Tritium in Laurentian Great Lakes surface waters

A basin-wide water quality survey for the radionuclide tritium during 2017 and 2019 provides an overview of levels in Great Lakes surface waters. All data, together with those from similar basin-wide surveys since the early 1990s, are included in the Supplemental Material. Values of tritium are lowest in Lake Superior and are highest within a region of northwestern Lake Ontario, as well as locally near a known source in Lake Huron. Twenty-year trends show declines in all of the lakes, and this is consistent with the decline in fallout from past nuclear weapons testing, the major source of tritium to the lakes. Longer-term trends, developed using values from the literature, demonstrate a marked overall reduction in tritium values since maxima in the late 1960s, with a slowing rate of decline in the most recent decade. As atmospheric fallout is reduced, the relative importance of other sources is increasing. Known releases, primarily from nuclear generating stations using heavy water, could therefore drive any future changes in Great Lakes tritium levels.     

Fatty acids in Great Lakes lake trout and whitefish

Fish are an excellent source of lean protein and omega-3 polyunsaturated fatty acids (PUFAs) but there is inadequate information on the levels of PUFAs in freshwater fish and specifically Great Lakes fish. Knowledge of PUFAs is necessary to make informed decisions regarding the balance between the benefits of fish consumption due to these factors versus risks of adverse health effects associated with elevated levels of contaminants known to be present in some Great Lakes fish and linked to increased risk of cancer and adverse neurological effects to both infants and adults. Our goal was to determine the lipid profiles in two species of Great Lakes fish, lake trout and whitefish. Total fat and the percentage of total and omega-3 PUFAs were with one exception significantly higher in lake trout than whitefish. Average concentrations of EPA + DHA were 11.2 and 9.7 g/100 g lipid in lake trout and whitefish, respectively. The concentrations of EPA + DHA in fatty marine fish (22.7, 23.9 and 30.2 g/100 g lipid, respectively) are about double those found in Great Lakes lake trout and whitefish. Nevertheless a 100 g serving of Great Lakes lake trout provides more than 500 mg of EPA + DHA, which is the daily intake level recommended by the American Dietetics Association for the prevention of coronary heart disease.     

Historical changes and current status of crayfish diversity and distribution in the Laurentian Great Lakes

Despite increasing recognition of the importance of invertebrates, and specifically crayfish, to nearshore food webs in the Laurentian Great Lakes, past and present ecological studies in the Great Lakes have predominantly focused on fishes. Using data from many sources, we provide a summary of crayfish diversity and distribution throughout the Great Lakes from 1882 to 2008 for 1456 locations where crayfish have been surveyed. Sampling effort was greatest in Lake Michigan, followed by lakes Huron, Erie, Superior, and Ontario. A total of 13 crayfish species occur in the lakes, with Lake Erie having the greatest diversity (n = 11) and Lake Superior having the least (n = 5). Five crayfish species are non-native to one or more lakes. Because Orconectes rusticus was the most widely distributed non-native species and is associated with known negative impacts, we assessed its spread throughout the Great Lakes. Although O. rusticus has been found for over 100 years in Lake Erie, its spread there has been relatively slow compared to that in lakes Michigan and Huron, where it has spread most rapidly since the 1990s and 2000, respectively. O. rusticus has been found in both lakes Superior and Ontario for 22 and 37 years, respectively, and has expanded little in either lake. Our broad spatial and temporal assessment of crayfish diversity and distribution provides a baseline for future nearshore ecological studies, and for future management efforts to restore native crayfish and limit non-native introductions and their impact on food web interactions.     

The flathead catfish invasion of the Great Lakes

A detailed review of historical literature and museum data revealed that flathead catfish were not historically native in the Great Lakes Basin, with the possible exception of a relict population in Lake Erie. The species has invaded Lake Erie, Lake St. Clair, Lake Huron, nearly all drainages in Michigan, and the Fox/Wolf and Milwaukee drainages in Wisconsin. They have not been collected from Lake Superior yet, and the temperature suitability of that lake is questionable. Flathead catfish have been stocked sparingly in the Great Lakes and is not the mechanism responsible for their spread. A stocking in 1968 in Ohio may be one exception to this. Dispersal resulted from both natural range expansions and unauthorized introductions. The invasion is ongoing, with the species invading both from the east and the west to meet in northern Lake Michigan. Much of this invasion has likely taken place since the 1990s. This species has been documented to have significant impacts on native fishes in other areas where it has been introduced; therefore, educating the public not to release them into new waters is important. Frequent monitoring of rivers and lakes for the presence of this species would detect new populations early so that management actions could be utilized on new populations if desired.     

Assessment of nitrogen fixation rates in the Laurentian Great Lakes

Nitrogen fixation (N_Fix) is an important, yet understudied, microbial process in aquatic ecosystems, especially in the Laurentian Great Lakes (LGL). To date, a dearth of nitrogen fixation rate measurements exists in the LGL, are from temporally isolated studies, and were collected primarily from near-shore and surface water environments. Evidence of nitrogen accumulation across the Laurentian Great Lakes suggest that we do not have a firm grasp on nitrogen cycling in large lakes. Thus, we sought to quantify the spatial variability of N_Fix in the LGL. We found lakes are significantly different in N_Fix rates from one another and that rates are depth dependent. Overall mean surface N_Fix rates of Lakes Superior, Michigan, Huron, Erie and Ontario were 0.024, 0.020, 0.069, 0.145, and 0.078 (nmol N₂/L/hr), respectively. Likewise, we found the Western, Central and Eastern basins of Lake Erie are significantly different in N_Fix rates (0.1540, 0.1032, 0.0738 nmol N₂/L/hr). However, we found no significant difference in N_Fix rates between near and offshore sites in Lake Erie, which may have been biased due to a cyanobacterial bloom containing a nitrogen-fixing Dolichospermum sp. Linear regression models indicate N_Fix is generally positively correlated with chlorophyll-a concentration and negatively correlated with oxidized nitrogen species concentrations. However, Lakes Erie and Huron exhibited a positive linear relationship with oxidized nitrogen, suggesting that N_Fix may persist to meet cellular and community nitrogen demands. Together, our data highlight N_Fix is important despite the presence of abundant nitrogen in all LGL.     

Perchlorate in environmental waters of the Laurentian Great Lakes watershed: Evidence for uneven loading

A database of nearly 500 analyses of perchlorate in water samples from the Laurentian Great Lakes (LGL) watershed is presented, including samples from streams, from the Great Lakes and their connecting waters, with a special emphasis on Lake Erie. These data were assessed to test an earlier hypothesis that loading of perchlorate to the LGL watershed is relatively uniform. Higher perchlorate concentrations in streams in more developed and urban areas appear to indicate higher rates of loading from anthropogenic sources in these areas. Variable perchlorate concentrations in samples from Lake Erie indicate transient (un-mixed) conditions, and suggest loss by microbial degradation, focused in the central basin of that lake. Interpretation of the data included estimation of annual loading by streams in various sub-watersheds, and simulations (steady state and transient state) of the mass balance of perchlorate in the Great Lakes. The results suggest uneven loading from atmospheric deposition and other sources.     

Fatty acids in ten species of fish commonly consumed by the Anishinaabe of the upper Great Lakes

The Chippewa Ottawa Resource Authority (CORA) in Sault Ste. Marie, Michigan, has been monitoring contaminant concentrations in the fillet portions of fish from the 1836 treaty-ceded waters of lakes Superior, Huron, and Michigan since 1991. The goal is to provide up to date consumption advice for their CORA member tribes. For the first time since the program started, CORA has included fatty acid analysis in 2016 monitoring of fish in Lake Superior. Ten species were targeted by CORA based on 25 years of experience and regular discussions with Anishinaabe fish consumers. This paper reports these results and presents some preliminary discussion of the consequences for consumption advice for the CORA member tribes who inhabit the Great Lakes region. Six of the species were sampled from Lake Huron and Lake Superior and four were sampled from supermarkets. Wild caught fish are an important link to the culture of Great Lakes Native American tribes and important sources of food and omega-3 polyunsaturated fatty acids (PUFA N-3). While some PUFA N-3 data from the Great Lakes is available, this dataset provides an important supplement and is specific to the 1836-treaty ceded waters of CORA. This paper confirms the presence of PUFA N-3s in Great Lakes fish traditionally harvested by the CORA tribes.     

Fate and transport of tritium in the Laurentian Great Lakes system

A mass balance modelling approach was used to help understand the movement and impacts of tritium discharged from Canada Deuterium Uranium (CANDU) reactor facilities into Lake Ontario. A concentration-time model, previously developed, is updated in this study. Historical and projected tritium concentrations for Lake Ontario waters are presented. A model calculated accident scenario (10 times highest accidental release) indicates the importance of dilution to the dispersion of tritium; a “modelled” release in 2016 has tritium levels declining by the year 2030 to “previous 2016 levels”. As part of the mass balance approach, lake-bottom sediments were considered as potential radionuclide “sinks”. Tritium porewater results were noted as perturbations at depth in both short (30–50 cm cores) and long sediment core profiles (to 300 cm). These change in tritium concentrations with depth may have been due to CANDU emissions (as the most likely source) over time, based on records of accidental releases of tritiated coolant water. However, the exact process (advection and/or diffusion) responsible for the penetration of tritium into the lake bottom requires additional physical and hydrogeological characterization of the lake bottom sediments.     

Stable isotope mass balance of the Laurentian Great Lakes

    We investigate the physical limnology of the Laurentian Great Lakes of North America using a new dataset of ¹⁸O/¹⁶O and ²H/¹H ratios from over 500 water samples collected at multiple depths from 75 stations during spring and summer of 2007. δ¹⁸O and δ²H values of each lake plot in distinct clusters along a trend parallel to, but offset from, the Global Meteoric Water Line, reflecting the combined effects of evaporative enrichment and the addition of precipitation and runoff along the chain lake system. We apply our new dataset to a stable-isotope-based evaporation model that explicitly incorporates downwind lake effects, including humidity build-up and changes to the isotope composition of atmospheric vapor. Our evaporation estimates are consistent with previous mass transfer results for Michigan, Huron, Ontario and Erie, but not for Superior, which has a much longer residence time. Calculated evaporation from Superior is ~300 mm per year, less than previous estimates of ~500 mm per year, likely arising from integration of the ‘isotopic memory' of lower evaporation rates under cooler climatic conditions with greater ice-cover than the present. Uncertainties in the estimates from the stable-isotope-based model are comparable to mass transfer results, offering an independent technique for evaluating evaporation fluxes.     

Coexistence of the native benthic amphipod Diporeia spp. and exotic dreissenid mussels in the New York Finger Lakes

Populations of the benthic amphipod Diporeia spp. have sharply declined since the early 1990s in all North America's Great Lakes except Lake Superior. The onset and continued decline coincides with the invasion of these lakes by zebra (Dreissena polymorpha) and quagga (Dreissena rostriformis bugensis) mussels and the spread of quagga mussels to deep habitats. The six deepest Finger Lakes of central New York (Seneca, Cayuga, Skaneateles, Canandaigua, Keuka, and Owasco) have historically been Diporeia habitat and have had dreissenids for more than a decade. These lakes represent a wide range of trophic state, maximum depth, and dreissenid invasion history. We hypothesized that Diporeia abundance would be negatively impacted by dreissenid mussel expansion in the Finger Lakes. During 2006–2010, we sampled Diporeia and mussel populations in these six lakes. Diporeia was present in all six lakes, and was abundant (2000/m²) in Owasco Lake that has only zebra mussels and in Cayuga and Seneca Lakes that have had zebra and quagga mussels since 1994. Diporeia abundance was lowest (1000/m²) in Skaneateles, Canandaigua, and Keuka Lakes where quagga mussels have recently expanded. Productivity indicators explained much of the variability of Diporeia abundance. The persistence of Diporeia with quagga mussels in these lakes may be because of available alternative food resources. Fatty acid tracers indicate that Diporeia from Owasco Lake, the lake without quagga mussels, utilize diatoms, but Diporeia from Cayuga Lake that coexist with abundant quagga mussels also use food resources associated with terrestrial detritus that cannot be intercepted by dreissenids.     

Fish movement and migration studies in the Laurentian Great Lakes: Research trends and knowledge gaps

Resource management agencies in the Laurentian Great Lakes routinely conduct studies of fish movement and migration to understand the temporal and spatial distribution of fishes within and between the lakes and their tributaries. This literature has never been summarized and evaluated to identify common themes and future research opportunities. We reviewed 112 studies, published between 1952 and 2010, with the goal of summarizing existing research on the movement and migration of fishes in the Laurentian Great Lakes. The most commonly studied species were Lake Trout (Salvelinus namaycush), Walleye (Sander vitreus), and Lake Sturgeon (Acipenser fulvescens). Studies relied mainly on mark-recapture techniques with comparatively few using newer technologies such as biotelemetry, hydroacoustics, or otolith microchemistry/isotope analysis. Most movement studies addressed questions related to reproductive biology, effects of environmental factors on movement, stocking, and habitat use. Movement-related knowledge gaps were identified through the literature synthesis and a survey distributed to Great Lakes fisheries managers. Future studies on emigration/immigration of fishes through lake corridors, the dispersal of stocked fishes and of stock mixing were identified as being particularly important given their potential for developing lake- or region-wide harvest regulations and stocking strategies. The diversity of tools for studying fish movement across multiple years and various spatial scales gives researchers new abilities to address key science questions and management needs. Addressing these needs has the potential to improve upon existing fisheries management practices within the complexity of multi-jurisdictional governance in the Laurentian Great Lakes.     

The benthic community of the Laurentian Great Lakes: Analysis of spatial gradients and temporal trends from 1998 to 2014

We used the results of seventeen years of Great Lakes benthic monitoring conducted by the U.S. EPA's Great Lakes National Program Office to describe the spatial and temporal patterns of benthic communities, assess their status, trends, and main drivers, and to infer the potential impact of these community changes on ecosystem functioning. Benthic abundance and diversity were higher at shallow (<70?m in depth) stations with chlorophyll concentrations above 3?μg/L than at deeper sites (<1?μg/L). We infer that lake productivity, measured by chlorophyll was likely the major driver of benthic abundance and diversity across lakes. Consequently, benthic diversity and abundance were the highest in the most productive Lake Erie, followed by lakes Ontario, Michigan, Huron, and Superior. Multivariate analysis distinguished three major communities shared among lakes (littoral, sublittoral, and profundal) that differed in species composition and abundance, functional group diversity, and tolerance to organic pollution. Analysis of temporal trends revealed that the largest changes occurred in profundal communities, apparent in significant shifts in dominant taxa across all lakes except Lake Superior. In lakes Michigan, Huron, and Ontario, the former dominant Diporeia was replaced with Dreissena and Oligochaeta. Profundal species, with the exception of dreissenids, became less abundant, and their depth distribution has shifted. In contrast, density and diversity of native littoral and sublittoral communities increased. The invasion of dreissenids was among the most important drivers of changes in benthic communities. Continued monitoring is critical for tracking unprecedented changes occurring in the Great Lakes ecosystem.     

Widespread prevalence of hypoxia and the classification of hypoxic conditions in the Laurentian Great Lakes

Aquatic hypoxia within the Laurentian Great Lakes has contributed to various adverse ecological consequences and stimulated research interest in recent decades. An analysis of published peer-reviewed journal articles from 2000 to 2020 demonstrates an increasing trend of studies related to hypoxia in the Laurentian Great Lakes. However, the majority of these studies (78%) focus on Lake Erie and in particular the well-documented hypolimnetic hypoxic conditions that develop in the central basin of Lake Erie. This hypoxic zone is relatively large (up to 1.5 million ha), has substantial ecological effects, and motivates monitoring programs and water quality improvement initiatives. Nonetheless, the hypoxic zone in the central basin of Lake Erie is only one of over twenty documented hypoxic zones in the Laurentian Great Lakes. Moreover, hypoxic conditions in the Great Lakes are quite diverse. Here, we define and characterize a four-fold classification of Great Lakes hypoxic conditions: 1) hypolimnetic hypoxia, 2) over-winter hypoxia, 3) diel hypoxia, and 4) episodic hypoxia. We suggest that Great Lakes research and monitoring programs should seek to more broadly document hypoxic conditions and develop models to predict the temporal and spatial occurrence of hypoxia. Such efforts are particularly timely as future climatic conditions contributing to warmer temperatures, longer and more intense stratified periods, increased spring nutrient loading and more variable allocthonous inputs are expected to exacerbate three of the four hypoxic conditions described for the Great Lakes (hypolimnetic, diel, and episodic hypoxia).     

Checklist of Diatoms from the Laurentian Great Lakes. II

An updated diatom (Bacillariophyta) checklist for the Great Lakes is provided. The present checklist supplants the preliminary checklist published in The Journal for Great Lakes Research in 1978 and effectively represents a 20-year update. A series of procedures were used in this update which included: a reexamination of taxa reported in the 1978 list, additions of taxa reported from the Great Lakes during the past 20 years, and a revision of taxonomy, commensurate with systematic and nomenclatural changes which have occurred primarily during the past 8 years. 1488 diatom species or subordinate taxa are considered to be correct reports from the Great Lakes out of the 2188 diatom entities reported in the list. Of the 124 genera reported 105 are considered to be names in current use. The number of diatom species reported represents a 16.5% increase and the number of genera reported represents a 78% increase over those reported in the 1978 checklist. 13% of the species reported and 32% of the genera reported are due solely to nomenclatural changes. Results indicate that Great Lakes diatoms are a biodiverse component of the ecosystem, commensurate with the wide range of habitats found in the system. The present checklist indicates that most of the newly added species are primarily benthic or periphytic in nature and these represent largely understudied habitats. These results suggest that the present checklist may only represent approximately 70% or less of the extant diatom flora of the Great Lakes system.     

Coastal geomorphic response to seasonal water-level rise in the Laurentian Great Lakes: An example from Illinois Beach State Park,USA

Hydrodynamic processes, such as fluctuating water levels, waves, and currents, shape coastlines across timescales ranging from minutes to millennia. In large lacustrine systems, such as the Laurentian Great Lakes, the role of water level in driving long-term (centuries to millennia) coastal evolution is well understood. However, additional research is needed to explore short-term (weeks to months) beach geomorphic response to fluctuating water level. Developing a process-focused understanding of how water level fluctuations shape coastal response across these shorter time scales is imperative for coastal management. Here, we present measurements of geomorphic response along a lacustrine beach ridge plain to seasonal water level fluctuations during a decadal high-stand in Lake Michigan water level. Frequent topographic change measurements revealed high spatial and temporal variability in geomorphic response to rising lake level. Sites immediately downdrift of shore protection began to erode immediately as lake level increased. The co-occurrence of peak seasonal lake levels and a modest increase in wave energy resulted in erosion and overwash at sites that resisted erosion during the initial seasonal rise in lake level. None of the sites in this study returned to their initial morphology following seasonal lake level rise. Given that peak water levels were nearly identical in 2017 and 2018, yet the majority of erosion at our sites occurred in 2017, we postulate that erosion associated with seasonal lake level rise is primarily a function of the change in annual maximum water level from year to year, rather than solely the elevation of the water level.     

Ecological data as a resource for invasive species management in U.S. Great Lakes coastal wetlands

The coordinated use of ecological data is critical to the proper management of invasive species in the coastal wetlands of the Laurentian Great Lakes. Researchers and government programs have been increasingly calling for the use of data in management activities to increase the likelihood of success and add transparency in decision making. Web-enabled databases have the potential to provide managers working in Great Lakes coastal wetlands with relevant data to support management decisions. To assess the potential value of these databases to managers in Laurentian Great Lakes states, we surveyed wetland managers to determine their current data usage as well as their future data interests and catalogued the online databases currently available. Surveys were disseminated via email to managers in 56 different organizations overseeing invasive species management efforts in Great Lakes coastal wetlands; 46 responses were included in this analysis. Of the survey respondents, all reported using raw biotic data for decision making, (i.e. presence of target species) but many indicated that they would prefer to incorporate a greater variety of data, as well as more complex information. Our survey found that managers used web-enabled databases, but most databases that we catalogued only provided presence data for wetland biota. We concluded that databases can provide the types of data sought by invasive species managers but have unmet potential to be integrated into responsive management processes.     

Alkalinity,pH, and pCO2 in the Laurentian Great Lakes: An initial view of seasonal and inter-annual trends

Ongoing human perturbations to the global inorganic carbon cycle can cause various changes in the pH and alkalinity of aquatic systems. Here seasonal and inter-annual trends in these variables were investigated in the five Laurentian Great Lakes using data from the U.S. EPA GLENDA database. These observations, along with temperature, were also used to predict the partial pressure of carbon dioxide in surface water (pCO₂). There are strong seasonal differences in pH in all five lakes, with higher pH levels in summer than in spring. All lakes show significantly higher pCO₂ values in spring than in summer. Michigan and Ontario show higher alkalinity values in spring; Huron shows lower spring values. Inter-annually, open-lake pH shows the highest values in all lakes around 2010, the time frame of lowest lake water levels, though only lakes Superior and Erie show statistically significant inflection points at that time. Inter-annual alkalinity trends differ considerably from those of pH. Superior’s alkalinity increases until ～2008 and then begins dropping; Ontario’s alkalinity decreases until ～2004 and then begins increasing, with the decrease coinciding with the introduction and establishment of Dreissenid mussels. The other lakes show much less clear inter-annual alkalinity trends. For pCO₂, inter-annual trends in each lake show either increases from 1992 to 2019 (for Superior, Michigan, and Huron) or shifts from slightly decreasing values to increasing values for the other lakes. The timing of this shift is from 2008 to 2010.     

Researcher disciplines and the assessment techniques used to evaluate Laurentian Great Lakes coastal ecosystems

The Laurentian Great Lakes of North America have been a focus of environmental and ecosystem research since the Great Lakes Water Quality Agreement in 1972. This study provides a review of scientific literature directed at the assessment of Laurentian Great Lakes coastal ecosystems. Our aim was to understand the methods employed to quantify disturbance and ecosystem quality within Laurentian Great Lakes coastal ecosystems within the last 20 years. We focused specifically on evidence of multidisciplinary articles, in authorship or types of assessment parameters used. We sought to uncover: 1) where Laurentian Great Lakes coastal ecosystems are investigated, 2) how patterns in the disciplines of researchers have shifted over time, 3) how measured parameters differed among disciplines, and 4) which parameters were used most often. Results indicate research was conducted almost evenly across the five Laurentian Great Lakes and that publication of coastal ecosystems studies increased dramatically ten years after the first State of the Great Lakes Ecosystem Conference in 1994. Research authored by environmental scientists and by multiple disciplines (multidisciplinary) have become more prevalent since 2003. This study supports the likelihood that communication and knowledge-sharing is happening between disciplines on some level. Multidisciplinary or environmental science articles were the most inclusive of parameters from different disciplines, but every discipline seemed to include chemical parameters less often than biota, physical, and spatial parameters. There is a need for an increased understanding of minor nutrient, toxin, and heavy metal impacts and use of spatial metrics in Laurentian Great Lakes coastal ecosystems.     

Great Lakes coastal wetlands as suitable habitat for invasive mute swans in Michigan

Great Lakes coastal wetlands provide critical habitat and food resources for more species than any other Great Lakes ecosystem. Due to past and current anthropogenic disturbances, coastal wetland area has been reduced by >50% while remaining habitat is frequently degraded. Invasive mute swans have contributed to the degradation of coastal wetlands by removing submergent vegetation and competitively excluding native species from breeding areas and food resources. Despite current control practices, mute swan population estimates in Michigan are ~8000, comparable to population estimates in the entire Atlantic Flyway of North America. We collected local abiotic data and adjacent land cover data at 3 scales from 51 sites during 2010 and 2011 and conducted 2 mute swan detection surveys each year during the summer and fall. We developed a single-species, single-season occupancy-based habitat suitability model to determine current and potential mute swan habitat among Great Lakes coastal wetlands. We found mute swans occupied heterotrophic coastal wetlands adjacent to urban areas, which were high in ammonium and oxidation-reduction potential and low in nitrates, dissolved oxygen, and turbidity. Our model provides managers with a valuable tool for rapidly identifying mute swan habitat areas for control efforts, particularly the need for targeting mute swan populations in or near urbanized areas. Our model will also aid managers in monitoring areas that mute swans may invade and prioritizing coastal wetland areas for restoration efforts.