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.     

Spectral Attenuation and Irradiance in the Laurentian Great Lakes

Optical data collected between 1973 and 1979 are utilized to study spectral attenuation and irradiance in the midlake waters of four Laurentian Great Lakes. Particular attention is given to the photosynthetic available radiation (PAR), its incident spectral distribution, its spectral attenuation, and its qualitative change with depth. Curves are shown illustrating these spectral properties as well as the relationships between PAR and the photosynthetic usable radiation (PUR) for each of the lakes. These curves, along with the included mathematical relationships, enable quantitative estimates to be obtained of incident PAR, subsurface PAR, and subsurface PUR, and qualitative information to be obtained on subsurface irradiance levels for the Laurentian Great Lakes.     

Potential for fisheries-induced evolution in the Laurentian Great Lakes

Fisheries are selective, capturing fish based on their body size, behaviour, life stage, or location. Over time, if harvest pressure is strong enough and variation in traits heritable, evolution can occur that affects key aspects of the ecology of fish stocks. Most compelling examples of rapid evolution in response to harvest have come from marine systems. Here, we review the state of knowledge on fisheries-induced evolution (FIE) in the Laurentian Great Lakes where subsistence, commercial, and recreational fisheries have operated for centuries. We conclude that stocks experienced harvest rates high enough and for long enough to undergo evolution. While historical fisheries exploited more juveniles, some contemporary Great Lakes fisheries target primarily adult size-classes thus reducing current selection for earlier maturation; however, other traits and behaviours could evolve (e.g., growth, timing of spawning, boldness). While commercial harvest previously dominated, recreational fishing is now expected to be a strong contributor to harvest selection in the Great Lakes. Environmental variation, density-dependence, invasive species, and the genetic legacy of population bottlenecks and stocking interact with, and make it more challenging to detect, FIE in the Great Lakes than in marine systems. Case studies are presented for Great Lakes stocks of yellow perch Perca flavescens and lake whitefish Coregonus clupeaformis for which FIE has been investigated. The evidence for FIE in the Great Lakes is currently sparse, potentially because of the low research focus on this topic or because of the interacting influence of environmental variation and anthropogenic stressors.     

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.     

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.     

Feeding ecology of Limnocalanus macrurus in the Laurentian Great Lakes

The zooplankton communities of several Laurentian Great Lakes have shifted toward greater biomass of calanoid copepods, particularly Limnocalanus macrurus, since the 1990s. Limnocalanus is an omnivore that feeds on large phytoplankton cells, ciliates, rotifers, and small crustacean zooplankton, especially copepod nauplii, and it may be an increasingly important zooplanktivore in these systems. Although there is previous research examining Limnocalanus predation rates on nauplii, we do not know if the presence of phytoplankton affects predation rates. Our initial experiments confirmed Limnocalanus preference for nauplii over small copepodites. Additional experiments showed that Limnocalanus feeding rates on nauplii decreased by 50% at the highest phytoplankton concentrations tested. Limnocalanus fed more on the larger algae tested (Cryptomonas, Cryptophyta, 40 µm) than on the smaller taxa (Scenedesmus, Chlorophyta, 10 µm). We used stable isotope analysis to infer Limnocalanus trophic position in the five Laurentian Great Lakes by comparing Limnocalanus with simultaneously captured Leptodiaptomus sicilis, another calanoid copepod known to feed on phytoplankton and microzooplankton. This analysis showed Limnocalanus at higher trophic positions in the more oligotrophic lakes Huron, Michigan, and Superior than in lakes Ontario and Erie. Summer Limnocalanus trophic position was inversely related to both the site-specific concentration of algae in the deep chlorophyll layer and a trophic state index based on spring chlorophyll and total phosphorus. Our results indicate that predation by Limnocalanus on zooplankton depends on lake algal abundance, and that feeding rates on nauplii by an individual Limnocalanus adult are likely higher in the more oligotrophic lakes.     

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.     

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.     

A reference inventory for aquatic fauna of the Laurentian Great Lakes

The Laurentian Great Lakes encompass an expansive and diverse set of freshwater ecosystems that contain a concordantly large and diverse vertebrate and invertebrate fauna. Although numerous publications exist concerning the composition and distribution of this fauna, there is at present no single readily available resource that brings all this information together. Here, we present and describe the compilation process for a comprehensive Great Lakes aquatic fauna inventory covering fishes, reptiles, amphibians, zooplankton, mollusks, annelids, insects, mites, and various other aquatic invertebrates. Inventory entries were developed via an extensive search of literature and internet sources and are attributed with detailed nomenclature information, general lake and habitat occurrences, and supporting citations and links to life history and genetic marker information. The inventory scope is the Laurentian Great Lakes proper and their connecting rivers, and their fringing coastal wetlands and lower tributaries. Over 2200 unique taxa are contained in the inventory – 85% resolved to species and 14% to genus. The listing substantially expands previous richness estimates for invertebrates in the Great Lakes, but taxonomic resolution and spatial distribution information for them remains quite uneven. Example pattern analyses for fauna in this inventory show that aquatic vertebrates are generally more widely distributed than invertebrates, and that biodiversity is concentrated in the coastal margins. The inventory is being packaged into a public, searchable database that showcases the biodiversity of the Great Lakes aquatic fauna and can assist the research and management community in their biological investigations.     

Variation of cisco egg size among Laurentian Great Lakes populations

Many fish species display inter-population and inter-individual egg size variation. Intra-specific differences in egg size seemingly reflect both energetic experiences of individual spawning fish and long-term population responses to differing ecosystems. Optimal egg size theory implies that selection influences a population’s mean egg size in response to its early-life environment, given the well-established trade-off between egg size and fecundity. Currently, there is strong interest in rehabilitation of Laurentian Great Lakes cisco, Coregonus artedi, which is characterized by inter-population variation of morphological and behavioral traits. However, the extent of cisco egg size variation is under-described. In fall 2018 and 2019, we collected egg samples by stripping ripe females at seven total locations in four Great Lakes. We measured unfertilized egg diameters using imaging software and compared mean egg diameters among locations with and without including maternal total length as a covariate. Lake Michigan females produced the largest eggs overall but were excluded from analyses using the total length covariate because of their significantly larger body sizes. Maternal length had a positive effect on egg size, and when accounting for this effect, females in Lake Huron produced the largest eggs followed by Lake Ontario and Lake Superior. We also found that egg size varied among locations within Lake Superior. These findings aligned with observations of morphological and behavioral differences among populations and suggest that cisco phenotypic variation at a fine spatial scale extends to reproductive biology. Consideration of cisco reproductive traits, such as egg size, may inform restoration strategies, including supplemental stocking.     

Contaminant Trends in Lake Trout and Walleye From the Laurentian Great Lakes

Trends in PCBs, DDT, and other contaminants have been monitored in Great Lakes lake trout and walleye since the 1970s using composite samples of whole fish. Dramatic declines have been observed in concentrations of PCB, ΣDDT, dieldrin, and oxychlordane, with declines initially following first order loss kinetics. Mean PCB concentrations in Lake Michigan lake trout increased from 13 μg/g in 1972 to 23 μg/g in 1974, then declined to 2.6 μg/g by 1986. Between 1986 and 1992 there was little change in concentration, with 3.5 μg/g observed in 1992. ΣDDT in Lake Michigan trout followed a similar trend, decreasing from 19.2 μg/g in 1970 to 1.1 μg/g in 1986, and 1.2 μg/g in 1992. Similar trends were observed for PCBs and ΣDDT in lake trout from Lakes Superior, Huron and Ontario. Concentrations of both PCB and ΣDDT in Lake Erie walleye declined between 1977 and 1982, after which concentrations were relatively constant through 1990. When originally implemented it was assumed that trends in the mean contaminant concentrations in open-lake fish would serve as cost effective surrogates to trends in the water column. While water column data are still extremely limited it appears that for PCBs in lakes Michigan and Superior, trends in lake trout do reasonably mimic those in the water column over the long term. Hypotheses to explain the trends in contaminant concentrations are briefly reviewed. The original first order loss kinetics used to describe the initial decline do not explain the more recent leveling off of contaminant concentrations. Recent theories have examined the possibilities of multiple contaminant pools. We suggest another hypothesis, that changes in the food web may have resulted in increased bioaccumulation. However, a preliminary exploration of this hypothesis using a change point analysis was inconclusive.     

Molecular characterization of bacterial communities associated with sediments in the Laurentian Great Lakes

The Laurentian Great Lakes freshwater ecosystem comprises a series of interconnected lakes exhibiting wide spatial differences in hydrogeochemistry, productivity, and allochthonous inputs of nutrients and xenobiotics. Bacterial communities associated with offshore sediments in this ecosystem have been rarely studied. We evaluated bacterial communities associated with sediments from 10 locations in four of the five Great Lakes (Lakes Superior, Michigan, Huron, and Ontario) by generating16S rRNA pyrosequence libraries. Pyrosequencing analysis revealed the presence of 26 bacterial phyla and proteobacterial classes among Great Lakes sediment samples. Actinobacteria, Acidobacteria, Betaproteobacteria, and Gammaproteobacteria were the most abundant groups of bacteria. Redundancy analysis was used to examine the role of sediment properties, including depth and chemical composition, in shaping bacterial community structure. One sample from Lake Huron was distinctly different from all other samples. Phylogenetic analysis revealed that the sample contained greater abundances of groups of bacteria associated with polluted environments. This study constitutes the most extensive examination of bacteria associated with Laurentian Great Lakes sediments and sheds useful insight into the microbial ecology of the Great Lakes.     

Preliminary Checklist of Diatoms (Bacillariophyta) from the Laurentian Great Lakes

A preliminary compilation indicates the occurrence of at least 1611 diatom taxa in the Great Lakes. Included are 81 genera, 1059 species, and 471 subordinate taxa. Although the present list is known to be incomplete, it furnishes some indication of the diversity of the flora and indicates problem areas which deserve further attention.     

Status of the major aquaculture carps of China in the Laurentian Great Lakes Basin

There is concern of economic and environmental damage occuring if any of the four major aquacultured carp species of China, black carp Mylopharyngodon piceus, bighead carp Hypophthalmichthys nobilis, silver carp H. molitrix, or grass carp Ctenopharyngodon idella, were to establish in the Laurentian Great Lakes. All four are reproducing in the Mississippi River Basin. We review the status of these fishes in relation to the Great Lakes and their proximity to pathways into the Great Lakes, based on captures and collections of eggs and larvae. No black carp have been captured in the Great Lakes Basin. One silver carp and one bighead carp were captured within the Chicago Area Waterway System, on the Great Lakes side of electric barriers designed to keep carp from entering the Great Lakes from the greater Mississippi River Basin. Three bighead carp were captured in Lake Erie, none later than the year 2000. By December 2019, at least 650 grass carps had been captured in the Great Lakes Basin, most in western Lake Erie, but none in Lake Superior. Grass carp reproduction has been documented in the Sandusky and Maumee rivers in Ohio, tributaries of Lake Erie. We also discuss environmental DNA (eDNA) results as an early detection and monitoring tool for bighead and silver carps. Detection of eDNA does not necessarily indicate presence of live fish, but bigheaded carp eDNA has been detected on the Great Lakes side of the barriers and in a small proportion of samples from the western basin of Lake Erie.     

Formation of the St. Clair River Delta in the Laurentian Great Lakes System

The St. Clair River delta, the largest in the Laurentian Great Lakes system, is located in Lake St. Clair at the mouth of the St. Clair River. It straddles the border between the state of Michigan in the USA and the province of Ontario in Canada. The current study aims at characterizing and explaining the formation and evolution of the distributary channels in the St. Clair River delta and the delta itself. The delta is classified as a river dominated feature with classic “bird's foot” structure similar to the Mississippi delta model. The delta is composed of two surfaces. The older surface at the delta apex was deposited at a higher lake level some 3,500 to 5,000 years B.P. The current delta started to form following a drop in lake level some 3,500 years B.P. and continues to build to the present day. The delta consists of seven active deep channels averaging 11 m in depth entering a lake with a mean depth of 3 m with much shallower water in the delta front region. The channels are stable and are actively eroding into the sediments of the lake creating both sub-aqueous and sub-aerial levee deposits with crevasses. The interdistributary bays are being filled with sandy deposits created by wave energy and by crevasse deposits. At the erosional front of each distributary a narrow erosional notch “leading channel” is being formed which appears to control the direction of lakeward erosion of each deep distributary channel. The emplacement of the delta body in the shallow receiving water body has been termed “burrowing” delta formation and is the mechanism controlling the formation of this sedimentary feature.     

Tweeting the Laurentian Great Lakes: A community opinion analysis about Great Lakes areas as assessed through mentions on Twitter

Accounting for community opinions of environmental restoration is critical both for planning and evaluating these initiatives. While considerable research assesses the value of restoration through economic metrics focusing on expenditures or preferences for ecosystem services, these metrics may not adequately account for the sociocultural services that ecosystems provide communities, such as mental and physical health or recreational opportunities. To address this challenge, we explored the use of social media data to assess online discourse communities’ opinions about ecosystem services through a case study of Twitter mentions of sites targeted for restoration through the Great Lakes Restoration Initiative (GLRI). While there is evidence of the economic and ecological benefits of GLRI, little is known about how these benefits at sites targeted for funding are perceived by the public. From April through October 2019, we collected 40,000 tweets that mentioned an Area of Concern or a Great Lakes National Park that received GLRI funding. We used a mixed-methodological approach combining tweet content and sentiment analysis to determine themes of discussion and characterize online discourse communities’ opinions around these topics. Half of all tweets were about one of three Areas of Concern, and recreation was the most discussed theme with an overall positive sentiment. A metric accounting for the number of tweets and the sentiment of tweets was derived to understand community opinions of restoration at these areas. Our findings demonstrate the potential of social media data mining as a tool for examining online conversations about and engagement with the Great Lakes.     

Genetic population structure of cisco,Coregonus artedi,in the Laurentian Great Lakes

Management of a widely distributed species can be a challenge when management priorities, resource status, and assessment methods vary across jurisdictions. For example, restoration and preservation of coregonine species diversity is a goal of management agencies across the Laurentian Great Lakes. However, management goals and the amount of information available varies across management units, making the focus for management efforts challenging to determine. Genetic data provide a spatially consistent means to assess diversity. Therefore, we examined the genetic stock structure of cisco (Coregonus artedi) in the Great Lakes where the species is still extant. Using genotype data from 17 microsatellite DNA loci, we observed low levels of population structure among collections with most contributions to overall diversity occurring among lakes. Cisco from lakes Superior, Michigan, Ontario, and the St. Marys River could be considered single genetic populations while distinct genetic populations were observed among samples from northern Lake Huron. Significant within-lake diversity in Lake Huron is supported by populations found in embayments in northern Lake Huron. The Grand Traverse Bay population in Lake Michigan represents a distinct population with reduced levels of genetic variation when compared to other lakes. The different levels of within lake population structure we observed will be important to consider as future lake-specific management plans are developed.     

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).