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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]

We analyzed nutrient data from the National Lakes Assessment (NLA), a probability survey of 1028 lakes .4 ha in lake area across the conterminous USA to quantify and contrast different methods of setting nutrient criteria. We calculated potential nutrient criteria for total P (TP), total N (TN), and chlorophyll a (Chl a) by 4 methods (25th percentile of population, 75th percentile of least-disturbed reference sites, diatom-based paleolimnological reconstruction, and stressor modeling) and compared them to existing draft US Environmental Protection Agency (EPA) criteria within national nutrient ecoregions. At the national scale, the ecoregional criteria derived from the different approaches were highly correlated. However, absolute values differed widely among approaches within ecoregions. Population 25th percentiles were lower (often by a factor of 2-6) than values obtained with other approaches in almost all ecoregions, results indicating that population 25th percentiles cannot be used as a surrogate for reference-site or paleolimnological approaches. Stressor regression models often did not explain much of the variance in nutrient concentration, especially in the less-disturbed ecoregions. For TP, diatom-inferred paleolimnological criteria were higher than reference-site-based criteria, which were higher than stressor-model criteria. For TN, these 3 approaches were very comparable. For Chl a, the reference-site and stressor-model approaches gave similar criterion values in low-nutrient ecoregions, but the reference-site 75th-percentile approach had much higher criterion values than the stressor-modeling approach in the high-nutrient ecoregions. Use of NLA reference-site 75th percentiles as nutrient criteria showed that 42% of the assessed lakes exceeded TP criteria, 47% exceeded TN criteria, and 32% exceeded Chl a criteria. Survey results also suggest that most lakes are P limited. Ninety-three percent of the lakes in the population had molar TN:TP ratios > 16, and 52% had TN:TP > 50. [ABSTRACT FROM AUTHOR]