U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2011–5233 

Prepared in cooperation with the Michigan Department of Natural Resources and Environment 

Water-Quality Characteristics of Michigan’s Inland Lakes, 

2001–10 

Lake Huron

Lake Michigan

Lake Erie

Lake Huron

Lake Michigan

Lake Superior
Cover: Map showing inland lakes greater than 25 acres in Michigan. 
Water-Quality Characteristics of Michigan’s 

Inland Lakes, 2001–10 

By L.M. Fuller and C.K. Taricska 

Prepared in cooperation with the Michigan Department of Natural Resources and Environment 

Scientific Investigations Report 2011–5233

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012 

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Fuller, L.M., and Taricska, C.K., 2012, Water-quality characteristics of Michigan’s inland lakes, 2001–10: 

U.S. Geological Survey Scientific Investigations Report 2011–5233, 53 p., plus CD–ROM. 
iii

Acknowledgments

The authors acknowledge the technical support from the Michigan Department of Environmental 

Quality (MDEQ), the financial support from the Clean Michigan Initiative (CMI), and the personal 

contributions of Ralph Bednarz (retired from MDEQ) associated with this report. 

Many U.S. Geological Survey (USGS) Michigan Water Science Center employees were involved 

in the collection and analysis of data for this study and in the preparation and review of this 

report. They include J.A. Wilkinson, R.G. Nettleton, T.A. Dewitt, C.E. Lipinski, D.A. James, 

and E. Carter (USGS, Grayling, Michigan); D. Burdett, J.M. Ellis, M. Holmio, S.B. Horton, 

J. Knudsen, and D.G. Wydra (USGS, Escanaba, Mich.); and S. Beeler, R.W. Howell, R. Hubbell, 

R.S. Jodoin, D. Obenauer, and C.K. Taricska (USGS, Lansing, Mich.). Technical reviews were 

provided by Michael Rosen, USGS Carson City, Nevada; Tim Willoughby, USGS Indianapolis, 

Indiana; and Ralph Bednarz, retired since 2010 from the Michigan Department of Environmental 

Quality, Lansing, Michigan. Editorial support was provided by Bonnie Stich Fink, USGS Science 

Publishing Network (SPN), Louisville, Kentucky, and by Michael Eberle, USGS-SPN, Columbus, 

Ohio. 
iv

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

History of Monitoring on Michigan’s Inland Lakes..........................................................................2

Purpose and Scope ..............................................................................................................................2

Water-Quality Data-Collection Methods....................................................................................................5

Lake and Site Selection .......................................................................................................................5

Site Identification..................................................................................................................................6

Sampling Strategy.................................................................................................................................6

Field and Laboratory Methods............................................................................................................6

Water Sampling............................................................................................................................6

Trophic-Status Evaluation ..........................................................................................................7

Quality Assurance of Data, Treatment of Censored Data, and Access to Data.........................8

Statewide Water Quality of Inland Lakes ..................................................................................................9

Index Station (Deepest-Basin) Data Comparison ...........................................................................9

Vertical Profile............................................................................................................................13

Trophic Status: Secchi-Disk Transparency, Chlorophyll a, and Total Phosphorus.........20

Color.............................................................................................................................................21

Nutrients......................................................................................................................................23

Major Ions and Physical Properties .......................................................................................28

Examination of Multibasin Lakes .....................................................................................................28

Comparisons With Other Lake-Monitoring Data for Michigan ...................................................35

Comparison with Historical Measurements...................................................................................37

Relation of Lake Water Quality to Land Cover ...............................................................................40

Summary and Conclusions.........................................................................................................................43

References Cited..........................................................................................................................................44

Appendix 1. List of Michigan lake basins sampled during 2001–10............................... on CD-ROM

Appendix 2. Trilinear (Piper) diagrams..................................................................................................47

Appendix 3. Boxplots of ammonia plus organic nitrogen and total phosphorus 

near-surface measurements ...........................................................................................51 
v

Figures

1A. Map showing inland lakes greater than 25 acres in the Lower Peninsula of 

Michigan ........................................................................................................................................3

1B. Map showing inland lakes greater than 25 acres in the Upper Peninsula of 

Michigan ........................................................................................................................................4

2. Map showing watershed-management units and 5-year rotational cycle for 

Lake Water-Quality Assessment in Michigan..........................................................................5

3. Photograph showing U.S. Geological Survey technician preparing to collect a 

chlorophyll a measurement ........................................................................................................8

4. Photograph showing Secchi disk, which is lowered into the water attached to 

a measuring tape to determine the Secchi-disk transparency (depth at which 

the Secchi disk disappears)........................................................................................................8

5. Graph showing statistical distribution of dissolved oxygen for spring and summer 

for near surface, mid-depth or metalimnion, and near bottom for the index 

stations (deepest basins) in Michigan lakes measured during 2001–10...........................14

6. Graph showing statistical distribution of water temperature for spring and 

summer for near surface, mid-depth or metalimnion, and near bottom for the 

index stations (deepest basins) in Michigan lakes measured during 2001–10 ................14

7. Graph showing statistical distribution of specific conductance for spring and 

summer for near surface, mid-depth or metalimnion, and near bottom for the 

index stations (deepest basins) in Michigan lakes measured during 2001–10 ................15

8. Graph showing statistical distribution of pH for spring and summer for near 

surface, mid-depth or metalimnion, and near bottom for the index stations 

(deepest basins) in Michigan lakes measured during 2001–10 ..........................................15

9. Maps showing spatial distribution of dissolved oxygen and the percentage of 

the water column below 0.5 and 2.0 milligrams per liter in spring and summer 

measurements for Michigan inland lakes during 2001–10 ..................................................17

10. Maps showing spatial distribution of specific conductance and pH in spring 

and summer mid-depth or metalimnion measurements for Michigan inland 

lakes during 2001–10 ..................................................................................................................18

11. Graph showing trophic-status distribution using Secchi-disk transparency, 

chlorophyll a, or total phosphorus as the indicator..............................................................20

12. Graphs showing statistical distribution of Secchi-disk transparency and 

chlorophyll a in spring and summer measurements and near-surface 

phosphorus in summer measurements at the index stations (deepest basins) 

in Michigan lakes during 2001–10............................................................................................21

13. Maps showing spatial distribution of trophic status for Michigan inland lakes 

during 2001–10.............................................................................................................................22

14. Graph showing Trophic State Index (TSI) values compared to lake depth for 

Michigan inland lakes during 2001–10 ....................................................................................23

15. Graph showing statistical distribution of true color in the summer near-surface 

measurements at the index stations (deepest basins) in Michigan lakes during 

2007–10 .........................................................................................................................................23

16. Graph showing statistical distribution of Kjeldahl nitrogen (ammonia plus 

organic nitrogen) in spring and summer near-surface, mid-depth or metalimnion, 

and near-bottom measurements at the index stations (deepest basins) in 

Michigan lakes during 2001–10 ................................................................................................24
vi

17. Graph showing statistical distribution of ammonia in spring and summer 

near-surface, mid-depth or metalimnion, and near-bottom measurements at 

the index stations (deepest basins) in Michigan lakes during 2001–10 ............................24

18. Graph showing statistical distribution of total phosphorus in spring and summer 

for near-surface, mid-depth or metalimnion, and near-bottom measurements at 

the index stations (deepest basins) in Michigan lakes during 2001–10 ............................25

19. Graph showing statistical distribution of nitrate plus nitrite in spring and summer 

for near-surface, mid-depth or metalimnion, and near-bottom measurements at 

the index stations (deepest basins) in Michigan lakes during 2001–10 ............................25

20. Maps showing spatial distribution of total nitrogen and total phosphorus for 

both spring and summer near surface measurements for Michigan inland lakes 

during 2001–10.............................................................................................................................26

21. Graphs showing statistical distribution of alkalinity, sulfate, chloride, and 

hardness in spring measurements at the index stations (deepest basins) in 

Michigan lakes during 2001–10 ................................................................................................29

22. Graphs showing statistical distribution of potassium, magnesium, sodium, and 

calcium in spring measurements at the index stations (deepest basins) in 

Michigan lakes during 2001–10 ................................................................................................29

23A. Maps showing spatial distribution of alkalinity, sulfate, chloride, and hardness 

measured at Michigan inland lakes during 2001–10 in relation to 

U.S. Environmental Protection Agency Level III ecoregions ..............................................30

23B. Maps showing spatial distribution of potassium, magnesium, sodium, and 

calcium measured at Michigan inland lakes during 2001–10 in relation to 

U.S. Environmental Protection Agency Level III ecoregions ..............................................31

24. Multiple-basin lake examples in Michigan shown over Landsat satellite imagery 

for September 11, 2009. A, Higgins Lake in Roscommon County. B, Martiny Lake in 

Mecosta County. ........................................................................................................................32

25. Graphs showing trophic status based on data from the Cooperative Lakes 

Monitoring Program for one date per lake during the month of August for 2001–10 

calculated from A, chlorophyll a and B, Secchi-disk transparency ..................................36

26. Graph showing statewide predictions of trophic status, by time period, produced 

by relating existing Secchi-disk transparency measurements to satellite imagery 

to produce predictions for all Michigan lakes greater than 20 acres ...............................36

27. Graph showing National Lakes Assessment trophic status (statistically valid, 

probability-weighted design) comparing Michigan and National data using 

total phosphorus, chlorophyll a, and Secchi-disk transparency........................................37

28. Maps showing increase (higher values) or decrease (smaller values) in property 

and constituent values from historical to current measurements for Michigan 

inland lakes..................................................................................................................................38

29. Graph showing historical and current trophic status for comparable lakes based 

on total phosphorus, chlorophyll a, and Secchi-disk transparency ..................................39

30. Map showing spatial distribution of dominant National Land Cover Data 2001 

class for Michigan inland lakes drainage areas for 2001–10..............................................40

31. Trilinear (Piper) diagram analysis, color coded by National Land Cover Data 2001 

majority class for lake-drainage areas ...................................................................................41

32. Graphs showing chloride and calcium mid-depth measurements made in 

spring compared to percent urban National Land Cover Data 2001 for 

lake-drainage areas ...................................................................................................................42
vii

33. Graph showing chloride mid-depth measurements made in spring compared 

to percent urban National Land Cover Data 2001 for lake-drainage areas, 

excluding Oakland, Livingston, or Genesee County data from the regression 

analysis.........................................................................................................................................42

2–1. Trilinear (Piper) diagram analysis, color coded by U.S. Environmental Protection 

Agency Level III Ecoregions, Michigan glacial landsystems, 6-digit hydrologic 

unit codes, and ecological drainage units .............................................................................48

2–2. Trilinear (Piper)diagram analysis, color coded by Trophic State Index classes 

using Secchi-disk transparency, chlorophyll a, or total phosphorus as the 

primary indicator.........................................................................................................................49

3–1. Graphs showing ammonia plus organic nitrogen and total phosphorus nearsurface measurements made in summer compared to lake drainage-area size............52

3–2. Graphs showing ammonia plus organic nitrogen and total phosphorus nearsurface measurements made in summer compared to dominant percent 

National Land Cover Data 2001 for lake drainage areas......................................................53 

Tables

1. Properties and constituents, laboratory analytical methods, and reporting levels 

of water-quality data samples collected from Michigan lakes during 2001–10.................7

2. Lake trophic state and classification ranges of Trophic State Index for total 

phosphorus, Secchi-disk transparency, and chlorophyll a...................................................8

3. Statistical summary, by physical property or consitutent, of Michigan lakes 

sampled in the index station (deepest basin) during 2001–10.............................................10

4. Results of Wilcoxon signed-rank test to determine statistical significance 

among Michigan lakes spring and summer measurements throughout the 

vertical profile at measured depths for 2001–10 ...................................................................19

5. Results of Wilcoxon signed-rank test to determine statistical significance among 

Michigan lakes in the vertical profile between measured depths for 2001–10................19

6. Results of Wilcoxon signed-rank test to determine statistical significance 

between Michigan lakes spring and summer measurements of Secchi-disk 

transparency and chlorophyll a for 2001–10 ..........................................................................23

7. Results of Wilcoxon signed-rank test to determine statistical significance 

among Michigan lakes nutrients at measured depths for 2001–10....................................27

8. Results of Wilcoxon signed-rank test to determine statistical significance among 

Michigan lakes spring and summer nutrients at measured depths for 2001–10 .............27

9. Results of Wilcoxon signed-rank test to determine statistical significance 

among Michigan lakes with multiple basins, by constituent, for 2001–10 ........................33

10. Results of Wilcoxon signed-rank test to determine statistical significance of 

differences among historical and current Michigan lakes property and 

constituent values for 2001–10 .................................................................................................39

11. Comparison of historical and current trophic status and percentage of lakes 

decreasing, not changing, or increasing classes for 2001–10............................................40 
viii

Conversion Factors and Abbreviations 

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

meter (m) 3.281 foot (ft)

Area

acre 4,047 square meter (m2

)

acre 0.004047 square kilometer (km2

)

Volume

milliliter (mL) 0.0338140227 ounce (oz)

Concentration

microgram per liter (μg/L) 0.001 milligram per liter (mg/L) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32

Vertical coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) or 

micrograms per liter (μg/L). The pore sizes of filters used in processing water samples for chemical 

analysis are given in micrometers (μm); a micrometer is one-thousandth of a millimeter (mm).

Specific conductance is given in microsiemens per centimeter (μS/cm). 

Abbreviations used in this report

AU/cm – Absorbance Units per centimeter

CaCO3 – calcium carbonate

CLMP – Cooperative Lakes Monitoring Program

CMI – Clean Michigan Initiative

DO – dissolved oxygen

GPS – Global Positioning System

Lat-Long – latitude-longitude

LWQA – Lake Water Quality Assessment 

MDEQ – Michigan Department of Environmental Quality

MDNRE – Michigan Department of Natural Resources and Environment

MRL – minimum reporting level

NLA – National Lakes Assessment

NPDES – National Pollutant Discharge Elimination System

NWIS – National Water Information System

R2

 –coefficient of determination

SDT – Secchi-disk transparency

SRS – Standard Reference Samples

STORET – Storage and Retrieval

TP – total phosphorus

TSI – Trophic State Index

USEPA – U.S. Environmental Protection Agency

USGS – U.S. Geological Survey
Water-Quality Characteristics of Michigan’s Inland Lakes, 

2001–10 

By L.M. Fuller and C.K. Taricska

Abstract 

The U.S. Geological Survey and the Michigan Department of Environmental Quality (MDEQ) jointly monitored 

for selected water-quality constituents and properties of inland 

lakes during 2001–10 as part of Michigan’s Lake Water-Quality Assessment program. During 2001–10, 866 lake basins 

from 729 inland lakes greater than 25 acres were monitored 

for baseline water-quality conditions and trophic status. This 

report summarizes the water-quality characteristics and trophic 

conditions of the monitored lakes throughout the State; the 

data include vertical-profile measurements, nutrient measurements at three discrete depths, Secchi-disk transparency (SDT) 

measurements, and chlorophyll a measurements for the spring 

and summer, with major ions and other chemical indicators 

measured during the spring at mid-depth and color during the 

summer from near-surface samples. 

In about 75 percent of inland lake deep basins (index 

stations), trophic characteristics were associated with oligotrophic or mesotrophic conditions; 5 percent or less were 

categorized as hypereutrophic, and 80 percent of hypereutrophic lakes had a maximum depth of 30 feet or less. Comparison of spring and summer measurements shows that water 

clarity based on SDT measurements were clearer in the spring 

than in the summer for 63 percent of lakes. For near-surface 

measurements made in spring, 97 percent of lakes can be 

considered phosphorus limited and less than half a percent 

nitrogen limited; for summer measurements, 96 percent of 

lakes can be considered phosphorus limited and less than half 

a percent nitrogen limited. Spatial patterns of major ions, alkalinity, and hardness measured in the spring at mid-depth all 

showed lower values in the Upper Peninsula of Michigan and 

a southward increase toward the southern areas of the Lower 

Peninsula, though the location of increase varied by constituent. A spatial analysis of the data based on U.S. Environmental 

Protection Agency Level III Ecoregions separated potassium, 

sulfate, and chloride concentrations fairly well, with a pattern 

of lower values in northern ecoregions trending toward higher 

values in southern ecoregions; lower and higher concentrations of magnesium, hardness, calcium, and alkalinity were 

well separated, but middle-range concentrations in central 

Michigan ecoregions were mixed. The highest concentrations 

of chloride and sodium were in the southeastern area of the 

Lower Peninsula. 

Lakes with multiple basins showed few statistically 

significant differences in constituent concentrations at the 

95-percent confidence level among combinations of depths 

between basins. The most statistically significant differences 

were found for water temperature, with significant differences 

in somewhat less than half the combinations in the spring and 

just a few combinations in the summer. The lack of significant 

differences between major basins of multibasin lakes indicates 

that monitoring of trophic characteristics in all major basins 

might not be necessary for the majority of constituents in 

future sampling programs.

Trophic characteristics based on the 2001–10 dataset 

were compared to trophic characteristics resulting from 

other Michigan sampling programs, including the volunteer 

Cooperative Lakes Monitoring Program coordinated by the 

MDEQ (measurements on 250 lakes in 2011), trophic-state 

predictions produced by relating existing measurements to 

remotely sensed data (measurements for about 3,000 lakes), 

and the National Lakes Assessment (NLA) statistically valid, 

probability-designed lakes program (measurements for 50 

lakes in Michigan and about 1,100 lakes nationally). A higher 

percentage of oligotrophic lakes resulted when using SDT 

from the volunteer data and the 2001–10 dataset than when 

using the predicted measurements from remotely sensed data 

or the NLA. Comparing trophic characteristics from differently designed programs provides multiple interpretations of 

lake water-quality status in Michigan lakes.

No directional statistically significant difference was 

found at the 95-percent confidence level among historical 

nutrients and trophic characteristics when comparing 445 lakes 

with historical data for 1974–84 with the 2001–10 dataset, 

though SDT did show statistically significant differences at the 

95-percent confidence level. Depending on the primary indicator, 50–66 percent of lakes did not change trophic-status class, 

13–23 percent moved towards the oligotrophic end of the TSI 

scale, and 20–25 percent moved a class towards the eutrophic 

end of the TSI scale. 
2 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Increasing percentages of urban-dominant land cover 

in the drainage areas of lakes had a more positive correlation 

with chloride concentration than did increased percentages 

of other land-cover classes; there was also a slight correlation of urban-dominant land cover and calcium concentration. 

Removing data for lakes in southeastern Lower Michigan, 

known from previous reports to be higher in chloride, still 

resulted in a positive relation even though the coefficient of 

determination (R2 value) decreased from 0.55 to 0.39. Dominant land-cover drainage areas were not strongly related 

to nutrients with respect to a linear relation, nor were lake 

drainage-area sizes.

Introduction

Michigan has more than 11,000 inland lakes. These 

resources provide numerous recreational opportunities for 

tourists and local residents, and they support a recreational 

industry in Michigan valued at $15 billion per year (Stynes, 

2002). Knowledge of water-quality characteristics in these 

lakes is essential for the effective management of these 

resources.

Historically, the U.S. Geological Survey (USGS) and 

the Michigan Department of Environmental Quality (MDEQ) 

have jointly monitored water quality in Michigan’s lakes and 

rivers. During the 1990s, however, funding for surface-waterquality monitoring was greatly reduced, and funds devoted 

to monitoring inland lakes through the Federal Clean Water 

Act Clean Lakes Program (Section 314) were eliminated. 

In 1998, citizens of Michigan passed the Clean Michigan 

Initiative (CMI) to clean up, protect, and enhance Michigan’s 

environmental infrastructure. MDEQ and USGS jointly redesigned and implemented the Lake Water-Quality Assessment 

(LWQA) monitoring program because of expanding waterquality data needs, resulting in part from the new CMI program (Michigan Department of Environmental Quality, 2001). 

Through the LWQA monitoring program, all Michigan 

lakes larger than 25 acres (with developed public boat-launch 

access) were monitored during 2001–10 (figs. 1A and 1B). The 

LWQA monitoring-program design incorporates the watershed-management units and 5-year rotational cycle currently 

being used by the MDEQ Ambient Surface Water Chemistry 

Monitoring Program (Michigan Department of Environmental 

Quality, 2000) to assess Michigan’s rivers, Great Lakes connecting channels, and bays (fig. 2).

The 5-year basin-monitoring cycle identifies 45 watershed-management units on the basis of statewide drainage to 

the Great Lakes. Each year, 7 to 10 of the major watersheds in 

Michigan are monitored and assessed. This is done to ensure 

that specific watersheds are monitored in the 5-year cycle 

to assist in (1) statewide water-quality assessments, (2) the 

National Pollutant Discharge Elimination System (NPDES) 

permitting process, and (3) resource-management decisions. 

The monitoring emphasizes data collection to classify each 

lake by its primary biological productivity (trophic status) and 

document its general chemical characteristics.

History of Monitoring on Michigan’s Inland 

Lakes

In 1973, the Michigan Department of Natural Resources 

(currently separate agencies: MDEQ and MDNR) began 

systematically monitoring the quality of Michigan inland lakes 

under Section 314 of the Clean Water Act. In previous years, 

few water-chemistry data had been collected on Michigan 

lakes, which hampered documentation of changes in lake 

water quality. Initially, it was expected that the “significant” 

lakes, defined as public lakes greater than or equal to 50 acres, 

could be sampled every 5 years. By 1979, however, more 

than half of the significant lakes had not been sampled. The 

U.S. Environmental Protection Agency (USEPA) increased 

funding under the Clean Water Act to assist states in assessing 

the quality of lakes. Also, a one-time grant from USEPA was 

awarded to the State of Michigan in June 1980 for the purpose 

of inventorying and classifying unsampled lakes. 

Carlson’s Trophic State Index (TSI) was chosen to 

classify lakes for five reasons: (1) the index works well over 

a broad range of trophic conditions; (2) lakes can be classified by total phosphorus (TP), transparency, and chlorophyll 

a; (3) the index is well suited for Michigan because it was 

developed by use of data from Michigan and Minnesota lakes; 

(4) the index is a continuum with divisions to distinguish 

general categories; and (5) TP, transparency, or chlorophyll a

data previously collected could be evaluated. Because TSI 

may underestimate the trophic condition in lakes dominated 

by macrophytes, the relative abundance of submergent macrophytes was observed and noted in the lakes to assist with the 

TSI classification. 

Purpose and Scope

This report summarizes the results of the 10-year datacollection project (2001–10) for the Michigan LWQA program. An explanation of sampled constituents is provided, and 

sample-collection methods used for the analysis are described. 

These constituents are summarized as a statewide deep-basin 

(index station) dataset and also as a comparison of multiplebasin lakes by season and sampling depth. Other Michigan 

sampling programs are compared with these data to determine 

whether all sampling programs provide the same results for 

Michigan inland lakes. Historical data are compared to current lake data to determine whether temporal changes have 

occurred, and the relation of land cover to lake water quality is 

discussed.
Introduction 3

Kent Ionia

Allegan

Oakland

Barry

Cass

Eaton

St. Clair

Jackson Wayne

Calhoun

Lenawee

Ottawa Clinton

Berrien

Ingham

Branch Hillsdale

Monroe

Cheboygan

Van Buren Washtenaw

Livingston

Macomb

Presque Isle

Kalamazoo

St. Joseph

Shiawassee

Huron

Sanilac

Lake

Bay

Clare

Iosco

Tuscola

Alcona

Lapeer

Saginaw

Newaygo

Alpena

Gratiot

Antrim

Mason

Isabella

Montcalm

Oscoda

Otsego

Genesee

Osceola

Oceana

Wexford

Emmet

Mecosta Midland

Ogemaw

Gladwin

Kalkaska Crawford

Manistee Missaukee

Arenac

Muskegon

Benzie

Roscommon

Montmorency

Leelanau

Charlevoix

Grand Traverse

84°W

86°W

45°N

43°N

0 25 50 MILES

0 25 50 KILOMETERS

Lake Huron

Lake Michigan

Lake Erie

Michigan

Northern

Lower

Peninsula

Southern

Lower

Peninsula

Northwestern

Lower

Peninsula

EXPLANATION

No public boat launch and not designated to be 

sampled for Lake Water-Quality Assessment

Has public boat launch and was sampled for Lake 

Water-Quality Assessment during 2001–10

INLAND LAKES GREATER THAN 25 ACRES (Breck, 2004)

Figure 1A. Inland lakes greater than 25 acres in the Lower Peninsula of Michigan. 
Iron

Delta

Marquette Luce

Alger

Chippewa

Gogebic

Baraga

Ontonagon

Schoolcraft

Houghton

Mackinac

Menominee

Dickinson

Keweenaw

Isle Royale

0 25 50 MILES

0 25 50 KILOMETERS

86°W

88°W

47°N

46°N

Lake Michigan

Lake Superior

Weste

Uppe

Penins

EXPLANATION

No public boat launch and not designated to be sampled for Lake Water-Quality AssessmentHas public boat launch and was sampled for Lake Water-Quality Assessment during 2001–10

INLAND LAKES GREATER THAN 25 ACRES (Breck, 2004)

Figure 1B. Inland lakes greater than 25 acres in the Upper Peninsula of Michigan. 
Water-Quality Data-Collection Methods 5

Monitoring cycle

Year 1

Year 2

Year 3

Year 4

Year 5

EXPLANATION

Figure 2. Watershed-management units and 5-year rotational cycle for Lake WaterQuality Assessment in Michigan. 

Water-Quality Data-Collection 

Methods

The sampling methodology was designed to replicate the 

methods and techniques historically used by MDEQ to sample 

inland lakes. Care was taken to minimize deviation from past 

sampling methods, locations, or laboratory-analysis methods 

that could create variability in the data owing to changes in 

techniques or methods rather than actual changes in lake water 

quality.

Lake and Site Selection

Lakes sampled in a given year were selected randomly 

from 7 to 10 major watersheds throughout Michigan on the 

basis of watershed-management units and 5-year rotational 

cycle currently being used by MDEQ Ambient Surface Water 

Chemistry Monitoring Program (Michigan Department of 

Environmental Quality, 2000) to assess Michigan’s rivers, 

Great Lakes connecting channels, and bays. Lakes targeted 

for monitoring under the LWQA monitoring program were 

25 acres or larger with public boat-launch access. Each 

watershed is to be sampled on a 5-year rotation until all lakes 

meeting these criteria are sampled. After the first 5-year rotation (2001–05), a selection of lakes had been sampled in all 

45 major watersheds in Michigan. 

The sampling site in each major lake basin was as close 

as possible to the known historical sampling location (index 

station) in the deepest basin of each lake. Geographic coordinates for each sampling site were established with a handheld 

Global Positioning System (GPS) unit. Once the sample site 

was located with a GPS unit, the site was verified by comparing the depth measured with an electronic depth finder to 

measured depths from previous visits.
6 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Site Identification

All previously sampled lake basins were identified with a 

USEPA Storage and Retrieval (STORET) number. Data stored 

in the USGS National Water Information System (NWIS) are 

referenced with a USGS station number, which is the latitudelongitude (Lat-Long) of the location where the sample was 

collected. A sequence number indicating the depth at which 

a sample was collected was then incorporated into the station 

number as the last two digits. The lake-station numbering 

system is summarized below:

Lat-Long-01 Vertical profile data

Lat-Long-02 Sample collected 3 ft above lake 

bottom

Lat-Long-03 Depth-integrated sample through the 

photic zone

Lat-Long-05 Sample collected 3 ft below lake 

surface

Lat-Long-06 Sample collected at mid-depth or 

metalimnion

Sampling Strategy

All lakes selected for monitoring within a given year 

were sampled once during spring turnover (usually April) and 

again in late summer (August or September), when they are 

typically thermally stratified. Samples collected during spring 

turnover, when water is well mixed, represent average water 

quality in the lake as a whole. Samples collected in late summer, when water is warmest and algae and aquatic macrophyte 

growth is at its peak, represent water-quality characteristics 

when biological productivity is the greatest specific to each 

zone of the lake.

Discrete lake-water samples were collected at several 

depths representing each zone of the lake water column 

(epilimnion, metalimnion, hypolimnion) at the index station 

(deepest basin) of each lake. Additional sample locations were 

used for those lakes that had multiple deep basins where complete interbasin mixing was unlikely.

Before sample collection, vertical profiles of dissolved 

oxygen (DO) concentration, water temperature, specific 

conductance, and pH were measured to determine thermal 

stratification. Water clarity was measured with a Secchi disk, 

and the photic zone was determined as twice the Secchi disk 

depth. During late-summer sampling, a qualitative macrophyte 

evaluation was made in the littoral zone of each lake to refine 

the trophic-state evaluation, and a measure of color was added 

during 2007–10.

Field and Laboratory Methods

Water Sampling

Standard MDEQ and USGS field methods were used to 

collect and preserve samples. During spring sampling, three 

discrete samples were collected. One sample was collected 3 ft 

below the surface, another 3 ft above the bottom, and the third 

at mid-depth. Summer sampling used the same sample depths 

except in stratified lakes, where the mid-depth sample was collected from the center of the metalimnion (thermocline). During spring sampling, only mid-depth samples were collected 

on shallow lakes when depth prohibited collection of three 

discrete samples. Samples were recorded in the database as 

the “3 ft below lake surface” samples when depth prohibited 

collection of three discrete samples during the summer. The 

depths at which the samples were recorded for these shallow 

lakes were noted for both spring and summer. 

Water samples collected near bottom, near surface, and at 

mid-depth were analyzed for nutrients; samples for analyses 

of all other water-quality characteristics were collected only 

in the spring from mid-depth. All water samples except for 

chlorophyll a samples were collected as discrete samples with 

a Van Dorn-style sampler. Water samples for chlorophyll a

analysis were depth-integrated composite samples collected by 

lowering a bottle sampler through the photic zone. On those 

lakes where the photic zone extended to the lake bottom, the 

bottle sampler was lowered to within 1 ft of the lake bottom 

with care taken not to disturb bottom sediments.

Water samples for nutrients were preserved with sulfuric 

acid (H2SO4) to a pH of less than 2, and water samples for 

selected ions (calcium, magnesium, sodium, and potassium) 

were preserved with nitric acid (HNO3) to a pH of less than 2. 

Chlorophyll a samples were filtered onsite through a 0.45 μm 

combination acetate, nitrate cellulose filter (filter type HAWP 

047 00). The filter then was placed in a vial containing 

10 milliliters (mL) of 90 percent acetone. All samples then 

were overnight shipped in coolers with ice to the MDEQ 

laboratory for analysis of chlorophyll a and ions. Nutrient 

analyses were completed either by the MDEQ or its contract 

laboratory. Standard analytical methods approved by the 

USEPA were used for sample analyses (table 1).
Water-Quality Data-Collection Methods 7

Table 1. Properties and constituents, laboratory analytical methods, and reporting levels of water-quality data samples collected from 

Michigan lakes during 2001–10. 

[MRL, minimum reporting level; mg/L, milligrams per liter; CaCO3, calcium carbonate; μg/L, microgams per liter; AU/cm, Absorbance Units per centimeter; 

USEPA, U.S. Environmental Protection Agency]

Property or constituent Units MRL1 Method Reference

Alkalinity, water unfiltered (acid neutralizing 

capacity)

mg/L as CaCO3 20 310.1 USEPA, 1983

Calcium, total recoverable mg/L 1 7140/215.1 USEPA, 1983

Chloride, dissolved mg/L 1 325 USEPA, 1983

Hardness, total mg/L as CaCO3 5 Calculated SM 2340 B Clesceri and others, 1998

Magnesium, total recoverable mg/L 1 7450/242.1 USEPA, 1983

Sodium, total recoverable mg/L 1 7770/273.1 USEPA, 1983

Sulfate mg/L 2 375.1 USEPA, 1983

Potassium, total recoverable mg/L .1 7610/258.1 USEPA, 1983

Nitrogen, ammonia plus organic, total mg/L .1 351.2 USEPA, 1983

Nitrogen, ammonia, total mg/L .01 350.1 USEPA, 1983

Nitrogen, nitrate plus nitrite, total mg/L .01 353.2 USEPA, 1983

Phosphorus, total mg/L .005 365.4 USEPA, 1983

Chlorophyll a μg/L 1 SM 10200 H Clesceri and others, 1998

Absorbance at 400 nanometers (color) AU/cm .007 SM 2004B Clesceri and others, 1998

1

Established reporting levels for various analytical procedures (Oblinger-Childress and others, 1999).

Trophic-Status Evaluation

A biologically productive lake is desirable for many 

activities such as fishing and maintaining a healthy wildlife 

population; however, a lake can be overly productive. Eventually, if excessive plant and algal growth goes unchecked, the 

lake may become impaired. The MDEQ classifies lakes on 

their level of primary biological productivity or trophic status. 

A lake with low productivity is classified as oligotrophic, and 

a lake with moderate productivity is classified as mesotrophic. 

A biologically productive lake is classified as eutrophic, and 

an excessively biologically productive lake is classified as 

hypereutrophic. The trophic status of lakes can be compared 

and tracked over time to help evaluate eutrophication resulting 

from nutrient enrichment, which may reflect changes in landuse practices. 

The primary biological productivity in each lake basin 

was evaluated by MDEQ with Carlson’s TSI (Carlson, 1977). 

Carlson’s TSI was developed for use with lakes that have few 

rooted aquatic plants and little non-algal turbidity (U.S. Environmental Protection Agency, 2007). Late-summer sample 

data are used in this evaluation because primary biological 

productivity is at its peak and lakes are typically at maximum 

thermal stratification. Carlson’s TSI was computed from TP 

concentrations (collected near the surface), chlorophyll a concentrations (collected in the photic zone; fig. 3), and Secchidisk measurements (fig. 4). 

Carlson’s TSI is a numerical scale ranging from 0 to 100. 

The low end of the scale represents low primary biological 

productivity (oligotrophy), the middle of the scale represents 

moderate biological productivity (mesotrophy), the high end 

of the scale represents a very biologically active lake (eutrophy), and the highest end of the scale represents excessive 

biological productivity (hypereutrophy). Carlson and Simpson (1996) suggest TSI ranges for northern temperate lakes 

whereby a TSI value less than 40 represents oligotrophic 

conditions; 40–50, mesotrophic; 50–70, eutrophic; and greater 

than 70, hypereutrophic. 

Although the concept of TSI ranges is simple, the interpretation can be complex because of their interdependency. 

The TSI values computed from any of the three indicators 

(chlorophyll a, TP, and Secchi-disk transparency (SDT)) will 

not necessarily be the same. There is a chemical and physical 

environment that can influence particular indicators and the 

interrelation with one another (Wetzel, 2001).

The MDEQ has adopted a modified scale and interpretation of the three indicators to account for regional characteristics (Michigan Department of Natural Resources, 1982). 

TSI values less than 38 represent oligotrophic conditions; 

38–48, mesotrophic; 49–61, eutrophic; and greater than 

61, hypereutrophic. 
8 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Figure 3. U.S. Geological Survey technician preparing to collect 

a chlorophyll a measurement. 

Figure 4. Secchi disk, which is lowered into the water attached 

to a measuring tape to determine the Secchi-disk transparency 

(depth at which the Secchi disk disappears). 

Walker (1979) proposed that averaging the TSI values 

from all three indicators would provide a means of reducing 

the effects of individual sampling and measurement errors, 

thus developing a robust estimate of the index. Carlson’s 

TSI may underestimate the trophic state of lakes dominated 

by macrophytes. The relative abundance of submergent 

macrophytes was used to indicate more productive conditions 

than indicated by TSI values. Walker assumed that “moderate” 

and “dense” growths of macrophytes were indicative of mesotrophic and eutrophic conditions, respectively. Therefore, if 

the TSI indicated mesotrophic conditions but “dense” growths 

of aquatic macrophytes were present, the lake would then be 

classified as eutrophic. For lakes with multiple basins, the TSI 

values from the index station (deepest basin) were used to 

determine the trophic state of these lakes.

The following equations were used to calculate the TSI 

values for each indicator, and were then averaged for lakes 

presented in appendix 1 and are summarized in table 2 for 

determining the trophic-status classification: 

TSI Secchi disk = 60 − 14.41 * ln (1)

(Secchi-disk transparency, in meters)

TSI chlorophyll a = 9.81 * ln (chlorophyll a, (2)

in micrograms per liter) + 30.6

TSI total phosphorus = 14.42 * ln (total (3)

phosphorus, in micrograms per liter) + 4.15

Table 2. Lake trophic state and classification ranges of Trophic 

State Index for total phosphorus, Secchi-disk transparency, and 

chlorophyll a. 

[Based on Michigan Department of Natural Resources (1982) and modified 

by the State of Michigan to account for regional characteristics; TSI, Trophic 

State Index; SDT, Secchi-disk transparency; Chl-a, chlorophyll a; TP, total 

phosphorus; m, meter; μg/L, micrograms per liter; mg/L, milligrams per liter; 

<, less than; >, greater than]

Lake trophic Carlson’s SDT Chl-a TP 

state TSI (m) (μg/L) (mg/L) 

Oligotrophic < 38 > 4.6 < 2.2 < 0.010 

Mesotrophic 38 – 48 2.3 – 4.6 2.2 – 6 .010 – .020 

Eutrophic 49 – 61 .9 – 2.2 6.1 – 22 .021 – .050 

Hypereutrophic > 61 < .9 > 22 > .050 

Quality Assurance of Data, Treatment of 

Censored Data, and Access to Data

In accordance with USGS policy, analytical laboratories 

that provide chemical, radiochemical, and biological analyses are regularly reviewed and evaluated to ensure that data 

quality is appropriately maintained (U.S. Geological Survey, 

1998). During the review process for the Michigan lakesample analyses, any samples that did not follow prescribed 

processing protocol were excluded from the database. In a 

few instances, sample bottles leaked and were not processed. 

For these reasons, some samples of various constituents were 

removed. The tables in this report note the number of samples 

analyzed for the respective constituents.
Statewide Water Quality of Inland Lakes 9

The MDEQ laboratory and their private contract laboratory provided analytical results with associated data-qualifier 

descriptions. All analytical methods used were matched 

with the appropriate USGS constituent codes and methods, 

in addition to the remarks codes for less-than and estimated 

data, before the data were stored in the USGS National Water 

Information System (NWIS) database. Both laboratories 

participated in the USGS Standard Reference Sample (SRS) 

program, and the laboratories were evaluated by using performance evaluation samples called SRS. The SRS were submitted to the laboratories semiannually for performance-comparison purposes. Statistical evaluation of the results provided 

information to compare the analytical performance between 

laboratories and to determine possible analytical deficiencies 

and problems. Although the SRS project is not a certification program, participation is required for all laboratories that 

provide water-quality data for the USGS. Any analyte measured by an individual laboratory that did not receive marginal 

or higher ratings during the evaluation period was not used in 

the data analysis for this report. Laboratory-evaluation results 

for the Michigan Department of Environmental Quality lab 

number 341 can be found under the results tab at http://bqs.

usgs.gov/srs/. 

In addition to the actual measurement done by the 

laboratories for each constituent, some data also were censored—that is, reported as “nondetect” or “less than (<)”—or 

estimated (E). Nondetects result when analysis for a specific 

constituent yields no evidence of the constituent being present in the sample. Less-than data result when a constituent 

is detected but the concentration is less than the minimum 

reporting level (MRL) for that constituent; for example, 

< 0.01 mg/L. This occurs when an exact value cannot be 

assigned to a constituent but can be assigned various ranges 

less than applicable reporting limits. Estimated values are 

assigned for a variety of reasons where the analysis deviates 

from strict protocol or ideal analytical conditions, such as 

extrapolation or minor loss of sample during preparation.

A conservative approach to the treatment of censored 

data was used from Data Interpretation Example number 1 

from Bonn (2008). Half the reporting limit was used for all 

nondetect and less-than data values, except when more than 

5 percent of the data values were nondetect and less-than 

values, in which case the Helsel and Cohn (1988) adjusted 

maximum likelihood method was used to assist in the creation 

of summary statistics. All LWQA monitoring data were 

archived in the USEPA data-management system (STORET), 

as well as in the USGS NWIS database. These data are 

available to the public on the Internet at http://mi.waterdata.

usgs.gov/nwis/qw, http://www.epa.gov/storet, and http://www.

michigan.gov/miswims. 

Statewide Water Quality of Inland 

Lakes

During 2001–10, lakes greater than 25 acres (with 

developed public boat-launch access) were sampled in all 45 

of Michigan’s watershed-management units. Of the 729 lakes 

sampled, 109 were lakes with more than 1 major basin. In all, 

866 lake basins were assessed and used in the analysis for this 

report. All lake basins included vertical-profile measurements, 

nutrient measurements at three discrete depths, SDT measurements (unless the Secchi disk hit the bottom of the lake, in 

which case no SDT depth was recorded), and depth-integrated 

chlorophyll a measurements all for spring and summer, with 

major ions and physical properties measured only for spring 

mid-depth and color only for summer during 2007–10.

In the spring, 52 lakes were deemed too shallow for the 

collection of near-lake-surface and near-lake-bottom measurements, thus only mid-depth samples were collected for all lake 

basins in the spring. In the summer, 58 lakes were deemed too 

shallow; thus, measurements were recorded in the near lake 

surface and not for mid-depth or lake bottom. Measurements 

for shallow lakes were made in the spring from 1 to 6 ft and 

in the summer from 1 to 5 ft. These measurements could be 

reflective of the near surface or mid-depth or metalimnion 

measurement depths. 

For summary statistics, the spring measurements deemed 

too shallow for three discrete measurements were moved from 

near surface to mid-depth to allow comparison between the 

two seasons. For other analyses where a near-surface sample 

was required (such as phosphorus for TSI calculation), the 

single measurement noted in the mid-depth or metalimnionin 

the spring, or near surface for the summer was used. Of the 

shallow lakes, 48 were determined to be too shallow in the 

spring and summer, 4 lakes were determined to be too shallow 

only in the spring, and 10 lakes were determined to be too 

shallow only in the summer. 

A few properties and constituents measured are reported 

as missing data values owing to leaking, improperly preserved 

samples, field meters not functioning correctly or losing battery power, and other similar circumstances. Four lakes, one 

with multiple basins, were resampled and are noted in appendix 1. The resampled data were used in this analysis, though 

available measurements for the original sample date are stored 

in NWIS. 

Index Station (Deepest-Basin) Data Comparison

During 2001–10, the 729 lakes sampled had measurements from the index station (deepest basin). A statistical summary of the index station (deepest-basin) water-quality data 

collected at various depths are listed in table 3. The data are 

summarized below by physical and chemical category.
10 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Table 3. Statistical summary, by physical property or consitutent, of Michigan lakes sampled in the index station (deepest basin) 

during 2001–10. (The Helsel and Cohn (1988) adjusted maximum liklihood method was used to create summary statistics for constituents 

with greater than 5 percent of the data as nondetects or less-than values; otherwise, half the reporting limit was used for all nondetect 

and less-than data values.)—Continued

[Min, minimum; 25 Per, 25 Percentile; Med, median; 75 Per, 75 Percentile; Max, Maximum; St. Dev, standard deviation; μg/L, micrograms per liter; 

mg/L, milligrams per liter; deg C, degrees Celsius; μS/cm, microsiemens per centimeter; std units, standard units; AU/cm, absorbance units per centimeter; 

CaCO3, calcium carbonate; <, less-than sign noting greater than 5 percent of data were nondetects or less-than values and adjusted maximum liklihood method 

was used]

Spring—continued

Constituent Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Secchi-disk 

transparency

meters 697 0.30 2.10 3.20 3.64 4.70 14.80 2.01

Chlorophyll a μg/L 724 <1.0 1.10 2.70 4.42 5.63 52.00 5.68

Dissolved oxygen Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 729 5.3 7.8 8.30 8.76 9.50 14.20 1.53

Mid-depth or 

metalimnion

mg/L 728 0 2.2 6.75 5.88 8.50 15.60 3.71

Near bottom mg/L 727 0 1 7.70 6.36 10.40 14.10 4.44

Water temperature Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface deg C 729 1.5 20 23.00 20.72 25.00 29.00 6.58

Mid-depth or 

metalimnion

deg C 728 2.5 9.5 16.75 15.84 22.00 28.50 6.98

Near bottom deg C 728 2.5 5 6.50 8.31 10.00 27.00 4.82

Specific conductance Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface μS/cm 728 12 164 276.50 274.01 359.00 1,230.00 158.20

Mid-depth or 

metalimnion

μS/cm 729 12 161 285.00 290.58 388.00 1,380.00 175.04

Near bottom μS/cm 729 13 172 312.00 309.74 419.00 1,390.00 182.72

pH Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface std units 727 5.1 7.9 8.20 8.12 8.40 9.60 0.55

Mid-depth or 

metalimnion

std units 724 4.8 7.3 7.70 7.65 8.10 9.50 0.66

Near bottom std units 723 4.5 7.1 7.50 7.49 8.00 9.90 0.66

Ammonia plus 

organic nitrogen Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 683 0.050 0.35 0.49 0.53 0.64 2.19 0.26

Mid-depth or 

metalimnion

mg/L 729 0.050 0.35 0.49 0.52 0.65 2.89 0.26

Near bottom mg/L 682 0.050 0.38 0.52 0.58 0.71 4.70 0.35

Ammonia Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 682 0.001 0.01 0.03 0.06 0.07 0.68 0.10

Mid-depth or 

metalimnion

mg/L 728 0.001 0.01 0.03 0.07 0.08 0.67 0.10

Near bottom mg/L 682 0.001 0.02 0.05 0.12 0.15 4.00 0.23
Water-Quality Data-Collection Methods 11

Table 3. Statistical summary, by physical property or consitutent, of Michigan lakes sampled in the index station (deepest basin) 

during 2001–10. (The Helsel and Cohn (1988) adjusted maximum liklihood method was used to create summary statistics for constituents 

with greater than 5 percent of the data as nondetects or less-than values; otherwise, half the reporting limit was used for all nondetect 

and less-than data values.)—Continued

[Min, minimum; 25 Per, 25 Percentile; Med, median; 75 Per, 75 Percentile; Max, Maximum; St. Dev, standard deviation; μg/L, micrograms per liter; 

mg/L, milligrams per liter; deg C, degrees Celsius; μS/cm, microsiemens per centimeter; std units, standard units; AU/cm, absorbance units per centimeter; 

CaCO3, calcium carbonate; <, less-than sign noting greater than 5 percent of data were nondetects or less-than values and adjusted maximum liklihood method 

was used]

Summer

Constituent Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Secchi-disk 

transparency

meters 693 0.3 2.1000 3.00 3.14 4.00 11.30 1.48

Chlorophyll a μg/L 727 0.5 2.0000 3.60 6.10 6.00 120.00 9.98

**Color AU/cm 285 <0.007 <0.007 0.01 0.01 0.01 0.15 0.01

Dissolved oxygen Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 729 5.3 7.6 8.10 8.12 8.50 15.80 1.01

Mid-depth or 

metalimnion

mg/L 729 0.1 2.2 6.20 5.49 7.90 15.50 3.44

Near bottom mg/L 728 0 0.3 0.50 2.22 3.40 13.10 2.96

Water temperature Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface deg C 729 16.5 22.5 24.00 23.91 25.50 29.00 2.21

Mid-depth or 

metalimnion

deg C 729 6.5 15 20.00 18.74 22.50 28.00 4.99

Near bottom deg C 727 3.5 7 11.00 13.05 19.50 28.00 6.62

Specific conductance Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface μS/cm 728 12 162.75 273.00 271.13 358.00 1,310.00 157.79

Mid-depth or 

metalimnion

μS/cm 729 12 167 288.00 288.59 382.00 1,380.00 171.93

Near bottom μS/cm 729 12 187 320.00 323.65 438.00 1,390.00 186.13

pH Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface std units 728 5.1 8 8.26 8.19 8.50 9.60 0.54

Mid-depth or 

metalimnion

std units 729 4.8 7.3 7.70 7.66 8.10 9.50 0.67

Near bottom std units 728 5.2 6.9 7.30 7.29 7.60 9.50 0.65

Ammonia plus 

organic nitrogen Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 678 0.05 0.38 0.52 0.58 0.69 2.64 0.32

Mid-depth or 

metalimnion

mg/L 729 0.05 0.39 0.54 0.61 0.73 3.10 0.34

Near bottom mg/L 678 0.05 0.53 0.86 1.07 1.36 7.58 0.82

Ammonia Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 678 0.0010 0.0050 0.01 0.02 0.03 0.82 0.05

Mid-depth or 

metalimnion

mg/L 729 0.0010 0.0060 0.02 0.05 0.04 4.00 0.18

Near bottom mg/L 678 0.0010 0.0343 0.22 0.48 0.68 6.79 0.68
12 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Table 3. Statistical summary, by physical property or consitutent, of Michigan lakes sampled in the index station (deepest basin) 

during 2001–10. (The Helsel and Cohn (1988) adjusted maximum liklihood method was used to create summary statistics for constituents 

with greater than 5 percent of the data as nondetects or less-than values; otherwise, half the reporting limit was used for all nondetect 

and less-than data values.)—Continued

[Min, minimum; 25 Per, 25 Percentile; Med, median; 75 Per, 75 Percentile; Max, Maximum; St. Dev, standard deviation; μg/L, micrograms per liter; 

mg/L, milligrams per liter; deg C, degrees Celsius; μS/cm, microsiemens per centimeter; std units, standard units; AU/cm, absorbance units per centimeter; 

CaCO3, calcium carbonate; <, less-than sign noting greater than 5 percent of data were nondetects or less-than values and adjusted maximum liklihood method 

was used]

Spring—continued

Total phosphorus Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 683 0.003 0.009 0.013 0.018 0.021 0.234 0.018

Mid-depth or 

metalimnion

mg/L 729 0.003 0.009 0.014 0.018 0.020 0.203 0.017

Near bottom mg/L 683 0.003 0.010 0.014 0.024 0.022 1.880 0.081

Nitrate plus nitrite Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 683 0.001 0.05 0.09 0.26 0.21 6.70 0.56

Mid-depth or 

metalimnion

mg/L 728 0.001 0.05 0.10 0.26 0.21 6.70 0.55

Near bottom mg/L 683 0.001 0.05 0.10 0.27 0.21 5.90 0.55

Total nitrogen (sum of 

ammonia + organic 

nitrogen and 

nitrate + nitrite)

Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 683 0.116 0.46 0.60 0.79 0.86 7.72 0.66

Mid-depth or 

metalimnion

mg/L 729 0.107 0.46 0.62 0.78 0.86 7.75 0.65

Near bottom mg/L 683 0.059 0.47 0.66 0.85 0.95 6.85 0.69

Mid-depth or 

metalimnion Units Count Min 25 Per Med Mean 75 Per Max St. Dev

*Alkalinity 

(as CaCO3

)

mg/L 729 <20 64.00 116.00 109.53 150.00 323.00 58.20

*Calcium - total mg/L 729 1.100 20.50 35.00 34.17 45.50 98.60 19.03

*Chloride mg/L 726 0.500 4.00 9.00 16.72 19.00 278.00 25.14

*Hardness - calculated mg/L 729 2.500 76.00 136.00 130.70 178.00 337.00 72.62

*Potassium - total mg/L 729 0.050 0.60 1.00 1.20 1.60 5.40 0.82

*Magnesium - total mg/L 729 0.500 5.40 11.30 11.08 16.00 28.20 6.57

*Sodium - total mg/L 728 <1.0 2.20 4.95 8.28 8.60 154.00 13.00

*Sulfate mg/L 727 1.000 3.00 6.00 10.50 14.00 142.00 12.58
Statewide Water Quality of Inland Lakes 13

Table 3. Statistical summary, by physical property or consitutent, of Michigan lakes sampled in the index station (deepest basin) 

during 2001–10. (The Helsel and Cohn (1988) adjusted maximum liklihood method was used to create summary statistics for constituents 

with greater than 5 percent of the data as nondetects or less-than values; otherwise, half the reporting limit was used for all nondetect 

and less-than data values.)—Continued

[Min, minimum; 25 Per, 25 Percentile; Med, median; 75 Per, 75 Percentile; Max, Maximum; St. Dev, standard deviation; μg/L, micrograms per liter; 

mg/L, milligrams per liter; deg C, degrees Celsius; μS/cm, microsiemens per centimeter; std units, standard units; AU/cm, absorbance units per centimeter; 

CaCO3, calcium carbonate; <, less-than sign noting greater than 5 percent of data were nondetects or less-than values and adjusted maximum liklihood method 

was used]

Summer—continued

Total phosphorus Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 678 0.00250 0.008 0.012 0.016 0.017 0.172 0.016

Mid-depth or 

metalimnion

mg/L 729 0.00250 0.011 0.015 0.021 0.022 0.420 0.024

Near bottom mg/L 678 0.00250 0.015 0.029 0.075 0.066 2.160 0.152

Nitrate plus nitrite Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 678 <0.01 0.0020 0.00 0.05 0.01 3.10 0.22

Mid-depth or 

metalimnion

mg/L 728 <0.01 0.0020 0.00 0.07 0.01 3.60 0.26

Near bottom mg/L 678 <0.01 0.0020 0.00 0.06 0.02 3.60 0.20

Total nitrogen (sum of 

ammonia + organic 

nitrogen and 

nitrate + nitrite)

Units Count Min 25 Per Med Mean 75 Per Max St. Dev

Near surface mg/L 678 <0.01 0.396 0.54 0.63 0.73 4.11 0.40

Mid-depth or 

metalimnion

mg/L 728 <0.01 0.425 0.59 0.68 0.79 4.44 0.44

Near bottom mg/L 678 <0.01 0.56 0.89 1.13 1.42 7.59 0.83

*Spring only.

**Summer only 2007–10.

Vertical Profile

Dissolved oxygen, water temperature, specific conductance, and pH were measured throughout the water column. 

Boxplots for comparison of individual constituents measured in the spring and summer at specific depths are shown 

in figures 5–8. (In these figures and similar figures later in 

the report, “adjacent” values represent up to one step above 

the 75th percentile for upper adjacent, or one step below the 

25th percentile for lower adjacent, “outside” values if present represent values between 1 and 2 steps above the 75th and 

below the 25th, and “detached” values if present represent 

values more than 2 steps above the 75th or below the 25th. One 

step is equal to 1.5 times the height of the box, or interquartile 

range. (Helsel and Hirsch, 2002); the other statistics should be 

self-explanatory.) 

Dissolved oxygen concentration is a critical factor in lake 

ecosystems; DO is a component of many chemical and biological processes and is important in determining the productivity 

of a lake. It is critical to many organisms for respiration and 

is an important controlling factor regarding the diversity of 

fish and other living organisms that lakes support. Dissolved 

oxygen is fairly consistent between spring and summer for the 

near surface and mid-depth or metalimnion but is lower in the 

summer than in the spring for the majority of measurements 

(fig. 5). This could be the result of incomplete lake turnover in 

the spring. If a lake does not mix completely, anoxic conditions develop in the hypolimnion during the summer months 

(Denys, 2009).
14 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10 Dissolved oxygen, in milligrams per liter

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

729 728 729 727 728

729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

729

Spring

near

surface

0

5

10

15

20

x

x

x

xx

x

x

xx

x

x

x

x

x

x

x

xx

x

xxx

x

x

x

x

x

x

x

xx

x

x

x

x

x

x

x

x

x

o

o

o

o

o

o

o

o

o

x

x

x

xx

x

xx

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xx

o

Figure 5. Statistical distribution of dissolved oxygen for spring and summer for near surface, mid-depth or metalimnion, 

and near bottom for the index stations (deepest basins) in Michigan lakes measured during 2001–10. Water temperature, in degrees celsius

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

729 728 729 728 727

729 number of values 

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

729

Spring

near

surface

0

5

10

15

35

x

x

x

x

x

x

x

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x

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x

x

o

o

20

25

30

Figure 6. Statistical distribution of water temperature for spring and summer for near surface, mid-depth or metalimnion, 

and near bottom for the index stations (deepest basins) in Michigan lakes measured during 2001–10. 
Statewide Water Quality of Inland Lakes 15

Specific conductance, in microsiemens per centimeter

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

728 729 729 729 729

729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

728

Spring

near

surface

0

400

200

800

1,200

600

1,000

1,400

1,600

x

x

x

x

x

x

x

x

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o

o

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xx

x

x

o

o

x

x

x

x

o

o

x

x

x

x

x

o

o

Figure 7. Statistical distribution of specific conductance for spring and summer for near surface, mid-depth or 

metalimnion, and near bottom for the index stations (deepest basins) in Michigan lakes measured during 2001–10. pH, in standard units

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

728 724 729 723 728

729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

727

Spring

near

surface

4

5

6

7

11

8

9

10

x

xxx

x

x

x

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x

x

x

x

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o

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Figure 8. Statistical distribution of pH for spring and summer for near surface, mid-depth or metalimnion, and near bottom 

for the index stations (deepest basins) in Michigan lakes measured during 2001–10. 
16 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Water temperature in lakes is important because of the 

role it plays in lake stratification and its relation to chemical 

and biological processes. Thermal stratification is a phenomenon found in most Michigan lakes, whereby a layer of less 

dense, warm water is isolated from mixing with a denser, 

colder layer as the thermocline is established in the summer. 

The extent of thermal stratification in lakes depends on the 

interaction between the size and depth of the lake basin, solar 

heating, and local wind characteristics. Water temperature for 

the majority of lakes undergoes the greatest change between 

seasons for near-bottom measurements, where temperature is 

greater in the summer than in the spring (fig. 6). Water temperature was the most different for the majority of lake’s near 

bottom measurements, where the temperature difference is 

greatest between the top and bottom measurements in the summer than in the spring. The smaller difference between top 

and bottom measurements in the spring is most likely because 

the lakes were close to complete turnover, and the greater 

difference in the summer was most likely owing to surface 

warming, which can result in lake stratification (though not all 

lakes stratified). 

Specific conductance is a measure of the ability of water 

to conduct an electrical current and is an indicator of the 

concentration of dissolved solids in water. As the concentration of dissolved minerals increases, specific conductance 

also increases. While a lake is stratified, specific conductance 

generally is higher near the bottom because of the release of 

dissolved materials (such as iron, manganese, and phosphorus) 

from the bottom sediments under anoxic conditions. Specific 

conductance is fairly similar between the spring and summer 

depth comparisons, though the mean does show a slight overall increase with depth (fig. 7). 

The pH of a lake is a measure of the hydrogen ion activity in the lake water; it is defined as the negative logarithm 

of the hydrogen ion concentration and varies over a 14-unit 

log scale. A pH of 7.0 is neutral, values less than 7.0 indicate 

acidic conditions, and values greater than 7.0 indicate alkaline 

conditions. The pH in most natural surface water ranges from 

6.5 to 8.5, but pH values outside this range do occur. Overall, 

the pH in measured lakes is slightly lower as the measurement depth increases (fig. 8). Many chemical and biological 

processes, including photosynthesis, nitrification, and calciumcarbonate dissolution, control pH in lake water. Algae and 

aquatic plants produce oxygen and consume carbon dioxide 

by the photosynthesis process during the day and produce carbon dioxide when they respire at night. Carbon dioxide then 

combines with water to form carbonic acid, thereby creating a 

diurnal fluctuation of pH. This fluctuation is important because 

it affects the solubility of many chemical constituents and 

because aquatic organisms have limited pH tolerances. It has 

been shown that pH values greater than 8.5 will accelerate the 

release of phosphorus from lake-bottom sediment (James and 

Barko, 1991). At 6 to 7 percent of lakes (the majority less than 

30 ft deep), measured pH was greater than 8.5 at mid-depth 

or metalimnion for spring or summer. Less than 2 percent of 

lakes had a pH greater than 8.5 for all three discrete sampling 

depths. Four percent of lakes had a pH value less than 6.5, 

with less than 2 percent of lakes with a pH less than 6.5 for all 

three discrete sampling depths; these latter lakes are solely in 

the Upper Peninsula. 

The spatial distribution of the percentage of DO in the 

water column below two critical levels is shown in figure 9. 

Available data indicate that a healthy warmwater fish population requires DO concentrations of at least 2–5 mg/L (Kalff, 

2002). Dissolved oxygen concentrations below 0.5 mg/L were 

categorized as anoxic for Michigan lakes according to Wehrly 

and others (2011). While some species have evolved to survive weeks to months with low DO, there can still be negative 

effects with longer periods of low DO such as inefficient food 

conversion for fish species. The portion of the water column 

with low DO can become unavailable to species, such as those 

requiring refuge during the day in deeper water from predators. Effects and stressors from low DO are dependent on 

the length of time of the low DO period and the lake system 

(Kalff, 2002). 

Although there were a greater percentage of lakes in the 

summer than in the spring with DO below 2 and 0.5 mg/L, the 

lakes seem to be somewhat evenly distributed spatially among 

the Michigan lakes measured. In the spring, 34 percent of 

lakes had 25–69 percent of the water column (25–69 percent 

of the total lake depth) below 2.0 mg/L and 12 percent had 

25–50 percent of the water column below 0.5 mg/L. In the 

summer, 58 percent of lakes had 25–86 percent of the water 

column below 2.0 mg/L and 30 percent had 25–83 percent of 

the water column below 0.5 mg/L. 

The spatial distribution of mid-depth or metalimnion specific conductance and pH measurements is shown in figure 10. 

While specific conductance is similar among the spring and 

summer measurements, it varies noticeably by lower values 

in the Upper Peninsula of Michigan (hereinafter, “Upper 

Peninsula”) compared to increasingly higher from the north 

to south of the Lower Peninsula of Michigan (hereinafter, 

“Lower Peninsula”). The highest values have been noted in the 

southeastern part of the Lower Peninsula. Lake-water pH does 

not vary noticeably among the spring and summer measurements, or spatially throughout Michigan, though most of the 

measurements below 6.5 seem to be solely in the Upper Peninsula. Lillie and Mason (1983) also found for Wisconsin that 

the majority of lakes with lower pH values were located in the 

northeast region of Wisconsin, and was “probably indicative 

of the large number of brown-stained lakes” in that region.

To determine whether differences among the verticalprofile measurements at different depth measurements in the 

deepest basin were statistically significant, and also whether 

differences between same depth measurements in the deepest 

basin between spring and summer were statistically significant, the Wilcoxon signed-rank test (Helsel and Hirsch, 2002) 

was used; results are listed in tables 4 and 5, respectively. The 

Wilcoxon signed-rank test is used for paired data to determine whether the median difference equals zero. There was a 

statistically significant difference at the 95-percent confidence 

level between spring and summer for all measurements except 

DO at mid-depth or metalimnion. There was also a statistically 

significant difference at the 95-percent confidence level among 

DO, water temperature, specific conductance, and pH between 

all measurement depths.
Statewide Water Quality of Inland Lakes 17

EXPLANATION

(Dissolved oxygen - spring)

 Percent of water column less 

 than 2.0 milligrams per liter

(Dissolved oxygen - summer)

 Percent of water column less 

 than 2.0 milligrams per liter

< 10 (36 percent)

10 - 24 (30 percent)

25 - 49 (32 percent)

50 - 69 (2 percent)

EXPLANATION

< 10 (30 percent)

10 - 24 (12 percent)

25 - 49 (29 percent)

50 - 86 (29 percent)

EXPLANATION

< 10 (67 percent)

10 - 24 (22 percent)

25 - 49 (11 percent)

50 (< 1 percent)

EXPLANATION

< 10 (58 percent)

10 - 24 (12 percent)

25 - 49 (20 percent)

50 - 83 (10 percent)

0 50 100 MILES

0 50 100 KILOMETERS

(Dissolved oxygen - spring)

 Percent of water column less 

 than 0.5 milligram per liter

(Dissolved oxygen - summer)

 Percent of water column less 

 than 0.5 milligram per liter

Figure 9. Spatial distribution of dissolved oxygen and the percentage of the water column below 0.5 and 2.0 milligrams per liter in 

spring and summer measurements for Michigan inland lakes during 2001–10. 
18 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

EXPLANATION

< 50

50 - 149

150 - 249

250 - 349

350 - 449

450 - 549

550 - 649

> 649

EXPLANATION

pH, in standard units - spring

< 6.5

6.5 - 7.4

7.5 - 8.4

> 8.4

EXPLANATION

pH, in standard units - summer

< 6.5

6.5 - 7.4

7.5 - 8.4

> 8.4

0 50 100 MILES

0 50 100 KILOMETERS

Specific conductance, in 

 microsiemens per 

 centimeter - spring 

EXPLANATION

< 50

50 - 149

150 - 249

250 - 349

350 - 449

450 - 549

550 - 649

> 649

Specific conductance, in 

 microsiemens per 

 centimeter - summer

Figure 10. Spatial distribution of specific conductance and pH in spring and summer mid-depth or metalimnion measurements 

for Michigan inland lakes during 2001–10. 
Statewide Water Quality of Inland Lakes 19

Table 4. Results of Wilcoxon signed-rank test to 

determine statistical significance among Michigan 

lakes spring and summer measurements throughout 

the vertical profile at measured depths for 2001–10. 

[Significant at 95-percent confidence level if p-value result is 

less than 0.05]

Constituent Sample 

size

Spring and 

summer

Dissolved oxygen p-value

Near surface 729 0.00

Mid-depth or metalimnion 728 .49

Near bottom 727 .00

Water temperature p-value

Near surface 729 0.00

Mid-depth or metalimnion 728 .00

Near bottom 727 .00

Specific conductance p-value

Near surface 728 0.00

Mid-depth or metalimnion 729 .00

Near bottom 729 .00

pH p-value

Near surface 727 0.00

Mid-depth or metalimnion 724 .01

Near bottom 723 .00

Table 5. Results of Wilcoxon signed-rank test to determine statistical significance among Michigan 

lakes in the vertical profile between measured depths for 2001–10. 

[Significant at 95-percent confidence level if p-value result is less than 0.05]

Sample size

Near 

surface and 

mid-depth

Sample size

Near 

surface and 

near bottom

Sample size

Mid-depth 

and 

near bottom

Spring p-value p-value p-value

Dissolved oxygen 728 0.00 727 0.00 727 0.02

Water temperature 728 .00 728 .00 728 .00

Specific conductance 728 .00 728 .00 729 .00

pH 724 .00 723 .00 723 .00

Summer p-value p-value p-value

Dissolved oxygen 729 0.00 729 0.00 729 0.00

Water temperature 729 .00 727 .00 727 .00

Specific conductance 728 .00 728 .00 729 .00

pH 728 .00 728 .00 728 .00
20 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Trophic Status: Secchi-Disk Transparency, 

Chlorophyll a, and Total Phosphorus

At about 75 percent of inland lake index stations (deep 

basins) measured, trophic characteristics were associated with 

oligotrophic or mesotrophic conditions—74 percent if SDT 

was used as the sole indicator, 78 percent if chlorophyll a 

was used, and 84 percent if TP was used to determine the TSI 

values and trophic classifications. Differences in the percentages separating the oligotrophic and mesotrophic classes were 

greater when the three indicators were compared (fig. 11). 

Five percent of the lakes or fewer were categorized as hypereutrophic when using any of the three indicators. Land-cover 

data (Homer, 2004) show that the dominant land-cover type 

for lakes in northern Lower Michigan is mostly forested, with 

urban and agriculture as the dominant land cover for southern 

Lower Michigan; these land-cover differences could play a 

role in the trophic-classification spatial difference in Lower 

Michigan.

Fewer lakes were available for using SDT as the sole 

indicator because the Secchi disk hit the bottom of some lakes. 

In these instances, recording the depth of the lake would give 

a false sense of the TSI class. For example, if a lake has a 

maximum depth of about 6 to 9 ft (2 to 3 m) and the Secchi 

disk hit bottom, recording the lake depth for the SDT would be 

misleading because it would be associated with a mesotrophic 

to eutrophic TSI category. If a Secchi disk hit bottom, it might 

be indicative of a clear oligotrophic lake. Of the 36 lakes 

where the Secchi disk hit bottom, all but one lake might be 

considered shallow: 28 of the lakes were considered too shallow to allow for three discrete-depth measurements; in 7 lakes, 

maximum depth ranged from 7.5 to 20 ft; the last lake had 

a depth of 48 ft. For the lakes too shallow for three discretedepth measurements, the single TP measurement from summer 

was used to compute the TSI and trophic status. Statistical 

distributions of individual measurements made in the spring 

and summer for SDT and chlorophyll a in the summer total for 

phosphorus used to compute TSI are shown in figure 12.

0 20 40 60 80 100

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

EXPLANATION

Phosphorus

(729)

Chlorophyll a

(727)

Percent

Secchi-disk

transparency

(693)

37 45 15

4

26 52 19

4

13 61 22

5

Figure 11. Trophic-status distribution using Secchi-disk transparency, chlorophyll a, or total phosphorus as the indicator. 
Statewide Water Quality of Inland Lakes 21

Meters

Feet

Reporting limit = 1.0 

Summer

692

729 number of values 

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

696

Spring

0

5

10

15

20

xxxx

x

xx

xx

x

xx

x

x

xx

oo

o

o

o

o

o

o

o

o

o

o

o

o

xx

xx

x

x

x

x

o

o

o

o

o

o

o

o

oo

o

oo

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

Micrograms per liter

Summer

723 726

Spring

0

20

40

60

80

100

120

140

Secchi-disk transparency Chlorophyll a

x

x

x

x

x

x

x

x

o

o

xxx

x

x

x

x

o

xxxxxxxx

xx

oooooooo

ooooo

ooo

o

oo

o

o

o

o

o

o

o

o

o

o

o

o

o

Reporting limit = 0.005

Summer

728

Phosphorus

0.30

0

0.05

0.10

0.15

0.20

0.25

Milligrams per liter

5

10

15

20

25

30

35

40

45

50

55

60

65

Figure 12. Statistical distribution of Secchi-disk transparency and chlorophyll a in spring and summer measurements and nearsurface phosphorus in summer measurements at the index stations (deepest basins) in Michigan lakes during 2001–10. 

The spatial distribution of mesotrophic and eutrophic 

lakes in Michigan is fairly even around the State. Oligotrophic lakes are more numerous and dense in the northern part 

of the Lower Peninsula, and hypereutrophic lakes are more 

numerous in the southern part of the Lower Peninsula, as is 

evident in figure 13. As lake depth decreases, the TSI values 

increase, as can be seen in figure 14. More than 80 percent 

of hypereutrophic lakes, regardless of the sole indicator used 

to compute the TSI, were 30 ft deep or less, with the deepest hypereutrophic lake measured at 52 ft. Bottom sediments 

in shallow lakes can play a major role in nutrient releases 

throughout the water column during wind-induced sediment 

resuspension and/or biological processes. The resuspension of 

sediment results in high levels of turbidity and nutrients in the 

lake. These conditions affect light penetration and influence 

algal productivity (Søndergaard and others, 2003).

There was a statistically significant difference at the 

95-percent confidence level among spring and summer SDT 

and chlorophyll a measurements for individual lakes (table 6). 

However, there were fairly even counts of lakes with increases 

and decreases in the SDT and Chlorophyll a measurements 

among spring and summer in comparing all lakes; thus, an 

overall positive or negative relation between seasons was 

not found. For SDT measurements, there were slightly more 

clearer lakes in the spring than summer for 63 percent of 

lakes measured. Water clarity can be reduced by the presence 

of suspended sediment, dissolved organic substances, freefloating algae, and zooplankton, but algae commonly are the 

dominant affect on the clarity of lake water. Therefore, as a 

lake becomes more productive during summer months, it generally is expected that water clarity would decrease. 

Color

True color, as by the spectrophotometric method, was 

added to the LWQA monitoring program during summer 

2007–10 (fig. 15), and it provides a standardized means to 

assess lake-water color. Although color is a measure of the 

appearance of water, true color measures of water result when 

dissolved substances have been removed. Suspended matter, 

which includes algae and non-algal matter, can affect water 

color, as can climatic events such as increased rainfall (and 

thus runoff) and drought conditions. High levels of color in 

water can have a negative effect on lake biota if they prevent 

light from reaching necessary depths (Florida LAKEWATCH, 

2004). A study by Webster and others (2008) showed both 

color and TP had a strong positive effect on chlorophyll a and 

a negative effect on Secchi transparency. Webster and others 

(2008) suggest misinterpretation of these widely used trophicstatus indicators could occur without corresponding color 

measurements.
22 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

EXPLANATION

EXPLANATION

Trophic State Index,

 phosphorus

0 50 100 MILES

0 50 100 KILOMETERS

Trophic State Index,

 Secchi-disk transparency

EXPLANATION

Trophic State Index,

 chlorophyll a

No data

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

No data

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

No data

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

Figure 13. Spatial distribution of trophic status for Michigan inland lakes during 2001–10. 
Statewide Water Quality of Inland Lakes 23

0

10

20

30

40

50

60

70

80

90

0 50 100 150 200 250 300

TSI (Secchi-disk transparency)

TSI (chlorophyll a)

TSI (phosphorus)

Trophic State Index (TSI) value

Lake depth, in feet

Figure 14. Trophic State Index (TSI) values compared to lake 

depth for Michigan inland lakes during 2001–10. 

Table 6. Results of Wilcoxon signed-rank test 

to determine statistical significance between 

Michigan lakes spring and summer measurements 

of Secchi-disk transparency and chlorophyll a for 

2001–10. 

[Significant at 95-percent confidence level if p-value result 

is less than 0.05]

Measurement Sample 

size p-value

Secchi-disk transparency 693 0.00

Chlorophyll a 724 .00

Nutrients 

Total phosphorus, Kjeldahl nitrogen (ammonia plus 

organic nitrogen), ammonia, and nitrate plus nitrite all were 

sampled near surface, at mid-depth or metalimnion, and near 

bottom during spring and summer. Total nitrogen was calculated by summing Kjeldahl nitrogen and nitrate plus nitrite. 

For those lakes considered too shallow to allow for three 

discrete-depth measurements, the available measurements 

were used in this analysis. Individual lake measurements for 

spring and summer at the three discrete depths are shown 

in figures 16–19. Nitrate plus nitrite values were higher in 

the spring than summer for the majority of lakes, whereas 

ammonia and ammonia plus organic nitrogen were higher in 

the summer near-bottom measurements. This pattern, along 

with the DO being lower in the summer near bottom (fig. 5), 

suggests that nutrient cycling and conversion was occurring in 

the summer at near-bottom depths.

30 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

Absorbance units, per centimeter

Reporting limit = 0.007

286

Color absorbance, 

at 400 nanometers

0

0.05

0.10

0.15

0.20

xx

x

x

o

o

o

Figure 15. Statistical distribution of true color in the 

summer near-surface measurements at the index 

stations (deepest basins) in Michigan lakes during 

2007–10. 

Phosphorus and nitrogen are required for algae and 

aquatic macrophyte growth. The concentration of these 

nutrients will determine the quantity of plants that a lake can 

support. The quantity and diversity of the algae and aquatic 

macrophyte communities in turn plays an important role in 

the quantity and diversity of fish and other living organisms in 

the environment. However, if excessive amounts of nutrients 

are present, algal blooms and excessive growth of aquatic 

macrophytes can occur. Commonly, it requires only small 

additional quantities of nutrients above those that are naturally 

present to increase the primary productivity of a lake to the 

point where eutrophication becomes a concern. The nutrient in 

shortest supply tends to be the limiting control on production 

(Kalff, 2002). The spatial distribution of total nitrogen and TP 

for near-surface measurements in the spring and summer are 

shown in figure 20.
24 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10 Ammonia plus organic nitrogen, In milligrams per liter

Reporting limit = 0.1 

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

677 728 728 681 677

729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

682

Spring

near

surface

0

2

4

6

8

10

x

x

x

xx

x

x

xxx

x

xx

x

o

o

oo xxx

x

xxx

x

x

x

x

xx

oo

o

o

o

oo

o

oo

x

x

xxx

x

xxx

xx

x

xx

o

o

o

o x

x

x

xx

xx

x

xxx

x

x

x

x

xx

xx

x

xxx

o

oo

o

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x

x

x

x

xx

x

x

xx

x

xx

o

o

o

o

o

xx

x

xxx

x

x

x

x

xx

x

x

x

x

xx

o

o

o

o

o

o

Figure 16. Statistical distribution of Kjeldahl nitrogen (ammonia plus organic nitrogen) in spring and summer near-surface, mid-depth 

or metalimnion, and near-bottom measurements at the index stations (deepest basins) in Michigan lakes during 2001–10. Ammonia, in milligrams per liter

682 678 728 729 682 678

0

2

4

6

8

xxxx

ox

o

o

o

oo

oo

o

o

o

o

oo

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o

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x

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x

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x

x

x

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x

x

x

x

o

o

o

oo

o

o

o

o

o

30 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

Spring

near

surface

Reporting limit = 0.01 

Figure 17. Statistical distribution of ammonia in spring and summer near-surface, mid-depth or metalimnion, and near-bottom 

measurements at the index stations (deepest basins) in Michigan lakes during 2001–10. 
Statewide Water Quality of Inland Lakes 25

Phosphorus, in milligrams per liter

Reporting limit = 0.005

683 678 729 729 683 678

0

0.25

0.50

0.75

1.00

2.50

xxxx

xxx

xx

o

ooo

o

o

o

o

o

o

o

o

o

o ooxxx

ooooooooo

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729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

Spring

near

surface

Figure 18. Statistical distribution of total phosphorus in spring and summer for near-surface, mid-depth or metalimnion, and 

near-bottom measurements at the index stations (deepest basins) in Michigan lakes during 2001–10. Nitrate plus nitrite, in milligrams per liter

Reporting limit = 0.01 

683 678 728 728 683 678

0

2

4

6

8

x

xxxxxx

x

x

x

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729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

Summer

near

surface

Spring

mid-depth

Summer

center of

metalimnion

Spring

near

bottom

Summer

near

bottom

Spring

near

surface

Figure 19. Statistical distribution of nitrate plus nitrite in spring and summer for near-surface, mid-depth or metalimnion, and 

near-bottom measurements at the index stations (deepest basins) in Michigan lakes during 2001–10. 
26 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

0 50 100 MILES

0 50 100 KILOMETERS

U.S. Environmental Protection Agency Level III Ecoregions (U.S. Environmental Protection Agency, 2011)

55 Eastern Corn Belt Plains

57 Huron/Erie Lake Plains

51 North Central Hardwoods

50 Northern Lakes and Forests

56 Southern Michigan/Northern Indiana Drift Plains

EXPLANATION

Summer total nitrogen,

 in milligrams per liter

< 0.5

0.5 - 0.9

1.0 - 1.5

> 1.5

EXPLANATION

Spring total nitrogen,

 in milligrams per liter

< 0.5

0.5 - 0.9

1.0 - 1.5

> 1.5

EXPLANATION

Summer total phosphorus,

 in milligrams per liter

< 0.01

0.01 - 0.02

0.021 - 0.05

> 0.05

EXPLANATION

Spring total phosphorus,

 in milligrams per liter

< 0.01

0.01 - 0.02

0.021 - 0.05

> 0.05

Figure 20. Spatial distribution of total nitrogen and total phosphorus for both spring and summer near surface measurements 

for Michigan inland lakes during 2001–10. 
Statewide Water Quality of Inland Lakes 27

Lakes with total nitrogen to TP ratios greater than 15:1 

near surface generally are considered phosphorus limited; 

ratios ranging from 10:1 to 15:1 indicate a transition situation; and a ratio less than 10 to 1 indicates nitrogen limitation 

(Vollenweider, 1968, 1969, 1976). On the basis of near-surface 

measurements from Michigan lakes during 2001–10 in the 

spring, 97 percent of the lakes can be considered phosphorus 

limited and less than a half percent nitrogen limited; for summer measurements, 96 percent would be phosphorus limited, 

and less than a half percent nitrogen limited. Following these 

same guidelines for the mid-depth or metalimnion measurements, similar percentages of phosphorus-limited lakes 

(around mid-90 percents) and nitrogen-limited lakes (less than 

1 percent) occurred during both seasons, as well as for the 

spring near bottom; on the basis of summer near-bottom measurements, 75 percent of the lakes were phosphorus limited 

and 14 percent nitrogen limited.

To determine whether differences among the nutrient 

measurements at different depths and also between spring 

and summer were statistically significant, the Wilcoxon 

signed-rank test was used. Results are listed in tables 7 and 8, 

respectively. For the spring, all nutrients measured were statistically significant at the 95-percent confidence level among 

all depths except for ammonia plus organic nitrogen and TP 

between the near surface and mid-depth measurements. For 

summer, differences among all nutrients measured were statistically significant at the 95-percent confidence level among 

all depths except for nitrate plus nitrite between mid-depth or 

metalimnion and near bottom. On a lake-by-lake basis, there 

were more differences in nutrients among depths in the spring 

than in the summer between the near surface and mid-depth. 

All differences in nutrient concentrations were determined 

to be statistically significant at the 95-percent confidence 

level among spring and summer measurements at the discrete 

depths. 

Table 7. Results of Wilcoxon signed-rank test to determine statistical significance among Michigan lakes nutrients 

at measured depths for 2001–10. 

[Significant at 95-percent confidence level if p-value result is less than 0.05]

Sample size

Near 

surface and 

mid-depth

Sample size

Near 

surface and 

near bottom

Sample size

Mid-depth 

and 

near bottom

Spring p-value p-value p-value

Ammonia plus organic nitrogen 683 0.96 682 0.00 682 0.00

Ammonia 682 .00 682 .00 682 .00

Total phosphorus 683 .95 683 .00 683 .00

Nitrate plus nitrite 683 .00 683 .00 683 .03

Summer p-value p-value p-value

Ammonia plus organic nitrogen 678 0.00 678 0.00 678 0.00

Ammonia 678 .00 678 .00 678 .00

Total phosphorus 678 .01 678 .00 678 .00

Nitrate plus nitrite 678 .00 678 .00 678 .47

Table 8. Results of Wilcoxon signed-rank test to 

determine statistical significance among Michigan lakes 

spring and summer nutrients at measured depths for 

2001–10. 

[Significant at 95-percent confidence level if p-value result is less 

than 0.05]

Constituent Sample 

size

Spring and 

summer

Ammonia plus organic nitrogen p-value

Near surface 678 0.00

Mid-depth or metalimnion 729 .00

Near bottom 678 .00

Ammonia p-value

Near surface 678 0.00

Mid-depth or metalimnion 728 .00

Near bottom 678 .00

Total phosphorus p-value

Near surface 678 0.01

Mid-depth or metalimnion 729 .00

Near bottom 678 .00

Nitrate plus nitrite p-value

Near surface 678 0.00

Mid-depth or metalimnion 728 .00

Near bottom 678 .00
28 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Major Ions and Physical Properties

In the spring, major ions and various physical properties 

of water were measured at mid-depth to establish “whole lake” 

baseline water-quality conditions after mixing and before 

potential summer stratification. Catchment runoff contributes 

to the concentration of major ions, along with those naturally 

present that together yield the salinity in inland lakes. Sources 

of major ions can include local geology, human activities, and 

climatic variation, and the concentration and mix of major 

ions influence lake biota (Kalff, 2002); statistical distributions 

of measurements of major ions and associated properties are 

shown in figures 21 and 22. There were spatial patterns of 

lower values in the Upper Peninsula, with increasing values 

towards the southern part of Michigan, for all major ions and 

physical properties, although the location of increase and 

intensity varied by constituent and property. 

U.S. Environmental Protection Agency level III ecoregions (U.S. Environmental Protection Agency, 2011), which 

were created on the basis of land cover, geology, physiography, vegetation, climate, soils, wildlife, and hydrology, are 

shown in figures 23A and 23B along with constituent values. 

Potassium, sulfate, and chloride concentrations fairly closely 

followed the level III ecoregion boundaries. Magnesium, 

hardness, calcium, and alkalinity middle-range values were 

spatially diverse, though low to middle values were found in 

the Northern Lakes and Forest ecoregion covering the Upper 

Peninsula and the northern Lower Peninsula, and middle to 

higher values were found in the southern Michigan/Northern 

Indiana Drift Plains ecoregion. Alkalinity and magnesium 

concentrations were the lowest in the Upper Peninsula, but the 

higher concentrations started in the eastern end of the Upper 

Peninsula and continued to increase southward, with the highest measurements in the southern Lower Peninsula. Sodium 

and chloride concentrations were lower in the Upper Peninsula 

and in most of the Lower Peninsula, but higher concentrations 

were clustered in the southeastern Lower Peninsula, especially for lakes in Genesee, Livingston, and Oakland Counties. Higher concentrations of sodium and chloride observed 

in the southeastern Lower Peninsula are most likely a result 

from the use of road salt for deicing roads, parking lots, and 

other impervious surfaces during the winter months. Elevated 

concentrations of chlorides can inhibit plant growth, impair 

reproduction, and reduce the diversity of organisms in streams 

(Mullaney and others, 2009).

Examination of Multibasin Lakes

Of the 729 Michigan lakes measured, 109 have at least 

two deep basins, 26 have three deep basins, and 2 have four 

deep basins where thermal stratification is likely to occur during the summer. All these secondary basins were measured, 

replicating the same water-quality parameters measured at 

the index stations (deepest basins). The basins were ranked 

1 through 4 by their maximum depth; examples of multiplebasin lakes are shown in figure 24.

The Wilcoxon signed-rank test was used to determine 

statistical significance of differences in measurements among 

multiple basins. When basins 1 and 2 were compared for the 

spring, differences in SDT, water temperature for mid-depth 

and near bottom, pH for near surface, and nitrate plus nitrite 

for near surface and mid-depth were determined to be statistically significant at the 95-percent confidence level. Differences in chlorophyll a, water temperature, potassium, and 

magnesium were also statistically significant at the 95-percent confidence level between other basin combinations. In 

the summer, differences in water temperature near-bottom 

measurements, pH for near-surface measurements, and nitrate 

plus nitrite for near-bottom measurements were statistically 

significant at the 95-percent confidence level between basins 1 

and 2. Differences in DO and water temperature were statistically significant at the 95-percent confidence level between 

other basin combinations. There were not enough lakes with 

four basins to test for significant difference between the fourth 

basins. The resulting p-values are listed in table 9. Owing 

to the lack of statistical significance overall for differences 

between constituents in lakes with multiple basins, one basin 

measurement per lake would be sufficient, perhaps with some 

exceptions depending on desired constituents.
Statewide Water Quality of Inland Lakes 29

Reporting limit = 20

Milligrams per liter

729

Alkalinity, as 

calcuim carbonate

0

50

100

150

200

250

300

350

400

x

Reporting limit = 1

726

Chloride

0

50

100

150

200

250

300

350

400

Reporting limit = 5

729

Hardness, as

calcuim carbonate

0

50

100

150

200

250

300

350

400

x

x

xx

xxx

x

xxx

x

x

x

x

x

xxx

x

xx

x

o

o

o

oo

o

o

o

o

o

o

o

o

o

oo

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

Reporting limit = 2

727

Sulfate

x

x

x

x

x

x

x

x

x

x

x

x

xx

x

x

x

x

x

x

x

xx

x

x

o

o

o

o

o

oo

o

o

o

o

0

25

50

75

100

125

150

175

200

729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

Figure 21. Statistical distribution of alkalinity, sulfate, chloride, and hardness in spring measurements at the index stations 

(deepest basins) in Michigan lakes during 2001–10. 

Reporting limit = 0.1

Milligrams per liter

729

Potassium

0

1

2

3

4

5

6

Reporting limit = 1

729

Magnesium

0

5

10

15

20

25

30

35

Reporting limit = 1

728

Sodium

0

25

50

75

100

125

150

175

200

x

x

x

x

x

x

x

x

x

x

x

o

o

o

x

x

xx

xx

x

x

xx

xx

x

x

x

x

x

o

o

o

o

o

o

o

oo

o

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o

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o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

o

Reporting limit = 1

729

Calcium

0

50

100

150

200

250

300

350

400

xx

x

xx

x

729 number of values

EXPLANATION 

x

x

o

o

upper adjacent

75th percentile

median (50th percentile)

25th percentile

lower adjacent

lower outside

lower detached

upper detached

upper outside

Figure 22. Statistical distribution of potassium, magnesium, sodium, and calcium in spring measurements at the index stations 

(deepest basins) in Michigan lakes during 2001–10. 
30 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

EXPLANATION

Alkalinity, in milligrams per 

 liter as calcium carbonate

5 - 24

25 - 74

75 - 124

125 - 199

200 - 323

EXPLANATION

Chloride, in milligrams per liter

0 - 14

15 - 29

30 - 75

76 - 278

EXPLANATION

Hardness, as calcium carbonate

1 - 74

75 - 149

150 - 225

226 - 337

0 50 100 MILES

0 50 100 KILOMETERS

U.S. Environmental Protection Agency Level III Ecoregions (U.S. Environmental Protection Agency, 2011)

55 Eastern Corn Belt Plains

57 Huron/Erie Lake Plains

51 North Central Hardwoods

50 Northern Lakes and Forests

56 Southern Michigan/Northern Indiana Drift Plains

EXPLANATION

Sulfate, in milligrams per liter

0 - 10

11 - 19

20 - 75

76 - 142

Figure 23A. Spatial distribution of alkalinity, sulfate, chloride, and hardness measured at Michigan inland lakes during 2001–10 in 

relation to U.S. Environmental Protection Agency Level III ecoregions. 
Statewide Water Quality of Inland Lakes 31

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Potassium, in milligrams per liter

0.0 - 0.9

1.0 - 2.4

2.5 - 4.0

4.1 - 5.4

EXPLANATION

Magnesium, in milligrams per liter

0 - 5

6 - 11

12 - 20

21 - 28

EXPLANATION

Sodium, in milligrams per liter

0 - 10

11 - 24

25 - 75

76 - 154

U.S. Environmental Protection Agency Level III Ecoregions (U.S. Environmental Protection Agency, 2011)

55 Eastern Corn Belt Plains

57 Huron/Erie Lake Plains

51 North Central Hardwoods

50 Northern Lakes and Forests

56 Southern Michigan/Northern Indiana Drift Plains

EXPLANATION

Calcium, in milligrams per liter

1 - 24

25 - 49

50 - 75

76 - 99

Figure 23B. Spatial distribution of potassium, magnesium, sodium, and calcium measured at Michigan inland lakes during 2001–10 

in relation to U.S. Environmental Protection Agency Level III ecoregions. 
32 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

10

20

80

80

90

90

110

100

120

100

130

70

20

20

100

20

40 30 50

60

70

70

100

90

0

50

40

30

0

130

40 40 30

90

80

90

70

90

100

80

70

100

80

0 0.5 1 MILE

0 0.5 1 KILOMETER

30

40

30

30

0

40

20 10

0

20

20 30

20

20

30

30

30

30

0

20

30

20

30

40

40

0

10

0

0

0

0

0

0

0

0

0 0.5 1 MILE

0 0.5 1 KILOMETER

Roscommon

County

A B

Mecosta

County

 EXPLANATION

Bathymetric contour—depth from surface, in feet 

(Michigan Center for Geographic Information, 2005)

• Measurement location

•

•

•

•

•

Figure 24. Multiple-basin lake examples in Michigan shown over Landsat satellite imagery for September 11, 2009. A, Higgins Lake 

in Roscommon County. B, Martiny Lake in Mecosta County. 
Statewide Water Quality of Inland Lakes 33

Table 9. Results of Wilcoxon signed-rank test to determine statistical significance among 

Michigan lakes with multiple basins, by constituent, for 2001–10.

[CaCO3, calcium carbonate; ~, not enough measurements to produce p-value; significant at 95-percent confidence 

level if p-value result is less than 0.05]

Basin

Spring 1 and 2 1 and 3 2 and 3

Constituent Sample size p-value Sample size p-value Sample size p-value

Secchi-disk transparency 109 0.70 26 0.20 26 0.71

Chlorophyll a 108 0.88 26 0.03 26 0.07

Dissolved oxygen Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.25 26 0.78 26 0.51

Mid-depth or metalimnion 109 0.70 26 0.48 26 0.98

Near bottom 108 0.88 26 0.65 26 0.56

Water temperature Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.68 26 0.22 26 0.57

Mid-depth or metalimnion 109 0.00 26 0.03 26 0.05

Near bottom 108 0.00 26 0.07 26 0.38

Specific conductance Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.51 26 0.88 26 0.32

Mid-depth or metalimnion 109 0.67 26 0.95 26 0.47

Near bottom 109 0.53 26 0.57 26 0.57

pH Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.04 26 0.17 26 0.95

Mid-depth or metalimnion 108 0.26 26 0.19 26 0.17

Near bottom 108 0.79 26 0.72 26 0.81

Ammonia plus 

organic nitrogen Sample size p-value Sample size p-value Sample size p-value

Near surface 106 0.79 23 0.81 23 0.88

Mid-depth or metalimnion 109 0.13 26 0.66 26 0.06

Near bottom 106 0.81 23 0.48 23 0.84

Ammonia Sample size p-value Sample size p-value Sample size p-value

Near surface 105 0.98 23 0.66 23 0.65

Mid-depth or metalimnion 108 0.10 26 0.90 26 0.50

Near bottom 105 0.06 23 0.47 23 0.64

Total phosphorus Sample size p-value Sample size p-value Sample size p-value

Near surface 104 0.27 23 0.74 23 0.34

Mid-depth or metalimnion 109 0.79 26 0.64 26 0.29

Near bottom 106 0.32 23 0.93 23 0.84

Nitrate plus nitrite Sample size p-value Sample size p-value Sample size p-value

Near surface 106 0.03 23 0.58 23 0.60

Mid-depth or metalimnion 109 0.04 26 0.74 26 0.39

Near bottom 106 0.25 23 0.70 23 0.41
34 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Table 9. Results of Wilcoxon signed-rank test to determine statistical significance among 

Michigan lakes with multiple basins, by constituent, for 2001–10.—Continued

[CaCO3, calcium carbonate; ~, not enough measurements to produce p-value; significant at 95-percent confidence 

level if p-value result is less than 0.05]

Basin

Summer 1 and 2 1 and 3 2 and 3

Constituent Sample size p-value Sample size p-value Sample size p-value

Secchi-disk transparency 107 0.58 25 0.17 25 0.29

Chlorophyll a 109 0.18 26 0.37 26 0.84

**Color 45 0.13 8 ~ 8 ~

Dissolved oxygen Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.70 26 0.28 26 0.16

Mid-depth or metalimnion 109 0.41 26 0.28 26 0.03

Near bottom 108 0.81 26 0.67 26 0.57

Water temperature Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.41 26 0.30 26 0.79

Mid-depth or metalimnion 109 0.19 26 0.13 26 0.14

Near bottom 107 0.00 26 0.03 26 0.24

Specific conductance Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.30 26 0.40 26 0.77

Mid-depth or metalimnion 109 0.10 26 0.43 26 0.94

Near bottom 109 0.42 26 0.12 26 0.29

pH Sample size p-value Sample size p-value Sample size p-value

Near surface 109 0.01 26 0.24 26 0.63

Mid-depth or metalimnion 109 0.25 26 0.42 26 0.95

Near bottom 109 0.63 26 0.25 26 0.51

Ammonia plus 

organic nitrogen Sample size p-value Sample size p-value Sample size p-value

Near surface 105 0.54 23 0.56 23 0.92

Mid-depth or metalimnion 109 0.45 26 0.40 26 0.37

Near bottom 105 0.45 23 0.47 23 0.41

Ammonia Sample size p-value Sample size p-value Sample size p-value

Near surface 105 0.60 23 0.14 23 0.96

Mid-depth or metalimnion 109 0.54 26 0.22 26 0.84

Near bottom 105 0.17 23 0.76 23 0.85

Total phosphorus Sample size p-value Sample size p-value Sample size p-value

Near surface 105 0.76 23 0.33 23 0.15

Mid-depth or metalimnion 109 0.33 26 0.40 26 0.77

Near bottom 105 0.08 23 0.55 23 0.27

Nitrate plus nitrite Sample size p-value Sample size p-value Sample size p-value

Near surface 105 0.28 23 0.49 23 0.66

Mid-depth or metalimnion 109 0.47 26 0.40 26 0.55

Near bottom 105 0.03 23 0.36 23 0.31
Statewide Water Quality of Inland Lakes 35

Table 9. Results of Wilcoxon signed-rank test to determine statistical significance among 

Michigan lakes with multiple basins, by constituent, for 2001–10.—Continued

[CaCO3, calcium carbonate; ~, not enough measurements to produce p-value; significant at 95-percent confidence 

level if p-value result is less than 0.05]

Basin

Spring—continued 1 and 2 1 and 3 2 and 3

Mid-depth or metalimnion Sample size p-value Sample size p-value Sample size p-value

*Alkalinity (as CaCO3) 109 0.59 26 0.26 26 0.33

*Calcium - total 109 0.95 26 0.10 26 0.30

*Chloride 108 0.82 25 0.35 25 0.86

*Hardness - calculated 109 0.98 26 0.24 26 0.16

*Potassium - total 109 0.97 26 0.05 26 0.30

*Magnesium - total 109 0.63 26 0.69 26 0.01

*Sodium - total 109 0.58 26 0.85 26 0.29

*Sulfate 108 0.07 25 1.00 26 0.11

*Spring only.

**Summer only 2007–10.

Comparisons With Other Lake-Monitoring Data 

for Michigan 

Volunteers coordinated by the original MDNR (now 

the MDEQ) began sampling lakes in 1974 and continue to 

sample (in 2010) approximately 250 inland lakes each year 

through the Michigan Cooperative Lakes Monitoring Program 

(CLMP). The objectives of the CLMP are to help citizen volunteers monitor indicators of water quality in their lakes such 

as SDT, chlorophyll a, phosphorus, DO, and temperature; to 

perform aquatic-plant surveys along with identifying whether 

exotic species are present; and to document changes in lake 

quality over time (http://www.micorps.net/lakeoverview.html).

Chlorophyll a and SDT data were used from the CLMP 

to determine trophic-status conditions from those lakes 

monitored for 2001–10. Lake data were chosen for the month 

of August of each year, and only one measurement for that 

month in the primary sampling station (deepest basin) was 

used to determine trophic-status conditions. The month of 

August corresponded to when measurements were made for 

the summer sampling season, which allowed a comparable 

time period between the two datasets. The majority of lakes 

have been sampled for multiple years, but some lakes are or 

are not measured each year for various reasons, which is why 

the number of measurements per year has varied. The resulting 

trophic-status conditions by percent for each year are shown in 

figure 25.

Data from the CLMP in addition to supplemental data 

specific to the Upper Peninsula, which was jointly measured 

by USGS and MDEQ, were used to extend the existing SDT 

measurements to produce TSI predictions for Michigan 

lakes greater than 20 acres based on satellite imagery. The 

remote sensing processes to produce the predicted TSI values 

are summarized in Fuller and others (2011). The statewide 

predictions for Michigan inland lakes greater than 20 acres 

without interference from clouds, cloud shadows, haze, dense 

vegetation, or shoreline for the periods 2003–05, 2007–08, and 

2009–10 are shown in figure 26.

The National Lakes Assessment (NLA)—based on chemical, physical, and biological data—measured lakes nationwide during 2007 to provide a statistically valid, probabilitydesigned estimate of the condition of lakes on a national and 

regional scale (U.S. Environmental Protection Agency, 2009). 

Michigan was one of the States that opted to increase the 

number of measurements collected to provide State-specific 

estimates. The calculated TSI values were weighted on the 

basis of lake size and ecological region by use of a mapping 

and analysis tool (NLA Data Viewer) developed by USEPA 

(Kiddon, 2009). The tool was used to obtain the data and produce the percentages of trophic-status categories for Michigan 

data and also for the Nation for comparison to those produced 

using trophic-status criteria specific to Michigan (from table 2) 

using SDT, chlorophyll a, and phosphorus; results are shown 

in figure 27. 

When SDT was compared for the current (2001–10) 

729 deepest basin in lakes greater than 25 acres with public 

boat launch (fig. 11) to the CLMP from Michigan’s volunteer 

monitoring network for years 2001 through 2010 (fig. 25), 

similar percentages for trophic-status categories emerged; on 

average, however, the CLMP percentage for the oligotrophic 

class was slightly higher and for the hypereutrophic class was 

slightly lower. It makes sense that more volunteers might be 

living on or having access to clearer lakes than impaired lakes. 

When the SDT data for 2001–10 were compared to the remote 

sensing statewide predicted layer with predictions for all lakes 

greater than 20 acres (fig. 26) regardless of public or volunteer 

access, the percentage for the oligotrophic class was lower. 
36 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

EXPLANATION

A B 

16%

27%

32%

26%

29%

14%

25%

18%

24%

26%

56%

59%

51%

56%

53%

56%

59%

64%

50%

52%

22%

10%

17%

17%

17%

27%

16%

14%

21%

18%

5

4

0

1

1

3

0

3

4

4

16%

20%

21%

19%

17%

19%

16%

13%

19%

19%

64%

61%

60%

66%

65%

58%

62%

67%

62%

63%

18%

17%

18%

14%

16%

22%

21%

20%

18%

18%

2

2

1

1

1

1

1

0

1

1

0 20 40 60 80 100 0 20 40 60 80 100

2001 (93)

2002 (98)

2003 (76)

2004 (83)

2005 (90)

2006 (98)

2007 (100)

2008 (104)

2009 (78)

2010 (98)

2001 (177)

2002 (177)

2003 (163)

2004 (178)

2005 (196)

2006 (202)

2007 (207)

2008 (180)

2009 (186)

2010 (194)

2001 (93) Sampling year and number of lakes sampled

Percent Percent

Figure 25. Trophic status based on data from the Cooperative Lakes Monitoring Program for one date per lake during the month of 

August for 2001–10 calculated from A, chlorophyll a and B, Secchi-disk transparency. 

0 20 40 60 80 100

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

EXPLANATION

2009–10

(2,591 lakes)

2007–08

(3,024 lakes)

2003–05

(3,121 lakes)

8 68 20

4

7 74 17

2

8 64 25

3

Percent

Figure 26. Statewide predictions of trophic status, by time period, produced by relating existing 

Secchi-disk transparency measurements to satellite imagery to produce predictions for all Michigan 

lakes greater than 20 acres. 
Statewide Water Quality of Inland Lakes 37

0 20 40 60 80 100

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

No data

EXPLANATION

Secchi-disk 

transparency, 

National (1,091)

Secchi-disk 

transparency, 

Michigan (50)

6 24 30

7 35 32

31

12

9

14

15 28 25

< 1

32

8 67 3

1

21

Chlorophyll a,

National (1,152)

Chlorophyll a,

Michigan (50)

Phosphorus,

National (1,157)

Phosphorus,

Michigan (50)

25 25 19 31

59 24 17

(1,157) Number of lakes sampled

Percent

Figure 27. National Lakes Assessment trophic status (statistically valid, probability-weighted design) 

comparing Michigan and National data using total phosphorus, chlorophyll a, and Secchi-disk transparency. 

This finding might suggest that Michigan lakes with public 

boat launch access and volunteer access might be clearer 

than Michigan lakes overall, regardless of public or volunteer 

access. The percentages for NLA (fig. 27) SDT Michigan-specific trophic classes and National classes are lower than those 

for the current 2001–10 dataset but more consistent with the 

remote sensing statewide predicted layer. The percentage for 

the mesotrophic class is much lower than for any of the other 

three datasets and higher for the eutrophic and hypereutrophic 

classes than for any of the other three datasets. Also, there was 

a substantial amount of unavailable data in the NLA set, which 

might be the cause of the difference between classes. 

When chlorophyll a was compared for the current 

(2001–10) 729 deepest basin in lakes greater than 25 acres 

with public boat launch (fig. 11) to the CLMP from Michigan’s volunteer monitoring network for years 2001 through 

2010 (fig. 25) similar percentages for trophic-status categories emerged. Chlorophyll a data were not available from 

remote sensing statewide predicted layer. The NLA (fig. 27) 

chlorophyll a data resulted in a lower percentage of oligotrophic lakes and a slightly higher percentage for mesotrophic 

lakes, but results were more comparable for the eutrophic and 

hypereutrophic classes. The trophic category percentages for 

the National lakes are not as comparable for any of the trophic 

classes, which could suggest lake differences nationally are 

not as comparable for chlorophyll a.

Comparison with Historical Measurements

Historical near-surface summer measurements made 

during 1974–84 for 445 lakes using standard protocols of 

the MDRNE were compared to their current (2001–10) lake 

measurements. Selection of the subset of 445 historical lake 

measurements was based on comparable sampling methods. 

The constituents compared were total nitrogen, TP, chlorophyll a, and SDT; increases and decreases for each constituent 

are shown in figure 28.
38 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

EXPLANATION

Nitrogen, in milligrams per liter

Increase 0.2 - 2.1

Increase < 0.2

Decrease < 0.2

Decrease 0.2 - 1.6

EXPLANATION

Phosphorus, in milligrams per liter

Increase 0.02 - 0.10

Increase < 0.02

Decrease < 0.02

Decrease 0.02 - 0.07

EXPLANATION

Secchi-disk transparency, meters

Increase 2.0 - 4.3

Increase < 2.0

Decrease < 2.0

Decrease 2.0 - 5.2

EXPLANATION

Chlorophyll a, in micrograms per liter

Increase 5.0 - 118.3

Increase < 5.0

Decrease < 5.0

Decrease 5.0 - 53.0

0 50 100 MILES

0 50 100 KILOMETERS

Figure 28. Increase (higher values) or decrease (smaller values) in property and constituent values from historical to current 

measurements for Michigan inland lakes. 
Statewide Water Quality of Inland Lakes 39

The Wilcoxon signed-rank test was used to determine 

statistical significance of differences among the historical and 

current measurements, as listed in table 10. SDT showed statistically significant differences among historical and current 

measurements at the 95-percent confidence level; however, 

as is evident in figure 28, the number and distribution around 

the State is fairly even between the increasing and decreasing 

values. Even though comparison of historical to current values 

showed statistically significant differences at the 95-percent 

confidence level, these difference did not indicate an overall 

trend towards clearer lakes or impaired lakes.

The trophic-status conditions are fairly comparable 

among current and historical measured lakes, with 87 percent 

of current lakes and 84 percent of historical lakes classified as 

oligotrophic or mesotrophic, using TP as the primary indicator; 80 percent of current lakes and 76 percent of historical 

lakes classified as oligotrophic or mesotrophic, using chlorophyll a as the primary indicator; and 78 percent of both current 

and historical lakes classified as oligotrophic or mesotrophic 

using SDT as the primary indicator (fig. 29). Although the 

percentage of lakes classified as oligotrophic and mesotrophic 

are comparable between historical and current measurements 

using any of the three primary indicators, this result does not 

necessarily mean that the same lakes were in these categories 

between historical and current measurements. Depending on 

the primary indicator, 50–66 percent of lakes did not change 

trophic-status conditions between the historical and current 

measurements, 13–23 percent moved towards the oligotrophic end of the TSI scale, and 20–25 percent moved towards 

the eutrophic end of the TSI scale (table 11). Further spatial 

analysis might help to identify patterns, hotspots, or certain 

lake characteristics (such as lake depth or land cover) where 

potential positive or negative trends for nutrients or trophic 

classes could be identified, but the current analysis comparing 

the 445 lakes did not seem to show patterns spatially, nor does 

it indicate overall increases or decreases.

Table 10. Results of Wilcoxon signed-rank test 

to determine statistical significance of differences 

among historical and current Michigan lakes 

property and constituent values for 2001–10. 

[Significant at 95-percent confidence level if p-value result 

is less than 0.05]

Property or constituent Sample 

size p-value

Secchi-disk transparency 434 0.02

Chlorophyll a 444 .60

Total nitrogen 445 .55

Total phosphorus 445 .30

Figure 29. Historical 

and current trophic 

status for comparable 

lakes based on 

total phosphorus, 

chlorophyll a, 

and Secchi-disk 

transparency. 

0 20 40 60 80 100

Oligotrophic

Mesotrophic

Eutrophic

Hypereutrophic

EXPLANATION

Secchi-disk 

transparency 

2001–10 (434)

Secchi-disk 

transparency 

1974–84 (434)

15 63 19

10 68 21

3

4

27 53 17 3

32 44 321

Chlorophyll a

2001–10 (445)

Chlorophyll a

1974–84 (445)

Phosphorus

2001–10 (445)

Phosphorus

1974–84 (445)

38 49 10

42 42 11

3

2

Percent

(445) Number of lakes sampled
40 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Table 11. Comparison of historical and current trophic 

status and percentage of lakes decreasing, not changing, or 

increasing classes for 2001–10. 

[TSI, trophic state index]

Towards 

oligotrophic Same Towards eutrophic

Percent

Down class Same class Up class

TSI (Secchi-disk 

transparency)

13 66 20

TSI (Chlorophyll a) 24 50 25

TSI (Phosphorus) 23 57 20

Relation of Lake Water Quality to Land Cover

The dominant land cover for each lake was found by first 

selecting catchments from the National Hydrography Dataset 

Plus, using version 1 for lakes in hydrologic region 7 and 

version 2 for lakes in hydrologic region 4 (Horizon Systems 

Corporation, 2011). Catchments that touched or intersected 

each lake were selected as the lake drainage basin, and the 

dominant National Land Cover Database 2001 class was 

chosen for each lake drainage basin (Homer and others, 2004). 

The five dominant land-cover classes that resulted for the 

729 lakes were agriculture (185 lakes), forest (433 lakes), 

urban (59 lakes), wetlands (51 lakes), and barren (1 lake). 

The forest- and wetland-dominant land-cover drainage 

areas are the only two in the Upper Peninsula. The majority 

of agriculture-dominated drainage areas are in the southwest 

area of the Lower Peninsula. Although most of the urban-dominated drainage areas are in the southeast area of the Lower 

Peninsula, some are in larger city areas on the west side of the 

Lower Peninsula, especially along Lake Michigan. The spatial 

distribution of dominant land-cover drainage areas is shown in 

figure 30. 

Water type for the 729 measured lakes was color coded 

by dominant land cover on a trilinear (Piper) diagram analysis (fig. 31). For lake-drainage areas where the dominant 

land-cover class was less than 40 percent, the next dominant 

land-cover class was added to the figure. Although the patterns 

for agriculture- and forest-dominated lake-drainage areas are 

somewhat similar, patterns for urban- and potentially wetlanddominated lake-drainage areas had some differences. Urban 

lakes differ with respect to chloride and nitrate plus nitrite, 

which are higher than for the other dominant land-cover 

classes. Calcium also is somewhat lower than for the other 

classes. If wetland-dominated classes are representative of 

more natural waters, then lakes in urban-dominated landcover classes could have lower calcium and higher chloride 

and nitrate plus nitrite than lakes with more natural water and 

wetland drainage areas. 

EXPLANATION

Agriculture

Barren

Forest

Urban

Wetlands

0 50 100 MILES

0 50 100 KILOMETERS

Figure 30. Spatial distribution of dominant National Land Cover 

Data 2001 class for Michigan inland lakes drainage areas for 

2001–10. 
PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate (Alkalinity)

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

AgricultureAgriculture/ForestAgriculture/UrbanAgriculture/Wetlands

PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrite 

 0 

 20 

 40 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate (Alkalinity)

 1

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

ForestForest/AgricultureForest/GrasslandForest/UrbanForest/Wetlands

PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrite plus Nitrate

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate (Alkalinity)

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

Urban

PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitri

 0 

 20 

 40 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate (Alkalinity)

 

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

P

Barren/WetlandsWetlands

EXPLANATION EXPLANATION

EXPLANATION EXPLANATION

Figure 31. Trilinear (Piper) diagram analysis, color coded by National Land Cover Data 2001 majority class for lake-drainage areas. 
42 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

Water type also was color coded by USEPA level III 

ecoregions (U.S. Environmental Protection Agency, 2011); 

Michigan glacial landsystems (Michigan Department of 

Environmental Quality and others, 2005); 6-digit hydrologic 

unit code boundaries (Steeves and Nebert, 1994); ecological 

drainage units (Higgins and others, 2005); and trophic status 

calculated from lake measurements using SDT, chlorophyll a, 

and TP as the primary indicators. These classification systems 

were used to test whether any noticeable driving factors were 

separating lake-water type. Although some properties and constituents individually showed separation between groups, these 

classification systems did not result in the degree of separation 

that is seen in the land-cover data between all constituents 

used to determine water type. These additional trilinear (Piper) 

diagram analyses are presented in appendix 2.

Linear regression graphs for chloride and calcium for 

spring mid-depth measurements (fig. 32) show a positive 

relation with lakes that have urban land cover as the dominant 

class, though the relation is stronger for chloride than calcium. 

Positive chloride trends were found by Syed and Fogarty 

(2005) for the Clinton River at Mount Clemens, which drains 

four southeastern Michigan counties, with the southern part of 

the watershed being composed of more than 50 percent urban 

land cover in 2001. Chloride has been found in elevated levels 

in groundwater in southeastern Michigan, including Oakland 

County (Aichele, 2004; Myers and others, 2000). Also in 

Oakland County, a strong positive relation was found between 

chloride in stream water and degree of urban development in 

the watershed (Aichele, 2005). By removing chloride measurements for Oakland, Livingston, and Genesee Counties 

in southeastern Michigan, there still was a positive relation 

between chloride and percent land cover (fig. 33), though the 

coefficient of determination (R2) value decreased from 0.55 to 

0.39. The relations are weak between lake-drainage area size 

and nutrients when using linear relations. These results can be 

viewed in appendix 3. Further spatial analysis might help to 

identify patterns, hotspots, or certain lake characteristics (such 

as lake depth or landcover) where overall potential positive or 

negative relations could be identified.

y = 0.4832x + 6.4026

R2 = 0.55

0

20

40

60

80

100

0 50 100 150 200 250 300

y = 0.3284x + 3.2304

R2 = 0.14

0 20 40 60 80 100 120

Percent urban land cover

in lake-drainage area

Spring mid-depth measurements of chloride, 

in milligrams per liter

0

20

40

60

80

100

Percent urban land cover

in lake-drainage area

Spring mid-depth measurements of calcium, 

in milligrams per liter

Figure 32. Chloride and calcium mid-depth measurements made in spring compared to percent urban National Land Cover Data 2001 

for lake-drainage areas. 

0

20

40

60

80

100

0 50 100 150 200 250 300

Percent urban land cover

in lake-drainage area

Spring, mid-depth measurements of chloride,

in milligrams per liter

y = 0.4643x + 6.3511

R2 = 0.39

Figure 33. Chloride mid-depth measurements made in spring 

compared to percent urban National Land Cover Data 2001 for lakedrainage areas, excluding Oakland, Livingston, or Genesee County 

data (shown in red) from the regression analysis. 
Summary and Conclusions 43

Summary and Conclusions 

During 2001–10, 729 lakes greater than 25 acres (with 

public boat-launch access) were jointly monitored by the 

U.S. Geological Survey (USGS) and the Michigan Department 

of Environmental Quality (MDEQ) as part of Michigan’s Lake 

Water-Quality Assessment program. Of the 729 lakes sampled, 

109 have more than one major basin, 26 had three, and 2 have 

four major basins, for a total of 866 basins that were assessed 

and used in the analyses for this report. All lake basins 

included vertical-profile measurements; nutrient measurements at three discrete depths; Secchi-disk transparency (SDT) 

measurements (unless the Secchi disk hit the bottom of the 

lake in which case no SDT depth was recorded); and depthintegrated chlorophyll a measurements for the spring and 

summer, with major ions and physical properties measured for 

spring mid-depth and true color for summer for 2007–10. In 

the spring, 52 lakes were deemed too shallow for the collection of 3 discrete-depth measurements, with 58 lakes being too 

shallow in the summer.

Deep-basin measurements in the vertical profile for dissolved oxygen (DO) showed that in the spring, 12 percent of 

lakes had 25-50 percent of the water column (25–50 percent 

of the total lake depth) below below 0.5 mg/L. In the summer, 

30 percent of lakes had 25–83 percent of the water column 

below 0.5 mg/L Specific conductance showed a spatial pattern 

of lower values in the Upper Peninsula and increasingly higher 

values from the north to south in the Lower Peninsula. Lakewater pH did not vary as noticeably throughout the State, but 

measurements below 6.5 were mostly in the Upper Peninsula. 

Comparisons of the vertical profile at discrete depths corresponding to near surface, mid-depth or metalimnion, and near 

bottom show statistically significant differences at the 95-percent level among all depths and constituents except DO for 

mid-depth between spring and summer.

In about 75 percent of inland lake deep basins measured, 

depending whether SDT, chlorophyll a, or TP was used as the 

primary indicator, trophic characteristics were associated with 

oligotrophic or mesotrophic conditions. Five percent or fewer 

were categorized as hypereutrophic when using any of the 

three indicators, with more than 80 percent of hypereutrophic 

lakes having a maximum depth of 30 ft or less. Deeper lakes 

were correlated with lower Trophic State Index (TSI) values 

reflecting towards the oligotrophic end of the scale. There was 

a statistically significant difference at the 95percent confidence 

level between spring and summer SDT and chlorophyll a measurements. There were somewhat even increases and decreases 

in the SDT measurements among spring and summer when 

comparing all lakes; an overall statistical increase or decrease 

was not found. SDT measurements were clearer in the spring 

than in the summer in 63 percent of lakes.

During 2001–10, on the basis of near-surface measurements made in the spring, 97 percent can be considered phosphorus limited and less than half a percent nitrogen limited; 

for the summer measurements, 96 percent can be considered 

phosphorus limited and less than half a percent nitrogen 

limited. Differences in concentrations of the majority of nutrients were statistically significant at the 95-percent confidence 

level between discrete sampling depths, and differences in 

concentrations of all nutrients were statistically significant at 

the 95-percent confidence level between spring and summer 

measurements. 

All major ions and physical properties measured in the 

spring at mid-depth showed spatial patterns of lower values in 

the Upper Peninsula that increased southward to the southern 

areas in the Lower Peninsula, though the location of increase 

varied by constituent. U.S. Environmental Protection Agency 

Level III Ecoregions separated potassium, sulfate, and chloride values fairly well; separated lower and higher values for 

magnesium, hardness, calcium, and alkalinity; and were mixed 

spatially for middle-range values. The highest concentrations 

of chloride and sodium were in the southeastern part of the 

Lower Peninsula. 

For the majority of constituents measured, differences in 

concentrations for lakes with more than one major basin did 

not prove to be overall statistically significant at the 95-percent confidence level between basins. There were more statistically significant differences in the spring than in the summer. 

However, it is notable that the most statistically significant 

differences at the 95-percent confidence level were found for 

water temperature at various depth and basin combinations; 

this finding indicates that water temperature could be a useful 

single measure of a multiple-basin lake for gaining an understanding of the lake as a whole.

Comparison of other Michigan lake-sampling programs 

producing trophic-status determinations for Michigan inland 

lakes revealed a few interesting relations to the current 

(20001–10) dataset. For example, volunteers coordinated by 

the MDEQ (former MDNR) started sampling in 1974 and 

continue to sample to date (2010) approximately 250 inland 

lakes each year through the Cooperative Lakes Monitoring Program (CLMP). When the primary sampling station 

(deepest-basin) measurement per lake for TSI from SDT in the 

month of August during 2001–10 is compared, the percentage 

of lakes in the oligotrophic class is higher than in the 2001–10 

dataset. The hypereutrophic class has a lower percentage of 

lakes than the 2001–10dataset, whereas the mesotrophic and 

eutrophic classes are fairly comparable. This difference might 

result from more CLMP volunteers living near or otherwise 

having access to clearer lakes. Results from comparing TSI 

from chlorophyll a, the are more varied by year for the CLMP 

data. 

Data from the CLMP, in addition to supplemental data 

specific to the Upper Peninsula (jointly collected by USGS 

and MDEQ), were used to extend the existing SDT measurements to produce TSI predictions using remotely sensed data 

for Michigan lakes greater than 20 acres. The three time periods of available TSI predictions from SDT include 3,121 lakes 

for 2003–05; 3,024 lakes for 2007–08; and 2,591 lakes for 

2009–10. The predictions on average for the three periods 

result in a lower percentage of lakes in the oligotrophic category, a higher percentage in the mesotrophic category, about 
44 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

the same percentage in the eutrophic category, and a slightly 

lower percentage in the hypereutrophic category compared to 

the 2001–10 dataset. The predictions are an interesting comparison and extension to program monitoring based on public 

boat-launch access or volunteer residence or access, with some 

noticeable differences in percentages for trophic classes. 

The National Lakes Assessment (NLA) is a statistically 

valid, probability-designed estimate of the condition of lakes 

on a national and regional scale, and it includes 50 lakes measured in Michigan during 2007. Percentages Michigan NLA 

lakes in the oligotrophic and mesotrophic trophic classes were 

larger than those in the 2001–10 lake data, though combining the two classes produced similar results. Of the Michigan 

lakes from the 2001–10 data that were greater than 25 acres 

(with public boat-launch access), a higher percentage of lakes 

were in the oligotrophic class (except when using phosphorus 

as the sole indicator) than the Michigan NLA lakes. When 

trophic status was determined by using SDT was the sole indicator for lakes for 2001–10, 74 percent of lakes were classified 

as oligotrophic or mesotrophic, compared to only 42 percent 

from the NLA Michigan lakes (though SDT depth was not 

recorded for 14 percent of the NLA Michigan lakes); 5 percent 

were classified as hypereutrophic, compared to 12 percent of 

the NLA Michigan lakes. 

Data for 445 lakes measured historically by the MDNRE 

during 1974–84 were compared to 2001–10 lake measurements. Four constituents were comparable between the two 

time periods: total nitrogen, TP, chlorophyll a, and SDT. Of 

the four, only SDT was found to have statistically significant 

differences between datasets at the 95-percent confidence 

level, though no overall historical increasing or decreasing 

trend in SDT is evident. Depending on the primary indicator, 

50–66 percent of lakes did not change trophic-status class, 

13–23 percent moved down a class towards the oligotrophic 

end of the TSI scale, and 20–25 percent moved up a class 

towards the eutrophic end of the TSI scale. 

Dominant land cover was calculated for each lake drainage area, resulting in five dominant land-cover classes of 

agriculture, forest, urban, wetlands, and barren. Lake water 

types were shown on a tri-linear (Piper) diagram analysis that 

assisted in the identification of higher chloride concentrations in urban-dominant drainage areas and somewhat lower 

calcium than in other land-cover classes. Although previous 

reports document high chloride concentrations in southeastern Lower Michigan, removing data from lakes in this area 

still resulted in a positive relation between percent urban land 

cover and chloride, though the coefficient of determination 

(R2

) value decreased from 0.55 to 0.39. 

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streamflow and water quality in Oakland County, Michigan, 

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analysis: U.S. Geological Survey Scientific Investigations 

Report 2005–5016, 22 p.

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Appendix 2 
PERCENT Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrate plus Nitrite

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

U.S. Environmental ProtectionAgency Level III Ecoregions

EXPLANATION EXPLANATION

EXPLANATION EXPLANATION

Eastern Corn Belt PlainsHuron/Erie Lake PlainsNorth Central hardwoodNorthern Lakes and ForestsSouthern Michigan/Northern Indiana Drift Plains

PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrate plu

 0 

 20 

 40 

 60 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PER

Ice-contact-outwashLacustrine-fineLacustrine-courseLakesLodge-Till-or-Fine-supraglacial-driftProglacial-outwashThin-drift-over-bedrock

Ice-marginal-till

PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrate plus Nitrite

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

PERCENT

4020140202403014050040601406024070040801408024090041000

PERCENT

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrat

 0 

 20 

 40 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

19232627

3

33404243

8

Michigan glacial landsystems

6-digit hydrologic unit code Ecological drainage units

Figure 2–1. Trilinear (Piper) diagram analysis, color coded by U.S. Environmental Protection Agency Level III Ecoregions, Michigan gla6-digit hydrologic unit codes, and ecological drainage units. 
Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrate plus Nitrite

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrate plus 

 0 

 20 

 40 

 60 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

OligotrophicMesotrophicEutrophicHypereutrophic

Calcium

 100 

 80 

 60 

 40 

 20 

 0 0 

 20 

 40 

 60 

 80 

 100 

Magnesium

 0 

 20 

 40 

 60 

 80 

 100 

Sodium plus Potassium

Chloride, Nitrate plus Nitrite

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 

Bicarbonate

 100 

 80 

 60 

 40 

 20 

 0 

Sulfate

 0 

 20 

 40 

 60 

 80 

 100 

Sulfate plus Chloride 0 

 20 

 40 

 60 

 80 

 100 

Calcium plus Magnesium

 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

Trophic State Index classesSecchi-disk transparency

Trophic State Index classesChlorophyll a

Trophic State Index classesPhosphorus

EXPLANATION EXPLANATION

EXPLANATION

OligotrophicMesotrophicEutrophicHypereutrophicOligotrophicMesotrophicEutrophicHypereutrophic

PERCENT

PERCENT

PERCENT

PERCENT

PERCENT

PERCENT

PERCENT

PERCENT

PERCENT

Figure 2–2. Trilinear (Piper)diagram analysis, color coded by Trophic State Index classes using Secchi-disk transparency, chlorophyltotal phosphorus as the primary indicator. 

Appendix 3 
52 Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10

y = −1710.9x + 6142.2

R2

 = 0.0115

0 0

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

y = −13207x + 5345

R2

 = 0.0016

0 0.05 0.10 0.15 0.20

Drainage area, in acres

Drainage area, in acres

5,000

10,000

15,000

20,000

25,000

30,000

35,000

40,000

45,000

Summer near-surface measurements of total phosphorus,

 in milligrams per liter

Summer near-surface measurements of ammonia plus 

organic nitrogen, in milligrams per liter

Figure 3–1. Ammonia plus organic nitrogen and total phosphorus near-surface measurements made in summer compared to lake 

drainage-area size. 
Appendix 3 53

y = 13.695x + 10.08

R2

 = 0.0489

y = 5.2797x + 11.309

R2

= 0.0122

y = 0.3105x + 18.683

R2

 = 0.0000 

y = −17.847x + 52.773

R2

 = 0.0739

y = 181.04x + 15.197

R2

 = 0.0195

y = 240.59x + 10.416

R2 = 0.0577

y = 240.59x + 10.416

R2 = 0.0577

y = −331.95x + 47.715

R2 = 0.0585

Percent agriculture land cover

in lake-drainage area

0

20

40

60

80

100

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Percent urban land cover

in lake-drainage area

Summer near-surface measurements of ammonia plus 

organic nitrogen, in milligrams per liter

Summer near-surface measurements of ammonia plus 

organic nitrogen, in milligrams per liter

Percent wetlands land cover

in lake-drainage area

Percent forest land cover

in lake-drainage area

Summer near-surface measurements of ammonia plus 

organic nitrogen, in milligrams per liter

Summer near-surface measurements of ammonia plus 

organic nitrogen, in milligrams per liter

0

20

40

60

80

100

Percent agriculture land cover

in lake-drainage area

Percent urban land cover

in lake-drainage area

Percent wetlands land cover

in lake-drainage area

Percent forest land cover

in lake-drainage area

Summer near-surface measurements of total phosphorus,

 in milligrams per liter

0 0.05 0.10 0.15 0.20

0

20

40

60

80

100

0 0.05 0.10 0.15 0.20

0

20

40

60

80

100

0 0.05 0.10 0.15 0.20

0

20

40

60

80

100

0 0.05 0.10 0.15 0.20

Summer near-surface measurements of total phosphorus,

 in milligrams per liter

Summer near-surface measurements of total phosphorus,

 in milligrams per liter

Summer near-surface measurements of total phosphorus,

 in milligrams per liter

0

20

40

60

80

100

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

20

40

60

80

100

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

20

40

60

80

100

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Figure 3–2. Ammonia plus organic nitrogen and total phosphorus near-surface measurements made in summer compared to 

dominant percent National Land Cover Data 2001 (Homer, 2004) for lake drainage areas. 


Fuller and Taricska —Water-Quality Characteristics of Michigan’s Inland Lakes, 2001–10—Scientific Investigations Report 2011–5233

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