Significant resources are used each year to manage the abundance of invasive, submersed aquatic plants, mostly in disturbed systems. Only recently has low-cost, automated hydroacoustic methods been developed and used for mapping aquatic plant abundance and conducting "before-after" assessments of aquatic plant management (APM) activities. Recent studies suggest that macrophyte abundance is highly variable in disturbed systems even in the absence of management. Therefore, detecting an APM management "signal" through the background "noise" inherent in disturbed aquatic systems is a challenge. I repeated hydroacoustic and species surveys during the course of 5 yr (2011 to 2015) in one eutrophic Minnesota glacial lake that experienced minimal APM. Basic geographic information system raster comparison methods were used to compare aquatic plant biovolume (percentage of water column filled with vegetation) maps across years and created a benchmark map based on the time series average. Deviation maps displayed spatial and temporal variability, with average lakewide plant biovolume ranging from 53% in 2013 to 31% in 2015. There were no consistent interannual trends or correlations with other meteorological variables (e.g., ice-out, average temperature, or precipitation amount). Although aquatic plant biovolume was highly variable, percentage of cover and other indicators (dominance and index of biotic integrity) remained relatively similar during the course of the study. To be more confident in APM prescriptions and outcomes, my results suggest that greater investment into long-term aquatic plant abundance monitoring programs will be necessary to establish benchmarks and ranges of variability in different types of lakes.

Accurate information on the location and condition of spawning grounds of environmentally-demanding lithophilic fish species, which may use only a very small area of their habitat for spawning, is critical to their conservation and fisheries management but is frequently lacking. Here, the new hydroacoustic system BioBase, which enables the rapid characterisation of features including lake bottom hardness (with soft, medium hard and hard bottoms represented by values of 0 to 0.25, 0.25 to 0.40, and 0.40 to 0.50, respectively), was applied to known spawning grounds of Arctic charr (Salvelinus alpinus) in the north basin of the eutrophicated lake of Windermere, U.K. The output of BioBase was successfully ground-truthed using an independent video-based system (r2 = 0.48, F = 17.705, p < 0.001) and depth and bottom hardness descriptive statistics were produced for six spawning grounds. Average depth ranged from 9.4 m (North Thompson Holme) to 38.5 m (Balla Wray), while average bottom hardness ranged from 0.254 (Low Wray Bay) to 0.303 (North Thompson Holme). Detailed visual outputs were also produced for contrasting shallow (North Thompson Holme) and deep (Holbeck Point) spawning grounds, both of which showed high within-site spatial variation in bottom hardness and thus in suitability for spawning. Findings were consistent with earlier, less quantitative, interpretations of the possible effects of eutrophication and associated increased deposition of fine sediments on local Arctic charr reproduction.

Biological indicators that signal changes in lake condition are essential tools for guiding resource management decisions. Macrophyte-based indicators have traditionally been selected and evaluated in the context of nutrient-based stressors, although the need to evaluate indicators that are sensitive to climate stressors has been increasingly relevant. Moreover, indicators should ideally exhibit minimal sampling variation and have low natural temporal variation so there is high power to detect changes in the mean value over time. Eight macrophyte indicators were estimated in 23 Minnesota (USA) lakes using four years of repeated surveys to estimate sampling and temporal variation, response to development (phosphorus concentration) and climate stress (annual growing degree days), and power to detect significant change at various annual sampling intervals. Indicators included a macrophyte index of biotic integrity, floristic quality index, maximum depth of growth, total species richness, common species richness, mean richness, and frequency occurrence of rooted species and Chara sp. Overall, regression and smoothed additive models indicated significant relationships of indicators to total lake phosphorus and mean annual growing degree days. The macrophyte index of biotic integrity, floristic quality index, and the frequency rooted species had minimal sampling variation in this study, were responsive to development or climate stress, and had low annual variation (coefficients of variation 0.08, 0.10, and 0.19, respectively) resulting in high to moderate power (>50%) for detecting significant change over a 20 year period. Results from these analyses will facilitate the use of precise and powerful indicators that respond to stressors that are of concern for the management of freshwater glacial lakes. (C) 2014 Elsevier Ltd. All rights reserved.

Many ecosystem goods and services are derived from aquatic plant–dominated environments and the abundance and composition of aquatic plant communities affects habitat, recreation, angling, aesthetics, and commerce. We describe standardized hydroacoustic methodology that complements species composition surveys and generates comprehensive aquatic plant abundance data with little additional assessment or analysis effort than is already put forth for species surveys. Using data from 22 lakes across the United States, collected by biologists with varying levels of expertise, we compare hydroacoustically derived biovolume with two other semiquantitative measures of whole-lake abundance (frequency of occurrence and ‘‘rake fullness’’). Although we documented some significant correlations between hydroacoustically derived biovolume and frequency and rake fullness, frequency or rake fullness was difficult to interpret biologically on a lakewide scale. We also describe a dominance index that incorporates both species composition and vegetation biovolume to evaluate the degree that a species dominates a local assemblage. We found that the extent of aquatic plant growth and invasive dominance was related to lake productivity with highest biovolume and dominance occurring in mesotrophic to eutrophic study lakes. Using both empirical and simulated data, we also found no significant differences between dominance calculated from a simple metric that gives equal weight to all species at a survey site and a metric that incorporated rake fullness for each species.

Curlyleaf pondweed (Potamogeton crispus) is a long-established, nonnative aquatic plant common throughout southern and central Minnesota that is thought to be expanding northward. Curlyleaf pondweed typically grows abundantly in spring in productive lakes and then senesces in midsummer, often followed by algae blooms. We report observations of widespread, short-term declines in curlyleaf pondweed cover that appear linked to winter snow depth on frozen lakes. These findings suggest that climate change may greatly affect habitat suitability for curlyleaf pondweed. As Minnesota lakes warm with less snow cover limiting light penetration, curlyleaf pondweed growth will likely increase. These observations form the foundation for targeted follow up studies that more precisely describe conditions limiting the growth and expansion of curlyleaf pondweed in north-temperate, North American lakes.

─ Curly-leaf pondweed Potamogeton crispus is a long-established invasive in Minnesota. It is common throughout southern and central Minnesota and is thought to be expanding northward. Curly-leaf pondweed typically grows abundantly in spring in productive lakes and then senesces in mid-summer followed by algal blooms. Results from the literature and Sentinel Lakes suggest the relationships between curly-leaf senescence and water quality vary substantially among lakes. The post-senescence decreases in water clarity are most pronounced in shallow lakes with minimal native vegetation. In these lakes, abundant growth of curly-leaf is followed by severe algal blooms as described above. However, in shallow lakes with abundant native vegetation, the post-senescence decreases in water clarity are muted. In deep lakes by comparison, we saw fewer decreases in water clarity following curly-leaf senescence. Where curly-leaf density is low to moderate, observations of decreases in water clarity following curlyleaf senescence were minimal. We also share unexpected findings of widespread, short-term declines in curly-leaf pondweed and evaluate possible reasons for these declines. Evidence suggests that high early-season snowfall may decrease curly-leaf pondweed production in lakes even if ice-out occurs earlier. These findings provide a basis for more targeted follow-up investigations that seek to predict what lake environments should be most favorable for curly-leaf pondweed growth and where the plant is most likely to have negative consequences on water quality and fish habitat. This will facilitate better invasive plant management decisions and actions and more efficient targeting of management resources. Finally, our findings also suggest that climate change may significantly affect habitat viability for curly-leaf pondweed. If more winter precipitation falls as snow, this could reduce curly-leaf pondweed abundance across lakes. If more winter precipitation falls as rain, curly-leaf pondweed populations could expand. Current climate models clearly place Minnesota in the transition between winter precipitation that falls as rain or snow, with the line potentially moving north over the long-term as the climate warms. 1 This project was funded in part by the Federal Aid in Sport Fish Restoration (Dingell-Johnson) Program, in part by the Minnesota Environment and Natural Resources Trust Fund, and in part by the Minnesota Pollution Control Agency. Completion Report, Study 605, D-J Project F-26-R Minnesota.

Traditional approaches for managing aquatic resources have often failed to account for effects of anthropogenic disturbances on biota that are not directly reflected by chemical and physical proxies of environmental condition. The index of biotic integrity (IBI) is a potentially effective assessment method to integrate ecological, functional, and structural aspects of aquatic systems. A macrophyte-based IBI was developed for Minnesota lakes to assess the ability of aquatic plant communities to indicate environmental condition. The index was developed using quantitative point intercept vegetation surveys for 97 lakes that represent a range of limnological and watershed characteristics. We followed an approach similar to that used in Wisconsin to develop the aquatic macrophyte community index (AMCI). Regional adaptation of the AMCI required the identification of species representative of macrophyte communities in Minnesota. Metrics and scaling methods were also substantially modified to produce a more empirically robust index. Regression analyses indicated that IBI scores reflected statewide differences in lake trophic state (R2=0.57, F=130.3, df=1, 95, p<0.005), agricultural (R2=0.51, F=83.0, df=1, 79, p<0.005), urban (R2=0.22, F=23.0, df=1, 79, p<0.005), and forested land uses (R2=0.51, F=84.7, df=1, 79, p<0.005), and county population density (R2=0.14, F=16.6, df=1, 95, p<0.005). Variance partitioning analyses using multiple regression models indicated a unique response of the IBI to human-induced stress separate from a response to natural lake characteristics. The IBI was minimally affected by differences in sample point density as indicated by Monte Carlo analyses of reduced sampling effort. Our analysis indicates that a macrophyte IBI calibrated for Minnesota lakes could be useful for identifying differences in environmental condition attributed to human-induced stress gradients.

We explored patterns of habitat use and movement of three declining fish species intolerant to eutrophication in a north-temperate (Minnesota, USA) glacial lake: the blackchin shiner Notropis heterodon , blacknose shiner Notropis heterolepis , and banded killifish Fundulus diaphanus . We marked individuals with elastomer tags and estimated movement distances of recaptured individuals. Estimated home ranges for all species ranged from 3,264 to 19,525 m 2 , which covered 0.8 and 5.0% of our study lake’s total littoral area. Individuals of all species traveled to opposite ends of the lake over periods of time as short as 24 h. Using Geographic Information System (GIS) overlays and generalized additive models, we found fish species occurrences to be positively associated with macrophyte biovolume greater than 20% and with a high probability of occurrence of Chara. The magnitude of main and interaction effects varied among years and species. Overall, blackchin shiner occurrence was most strongly associated with biovolume. In other species by year combinations, biovolume and Chara explained varying degrees of variance in fish probability of occurrence. Our results suggest that controlling lake eutrophication and protecting of refuge areas of dense macrophytes and Chara may be needed to conserve these species.

Short-term variability of spatial heterogeneity of submersed macrophyte biovolume (percent of water column occupied by vegetation) was evaluated over 3 years along a gradient of productivity in four north temperate glacial lakes in Minnesota, USA. We hypothesized we would observe the lowest among-year variability in spatial heterogeneity of biovolume in our undisturbed, moderately productive lake and high variability in our more locally disturbed productive lakes. Our analysis involved three major steps: first, we removed negative trends of biovolume across depth with non-parametric regression smoothers; second, we examined spatial pattern in residuals using variograms; finally, we compared spatial pattern of biovolume among lakes seasonally, over 3 years. Lake productivity negatively correlated with water clarity and the depth range of macrophyte growth, and positively correlated with the variability of spatial patterns. In the least disturbed moderately productive lake, vegetation grew over a large range of depths (up to 7.5 m), and spatial pattern across the littoral zone was similar for each survey. In contrast, in the more turbid, productive lakes, depth and spatial patterns of biovolume varied greatly from survey to survey. Factors that increase productivity and weaken resilience in lakes may lead to unstable spatial patterns of macrophyte biovolume.

Hydroacoustics, coupled with GPS and GIS represents a promising tool in monitoring changes to submersed vegetation biovolume, which is important for many Minnesota fish species. However, prior to establishing operational survey programs using these technologies, the performance of the equipment, software, and survey methodology must be rigorously evaluated. Accordingly, we conducted ground-truth experiments with a BioSonics Inc. digital echosounder by comparing estimates of bottom depth, plant height, and depth to the top of the plant made with EcoSAV vegetation analysis software with measurements made with divers. EcoSAV-estimated and diver-measured plant heights did not differ significantly, however, the EcoSAV-estimated position of the plant in the water column did differ from the diver-measured position. On average, EcoSAV over-estimated bottom depth by 0.18 m and over-estimated the depth from the surface to the top of the plant by 0.23 m. As a result, the EcoSAV estimates indicated that plants occupied less of the water column than divermeasured values. Bias in bottom measurements was likely due to signal penetration of the soft sediments in Square Lake by the echosounder. Bias in top of plant measurements was likely a result of difficulty placing the transducer directly over the marker buoys, so the top of the plant sometimes fell outside and above the acoustic cone. We also evaluated whether boat navigation error affected the accuracy and precision of vegetation maps, and the repeatability of whole-lake surveys. To do so, we conducted surveys on three consecutive days in two diversely vegetated lakes. Boat navigation RMSE averaged 3.5 to 4.0 meters; however, GPS location error was only ± 1.06 m. These errors had little effect on the overall accuracy and precision of maps of biovolume in both lakes. Precision of biovolume estimates was lower at depths less than 2 meters than at deeper depths. *Corresponding author: phone 651-793-6539, fax 651-772-7974, e-mail ray.valley@dnr.state.mn.us 1 This project was funded in part by the Federal Aid in Sport Fish Restoration (Dingell-Johnson) Program. Completion Report, Study 639, D-J Project F-26-R Minnesota.

Whole-lake techniques are increasingly being used to selectively remove exotic plants, including Eurasian watermilfoil (Myriophyllum spicatum L.). Fluridone (1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone), a systemic whole-lake herbicide, is selective for Eurasian watermilfoil within a narrow low concentration range. Because fluridone applications have the potential for large effects on plant assemblages and lake food webs, they should be evaluated at the whole-lake scale. We examined effects of low-dose (5 to 8 ppb) fluridone applications by comparing submersed plant assemblages, water quality and largemouth bass (Micropterus salmoides) growth rates and diets between three reference lakes and three treatment lakes one- and two-years post treatment. In the treatment lakes, fluridone reduced Eurasian watermilfoil cover without reducing native plant cover, although the duration of Eurasian watermilfoil reduction varied among treatment lakes. Large pre-treatment differences among lakes in plant cover persisted, reflecting morphometric differences. We detected no treatment effects on water quality, and estimated the probability of treatment converting a lake to eutrophic conditions as being <= 0.10. Growth of largemouth bass > 200 mm total length did not change in treatment lakes and we detected few treatment effects on their diet. However, in two treatment lakes, growth of smaller largemouth bass increased modestly following treatment. Overall, the extent of effects of low-dose fluridone treatment differed among lakes, apparently reflecting differences in initial conditions that, in part, were driven by lake morphometry. Therefore, future evaluations should include more lakes, chosen to represent a range of morphometry, allowing extrapolation of findings to regions containing many diverse lakes.