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i
Lake Data Statistical Analysis Report
Prepared for
Minnehaha Creek Watershed District
by
HDR Engineering, Inc.
August 2013
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
1
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/
2
Figure 1. Lake Minnetonka Water Quality Monitoring Stations*
*from MCWD 2011 Hydrologic Data Monitoring Report
3
Figure 2. Upper Watershed Lakes Water Quality Monitoring Stations*
*from MCWD 2011 Hydrologic Data Monitoring Report
4
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
5
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
6
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.
7
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
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
9
Table 3. Sample Sizes for Upper Watershed Lakes (June through September)
10
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.
11
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.
12
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
13
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
14
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
15
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
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
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
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
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
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
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
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.
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
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
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
26
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,
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
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.
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
30
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
31
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
34
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.