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Diatoms have been successfully used as indicators of past and present environmental conditions in freshwater and marine habitats, but their diversity and indicative properties in microtidal coastal wetlands of temperate zone are relatively poorly studied. The goal of this study was to determine whether diatoms can be used as indicators of sea level and nutrient content in coastal wetlands of the mid-Atlantic region of the USA. Our exploratory analyses indicated that benthic and marsh soil diatom assemblages were jointly controlled by tidal elevation, salinity, sediment texture, and nutrient content, while the relative importance of these factors varied among datasets targeting different environmental gradients. The habitat type could be correctly inferred from diatom assemblage composition in 79% of samples. A diatom-based model developed for inferring sediment nitrogen content had an average accuracy of prediction of 12.5% of observed nitrogen range. Models for inferring tidal exposure from diatom assemblage data provided assessment with 11–16% accuracy as estimated by the bootstrapped root mean square errors of prediction, which is similar to the accuracy of models based on other microfossils. These results show that despite inherent challenges, such as tidal redistribution of diatom frustules across the intertidal zone, diatoms can be successfully used as an independent source of evidence in paleoreconstructions of sea-level change and nitrogen enrichment, as well as for monitoring current nutrient pollution in mid-Atlantic wetlands. [ABSTRACT FROM AUTHOR]

Diatom assemblages in recent versus pre-industrial sediments were examined in 40 relatively undisturbed lakes from the Experimental Lakes Area (ELA). The ELA region of northwestern Ontario receives low amounts of acidic deposition and the lakes have been minimally disturbed by watershed development or other human activities. Consequently, this region represents an important location to detect possible changes in lakes due to climate change. In over half of the lakes, planktonic taxa (especially Discostella stelligera) increased between 10 and 40% since pre-industrial times. Changes in diatom assemblages are consistent with taxa that would benefit from enhanced stratification and a longer ice-free season. We hypothesized that there should be a relationship between stratification and measured chemical and physical characteristics of the study lakes. Multiple correlation analysis was undertaken to see the relationship between planktonic taxa and D. stelligera since pre-industrial times and the physical and chemical characteristics of the study lakes. Lake depth was consistently identified as an important variable. The timing of the increase in planktonic taxa within cores from these lakes will be needed to rule out other possible regional changes that may also be occurring in the ELA region. [ABSTRACT FROM AUTHOR]

Volcanic ash layers in sediment cores are valuable geochronological markers in paleolimnological research. The composition of volcanic glass is related to identifiable, chronologically distinctive volcanic eruptions. Consequently, tephra layers provide time horizons allowing regional-scale correlations for lake sediments. Volcanic glass is often present in samples routinely prepared by paleolimnologists such as diatom slides and thin sections. Knowledge of the morphological and optical properties of volcanic glass allows for its identification. This is essential for the identification of ash layers that are not macroscopically visible or to track their vertical migration in soft organic sediments. The purposes of this note are to: (1) describe how the morphological (i.e., shape, vesicularity) and optical (i.e., refractive index and birefringence) properties can be used to identify volcanic glass in preparations from lake sediments; and (2) show how the quantification of volcanic glass from diatom slides is used to quantify the density-induced displacement of a 4.5 cm-thick Mazama ash-layer through organic sediments and to approximate its timing of initial deposition. [ABSTRACT FROM AUTHOR]