2002 LAKE

WATER QUALITY REPORT

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INTRODUCTION                     (Next Section)                                 (Top of Page)

          The City of Orlando currently has 104 named water bodies that lie within 5 major drainage basins.  Of those water bodies, 81 are lakes with natural origins and the remaining 23 were excavated as borrow pits or stormwater ponds.  In 1990, a lake-monitoring program was instituted in which water quality analysis and field data collection was performed on 78 City lakes.  The City has continued to collect quarterly data on each City lake since 1990 as an ongoing lake monitoring program, which has expanded in the number of lakes sampled due to annexations.  The current policy for sampling lakes is to collect data on all natural lakes.  Some manmade water bodies are also sampled if there is significant public access such as an adjacent park. This is the 13^th annual water quality report presented since the monitoring program began.

           The purpose of the monitoring program is to establish ambient water quality conditions, identify lakes with potential water quality problems, and to increase our knowledge of the chemical/physical processes that occur in our lakes.  Water quality data is used to detect water quality trends and to evaluate the effectiveness of lake restoration projects and stormwater management practices.

          The water quality database was greatly expanded because of the City’s involvement with the University of Florida's LAKEWATCH program, which began in 1991.  The LAKEWATCH program involves citizen volunteers who become involved in their lake’s management by collecting monthly field data and water samples.  LAKEWATCH volunteers collected data from 32 City lakes during 2002.  Orange County Environmental Protection Department provided quarterly data for 13 City lakes, which is also used in this report.  Data presented in the 2002 Water Quality Report is based on a total of 633 sampling events on 93 lakes, which is an average of 6.8 sampling events per lake.   

          As part of the monitoring program, a database for water quality was created in 1990. Many of the City lakes had never been monitored prior to the 1990 City’s monitoring program. Other lakes had historical data that was collected by Orange County Environmental Protection Department, but no recent data.  All current data is entered into the database that contains historical data collected by the City of Orlando, Orange County and LAKEWATCH volunteers.  A Lake Water Quality database is maintained by the Street, Drainage and Stormwater Utility Bureau’s Lake Enhancement Section and is made available to anyone interested in the historical and current water quality of any City lake. 

          An individual data sheet for each lake can be found in Appendix III.  Information provided for each lake includes: surface area, drainage basin area, mean depth, lake volume, historic and current water levels, drainage basin land uses, if the lake contains aeration and/or grass carp and whether the lake is natural or manmade.  In addition to physical data, the lake data sheets provide current and long-term water quality data including a graph, which indicates water quality trends over time for each lake.  Bathymetric maps, which detail the contours of the lake bottoms, are also provided for 60 City lakes.

METHODS AND MATERIALS      (Next Section)  (Previous Section)  (Top of Page)

Water quality samples taken by the City were collected in the pelagic (open water) zone.  All samples were collected by submerging a sample bottle to elbow depth (approximately 0.5 m). Approximately half the samples were collected using a composite method.  This means that a lake would be divided into stations and half of the sample would be collected at one station while the other half would be collected at another station.  This method produces more representative results than collecting a single sample for each lake and is more cost effective than collecting multiple samples and averaging the data.  Lake size and configuration were considered when deciding the number of samples to collect per lake.  Some lakes were sampled and treated as two separate lakes in this report. The reasoning was that even though the water bodies had the same name, they were sufficiently isolated to have different chemical characteristics.  For example, values for Lake Estelle were reported as Lake Estelle East and Lake Estelle West.  Lake Ivanhoe, Lake Cay Dee, and Lake Lucerne were also treated in this manner. 

          LAKEWATCH samples were collected by citizen volunteers at 3 open water stations per sampling event, then the data was averaged for each event.  LAKEWATCH samples were collected by submerging a sample bottle to elbow depth.  Orange County samples were collected at a single station in the lake’s center by submerging a sample bottle to elbow depth.

          A state-certified private laboratory using EPA approved methods performed water quality analyses on all City-collected samples.  The quarterly parameters analyzed for each lake and method reference are as follows: pH (EPA 150.1), alkalinity (EPA 310.1), biochemical oxygen demand (EPA 405.1), total phosphorus (EPA 365.2), ortho phosphorus (EPA 365.2), total nitrogen (TKN + NO3/NO2), ammonia nitrogen (EPA 350.1), nitrate nitrogen (EPA 300.A), nitrite nitrogen (EPA 300.A), total Kjeldahl nitrogen (EPA 351.2), total suspended solids (EPA160.2), volatile suspended solids (EPA 160.4), total dissolved solids (EPA 160.1), chlorophyll a (SM 10200H-Acid Corrected), and fecal coliform (SM 9222D).      

          Orange County samples were analyzed for the above-mentioned parameters by their state certified Environmental Protection Department laboratory. LAKEWATCH samples were analyzed at the University of Florida's laboratory for total phosphorus, total nitrogen, and chlorophyll a.

          Field data was collected using a model 600XL Multi-Parameter YSI Sonde.  Water quality parameters measured were temperature, pH, dissolved oxygen, specific conductivity, and oxidation‑reduction potential.  These parameters were measured at 0.5 meters, and then at one (1) meter intervals in the water column until the bottom was reached.  The YSI was calibrated before and after each day’s sampling event.

     Mean depth data was determined for approximately half of the lakes by running a transect along the lake’s maximum berth with a Lowrance X -16 Recording Fathometer and averaging the depths.  Sufficient bathymetric data was available for the remaining lakes to calculate a more accurate mean depth by dividing the lakes volume by surface area.

          Water elevation data was retrieved from City Survey records that contain a long-term database of monthly lake elevation measurements.  The mean high water mark was derived statistically from monthly data by calculating one standard deviation above the mean.

          Lake origin information was determined from the United States Department of Interior National Wetland Inventory Maps. 

 CURRENT TROPHIC STATES    (Next Section)   (Previous Section)   (Top of Page)

          The water quality of a Florida lake is commonly assessed by determining the trophic state of a lake.  The trophic state is a measure of the degree of productivity in the water column.  Eutrophication is the process in which a lake becomes nutrient enriched, which in turn, alters the chemical, physical, and biological characteristics of the lake.  Most of the changes in a lake’s characteristics result from increased plant production, which is in turn caused by increased levels of nutrients.

          Since most water quality problems associated with Florida lakes can be attributed to eutrophication, trophic state measurements are used by the City as the primary indicator to detect changes over time and to rank lakes by water quality.  The City uses the Florida Trophic State Index (Brezonik 1984), which was derived using data from 313 Florida lakes.  A lake is assigned a Trophic State Index (TSI) by entering key water quality parameters [total nitrogen (TN – mg/l), total phosphorous (TP – ug/l), chlorophyll a (chl a – mg/m³) for measuring planktonic algae densities, and secchi depth (SD - m) for measuring water transparency] into an empirical formula (see below) resulting in a better overall trophic state measurement rather than using individual indicators.

             Empirical procedure for calculating the Florida Trophic State Index:

            I.            Phosphorus-Limited Lakes (TN/TP >30)

                  TSI (AVG) = 1/3 [TSI (chl a) + TSI (SD) + TSI (TP)],

                           Where TSI (chl a) = 16.8 + 14.4 ln chl a, TSI (SD) = 60.0 - 30.0 ln SD, TSI (TP) = 23.6 In TP - 23.8

            II.            Nitrogen-Limited Lakes (TN/TP<10)

                  TSI (AVG) = 1/3 [TSI (chl a) + TSI (SD) + TSI (TN)]    

      Where TSI (chl a) = 16.8 + 14.4 In chl a, TSI (SD) = 60.0 - 30.0 ln SD, TSI (TN) = 59.6 + 21.5 ln TN

            III.            Nutrient-Balanced Lakes  (10 <TN/TP <30)

                  TSI (AVG) = 1/3 [TSI (chl a) + TSI (SD) + 0.5 (TSI (TP) + TSI (TN))]

      Where TSI (chl a) = 16.8 + 14.4 ln chl a, TSI (TN) = 56 + 19.8 ln TN, TSI (TP) = 18.6 In TP - 18.4,    

      TSI (SD) = 60.0 - 30.0 ln SD

          Based on the Trophic State Index value, the lake is then characterized into one of the 4 trophic states:  oligotrophic, mesotrophic, eutrophic, or hypereutrophic.

          A lake, which has low nutrient concentrations in the water column, will typically have good water transparency because the algae densities are low.  These lakes are termed oligotrophic and are generally considered as having excellent water quality.  Oligotrophic lakes tend to be deep with abundant oxygen and a small amount of organic material on the bottom.  Lakes were assigned an oligotrophic status if their Trophic State Index was less than 50.  During 2002, 34.4% of City lakes were considered oligotrophic using this criterion.

          As nutrient concentrations increase in a lake, algae density increases and water transparency decreases.  A lake with high levels of nutrients and planktonic algae is termed eutrophic.  Eutrophic lakes are highly productive and will have the potential for water quality problems resulting from severe algae blooms that can deplete oxygen and form mucky organic layers on the lake bottom.  Lakes were assigned a eutrophic status if Trophic State Index values were between 61 and 70.  During 2002, the percent of City lakes with eutrophic conditions was 20.4%.

          Lakes with moderate nutrient levels and water quality characteristics between oligotrophic and eutrophic conditions are termed mesotrophic.  Mesotrophic lakes will typically only have occasional water quality problems and are generally considered to have good water quality.  Lakes were assigned a mesotrophic status if Trophic State Index values were between 50 and 61.  During 2002, the percent of City lakes with mesotrophic conditions was 39.8%.

          Lakes that have reached advanced stages of the eutrophication process are termed hypereutrophic.  Persistent algae blooms, extreme fluctuations in dissolved oxygen and deep organic muck layers, characterize these lakes.  Hypereutrophic lakes in the City tend to be shallow and have mucky sediments from organic material, which is produced in the lake faster than it can be removed by decomposition processes.  Lakes were assigned a hypereutrophic status if Trophic State Index values were greater than 70.  During 2002, the percent of City lakes exhibiting hypereutrophic conditions was 5.4%.

          The Florida Department of Environmental Protection uses Trophic State Index values to assign a good, fair or poor rating to Florida lakes.  Trophic State Index values between 0 and 59 correspond to good water quality, 60-69 is fair and values 70 or greater indicate poor water quality (Hand et al, 1990).  Using these criteria, the majority (69.9%) of City lakes have good water quality, 24.7% have fair water quality and 5.4% have poor water quality.

           Trophic State Index values resulting from the monitoring program indicates that a wide variability in trophic conditions is present in City lakes.  Trophic State Index values ranged from 24 for Lake Nona to 93 for Lake Kozart, based on average annual data for the 93 lakes surveyed (Table 1).

          While the process of a lake going from an oligotrophic to a eutrophic state is a natural occurrence, human impact on the lake drainage basin can greatly accelerate this process.  When the eutrophication process is accelerated due to human activity such as urbanization, the process is termed cultural eutrophication.  Since most water quality problems in Florida can be attributed to cultural eutrophication, the majority of pollution abatement and lake restoration methods attempt to reverse this process through nutrient removal or inactivation. In the City of Orlando, the major source of nutrients and other pollutants entering lakes originates from stormwater runoff.  Since stormwater is recognized as a major source of pollution, Orlando has adopted strict requirements for on-site stormwater treatment and is active in retrofitting public stormwater systems to provide treatment of stormwater runoff.  

          The fact that a lake is eutrophic does not necessarily mean the condition is man-made and/or undesirable.  Many Florida lakes are eutrophic because of naturally occurring high nutrient concentrations in the soils of the watershed.  Water quality can be a matter of individual perspective and depend on primary uses for the lake.  For example, an oligotrophic lake would be desirable to individuals who swim or ski while a eutrophic lake is more productive and will tend to support larger populations of fish and wildlife for anglers and bird watchers.  

          One factor that the Florida Trophic State Index does not use in determining the trophic state of a lake is the biomass of aquatic macrophytes (vascular plants and large non-planktonic algae).  This is a missing feature since the trophic state of a lake is meant to express overall productivity, and aquatic macrophytes can comprise a substantial amount of the productivity in a lake.   Lakes that have large densities of aquatic macrophytes may have excellent water quality but may not be truly oligotrophic because they have high productivity in the form of macrophytes instead of algae.  For this reason the Trophic State Index values should only be used as an indicator of water quality, not the overall trophic condition in lakes that have abundant aquatic macrophytes.  Aquatic macrophytes are generally considered desirable because they improve wildlife habitat and can have a positive effect on water quality.  Macrophytes are considered undesirable when densities reach excessive levels that interfere with lake uses.  Also, some macrophytes, especially exotics such as hydrilla and water hyacinths, have the potential to cause severe water quality problems such as low dissolved oxygen and high organic loading, if densities are not monitored and controlled.

CHEMICAL CHARACTERISTICS  (Next Section)  (Previous Section)  (Top of Page)

          In addition to using Trophic State Index values as indicators of water quality, there are numerous other individual chemical constituents that impact water quality.  The parameters the City includes in its monitoring program and the results of the 2002 lake survey are as follows:

          Phosphorus is an essential nutrient but in high concentrations can lead to rapid lake eutrophication that results in excessive algae and aquatic plant growth.  A major source of phosphorus in urban lakes is from stormwater runoff that transports phosphorus to lakes from lawns, driveways and streets.  High levels of phosphorus in a lake may also result from naturally occurring deposits in the soils of a lake watershed.  The total phosphorus concentrations in City lakes ranged from an absolute minimum value of <0.005 mg/l in Lake Nona to an absolute maximum value of 0.333 mg/l in Lake Fairhope.  The median value was 0.034 mg/l based on average annual data.  A commonly used critical level for phosphorus, above, which a lake is considered to have the potential for trophic related problems, is 0.050 mg/l (Brezonik 1984).  Based on these criteria, 35.5% of City lakes had phosphorus levels, which can cause trophic related water quality problems (Table 2).

          Orthophosphate is a soluble form of phosphorus that can be directly utilized by algae and aquatic plants, as opposed to total phosphorus, which also measures forms of phosphorus that are temporarily unavailable for plant growth.  Orthophosphate levels ranged from an absolute minimum value of <0.005 mg/l to an absolute maximum value of 0.104 mg/l in Lake Como. Orthophosphate concentrations were generally low with a median value of 0.002 mg/l. 

          Nitrogen is a nutrient, which along with phosphorus can have a significant impact on the productivity of a lake.  Nitrogen can be introduced into a lake from pollution such as stormwater runoff or from natural sources such as rainfall or groundwater.  Some types of algae and bacteria are capable of nitrogen fixation, which converts atmospheric nitrogen gas to forms of nitrogen that can be utilized by plants.  Total nitrogen values ranged from an absolute minimum value of 0.31 mg/l in Lake Baldwin to an absolute maximum of 3.53 mg/l in Lake Fairhope.  The median value was 0.75 mg/l.  A commonly used critical value for nitrogen is 1.00 mg/l (Brezonik 1984).  Using this criterion, 25.8% of City lakes have average nitrogen levels that can cause trophic related water quality problems.

          Ammonia is a nitrogen compound that can be directly utilized by algae and larger aquatic plants.  Ammonia levels were generally low with a median value of 0.010 mg/l.  The maximum value for ammonia was 0.104 mg/l in Lake Greenwood.

          Nitrates and nitrites are inorganic forms of nitrogen that can be utilized by algae, but generally to a lesser degree than ammonia (Wetzel 1983).  Nitrate values ranged from <0.01 mg/l to 0.89 in Lake Farrar.  The median value for nitrate was 0.01 mg/l.  Nitrite values were generally low with the majority of lakes having average values below the detection limit of <0.005 mg/l.  The maximum nitrite value was 0.038 mg/l in Lake Como.

          The ratio of total nitrogen to total phosphorus can be used to determine the limiting nutrient in a lake.  The limiting nutrient is the nutrient in low concentration with respect to other nutrients.  For example, if algae needs ten (10) parts nitrogen to one (1) part phosphorus and there is an excess of nitrogen with respect to phosphorus, then the addition of nitrogen should not increase growth as long as more phosphorus does not become available.  In this case phosphorus is the limiting nutrient and efforts to decrease productivity should involve phosphorus removal.  Determination of the limiting nutrient is generally done using the criteria that lakes with nitrogen to phosphorus ratios >30 are phosphorus limited.  Lakes with nitrogen to phosphorus ratios <10 are nitrogen limited and lakes with ratios between 10 and 30 are balanced.  Based on these criteria, the majority (66.7%) of City lakes are balanced, so reductions in phosphorus or nitrogen should reduce productivity. Phosphorus and nitrogen limited lakes comprised 29.0% and 4.3% of the lakes respectively.

          Chlorophyll-a is a pigment present in all green plants and is used to measure the densities of planktonic algae.  High chlorophyll-a values indicate high planktonic algae densities, which is a result of excessive nutrients in the water. Algae blooms are the most common cause of water quality problems in City lakes.  Algae have the potential to change the apparent color of water and result in undesirable appearances such as surface scum.  In addition to creating unaesthetic appearances, high algae densities can alter water chemistry such as depleting dissolved oxygen during low light conditions and elevating pH values due to high rates of photosynthesis.  Chlorophyll-a values were extremely variable in City lakes with values ranging from <0.5 mg/m³ in multiple lakes to 97.5 mg/m³ in Lake Fairhope.  The median value of chlorophyll-a was 15.8 mg/m³.  Chlorophyll-a values greater than 20 mg/m³ are commonly used to identify problem lakes.  Using these criteria, 33.3% of the City lakes have algae associated problems.

          Secchi depth is a measure of the water transparency in a lake. Even though transparency is not a chemical measurement, it is influenced by water chemistry.  In general, increasing nutrient levels result in increased planktonic algae densities, which in turn decrease water transparency.  Other factors that affect transparency are water color and suspended solids.  The secchi depth is obtained by lowering a standard size white and black disc into the water column until it is no longer visible.  Secchi depth is a very good indicator of water quality and anyone can easily collect this data to monitor lake water for changes.  Secchi depths range from 0.1 m (0.3 ft) in Lake Kozart to 4.6 m (15.1 ft) in lake Shannon. The median value for secchi depth was 1.0 m (3.3 ft).

          The Biochemical Oxygen Demand (BOD) is a measure of the potential for oxygen uptake by organic and inorganic materials in the water.  Elevated BOD values indicate the presence of high concentrations of oxygen demanding substances in a lake. High BOD concentrations are undesirable because there is a potential for oxygen depletion, which will result in water quality problems such as fish kills and increased nutrient release from the sediments.  BOD values ranged from <1.5 mg/l to 14.4 mg/l in Lake Angel.  The median value was 3.2 mg/l.               

          The pH value of water is a measure of acidity or alkalinity.  While pure water has a pH value of 7.0, surface waters can vary considerably into the acid or alkaline range from natural causes.  The majority of lakes were alkaline with 94.6% having average pH values greater than 7.0.  The range of pH values was from 5.8 in Red Lake to 9.4 in Lake Angel.  The median value for pH was 7.9.  Lakes with high pH values tended to be eutrophic with the highest values occurring in hypereutrophic lakes.  This is to be expected because carbon dioxide concentrations can be reduced in the water column due to high rates of photosynthesis by planktonic algae, which in turn increases the pH.

          The alkalinity of a lake refers to the quantity of compounds that shift the pH to the alkaline side of the pH range.  The higher the alkalinity, the more resistance exists for pH shifts to the acidic range.  Generally, lakes with an alkalinity below 40 mg/l are considered softwater lakes and lakes with an alkalinity over 40 mg/l are considered hardwater lakes.  Softwater lakes have reduced resistance to pH changes because of low concentrations of bicarbonates, carbonates, and hydroxides that function as buffering agents.  Hardwater lakes tend to be more productive (eutrophic) than softwater lakes.  The majority of Orlando's lakes can be considered hardwater lakes with 55.9% having alkalinity values over 40 mg/l.  Orlando's lakes have higher alkalinity than Florida lakes in general.  A study of 165 Florida lakes found that less than 25% had alkalinity values greater than 40 mg/l (Canfield and Hoyer, 1981).  The range of alkalinity values for Orlando lakes was 1.2 mg/l in Lake Red to 133.0 mg/l in Lake of the Woods.  The median value for alkalinity was 44.1 mg/l. 

          Alkalinity is a good indicator of a lake’s susceptibility to acidification from atmospheric depositions such as acid rain.  Lakes with total alkalinity values below concentrations of 10 mg/l are considered sensitive to acid deposition, where as lakes with values between 10 mg/l and 20 mg/l are considered moderately susceptible (Brenner et. al., 1990).  Using this criterion only 5.4% of City lakes are highly susceptible to acidification, 6.5% are moderately susceptible and 88.1% have a low susceptibility to acidification.  The Orlando lakes with the lowest alkalinity are Red Lake, Lake Nona, Turkey Lake, Mud Lake and Buck lake which all have values < 10 mg/l. with the exception of Turkey Lake, these lakes are a group of almost pristine softwater lakes located in southeast Orlando.

          Dissolved solids are a measure of the amount of organic and inorganic material in solution.  These materials are mainly bicarbonates, chlorides, sulfates, magnesium and sodium with concentrations generally less than 200 mg/l in Central Florida lakes (Swihart et al., 1984).  Orlando lakes are typical of Central Florida lakes with all but five lakes having average dissolved solid values below 200 mg/l.  Lake Sandy had the highest average dissolved solids at 231 mg/l. The absolute range for dissolved solids was 54 mg/l in Lake Kelly to 288 mg/l in lake Sandy.  The median dissolved solids value was 128 mg/l.

          Total suspended solids and volatile suspended solids are measures of the solid material suspended in water.  Volatile solids indicate the organic fraction of total solids.  The suspended solid in all the City lakes was predominantly organic.  The higher values for solids were found in lakes with high planktonic algae densities which indicates that most of the suspended solids materials is composed of algal cells.  Total suspended solids ranged from <1.0 mg/l to a maximum value of 90 mg/l in Lake Kozart.  The median value for total suspended solids was 3.5 mg/l.  Suspended solids are important indicators of water quality because high levels entering a lake have the potential to cause severe water quality problems.  Pollutants such as heavy metals and phosphorus tend to bind to solids and are carried into lakes via stormwater runoff or from construction activities.  Erosion from construction activity can cause severe water quality problems resulting from solids entering a lake.

          Specific conductivity is a measurement of the total amounts of ionized substances dissolved in water.  High values for conductivity can be natural or caused by land use activities or discharges of contaminated water.  Orlando's lakes tend to have higher conductivity than Florida lakes in general.  A survey of 165 Florida lakes conducted by Canfield and Hoyer (1981) found a median conductivity value of 97 umhos@ 25⁰C.  The median value for Orlando was 196 umhos@ 25⁰C with a range of 71 umhos@ 25⁰C in Lake Kelly to 474 umhos@ 25⁰C in Lake Sandy.

          Water quality data collected for the past thirteen years indicate the overall water quality is fairly constant in City lakes (Table 3).  A comparison of median concentrations from 1990-2002 does indicate annual variation but no overall increasing or decreasing trends.

PHYSICAL CHARACTERISTICS   (Next Section)   (Previous Section)   (Top of Page)

          Morphological features such as size, shape, and depth have significant effects on the water quality of a lake.  Most Florida lakes are solution lakes, which were formed through sinkhole activity (Swihart et al., 1984).  This is the reason that the majority of the City's lakes are circular in shape.  Other characteristics of solution lakes are highly variable depths and tendencies to undergo fluctuations in water level in response to changes in the groundwater table (Wetzel, 1983).

          Size and depth are factors that determine if a lake will develop thermal stratification, which in turn can have an impact on nutrient and dissolved oxygen levels.  Thermal stratification is the process in which a lake develops a layer of denser cooler water that underlies a surface layer of less dense, warmer water.  The lower layer of water is termed the hypolimnion, the upper warmer layer is the epilimnion and the transitional layer, which acts as a barrier between the two, is the metalimnion.  The metalimnion, which is also called a thermocline, is usually identified by a temperature change of 1⁰C per meter (Wetzel, 1983).

          The hypolimnion does not mix with surface waters and tends to become stagnant.  The hypolimnion will become anoxic with time, which can cause sediment bound phosphorus to be released to the water column.  Water quality problems can occur when stratification occurs in lakes with nutrient rich bottom sediments.  When the metalimnion is eliminated and the nutrient-rich anoxic waters of the hypolimnion mix throughout the lake, algal blooms can occur. This is a common phenomenon in northern temperate lakes, which tend to develop stable stratification that persist throughout the winter and summer and mix in the fall and spring.  Stable stratification occurs much less frequently in Florida's semi‑tropical lakes because most of our lakes are shallow enough to be mixed by wind and the temperature differences between seasons are relatively small.

          Field data collected during the past several years indicates that stratification is somewhat common in City lakes.  Using the criteria of the presence of a metalimnion, identified by a temperature difference of 1⁰C per meter, 60% of Orlando's lakes exhibited thermal stratification during at least one quarterly sampling event in 2002 and 34% were stratified on more than one sampling event. 

          Small differences in temperature between the top and bottom of the water column indicate that most of the City's lakes are either not stratified or very weakly stratified the majority of the year.  A total of 55% of the lakes had average temperature differences of <2⁰C between top and bottom.  The range for average temperature difference between top and bottom was 0.02⁰C to 10.0⁰C and the median value was 1.4⁰C (Table 4). Only a few of the deeper lakes develop stable stratification.  Average temperature differences >5⁰C between top and bottom were present in 11% of the lakes in 2002.          

          The majority of the lakes that exhibited thermal stratification could be classified as polymitic (the lake stratifies and mixes multiple times throughout the year).  Lakes with weak stratification mix with a minimum amount of disturbance such as a moderate wind. Data indicated that stratification can be present any time of year and the time of year it occurs, fluctuates on an annual basis.  During 1994-1996, and 2000 stratification was observed most frequently in the winter during the month of February.   During 1997–1999 stratification was observed most frequently during the summer May - July time period.  In 2002, stratification occurred the most in March.  Wintertime stratification occurs when cold-water temperatures are followed by calm warm conditions that only heats lakes surface. 

          Even though stratification was usually unstable and did not persist for long periods of time, the hypolimnion tended to become low in oxygen.  During periods of stratification, bottom dissolved oxygen below 1.0 mg/l was observed 56% of the time.  Dissolved oxygen values of <3.0 mg/l occurred in 95% of the lakes at an average depth of 4.6 m during periods of stratification.  Even though the majority of the lakes did not exhibit stable thermal stratification, low dissolved oxygen was common in the lower water columns even during un-stratified conditions.  Anoxic conditions on the bottom of lakes were common with bottom dissolved oxygen values of <1.0 mg/l occurring in 71% of the lakes.  Most game fish could not survive in the lower levels of these lakes.  The range and median value of dissolved oxygen on the bottom was 0.1 mg/l to 9.5 mg/l and 2.0 mg/l respectively, as opposed to surface values ranging from 6.7 mg/l to 19.8 mg/l with a median value of 8.3 mg/l.  Even though low dissolved oxygen values were more prevalent in lakes that exhibited thermal stratification, low values were also common in lakes that were not stratified.

          Mechanical aeration units, which consist of a compressor and diffuser, were present in 24 of the lakes surveyed.  Of the lakes with aeration, 50% had average temperature differences between top and bottom of  <1⁰C as opposed to 28% for non-aerated lakes.  This indicates that aeration tends to reduce stratification. 

          Surface temperatures in City lakes ranged from 16.5⁰C (61.7⁰F) to 36.8⁰C (98.2 ⁰F) with a median value of 26.7⁰C (80.1⁰F).  Maximum water temperatures occurred in July, and minimum water temperatures occurred in February.

WATER QUALITY TRENDS           (Next Section)  (Previous Section)   (Top of Page)

          The most important reason for having an ongoing water quality program is for the determination of changes in water quality over time.  Lakes naturally fluctuate in water chemistry and quality between seasons and from year to year.  Differences in factors such as water temperature, sunlight intensity and rainfall patterns are responsible for these fluctuations.  Because of natural and random fluctuations, long term monitoring data is required to identify overall changes in water quality over time.  The City has had a comprehensive monitoring program since 1990, so we are becoming increasingly accurate in detecting trends in water quality.

Changes in water quality over time are determined using regression analysis, which is a simple model for the relationship between two variables.  The relationships of changes in Florida Trophic State Index values over time are used to determine water quality trends. This analysis generates a regression line, which indicates whether there is a positive or negative relationship between time and water quality, by the slope of the line.  Positive slopes indicate increasing trophic states over time, which is undesirable.  Negative slopes indicate decreasing trophic states, which indicate improvements.

Regression analysis generates a value called the coefficient of determination (R²), which is used to measure the percent change, which is attributable to time.  For example a R² of 0.40 would indicate that 40% of the change observed is correlated to changes in time and the other 60% of change is due to other factors such as random fluctuations.  Coefficient of determination values greater than 0.20 are considered significant by the State of Florida for surface water trends (Swithart et. al. 1984). 

Trend determinations for Orlando lakes were done by assigning each lake a status of improving, no trend, or degrading, depending on the line slope and R² value for each regression analysis of trophic state versus time.  Improving trends were assigned to lakes with R² values of > 0.20 and decreasing trophic states.  No trend was assigned if the R² value was <0.20.  A degrading trend was assigned for increasing trophic states and R² values > 0.20.  The majority of lakes (83 out of 93) had at least 12 years of data with a minimum of 4 sample events per year.  Analysis was not performed on lakes with less than 5 years of data.

Regression analysis indicated that 10 (11.3%) of 88 City lakes have improving water quality trends and 6 lakes (6.8%) have degrading water quality trends (Table 5).  The remaining 65 lakes (81.8%) did not show any significant change in water quality over time. Lakes Beauty, Sandy, George, and Rabama had the largest improvements in water quality.  Lakes Mann, Bay Pamela and Red had the highest rates of declining water quality. 

Even though trend analysis indicates that some lakes are degrading, it is encouraging that the vast majority (93.1%) of City lakes are maintaining their water quality or improving. Data indicated Lakes Fairview and Lancaster benefited from recent restoration work with removal from last years list of degrading lakes.  Lake Weldona remains on the degrading list but appears to be improving following a muck removal project and should be able to de-list next year.  Further research should be done on the remaining lakes that are showing negative trends, to determine the factors responsible for the changes, so that corrective actions can be taken.

SUMMARY                                (Next Section)  (Previous Section)  (Top of Page)

          During 2002, the majority (69.9%) of City lakes had good water quality using State criteria that Trophic State Index values (measures productivity of a lake) of less than 60 indicate good water quality.  The percent of lakes with Trophic State Index values between 60‑69 was 24.7%, which is commonly used to indicate eutrophic conditions and fair water quality. Hypereutrophic or poor water quality conditions were found in 5.4% of the lakes.

          The five lakes with the best water quality as measured by low Trophic State Index values were Lake Nona, Lake Shannon, Lake George, Lake Beauty and Spring Lake (Southwest).  Lake George, Lake Nona and Spring Lake (Southwest) have been ranked in the top five since 1998.  The five lakes with the highest Trophic State Values were Lake Kozart, Lake Richmond, Lake Fairhope, Lake Lawne, and Lake of the Woods.

          The majority (66.7%) of City lakes are nutrient balanced, so reductions in phosphorus or nitrogen should reduce productivity. Phosphorus limited lakes comprised 29.0% of the lakes sampled, and 4.3% were nitrogen limited.

          The results of the monitoring program revealed that the majority of City lakes are hardwater lakes with 94.6% of the lakes having average pH values above 7.0 and 55.9% having mean total alkalinity values greater than 40 mg/l.  City lakes had higher alkalinity and specific conductivity than Florida lakes in general.  High alkalinity is desirable when acidification is a concern since these lakes are less susceptible to pH changes resulting from atmospheric deposition of acid.  Only 5.4% of City lakes are highly susceptible to acidification, 6.5% are moderately susceptible, and 88.1% have a low susceptibility to acidification.    

          Dissolved solids levels were typical of Central Florida lakes with a median value of 128 mg/l.  Total suspended solids were generally low with a median value of 3.5 mg/l.

          Temperature data from 2002 indicated that a substantial percentage (60%) of the lakes developed thermal stratification.  Most of these lakes could be considered polymitic, which means the lake stratifies and mixes