Changes in Aquatic Macrophytes after Liming Thrush Lake, Minnesota

Thrush Lake, Minnesota, was treated with limestone in 1988 to evaluate the efficacy of protective base addition against the loss of sport fisheries in a sensitive, mildly acidic lake. Prior to treatment, the lake was stressed (pH 6.46, ANC 64 μeq/L) but not severely degraded by acidic deposition and had a macrophyte community typical of lakes in northeastern Minnesota with low acid-neutralizing capacity (ANC). This paper describes the changes observed in aquatic plant communities during the 5 years after treatment, as pH and ANC slowly returned to pretreatment levels. Sphagnum platyphyllum, intolerant of non-acid conditions, was completely eliminated from the lake. The charo-phyte, Nitella, that originally shared dominance in the deep littoral zone with S. platyphyllum, decreased in importance during the first 2 years after treatment. Two vascular plants, Potamogeton pusillus and Najas flexilis, were first found in the lake the year after treatment and were abundant for 2 years after liming, probably in response to a combination of more neutral pH and reduced cover of Nitella. As the ANC and pH slowly returned to pretreatment conditions, Nitella again increased in coverage and depth range, with a concomitant decrease in P. pusillus and N. flexilis. The moss, S. platyphyllum, had not reinvaded the lake by 1993, 2 years after its dramatic decline.     

Fish-Community Response To Protective Liming in Thrush Lake, Minnesota

A protective limestone treatment was applied to an acid-sensitive lake in northeastern Minnesota as part of the Acid Precipitation Mitigation Program. This 6–year study evaluated the impact of that treatment on lakes in the upper Midwest that experience episodes of acid stress but have not lost basic species integrity and community structure. Several changes in the fish community can be directly or indirectly attributed to the addition of 4.6 tonnes of calcium carbonate early in the third year of the study. An almost 30–fold increase in the population of Pimephales promelas(fathead minnow) a year after liming, based on mark-recapture estimates from trap netting and snorkeling, was attributed to a pH increase and a three-fold increase in the calcium concentration of the epilimnion. After the initial increase, the abundance of fathead minnows declined in subsequent years, as did the elevated pH and calcium concentrations. The Salvelimis fontinalis(brook trout) population also increased in the lake following application of limestone, but this was due in part to closing the lake to fishing. An increase in survival of stocked brook trout to age 1+ and an increase in growth of older brook trout after liming were attributed to the increased forage that the fathead minnows provided. Fathead minnows may have also reduced predation pressure on young brook trout by older brook trout. This study demonstrated that liming of a slightly acidic lake did not adversely affect the integrity of the fish community, and in fact may have increased the abundance and biomass of the forage fish community and indirectly increased the survival, abundance, and growth of brook trout.     

Impact of a Protective Limestone Treatment on the Water Chemistry and Zooplankton Community of Thrush Lake, Minnesota

A limestone slurry was sprayed on the surface of Thrush Lake, a small headwater lake in northeastern Minnesota, to test a treatment designed to protect acid-sensitive waters from anthropogenic acidification. The 6-year study, consisting of pretreatment, transition, and post-treatment phases, was part of the four-state Acid Precipitation Mitigation Program directed by the U.S. Fish and Wildlife Service. Measured water-chemistry parameters, including acid-neutralizing capacity, pH, dissolved calcium, and dissolved inoroganic carbon, increased following treatment, although local climatic conditions influenced the magnitude and duration of the chemical changes. Physical changes to the lake, other than an increase in conductivity and a short-term alteration of water clarity subsequent to treatment, were not documented. The composition of the zooplankton community was altered, with the proportion of rotifers increasing after treatment. Individual zooplankton species showed a variety of changes in abundance that were associated with treatment over both seasonal and multi-year intervals. For example, Holopedium gibbemm was absent from lake samples immediately following treatment and recovered within a season, whereas Diaptomus minutus and Keratella taurocephala populations were reduced after treatment and had not recovered by the end of the study. Alternately, Asplanchna priodonta increased in abundance after treatment. These observed abundance patterns were generally consistent with previzous acidification or base-addition studies. In contrast, the changes in community composition of zooplankton did not consistently fit patterns developed from regional studies across water-chemistry gradients. These differences emphasize the importance of biotic as well as abiotic factors in controlling zooplankton community composition.     

Limnology of Shagawa Lake,Minnesota, prior to reduction of phosphorus loading

Various limnological parameters have been measured in Shagawa Lake, a culturally eutrophic lake in northeastern Minnesota, from 1970–1972. These included temperature; specific conductance; dissolved oxygen; alkalinity; pH; total and orthophosphate phosphorus; nitrate, nitrite and ammonia nitrogen; chlorophyll a; transparency; major cations; and selected trace elements. The lake exhibited many characteristics of high productivity. Concentrations of chlorophyll a reached 60 µg/l during summer months and were reflected in pH values in excess of 9. Anaerobic conditions developed during both summer and winter and, during these intervals, large pools of available nutrients developed in the lower waters. Manganese and iron concentrations varied greatly as a consequence of changes in the oxygen regime. Major cations and other trace elements showed no yearly pattern.     

Lake sturgeon population attributes and reproductive structure in the Namakan Reservoir,Minnesota and Ontario

Quantified were the age, growth, mortality and reproductive structure of lake sturgeon (Acipenser fulvescens) collected in the US and Canadian waters of the Namakan Reservoir. The hypotheses were tested that (i) age and growth of lake sturgeon in the Namakan Reservoir would differ by sex and reproductive stage of maturity, and (ii) that the relative strength of year‐classes of lake sturgeon in the reservoir would be affected by environmental variables. To quantify age, growth and mortality of the population, existing data was used from a multi‐agency database containing information on all lake sturgeon sampled in the reservoir from 2004 to 2009. Lake sturgeon were sampled in the Minnesota and Ontario waters of the Namakan Reservoir using multi‐filament gillnets 1.8 m high and 30–100 m long and varying in mesh size from 178 to 356 mm stretch. Reproductive structure of the lake sturgeon was assessed only during spring 2008 and 2009 using plasma testosterone and estradiol‐17β concentrations. Ages of lake sturgeon >75 cm ranged from 9 to 86 years (n = 533, mean = 36 years). A catch‐curve analysis using the 1981–1953 year classes estimated total annual mortality of adults to be 4.8% and annual survival as 95.2%. Using logistic regression analysis, it was found that total annual precipitation was positively associated with lake sturgeon year‐class strength in the Namakan Reservoir. A 10 cm increase in total annual precipitation was associated with at least a 39% increase in the odds of occurrence of a strong year class of lake sturgeon in the reservoir. Plasma steroid analysis revealed a sex ratio of 2.4 females: 1 male and, on average, 10% of female and 30% of male lake sturgeon were reproductively mature each year (i.e. potential spawners). Moreover, there was evidence based on re‐captured male fish of both periodic and annual spawning, as well as the ability of males to rapidly undergo gonadal maturation prior to spawning. Knowledge of lake sturgeon reproductive structure and factors influencing recruitment success contribute to the widespread conservation efforts for this threatened species.     

Timing of predation by rainbow trout controls Daphnia demography and the trophic status of a Minnesota lake

1. Stocking of lakes with rainbow trout is a common practice that presents a potential conflict for lake managers who must balance the interests of anglers with those concerned that zooplanktivory by trout may trigger a trophic cascade and result in decreased water clarity. 2. This study examined how the timing of trout stocking (autumn versus spring) in a Minnesota (U.S.A.) lake affected (i) the population dynamics of their zooplankton food supply (Daphnia pulicaria), (ii) phytoplankton biomass and water clarity and (iii) trout survival. Sizes of both Daphnia and trout populations were estimated acoustically with high‐frequency (192 kHz) sonar. 3. Daphnia were nearly eliminated from the lake during winters after trout were stocked in autumn. In both of these years (1996 and 1997), the Daphnia population was small in the spring, and grew during the summer and into the autumn as the trout population diminished. 4. The lake was then stocked in spring for 2 years (1998 and 1999). This fisheries manipulation alleviated predation over the winter, but increased predation on D. pulicaria during the spring, summer and autumn. However, the high mortality caused by the spring‐stocked trout was offset by even higher rates of reproduction by the relatively large populations of fecund Daphnia that survived the winter in 1998 and 1999. 5. Grazing by these dense populations of Daphnia produced clear‐water phases during May and June that were inhibited in autumn stocking years. In addition, the large Daphnia populations present during the spring and early summer of 1998 and 1999 provided abundant forage for trout. 6. This fisheries manipulation achieved seemingly mutually exclusive management objectives: a robust planktivorous sport fishery, and clear water for other forms of recreation.     

The spatial distribution of desmids in lakes in northern Minnesota, U.S.A.

As a result of studies in certain lakes in northern Minnesota it has been found that the spatial distribution of desmids in them depends especially on the following factors: the constitution, abundance and distribution of the lakes' submerged aquatic flora and the morphometry and consequent patterns of their water circulation. The study has also shown a positive relationship between the size of desmid cells present in the plankton and the extent of water circulation. The latter factor may determine the composition and hence the diversity of lake phytoplankton.     

Lake sturgeon population characteristics in Rainy Lake, Minnesota and Ontario

Rainy Lake contains a native population of lake sturgeon Acipenser fulvescens that has been largely unstudied. The aims of this study were to document the population characteristics of lake sturgeon in Rainy Lake and to relate environmental factors to year‐class strength for this population. Gill‐netting efforts throughout the study resulted in the capture of 322 lake sturgeon, including 50 recaptures. Lake sturgeon in Rainy Lake was relatively plump and fast growing compared with a 32‐population summary. Population samples were dominated by lake sturgeon between 110 and 150 cm total length. Age–structure analysis of the samples indicated few younger (<10 years) lake sturgeon, but the smallest gill net mesh size used for sampling was 102 mm (bar measure) and would not retain small sturgeon. Few lake sturgeon older than age 50 years were captured, and maximum age of sampled fish was 59 years. Few correlations existed between lake sturgeon year‐class indices and both annual and monthly climate variables, except that mean June air temperature was positively correlated with year‐class strength. Analysis of Rainy Lake water elevation and resulting lake sturgeon year‐class strength indices across years yielded consistent but weak negative correlations between late April and early June, when spawning of lake sturgeon occurs. The baseline data collected in this study should allow Rainy Lake biologists to establish more specific research questions in the future.     

Limnology of Botos Lake, a tropical crater lake in Costa Rica

Botos Lake, located at the Poas Volcano complex (Costa Rica) was sampled eight times from 1994 to 1996 for physicochemical conditions of the water column and phytoplanktonic community composition. Depth was measured at fixed intervals in several transects across the lake to determine its main morphometric characteristics. The lake has an outlet to the north. It is located 2580 m above sea level and is shallow, with a mean depth of 1.8 m and a relative depth of 2.42 (surface area 10.33 ha, estimated volume 47.3 hm3). The lake showed an isothermal water column in all occasions, but it heats and cools completely according to weather fluctuations. Water transparency reached the bottom on most occasions (> 9 m). The results support the idea that the lake is polymictic and oligotrophic. The lake has at least 23 species of planktonic algae, but it was always dominated by dinoflagellates, especially Peridinium inconspicuum. The shore line is populated by a sparse population of Isoetes sp. and Eleocharis sp. mainly in the northern shore where the bottom has a gentle slope and the forest does not reach the shore.     

A LIMNOLOGICAL INVESTIGATION OF LAKE LIAMBEZI,CAPRIVI

SUMMARY

A general investigation was carried out on Lake Liambezi to characterize the limnological features of the lake. The lake, 101 km² in area and situated within the tropics, was found to be shallow and bordered by reed swamps, particularly on the south-western shore. It was polymictic and consequently oxygen levels were high at all depths. Transparency was such that photosynthesis occurred to the bottom. Conductivity ranged from 17,3 to 41,2 mSm^?1 and the waters were moderately alkaline. Transfer of organic detritus from the reed swamp into the open water has resulted in the presence of high levels of organic nitrogen and phosphorus whilst ambient levels of inorganic forms were low. Planktonic chlorophyll a concentrations ranged between 1 and 104 μg l^?1 and Microcystis was the dominant phytoplankter. The dominant zooplankter was Bosmina. The important species of phytoplankton and zooplankton are listed and include two new species of blue-green algae which are described. The lake was found to be generally similar to most shallow tropical African lakes with a wide diversity of food sources for fish. Progressive encroachment by Salvinia appears to hamper access to fishing areas and the fishery potential is threatened.     

Effectiveness of Lethal,Directed Wolf-Depredation Control in Minnesota

Abstract: Wolf (Canis lupus) depredations on livestock in Minnesota, USA, are an economic problem for many livestock producers, and depredating wolves are lethally controlled. We sought to determine the effectiveness of lethal control through the analysis of data from 923 government-verified wolf depredations from 1979 to 1998. We analyzed the data by 1) assessing the correlations between the number of wolves killed in response to depredations with number of depredations the following year at state and local levels, and 2) the time to the next depredation. No analysis indicated that trapping wolves substantially reduced the following year's depredations at state or local levels. However, more specific analyses indicated that in certain situations, killing wolves was more effective than no action (i.e., not trapping). For example, trapping and killing adult males decreased the re-depredation risk. At sheep farms, killing wolves was generally effective. Attempting to trap, regardless of the results, seemed more effective at reducing depredations than not trapping, suggesting that mere human activity near depredation sites might deter future depredations. (JOURNAL OF WILDLIFE MANAGEMENT 72(3):778–784; 2008)     

Field study of Haplochromis burtoni: habitats and co-habitant

Synopsis This report describes the natural habitat of the cichlid fish Haplochromis burtoni along the Burundi coast of Lake Tanganyika in central Africa. Parameters of the physical environment as well as more general biological factors are described qualitatively and quantitatively. Temporary shore pools, a previously undescribed habitat, are examined in detail. These pools, appear to play an important role in the life cycle of Haplochromis burtoni and other mouthbrooding cichlids by providing, for a large part of each year, a protected area 0 which is particularly suited for reproduction, possibly at the cost of high predation. As a result of circaannual changes in the lake level, the shore pools are alternately isolated from and rejoined with the lake, allowing a regular exchange of fauna.     

Traffic Noise and Inequality in the Twin Cities,Minnesota

It is widely known that prolonged exposure to high levels of traffic noise has several health effects. While scholarship in environmental justice has explored the environmental equity hypothesis in a wide range of areas, whether the spatial distribution of traffic noise is equitable among different racial and socioeconomic groups has rarely been explored, especially in the United States. This article addresses this lacuna by examining this relationship in the Twin Cities Metro Region, Minnesota. Traffic data from the Minnesota Department of Transportation were used to model the propagation of traffic noise over the study area and aircraft noise contour lines were added to account for aircraft noise. Inequities associated with exposure to chronic traffic noise were investigated using selected demographic and socioeconomic variables from the U.S. Census 2000. Statistical analysis was based on a regression model that addressed spatial autocorrelation. Results indicate that there is an association between noise levels and household income, median household value, the percentage of non-white residents, and the percentage of the population less than 18 years of age.     

Maintenance of zonation patterns in two species of flat periwinkle,Littorina obtusata and L. mariae

The zonation patterns of Littorina obtusata (L.) and Littorina mariae Sacchi et Rastelli were shown to be quite distinct on a sheltered rocky shore. L. obtusata was found at all the heights sampled; it reached peak numbers at mid shore on the alga Ascophyllum nodosum L. (Le Jol). There was no difference in the tidal height occupied by adults or juveniles; or in the mean size of L. obtusata along the vertical gradient of the shore. In contrast L. mariae occurred exclusively low on the shore, on Fucus serratus L. Translocation of the two species within their respective levels resulted in random movement after 4 days, although initial movements after 1 and 2 days were sometimes directional. Animals transplanted to the normal level of the other species showed directional movement towards their home zone; this was most pronounced after 4 days. There was no difference in the distance moved by the two species, although the distance moved did vary with tidal height, both species moving further at mid shore than low shore. Distances moved by littorinids at replicate areas in the low shore were similar but those at mid shore did vary. There was an interaction between the species and the different tidal heights which revealed that transplanted species moved further than translocated species at the same tidal level. However, this was only significant in the case of L. mariae. It is suggested that the close relationship between the winkles and their host algae may direct the homing behaviour of displaced individuals.     

Modeling the potential natural vegetation of Minnesota,USA

Assessing the effects of human land use and management decisions requires an understanding of how temporal changes in biodiversity influence the rate of ecosystem functions and subsequent delivery of ecosystem services. In highly modified anthromes, the spatial distribution of natural vegetation types is often unknown or coarsely represented challenging comparative analyses seeking to assess changes in biodiversity and potential downstream effects on ecosystem processes and functions. In this context, the objectives of this study were to construct a multi-resolution representation of potential natural vegetation at four hierarchical classification levels of increasing floristic and physiognomic detail for the state of Minnesota, USA. Using a collection of natural/near-natural vegetation relevés, a series of Random Forest classification models were used to project the potential distribution of natural vegetation types based on their association with a variety of environmental variables.Model performance varied within and between classification levels with overall accuracy ranging between 64–99% (kappa 0.44–0.99). Model performance tended to decrease and become more variable with increasing floristic complexity at finer classification levels. Classwise performance metrics including precision and sensitivity were also reported. A method for exploring potential class confusion resulting from niche overlap using Random Forest proximities and Nonmetric Multidimensional Scaling is demonstrated.Collectively, the results presented here provide an analytically supported baseline representation of potential natural vegetation for the state of Minnesota, USA. These data can provide a backdrop to further analyses surrounding the influence of human activity on ecosystem processes and services as well as inform future conservation and restoration efforts.     

Phylogeographic history of the genus Tropheus,a lineage of rock-dwelling cichlid fishes endemic to Lake Tanganyika

Lake Tanganyika contains the oldest and most complex flock of cichlid fishes counting about 200 endemic species. It is comprised of 16 ecologically, morphologically and genetically highly distinct tribes. Many species are further subdivided into arrays of geographic morphs, each colonizing particular sections of the shore line. The genus Tropheus represents the most spectacular and best studied example for this phenomenon, counting more than 100 distinctly colored populations and sister species, some living in sympatry. Their present distribution and genetic structure was shaped by a series of lake level fluctuations which caused cycles of isolation and secondary admixis. The present study extends previous work on the phylogeography of Tropheus and aims at the fine-scale reconstruction of the origin and spread of lineages in the central and southern basin of the lake. The previously defined mtDNA lineages were evaluated on the basis of statistical parsimony networks. Haplotype networks were created for each lineage and related to their centers of diversity in terms of present distribution. A linearized tree analysis and a mismatch distribution analysis corroborate two of the three radiation waves suggested in earlier works, but the new data suggest a different primary colonization scenario for the southern basin.     

The impact of the River Ntahangwa, the most polluted Burundian affluent of Lake Tanganyika, on the water quality of the lake

Sources of pollution at the north end of Lake Tanganyika are concentrated around Bujumbura, Burundi. The impact of River Ntahangwa on the water quality of this part of Lake Tanganyika was compared with that of an unpolluted river, the Mugere. A shallow bay, not influenced by a river, was used as a reference station. Chemical analyses were carried out at four week intervals during nearly a year. Samples were taken at different distances from the shore and at different depths. River impact was detected up to 100 m from both river mouths. Even at 50 m from the mouth of the polluted River Ntahangwa, no important decrease of dissolved oxygen was detected. Chlorophyll-a maxima were found 100 m from the Ntahangwa mouth, but this was also the case 100 m from shore in the unpolluted bay. Lake water at 60 m depth was saturated with oxygen during the dry season, as it used to be in 1956. The rivers studied descent initially within the lake due to greater sediment load and lower water temperature. Nevertheless, it appears that these rivers mix completely in the lake within 100–300 m from the river mouths. The direct effects from organic pollution of the Ntahangwa on the lake seem to be rather negligible. Appropriate anti-erosion measures should reduce massive sediment discharge into the lake.     

An atmospheric pollen survey conducted during the winter in Minneapolis,Minnesota, USA

Few atmospheric pollen surveys have been conducted during the winter months in cities located in the midlatitudes. The purpose of this present investigation was to study pollen conditions during the winter in Minneapolis, Minnesota, a city that experiences cold temperatures and develops substantial snow cover. One hundred thirty‐three atmospheric samples were collected with a Rotorod Sampler between 11 October 1995 and 1 May 1996. Pollen was recovered in 109 samples, including 27 of the 40 samples collected in December and January. Twenty‐five types of pollen were observed during the study period; Cupressaceae (cedar/juniper), Ambrosia (ragweed) and Artemisia (sage/sagebrush) grains were most abundant during the period when local plants were not flowering. The weed pollen was most likely contributed by local sources, perhaps by dried flowers on dead plants. In contrast, the Cupressaceae pollen recovered in October and the early winter months is thought to have originated in the southern United States where several species of Juniperus flower late in the year. This study demonstrated that low but measurable concentrations of pollen occurred in the atmosphere throughout the winter.     

Socioeconomic and ecological correlates of bobcat harvest in Minnesota

Management of game animals requires understanding of factors that affect harvest levels. Although influenced by international law, bobcat (Lynx rufus) management is the responsibility of state or provincial agencies, and jurisdictional environmental, ecological, and regulatory differences may alter which variables influence harvest. Consequently, our understanding of the factors driving bobcat harvest should be at a scale similar to that at which they are managed. We associated 32 years of bobcat harvest data from Minnesota with socioeconomic (e.g., pelt prices, license sales) and ecological variables (e.g., prey abundance, bobcat-specific index of winter severity) to determine what variables most strongly influenced annual bobcat harvest. We constructed candidate negative binomial generalized linear models based on an information–theoretic approach and used quasi-likelihood Akaike's Information Criterion adjusted for small sample size to assess the relative performance of each model. Our best model suggested that annual bobcat harvest in Minnesota was positively related to the proportion of scent stations visited by bobcats and season length, and negatively related to the proportion of days when the maximum temperature did not exceed the bobcat's lower critical temperature. Our results differ from those of other studies examining factors influencing furbearer harvest that have suggested furbearer harvest is driven primarily by pelt price, and suggest that managers can influence the annual harvest of bobcats by changing season length. © 2011 The Wildlife Society.     

Assessment of the Feasibility of Preventing Reproduction of Lake Charr, Salvelinus Namaycush, in Shallow Areas of Reservoirs Affected by Drawdowns

Lake charr, Salvelinus namaycush, spawn almost exclusively in lakes, in the fall. Spawning sites in inland lakes are generally located close to the shore in areas with coarse-textured substrate, and in depths of less than 2m. Because of the type of water management system applied in reservoirs where water levels are lowered during the winter, eggs deposited during the fall around the shallow shoreline of reservoirs can be exposed. The depth of spawning sites relative to drawdowns may be a limiting factor for lake charr survival in reservoirs. In 1995 and 1996, we evaluated the ability to induce lake charr reproduction in deeper water, below the range of fluctuating water levels by the sequential construction of artificial reefs and limiting access to natural spawning habitat. The creation of artificial deeper water spawning areas, adjacent to natural sites, produced a partial transfer of reproduction. However when natural spawning sites were covered with tarpaulins almost all the eggs were laid in the artificial deeper water spawning areas. Complete or partial transfer of reproduction activities to sites below the depth not affected by water level fluctuations is therefore a potential way of alleviating the effects of hydroelectric reservoir management.