A Beginner’s Guide To Water Management — Color

Part 1Two Ways to Define Color in a WaterbodyThere are two basic terms commonly usedwhen referring to the color of the water ina lake or waterbody. One is known asapparent color and the other as true color. Whencommunicating with lake residents, lake managers and others, it’s helpful to know the differencebetween the two. In Part 1, we’ll begin ourdiscussion by defining these two types of lakecolor and in Part 2, we will go into greater detailabout what influences both types. In Part 3, we’lldelve even deeper to see how color can influencethe biology of a lake (i.e., the amount of algae,aquatic plants and wildlife).Suspended substances can include algal matter(i.e., floating in the water column or stirred upbottom sediments due to wind mixing or boatingactivity) or non-algal matter such as finely groundcalcium carbonate particulates from limestone.Depending on the source, these substances mayimpart any number of colors to the water, includinga blue-green tint, various shades of brown, gray,green or even orange.Dissolved substances often include metallicions of iron and manganese from natural sources(e.g., rocks and soils) as well as humic acids andtannins derived from organic matter (e.g., deadleaves and plants, etc.). These substancesusually impart a reddish or brown stain to thewater.Apparent ColorApparent color is the color of the water asseen by the human eye. For example, when aperson looks into a lake, the water may appear tobe colorless (i.e., clear), blue, green, yellow, red,brown, black or somewhere in between.Most of the time, apparent colors are theresult of substances that are either suspended ordissolved in the water column. However, thereare other factors that can affect the apparent colorof a lake including the color of the lake bottom(i.e., is it light or dark?); the depth of the lake;reflections from the sky, trees or structures surrounding the waterbody; and the presence orabsence of aquatic plants. Some of these factorscan be difficult to measure, which is why lakemanagement professionals prefer to use true colormeasurements when assessing a lake.Forel-Ule color scale, a system developed byEuropean lake scientists. The scale classifies lakecolor into 22 categories ranging from blue, greenish blue, bluish green, green, greenish yellow,yellow and brown. Using the scale, one candetermine the apparent color of a lake by visuallymatching (i.e.,, with the naked eye) the color ofthe water with the Forel-Ule color spectrum.In Europe, this system has been used as away of defining the various levels of a lake’sproductivity: lakes with water that appears to beblue are considered to be less productive. Green,yellow and brown lakes are considered to be moreproductive. In the United States, the Forel-Ulesystem is seldom used for measuring the apparentcolor of water because it is cumbersome anddifficult to use.See page 2 for information on true color.Measuring Apparent ColorSome water management professionalsassess the apparent color of lake water using the1

Criteria used to determine apparent color isconsidered to be somewhat subjective whereastrue color is based on actual water analysismeasurements.Illustrating the Difference BetweenSuspended and Dissolved SubstancesTrue ColorThe following activities provide a handy visualaid for understanding the difference betweensuspended and dissolved substances:True color is defined as the color of waterresulting from dissolved substances only; allsuspended substances have been removed andare therefore not allowed to “conceal” or influencethe color of the water. In the United States,when lake management professionals talk about“color,” they are generally referring to truecolor.Suspended substancesFill a clear glass jar with water. Next, find severalpieces of chalk and break them up or grind them into acoarse powder. Add this mixture to the water. If youwere to stir the mixture vigorously with a spoon, thechalk would become suspended into the water,changing the overall color. Depending on how muchagitation is used, the chalk may stay in the watercolumn, sink to the bottom or float to the top.Measuring True ColorTrue color is determined by first filtering awater sample to remove all suspended substances.After the samples have been filtered, they arecompared to a specific color scale. This comparisonis generally done in a laboratory with a spectrophotometer.In the United States, the most commonlyused color scale is the platinum-cobalt color scale.This system is comprised of 1,000 color units orplatinum-cobalt units (PCU or Pt-Co units). Ifone were to use the platinum-cobalt color scale tomeasure lake water that is especially clear (i.e.,colorless), the color readings would probably beless than 10 PCU, whereas lakes that have a littlecolor will have a true color measurement rangingfrom 20 to 50 PCU. On the far end of the spectrum, lake water that is extremely dark in colorwill have a color reading of 500 PCU or higher.In Florida lakes, true color generally rangesfrom 5 PCU to 600 PCU. Of course, there arealways exceptions: Lake Charles in MarionCounty has shown true color readings approaching 700 PCU! To the naked eye, such a lake appearsto be almost black in color.Dissolved SubstancesNow fill a clear jar with water and place three to fourtea bags into it and place it out in the sun for about 30minutes. Tannins from the tea leaves will begin to stainor color the water. This is an example of dissolvedsubstances. The same process can be found in lakesthat are colored by tannins from decaying leaf material,plant stems or roots found within the waterbody and/orsurrounding watershed.Important Points to Remember AboutApparent Color Most of the recognizable apparent color in alake is the direct result of suspended substancesin the water (i.e., both algal and non-algal matter). Apparent color can change significantly oncesuspended substances are filtered out of a watersample. That is why scientists rely less on apparentcolor when studying lakes and instead, usuallyinsist on taking true color measurements beforecoming to any conclusions about lake color.Important Points to Remember About True Color True color measurements are especially helpful Nutrients can influence the apparent color ofwater indirectly by increasing algal populations.(e.g., Once algal populations increase, the algaethemselves are known to release organic substancesthat can tint the water various shades of green oreven brown.)to lake scientists because they provide a standardized way of assessing the color of a waterbody. True color is a component of apparent color. True color does not stay constant; it can increase2

dramatically, especially after prolonged rainevents or it can decrease under severe droughtconditions.suspended particles, true color measurements willnot necessarily coincide with apparent color. Forexample, algae or clay particles can make the waterappear to be a certain color, but once the particlesare filtered out, its appearance may change significantly. That’s one reason why lake managementprofessionals prefer to use true color measurementsinstead of apparent color. Waterbodies with limited suspended particleswill generally have true color measurements thatcoincide with the visual appearance of the water.However, in waterbodies with an abundance ofHow Does Florida LAKEWATCH Measure True Color?Florida LAKEWATCH measures the true color of lake water using the platinum-cobalt color scale.These measurements are processed two to four times a year from water samples that volunteers provide(i.e., the same samples that are being processed for total nitrogen and total phosphorus analysis eachmonth). Usually, an equal amount of water is taken from each of the bottles collected for a lake, for a givenmonth. The water is then combined to form an ‘average’ color sample for that lake. In some instances,water samples from individual stations are analyzed separately, particularly if there are obvious visible colordifferences between the sampling stations.Water is filtered to remove any particulate material.Afterwards, the sample is spun in a centrifuge,*measured on a spectrophotometer,** and comparedto a series of platinum-cobalt standards(i.e., standards that simulate the color of lake water).At the end of the year, color measurements are averagedfor each lake and that number is included as part of theannual periodic water chemistry data.Color data from Florida LAKEWATCH lakes can beobtained by: accessing the annual data report on the FLWwebsite: http://lakewatch.ifas.ufl.edu/ calling 1-800-LAKEWATch (1-800-525-3928) toobtain a copy of the printed page from the annualreport; requesting a printout of the annual data packet,which now includes color data in the form oftables and graphs.* A centrifuge is a machine that uses centrifugal force(i.e., intense spinning) for separating substances ofdifferent densities.Note: Florida LAKEWATCH evaluated frozen watersamples among a wide range of true color valuesand they did not change significantly over time.** A spectrophotometer is an instrument that is used formeasuring the relative intensities of light found in differentparts of a light spectrum.3

David WatsonGrasshopper Lake in Lake County.An Anecdote About True ColorTrue color has been found to be strongly linked to the amount of seasonal rainfall awatershed receives and the amount of runoff that seeps into a waterbody. This phenomenon has been documented with LAKEWATCH data numerous times. For example, in1993 -1994, Grasshopper Lake, in LakeCounty, had Secchi depth values greaterthan 12 feet. This happened to be during atime of extremely dry weather. Followingheavy rains in 1995 - 1996, the same lakeDavid Watsonhad Secchi depth measurements of lessthan three feet. The difference in waterclarity was associated with a change inBladderwort (Utricularia spp.), a submersedplant, grows abundantly in Grasshopper Lakewhen water clarity is sufficient.true color from 0 PCU in the dry years tomore than 50 PCU after the heavy rains.4

Figure 1 Color Frequency in Florida LakesThe graph below is an illustration of the frequency of occurrence that color valuesoccurred in lakes throughout Florida. To be more specific, 3,223 true color measurements have been collected and plotted from 670 waterbodies, located within 48 Floridacounties. Color measurements ranged from 0 PCU to 930 PCU, with a median of 18 PCU.Because there are very few lakes that had color measurements exceeding 500, they-axis in this graph (i.e., Frequency of Occurrence) stops at 500. Notice the left portionof the graph clearly shows that the lion’s share of lakes in this data set (i.e., about 79percent) had color measurements that were less than 50 PCU.The highest true color value sampled withinthe LAKEWATCH database was Charles Lakein Marion County, which had a true colormeasurement of 930 PCU! 5

Amy RichardSuspended algal matter can easily be seen in this glass beaker. The sample was pulled from theadjacent pond at the UF/IFAS Department of Fisheries and Aquatic Sciences in Gainesville.6

Part 2More About Suspended and Dissolved SubstancesNow that we’ve learned about the twobasic ways that lake scientists definecolor, we will discuss suspended substances and dissolved substances in greater detail,as they are particularly important to understandingthe color of a waterbody.A Mystery Color?During the months of March, April or May, manyFlorida lakes have been known to take on a brightyellow hue. As a result, FLW has heard from manypeople who are under the impression that an algalbloom is occurring in their lake when, in fact, whatthey are seeing is pine and oak pollen floating onthe surface or suspended in the water column.A clue for determining whether or not it’s algae:If a yellow powdery substance has collected oncars, windows and other outdoor objects in thearea, there’s a good chance it’s pollen.Suspended SubstancesThere are any number of naturally occurringsuspended substances that can be found in Floridalakes or waterbodies. In lake management circles,they are also referred to as suspended matter orparticulates and they are usually classified intotwo basic groups: algal matter, which consist ofalgae cells suspended in the water column andnon-algal matter which includes fine soil particlesor non-living plant material. Both are described ingreater detail below. Yellowish-brown colors are frequently noticedin waterbodies where diatoms dominate the algalpopulation. Botryococcus (pronounced Ba - TREE - o - cockus)is a type of algae that gives many Florida lakes arusty or orange-brown color. It is often mostvisible during afternoon hours when it tends tofloat to the surface. At times, Botryococcus producesan oily sheen on the water, fooling people intothinking there’s been a gasoline or oil spill.Algal MatterIn many cases, apparent color in Floridalakes is due to large concentrations of algaesuspended in the water. In other words, if thereare enough algae in the water column, lake waterwill appear to be the same color of the actual algalcells. Sometimes, this results in a short-term event— an algae bloom for example — or sometimeslakes maintain a particular color for many monthsor years, due to the presence of a dominant algalspecies. Depending on the species and theamount of algal cells in the water, such bloomscan impart a variety of colors to the water: Many turbid lakes display a green hue due togreen chlorophyll pigments within the algae.However, at times, some waterbodies have beenknown to develop a blood-red color. The cause ofthis red coloration is the alga Euglena, whichproduces a red pigment during intense periods ofsunlight to protect its green chlorophyll pigment. Blue-green algae are dominant in the moreeutrophic lakes and impart a dull-green appearanceto the water. When large amounts of blue-greenalgae float to the surface, it may look like someonedumped a bucket of blue-green paint into the water.Non-algal MatterSuspended particulate matter that is not ofalgal origin can also influence the apparent colorof water. This includes both organic matter (e.g.,tiny particulates from dead aquatic and terrestrial7

plants) or inorganic particles (e.g., clay, sand, soils).These materials are usually introduced to a lakefrom storm-water runoff or erosion of the shoreline.In Florida, these lakes, which are often describedas “muddy,” are in the minority. However, they doexist. In the northern part of the state, some lakesreceive large amounts of red clay resulting in adistinctive reddish “Georgia clay” appearance.Lake Talquin and Lake Seminole are good examples.Other lakes receive inputs of grayish-white coloredclay, giving them a milky white appearance.In flatter parts of the state, erosion or runoffrelated color is rare. However, the central andsouthern portion of Florida does have its share oflakes that are influenced by non-algal suspendedsediments from within the lake itself. Lake Okeechobee and Lake Apopka are prime examples. InOkeechobee, water depth at numerous mid-lakelocations is typically 2.7 meters (8.8 feet) and inLake Apopka, the average mid-lake depth is 1.7meters (5.6 feet). Because of the shallow depthand large amount of fetch in both lakes, wind isable to constantly re-suspend sediments from thebottom and mix them throughout the watercolumn causing changes in apparent color.1 This isknown as turbidity-related color. Needless to say,water clarity at these same mid-lake locations isquite low (i.e., as measured by a Secchi disk),typically less than 0.33 meters (one foot).There are also instances in which colloidalsubstances (i.e., particles tiny enough to passthrough a filter) remain in a sample and affectwater color. This is particularly true in limerockpits where inorganic materials such as calcium andmagnesium carbonates will give water a green oremerald hue.Can nutrients, such as nitrogen andphosphorus, add color to water in a lake?Phosphorus and nitrogen are nutrients found invirtually every lake or waterbody. They are alsonaturally occurring in plants and soils. In fact,Florida’s phosphorus-rich soils are what motivatedmany farmers to move to Florida in the early1900s. Phosphate mines are also prevalent invarious regions of the state, for the same reason.As far as color is concerned, when nitrogenand phosphorus are dissolved in water, theinorganic compounds are generally colorless sothey don’t really add to the apparent and/or truecolor of a waterbody directly. However nutrientscan affect color indirectly by influencing the growthof algae.Example 1: In lakes where algae are abundant,the apparent color of the lake is affected becauseyou are seeing the color of algal cells in highdensities.Example 2: Should algae concentrations beginto increase in lakes that previously had low algalabundance, one of the first things people notice isa shift in from a bluish color to various shades ofgreen. This change is largely due to the release oforganic matter from within the algal cells, whichwill be evident in a true color measurement.For more on nutrients and algae, see A Beginner’sGuide to Water Management — Nutrients (Circular 102).types of terrestrial and aquatic plants. There areliterally hundreds of lakes in Florida that are colored due to the presence of these substances. (SeeFigure 1 on page 5.) As mentioned in Part 1, LakeCharles, in the Ocala National Forest, is a goodexample of this type of lake; the clear brown teacolored water is the result of humic acids enteringthe waterbody from the surrounding watershed.Lakes with small amounts of these substanceswill generally appear green in color. (In thisinstance, the color is not related to algae.) As thewaterbody receives more dissolved organic matter,the color will begin to shift from green to yellowgreen, to a yellow-brown and then a “clear” brown.In addition to the compounds described above,algae can be another source of dissolved organicmatter in water. The substances are releasedDissolved SubstancesDissolved substances can affect both true andapparent color. These substances enter lakes via avariety of pathways including surface water runofffrom the surrounding watershed, following rainevents, and the leaching of organic compounds fromdecomposing plant material within the lake itself.OrganicThe dominant dissolved substances found inwater are typically organic compounds includinghumic acids and tannins that originate from many1 Fetch is the distance that wind can travel over waterbefore intersecting a land mass.8

directly into the water from the algal cells. This typeof organic matter can change the true color of alake by affecting light absorption. (See Part 3 formore on this.) However, when algae are veryabundant, they can also affect apparent color asdescribed on page 7. (See section on Algal Matter.)InorganicDisso

A Beginner’s Guide to Water Management – Water Clarity (Circular 103) Anyone interested in the subject of water clarity can benefit from reading this 36-page circular. Topics include the many factors that can affect water