Lidar (light detection and ranging) and benthic invertebrate investigations: Migrating tailings threaten Buffalo Reef in Lake Superior

The low turbidity of northern Great Lakes waters allows lidar sufficient penetration to greatly aid environmental studies of coastal environments. On the Keweenaw Peninsula of Lake Superior, Big Traverse Bay provides an excellent example of mine tailings spreading from an old coastal release site. Between 1901 and 1932, two stamp mills (Mohawk and Wolverine) discharged 22.7 million metric tonnes of tailings (stamp sands) off the town of Gay. Along beaches, migrating stamp sands have dammed stream outlets, encroached upon wetlands, and contaminated recreational beaches. The tailings are now threatening benthic environments and critical commercial fish breeding grounds. Buffalo Reef is important for commercial and recreational lake trout and lake whitefish production (32% of the commercial catch in Keweenaw Bay, 22% of southern Lake Superior). Aerial photographs and five lidar over-flights emphasize: 1) the enormous amounts of stamp sands moving along the shoreline, and 2) large amounts migrating underwater across the bay towards Buffalo Reef. Differences between 2008 and 2016 lidar over-flights are used to quantify underwater stamp sand movement. For years, tailings have accumulated in an ancient riverbed cut (“trough”) just north of Buffalo Reef. Stamp sand overflow out of the “trough” is now moving into Buffalo Reef cobble fields, where fish drop eggs. Ponar sediment studies quantify % stamp sand in sand mixtures around Buffalo Reef, copper concentrations, and quantify impacts on benthic invertebrate taxa. Our study emphasizes that when large amounts of mine tailings are discharged into coastal environments, temporal and spatial impacts are progressive, threatening benthic organisms and fish.     

Sediment Toxicity from Copper in the Torch Lake (MI) Great Lakes Area of Concern

Torch Lake (MI) is a Federal Superfund Site and a Great Lakes Area of Concern. Torch Lake was impacted by over 200 million tons of copper rich mine tailings that were deposited in and along the shores of the lake. Twenty percent of the volume of the lake was displaced and the sediments have high concentrations of copper (1,000 mg/kg dry weight on average).Pore water from four sediment cores was analyzed at incremental depths for copper, total organic carbon, and toxicity using the Microtox^® 90% Comparison Test. Cores were also analyzed for copper and organic matter in the dry sediments. Statistical evaluation of data indicated that the upper sediments compared to the deeper sediments: 1) were less toxic (49% light loss versus 68%); 2) contained less pore water copper (0.59 mg/L vs. 0.81 mg/L); 3) had a higher percent organic matter (2.2% vs. 1.6%); and, 4) had no difference in the solid phase copper concentrations.Further evaluation of the sediment toxicity through direct comparison to copper chloride standards demonstrated that all pore water samples had reduced toxicity. The reduced toxicity of the pore water samples was reproduced by adding synthetic organic carbon to the copper chloride standards. These findings have implications for the EPA's No Action alternative for the sediments of Torch Lake. In making their recommendation, the EPA cites that preliminary data (believed to be obtained from nearby Portage Lake) shows that the sediments of Torch Lake are being covered and detoxified by natural sedimentation. However, total copper concentrations in the sediments from the south basin of Torch Lake do not indicate that the sediments are being covered and diluted by natural organic matter laden particles. Also, detoxification may be difficult to demonstrate without a baseline of sedimentation readings for comparison. In fact, solid phase copper concentrations may remain high due to scouring and erosion of surrounding stamp sand beaches or the steep side walls of the lake. This instability of particles has important implications for future restoration and monitoring activities in Torch Lake.     

Feeding ecology of age-0 lake whitefish in Saginaw Bay,Lake Huron

Age-0 lake whitefish Coregonus clupeaformis (11–160 mm total length) were collected from Saginaw Bay, Lake Huron during April–November 2009 and 2010 for diet analysis and for the evaluation of ontogenetic changes in feeding ecology. Lake whitefish ≤ 50 mm ate mainly zooplankton, after which their diets switched mainly to benthic macroinvertebrates. Cyclopoida were the dominant prey consumed by very small lake whitefish (< 17 mm) and the most frequently selected zooplankton type for individual small fish. Once lake whitefish reached 18–19 mm, Cyclopoida in the diet declined and cladocerans emerged as an important diet item. Daphnia were the most common cladoceran in the diets, but for fish 31–50 mm Bosminidae were also relatively important. Although the shift to Daphnia could represent an ontogenetic point when lake whitefish were large enough to effectively handle this prey, it also took place when the relative availability of Daphnia increased. Once lake whitefish were > 50 mm, Chironomidae larvae became a dominant prey item and this shift to benthivory coincided with a 55% increase in length-adjusted energy content between June and July. However, as fish grew (around 110–120 mm), Sphaeriidae and the benthic zooplankton Chydoridae became increasingly important in the diet. As these less energetically rich prey were incorporated into the diet, there were corresponding 21 and 15% decreases in length-adjusted energy content from July to August and September, respectively.     

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.     

Measures of larval lake whitefish length and abundance as early predictors of year-class strength in Lake Michigan

Many lake whitefish stocks in Lake Michigan have experienced substantial declines in growth and condition since the 1990s. Reduced growth and condition could result in reduced quality or quantity of eggs produced by spawning females, which in turn could negatively impact recruitment. We evaluated the potential for reduced recruitment by measuring early life stage density and length, and we discuss the utility of these measures as early indicators of lake whitefish year-class strength. Overall, mean larval density (number per 1000 m³ ± SE) in Lake Michigan was greater in 2006 (373.7 ± 28.3) than in 2005 (16.6 ± 24.8); whereas, mean length (mm ± SE) of larval lake whitefish was smaller in 2006 (12.87 ± 0.07) than in 2005 (14.38 ± 0.13). The ratio of zooplankton to fish density did not show an expected relationship with larval fish density or length. Rather, variation in larval density was best explained by a multiple-regression model that included larval length, spring wind intensity, and adult stock density as predictor variables. Our results suggest that the density of larval lake whitefish is not directly regulated by temperature or zooplankton density at the time of emergence, but that a potential for density-dependent regulation exists when larval emergence rates are high. We conclude that the observed declines in growth and condition of adult lake whitefish are not resulting in substantial reductions in recruitment.     

An initial view of subsurface Lagrangian observations in Lake Champlain: General patterns,cross-lake flow and coastal currents

Subsurface free-drifting floats operating in the Main Lake of Lake Champlain in 2002 and 2004 showed the presence of 1) small circular to elliptical motion away from coastal boundaries, 2) linear to curvilinear motion associated with alongshore currents and 3) subsurface westward cross-lake flow located within the base of the epilimnion and upper metalimnion (10–16 m) followed by large displacements up to 48 km by a coastal current. This subsurface westward flow is believed to be driven by linked upwelling and downwelling regimes on either side of the lake. Subsurface moorings deployed for ~ 34 days in 2007 and within 1 km of Whallon Bay's (NY) western shore showed that Kelvin wave dynamics and a high-speed subsurface jet located above the metalimnic core and > 600 m offshore could account for these large southerly transports. This subsurface jet may be linked to the cross-lake flow due to similar vertical positioning. The southern concave shape of Whallon Bay also appears to be the cause of high-speed northward-flowing currents within 400 m of the shore.     

Space-for-time proxy for climate change in deep lakes in the Canadian cordillera: Seasonality along a latitudinal climate gradient

A comparison of the annual thermal histories in seven deep, intermontane lakes in western Canada along a north–south transect from 49.67° to 61.25° N reveals latitudinal trends in thermal structure; specifically: 1) summer maximum and winter minimum mean temperatures decrease ~  0.2°C per degree latitude, 2) the dates of maximum and minimum heat content lag ~ 1 and ~  6 days per degree latitude, respectively, 3) the lengths of summer and winter stratification lag ~  8 days per degree latitude, 4) the timing of the spring and autumn turnovers lags ~  4 days per degree latitude, 5) the timing of the onset and break-up of ice cover lags ~ 6 days per degree latitude, and 6) the rate of response to atmospheric heating and cooling is ~  1MJ m^− 2 day^− 1 per degree latitude. We propose that insight into the response of a lake at a specific latitude to climate change can be acquired by examining the present conditions of a morphologically and dynamically similar lake at an appropriate latitude and altitude further south. Of interest is the potential transition of a lake from one mictic state to another, with consequences to the resupply of nutrients to the euphotic zone and to the phenology of spring and fall phytoplankton blooms. The absence or presence of ice cover and timing of freeze-up and break-up also affect a lake's heat budget and thermal history. The full interpretation of climate signals in lake systems requires an understanding of lake-specific physical forcing and response mechanisms.     

Increased Abundance and Depth of Submersed Macrophytes in Response to Decreased Turbidity in Saginaw Bay,Lake Huron

Submersed macrophyte communities and turbidity near shore were measured from 1991 to 1993 to determine if more light resulting from colonization of zebra mussels (Dreissena polymorpha Pallas) into Saginaw Bay in 1990 corresponded with changes in macrophyte distribution. Turbidity was sampled along five transects distributed at intervals perpendicular to the perimeter of inner Saginaw Bay to monitor changes in light available to plants in Saginaw Bay. Vegetation was sampled in July along these transects to determine the distribution and composition of the macrophyte communities each summer. We also measured the maximum depth of colonization and the area of plant coverage by use of 31 transects evenly distributed around Saginaw Bay in August. Turbidity decreased (P ≤ 0.097) at transects in northern littoral regions from 1991 to 1993 over submersed plant communities and uncolonized sediments, but not in southern littoral regions. The relative abundance of submersed macrophytes increased (P ≤ 0.0001) at all transects from 1991 to 1993, especially at transects where turbidity decreased significantly. Maximum depth of colonization (2.0 m) and the area of macrophyte coverage (101.3 km²) increased in Saginaw Bay, especially in the northwestern littoral region of the bay. Macrophytic chlorophytes, charophytes, and Vallisneria americana increased (P ≤ 0.003) in relative abundance most at transects where turbidity decreased significantly. These results demonstrate that even in a large well-mixed lacustrine environment, zebra mussels have the capacity to reduce turbidity sufficiently to allow submersed macrophytes to expand their distribution and abundance.     

Diets of Siscowet Lake Trout from the Apostle Islands Region of Lake Superior, 1993

Stomach contents of 384 siscowet lake trout (Salvelinus namaycush siscowet) from the Apostle Islands region of Lake Superior were examined to determine if diets exhibited seasonal trends or shifted with predator size. The information was also used to qualitatively compare siscowet and lean lake trout (S. namaycush) diets. Stomachs from siscowet less than 500-mm total length (TL) contained primarily macroinvertebrates (clams, Mysis relicta, and insects), sculpins (Cottidae; mean TL = 45 mm), and coregonines (Coregonus spp.; mean TL = 120 mm). Stomachs from siscowet between 500- and 699-mm TL contained items similar to smaller siscowet, but burbot (Lota lota; mean TL = 162 mm and a 254-mm siscowet were also present. Stomachs from siscowet longer than 699-mm TL contained fishes such as burbot (mean TL = 246 mm) and coregonines (mean TL = 240 mm). Dietary proportions of coregonines increased significantly with increasing siscowet length groups. The proportion of the total number of identified taxa and frequency of occurrence of terrestrial insects and sculpins declined significantly across all length groups between June and December. Siscowet and lean lake trout diets were not found to be substantially different and a potential for competition exists.     

A status assessment and review of the herpetofauna within the Saginaw Bay of Lake Huron

Amphibians and reptiles are key bioindicators of environmental health and habitat quality and can be used to provide baseline information to help assess habitat conditions and evaluate restoration success. In 2011 and 2012 we conducted comprehensive herpetological surveys throughout the Saginaw Bay area and assessed community composition, species richness, and spatial distribution. We also compared current distributions to historic observations and habitat conditions. A total of 25 taxa (13 species of reptiles and 12 species of amphibians) were observed within the Saginaw Bay area during this study. Herpetofauna were conspicuously unobserved in areas where Phragmites australis dominates the vegetation community and were concentrated in remaining areas of suitable habitat adjacent to Phragmites. Herpetofauna observations were clustered in areas where Phragmites and other invasive plant species were rare or absent. We were able to relate categorization of Phragmites invasion (i.e., 1 = 0–10% colonization detected, 2 = 10–50%, 3 > 50%) to both a biologically and statistically significant decrease in amphibian and reptile species richness. Our results indicate that Saginaw Bay can support a diverse herpetofauna community and there is potential to restore and improve this region for rare and common amphibian and reptile species. Removal of invasive plant species would greatly improve herpetofaunal communities within Saginaw Bay. Our results will help this region's resource professionals assess the quality of habitat and set goals for restoration of amphibian and reptile habitats.     

Lake Michigan sediment lead storage and history of loads

Dated sediment box cores collected in 1994–1996 from 52 locations in Lake Michigan were analyzed to assess storage, trends, and loading history of lead. The results of this study provide information of historic lead loads to the lake for a time period (pre-1960) for which no reliable lead measurements exist. The information can be utilized by those wishing to model lead and to access lead loading trends. Anthropogenic lead storage in the lake's sediments totaled 143,000 t as of 1994. Storage of acid-extractable total (anthropogenic + background) lead totaled 171,000 t between 1850 and 1994. The date of 1850 is the time at which lead loads increased above background loads (219 t/y) to the lake. Anthropogenic loads peaked between 1959 and 1962 at 2440 t/y and were 1170 t/y between 1994 and 1995, illustrating that at the time of collection in 1994, loads were decreasing from previous highs. The load in 1994 to 1995 was equivalent to the load during the time frame of 1922 to 1925. Largest lead loads were to southeastern Lake Michigan in a region downwind of Chicago, illustrating the impact of large populated areas utilizing coal and gasoline on lead loads to the lake. Loading trends were impacted by coal and gasoline consumption, increased industrial activity during World War II, the Clean Air Act of 1970, and the phase-out of leaded gasoline.     

Thirty-year update: Changes in biological characteristics of degraded muskellunge nursery habitat in southern Georgian Bay,Lake Huron,Canada

Aquatic vegetation is a critical component of nursery habitat for young-of-the-year (YOY) muskellunge. The trophy status of the muskellunge fishery in southeastern Georgian Bay owes its reputation to the widespread distribution of aquatic vegetation in coastal marshes of this region. Unfortunately, wetland habitat has been in decline because of an unprecedented period of sustained low water levels since 1999. In this study, we strategically re-sampled 16 historic sites that supported YOY muskellunge in 1981. The sustained low water levels and increased shoreline modifications experienced by southeastern Georgian Bay may have contributed to the current disappearance of YOY muskellunge at those sites. These physical stressors appeared to have altered the habitat structure of the plant community and led to changes in fish communities, making them no longer suitable for YOY muskellunge. The precise mechanisms limiting survival to the YOY stage are unknown because spawning adults have been observed in the area in the spring of 2012 and 2013. These results corroborated previous sampling programs at the historic sites (2004–2005: n = 8 and 2007: n = 16) that employed other fishing gears and protocols as well as a supplemental YOY sampling in 2013 (n = 26 additional sites). If this muskellunge population is to remain self-sustaining, a complementary management strategy specifically developed for Georgian Bay is required. The strategy should identify and ultimately protect suitable muskellunge breeding habitat by accounting for the unique geomorphology, current physical stressors affecting Georgian Bay, and the biological links between suitable spawning and nursery habitats.     

Reproductive Potential and Fecundity of Lake Trout Strains in Southern and Eastern Waters of Lake Ontario, 1977–1994

We assessed the reproductive potential of various genetic strains of hatchery lake trout (Salvelinus namaycush) in southern and eastern Lake Ontario from indices of fecundity and indices of male abundance. Indices were constructed from catches of mature lake trout in gill nets during September 1980 to 1994 after correcting for mortality from sea lampreys (Petromyzon marinus) which occurred between September sampling and late fall spawning. Strain and age were assigned to individual lake trout based on clipped fins and maxillary bones or coded wire tags. Fecundity-length relationships for fish of the same age, determined from mature females collected in 1977 to 1981 and 1994, were not different (P > 0.05) among genetic strains. For all strains combined, fecundity-length relationships in 1977 to 1981 were not different among fish of various ages but in 1994, age-5 and -6 fish had fewer eggs (P < 0.003) than age-7 fish, and age-7 fish had fewer eggs (P < 0.003) than fish of age 8, 9, or 10. Annual indices of fecundity varied 19 fold and indices of mature males varied 11 fold; both indices were low in the early 1980s, increased sharply in the mid 1980s, and peaked in 1993. The strain which dominated fecundity and mature male indices shifted during the study from Seneca Lake strain to Lake Superior strain and then back to Seneca Lake strain. However, changes in either reproductive potential or genotypes do not appear responsible for the abrupt appearance of naturally-produced yearling lake trout throughout southern and eastern Lake Ontario in 1994–1995, the first widespread occurrence of juveniles produced by hatchery lake trout in Lake Ontario.     

Association of Low Reproductive Rates and High Contaminant Levels in Bald Eagles on Green Bay,Lake Michigan

Bald eagles (Haliaeetus leucocephalus) nesting on the shores of Green Bay, Lake Michigan, had reproductive rates significantly lower than those of neighboring eagles nesting in inland Wisconsin (0.55 vs. 1.1 young per occupied territory). This study investigated effects of two factors which have depressed eagle reproductive rates at other locations: exposure to organochlorine contaminants and low food availability. Levels of dichloro-diphenyl-dichloroethylene (DDE) and total polychlorinated biphenyls (PCBs) in addled eggs and eaglet blood samples from Lake Michigan and inland Wisconsin reference sites were measured. An index to food availability, the food delivery rates by parent eagles to nestlings, and other behavioral indices that have been associated with food availability, were measured and compared to inland reference data. Mean contaminant concentrations in addled eggs from Green Bay were 8.3 μg/g wet weight DDE and 31.3 μg/g wet weight total PCBs (1987 to 1996, n = 9). Mean concentrations in nestling blood plasma were 53 μg/kg wet weight DDE and 207 μg/kg wet weight total PCBs (1987 to 1995, n = 8). Indices of food availability were generally normal in comparison to inland reference data. Mean food delivery rate to the nestlings was 2.55 items per nestling per day (n = 7 nests). None of the behavioral measures differed significantly from inland reference values, suggesting that prey was adequate. It was concluded that organochlorine contaminants caused all or most of the depression in reproductive rates of Green Bay bald eagles.     

Modelling of nearshore microbial water quality at confluence of a local tributary in Lake St. Clair

Microbial water quality, measured as Escherichia coli (E. coli) concentration, at beaches along the southern shore of Lake St. Clair in Canada, often exceeds public safety guidelines. Belle River, located near a public beach and a drinking water intake, is one of the several smaller tributaries of the lake whose contribution to nearshore microbial water quality is currently unknown. A flexible mesh 3D coupled TUFLOW-FV and Aquatic Ecodynamic (AED2+) model was used to simulate the hydrodynamics and microbial water quality in Lake St. Clair. A higher resolution nested model was developed within the lake-wide TUFLOW-FV model for better spatial and temporal resolution in the local region surrounding Belle River. Regular and up to a factor of four difference in predicted E. coli concentrations were observed with the nested and lake-wide models at the public beach next to Belle River, whereas the difference was marginal at the drinking water intake about a kilometre away from the shore. While the E. coli loading to Lake St. Clair from Belle River is considered negligible, >90% of the predicted daily E. coli concentration at the beach and > 50 % at the water intake were attributed to Belle River from amongst all watershed sources to Lake St. Clair considered in the model. The model results also show that the construction of a new 150 m jetty in 2018, replacing the older 25 m jetty separating Belle River from the public beach, is expected to significantly reduce E. coli concentrations observed at the beach.     

Characteristics of a Lake Sturgeon Spawning Population Sampled a Half Century Apart

A lake sturgeon spawning assessment study conducted in the late 1940s below a dam on the Ottawa River, Canada was repeated over a 3-year period in 2001–2004. The objectives of the survey were to determine whether lake sturgeon, a long-lived species, continue to congregate at this location during the spawning period and to assess changes in the characteristics of the spawning population since 1949. Eighty-three lake sturgeon were caught, including 10 recaptures, over the 3-year survey with the majority of sturgeon sampled in 2003. The Schnabel population estimate for the 2003 spawning stock was 202 (93–378; 95% C.I.). Mean size of lake sturgeon sampled in the current survey (118.0 ± 12.8 S.D.) was greater than in the historical survey (101.7 ± 11.5 S.D.). However, lake sturgeon < 110 cm TL comprised only 31.1% of the sturgeon sampled in this survey whereas they comprised the majority of the catch in 1949 (69.9%), suggesting the population is experiencing a recruitment problem. Weight-length relationships of lake sturgeon did not vary between studies. Growth differed between studies which may be a function of aging error.     

Ontogenetic and Seasonal Patterns in the Energy Content of Piscivorous Fishes in Lake Superior

Ontogenetic, seasonal, and sex-related patterns in energy density (J/g wet mass) was studied in burbot (Lota lota), lean lake trout (Salvelinus namaycush namaycush) and siscowet lake trout (Salvelinus namaycush siscowet) collected from Lake Superior in 1996 and 1997. Energy density was strongly negatively correlated with water content for all fishes (r² = 0.86 to 0.99). For fishes of comparable size, energy density of siscowet (10.8 kJ/g) was significantly higher than lean lake trout (7.8 kJ/g) and both were significantly higher than burbot (5.1 kJ/g). Energy density in spring was higher in lean and siscowet lake trout, and lower in burbot than in other seasons. No significant differences were apparent between sexes within a species. Energy density increased in a predictable fashion with body mass for lean and siscowet lake trout, but varied without trend for burbot. The regression common to the three species (kJ/g wet mass = 36.78 – 0.41(% water), p < 0.001) provides a straightforward method for estimating ontogenetic and seasonal energy density in these fishes.     

Enterococcus phages as potential tool for identifying sewage inputs in the Great Lakes region

Bacteriophages are viruses living in bacteria that can be used as a tool to detect fecal contamination in surface waters around the world. However, the lack of a universal host strain makes them unsuitable for tracking fecal sources. We evaluated the suitability of two newly isolated Enterococcus host strains (ENT-49 and ENT-55) capable for identifying sewage contamination in impacted waters by targeting phages specific to these hosts. Both host strains were isolated from wastewater samples and identified as E. faecium by 16S rRNA gene sequencing. Occurrence of Enterococcus phages was evaluated in sewage samples (n = 15) from five wastewater treatment plants and in fecal samples from twenty-two species of wild and domesticated animals (individual samples; n = 22). Levels of Enterococcus phages, F + coliphages, Escherichia coli and enterococci were examined from four rivers, four beaches, and three harbors. Enterococcus phages enumeration was at similar levels (Mean = 6.72 Log PFU/100 mL) to F + coliphages in all wastewater samples, but were absent from all non-human fecal sources tested. The phages infecting Enterococcus spp. and F + coliphages were not detected in the river samples (detection threshold < 10 PFU/100 mL), but were present in the beach and harbor samples (range = 1.83 to 2.86 Log PFU/100 mL). Slightly higher concentrations (range = 3.22 to 3.69 Log MPN/100 mL) of E. coli and enterococci when compared to F + coliphages and Enterococcus phages, were observed in the river, beach and harbor samples. Our findings suggest that the bacteriophages associated with these particular Enterococcus host strains offer potentially sensitive and human-source specific indicators of enteric pathogen risk.     

Values of Sediment Oxygen Demand Measured in the Central Basin of Lake Erie, 1979

The results of sediment oxygen demand (SOD) measurements for the central basin of Lake Erie, 1979, are presented. Two chambers were used. One, a triangular chamber, has a mixing velocity of 5 cm/sand gives values for SOD of 0.86 ± 0.42 gm²/d (n = 52). The second chamber, a hemispherical dome with gentle mixing, gives values of 0.32 ± 0.11 g/m²/d (n = 13). There are no significant differences in measured values between two stations, located 50 km apart, when measurements from the same chamber design are compared. There are no measurable effects of photosynthesis when daytime values are compared with nighttime values or when light and dark chambers are compared. A comparison of these two SOD values with rates observed for hypolimnetic oxygen decline in the water column shown that the value measured by the dome (0.32 gm/m²/d) is the most plausible value. It is hypothesized that the fluid mechanics of the triangular chamber do not properly emulate the hydrodynamics of the lake, causing the higher values.     

Evaluation of Bottom Trawls as Compared to Acoustics to Assess Adult Lake Herring (Coregonus artedi) Abundance in Lake Superior

We compared density estimates from day bottom trawl tows against night midwater trawl tows and acoustic gear to test the hypothesis that adult lake herring (≥ 250 mm) are underestimated by day bottom trawl tows during the annual USGS spring fish community survey in Lake Superior. We found average density at nine nearshore stations was significantly higher at night (21.3 adult fish/ha) compared to day (1.0 adult fish/ha; p = 0.0119). At nine offshore stations, no lake herring were captured during the day but density averaged 39.6 adult fish/ha at night. At a lakewide scale (n = 18 stations), precision (relative standard error) was much better using night midwater trawls and acoustic gear (37%) compared to day bottom trawls (100%). Moderate sample size increases using the former methodology would likely bring precision within recommended levels (≤ 30%) for stock-recruit data sets. Our results suggest that 1) population abundances of adult lake herring in Lake Superior are much higher than previously considered, 2) the annual spring fish community survey may not provide a relative index of abundance of adult lake herring, 3) night midwater trawls and acoustic gear are necessary for assessing adult lake herring abundance, and 4) previous studies using lake herring data from the annual spring fish community survey need to be re-evaluated in light of these results. Lake herring appear to become progressively more pelagic and less susceptible to bottom trawling as they mature. Day bottom trawls appear to be an adequate tool for estimating relative density of age-1 recruits, although this method still suffers from relatively poor precision.