Wisconsin Lakes Research Articles

Large-scale modeled contemporary and future water temperature estimates for 10774 Midwestern U.S. Lakes

Climate change has already influenced lake temperatures globally, but understanding future change is challenging. The response of lakes to changing climate drivers is complex due to the nature of lake-atmosphere coupling, ice cover, and stratification. To better understand the diversity of lake responses to climate change and give managers insight on individual lakes, we modelled daily water temperature profiles for 10,774 lakes in Michigan, Minnesota, and Wisconsin for contemporary (1979–2015) and future (2020–2040 and 2080–2100) time periods with climate models based on the Representative Concentration Pathway 8.5, the worst-case emission scenario. In addition to lake-specific daily simulated temperatures, we derived commonly used, ecologically relevant annual metrics of thermal conditions for each lake. We include all supporting lake-specific model parameters, meteorological drivers, and archived code for the model and derived metric calculations. This unique dataset offers landscape-level insight into the impact of climate change on lakes.

Fillet Weight and Fillet Yield: New Metrics for the Management of Panfish and Other Consumption‐Oriented Recreational Fisheries

AbstractProposals for regulations to improve panfish size structure often lack angler support, in part because of a perceived loss of panfish harvest. We propose that fillet weight and fillet yield, the percentage of the body weight available as fillets, are useful metrics for exploring regulation options and potential outcomes. We determined fillet weight and fillet yield for 321 Bluegill Lepomis macrochirus, 139 Black Crappie Pomoxis nigromaculatus, and 137 Yellow Perch Perca flavescens and developed equations to predict fillet weight from total length. Mean fillet yield was 29.5% for Bluegills, 28.6% for Black Crappies, and 33.1% for Yellow Perch. We used Bluegill size increases associated with a reduction in daily bag limits from 25 fish to 10 fish in seven Wisconsin lakes and then estimated fillet weights to compare the potential total weight of Bluegill fillets available to anglers under both a 10‐ and a 25‐fish daily bag limit. We projected that many anglers might obtain a greater weight of Bluegill fillets per fishing trip under a 10‐fish bag limit. We conclude that for consumption‐oriented fisheries, fillet weight and fillet yield have value in demonstrating how more restrictive harvest regulations may sometimes lead to a greater weight of fillets for most anglers.Received April 19, 2016; accepted February 10, 2017 Published online April 11, 2017

Lessons from a decade of lake management: effects of herbicides on Eurasian watermilfoil and native plant communities

AbstractEurasian watermilfoil (Myriophyllum spicatum) is a non‐native and invasive aquatic macrophyte with a broad North American distribution. It can have significant negative effects on invaded waterbodies, including decreased native macrophyte diversity, formation of recreational nuisances, and lowered lakefront property values. Previous research suggests that M. spicatum decreases in response to herbicide treatment, but most studies are spatially and temporally limited, usually focusing on a single waterbody for a single year. The long‐term effects of herbicides remain relatively unknown. Here, we share the results of an 11‐yr observational study of aquatic macrophyte diversity, dynamics, and response to herbicide treatment on 28 Wisconsin lakes (15 of which were adaptively managed with herbicide for M. spicatum and 13 of which acted as unmanaged reference lakes). We found that overall, adaptive management decreases M. spicatum abundance over time, but that the efficacy of individual herbicide treatments can vary. We also found that lakes with relatively new M. spicatum populations (discovered within the last decade) treated smaller areas with lower frequency than lakes with established populations, and were able to maintain lower M. spicatum abundance. This suggests that using adaptive, science‐based aquatic plant management strategies, including early detection and response, may increase invasive species management success. Finally, we show that the effect of herbicide treatment on native macrophytes is variable and can be significant. Overall, our results suggest that while herbicide treatment can be an effective adaptive management tool, particularly in lakes with relatively recent M. spicatum invasions, the specific effects of individual treatments can be unpredictable. This study allows lake stakeholders to better understand the efficacy of herbicide treatment, in addition to the possible non‐target effects on native macrophyte species.

A field test on the effectiveness of milfoil weevil for controlling Eurasian watermilfoil in Wisconsin lakes

We tested the effectiveness of milfoil weevils (Euhrychiopsis lecontei) for reducing biomass of Eurasian watermilfoil (Myriophyllum spicatum, EWM) under natural lake conditions in a 3-year field experiment. In each of four lakes, we randomly chose two EWM beds for stocking and two beds as controls. A total of ca. 40,000 weevils were added to the eight stocked beds. During June and August 2013–2015, we measured plant diversity, biomass of EWM and native plants, and weevil abundance in the 16 study beds. Background weevil densities varied widely among beds and were often greater than the densities stocked. Weevil stocking had no significant effect on EWM or native plant biomass. Nevertheless, weevil damage to EWM was common and its extent appeared strongly related to observed densities of the weevil. ANCOVA results indicated that weevil density was a significant predictor of EWM biomass in both June and August, but not on growth during summer. Overall, our study found that weevil density is an important factor for predicting EWM biomass, while weevil density is likely affected by a large number of environmental factors. This work highlights the importance of carefully considering lake conditions that may influence the efficacy of stocking for biological control.

Chill out: physiological responses to winter ice-angling in two temperate freshwater fishes.

A large body of research has documented the stress response of fish following angling capture. Nearly all of these studies have taken place during the open-water season, with almost no work focused on the effects of capture in the winter via ice angling. We therefore conducted a study to examine physiological disturbance and reflex impairment following capture by ice-angling in two commonly targeted species, bluegill Lepomis macrochirus and yellow perch Perca flavescens. Fish were captured from a lake in eastern Wisconsin (USA) and sampled either immediately or after being held in tanks for 0.5, 2 or 4 h. Sampling involved the assessment of reflex action mortality predictors (RAMP) and a blood biopsy that was used to measure concentrations of plasma cortisol and lactate. The capture-induced increase in plasma cortisol concentration was delayed relative to responses documented in previous experiments conducted in the summer and reached a relative high point at 4 h post-capture. Reflex impairment was highest at the first post-capture time point (0.5 h) and declined with each successive sampling (2 and 4 h) during recovery. Bluegill showed a higher magnitude stress response than yellow perch in terms of plasma cortisol and RAMP scores, but not when comparing plasma lactate. Overall, these data show that ice-angling induces a comparatively mild stress response relative to that found in previous studies of angled fish. While recovery of plasma stress indicators does not occur within 4 h, declining RAMP scores demonstrate that ice-angled bluegill and yellow perch do recover vitality following capture.

Geology and Wine 14. Terroir of Historic Wollersheim Winery, Lake Wisconsin American Viticultural Area

The viticultural history of Wisconsin started in the 1840s, with the very first vine plantings by Hungarian Agoston Haraszthy on the Wollersheim Winery property located in the Lake Wisconsin American Viticultural Area (AVA). This study examines the terroir of historic Wollersheim Winery, the only winery within the confines of the Lake Wisconsin AVA, to understand the interplay of environmental factors influencing the character and quality as well as the variability of Wollersheim wines. Soil texture, chemistry, and mineralogy in conjunction with precision viticulture tools such as electromagnetic induction and electrical resistivity tomography surveys, are utilized in the Wollersheim Winery terroir characterization and observation of spatially variable terroir at the vineyard scale. Establishing and comparing areas of variability at the plot level for two specific vineyard plots (Domaine Reserve and Lot 19) at Wollersheim Winery provides insight into the effects of soil properties and land characteristics on grape and wine production using precision viticulture tools. The viticultural future of Wisconsin looks quite favorable, as the number of wineries keeps rising to meet the demand for Wisconsin wine and local consumption. As climate change continues to affect the grape varieties cultivated across the world’s wine regions, more opportunities arise for Wisconsin to cultivate cool-climate European varieties, in addition to the American and French–American hybrid varieties currently dominating grape production in this glacially influenced wine region.RÉSUMÉL'histoire viticole du Wisconsin a commencé dans les années 1840, avec les premières plantations de vigne par le Hongrois Agoston Haraszthy sur la propriété du vignoble Wollersheim situé dans la région de l’American Viticultural Area (AVA) du lac Wisconsin. Cette étude porte sur le terroir historique du vignoble Wollersheim, le seul à l'intérieur de l’AVA du lac Wisconsin, qui soit soumis à l'interaction des facteurs environnementaux qui influencent le caractère, la qualité et la variabilité des vins Wollersheim. La caractérisation et l’observation des variations spatiales du terroir à l’échelle du vignoble Wollersheim se font par l’étude de la texture du sol, sa chimie et sa minéralogie en conjonction avec des outils de viticulture de précision comme l'induction électromagnétique et la tomographie par résistivité électrique. En définissant des zones de variabilité au niveau de la parcelle et en les comparant pour deux parcelles de vignobles spécifiques (domaine Reserve et lot 19) du vignoble Wollersheim on peut mieux comprendre les effets des propriétés du sol et des caractéristiques du paysage sur la production de raisin et de vin. Le nombre de vignoble augmentant pour répondre à la demande de vin du Wisconsin et à la demande locale, l'avenir viticole du Wisconsin semble assez prometteur. Comme le changement climatique continue d'influer sur la variétés des cépages cultivés dans les régions viticoles du monde, c’est l’occasion pour le Wisconsin de cultiver des variétés européennes de climat frais, en plus des variétés hybrides américaines et franco–américaines qui dominent actuellement la production de raisin dans ce vin glaciaire région.

Relationships between water chemistry and herbicide efficacy of Eurasian watermilfoil management in Wisconsin lakes

ABSTRACTFrater P, Mikulyuk A, Barton M, Nault M, Wagner K, Hauxwell J, Kujawa E. 2017. Relationships between water chemistry and herbicide efficacy of Eurasian watermilfoil management in Wisconsin lakes. Lake Reserve Manage. 33:1–7.Eurasian watermilfoil (EWM) is an invasive aquatic plant with the potential to disrupt native aquatic plant communities, decrease property values, and affect recreational uses of lakes. Much time, effort, and money is put forth every year to combat the spread of this invader. We examined the long-term efficacy of EWM management using herbicide and derived an empirical equation to quantify long-term treatment success for the lakes in this study. We found that lakes with lower pH (<7.5), conductivity (<0.1 mS/cm), and total dissolved solids (TDS <0.1 g/L) exhibited more successful long-term management. Our results suggested that lake water chemistry may play a role in long-term herbicide management success of EWM.

Projected shifts in fish species dominance in Wisconsin lakes under climate change.

Temperate lakes may contain both coolwater fish species such as walleye (Sander vitreus) and warmwater fish species such as largemouth bass (Micropterus salmoides). Recent declining walleye and increasing largemouth bass populations have raised questions regarding the future trajectories and management actions for these species. We developed a thermodynamic model of water temperatures driven by downscaled climate data and lake-specific characteristics to estimate daily water temperature profiles for 2148 lakes in Wisconsin, US, under contemporary (1989-2014) and future (2040-2064 and 2065-2089) conditions. We correlated contemporary walleye recruitment and largemouth bass relative abundance to modeled water temperature, lake morphometry, and lake productivity, and projected lake-specific changes in each species under future climate conditions. Walleye recruitment success was negatively related and largemouth bass abundance was positively related to water temperature degree days. Both species exhibited a threshold response at the same degree day value, albeit in opposite directions. Degree days were predicted to increase in the future, although the magnitude of increase varied among lakes, time periods, and global circulation models (GCMs). Under future conditions, we predicted a loss of walleye recruitment in 33-75% of lakes where recruitment is currently supported and a 27-60% increase in the number of lakes suitable for high largemouth bass abundance. The percentage of lakes capable of supporting abundant largemouth bass but failed walleye recruitment was predicted to increase from 58% in contemporary conditions to 86% by mid-century and to 91% of lakes by late century, based on median projections across GCMs. Conversely, the percentage of lakes with successful walleye recruitment and low largemouth bass abundance was predicted to decline from 9% of lakes in contemporary conditions to only 1% of lakes in both future periods. Importantly, we identify up to 85 resilient lakes predicted to continue to support natural walleye recruitment. Management resources could target preserving these resilient walleye populations.

Seasonal shifts in the relative importance of local versus upstream sources of phosphorus to individual lakes in a chain

Water quality in the Yahara chain of lakes in southern Wisconsin has been degraded significantly since European settlement of the region, primarily as a result of anthropogenic nutrient inputs. While all four main lakes (Mendota, Monona, Waubesa, and Kegonsa) have undergone eutrophication, elevated phosphorus and chlorophyll concentrations are particularly pronounced in the smaller lakes at the bottom of the chain (Waubesa and Kegonsa). Due to their short water residence times (2–3 months), these lakes are more responsive to seasonal variability in magnitude and source of phosphorus loading compared with the larger upstream lakes. In 2014, more than 80 % of the phosphorus load to Lake Waubesa passed through the outlet of Lake Monona (situated immediately upstream). However, between mid-May and late October when phosphorus concentrations in Lake Monona were reduced as a result of thermal stratification the upstream load dropped to ~40 % of the total, with the majority of loading during this period coming from Lake Waubesa’s local watershed. Correspondingly, seasonal phosphorus trends in Lake Waubesa during summer are correlated with precipitation, rather than phosphorus concentrations leaving upstream lakes. While phosphorus export from the local watersheds of Lakes Waubesa and Kegonsa is relatively small on an annual time scale, targeted loading reductions in these areas during the summertime will most effectively reduce summertime phosphorus concentrations in these fast-flushing lakes. Understanding the interaction of landscape position, water residence time, and mixing regime can help guide watershed management for water quality improvements in lake chains.

Do Higher Size and Reduced Bag Limits Improve Northern Pike Size Structure in Wisconsin Lakes?

AbstractMinimum length limits (MLLs) combined with reduced bag limits are frequently used by fisheries managers to improve size‐structure in Northern Pike Esox lucius populations. We evaluated the effectiveness of a relatively moderate 660‐mm MLL and a two‐fish‐daily bag limit (hereafter, “moderate limit”) and a relatively conservative 813 mm MLL and one‐fish‐daily bag limit (hereafter, “conservative limit”) on size structure of Northern Pike in Wisconsin lakes. We used a before–after‐control–impact design on a series of treatment and reference lakes to test the efficacy of each regulation. Across seven moderate‐limit lake pairs, mean and mean maximum total length (i.e., mean of the 10 longest fish) in treatment lakes did not differ relative to reference lakes, where there was no MLL and a five‐fish‐daily bag limit (relatively liberal; hereafter, “liberal limit”). Across five conservative treatment lakes, there was a significant reduction in mean size relative to references, but mean maximum size increased significantly by an average of 104 mm. A linear mixed‐effect regression of 990 Northern Pike populations similarly suggested that higher size and reduced bag limits were related to improved size structure. Northern Pike populations in the moderate‐limit lakes and conservative‐limit lakes increased by an average of 35 mm and 45 mm, respectively, compared with the liberal‐limits lakes. Size structure was significantly and positively related to increased angler effort and reduced Northern Pike densities and was negatively related to longitude and mean Secchi depth, revealing the importance of lake characteristics in mediating regulation efficacy. Proportional size structure differed significantly between the southern and northern management zones of Wisconsin for the 20 years before and after implementation of regional regulations (moderate limit in south versus conservative limit in north). Analysis of long‐term creel data revealed that catch rates of Northern Pike have been increasing over time, and anglers have focused on fewer, larger fish. We conclude that higher MLLs and reduced bag limits are effective at improving availability of large Northern Pike in Wisconsin lakes but are best utilized in certain lake and population types.Received September 4, 2015; accepted April 11, 2016 Published online August 23, 2016

Validation of a Side‐Scan Sonar Method for Quantifying Walleye Spawning Habitat Availability in the Littoral Zone of Northern Wisconsin Lakes

AbstractPrevious research has generally ignored the potential effects of spawning habitat availability and quality on recruitment of Walleye Sander vitreus, largely because information on spawning habitat is lacking for many lakes. Furthermore, traditional transect‐based methods used to describe habitat are time and labor intensive. Our objectives were to determine if side‐scan sonar could be used to accurately classify Walleye spawning habitat in the nearshore littoral zone and provide lakewide estimates of spawning habitat availability similar to estimates obtained from a transect–quadrat‐based method. Based on assessments completed on 16 northern Wisconsin lakes, interpretation of side‐scan sonar images resulted in correct identification of substrate size‐class for 93% (177 of 191) of selected locations and all incorrect classifications were within ± 1 class of the correct substrate size‐class. Gravel, cobble, and rubble substrates were incorrectly identified from side‐scan images in only two instances (1% misclassification), suggesting that side‐scan sonar can be used to accurately identify preferred Walleye spawning substrates. Additionally, we detected no significant differences in estimates of lakewide littoral zone substrate compositions estimated using side‐scan sonar and a traditional transect–quadrat‐based method. Our results indicate that side‐scan sonar offers a practical, accurate, and efficient technique for assessing substrate composition and quantifying potential Walleye spawning habitat in the nearshore littoral zone of north temperate lakes.Received December 1, 2015; accepted March 24, 2016 Published online July 20, 2016

Effect of DOC on evaporation from small Wisconsin lakes

Summary Evaporation (E) dominates the loss of water from many small lakes, and the balance between precipitation and evaporation (P–E) often governs water levels. In this study, evaporation rates were estimated for three small Wisconsin lakes over several years using 30-min data from floating evaporation pans (E-pans). Measured E was then compared to the output of mass transfer models driven by local conditions over daily time scales. The three lakes were chosen to span a range of dissolved organic carbon (DOC) concentrations (3–20 mg L −1 ), a solute that imparts a dark, tea-stain color which absorbs solar energy and limits light penetration. Since the lakes were otherwise similar, we hypothesized that a DOC-mediated increase in surface water temperature would translate directly to higher rates of evaporation thereby informing climate response models. Our results confirmed a DOC effect on surface water temperature, but that effect did not translate to enhanced evaporation. Instead the opposite was observed: evaporation rates decreased as DOC increased. Ancillary data and prior studies suggest two explanatory mechanisms: (1) disproportionately greater radiant energy outflux from high DOC lakes, and (2) the combined effect of wind speed ( W ) and the vapor pressure gradient ( e s − e z ), whose product [ W ( e s − e z )] was lowest on the high DOC lake, despite very low wind speeds ( −1 ) and steep forested uplands surrounding all three lakes. Agreement between measured (E-pan) and modeled evaporation rates was reasonably good, based on linear regression results ( r 2 : 0.6–0.7; slope: 0.5–0.7, for the best model). Rankings based on E were similar whether determined by measured or modeled criteria (high DOC −1 (C.V. 9%), but statistically significant differences between lakes resulted in substantial differences in cumulative E that were consistent from year to year. Daily water budgets for these lakes show that inputs were dominated by P and outputs by E; and our findings indicate that subtle changes in the variables that drive E can have measurable effects on water levels by shifting the balance between P and E.

Diet Overlap and Predation Between Largemouth Bass and Walleye in Wisconsin Lakes Using DNA Barcoding to Improve Taxonomic Resolution

AbstractOver the last decade, the abundance of Largemouth Bass Micropterus salmoides has increased in many northern Wisconsin lakes, causing concern among anglers and biologists regarding the potential for Largemouth Bass to negatively affect populations of Walleye Sander vitreus through predation or competition for prey. Our objectives were to determine whether (1) diet overlap and predation occurred between adult Walleyes and Largemouth Bass in four northern Wisconsin lakes and (2) the use of DNA barcoding to reduce unidentifiable fish in diet samples affected conclusions regarding diet overlap. A single occurrence of Walleye predation was observed in the diets of 945 Largemouth Bass. Moderate to high diet overlap was observed between Largemouth Bass and Walleyes throughout much of the study period. The use of DNA barcoding reduced the amount of unidentified fish in diets to &lt;1% and showed that failure to identify fish or fish parts can affect conclusions regarding diet overlap. Largemouth Bass predation is probably not a primary factor affecting Walleye abundance in the lakes we selected, but observed diet overlap suggests the potential for competition between the two species.Received June 15, 2015; accepted January 18, 2016 Published online May 26, 2016

Response of temperate lakes to drought: a paleolimnological perspective on the landscape position concept using diatom-based reconstructions

The hydrological position of a lake within the landscape can affect a number of lake chemical, physical, and biological features, as well as how lakes respond to environmental change. We present a paleolimnological test of the model for landscape position and lake response to climate change proposed by Webster et al. (2000). To investigate how diatom communities have responded to drought relative to landscape position, we examined sedimentary diatom profiles extending through the twentieth century from an upland site (Crystal Lake) and a lowland site (Allequash Lake) in the Northern Highlands region of north-central Wisconsin (USA). To explore changes in diatom communities at each site, we developed a calibration set and transfer functions from 48 lakes in Wisconsin’s Northern Highland Lake District. We further determined planktic:benthic ratios in the two target lakes, developed lake level models, and investigated the sensitivity of planktic:benthic diatom ratios to climatic variability over the past century. In the upland lake, diatom communities responded indirectly to climate via drought-induced changes in lake level, which resulted in shifts in planktic versus benthic habitat availability. This response of diatoms to changes in habitat availability provides an alternative approach for tracking climate change in upland lakes, though careful consideration must be given to the effect of the bathymetry and its relationship to lake level change and habitat zonation at individual sites. In the lowland lake, changes in diatom communities were related to temperature (and possibly lakewater chemistry) and physical changes secondarily. These results are consistent with the model by Webster et al. (2000), with chemical changes occurring in the lowland system and little chemical response in the upland system. However, the biological changes in sediment records presented here provide additional insight into how lake response to climatic change is shaped by landscape position, contributing to a clearer understanding of potential changes in ecosystem structure and function during drought conditions.

Spatial heterogeneity in invasive species impacts at the landscape scale

AbstractInvasive species have substantial impacts across the globe. While management efforts should aim to minimize undesirable impacts, we have a poor understanding of how impacts of a given invasive species vary spatially. Here, we develop a framework for considering heterogeneity of invasive species impacts that allows us to explore the range of possible spatial patterns of impact. This framework incorporates two factors—how invasive species abundance varies among sites (i.e., abundance distributions) and how invasive species impact varies as a function of abundance (i.e., abundance–impact curves). Combining these two factors allows for the creation of probability distributions that represent how invasive species impacts may vary spatially among sites. We used published abundance distributions and inferred abundance–impact curves to generate impact distributions for two problematic invasive species—zebra mussel and Eurasian watermilfoil—across lakes in Wisconsin, USA. Impact distributions of these species tended to be right‐skewed (i.e., the majority of sites had low impacts), although the tail thickness varied. We also simulated how a broader range of combinations of invasive species abundance distributions and abundance–impact curves produce different patterns of invasive species impact. These simulations illustrate a remarkable diversity of invasive species spatial impact patterns—probability distributions of impact were left‐skewed, right‐skewed, bimodal, and normal. Total landscape‐level impacts, estimated by summing site‐level impacts, were similarly variable depending on the distribution of site‐level impacts. Our results indicate that invasive species abundance and abundance–impact curves ultimately affect how invasive species impacts are distributed across the landscape, which has important implications for invasive species management.

Using maximum entropy to predict the potential distribution of an invasive freshwater snail

SummaryEcological niche modelling is a technique used to estimate potential distributions of invasive species based on available occurrence data and associated environmental conditions. Maximum entropy (Maxent) is a powerful method for ecological niche modelling and yet has only rarely been applied to aquatic species.Here we applied Maxent to estimate the potential distribution of the invasive Chinese mystery snail (Cipangopaludina chinesis) in Wisconsin and analysed several methodological issues associated with using Maxent for lentic species.To generate Maxent estimates of the potential distribution ofC. chinesis, we used presence records from 292 lakes, five spatially explicit climatic variables, and two lake‐specific environmental data sets (area, conductivity) from 7995 lakes. Our investigations included three aspects that could affect model prediction accuracy and transferability: (i) combining climate and lake variables into a single data set in two different ways, using each lake as a single observation and as a grid of 1 ha cells; (ii) varying the size of the background data set (locations without presences); and (iii) contrasting environmental conditions between locations with and withoutC. chinesis.The lake‐based model had higher accuracy than the grid‐based model, although both models had accuracy values indicative of good performance. Conductivity and lake area were important predictor variables for both models, but had higher contribution to the lake‐based model accuracy. Decreasing the background sample size minimally affected model accuracy and thus Maxent can be used even when background sampling does not meet the algorithm's default settings. Lastly, lakes that were environmentally dissimilar from lakes with knownC. chinesisrecords were more likely to be predicted unsuitable by both grid‐based and lake‐based models.Overall, the models predicted high potential suitability across Wisconsin lakes forC. chinesis, especially in lakes ≥60 ha. Our study provides evidence that small or environmentally biased presence data sets may underestimate the number of environmentally suitable locations of invasive species.

Interannual and long-term changes in the trophic state of a multibasin lake: effects of morphology, climate, winter aeration, and beaver activity

Little St. Germain Lake (LSG), a relatively pristine multibasin lake in Wisconsin, USA, was examined to determine how morphologic (internal), climatic (external), anthropogenic (winter aeration), and natural (beaver activity) factors affect the trophic state (phosphorus, P; chlorophyll, CHL; and Secchi depth, SD) of each of its basins. Basins intercepting the main flow and external P sources had highest P and CHL and shallowest SD. Internal loading in shallow, polymictic basins caused P and CHL to increase and SD to decrease as summer progressed. Winter aeration used to eliminate winterkill increased summer internal P loading and decreased water quality, while reductions in upstream beaver impoundments had little effect on water quality. Variations in air temperature and precipitation affected each basin differently. Warmer air temperatures increased productivity throughout the lake and decreased clarity in less eutrophic basins. Increased precipitation increased P in the basins intercepting the main flow but had little effect on the isolated deep West Bay. These relations are used to project effects of future climatic changes on LSG and other temperate lakes.

Comparing the diel cycles of dissolved organic matter fluorescence in a clear-water and two dark-water Wisconsin lakes: potential insights into lake metabolism

The cycling of organic carbon is fundamental to aquatic ecosystems, reflecting processes that extend from terrestrial watersheds to fish. Here, we use embedded fluorescence sensors that sample at high frequency to investigate the daily dynamics of a proxy for the major pool of organic carbon (chromophoric dissolved organic matter, CDOM) in a clear-water Wisconsin lake (∼3 mg C·L−1). We compare the diel CDOM cycle in this lake with cycles observed previously in two dark-water lakes (10 to 20 mg C·L−1). Despite differences in DOM quality and quantity, diel fluorescence cycles were evident in the epilimnia and hypolimnia of all three lakes. The amplitude differed among lakes, but the timing of the diel cycles was similar, with increases in fluorescence during nighttime and decreases during daylight (except in the aphotic hypolimnion of the darkest lake). The amplitude of the diel cycle increased with increasing DOM concentration, and estimates of DOM turnover based on the magnitude of oscillation ranged from 0.28 mg C·L−1·day−1in the darkest lake to 0.14 mg C·L−1·day−1in the clear lake. Independent estimates of free water metabolism based on the daily dynamics of O2or CO2were in general agreement, ranging from 0.32 to 0.06 mg C·L−1·day−1. Although absolute rates of turnover varied directly with DOM concentration, relative rates were highest in clear waters (∼5%·day−1). We conclude that these daily oscillations may be a common property of lakes and that they may provide insights into internal DOM processing over short time scales.

Improved Models for Predicting Walleye Abundance and Setting Safe Harvest Quotas in Northern Wisconsin Lakes

AbstractIn Wisconsin, the management of Walleyes Sander vitreus relies on a set of log‐linear regressions to predict Walleye abundance and to set safe harvest. The regression models predict mean Walleye abundance from lake area, but they ignore variability among years; they also predict equal Walleye populations in lakes with the same size and recruitment source. We evaluated three alternative models in terms of predictive accuracy and the risk of overharvest. We used 899 mark–recapture population estimates (collected between 1953 and 2013) from 219 lakes to develop and evaluate (1) a log‐linear mixed‐effects model that used all individual observations and estimated adult Walleye abundance from lake area and lake‐specific deviations from the overall intercept; (2) a mixed‐effects model that builds on model 1 by adding a linear fixed effect of sampling year; and (3) a mixed‐effects model that builds on model 1 by adding a random year effect. Walleye abundance was positively correlated with lake area in all models and was negatively correlated with sampling year (when included). Alternative models improved predictive accuracy by 17–22% over the current regression model. Restricting data to those collected during the most recent 20 years improved model responsiveness to new data and reduced the value of including a linear time trend. When all data were used for model construction, the relative risk of overharvest was lowest under the mixed‐effects model with a linear time trend; when the most recent 20 years of data were used, the risk was lowest under the mixed‐effects model with a random year effect. Accounting for variability among years would allow harvest to track changing Walleye populations and would allow management to be more adaptive. We recommend using the mixed‐effects model with a random year effect and restricting the data inputs to the most recent 20 years.Received May 20, 2015; accepted September 17, 2015

Relationships between redox-sensitive phosphorus concentrations in sediment and the aluminum:phosphorus binding ratio

Aluminum (Al) sulfate dosage to control internal phosphorus (P) loading in lakes can be estimated by determining the mass of Al required to bind redox-sensitive P (loosely bound and iron-bound P fractions, redox-P) in sediment (Al:P binding ratio). Recent research found that the Al:P binding ratio varied negatively with redox-P, suggesting competition for binding sites between P and other constituents. We examined relationships between redox-P and the Al:P binding ratio over a broad range of redox-P to delineate general patterns that might be used to improve Al dosing calculations. Surface sediments collected from lakes in eastern Minnesota and western Wisconsin were subjected to a range of precipitated Al(OH)3 concentrations to determine the Al required to bind redox-P (Rydin and Welch 1999). Although sediments exhibited similar physical-textural characteristics, redox-P, dominated by iron-bound P, ranged widely between <0.1 and 8.8 mg/g. The Al:P binding ratio exceeded 100:1 as the redox-P concentration declined to <0.15 mg/g and approached 10:1 for redox-P concentrations exceeding 5 mg/g. These relationships were attributed to (1) competition for binding sites by other constituents at lower redox-P and (2) increased competition for binding sites by PO43− relative to other constituents at higher redox-P. Regression relationships between redox-P concentration and the Al:P binding ratio may be used with vertical sediment P profiles to estimate Al dosage required to reduce internal P loading, particularly for stratified or polymictic lakes where iron-bound P is the dominant fraction.