Lake Pepin Watershed TMDLSite Specific Eutrophication

Criteria Development

September 2008

Lake Pepin Photo by Guy Schmickle

Lake Pepin Compared to North Central Hardwoods Forest & Western Corn Belt Plains Thresholds. Listed in 2002

< 0.7> 32> 90WCP

1.025198Pepin mean1991-2000

< 1.1> 18> 45CHF

Secchi m

Chl-a ppb

TP ppb

Ecoregion & Lake

#

#

#

#

Lake PepinM781.2O

M775.6Q

M771.2P

M766.0I

Two somewhat distinct segments

1. “Upper” – shallow inflow segment: 40% of lake by area, 28% by volume short Tw -2-3 days

2. “Lower” – deeper segment: 60% by area, 72 % by volume

LTRMP monitoring is primary source of data.

Consists of 4 “fixed sites” with 1 in upper and 3 in lower along thalweg of lake.

Lake Pepin characteristics compared to ecoregion reference lakes

Table 2. Lake Pepin morphometric and watershed characteristics as compared to interquartile range for CHF (n=38) & WCP (n=12) ecoregion reference lakes. Parameter Pepin CHF (25th-75th) WCP (25th-75th) Surface area (mi2) 39.7 0.62 – 1.38 0.43 – 0.59 Mean depth (ft) 17.7 21 - 26 8 - 11 Maximum depth (ft) 56 43 - 73 10 - 27 Maximum width (mi) 1-2 -- -- Length (mi) 20.8 -- -- Volume (acre-ft) 448,340 49,027 – 142,090 7,547 – 22,152 Watershed Area (mi2) 48,634 4 - 12 2 - 3 Watershed: lake surface area 1,225:1 6 - 9 4 - 7 Mean Hydraulic Retention Time 0.04 year

(16 days) 9.3 years (mean)

4.8 years (mean)

(Heiskary & Vavricka 1993)

(Heiskary & Wilson 2005)

Table 4a. Summer-mean flow statistics based on long-term (80-year) and recent (30-year) record (based on USGS June-Sept. readings at Prescott). Most recent year in that flow range noted.

Percent exceedance 95th

90th

75th

50th

25th

10th

Long (1928-2007)

5,296 7,610 10,600 17,595 24,700 31,109

Example year 1932 1961 2006 1967 1997 1984 Recent (1978-2007) 9,330 10,145 16,411 22,358 25,886 31,374 Example year 1989 2006 1982 1994 1995 1984

Table 4b. Comparison of 10th percentile flows based on continuous 72-year (1936-2007) and 30-year (1978-2007) records for three primary tributaries to Pepin.

River 30-year 72-year Upper Mississippi 4,499 3,688 St. Croix 2,174 2,149 Minnesota 3,236 1,033

Flow Record

Northern Lakes and Forests

North-Central Hardwood Forests

Western Corn Belt Plains

Northern Glaciated

Plains

Driftless Area

Northern Minnesota Wetlands

Minnesota Pollution Control Agency

Ecoregions of the Lake Pepin Watershed

Ecological Regions2.0% Driftless Area

32.0% North Central Hardwood Forests

13.4% Northern Glaciated Plains

30.8% Northern Lakes and Forests

<0.1% Northern Minnesota Wetlands

21.7% Western Corn Belt Plains

Lake Pepin

¯

Table 3. Lake Pepin watershed ecoregion composition. Estimated areal composition, flow contribution, and 25th percentile stream TP based on minimally-impacted streams for each ecoregion (McCollor and Heiskary, 1993). Estimated “background” TP for Lake Pepin based on ecoregion area and flow-based estimates.

Region Area % Flow % TP µg/L

area-based

flow-based

NLF 31% 42% 30 9 13 CHF 32% 38% 70 22 27 WCP 22% 16% 210 46 34 NGP 13% 4% 160 21 6

Mean 99 79

Lake Pepin residence time. Estimated based on summer-mean flow at Prescott

0

10

20

30

40

50

60

0

5,000

10,00

0

15,000

20,00

0

25,00

0

30,00

0

35,00

0

40,00

0

45,000

50,00

0

55,00

0

60,00

0

65,00

0

cfs

days

1976 & 1988

1990

2006

1993

2000

View as a continuum, “lake or reservoir-like” over ~60% of flows

Flow Duration Frequency vs. Pepin Residence Time

0

10

20

30

40

50

60

70

80

0 10 20 30 40 50 60 70 80 90 100

Flow Duration Frequency (%)

Res

. tim

e da

ys

1928 1976

Lake-likeRiver-like

2006

1988

FlowTarget

~Reservoir-like~

Lake Pepin Chl-a vs Flow Duration Frequency: 1978 - 2006 (n=21 summers)

R2 = 0.68

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70 80 90 100

Flow Duration Interval (%)

Chl

-a u

g/L

High Low

Chlorophyll (algal production) is inversely related to river flow

1990-2002 LTRMP Data (relatively high flow years)

Recent (1990-2002) mean & median chlorophyll-a by site

0

5

10

15

20

25

30

35

40

45

50

Dam Upper Lower Outlet

ppb

Mean Med Max

Regression equation based on rivers is a better descriptor of TP & chlorophyll-a relationship for Pepin as compared to “glacial” lake regression

TP vs.chl-a regressions for reference lakes & rivers relative to Upper & Lower Lake Pepin

Rivers: y = 1.66x - 2.21R2 = 0.87

Lakes: y = 1.27x - 0.90R2 = 0.87

0.0

0.5

1.0

1.5

2.0

2.5

0.5 0.8 1.0 1.3 1.5 1.8 2.0 2.3 2.5 2.8

Log TP ug/L

Log

chl-a

ug/

L

Lakes Rivers Upper Pepin Lower Pepin Linear (Rivers) Linear (Lakes)

(100 200 316)

32

10

Algal composition available for model development

•Lake users respond to BG blooms;

•Blue-greens low % of algal community in most summers

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1992 (47%) 1994 (70%) 1995 (83%) 1996 (59%) 1997 (75%) 1998 (53%) 2006 (24%)

Upper L.P. - yes - Average of % BG Upper L.P. - yes - Average of % Greens

Upper L.P. - yes - Average of %Diatoms

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

1992(47%)

1994(70%)

1995(83%)

1996(59%)

1997(75%)

1998(53%)

2005(65%)

2006(24%)

Lower L.P. - yes - Average of % BG Lower L.P. - yes - Average of % Greens

Lower L.P. - yes - Average of %Diatoms

Lower Lake

Table 4. Diatom-inferred pre-European phosphorus concentrations for Lake Pepin and lakes from adjacent ecoregions Pepin CHF-deep CHF-shallow WCBP-deep WCBP-shallow 35-40 ppb 1 25 ± 3 39 ± 4 47 ± 6 67 ± 11 # of lakes 35 6 5 6 1 Engstrom et al. (in press)

0

20

40

60

80

100

1762 1800 1860 1904 1921 1940 1964 1976 1988 1996

Year

Perc

ent o

f Dia

tom

s

0.00

0.05

0.10

0.15

0.20

0.25

Tota

l Pho

spho

rus

(mg/

L)

Planktonic Diatoms Benthic Diatoms TP Estimate #1 TP Estimate #2

Limno Tech’s RCA Model

Scenario viewer allows for an overview of in-lake & river response to various load reduction scenarios;Variable response noted among low flow years;Focus on range of response for 3 low flow years: 1987, 1989, 2006 and one median flow year: 1998;Use to inform criteria selection & frame necessary reductions;

RCA Scenario viewer

Summer-mean Chl-a as a function of TP

0

5

10

15

20

25

30

35

40

0.000 0.050 0.100 0.150 0.200 0.250

TP ug/L

Chl

-a u

g/L

1998 (28%) 1987 (14%) 2006 (10%) 1989 (3%)

In-lake response as a function of 3 low flow years& 1 median flow year; with 19 reduction scenarios per year

Days with Chl-a >50 ug/L as a function of summer-mean TP

0

10

20

30

0.000 0.050 0.100 0.150 0.200 0.250

TP ug/L

days

1998 (28%) 1987 (14%) 2006 (10%) 1989 (3%)

Days with Chl-a >50 ug/L as a function of summer-mean chl-a

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40

Summer-mean ug/L

Day

s >5

0 ug

/L

1998 (28%) 1987 (14%) 2006(10%) 1989 (3%)

Blue-green algal biomass as a function of summer-mean TP

0

5

10

15

20

25

0.000 0.050 0.100 0.150 0.200 0.250

TP ug/L

BG

bio

mas

s %

1998 (28%) 1987 (14%) 2006 (10%) 1989 (3%)

Summer-mean Secchi as a function of TP

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

0.00 0.05 0.10 0.15 0.20 0.25

TP ug/L

SD m

1998 (28%) 1987 (14%) 2006 1989 (3%)

Summer-mean Secchi as a function of chlorophyll-a

0.00.20.40.60.81.01.21.41.61.8

0 5 10 15 20 25 30 35 40

Chl-a ug/L

Secc

hi m

1998 (28%) 1987 (14%) 2006 (10%) 1989 (3%)

Table 7. Draft Lake Pepin site specific criteria as compared to Minnesota’s lake eutrophication criteria for adjacent ecoregions. Ecoregion – lake type (use classification1) TP Chl-a Secchi

µg/L µg/L meters

Lake Pepin site specific goals: whole lake 100 32 0.8CHF – Aquatic Rec. Use – Deep (Class 2B) 40 14 1.4CHF – Aquatic Rec. Use – Shallow (Class 2B) 60 20 1.0WCP&NGP – Aquatic Rec. Use – Deep (Class 2B)

65 22 0.9

WCP&NGP – Aquatic Rec. Use - Shallow (Class 2B)

90 30 0.7

Site-specific Considerations

TP – 100 ug/L is in range of diatom-inferred TP c1900-1960. Slightly above 90 ug/L WCP criteria; however results in lower chl-a because Pepin produces less chl-a per unit TP;Chl-a 32 ug/L – slightly above 30 ug/L but deemed “protective of aquatic recreational use” since it results in a lower frequency of severe nuisance blooms & BG algae comprise a small proportion of blooms;Secchi – 0.8 m should ensure Secchi of 0.5-0.6 in upper (allow for growth of SAV) & in lower Secchi should average 0.9-1.0 m – consistent with criteria for deep WCP and shallow CHF lakes;

Evaluating Lake Pepin for attainment of criteria

Use LTRMP fixed site network (4 sites) as primary basis for assessment;Assess on a biennial basis as a part of 303(d) assessment; use most recent 10 years of data;Assess for summers when “reservoir-lake-like” – corresponds to flows of ~10,145-20,000 cfs;

Pepin Draft Site Specific Criteria (Eutrophication Impairment)

In-lake TP 100 μg/L ~ c1900-1960 based on diatom-reconstructed P;Viable chl-a 32 μg/L - met over majority of flows, seeks to minimize chlorophyll-a >50 ug/L;Secchi 0.8 m (upper & lower), allow for SAV in upper;Summer average June-Sept.Flow range ~10,145 cfs (10% recurrence) –20,000 cfs (Res. Time ~11 days) based on recent 30-year record