Exploring potential drivers of spatiotemporal variation in length-at-age and condition of two common nearshore fishes in southwestern Lake Michigan

Understanding factors like fish size and condition can be crucial to fisheries management. In the Great Lakes, long-term observations of these factors are used to describe the effects of ecosystem dynamics on fish ecology. Considering the diversity of available benthic habitat and dynamic environmental conditions in the nearshore zone of Lake Michigan, we sought to understand the variability in length-at-age and condition of yellow perch and round goby, two important members of the nearshore fish community. We measured these metrics from both species captured in gillnets across three locations in Illinois waters of southwest Lake Michigan from June through October 2008–2012 and related the spatiotemporal differences seen to observed variation in temperature, prey abundance, and competitor abundance. Yellow perch length-at-age varied annually for all three age-classes, with differences relating to thermal regime (age-0 and age-2) and alewife abundance (age-1). Yellow perch condition, measured as residual weight, varied spatially and annually, showing a negative relationship with round goby abundance. Round goby length-at-age was greater at the southern location, likely due to warmer temperatures, while condition was greater at the rockier location. In general, model selection analysis revealed several potential relationships between the relative abundances of competitor species and length and condition of yellow perch and round goby, while prey abundance may be related to round goby condition. These results demonstrate the importance of understanding the diversity in nearshore habitats and the influence of this diversity on factors affecting fish production, including size and condition, in southwest Lake Michigan.     

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.     

Diet composition and overlap for adult walleye,lake whitefish,and yellow perch in Green Bay,Lake Michigan

Interspecific interactions among walleye Sander vitreus, lake whitefish Coregonus clupeaformis, and yellow perch Perca flavescens in Green Bay could influence the population status of each species, but potential trophic interactions are poorly understood. Our objectives were to determine if diet assemblages for each species and diet overlap among species varied spatially and temporally within Green Bay. Adult walleye (≥381 mm total length (TL); N = 981), lake whitefish (≥432 mm TL; N = 1507), and yellow perch (≥150 mm TL; N = 1174) were collected during May-October of 2018 and 2019 from multiple locations in southern and northern Green Bay. Diet assemblages of each species varied between northern and southern Green Bay, but walleye diets were more temporally variable (among months within zones and between years) than diets of lake whitefish or yellow perch. Lake whitefish represented a seasonally important prey item for walleye in southern Green Bay, composing 10 % and 41 % of walleye diets by weight in May and June, respectively. Yellow perch generally composed <15 % of walleye diets by weight but were consumed at a broader spatiotemporal scale than lake whitefish. Diet overlap between walleye and both lake whitefish and yellow perch was generally weak or moderate, whereas diet overlap between whitefish and perch was generally strong. Our assessment of adult trophic interactions suggests that changes in the population status of one species could influence fisheries for all three, and we identify additional research questions to address potential population-level effects of these trophic interactions.     

Assessment of landed and non-landed by-catch of walleye, yellow perch and white perch from the commercial gillnet fisheries of Lake Erie, 1994-2007

Theoretical by-catch (including landed and non-landed bycatch) of walleye (S. vitreus), yellow perch (Perca flavescens), and white perch (Morone americana) from the Lake Erie commercial gillnet fisheries during 1994-2007, was predicted by a delta model developed on the fishery-independent survey data (Lake Erie Partnership Index Fishing Survey). The delta model consisted of one generalized additive model and one AdaBoost model. The generalized additive model was used to predict non-zero catches of the by-catch species, and the AdaBoost model was used to predict the probability of obtaining non-zero catches. Non-landed by-catch was estimated as the difference between the theoretical by-catch predicted from the delta model and the landed by-catch recorded in the commercial fishery data. The theoretical by-catch of walleye was relatively higher in the west basin in October. A higher theoretical by-catch of yellow perch occurred in the west central basin in November, and a higher theoretical by-catch of white perch occurred in the west central basin in October. We observed higher levels of non-landed by-catch of walleye in the west basin during August to September, higher levels of non-landed by-catch of yellow perch in the west central and east central basins in November, and higher levels of non-landed by-catch of white perch in the west basin in August and November. The combination of the AdaBoost model with the delta model provided an alternative model in by-catch analyses when the percentage of zero observations was high.     

Differential habitat use patterns of yellow perch Perca flavescens in eastern Lake Michigan and connected drowned river mouth lakes

Understanding stock structure and habitat use is important for sustainable fisheries management and conservation of genetic and phenotypic diversity. In eastern Lake Michigan, yellow perch Perca flavescens is found in both the nearshore region of Lake Michigan proper and drowned river mouth (DRM) lakes, small estuary-like systems directly connected to Lake Michigan. Recent genetic analyses suggest complex stock structure between these habitats and the potential migration of Lake Michigan yellow perch into the profundal zone of DRM lakes. We quantified carbon (δ¹³C) and oxygen (δ¹⁸O) stable isotope ratios of yellow perch otolith cores to index natal origins, and measured muscle δ¹³C, nitrogen (δ¹⁵N), δ¹⁸O, and hydrogen (δ²H) isotope ratios to reflect recent diet and habitat use. Stable isotope ratios of otolith cores and muscle samples support the existence of resident populations in nearshore Lake Michigan and DRM lakes, as well as Lake Michigan migrants using DRM lakes. Most fish caught in DRM lakes that had natal and recent stable isotope values similar to Lake Michigan fish were collected during fall in DRM lake profundal zones. Comparison of otolith core and muscle stable isotope ratios of individual yellow perch suggest that individuals that recently migrated to DRM lakes also spent early life in Lake Michigan. Differential habitat use patterns of yellow perch in eastern Lake Michigan may have important implications for harvest estimates and fishing regulations. Migration by Lake Michigan fish into DRM lakes does not appear to be related to reproduction, and the underlying benefits of these migrations remain unclear.     

Effects of water clarity on the length and abundance of age-0 yellow perch in the Western Basin of Lake Erie

Water clarity is an important environmental variable that may affect fish populations by altering the visual environment. Effects can change feeding ability, as well as alter predation risk. The western basin of Lake Erie provides a valuable model system for studying the effects of transparency because the two main tributaries, the Maumee and Detroit rivers, differ substantially in clarity. We used Generalized Additive Models (GAMs) to quantify the relationship between transparency and the observed abundance and length of age-0 yellow perch (Perca flavescens) in August, based on surveys from 1986 to 2006. Secchi data from June to August were included in the models that best explained the variation in yellow perch abundance and length. August values for bottom oxygen and bottom temperature also increased model fit for abundance, whereas only bottom temperature improved model fit for length. Our models indicate that transparency was positively related to the August length while abundance of age-0 yellow perch was inversely related to transparency. Highest abundance was observed in areas with the lowest transparency, with peak abundances observed in areas with less than 1 m of Secchi depth. This is in contrast to August length, which increased as transparency increased, to an asymptote at around 3 m of Secchi depth. The split nature of water clarity conditions in the western basin of Lake Erie has resulted in areas with higher growth potential, versus areas with higher apparent survival.     

Diet partitioning, habitat preferences and behavioral interactions between juvenile yellow perch and round goby in nearshore areas of Lake Erie

Ecological interactions between native and non-indigeneous species depend on interspecies dietary and habitat overlap and species-specific behavior. In the Great Lakes, the exotic round goby (Apollonia melanostoma) is very abundant in littoral areas used by the native yellow perch (Perca flavencens). We examined yellow perch-round goby interactions using multiple approaches. Field surveys analyzing dietary overlap among three size classes of yellow perch and round goby detected significant overlap only between juvenile perch (< 95 mm TL) and gobies (< 60 mm TL). Laboratory experiments using juvenile stages tested for habitat preference differences (open sand, macrophytes and dreissenids) in solitary, intraspecific (2 perch) and interspecific (1 perch, 1 goby) treatments. In macrophyte and dreissenid habitats, we tested for treatment differences in fish behavior (intraspecific vs. interspecific) and yellow perch growth (solitary, intraspecific and interspecific). Round goby consistently preferred complex habitats. Yellow perch showed diurnal preference of complex habitats, but increased nocturnal use of sand in the solitary and interspecific treatments. Activity was greater in dreissenid than macrophyte habitat, but prey attacks showed the opposite trend. Activity and prey attacks were greater in the intraspecific than interspecific treatments. The trend was due to lower prey attacks executed by round goby. In macrophytes, individual yellow perch growth was lower in the intraspecific than in the solitary and interspecific treatments. In dreissenids, intraspecific and interspecific competitors equally decreased yellow perch growth. Our results suggest differences in diet, habitat preference and behavior between juvenile round goby and yellow perch may allow their coexistence in nearshore areas.     

Comparison of catch in multifilament and monofilament gill nets in a long-term survey on Lake Michigan

Long-term fishery independent surveys provide metrics of relative abundance and contribute biological information critical to effective fisheries management. Improvements in technology and manufacturing processes have had profound effects on gear efficiency. In Lake Michigan, a standardized multi-agency fish community survey was adopted in 1998 which used multifilament nets to survey nearshore fish communities. Contemporary commercial gill netting operations largely utilize monofilament nets because they’re less expensive, more durable and thought to be less visible and therefore more efficient. As such, interest in converting long-term agency surveys to monofilament nets has grown. To compare catch rates and the size of fish collected, we set similarly configured nets of both mesh materials on the same day at standard survey locations. The most common species captured in the survey were lake trout (52%), yellow perch (19%), lake whitefish (15%), longnose sucker (8%), white sucker (4%) and burbot (1%). Monofilament nets were more efficient in capturing lake whitefish and longnose sucker and equally efficient in capturing other species. On average, nearly three times (2.85) as many lake whitefish were captured in monofilaments nets, and two times as many longnose suckers (2.13). Larger lake whitefish and yellow perch were captured in monofilament nets, whereas lake trout were slightly smaller. For other fish species, similar sizes were captured in each net material. This evaluation incorporated broad replication (298 paired nets) and representation from diverse locations and depth strata in Lake Michigan to inform gear conversion efforts and allow for adjustments as needed in gear conversion efforts.     

Evaluation of anal fin spines,otoliths, and scales for estimating age and back-calculated lengths of yellow perch in southern Green Bay

Southern Green Bay supports important fisheries for yellow perch Perca flavescens and valid estimates of age structure and growth are critical to effective management. Anal fin spines and scales are used by the Wisconsin Department of Natural Resources for age estimation, but these structures may provide lower precision and accuracy than otoliths. The primary objective of our assessment was to determine if age estimates, among-reader precision, and mean back-calculated total lengths (TLs) at age differed among scales, anal fin spines, and otoliths. Ages estimated from anal fin spines were more precise than scale ages, were as precise as otolith-based ages, and generally agreed with consensus ages estimated from sectioned otoliths. Relationships between TL and radii of calcified structures were linear for scales, anal spines, and otoliths along two different transects. Mean back-calculated TLs were generally similar between intercept-corrected direct proportion (ICDP) and linear regression (LR) models, but otolith-based direct proportion models (no intercept correction) generally provided higher back-calculated mean TLs at ages 1 and 2 than ICDP and LR models. Mean back-calculated TLs at age estimated from whole otoliths were higher than estimates for other structures; but differences among anal fin spines, scales, and sectioned otoliths were <10?mm. Our results suggest biologists have little to gain by switching to otoliths when assessing age structure and growth for this fast-growing yellow perch population with relatively few fish ≥age 6, but additional analyses are warranted for slower-growing perch populations in the Great Lakes where older fish are more common.     

Response of yellow perch in Les Cheneaux Islands,Lake Huron to declining numbers of double-crested cormorants stemming from control activities

The yellow perch (Perca flavescens) fishery of the Les Cheneaux Islands region of Lake Huron experienced an unprecedented collapse in 2000. Immediately prior to the collapse was the proliferation of double-crested cormorants (Phalacrocorax auritus) in the area. Subsequent investigations established that double-crested cormorant predation was chief among the forces shaping the local yellow perch population and contributing to the collapse of the fishery. A double-crested cormorant control program was implemented in 2004 with the objective of benefiting the yellow perch population and fishery. This study used creel survey and gillnet fish community assessment data to evaluate the response of the yellow perch population and fishery. In all, seven key yellow perch metrics were analyzed using regression analysis with double-crested cormorant abundance as the independent variable. As double-crested cormorant abundance declined, yellow perch abundance increased, total mortality rate decreased, the angler catch rate and harvest in the recreational fishery improved, yellow perch growth rate declined and mean age increased. Increased yellow perch recruitment was documented since 2003 but it was the longevity of these year classes, (improved survival) as much or more than their magnitude of the year class, that allowed for the progress towards recovery. Questions facing managers are the sustainable level of double-crested cormorants in the region and the long term prognosis for the yellow perch fishery to fully recover to pre-double-crested cormorant levels.     

Effects of walleye predation on variation in the stock-recruitment relationship of Lake Erie yellow perch

Stock-recruitment relationships (SRRs) may vary over time due to temporal variation in ecological conditions, reducing confidence in projections from stock-recruitment models. We examined whether the time-varying SRRs detected for yellow perch (Perca flavescens) in the western basin of Lake Erie between 1977 and 2013 could be attributed to variation in yearling walleye (Sander vitreus) predation, indexed by variation in density. Annual variation in yearling walleye density was strongly associated with yellow perch recruitment dynamics, and positively correlated with temporal variation in density-dependence of yellow perch SSRs. However, non-stationary SRRs persisted after accounting for effects of yearling walleye density, and the extent of temporal variation in SRRs actually increased. In simulations, we showed that time-varying SRRs may result more from variation in low-frequency ecological factors on the order of decades, than from variation in high-frequency ecological factors on the order of years (e.g., yearling walleye density) and thus may not be distinguishable from noise. Hence, of these two types of factors, the systematic identification, characterization and incorporation of those of low-frequency factors into stock-recruitment models (e.g., exotic mussel invasions and eutrophication, in the case of Lake Erie) may offer greater promise to improve the reliability of long-term forecasts for sustainable harvests in this and other fisheries in dynamic ecosystems.     

Wild juvenile salmonid abundance in Wisconsin tributaries indicates limited contributions to Lake Michigan fisheries

Natural reproduction of salmonids occurs in many Lake Michigan tributaries, yet little is known about abundance and the potential contribution of wild fish hatching in Wisconsin tributaries. The objectives of our study were to determine if: 1) abundance of wild juvenile salmonids (primarily adfluvial rainbow trout, Oncorhynchus mykiss, referred to as steelhead) varied among selected Wisconsin streams based on available spawning and age-0 habitat; 2) stream temperature regimes could limit survival of juvenile salmonids, and 3) wild juvenile salmonids outmigrate from Wisconsin tributaries into Lake Michigan or larger tributaries. In 2016 and 2017, juvenile salmonid abundance was estimated in six Wisconsin tributaries to Lake Michigan by multiple-pass depletion sampling using backpack electrofishing. Habitat assessments included steelhead redd surveys, age-0 habitat surveys, and stream temperatures were monitored using in-stream loggers. Passive integrated transponder (PIT) tagging and PIT antennas were used to detect outmigration from three streams (Willow, Stony and Hibbard creeks). Population estimates for individual streams ranged from 75 to 2276 for juvenile steelhead and from 0 to 243 for juvenile coho salmon, Oncorhynchus kisutch. No correlation was detected between juvenile steelhead abundance and quality age-0 habitat. Stream temperatures rarely exceeded the thermal limit for steelhead (27 °C). Outmigration rates for three streams ranged from 0.6% to 3.1%, but these estimates were considered minimum values. Low abundance of wild juvenile steelhead and coho salmon alone suggest that the contributions of these tributaries to Lake Michigan fisheries are likely small. Furthermore, relying on returns of wild steelhead produced in these streams is probably insufficient to maintain stream fisheries.     

Lead in Lake Michigan and Green Bay surficial sediments

Sediment samples were collected in 1987–1990 from Green Bay and in 1994–1996 from Lake Michigan. Surficial sediments (0–1 cm) from both locations were analyzed for lead for the purpose of describing the horizontal variation of lead in 1994–1996 Lake Michigan and 1987–1990 Green Bay sediments, estimating lead fluxes to surficial sediments, and comparing results to earlier studies. With Lake Michigan concentrations ranging from below the method detection limit to 180 μg/g, the surficial sediments had mean and median lead concentrations of 70 μg/g and 64 μg/g, respectively. Lead concentrations in Green Bay surficial sediments were similar to those in Lake Michigan and ranged between the method detection limit and 160 μg/g. For the bay, mean and median concentrations were 58 and 59 μg/g, respectively. Surficial lead concentrations were highest in the Southern, Waukegan, and Grand Haven basins of Lake Michigan and in the central region of Green Bay in the vicinity of Chambers Island. For Lake Michigan and Green Bay, dated sediment cores illustrate the decline in lead concentrations during the last 30 and 10 years, respectively. Lead fluxes ranged between < 0.049 and 7.2 μg/cm²/yr for Green Bay and between 0.47 and 20 μg/cm²/yr for Lake Michigan. Lead fluxes to Lake Michigan were lower than those reported for 1972. These are the most comprehensive fluxes of lead to Lake Michigan and Green Bay surficial sediments reported to date.     

A comparative examination of recent changes in nutrients and lower food web structure in Lake Michigan and Lake Huron

The lower food webs of Lake Huron and Lake Michigan have experienced similar reductions in the spring phytoplankton bloom and summer populations of Diporeia and cladocerans since the early 2000s. At the same time phosphorus concentrations have decreased and water clarity and silica concentrations have increased. Key periods of change, identified by using a method based on sequential t-tests, were 2003–2005 (Huron) and 2004–2006 (Michigan). Estimated filtration capacity suggests that dreissenid grazing would have been insufficient to directly impact phytoplankton in the deeper waters of either lake by this time (mid 2000s). Despite some evidence of decreased chlorophyll:TP ratios, consistent with grazing limitation of phytoplankton, the main impact of dreissenids on the offshore waters was probably remote, e.g., through interception of nutrients by nearshore populations. A mass balance model indicates that decreased phosphorus loading could not account for observed in-lake phosphorus declines. However, model-inferred internal phosphorus dynamics were strongly correlated between the lakes, with periods of increased internal loading in the 1990s, and increased phosphorus loss starting in 2000 in Lake Michigan and 2003 in Lake Huron, prior to dreissenid expansion into deep water of both lakes. This suggests a limited role for deep populations of dreissenids in the initial phosphorus declines in the lakes, and also suggests a role for meteorological influence on phosphorus dynamics. The high synchrony in lower trophic level changes between Lake Michigan and Lake Huron suggests that both lakes should be considered when investigating underlying causal factors of these changes.     

Nitrogen dry deposition to Lake Superior and Lake Michigan

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

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.     

Viral hemorrhagic septicemia virus infection in Yellow perch, Perca flavescens, in Lake Erie

Viral hemorrhagic septicemia virus (VHSV) infects wild and hatchery fish in Europe, Japan, and the Great Lakes and Pacific regions of North America. The virus was associated with a large die-off of yellow perch, Perca flavescens, in Lake Erie in 2006. To determine the infection pattern of VHSV, we sampled yellow perch during the spring, summer, and fall of 2007 and 2008 in the central basin of Lake Erie during routine sampling by the Ohio Division of Wildlife with bottom trawls in nearshore, mid-depth, and offshore locations near the Chagrin River. The Ohio Department of Agriculture's Diagnostic Laboratories and the U.S. Fish and Wildlife Service's La Crosse Fish Health Center tested for VHSV from homogenized samples obtained from yellow perch kidney, spleen, and brain. At each lake sample location, we also measured temperature, dissolved oxygen, and conductivity. In both years, we found yellow perch infected with VHSV during a three-week period starting in the last week of spawning to early June. A high proportion of adult male and female yellow perch tested positive for VHSV during the infection period in our sample population. Infection appeared to be associated with temperatures between 12 and 18 °C and with significantly higher yellow perch densities during spawning. No large mortalities of yellow perch were observed during the VHSV infection period in 2007 and 2008.     

Variability in fish and water hydrogen and oxygen stable isotope values in the nearshore region of a large water body

Incorporating organismal hydrogen (δ²H or δD) and oxygen (δ¹⁸O) stable isotope ratios into aquatic food web research can help elucidate relative reliance on shoreline inputs and/or feeding at different trophic levels; however, their interpretation is complicated by the fact that aquatic organisms derive hydrogen and oxygen from both their diet and the ambient water in which they reside. We examined spatial and seasonal patterns in δ²H and δ¹⁸O of field-collected water and small-bodied fishes. Samples were gathered from nearshore Lake Michigan, a dynamic region of a large, increasingly oligotrophic freshwater lake. We examined seasonal and spatial δ²H and δ¹⁸O values of surface and bottom water, and compared spatial δ²H and δ¹⁸O values of young-of-year yellow perch (Perca flavescens) and seasonal δ²H and δ¹⁸O values of round goby (Neogobius melanostomus). Nearshore δ²H and δ¹⁸O water values were more variable than previously-described offshore values. Variation in nearshore water δ²H and δ¹⁸O values was likely related to differential precipitation and/or discharges from tributaries, and there may be a surface dilution effect which led to lower δ¹⁸O values in nearshore surface waters than bottom waters. In our study, fish tissue δ²H and δ¹⁸O also varies more spatially than seasonally, and some findings suggest that fish tissue δ²H may reflect feeding at higher trophic levels. Though characteristics of the study system affect their interpretation, we suggest that δ²H and δ¹⁸O can be another tool to assess food web structure, with δ²H in particular having potential to resolve questions when δ¹³C or δ¹⁵N are inconclusive.     

Resurrection of the Lake Michigan Eutrophication Model,MICH1

The Lake Michigan model, MICH1, was developed more than 30 years ago. This framework was evaluated using field data collected in 1976 and was later applied to predict total phosphorus and phytoplankton concentrations in Lake Michigan during the 1980s and early 1990s. With a renewed interest in the interaction of phytoplankton with toxics and the applicability to Total Maximum Daily Load studies, several new models have been developed and older models have been revived. As part of our interest in plankton dynamics in Lake Michigan, the MICH1 model was resurrected. The model was evaluated over the 1976–1995 period, with a surprisingly good model fit to lake-wide average total phosphorus (TP) field data. However, the model was less successful in mimicking the chlorophyll-a measurements, especially in the hypolimnion. Given the results, the model was applied to perform a few long-term TP model simulations. Using the model with average 1994–95 phosphorus loadings, a steady state was reached within approximately 20 years, and the lakewide phosphorus concentration was below the International Joint Commission water quality guideline of 7 μg/L. This exercise demonstrated that a relatively simple, four-segment model was able to mimic the TP lake-wide data well. However, this model was less suitable to predict future chlorophyll-a concentrations due to the limitation in the representation of the foodchain and the difficulty of the coarse segmentation of the model to capture the deep chlorophyll-a layer. Strengths and limitations of this model can guide future development of eutrophication models for Lake Michigan and the other Great Lakes.     

Foredune dynamics at a Lake Michigan site during rising and high lake levels

On Great Lakes dunes, the link between foredune dynamics and coastal processes is seen in dune responses to changing lake levels. This paper investigates foredune dynamics during a recent period of rising and high lake levels. The study location was an active foredune in P.J. Hoffmaster State Park on the east coast of Lake Michigan, where field data were collected from 2000 through the final destruction of the foredune by wave removal in November 2019. Foredune dynamics were studied with erosion pins, direct observations, photographs, mapping, and on-site wind measurements. Regional climate and lake-level data were obtained from established data collection programs. The response of the foredune to rising lake levels was compared to several models of foredune behavior. During the study, the Lake Michigan-Huron level rose 1.89 m from January 2013 to July 2020. After an early transitional period, foredune activity was characterized by scarp retreat (4–19 m per year) and dune narrowing from 2014 to 2019. When the foredune completely disappeared in November 2019, erosion/scarping began on the next landward dune. The foredune activity fits Olson’s (1958) model for foredune growth and erosion through lake-level cycles. The foredune migration predicted by the revised Davidson-Arnott (2021) model of foredune response to relative water level rise did not occur, most likely because the rate of lake-level rise was too high. The six years of foredune narrowing before wave erosion started affecting the next landward dune represent a time-lag in Lake Michigan dune history models of increased dune activity during high lake-level stands.