Lake Superior Water Research Articles

Coastal wetland plant community responses to record-high Lake Superior water levels: An Allouez Bay case study

Here we present findings from a natural experiment to better understand coastal wetland plant community responses to rising water levels. Plant communities were monitored in three vegetation zones (submergent, emergent, and wet meadow) at Allouez Bay, a lacustrine coastal marsh, six times over years 2011–2020. Lake Superior water levels reached record-highs in 2017, and again in 2019. During our six sampling campaigns, we encountered eighty-four vascular plant species, seven of which were non-native. Except for reductions in total plant cover in the wet meadow zone, emergent and wet meadow plant communities were only marginally affected by rising water. Percent cover of non-native species did not increase in a clear pattern. Temporal changes in floristic quality were non-significant at the whole site level, and mean coefficient of conservatism values ranged from 5.3 to 6.0. Aquatic vegetation in the submergent zone was most affected by rising water. Submergent zone richness declined from sixteen plant species in 2011 to zero in 2020. Multivariate PERMANOVA analysis showed significant effects of year on site-wide plant composition. Temporal composition changes were predominately driven by species turnover in the submergent vegetation zone, whereby floating aquatic species were replaced by non-floating species from 2011 to 2017, and an absence of aquatic vegetation along research transects in 2020. Tracking regeneration of aquatic vegetation is a focus of future research as unknown effects from prolonged exposure to record-high water levels may affect natural regenerative processes at Allouez Bay, and potentially at other lacustrine Great Lakes wetlands throughout the basin.

Lake Superior Has Lost over 90% of Its Pesticide HCH Load since 1986.

The time trend of α- and γ-hexachlorocyclohexane (HCH) isomers in Lake Superior water was followed from 1986 to 2016, the longest record for any persistent organic pollutant (POP) in Great Lakes water. Dissipation of α-HCH and γ-HCHs was first order, with halving times (t1/2) of 5.7 and 8.5 y, respectively. Loss rates were not significantly different starting a decade later (1996–2016). Concentrations of β-HCH were followed from 1996–2016 and dissipated more slowly (t1/2 = 16 y). In 1986, the lake contained an estimated 98.8 tonnes of α-HCH and 13.2 tonnes of γ-HCH; by 2016, only 2.7% and 7.9% of 1986 quantities remained. Halving times of both isomers in water were longer than those reported in air, and for γ-HCH, they were longer in water than those reported in lake trout. Microbial degradation was evident by enantioselective depletion of (+)α-HCH, which increased from 1996 to 2011. Volatilization was the main removal process for both isomers, followed by degradation (hydrolytic and microbial) and outflow through the St. Mary’s River. Sedimentation was minor. Major uncertainties in quantifying removal processes were in the two-film model for predicting volatilization and in microbial degradation rates. The study highlights the value of long-term monitoring of chemicals in water to interpreting removal processes and trends in biota.

Chronic toxicity of aluminum, at a pH of 6, to freshwater organisms: Empirical data for the development of international regulatory standards/criteria.

The chemistry, bioavailability, and toxicity of aluminum (Al) in the aquatic environment are complex and affected by a wide range of water quality characteristics (including pH, hardness, and dissolved organic carbon). Data gaps in Al ecotoxicology exist for pH ranges representative of natural surface waters (pH 6-8). To address these gaps, a series of chronic toxicity tests were performed at pH 6 with 8 freshwater species, including 2 fish (Pimephales promelas and Danio rerio), an oligochaete (Aeolosoma sp.), a rotifer (Brachionus calyciflorus), a snail (Lymnaea stagnalis), an amphipod (Hyalella azteca), a midge (Chironomus riparius), and an aquatic plant (Lemna minor). The 10% effect concentrations (EC10s) ranged from 98 μg total Al/L for D. rerio to 2175 μg total Al/L for L. minor. From these data and additional published data, species-sensitivity distributions (SSDs) were developed to derive concentrations protective of 95% of tested species (i.e., 50% lower confidence limit of a 5th percentile hazard concentration [HC5-50]). A generic HC5-50 (not adjusted for bioavailability) of 74.4 μg total Al/L was estimated using the SSD. An Al-specific biotic ligand model (BLM) was used to develop SSDs normalized for bioavailability based on site-specific water quality characteristics. Normalized HC5-50s ranged from 93.7 to 534 μg total Al/L for waters representing a range of European ecoregions, whereas a chronic HC5 calculated using US Environmental Protection Agency aquatic life criteria methods (i.e., a continuous criterion concentration [CCC]) was 125 μg total Al/L when normalized to Lake Superior water in the United States. The HC5-50 and CCC values for site-specific waters other than those in the present study can be obtained using the Al BLM. Environ Toxicol Chem 2018;37:36-48. © 2017 SETAC.

Institute Profile: The Large Lakes Observatory and the Scientific Study of the Large Lakes of Earth

Institute Profile: The Large Lakes Observatory and the Scientific Study of the Large Lakes of Earth

Hydrological drivers of record‐setting water level rise on Earth's largest lake system

AbstractBetween January 2013 and December 2014, water levels on Lake Superior and Lake Michigan‐Huron, the two largest lakes on Earth by surface area, rose at the highest rate ever recorded for a 2 year period beginning in January and ending in December of the following year. This historic event coincided with below‐average air temperatures and extensive winter ice cover across the Great Lakes. It also brought an end to a 15 year period of persistently below‐average water levels on Lakes Superior and Michigan‐Huron that included several months of record‐low water levels. To differentiate hydrological drivers behind the recent water level rise, we developed a Bayesian Markov chain Monte Carlo (MCMC) routine for inferring historical estimates of the major components of each lake's water budget. Our results indicate that, in 2013, the water level rise on Lake Superior was driven by increased spring runoff and over‐lake precipitation. In 2014, reduced over‐lake evaporation played a more significant role in Lake Superior's water level rise. The water level rise on Lake Michigan‐Huron in 2013 was also due to above‐average spring runoff and persistent over‐lake precipitation, while in 2014, it was due to a rare combination of below‐average evaporation, above‐average runoff and precipitation, and very high inflow rates from Lake Superior through the St. Marys River. We expect, in future research, to apply our new framework across the other Laurentian Great Lakes, and to Earth's other large freshwater basins as well.

Nitrogen cycling in a freshwater estuary

Freshwater estuaries may be important control points but have received limited research attention, emblematic of a general under-appreciation of these ecosystems and the services they provide. These ecotone environments exist at the interface of rivers flowing into large lakes, where seiches cause mixing of lotic and lentic waters within flooded river deltas. We assessed the dissolved inorganic nitrogen (DIN) retention and processing controls in the Saint Louis River Estuary (SLRE), which receives inputs from rivers, urban sources, and Lake Superior. Nitrate (NO3–N) was the dominant form of DIN and was consistently highest in the lower estuary due to seiche-delivered Lake Superior water and nitrification of ammonium from urban sources. The estuary transitioned from a net NO3–N source during high flows to a net sink during summer baseflow conditions. NO3–N availability controlled site-specific denitrification rates while sediment organic matter explained the spatial pattern in denitrification potential. As the estuary shifted from a riverine state to one with more lake influence, seiches delivered Lake Superior NO3–N to the lower portion of the estuary, alleviating the final denitrification control and activating the estuary’s ‘denitrification pump’. This amplified removal condition is maintained by critically delivered NO3–N during periods of warm temperatures and long residence times. Often these controls are unsynchronized in streams where NO3–N is typically lowest during summer baseflow. Similar ephemeral biogeochemical processes are likely found within other seiche-prone lakes where organic-rich sediments accumulate at river mouths and are supplied with chemically distinct lake water during low flow periods.

A regime shift in Lake Superior ice cover, evaporation, and water temperature following the warm El Niñ winter of 1997–1998

Significant trends in Lake Superior water temperature and ice cover have been observed in recent decades, and these trends have typically been analyzed using standard linear regression techniques. Although the linear trends are statistically significant and contribute to an understanding of environmental change, a careful examination of the trends shows important nonlinearities. We identify a pronounced step change that occurred in Lake Superior following the warm El Niño winter of 1997–1998, resulting in a “regime shift” in summer evaporation rate, water temperature, and numerous metrics of winter ice cover. This statistically significant step change accounts for most of the long‐term trends in ice cover, water temperature, and evaporation during the period 1973–2010, and it was preceded (and followed) by insignificant linear trends in nearly all of the metrics examined. The 1998 step change is associated with a decrease in winter ice duration of 39 d (a 34% decline), an increase of ∼ 2–3°C in mean surface water temperature (July–September averages), and a 91% increase in July–August evaporation rates, reflecting an earlier start to the summer evaporation season. Maximum wintertime ice extent decreased by nearly a factor of two, from an average of 69% of the lake surface area (before 1997–1998) to 36% after the step change. This reassessment of long‐term trends highlights the importance of nonlinear regime shifts such as the 1997–1998 break point—an event that may be related to a similar shift in the Pacific Decadal Oscillation that occurred around the same time. These pronounced changes in Lake Superior physical characteristics are likely to have important implications for the broader lake ecosystem.

Phototoxicity of TiO2 nanoparticles to a freshwater benthic amphipod: Are benthic systems at risk?

This study investigated phototoxicity of TiO2 nanoparticles (nano-TiO2) to a freshwater benthic amphipod (Hyalella azteca) using 48-h and 96-h bioassays. Thorough monitoring of particle interactions with exposure media (Lake Superior water, LSW) and the surface of organisms was performed using dynamic light scattering, UV/Vis spectroscopy, and Scanning Electron Microscopy. Large agglomeration and sedimentation (>77%) in LSW was observed after 0.5h. A simulated solar radiation (SSR)-favored surface attachment of nanoparticles was observed, indicating enhanced phototoxicity with the increased attachment. A 96-h median lethal concentration (LC50) of 29.9mg/L in H. azteca was calculated, with a daily 4-h UV exposure of 2.2W/m2. Phototoxicity of nano-TiO2 under SSR had a 21-fold increase as compared to that under ambient laboratory light. This phototoxicity was also dependent on UV dose, with calculated LC50s around 22.9 (95% CI, 20.5–23.3)Wh/m2 when exposed to 20mg/L nano-TiO2. Also, H. azteca exhibited negative phototaxis in the presence of shelters, indicating that other factors might play a role in environmental systems. Finally, the environmental implications of nano-TiO2 to benthic organisms were illustrated, emphasizing the importance of various environmental factors in the ultimate phototoxicity. This increased phototoxicity and its complex interactions with various environmental factors suggest further investigations are needed for future risk assessment of photoactive nanomaterials to benthic organisms.

Climate Change Expands the Spatial Extent and Duration of Preferred Thermal Habitat for Lake Superior Fishes

Climate change is expected to alter species distributions and habitat suitability across the globe. Understanding these shifting distributions is critical for adaptive resource management. The role of temperature in fish habitat and energetics is well established and can be used to evaluate climate change effects on habitat distributions and food web interactions. Lake Superior water temperatures are rising rapidly in response to climate change and this is likely influencing species distributions and interactions. We use a three-dimensional hydrodynamic model that captures temperature changes in Lake Superior over the last 3 decades to investigate shifts in habitat size and duration of preferred temperatures for four different fishes. We evaluated habitat changes in two native lake trout (Salvelinus namaycush) ecotypes, siscowet and lean lake trout, Chinook salmon (Oncorhynchus tshawytscha), and walleye (Sander vitreus). Between 1979 and 2006, days with available preferred thermal habitat increased at a mean rate of 6, 7, and 5 days per decade for lean lake trout, Chinook salmon, and walleye, respectively. Siscowet lake trout lost 3 days per decade. Consequently, preferred habitat spatial extents increased at a rate of 579, 495 and 419 km2 per year for the lean lake trout, Chinook salmon, and walleye while siscowet lost 161 km2 per year during the modeled period. Habitat increases could lead to increased growth and production for three of the four fishes. Consequently, greater habitat overlap may intensify interguild competition and food web interactions. Loss of cold-water habitat for siscowet, having the coldest thermal preference, could forecast potential changes from continued warming. Additionally, continued warming may render more suitable conditions for some invasive species.

Water Quality Assessment of Newton Creek and Its Effect on Hog Island Inlet of Lake Superior

A study of water quality was conducted for Newton Creek that flows in an urban area in the city of Superior and discharges into Hog Island Inlet (HII). Water quality data were collected from five locations located along the creek flow from its head to its mouth. Throughout the year there was a wide range in measured values for the temperature, dissolved oxygen (DO), electro conductivity (EC), and pH. These ranges showed there were numerous factors affecting these properties of the creek, with the impact from oil refinery being the greatest. Some of the recorded salinity was observed to be much greater than the acute criteria. The high salinity of the creek water is attributed to the chloride and sodium concentrations with relatively elevated concentrations of sulfate and nitrate. A geochemical model revealed that the water chemical composition of HII was the result of a mixing ratio of about 90 % of Lake Superior water and 10 % of Newton Creek water. Faxon Creek, which was monitored for comparison purposes, recorded salinity concentrations much higher than the salinity of Newton Creek. Both Newton and Faxon creeks revealed different chemical makeup. Faxon Creek revealed that all major cation and anion concentrations in its water were higher than in Newton Creek waters with the exception of potassium, sulfate and nitrate. Nitrate and phosphorous concentrations in Newton Creek are much higher than in Faxon Creek and all the surface water in the area, and they exceed the recommended concentration values set by U.S. Environmental Protection Agency (EPA).

Denitrification rates in a Lake Superior coastal wetland

The amount of excess fixed nitrogen removed from the freshwater aquatic nitrogen cycle, particularly by freshwater wetlands, through denitrification (DNF) is largely unknown. Typically, DNF rates increase within sediments that have higher organic content and a source of sufficient NO3−, in this context we measured DNF in organic-rich sediments of Lost Creek wetland on the south shore of Lake Superior, where NO3− concentrations have increased dramatically over the last century. The concentrations of N2, O2, and Ar were determined on intact water-sediment cores. Denitrification and respiration rates were determined using membrane inlet mass spectrometry and N2:Ar and O2:Ar ratios. Nitrogen flux rates measured in August 2000 and 2001 using overlying ambient wetland water, Lake Superior water, and nitrate augmented wetland water ranged from <10 to 78 μmol N m−2 h−1. These rates are low compared to those published for a variety of wetland and aquatic ecosystems. Nonetheless these are the first DNF measurements we know of to assess natural rates in the Lake Superior Basin and they help quantify a missing piece of wetland and lake nitrogen transformations.

Perfluoroalkyl acids in Lake Superior water: Trends and sources

Perfluoroalkyl acids (PFAs) are a family of highly persistent compounds which are present in the environment as a result of degradation of polyfluorinated precursors, from use as processing aids for production of fluoropolymers, and use in fire fighting foams. The purpose of this study was to investigate prevailing concentrations and possible sources of PFAs in Lake Superior, as well as in Siskiwit Lake on Isle Royale. Between 2001 and 2005, replicate water samples were taken from lake surface waters, and from depth profiles, as well as from major tributaries including municipal waste water treatment plants (WWTPs) at three major population centers. Perfluorooctanoate (PFOA) was the predominant PFA in Lake Superior, with concentrations ranging from 0.07 to 1.2 ng/L in surface waters. PFOA concentrations were generally 1.5 to 2-fold greater than perfluorooctanesulfonate (PFOS) levels. WWTPs were found to contribute up to 20 fold higher concentrations of PFOA (22 ng/L) relative to the intake water from Lake Superior, while most tributaries contained lower concentrations of perfluorocarboxylates (PFCA) and perfluoroalkylsulfonates (PFSs) (< 0.1 ng/L). Overall tributaries and precipitation were estimated to be the major sources of PFCAs and PFSs to Lake Superior. Tributaries were estimated to be the largest source contributing 59% of PFOA and 57% of PFOS inputs to the lake. Profiles conducted over the deepest points in the lake showed that PFAs were found throughout the water column, however, there was no distinctive trend with depth.

Lake Superior water level fluctuation and climatic factors: A dynamic linear model analysis

We use Dynamic Linear Models (DLM) to analyze the time series of annual average Lake Superior water levels from 1860 to 2007, as well as annual averages of climate drivers including precipitation (1900–2007), evaporation and net precipitation (1951–2007). Our results indicate strong evidence favoring the presence of a systematic trend over a random walk for Lake Superior water levels, and this trend has been negative in recent decades. We then show decisive evidence, in terms of improved predictive performance, favoring a model in which the trend component is replaced with regression components consisting of climatic drivers as predictor variables. Because these models use lagged values of precipitation or net precipitation as predictors, the models can be used to forecast water levels, with the associated uncertainty, several years into the future. We use several of the best fit models and compare one (2008) and two step-ahead (2009) forecasts. The 2008 forecasts compare very well with the observed 2008 water level; the two step-ahead 2009 forecasts are offered as testable hypotheses. The Bayesian context in which these models are developed provides a rigorous framework for data assimilation and regular model updating.

An evaluation of iron bioavailability and speciation in western Lake Superior with the use of combined physical, chemical, and biological assessment

An iron‐dependent cyanobacterial bioreporter (Synechococcus strain KAS101) was used in unison with size‐fractionated iron content (&gt;0.45, &lt;0.45, &lt;0.02 µm), and chemical characterization of iron complexation (C18 resin column) to elucidate the bioavailable forms of iron present in Lake Superior during periods of inverse thermal stratification (May) and strong thermal stratification (September) of the water column. The results provide evidence of organic complexation of iron in Lake Superior waters. Iron in most sampled water was complexed by organic compounds that behaved like fulvic acids, whereas some samples showed evidence for the presence of siderophore‐like compounds. The presence of dissolved organic matter suppressed the cellular luminescence of the bioreporter, indicating an increased iron bioavailability. This effect could result either from the presence of siderophores forming iron complexes that are bioavailable to the bioreporter, or from more indirect effects because of the presence of other organic compounds, such as fulvic acids or polysaccharides. Model ligand additions, iron bioaccumulation, and photo‐oxidation of dissolved organic matter were used to assess the bioavailability of organically complexed iron to the bioreporter. A significant fraction of the iron (40‐100%) was bioavailable to the bioreporter. Iron bioavailability was high enough for the bioreporter not to be iron limited in the water collected from Lake Superior. This measure of bioavailability to picocyanobacteria is relevant because picoplankton accounted for the majority of chlorophyll a in Lake Superior during this study.

Cold, Clear, and Deadly: Unraveling a Toxic Legacy by Melvin J. Visser

162 Michigan Historical Review Melvin J. Visser. Cold, Clear, and Deadly: Unraveling a Toxic Legacy. East Lansing: Michigan State University Press, 2007. Pp. 192. Bibliography. Index. Cloth, $24.95. Cold, Clear, and Deadly chronicles Michigan engineer Melvin Visser's efforts to understand the ongoing pollution of Lake Superior and other northern waters by persistent organic pollutants (POPs). Pollutants, such as DDT, hexachlorane, benzene, chlordane, hexachlorocyclohexane (HCH), toxaphene, dieldrin, aldrin, heptachlor, mirex, polychlorinated biphenyls (PCBs), and dioxin, persist despite bans on the use of these chemicals in the United States and many developed nations. Visser argues that POPs are an ongoing problem because developing countries, such as India and China, continue to use them, particularly in agriculture; atmospheric processes transport them globally; and once deposited in northern waters, these pollutants degrade slowly. The book begins with Visser's account of attempts to reduce POPs in the Great Lakes in the 1980s and 1990s. The narrative then transitions to a broader analysis of the scientific literature on POPs and particularly POPs in the Arctic. The book concludes with a plea for readers to advocate aworldwide ban on POPs. Visser does an admirable job of making complex science approachable. He explains the sources, transport, and fate of POPs at a level understandable to readers who are not experts in these matters. For example, to communicate how POPs get from the air into the water, Visser poses a thought experiment. He asks readers to imagine a gallon jug filled with water, air, sediment, and a small amount of PCBs. He explains that, after vigorous shaking, the amount of PCBs in each substance reaches equilibrium; a fixed portion of the PCBs is in each layer: water, air, and sediment. The earth works the same way. PCBs and POPs accrue equally in the air, water, and soil. So, if POPS are added to the global air in one place, such as in developing countries, they are then deposited inwater (and son) all over the globe, including bodies of water such as Lake Superior, thus maintaining the accrual of equal portions of pollutants in air,water, and soil (pp. 80-81). Visser also contributes to an important area of environmental scholarship by illustrating the shortcomings inU.S. environmental policies that seek local, regional, and national solutions to global problems. For example, Visser recounts that in the late 1990s, the United States Environment Protection Agency encouraged research on the relationship between indoor air quality in Chicago and POPs in the Great Lakes. At Book Reviews 163 the same time, however, scientists were concluding that global air pollution controlled POP levels in the Great Lakes (pp. 83-87). Overall, Clear, Cold, and Deadly is an elegant contribution toMichigan environmental history and environmental studies, as well as U.S. and global environmental history and studies. It is enjoyable and informative to read, despite the sober topic, and it could be used in undergraduate teaching. Excerpts could illustrate the dynamics of the environment or of science policy. The entire book could be assigned in order to give students an in-depth understanding of the specific topic of POPs in northern waters. Karin Ellison, Associate Director Center for Biology and Society, Arizona State University Edward Watts. In This Remote Country: French Colonial Culture in the Anglo American Imagination, 1780-1860. Chapel Hill: University of North Carolina Press, 2006. Pp. 288. Bibliography. Index. Notes. Cloth, $59.95; paper, $19.95. In this excellent book, Edward Watts examines representations of eighteenth-century French colonists in the writings of nineteenth-century Anglo-American authors. These representations were ubiquitous, Watts shows, and were a key site for waging what Watts terms an "antebellum culture war" (p. 223) over the identity of the young American nation. For many writers, the retrograde and "failed" French colonial culture provided a foil against which to valorize a heroic, expansionist, and capitalist culture of Anglo Americans. For others, however, the French colonial way of life was a road not taken, an attractive alternative to the aggressiveness of American capitalism, racism, patriarchy, Indian removal, and manifest destiny. Using insights and tools from postcolonial theory, Watts brings to light an important set of critical voices and a lost dissenting discourse from nineteenth-century...

Synthetic organic toxicants in Lake Superior

Numerous synthetic organic toxicants have been reported in Lake Superior in the past quarter century although relatively few industrial centers are located on its shores. The chemicals enter the lake primarily through atmospheric deposition via transport from regional and distant sources. This contribution discusses research issues regarding the processes by which the chemicals enter and exit the lake, their in-lake cycling and bioaccumulation, and recently reported potential toxicological effects. Research issues that remain for historically important synthetic organic toxicants are discussed as well as those of emerging chemicals of concern. Although concentrations of some historically important toxicants are decreasing in Lake Superior's waters through volatilization and sedimentation and burial, abiotic and biotic in-lake cycling opens routes of entry into the lake's lower food web, contributing to concentrations in fish that warrant consumption advisories in certain cases. Concentrations of some non-...

Hydroclimatic Factors of the Recent Record Drop in Laurentian Great Lakes Water Levels

Abstract An extreme low-water supply episode from 1997 to 2000 resulted in the largest 1-yr drop in Lakes Michigan–Huron and Lake Erie water levels (0.92 and 1.03 m, respectively) recorded since measurements began in the early 1800s. Lake Superior water levels were the lowest since 1925. Lakes Erie and Ontario also had relatively low levels. The episode was unusual, particularly when compared to the record-low water episode of the mid-1960s, in that the primary hydroclimatological driver was high air temperatures and not extremely low precipitation. The high air temperatures resulted in unusually high lake evaporation rates and decreased basin runoff. The drop in levels during this episode was compared to other 1–3-yr decreases throughout the period of record. A comparison of the 1997–2000 episode for Lakes Michigan–Huron with the 1960–64 episode, which led to record-low lake levels in 1964, shows that the various elements of the water balance have differing importance in the two episodes.

Bioconcentration of inorganic and organic thallium by freshwater phytoplankton.

Uptake of inorganic Tl(I) and dimethylthallium, (CH3)2Tl+, by Chlorella spp. (Chlorophyta) and the diatom Stephanodiscus hantzschii (Heterokontophyta) were measured using radio-tracer techniques in water from Lakes Erie and Superior (North America). Uptake of both Tl(I) and dimethylthallium was bioactive. Uptake of [204Tl]-Tl(I) was greater in Lake Superior water than in Lake Erie water due to the greater K content in Lake Erie that inhibits Tl(I) uptake by phytoplankton but not that of [204Tl]-dimethylthallium. Volume-based bioconcentration factors for Tl(I) after 72 h of exposure were 5 x 10(4) and 1.1 x 10(4) for Chlorella sp. and S. hantzschii; for dimethylthallium they were 7.8 x 10(2) and 8.3 x 10(3). Both Tl(I) and Tl(III) were concentrated similarly by Chlorella spp. These results suggest that chlorophytes, but not diatoms, accumulate Tl(I) to a greater extent than dimethylthallium. Greater bioaccumulation factors of inorganic Tl are possible in waters containing low amounts of K+; water quality guidelines seeking to protect biota from deleterious effects of Tl should consider the role of K.

Trends in the Lake Superior Water Budget Since 1948: A Weakening Seasonal Cycle

An analysis of Lake Superior water levels from 1948–1999 reveals that the seasonal cycle has decreased in amplitude by 20% (from 40 cm to 32 cm). This change is manifested as a downward trend in summer and autumn lake levels (when levels are typically highest) accompanied by roughly no change in winter and spring lake levels (and an overall 4-cm drop in annual mean levels). The decreased rates of seasonal rise and fall in lake level over the 51-year interval reflect a large decrease in the net monthly influx of water during the late spring (up to –1,360 m 3/s) coupled with a nearly compensating increase in net influx during late autumn (up to +1,100 m 3/s). Analysis of the Lake Superior water budget indicates that these changes are primarily the result of trends in runoff and over-lake precipitation. A systematic decrease in outflow through the St. Marys River is also evident during July-December (in association with the lower lake levels), as well as a moderate shift in the seasonal pattern of lake evaporation (but not the annual mean). The observed water budget trends are primarily related to variations in climate, rather than lake regulation. Land surface effects are also important, as suggested by a 20% increase in annual mean evapotranspiration during the 51-year interval and large changes in monthly storage (e.g., snowmelt, groundwater, etc.). Significant uncertainties are present in the calculated water budget, and it is suggested that a likely source of error is in measured precipitation and (especially) runoff.

Photochemical Transformation of Dissolved Organic Carbon in Lake Superior—An In-situ Experiment

A direct solar irradiation experiment was conducted for Lake Superior water (initial [DOC] = 2.49 mgC/L) and Sturgeon River water (initial [DOC] = 33.95 mgC/L) in Lake Superior on 22–23 August 1999. Water samples were sealed in quartz tubes and suspended in the lake at 0 m, 6 m, and 24 m. Samples were collected after 5.5 h and 15.5 h of exposure to sunlight. The dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) concentrations, UV-vis absorbance, and 3D fluorescence were measured on all samples. For irradiated lake water, DIC was photoproduced at a rate of 1.3 to 5.6 M DIC hr −1 for the first day and 1.3 to 2.1 × 10 2 nM DIC hr −1 for the second day, normalized to 1 mg DOC/L, and long wavelength absorbance decreased as a function of irradiation time. For both river and lake samples, integral fluorescence also decreased systematically and the fluorescence quantum yield decreased after photoirradiation. In addition, new fluorescence peaks at short wavelengths appeared in irradiated lake samples, suggesting the formation of new chromophores. After irradiation, DOC concentrations in surface samples were lower than those in deep samples. Rate constants for all processes measured decreased during the irradiation time. Rate constants for loss of fluorophores were greater than those for chromophores, which, in turn, were greater than the rate constants for loss of DOC.