Interrelationships Among Water Quality, Lake Morphometry, Rooted ...

Interrelationships Among Water Quality, Lake Morphometry, Rooted Plants and Related Factors for Selected Shallow Lakes of West-Central Minnesota Part of a Series on Minnesota Lake Water Quality Assessment

March 2005

Interrelationships Among Water Quality, Lake Morphometry, Rooted Plants and Related Factors for Selected Shallow Lakes of West-Central Minnesota Part of a Series on Minnesota Lake Water Quality Assessment Steven Heiskary and Matt Lindon

Minnesota Pollution Control Agency Environmental Analysis & Outcomes Division

March 2005

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Acknowledgments

Study design – This study was a collaboration among MPCA staff (Steve Heiskary, Matt Lindon, Mark Gernes, and Howard Markus; MDNR staff (Donna Perleberg and Nicole Hansel-Welch) and Science Museum of Minnesota (Mark Edlund) Report contributors - Portions of the document were drawn from analyses and reports from the following persons who collaborated on the production of this report: Lauren Anderson - MPCA student intern – data analyses, charts; Nicole Hansel-Welch – Wildlife Section, Minnesota Department of Natural Resources & Donna Perleberg – Ecological Services Section, Minnesota Department of Natural Resources Assessment of aquatic plant communities of study lakes, preparation of related statistics and general lake-specific plant community descriptions; Dr. Mark Edlund, St. Croix Research Station, Science Museum of Minnesota – diatom reconstruction for study lakes; Manuscript Review John Barten – Three Rivers Park District, Hennepin County Mark Gernes – MPCA, Environmental Analysis and Outcomes Division Doug Hall - MPCA Environmental Analysis and Outcomes Division Nicole Hansel-Welch - MDNR Dr. Howard Markus – MPCA Environmental Analysis and Outcomes Division Donna Perleberg – MDNR Dr. David Wright – MDNR Ecological Services

Word Processing – Jan Eckart Funding: This project was funded in part by an U.S. Environmental Protection Agency Nutrient Criteria Development grant. The study represents a collaborative effort among the Minnesota Pollution Control Agency, Minnesota Department of Natural Resources and Science Museum of Minnesota.

Table of Contents Page List of Tables .........................................................................................................................ii List of Figures ........................................................................................................................ii Executive Summary and Recommendations………………………………………………...iii Introduction............................................................................................................................1 Background ............................................................................................................................4 Precipitation ..................................................................................................................9 Lake levels ....................................................................................................................10 Methods.........................................................................................................................13 Results...........................................................................................................................15 Lake-specific summaries for 2003 lakes................................................................................20 Comparisons among lakes for 2003.......................................................................................82 Total phosphorus, chlorophyll-a and Secchi relationships ....................................................88 Association between submerged macrophytes and water quality..........................................103 Sediment Diatom reconstruction – statewide and shallow lakes studies...............................112 Discussion ..............................................................................................................................115 Deriving nutrient criteria for shallow lakes ...............................................................115 References..............................................................................................................................123 Appendix ...............................................................................................................................129

i

List of Tables Page 1. 2. 3. 4a 4b 5. 6.

Lake morphometry and watershed data ..............................................................................6 Ecoregion percentile distributions based on assessed lakes for 2004.................................8 Ecoregion reference lake typical range for summer mean water quality............................8 Summer-mean water quality measurements for 2003 ........................................................16 Summer-mean, minimum and maximum measurements for field parameters for 2003 ....18 Comparison of observed vs. predicted TP, chlorophyll-a and Secchi for CHF lakes….. ..98 Summary of morphometric and water quality characteristics for FQI classes ...................117

List of Figures 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

Total phosphorus concentration as a function by lake mixing type & ecoregion ...........5 Location of study lakes overlain on ecoregion map........................................................7 Water Year Precipitation: 2003.......................................................................................9 Lake level measurement for selected lakes .....................................................................11 Total phosphorus concentrations for summer 2003 ........................................................84 Comparisons of a) TKN + TP and b) TN:TP ratios and TP for 2003 study lake............86 Chlorophyll-a concentrations for summer 2003..............................................................87 Monthly mean total phosphorus and chlorophyll-a for west central lakes for 2003.......89 Carlson’s TSI...................................................................................................................90 Summer-mean TP vs. chlorophyll-a (log-log) and Secchi a) all west-central lakes (n=31 lakes) compared to statewide and b) CHF WC lakes only (n=19) compared to SW lakes (2002 study).................................................................................................91 Summer-mean TKN versus chlorophyll-a for west-central lakes:2003..........................92 Bloom frequency relative to TP for west-central lakes...................................................93 Summer-mean Secchi versus total phosphorus and chlorophyll-a for west-central lakes .......................................................................................................95 Observed a) TP, b) chlorophyll-a and c) Secchi versus predicted for CHF lakes...........96 Summary of late-summer zooplankton populations........................................................99 Floristic Quality Index (FQI) values for west-central lakes............................................104 Number of submerged and floating-leaf plants species for west-central lakes...............104 Number of plant species (a) & FQI (b) relative to total phosphorus..............................105 Number of plant species (a) & FQI (b) relative to TKN .................................................106 Number of a) plant species and b) FQI relative to TN:TP ratio......................................108 Submergent and floating-leaf plants relative to a) Secchi and b) maximum depth of native plant colonization relative to Secchi transparency..................................110 Number of plant species relative to TP, chlorophyll-a and Secchi .................................111 FQI relative to total phosphorus and chlorophyll-a ........................................................112 Comparison of diatom-inferred pre-European and modern-day TP for a) west-central shallow lakes and b) among ecoregions .................................................114 Summer-mean TP distribution (IQ range) for CHF ecoregion .......................................119 Summer-mean Chlorophyll-a distribution (IQ range) for CHF ecoregion .....................119 Summer-mean Secchi distribution (IQ range) for CHF ecoregion .................................120 ii

Executive Summary and Recommendations The Minnesota Pollution Control Agency (MPCA) is developing eutrophication standards for Minnesota lakes and reservoirs. This effort is in response to a U.S. Environmental Protection Agency (EPA) requirement that states develop nutrient criteria for lakes, rivers, wetlands and estuaries. In the course of work on ecoregion reference lakes, assessment of statewide data sets for 305(b), and the development of guidance for listing of nutrient-impaired lakes on Minnesota’s 303(d) list it was apparent that there are some distinct differences in trophic status and potentials of shallow, well-mixed lakes as compared to deeper, stratified lakes. Previous work documented, for instance, the distinct difference in total phosphorus concentrations among dimictic, intermittently mixed, and well-mixed lakes (Fig. 1; Heiskary and Wilson, 1988). From this work it was evident that differences were particularly marked among deep and shallow lakes of the North Central Hardwoods Forests (CHF) and Western Corn Belt Plains (WCP) ecoregions. Also during public comment periods and hearings associated with the establishment of guidance for the listing of nutrient-impaired waters, concerns were expressed that swimming may not be the primary use in many of the states shallow lakes. Among their contentions was shallowness of the lakes, highly organic substrates, and often times over-abundance of rooted submergent and emergent plants. Because of these factors it was recommended that the MPCA consider separate nutrient criteria for shallow lakes that would take these factors into account. This would result in criteria that were more closely attuned to the actual uses of these shallow lakes, which are commonly boating, fishing, aesthetics, wading and waterfowl production, rather than an emphasis on swimming (primary body contact). The current study of shallow lakes in west central Minnesota built on a previous effort that focused on shallow lakes of southwestern Minnesota (Heiskary et al. 2003). While that study was valuable for characterizing water quality patterns among lakes in that portion of the state it provided minimal insight into relationships among nutrients and rooted plants since most of the lakes were highly eutrophic and had very minimal submerged plant populations. However, that study did provide insights into pre-European trophic status of shallow lakes in that part of the state. These concerns led us to seek a Nutrient Criteria Grant through the USEPA. The overall project, which involved water quality sampling, assessment of submerged vegetation, collection of lake sediment cores for selected lakes in west central Minnesota was a collaborative effort among the MPCA, MDNR and Science Museum of Minnesota. Of nine lake-sediment cores taken in 2004, from the west-central lakes, diatoms were adequately preserved or in sufficient numbers to allow for estimates of pre-European TP in seven lakes (Fig. 24a). Of these, six are from the CHF ecoregion and one, Red Sand, is from the NLF ecoregion. Based on this comparison CHF pre-European TP ranged from a low of 27 µg/L in Fremont Lake to 51 µg/L in Quamba Lake (Fig. 24a). The average change (modernday TP minus pre-European) was 60 µg/L (184% increase) and ranged from 20 µg/L (50%) in Johanna up to 132 µg/L (469 %) in Silver Lake. When diatom-inferred TP from these six lakes was compared to those from deeper CHF lakes (Fig. 24b) it is evident that pre-European TP was higher in the shallow lakes and the magnitude of change from pre-European to modernday was greater as well.

iii

This study did not develop a predictive model; rather we characterized linkages among nutrient concentration, algal abundance and composition, macrophyte (submergent and floating-leaf) composition and coverage, fishery composition and management and related factors based on a set of representative shallow lakes from across west central Minnesota. These linkages combined with region-wide patterns in lake trophic status (both pre-European and modernday), user perception and literature review, provide a basis for establishing nutrient criteria to protect uses such as secondary contact (boating and aesthetics) and fish and waterfowl habitat. In summary, based on the various interrelationships among trophic status variables, rooted plant metrics and other considerations it appears that appropriate ranges for selecting eutrophication criteria values for shallow lakes in the CHF ecoregion are: • Secchi transparency - greater than 0.7 to 1.0 meters; • Chlorophyll-a - less than 20 – 30 µg/L; • Total phosphorus – less than 60 – 80 µg/L; Given this range of values, and acknowledging that other biotic and abiotic factors can be very significant in determining whether a lake can support a healthy and diverse population of rooted macrophytes, we are inclined to recommend criteria be set at the lower end of each range of the aforementioned values, i.e. maintain summer average Secchi of 1.0 m or greater, summer average chlorophyll-a of 20 µg/L or lower, and summer average total phosphorus of 60 µg/L or lower. While we are not offering nitrogen criteria at this time, it would appear to be beneficial to keep TKN below 2.0 mg/L when possible. Based on the relationship between TP and TKN, maintaining TP below 60-80 µg/L should yield TKN 30% of the time in summer. Scheffer (1998) also describes the feedback effects of macrophytes on turbidity whereby as plant biomasss increases sediment resuspension declines resulting in lower turbidity causing hysterisis in plant mass – water turbidity relationships. Moss (1998) notes further that macrophytes store nutrients (luxury uptake) which prevents algal growth, provide zooplankton refuge from predators which increases algal grazing, may also produce allelopathic compounds which inhibit algal growth, and stabilize the sediment which lessens resuspension of nutrients. Radomski and Goeman (2001) describe multiple benefits of macrophytes to the overall ecology of the lake with a particular emphasis on fish spawning and habitat and demonstrated that shoreline areas with diverse submergent vegetation exhibited significantly more fish than shorelines which had little or no vegetation. Bird use of shallow lakes is often correlated with the abundance of aquatic macrophytes (Mitchell and Perrow, 1998). Changes in waterfowl use have been correlated with presence or absence of aquatic macrophytes in many shallow lakes, including Lake Christina in Minnesota (Hanson and Butler 1994). Many species of aquatic vegetation are consumed by waterfowl. Certain species of Potamogeton are especially important food sources for migrating waterfowl. Submerged aquatic vegetation usually supports a wide array of invertebrates that are consumed 2

by waterfowl. These invertebrates are especially important protein sources for female and juvenile waterfowl. Emergent vegetation such as wild rice can also provide brood cover for waterfowl in addition to being a food source for migratory waterfowl. Aquatic vegetation also provides habitat for other types of aquatic birds such as grebes which nest in dense stands of bulrush. Numerous attempts have been made to model factors that contribute to macrophytes growth. Herb and Stefan (2003) summarize several of these but note that “…however there is a lack of relationships that characterize the response of macrophyte growth to varying physical conditions.” They go on to present a growth model that relates the rate of production of rooted aquatic macrophytes to basic physiological parameters (growth and respiration rates) and controlling physical parameters (incident irradiance, water temperature, light attenuation by water and phytoplankton). While this work is quite valuable it does not offer specific eutrophication-related thresholds that can be translated into nutrient criteria for shallow lakes. Our study does not seek to develop a predictive model; rather we will characterize linkages among nutrient concentration, algal abundance and composition, macrophyte (submergent and floating-leaf) abundance and composition, fishery composition and management and related factors based on a set of representative shallow lakes from across west central Minnesota. These linkages combined with region-wide patterns in lake trophic status (both pre-European and modern-day), user perception and literature review, will provide a basis for establishing nutrient criteria to protect uses such as secondary contact (boating and aesthetics) and fish and waterfowl habitat. A general description of the study that was conducted in conjunction with the Minnesota Department of Natural Resources and Science Museum of Minnesota follows: a) Sampled 31 lakes monthly for water quality during the summer of 2003; b) Collected phytoplankton and zooplankton for qualitative and semi-quantitative measurement; c) Assessed macrophyte (submergent and floating-leaf primarily) composition and distribution were based on existing data or current field assessments by MDNR; d) Compiled fishery composition and management from MDNR records; e) Estimated watershed areas based on USGS web-based minor watershed maps; f) Collected surface sediment samples from all lakes and deep (long) cores from eight lakes to help document pre-European condition. This report will: • Provide lake-specific descriptions of trophic status and plant communities for shallow west-central Minnesota lakes based on the aforementioned monitoring. • Examine interrelationships among trophic status variables and plant metrics; • Describe pre-European trophic status based on diatom reconstruction from these lakes; • Combine data from this study with that from the prior study of shallow southwest Minnesota lakes and propose potential nutrient criteria ranges for shallow lakes in the CHF, WCP and NGP ecoregions.

3

Background Lake condition has been described for four of the seven Minnesota ecoregions, based on data from a set of reference lakes and a review of the overall data (water quality, watershed and morphometric) for the larger population of lakes in each region (MPCA, 2004). The reference lakes were deemed to be representative of the region they were located in and minimally impacted by human activities. In some ecoregions, such as the Northern Lakes and Forests (NLF) this was much easier to accomplish than it was for regions such as the Western Corn Belt Plains (WCP) or Northern Glaciated Plains (NGP) where agriculture may comprise over 70-80 percent of the watershed landuse – even for the reference lakes. This approach has provided a basis for comparing lake condition within and across ecoregions. As is evident from land use data from the reference lakes (Heiskary and Wilson, 1988) and ecoregions as a whole, cultivated land use predominates across both regions. This is followed by wetland/marsh land use and pasture/grass land uses. The percentage of pasture and grass uses was slightly higher in the NGP as compared to the WCP ecoregion. While specific data on landuses was not compiled for lakes in this study field observation suggests that land use in these watersheds is quite typical for the given ecoregion (Fig. 2). As a part of the current (2005) rulemaking effort various definitions are included in the rule language. One definition that is pertinent to this study is “shallow lakes” that is defined as follows: “Shallow lakes are lakes with a maximum depth of 15 feet or less or where 80% or more of the lake is 15 feet or less in depth. Shallow lakes are not generally considered wetlands which are already defined in rule.” As such most of the lakes in this study fit that category, although a few are deeper than 15 feet (Table 1). The lakes in this study were located in three ecoregions in west central Minnesota (Fig. 2). The majority of the lakes studied in 2003 are located in the North Central Hardwood Forests (CHF); however a few from the Western Corn Belt Plains (WCP), Northern Glaciated Plains (NGP), and Northern Lakes and Forests (NLF) were included as well to ensure that we obtained a range in lake trophic status. A previous study in 2002 focused primarily on shallow lakes in the NGP and a few in the WCP (Heiskary et al. 2003) and these data will be used in the discussion of regional patterns and considered as a part of criteria development for shallow lakes in those regions. Distinct differences in water quality among ecoregions (Table 3) have previously been documented (e.g., Heiskary and Wilson, 1988). These differences are typically a function of lake morphometry and watershed characteristics including landform, soil type and land use. The lakes of the WCP and NGP ecoregions are much more nutrient rich than those of the NLF and CHF ecoregions as is evident in both the assessed and reference lake data (Tables 2 and 3). These data will provide one basis for comparing the trophic status of lakes in this study to other lakes in the ecoregion. Within-region differences in summer-mean TP concentrations have also been described for each ecoregion based on the mixing (temperature stratification) status of lakes: dimictic (deep lake, fully mixes in spring and fall but remains stratified in summer); polymictic (shallow lake, remains well mixed from spring through fall); intermittent (lake with moderate depths, may stratify temporarily during summer, but may mix with strong wind action). The lack of stratification during the summer months contributes to frequent nutrient exchange between the sediments and the overlying water, thus internal nutrient loading in polymictic or intermittently stratified lakes can be high compared to deeper lakes and contribute to elevated epilimnetic phosphorus (illustrated in Fig. 1). Often even when nutrient loading is reduced in the watersheds, water quality in shallow lakes does not improve 4

for some time due to the internal nutrient loading (Scheffer, 1998 and Sondergaard et al. 1993). In this sense it is very important to protect these lakes from excessive loading because recovery can be a long and difficult process. In general differences in TP were modest among the dimictic, intermittent, and polymictic lakes of the NLF ecoregion but were quite marked in the other two regions (Fig. 1). This and other factors previously noted led us to concentrate most of our shallow lakes efforts in the CHF, WCP and NGP ecoregions (2002 and 2003 studies). Figure 1. Total phosphorus concentration as a function of lake mixing type and ecoregion. Based on assessed lake data (Heiskary and Wilson, 1988). Median TP by ecoregion & lake mixing type (Heiskary and Wilson, 1988)

160 140 120 TP ppb

100 80 60 40 20 0 Deep

Inter

NLF

CHF

5

Shallow

WCP

Table 1. Lake Morphometry and Watershed Data. Lake Name Platte Clark Red Sand Tiger Diamond French Prairie Quamba Ringo Florida Slough Johanna Nelson Fremont Silver Cedar Cedar McCormic Monson Trace Pelican Cedar Smith Jennie

Shaokotan West Twin East Twin Hattie Hollerberg Hassel East Solomon Titlow

Lake ID 18-0088 18-0374 18-0386 10-0108 27-0125 27-0127 27-0177 33-0015 34-0172 34-0204

County Crow Wing Crow Wing Crow Wing Carver Hennepin Hennepin Hennepin Kanabec Kandiyohi Kandiyohi

61-0006 61-0101 71-0016 72-0013 73-0226 73-0255 73-0273 76-0033 77-0009 86-0031 86-0073 86-0250 21-0323 41-0089 41-0102 41-0108 75-0200 76-0057 76-0086 34-0246 72-0042

Ecoregion NLF NLF NLF CHF CHF CHF CHF CHF CHF CHF

Area in Acres 1746 343 502 575 406 352 34 214 716 772

Depth Mean Ft. 10.0 15.0 7.0 3.0 6.0 2.0 3.0 6.0 5.0 2.5

Depth Max Ft. 23.0 31.0 23.0 8.0 8.0 3.0 6.0 11.0 10.0 4.0

Pope Pope Sherburne Sibley Stearns Stearns Stearns Swift Todd Wright Wright Wright Douglas Lincoln Lincoln Lincoln Stevens Swift Swift Kandiyohi

CHF CHF CHF CHF CHF CHF CHF CHF CHF CHF CHF CHF NGP NGP NGP NGP NGP NGP NGP WCP

1584 403 484 621 90 210 211 153 277 2793 147 226 316 995 216 215 477 260 706 706

7.0 6.0 7.0 4.5 20.0 5.0 7.0 12.0 6.0 5.0 15.0 3.0 3.0 7.0 2.3 2.4 6.0 3.5 4.0 9.5

12.0 9.0 10.0 9.0 36.0 8.0 12.0 21.0 8.5 9.0 47.0 5.0 5.0 12.0 4.4 4.5 9.0 5.0 5.0 13.0

6987 820 2458 3747 1428 1225 984 1164 672 7705 533 759 2113 8400 2026 2026 11662 3233 23335 12907

4 2 5 6 16 6 5 8 2 3 4 3 7 8 9 9 24 12 33 18

Sibley

WCP

924

2.0

4.0

35393

38

570 216

6.3 3.0

12.1 5.0

7813 1195

15 3

664

7.0

12.0

7705

12

Mean 25th percentile of study 75th percentile of study

Watershed Watershed Area Acres Lake Ratio 19547 11 12722 37 4550 9 4442 8 811 2 3712 3 92 3 23241 109 1471 2 42044 54

1. Lake area from MDNR bathymetric maps; mean and maximum depth based on maps and/or field measurements during water quality or plant surveys. 2. Based on USGS minor subwatershed maps at: http://gisdmnspl.cr.usgs.gov/.

6

Figure 2. Location of study lakes overlain on ecoregion map.

7

Table 2. Ecoregion Percentile Distributions based on Assessed Lakes for 2004. Ecoregion

Parameter

90

95

N

15

22

49

129

347

835

1,654

1,809

NLF & NMW Depth-max. (feet)

7

10

19

33

54

80

100

1,519

NLF & NMW TP ppb

7

9

13

21

30

45

58

863

NLF & NMW chlorophyll-a ppb

2

2

3

5

8

14

22

521

NLF & NMW Secchi (m)

0.9

1.2

1.8

2.8

4

5.1

5.9

1,394

CHF & RRV

Area (acres)

13

22

58

165

400

984

1,754

976

CHF & RRV

Depth-max. (feet)

6

8

16

28

46

68

82

829

CHF & RRV

TP ppb

15

18

28

51

112

229

351

691

CHF & RRV

chlorophyll-a ppb

3

4

8

21

45

89

131

622

CHF & RRV

Secchi (m)

0.4

0.5

1

1.6

2.6

3.5

4.2

968

WCP

Area (acres)

32

61

143

322

694 1,776

2,222

110

WCP

Depth-max. (feet)

4

6

7

10

16

25

33

87

WCP

TP ppb

54

62

99

159

234

404

609

89

WCP

chlorophyll-a ppb

11

14

32

50

83

125

173

79

WCP

Secchi (m)

0.2

0.3

0.4

0.6

1

1.5

2.2

109

NGP

Area (acres)

80

108

150

364

658 2,091

4,700

38

NGP

Depth-max. (feet)

4

5

8

10

15

18

25

28

NGP

TP ppb

46

54

104

148

194

396

405

30

NGP

chlorophyll-a ppb

4

9

25

36

52

64

66

27

NGP

Secchi (m)

0.3

0.4

0.5

0.7

1.6

1.9

2.1

37

NLF & NMW Area (acres)

5

10

25

50

75

Table 3. Ecoregion reference lake typical range for summer-mean water quality. Parameter Total Phosphorus (ug/l) Chlorophyll mean (ug/l) Chlorophyll max (ug/l) Secchi Disk (feet) (meters) Total Kjeldahl Nitrogen (mg/l) Nitrite + Nitrate-N (mg/l) Alkalinity (mg/l) Color (Pt-Co Units) pH (SU) Chloride (mg/l) Total Sus. Solids (mg/l) Total Sus. Inorganic (mg/l) Turbidity (NTU) Conductivity (umhos/cm) TN:TP ratio

Northern Lakes and Forests

North Central Hardwood Forests

Western Corn Belt Plains

Northern Glaciated Plains

14 – 27 4 – 10 < 15 8 – 15 (2.4 - 4.6) 0.4 – 0.75 2.0 mg/L had low FQI; however there was no distinct correlation between FQI and TKN for those lakes > 2.0 mg/L. Sagrario et al. (2004) noted, based on enclosure experiments, that a shift to a turbid state with low plant coverage occurred at TN >2.0 mg/L. They state further that this concurs with findings for Danish shallow lakes as well. In an attempt to better understand the relative roles of TP and TN we reviewed relationships between the number of plant species and FQI relative to TN:TP ratios (Fig. 20). No linear relationship was evident in the comparison of number of species and TN:TP ratios (Fig. 20). However, we note that all lakes with 15 or more plant species would be considered “P-limited.” Likewise all lakes with a high FQI were “P-limited as well (Fig. 20). All lakes with a medium FQI were either “P-limited” or were intermediate between P and N limitation. As noted previously, Fremont and Nelson Lakes tend to be “outliers” suggesting that factors other than nutrients contribute to the higher than expected number of species in Fremont and the lower than expected number of species in Nelson. We pursued similar comparisons with summer-mean Secchi transparency (Fig. 21a). In this case floating-leaf species were generally absent at summer-mean Secchi values of less than one meter. Further, the only lakes that supported 15 or more species were those with Secchi values of one meter or greater (Fig. 21a). However, as was the case with TP, a low TP and relatively high Secchi does not guarantee a diverse plant population. For example, Nelson Lake with a TP of 50 µg/L and Secchi of 1.2 m had only 6 species and a FQI of 13. However, Perleberg (2004) noted that lake level was rather high at the time of the plant survey (average depth about 11 feet as compared to an estimated maximum depth of 9 feet based on a 1997 MDNR fisheries survey). Increased depth and low transparency (2 feet, 0.6 m), at the time of the 2003 survey, likely 107

Figure 20. Number of a) plant species and b) FQI relative to TN:TP a) Number of plant species relative to TN:TP N-limited

P-lim ited

25

20 # of species

Fremont

15

10

5 Nelson

0 0

10

20

30

40

50

TN:TP

b) FQI vs. TN:TP ratio N-lim ited

P-lim ited

30 25 high Fremont

FQI

20

m edium

15 Nelson

low

10 5 Titlow Hattie

0 0

10

20

30

TN:TP

108

40

50

combined to limit the number of species and extent of plant growth in the lake. With respect to floating-leaf plants, spring transparency may play an important role in floating-leaf survival, since they are in a submerged state at that time. This might explain why lakes such as Fremont, Pelican and Titlow, with summer-mean Secchi of 1.1, 0.5 and 0.2 m each had floating-leaf plants (Fig. 21a); whereas lakes with similar Secchi values did not. Fremont, for example, had a maximum Secchi of 2.1 m (Table 4b) which would allow light to reach the bottom of the lake throughout the basin. Secchi transparency can also influence the depth to which rooted plants may be found in the lake. Canfield et al. (1985) and Chambers and Kalff (1985) offer equations as follows for predicting depth of submersed aquatic vegetation (SAV) colonization as a function of Secchi depth: Eq. (5) Log (y) = 0.79 log (Secchi) + 0.25 (Canfield et al. 1985) Eq. (6) Y0.5 = 1.33 log Secchi + 1.4 (Chambers and Kalff 1985) Eq. (7) Y = 1.6 Secchi + 1.10 (MPCA, based on 27 west-central lakes) As with these two studies a significant relationship was noted (R2=0.78) for the west-central MN lakes and a simple linear regression is offered (Fig. 21b). The Canfield et al. (1985) relationship was included on Fig. 21b as a basis for comparison. Based on Eq. 7, as Secchi falls below about 0.7 m the depth for rooted plant colonization will be limited to about 2 m (6.6 feet) or less in most lakes, which for many of the shallow lakes in this study could constitute an appreciable portion of the lake basin (Table 1). Charting the number of plant species (Fig. 22) and FQI (Fig. 23) relative to TP, Chlorophyll-a, and Secchi provides a further basis for examining patterns and relationships. In general, the total number of submerged and floating-leaf plant species tended to decline as TP increased and Secchi decreased (Fig. 22b). At TP concentrations less than 60-70 µg/L there were typically 10 or more species found in each lake; whereas as TP increased to about 90 µg/L or greater there were generally less than 10 species and in several instances five or fewer species were found (Fig. 22). Likewise, as Secchi fell below about 1 meter there were generally 10 or fewer species found in the lakes (Fig. 22b). Further, floating-leaf species were quite uncommon in lakes where TP was 90 µg/L or greater and Secchi was below one meter. As TP increases from about 60 to 90 µg/L chlorophyll-a often averages 30 µg/L or greater, a concentration typically characterized as severe nuisance blooms. Secchi generally declines below one meter as TP increases above 60 µg/L; however some variability is noted over the 60-80 µg/L range, which is often attributed to the blue-green algae that tend to dominate as TP and chlorophyll-a increase. The blue-greens often form colonies at the surface of the lake that allow for deeper than anticipated transparency. Based on Fig. 23 lakes with high FQI had TP less than 90 µg/L and chlorophyll-a less than 30 µg/L, with the exception of Fremont Lake. We believe several factors contribute to Fremont Lake’s high FQI. As noted in the vegetation survey the shallowness of the lake (~ 2 m mean depth) and relatively high summer Secchi (~1.4 m) allow SAV to grow over essentially the whole basin. Perleberg (2004) describes this as a lake in transition whereby native species, while yet present are represented by very small numbers and increasingly curly-leaf pondweed is becoming the dominant plant being found at 87% of the sample stations.

109

Figure 21. Submerged and floating-leaf plants relative to a) Secchi & b) maximum depth of native plant colonization relative to Secchi transparency. a) Submerged and floating-leaf plants sorted by Secchi

30

# of plants

25 20 15 10 5

3. 1

2. 3

1. 5

1. 4

1. 2

0. 9

0. 8

0. 8

0. 7

0. 5

0. 4

0. 4

0. 2

0. 2

0. 1

0

Secchi m # Submerged Species

# floating-leaf species

b) Mean Secchi vs. Max. Depth of Native Plant Colonization Depth (m ) = 1.60x + 1.10 R2 = 0.78

7.0

Plant depth m

6.0 5.0 4.0 3.0 2.0 1.0 0.0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Secchi m

MPCA

Canfield

110

Linear (MPCA)

3.5

4.0

Figure 22. Number of plant species relative to TP, chlorophyll-a, and Secchi. A “benchmark” of 15 plant species is noted. a) Number of rooted plant species vs. chlorophyll (algae). Based on 2003 data from west-central MN lakes. Sorted by TP

200

30

180 160

20

140

Nelson

Frem ont

120

Johanna

100

15

80

10

Chl-a(ppb)

# of species

25

60 40

5

20 0

0 10

24

32

40

60

90

90

90

110 140 160 180 210 230 320 370

TP ppb # plants

Chl-a

b) Num ber of rooted plant species vs. Secchi. Sorted by TP.

30

4.0 3.5

25 20

2.5

15

2.0 1.5

10

1.0 5

0.5

0

0.0 10

24

32

40

60

90

90

90

110

140

TP ppb # plants

111

Secchi

160

180

210

230

320

370

meters

# of species

3.0

Figure 23. FQI relative to TP and chlorophyll-a. Lines indicate high, medium and low FQI. Floristic Quality Index vs. chlorophyll-a

30

200 180

25 20

FQI

120 15

100 80

10

60 40

5

Chl-a ppb

160 140

20 0

0 10

24

32

40

60

90

90

90 110 140 160 180 210 230 320 370

TP ppb FQI

Chl-a

Sediment diatom reconstruction – statewide and shallow lakes studies Diatom reconstruction provides an opportunity to examine both temporal and spatial trends in lake trophic status and related factors. For Minnesota we can draw on three studies to examine these patterns: 1) a study of 55 lakes distributed across Minnesota (Heiskary and Swain, 2002); 2) a study of nine southwest Minnesota lakes (Heiskary et al. 2003); and 3) the current study of nine west-central Minnesota lakes. Ecoregion-based patterns in lake trophic status have long been recognized in Minnesota (e.g., Heiskary et al., 1987). Lakes in the forested Northern Lakes and Forests (NLF) ecoregion are moderately deep and exhibit relatively low TP while the shallow lakes in the highly agricultural Western Corn Belt Plains (WCP) and Northern Glaciated Plains (NGP) exhibit high TP concentrations. The transitional North Central Hardwood Forests (CHF), characterized by moderately deep lakes and a mosaic of land uses -- intermediate between these two extremes. We have also demonstrated differences in shallow versus deeper lakes within ecoregions – with differences being most pronounced in the CHF and WCP ecoregions (Fig. 1). Of nine cores from the west-central lakes in 2004, diatoms were adequately preserved or in sufficient numbers to allow for estimates of pre-European TP in seven lakes (Fig. 24a). Of these, five are from the CHF ecoregion and two, Red Sand and Platte, are from the NLF ecoregion. Based on this comparison CHF pre-European TP ranged from a low of 27 µg/L in Fremont Lake to 51 µg/L in Quamba Lake (Fig. 24a). The average change (modern-day minus pre-European) was 19 µg/L (51 % increase) and ranged from 8 µg/L (21 %) in Johanna up to 53 µg/L (189 %) in Silver Lake. Red Sand, in the NLF, exhibited a 2 µg/L (10%) increase in TP. 112

Distinct differences among regions were evident in comparisons of pre-European and modernday diatom-inferred TP concentrations. The NLF lakes were significantly lower in TP as compared to the CHF, WCP and NGP lakes based on a comparison of group-means plus or minus standard error (SE) (Fig. 24b). Based on SE (typically 2-3 µg/L or less) and overall range of TP concentrations the NLF lakes were somewhat less variable as compared to lakes in the other ecoregions. Variability was slightly higher in the CHF lakes and no significant difference was evident in a comparison of rural and Metro-area CHF lakes (Fig. 24b). The shallow WCP and NGP lakes were much more variable by comparison, with a SE of 10 ug/L. However, if we express this as a percent off the mean the SE for all regions is about 10-20 percent of the corresponding means for each region. For the NLF lakes, as a group, there was no significant difference in modern-day vs. preEuropean TP (Fig. 24b). However, distinct increases in TP were noted for the CHF lakes and these increases exceeded the “natural variability” noted in comparisons of pre-European (1750 and 1800) TP concentrations (Ramstack et al., 2004). They also note that the degree of change in TP among the Metro CHF lakes is significantly correlated with the percent of the watershed in urbanized landuse, while those in the rural portion exhibit a significant correlation with the percent of landuse in agricultural uses or inversely the percent in forested uses. The five “deeper” WCP lakes also did not change significantly across the two time periods based on this analysis and no significant associations with landuse were noted by Ramstack et al. (2004) – presumably because of the small sample size (5 lakes) and the predominately agricultural landuse in these watersheds. It is important to note that the five “deep” WCP lakes were among the most eutrophic of the original 55 lakes and as such were on the fringe of the model development data set. Future models that include all diatom study lakes (71 lakes, Fig. 24b) could result in modified pre-European values for the deep WCP lakes. Also, while diatominferred values did not differ greatly between pre-European and modern-day samples in the deep WCP lakes, modern-day water quality samples indicate that three of five lakes have much higher TP as compared to pre-European diatom-inferred TP. The shallow WCP & NGP lakes added in the recent study of SW Minnesota lakes exhibited a significant increase when pre-European and modern-day TP are compared (Fig. 24b). Agricultural landuse predominates in all SW MN study lakes. The shallow CHF lakes exhibited higher pre-European TP as compared to the deeper CHF (metro and rural) lakes (Fig. 24b). In comparison to the deeper, rural CHF lakes (that were also from west-central MN) pre-European TP for the shallow lakes was on average 10 µg/L (37%) higher; however the relative change (on average) from pre-European to modern-day TP was similar at 51 %. However when diatom-inferred values are compared to modern-day measured (observed) TP the difference is much more pronounced.

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Figure 24. Comparison of diatom-inferred pre-European and modern-day TP for a) westcentral shallow lakes and b) among ecoregions. a. West Central Shallow Lakes: pre-E, m odern dia-P & m od WQ

100 90 80

TP ppb

70 60 50 40 30 20 10 0 Quamba

Johanna

Fremont

Silver

Pre-E Dia-P

McCormic

Platte

Red Sand

Mod Dia-P

b. Diatom-inferred TP: Pre-European vs. Modern-day 140 120

TP ppb

100 80 60 40 20 0 NLF (n=20)

CHF-Metro (n=20)

CHF-Rural (n=15)

Pre-E

114

CHFShallow (n=5) Modern

WCP - Deep WCP/NGP (n=5) Shallow (n=6)

Discussion Deriving nutrient criteria for shallow lakes Deriving nutrient, chlorophyll-a and transparency thresholds relative to lake uses is an acceptable approach for developing eutrophication (nutrient) criteria according to USEPA (2000a). Several examples are presented in the guidance manual that consider a range of uses including coldwater fisheries, swimming, boating and other uses. Consideration of lake uses is also consistent with our original approach for setting ecoregion-based TP goals (Heiskary and Wilson, 1988). Thus, as we approach criteria development for shallow lakes (which have been defined as lakes with a maximum depth of 15 feet or less or where 80% or more of the lake is 15 feet or less) it is reasonable to consider the actual or potential uses of these lakes and how eutrophication criteria (TP, chlorophyll-a and Secchi) might be used to protect these uses. In the case of shallow lakes included in this current study a wide range of uses was evident including boating, fishing, hunting, and fish and waterfowl propagation and while swimming is a potential use it is most likely not the primary use of these and other lakes that share the characteristics of these lakes. It is evident that the value of shallow lakes for these activities declines when the lakes are dominated by phytoplankton and lack aquatic macrophytes. As noted initially, Moss et al. (1996) and numerous other researchers who have studied shallow lake ecology help guide our understanding of how shallow lakes work. A well-accepted hypothesis is that shallow lakes exist in alternative stable states that range from plant dominance and clear water at low nutrient concentrations to algal-dominated, turbid conditions at high nutrient concentrations. The exact nutrient or chlorophyll-a concentration where this “switch” occurs is not explicitly stated but it is widely acknowledged that several factors including fish, zooplankton, lake depth and numerous other factors in addition to nutrients play a role. A recent effort by the Upper Mississippi River Conservation Committee (UMRCC) to develop light-related water quality criteria necessary to sustain submerged aquatic vegetation (SAV) in the Upper Mississippi River provides some insights as well. While their work focused on the backwaters and pools on the Mississippi River some of the same concepts can be applied to shallow lakes elsewhere. As such we would like to draw upon their discussion as follows (UMRCC, 2003): “The negative impact of high turbidity or suspended particulate matter on SAV is well known and has been documented in many systems including Lake Chatauqua, Illinois (Jackson and Starret 1959), Rice Lake Wisconsin, and Chesapeake Bay (Dennison et al., 1993). These impacts are expressed through a reduction in light energy on leaf surfaces, which contribute to reduced growth and reproduction (Korschgen et al., 1997 and Kimber et al., 1995). The maximum depth of colonization of SAV has been directly linked to the transparency of water (Chambers and Kalff, 1985 and Canfield et al., 1985). Their regression plots of the maximum colonization depth versus Secchi disk depth are similar and suggests the relationship may have broad application to many freshwater systems. For example, this simple relationship could be used to establish the target depth for SAV establishment in the UMR navigation pools. Water quality management efforts would then be directed at controlling turbidity or suspended particulate matter to provide the necessary underwater light conditions to support SAV growth and reproduction. A similar approach has been suggested for Chesapeake Bay (Dennison et al., 1993).”

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Their work focused directly on the pools and fairly specifically on the production of wild celery (Vallisneria), a very important food source for migrating waterfowl. It took into account light regimes in the pools, depth of pools and extent of colonization (depth) of wild celery both in current times as well as historically. Their analysis led them to suggest the following values as criteria for the system as follows: Recommended light-related water quality criteria necessary to support and sustain submersed aquatic vegetation in the Upper Mississippi River. Variable Light Extinction Coefficient

Secchi Disk Depth

Total Suspended Solids

Turbidity

Value* -1

Basis

3.42 m

Average growing season light extinction necessary to promote Vallisneria growth and reproduction at 0.8 m depth

0.5 m

Light extinction vs. Secchi depth regression, WDNR data for Pools 4-11

25 mg/L

Light extinction vs. TSS regression - WDNR data for Lock & Dam 8 & 9

20 ntu

Light extinction vs. turbidity regression - LTRMP data for Pools 8 & 13.

* Values should be applied as a growing season average (May 15 to September 15) based on bi-weekly measurements.

While in this case they focused principally on light they did acknowledge several other factors that also influence SAV growth and survival, e.g., “water level changes, waves, nutrients, floods, substrate composition and herbivore activity and other factors also play a role in governing the development and persistence of SAV communities on the river.” One point they make with respect to the negative impacts of excessive nutrient enrichment is on the enhancement of filamentous algae or epiphytic plant growth on SAV may be especially important since these attached plants have been implicated as a critical factor contributing to submersed aquatic macrophyte declines in freshwater systems (Phillips et al., 1978). Their work suggests excessive canopies of filamentous algae and other attached algae may lead to increased competition for light and nutrients and may promote the "switch" from a SAV dominated system to one dominated by algae. In the course of our study we did not specifically address this but it is just one more reason to minimize nutrient concentrations in these shallow systems. Based on the aforementioned analysis characteristics of lakes in each of the FQI classes were summarized (Table 6). The mean and median TP, chlorophyll-a Secchi and TSS are noted for each class. In general there is minimal difference among the lakes classified as medium to low FQI (Table 6). However those classified as high FQI had TP and chlorophyll-a values about three to four-fold lower than the medium class, while Secchi was about two-fold greater. TSS was extremely low as compared to the other two classes, with values that are fairly consistent with reference lake data for CHF lakes (Table 3). A similar pattern was evident as we compared lakes that support 15 or more plant species as compared to those that support less than 15. Trophic status characteristics were quite similar among the lakes supporting 15 or more species 116

and the lakes with “high” FQI, which is not too surprising since the number of plant species (richness) is factored into the FQI calculation. Lake morphometry can be an important consideration as well since the “high” FQI lakes and lakes supporting 15 or more species tended to be deeper than the lakes in the other three groups (Table 6). In general lakes exhibiting high FQI had on average TP on the order of 50 µg/L or less, chlorophyll-a less than 20 µg/L and Secchi of 1.7 m as summer averages. Table 6. Summary of morphometric and water quality characteristics for Floristic Quality Index “classes” and lakes supporting 15 or more species of plants. Based on 27 westcentral Minnesota lakes. Depth feet

Area acres

FQI Mean

High Medium Low Plant species > 15 plant sp. < 15 plant sp.

11 5 5 Mean

Max.

23 9 8 Max.

Mean

792 555 502 Mean

TP ppb mean / median

Chl-a ppb mean / median

49 / 34 142 / 130 194 / 210 mean / median

19 / 10 54 / 48 74 / 45 mean / median

Secchi m mean / median

2.0 / 1.9 0.7 / 0.7 0.5 / 0.4 mean / median

TSS ppm mean / median

8/3 33 / 15 43 / 38 mean / median

10

20

708

47 / 38

17 / 10

1.9 / 1.7

8/3

5

8

548

171 / 155

65 / 48

0.6 / 0.5

39 / 32

Moss (1998) makes several points regarding the switching of lakes from one “state” to another. And while not stating a specific threshold he notes that at very high nutrient concentrations algal dominance may be the only possibility in a lake as macrophytes will be absent. This would seem to be the case in several of the west-central lakes with low FQI and extremely high chlorophyll-a (Fig. 23) and based on our study of shallow SW Minnesota lakes (Heiskary et al., 2003). He does note that a switch from plant-dominated to algal-dominated is easier at high nutrient concentrations – which again seems to be the case for several of the lakes in this study (Figs. 18, 19 & 22) and argues for trying to keep this shift from occurring in the first place (as most literature point to the difficulty in reversing this process once it occurs). Internal recycling of phosphorus, in shallow lakes that are in the turbid water state, often serves to keep lakes from switching back and contributes to the difficulty in restoring these lakes. This effectively maintains the lake in a “state” where it may have minimal values for fish, wildlife and aquatic recreational use. It is also quite likely that nutrient reduction alone may not be sufficient to switching to plant dominance from algal dominance as most cases where this has occurred tends to involve biomanipulation of the fish community and Moss (1998) notes further many instances exist where control of nutrient loading did not switch community back to plant dominance, but biomanipulation did. While we have not established any precise empirical relationships upon which to base our criteria for shallow lakes we do have several pieces of information from this study, our previous work (e.g., Heiskary and Wilson, 1988), and review of the literature that allow for a weight-of evidence approach to setting these criteria that includes: 117

• • • • • •

Interrelationships among TP, chlorophyll-a, Secchi and nuisance bloom frequency Patterns in FQI and macrophyte richness and abundance relative to TP, chlorophyll-a and Secchi; Ecoregion-specific distributions for TP, chlorophyll-a and Secchi for all assessed lakes and TP for shallow lakes more specifically; Lake user perception data; Pre-European diatom-inferred TP concentrations for both deep and shallow CHF lakes; and Extensive literature on shallow lakes and factors that contribute to “alternate states” and the overall value of shallow lakes.

A comparison of distributions for TP, chlorophyll-a and Secchi, drawn from different populations of CHF lakes, (including USEPA criteria development manuals; USEPA 2000b) can help provide some perspective as we explore potential criteria values and what these values may mean relative to the various populations. For TP we have comparisons for pre-European (diatom-inferred) TP and modern-day distributions for all assessed CHF lakes and a subset of that “shallow lakes” as bases for comparison (Fig. 25). For this exercise the shallow lakes distribution represents all assessed lakes with a maximum depth of less than 20 feet, which should include most of the lakes that meet our definition of shallow lakes – “maximum depth of 15 feet or less or 80% or more of the lake is littoral.” As noted previously the shallow, polymictic lakes of the CHF ecoregion tend to be more nutrient-rich as compared to the overall population (Fig. 25) and in particular when compared to deeper, dimictic lakes of the same ecoregion (Fig. 1). For example the 25th percentile for the shallow lakes is about 60 ppb, which is slightly above the median for the MPCA assessed lakes (Fig. 25). The IQ range for the 2003 study lakes is fairly similar to the TP range for shallow lakes and would appear to be a reasonable representation of the range in TP found in shallow CHF lakes. It is also evident that pre-European TP was higher in the shallow lakes as compared to deeper CHF lakes (Fig. 24b). Similar distributions were prepared for chlorophyll-a and Secchi to provide perspective on these trophic state parameters as well (Figs. 26 & 27). As with TP the shallow lakes exhibit higher chlorophyll-a as compared to the overall MPCA assessed lakes. In this case the 25th percentile for the shallow lakes – 22 µg/L is near the median for the MPCA assessed lakes. Again the IQ range for the 2003 study lakes is quite similar to the assessed shallow lakes (Fig. 26). As anticipated, Secchi transparency for the assessed shallow lakes is less than the overall assessed lakes with a median value of 1.1 m corresponding to the 75th percentile for the assessed population (Fig. 27), i.e., 75 % of the assessed lakes have transparencies greater than 1.1 m. The 2003 study lakes exhibited a similar but slightly lower range of Secchi as compared to the assessed shallow lakes.

118

Figure 25. Summer-mean total phosphorus distributions (IQ range) for CHF ecoregion. CHF Ecoregion Lakes TP Distributions 200 180 160

TP ppb

140 120 100 80 60 40 20 0 Pre-E (deep, N=30)

Pre-E (shallow, N=5)

MPCA-Ref (N=43)

EPA-Assess PCA-Assess 2003 study (N=469) (shallow, lakes (N=27) N=192)

MPCAAssess (N=691) 25th

50th

75th

Figure 26. Summer-mean chlorophyll-a distributions (IQ range) for CHF ecoregion CHF Ecoregion Chlorophyll-a Distributions

80

Chl-a ppb

70 60 50 40 30 20 10 0 MPCA-Ref (N=43)

MPCA-Assess (N=691)

EPA-Assess (N=273)

50th

119

25th

75th

PCA-Assess (shallow, N=176)

2003 study lakes (N=27)

Figure 27. Summer-mean Secchi distributions (IQ range) for CHF ecoregion. Secchi Interquartile range for CHF ecoregion

3.5 3.0

Secchi m

2.5 2.0 1.5 1.0 0.5 0.0 MPCA-Ref (N=43)

MPCA-Assess (N=691)

EPA-Assess (N=559)

50th

25th

PCA-Assess (shallow, N=176)

2003 study lakes (N=27)

75th

A primary focus in setting eutrophication criteria for shallow lakes is to allow for a balanced population of macrophytes that helps support a broad array of aquatic life uses and aquatic recreation (Class 2b & 2c water quality standards; Minn. Rule Ch. 7050, 2002). As such maintaining adequate transparency to allow native plants to establish themselves over much of the basin, minimizing the chance that non-native species (e.g., curly-leaf) become dominant, minimizing the occurrence of nuisance algal blooms, and keeping TP concentrations below a range that promotes excessive algal growth are all important considerations upon which to base eutrophication criteria. And of these three variables (parameters), transparency may be the most important. In turn, transparency can be directly related to TP and chlorophyll-a, though several biotic factors, such as dominance of benthivorous (e.g., carp and bullhead) or planktivorous fish (fatheads, sunfish and crappies), and abiotic factors such as suspended sediments, lake depth, wind erosion and resuspension may also influence transparency and the ability of the lake to support macrophytes. Based on Table 6, and the figures that precede it, transparency should remain above about 0.7 m and ideally 1.0 m or more to minimize the likelihood of low FQI and a reduced number of species of rooted plants. Relative to the assessed shallow CHF lakes 0.7 m and 1.0 m correspond to about the 25th and 50th percentiles respectively (Fig. 24). A summer average transparency of 0.7 – 1.0 m should allow for SAV colonization to a depth of about 1.5 – 2.0 m (~5 - 6 ft.) based on Fig. 21b and equations developed by Canfield et al. (1985) and Chambers and Kalff (1985). This would represent an appreciable portion of the lake-basins included in this study (Table 1), which collectively had an average mean depth of 1.9 m (6.3 ft.).

120

Lake user perception can provide an additional perspective for determining an appropriate level of transparency – even though swimming may not be the primary use of shallow lakes. Based on user perception for CHF lakes a transparency of 0.7 m represents the average transparency associated with “severe nuisance blooms” and/or “swimming and aesthetic enjoyment nearly impossible,” whereas 1.0 m was the average transparency associated with “high algal levels” and/or “desire to swim reduced because of algae levels (Smeltzer and Heiskary, 1990). This would suggest a transparency closer to 1.0 m may be more desirable based on potential lake users. Chlorophyll-a is the next consideration and based on Carlson’s TSI (Fig. 9) and interrelationships developed in this study corresponding chlorophyll-a concentrations would be on the order of 20 µg/L (1 m Secchi) to 30 µg/L (0.7 m Secchi). However, based on a desire to minimize nuisance blooms a concentration closer to 20 µg/L would be more desirable since the frequency of nuisance blooms (chlorophyll-a > 30 µg/L) increases from about 15% at 20 µg/L up to about 45% at a chlorophyll-a of 30 µg/L (Heiskary and Wilson, 1988) and would likely lead to an algal-dominated system. Also the average chlorophyll-a associated with high FQI was 19 µg/L (Table 6). As a frame of reference, a chlorophyll-a concentration of about 20 µg/L ranks near the 25th percentile for shallow CHF lakes (Fig. 26). A corresponding range of TP concentrations to yield a transparency of 0.7-1.0 m would be on the order of 48-68 µg/L based on Carlson’s TSI (Fig. 9) and about 60-80 µg/L based on Fig. 10. TP concentrations greater than about 60-80 µg/L would be undesirable since the frequency of nuisance blooms increases substantially (Fig. 12) and the number of species of rooted plants declines (Fig. 18) and with perhaps a few notable exceptions signals a shift to algal-dominated systems. As noted earlier the average TP associated with high FQI or lakes supporting 15 or more species, was 47- 49 µg/L (Table 6). Lake response to increased TP over the range from 6090 µg/L is rather variable for these shallow lakes in terms of Secchi, chlorophyll-a and the number of plant species (Fig. 10, 11, & 18); however the general pattern is toward increased chlorophyll-a, declining transparency, and declining numbers of species of rooted plants. Also the potential for curly-leaf dominance appears to increase as mean in-lake TP increases above about 50 µg/L (John Barten, personal communication, 2005). As further frames of reference, a TP of 60 µg/L ranks near the 25th percentile based on assessed shallow lakes (Fig. 25) and based on MINLEAP model runs for lakes in this study, 60 µg/L was the average predicted TP for the 19 CHF lakes (Table 5). While we did not address nitrogen in detail, there appeared to be some relationship between TKN and the number of plant species and FQI. As with TP, as concentrations increased, there was a tendency toward reduced number of species and FQI (Fig. 19). And, consistent with some recent literature (Sagrario et al., 2005), there appeared to be a “threshold-type” response at 2.0 mg/L; whereby as TKN exceeded this concentration range, a marked decline in number of plant species and FQI were noted (Fig. 19).

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In summary, based on the various interrelationships among trophic status variables, rooted plant metrics and other considerations it appears that appropriate ranges for selecting eutrophication criteria values for shallow lakes in the CHF ecoregion are: • Secchi transparency - 0.7 to 1.0 meters; • Chlorophyll-a - 20 – 30 µg/L; • Total phosphorus – 60 – 80 µg/L; Given this range of values, and acknowledging that other biotic and abiotic factors can be very significant in determining whether a lake can support a healthy and diverse population of rooted macrophytes, we are inclined to recommend criteria be set at the lower end of each range of the aforementioned values, i.e. maintain summer average Secchi of 1.0 m or greater, summer average chlorophyll-a of 20 µg/L or lower, and summer average total phosphorus of 60 µg/L or lower. While we are not offering nitrogen criteria at this time, it would appear to be beneficial to keep TKN below 2.0 mg/L when possible. Based on the relationship between TP and TKN, maintaining TP below 60-80 µg/L should yield TKN