake depth, but of some other highly correlated variable, likely thermal conditions.     

Paleoecological investigation of recent lake acidification in the Adirondack Mountains,N.Y.

Paleoecological analysis of the sediment record of 12 Adirondack lakes reveals that the 8 clearwater lakes with current pH < 5.5 and alkalinity < 10 eq l^-1 have acidified recently. The onset of this acidification occurred between 1920 and 1970. Loss of alkalinity, based on quanitative analysis of diatom assemblages, ranged from 2 to 35 eq l^-1. The acidification trends are substantiated by several lines of evidence including stratigraphies of diatom, chrysophyte, chironomid, and cladoceran remains, Ca:Ti and Mn:Ti ratios, sequentially extracted forms of Al, and historical fish data. Acidification trends appear to be continuing in some lakes, despite reductions in atmospheric sulfur loading that began in the early 1970s. The primary cause of the acidification trend is clearly increased atmospheric deposition of strong acids derived from the combustion of fossil fuels. Natural processes and watershed disturbances cannot account for the changes in water chemistry that have occurred, but they may play a role. Sediment core profiles of Pb, Cu, V, Zn, S, polycyclic aromatic hydrocarbons, magnetic particles, and coal and oil soot provide a clear record of increased atmospheric input of materials associated with the combustion of fossil fuels beginning in the late 1800s and early 1900s. The primary evidence for acidification occurs after that period, and the pattern of water chemistry response to increased acid inputs is consistent with current understanding of lake-watershed acidification processes.This is the second of a series of papers to be published by this journal which is a contribution of the Paleoecological Investigation of Recent Lake Acidification (PIRLA) project. Drs. D.F. Charles and D.R. Whitehead are guest editors for this series.     

Littoral cladoceran community reassembly following the cessation of disturbance

The process of recovery from environmental impact provides insights into factors that drive community assembly. This study investigated community reassembly following perturbation, using littoral cladoceran microfossils. Whole-lake experimental manipulations of acids and nutrients in Lake 302N (L302N; 1972–1997) and acids in Lake 223 (L223; 1976–1994) at the Experimental Lakes Area (ELA; Ontario, Canada), provided an opportunity to use the sediment record to reconstruct the trajectory of recovery. Comparison was made to unmanipulated Lake 377 (L377) to estimate changes in regional baseline conditions. Recovery was evaluated at both the species and community level, using univariate and multivariate metrics. Up to 14 years after cessation of acid and nutrient additions, and following chemical recovery to pH > 6.5, both L302N and L223 littoral cladoceran communities failed to recover to their pre-manipulation states. Multivariate metrics demonstrated hysteresis along the recovery trajectory, and the movement to alternative states in littoral cladoceran communities in both lakes. The most recent littoral cladoceran community structure, reconstructed from the paleorecord, is driven by the persistence of species from the acidification period, suggesting biological resistance. Recovery states differed between L302N and L223, although their initial, pre-manipulation littoral cladoceran community structures were similar, reflecting multiple, independent recovery trajectories. Independent recovery trajectories are at odds with previous findings, which suggest that dispersal from an egg bank should support deterministic recovery trajectories. We conclude that littoral cladoceran microfossils can be used effectively to determine the community state with recovery and compare it to the pre-manipulation condition, particularly with the use of multivariate metrics.     

The importance of model choice on pH inferences from scaled chrysophyte assemblages in North America

The selection of a reliable inference model is a crucial step in developing ecologically sound reconstructions of environmental variables in the past. We compared intra- and inter-regional regression-based models, and an inter-regional Modern Analogue Technique (MAT) model in their ability to infer lakewater pH from scaled chrysophyte assemblages. The performance of each model was assessed by examining cross-validated coefficients of determination and prediction errors, and through reconstructing the pH of 50 modern and fossil samples in south-central Ontario, Canada. Using the intra- and inter-regional data sets, we found little difference in the ability of the regression-based models to infer present-day pH. Partial Least Squares (PLS) regression, Weighted Averaging (WA), and Weighted Averaging Partial Least Squares (WA-PLS) inference models showed similar values for jack-knifed coefficients of determination (r² _jack), root mean squared errors of prediction (RMSEP_jack), and mean and maximum biases. Based on an analogue matching approach, the inferred values from 48 fossil sediment samples suggested that the intra-regional model did not provide reliable reconstructions for approximately half of the fossil samples. However, inferences from the inter-regional MAT and regression-based models were found to have appropriate analogues and thus considered to be more reliable.     

The simulation of fabric development during plastic deformation and its application to quartzite: fabric transitions

Fabric transitions can arise in materials such as quartz in which more than one set of symmetrically equivalent glide systems must be considered. The external conditions, such as temperature and stress, affect the relative ability of different mechanisms to operate. Adopting the Taylor-Bishop-Hill analysis allows an approximation to the resulting effects in the choice of critical resolved shear stress (CRSS) values for glide on the different dislocation systems. Different CRSS values may be appropriate to simulating fabric development in different deformational environments.For any specific set of CRSS values, for a particular deformation, a set of reorientation trajectories can be defined for differently oriented crystals with respect to the instantaneous stretching axes. There is a basic number of pattern types, and deformation leads to c-axes populating specific end-orientations.The CRSS values on different glide systems can vary smoothly relative to one another, but abrupt changes result in the deformation fabrics at critical CRSS ratios. Quartz fabrics may thus be used to delineate regions subjected to particular conditions of temperature and strain-rate in deformed metamorphic terrains, provided that allowance can be made for other factors such as trace impurity content of quartz.     

Academic Performance Indicators for Departments of Geography in the United States and Canada

A common problem faced by geography departments, particularly during times of fiscal compression and mounting pressure for accountability, is how to compare themselves and their faculty with others. The recent revolution in bibliometrics provides a growing volume of data that can be used in benchmarking exercises. In this article, we assess the production and citation of journal articles and books by tenure-track and tenured faculty in selected U.S. and Canadian geography departments (n = 17) according to a set of readily derived and transparent performance indicators derived from publicly available data. Scopus was used to assess article production and citation; Google Scholar was used for book citation. Results point to significant heterogeneity in department characteristics, productivity, and citation of published work. The number of publications, citations, and h-Index scores among scholars in the sample (n = 369) is related strongly to academic age and subfield of enquiry (i.e., physical or human geography) but not—despite apparently marked differences in output and citations—to gender.     

Characterization of minerals and organic phosphorus species in marine sediments using soft X-ray fluorescence spectromicroscopy

Phosphorus Near Edge X-ray Fluorescence Spectroscopy (P-NEXFS) data were collected on phosphorus containing phases including organic and inorganic compounds and minerals. Although phases containing P in the plus five oxidation state P(V) in a tetrahedral PO₄ structure have similar primary fluorescence peak positions, the size, shape, and positions of secondary spectral features are diagnostic for different compounds and minerals. In particular, calcium phosphates exhibited a notable post-peak shoulder at 2154.5 eV, while oxidized iron phosphates had a distinctive pre-peak feature at 2148 eV. Polyphosphates have a broad secondary peak located approximately 2 eV higher in energy than a similar feature in phosphate esters and diesters. Compounds containing P(V) in structures other than PO₄ tetrahedra such as phosphonates have a primary peak shifted about 1 eV lower than corresponding organo-phosphates. Organo-phosphates with P in the plus three oxidation state P(III) such as phosphines had primary fluorescence peaks shifted still further down in energy (2–3 eV). The substitution of aromatic carbon groups in close proximity to P structures in organic compounds generated both pre- and post-peak features as well as a number of secondary peaks. In addition, X-ray fluorescence mapping of P, Si, Al, Mg, and Na was conducted on a marine sediment sample with sub-micron spatial resolution. Phosphorus was heterogeneously distributed in the sample and not correlated on a broad scale with any other element examined. Much of the P present in the sample was located in small, 0.6–8 μm size, P-rich domains. Several P-rich regions were examined with P-NEXFS using a focused beam with 60 nm resolution and were found to consist of either calcium phosphate or polyphosphate phases. The presence of significant polyphosphate-dominated regions in a marine sediment sample supports the recent observations that such phases can play an important role in marine P cycling. The combination of fluorescence mapping and P-NEXFS data collection on fine particles provides a powerful new tool for environmental phosphorus studies.     

Microbial activity in natural and organically enriched intertidal sediments near Nelson,New Zealand

Two measures of microbial activity were used to characterise a variety of sediment habitats in three intertidal inlets in the Nelson region, South Island, New Zealand. Rates of microbial mineralisation potential and epibenthic microalgal production were compared with sediment textures, concentrations of photosynthetic pigments, ATP concentrations, and organic and inorganic nutrients. Baseline ranges for these parameters were established for relatively undisturbed estuarine sites for assessing future environmental deterioration and for comparison with sites affected by organic enrichment. Sediment mineralisation rates were increased more than 1000‐fold by enrichment from a fruit processing plant and microalgal production was enhanced by more than 50‐fold at a site exposed to slaughterhouse wastes. The remaining variables, although often strongly correlated with activity measurements, were not as sensitive as measures of enrichment. Sediment microbial activity measurements are proposed as a means of detecting changes in nutrient status of estuarine environments.