Selected Features of the Distribution of Chlorophyll along the Southern Shore of Lake Superior

Monitoring was conducted over the April to October interval of 1999 and 2000, seeking signals in the spatiotemporal structure of algal biomass (chlorophyll) in Lake Superior. Sampling was conducted along three nearshore-offshore transects in the vicinity of Michigan's Keweenaw Peninsula. Levels of algal biomass in Lake Superior are the lowest in the Great Lakes, with chlorophyll concentrations ranging from ∼0.25 to 2.5 mg·m⁻³ and averaging less than 1 mg·m⁻³ in the surface waters. Several spatial signals were observed, including inter-transect differences, nearshore-offshore gradients, and vertical structure. These signals are thought to be mediated by several factors, including runoff from a major tributary, light availability and mixing depth, inter-transect variations in bathymetry and temperature gradients, e.g., the thermal bar in spring and vertical stratification in summer. Seasonally, surface water chlorophyll dynamics were characterized by an increase from late-winter concentrations in late April and early May, a continued increase in the nearshore and a decrease/stabilization at offshore sites from late May through July, a summer minimum in late July and August and an increase in September and October with the approach to turnover. These signals are striking given the modest levels of chlorophyll present in the lake and considering prevailing conditions of light, temperature and phosphorus availability. The spatial and temporal structure evidenced here likely resonates through the system, impacting the distribution of organisms both higher and lower in the food web. These results will find application in this and other similar systems for consideration of the vernal thermal bar as a factor mediating primary production and cross-margin transport of materials of biogeochemical significance, in developing a greater appreciation of the role of the microbial loop in fostering secondary production, in defining the relative roles of allochthonous and autochthonous contributions to the carbon budget and in supporting remote sensing studies of large scale transport phenomena in the lake.     

Pontoporeia Distribution Along the Keweenaw Shore of Lake Superior Affected by Copper Tailings

The sediment and benthic macroinvertebrates of a 540 km² area of Lake Superior were examined. The amphipod Pontoporeia hoyi was by far the most abundant animal collected. A 42 km² area devoid of P. hoyi was found near where 50 million tons of copper tailings containing metallic copper in concentrations averaging 3750 mg/kg had been dumped between 1895 and 1968. In the no-amphipod region sediment copper ranged from 395 to 1310 mg/kg, while elsewhere mean levels were 14 to 298 mg/kg. The study area was divided into five transects which had maximum densities of P. hoyi ranging from 470 to 4480/m². The depth of maximum density ranged from 40-45 to 60-65 m. Maximum density was associated with a mean grain size of ? 3.5-4.0 and elevated levels of total volatile solids, nitrogen, phosphorus, and chemical oxygen demand. Copper and dark color, characteristics of the copper tailings, showed significant negative correlations with P. hoyi density.     

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.     

Phosphorus Uptake Dynamics as Related to Mathematical Modeling of Cladophora at a Site on Lake Huron

Cladophora is a significant symptom of eutrophication in Lakes Erie and Ontario and is a local problem associated with nutrient perturbations in Lakes Huron, Michigan, and Superior. This paper presents results of measurements of phosphorus uptake rates as a function of internal phosphorus levels by Cladophora growing near Harbor Beach, Michigan. Cladophora collected near the Harbor Beach wastewater treatment plant had high levels of internal phosphorus and low (or even negative) phosphorus uptake rates. Cladophora distant from the wastewater treatment plant had low internal phosphorus levels and rapid phosphorus uptake rates. The experimental results are discussed in terms of quantitative kinetic formulations which may be incorporated into mathematical models useful for predicting the response of Cladophora to alternative management and control strategies. Preliminary model simulations of Cladophora biomass, internal phosphorus, and external phosphorus are qualitatively similar to observed field data.     

Bacterioplankton production in a water column of Lake Biwa

Seasonal production of bacterioplankton in a water column of Lake Biwa was examined based on cell‐specific growth rates at above (surface layer) and below the thermocline (deep layer). The growth rates were estimated by incubating bacterioplankton in situ with a dilution technique. The rates ranged from 0.05 per day in winter to 0.89 per day in summer, generally with much higher rates in the surface layer than in the deep layer. In an entire water column, bacterial production (in terms of carbon [C]) ranged from 0.217 to 0.811 gC/m² per day with a mean of 0.451 gC/m² per day, which, on average corresponded to 43% of primary production. However, no significant correlation was detected between bacterial production and primary production rates. Although the bacterial production rate correlated positively to water temperature, surface and deep layer rates were comparative for some dates because the deep layer shared a large fraction of the water column. These results suggest that, although specific bacterial activity was low in the deep layer due to the low temperature, bacteria in that layer play substantial roles in consumption of organic matter and material flows in Lake Biwa.     

E. coli at Lake Superior Recreational Beaches

Increased usage of public beaches and heightened awareness of the need to monitor water for potential microbial contamination have led to passage of the U.S. Beaches Environmental Assessment and Coastal Health Act (BEACH Act) in 2000. This legislation calls for bacterial monitoring of recreational waters along the U.S. coastline, including the Great Lakes. The State of Wisconsin implemented this legislation in summer 2003, triggering extensive microbial monitoring of Lake Superior beaches. E. coli were measured at 27 beaches along Lake Superior, Wisconsin by defined substrate analysis. While E. coli concentrations were relatively low at these “cold water” beaches, monitoring did result in a few swimming advisories and beach closures (0.8% of total samples collected had E. coli concentrations that exceeded standards). Increasing water temperatures were not associated with increasing concentrations of bacterial contaminants. Location of sampling site and depth of water at the location where samples were taken did have an effect on detection of E. coli. Greater E. coli levels consistently were detected in shallower water and varied depending on the location of the sampling site horizontally across the beach. These findings support the notion that the concentration of E. coli in chest deep water may not be representative of E. coli concentrations in shallower water, closer to shore.     

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.     

Observations of Hydra from a Submersible at Two Deepwater Sites in Lake Superior

During a series of submersible dives at depths of 300 m and 400 m in southeastern Lake Superior, Hydra were observed attached to a variety of hard substrates including individual rocks, the edges of large boulders, and the undersurfaces of sandstone ledges. Although widespread, populations were generally found in distinct clumps. Based strictly on morphological features, the species resembled H. littoralis or H. carnea. The finding of Hydra at depths of 400 m in Lake Superior, the deepest sounding in the Great Lakes, would indicate that Hydra are not limited by depth.     

Inshore-offshore Distribution of Larval Fishes in Lake Superior off the Western Coast of the Keweenaw Peninsula,Michigan

We surveyed the larval fish community in Lake Superior off the western coast of the Keweenaw Peninsula, Michigan, as a first component in understanding how the Keweenaw Current affects larval fish distribution and survivorship. On transects at Ontonagon, Houghton, and Eagle Harbor, we collected larval fishes with a 1-m diameter plankton net towed through surface and deep (below metalimnion) waters at an inshore location (1 km from shore) and an offshore location (5–9 km from shore) during day and night in 1998 and 1999. The most abundant larvae caught were lake herring (Coregonus artedii), rainbow smelt (Osmerus mordax), burbot (Lota lota), deepwater sculpin (Myoxocephalus thompsoni), and spoonhead sculpin (Cottus ricei). Lake herring was generally most abundant at the surface during the day, while the other four species avoided the surface by day but not at night. Overall, larval fish density was greater inshore than offshore, with exceptions for particular locations and seasonal periods (1.24x for lake herring, 12.93x for rainbow smelt, 1.27x for burbot, 1.25x for deepwater sculpin, and 4.26x for spoonhead sculpin). Differences in the sizes of larvae between inshore and offshore locations, in conjunction with density patterns, suggest a seasonal inshore to offshore movement. Despite the presence of the Keweenaw Current, the overall distribution patterns of larval fishes follow those of previous studies conducted in the Great Lakes, but with lower densities.     

Temporal Dynamics of Nutrients (N and P) and Hydrology in a Lake Superior Coastal Wetland

Coastal wetlands on Lake Superior are hydrologically complex ecosystems situated at the interface of upland catchments and the oligotrophic lake. Little is known about nutrient dynamics within coastal wetlands or their role in modifying or contributing to nutrient fluxes from watersheds to Lake Superior. We conducted an intensive study of Lost Creek Wetland (LCW) near Cornucopia, WI, with the objective of determining influences of temporal variability in hydrology on dynamics and retention of N and P. We measured hydrologic inputs and distributions of inorganic and organic forms of nitrogen and phosphorus within LCW under hydrologic conditions ranging from summer base flow to spring snow melt. Our study confirms that the interrelationship between hydrologic connections to lake and tributary and seasonal variations in hydrology can regulate internal nutrient dynamics of coastal wetlands. The strength of hydrologic linkage of LCW to Lake Superior and tributary varied greatly among seasons, resulting in shifts in the relative importance of these nutrient sources and influencing spatial distribution of nutrients within the wetland. Ratios of inorganic nitrogen and phosphorus in the wetland were consistently low (< 16) indicating a potential for nitrogen limitation. Retention of inorganic nitrogen ranged from 11% to 94% and was positively related to hydraulic residence time which ranged from < 1 day during snow melt to 26 days in summer. Retention of total and soluble reactive phosphorus was generally lower than retention of inorganic nitrogen and was not related to hydraulic residence time.     

Changes in Lake Superior ice cover at Bayfield,Wisconsin

This research documents a 150-year record pertaining to the duration of closed navigation for Bayfield harbor. Data were gathered recording the opening and closing of navigation in Bayfield, Wisconsin from 1857–2007. Data were primarily collected from the Madeline Island Ferry Line and microfilmed copies of the Bayfield County Press. Analysis of the data indicates that the duration of ice cover on Lake Superior at Bayfield, Wisconsin has decreased over the past 150 years at the rate of approximately 3 days/decade or 45 days over the course of the study. During the past 150 years, the date that the last boat is able to navigate in the Bayfield harbor indicates the onset of ice cover. This date has occurred an average of 1.6 days later every decade. Conversely, the date that the first boat is able to navigate in Bayfield harbor marking the break up of ice cover has come to an average of 1.7 days earlier every decade. Although this represents the overall trend for the past century and a half, the most dramatic changes have occurred since 1975. During this period the ice season has begun an average of 11.7 days later and ended 3.0 days earlier every decade. Bayfield's ice season was compared to the lake's annual maximum ice concentration (AMIC) as compiled in a study by [Assel, R.A., Cronk, K., and Norton, D. 2003. Recent trends in Laurentian Great Lakes ice cover, Climatic Change 57: 185–204, 2003.] The fraction of the potential closed navigation season that the Bayfield harbor is ice covered decreased at a rate of 0.77% a year while the AMIC decreased at a rate of 0.39%/year during the period from 1964–2001. In general, the decline in the ice cover at Bayfield mirrors the pattern shown by the AMIC, suggesting that Bayfield's ice season could be used as a nonspecific indicator of overall lake trends.     

Changes in Marsh Area Along the Canadian Shore of Lake Ontario

Baseline marsh area is approximated for the Canadian shoreline of Lake Ontario west of the Bay of Quinte using maps dated from 1789 to 1962. The measured difference between that and the area of marsh indicated on topographical maps dated from 1977 to 1979 reveals a net loss of about 4,000 acres or 57% of the baseline area for 38 marshes. Using extrapolation for another 24 marshes, the net loss for 62 marshes is estimated at approximately 4,745 acres or 43% of the baseline area. Losses were particularly high for marshes west of and including Toronto. Estimated net loss was lower in absolute terms, but similar in proportion to that from a previous study which was based on waterfowl production habitat. Relative losses of marsh area along the Canadian shoreline of Lake Ontario are consistently higher than previous estimates for inland wetlands in counties fronting the same shoreline. This and other studies suggest that heavily settled Great Lakes environments have generally lost up to 75% of their baseline wetland area and almost 100% in a few cases. The use of historic maps provides an early baseline estimate of wetland area, in some cases the only estimate possible, but it is less subject to standardization, probably reflects emergent vegetation, and is somewhat cumbersome.     

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.     

Contemporary diets of Lake Superior lake whitefish off the Keweenaw Peninsula and changes in condition from the 1980s to 2010s

Over the last two decades, declines in lake whitefish (Coregonus clupeaformis) recruitment and growth in many areas of the Laurentian Great Lakes have raised concerns about the status of this important species. Although Lake Superior populations have been less affected than those in other Great Lakes, these populations still face multiple threats. We characterized lake whitefish diets collected off the Keweenaw Peninsula between 2015 and 2017 and compared results to previous Lake Superior studies. We additionally estimated length-weight relationships to determine whether lake whitefish body condition (i.e., expected weight-at-length) had changed since the 1980s. Diet diversity was low, although individual specialization was moderate to high. Fish transitioned from consuming Diporeia in the spring to Mysis and fish eggs during fall and winter; sphaeriids composed 20–30% of diets across all seasons. Compared to findings for other Lake Superior regions, lake whitefish diets comprised lower percentages of high energy items (e.g., Diporeia, Mysis) and higher percentages of low energy items (e.g., sphaeriids). Expected weights in the 2000s and 2010s were lower in the 400- and 500-mm length groups but similar in larger lengths groups compared to the 1980s; condition was highest across all lengths in the 1990s. The observed decline in condition since the 1990s in the 400- and 500-mm length groups, in combination with possibly greater consumption of less energetically profitable items, suggests that lake whitefish <600 mm or preferred prey resources in this lake region may be experiencing stressors leading to condition declines, although what these stressors are remain unknown.     

The Quagga Mussel Invades the Lake Superior Basin

Prior studies recognized the presence of a single dreissenid species in Lake Superior—the zebra mussel Dreissena polymorpha. However, taxonomic keys based on traditional shell morphology are not always able to differentiate dreissenid species with confidence. We thus employed genetic and morphological analyses to identify dreissenids in a major river-embayment of Lake Superior—the lower St. Louis River/Duluth-Superior Harbor—during 2005–2006. Our results revealed the presence of a second dreissenid species—the quagga mussel D. bugensis (alternatively known as D. rostriformis bugensis). Both species occurred in mixed clusters, in which zebra mussels outnumbered quagga mussels (20–160:1). The largest quagga mussel collected in 2005 was 26.5 mm long and estimated to be two years old, suggesting that the initial introduction occurred no later than 2003. Further monitoring is necessary to determine whether the quagga mussel will colonize Lake Superior. Our results indicate that the coupling of conventional morphological and molecular approaches is essential for monitoring dreissenid species.     

Prey selection by the Lake Superior fish community

Mysis diluviana is an important prey item to the Lake Superior fish community as found through a recent diet study. We further evaluated this by relating the quantity of prey found in fish diets to the quantity of prey available to fish, providing insight into feeding behavior and prey preferences. We describe the seasonal prey selection of major fish species collected across 18 stations in Lake Superior in spring, summer, and fall of 2005. Of the major nearshore fish species, bloater (Coregonus hoyi), rainbow smelt (Osmerus mordax), and lake whitefish (Coregonus clupeaformis) consumed Mysis, and strongly selected Mysis over other prey items each season. However, lake whitefish also selected Bythotrephes in the fall when Bythotrephes were numerous. Cisco (Coregonus artedi), a major nearshore and offshore species, fed largely on calanoid copepods, and selected calanoid copepods (spring) and Bythotrephes (summer and fall). Cisco also targeted prey similarly across bathymetric depths. Other major offshore fish species such as kiyi (Coregonus kiyi) and deepwater sculpin (Myoxocephalus thompsoni) fed largely on Mysis, with kiyi targeting Mysis exclusively while deepwater sculpin did not prefer any single prey organism. The major offshore predator siscowet lake trout (Salvelinus namaycush siscowet) consumed deepwater sculpin and coregonines, but selected deepwater sculpin and Mysis each season, with juveniles having a higher selection for Mysis than adults. Our results suggest that Mysis is not only a commonly consumed prey item, but a highly preferred prey item for pelagic, benthic, and piscivorous fishes in nearshore and offshore waters of Lake Superior.     

Pelagic Occurrence of Benthic Animals Near Shore in Lake Michigan

Collections with towed nets showed that presumably sedentary benthic animals occurred continually in the water above bottom in small numbers over depths of 6 m and 9 m in southeastern Lake Michigan. Densities were usually higher at night and in the months from June through September. Pelagic individuals at both depths consisted mainly of two chironomids, Saetheria tylus and Chironomus sp., and two naidids, Stylaria lacustris and Nais pardalis, but a total of 71 species were collected over a seven-month period. About 0.1% of the benthic animals were present above bottom at any time during a night in July 1974. The dominant pelagic species were the same as the benthic dominants at 6 m, but were much less important in the benthos at 9 m. We propose that this behavior, analogous to drifting of stream invertebrates, serves to maintain benthic populations in the frequently disturbed sediments of shallow (6 m deep or less) areas in Lake Michigan, and probably many other large lakes.     

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.     

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.