Bio-optical properties and primary production of Lake Michigan: Insights from 13-years of SeaWiFS imagery

Thirteen years of SeaWiFS data (1998–2010) from the early spring isothermal period (March–April) were used to determine trends of water attenuation coefficient (K_dPAR), chlorophyll a (Chl a), Photosynthetic Available Radiation (PAR), and modeled primary production in southern Lake Michigan. Surface PAR values remained unchanged between 1998 and 2010, but there was an 18–22% drop in K_dPAR during the March/April isothermal period as water clarity increased. This transparency increase was accompanied by a 41–53% decline in Chl a concentration (μg · L^− 1) and a 42–46% decline in modeled primary production (Great Lakes Primary Production Model). These changes were most pronounced in 2001–2003 which coincided with the period of initial colonization of the quagga mussels. Statistically significant spatial differences were noted in Chl a (μg · L^− 1) concentrations between mid-depth (z = 30–90 m deep), and offshore (z > 90 m deep) waters. Chl a concentrations in the mid-depth region (30–90 m) decreased at a higher rate compared to offshore waters (> 90 m) likely as a result of filtration activities of quagga mussel.     

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.     

Sensitivity of a sediment transport model for Lake Michigan

A two-dimensional (vertical and cross-shore) sediment transport model was applied to several transects in southern Lake Michigan using observations of waves and currents recorded during the spring of 2000. Conditions during this period included several storms that are among the largest observed in the lake. The observations were used to examine the sensitivity of the model to variations in the input parameters (waves, currents, initial bottom sediment size distribution, settling velocity, and bottom stress required for erosion). The results show that changing the physical forcing (waves and currents) or the initial bottom sediment size distribution affected the results more than varying the particle properties (settling velocity and critical shear stress) or the size classes used to describe the size distribution. This indicates that for this model specification of input parameters are of first order importance and should be specified with some confidence before adding additional complexity by including processes such as flocculation and bed consolidation.     

长期预报应用分析与实况验证

本文较详细地阐述了长期洪水定量预测中的经验和教训,讨论了定性预报与定量预报二者之间的关系。     

The Lake Michigan Charter Fishing Industry: a Product of Love and Taxes

Little is known about the Lake Michigan charter fishing industry. This study was conducted to describe Lake Michigan charter fishing boat operators and their customers and to evaluate the industry's financial status and local economic impact. Field work in 1973 revealed 98 charter operators working on Lake Michigan from Wisconsin ports. From a geographically proportionate sample, 44 extensive interviews were completed. The industry generated $670,000 in sales in 1973. Low average sales and profits do not provide financial incentives to entrepreneurs. Financial opportunity, however, appears to have played a minor role in the decision to enter business. Major motivations appear to be tax advantages and life style rewards. A profile of charter customers was developed through a mail survey of a sample of 483 charter customers. Catch data were collected from operators surveyed. Catch varied with the proportion of operator income obtained from charter fishing, Port communities were affected economically by the industry. Applying a community multiplier of 2.16 to the $1.6 million of direct expenditures yields an economic impact of $3,456,000 on Wisconsin's Lake Michigan community in 1973. The industry is also important in that it provided fishermen with access to the Great Lakes lake trout and salmon fisheries. Without that access, these fisheries would be largely reserved for the wealthy.     

Monitoring Energetic Status of Lake Michigan Chinook Salmon Using Water Content as a Predictor of Whole-fish Lipid Content

Monitoring health indicators of fish populations can be an expensive and time consuming process. This study analyzed energy dynamics of Lake Michigan Chinook salmon using proximate composition analysis with the goal of determining an efficient method for monitoring the nutritional status of the population. Condition factor performed poorly as an indicator of whole-fish lipids (r² = 0.07). Water content in a dorsal muscle plug was found to be correlated with whole-fish lipids (r² = 0.50) for all samples. For the subset of samples that included small fish collected in the spring, the strength of the relationship between water content in a dorsal muscle plug and whole-fish lipids increased (r² = 0.70). The metric of water content in a dorsal muscle plug was determined to provide an adequate surrogate of whole-fish lipid content and, therefore, overall nutritional status. We propose a monitoring program that involves collecting small individuals in the spring and reporting the proportion of samples with over 78% water content in muscle tissue. Small individuals collected in the spring had the lowest whole-fish lipid levels of any segment of the population and would be the most prone to nutritional stress; therefore we recommend focusing on them for monitoring.     

An Economic Analysis of Integrated Fisheries Management: The Case of the Lake Michigan Alewife and Salmonid Fisheries

Intense predator-prey relationships exist between stocks of alewives and salmonids in Lake Michigan. In this paper, the economic implications of these biological interactions are explored and suggestions regarding efficient fisheries management options are made. A theoretical optimal control model is developed which delineates conditions for optimizing alewife and salmonid harvest rates. Model results indicate that the indirect effects which harvesting in one fishery have on harvest costs and returns in the other must be considered if economic efficiency in alewife/salmonid management is to be achieved. Furthermore, marginal net social values associated with alewife and salmonid harvests must be equated. It is estimated that alewives currently have a marginal net social value close to zero compared to $4.10 per salmonid. These findings strongly suggest that utilization of alewife biomass as a forage base for predatory salmonids is more efficient than direct harvesting of alewives in a commercial fishery.     

Mapping bottom substrate in Illinois waters of Lake Michigan: Linking substrate and biology

Habitat plays an essential role in shaping aquatic communities yet limited information on the type and distribution of bottom features is available in the form of detailed maps. This is especially apparent in large systems where obtaining such information can be both expensive and challenging. Current maps of Lake Michigan substrate are very general and lack suitable detail of substrate composition and geographic extent of rocky areas. This limits our ability to link biological processes, such as fish spawning, to nearshore habitat and makes it difficult to target structurally complex habitats for sampling. We compiled substrate information gathered over 72 years for Illinois waters and incorporated it into a GIS framework to generate more complete documentation of sediment type and particle size distribution in southwestern Lake Michigan. Sediment data for 1682 sites were standardized to phi units; natural neighbor interpolation was used during GIS analysis to predict sediment type in areas lacking data. Predicted sediment values based on this interpolation had a significant positive relationship with observed sediment values (R² = 0.92). Further, we linked existing fishery (yellow perch Perca flavescens) data with newly generated substrate information to identify potential associations between spawning site selection and habitat. Tagging data indicated that substrate quality affected the likelihood of yellow perch returning to their release sites in subsequent spawning seasons; higher fidelity was associated with larger, coarser substrate. The generated map will be a useful tool to enhance our understanding of habitat's importance in the Great Lakes, particularly when linked with fishery data.     

Resolution of optical gradients and pursuit of optical closure for Green Bay,Lake Michigan

Optical properties have fundamental importance to water quality, ecology, and remote sensing initiatives. Paired measurements of optically active constituents (OACs), and inherent optical properties (IOPs) and apparent optical properties (AOPs), were made in September 2010 across the optical gradients of Green Bay, extending from the Fox River to Sturgeon Bay (8 sites), and for three near-shore locations in the main basin of Lake Michigan. The array of laboratory and in situ measurements provided a robust characterization of the underwater and emergent light fields of these waters with respect to magnitudes and spectral features of the OACs, IOPs and AOPs. These measurements resolved the character and possible origins of the major gradients within the bay (5 to 10-fold differences) and the substantial differences between the bay and the main basin. The credibility of the characterizations was supported through closure analyses which demonstrated: (1) the approach to equivalence between various field and laboratory measurements, and (2) good matches of AOP observations by values predicted from measured IOPs using accepted radiative transfer expressions. The bay was demonstrated to be an optically complex case 2 system, with uncoupled variations along the spatial gradient(s) in OACs of phytoplankton biomass, colored dissolved organic material, and non-algal particulates. The documented spatial differences in optical properties rival those reported in much larger marine surveys. Radiative transfer expressions are used to predict changes in AOPs of the downwelling (underwater) attenuation coefficient and remote sensing signal in response to scenarios of changes in levels of OACs of potential ecological and management interest.     

Movement patterns of smallmouth and largemouth bass in and around a Lake Michigan harbor: The importance of water temperature

Smallmouth bass Micropterus dolomieu and largemouth bass M. salmoides in southwestern Lake Michigan use shallow, warm harbors for spawning during spring. After the reproductive period ends in early summer, catch rates from standardized sampling of smallmouth bass in harbors decrease. Fishes are presumed to use the main lake during summer but little is known about how black basses use main-lake habitat in Lake Michigan or mechanisms driving this transition. We tracked smallmouth bass (N = 26) and largemouth bass (N = 8) using radio and acoustic telemetry during 2005–2006 near North Point Marina, IL. A temperature difference persisted between inside and outside the harbor for much of May–October where harbor temperatures were generally warmer than those outside the harbor. Smallmouth bass responded to water temperature changes, inhabiting the harbor until temperatures outside the harbor approached 18.5 °C, at which time they left the harbor. Frequent temperature fluctuations of > 3 °C occurred outside the harbor within 24-hour periods. Sudden reductions in water temperature were associated with smallmouth bass temporarily returning to the harbor until the temperature outside the harbor again approached 18.5 °C. As water cooled during fall, smallmouth bass again returned to the harbor. Largemouth bass exhibited comparatively restricted movements during this time and rarely ranged outside the harbor. Thus, home range estimates for smallmouth bass (142.6 ha) were an order of magnitude greater than those of largemouth bass (12.9 ha). Both water temperature and species played a role in determining the degree of movement outside the harbor but both black basses used the harbor as a thermal refuge.     

Diet of lake trout and burbot in Northern Lake Michigan during spring: Evidence of ecological interaction

We used analyses of burbot (Lota lota) and lake trout (Salvelinus namaycush) diets taken during spring gill-net surveys in northern Lake Michigan in 2006–2008 to investigate the potential for competition and predator–prey interactions between these two species. We also compared our results to historical data from 1932. During 2006–2008, lake trout diet consisted mainly of alewives (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax), whereas burbot utilized a much wider prey base including round goby (Neogobius melanostomus), rainbow smelt, alewives, and sculpins. Using the Schoener's diet overlap index, we found a higher potential for interspecific competition in 1932 than in 2006–2008, though diet overlap was not significant in either time period. No evidence of cannibalism by lake trout or lake trout predation on burbot was found in either time period. In 2006–2008, however, lake trout composed 5.4% (by weight) of burbot diet. To determine whether this predation could be having an impact on lake trout rehabilitation efforts in northern Lake Michigan, we developed a bioenergetic-based consumption estimate for burbot on Boulder Reef (a representative reef within the Northern Refuge) and found that burbot alone can consume a considerable proportion of the yearling lake trout stocked annually, depending on burbot density. Overall, we conclude that predation, rather than competition, is the more important ecological interaction between burbot and lake trout, and burbot predation may be contributing to the failed lake trout rehabilitation efforts in Lake Michigan.     

Vertical distribution of larval fish in pelagic waters of southwest Lake Michigan: Implications for growth, survival, and dispersal

Due to variability in biotic and abiotic conditions along a vertical gradient within aquatic systems, the vertical distribution of larval fish can profoundly affect their growth and survival. In large systems such as the Great Lakes, vertical distribution patterns also can influence dispersal and ultimately settlement events. The objective was to describe the diel vertical distribution of the larval fish community in the pelagic waters of Lake Michigan and determine which biotic and abiotic factors most strongly influence their vertical distribution. To determine vertical distribution, the upper 27 m of the water column was divided into six discrete depth bins. Larval fish sampling was conducted within each of these depth bins on seven occasions during both day and night. Temperature, light intensity, and prey density also were recorded at depths corresponding to larval fish sampling. Larval fish from five species were collected during the study: alewife (Alosa pseudoharengus), bloater (Coregonus hoyi), burbot (Lota lota), deepwater sculpin (Myoxocephalus thompsonii), and yellow perch (Perca flavecens). Among the five species, we observed three general patterns of depth distribution. Alewife and yellow perch were restricted to the upper strata, whereas the opposite trend was observed for deepwater sculpin. Bloater and burbot larvae were more evenly distributed throughout the upper 27 m, and their pattern of vertical distribution changed between diel periods. Our analysis suggests abiotic factors were more important than biotic factors in structuring the vertical distribution of larval fish in southwestern Lake Michigan, with temperature having the largest influence on distribution of larvae.     

Estimates of fishing and natural mortality rates for four Lake Whitefish stocks in Northern Lakes Huron and Michigan

We analyzed tag-recovery data to estimate instantaneous fishing (F) and natural mortality (M) rates of four lake whitefish stocks in lakes Michigan and Huron during 2004–2007. We tagged and released 22,452 adult lake whitefish of which 8.7% were subsequently recovered. Annual tag-reporting rates ranged from 17.8% to 56.2%. Tag retention was high for the first 5–6 months after tagging, but tag loss increased substantially thereafter. Nine tag-recovery models were evaluated with respect to whether F and/or M varied among stocks, lakes, or years. There was support for three models based on Akaike information criteria. The best model had yearly and stock-specific estimates of F of 0.03 to 0.79 and lake-specific estimates of M of 0.35 for Lake Michigan and 0.60 for Lake Huron. The second best model had yearly and stock-specific estimates of F of 0.04 to 0.71 and a constant estimate for M of 0.52. The third model had yearly and stock-specific estimates of F of 0.04 to 0.85 and stock-specific estimates of M of 0.32 to 0.67. Model-averaged estimates of F ranged from 0.04 to 0.78 and were substantially different than statistical catch-at-age estimates of F. Model-averaged estimates of M ranged from 0.40 to 0.59 and were greater than estimates obtained from prediction equations, possibly due to sea lamprey-induced mortality. We recommend that tag-recovery estimates of F and M be used as Bayesian priors in future lake whitefish stock assessments to help refine mortality estimates for the stocks.     

平面网格结构拟膜分析法及其参数验证

建立了一种正交斜放网格结构的单胞模型,并将这种网格结构比拟成膜,利用能量等效计算出拟膜的弹性参数计算公式。同时,分析了将这种结构固结于各向同性基体材料中形成的复合材料的弹性参数的计算公式。用数值分析验证了公式的正确性。     

Paleomagnetism of Sediment Cores from Cedarburg Bog,Wisconsin, and a Comparison with Cores from Lake Michigan

Four 10-m sediment cores from the Cedarburg Bog, Wisconsin, were collected for paleomagnetic measurement and carbon-14 dating. The paleomagnetic records for these cores are self consistent, with the intensity record affording the best correlation. These results are compared with sediment cores from Lake Michigan. The two sets of cores are unmistakably correlatable on the basis of intensity. The inclination correlation also shows common long and short wavelength features.On the basis of this correlation, we date certain member boundaries of the Lake Michigan Formation as follows: Waukegan–Lake Forest, 5,500 years B.P.; Lake Forest–Winnetka, 7,800 years B.P.; and Winnetka–Sheboygan, 9,700 years B.P.     

Simulation of wind-driven circulation and temperature in the near-shore region of southern Lake Michigan by using a channelized model

To enhance the efficiency of a pathogen forecasting model in the beach areas of southern Lake Michigan and to reduce the computation time, the near-shore current is approximated as a channelized flow parallel to the shorelines in clockwise or anti-clockwise direction within the accuracy tolerance range. A channelized model with a curvilinear boundary can significantly reduce the computation effort, and at the same time achieve a good agreement between the predicted and measured water surface elevations, currents, and water temperatures. The sensitivity analysis results show that the suitable channel width for the near-shore region of southern Lake Michigan should be no less than 10 km. The modeling results of the water temperature are much less sensitive to the channel width than those of the current velocity and the water surface elevation. The modeling results also show a close correlation between the speeds of the wind and the near-shore current. The current may fully respond the wind stress with a time lag of several hours. The correlation may provide an approximate estimation of the lake circulation under some wind conditions for a practical forecasting purpose. More complex wind-current relationships need to be described with a more sophisticated hydrodynamic model. This verified model can be used for the pathogen forecasting in the near-shore regions of southern Lake Michigan in the future.     

Temperatures and Currents in a Stratified Lake: A Two-Dimensional Analysis

Two-dimensional, time-dependent numerical models are used to predict the temperatures and currents in a stratified lake. In this study, (1) essential features of the observed distributions of temperatures and currents in large, stratified lakes, especially Lake Erie, are reproduced, (2) the effects of various governing parameters such as heat flux to the water, wind stresses, and turbulence are examined, and (3) the effects of changes in various parameters appearing in the turbulent eddy coefficients are investigated. The emphasis is on the general characteristics of thermocline formation, maintenance, and decay and therefore the time scales of interest are weeks and months rather than hours or even days.     

SWAT模型在呼伦湖流域上的河网地形上的分析应用

为了高效简便地提取河网信息,文中以呼伦湖流域克鲁伦河为例,运用SWAT模型,对克鲁伦河进行河网划分,并对克鲁伦河国内部分进行地形分析。结果表明:当集水面积阈值为100 km2时,划分结果与流域实际最为接近;克鲁伦河国内部分大部分地势平坦,不易造成水土流失;北岸草原腹地太阳辐射较强,促进植物生长,同时也会引起土壤干旱。