Peter C. Jacobson

Managing ecological systems for resilience can increase their capacity to maintain key functions even under global change. Oxygenated coldwater (oxythermal) habitat in lakes is an important ecological resource that is threatened by both climate change and eutrophication. Here, we quantify the resilience of oxythermal habitat in over 10,000 glacial lakes in the upper Midwestern United States to climate change and watershed disturbance and classify lakes for conservation prioritization based on their current conditions and resilience. Oxythermal habitat was predicted by lake morphometry, July air temperatures, and watershed land use. Temperatures are projected to increase by mid‐century, and the magnitude of warming, its effect on oxythermal habitat, and the uncertainty surrounding that effect varied among lakes. Under mid‐century climate conditions, the number of lakes containing suitable coldwater habitat was predicted to decline by 67%, while the number of lakes with unsuitable habitat was predicted to increase by over 200%. Lakes varied in the amount of temperature increase that they could sustain without a resultant change in habitat tier (i.e., their climate resilience). Median climate resilience was 4.3°C, with some lakes capable of remaining in their habitat tier even with temperature increases up to 14°C. Changing watershed land use was predicted to influence oxythermal habitat in 24% of lakes (n = 2391). We used the magnitude of increase in watershed development that a lake could sustain while remaining in its current habitat class as a measure of its resilience to watershed disturbance. Conversely, decreased watershed development may improve oxythermal habitat conditions and push a lake into an improved condition, and this value represented a lake's restoration potential. We classified lakes into seven management classes based on their current oxythermal habitat conditions and the resilience of oxythermal habitat to climate and watershed disturbance. To facilitate management on individual lakes, we also assessed the vulnerability and resilience of individual lakes and the uncertainty surrounding these estimates. By quantifying the resilience of lakes and how it is influenced by local action across a multistate region, we can prioritize conservation action across multiple scales to maintain the critical habitat and ecosystem function of glacial lakes.

The effects of lake productivity and late‐summer hypolimnetic oxygen on the size and shape of Cisco Coregonus artedi were examined in 27 Minnesota lakes. Geometric morphometry analyses of specimens captured in vertical gill nets indicated that Cisco in more productive lakes with hypoxic hypolimnia were larger, with deeper bodies, shorter snouts and caudal peduncles, and longer fins. Cisco in unproductive lakes with oxygenated hypolimnia were smaller, with slender bodies, longer snouts and caudal peduncles, and shorter fins. The effect of hypolimnetic oxygen on depths utilized by Cisco and the resulting vulnerability to predation was an important driver of shape and fin length. Deeper body depths and longer fin lengths were possibly a response to greater vulnerability to predation when hypoxic hypolimnia limit coldwater refugia for Cisco, which are forced to inhabit shallower depths near warmwater predators such as Walleye Sander vitreus and Northern Pike Esox lucius. Differences in size and shape relationships of Cisco in lakes on and off the Canadian Shield were also detected. The availability of late‐summer habitat, depth‐mediated predation risk, and growth potential appear to be important ecological drivers of morphological variation in Cisco.

Eutrophication and climate warming are profoundly affecting fish in many freshwater lakes. Understanding the specific effects of these stressors is critical for development of effective adaptation and remediation strategies for conserving fish populations in a changing environment. Ecological niche models that incorporated the individual effects of nutrient concentration and climate were developed for 25 species of fish sampled in standard gillnet surveys from 1,577 Minnesota lakes. Lake phosphorus concentrations and climates were hindcasted to a pre-disturbance period of 1896–1925 using existing land use models and historical temperature data. Then historical fish assemblages were reconstructed using the ecological niche models. Substantial changes were noted when reconstructed fish assemblages were compared to those from the contemporary period (1981–2010). Disentangling the sometimes opposing, sometimes compounding, effects of eutrophication and climate warming was critical for understanding changes in fish assemblages. Reconstructed abundances of eutrophication-tolerant, warmwater taxa increased in prairie lakes that experienced significant eutrophication and climate warming. Eutrophication-intolerant, warmwater taxa abundance increased in forest lakes where primarily climate warming was the stressor. Coolwater fish declined in abundance in both ecoregions. Large changes in modeled abundance occurred when the effects of both climate and eutrophication operated in the same direction for some species. Conversely, the effects of climate warming and eutrophication operated in opposing directions for other species and dampened net changes in abundance. Quantifying the specific effects of climate and eutrophication will allow water resource managers to better understand how lakes have changed and provide expectations for sustainable fish assemblages in the future.

Lakes in Minnesota face a number of large-scale ecological stressors that threaten critical aquatic habitat and fish populations. We developed a fish habitat conservation framework to guide protection and restoration efforts for lakes of the state. Surrogate measures of habitat quality were used to assess fish habitat conditions in more than 1,800 Minnesota lakes. Two fundamental fish habitat types in lakes were described (physical and water quality) and geographic information system-based surrogate measures of habitat condition (shoreline and watershed disturbance) were quantified for each habitat type. Simultaneous consideration of the two habitat types were used to develop a bivariate classification of habitat condition. Habitat condition classifications were identified using data from previous studies to categorize lakes into protection and restoration classes. Appropriate protection and restoration actions was then tailored for each classification of habitat condition. The conservation framework is actively being used to protect and restore habitat in lakes throughout Minnesota and is potentially useful for other regions and spatial scales where anthropogenic disturbances affect fish habitat. Los lagos en Minnesota enfrentan cantidad de factores de estrés ecológico de gran escala, que amenazan hábitats acuáticos críticos para las poblaciones de peces. En este trabajo se desarrolla un marco de referencia de conservación de hábitats para peces, con la finalidad de guiar los esfuerzos de protección y restauración que se lleven a cabo en los lagos del estado. Se utilizaron mediciones representativas de la calidad del hábitat con el objetivo de evaluar las condiciones del hábitat en más de 1,800 lagos de Minnesota. Se describieron dos tipos fundamentales de hábitat de peces (en términos físicos y de calidad del agua) y para cada tipo de hábitat, se cuantificaron medidas representativas de información geográfica (línea de costa y perturbaciones en la cuenca hidrográfica). Se consideraron simultáneamente dos tipos de hábitats para desarrollar una clasificación bivariada de la condición del hábitat. Con el fin de categorizar a los lagos en distintos grados de protección y restauración, se identificaron las clasificaciones de la condición del hábitat a partir de datos de estudios previos. Posteriormente se diseñaron acciones de protección y restauración para cada clasificación de condición de hábitat. El marco de referencia de conservación se utiliza activamente para proteger y restaurar hábitats de los lagos a lo largo de Minnesota, y puede ser potencialmente útil para otras regiones y escalas espaciales donde existan disturbios de origen humano que afecten el hábitat de los peces. Les lacs du Minnesota font face à un certain nombre de facteurs de stress écologiques à grande échelle qui menacent le milieu aquatique vital et les populations de poissons. Nous avons développé un cadre de conservation de l'habitat du poisson pour guider les efforts de protection et de restauration pour les lacs de l'État. Des mesures substitutives de qualité de l'habitat ont été utilisées pour évaluer les conditions de l'habitat du poisson dans plus de 1800 lacs du Minnesota. Deux types fondamentaux d'habitats du poisson dans les lacs ont été décrits (physique et en fonction de la qualité de l'eau) et des mesures de substitution de l'état de l'habitat basées sur un système d'information géographique (la perturbation du rivage et des bassins versants) ont été quantifiés pour chaque type d'habitat. Un examen simultané des deux types d'habitats a été utilisé pour élaborer une classification bivariée de l'état de l'habitat. Les classifications des conditions de l'habitat ont été identifiées à partir des données provenant d'études précédentes pour classer les lacs dans les classes de protection et de restauration. Les actions de protection et de restauration appropriées ont ensuite été adaptées pour chaque classification de l'état de l'habitat. Le cadre de conservation est activement utilisé pour protéger et restaurer l'habitat dans les lacs à travers tout le Minnesota et est potentiellement utile pour d'autres régions et échelles spatiales où les perturbations anthropiques affectent l'habitat du poisson.

Climate warming would alter water temperature and dissolved oxygen (DO) characteristics of lakes. These changes can be expected to have an effect on fish populations. A process-oriented, dynamic, one-dimensional year-round lake water quality model was developed and applied to simulate daily temperature and DO profiles in Minnesota lakes over 48 (1961 to 2008) or 18 (1991 to 2008) years, depending on weather data availability. The model was calibrated with data from 28 study lakes, and the average standard error of estimate against measured data was 1.47 °C for water temperature and 1.50 mg/L for DO. Projected future climate scenarios were based on the output of the Coupled Global Climate Model (CGCM), CCCma CGCM 3.1 from the Canadian Climate Centre for Climate Modeling and Analysis (CCCma), and the Model for Interdisciplinary Research on Climate, MIROC 3.2 developed in Japan. Simulated water temperature and DO profiles under past and projected future climate conditions were used to identify coldwater fish habitat in the 28 Minnesota lakes. The purpose of the study was to identify refuge lakes for cisco (tullibee), a coldwater fish species, under climate warming scenarios. Cisco is a food source for predatory sport fish species such as walleye and northern pike. Suitable coldwater fish habitat was identified by two methods: (1) a constant upper (lethal) temperature and a lower DO survival limit over the entire simulation period, or (2) variable temperature and DO survival limits based on observed fish mortality. Results obtained by both fish habitat models indicate that 19 deep, seasonally stratified, mesotrophic or oligotrophic lakes located in north or north-central Minnesota can support coldwater fish habitat under both past and future climate scenarios, and these lakes are candidates of refuge lakes. Once refuge lakes are identified, lake watershed protection efforts can be initiated at refuge lakes to prevent deterioration of water quality in these lakes by anthropogenic activities. The presentation will cover the lake water quality model development and calibration, the fish habitat model development, the application of both models to 28 lakes, the simulation results and the implications for watershed management to prevent other deteriorations of coldwater fish habitat.