Southern Lake Michigan Research Articles

Abiotic and biotic correlates of yellow perch recruitment to age‐2 in southern Lake Michigan, 1984–2007

Abstract Seven abiotic and biotic variables were tested to determine whether they influence the recruitment to age two of yellow perch,Perca flavescens(Mitchill), in waters of southern Lake Michigan, USA. Recruitment was analysed using a Ricker stock‐recruitment relationship that accounts for density dependency. Significant model variables tested individually and ranked by total variance explained included the abundance of sexually mature yellow perch, alewife,Alosa pseudoharengus(Wilson) and spottail shiner,Notropis hudsonius(Clinton). Variables found unrelated to recruitment or generally less explanatory included the abundance of round goby,Neogobius melanostomus(Pallus), mean water temperature, variation in water temperature and water clarity. The best‐fitting multivariable model explained 69% of the recruitment variability and included abundances of sexually mature yellow perch, alewife and spottail shiner. These results suggest that yellow perch recruitment in southern Lake Michigan is regulated in part by biotic interactions with other species of the near‐shore community, including alewife and spottail shiners, in addition to the abundance of reproductively mature yellow perch.

A year of internal Poincaré waves in southern Lake Michigan

A unique set of full year, deep water observations from the middle of Lake Michigan's southern basin are analyzed to quantify the seasonal variability of the dominant near‐inertial internal Poincaré wave. At this mid‐lake location, the Poincaré wave is seen to describe more than 80% of the observed surface current variability for much of the year, with characteristic near‐inertial frequency and clockwise‐rotating velocities. The dominance of the near‐inertial seiche on the flow decreases with depth. The wave persists during the “stratified period,” roughly May through late December, and is supported by as few as 1–2 degrees of thermal stratification over 150 m; only after complete water column mixing does the wave go dormant for January through April. The strongest Poincaré wave activity is seen to correspond to the period of strongest summer thermal stratification (August), in spite of the relatively weak winds at this time. A simple inertial slab model optimized with linear friction is shown to capture the seasonal variability of the near‐inertial energy at this location reasonably well. The vertical structure of the wave shows good agreement with that calculated with a standard normal modes formulation, which is in turn used to characterize the potential shear and mixing caused by the wave. Late‐spring and summer events of elevated Poincaré wave activity are shown to generate sufficiently strong shear with persistent periods of sub‐1 Richardson numbers within the thermocline, suggesting that the near‐inertial seiche is likely generating thermocline instabilities in the lake's interior.

The sedimentary and palynological records of Serpent River Bog, and revised early Holocene lake-level changes in the Lake Huron and Georgian Bay region

Serpent River Bog lies north of North Channel, 10 m above Lake Huron and 15 m below the Nipissing Great Lake level. A 2.3 m Holocene sequence contains distinct alternating beds of inorganic clastic clay and organic peat that are interpreted as evidence of successive inundation and isolation by highstands and lowstands of the large Huron-Basin lake. Lowstand phases are confirmed by the presence of shallow-water pollen and plant macrofossil remains in peat units. Twelve 14C dates on peat, wood and plant macrofossils combined with previously published 14C ages of lake-level indicators confirm much of the known early Holocene lake-level history with one notable exception. A new Late Mattawa highstand (8,390 [9,400 cal]–8,220 [9,200 cal] BP) evidenced by a sticky blue-grey clay bed is tied to outburst floods of glacial Lake Minong during erosion of the Nadoway drift barrier in the eastern Lake Superior basin. A subsequent Late Mattawa highstand (8,110 [9,040 cal]–8,060 [8,970 cal] BP) is attributed to enhanced meltwater inflows that first had deposited thick varves throughout Superior Basin. Inundation by the Nadoway floods and possibly the last Mattawa flood were likely responsible for termination of the Olson Forest (southern Lake Michigan). A pollen diagram supports the recognized progression of Holocene vegetation, and defines a subzone implying a very dry, cool climate about 7.8–7.5 (8.6–8.3 cal) ka BP based on the Alnus crispa profile during the Late Stanley lowstand. A new date of 9,470 ± 25 (10,680–10,750 cal) BP on basal peat over lacustrine clay at Espanola West Bog supports the previous interpretation of the Early Mattawa highstand at ca. 9,500 (10,740 cal) BP. The organic and clastic sediment units at these two bogs are correlated with other records showing coherent evidence of Holocene repeated inundation and isolation around northern Lake Huron. Taken together the previous and new lake-level data suggest that the Huron and Georgian basin lakes were mainly closed lowstands throughout early Holocene time except for short-lived highstands. Three of the lowstands were exceptionally low, and likely caused three episodes of offshore sediment erosion which had been previously identified as seismo-stratigraphic sequence boundaries.

Cross‐shelf thermal variability in southern Lake Michigan during the stratified periods

Results from a field experiment in southern Lake Michigan are used to quantify the cross‐shelf nearshore variability in Great Lakes temperatures during the stratified season. The experiment was conducted along the Indiana coast of southern Lake Michigan, with temperature and velocity moorings arranged in a cross‐shelf transect that extended to approximately 20 km from shore (40 m depth). The field site is noteworthy because of its location at the end of a major axis of an elliptical Great Lake, the relatively mild bathymetric slope, and local shoreline orientation that is perpendicular relative to the dominant summer winds. Measurements demonstrate that the location of the thermocline‐bottom intersection is highly variable, causing a wide zone of extreme thermal variability in the nearshore region with time scales of variability ranging from hours to months. Near‐inertial internal Poincaré waves are shown to cause large thermocline excursions but primarily only during periods of elevated activity. Several full upwelling events were observed, but in general, they were brief, lasting only 1–2 days, and had very limited spatial extent (2.5 km or less). Nonetheless, the offshore extent of the upwelling front was shown to be reasonably estimated with a simple estimate of the cross‐shelf transport caused by alongshore wind events. A persistent feature that determined the zone of elevated thermal variability (the thermocline‐shelf intersection point) was the strongly tilted thermocline, which resulted in the thermocline being located very close to shore. No evidence was found to support the hypothesis that internal Kelvin waves affect thermal variability at the study location.

Lengths and correlates of lag phases in upper-Midwest plant invasions

There are often lag phases in plant invasions, seemingly dormant periods between arrival in a new range and rapid population growth. Lags impede prioritization of invasive-species control efforts: when eradication is most feasible, it is often unclear whether a species is benign or a potentially harmful “sleeper weed.” I used herbarium records to estimate lag phases for invasive or potentially invasive plant species in three regions of the upper Midwest. I tested whether factors related to species’ invasion epidemiology, traits, or the habitats they invade were correlated with lag lengths. From an initial pool of 151 species, there were sufficient records to test for lags in 76 for northern Wisconsin, 90 for southern Wisconsin, and 91 for the southern Lake Michigan region. Lags were identified in 77% (197) of these 257 datasets and ranged from 3–140 years with a mean of 47.3 ± 34.6 (SD). Lags differed by native range, introduction pathway, growth form and habit, dispersal mode, flowering phenology, pollination mode for a subset of species, and breadth, light availability, and water availability of invaded habitats. However, estimated lags were highly variable and tested factors did not have strong explanatory power. Exotic species comprised an increasing proportion of total herbarium records. Of the species with known introduction pathways, 85% were intentionally introduced, mainly as ornamentals. The long durations, high variability, and low predictability of lags, along with human culpability for an increasingly non-native flora, support a cautious approach to species introductions.

Record of PCB congeners, sorbents and potential toxicity in core samples in Indiana Harbor and Ship Canal

Indiana Harbor and Ship Canal (IHSC) is an active navigational system that serves a heavily industrial area of southern Lake Michigan. We have determined the amount of polychlorinated biphenyls (PCBs), congener distributions, sorbent types and potential for dioxin-like PCB toxicity from two IHSC sediment cores. Vertical distributions of ΣPCBs (sum of 161 individual or coeluting congeners) ranged from 410 to 91000 and 1800 to 41000ngg−1 dry weight (d.w.) for cores 1 and 2, respectively. Core 1 showed its highest accumulation rate for the year ∼1979 and exhibits a strong Aroclor 1248 signal in sediments accumulating over the last 60years. It appears that from the late 1930s until the beginning of the 1980s there was a large and constant input of PCBs into this system. This pattern differs from lake cores from the Great Lakes region which commonly exhibit a rapid increase, a peak, followed by a sharp decrease in the PCB accumulation rates. Core 2 also has a strong Aroclor 1248 signal in the top layers, but deeper layers show evidence of mixtures of Aroclors and/or weathering processes. High levels of black carbon as a fraction of total organic carbon were found in both cores (median ∼30%), which reflect the long history of local combustion sources. No strong relationship was found between ΣPCB concentration and sorbents. Both cores contain dioxin-like PCBs that are highest in concentration below the surface. The high levels of PCBs in the deep sediments are of concern because of plans to dredge this system.

Are temperate mature forests buffered from invasive lianas?1

Mature and old-growth forests are often thought to be buffered against invasive species due to low levels of light and infrequent disturbance. Lianas (woody vines) and other climbing plants are also known to exhibit lower densities in older forests. As part of a larger survey of the lianas of the southern Lake Michigan region in mature and old-growth forests, the level of infestation by invasive lianas was evaluated. The only invasive liana detected in these surveys was Celastrus orbiculatus Thunb. (Celastraceae). Although this species had only attached to trees and reached the canopy in a few instances, it was present in 30% of transects surveyed, mostly as a component of the ground layer. Transects with C. orbiculatus had higher levels of soil potassium and higher liana richness than transects without. In contrast, transects with the native C. scandens had higher pH, sand content, and soil magnesium and lower organic matter compared to transects where it was absent. Celastrus orbiculatus appears to be a generalist liana since it often occurs with native lianas. Celastrus orbiculatus poses a substantial threat to mature forests as it will persist in the understory until a canopy gap or other disturbance provides the light and supports necessary for it to ascend to the canopy and damage tree species. As a result, these forests should be monitored by land managers so that C. orbiculatus eradication can occur while invasions are at low densities and restricted to the ground layer.

Linking non-culturable (qPCR) and culturable enterococci densities with hydrometeorological conditions

Quantitative polymerase chain reaction (qPCR) measurement of enterococci has been proposed as a rapid technique for assessment of beach water quality, but the response of qPCR results to environmental conditions has not been fully explored. Culture-based E. coli and enterococci have been used in empirical predictive models to characterize their responses to environmental conditions and to increase monitoring frequency and efficiency. This approach has been attempted with qPCR results only in few studies. During the summer of 2006, water samples were collected from two southern Lake Michigan beaches and the nearby river outfall (Burns Ditch) and were analyzed for enterococci by culture-based and non-culture-based (i.e., qPCR) methods, as well as culture-based E. coli. Culturable enterococci densities (log CFU/100 ml) for the beaches were significantly correlated with enterococci qPCR cell equivalents (CE) ( R = 0.650, P < 0.0001, N = 32). Enterococci CE and CFU densities were highest in Burns Ditch relative to the beach sites; however, only CFUs were significantly higher ( P < 0.0001). Culturable enterococci densities at Burns Ditch and the beaches were significantly correlated ( R = 0.565, P < 0.0001, N = 32). Culturable E. coli and enterococci densities were significantly correlated ( R = 0.682, P < 0.0001, N = 32). Regression analyses suggested that enterococci CFU could be predicted by lake turbidity, Burns Ditch discharge, and wind direction (adjusted R 2 = 0.608); enterococci CE was best predicted by Burns Ditch discharge and log-transformed lake turbidity × wave height (adjusted R 2 = 0.40). In summary, our results show that analytically, the qPCR method compares well to the non-culture-based method for measuring enterococci densities in beach water and that both these approaches can be predicted by hydrometeorological conditions. Selected predictors and model results highlight the differences between the environmental responses of the two method endpoints and the potentially high variance in qPCR results.

The influence of abiotic factors on gillnet catch rates of yellow perch in southern Lake Michigan, 1989–2006

Abstract The difference in yellow perch, Perca flavescens (Mitchill), catch rates were evaluated for gillnets set on the bottom at 10‐ and 15‐m depths in southern Lake Michigan during June, July and August from 1989 to 2006. More yellow perch were captured in gillnets set at 10 m than at 15 m. Differences in water temperature were significant in explaining variation in catch rates, but differences in Secchi depth and wave height were not significant. These results suggest that yellow perch may associate with habitat based on water temperature during the summer and move in response to thermal changes. In addition, this portion of the lake lacks noticeable heterogeneous structural habitat features that normally influence yellow perch behaviour. Fisheries managers should consider sampling based on temperature in addition to depth or other habitat features when designing sampling protocols.

Spatial and temporal variations of persistent organic pollutants impacted by episodic sediment resuspension in southern Lake Michigan

The impacts of large-scale, episodic sediment resuspension on the cycling of polychlorinated biphenyl congeners (PCBs) were examined using a spatially coordinated air and water sampling strategy conducted in southern Lake Michigan in the late winters of 1998, 1999, and 2000. We found no significant temporal changes in gas phase, dissolved phase, or suspended sediment PCB concentrations despite large-scale seasonal storms occurring before and after sampling campaigns. Only gas phase and suspended sediment PCBs varied spatially. Higher total suspended material ( TSM) concentrations and fraction organic carbon ( f oc ) were measured at sampling stations located in the near-shore region of southern Lake Michigan than at open-water sampling stations. Gas phase concentrations (ΣPCB g) were higher in the west (0.436 ± 0.200 ng/m 3, n = 11) and south (0.408 ± 0.286 ng/m 3, n = 5) than the east (0.214 ± 0.082 ng/m 3, n = 10) and central (0.253 ± 0.145 ng/m 3, n = 8) regions of southern Lake Michigan. Dissolved phase concentrations (ΣPCB d) averaged 0.18 ± 0.024 ng/L ( n = 52); suspended sediment concentrations (ΣPCB s) accounted for between 4% and 72% (23 ± 4%, n = 52) of the total ΣPCB concentrations (ΣPCB T = ΣPCB d + ΣPCB s). Despite no consistent temporal variations in both dissolved phase or suspended sediment ΣPCB concentrations, there were temporal and spatial variations in the distribution shift between phases that can be linked to sediment resuspension, not a state of equilibrium. Specifically, our analysis suggests sediment resuspension results in preferential sorption of heavier, more chlorinated PCB congeners.

Long-term and recent changes in southern Lake Michigan water quality with implications for present trophic status

Southern Lake Michigan has changed in response to alterations in nutrients and invasive species. NOAA and EPA monitoring results are used to examine those changes. NOAA provides detailed seasonal resolution, but limited spatial coverage, whereas the EPA provides more spatial coverage, but limited seasonal resolution. We compare changes in total phosphorus (TP), silica, nitrate plus nitrite, and chlorophyll concentrations from before and after the invasion by the quagga mussel ( Dreissena rostriformis bugensis). Although TP at NOAA stations was consistently higher than at EPA stations, both confirm declines in spring and summer surface mixed layer (SML) conditions. Chlorophyll differed at EPA and NOAA stations before quagga mussel invasion, but not after the invasion. Spring chlorophyll decreased at NOAA stations after the invasion, but summer conditions did not change at either set of stations. Pre-invasion silica at NOAA stations was slightly higher than at EPA stations, and the lake's Si reservoir increased over the study period. Basin-scale spring Si increased gradually, whereas summer SML Si increased dramatically after 2003, likely reflecting reduced diatom production. Basin-scale nitrate increased significantly from pre- to post-invasion in both spring and summer. Summer nitrate utilization declined drastically in recent years, likely reflecting reduced phytoplankton production. TP loads decreased; however, the timing of changes in chlorophyll and Si and nitrate utilization suggest the recent increase in dreissenid filtering dramatically reduced spring phytoplankton abundance and production across the entire southern basin. The offshore pelagic zone of the historically mesotrophic southern Lake Michigan is now similar to oligotrophic Lake Superior.

Recent changes in density, biomass, recruitment, size structure, and nutritional state of Dreissena populations in southern Lake Michigan

Trends in density, biomass, population structure, and nutritional state of Dreissena polymorpha and Dreissena rostriformis bugensis were examined in southern Lake Michigan between the 1990s and 2008. Density and biomass of D. polymorpha increased to a peak in the early 2000s and then declined. In contrast, D. rostriformis bugensis was first found in the southern basin in 2001 and has continued to increase in density or biomass at all depths ever since. In 2008, maximum mean density of D. rostriformis bugensis occurred at 16–30 m (19,000/m 2), but maximum biomass (AFDW) occurred at 31–50 m (43.9 g/m 2). D. rostriformis bugensis has only recently (since 2005) began to increase at depths > 50 m. When both species were present in 2004 at depths < 50 m, a condition index (CI) for D. rostriformis bugensis was 27% higher, and shell weight per shell length was 48% lower compared to D. polymorpha. For D. rostriformis bugensis, CI decreased in 2008 compared to 2004 at 25 m and 45 m, but biochemical content (lipid, glycogen) did not. Seasonal changes in both RNA/DNA ratio (growth) and ETS (metabolic activity) in D. rostriformis bugensis were unaffected by reproductive activity, and only ETS appeared to change seasonally relative to bottom temperatures. Spawning of D. rostriformis bugensis occurred in late summer at 25 m, but occurred in spring at 45 m. Veliger densities peaked in both spring and late summer at both depths. Future population expansion (biomass) is expected to be most rapid at depths > 50 m.

Lake Michigan lower food web: Long-term observations and Dreissena impact

Lake Michigan has a long history of non-indigenous introductions that have caused significant ecological change. Here we present a summary of eight papers that document recent changes and the current state of the lower food web of southern Lake Michigan after the establishment of large dreissenid populations. Results are based on long-term data sets collected by federal and academic research and monitoring programs that place recent changes into a historic context. Dramatic and significant changes in the lower food web, such as the loss of the spring diatom bloom, large declines in phytoplankton productivity, and a decline of Mysis populations, were directly or indirectly attributed to the expansion of Dreissena rostriformis bugensis. Total phosphorus concentrations and loadings also have decreased in the last 20 years. Changes in the Lake Michigan ecosystem induced by D. r. bugensis have produced conditions in the offshore pelagic region that are similar to oligotrophic Lake Superior. The future state of the lower food web in southern Lake Michigan is difficult to predict, mainly because population trends of D.r. bugensis in cold, offshore regions are unknown. Hence, monitoring programs designed to collect long-term, consistent data on the lower food web of Lake Michigan are essential.

Approaching storm: Disappearing winter bloom in Lake Michigan

Between 1990 and 2001, late-winter phytoplankton blooms were common in parts of the lower Great Lakes (southern Lake Michigan, Saginaw Bay and southern Lake Huron, and western Lake Erie), providing resources for over-wintering zooplankton. In Lake Michigan up to 2001, detailed remote sensing and ship studies documented well-developed late-winter blooms in the southern gyre (circular bloom termed the ‘doughnut’). However, from 2001 to 2008, the winter blooms in Lake Michigan also supported early season veliger larvae from the introduced, cold-water adapted “profunda” morph of quagga mussels ( Dreissena rostriformis bugensis). Remote sensing and ship studies revealed that settled mussels caused an extraordinary increase in water transparency and a simultaneous decrease of Chl a in the late-winter bloom. Before quagga mussels in 2001, water transparency was 74–85% at deep-water sites, whereas it increased progressively to 89% by 2006 and 94–96% by 2008. Chlorophyll a concentrations in the gyre rings were 1.1–2.6 µg/L in 2001, declining to 0.5–1.7 µg/L by 2006 and 0.4–1.5 µg/L by 2008. The reduction of Chl a in the winter bloom rings from 2001 to 2008 was 56–78% for the western limb and 74–75% for the eastern limb. Zooplankton species abundance, composition and abundance also changed, as cyclopoid copepods became very scarce and over-wintering omnivorous calanoid copepods declined. Reduction in late-winter phytoplankton and zooplankton poses a serious threat to open-water food webs.

Climate change impacts on terrestrial ecosystems in metropolitan Chicago and its surrounding, multi-state region

This paper describes the potential impacts of warming temperatures and changing precipitation on plants, wildlife, invasive species, pests, and agricultural ecosystems across the multi-state region centered on Chicago, Illinois. We examine a geographic area that captures much of Lake Michigan, including a complex mosaic of urbanization and agriculture surrounding southern Lake Michigan. We consider species currently present within this broad region as well as species that are expected to move into or out of the area as climate zones shift northward through the coming century. Our analysis draws upon disparate data sources to compile projections. We conclude that a complex mixture of land use poses particular challenges to natural ecosystems in this region under climate change. Dispersal is likely to be limited for some species, and some populations of native taxa may already be reduced due to habitat loss. Other species can persist, even thrive, within a mixed landscape mosaic, provided natural areas and green spaces are available. If such spaces are somehow connected, they can provide opportunities for native species to inhabit and move through the metropolitan region (perhaps even better than the landscapes previously dominated by agriculture). Strategies for adapting regional agriculture and minimizing pest outbreaks also call for creative management intervention. With additional research, Chicago and its surrounding environs have an opportunity to provide leadership on effective management of natural resources under climate change.

Spatio-temporal trends in the food habits of age-0 lake whitefish

We compared diets of juvenile lake whitefish among six sites in Lake Michigan and one in Lake Superior during 2005 and 2006 to assess spatial and temporal patterns in food habits and evaluate if ontogenetic diet shifts occur that may influence growth and survival. A total of 262 and 496 juveniles were captured in 2005 and 2006, the majority of which were captured during June and July. Sites in southern Lake Michigan tended to have larger juveniles, and the smallest juveniles were observed at Naubinway, northern Lake Michigan, and Whitefish Point, Lake Superior. The mean number of prey items per stomach differed among sampling sites and years. Copepods were the most prevalent prey item, and were present in greater than 70% of juvenile stomachs from most sites. However, the percent by number of copepods decreased during July as chironomids and other benthic macroinvertebrates increased in number. There was a significant positive relationship between percent of benthic prey items and mean length of juvenile lake whitefish. A substantial increase in the percent of benthic prey consumed after 40 mm (total length) was observed and likely resulted from juvenile lake whitefish crossing a size threshold for benthic feeding relating to morphological changes (i.e., transition of mouth opening from terminal to sub-terminal) in addition to a potential increase in the availability of emergent macroinvertebrates. Timing of the transition to benthic feeding is likely regulated by the number of prey per juvenile and the overlap with peak emergence of important benthic aquatic invertebrates such as chironomids. A better understanding of these factors will increase our understanding of juvenile lake whitefish growth and survival, which are necessary for improving year-class strength predictions.

Budget Analysis of Escherichia coli at a Southern Lake Michigan Beach

Escherichia coli (EC) concentrations at two beaches impacted by river plume dynamics in southern Lake Michigan were analyzed using three-dimensional hydrodynamic and transport models. The relative importance of various physical and biological processes influencing the fate and transport of EC were examined via budget analysis and a first-order sensitivity analysis of model parameters. The along-shore advective flux of EC (CFU/m(2).s) was found to be higher compared to its cross-shore counterpart; however, the sum of diffusive and advective components was of a comparable magnitude in both directions showing the importance of cross-shore exchange in EC transport. Examination of individual terms in the EC mass balance equation showed that vertical turbulent mixing in the water column dominated the overall EC transport for the summer conditions simulated. Dilution due to advection and diffusion accounted for a large portion of the total EC budget in the nearshore, and the net EC loss rate within the water column (CFU/m(3).s) was an order of magnitude smaller compared to the horizontal and vertical transport rates. This result has important implications for modeling EC at recreational beaches; however, the assessment of the magnitude of EC loss rate is complicated due to the strong coupling between vertical exchange and depth-dependent EC loss processes such as sunlight inactivation and settling. Sensitivity analysis indicated that solar inactivation has the greatest impact on EC loss rates. Although these results are site-specific, they clearly bring out the relative importance of various processes involved.

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.

Tracking bioaccumulation in aquatic organisms: A dynamic model integrating life history characteristics and environmental change

We have developed a dynamic model to track the evolution of contaminant concentration in an aquatic organism as a function of season and ontogeny throughout its life cycle. We have focused our analysis on the round goby ( Apollonia melanostoma), a globally distributed invasive forage fish. By integrating bioenergetics with a bioaccumulation model, we illustrate how life history characteristics interact to influence contaminant accumulation. We use uncertainty and sensitivity analyses to assess how the model output is affected by uncertainty and variability in model parameters. We then demonstrate the influence of important physiological characteristics on contaminant accumulation with two scenarios. First, we probe the influence of sexual dimorphism by comparing gender-specific accumulation of a standard polychlorinated biphenyl congener, PCB153, in male and female round gobies. We hypothesize that lipid loss in female gobies during spawning season leads to a decrease in the PCB body burden compared to male gobies. Second, we compare PCB accumulation in the round goby and in the mottled sculpin ( Cottus bairdi), the native forage fish that the round goby displaced in southern Lake Michigan, to determine whether the invasive species has an intrinsically different bioaccumulation potential than the native one. Our non-intuitive findings from these simulations illustrate how the interaction of growth rate with other life history characteristics lead to unexpected bioaccumulation patterns. The model we present is a flexible tool that integrates complex and dynamic interactions among environmental parameters, thus providing a means to better assess the potential for chemical accumulation in human and wildlife populations, and aiding the development of ecological forecasts.

Acoustic estimates of abundance and distribution of spawning lake trout on Sheboygan Reef in Lake Michigan

Efforts to restore self-sustaining lake trout ( Salvelinus namaycush) populations in the Laurentian Great Lakes have had widespread success in Lake Superior; but in other Great Lakes, populations of lake trout are maintained by stocking. Recruitment bottlenecks may be present at a number of stages of the reproduction process. To study eggs and fry, it is necessary to identify spawning locations, which is difficult in deep water. Acoustic sampling can be used to rapidly locate aggregations of fish (like spawning lake trout), describe their distribution, and estimate their abundance. To assess these capabilities for application to lake trout, we conducted an acoustic survey covering 22 km 2 at Sheboygan Reef, a deep reef ( < 40 m summit) in southern Lake Michigan during fall 2005. Data collected with remotely operated vehicles (ROV) confirmed that fish were large lake trout, that lake trout were 1–2 m above bottom, and that spawning took place over specific habitat. Lake trout density exhibited a high degree of spatial structure (autocorrelation) up to a range of ∼ 190 m, and highest lake trout and egg densities occurred over rough substrates (rubble and cobble) at the shallowest depths sampled (36–42 m). Mean lake trout density in the area surveyed (∼ 2190 ha) was 5.8 fish/ha and the area surveyed contained an estimated 9500–16,000 large lake trout. Spatial aggregation in lake trout densities, similarity of depths and substrates at which high lake trout and egg densities occurred, and relatively low uncertainty in the lake trout density estimate indicate that acoustic sampling can be a useful complement to other sampling tools used in lake trout restoration research.