Understanding Lakes

ecoregions



Figure 23

Observations of lake characteristics that are geographically-distinct have led to the ecoregion approach of lake classification developed by the U.S. Environmental Protection Agency (EPA) Research Laboratory at Corvallis, Oregon. Ecoregions are areas of relative homogeneity based on landuse, soils, land and surface form, and potential natural vegetation. Minnesota is divided into seven ecoregions (Figure 23), but most of its lakes are found in four of these. Minnesota Pollution Control Agency (MPCA) researchers found regional patterns in numbers of lakes, lake water quality, morphometry, and watershed characteristics (Table 5) among these ecoregions. For example, lakes of the Northern Lakes and Forests ecoregion have significantly lower total phosphorus and chlorophyll than lakes in the Western Corn Belt Plains ecoregion. Furthermore, the MPCA discovered through lake-user surveys that user perception of water quality varied by ecoregions. This has led to ecoregion-specific criteria for phosphorus and, in general, helped to clarify expectations and goals for protecting lakes in Minnesota.

 

Table 5
Ecoregion lake data summary
Values are typical for surface water (0-2 m) in summer (June-
September).

PARAMETER

Northern
Lakes
and Forests

North Central Hardwood Forests

Western Corn Belt Plains

Northern
Glaciated
Plains

Total Phosphorus (µg/L)

14 - 17

23 - 50

65 - 150

130 - 250

Chlorophyll-mean (µg/L)

< 10

5 -22

30 - 80

30 - 55

Chlorophyll-max (µg/L)

< 15

7 - 37

60 - 140

40 - 93

Secchi Disk (m)

2.4 - 4.6

1.5 - 3.2

0.5 - 1.0

0.3 - 1.0

Total Kjeldahl - N (mg/L)

< 0.75

< 0.60 - 1.2

1.3 - 2.7

1.8 - 2.3

[Nitrite + Nitrate]-N (mg/L)

< 0.01

< 0.01

0.01 - 0.02

0.01 - 0.1

Alkalinity (mg/L)

40 - 140

75 - 150

125 - 165

160 - 260

Color (Pt-Co units)

10 - 35

10 - 20

15 - 25

20 - 30

pH

7.2 - 8.3

8.6 - 8.8

8.2 - 9.0

8.3 - 8.6

Chloride (mg/L)

< 2

4 - 10

13 - 22

11 - 18

TSS (µg/L)

< 1 - 2

2 - 6

7 - 18

10 - 30

Turbidity (NTU)

< 2

1 - 2

3 - 8

6 - 17

EC (uS/cm)

50 - 250

300 - 400

300 - 650

640 - 900

TN:TP ratio

25:1 - 35:1

25:1 - 35:1

17:1 - 27:1

7:1 - 18 - 1

* Summer averages by ecoregion, based on interquartile range (25th - 75th percentile) for ecoregion reference lakes. Derived in part from Heiskary, S.A. and C.B. Wilson (1990).

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