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Lamborg C.H.,Woods Hole Oceanographic Institution | Engstrom D.R.,St. Croix Research | Fitzgerald W.F.,University of Connecticut | Balcom P.H.,University of Connecticut
Science of the Total Environment | Year: 2013

Our previous work has documented a correlation between Hg concentrations and 210Pb activity measured in wet deposition that might be used to help apportion sources of Hg in precipitation. Here we present the results of a 27-month precipitation collection effort using co-located samplers for Hg and 210Pb designed to assess this hypothesis. Study sites were located on the east and west coasts of North America, in the continental interior, and on the Florida Peninsula. Relatively high variability in Hg/210Pb ratios was found at all sites regionally and seasonally (e.g., overall: 0.99-9.13ngdpm-1). The ratio of average volume-weighted Hg concentrations and 210Pb activities showed consistent trends (higher in impacted area), with Glacier Bay in southeast Alaska, exhibiting the lowest value. Assuming that Glacier Bay represents a benchmark for a site with no regional contribution, we estimate less than 50% of the Hg input was "global" at the Seattle and Florida sites. Differences in Hg/210Pb in wet deposition could be due to either a regional/local source contribution of Hg, or a regional/local enhancement in the removal of Hg from the atmosphere (i.e., oxidants), however, this approach is not capable of discerning between these two possibilities. Thus, this method of source apportionment represents an estimate of the maximal amount of Hg contributed by regional sources and may be limited in regions of deep convective mixing. © 2012 Elsevier B.V.

Almendinger J.E.,St. Croix Research | Murphy M.S.,Three Rivers Park District | Ulrich J.S.,University of Minnesota
Journal of Environmental Quality | Year: 2014

For two watersheds in the northern Midwest United States, we show that landscape depressions have a significant impact on watershed hydrology and sediment yields and that the Soil and Water Assessment Tool (SWAT) has appropriate features to simulate these depressions. In our SWAT models of the Willow River in Wisconsin and the Sunrise River in Minnesota, we used Pond and Wetland features to capture runoff from about 40% of the area in each watershed. These depressions trapped considerable sediment, yet further reductions in sediment yield were required for calibration and achieved by reducing the Universal Soil Loss Equation (USLE) cropping-practice (P) factor to 0.40 to 0.45. We suggest terminology to describe annual sediment yields at different conceptual spatial scales and show how SWAT output can be partitioned to extract data at each of these scales. These scales range from plot-scale yields calculated with the USLE to watershed-scale yields measured at the outlet. Intermediate scales include field, upland, pre-riverine, and riverine scales, in descending order along the conceptual flow path from plot to outlet. Sediment delivery ratios, when defined as watershed-scale yields as a percentage of plot-scale yields, ranged from 1% for the Willow watershed (717 km2) to 7% for the Sunrise watershed (991 km2). Sediment delivery ratios calculated from published relations based on watershed area alone were about 5 to 6%, closer to pre-riverine-scale yields in our watersheds. © American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.

Engstrom D.R.,St. Croix Research | Rose N.L.,University College London
Journal of Paleolimnology | Year: 2013

Lake sediments record the flux of materials (nutrients, pollutants, particulates) through a lake system both qualitatively, as changes in the composition of geochemical and biological tracers, as well as quantitatively, through changes in their rate of burial. Burial rates provide a direct link to contemporary (neo-) limnological studies as well as management efforts aimed at load reductions, but are difficult to reconstruct accurately from single cores owing to the spatial and temporal variability of sediment deposition in most lakes. The accurate determination of whole-lake burial rates from analysis of multiple cores, though requiring more effort per lake, can help resolve such problems and improve our understanding of sediment heterogeneity at multiple scales. Partial solutions to these problems also include focusing corrections based on 210Pb flux, co-evaluation of concentration profiles, trend analysis using multiple lakes, and trend replication based on a small number of cores from the same lake. Recent multi-core studies demonstrate that no single core site faithfully records the whole-lake time-resolved input of materials, but that as few as five well-placed cores can provide a reliable record of whole-lake sediment flux for morphometrically simple basins. Lake-wide sediment fluxes can be coupled with reconstructed outflow losses to calculate historical changes in watershed and atmospheric loading of nutrients, metals, and other constituents. The ability of paleolimnology to accurately assess the sedimentary flux and extend the period of reference into the distant past represents an important contribution to the understanding of biogeochemical processes and their response to human and natural disturbance. © 2013 Springer Science+Business Media Dordrecht.

Reavie E.D.,University of Minnesota | Edlund M.B.,St. Croix Research
Journal of Paleolimnology | Year: 2013

Paleolimnological information is often extracted from diatom records using weighted averaging calibration and regression techniques. Larger calibration sample sets yield better inferences because they better characterize the environmental characteristics and species assemblages of the sample region. To optimize inferred information from fossil assemblages, however, it is worth knowing if fewer calibration samples can be used. Furthermore, confidence in environmental reconstructions is greater if we consider the relative importance of (A) similarity between fossil and calibration assemblages and (B) how well fossil taxa respond to the environmental variable of interest. We examine these issues using ~200-year sediment profiles from four Minnesota lakes and a 145-lake surface sediment training set calibrated for total phosphorus (TP). Training set sample sizes ranging from 10 to 145 were created through random sample selection, and models based on these training sets were used to calculate diatom-inferred (DI) TP data from fossil samples. Relationships between DI-TP variability and sample size were used to determine the minimum sample size needed to optimize the model for paleo-reconstruction. Similarly, similarities between fossil and modern assemblages were calculated for each size training set. Finally, fossil and modern assemblages were compared to determine whether older fossil samples had poorer similarity with modern analogs. More than 50-80 samples, depending on lake, were needed to stabilize variability in DI-TP results, and >110 training set samples were needed to minimize modern-fossil assemblage dissimilarities. Dissimilarities appeared to increase with sample age, but only one of the four studied cores displayed a significant trend. We have two recommendations for future studies: (1) be cautious when dealing with smaller training sets, especially if they are used to interpret older fossil assemblages and (2) understand how well fossil taxa are attuned to the variable of interest, as it is critical to evaluating the quality of the diatom-inferred data. © 2013 Springer Science+Business Media Dordrecht.

Siver P.A.,Connecticut College | Wolfe A.P.,University of Alberta | Edlund M.B.,St. Croix Research
Plant Ecology and Evolution | Year: 2010

Background and aims - The majority of freshwater diatom lineages appear to have evolved before or during the Eocene, with rapid radiation occurring in the Miocene. However, only a few accurately dated fossil localities are available to test this assertion, resulting in an overall poor understanding of the biostratigraphy and early evolution of these organisms. Exquisitely preserved diatoms have been uncovered from Eocene lake sediments deposited post-eruptively in the Giraffe kimberlite pipe (Northwest Territories, Canada). We describe three new pennate diatoms from this deposit and discuss their evolutionary implications. Methods - Both oxidized preparations and whole-rock mudstone samples from the Giraffe Pipe core were examined with light and scanning electron microscopy for diatom remains. Key results - Species belonging to the genera Actinella F.W.Lewis, Oxyneis Round and Nupela Vyverman & Compere are described from the Giraffe Pipe sediments. Oxyneis apporrecta is a unique species with valves that are centrally constricted, infations midway between the center and apices, and protracted and rostrate apices. Actinella giraffensis is a small taxon that lacks a highly complex head pole, and is most closely related to a species known today only from the highlands of New Zealand. Nupela mutabilis has variably shaped valves possessing a raphe and unique areolae characteristic of this genus. These fndings represent the first known fossils for both Oxyneis and Nupela, and the oldest record for Actinella. Conclusions - None of the species are known from extant foras and are therefore considered extinct. However, the morphological features they present, including the structure of the areolae, raphe, rimoportula and girdle bands, share pronounced affinities with modern congeneric taxa. The well-developed raphe found on Nupela mutabilis confrms that the evolution of this structure dates to at least the Middle Eocene, and supports the hypothesis that it may be older than the Paleocene. © 2010 National Botanic Garden of Belgium and Royal Botanical Society of Belgium.

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