Sediment Trap Studies in Lake Superior: Insights into Resuspension,Cross-margin Transport,and Carbon Cycling

As part of the Keweenaw Interdisciplinary Transport Experiment in Lake Superior (KITES) project, sediment traps were deployed at multiple locations along the northern coast of the Keweenaw Peninsula in Lake Superior. Traps were deployed at multiple depths (25 m below water surface to 5 m above sediments) at varying distances from shore (0.5–21 km) during the months May-October over a 3-year period. Material captured in the sediment traps was analyzed for total carbon (C), total nitrogen (N), total phosphorus (P), total copper (Cu), ²¹⁰Pb, and stable isotope ratios of C and N. Copper in sediment trap material, a tracer for mine residues, indicated transport of material in both directions along the peninsula. Cross-margin transport of material occurred at all trap locations along the coast and in all seasons. The most significant finding was that sediment traps suspended just below the thermocline collected large amounts of resuspended sediments even when the trap was moored in 120–220 m of water (9–21 km from shore). Estimates of the contribution to settling fluxes of organic carbon from resuspended sediments ranged from 10–30% in offshore traps; estimates based on ²¹⁰Pb agreed well with estimates based on a carbon mixing model. Element ratios suggest that resuspended material in offshore traps originated in near-shore regions. Despite the strong influence of resuspension on the composition of sedimenting material, stable isotope ratios were controlled by processes occurring in the water column. Seasonal variations in isotope ratios may reflect seasonal shifts between predominance by autotrophic and heterotrophic processes in the water column.     

Distribution of Diatoms in Lake Superior Sediments

Four maps showing semiquantitative abundance patterns of diatoms throughout Lake Superior were made by examination of 170 cores at sediment depths of 0–1, 10, 20, and 50 cm. These maps show a decrease in diatom abundance with sediment depth and an absence of diatoms in most open-lake sediments. Diatoms in surficial sediments are most abundant in Jive regions in Lake Superior: Thunder Bay, Keweenaw Bay, the Thunder Bay and northshore troughs, and a region to the southwest of Isle Royale. Only three of these five regions still show relatively abundant diatoms at the 50-cm sediment depth. Diatom abundance in the sediments is positively correlated with sedimentation rates, water depth, and proximity to shore. Nearshore deep-water troughs act as depositional traps for diatoms in western Lake Superior and Keweenaw Bay. Later transport of diatoms by waves and currents generally erases any relationship between diatom abundance in the sediments and diatom biomass in the overlying water. Diatom abundances at 10- and 20-cm depths correlate positively with sedimentation rates, suggesting that biogenous and terrigenous components are eroded, transported, and redeposited as a mixture, rather than being hydraulically separated. Diatoms are abundant at 0–1 cm in regions of both low and high sedimentation rates.     

Distribution of Photosynthetic Pigments in Nearshore Sediments of Lake Michigan

To characterize nearshore detrital deposits and to identify potential input sources, photosynthetic pigments were examined in sediment cores taken from three stations in nearshore Lake Michigan between May and October 1979. Two stations were located at 11 m and one was at 23-m depth. At the 11-m stations, total pigment concentrations in the upper 0–1 cm layer were greatest in May and then declined, while seasonal changes at the 23-m station were not apparent. A high proportion of the spring influx of pigment at the 11-m depth was undegraded, indicating freshly settled material from the water column as a likely source. Sediment chlorophyll concentrations in the spring were as high as 212 μg/g, but typical summer values were 1–7 μ/g. Although these pelagic inputs are temporary, strong links exist between this material and benthic invertebrate distributions.     

Toxicity of Cuyahoga River and Nearshore Lake Erie Sediments to Photobacterium phosphoreum

The Cuyahoga River Basin and Lake Erie (nearshore of Cleveland) sediments were assayed with Photobacterium phosphoreum during the summer of 1991. Of the 33 sediments tested, 23 exhibited toxicity using either interstitial or extract water. Nearly all (93%) of the “navigation channel” (i.e., the lower 8 km of the river) and nearshore lake samples were toxic. No statistical difference was observed between river sediment interstitial and extract waters. With the limited number of lake samples (seven), more toxicity was observed with extract water than with interstitial water.     

Vertical Distribution of Profundal Benthos in Lake Superior Sediments

Layers of sediment in box cores from 10 Lake Superior open lake sites were sieved at 250 μm to retain benthos. The average density of benthic organisms, 3,055/m², was higher than has previously been reported for profundal regions of the lake, suggesting that biological mixing is important in the sediments. Bottom fauna were distributed from the water-sediment interface to a depth of 1.7 cm in firm glacial till and to a depth of 15 cm in soft clay. There was variability between sampling sites: oligochaetes and nematodes penetrated further into loose sediments than into compacted sand or clay. Ninety-six percent of the profundal benthos was found within the first four centimeters of sediment, with 47% between 0 and 0.5 cm (mostly Pontoporeia hoyi, naidids, sphaeriids, copepods, ostracods, and neorhabdocoels); 49% between 0.5 and 4 cm (mostly nematodes and oligochaetes); and 4% below 4 cm. The location of oligochaete cocoons containing embryos indicated that enchytraeid positions in the cores often represented their in situ vertical distribution, whereas some of the lumbriculids and tubificids migrated downward during the sampling procedures. The density and vertical distribution of Pontoporeia indicated that the potential depth and rate of biological mixing of sediment may exceed the sediment accumulation rate in Lake Superior.     

Anthropogenic Copper Inventories and Mercury Profiles from Lake Superior: Evidence for Mining Impacts

During the past 150 years, the mining industry discharged more than a billion tons of tailings along Lake Superior shorelines and constructed numerous smelters in the watershed. Given the vast size of Lake Superior, were sediment profiles at locations far offshore impacted by nearshore activities? Did copper and associated precious metal mining modify regional fluxes for copper and mercury? Samples from thirty sediment cores document that background concentrations of copper are high (mean 60.9 ± 7.0 μg/g), due to the proximity of natural ore sources. Anthropogenic inventories uncorrected for focusing also are high, ranging from 20 to 780 μg/cm² (mean 187 ± 54 μg/cm²). Focusing factor corrections decrease the mean estimate and reduce variance (144 ± 24 μg/cm²). Several approaches to estimating inputs suggest that only 6 to 10% of historic copper deposition originated directly from atmospheric sources, emphasizing terrestrial sources. Moreover, coastal sediment cores often show synchronous early increases in copper and mercury with buried maxima. Around the Keweenaw Peninsula, twenty-two cores trace high copper and mercury inventories back to mill and smelting sources. Direct assays of ores from thirteen mine sites confirm a natural amalgam source of mercury in the stamp mill discharges. Core records from inland lakes (Michigamme Project) also reveal patterns of copper and mercury inputs from a variety of mining sources: historic tailing inputs, amalgam assay releases, and atmospheric smelter plumes.     

A Preliminary Study of Recent Diatom Assemblages in Lake Superior Sediments

Nine fossil planktonic diatom species were used for quantitative stratigraphic analysis of four cores from Lake Superior. Few changes in relative abundances of diatom species can be correlated among the cores. However, a consistent increase in Melosira islandica within the upper 20 cm of sediment is evident in all four cores. This increase occurs before an increase in Ambrosia (ragweed) pollen that marks the postsettlement horizon. Therefore, the Melosira islandica increase cannot be attributed to nutrient enrichment by man but may be a response to gradual climatic cooling over the last few thousand years, to differential dissolution, or to some other unknown cause.     

Short-term Water Mass Movements in Lake Michigan: Implications for Larval Fish Transport

Water mass movement within the Great Lakes may rapidly transport fish larvae from favorable nursery areas to less favorable habitats, thereby affecting recruitment success. During 2001 and 2002, we released satellite-tracked drifting buoys in eastern Lake Michigan to follow discrete water masses, and used ichthyoplankton nets to repeatedly sample larval fish within these water masses. Observed nearshore water currents were highly variable in both direction and velocity. Current velocities far exceeded potential larval fish swimming speeds, suggesting that currents can potentially rapidly advect fish larvae throughout the lake. Evidence suggests that while paired drifters released during 2002 were able to track relatively small alewife (Alosa pseudoharengus) and yellow perch (Perca flavescens) larvae within an alongshore coastal current, paired drifters released during 2001 failed to track larger alewife larvae when flow was more offshore and highly variable. These results are consistent with the decorrelation scales associated with alongshore and offshore transport.     

Organochlorine Insecticides and PCB in the Surficial Sediments of Lake Superior (1973)

Analysis of 405 Lake Superior surficial sediments (0-3 cm depth) collected in 1973 were found to contain low levels of DDE, HEOD, and PCB, but no detectable mirex, heptachlor expoxide, endrin, endosulfan, or chlordane. p,p’-DDE was detected in 50%, HEOD in 9%, and PCB in 44% of the sediment samples at trace or low measurable levels. Mean residues for the whole lake were p,p’-DDE 0.71 ± 1.65 ng/g, HEOD below the detection level of 0.25 ng/g, and PCB 3.3 ± 5.7 ng/g. p,p’-DDE was more concentrated in depositional basins than in non-depositional zones while PCB levels were similar for the two zones.     

Annual and Offshore Changes in Bacterioplankton Communities in the Western Arm of Lake Superior during 1989 and 1990

A shallow site in the western arm of Lake Superior near Duluth, Minnesota was sampled bimonthly from May to October during 1989 and 1990 to identify seasonal and annual changes in bacterioplankton communities. The greatest change in bacterioplankton abundance was between 1989 (1.48 × 10⁹/L ± 0.06 SE) and 1990 (1.14 × 10⁹/L ± 0.06 SE). The majority of bacterial cells (65%) were cocci. Individual cells were larger during 1989 (0.067 μm³ ± 0.007 SE) than 1990 (0.025 μm³ ± 0.002 SE). Although the rate of thymidine incorporation varied from 0.2 to 47.0 pmol/L/h over both years (mean = 12.1 pmol/L/h ± 1.3 SE), no consistent temporal or spatial changes were detected. Bacteria were more abundant (∼2×) and productive (∼10×) at the mouth of the Lester River than offshore of this site. During July and August, a benthic nepheloid layer (BNL) formed at shallow offshore sites but bacterioplankton abundance and production in this BNL were usually similar to values measured in the hypolimnion. Three additional sites from the Duluth basin northeast to the Chefswet basin were sampled during late summer (Aug-Sept) 1990 to identify spatial differences in bacterioplankton communities. Although the number of bacteria was often greater at shallower sites compared to deeper sites further offshore, a strong gradient was not found and bacterial production was similar at all sites. These results may be due in part to the lake basin morphology in this region of Lake Superior, as well as the time when these additional offshore sites were investigated.     

A Comparison of Total Phosphorus,Chlorophyll a,and Zooplankton in Embayment,Nearshore, and Offshore Habitats of Lake Ontario

Lower trophic levels were compared in embayment, nearshore, and offshore habitats of Lake Ontario, 1995 to 1997, in the context of oligotrophication and invasion of dreissenid mussels. Total phosphorus (TP), chlorophyll a (chl a), Secchi disk depth, temperature, and zooplankton were measured to spatially and temporally contrast these habitats and to test for recently hypothesized “decoupling” of chl a from TP (lower than predicted chl a per unit TP, consistent with dreissenid mussel grazing). The embayment habitat had higher concentrations of TP and chl a, greater volumetric zooplankton density and biomass, and higher springtime water temperatures than both nearshore and offshore habitats, while overall areal zooplankton biomass was highest in the offshore. Furthermore, concentrations of TP and volumetric zooplankton density in nearshore habitats are now more similar to the offshore pelagia than they were three decades ago. Finally, a lower yield of chl a per unit TP was found in nearshore habitat compared to offshore and embayment habitats. The current lower yield of chl a per unit TP in nearshore habitat can be attributed more to Dreissena than to erosion and/or resuspension of sediments.     

Horizontal and Vertical Distributions of Mercury in 1983 Lake Superior Sediments with Estimates of Storage and Mass Flux

Persistent elevated mercury concentrations in some species of Lake Superior fish and the general lack of information on mercury storage in and fluxes to the lake's sediments prompted the analysis of samples that were collected in 1983. Results of the analyses support the conclusion that Lake Superior sediments have mercury concentrations above background levels at all sites sampled. For those cores which penetrated the sediment deeply enough (the majority of the cores), background mercury concentrations ranged between 0.016 and 0.048 mg/kg. Mercury concentrations in surficial sediments ranged between 0.027 and 0.96 mg/kg. The maximum mercury concentration found in sub-surface sediments (2 to 20 cm deep) was 6.5 mg/kg. The surficial 20 cm of sediment contained 342 metric tons of mercury of which 51% or 174 metric tons was anthropogenic. The surface 2 cm of sediment contained 29 metric tons of mercury of which 76% or 22 metric tons was anthropogenic. Estimated total mercury fluxes to surficial sediments ranged between 0.1 and 10 ng/cm²/y with a mean of 3.2 ng/cm²/y. Background total mercury fluxes to the lake ranged between 0.20 and 0.72 ng/cm²/y with a mean of 0.48 ng/cm²/y. Estimated fluxes of anthropogenic mercury to surficial sediments ranged between –0.42 and 10 ng/cm²/y with a mean of 2.7 ng/cm²/y. The fluxes reported here are only the second known reporting of mercury fluxes to Lake Superior sediments. The inventory of mercury in the sediments is the first reported.     

Nearshore Regions of Lake Superior: Multi-element Signatures of Mining Discharges and a Test of Pb-210 Deposition under Conditions of Variable Sediment Mass Flux

Around the turn-of-the century, mining activities greatly increased sediment accumulation and metal fluxes in nearshore regions of Lake Superior. In the low-energy environment of Portage Lake, within the Keweenaw Waterway estuary, sediment accumulation increased 33X, whereas elemental Cu flux increased 312X. One difficulty in establishing the dispersion of mining discharges is that stamp sands were derived from local ore deposits, hence few elements are “unique” to the source materials. One approach is to search for multi-elemental “signatures” in concentration and flux profiles. For example, several rare earth elements of the lanthanide series are characteristic of source materials and have the potential to identify stamp sand material across Lake Superior. Although conditions of variable mass loading from multiple sources can produce complicating dilution effects in concentration profiles, multivariate techniques are capable of deciphering original source signals. Here non-destructive neutron activation analysis was utilized to construct elemental flux and concentration profiles, then multivariate techniques (Factor Analysis, End-member Analysis) were used to illustrate how partial mass flux signatures can be assigned to two different types of ore lodes (conglomerate, amygdaloid) and to background (erosional) sedimentation. Temporal patterns were verified through archived company discharge records. Also exploited were the varve-like deposition of slime clays to independently check ²¹⁰Pb determinations under conditions of variable sediment mass flux and to demonstrate constant excess ²¹⁰Pb delivery to sediments in the presence of massive slime clay loading. The results suggest assumptions of ²¹⁰Pb dating may apply under conditions where sediment accumulation is highly variable.     

Perfluoroalkyl acids in Lake Superior water: Trends and sources

Perfluoroalkyl acids (PFAs) are a family of highly persistent compounds which are present in the environment as a result of degradation of polyfluorinated precursors, from use as processing aids for production of fluoropolymers, and use in fire fighting foams. The purpose of this study was to investigate prevailing concentrations and possible sources of PFAs in Lake Superior, as well as in Siskiwit Lake on Isle Royale. Between 2001 and 2005, replicate water samples were taken from lake surface waters, and from depth profiles, as well as from major tributaries including municipal waste water treatment plants (WWTPs) at three major population centers. Perfluorooctanoate (PFOA) was the predominant PFA in Lake Superior, with concentrations ranging from 0.07 to 1.2 ng/L in surface waters. PFOA concentrations were generally 1.5 to 2-fold greater than perfluorooctanesulfonate (PFOS) levels. WWTPs were found to contribute up to 20 fold higher concentrations of PFOA (22 ng/L) relative to the intake water from Lake Superior, while most tributaries contained lower concentrations of perfluorocarboxylates (PFCA) and perfluoroalkylsulfonates (PFSs) (< 0.1 ng/L). Overall tributaries and precipitation were estimated to be the major sources of PFCAs and PFSs to Lake Superior. Tributaries were estimated to be the largest source contributing 59% of PFOA and 57% of PFOS inputs to the lake. Profiles conducted over the deepest points in the lake showed that PFAs were found throughout the water column, however, there was no distinctive trend with depth.     

High Resolution Bathymetry and Lakebed Characterization in the Nearshore of Western Lake Michigan

The nearshore zone of western Lake Michigan is poorly characterized both in terms of its fine scale bathymetry and its lakebed characteristics. Difficulties in characterizing the lakebed of this region arise in part because of its patchiness as well as the fact that much of the lakebed is not amenable to conventional sediment sampling techniques. With this in mind, high precision bathymetry and the lakebed characteristics of ∼17.5 km2 of the nearshore of western Lake Michigan (near Oak Creek, WI) were mapped using multi-beam and single beam sonar in conjunction with lakebed characterization and surface mapping software. Lakebed features showing nearshore bluff erosion material, glacial scouring, and isolated ridges were revealed in a 1.0 × 1.5 meter resolution map of the survey area. A map of the bottom characteristics, meanwhile, showed a patchwork of four distinct bottom types (area %): mostly rock (12.8%), cobble and sand (21.0%), mostly sand (60.3%), and clay outcrops (5.9%). All predicted bottom types were shown to be accurately characterized by direct observation with an ROV.     

Phytoplankton Composition,Abundance, and Distribution: Nearshore Lake Ontario and Oswego River and Harbor

Phytoplankton samples were collected during July, August, and October of 1981 from 16 stations located in the nearshore of Lake Ontario and in the Oswego Harbor and river. Phytoplankton assemblages observed represented many species widely recognized as associated with eutrophic environments and environments possessing high chloride levels. Picoplankton and diatoms were the dominant groups on a numerical and bioyolume basis, respectively. The eutrophic species Stephanodicus tenuis, Fragilaria capucina, and Scenedesmus spp. were present in the Oswego Harbor and river with substantially higher abundances than in the nearshore region of Lake Ontario. A decrease in abundance of the historically prevalent Asterionella and Tabellaria and an increase in cryptomonads is suggested for the Oswego area. Halophilic species dominated the diatom assemblage of the Oswego Harbor and mouth of the Oswego River. The abundance of halophilic species was correlated with high conductivity and chloride levels. Cyclotella atomus, Cyclotella cryptica, and Cyclotella meneghiniana, the dominant halophilic species at the harbor and river stations, accounted for 36.6% of the total diatom abundance.     

Silica Depletion in Lake Michigan: Verification Using Lake Superior as an Environmental Reference Standard

A mixing model based on the regression of silica concentrations on specific conductance was used to predict silica concentrations in the outflowing waters of Lake Michigan and Lake Superior on 21 cruises conducted from 1954 to 1980 in northern Lake Huron where chemical characteristics are determined by mixtures of the two source waters. Specific conductance @ 25°C can be used in the mixing model because it varies from 95 μS?cm^?1 in Lake Superior waters to 267 μS?cm^?1 in Lake Michigan waters. Results show that silica concentrations in the outflow of Lake Michigan in 1954 were comparable to the Lake Superior concentrations (ca. 2.0 mg?L^?1) but were reduced to <0.4 mg?L^?1 in summer cruises beginning in 1968. These results provide strong evidence that silica concentrations in the outflowing waters of Lake Michigan decreased at least 2.0 mg?L^?1 from 195 to 1971. Confidence can be given to this postulated decrease in concentration because the accuracy of data used in these analyses was verified by comparisons with the relatively invariate silica concentration in Lake Superior.     

Sediment nitrification and denitrification in a Lake Superior estuary

Inorganic nitrogen (N) transformations and removal in aquatic sediments are microbially mediated, and rates influence N-transport. In this study we related physicochemical properties of a large Great Lakes embayment, the St. Louis River Estuary (SLRE) of western Lake Superior, to sediment N-transformation rates. We tested for associations among rates and N-inputs, vegetation biomass, and temperature. We measured rates of nitrification (NIT), unamended base denitrification (DeNIT), and potential denitrification [denitrifying enzyme activity (DEA)] in 2011 and 2012 across spatial and depth zones. In vegetated habitats, NIT and DeNIT rates were highest in deep (ca. 2 m) water (249 and 2111 mg N m^− 2 d^− 1, respectively) and in the upper and lower reaches of the SLRE (> 126 and 274 mg N m^− 2 d^− 1, respectively). Rates of DEA were similar among zones. In 2012, NIT, DeNIT, and DEA rates were highest in July, May, and June, respectively. System-wide, we observed highest NIT (223 and 287 mg N m^− 2 d^− 1) and DeNIT (77 and 64 mg N m^− 2 d^− 1) rates in the harbor and from deep water, respectively. Amendment with NO₃⁻ enhanced DeNIT rates more than carbon amendment; however, DeNIT and NIT rates were inversely related, suggesting the two processes are decoupled in sediments. Average proportion of N₂O released during DEA (23–54%) was greater than from DeNIT (0–41%). Nitrogen cycling rates were spatially and temporally variable, but we modeled how alterations to water depth and N-inputs may impact DeNIT rates. A large flood occurred in 2012 which temporarily altered water chemistry and sediment nitrogen cycling.     

Spring pH and Alkalinity Depressions in Lake Superior Tributaries

Between October 1980 and October 1981, the pH and alkalinity of 21 Canadian Lake Superior tributaries were measured. All tributaries studied exhibited reductions in pH and alkalinity during the spring runoff period of 1981; two showed pH minima of near 5.0 and an additional five showed pH minima in the range of 5.5 to 6.0. Since most of these waters constitute valuable fishery resources, particularly for salmonids, the observed pH depressions prompt concern over possible effects on fish populations.