Lake Water Quality Assessment Data - Acronym Definitions and Database Summary and Notes -- Minnesota Pollution Control Agency

The following notes are included to clarify how this data base was assembled, how to interpret the information, and cautions for its use.

LakeID numbers are taken from the Minnesota Department of Natural Resources, 1968 publication "Bulletin 25: An Inventory of Minnesota Lakes" or its current equivalent “Public Waters Inventory (PWI). It is a six-digit code with the first two digits indicating the county and the next four the lake number. These numbers are used for storing/retrieving data in STORET. Lakes with a 01, 02, etc., suffix indicates that the lake has been divided into "bays" for data storage and assessment purposes. The noted surface area and depth is an estimate for that bay (Example: Lake ID = 01-0123-01 indicates - Anoka County, Cedar Lake, main basin.).
WBT – Refers towater body type as indicated in MDNR’s PWI. PWI serves to document all public waters (lakes and wetlands) and assign individual ID numbers. (consistent with Bulletin 25). However some lakes in Bulletin 25 are really wetlands and are coded as “LW” whereas lakes have the codes “LP” or “L” in the PWI. Lakes that are “wetlands” based on the PWI were not assessed for the 303(d) list, however this differentiation was not made for the 305(b) assessments noted herein.
ME - Any lake with total phosphorus (TP), chlorophyll-a, or Secchi transparency data collected between 1970-2004, and stored under MPCA's agency code 21 MINNL in STORET is included in this assessment. Lakes were classified into one of two categories for this data assessment. "M" implies monitored (recent) and indicates that summer (June through September) data collected between 1995 and 2004 was available for that lake. "E" implies evaluated (old). This indicates that data for this lake was collected between 1970-1994. Non-summer total phosphorus measurements may also be included in this category.

Qual – This refers to the “quality” of the data used in the assessment and refers to the number and extent of observations available for the assessment. The terms used were derived from USEPA guidance but actual classifications were developed by MPCA as follows: Poor - < 4 TP observations; Fair – 4 ≤ TP< 8, some chlorophyll-a and Secchi; Good – 8<TP<12, some chlorophyll-a and Secchi; and Excellent – 12 TP, 12 chlorophyll-a and 12 Secchi observations.

Table 1. Data quality characterizations for 305(b) and 303(d) assessments.

Quality	“Monitored data”	“Evaluated data”
Poor	< 4 TP measurements	< 4 TP measurements
Fair	4 ≤ TP < 8, some chl-a & Secchi	4 ≤ TP < 8, some chl-a & Secchi
Good	8 < TP < 12, some chl-a & Secchi	8 < TP < 12, some chl-a & Secchi
Excellent	12 TP, 12 chlorophyll-a & 12 Secchi	NA

Statistics – Summer- mean TP, chlorophyll-a, and Secchi was calculated for each lake where data was available. “Summer” includes any observations collected from June through September. The number of observations (N) represents the individual sample dates used to calculate the summer-mean. The standard error of the mean (SE) is calculated as the standard deviation divided by the square root of the number of observations. In addition, the minimum and maximum values for the assessment period are included to assist in the data assessment. Considering the N and SE values, in conjunction with the mean, provides a basis for assessing the “confidence” we have that the mean value accurately characterizes the trophic status of the lake. Likewise the minimum and maximum values provide an indication of how variable the particular parameter may be for a given lake. In general, means calculated from a small number of observations, or highly variable observations, often have a large standard error. Alternately, means calculated from a large number of observations, or relatively uniform observations, often have a small standard error. These statistics (SE in particular) can also be used as a basis for comparing summer-mean values among lakes to determine if the means are significantly different.
Trophic State Index – Carlson’s Trophic State Index (TSI) is used as the basis for estimating the trophic status of Minnesota lakes (Figure 1). Trophic status ranges from oligotrophic to hypereutrophic (and is viewed as a continuum) on this scale. Carlson’s TSI is based on the interrelationships of TP, chlorophyll-a, and Secchi transparency. The individual TSIs are very useful for understanding the relationship of TP, chlorophyll, and transparency for a given lake and provide the best information on the trophic condition of the lake. If the individual TSI values for a lake do not correspond fairly closely (e.g., within 5 TSI units), then the individual values should be inspected and particular attention should be paid to the number of observations used to calculate the mean values and to determine which parameter might be the more accurate predictor of trophic state. The following notes may be helpful in this regard:
- If one index value is based on numerous measures while the others are based on a single measure, then the former is probably the better indicator of trophic state.
- If there is only a single measurement for each index value, the phosphorus TSI should be favored as it provides an estimate of the "potential" trophic status of a lake.
- Secchi or chlorophyll-a TSIs based on single observations should be viewed with caution.
- Secchi TSI values in highly colored waters (see note 7) or waters high in inorganic suspended solids (e.g., clay) may provide a poor estimate of trophic state. This is because the dark coloration or high suspended sediments may limit the amount of algae produced and often will be the primary factor limiting transparency.
- Lakes dominated by large colonial algae, such as Aphanizomenon sp. (look like clumps of grass clippings), may have high transparencies (low TSI) relative to the phosphorus concentration. This is because these colonies of algae may form “rafts” or scums at the surface of the water which are easily displaced by wind or lowering of a Secchi disk and hence Secchi readings may be deeper than if the algae were dispersed evenly throughout the water column. This is very common in hypereutrophic lakes and hence Secchi may not be the best indicator of trophic status in highly nutrient-rich lakes.
- Lakes with extensive macrophyte (rooted submergent and emergent plants) growth may have higher transparency and lower chlorophyll-a (lower TSIs) than expected based on the phosphorus concentration. These plants may compete with algae for available nutrients like phosphorus.
- Ecoregion patterns may also give an indication as to which TSI value is a better reflection of the trophic status of a lake.
- This data base may not provide an accurate estimate of the current trophic status of a given lake because of the number of observations or the age of the data. In particular, those labeled as "evaluated" should be viewed with caution since the data may be over ten years old.
- If more current data is available for a lake, or if data is available for a lake that is not included in the appendix, then the appendix data and Table 5 can be used to place the condition of the lake in perspective relative to other lakes in a given ecoregion.
  
  Carlson Trophic State Index
  
  Trophic status ranges from oligotrophic, as shown below:
  
  to hypereutrophic (and is viewed as a continuum) on this scale.
  
  Carlson's TSI is based on the interrelationships of TP, chlorophyll-a, and Secchi transparency as depicted in Figure 4.
  Figure 4. Total Phosphorus, Chlorophyll-a, and Secchi Scatterplots and Regressions. Based on ecoregion reference lake data.
  
  Table 5. Minnesota Lake Water Quality Assessment Data Base Summary (2006).
  Water quality values represent summer means
  
  ¹NLF summary includes lakes from Northern Minnesota Wetlands (NMW) and NCHF includes lakes from Red River Valley (RRV).
Color - as measured against a platinum-cobalt standard (PCU or Pt-Co units), can give an indication of the relative amount of dissolved organic matter in the water. High coloration, or "bog stain" as it is referred to, is usually caused by runoff from wetlands or forested lands. At high values, color may interfere with the expected phosphorus - chlorophyll - Secchi relationship of a lake. In general, values between 0-20 are considered clear, 21-50 are considered moderately colored, 51-100 are considered highly colored, and values greater than 100 can be considered very highly colored. Based on data from the ecoregion reference lakes color begins to influence the phosphorus - chlorophyll - Secchi relationships above about 50 Pt-Co units, and may strongly influence the relationship above 100 Pt-Co units. In highly colored lakes, transparency and chlorophyll values are often lower than expected based on the phosphorus values.
Alkalinity - is measured in mg/l as calcium carbonate (CaCO 3). It represents a measure of a solution's ability to buffer or neutralize acids. Lakes located in areas of calcareous glacial till (common throughout central and southern Minnesota) will have higher alkalinity than lakes formed on non-calcareous bedrock (common in northeastern Minnesota). Water with alkalinity less than about 75 mg/L could be considered soft, 76-150 moderately hard, 151-300 hard, and greater than 300 very hard. Alkalinity has also been used as a basis for estimating sensitivity to acid precipitation. For this purpose, lakes with alkalinity values less than 5 to 10 mg/L could be considered potentially sensitive to acid precipitation based on current levels of deposition across Minnesota. At this point we have identified no “culturally acidified” lakes in Minnesota.

Ecoregion Reference Lakes - One means for placing lake water quality information in perspective is to compare summer mean values to those found in reference lakes from the same ecoregion in which the lake is located in. The U.S. Environmental Protection Agency mapped ecoregions for the United States from information on soils, landform, potential natural vegetation, and land use. For Minnesota, within-ecoregion similarities in lake chemistry and lake morphometry (depth and surface area) have been noted. Reference lakes, deemed to be representative and minimally impacted by man (e.g., no point source wastewater discharges, no large urban areas in the watershed, etc.), were sampled in each ecoregion by the MPCA from 1985 through 1988. These lakes are not necessarily the most “pristine” for the region, as is evident from the data in Table 4. The reference lake data base consists of approximately 90 lakes distributed as follows among the four ecoregions with the majority of Minnesota's lakes: Northern Lakes and Forests (NLF)- 30, North Central Hardwood Forest (CHF)- 38, Western Corn Belt Plains (WCP) - 12, and Northern Glaciated Plains (NGP)- 10. Data from the reference lakes can be used as a "yardstick" to compare other data against. Table 4 provides a range of summer-mean values for each parameter and each ecoregion. These values were taken from the "inter-quartile range" (25 th to 75 th percentile) of the reference lakes for each region. By using these values, we have excluded the very low values (lower 25 percent) and the very high values (upper 25 percent) and thus, have a range of values that represent the central tendency of the reference lake's water quality. If your lake is near the transition zone of two ecoregions it is often useful to make comparisons to reference lakes from both ecoregions.

Figure 2. Minnesota's Seven Ecoregions

Figure 2. Minnesota's Seven Ecoregions

Table 4. Ecoregion reference lake data summary. Based on the interquartile (25th – 75th percentile) range for reference lakes in each ecoregion. Also referred to as “typical range.”

Parameter	Northern Lakes and Forests	North Central Hardwood Forests	Western Corn Belt Plains	Northern Glaciated Plains
# of reference lakes	30	35	12	10
Total Phosphorus (µg/L)	14 - 27	23 - 50	65 - 150	122 - 160
Chlorophyll mean (ug/l)	4 - 10	5 - 22	30 - 80	36 - 61
Chlorophyll maximum (ug/l)	< 15	7 - 37	60 - 140	66 - 88
Secchi Disk (feet) (meters)	8 - 15 (2.4 - 4.6)	4.9 - 10.5 (1.5 - 3.2)	1.6 - 3.3 (0.5 - 1.0)	1.3 – 2.6 (0.4 – 0.8)
Total Kjeldahl Nitrogen (mg/l)	0.4 – 0.75	< 0.60 - 1.2	1.3 - 2.7	1.8 - 2.3
Nitrite + Nitrate-N (mg/l)	<0.01	<0.01	0.01 - 0.02	0.01 - 0.1
Alkalinity (mg/l)	40 – 140	75 - 150	125 - 165	160 - 260
Color (Pt-Co Units)	10 – 35	10 - 20	15 - 25	20 - 30
pH (SU)	7.2 - 8.3	8.6 - 8.8	8.2 - 9.0	8.3 - 8.6
Chloride (mg/l)	0.6 – 1.2	4 - 10	13 - 22	11 - 18
Total Suspended Solids (mg/l)	< 1 – 2	2 - 6	7 - 18	10 - 30
Total Suspended Inorganic Solids (mg/l)	< 1 – 2	1 - 2	3 - 9	5 - 15
Turbidity (NTU)	< 2	1 - 2	3 - 8	6 - 17
Conductivity (umhos/cm)	50 – 250	300 - 400	300 - 650	640 - 900
TN:TP ratio	25:1 - 35:1	25:1 - 35:1	17:1 - 27:1	7:1 - 18:1

Major Drainage Basins - Minnesota is characterized by nine major drainage basins (Figure 3). The MPCA is using the major river basins as a basis for focusing permitting, monitoring, and other water quality activities. Basin information documents (BID’s) and plans will be developed for each basin over the next several years. The ecoregion framework will be used in conjunction with the basin approach to evaluate lake condition.
Figure 3. Minnesota's Ecoregions and Basins

Use Support (Aquatic recreation) - Formerly referred to as “swimmable use “ support, this use support classification considers not only swimming, but also wading, aesthetics, and other related uses. The various classes of full, partial and non support are defined in the text of this report. The current threshold values for the 2006 305(b) and 303(d) assessments are noted on Table 3. The draft eutrophication criteria (Table 5) will become the new basis for these assessments once they are adopted into water quality standards.

Table 3. Draft eutrophication criteria by ecoregion and lake type(Heiskary and Wilson, 2005)

Ecoregion	TP ppb	Chl-a ppb	Secchi meters
NLF – Lake trout (Class 2a)	< 12	< 3	> 4.8
NLF – Stream trout (Class 2a)	< 20	< 6	> 2.5
NLF – Aquatic Rec. Use (Class 2b)	< 30	< 9	> 2.0
CHF – Stream trout (Class 2a)	< 20	< 6	> 2.5
CHF – Aquatic Rec. Use (Class 2b)	< 40	< 14	> 1.4
CHF – Aquatic Rec. Use (Class 2b) Shallow lakes	< 60	< 20	> 1.0
WCP & NGP – Aquatic Rec. Use (Class 2b)	< 65	< 22	> 0.9
WCP & NGP – Aquatic Rec. Use (Class 2b) Shallow lakes	< 90	< 30	> 0.7

Figure 5. Algal Bloom Frequency as a Function of Mean Chlorophyll-a and Total Phosphorus

Figure 5. Algal Bloom Frequency as a Function of Mean Chlorophyll-a

Figure 5. Algal Bloom Frequency as a Function of Mean Chlorophyll-a and Total Phosphorus

Further Reading - The following papers/reports are suggested for further information on the following topics: trophic state index, use of ecoregion framework, and lake water quality assessment in Minnesota. Several of these may be found on the MPCA lake water quality assessment Web page.

Hydrologic Unit Code The 8-character federal code identifying the Cataloguing Unit, the smallest of the different hydrologic units. The coding is hierarchical with a 2-character region, a 4-character subregion, a 6-character accounting unit and an 8-character cataloguing unit.

References

Carlson, R.E. 1977. A trophic state index for lakes. Limnol. Oceangr. 22:361-369.

Heiskary, S.A. 1997. Lake prioritization for protecting swimmable use. Part of a series on Minnesota Lake Water Quality Assessment. MPCA. St. Paul, MN

Heiskary, S.A. and J. Lindbloom. 1993. Lake water quality trends in Minnesota. Part of a series on Minnesota Lake Water Quality Assessment. MPCA. St. Paul, Minnesota.

Heiskary, S.A. and C.B. Wilson. 1989. The regional nature of lake water quality across Minnesota: an analysis for improving resource management. Jour. Minn. Acad. Sci. 55(1):71-77.

Heiskary, S.A. and C.B. Wilson. 2005. Minnesota lake water quality assessment report: Developing nutrient criteria. Minnesota Pollution Control Agency. St. Paul, Minnesota.

Heiskary, S.A. and W.W. Walker Jr. 1988. Developing phosphorus criteria for Minnesota lakes. Lake and Reserve. Manage. 4(1):1-9

Omernik, J.M. 1987. Ecoregions of the continuous United States. Annals. Assoc. Amer. Geogr. 77(1):118-125.

Osgood, R.A. 1982. Using differences among Carlson's Trophic State Index values in regional water quality assessment. Water Resour. Bull. 18:67-73.

Smeltzer, E. and S. Heiskary. 1990. Analysis and application of lake user survey data. Lake and Reserve Manage. 6(1):109-118.

Vighi, M. and G. Chiaudani. 1985. A sample method to estimate lake phosphorus concentrations resulting from natural background loading. Wat. Res. 19:987-991.

Wilson, C.B. and W.W. Walker. 1989. Development of lake assessment method based upon aquatic ecoregion concept. Lake and Reserv. Manage. 5(2):11-27.

Glossary

Acid Rain: Rain or other precipitation with a higher than normal acid range. Caused when polluted air mixes with cloud moisture. High acidity (low pH) can make lakes devoid of fish.

Bioaccumulation: Build-up of toxic substances in fish flesh. Toxic effects may be passed on to humans eating the fish.

Biomanipulation: Adjusting the fish species composition in a lake as a restoration technique.

Chlorophyll-a: A pigment produced by algae (and other plants). Chlorophyll-a is measured in a water sample and is used as an estimate of the amount (biomass) of algae in water.

Dimictic: Lakes which thermally stratify and mix (turnover) once in spring and fall.

Ecoregion: Areas of relative homogeneity. EPA ecoregions have been defined for Minnesota based on land use, soils, landform, and potential natural vegetation.

Ecosystem: A community of interaction among animals, plants, and microorganisms, and the physical and chemical environment in which they live.

Epilimnion: Most lakes form three distinct layers of water during summertime weather. The epilimnion is the upper layer and is characterized by warmer and lighter water.

Eutrophication: The aging process by which lakes are fertilized with nutrients. Natural eutrophication will very gradually change the character of a lake. Cultural eutrophication is the accelerated aging of a lake as a result of human activities.

Eutrophic Lake: A nutrient-rich lake - usually shallow, "green" and with limited oxygen in the bottom layer of water.

Fall Turnover: Cooling surface waters, activated by wind action, sink to mix with lower levels of water. As in spring turnover, all water is now at the same temperature.

Hypereutrophic: A very nutrient-rich lake characterized by frequent and severe nuisance algal blooms and low transparency.

Hypolimnion: The bottom layer of lake water during the summer months. The water in the hypolimnion is denser and much colder than the water in the upper two layers.

Lake Management: A process that involves study, assessment of problems, and decisions on how to maintain a lake as a thriving ecosystem.

Lake Stewardship: An attitude that recognizes the vulnerability of lakes and the need for citizens, both individually and collectively, to assume responsibility for their care.

Limnetic Community: The area of open water in a lake providing the habitat for phytoplankton, zooplankton and fish.

Littoral Community: The shallow areas around a lake's shoreline, dominated by aquatic plants. The plants produce oxygen and provide food and shelter for animal life.

Mesotrophic Lake: Midway in nutrient levels between the eutrophic and oligotrophic lakes.

Nonpoint Source: Polluted runoff - nutrients and pollution sources not discharged from a single point: e.g. runoff from agricultural fields or feedlots.

Oligotrophic Lake: A relatively nutrient- poor lake, it is clear and deep with bottom waters high in dissolved oxygen.

Phosphorus: An essential plant nutrient. Excess quantities promote excessive growth of algae and plants in lakes and streams. Total phosphorus refers to the most common form measured in water and includes both dissolved and particulate phosphorus.

Photosynthesis: The process by which green plants produce oxygen from sunlight, water and carbon dioxide.

Phytoplankton: Algae - the base of the lake's food chain, it also produces oxygen.

Point Sources: Specific sources of nutrient or polluted discharge to a lake: e.g. stormwater outlets.

Polymictic: A lake which does not thermally stratify in the summer. Tends to mix periodically throughout summer via wind and wave action.

Profundal Community: The area below the limnetic zone where light does not pentrate. This area roughly corresponds to the hypolimnion layer of water and is home to organisms that break down or consume organic matter.

Sedimentation: The addition of soils to lakes, a part of the natural aging process, makes lakes shallower. The process can be greatly accelerated by human activities.

Spring Turnover: After ice melts in spring, warming surface water sinks to mix with deeper water. At this time of year, all water is the same temperature.

Thermocline: During summertime, the middle layer of lake water. Lying below the epilimnion, this water rapidly loses warmth.

Trophic Status: The level of growth or productivity of a lake as measured by phosphorus content, algae abundance, and depth of light penetration.

Turbidity: Particles in solution (e.g. soil or algae) which scatter light and reduce transparency.

Water Density: Water is most dense at 39 degrees F (4 degrees C)and expands (becomes less dense) at both higher and lower temperatures.

Watershed: The surrounding land area that drains into a lake, river or river system.

Lake Water Quality Assessment Data - Acronym Definitions and Database Summary and Notes

Lake Water Quality Assessment Data Acronym Definitions

Lake Water Quality Assessment Database Summary and Notes

References

Glossary