36
i Lake Data Statistical Analysis Report Prepared for Minnehaha Creek Watershed District by HDR Engineering, Inc. August 2013

Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

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Page 1: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

i

Lake Data Statistical Analysis Report

Prepared for

Minnehaha Creek Watershed District

by

HDR Engineering, Inc.

August 2013

Page 2: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

ii

Table of Contents

I. Introduction 1

II. Data Review 5

III. Optimal Number of Lake Minnetonka Monitoring Stations 10

IV. Temporal Sampling Frequency 22

V. Trend Analysis 26

VI. Discussion and Conclusions 32

List of Figures

Figure 1. Lake Minnetonka Water Quality Monitoring Stations Figure 2. Upper Watershed Lakes Water Quality Monitoring Stations Figure 3. Southwestern Area Waterbody Comparisons Figure 4. South Central Area Waterbody Comparisons Figure 5. Northwestern Area Waterbody Comparisons Figure 6. Northeastern Area Waterbody Comparisons Figure 7. North Central Area Waterbody Comparisons Figure 8. Southeastern Area Waterbody Comparisons

List of Tables

Table 1. Lake Minnetonka Bays and Upper Watershed Lakes Table 2. Sample Sizes for Lake Minnetonka Bays Table 3. Sample Sizes for Upper Watershed Lakes (June through September) Table 4. One-Way ANOVA Comparison Levels of Significance, Southwestern and South Central Area Table 5. One-Way ANOVA Comparison Levels of Significance, Northwestern and Northeastern Area Table 6. One-Way ANOVA Comparison Levels of Significance, North Central Area Table 7. One-Way ANOVA Comparison Levels of Significance, Southeastern Area Table 8. Sampling Frequency Comparisons, Lake Minnetonka Bays Table 9. Sampling Frequency Comparisons, Upper Watershed Lakes Part 1 Table 10. Sampling Frequency Comparisons, Upper Watershed Lakes Part 2 Table 11. Average Annual Runoff, Precipitation, and Air Temperatures Table 12. Trend Analysis for Raw and Weighted Data, Upper Watershed Lakes Table 13. 1997-2012 Trend Analysis for Raw and Weighted Data, Lake Minnetonka Bays

Appendix

Graphs of Lake Water Quality Annual Summer Means

Page 3: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

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Introduction

Monitoring of lakes has occurred in the Minnehaha Creek Watershed District (MCWD) since 1968, with

significant expansions in monitoring intensity occurring since 1997. Two of the goals of the MCWD

Hydrodata Program (as noted in the MCWD 2013 5-Year Monitoring Plan) are to provide data to 1)

identify long-term trends in water quality and 2) maximize efficiencies in monitoring frequencies,

locations, and events. These goals are addressed in this report for Lake Minnetonka (Figure 1) and Upper

Watershed lakes (Figure 2) with respect to selected water quality parameters.

Monitored lake surface parameters of concern include total phosphorus (TP), chlorophyll a (CHLA), and

Secchi disk transparency (SECC). In addition, an overall trophic state index (TSI) value is calculated every

year for each monitoring station. These parameters are typical for lake surface sampling, and go back to

1968 for several stations. Lake above-bottom TP has been measured in many MCWD-monitored lakes

beginning in 2004, but these are not part of the analysis. In addition, profiles are typically taken every

meter of depth for temperature, dissolved oxygen (DO), conductivity, and pH (again, not included in the

analysis). MCWD lake sampling frequency is typically every two weeks from late April to early October.

A brief summary of MCWD lake data available for analysis is presented in Table 1. Approximate locations

of the sampling stations are shown in Figures 1 and 2. Note that the table does not indicate how many

sampling dates occurred during a given year, or how many TP, CHLA, and SECC data points exist for each

year. Gideon’s Bay (Lake Minnetonka) is not included in the analysis because only a few years of data

exist. Therefore, 26 Lake Minnetonka stations and 11 Upper Watershed lake stations are part of the

analysis. The specific objectives of the Project are:

1. To assess the number of monitoring stations on Lake Minnetonka and the required sampling

frequency to determine the health of Lake Minnetonka and of the lakes in the upper watershed,

and

2. To perform trend analysis to determine statistically-significant improvement or degradation

of water quality at the sampling stations.

SPSS statistical software is used for the analyses because of its ability to provide probability values (i.e.

p-values) and the quality and acceptability of its analytical methods as compared to MS Excel programs.

http://www-01.ibm.com/software/analytics/spss/products/statistics/

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Figure 1. Lake Minnetonka Water Quality Monitoring Stations*

*from MCWD 2011 Hydrologic Data Monitoring Report

Page 5: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

3

Figure 2. Upper Watershed Lakes Water Quality Monitoring Stations*

*from MCWD 2011 Hydrologic Data Monitoring Report

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Table 1. Lake Minnetonka Bays and Upper Watershed Lakes

Site

CodeLake Sampling Sites

pre

-1995

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

2012

LBL01 Black Lake X X X X X X X X

LCM01 Carman Bay X X X X X X X X X

LCS01 Carsons Bay X X X X X X X X X X

LCO01 Cooks Bay X X X X X X X X X X X X X X X X X

LCR01 Crystal Bay X X X X X X X X X X X X X X X X

LFO01 Forest Lake X X X X X X X X X X X X X X X X X

LGB01 Grays Bay X X X X X X X X X X X

LHL01 Halsted Bay X X X X X X X X X X X X X X X X X X

LHR01 Harrison Bay X X X X X X X X X X X X X X X X

LJE01 Jennings Bay X X X X X X X X X X X X X X X X X X X

LLF01 Lafayette Bay X X X X X X X X X X X X

LMU01 Low er Lake North X X X X X X X X X X X

LGI01 Low er Lake South (Gale Island) X X X X X X X X X X X X X X X X X X X

LMA01 Maxw ell Bay X X X X X X X X X X X X X X X X

LNR01 North Arm X X X X X X X X X X X X X X X X

LPE01 Peavey Lake X X X X X X X X X X X X X X X X X

LPH01 Phelps Bay X X X X X X X X

LPT01 Priests Bay X X X X X X X X X

LSM01 Smithtow n Bay X X X X X X X X X X X

LSP01 Spring Park Bay # X X X X X X X X X # # # # # # #

LAL01 St. Alban’s Bay X X X X X X X X X X X X X X X X

LSU01 Stubbs Bay * X X X X X X X * X * * * * * * *

LTG01 Tanager Lake X X X X X X X X X X X X

LWA01 Wayzata Bay X X X X X X X X X X X X X X X X X X X

LWE01 West Arm (East Basin) X X X X X X X X X X X X X X X X X

LCI01 West Upper Lake (Crane Island) X X X X X X X X X X X X X X X X X X X

LCH01 Christmas Lake X X X O X O O X X X X X X X X X X X X

LDU01 Dutch Lake X X X X X X X X X X X X X X X X X

LGL01 Gleason Lake X X X X X X X X X X X X X X X X X X X

LMW01 Lake Minnew ashta X X X X X X X X X X X X X X X X X X X

LVI01 Lake Virginia X X X @ X X X X X X X X X

LLA01 Langdon Lake X X X X X X X X X X X X X X X X X X X

LLO01 Long Lake X X X X X X X X + X X X X X X X X X X

LPR01 Parley Lake X X X X X X X X X X X X X X X X

LPI01 Piersons Lake X X X X X X X X X X X X X X X X

LSC01 Schutz Lake X X X X X X X X X X X

LWS01 Wassermann Lake X X X X X X X X X X X X X X X X X

NOTE: MCWD began in-house monitoring in 2005 # Site LSP02 * Site LSU03 O Site LCH02

@ Site LVI02 + Site LLO02

Lake M

inn

eto

nka S

ites

Up

per

Wate

rsh

ed

Sit

es

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I. Data Review

Water quality data were provided by MCWD Water Quality staff in spreadsheet format. HDR screened

the data and confirmed several procedures with MCWD Water Quality staff. First, it was noted that

there were many instances of CHLA being reported as “< 0.2 ppb”, indicating that the sample was below

detection limits. These values were changed to 0 (zero) for the purposes of these statistical analyses.

Second, there were many instances of sampling dates where only SECC was reported which coincided

with reported volunteer SECC monitoring programs for certain lakes/bays. These values were removed

from the data set for the purposes of these statistical analyses. Third, part of the MCWD monitoring

program involved duplicate sampling for the purposes of their quality assurance program. Where

present, the duplicate and regular sample values were simply averaged for the purposes of these

statistical analyses.

One goal of the screening was to make sure that samples were taken at least once during each of the

following months: June, July, August, and September. The reason for this concern is that these months

represent the averaging period used by the Minnesota Pollution Control Agency (MPCA) for the

purposes of determining lake impairment for nutrients.

For the Lake Minnetonka bays, most sampling began in 1996, although there are six bays that have data

as far back as the early 1970s (Table 2). It is apparent that 1997 represents the year in which most

locations sampled the three primary water quality parameters used in these analyses at least four times

during the summer, although some bays did not have data collected until the 2000s. For the six bays

with water quality data from the 1970s and 1980s, 1981-1986 represents the most consistent period in

which 4 or more sampling events occurred each year.

For the Upper Watershed lakes, data collected between 1971 and 1981 represents samples taken only

in September (Table 3). For Christmas Lake, Gleason Lake, Langdon Lake, Lake Minnewashta, Parley

Lake, Piersons Lake, and Lake Virginia, any 1982-1996 samples were taken during July. For Long Lake,

any 1982-1989 and 1991-1995 samples were taken during July; all months were sampled in 1990, and

1996 samples were taken during the June-August period. As seen with the Lake Minnetonka bays, it is

apparent that 1997 represents the year in which most Upper Watershed lake stations sampled the three

primary water quality parameters at least 4 times during the summer, although some lakes did not have

data collected until the 2000s.

Initial graphical analysis revealed a noticeable shift in mean annual TP concentrations for several

monitoring locations between the years 2000 and 2001. Upon review, the annual Hydrodata Reports

depicted the same behavior. All data were compared against a different set of data (that used to

calculate lake grades each year), and the following corrections or additions were made:

Christmas Lake: 1997-2000 CHLA and TP

Long Lake: 2007 SECC, CHLA, and TP

Cooks, Forest Lake, Harrisons, Lafayette, LLSouth, Spring Park, Stubbs, St. Albans, West Arm:

1997-2000 TP

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West Upper: 1979-1986, 1997-2000 TP

Wayzata: 1983-1986, 1997-2000 TP

Peavey: 2008 SECC

Tanager: 2000 TP

Halsted: 1997-1998 TP data (originally not available).

After these corrections or additions were completed, the statistical analyses were conducted as

described in the following sections.

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Table 2. Sample Sizes for Lake Minnetonka Bays

Year

SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP

1996 3 3 3 4 4 4 2 2 1 3 3 3 3 2

1997 9 9 9 9 8 9 9 9 9 9 9 1 9 8 9 9 8 9

1998 8 9 8 9 7 9 9 8 9 9 7 8 1 8 8 9 8 8 9

1999 9 9 9 8 9 9 9 9 9 8 9 9 9 9 9 9 8 9

2000 8 8 7 8 8 8 8 8 8 8 8 8 8 8 8

2001 8 8 8 8 8 8 8 8 8 8 8 9 8 8 8

2002 7 7 7 7 7 7 7 7 7 7 9 10 7 7 7

2003 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2004 5 5 5 5 5 5 5 5 5 8 8 8 8 8 8 8 8 8 5 5 5 8 8 8 8 8 8

2005 4 4 4 8 8 8 8 8 8 8 8 8 4 4 4 10 10 9 8 8 8 4 4 4

2006 5 5 5 7 7 7 7 7 7 5 5 5 6 6 6 6 6 6 8 8 8 7 7 7 5 5 5 8 8 8

2007 7 7 7 8 8 8 8 8 8 8 8 8 7 7 7 8 8 8 7 7 7 7 7 7 8 8 8

2008 9 9 9 9 9 9 9 9 9 9 9 9 8 8 8 8 8 8 9 9 9 9 9 9 8 8 8 9 9 9

2009 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

2010 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

2011 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 7 8 8

2012 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 7 7 7 8 8 8 8 8 8 8 8 8 8 8 8

June through September values only

Lafa

yett

e

Bay

Hal

ste

d

Bay

Co

oks

Bay

Fore

st

Lake

Har

riso

ns

Bay

Gra

ys B

ay

Car

son

s

Bay

Cry

stal

Bay

Bla

ck

Lake

Car

man

Bay

Year

SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP

1996 3 3 2 3 3 3

1997 9 9 9 9 9 9 8 8 8 13 8 9 8 9 9 8 9 9 9 9

1998 9 8 8 9 8 9 9 7 9 13 8 7 7 9 7 9 9 8 9

1999 9 9 9 9 9 9 9 8 9 9 9 9 9 9 9 9 9 9

2000 8 8 8 8 8 8 7 7 7 7 7 7 8 8 8 8 8 8

2001 8 8 8 8 8 8 8 7 8 8 7 8 8 8 8 8 8 8

2002 7 7 7 7 7 7 5 7 7 6 7 7 7 7 9 8 8 8

2003 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 9 8 9

2004 8 8 8 8 8 8 5 5 5 4 4 4 8 8 8 8 8 8 8 8 8 7 7 7

2005 4 4 4 8 8 8 8 8 8 3 3 2 5 5 5 12 12 12 10 10 10 12 11 11

2006 8 8 8 6 6 6 6 6 6 7 7 7 6 6 6 6 6 6 6 6 6 7 7 7 7 7 7 7 7 7

2007 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2008 9 9 9 8 8 8 8 8 8 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 7 8 8 9 9 9

2009 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 8 9 9 9 9 9 9 9 9 9 9 9

2010 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

2011 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2012 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 4 8 8 8 8 9 9 8 8 8 8 8 8

June through September values only

Tan

age

r

Lake

Ph

elp

s B

ay

Pri

est

s B

ay

Smit

hto

wn

Bay

Spri

ng

Par

k

Bay

St.

Alb

ans

Bay

Stu

bb

s B

ay

Low

er

Lake

No

rth

Max

we

ll

Bay

No

rth

Arm

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8

Table 2, Continued

Year

SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP SEC

C

CH

LA

TP

1971 1 1 1 1

1972 1

1973

1974 3 3 3 3 3 2 3 3 3

1975 1 1 1 1

1976

1977 1 1 1 1 1 1 1 2 1 1 1 1 1 1

1978 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1979 4 4 4 4 4 4 3 4 3 4 4 4 4 4 4

1980 3 3 2 3 3 3 3 3 2 3 3 3 3 3 3

1981 4 4 4 3 4 4 3 4 4 4 4 4 4 4 4

1982 4 4 4 4 4 4 3 4 4 4 1 4 4 4 4

1983 4 4 4 4 3 4 4 2 4 4 3 4 4 2 4

1984 4 4 4 4 1 4 3 2 3 2 1 2 4 3 4

1985 4 3 4 4 3 4 4 3 4 4 2 4 4 3 4

1986 4 2 4 4 3 4 4 3 4 4 3 4 4 3 4

1987 2 2 2 2 1 2 2 2 2 2 1 3 2 2 2

1988 2 2 2 2 2 3 2 2 2 2 2 3 2 2 2

1989 2 2 2 2 2 2

1990 2 2 2 4 4 4

1991 2 2 2 2 2 2

1992 1 1 1 1 1 1 1 1 2 2 1 1

1993 2 2 2 2 1 2 2 2 1 2 2

1994 1 2 1 1 1 1 1 1 1 1

1995 3 3 2 2 1 1 2 2 2 2 2 1 2 1 2

1996 2 2 3 2 1 3 3 3 3 3 3 3 3

1997 9 8 9 9 9 9 9 8 9 8 8 8 9 8 9

1998 8 9 9 9 8 8 9 9 7 9 9 9 8 9 9

1999 9 9 9 9 9 8 9 9 9 9 9 9 9 9 9 9 9 9

2000 8 8 8 8 8 8 8 8 7 8 8 8 8 8 8 8 8 8

2001 8 8 8 8 7 8 8 8 8 8 6 8 9 9 9 8 8 8

2002 7 7 7 7 7 7 7 7 10 7 7 7 6 6 6 7 6 7

2003 7 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2004 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2005 10 10 10 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2006 7 7 7 8 8 8 7 7 7 7 7 7 6 6 6 6 7 7

2007 7 7 7 8 8 8 8 8 8 8 8 8 7 7 7 8 8 8

2008 8 8 8 9 9 9 8 9 9 9 9 9 8 8 8 9 9 9

2009 9 9 9 9 9 9 9 9 9 9 10 10 9 9 9 9 9 9

2010 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

2011 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8

2012 8 8 8 8 8 8 9 9 9 8 8 8 8 8 8 8 8 8

Way

zata

Bay

Pe

ave

y

Lake

We

st A

rm

Low

er

Lake

Sou

th

Jen

nin

gs

Bay

We

st

Up

pe

r

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Table 3. Sample Sizes for Upper Watershed Lakes (June through September)

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II. Optimal Number of Lake Minnetonka Monitoring Locations

The inter-connectedness of many bays on Lake Minnetonka suggests that water quality monitoring

stations in reasonably-close proximity to one another might be hydraulically-connected, and therefore

have similar water quality. As seen in Table 1, sampling has taken place at all 26 Lake Minnetonka

stations since 2006, with a few stations not sampled in 2004 and 2005. Prior to 2004 nearly a third of the

stations were not sampled. The mean annual SECC, CHLA, and TP values from 2004-2012 were used for

direct comparisons between monitoring stations for each of the available years.

The following spatial clusters of monitoring stations in Lake Minnetonka were selected for analysis:

Southwestern: Halsted Bay – Priests Bay – Cooks Bay – West Upper Lake – Smithtown Bay –

Phelps Bay

South Central: Black Lake – Carman Bay – Phelps Bay – Spring Park Bay – Lafayette Bay

Northwestern: Jennings Bay – West Arm – Harrisons Bay – Crystal Bay – Forest Lake

Northeastern: Lower Lake North – Wayzata Bay – Grays Bay – Tanager Lake – Peavey Lake

North Central: Maxwell Bay – Crystal Bay – Stubbs Bay – North Arm – Lower Lake North

Southeastern: Lower Lake North – Lafayette Bay – Lower Lake South – Carsons Bay – St. Albans

Bay

Southwestern Area

Comparisons between these stations suggest that Halsted Bay is unique with respect to SECC, CHLA, and

TP, which is understandable given its physical separation (small inlet to Priests Bay) from the other

locations (Figure 3, Table 4). Only 4 of 27 comparisons between Halsted Bay and Priests Bay yielded

statistically-significant differences. Priests Bay and Cooks Bay represent a transitional area, from the

highly-eutrophic Halsted Bay to the more mesotrophic open expanses (e.g. West Upper Lake). Priests-

Cooks comparisons and Cooks-West Upper Lake comparisons yielded 10 and 15 instances, respectively,

of water quality means not showing statistically-significant differences. West Upper Lake had very few

statistically-significant differences between either Phelps Bay or Smithtown Bay, suggesting that these

three bays may be strongly hydraulically connected.

South Central Area

Black Lake to Spring Park Bay comparisons yielded 17 of 24 instances of water quality means showing

statistically-significant differences, indicating the uniqueness of Black Lake’s water quality (Table 4,

Figure 4). Phelps Bay showed no statistically-significant differences when compared to Spring Park Bay.

Carman Bay to Spring Park Bay comparisons yielded 18 of 21 instances of water quality means showing

statistically-significant differences. A comparison of Carman Bay to Lafayette Bay, which are connected

via a channel locally known as “The Narrows”, yielded 5 of 18 statistically-significant differences.

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Northwestern Area

These comparisons yielded three major findings. First, Crystal Bay, which is connected to the other bays

in the northwestern area by a small passage, had 26 of 27 comparisons to West Arm yield statistically-

significant differences (Figure 5, Table 5). Second, Jennings Bay, West Arm, Harrisons Bay, and Forest

Lake had no statistically-significant differences among each other for SECC comparisions, and only 4 of

36 statistically-significant differences for CHLA comparisons. Third, Jennings Bay appears unique with

respect to TP, showing 15 of 18 statistically-significant differences when compared to West Arm or

Harrisons Bay. West Arm, however, had only 2 of 9 statistically-significant TP differences when

compared to Harrisons Bay.

Northeastern Area

Results indicate that Peavey Lake and Tanager Lake are quite unique to each other and to Lower Lake

North (Table 5, Figure 6). Lower Lake North had only 1 of 24 statistically-significant differences when

compared to Wayzata Bay, and Wayzata Bay had 8 of 27 statistically-significant differences when

compared to Grays Bay.

North Central Area

Lower Lake North is rather distinct from Crystal Bay, showing only 4 of 24 statistically-significant

differences (Figure 7, Table 6). Stubbs Bay had 27 of 29 statistically-significant differences when

compared to Maxwell Bay, and Maxwell Bay had 13 of 27 statistically-significant differences when

compared to Crystal Bay. North Arm had only one statistically-significant difference when compared to

Maxwell Bay (1 of 27) and only 9 of 27 when compared to Crystal Bay.

Southeastern Area

Comparisons among these bays yield very few statistically-significant differences (Figure 8, Table 7).

These results are more likely due to the fact that their water quality is quite high. For example, CHLA

means are often in the 4 to 8 ppb range for these bays. Combined with the fact that standard deviations

can be up to 50% of the mean value, it makes it unlikely that a statistically-significant difference will

occur.

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Figure 3. Southwestern Area Waterbody Comparisons

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2004 2005 2006 2007 2008 2009 2010 2011 2012

SEC

C (

m)

Halsted Priests Cooks West Upper Smithtown Phelps

0

10

20

30

40

50

60

70

80

90

2004 2005 2006 2007 2008 2009 2010 2011 2012

CH

LA (

pp

b)

Halsted Priests Cooks West Upper Smithtown Phelps

0

20

40

60

80

100

120

140

2004 2005 2006 2007 2008 2009 2010 2011 2012

TP (

pp

b)

Halsted Priests Cooks West Upper Smithtown Phelps

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Table 4. One-Way ANOVA Comparison Levels of Significance, Southwestern and South Central Area

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

Year TP CHLA SECC TP CHLA SECC

2004 ** * ns ** ** **

2005 ** ** ns ns ns ns

2006 ** ** ** ns * ns

2007 ** ** ** ns ** ns

2008 ** ** * ** * **

2009 ** ** ** ns ** *

2010 ** ** ** ** ** ns

2011 ** * ns ** ** ns

2012 ** ns ** ** * **

TP CHLA SECC TP CHLA SECC TP CHLA SECC

2004 ns ** * ns ** ns

2005 * * ns ns ns ns

2006 ns ns ns ns ns ns ns ns ns

2007 ns ns ns ns ns ns ns ns ns

2008 ns * ns ns ns ns ns ns ns

2009 ** ** ** ns ns ns ns ns ns

2010 * ns ns ns ns ns ns ns ns

2011 ns ** ns ns ns ns ns ns ns

2012 ** * ns ns * ns ns ns ns

Year TP CHLA SECC TP CHLA SECC TP CHLA SECC

2004 ns ns ** ** ** **

2005

2006 * ns ns ns ns ns ns ns ns

2007 * ** ns ns ns ns

2008 ns ns ns ** ns ns ns ns ns

2009 ns ns ns ** ** ** ns ns ns

2010 ns ns ns ** * ns ns ns ns

2011 ns ns ns ** * * ns ns ns

2012 ** ns ns ** ** ** ns ns ns

2004

2005

2006 ns ns ns

2007

2008 ns * **

2009 * ns ns

2010 ns ns ns

2011 ns ns ns

2012 ns ** **

La

faye

tte

Ba

yP

rie

sts

Ba

yS

pri

ng

Pa

rk B

ay

Carman Bay Black Lake Phelps Bay

Cooks Bay Phelps Bay Smithtown Bay

We

st U

pp

er

La

ke

Halsted Bay Cooks Bay

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Figure 4. South Central Area Waterbody Comparisons

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

2004 2005 2006 2007 2008 2009 2010 2011 2012

SEC

C (

m)

Black Carman Phelps Spring Park Lafayette

0

2

4

6

8

10

12

14

16

18

20

2004 2005 2006 2007 2008 2009 2010 2011 2012

CH

LA (

pp

b)

Black Carman Phelps Spring Park Lafayette

0

5

10

15

20

25

30

35

40

45

2004 2005 2006 2007 2008 2009 2010 2011 2012

TP (

pp

b)

Black Carman Phelps Spring Park Lafayette

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Figure 5. Northwestern Area Waterbody Comparisons

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

2004 2005 2006 2007 2008 2009 2010 2011 2012

SEC

C (

m)

Jennings West Arm Harrisons Crystal Forest

0

20

40

60

80

100

120

140

2004 2005 2006 2007 2008 2009 2010 2011 2012

CH

LA (

pp

b)

Jennings West Arm Harrisons Crystal Forest

0

20

40

60

80

100

120

140

2004 2005 2006 2007 2008 2009 2010 2011 2012

TP (

pp

b)

Jennings West Arm Harrisons Crystal Forest

Page 18: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

16

Table 5. One-Way ANOVA Comparison Levels of Significance, Northwestern and Northeastern Area

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

Year TP CHLA SECC TP CHLA SECC TP CHLA SECC TP CHLA SECC

2004 ** ns ns ns ** ns ns ns ns ** ** **

2005 ** * ns ns ns ns ns ns ns ** ** **

2006 ns ns ns ns ns ns ns ns ns ** ** **

2007 ** ns ns ns ns ns ns ns ns ** ** **

2008 ** ns ns ns ns ns ns ns ns ** ** **

2009 ** ns ns ** ns ns ** ns ns ** ** **

2010 ns ns ns ns ns ns ns ns ns * ** **

2011 * ns ns ns ns ns ns ns ns ** ** ns

2012 ** ns ns ** ns ns ** ns ns ** ** **

2004 ** ns ns

2005 ** ** ns

2006 ns ns ns

2007 ** ns ns

2008 ** ns ns

2009 ** ns ns

2010 ** * ns

2011 ** ns ns

2012 ** ns ns

Jennings Bay Forest Lake Harrisons Bay Crystal Bay

Ha

rris

on

s B

ay

We

st A

rm

Year TP CHLA SECC TP CHLA SECC TP CHLA SECC

2004

2005 ns ns ns ** ** **

2006 ns ns ns ** ** * ** ** **

2007 ns ns ns ** ** ** ** ** **

2008 ns ns ns ** ** ** ** ** **

2009 ns ** ns ** ** ** ** ** **

2010 ns ns ns ** ** ** ** ** **

2011 ns ns ns ** ns ** ** ** **

2012 ns ns ns ** ** ** ** ** **

2004 ns ** ns 2004 ** ** **

2005 ns ns ns 2005

2006 ns ns ns 2006 ns ns **

2007 ns ns ns 2007 ns ** **

2008 ns ns ns 2008 ** ** **

2009 ** ** ** 2009 ** ** **

2010 ns ns ** 2010 ns ** **

2011 ns ns ns 2011 ns ** **

2012 ** * * 2012 ns ** **

Tanager Lake

Gra

ys B

ay

Pe

ave

y L

ake

Wayzata Bay Peavey Lake

Lo

we

r L

ake

No

rth

Page 19: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

17

Figure 6. Northeastern Area Waterbody Comparisons

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

2004 2005 2006 2007 2008 2009 2010 2011 2012

SEC

C (

m)

LLNorth Wayzata Grays Tanager Peavey

0

10

20

30

40

50

60

70

80

90

100

2004 2005 2006 2007 2008 2009 2010 2011 2012

CH

LA (

pp

b)

LLNorth Wayzata Grays Tanager Peavey

0

20

40

60

80

100

120

140

2004 2005 2006 2007 2008 2009 2010 2011 2012

TP (

pp

b)

LLNorth Wayzata Grays Tanager Peavey

Page 20: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

18

Figure 7. North Central Area Waterbody Comparisons

0.0

1.0

2.0

3.0

4.0

5.0

6.0

2004 2005 2006 2007 2008 2009 2010 2011 2012

SEC

C (

m)

Maxwell Crystal Stubbs North Arm LLNorth

0

10

20

30

40

50

60

70

80

2004 2005 2006 2007 2008 2009 2010 2011 2012

CH

LA (

pp

b)

Maxwell Crystal Stubbs North Arm LLNorth

0

10

20

30

40

50

60

70

80

2004 2005 2006 2007 2008 2009 2010 2011 2012

TP (

pp

b)

Maxwell Crystal Stubbs North Arm LLNorth

Page 21: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

19

Table 6. One-Way ANOVA Comparison Levels of Significance, North Central Area

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

Year TP CHLA SECC TP CHLA SECC TP CHLA SECC

2004 ns ** ns ns ns ns ** ** **

2005 ns * ns ns ns ns ** ** **

2006 ns ns ns ns ns ns ** ** ns

2007 ns ** ns ns * ns ** ** **

2008 ns ** * ns ns ns ** ** **

2009 ns ** ** ns ns ns ns ** **

2010 ** ** ** ns ns ns ** * *

2011 ns ns ns ns ns ns ** ** *

2012 ** ** ** ns ns ns ** ** **

2004

2005 ns ** *

2006 ns ** **

2007 ns ** **

2008 ** * **

2009 ** ** **

2010 ** ** ns

2011 ** ** **

2012 ** ** **

2004 ns ns ns

2005 ns ns ns

2006 ns ns ns

2007 ns ns ns

2008 ns ns ns

2009 ** ** **

2010 ** ** *

2011 ns ns ns

2012 ** ** **

North Arm Stubbs BayCrystal Bay

Lo

we

r L

ake

No

rth

No

rth

Arm

Ma

xw

ell B

ay

Page 22: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

20

Figure 8. Southeastern Area Waterbody Comparisons

0.0

1.0

2.0

3.0

4.0

5.0

6.0

2004 2005 2006 2007 2008 2009 2010 2011 2012

SEC

C (

m)

LLNorth Lafayette LLSouth Carsons St. Albans

0

2

4

6

8

10

12

2004 2005 2006 2007 2008 2009 2010 2011 2012

CH

LA (

pp

b)

LLNorth Lafayette LLSouth Carsons St. Albans

0

5

10

15

20

25

30

35

40

2004 2005 2006 2007 2008 2009 2010 2011 2012

TP (

pp

b)

LLNorth Lafayette LLSouth Carsons St. Albans

Page 23: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

21

Table 7. One-Way ANOVA Comparison Levels of Significance, Southeastern Area

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

Year TP CHLA SECC TP CHLA SECC TP CHLA SECC

2004

2005 ns ns ns ns ns ns ns ns ns

2006 ns ns ns ns ns ns ns ns ns

2007 ns ns ** ns ns * ns ns ns

2008 ns ns ns * ns ns ** ns ns

2009 * ns ns ** ns ns ** ns ns

2010 ns ns ns * ns ns * ns ns

2011 ns ns ns ** ns ** ns ns ns

2012 ns ns ns ns * * ns ns ns

2004 2004 ** ** ns

2005 ns ns ns 2005 ns ns ns

2006 ns ns ns 2006 ns ns ns

2007 ns ns ns 2007 ns ns ns

2008 ns ns ns 2008 * ns ns

2009 ns ns * 2009 * ns ns

2010 ns ns ns 2010 * ns ns

2011 ns * ns 2011 ns ns ns

2012 ** * * 2012 ns ns *

2004 ns ns ns

2005 ns ns ns

2006 * ns ns

2007 ns ns ns

2008 ns ns ns

2009 ns ns ns

2010 ns ns ns

2011 ns ns ns

2012 ** ** **

Lo

we

r L

ake

No

rth

Lower Lake South Lafayette Bay Carsons Bay

St. A

lba

ns B

ay

St. A

lba

ns B

ay

La

faye

tte

Ba

y

Page 24: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

22

III. Temporal Sampling Frequency

Six Lake Minnetonka stations and all eleven Upper Watershed lake stations were examined for the

effects of monitoring frequency. Analysis consisted of comparing twice-monthly monitoring to once

monthly monitoring to detect any statistically-significant differences in summer mean values. Available

data from 1997 to 2012 for each of the stations were chosen and the data for each station were

examined to ensure that there were two data points for each summer month (June, July, August, and

September) for a total of eight samples per year per water quality parameter. For stations with three

data points in a given month, the latest date was dropped; this was done to account for higher water

quality readings at the start of June prior to summer algal blooms, so eliminating the first data point in

each month would have excluded the early June readings and skewed the overall results. Exceptions

included the rare occasions where there were three data points in one month but only one data point

the next month. For example, if there were data points for August 2nd, 20th, 31st, and September 15th;

August 2nd and 20th were assigned to August and August 31st and September 20th were assigned to

September.

Annual means for each water quality parameter were calculated for each station and year using three

different methods:

“All”: All eight (8) data points for a given year were used to calculate the summer mean.

“1”: Data points from the first half of a given month for a given year (e.g. 1st-15th of month) were

used (4 data points).

“2”: Data points from the second half of a given month for a given year (e.g. 16th to 31st of

month) were used (4 data points)

Both “All” versus “1” and “All” versus “2” comparisons were made to avoid bias towards using the

beginning or end of the month for the sampling twice-monthly to once-monthly question. The “1”

versus “2” comparison was made to see if there was any difference due to sampling during the first or

second half of the month.

Results indicate that there are no statistically-significant differences between sampling twice monthly

versus sampling once a month for the six Lake Minnetonka stations (Table 8). However, there were

fourteen (14) statistically-significant differences between means calculated using data from the first half

of the month versus the second half of the month.

Results indicate only one (1) statistically-significant difference between sampling twice monthly versus

sampling one a month for the eleven Upper Watershed lake stations (Tables 9 and 10). This occurred in

2009 for Dutch Lake SECC. However, there were thirteen (13) statistically-significant differences

between means calculated from data from the first half of the month versus the second half of the

month.

Page 25: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

23

Table 8. Sampling Frequency Comparisons, Lake Minnetonka Bays

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

1997 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1999 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2000 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2001 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2002

2003 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2004

2005 ns ns ns ns ns ns ns ns * ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2009 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns **

1997 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns ns ns * ns ns ** ns ns ns

1999 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2000 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns *

2001 ns ns ** ns ns * ns ns * ns ns ns ns ns ns ns ns ns

2002

2003 ns ns ns ns ns ns ns ns * ns ns ns ns ns ns ns ns ns

2004

2005 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2009 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1997 ns ns * ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ** ns ns ns

1999 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2000 ns ns ns ns ns ns ns ns ns

2001 ns ns * ns ns ns ns ns ns ns ns * ns ns ns ns ns ns

2002

2003 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2004

2005 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns *

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns * ns ns ns

2009 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

SPR

ING

PA

RK

BA

Y

SECC CHLA TP SECC

WA

YZA

TA B

AY

CHLA TP

JEN

NIN

GS

BA

YLO

WER

LA

KE

SOU

TH

WES

T U

PP

ER L

AK

EST

UB

BS

BA

Y

Page 26: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

24

Table 9. Sampling Frequency Comparisons, Upper Watershed Lakes Part 1

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

1997 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1999 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2000 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2001 ns ns ns ns ns ns ns ns ns

2002 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2003 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2004 ns ns ns ns ns ns ns ns ns ns ns ns

2005 ns ns ns ns ns * ns ns * ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns

2008 ns ns ns ns ns ns ns ns * ns ns ns ns ns ns ns ns ns

2009 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1997 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1999 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2000 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2001 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2002 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2003 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns *

2004 ns ns ns ns ns ns ns ns ns

2005 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2006

2007

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2009 * ns ** ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1997 ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns

1999 ns ns ns ns ns ** ns ns ns ns ns ns ns ns ns ns ns **

2000 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2001 ns ns ns ns ns ns ns ns ns

2002 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2003 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2004 ns ns ns ns ns ns ns ns ns

2005 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2009 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

LON

G L

AK

E

PA

RLE

Y L

AK

E

TP

CH

RIS

TMA

S LA

KE

GLE

ASO

N L

AK

E

DU

TCH

LA

KE

LAN

GD

ON

LA

KE

SECC CHLA TP SECC CHLA

Page 27: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

25

Table 10. Sampling Frequency Comparisons, Upper Watershed Lakes Part 2

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

All

vs 1

All

vs 2

1 v

s 2

1997 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1999 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2000 ns ns ns ns ns ns ns ns ns

2001

2002 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2003 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2004 ns ns ns ns ns ns ns ns ns

2005 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2009 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ** ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1997 ns ns ns ns ns ns ns ns ns

1998 ns ns ns ns ns ns ns ns ns

1999 ns ns ns ns ns ns ns ns ns

2000

2001

2002 ns ns ** ns ns ns ns ns ns

2003

2004 ns ns ns ns ns ** ns ns ns

2005 ns ns ns ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns **

2008 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2009 ns ns ns ns ns ns ns ns ns

2010 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

1997

1998

1999

2000

2001

2002 ns ns ns

2003 ns ns ns ns ns ns ns ns ns

2004

2005 ns ns ns ns ns ns

2006

2007 ns ns ns ns ns ns ns ns ns

2008 ns ns ns ns ns ns ns ns ns

2009 ns ns ** ns ns ns ns ns *

2010 ns ns ns ns ns ns ns ns ns

2011 ns ns ns ns ns ns ns ns ns

2012 ns ns ns ns ns ns ns ns ns

TP

SCH

UTZ

LA

KE

WA

SSER

MA

N L

AK

E

VIR

GIN

IA L

AK

E

SECC CHLA TP

LAK

E M

INN

EWA

SHTA

PIE

RSO

NS

LAK

E

SECC CHLA

Page 28: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

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IV. Trend Analysis

This analysis focused on trends displayed in the mean annual water quality data (June-September) to

determine if water quality is improving, decreasing, or remaining the same over the period of record.

Four approaches were utilized for each station: raw water quality data, water quality data adjusted for

annual precipitation, water quality data adjusted for annual runoff, and water quality data adjusted for

annual average air temperature (during sampling season). TSI analysis was included in this analysis.

Available data from the 1997 to 2012 period were used for each monitoring station (26 Lake

Minnetonka bays, 11 Upper Watershed lakes; refer to Table 3). In addition to analysis of SECC, CHLA,

and TP, the trophic state index (TSI) was calculated as:

TSISECC = 60 – (14.41*(LN(SECC)))

TSICHLA = (9.81*(LN(CHLA))) + 30.6

TSITP = (14.42*(LN(TP))) + 4.15

TSI = (TSISECC + TSICHLA + TSITP)/3

where SECC, CHLA, and TP represent the mean summer values for a given year and location (Carlson,

R.E. 1977. A trophic state index for lakes. Limnology and Oceanography 22:361-369).

MCWD staff calculation of annual runoff to Lake Minnetonka was based on lake levels and the amount

of discharge passing through the dam at Grays Bay. Based on dam operation records, the dam was

typically opened in April and closed in September, although exceptions exist (note: dam closed entire

year in 2000). Hence the period of April through September was chosen when adjusting the lake water

quality data for runoff, precipitation, and air temperature. For example, monthly precipitation totals

from the MSP airport data were averaged for each year. For air temperature, monthly averages were

based on MSP daily averages, and these monthly values were averaged over the April through

September period for each year. Values for runoff, precipitation, and air temperature are presented in

Table 11. Runoff, precipitation, and air temperature were used as weighting variables in addition to

analyzing the raw data.

Upper Watershed Lakes

Analysis of the Christmas Lake raw data showed statistically-significant decreases in CHLA, TP, and TSI

(Table 12), due mainly to higher values during the 1997-2000 period (see Appendix). Weighting added

little improvement because correlation coefficients did not greatly increase and p-values did not greatly

decrease. Because of the relatively higher values during the 1997-2000 period, analysis of data from

2001-2012 was also performed; this analysis yielded no statistically-significant changes, suggesting that

the lake may have entered a new type of equilibrium beginning in 2001.

Dutch Lake showed a statistically-significant decrease in SECC when weighted for precipitation, but

again this was mainly due to higher values during the 1997-2000 period (see Appendix). Analysis of data

from 2001-2012 yielded statistically-significant increases in SECC and decreases in CHLA, TP, and TSI,

Page 29: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

27

Table 11. Average Annual Runoff, Precipitation, and Air Temperatures

suggesting that a change between 2000 and 2001 may have put Dutch Lake onto a path of improving

water quality.

Gleason Lake showed a statistically-significant decrease in TP, while Langdon Lake showed statistically-

significant decreases in CHLA and TSI.

Long Lake showed a statistically-significant decrease in SECC and TSI when weighted for runoff, but

again this was mainly due to higher values during the 1997-2000 period (see Appendix). Analysis of data

from 2001-2012 yielded statistically-significant increases in SECC and decreases in CHLA and TSI,

suggesting that a change between 2000 and 2001 may have put Long Lake onto a path of improving

water quality.

Lake Minnewashta showed a statistically-significant decrease in SECC when weighted, while Parley Lake

showed no statistically-significant changes. Piersons Lake showed a statistically-significant increase in

SECC and decreases in CHLA, TP, and TSI. Schutz Lake showed a statistically-significant increase in CHLA

when weighted for runoff, while Lake Virginia showed statistically-significant increases in CHLA and TSI.

Wasserman Lake yielded no statistically-significant changes.

Lake Minnetonka Bays

There were statistically-significant decreases in SECC readings for Halsted Bay, and statistically-

significant increases in SECC readings for Peavey Lake and Wayzata Bay (Table 13). Black Lake, Carsons

Bay, Crystal Bay, Grays Bay, Peavey Lake, Phelps Bay, Priests Bay, Smithtown Bay, Tanager Lake, and

Wayzata Bay had no statistically-significant changes for CHLA, while the remaining Lake Minnetonka

bays had statistically-significant increases in CHLA.

Runoff Precip Air Temp

Year (in) (in) (F)

1997 9.60 4.70 61.43

1998 4.10 3.74 64.97

1999 4.50 3.93 64.02

2000 0.00 3.61 63.32

2001 5.80 4.30 64.70

2002 11.70 5.29 64.13

2003 3.20 3.10 64.28

2004 4.00 3.38 62.97

2005 4.20 3.65 66.10

2006 6.60 3.51 66.27

2007 2.58 3.97 66.00

2008 2.96 2.60 63.45

2009 0.25 2.34 63.67

2010 3.68 4.09 66.38

2011 8.85 3.46 64.90

2012 0.86 3.76 67.02

Page 30: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

28

Carman Bay, Cooks Bay, Crystal Bay, Forest Lake, Jennings Bay, Lafayette Bay, Lower Lake North, Lower

Lake South, Maxwell Bay, North Arm, Priests Bay, Smithtown Bay, Spring Park Bay, St. Albans Bay,

Tanager Lake, Wayzata Bay, West Arm, and West Upper Lake had no statistically-significant changes for

TP (Table 13). The remaining Lake Minnetonka bays had statistically-significant decreases in TP with the

exception of Peavey Lake which had a statistically-significant increase in TP.

Black Lake, Carsons Bay, Cooks Bay, Crystal Bay, Grays Bay, Halsted Bay, Harrisons Bay, Lafayette Bay,

Lower Lake North, Lower Lake South, Maxwell Bay, Priests Bay, Smithtown Bay, St. Albans Bay, Stubbs

Bay, Tanager Lake, Wayzata Bay, and West Upper Lake had no statistically-significant changes for TSI

(Table 13). The remaining Lake Minnetonka bays had statistically-significant increases in TSI with the

exception of Phelps Bay which had a statistically-significant decrease in TSI.

Page 31: Lake Data Statistical Analysis Report...2. To perform trend analysis to determine statistically-significant improvement or degradation of water quality at the sampling stations. SPSS

29

Table 12. Trend Analysis for Raw and Weighted Data, Upper Watershed Lakes

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level.

Lake Weighting? SECC CHLA TP TSI SECC CHLA TP TSI

Raw ns ** * * ns ns ns ns

Runoff ns ** * ns ns ns ns ns

Precip ns ** * ns ns ns ns ns

AirTemp ns ** * ns ns ns ns ns

Raw ns ns ns ns ns ** * **

Runoff ns ns ns ns ** ** ** **

Precip * ns ns ns ns ** * **

AirTemp ns ns ns ns ns ** * **

Raw ns ns ** ns

Runoff ns ns * ns

Precip ns ns ** ns

AirTemp ns ns ** ns

Raw ns ns ns ns

Runoff ns ** ns **

Precip ns * ns ns

AirTemp ns ns ns ns

Raw ns ns ns ns ** ** ns **

Runoff * ns ns * ** ** ns **

Precip ns ns ns * ** ** ns **

AirTemp ns ns ns ns ** ** ns **

Raw ns ns ns ns

Runoff ** ns ns ns

Precip ** ns ns ns

AirTemp * ns ns ns

Raw ns ns ns ns

Runoff ns ns ns ns

Precip ns ns ns ns

AirTemp ns ns ns ns

Raw ** ** ** **

Runoff ** ** ** ** degraded water quality

Precip ** ** ** **

AirTemp ** ** ** ** improved water quality

Raw ns ns ns ns

Runoff ns ** ns ns

Precip ns ns ns ns

AirTemp ns ns ns ns

Raw ns ** ns *

Runoff ns ** ns ns

Precip ns ** ns *

AirTemp ns ** ns *

Raw ns ns ns ns

Runoff ns ns ns ns

Precip ns ns ns ns

AirTemp ns ns ns ns

Parley

Piersons

Schutz

Virginia

Wasserman

Dutch

Gleason

Langdon

Long

Minnewashta

Christmas

1997-2012 Data 2001-2012 Data

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Table 13. 1997-2012 Trend Analysis for Raw and Weighted Data, Lake Minnetonka Bays

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level; blue: improved water quality, orange:

degraded water quality

Lake or Bay Weighting? SECC CHLA TP TSI Lake or Bay Weighting? SECC CHLA TP TSI

Raw ns ns ns ns Raw ns * ns ns

Runoff ns ns ** ns Runoff ns ns ns ns

Precip ns ns ns ns Precip ns ** ns ns

AirTemp ns ns ns ns AirTemp ns ** ns ns

Raw ns ns ns ns Raw ns ** ns *

Runoff ns ** ns * Runoff ns ** ns ns

Precip ns ns ns ns Precip ns ** ns **

AirTemp ns ns ns ns AirTemp ns ** ns *

Raw ns ns * ns Raw ns ns * ns

Runoff ns ns * ns Runoff * ns * ns

Precip ns ns * ns Precip ns ns ** *

AirTemp ns ns * ns AirTemp ns ns ** ns

Raw ns ** ns ns Raw ns ns ns ns

Runoff ns * ns ns Runoff ns ns ** *

Precip ns ** ns ns Precip ns ns ns ns

AirTemp ns ** ns ns AirTemp ns ns ns ns

Raw ns ns ns ns Raw ns ns ns ns

Runoff ns ns ns ns Runoff ns ns ns ns

Precip ns ns ns ns Precip ns ns ns ns

AirTemp ns ns ns ns AirTemp ns ns ns ns

Raw ns ** ns ns Raw ns ns ns ns

Runoff ns ** ns ns Runoff ns ns ns ns

Precip ns ** ns * Precip ns ns ns ns

AirTemp ns ** ns ns AirTemp ns ns ns ns

Raw ns ns ns ns Raw ns ** ns ns

Runoff ns ns * ns Runoff ns ** ns *

Precip ns ns ns ns Precip ns ** ns ns

AirTemp ns ns ns ns AirTemp ns ** ns ns

Raw ** ** ** ns Raw ns ns ns ns

Runoff ** ** ** ns Runoff ns ** ns ns

Precip ** ** ** ns Precip ns ns ns ns

AirTemp ** ** ** ns AirTemp ns ns ns ns

Raw ns ** ns ns Raw ns ** ns ns

Runoff ns ** * ns Runoff ns ** ns ns

Precip ns ** ns ns Precip ns ** ns ns

AirTemp ns ** ns ns AirTemp ns ** * ns

Raw ns ** ns * Raw ns ns ns ns

Runoff ns ** ns ns Runoff ns ns ns ns

Precip ns ** ns * Precip ns ns ns ns

AirTemp ns ** ns * AirTemp ns ns ns ns

Raw ns ns ns ns Raw * ns ns ns

Runoff ns * ns ns Runoff ns ns ns ns

Precip ns ns ns ns Precip * ns ns ns

AirTemp ns ns ns ns AirTemp * ns ns ns

Raw ns ns ns ns Raw ns ** ns **

Runoff ns ** ns ns Runoff ns ** ns ns

Precip ns ns ns ns Precip ns ** ns **

AirTemp ns ns ns ns AirTemp ns ** ns **

Raw ns ns ns ns Raw ns ** ns ns

Runoff ns ** ns ns Runoff ns ** ns ns

Precip ns ns ns ns Precip ns ** ns ns

AirTemp ns ns ns ns AirTemp ns ** ns ns

Lafayette

Bay

Wayzata

Bay

Lower Lake

NorthWest Arm

Lower Lake

SouthWest Upper

Halsted BaySt. Albans

Bay

Harrisons

BayStubbs Bay

Jennings

Bay

Tanager

Lake

Crystal Bay Priests Bay

Forest LakeSmithtown

Bay

Grays BaySpring Park

Bay

Carman

BayNorth Arm

Carsons

BayPeavey Lake

Cooks Bay Phelps Bay

Black Lake Maxwell Bay

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Finally, there are five Lake Minnetonka bays that have 3 to 4 summer sampling events during the 1979-

1986 period. A comparison of this eight year period with the most recent eight year period (2005-2012)

was done to gain some insight into potential long-term trends in water quality in Lake Minnetonka.

Statistically-significant increases in SECC were noted for Lower Lake South (from 1.9 to 3.4 m), Wayzata

Bay (from 1.9 to 3.6 m), and West Upper Lake (from 1.7 to 2.2 m)(Table 14). There was a statistically-

significant increase in CHLA for Peavey Lake (from 11 to 29 ppb). Statistically-significant decreases in TP

were noted for Jennings Bay (from 179 to 102 ppb), Lower Lake South (from 52 to 20 ppb), Peavey Lake

(from 171 to 83 ppb), Wayzata Bay (from 50 to 22 ppb), and West Upper Lake (from 59 to 28 ppb).

Statistically-significant decreases in TSI were noted for Lower Lake South (from 53 to 45) and Wayzata

Bay (from 52 to 45).

Table 14. 1979-1986 Means vs 2005-2012 Means for Five Lake Minnetonka Stations

ns: not significant at 0.10 level; *: significant at 0.10 level; **: significant at 0.05 level; blue: improved water quality, orange:

degraded water quality

Lake or Bay SECC CHLA TP TSI

Jennings ns ns ** ns

LLSouth ** ns ** **

Peavey ns ** * ns

Wayzata ** ns ** **

West Upper ** ns ** ns

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V. Discussion and Conclusions

Spatial Sampling Frequency

The statistical analyses in this report indicate that several Lake Minnetonka bays do not show

statistically-significant differences to adjacent bays, suggesting that select adjacent bays are interacting

hydraulically to the point that their water quality is quite similar. Given these results, MCWD should

consider reducing sampling frequency (e.g. once every three years) for the following bays: Smithtown

Bay, Phelps Bay, Carman Bay, Harrisons Bay, Forest Lake, Lower Lake North, and North Arm. Continuing

these bays on a three-year schedule is important to assess any highly-localized changes that might

occur.

Although Jennings Bay did not show many statistically-significant differences to adjacent bays in regards

to SECC and CHLA, Jennings Bay did show statistically-significant difference with respect to TP. Because

reduction in watershed TP inputs is a major goal of MCWD, and Jennings Bay is the receiving waterbody

for Painters Creek, it is advised to continue sampling Jennings Bay on an annual basis.

While Wayzata Bay did not show many statistically-significant differences in comparisons to Lower Lake

North, Wayzata Bay has one of the longest records of water quality in Lake Minnetonka which justifies

continuing to sample Wayzata Bay on an annual basis.

The Southeastern area bays in general have very high water quality, meaning very low CHLA and TP

readings along with relatively high SECC values. Although there were few statistically-significant

differences among these bays, the reason is more likely due to standard deviations being of similar

magnitude to the means, and not due to a strong hydraulic connection to adjacent bays. Sampling in all

these bays should be continued.

Temporal Sampling Frequency

Based on the statistical analyses presented in this report, there is no benefit to sampling twice monthly

with respect to summer SECC, CHLA, and TP means, and MCWD should consider reducing sampling

frequency from twice monthly to once monthly for all lakes and bays. While there were several

instances of statistically-significant differences between “first half of the month” versus “second half of

the month” sampling, their occurrences were inconsistent in the data. Hence, no preference should be

given to time of the month that a bay or lake is sampled, but it is recommended that MCWD commit to

sampling a lake or bay at the same time of month (e.g. always sample Lake X during the first half of the

month). This is recommended for consistency and will aid MCWD staff in scheduling summer fieldwork.

Increasing sampling frequency (e.g. weekly) would not reduce the relatively large variance seen for all

variables enough to result in statistically-significant differences. Although averages may appear far

enough apart, the relatively large variations (i.e. standard deviations, not presented) compared to

average values makes it unlikely that a statistically-significant difference in averages is occurring.

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Trend Analysis

Overall, the trend analysis shows a mixture of increasing, decreasing, and stable water quality trends

through time. Several interesting patterns are noted below.

Upper Watershed Lakes

Review of the statistical analysis (Table 12) and the plotted data (see Appendix) suggests that several

lakes have remained relatively stable with respect to long-term water quality. These lakes include

Christmas Lake, Lake Minnewashta, Parley Lake, Schutz Lake, and Wasserman Lake. Lakes showing

trends of improving water quality include Dutch Lake, Gleason Lake, Langdon Lake, Long Lake, and

Piersons Lake. The lake showing the most evident trend of decreasing water quality is Lake Virginia.

Lake Minnetonka

There were few instances of statistically-significant changes in SECC, with decreases noted for Halsted

Bay and increases for Peavey Lake and Wayzata Bay (Table 13). Standard deviations for SECC were

relatively comparable in magnitude to their means, making it difficult to show statistically-significant

changes.

Statistically-significant increases in CHLA were noted in several bays, notably the Southwestern area

bays (Halsted Bay, Cooks Bay, West Upper Lake), the South Central area bays (Carman Bay, Spring Park

Bay), the Northwestern area bays (Jennings Bay, Harrisons Bay, West Arm, Forest Lake), and the North

Central area bays (Stubbs Bay, Maxwell Bay, North Arm).

Several of the bays showing statistically-significant CHLA increases have relatively low CHLA (< 10 ppb);

these bays include Carman Bay, Lafayette Bay, Lower Lake North, Lower Lake South, Spring Park Bay, St.

Albans Bay, and West Upper Lake. When bays with very low CHLA show small, statistically-significant

increases, this may represent more long-term systemic changes in the Lake Minnetonka system and

should continue to be monitored.

Improving water quality with respect to TP was demonstrated with statistically-significant decreases for

Black Lake, Carsons Bay, Grays Bay, Halsted Bay, Harrisons Bay, Phelps Bay, and Stubbs Bay. A

statistically-significant increase in TP was noted for Peavey Lake. Any statistically-significant changes in

TSI were associated with comparable changes in either CHLA or TP.

A comparison of early 1980s water quality to recent water quality (Table 14) for five Lake Minnetonka

Stations shows marked increases in water quality, in both high quality bays (e.g. Wayzata Bay) and more

eutrophic bays (e.g. Jennings Bay). These results are encouraging, and may be due to both natural long-

term lake cycles and human actions.

Future Directions

The results of this study provide an initial look into the complexity of Lake Minnetonka and Upper

Watershed Lakes dynamics. Additional understanding of how these lakes and bays are changing or not

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changing could be found by performing trend analysis on a monthly basis (e.g. Lower Lake South, June

averages through the years). Finally, there is some potentially unusual behavior taking place in the Lake

Minnetonka systems associated with poor water quality (Halsted Bay, Jennings Bay, and Stubbs Bay).

While the trend analyses (Table 13) and graphical analyses (see Appendix) suggest that TP in decreasing

in these bays, CHLA is increasing in these bays and the bays in close proximity. This is counter-intuitive,

and additional statistical investigation is warranted.