Understanding Lake Data

8/3/2019 Understanding Lake Data

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G3582

Understandinglake data

by Byron ShawChristine Mechenichand Lowell Klessig


2/20

THIS GUIDE WAS WRITTENto help people understand information aboutlake water quality and to interpret lake data.Each lake possesses a unique personality, orset of physical and chemical characteristicswhich may change over time. Lakes exhibitchemical changes on a daily basis while otherchanges, such as plant and algae growth, occurseasonally.

Year-to-year changes in a lake are commonbecause surface runoff, groundwater inflow,precipitation, temperature and sunlight vary.For example, the loss of dissolved oxygen candestroy a lakes fish population, but mayimprove water clarity. Eliminating fish allowsalgae-eating zooplankton (microscopic animals)to increase, which might reduce algaepopulations. Because of changes like these, data

from several years are needed to show whethera lake is experiencing significant changes inwater quality.

This publication explains the physical andchemical compositions of different types oflakes. It covers lakes nutrient status (trophiccondition), and their susceptibility to acid rain.It discusses toxic metals that accumulate in fishand tells how to use general water chemistryprinciples to document potential changes inwater quality. A glossary of technical terms isincluded to help the reader understand thelanguage used in the study of lakes (limnology).

CONTENTSPHYSICAL CHARACTERISTICS..................................3

Lake types............................................................3

Water source........................................................3

Mixing and stratification...................................4

Retention time.....................................................5

Drainage basin/lake area ratio (DB:LA).........5

Lake water levels ................................................6

Water clarity........................................................6

Trophic state........................................................7

CHEMICAL PROPERTIES ...........................................8

Phosphorus..........................................................8

Nitrogen...............................................................9

Chloride ...............................................................11

Sulfate...................................................................11

Sodium and potassium......................................12

DISSOLVED GASES....................................................12

Oxygen.................................................................12

Carbon dioxide ...................................................13

Nitrogen gas ........................................................14

Other gases ..........................................................14

CARBONATE SYSTEMS..............................................14pHacidity.........................................................14

Alkalinity and hardness ....................................15

Alkalinitya lake's buffer against acid rain ..15

Marl deposits.......................................................16

SUMMARY ................................................................16

BIBLIOGRAPHY ANDADDITIONAL REFERENCES .......................................16

GLOSSARY................................................................17


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1

1. SEEPAGE LAKEa natural lake fed byprecipitation, limited runoff and groundwater. Itdoes not have a stream outlet.

FIGURE 1.LAKE TYPES.MAJOR WATER INPUTSAND OUTFLOWS OF

DIFFERENT LAKE TYPES.(LARGE ARROWSINDICATE HEAVY

WATER FLOW.)

PHYSICAL CHARACTERISTICS

Lake types

Lakes are often classified into four types based onwater source and type of outflow (see Figure 1

below).

Water source

The source of a lakes water supply is veryimportant in determining its water quality and in

choosing management practices to protect thatquality. If precipitation is the major water source,the lake will be acidic, low in nutrients, andsusceptible to acid rain. (This includes manyseepage lakes.)

If groundwater is the major water source, the lake

is usually well buffered against acid rain andcontains low to moderate amounts of nutrients.(This includes all groundwater drainage lakesand some seepage lakes.) Local septic systems orother groundwater contamination could causeproblems. Water exchange is fairly slow.

RUNOFF

GROUNDWATER

PRECIPITATION

RUNOFF OUTFLOW

EVAPORATION

GROUNDWATER

PRECIPITATION

RUNOFFINFLOW

OUTFLOW

EVAPORATION

GROUNDWATER

2. GROUNDWATER DRAINAGE LAKEanatural lake fed by groundwater, precipitation andlimited runoff. It has a stream outlet.

4. IMPOUNDMENTa manmade lake created bydamming a stream. An impoundment is also drained by astream.

3. DRAINAGE LAKEa lake fed by streams,groundwater, precipitation and runoff and drained by astream.

RUNOFFINFLOW

EVAPORATION

GROUNDWATER

PRECIPITATION

EVAPORATION

w


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If streams are the major source of lake water,nutrient levels are often high and water exchangetakes place more rapidly. These lakes have themost variable water quality depending on theamount of runoff and human activity in thewatershed (land that drains toward the lake).

Managing the watershed to control nutrients andsoil that enter the lake is essential to protectingwater quality. Controlling water that runs fromthe lands surface into the lake is important fordrainage lakes and impoundments, and someseepage and groundwater lakes. Protectinggroundwater quality is particularly important forseepage and groundwater drainage lakes.

Watershed management becomes especiallycritical in impoundment lakes. If a stream isdammed the natural movement of water will berestricted, causing soil and nutrients to collect inthe impoundment.

Lake managers measure inflow and outflow todetermine a lakes water budget. As shown in theformula below, a water budget consists of severalelements. Precipitation in Wisconsin averages 30inches per year. Evaporation depends on thetype of summer weather, but is usually about 21inches. Groundwater flow is more difficult tomeasure, but can be estimated .

The water budget can be expressed in percent orin volume. A typical water budget for a drainagelake follows:

Mixing and stratification

A lakes water quality and ability to support fishare affected by the extent to which the watermixes. The depth, size and shape of a lake are themost important factors influencing mixing,though climate, lakeshore topography, inflowfrom streams, and vegetation also play a role.

Water density peaks at 39oF. It is lighter at bothwarmer and colder temperatures. Variations indensity caused by different temperatures canprevent warm and cold water from mixing.

When lake ice melts in early spring, thetemperature and density of lake water will besimilar from top to bottom. The uniform waterdensity allows the lake to mix completely,recharging the bottom water with oxygen and

bringing nutrients up to the surface. This is

called spring overturn. As surface water warmsin the spring, it loses density. Wind and wavescan circulate the warmed water only 20 to 30 feetdeep, so deeper areas are not mixed. If the lake isshallow (less than 20 feet), however, the watermay stay completely mixed all summer.

During the summer, lakes more than 20 feet deepusually experience a layering calledstratification. Depending on their shape, smalllakes can stratify even if they are less than 20 feetdeep. In larger lakes, the wind may continuouslymix the water to a depth of 30 feet or more. Lakeshallows do not form layers, though deeper areasmay stratify.

Summer stratification, as shown in Figure 2,divides a lake into three zones: epilimnion(warm surface layer), thermocline ormetalimnion (transition zone between warm andcold water), and hypolimnion (cold bottom

water). Stratification traps nutrients releasedfrom bottom sediments in the hypolimnion. Inthe fall, the surface cools until the watertemperature evens out from top to bottom, whichagain allows mixing (fall overturn). A fall algae

bloom often appears when nutrients mix and riseto the surface.

Winter stratification, with a temperaturedifference of only 7oF (39o on the lake bottomversus 32o right below the ice), remains stable

because the ice cover prevents wind from mixingthe water.

The lakes orientation to prevailing winds canaffect the amount of mixing that occurs. Somesmall, deep lakes may not undergo completemixing in the spring or fall if there is not enoughwind action. The mixing that takes place in the

bays of a large lake will more closely resemblethat of a small lake because the irregularshoreline blocks the wind .

Because mixing distributes oxygen throughout alake, lakes that dont mix may have low oxygenlevels in the hypolimnion, which can harm fish.Some fish species require lake stratification. Thecold water in the hypolimnion (bottom) can hold

more oxygen than warmer water in theepilimnion (top) and thus provide a summerrefuge for cold water fish such as trout. But if thelake produces too much algae, which fall into thehypolimnion to decay, oxygen becomes depleted.The steep temperature gradient of themetalimnion prevents any surface water withdissolved atmospheric oxygen from reaching the

bottom waters.

2

30% + 10% + 60% = 5% + 11% + 84%Groundwater Precipitation Surface Groundwater Evaporation Streaminflow runoff outflow outlet


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SPRING OVERTURN(March-April)

WIND

39

3939

3939

39

SUMMER STRATIFICATION

75

68

50

46

4343

Epilimnion

MetalimnionHypolimnion

50 feet

FALL OVERTURN(September- October)

39

41

4646

43

WIND

LATE FALL(November)

39

39

3939

3939

WINTER STRATIFICATION

32

3436

3939

39

ICE

Retention time

The average length of time water remains in alake is called the retention time or flushing rate.The lakes size, water source, and watershed sizeprimarily determine the retention time.

Rapid water exchange rates allow nutrients to beflushed out of the lake quickly. Such lakesrespond best to management practices that

decrease nutrient input. Impoundments, smalldrainage lakes, and lakes with large volumes ofgroundwater inflow and stream outlets(groundwater drainage lakes) fit this category.

Longer retention times occur in seepage lakeswith no surface outlets. Average retention timesrange from several days for some smallimpoundments to many years for large seepagelakes. Lake Superior has the longest retentiontime of Wisconsin lakes500 years!

Nutrients that accumulate over a number ofyears in lakes with long retention times can berecycled annually with spring and fall mixing.Reserve nutrients in lake sediments can continueto recirculate, even after the source of nutrients inthe watershed has been controlled. Thus, theeffects of watershed protection may not beapparent for a number of years. Nevertheless,lakes with long retention times tend to have the

best water quality as shown by the lower levelsof the plant nutrient phosphorus in Table 1.Better water quality results from both theirgreater depth and relatively smaller watersheds.

Drainage basin/lake area ratio(DB:LA)

The size of the watershed (drainage basin)feeding a lake relative to the lakes size (area) isan important factor in determining the amount of

3

FIGURE 2. Annual temperature cycles in stratified lakes.w


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TABLE 1. Several characteristics of lakes with different retention times. (Adapted from Lillie and Mason, 1983.)

Retention time in days 0-14 15-60 61-180 181-365 366-730 >730

Mean depth (ft.) 6 8 11 11 13 23

Max. depth (ft.) 16 21 25 27 35 57

Mean total phosphorus (g/l)* 94 85 56 48 33 25

Mean DB:LA ratio** 1166 142 42 15 8 6

*Summer values; g/l = micrograms per liter or parts per billion**DB:LA = Drainage basin/lake area

nutrients in a lake. Table 1 shows thisrelationship for a sample of Wisconsin lakes.

Lakes with relatively large drainage basinsusually have significant surface water inflow.This inflow carries more nutrients and sedimentsinto these drainage lakes or impoundments. By

definition, seepage lakes have small drainagebasins, more groundwater flow, and fewernutrients from runoff. Groundwater drainagelakes typically have an intermediate-sizeddrainage basin.

Table 1 shows the relationship between retentiontime and the drainage basin:lake area ratio. Lowratio lakes (small drainage basin and large lakearea) have high retention times while high-ratiolakes have short retention times. Drainage

basin:lake area ratios can be used to estimate alakes retention time.

Lake water levels

Lake levels fluctuate naturally due toprecipitation which varies widely from season toseason and year to year. While some lakes withstream inflows show the effect of rainfall almostimmediately, others, such as seepage lakes, donot reflect changes in precipitation for months.For example, heavy autumn rains often causewater levels to rise in the winter when rain entersthe lake as groundwater.

Water level fluctuations significantly affect lakewater's quality. Low levels may cause stressfulconditions for fish and increase the number ofnuisance aquatic plants. High water levels can

boost the amount of nutrients from runoff andflooded lakeshore soils. Older septic systems,located near lakes, may flood when groundwaterlevels are high. Yet another consequence offluctuating water levels is shoreline erosion.

Water clarity

Strictly speaking, clarity is not a chemicalproperty of lake water. More accurately, it is anindicator or measure of water quality related tochemical and physical properties.

Water clarity has two main components: truecolor (materials dissolved in the water) andturbidity (materials suspended in the water such asalgae and silt). The algae population is usuallythe largest and most variable component.

Water clarity often indicates a lakes overallwater quality, especially the amount of algaepresent. Algae are natural and essential, but toomuch of the wrong kind can cause problems.Table 2 shows the inverse relationship betweenSecchi disc depth (a measure of clarity) andchlorophyll a (a measure of algae) for differenttypes of lakes.

Secchi disc readings are taken using an 8-inchdiameter weighted disc painted black and white.The disc is lowered over the downwind, shadedside of the boat until it just disappears from sight,then raised until it is just visible. The average ofthe two depths is recorded. Secchi disc readingsshould be taken on calm, sunny days between 10a.m. and 2 p.m. since cloud cover, waves, and thesuns angle can affect the reading.

4

TABLE 2. Water clarity index.

Water clarity Secchi depth (ft.)

Very poor 3

Poor 5

Fair 7

Good 10

Very good 20

Excellent 32


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Secchi disc values vary throughout the summeras algal populations increase and decrease.Measuring several sites may be useful in somelakes, depending upon the uniformity of the lake.Year to year changes result from weather andnutrient accumulation. Weekly or biweekly Secchirecords (April-November) over a number of years

provide an excellent and inexpensive way to documentlong-term changes in water clarity.

The color of lake water reflects the type andamount of dissolved organic chemicals itcontains. Measured and reported as standardcolor units on filtered samples, colors mainsignificance is aesthetic. Color may also reducelight penetration, slowing weed and algaegrowth. Many lakes possess natural, tan-coloredcompounds (mainly humic and tannic acids)from decomposing plant material in thewatershed. Brown water can result from bogs

draining into a lake. Before or duringdecomposition, algae may impart a green, brownor even reddish color to the water.

Color can affect the Secchi disc reading. Table 3lists color values associated with varying degreesof water color.

Another measure of water clarity, turbidity iscaused by particles of matter rather thandissolved organic compounds. Suspended

particles dissipatelight, which affectsthe depth at whichplants can grow.

Turbidity affects theaesthetic quality ofwater. Lakesreceiving runoff fromsilt or clay soils oftenpossess high

turbidities. These values vary widely with thenature of the seasonal runoff.

Suspended plants and animals also produceturbidity. Many small organisms have a greatereffect than a few large ones. Turbidity caused byalgae is the most common reason for low Secchi

disc readings.

Trophic state

Trophic state is another indicator of waterquality. Lakes can be divided into threecategories based on trophic stateoligotrophic,mesotrophic, and eutrophic. These categoriesreflect a lakes nutrient and clarity levels.

Oligotrophic lakes are generally clear, deep andfree of weeds or large algae blooms. Though

beautiful, they are low in nutrients and do notsupport large fish populations. However,

oligotrophic lakes often develop a food chaincapable of sustaining a very desirable fishery oflarge game fish.

Eutrophic lakes are high in nutrients and supporta large biomass (all the plants and animals livingin a lake). They are usually either weedy orsubject to frequent algae blooms, or both.Eutrophic lakes often support large fishpopulations, but are also susceptible to oxygendepletion. Small, shallow, eutrophic lakes areespecially vulnerable to winterkill which canreduce the number and variety of fish. Roughfish are commonly found in eutrophic lakes.

Mesotrophic lakes lie between the oligotrophicand eutrophic stages. Devoid of oxygen in latesummer, their hypolimnions limit cold water fishand cause phosphorus cycling from sediments.

A natural aging process occurs in all lakes,causing them to change from oligotrophic to

5

The Wisconsin

Department of

Natural Resources

(DNR) operates a

Self-Help Monitoring

Program for lakes.Local volunteers take

Secchi disc and othe

readings and the

DNR provides

computer data

storage and annual

reports. For more

information, contact

district DNR office or

write to:

DNR Lake Manage-ment Program

WRM/2

P.O. Box 7921

Madison, WI 53707

wFIGURE 3. Lake aging process.

OLIGOTROPHIC

Clear water, low productivity Very desirable fishery of large

game fish

MESOTROPHIC

Increased production Accumulated organic matter Occasional algal bloom Good fishery

EUTROPHIC

Very productive May experience oxygen depletion

Rough fish common

TABLE 3. Water color.(Adapted from Lillie and

Mason, 1983.)0--40 units Low

40-100 units Medium

>100 units High


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TABLE 4. Trophic classification of Wisconsin lakes based on chlorophyll a, water clarity measurements,and total phosphorus values. (Adapted from Lillie and Mason, 1983.)

Trophic class Total phosphorus g/l Chlorophyll a g/l Secchi Disc feet

Oligotrophic 3 2 1210 5 8

Mesotrophic 18 8 627 10 6

Eutrophic 30 11 550 15 4

eutrophic over time, and eventually to fill in(Figure 3). People can accelerate theeutrophication process by allowing nutrientsfrom agriculture, lawn fertilizers, streets, septicsystems, and urban storm drains to enter lakes.

In nutrient-poor areas, the aging process may

lead instead to dystrophic and bog lakes whichare highly colored, acid, and not as productive aseutrophic lakes.

Researchers use various methods to calculate thetrophic state of lakes. Common characteristicsused to make the determination are:

total phosphorus concentration(important for algae growth) chlorophyll a concentration (a measure ofthe amount of algae present) Secchi disc readings (an indicator of waterclarity).

The trophic states associated with these threemeasures are shown in Table 4. Clearly, lowlevels of phosphorus are associated with lowlevels of algae (chlorophyll a), which areassociated with high Secchi disc readings.

CHEMICAL PROPERTIES

Phosphorus

Phosphorus promotes excessive aquatic plantgrowth. In more than 80% of Wisconsins lakes,

phosphorus is the key nutrient affecting theamount of algae and weed growth.

Phosphorus originates from a variety of sources,many of which are related to human activities.Major sources include human and animal wastes,soil erosion, detergents, septic systems and runofffrom farmland or lawns.

Phosphorus provokes complex reactions in lakes.An analysis of phosphorus often includes bothsoluble reactive phosphorus and total phosphorus.

Soluble reactive phosphorus dissolves in thewater and readily aids plant growth. Itsconcentration varies widely in most lakes overshort periods of time as plants take it up andrelease it.

Total phosphorus is considered a better indicator

of a lakes nutrient status because its levelsremain more stable than soluble reactivephosphorus. Total phosphorus includes solublephosphorus and the phosphorus in plant andanimal fragments suspended in lake water.

Ideally, soluble reactive phosphorusconcentrations should be 10 g/l (micrograms

6

CONCENTRATION

UNITS express the

amount of a chemical

dissolved in water.

The most common

ways chemical data is

expressed is in

milligrams per liter

(mg/l) and

micrograms per liter

(g/l). One milligram

per liter is equal to

one part per million

(ppm). To convert


(g/l) to milligrams

per liter (mg/l), divide

by 1000 (e.g., 30

g/l = 0.03 mg/l).

To convert milligrams

per liter (mg/l) to


(g/l), multiply by

1000 (e.g., 0.5 mg/l

= 500 g/l).

Microequivalents per

liter (eq/l) is also

sometimes used,especially for

alkalinity. It is

calculated by dividing

the equivalent weight

of the compound by

1000 and then

dividing that number

into the milligrams

per liter.

FIGURE 4. Total phosphorus concentrations forWisconsin's natural lakes and impoundments.(Adapted from Lillie and Mason, 1983.)

Water Quality Total PhosphorusIndex (g/l)

150140130120110100908070605040

30201001

Very poor

Poor

Fair

GoodVery good

Excellent

Average for

impoundments

Average for

natural lakes


9/20

per liter) or less at spring turnover to preventsummer algae blooms. A concentration of 10micrograms per liter is equal to 10 parts per

billion (ppb) or 0.01 milligrams per liter (mg/l).A concentration of total phosphorus below 20g/l for lakes and 30 g/l for impoundmentsshould be maintained to prevent nuisance algal

blooms (Figure 4).

Phosphorus does not dissolve easily in water. Itforms insoluble precipitates (particles) withcalcium, iron, and aluminum. In hard waterareas of Wisconsin, where limestone is dissolvedin the water, marl (calcium carbonate)precipitates and falls to the bottom. Marlformations absorb phosphorus, reducing itsoverall concentration as well as algae growth.Aquatic plants with roots in the marl bottom stillget phosphorus from sediments. Hard waterlakes often have clear water, but may be weedy.

Iron also forms sediment particles that storephosphorusbut only if oxygen is present.When lakes lose oxygen in winter or when thedeep water (hypolimnion) loses oxygen insummer, iron and phosphorus again dissolve inwater. Strong summer winds or spring and fallturnover may mix iron and phosphorus withsurface water. For this reason, algae blooms maystill appear in lakes for many years even ifphosphorus inputs are controlled.

Figure 5 shows the increase in totalphosphorus for stratified lakesfollowing fall turnover. Sinceshallow and windswept lakes thatstay mixed do not experienceoxygen depletion, they have thehighest total phosphorus levels insummer following spring turnoverand early summer runoff.

The amount of iron that might reactwith phosphorus varies widely inWisconsin lakes. Lakes in thesouthern part of the state are oftenlow in iron due to a higher pH andmore sulfur, both of which limit iron

solubility. This in turn affectswhether phosphorus mixed intolakes during fall turnoverprecipitates or stays in solutionduring the winter.

Lakes with low iron and insufficientcalcium to form marl are most likelyto retain phosphorus in solutiononce it is released from sediments or

brought in from external sources.These lakes are the most vulnerable

to naturally occurring phosphorus or tophosphorus loading from human activities

because the phosphorus remains dissolved in thewaternot pulled down into the sediments.Such lakes often respond with greater algaeproblems.

Figure 5 also shows that impoundments have thehighest phosphorus levels. Mixed drainage lakessustain intermediate levels, while seepage andstratified drainage lakes have the lowest. Evenwith the potential for internal phosphoruscycling caused by oxygen depletion, deepstratified lakes tend to have lower phosphoruslevels than their mixed counterparts.

Phosphorus control has been attempted in somelakes by using alum (aluminum sulfate) toprecipitate phosphorus. Sewage treatment plantsuse the same process to remove phosphorus.Aluminum phosphate precipitate, unlike iron

phosphate, is not redissolved when oxygen isdepleted.

Nitrogen

Nitrogen is second only to phosphorus as animportant nutrient for plant and algae growth. Alakes nitrogen sources vary widely. Nitrogencompounds often exceed 0.5 mg/l in rainfall, so

7

110

100

90

80

70

60

50

40

30

10

20

Stratifiedimpoundments

Mixedimpoundments

Mixeddrainage

lakes

Mixedseepage

lakes

Stratifieddrainage

lakes

Stratifiedseepage

lakes

TOTALPH

OSPHORUS(

g/l)

Winter Spring Summer Fall

wFIGURE 5.Seasonal totalphosphorusaverages for sixlake types byseason. (Adaptedfrom Lillie andMason, 1983).


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FIGURE 6.Sources and

cycling ofnitrogen in lake.

that precipitation may be the main nitrogensource for seepage and some drainage lakes.

In most cases, however, the amount of nitrogenin lake water corresponds to local land use.Nitrogen may come from fertilizer and animalwastes on agricultural lands, human waste from

sewage treatment plants or septic systems, andlawn fertilizers used on lakeshore property.Nitrogen may enter a lake from surface runoff orgroundwater sources.

Nitrogen exists in lakes in several forms.Analysis usually includes nitrate (NO3

) plusnitrite (NO2

), ammonium (NH4+), and organic

plus ammonium (Kjeldahl nitrogen). Totalnitrogen is calculated by adding nitrate andnitrite to Kjeldahl nitrogen. Organic nitrogen isoften referred to as biomass nitrogen.

Nitrogen does not occur naturally in soil

minerals, but is a major component of all organic(plant and animal) matter. Decomposing organicmatter releases ammonia, which is converted tonitrate if oxygen is present. This conversionoccurs more rapidly at higher watertemperatures. All inorganic forms of nitrogen(NO3

, NO2 and NH4

+) can be used by aquaticplants and algae. If these inorganic forms ofnitrogen exceed 0.3 mg/l (as N) in spring, there issufficient nitrogen to support summer algae

blooms.

Figure 6 shows the various ways that nitrogenenters and cycles within a lake. Sediments clearlycause nitrogen to undergo a number of changes.Nitrogen recycled back into overlying water atspring and fall turnover will often increaseammonia levels in samples taken duringturnover. Nitrogen can be lost from the lake tothe atmosphere by denitrification as shown in thefigure. This only occurs if oxygen is depleted,allowing nitrate to be converted back to nitrogengas.

In about 10% of Wisconsins lakes, nitrogen(rather than phosphorus) limits algae growth.This occurs when the ratio of total nitrogen tototal phosphorus is less than 10:1. Values

between 10:1 and 15:1 are considered transitional,while lakes with values greater than 15:1 areconsidered phosphorus limitedalgae growth iscontrolled by the amount of phosphorus.

Low nitrogen levels do not guarantee limitedalgae growth in the same way low phosphoruslevels do. Nuisance blue-green algae blooms areoften associated with lakes that have lownitrogen to phosphorus (N:P) ratios. These algaeuse atmospheric nitrogen gas (N2) dissolved inlake waters as a nitrogen source; other moredesirable types of algae and plants depend on theinorganic nitrate and ammonium forms ofnitrogen.

8

w

RAIN containing NO3, NH4

+

Runoff containing NO3,

NH4+,Organic N

Water table

Water table

Groundwater NO3

GroundwaterNO3

NO3 Biomass N

Biomass N

NO3

NH4+

NO3

NO2

N2 (Denitrification)


11/20

CHLORIDE CONCENTRATIONS (mg/l)

>10 >3 - 10 40 20 - 40 10 - 20


12/20

is toxic to aquatic organisms. The sulfide ion (S=)produced under these conditions can also affectthe amount of metal ions in the lake since mostmetals, including iron and mercury, forminsoluble sulfide precipitates. As a result of thehigh sulfate content (Figure 8), iron often exists in

lower concentrations in southern lakes because itprecipitates and settles out in sediments as ironsulfide.

Sodium and potassium

Since natural levels of sodium and potassiumions in soil and water are very low, their presencemay indicate lake pollution caused by humanactivities. Sodium is often associated withchloride. It finds its way into lakes from roadsalt, fertilizers, and human and animal waste.Potassium is the key component of commonly-used potash fertilizer, and is abundant in animalwaste.

Soils retain sodium and potassium to a greaterdegree than chloride or nitrate; therefore, sodiumand potassium are not as useful as pollutionindicators. Increasing sodium and potassiumvalues over time can mean there are long-termeffects caused by pollution. Although notnormally toxic themselves, these compoundsstrongly indicate possible contamination frommore damaging compounds.

DISSOLVED GASESThree gases found in the airoxygen, carbondioxide and nitrogenare very important to lakeecosystems. Three main factors determine theamount of gases present in a lake: wind mixing that brings water into contact withthe atmosphere; the biological activity that consumes orproduces gases within a lake; and gas composition of groundwater and surfacewater entering a lake.

Oxygen

Oxygen (O2) is undoubtedly the most importantof the gases, since most aquatic organisms need itto survive. The solubility of oxygen and othergases depends on water temperature. The colderthe water, the more gases it can hold. Boilingwater removes all gases. Table 5 shows this effectfor oxygen in typical lake water temperatures.

The values in Table 5 are found in lakes wherecontinuous mixing occurs, allowing free oxygen

exchange between water and the atmosphere.(The atmosphere contains about 21% oxygen.)However, the levels often differ greatly from thevalues found in Table 5 because mixing is seldomcomplete. Ice cover dramatically reduces mixing.In addition, biological reactions in the lakeconsume or release oxygen.

Oxygen is produced whenever green plantsgrow. Plants use carbon dioxide and water toproduce simple sugars and oxygen, usingsunlight as the energy source. Chlorophyll, thegreen pigment in plants, absorbs sunlight andserves as the oxygen production site. Thisprocess is called photosynthesis (Equation 1).

Photosynthesis occurs only during daylight hoursand only to the depths where sunlight penetrates.The amount of photosynthesis depends on the

quantity of plants, nutrient availability, andwater temperature. Higher temperatures speedup the process. Plants and animals alsoconstantly use oxygen to break down sugar andobtain energy by a process called respiration,

basically the reverse of the photosyntheticreaction as shown in Equation 2. Burning fossilfuels or other organic matter produces the samechemical reactions shown for respiration,releasing more carbon dioxide (CO2) to theatmosphere.

10

EQUATION 1. PHOTOSYNTHESIS.6 CO2 + 6 H2O -> C6H12O6 + 6O2

carbon water chlorophyll sugar oxygendioxide sunlight

EQUATION 2. RESPIRATION.

C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O

sugar oxygen carbon waterdioxide

TABLE 5. Oxygen solubility at differenttemperatures.

Temperature Oxygen solubilityoC oF (mg/l)

0 32 15

5 41 13

10 50 11

15 59 10

20 68 9

25 77 8


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The combination of these two reactions largelydetermines the amount of oxygen and carbondioxide present in lakes at different times of dayand at different depths. During daylight hours, itis not uncommon to find oxygen values insurface waters that exceed those listed in Table 5(supersaturation), while at night or earlymorning before photosynthesis begins they mayfall below those values. At lake depths below thereach of sunlight, the only reaction that occurs is

oxygen-consuming respiration. The deephypolimnic waters of productive lakes oftenexperience oxygen depletion. Lakes with high

biological activity undergo greater fluctuationsthan lakes with few plants and animals.

Typical oxygen levels in a productive lakefollowing summer stratification are shown inFigure 9. Low oxygen levels in the hypolimnionmean that fish must live in the epilimnion andmetalimnion. Fish (trout) that need high oxygenlevels and cool water disappear from such lakes.

Winter oxygen depletion (winterkill) is a

common problem in many shallow Wisconsinlakes. It happens in years when at least fourinches of snow cover the lake, which preventssunlight from reaching the water. Allphotosynthesis stops and plants begin to die anddecompose. The extent of oxygen loss dependson the total amount of plant, algae and animalmatter that decays. Drought increases the chanceof winterkill by reducing the volume of water inthe lake.

The water quality standard for oxygen in warmwater lakes and streams is 5 mg/l. This is theminimum amount of oxygen needed for fish tosurvive and grow. The standard for trout watersis 7 mg/l. A smart angler would know that thelake in Figure 9 contains no trout and that itwould be silly to fish for walleye in the deepholes in late summer. (See Equation 3.)

Carbon dioxideCarbon dioxide (CO2), like oxygen, is affected byphotosynthesis, respiration and contact with theatmosphere. It is also affected by a third reactioninvolving the amount of carbonate minerals, oralkalinity, present in lake water. Alkalinity isdiscussed in another section.

Carbon dioxide is essential to plant growth. It isthe basic carbon source from which plantsproduce sugar and more complex organic matter.Values often fluctuate, being highest late at nightand lowest early in the evening.

11

FIGURE 9.Typical oxygenand nutrientstatus ofmesotrophicand eutrophiclakes aftersummerstratification.

Metalimnion(thermocline)

Epilimnion

Hypolimnion

Wind

O2 mg/l75

75

65

55

45

45

45

45

Temp F

Organic matter

8

8

7

5

4

2

0

0 NH4 - Phos - Fe

Respiration and decomposition oforganic matter uses oxygen andreleases nutrients

Sediment nutrient release

Photosynthesis and wind add oxygen faster thanused by respiration

Nutrients tied up in weeds, algae

EQUATION 3. CARBON DIOXIDE REACTIONS.

CO2 + H2O > H2CO3 H+ + HCO3

-

carbon water carbonic hydrogen bicarbonatedioxide acid

HCO3 H+ + CO3

=

bicarbonate hydrogen carbonate

w


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As carbon dioxide changes from morning toevening, so does waters pH, especially in low-alkaline, productive lakes.

When carbon dioxide reacts with water, it formscarbonic acid. This in turn affects the pH(acidity) of water. Acidity regulates the solubility

of many minerals.

Nitrogen gas

Nitrogen comprises 78% of the gas in theatmosphere. Like other gases, it is more solubleat cooler temperatures. Most aquatic plants donot derive nutritional value from nitrogen gas,though blue-green algae is an exception.Nitrogen gas is important in lakes containingsuch algae.

Some bacteria convert nitrate back to nitrogen gasunder anaerobic conditions when soluble organic

matter is present. This reaction, calleddenitrification, is one of the main ways nitrogenis lost from certain lakes and some soils. Thisreaction is being investigated as a means ofreducing pollution from septic systems.

Other gases

Under anaerobic conditions, hydrogen sulfide(H2S) and methane gas (CH4) may form anddisperse into lake water from underlyingsediments. Commonly referred to as swampgases, hydrogen sulfide and methane can be

seen bubbling out when an oar pierces shallow,mucky sediments. Hydrogen sulfide (H2S) smellslike rotten eggs and is toxic to fish.

CARBONATE SYSTEMSA lakes carbonate system contains a number ofnaturally-occurring chemicals that affect basic

biological productivity, determine the lakes acidbuffering capacity, and regulate the solubility ofmany toxic chemicals. The complex carbonatesystem undergoes constant change in response to

biological activity, temperature change, sunlight,

and even wave action. The previous discussionon oxygen and carbon dioxide introduced someof these reactions.

pHacidity

An index of lake waters acid level, pH is animportant component of the carbonate system. Itis the negative logarithm of the hydrogen ion(H+) concentration and therefore inversely

related to the amount of hydrogen ion in thewater. Lower pH waters have more hydrogen ionsand are more acidic than higher pH waters.

A pH of 7 is neutral. Water with a pH of 7 hasequal amounts of hydrogen ions and hydroxideions (OH) from the natural separation of a tiny

fraction of water molecules as shown in Equation4. Pure, distilled water without any carbondioxide has a pH value of 7.

In Wisconsin, pH ranges from 4.5 in some acidbog lakes to 8.4 in hard water, marl lakes. For

every 1.0 pH unit, the hydrogen ion concen-tration changes tenfold. Therefore, a lake with apH of 6 is ten times more acid (ten times as muchH+) than a lake with a pH of 7. Water with a pHof 5 has 100 times as many hydrogen ions (H+) aspH 7. Lakes with a pH of 8 have one-tenth asmany hydrogen ions as water with a pH of 7.

While moderately low pH does not usually harmfish, the metals that become soluble under lowpH can be important. In low pH water,aluminum, zinc and mercury concentrationsincrease if they are present in lake sediment orwatershed soils. Table 6 shows the effectscommonly found in lakes acidified by acid rain orexperimentally acidified.

12

0

1

23

4

5

6

7

8

9

10

1112

13

14

Acid

NormalRange

Alkaline

EQUATION 4. SEPARATION OF WATERMOLECULES.

H2O H+ + OH

water hydrogen ion hydroxide ion

pH

The measure of thehydrogen ion (acid)

concentration inwater is called pH. A

pH of 7 is neutral.Values above 7 are

alkaline or basic.Those below 7 areacidic. A change of

1 pH unit is a tenfoldchange in acid level.

Iron may also befound in high levels

in acidic water.

TABLE 6. Effects of acidity on fish species.(Olszyk, 1980).

Water pH Effects

6.5 Walleye spawning inhibited

5.8 Lake trout spawning inhibited

5.5 Smallmouth bass disappear

5.2 Walleye, burbot, lake troutdisappear

5.0 Spawning inhibited in manyfish

4.7 Northern pike, white sucker,brown bullhead, pumpkinseed,sunfish and rockbass disappear

4.5 Perch spawning inhibited

3.5 Perch disappear

3.0 Toxic to all fish


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Aluminum has beenblamed for many ofthe problemsassociated withacidification of lakesand streams incertain areas ofNorth America andEurope. Mercurylevels in fish arehigh in acidifiedlakes. While notusually toxic to fish,high aluminum and

mercury levels pose a health problem for loons,eagles, osprey and humans who eat chemicallytainted fish. Some aquatic organisms appearunable to maintain calcium levels when pH islow, and consequently develop weak bones andshells.

Rainfall in Wisconsin varies from a pH of 4.4 insoutheastern Wisconsin to nearly 5.0 innorthwestern Wisconsin. Natural rainfall,exposed to CO2 in the atmosphere, maintains apH of 5.6. Thus, most fish could not reproduce ineven the best rainfall if rainwater pH were notraised by the chemical buffering of the carbonatesystem in streams, lakes and the surroundingwatershed.

Alkalinity and hardness

The carbonate system provides acid bufferingthrough two alkaline compounds: bicarbonate(HCO3

) and carbonate (CO3=). These

compounds are usually found with two hardnessions: calcium (Ca++) and magnesium (Mg++).

A lakes hardness and alkalinity are affected bythe type of minerals in the soil and watershed

bedrock, and by how much the lake water comesinto contact with these minerals. If a lake getsgroundwater from aquifers containing limestoneminerals such as calcite (CaCO3) and dolomite(CaMgCO3), hardness and alkalinity (Table 8)will be high.

High levels of hardness (greater than 150 mg/l)and alkalinity can cause marl (CaCO3) toprecipitate out of the water. Hard water lakes

tend to produce more fish and aquatic plantsthan soft water lakes. Such lakes are usuallylocated in watersheds with fertile soils that addphosphorus to the lake. As a balancingmechanism, however, phosphorus precipitateswith marl, thereby controlling algae blooms.

If the soils are sandy and composed of quartz or

other insoluble minerals, or if direct rainfall is amajor source of lake water, hardness andalkalinity will be low. This is the case in much ofnorthern Wisconsin, where glacial depositscontain little limestone or other soluble minerals.Lakes with low amounts of alkalinity are moresusceptible to acidification by acid rain and aregenerally unproductive.

Alkalinitya lake's buffer againstacid rain

Alkalinity acts to buffer lakes from the effects of

acid rain because bicarbonate (HCO3

) andcarbonate (CO3=) neutralize hydrogen ions from

the acid inputs. Buffering occurs when excesshydrogen ions are removed from the watersolution as shown in Equation 5. As thehydrogen ions are removed, pH goes up or haltsits decline.

Alkalinity results are reported in two differentways: as milligrams per liter (mg/l) ormicroequivalents per liter (eq/l). Table 9 lists

13

EQUATION 5. HARDNESS AND ALKALINITY.

CaMg(CO3)2 Ca++ + Mg++ + 2CO3

=

limestone (hardness) (alkalinity)

TABLE 7. Solubility ofaluminum at various pHlevels.

pH Aluminum (mg/l

4 4.8

5 .0048

6 .0000048

7 .0000000048

8 .0000000000048

TABLE 9. Sensitivity of lakes to acid rain. (Adapted from Taylor,1984.)

Sensitivity to Alkalinity Valuesacid rain ppm CaCO3 eq/l CaCO3

High 0-2 0-39

Moderate 2-10 40-199

Low 10-25 200-499

Nonsensitive >25 >500

TABLE 8. Categorization of hardness by mg/l of calciumcarbonate (CaCO3).

Level of hardness Total hardness as mg/l CaCO3

soft 0-60 mg/l

moderately hard 61-120 mg/lhard 121-180 mg/l

very hard >180 mg/l


16/20

alkalinity values by these two methods fordifferent degrees of acid rain susceptibility basedon 1 mg/l = 20 eq/l.

As can be seen in Equation 6, alkalinity is alsoconnected to the carbon dioxide reactionsdiscussed earlier.

The amount of alkalinity largely determines alake waters pH. Water with low alkalinity haslow pH value (high acid) and all of its alkalinity

in the bicarbonate (HCO3) form. Highly alkalinelakes have pH values above 7 and somealkalinity in the carbonate form (CO3

=). Eachbicarbonate ion can neutralize one hydrogen (H+)ion. The carbonate form is a better buffer, since itneutralizes two hydrogen ions.

Marl deposits

If the amount of carbonate (CO3=) is high enough,

it will react with calcium in the water to formCaCO3 (marl). Marl precipitates out, leaving awhite substance in the sedimentsometimes

even producing elaborate underwaterformations. Marl can often be observed as awhite precipitate on plant leaves in hard waterlakes. Plants speed up marl deposition by usingcarbon dioxide (CO2), which raises the pH andconverts most alkalinity to the carbonate (CO3

=)form. By precipitating phosphorus, marlformations help control algae growth in marllakes.

SUMMARYThe primary purpose of this publication is to helppeople understand the elements affecting lakewater quality. Another goal is to show the

benefits of keeping a long-term record of waterquality data. Such a record documents changesand helps to distinguish between a lakes naturalvariability and the impacts of human activity.

Lake water quality changes over time, sointerpreting data based on one or two samples isnot enough. Data collected during spring and falloverturn represent a lakes most uniform waterquality conditions and are most valuable forcomparing year-to-year changes. More extensivesampling provides additional information. Along-term commitment to continue a modest sampling

program is better than an extensive program whichcannot be sustained because of a lack of funds or

volunteers.The Environmental Task Force Program at theUniversity of WisconsinStevens Point provideslaboratory analysis and long-term data storage ofspring and fall turnover sample results.

If you have comments about this publication,would like to receive a free quarterly newsletter(Lake Tides), or simply want more information,contact your local University ofWisconsinExtension or DNR office, or theExtension lake management specialists at theCollege of Natural Resources, University ofWisconsin, Stevens Point WI 54481.

14

EQUATION 6. CARBONATE BUFFERING OF pH.

CO3= + H+ HCO3

carbonate hydrogen bicarbonate

HCO3 + H H2CO3 H2O + CO2

bicarbonate bicarbonate water carbondioxide


17/20

BIBLIOGRAPHY ANDADDITIONAL REFERENCESBartsch, A.F. and J. Gakstatter. 1978.

Management decisions for lake systems based on asurvey of trophic status, limiting nutrients, and

nutrient loading. U.S. EPA Amer.-Sov. Symp. onuse of Math. Models to Optimize Water Qual.Manage., Washington D.C., U.S. EPA Rep. 600/9-78-024, pp. 372-96.

Klessig, L. L., N. W. Bouwes and D. A. Yanggen.1986. The Lake in Your Community. UW-Extension, Madison. 24 pp.

Lillie, R. A. and J. W. Mason. 1983. LimnologicalCharacteristics of Wisconsin Lakes. Wis. Dept. ofNatural Resources Tech. Bull. 138, Madison.

Lundquist, J. B. 1975. A Primer on Limnology.Limnological Research Center, St. Paul, Univ. ofMinnesota. 28 pp.

Moore, L. and K. Thornton, eds. 1988. Lake andReservoir Restoration Guidance Manual. U.S. EPA,Washington DC.

Olszyk, D. 1980. Biological Effects of Acid Rain.Testimony, Wis. Public Service CommissionDocket No. 05-EP-2. 5 pp.

Sawyer, C.N. 1947. Fertilization of lakes byagricultural and urban drainage. J. New Engl.Waterworks Assoc. 61(2):109-127.

Shaw, B.H. and N. Nimphius. 1985. Acid Rain inWisconsin: Understanding Measurements in AcidRain Research (#2). UW-Extension, Madison. 4pp.

Stumm, W. and J. J. Morgan. 1981. AquaticChemistry: An Introduction Emphasizing ChemicalEquilibria in Natural Waters. 2nd Ed. Wiley-Interscience. 780 pp.

Taylor, J. W. ed. 1984. The Acid Test. NaturalResources Magazine. Wis. Dept. of NaturalResources. 40 pp.

Wisconsin Dept. of Natural Resources Acid

Deposition Task Force. 1980. A Review of AcidDeposition in Wisconsin: Recommendations forStudying and Solving the Problem. 46 pp.

GLOSSARYAlgae: One-celled (phytoplankton) ormulticellular plants either suspended in water(plankton) or attached to rocks and othersubstrates (periphyton). Their abundance, asmeasured by the amount of chlorophyll a (greenpigment) in an open water sample, is commonlyused to classify the trophic status of a lake.Numerous species occur. Algae are an essentialpart of the lake ecosystem and provides the food

base for most lake organisms, including fish.Phytoplankton populations vary widely fromday to day, as life cycles are short.

Alkalinity: A measure of the amount ofcarbonates, bicarbonates, and hydroxide presentin water. Low alkalinity is the main indicator ofsusceptibility to acid rain. Increasing alkalinity isoften related to increased algae productivity.

Expressed as milligrams per liter (mg/l) ofcalcium carbonate (CaCO3), or asmicroequivalents per liter (eq/l).20 eq/l = 1 mg/l of CaCO3.

Ammonia: A form of nitrogen found in organicmaterials and many fertilizers. It is the first formof nitrogen released when organic matter decays.It can be used by most aquatic plants and istherefore an important nutrient. It convertsrapidly to nitrate (NO3

) if oxygen is present.The conversion rate is related to watertemperature. Ammonia is toxic to fish atrelatively low concentrations in pH-neutral or

alkaline water. Under acid conditions, non-toxicammonium ions (NH4

+) form, but at high pHvalues the toxic ammonium hydroxide (NH4OH)occurs. The water quality standard for fish andaquatic life is 0.02 mg/l of NH4OH. At a pH of 7and a temperature of 68oF (20oC), the ratio ofammonium ions to ammonium hydroxide is250:1; at pH 8, the ratio is 26:1.

Anion: Refers to the chemical ions present thatcarry a negative charge in contrast to cations,which carry a positive charge. There must beequal amounts of positive and negative chargedions in any water sample. Following are the

common anions in their order of decreasingconcentration for most lakes: bicarbonate(HCO3

), sulfate (SO4=), chloride (Cl), carbonate

(CO3=), nitrate (NO3

), nitrite (NO2), and

phosphates (H2PO4, HPO4=, and PO4

=).

Aquatic invertebrates: Aquatic animals withoutan internal skeletal structure such as insects,mollusks, and crayfish.

Bioaccumulation: see Food chain.

15


18/20

Biomass: The total quantity of plants andanimals in a lake. Measured as organisms or drymatter per cubic meter, biomass indicates thedegree of a lake systems eutrophication orproductivity.

Blue-green algae: Algae that are often associated

with problem blooms in lakes. Some producechemicals toxic to other organisms, includinghumans. They often form floating scum as theydie. Many can fix nitrogen (N2) from the air toprovide their own nutrient.

Calcium (Ca++): The most abundant cationfound in Wisconsin lakes. Its abundance isrelated to the presence of calcium-bearingminerals in the lake watershed. Reported asmilligrams per liter (mg/l) as calcium carbonate(CaCO3), or milligrams per liter as calcium ion(Ca++).

Cation: Refers to chemical ions present that carrya positive charge. The common cations present inlakes in normal order of decreasingconcentrations follow: calcium (Ca++),magnesium (Mg++), potassium (K+), sodium(Na+), ammonium (NH4+), ferric iron (Fe+++) orferrous iron (Fe++), manganese (Mn++), andhydrogen (H+).

Chloride (Cl-): Chlorine in the chloride ion (Cl)form has very different properties from chlorinegas (Cl2), which is used for disinfecting. Thechloride ion (Cl) in lake water is commonlyconsidered an indicator of human activity.

Agricultural chemicals, human and animalwastes, and road salt are the major sources ofchloride in lake water.

Chlorophyll a: Green pigment present in allplant life and necessary for photosynthesis. Theamount present in lake water depends on theamount of algae and is therefore used as acommon indicator of water quality.

Clarity: see Secchi disc.

Color: Measured in color units that relate to astandard. A yellow-brown natural color isassociated with lakes or rivers receiving wetland

drainage. The average color value for Wisconsinlakes is 39 units, with the color of state lakesranging from zero to 320 units. Color also affectslight penetration and therefore the depth atwhich plants can grow.

Concentration units express the amount of achemical dissolved in water. The most commonways chemical data is expressed is in milligramsper liter (mg/l) and micrograms per liter (g/l).One milligram per liter is equal to one part per

million (ppm). To convert micrograms per liter(g/l) to milligrams per liter (mg/l), divide by1000 (e.g. 30 g/l = 0.03 mg/l). To convertmilligrams per liter (mg/l) to micrograms perliter (g/l), multiply by 1000 (e.g. 0.5 mg/l = 500g/l). Microequivalents per liter (eq/l) is alsosometimes used, especially for alkalinity; it iscalculated by dividing the weight of thecompound by 1000 and then dividing thatnumber into the milligrams per liter.

Conductivity (specific conductance): Measureswaters ability to conduct an electric current.Conductivity is reported in micromhos percentimeter (mhos/cm) and is directly related tothe total dissolved inorganic chemicals in thewater. Values are commonly two times the waterhardness unless the water is receiving highconcentrations of contaminants introduced byhumans.

Drainage basin: The total land area that drainstoward the lake.

Drainage lakes: Lakes fed primarily by streamsand with outlets into streams or rivers. They aremore subject to surface runoff problems butgenerally have shorter residence times thanseepage lakes. Watershed protection is usuallyneeded to manage lake water quality.

Dystrophic lake: A typically brownish-coloredlake high in dissolved organic substancesassociated with bog vegetation. Does not followeutrophications normal pattern because of

natural acidity or other chemical imbalances.Epilimnion: see Stratification.

Eutrophication: The process by which lakes areenriched with nutrients, increasing theproduction of rooted aquatic plants and algae.The extent to which this process has occurred isreflected in a lakes trophic classification:oligotrophic (nutrient poor), mesotrophic(moderately productive), and eutrophic (veryproductive and fertile).

Filamentous algae: Algae that forms filaments ormats attached to sediment, weeds, piers, etc.

Flushing rate: see Retention time.

Food chain: The sequence of algae being eatenby small aquatic animals (zooplankton) which inturn are eaten by small fish which are then eaten

by larger fish and eventually by people orpredators. Certain chemicals, such as PCBs,mercury, and some pesticides, can beconcentrated from very low levels in the water totoxic levels in animals through this process.

16


19/20

Groundwater drainage lake: Often referred to asspring-fed lake; has large amounts ofgroundwater as its source, and a surface outlet.Areas of high groundwater inflow may be visibleas springs or sand boils. Groundwater drainagelakes often have intermediate retention timeswith water quality dependent on groundwaterquality.

Hardness: The quantity of multivalent cations(cations with more than one +), primarily calcium(Ca++) and magnesium (Mg++) in the waterexpressed as milligrams per liter of CaCO3.Amount of hardness relates to the presence ofsoluble minerals, especially limestone, in the lakewatershed.

Hypolimnion: see Stratification.

Impoundment: Manmade lake or reservoirusually characterized by stream inflow and

always by a stream outlet. Because of nutrientand soil loss from upstream land use practices,impoundments ordinarily have higher nutrientconcentrations and faster sedimentation ratesthan natural lakes. Their retention times arerelatively short.

Ion: A charged atom or group of atoms that hasseparated from an ion of the opposite charge. Inwater, some chemical molecules separate intocations (positive charge) and anions (negativecharge). Thus the number of cations equals thenumber of anions.

Insoluble: incapable of dissolving in water.

Kjeldahl nitrogen: The most common analysisrun to determine the amount of organic nitrogenin water. The test includes ammonium andorganic nitrogen.

Limiting factor: The nutrient or condition inshortest supply relative to plant growthrequirements. Plants will grow until stopped bythis limitation; for example, phosphorus insummer, temperature or light in fall or winter.

Macrophytes: see Rooted aquatic plants.

Marl: White to gray accumulation on lake

bottoms caused by precipitation of calciumcarbonate (CaCO3) in hard water lakes. Marlmay contain many snail and clam shells, whichare also calcium carbonate. While it graduallyfills in lakes, marl also precipitates phosphorus,resulting in low algae populations and goodwater clarity. In the past, marl was recoveredand used to lime agricultural fields.

Metalimnion: see Stratification.

Nitrate (NO3-): An inorganic form of nitrogen

important for plant growth. Nitrogen is in thisstable form when oxygen is present. Nitrateoften contaminates groundwater when wateroriginates from manure pits, fertilized fields,lawns or septic systems. High levels of nitrate-

nitrogen (over 10 mg/l) are dangerous to infantsand expectant mothers. A concentration ofnitrate-nitrogen (NO3

N) plus ammonium-nitrogen (NH4

N) of 0.3 mg/l in spring willsupport summer algae blooms if enoughphosphorus is present.

Nitrite (NO2-): A form of nitrogen that rapidly

converts to nitrate (NO3) and is usually included

in the NO3 analysis.

Overturn: Fall cooling and spring warming ofsurface water increases density, and graduallymakes temperature and density uniform fromtop to bottom. This allows wind and wave actionto mix the entire lake. Mixing allows bottomwaters to contact the atmosphere, raising thewaters oxygen content. However, warming mayoccur too rapidly in the spring for mixing to beeffective, especially in small sheltered kettlelakes.

Phosphorus: Key nutrient influencing plantgrowth in more than 80% of Wisconsin lakes.Soluble reactive phosphorus is the amount ofphosphorus in solution that is available to plants.Total phosphorus includes the amount ofphosphorus in solution (reactive) and in

particulate form.Photosynthesis: Process by which green plantsconvert carbon dioxide (CO2) dissolved in waterto sugar and oxygen using sunlight for energy.Photosynthesis is essential in producing a lakesfood base, and is an important source of oxygenfor many lakes.

Phytoplankton: see Algae.

Precipitate: A solid material which forms andsettles out of water as a result of certain negativeions (anions) combining with positive ions(cations).

Retention time (turnover rate or flushing rate):The average length of time water resides in alake, ranging from several days in smallimpoundments to many years in large seepagelakes. Retention time is important indetermining the impact of nutrient inputs. Longretention times result in recycling and greaternutrient retention in most lakes. Calculateretention time by dividing the volume of waterpassing through the lake per year by the lakevolume.

17


20/20

Authors: Byron Shaw is a professor emeritus of soil and water science, University of WisconsinStevens Point, and awater quality specialist with the University of WisconsinExtension, Cooperative Extension. Lowell Klessig is a professoremeritus of resource management at the University of WisconsinStevens Point and a lake management specialist withthe University of WisconsinExtension, Cooperative Extension. Christine Mechenich was formerly an Extensiongroundwater education specialist with the Central Wisconsin Groundwater Center, University of WisconsinStevensPoint.

Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S.Department of Agriculture, University of WisconsinExtension, Cooperative Extension. University ofWisconsinExtension provides equal opportunities in employment and programming, including Title IX and ADArequirements. If you need this information in an alternative format, contact the Office of Equal Opportunity and DiversityPrograms of call Extension Publishing at 608-262-2655.

2004 by the Board of Regents of the University of Wisconsin System. Send inquiries about copyright permission to:Cooperative Extension Publishing Operations, 103 Extension Building, 432 N. Lake St., Madison, WI 53706.

To see more publications or to order copies of this publication, visit our web site at http://cecommerce.uwex.eduor call toll-free: 877-WIS-PUBS (947-7827).

Understanding Lake Data (G3582) RP-03/2004

Respiration: The process by which aquaticorganisms convert organic material to energy. Itis the reverse reaction of photosynthesis.Respiration consumes oxygen (O2) and releasescarbon dioxide (CO2). It also takes place asorganic matter decays.

Rooted aquatic plants (macrophytes): Refers tohigher (multi-celled) plants growing in or nearwater. Macrophytes are beneficial to lakes

because they produce oxygen and providesubstrate for fish habitat and aquatic insects.Overabundance of such plants, especiallyproblem species, is related to shallow waterdepth and high nutrient levels.

Secchi disc: An 8-inch diameter plate withalternating quadrants painted black and whitethat is used to measure water clarity (lightpenetration). The disc is lowered into water untilit disappears from view. It is then raised until

just visible. An average of the two depths, takenfrom the shaded side of the boat, is recorded asthe Secchi disc reading. For best results, thereadings should be taken on sunny, calm days .

Sedimentation: Accumulated organic andinorganic matter on the lake bottom. Sedimentincludes decaying algae and weeds, marl, andsoil and organic matter eroded from the lakeswatershed.

Seepage lakes: Lakes without a significant inletor outlet, fed by rainfall and groundwater.Seepage lakes lose water through evaporation

and groundwater moving on a down gradient.Lakes with little groundwater inflow tend to benaturally acidic and most susceptible to theeffects of acid rain. Seepage lakes often have longresidence times. and lake levels fluctuate withlocal groundwater levels. Water quality isaffected by groundwater quality and the use ofland on the shoreline.

Soluble: capable of being dissolved.

Stratification: The layering of water due todifferences in density. Waters greatest densityoccurs at 39oF (4oC). As water warms during thesummer, it remains near the surface while colderwater remains near the bottom. Wind mixing

determines the thickness of the warm surfacewater layer (epilimnion), which usually extendsto a depth of about 20 feet. The narrow transitionzone between the epilimnion and cold bottomwater (hypolimnion) is called the metalimnion orthermocline.

Sulfate (SO4=): The most common form of sulfur

in natural waters. The amounts relate primarilyto soil minerals in the watershed. Sulfate (SO4)can be reduced to sulfide (S=) and hydrogensulfide (H2S) under low or zero oxygenconditions. Hydrogen sulfide smells like rotteneggs and harms fish. Sulfate (SO4

=) input from

acid rain is a major indicator of sulfur dioxide(SO2) air pollution. Sulfate concentration is usedas a chemical fingerprint to distinguish acid lakesacidified by acid rain from those acidified byorganic acids from bogs.

Suspended solids: A measure of the particulatematter in a water sample, expressed inmilligrams per liter. When measured oninflowing streams, it can be used to estimate thesedimentation rate of lakes or impoundments.

Thermocline: see Stratification.

Trophic state: see Eutrophication.

Turnover: see Overturn.

Watershed: see Drainage basin.

Zooplankton: Microscopic or barely visibleanimals that eat algae. These suspendedplankton are an important component of the lakefood chain and ecosystem. For many fish, theyare the primary source of food.

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Understanding Lake Data