Noel Urban - Profile on Academia.edu (original) (raw)

Papers by Noel Urban

Does Terrestrial Carbon Explain Lake Superior Model-Data pCO2 Discrepancy?

AGU Fall Meeting Abstracts, Dec 1, 2008

ABSTRACT As part of the CyCLeS project, a three-dimensional hydrodynamic model (MITgcm) was coupl... more ABSTRACT As part of the CyCLeS project, a three-dimensional hydrodynamic model (MITgcm) was coupled to a medium- complexity ecosystem model and applied to Lake Superior in order to constrain the seasonal cycle of lake pCO2 and air-lake fluxes of CO2. Previous estimates of CO2 emissions from the lake, while very large, were based on field measurements of very limited spatial and temporal extent. The model allows a more realistic extrapolation from the limited data by incorporation of lake-wide circulation and food web dynamics. A large discrepancy (200 uatm) between observations and model-predicted pCO2 during spring suggests a significant input of terrestrial carbon into the lake. The physical model has 10-km horizontal resolution with 29 vertical layers, ten of which are in the top 50 m of the water column. The model is forced by interpolated meteorological data obtained from land-based weather stations, buoys, and other measurements. Modeled surface temperatures compare well to satellite- based surface water temperature images derived from NOAA AVHRR (Advanced Very High Resolution Radiometer), though there are regional patterns of bias that suggest errors in the heat flux forcing. Growth of two classes of phytoplankton is modeled as a function of temperature, light, and nutrients. One grazer preys upon all phytoplankton. The cycles of carbon and phosphorous are explicitly modeled throughout the water column. The model is able to replicate the observed seasonal cycle of lake chlorophyll and the deep chlorophyll maximum. The model is unable to capture the magnitude of observed CO2 super-saturation during spring without considering external carbon inputs to the lake. Simple box model results suggest that the estimated pool of terrestrial carbon in the lake (17 TgC) must remineralize with a timescale of months during spring in order to account for the model/data pCO2 difference. River inputs and enhanced remineralization in spring due to photo-oxidation are other mechanisms considered to explain the discrepancy between model predictions and observations of pCO2. Model results suggest that year-round and lake-wide direct measurements of pCO2 would help to better constrain the lake carbon cycle.

Environmental Science & Technology, Oct 30, 2018

Mercury (Hg) is emitted to air by natural and anthropogenic sources, transports and deposits glob... more Mercury (Hg) is emitted to air by natural and anthropogenic sources, transports and deposits globally, and bioaccumulates to toxic levels in food webs. It is addressed under the global 2017 Minamata Convention, for which periodic effectiveness evaluation is required. Previous analyses have estimated the impact of different regulatory strategies for future mercury deposition. However, analyses using atmospheric models traditionally hold legacy emissions (recycling of previously deposited Hg) constant, and do not account for their possible future growth. Here, using an integrated modeling approach, we investigate how delays in implementing emissions reductions and the associated growing legacy reservoir affect deposition fluxes to ecosystems in different global regions. Assuming nearly constant yearly emissions relative to 2010, each 5-year delay in peak emissions defers by additional extra ca. 4 years the return to year 2010 global deposition. On a global average, each 5-year delay leads to a 14% decrease in policy impacts on local-scale Hg deposition. We also investigate the response of fish contamination in remote lakes to delayed action. We quantify the consequences of delay for limiting the Hg burden of future generations and show that traditional analyses of policy impacts provide best-case estimates.

What can we learn from 28 years of monitoring of fish tissue polychlorinated biphenyls in Michigan's rivers?

Integrated Environmental Assessment and Management, May 17, 2022

Polychlorinated biphenyls (PCBs) are an important part of chemical legacies in the Laurentian Gre... more Polychlorinated biphenyls (PCBs) are an important part of chemical legacies in the Laurentian Great Lakes basin. Used in industrial products worldwide, PCBs are now extensively monitored because of their potential toxicity to humans. Fish consumption is a major pathway for exposure. Edible portion (i.e., fish fillet) data from Michigan's fish tissue PCB monitoring program were evaluated using regression statistics, principal component analysis, and t‐tests to answer three questions: (1) How do fish tissue total PCB concentrations vary across Michigan's rivers? (2) Are the PCB congener patterns uniformly distributed among tested sites and species? (3) Do monitoring methods limit our ability to discern trends in fish tissue PCB concentrations? Our results indicate that although contaminated sites have been successfully identified, based on higher PCB concentrations in samples from Areas of Concern (AOCs) compared to non‐AOC sites, 77% of fish samples from 2010 to 2015 exceeded the safe fish tissue PCB concentration for unrestricted consumption (97 g/day) by sensitive populations. The PCB congener profiles vary among species and locations. Results demonstrate that these data are not useful for supplementing ongoing spatial and temporal trend analysis. Only 15 of the 83 species + waterbody pairs had adequate data for evaluating temporal trends with more than three data points. In general, the trends at each location varied based on the analytical method. Conclusions from this work can inform revisions to existing monitoring programs and improve our ability to protect human health. Integr Environ Assess Manag 2023;19:152–162. © 2022 SETAC

Temporal and spatial variability of PCB concentrations in lake trout (Salvelinus namaycush) in Lake Superior from 1995 to 2016

Journal of Great Lakes Research, Apr 1, 2020

Abstract Production of polychlorinated biphenyl compounds (PCBs) was banned over 40 years ago in ... more Abstract Production of polychlorinated biphenyl compounds (PCBs) was banned over 40 years ago in the United States and Canada, but they remain a dominant contaminant in Great Lakes fish. Although PCB concentrations in Great Lakes fish have declined since the 1980s, there is concern that the rate of decline has slowed in recent years. Canadian and U.S. national agencies monitor trends in fish contaminants at only a few sites in each lake, while states and provinces monitor fish at more locations. In this study, we compare fish PCB measurements made by five agencies at multiple locations in order to evaluate spatial as well as temporal patterns in Lake Superior. For several monitoring locations, rapid increases and decreases in concentrations were observed. The wide range of concentrations (up to 1000 ng/g) reported among all stations in any single year is unlikely to result solely from differences in fish preparation or analytical techniques. Recent measurements indicate that spatial variation in fish PCB concentrations exists with peak concentrations 30-fold higher than lowest concentrations. After 1995, statistically significant changes, all declines, in PCB concentrations were observed at only three of seven locations; half-lives in these locations ranged from 9 to 14 years. Differences in diet and food web structure likely contribute both to the spatial variability of concentrations within the lake as well as to the rapid, short-term changes in concentrations at single sites that make long-term trends difficult to discern and to predict.

Journal of Great Lakes Research, 2004

As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, measu... more As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, measurements were made of the extent and the composition of the BNL in Lake Superior off the northwest coast of the Keweenaw Peninsula. Between 1998 and 2000, transmissometer profiles were obtained frequently along three transects oriented perpendicularly to the coast. Large-volume water samples were filtered, and the BNL particles were analyzed for major elements (C, N, P) and 210 Pb. In 1998 and 1999, an ROV was deployed to facilitate sampling of this layer. These measurements together with ancillary data from the KITES project show that the BNL develops each summer when the lake is stably stratified and disappears in early fall as the thermocline erodes. Particles within the BNL are enriched in biogenic elements and 210 Pb relative to material in surface sediments and relative to material collected in sediment traps, but are enriched in phosphorus relative to epilimnetic seston. There is a large inventory of solids suspended within the BNL (approximately 100,000 metric tons in a 20-km band along the Keweenaw Peninsula); this inventory appears quickly (1-2 weeks) in summer and disappears equally quickly via settling in fall. The large inventory of organic carbon indicates that particles are concentrated from a large area of surface waters into a considerably smaller region where the BNL exists.

Sed2K: Modeling Lake Sediment Diagenesis in a Management Context

Journal of Environmental Engineering, Mar 1, 2015

ABSTRACT The development of a vertically-segmented, mechanistic mass balance model (Sed2K) for pa... more ABSTRACT The development of a vertically-segmented, mechanistic mass balance model (Sed2K) for particulate organic matter (POM) diagenesis in lake sediments is described. The model is parsimonious in its requirements for input data and versatile in its accommodation of kinetic formulations. An application is provided for hypereutrophic Lake Alice, Minnesota, which includes a well-constrained calibration to downcore POM constituents (carbon, nitrogen, and phosphorus) and their efflux at the sediment-water interface. The application then considers the system response to a reduction in POM deposition to the sediments (i.e.,addressing the critical when and to what extent question fundamental to lake restoration programs).

Journal of Great Lakes Research, 1999

Over 500 million tons of copper-rich mine tailings were dumped into lakes, rivers, wetlands, and ... more Over 500 million tons of copper-rich mine tailings were dumped into lakes, rivers, wetlands, and along the shore of Lake Superior between 1850 and 1968. Metals leaching from mine residues have impacted ecosystems throughout the Keweenaw Peninsula as well as Lake Superior. The objective of this study was to elucidate the chemical processes that release Cu from mine tailings into the water. Copper in mining residues from three contrasting environments (lake sediments, wetland stamp stands, and exposed lakeshore tailings piles) was fractionated with a sequential extraction technique (SET) to identify and quantify the labile pools of copper. The SET revealed that the carbonate and oxide fractions were the largest pools of Cu (ca. 50 ~ 80%) in lakeshore and wetland stamp sands whereas the organic matter fraction was the largest reservoir (ca. 32%) in the lake sediments. X-ray diffraction and SEM confirmed the presence of the copper-bearing minerals cuprite, tenorite, malachite, and elemental Cu. Size fractionation studies suggested that weathering of native (elemental) Cu results in enrichment of particle surfaces with Cu oxides and carbonates; fine particles also are enriched in these phases. Both laboratory titrations and computer modeling suggested that aqueous Cu concentrations are limited by mineral (malachite and copper(II) oxides) dissolution and precipitation reactions. Concentrations of DOC and pH depressions caused by microbial activity strongly affect the dissolved Cu concentrations. At some sites, aqueous concentrations of copper approach equilibrium with a Cu oxyhydroxide that has a solubility intermediate between that of cupric hydroxide (Cu(OH) 2 ) and tenorite (CuO).

Nutrient Cycling in Lake Superior

Michigan State University Press eBooks, Jul 1, 2009

Canadian Journal of Fisheries and Aquatic Sciences, Jul 1, 2001

Lake sediments often are regarded as accurate records of changes in climatic conditions, rates of... more Lake sediments often are regarded as accurate records of changes in climatic conditions, rates of atmospheric deposition of substances to the lake, or other processes occurring within lakes. In this study, the sedimentary record of sulfur was examined in Little Rock Lake, Wisconsin. This lake received experimental sulfate additions from 1985 through 1990, and a mass balance showed that 35% (425 kg) of experimental sulfur additions were buried in the sediments. Sediment cores collected before and after sulfur additions confirmed that burial of 180-360 kg of sulfur occurred during the 6 years of experimental sulfur additions. However, cores collected in 1992 and 1996 contained less sulfur than cores taken in 1990 and suggested that sulfur incorporation in sediments is not permanent. To examine seasonal sulfur loss from sediments, sulfur inventories in the top 4 cm of sediments were measured in multiple cores before and after fall overturn. At water depths greater than 5 m, surface sediments retrieved after fall overturn had significantly less sulfur than did those collected before overturn. The small seasonal release from the sediments (25 kg S in 1996) might be caused by organic matter decomposition or sulfide oxidation. Résumé : Les sédiments lacustres sont souvent considérés comme des témoins fidèles des changements dans les conditions climatiques, dans les taux de précipitation atmosphérique de substances et dans plusieurs autres processus en opération dans les lacs. Notre étude examine la présence du soufre dans les sédiments du lac Little Rock, au Wisconsin. Ce lac a reçu des apports expérimentaux de sulfates de 1985 à 1990 et un bilan massique montre que 35% (425 kg) du soufre ajouté est enfoui dans les sédiments. Une comparaison de carottes de sédiments récoltées avant et après l'ajout de soufre confirme l'enfouissement de 180-360 kg de soufre durant les 6 ans de l'expérience. Cependant, des carottes récoltées en 1992 et en 1996 contiennent moins de soufre que celles prises en 1990, ce qui laisse croire que l'incorporation du soufre dans les sédiments n'est pas permanente. Des mesures du soufre dans les 4 cm supérieurs des sédiments dans un grand nombre de carottes avant et après le brassage d'automne ont permis d'évaluer la perte saisonnière de soufre des sédiments. Aux profondeurs supérieures à 5 m, les sédiments superficiels contiennent significativement moins de soufre après le brassage d'automne qu'avant. La faible libération saisonnière de soufre par les sédiments (25 kg S en 1966) est peut-être le résultat de la décomposition de la matière organique ou de l'oxydation des sulfures. [Traduit par la Rédaction] Urban and Monte 1355

Limnology and Oceanography, Mar 31, 2011

We present and compare direct and indirect pCO 2 observations taken in Lake Superior in the last ... more We present and compare direct and indirect pCO 2 observations taken in Lake Superior in the last decade and use them to understand temporal and spatial variability in lake carbon cycle processes. In situ observations from 2001 and biannual survey data for 1996-2006 indicate that Lake Superior was, on average, supersaturated (annual mean 5 46.7 6 17.3 Pa [461 6 171 matm]) with respect to atmospheric pCO 2 (mean 5 38.3 6 0.6 Pa) in April and close to equilibrium (mean 5 37.5 6 6.7 Pa) with respect to atmospheric pCO 2 (mean 5 36.4 6 0.7 Pa) in August. Both data sets indicate that temporal variability in surface lake pCO 2 from weekly to interannual timescales was predominantly controlled by changing dissolved inorganic carbon and associated changes in pH. An unstratified water column appears to have limited pCO 2 fluctuations in spring. Through summer and into early fall, pCO 2 variability on a daily timescale at 12 m increased with time to a maximum amplitude of 19 Pa, likely as a result of internal waves on the thermocline. Year-to-year changes in mean surface lake pCO 2 and temperature were of the same sign and approximate magnitude at all observed points, consistent with the lake's small size relative to the synoptic-scale meteorological systems that force it. Variability in pCO 2 was not correlated with major climate indices. While these data provide a first large-scale overview of Lake Superior's pCO 2 and its temporal variability, their time-space resolution and accuracy are not sufficient to further refine previously imbalanced lake-wide carbon budgets.

Journal of Great Lakes Research, Mar 1, 2013

Diporeia, formerly the dominant benthic macroinvertebrate in the Great Lakes, remains a keystone ... more Diporeia, formerly the dominant benthic macroinvertebrate in the Great Lakes, remains a keystone species in Lake Superior. Little is known, however, about fine scale amphipod distributions, especially as influenced by the production, transport and transformation of energy resources. Here, we document the distribution and abundance of Diporeia along 19 transects around the lake's perimeter. Regions of elevated density, averaging 958 ± 408 Diporeia/m 2 (mean ± S.D.) were observed along all transects, typically within slope habitat (depth of 30-125 m). Waters shoreward (shelf habitat, b 30 m) and lakeward (profundal habitat, >125 m) of these regions supported significantly lower densities, averaging 239±178/m 2 and 106±59/m 2 , respectively. Amphipods within regions of elevated density, termed here the Ring of Fire, account for two-thirds of the lakewide population while occupying only one-quarter of the benthic habitat. The Ring of Fire, observed lakewide as a band averaging 14.2±9.4 km in width, is characterized as a region of transitional sediment deposition with gentle slope, proximate to nearshore locations of elevated primary production. Within the Ring of Fire exceptionally high densities are found in the south central region, where the Keweenaw Current and slope bathymetries serve to funnel production from adjoining regions of high production. Density measurements for the 173 stations sampled here are used to estimate lakewide Diporeia standing stock (22.5-37.7 trillion individuals, 4.4-7.4 Gg dry weight, 2.1-3.5 Gg C), individual and biomass density (274-460/m 2 , 0.05-0.09 gDW/m 2 , 0.03-0.04 gC/m 2 ) and areal (0.02-0.03 g C/m 2 /yr) and total (1.6-2.6 Gg C/yr) production.

Journal of Geophysical Research, 2004

A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on the fraction of ... more A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on the fraction of O 2 saturation and the isotopic composition of O 2 , was undertaken to evaluate spatial and temporal variations in the trophic status of a large oligotrophic freshwater environment. The lake was predominantly net heterotrophic from April to October 2000 (R:P ratios: 1.2-2.5). Uniform R:P ratios of $1.5 with depth and across the lake in April 2000 and 2001 revealed the homogeneity of the water column during spring. A brief period of net autotrophy was observed during summer thermal stratification in 2000 and 2001, and surveys showed this condition to be prevalent and lake-wide in August 2001 (R:P ratios: 0.5-0.9). Strong net autotrophy (R:P ratios: 0.6) was found near Duluth, Minnesota, and suggested the potential for the formation of mesotrophic conditions within areas of increased nutrient loadings from urbanization. Respiration and photosynthesis were shown to exert a strong control on O 2 gas exchange within Lake Superior, as evidenced by significant correlations between R:P ratios and O 2 gas exchange during periods of net heterotrophy and autotrophy. This observation was unexpected since [O 2 ] in the lake appears to be dominated by atmospheric O 2 gas exchange, given that the fraction of O 2 saturation is continuously near levels expected for equilibration with the atmosphere. Furthermore, the relationship between the biological and physical O 2 fluxes may enable the use of R:P ratios to calculate O 2 gas exchange and ultimately estimate CO 2 fluxes between lakes and the atmosphere.

Verhandlungen, Apr 1, 2010

Limnology and Oceanography, Nov 1, 1990

Concentrations of Al, Fe, Mn, major ions, and dissolved organic C (DOC) were measured in 37 lakes... more Concentrations of Al, Fe, Mn, major ions, and dissolved organic C (DOC) were measured in 37 lakes in Halifax County, Nova Scotia, to determine the factors controlling the behavior of these metals in acidic waters with relatively high concentrations of organic matter. Concentrations of trace metals ranged from < 50 to 700 pg liter-l (Al), ~20 to 960 fig liter-' (Fe), and < 10 to 240 pg liter-l (Mn). Concentrations of Fe and Al were highly correlated with H+ ion and DOC concentration, whereas Mn was correlated most strongly with H+ ion concentration and size of catchment relative to lake surface area. Computation of trace-metal speciation indicated that lakes with pH > 5 were in equilibrium with gibbsite and amorphous iron hydroxide but highly supersaturated with respect to manganite and Mn(IV) oxides. Concentrations of Mn appear to be determined by the magnitude of the source (i.e. the catchment area) relative to the size of the lake. In lakes with pH < 5, concentrations of Al and Fe seem to be regulated by interactions with DOC. Although the metals and organic C enter the lake separately, we propose that precipitation of metal humates is the major process regulating concentrations of Al and Fe and, to a lesser extent, DOC in these Apart from lowering the pH of surface waters, one of the most harmful effects of acid deposition is to mobilize trace metals in the environment. High concentrations of Al in acidic waters have been shown to be toxic to fishes (e.g. , and elevated concentrations of Mn are thought to cause deformities in fishes . Dieback of forests in Germany and the eastern U.S.

Journal of Soils and Sediments, May 22, 2018

Purpose Select South Dakota, USA water bodies, including both natural lakes and man-made impoundm... more Purpose Select South Dakota, USA water bodies, including both natural lakes and man-made impoundments, were sampled and analyzed to assess mercury (Hg) dynamics and historical patterns of total Hg deposition. Materials and methods Sediment cores were collected from seven South Dakota lakes. Mercury concentrations and flux profiles were determined using lead ( 210 Pb) dating and sedimentation rates. Results and discussion Most upper lake sediments contained variable heavy metal concentrations, but became more consistent with depth and age. Five of the seven lakes exhibited Hg accumulation fluxes that peaked between 1920 and 1960, while the remaining two lakes exhibited recent (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) Hg flux spikes. Historical sediment accumulation rates and Hg flux profiles demonstrate similar peak and stabilized values. Mercury in the sampled South Dakota lakes appears to emanate from watershed transport due to erosion from agricultural land use common to the Northern Great Plains. Conclusions For sampled South Dakota lakes, watershed inputs are more significant sources of Hg than atmospheric deposition.

Aquatic Sciences, Mar 1, 1997

Geochimica et Cosmochimica Acta, Jul 1, 1999

Enrichment of organic matter with sulfur during early diagenesis has been amply documented in mar... more Enrichment of organic matter with sulfur during early diagenesis has been amply documented in marine sediments. The importance of such reactions in lacustrine sediments is not as well appreciated. In this study the organic sulfur contents of sediments from seven lakes were compared, and the speciation of the sulfur in the humic acid fraction of the sedimentary organic matter was examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of organic matter with sulfur occurred in five of seven lakes as evidenced by C:S ratios and stable isotope ratios. The availability of sulfide, organic matter, and reactive iron were not important determinants of the extent of organic S enrichment. The only environmental parameter that appeared to be related to S enrichment was lake trophic state. Together with data from the literature, our results suggest that sediments in most eutrophic lakes are enriched in organic S, while organic matter is enriched in only few oligotrophic lakes. Organic sulfides or thiols are the dominant forms of reduced organic S in the humic acids, and sulfur enrichment occurs primarily by formation of organic sulfides or thiols. Addition of S to polyunsaturated molecules could account for 5-10% of the S enrichment. Di-and polysulfides did not comprise a significant fraction of organic S in any of the sediment humic acids. Thiophenes could be identified tentatively only in the oldest (60 yr) sediment sample analyzed. Sulfoxides were observed in several samples. Lake trophic state and exposure to oxygen appear to be major factors influencing the extent and pathways of S addition to organic matter.

Proton Cycling in Bogs: Geographic Variation in Northeastern North America

Effects of Atmospheric Pollutants on Forests, Wetlands and Agricultural Ecosystems, 1987

A detailed hydrogen ion budget has been constructed for the Marcell bog in north-central Minnesot... more A detailed hydrogen ion budget has been constructed for the Marcell bog in north-central Minnesota based on a 5-year, intensive study of element cycles. Major features of the acidity balance for this site include the following: (1) production of organic acids (263 meg.m−2.y−1) is the dominant source of acidity and serves to buffer the bog water at pH 4; (2) seguestering of elements in peat is also a significant source of acidity (42.9 meg.m−2.y−1); (3) weathering of dustfall inputs is an important source of alkalinity (<76 meg.m−2.y−1) at this site which is situated near the major agricultural area of the plains; (4) nitrate and sulphate reduction contribute little alkalinity (<39.2 meg.m−2.y−1) because inputs (NO3 and SO4) to this bog are low. Analysis of peat and surface water from bogs across northeastern North America (Manitoba to Newfoundland) reveals the following: (1) production of organic acids across this region varies between 104 and 263 meg.m−2.y−1; (2) acidity-generation associated with net biological uptake (NBU, excluding nitrogen = 20–117 meg.m−2.y−1) varies in proportion to the rate of peat accumulation; (3) NBU-acidity exhibits high values in maritime bogs and lower values in mid-continental bogs; (4) bogs have a large capacity for sulphate reduction, and sulphate reduction becomes an increasingly important source of alkalinity as rates of sulphate deposition increase. From 60 to 93% of annual sulphate loadings are retained as reduced sulphur in bogs across eastern North America.

Journal of Marine Science and Engineering

Over a century ago, copper mills on the Keweenaw Peninsula of Lake Superior sluiced 64 million me... more Over a century ago, copper mills on the Keweenaw Peninsula of Lake Superior sluiced 64 million metric tonnes (MMT) of tailings into coastal waters, creating a metal-rich “halo”. Here we show that relatively small discharges can spread widely in time and space. Mass Mill (2.9 MMT) dumping into Lake Superior also illustrates the complexity of interactions with Indigenous Peoples. A combination of aerial photos, LiDAR, and a microscope technique for distinguishing end-member particles traces the migration of tailings. The clay fraction spread rapidly across Keweenaw Bay and curled into terminal L’Anse Bay, within tribal Reservation boundaries. The coarse stamp sand fraction moved more slowly southward as a beach sand deposit onto Sand Point, a sacred burial ground. Despite the partial recovery of northern beaches and southern sediments, concerns continue about chemical contamination. Mass Mill provides an excellent example of Indigenous Peoples’ territorial and resource issues with min...

Michigan State University Press eBooks, Jul 1, 2009

Carbon (C) cycling in Lake Superior was studied within the Keweenaw Interdisciplinary Transport E... more Carbon (C) cycling in Lake Superior was studied within the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project to assess (1) whether the lake is net heterotrophic, (2) sources, sinks and residence time for dissolved organic carbon (DOC), (3) importance of terrigenous organic C subsidies, and (4) factors limiting C flow through bacteria. During 3 years of fieldwork, measurements were made of spatial and temporal distributions of C pools and rates of photosynthesis, community respiration, and bacterial production. Measurements were made of the composition of dissolved organic matter (DOM), rates of DOM photolysis, lability of DOM toward microbial consumption, and river inputs of DOM. All measurements suggest the lake is net heterotrophic. The C:N ratios of DOM suggest that it is primarily of terrigenous origin, but other characteristics (size distribution, UV absorption) point to the presence of autochthonous DOM and to alteration of terrigenous material. The lake mass balance indicates that the residence time ($8 years) of the DOC pool (17 Tg) is short relative to the hydrologic residence time (170 years). The known flux of terrigenous DOC ($1 Tg/yr) is too low to support annual bacterial carbon demand (6-38 Tg/yr), but microbial respiration is the major sink for terrigenous DOC. A rapidly cycling, autochthonous DOC pool must exist. Microbial activity was correlated with temperature, phosphorus availability, and DOC concentration but not with photosynthesis rates. Measurements of respiration ($40 Tg/yr), photosynthesis (2-7 Tg/yr), and bacterial production (0.5-2 Tg/yr) are not all mutually compatible and result in a discrepancy in the organic carbon budget.

Does Terrestrial Carbon Explain Lake Superior Model-Data pCO2 Discrepancy?

AGU Fall Meeting Abstracts, Dec 1, 2008

ABSTRACT As part of the CyCLeS project, a three-dimensional hydrodynamic model (MITgcm) was coupl... more ABSTRACT As part of the CyCLeS project, a three-dimensional hydrodynamic model (MITgcm) was coupled to a medium- complexity ecosystem model and applied to Lake Superior in order to constrain the seasonal cycle of lake pCO2 and air-lake fluxes of CO2. Previous estimates of CO2 emissions from the lake, while very large, were based on field measurements of very limited spatial and temporal extent. The model allows a more realistic extrapolation from the limited data by incorporation of lake-wide circulation and food web dynamics. A large discrepancy (200 uatm) between observations and model-predicted pCO2 during spring suggests a significant input of terrestrial carbon into the lake. The physical model has 10-km horizontal resolution with 29 vertical layers, ten of which are in the top 50 m of the water column. The model is forced by interpolated meteorological data obtained from land-based weather stations, buoys, and other measurements. Modeled surface temperatures compare well to satellite- based surface water temperature images derived from NOAA AVHRR (Advanced Very High Resolution Radiometer), though there are regional patterns of bias that suggest errors in the heat flux forcing. Growth of two classes of phytoplankton is modeled as a function of temperature, light, and nutrients. One grazer preys upon all phytoplankton. The cycles of carbon and phosphorous are explicitly modeled throughout the water column. The model is able to replicate the observed seasonal cycle of lake chlorophyll and the deep chlorophyll maximum. The model is unable to capture the magnitude of observed CO2 super-saturation during spring without considering external carbon inputs to the lake. Simple box model results suggest that the estimated pool of terrestrial carbon in the lake (17 TgC) must remineralize with a timescale of months during spring in order to account for the model/data pCO2 difference. River inputs and enhanced remineralization in spring due to photo-oxidation are other mechanisms considered to explain the discrepancy between model predictions and observations of pCO2. Model results suggest that year-round and lake-wide direct measurements of pCO2 would help to better constrain the lake carbon cycle.

Environmental Science & Technology, Oct 30, 2018

Mercury (Hg) is emitted to air by natural and anthropogenic sources, transports and deposits glob... more Mercury (Hg) is emitted to air by natural and anthropogenic sources, transports and deposits globally, and bioaccumulates to toxic levels in food webs. It is addressed under the global 2017 Minamata Convention, for which periodic effectiveness evaluation is required. Previous analyses have estimated the impact of different regulatory strategies for future mercury deposition. However, analyses using atmospheric models traditionally hold legacy emissions (recycling of previously deposited Hg) constant, and do not account for their possible future growth. Here, using an integrated modeling approach, we investigate how delays in implementing emissions reductions and the associated growing legacy reservoir affect deposition fluxes to ecosystems in different global regions. Assuming nearly constant yearly emissions relative to 2010, each 5-year delay in peak emissions defers by additional extra ca. 4 years the return to year 2010 global deposition. On a global average, each 5-year delay leads to a 14% decrease in policy impacts on local-scale Hg deposition. We also investigate the response of fish contamination in remote lakes to delayed action. We quantify the consequences of delay for limiting the Hg burden of future generations and show that traditional analyses of policy impacts provide best-case estimates.

What can we learn from 28 years of monitoring of fish tissue polychlorinated biphenyls in Michigan's rivers?

Integrated Environmental Assessment and Management, May 17, 2022

Polychlorinated biphenyls (PCBs) are an important part of chemical legacies in the Laurentian Gre... more Polychlorinated biphenyls (PCBs) are an important part of chemical legacies in the Laurentian Great Lakes basin. Used in industrial products worldwide, PCBs are now extensively monitored because of their potential toxicity to humans. Fish consumption is a major pathway for exposure. Edible portion (i.e., fish fillet) data from Michigan's fish tissue PCB monitoring program were evaluated using regression statistics, principal component analysis, and t‐tests to answer three questions: (1) How do fish tissue total PCB concentrations vary across Michigan's rivers? (2) Are the PCB congener patterns uniformly distributed among tested sites and species? (3) Do monitoring methods limit our ability to discern trends in fish tissue PCB concentrations? Our results indicate that although contaminated sites have been successfully identified, based on higher PCB concentrations in samples from Areas of Concern (AOCs) compared to non‐AOC sites, 77% of fish samples from 2010 to 2015 exceeded the safe fish tissue PCB concentration for unrestricted consumption (97 g/day) by sensitive populations. The PCB congener profiles vary among species and locations. Results demonstrate that these data are not useful for supplementing ongoing spatial and temporal trend analysis. Only 15 of the 83 species + waterbody pairs had adequate data for evaluating temporal trends with more than three data points. In general, the trends at each location varied based on the analytical method. Conclusions from this work can inform revisions to existing monitoring programs and improve our ability to protect human health. Integr Environ Assess Manag 2023;19:152–162. © 2022 SETAC

Temporal and spatial variability of PCB concentrations in lake trout (Salvelinus namaycush) in Lake Superior from 1995 to 2016

Journal of Great Lakes Research, Apr 1, 2020

Abstract Production of polychlorinated biphenyl compounds (PCBs) was banned over 40 years ago in ... more Abstract Production of polychlorinated biphenyl compounds (PCBs) was banned over 40 years ago in the United States and Canada, but they remain a dominant contaminant in Great Lakes fish. Although PCB concentrations in Great Lakes fish have declined since the 1980s, there is concern that the rate of decline has slowed in recent years. Canadian and U.S. national agencies monitor trends in fish contaminants at only a few sites in each lake, while states and provinces monitor fish at more locations. In this study, we compare fish PCB measurements made by five agencies at multiple locations in order to evaluate spatial as well as temporal patterns in Lake Superior. For several monitoring locations, rapid increases and decreases in concentrations were observed. The wide range of concentrations (up to 1000 ng/g) reported among all stations in any single year is unlikely to result solely from differences in fish preparation or analytical techniques. Recent measurements indicate that spatial variation in fish PCB concentrations exists with peak concentrations 30-fold higher than lowest concentrations. After 1995, statistically significant changes, all declines, in PCB concentrations were observed at only three of seven locations; half-lives in these locations ranged from 9 to 14 years. Differences in diet and food web structure likely contribute both to the spatial variability of concentrations within the lake as well as to the rapid, short-term changes in concentrations at single sites that make long-term trends difficult to discern and to predict.

Journal of Great Lakes Research, 2004

As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, measu... more As part of the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project, measurements were made of the extent and the composition of the BNL in Lake Superior off the northwest coast of the Keweenaw Peninsula. Between 1998 and 2000, transmissometer profiles were obtained frequently along three transects oriented perpendicularly to the coast. Large-volume water samples were filtered, and the BNL particles were analyzed for major elements (C, N, P) and 210 Pb. In 1998 and 1999, an ROV was deployed to facilitate sampling of this layer. These measurements together with ancillary data from the KITES project show that the BNL develops each summer when the lake is stably stratified and disappears in early fall as the thermocline erodes. Particles within the BNL are enriched in biogenic elements and 210 Pb relative to material in surface sediments and relative to material collected in sediment traps, but are enriched in phosphorus relative to epilimnetic seston. There is a large inventory of solids suspended within the BNL (approximately 100,000 metric tons in a 20-km band along the Keweenaw Peninsula); this inventory appears quickly (1-2 weeks) in summer and disappears equally quickly via settling in fall. The large inventory of organic carbon indicates that particles are concentrated from a large area of surface waters into a considerably smaller region where the BNL exists.

Sed2K: Modeling Lake Sediment Diagenesis in a Management Context

Journal of Environmental Engineering, Mar 1, 2015

ABSTRACT The development of a vertically-segmented, mechanistic mass balance model (Sed2K) for pa... more ABSTRACT The development of a vertically-segmented, mechanistic mass balance model (Sed2K) for particulate organic matter (POM) diagenesis in lake sediments is described. The model is parsimonious in its requirements for input data and versatile in its accommodation of kinetic formulations. An application is provided for hypereutrophic Lake Alice, Minnesota, which includes a well-constrained calibration to downcore POM constituents (carbon, nitrogen, and phosphorus) and their efflux at the sediment-water interface. The application then considers the system response to a reduction in POM deposition to the sediments (i.e.,addressing the critical when and to what extent question fundamental to lake restoration programs).

Journal of Great Lakes Research, 1999

Over 500 million tons of copper-rich mine tailings were dumped into lakes, rivers, wetlands, and ... more Over 500 million tons of copper-rich mine tailings were dumped into lakes, rivers, wetlands, and along the shore of Lake Superior between 1850 and 1968. Metals leaching from mine residues have impacted ecosystems throughout the Keweenaw Peninsula as well as Lake Superior. The objective of this study was to elucidate the chemical processes that release Cu from mine tailings into the water. Copper in mining residues from three contrasting environments (lake sediments, wetland stamp stands, and exposed lakeshore tailings piles) was fractionated with a sequential extraction technique (SET) to identify and quantify the labile pools of copper. The SET revealed that the carbonate and oxide fractions were the largest pools of Cu (ca. 50 ~ 80%) in lakeshore and wetland stamp sands whereas the organic matter fraction was the largest reservoir (ca. 32%) in the lake sediments. X-ray diffraction and SEM confirmed the presence of the copper-bearing minerals cuprite, tenorite, malachite, and elemental Cu. Size fractionation studies suggested that weathering of native (elemental) Cu results in enrichment of particle surfaces with Cu oxides and carbonates; fine particles also are enriched in these phases. Both laboratory titrations and computer modeling suggested that aqueous Cu concentrations are limited by mineral (malachite and copper(II) oxides) dissolution and precipitation reactions. Concentrations of DOC and pH depressions caused by microbial activity strongly affect the dissolved Cu concentrations. At some sites, aqueous concentrations of copper approach equilibrium with a Cu oxyhydroxide that has a solubility intermediate between that of cupric hydroxide (Cu(OH) 2 ) and tenorite (CuO).

Nutrient Cycling in Lake Superior

Michigan State University Press eBooks, Jul 1, 2009

Canadian Journal of Fisheries and Aquatic Sciences, Jul 1, 2001

Lake sediments often are regarded as accurate records of changes in climatic conditions, rates of... more Lake sediments often are regarded as accurate records of changes in climatic conditions, rates of atmospheric deposition of substances to the lake, or other processes occurring within lakes. In this study, the sedimentary record of sulfur was examined in Little Rock Lake, Wisconsin. This lake received experimental sulfate additions from 1985 through 1990, and a mass balance showed that 35% (425 kg) of experimental sulfur additions were buried in the sediments. Sediment cores collected before and after sulfur additions confirmed that burial of 180-360 kg of sulfur occurred during the 6 years of experimental sulfur additions. However, cores collected in 1992 and 1996 contained less sulfur than cores taken in 1990 and suggested that sulfur incorporation in sediments is not permanent. To examine seasonal sulfur loss from sediments, sulfur inventories in the top 4 cm of sediments were measured in multiple cores before and after fall overturn. At water depths greater than 5 m, surface sediments retrieved after fall overturn had significantly less sulfur than did those collected before overturn. The small seasonal release from the sediments (25 kg S in 1996) might be caused by organic matter decomposition or sulfide oxidation. Résumé : Les sédiments lacustres sont souvent considérés comme des témoins fidèles des changements dans les conditions climatiques, dans les taux de précipitation atmosphérique de substances et dans plusieurs autres processus en opération dans les lacs. Notre étude examine la présence du soufre dans les sédiments du lac Little Rock, au Wisconsin. Ce lac a reçu des apports expérimentaux de sulfates de 1985 à 1990 et un bilan massique montre que 35% (425 kg) du soufre ajouté est enfoui dans les sédiments. Une comparaison de carottes de sédiments récoltées avant et après l'ajout de soufre confirme l'enfouissement de 180-360 kg de soufre durant les 6 ans de l'expérience. Cependant, des carottes récoltées en 1992 et en 1996 contiennent moins de soufre que celles prises en 1990, ce qui laisse croire que l'incorporation du soufre dans les sédiments n'est pas permanente. Des mesures du soufre dans les 4 cm supérieurs des sédiments dans un grand nombre de carottes avant et après le brassage d'automne ont permis d'évaluer la perte saisonnière de soufre des sédiments. Aux profondeurs supérieures à 5 m, les sédiments superficiels contiennent significativement moins de soufre après le brassage d'automne qu'avant. La faible libération saisonnière de soufre par les sédiments (25 kg S en 1966) est peut-être le résultat de la décomposition de la matière organique ou de l'oxydation des sulfures. [Traduit par la Rédaction] Urban and Monte 1355

Limnology and Oceanography, Mar 31, 2011

We present and compare direct and indirect pCO 2 observations taken in Lake Superior in the last ... more We present and compare direct and indirect pCO 2 observations taken in Lake Superior in the last decade and use them to understand temporal and spatial variability in lake carbon cycle processes. In situ observations from 2001 and biannual survey data for 1996-2006 indicate that Lake Superior was, on average, supersaturated (annual mean 5 46.7 6 17.3 Pa [461 6 171 matm]) with respect to atmospheric pCO 2 (mean 5 38.3 6 0.6 Pa) in April and close to equilibrium (mean 5 37.5 6 6.7 Pa) with respect to atmospheric pCO 2 (mean 5 36.4 6 0.7 Pa) in August. Both data sets indicate that temporal variability in surface lake pCO 2 from weekly to interannual timescales was predominantly controlled by changing dissolved inorganic carbon and associated changes in pH. An unstratified water column appears to have limited pCO 2 fluctuations in spring. Through summer and into early fall, pCO 2 variability on a daily timescale at 12 m increased with time to a maximum amplitude of 19 Pa, likely as a result of internal waves on the thermocline. Year-to-year changes in mean surface lake pCO 2 and temperature were of the same sign and approximate magnitude at all observed points, consistent with the lake's small size relative to the synoptic-scale meteorological systems that force it. Variability in pCO 2 was not correlated with major climate indices. While these data provide a first large-scale overview of Lake Superior's pCO 2 and its temporal variability, their time-space resolution and accuracy are not sufficient to further refine previously imbalanced lake-wide carbon budgets.

Journal of Great Lakes Research, Mar 1, 2013

Diporeia, formerly the dominant benthic macroinvertebrate in the Great Lakes, remains a keystone ... more Diporeia, formerly the dominant benthic macroinvertebrate in the Great Lakes, remains a keystone species in Lake Superior. Little is known, however, about fine scale amphipod distributions, especially as influenced by the production, transport and transformation of energy resources. Here, we document the distribution and abundance of Diporeia along 19 transects around the lake's perimeter. Regions of elevated density, averaging 958 ± 408 Diporeia/m 2 (mean ± S.D.) were observed along all transects, typically within slope habitat (depth of 30-125 m). Waters shoreward (shelf habitat, b 30 m) and lakeward (profundal habitat, >125 m) of these regions supported significantly lower densities, averaging 239±178/m 2 and 106±59/m 2 , respectively. Amphipods within regions of elevated density, termed here the Ring of Fire, account for two-thirds of the lakewide population while occupying only one-quarter of the benthic habitat. The Ring of Fire, observed lakewide as a band averaging 14.2±9.4 km in width, is characterized as a region of transitional sediment deposition with gentle slope, proximate to nearshore locations of elevated primary production. Within the Ring of Fire exceptionally high densities are found in the south central region, where the Keweenaw Current and slope bathymetries serve to funnel production from adjoining regions of high production. Density measurements for the 173 stations sampled here are used to estimate lakewide Diporeia standing stock (22.5-37.7 trillion individuals, 4.4-7.4 Gg dry weight, 2.1-3.5 Gg C), individual and biomass density (274-460/m 2 , 0.05-0.09 gDW/m 2 , 0.03-0.04 gC/m 2 ) and areal (0.02-0.03 g C/m 2 /yr) and total (1.6-2.6 Gg C/yr) production.

Journal of Geophysical Research, 2004

A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on the fraction of ... more A study of respiration to photosynthesis (R:P) ratios in Lake Superior, based on the fraction of O 2 saturation and the isotopic composition of O 2 , was undertaken to evaluate spatial and temporal variations in the trophic status of a large oligotrophic freshwater environment. The lake was predominantly net heterotrophic from April to October 2000 (R:P ratios: 1.2-2.5). Uniform R:P ratios of $1.5 with depth and across the lake in April 2000 and 2001 revealed the homogeneity of the water column during spring. A brief period of net autotrophy was observed during summer thermal stratification in 2000 and 2001, and surveys showed this condition to be prevalent and lake-wide in August 2001 (R:P ratios: 0.5-0.9). Strong net autotrophy (R:P ratios: 0.6) was found near Duluth, Minnesota, and suggested the potential for the formation of mesotrophic conditions within areas of increased nutrient loadings from urbanization. Respiration and photosynthesis were shown to exert a strong control on O 2 gas exchange within Lake Superior, as evidenced by significant correlations between R:P ratios and O 2 gas exchange during periods of net heterotrophy and autotrophy. This observation was unexpected since [O 2 ] in the lake appears to be dominated by atmospheric O 2 gas exchange, given that the fraction of O 2 saturation is continuously near levels expected for equilibration with the atmosphere. Furthermore, the relationship between the biological and physical O 2 fluxes may enable the use of R:P ratios to calculate O 2 gas exchange and ultimately estimate CO 2 fluxes between lakes and the atmosphere.

Verhandlungen, Apr 1, 2010

Limnology and Oceanography, Nov 1, 1990

Concentrations of Al, Fe, Mn, major ions, and dissolved organic C (DOC) were measured in 37 lakes... more Concentrations of Al, Fe, Mn, major ions, and dissolved organic C (DOC) were measured in 37 lakes in Halifax County, Nova Scotia, to determine the factors controlling the behavior of these metals in acidic waters with relatively high concentrations of organic matter. Concentrations of trace metals ranged from < 50 to 700 pg liter-l (Al), ~20 to 960 fig liter-' (Fe), and < 10 to 240 pg liter-l (Mn). Concentrations of Fe and Al were highly correlated with H+ ion and DOC concentration, whereas Mn was correlated most strongly with H+ ion concentration and size of catchment relative to lake surface area. Computation of trace-metal speciation indicated that lakes with pH > 5 were in equilibrium with gibbsite and amorphous iron hydroxide but highly supersaturated with respect to manganite and Mn(IV) oxides. Concentrations of Mn appear to be determined by the magnitude of the source (i.e. the catchment area) relative to the size of the lake. In lakes with pH < 5, concentrations of Al and Fe seem to be regulated by interactions with DOC. Although the metals and organic C enter the lake separately, we propose that precipitation of metal humates is the major process regulating concentrations of Al and Fe and, to a lesser extent, DOC in these Apart from lowering the pH of surface waters, one of the most harmful effects of acid deposition is to mobilize trace metals in the environment. High concentrations of Al in acidic waters have been shown to be toxic to fishes (e.g. , and elevated concentrations of Mn are thought to cause deformities in fishes . Dieback of forests in Germany and the eastern U.S.

Journal of Soils and Sediments, May 22, 2018

Purpose Select South Dakota, USA water bodies, including both natural lakes and man-made impoundm... more Purpose Select South Dakota, USA water bodies, including both natural lakes and man-made impoundments, were sampled and analyzed to assess mercury (Hg) dynamics and historical patterns of total Hg deposition. Materials and methods Sediment cores were collected from seven South Dakota lakes. Mercury concentrations and flux profiles were determined using lead ( 210 Pb) dating and sedimentation rates. Results and discussion Most upper lake sediments contained variable heavy metal concentrations, but became more consistent with depth and age. Five of the seven lakes exhibited Hg accumulation fluxes that peaked between 1920 and 1960, while the remaining two lakes exhibited recent (1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) Hg flux spikes. Historical sediment accumulation rates and Hg flux profiles demonstrate similar peak and stabilized values. Mercury in the sampled South Dakota lakes appears to emanate from watershed transport due to erosion from agricultural land use common to the Northern Great Plains. Conclusions For sampled South Dakota lakes, watershed inputs are more significant sources of Hg than atmospheric deposition.

Aquatic Sciences, Mar 1, 1997

Geochimica et Cosmochimica Acta, Jul 1, 1999

Enrichment of organic matter with sulfur during early diagenesis has been amply documented in mar... more Enrichment of organic matter with sulfur during early diagenesis has been amply documented in marine sediments. The importance of such reactions in lacustrine sediments is not as well appreciated. In this study the organic sulfur contents of sediments from seven lakes were compared, and the speciation of the sulfur in the humic acid fraction of the sedimentary organic matter was examined with X-ray photoelectron spectroscopy (XPS). Diagenetic enrichment of organic matter with sulfur occurred in five of seven lakes as evidenced by C:S ratios and stable isotope ratios. The availability of sulfide, organic matter, and reactive iron were not important determinants of the extent of organic S enrichment. The only environmental parameter that appeared to be related to S enrichment was lake trophic state. Together with data from the literature, our results suggest that sediments in most eutrophic lakes are enriched in organic S, while organic matter is enriched in only few oligotrophic lakes. Organic sulfides or thiols are the dominant forms of reduced organic S in the humic acids, and sulfur enrichment occurs primarily by formation of organic sulfides or thiols. Addition of S to polyunsaturated molecules could account for 5-10% of the S enrichment. Di-and polysulfides did not comprise a significant fraction of organic S in any of the sediment humic acids. Thiophenes could be identified tentatively only in the oldest (60 yr) sediment sample analyzed. Sulfoxides were observed in several samples. Lake trophic state and exposure to oxygen appear to be major factors influencing the extent and pathways of S addition to organic matter.

Proton Cycling in Bogs: Geographic Variation in Northeastern North America

Effects of Atmospheric Pollutants on Forests, Wetlands and Agricultural Ecosystems, 1987

A detailed hydrogen ion budget has been constructed for the Marcell bog in north-central Minnesot... more A detailed hydrogen ion budget has been constructed for the Marcell bog in north-central Minnesota based on a 5-year, intensive study of element cycles. Major features of the acidity balance for this site include the following: (1) production of organic acids (263 meg.m−2.y−1) is the dominant source of acidity and serves to buffer the bog water at pH 4; (2) seguestering of elements in peat is also a significant source of acidity (42.9 meg.m−2.y−1); (3) weathering of dustfall inputs is an important source of alkalinity (<76 meg.m−2.y−1) at this site which is situated near the major agricultural area of the plains; (4) nitrate and sulphate reduction contribute little alkalinity (<39.2 meg.m−2.y−1) because inputs (NO3 and SO4) to this bog are low. Analysis of peat and surface water from bogs across northeastern North America (Manitoba to Newfoundland) reveals the following: (1) production of organic acids across this region varies between 104 and 263 meg.m−2.y−1; (2) acidity-generation associated with net biological uptake (NBU, excluding nitrogen = 20–117 meg.m−2.y−1) varies in proportion to the rate of peat accumulation; (3) NBU-acidity exhibits high values in maritime bogs and lower values in mid-continental bogs; (4) bogs have a large capacity for sulphate reduction, and sulphate reduction becomes an increasingly important source of alkalinity as rates of sulphate deposition increase. From 60 to 93% of annual sulphate loadings are retained as reduced sulphur in bogs across eastern North America.

Journal of Marine Science and Engineering

Over a century ago, copper mills on the Keweenaw Peninsula of Lake Superior sluiced 64 million me... more Over a century ago, copper mills on the Keweenaw Peninsula of Lake Superior sluiced 64 million metric tonnes (MMT) of tailings into coastal waters, creating a metal-rich “halo”. Here we show that relatively small discharges can spread widely in time and space. Mass Mill (2.9 MMT) dumping into Lake Superior also illustrates the complexity of interactions with Indigenous Peoples. A combination of aerial photos, LiDAR, and a microscope technique for distinguishing end-member particles traces the migration of tailings. The clay fraction spread rapidly across Keweenaw Bay and curled into terminal L’Anse Bay, within tribal Reservation boundaries. The coarse stamp sand fraction moved more slowly southward as a beach sand deposit onto Sand Point, a sacred burial ground. Despite the partial recovery of northern beaches and southern sediments, concerns continue about chemical contamination. Mass Mill provides an excellent example of Indigenous Peoples’ territorial and resource issues with min...

Michigan State University Press eBooks, Jul 1, 2009

Carbon (C) cycling in Lake Superior was studied within the Keweenaw Interdisciplinary Transport E... more Carbon (C) cycling in Lake Superior was studied within the Keweenaw Interdisciplinary Transport Experiment in Superior (KITES) project to assess (1) whether the lake is net heterotrophic, (2) sources, sinks and residence time for dissolved organic carbon (DOC), (3) importance of terrigenous organic C subsidies, and (4) factors limiting C flow through bacteria. During 3 years of fieldwork, measurements were made of spatial and temporal distributions of C pools and rates of photosynthesis, community respiration, and bacterial production. Measurements were made of the composition of dissolved organic matter (DOM), rates of DOM photolysis, lability of DOM toward microbial consumption, and river inputs of DOM. All measurements suggest the lake is net heterotrophic. The C:N ratios of DOM suggest that it is primarily of terrigenous origin, but other characteristics (size distribution, UV absorption) point to the presence of autochthonous DOM and to alteration of terrigenous material. The lake mass balance indicates that the residence time ($8 years) of the DOC pool (17 Tg) is short relative to the hydrologic residence time (170 years). The known flux of terrigenous DOC ($1 Tg/yr) is too low to support annual bacterial carbon demand (6-38 Tg/yr), but microbial respiration is the major sink for terrigenous DOC. A rapidly cycling, autochthonous DOC pool must exist. Microbial activity was correlated with temperature, phosphorus availability, and DOC concentration but not with photosynthesis rates. Measurements of respiration ($40 Tg/yr), photosynthesis (2-7 Tg/yr), and bacterial production (0.5-2 Tg/yr) are not all mutually compatible and result in a discrepancy in the organic carbon budget.