U.S. EPA Great Lakes National Program Office monitoring of the Laurentian Great Lakes: Insights from 40 years of data collection

The U.S. EPA Great Lakes National Program Office (GLNPO) implements long-term monitoring programs to assess Great Lakes ecosystem status and trends for many interrelated ecosystem components, including offshore water quality as well as offshore phytoplankton, zooplankton and benthos; chemical contaminants in air, sediments, and predator fish; hypoxia in Lake Erie's central basin; and coastal wetland health. These programs are conducted in fulfillment of Clean Water Act mandates and Great Lakes Water Quality Agreement commitments. This special issue presents findings from GLNPO's Great Lakes Biology Monitoring Program, Great Lakes Water Quality Monitoring Program, Lake Erie Dissolved Oxygen Monitoring Program, Integrated Atmospheric Deposition Network, Great Lakes Fish Monitoring and Surveillance Program, and Great Lakes Sediment Surveillance Program. These GLNPO programs have generated temporal and spatial datasets for all five Great Lakes that form the basis for assessment of the state of these lakes, including trends in nutrients, key biological indicators, and contaminants in air, sediments and fish. These datasets are used by researchers and managers across the Great Lakes basin for investigating physical, chemical and biological drivers of ongoing ecosystem changes; some of these analyses are presented in this special issue, along with discussion of new methods and approaches for monitoring.     

Using satellite observations to assess the spatial representativeness of the GLNPO water quality monitoring program

The U.S. EPA's Great Lakes National Program Office (GLNPO) annual water quality survey (WQS) collects data at a relatively small number of stations in each lake. The survey was designed to measure conditions in the open-water regions of the lakes where an assumption of spatial homogeneity was thought likely to be met and the measured variables could be characterized by simple statistics. Here we use satellite observations to assess how well statistics based on samples collected in the GLNPO sampling network represent the lake-wide values of two variables, surface chlorophyll concentration and Secchi depth. We find strong linear relationships between the mean values calculated from the samples and the corresponding averages based on the subsets of the full satellite images. Although overall the means of the values from the sample locations agree well with means calculated from most of the non-coastal regions of the lakes, in terms of water depth, the GLNPO station averages best represent the regions of Lake Huron deeper than 30?m, of Lakes Michigan and Superior deeper than 90?m, and of Lake Ontario deeper than 60?m. When the lake regions are defined by distance offshore rather than by depth, the GLNPO station chlorophyll means in Lakes Huron, Ontario, and Superior are closest to the means for the area of the lakes >10?km offshore. In Lake Michigan the closest correspondence is with the >20?km offshore region. On a whole-lake basis in Lake Erie the GLNPO station chlorophyll averages are closest to the average calculated from the entire lake.     

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: I. Introduction and Phytoplankton Results

The Great Lakes National Program Office of the U.S. EPA has been conducting biological monitoring of the Laurentian Great Lakes since 1983. This paper presents synoptic survey data of phytoplankton communities from all five lakes. These communities were highly diverse, each lake typi-cally supporting over 100 species during both the spring and summer surveys. Much of that diversity was contributed by diatoms, which dominated the plankton of all lakes except Lake Superior in the spring. Summer communities shifted away from diatoms, toward chrysophytes in the upper lakes and chloro-phytes in the lower lakes. Ordination analyses indicated the close similarity of communities in the upper lakes, in particular Lakes Huron and Michigan, and a diverse range of communities in Lake Erie. Floristically, Lake Ontario was fundamentally different from all other lakes.     

Great Lakes total phosphorus revisited: 1. Loading analysis and update (1994–2008)

Phosphorus load estimates have been updated for all of the Great Lakes with an emphasis on lakes Superior, Michigan, Huron and Ontario for 1994–2008. Lake Erie phosphorus loads have been kept current with previous work and for completeness are reported here. A combination of modeling and data analysis is employed to evaluate whether target loads established by the Great Lakes Water Quality Agreement (GLWQA, 1978, Annex 3) have been and are currently being met. Data from federal, state, and provincial agencies were assembled and processed to yield annual estimates for all lakes and sources. A mass-balance model was used to check the consistency of loads and to estimate interlake transport. The analysis suggests that the GLWQA target loads have been consistently met for the main bodies of lakes Superior, Michigan and Huron. However, exceedances still persist for Saginaw Bay. For lakes Erie and Ontario, loadings are currently estimated to be at or just under the target (with some notable exceptions). Because interannual variability is high, the target loads have not been met consistently for the lower Great Lakes. The analysis also indicates that, because of decreasing TP concentrations in the lakes, interlake transport of TP has declined significantly since the mid-1970s. Thus, it is important that these changes be included in future assessments of compliance with TP load targets. Finally, detailed tables of the yearly (1994–2008) estimates are provided, as well as annual summaries by lake tributary basin (in Supplementary Information).     

Building a CSMI database: Experiences from the Lake Ontario 2018 CSMI Field Year

The Cooperative Science and Monitoring Initiative (CSMI) instituted under the Science Annex of the 2012 Great Lakes Water Quality Agreement (GLWQA) provides an international framework to coordinate science and monitoring activities in one of the five Great Lakes. On a five-year cycle: (Y1) CSMI priorities are developed under GLWQA Annex 2 Lake Partnerships with input from managers, researchers, and other stakeholders, (Y2) projects are then planned to address those priorities, (Y3) projects are implemented during the field sampling year, (Y4) samples are analyzed, and (Y5) results are shared through reporting. Although CSMI has advanced understanding and management of the Great Lakes, such large-scale studies present unique logistical challenges. Specifically, there is a need to promote and enhance data management, coordination, and sharing efforts. Herein, we describe the process used to develop a database for the 2018 Lake Ontario Field Year and explore the challenges, successes, and lessons learned that could improve collaboration and data compilation in future CSMI cycles. The creation of an accessible and transparent database can encourage collaboration between researchers and scientists, provide insight into the state and health of Lake Ontario, and engage the public as to why monitoring the Great Lakes is so crucial. We suggest the following recommendations to be implemented in future CSMI database iterations: 1) early planning of the database development, 2) house the database in a centralized location with emphasis on metadata, 3) encourage development of summary products for various user groups, and 4) sustained collaboration and commitment on database requirements.     

A band-ratio algorithm for retrieving open-lake chlorophyll values from satellite observations of the Great Lakes

The U.S. Environmental Protection Agency's Great Lakes National Program Office (GLNPO) has collected water quality data from the five Great Lakes annually since 1993. We used the GLNPO observations made since 2002 along with coincident measurements made by the Sea-viewing Wide Field-of-View Sensor (SeaWiFS) and the Moderate-resolution Imaging Spectroradiometer (MODIS) to develop a new band-ratio algorithm for estimating chlorophyll concentrations in the Great Lakes from satellite observations. The new algorithm is based on a third-order polynomial model using the same maximum band ratios employed in the standard NASA algorithms (OC4 for SeaWiFS and OC3M for MODIS). The sensor-specific coefficients for the new algorithm were obtained by fitting the relationship to several hundred matched field and satellite observations. Although there are some seasonal variations in some lakes, the relationship between the observed chlorophyll values and those modeled using the new coefficients is fairly stable from lake to lake and across years. The accuracy of the satellite chlorophyll estimates derived from the new algorithm was improved substantially relative both to the standard NASA retrievals and to previously published algorithms tuned to individual lakes. Monte-Carlo fits to randomly selected subsets of the observations allowed us to estimate the uncertainty associated with the retrievals purely as a function of the satellite data. Our results provide, for the first time, a single simple band ratio method for retrieving chlorophyll concentrations in the offshore “open” waters of the Great Lakes from satellite observations.     

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: III. Crustacean Zooplankton

Open water zooplankton communities were sampled across all five Laurentian Great Lakes during spring and summer 1998. Spring communities were characterized by relatively low species numbers and densities. Crustacean communities in all lakes except Lake Ontario were dominated by diaptomid copepods in spring. During summer, both abundance and species richness increased, the latter owing largely to the appearance of populations of cladocerans. Crustacean communities in the upper lakes were dominated by diaptomid copepods, cyclopoid copepodites, and Daphnia galeata mendotae (codominant with Holopedium gibberum in Lake Superior), and showed a high degree of spatial homogeneity. Lake Erie supported a notably more species rich community, and also exhibited a high degree of spatial heterogeneity. Lake Ontario differed from the other lakes by its relative lack of calanoid copepods, being dominated instead by cyclopoid copepods, along with Bosmina and Daphnia. There was a clear distinction between community composition in the western and eastern portions of the lake, though the reasons for this are unclear.     

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.     

Mean Circulation in the Great Lakes

In this paper new maps are presented of mean circulation in the Great Lakes, employing long-term current observations from about 100 Great Lakes moorings during the 1960s to 1980s. Knowledge of the mean circulation in the Great Lakes is important for ecological and management issues because it provides an indication of transport pathways of nutrients and contaminants on longer time scales. Based on the availability of data, summer circulation patterns in all of the Great Lakes, winter circulation patterns in all of the Great Lakes except Lake Superior, and annual circulation patterns in Lakes Erie, Michigan, and Ontario were derived. Winter currents are generally stronger than summer currents, and, therefore, annual circulation closely resembles winter circulation. Circulation patterns tend to be cyclonic (counterclockwise) in the larger lakes (Lake Huron, Lake Michigan, and Lake Superior) with increased cyclonic circulation in winter. In the smaller lakes (Lake Erie and Lake Ontario), winter circulation is characterized by a two-gyre circulation pattern. Summer circulation in the smaller lakes is different; predominantly cyclonic in Lake Ontario and anticyclonic in Lake Erie.     

Changes in mid-summer water temperature and clarity across the Great Lakes between 1968 and 2002

In recent decades, three important events have likely played a role in changing the water temperature and clarity of the Laurentian Great Lakes: 1) warmer climate, 2) reduced phosphorus loading, and 3) invasion by European Dreissenid mussels. This paper compiled environmental data from government agencies monitoring the middle and lower portions of the Great Lakes basin (lakes Huron, Erie and Ontario) to document changes in aquatic environments between 1968 and 2002. Over this 34-year period, mean annual air temperature increased at an average rate of 0.037 °C/y, resulting in a 1.3 °C increase. Surface water temperature during August has been rising at annual rates of 0.084 °C (Lake Huron) and 0.048 °C (Lake Ontario) resulting in increases of 2.9 °C and 1.6 °C, respectively. In Lake Erie, the trend was also positive, but it was smaller and not significant. Water clarity, measured here by August Secchi depth, increased in all lakes. Secchi depth increased 1.7 m in Lake Huron, 3.1 m in Lake Ontario and 2.4 m in Lake Erie. Prior to the invasion of Dreissenid mussels, increases in Secchi depth were significant (p < 0.05) in lakes Erie and Ontario, suggesting that phosphorus abatement aided water clarity. After Dreissenid mussel invasion, significant increases in Secchi depth were detected in lakes Ontario and Huron.     

Lake Erie phosphorus targets: An imperative for active adaptive management

Management actions taken to meet the phosphorus load targets in the 1978 Great Lakes Water Quality Agreement proved highly successful, initially. Eutrophication symptoms abated, and attention was redirected toward other important water quality problems. However, in the early 2000s Lake Erie, in particular, began to re-experience severe algal blooms and other problems associated with excessive nutrient inputs. The 2012 GLWQA prompted the development of updated phosphorus targets, and endorsed the concept of adaptive management. We propose that an active adaptive management program that maximizes learning opportunities will be imperative to sustain any future improvements realized in response to the new targets. Every year offers natural, albeit uncontrolled experiments to exploit the adaptive management concept of “learning by doing." A carefully thought out plan of complementary monitoring and modeling, supported by stakeholder engagement, will promote an improved understanding the processes that influence lake behavior and guide essential refinements to management goals and appropriate actions to attain them. In 2019 the International Joint Commission released a set of recommendations regarding the use of modeling approaches to support adaptive management in Lake Erie. We have incorporated those recommendations herein to further inspire the Great Lakes community to invest in an active adaptive management strategy that will serve us into the future.     

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).     

Quantification of the Lake Trophic Typologies of Naumann (Surface Quality) and Thienemann (Oxygen) with Special Reference to the Great Lakes

Separate trophic scales and indices are developed for two of the most significant symptoms of eutrophication: surface water quality and hypolimnetic dissolved oxygen depletion. The scales are made comparable by expressing them in dimensionless form with a lower bound of zero and a mesotrophic range from 5 to 10. In this way, the two symptoms can be compared and their relative importance judged. This is done for the Great Lakes with the result that for both scales Lakes Superior, Huron, and Michigan are classified as oligotrophic. However, while central and eastern Lake Erie and Lake Ontario are classified as mesotrophic in terms of surface water quality, they range from eutrophic (central Lake Erie) to oligotrophic (Lake Ontario) on the oxygen scale. This is because, although these lakes are similar in surface water quality, their hypolimnion thicknesses range from approximately 4 m for central Erie to 70 m for Lake Ontario. Because of its shallowness, western Lake Erie does not have a persistent oxygen problem. In terms of surface quality it is classified as eutrophic.We have attempted to relate the two scales by correlating surface primary production and areal depletion rate. The results indicate that for lakes of similar primary production, areal oxygen depletion is directly proportional to hypolimnion thickness.     

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.     

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.     

Prevalence and distribution of Renibacterium salmoninarum in non-salmonid fishes from Laurentian Great Lakes and inland habitats

Renibacterium salmoninarum (RS), the causative agent of bacterial kidney disease, has been a serious threat to salmonid health in the Laurentian Great Lakes. Despite its wide spread presence in the Great Lakes basin, little is known about RS ecology and the potential role of non-salmonid species as one of the pathogen’s reservoirs. This information is of paramount importance to fishery managers in order to better understand RS distribution in the different biotic components of the Great Lakes watershed. In this study, non-salmonid species from lakes Michigan and Huron, and from 13 inland waters of the Great Lakes watershed were collected from 1999 to 2008. Out of 380 fish sampled from lakes Michigan and Huron, 42 (11.05%) tested positive for RS as determined by the nested polymerase chain reaction. Prevalence was lower in Lake Huron (5.71%) compared to Michigan (20.74%), but the difference was not statistically significant. Prevalence of RS was not found to be significantly different between species or sites; however, when species were grouped into demersal vs. pelagic categories, significant differences (P < 0.01) in prevalence were observed. Out of 607 fish sampled from inland waters, 111 (18.28%) tested positive for RS as determined by the sandwich enzyme-linked immunosorbent assay. Infection prevalence was highly variable across species and among localities. Our results indicate that many non-salmonid species can harbor this bacterium without progression to disease and may become a reservoir for infection.     

Environmental predictors of phytoplankton chlorophyll-a in Great Lakes coastal wetlands

Coastal wetlands of the Laurentian Great Lakes are diverse and productive ecosystems that provide many ecosystem services, but are threatened by anthropogenic factors, including nutrient input, land-use change, invasive species, and climate change. In this study, we examined one component of wetland ecosystem structure – phytoplankton biomass – using the proxy metric of water column chlorophyll-a measured in 514 coastal wetlands across all five Great Lakes as part of the Great Lakes Coastal Wetland Monitoring Program. Mean chlorophyll-a concentrations increased from north-to-south from Lake Superior to Lake Erie, but concentrations varied among sites within lakes. To predict chlorophyll-a concentrations, we developed two random forest models for each lake – one using variables that may directly relate to phytoplankton biomass (“proximate” variables; e.g., dissolved nutrients, temperature, pH) and another using variables with potentially indirect effects on phytoplankton growth (“distal” variables; e.g., land use, fetch). Proximate and distal variable models explained 16–43% and 19–48% of variation in chlorophyll-a, respectively, with models developed for lakes Erie and Michigan having the highest amount of explanatory power and models developed for lakes Ontario, Superior, and Huron having the lowest. Land-use variables were important distal predictors of chlorophyll-a concentrations across all lakes. We found multiple proximate predictors of chlorophyll-a, but there was little consistency among lakes, suggesting that, while chlorophyll-a may be broadly influenced by distal factors such as land use, individual lakes and wetlands have unique characteristics that affect chlorophyll-a concentrations. Our results highlight the importance of responsible land-use planning and watershed-level management for protecting coastal wetlands.     

Trends in Mysis diluviana abundance in the Great Lakes, 2006–2016

With the large Diporeia declines in lakes Michigan, Huron, and Ontario, there is concern that a similar decline of Mysis diluviana related to oligotrophication and increased fish predation may occur. Mysis density and biomass were assessed from 2006 to 2016 using samples collected by the Great Lakes National Program Office's biomonitoring program in April and August in all five Great Lakes. Summer densities and biomasses were generally greater than spring values and both increased with bottom depth. There were no significant time trends during these 10–11 years in lakes Ontario, Michigan, or Huron, but there was a significant increase in Lake Superior. Density and biomass were highest in lakes Ontario and Superior, somewhat lower in Lake Michigan, and substantially lower in Lake Huron. A few Mysis were collected in eastern Lake Erie, indicating a small population in the deep basin of that lake. On average, mysids contributed 12–18% (spring-summer, Michigan), 18–14% (spring-summer, Superior), 30–13% (spring-summer, Ontario), and 3% (Huron) of the total open-water crustacean biomass. Size distributions consisted of two peaks, indicating a 2-year life cycle in all four of the deep lakes. Mysis were larger in Lake Ontario than in lakes Michigan, Superior, and Huron. Comparisons with available historic data indicated that mysid densities were higher in the 1960s–1990s (5 times higher in Huron, 2 times higher in Ontario, and around 40% higher in Michigan and Superior) than in 2006–2016.     

Results from the U.S. EPA's Biological Open Water Surveillance Program of the Laurentian Great Lakes: II. Deep Chlorophyll Maxima

Deep chlorophyll maxima (DCM) were found in all five Laurentian Great Lakes during August, 1998. Chlorophyll profiles were consistent over large areas in Lakes Superior and Michigan, while distinct inter-site differences were apparent in the other three lakes. Shade adaptation appeared to be primarily responsible for increases in chlorophyll at depth in Lakes Huron and Ontario, while in Lake Superior increases in phytoplankton biovolume were also noted. Deep living phytoplankton populations in the latter lake exhibited improved nutrient status at depth, where concentrations of both soluble phosphorus and silica were higher. Phytoplankton community composition in the DCM differed from that previously reported for the lakes, most notably in the reduced populations of Cyclotella, relative to the epilimnion, seen at most sites. Filamentous chlorophytes were often more abundant at depth, as were certain species of Dinobryon.