Light-absorbing components in Lake Superior

Features of light absorption are critical to optical aspects of water quality and in regulating the signal available for remote sensing. Spectral characteristics and spatial patterns of light-absorbing components, and their relationships with optically active constituents, are documented for the Sturgeon River, Keweenaw Bay, and Lake Superior based on analyses of samples collected on two cruises (2006 and 2007, 20 sites). The absorption coefficient, a (m^− 1), is partitioned according to the additive components (a_x) of colored dissolved organic matter (a_CDOM), non-algal particles (a_NAP), phytoplankton (a_?), and water itself (a_w; known). The role of minerogenic particles and their iron content in regulating a_NAP is evaluated based on paired measurements by an individual particle analysis technique (Peng et al., 2009), through empirical analyses and Mie theory calculations of absorption by these particles (a_m). Spectral characteristics of a_NAP and a_? were consistent with those reported for other case 2 (i.e., phytoplankton not dominant) systems. However, the slope values that describe a_CDOM spectra for the bay and the lake were unusually low, suggesting an atypical composition for the lake's CDOM. The dominant absorbing component in the blue wavelengths was CDOM, representing ≥ 75% of a at a wavelength of 440 nm at all sites in the 2006 survey. A general gradient in both a_CDOM and a_NAP extended from the Sturgeon River, through the bay, into eastern Lake Superior in that survey. Relationships between a_x and optically active constituents were within the broad ranges reported for other case 2 systems. Minerogenic particles, related to their iron content, are demonstrated to be an important component of a_NAP.     

Nearshore-offshore trends in Lake Superior phytoplankton

Changes in phytoplankton community composition and structure can have broad-scale ecosystem effects; however, drivers of species diversity in planktonic systems are not well understood. In lakes, a common but not thoroughly tested assumption is that shallow, nearshore waters are much more diverse and productive, and contribute considerably more material and energy to pelagic food webs than deeper waters farther offshore. Lake Superior is a large, cold, oligotrophic freshwater system which can provide insight into community organization under oligotrophic conditions. We used epilimnion and deep chlorophyll layer phytoplankton data from a lake-wide sampling program conducted in 2011 and 2016 to test whether assemblage composition, total algal biovolume, cell concentrations, diversity, and richness vary with depth. Although lake depth was an important factor in structuring assemblage composition, there were no clear nearshore-offshore gradients in cell density or biovolume despite the exposure of nearshore areas to higher concentrations of watershed-derived nutrients. Shannon diversity increased slightly with increasing depth, whereas richness was uncorrelated. Understanding of the nearshore-offshore patterns in phytoplankton community characteristics in the Great Lakes has implications for designing monitoring strategies and for considering how further changes in climate and nutrient deposition would affect the base of the food web.     

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.     

The Stonefly Isoperla bilineata in Lake Superior

Large numbers of nymphs of the stonefly Isoperla bilineata were found on gill nets set on the bottom off the west shore of the Keweenaw Peninsula in May, 1978. A fisherman said they were common there every year, but extensive Ponar grab sampling in the area in 1974 yielded no stoneflies. Similar sampling experience in the Apostle Islands by Selgeby (1974) suggests that grab samplers do not capture this species. Observations of stonefly nymphs on gill nets at three well-separated locations indicate a wide distribution of this species in Lake Superior.     

Prey of Deep-water Hydra in Lake Superior

Specimens of Hydra attached to a soft-substrate bottom were collected from a manned submersible at a 275-m-deep station near the Keweenaw Peninsula in Lake Superior. Near-bottom zoo-plankton were collected to assess Hydra feeding preference. We identified and counted the zooplankton samples, and identified the zooplankton prey found in the Hydra. Of 86 Hydra, 21 contained remains of 23 prey of 4 zooplankton taxa. Compared with expected numbers based on the proportions of the net samples, these Hydra appeared to be feeding selectively, with a preference for larger zooplankton. The large calanoid copepod Limnocalanus macrurus was particularly selected. A very provisional estimate of the rate of Hydra predation on the bottom of Lake Superior is one prey per 4 days. The relatively constant environmental conditions, lack of commensals and predators, and the zooplankton food source suggest that the bottom of Lake Superior is a stable habitat for Hydra.     

Nitrogen dry deposition to Lake Superior and Lake Michigan

Nitrate (NO₃^?) levels in Lake Superior have increased from historic levels of about 5?μM to its current concentration of about 25?μM. The atmosphere makes a substantial contribution to the nitrogen budgets for Lake Superior and Lake Michigan. This study provides a more well-defined estimate of nitrogen dry deposition rates derived from the measurement of over-water concentrations, and in situ meteorological measurements, which were input into the Resistance Model. We obtained a nitrogen dry deposition rate of [(3.41?±?2.26)?×?10⁷?kg?N/yr; (5.90?±?3.91)?kg?N/ha/yr] over Lake Michigan, and [(1.54?±?1.06)?×?10⁷?kg?N/yr; (1.87?±?1.27)?kg?N/ha/yr] over Lake Superior. Nitric acid (HNO₃), which originates from the combustion of fossil fuels, contributes 84% of the total nitrogen dry deposition to Lake Michigan; and 66% to Lake Superior. Ammonia (NH₃), which originates from agricultural activities and gasoline combustion, is the second highest contributor of nitrogen dry deposition to both lakes: contributing 13% to Lake Michigan and 32% to Lake Superior. The nitrogen dry deposition is approximately 68% of the nitrogen wet deposition over Lake Superior, and approximately 80% of wet deposition over Lake Michigan. The over-water dry deposition velocity of HNO₃ and NH₃ were also evaluated. We obtained morning deposition velocities of 0.099?cm/s for NH₃ and 0.095?cm/s for HNO₃; and afternoon values of 0.137?cm/s for NH₃ and 0.132?cm/s for HNO₃. Another key finding is that the atmospheric concentrations of nitrogen compounds near Lake Michigan and Lake Superior have decreased since 2003.     

PCBs in Lake Superior, 1978–1980

Polychlorinated biphenyls (PCBs) were measured in the water column of Lake Superior during the summers of 1978 to 1980. The total PCB water concentrations were relatively uniform areally and vertically within any one year: 1978, 1.3 ± 1.3 ng/L; 1979, 3.8 ±1.9 ng/L; 1980, 0.9 ±0.4 ng/L. The volume-weighted abiotic PCB burden in the water column fluctuated with climatic conditions between ～10,000 and 40,000 kg, but would be in the range of 10,000 to 20,000 kg most of the time. Analysis of 24 large volume, filtered samples from 1980 showed that 27±12% of the PCBs were in the paniculate phase. The distribution coefficient for PCBs, defined by use of a glass-fiber filter, ranged from 10⁴ to 10⁷ L/kg, exhibiting a strong, inverse log relationship to the suspended solids concentration. This behavior is partially attributed to the differences in the organic carbon content of the sampled particles in different regions of the lake and to artifacts of the filtration process. The elevated total PCB concentrations in the summer of 1979 in conjunction with the previous winter's severe weather conditions are evidence of seasonal fluctuations of PCBs in the water column resulting mostly from sediment resuspension.     

Survey of Siscowet Lake Trout at Their Maximum Depth in Lake Superior

The siscowet Salvelinus namaycush is a deepwater morphotype of lake trout in Lake Superior. As part of a standardized lake-wide survey in 2006 to assess siscowet populations, bottom-set, multi-mesh gill nets were fished at 36.6 m depth intervals from near shore areas to the deepest waters in south-central Lake Superior. Siscowet length distributions, diet compositions, and sea lamprey wounding rates were compared for three depth zones: shallow (< 200 m), deep (200–394 m), and deepest (395–399 m). There were 39 siscowets collected in proximity to Lake Superior's greatest recorded depth of 405 m. To our knowledge, this is the greatest depth that fish have been collected in the Great Lakes. Higher proportions of siscowets ≤ 500 mm were caught in the shallow zone compared to deeper zones. Deepwater sculpins were the dominant prey for small siscowets (< 600 mm) across all depth zones. The diet of large siscowets (≥ 600 mm) among all depth zones comprised mostly of coregonines and burbot Lota lota. Terrestrial insects were observed in the diet of siscowets in all depth zones, indicating migration to the surface. Type A sea lamprey wounding rates were higher for large (≥ 600 mm) than small siscowets among all depth zones. The highest wounding rate was observed on large siscowets in the deep zone. Recent work indicates that siscowets are the most abundant lake trout form and this research indicates that siscowets use the maximum depths of Lake Superior.     

Airborne Mercury in Precipitation in the Lake Superior Region

Mercury was measured in accumulated snow (March 1982) sampled from around Lake Superior and in rainfall from Duluth, Minnesota (June–September 1982 and March–November 1983), Forbes Township, and Dorset in northwestern and central Ontario, respectively (May–September 1983). Methods of melting snow and collecting rain samples were investigated to avoid loss of mercury during the melting process and sample shipment and storage. Low concentrations in snow and rain required greater analytical sensitivity. A detection limit of 0.008 ± 0.004 μg/L of mercury (N = 26) was attained using the cold vapor technique; and by utilizing a gold gauze amalgam accessory for preconcentration, a detection limit of 0.005 ± 0.003 μg/L (N = 13) was attained. Regional comparisons of mercury accumulation in the snow pack across the northern parts of Minnesota, Wisconsin, Michigan, and Ontario show highest values, 0.10 ± 0.17 μg/L Hg, in the Grand Rapids, Minnesota, area and lowest values in remote areas of Minnesota and Ontario. Rainfall concentrations of total mercury were substantially higher than snow accumulation and were mainly of inorganic forms (73%). The highest amounts of mercury were measured in rainfall at Duluth, Minnesota (3.4 μg/L) during June 1982 and at Forbes Township, Ontario, (1.2 μg/L) during July 1983. Substantially lower concentrations were observed in the other months of 1982 and 1983. There is no unique association of the observed mercury concentration with the air parcel history. However, the single highest mercury concentration in rainfall observed at Duluth, Minnesota, occurred when the 850-mb air parcel back trajectory indicated upwind corridors to the N-NW and the highest concentration at Forbes Township, Ontario, was associated with an 850-mb trajectory from W-NW. Significant positive correlation coefficients (0.05) were obtained with other measured components in rain at Duluth, Minnesota, including S, B, Ca, Mg, Na, Fe, Mn, and Zn in 1983. Mercury concentrations in precipitation and in lake water for precipitation-dominated lakes are similar and indicate that atmospheric deposition can be the major source of mercury for some lakes in the Lake Superior region.     

Winter Diet of Lake Herring (Coregonus artedi) in Western Lake Superior

Lake herring (Coregonus artedi) and zooplankton samples were simultaneously collected through the ice in the Apostle Islands region of western Lake Superior to provide information on the winter feeding ecology of lake herring. Zooplankton constituted the entire diet of the 38 lake herring collected for this study. We found no evidence of piscivory, although it has been reported by anglers. Diet selectivities were calculated using a Wilcoxon signed-ranks test and showed a preference of lake herring for larger zooplankton, especially Diaptomus sicilis, whereas the smaller copepod, Cyclops bicuspidatus thomasi, and immature copepod stages were selected against. These data document that overwintering copepods are food for a broad size range of lake herring in winter.     

Potential Sources of Asbestos in Lake Michigan

Chrysotile and amphibole mineral fibers, recognized carcinogens, have been found in Lake Michigan water. Since the sources of these fibers in Lake Michigan are unknown, a study was undertaken to determine potential sources. Information was gathered by contacting governmental agencies and reviewing literature. Two industries that used to discharge asbestos-containing effluents are located in the Lake Michigan drainage basin. Geologic deposits of mineral fibers occur in Marinette County, Wisconsin and the Upper Peninsula of Michigan. Currents and Army Corps of Engineers dredging and disposal activities were investigated as potential mechanisms of asbestos distribution in Lake Michigan. Limited environmental monitoring indicates that atmospheric loading and wash-out from numerous point and non-point sources may be an important mechanism of asbestos contamination of Lake Michigan. More environmental monitoring will be required in order to determine the relative importance of these potential sources.     

Modeling historical trends in Lake Superior total nitrogen concentrations

Nitrate concentrations in Lake Superior increased fivefold between 1900 and 1980, and have remained nearly constant since that time. Such rapid changes in concentration in a lake with a long hydraulic residence time (~ 190 years) are surprising. We developed a model to better understand the causes of the historical changes and to predict future changes in nitrate concentrations. Historical loadings were reconstructed based on average national NO_x emissions estimates, recent (past ~ 30 years) atmospheric N deposition data, recent tributary concentration data, and basin-wide runoff estimates. Increases in atmospheric N deposition alone were insufficient to have resulted in the observed trends. However, model runs combining increased atmospheric deposition with increased tributary N loading and/or decreased burial + denitrification mid-century reproduced the observed accumulation of N. Because internal N fluxes are an order of magnitude greater than external fluxes, relatively small changes in the lake's internal N cycle may produce relatively large changes in total N concentrations. Land-use changes in the watershed, particularly increases in logging activity, may have altered riverine N inputs. Regardless of the historical mechanisms leading to the rise in nitrate concentrations, it appears as though the system is currently at or is approaching peak N content.     

Relative Role of Lake and Tributary in Hydrology of Lake Superior Coastal Wetlands

Despite the documented importance of hydrodynamics in influencing the structure and function of Great Lakes coastal wetlands, systematic assessments of coastal wetland hydrology are lacking. This paper addresses this gap by describing patterns in lake and tributary inputs, water residence times, and mixing regimes for a suite of western Lake Superior wetlands that differ in the amount of tributary and seiche flow they receive. We show that variability in tributary flows among wetlands and over time is far greater than variability in seiche-driven water movements, and that the amount of tributary flow strongly influences wetland hydrology via effects on water mixing and residence times, seiche size, mouth closures, and relative amounts of main and off-channel areas. Wetland seiche amplitudes were reduced in systems with small mouth openings and wetland mouth size was correlated with tributary flow. All wetlands experienced seiche-driven water level oscillations, but there was lake water intrusion only into those wetlands where tributary outflow was small relative to the seiche-driven inflow. Wetlands in settings exposed to long-shore sediment transport exhibited periodic mouth closures when stream flows were low. The absolute and relative size of lake and tributary inputs must be explicitly considered in addition to wetland morphology and landscape setting in studies seeking to understand determinants of coastal wetland structure, function, and response to anthropogenic stressors.     

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.     

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.     

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.     

In situ-measured primary production in Lake Superior

Water column primary production is a major term in the organic carbon cycle, particularly in large lakes with relatively reduced shoreline and near-shore influence. Presently, there is a large imbalance in the known inputs vs. outputs of organic carbon in Lake Superior. This study examined primary production in offshore Lake Superior using in situ incubations over a range of conditions representing an annual cycle. Primary producers were dominated by small (< 20 μm) cells and included a relatively large abundance of small, spherical flagellates. During conditions with a warm surface layer, chlorophyll concentrations were two- to three-fold higher within the deep chlorophyll maximum (DCM) than at the surface. Volumetric production (mass L^− 1 d^− 1) was maximal at 2-10 m depth, well above the typical DCM depth. On average, 22% of ¹⁴C label appeared in the dissolved pool at the end of the incubation period with the rest appearing in GF/F-strained particles. A statistical model for volumetric production explained 93% of the variance in individual measurements for depths > 2 m, using temperature and light as predictors. This model was applied to annual fields of temperature and light, and a new estimate for whole-lake annual primary production, 9.73 Tg y^− 1, was derived. This combination of new measurements and modeling results brings the organic carbon cycle of Lake Superior closer to being balanced.     

Spatial and Temporal Variation of Ammonium in Lake Superior

Many aspects of Lake Superior's nitrogen cycle are poorly described in spite of the fact that the lake's nitrate concentration has risen dramatically this past century. One important, yet underdescribed parameter is the concentration of ammonium. Here, we present data to resolve spatial and temporal variation along with vertical profiles of ammonium concentration in Lake Superior. Lake-wide average concentrations were low (0.21 μM, n =166) with considerable spatial and temporal variation. During the onset of summer, the western margin of the lake had higher average concentrations than open and eastern parts. Surface layer (<10 m) ammonium concentrations showed an increase from January to October. Relatively higher ammonium concentration in the near bottom waters at a number of sites during August indicated efflux from sediment to be an important process. Subsurface maxima near the thermocline were observed in late August and persisted until September-October suggesting that ammonium might be controlled by food web processes during warm, stratified conditions. The higher potential for ammonium uptake compared to external inputs suggested rapid turnover of ammonium in the lake.     

Postspawning Mortality of Rainbow Smelt in Western Lake Superior

Extensive die-offs of rainbow smelt, Osmerus mordax (Mitchell), have occurred in the western end of Lake Superior during each of the past several springs. Sex, age composition, and incidence of fungus infection in smelt from the 1977 die-off were compared with the same characteristics of smelt of the extant spawning population to determine the extent and possible causes of mortality. The die-off in 1977 began about 3 weeks after the end of the spawning run and continued for approximately 2 weeks. The occurrence of dead rainbow smelt was greatest near the Superior-Duluth harbor and decreased as distance from the harbor increased. Age structure of smelt that died was similar to that of the spawning population. Some factor or factors related to spawning may have caused the mortality. The most probable cause of the die-off was temperature stress on spawning smelt in the spawning areas which increased the susceptibility of smelt to the fungus Saprolegnia sp. and may have promoted osmoregulatory imbalance. Male smelt were more vulnerable to the cause of the die-off than female smelt; young fish, especially females, were more resistant than older fish.     

Population Characteristics and Movements of Lake Sturgeon in the Sturgeon River and Lake Superior

A 2.6-km reach of the Sturgeon River, containing two sets of rapids, is an important spawning site to a native population of lake sturgeon, Acipenser fulvescens, which ranges widely into southern Lake Superior. Similar spawning areas in other Great Lake tributaries may also be important to the protection and rehabilitation of lake sturgeon throughout this region. Information on range and habitat needs of this species, which is considered “threatened” in the State of Michigan, was obtained from the Sturgeon River spawning population from 1987 to 1995. Radio-tracking was employed to determine movements and habitat use by post-spawning lake sturgeon. Telemetry data from 25 fish were supplemented with data obtained through identification tag returns. During the study 925 lake sturgeon were handled; 86 returned to spawn 1 time and 12 returned 2 times. Spawning intervals for male lake sturgeon were commonly 2, 3, or 4 years; yearly spawning by males was never observed. Females returned to spawn after 3 to 7 years. From 1991 to 1995 the male:female sex ratio at the spawning site was 1.25 to 2.7. In 1990 13 of 18 adults fitted with transmitters moved out of the river within 9 days. Upon reaching Portage Lake nine individuals spent time in shallow (maximum depth, 6 m) Pike Bay. After 3 to 53 days (mean, 22) tagged fish moved into the deeper water of Portage Lake (maximum depth, 17 m) and ranged more widely. Three fish were located in Keweenaw Bay, Lake Superior by late August. Identification tag returns reveal that lake sturgeon traveled 70 to 280 km from the spawning site throughout southern Lake Superior.