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2000 LAKEWATER QUALITY REPORT
INTRODUCTION
The
City of Orlando currently contains 104 named water bodies that lie within
five (5) major drainage basins. Eighty-one
of the water bodies 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 eleventh 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 35 City lakes during 2000.
Orange County Environmental Protection Department provides
quarterly data for 18 City lakes, which is also used in this report.
Data presented in the 2000 Water Quality Report is based on a total
of 608 sampling events on 94 lakes, which is an average of
6.5 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 Stormwater
Utility Bureau 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
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 three
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 CORR) and fecal coliform (SM9222D).
Metals were collected by the City and analyzed during one sampling
quarter for each lake during 2000. The
following metals were analyzed in each lake and method references are as
follows: beryllium (EPA 210.2), cadmium (EPA 213.2), chromium (EPA 218.2),
copper (EPA 220.2/7211), iron (EPA 200.7), lead (EPA 239.2/7420), mercury
(EPA 245.1), nickel (EPA 200.7), selenium (EPA 270.2), silver (EPA 272.2),
and zinc (EPA 200.7).
Orange County samples were analyzed for the above-mentioned
parameters (except metals) 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 SVR3‑DL Hydrolab
Surveyor III. 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 Hydrolab was calibrated prior to, and following each day’s
sampling event. Mean depth data was determined for approximately half of lakes by running a transect for each lake along the 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. CURRENT
TROPHIC STATES 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 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. The trophic
state of a lake is determined by measuring key water quality parameters
and entering these values into an empirical formula, which calculates a
numeric value. The parameters
used to determine the trophic state of a lake are total nitrogen, total
phosphorus, chlorophyll‑a (measures planktonic algae densities), and
secchi depth (measures water transparency).
These parameters are all correlated, and the index allows for a
better measure of overall trophic state than using individual indicators.
The index calculates a number that can be assigned to a particular trophic
state.
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 Trophic State Index values were
less than 50. During 2000,
30.9% 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 2000, the percent of City lakes with eutrophic
conditions was 21.3%. 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 60. During
2000, the percent of City lakes with mesotrophic conditions was 40.4%.
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
2000, the percent of City lakes exhibiting hypereutrophic conditions was
7.4%.
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 (67.0%) of City lakes have good water quality, 24.5% have
fair water quality and 8.5% 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 George to 88 for Lake Davis, based on
average annual data for the 94 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 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 2000 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 multiple lakes to an absolute maximum
value of 0.639 mg/l in Lake Davis. 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, 31.2% 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
<0.004 mg/l to an absolute maximum value of 0.075 mg/l in Lake Como.
Orthophosphate concentrations were generally low with a median
value of <0.004 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 0.15 mg/l in Lake
Farrar to an absolute maximum of 5.37 mg/l in Lake Davis.
The median value was 0.77 mg/l.
A commonly used critical value for nitrogen is 1.00 mg/l (Brezonik
1984). Using this criterion,
29.8% 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 <0.03 mg/l.
The maximum value for ammonia was 0.53 mg/l in Lake Como.
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 0.82 in Lake Farrar. The median value for nitrate was <0.02 mg/l.
Nitrite values were generally low with 62.8% of the lake having
average values below the detection limit of <0.005 mg/l.
The maximum nitrite value was 0.200 mg/l in Lake Beauty.
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 (58.5%) of City lakes are
balanced, so reductions in phosphorus or nitrogen should reduce
productivity. Phosphorus and nitrogen limited lakes comprised 30.9% and
10.6% 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 739 mg/m3 in Lake
Davis. The median value of
chlorophyll‑a was 16.4 mg/m3.
Chlorophyll‑a values greater than 20 mg/m3 are
commonly used to identify problem lakes.
Using these criteria 45.7 % 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 Davis to 5.4 m
(17.7 ft) in Lake George. The
median value for secchi depth was 1.1 m (3.6 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 <2.0 mg/l to 15.8 mg/l in Lake Angel. The
median value was 3.0 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 97.9% having average pH
values greater than 7.0. The
range of pH values was from 6.0 in Lake Nona to 9.7 in Lake Davis.
The median value for pH was 7.87.
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 55.3% 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 < 2.0 mg/l to 141.0
mg/l in Lake Beardall. The
median value for alkalinity was 46.0 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 7.4% of City lakes are highly susceptible
to acidification, 6.4% are moderately susceptible and 86.2% have a low
susceptibility to acidification. The
Orlando lakes with the lowest alkalinity are Red Lake, Lake Nona, and Mud
Lake which all have values < 4.0 mg/l. 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 two lakes having average dissolved solid values below 200 mg/l.
Lake Sandy had the highest average dissolved solids at 302 mg/l
followed by Lake of the Woods with 206 mg/l. The absolute range for
dissolved solids were 42 mg/l in Lake Rowena to 302 mg/l in Lake Sandy.
The median dissolved solids value was 130 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 solids in the majority of the lakes are organic in
nature with only two lakes having less than one-half of the measured total
suspended solids in the volatile fraction.
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 110 mg/l in Lake Davis. The
median value for total suspended solids was 5.0 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 180 umhos@ 250C with a
range of 56 umhos@ 250C in Mud Lake to 561 umhos@ 250C
in Lake Sandy.
Water quality data collected for the past ten years indicate the
overall water quality is fairly constant in City lakes (Table
3). A comparison of median concentrations from 1990-2000 does
indicate annual variation but no overall increasing or decreasing trends. METAL CONCENTRATIONS (Next Section) (Previous Section) (Top of Page) Beginning in 1994, the lake-monitoring program was expanded by performing a metal scan for each City Lake. Metal concentrations were of interest because of the potential toxicity to aquatic organisms and the lack of data for these parameters in City lakes. While collecting samples for standard water quality parameters such as solids and nutrients is important to characterize our lakes, this data provides little information on toxicity. On the other hand, pollutants such as metals can seriously affect the health of a lake by exerting a toxic effect on wildlife such as fish and birds. Organisms such as zooplankton and invertebrates, which are important components of a lake's food chain, can be sensitive to contamination by pollutants such as heavy metals.
Potential sources of metals in surface waters are stormwater runoff,
illicit dumping, or industrial discharges.
The pH of a lake can also influence the concentrations of metals
found in the water column.
Metal concentrations in stormwater runoff can be significant.
Studies by the City of Orlando Stormwater Utility Bureau found that
detectable levels of cadmium, copper, lead and zinc are commonly found in
Orlando's stormwater runoff.
These heavy metals are generated from sources such as wear of
automobile parts or atmospheric deposition and are carried to lakes by
stormwater runoff.
Results
of metal scans for City lakes are as follows:
Beryllium
was detected in 2 of the 86 lakes sampled for metals (Table
4). Lake
Monterey had a beryllium concentration of 0.00091, which exceeded the
0.00013 mg/l State Class III water quality standards for this metal. State
Class III water bodies are designated for recreation, propagation and
maintenance of a healthy well balance population of fish and wildlife.
The detection limit for beryllium was 0.00010 mg/l, which is the
minimum concentration that the laboratory can measure a given parameter.
Results that are reported as below detectable limits should not be
interpreted to mean the metal was not present.
The metal may have been present but at a lower concentration than
the analytical procedures could detect.
Cadmium was present in 15 lakes above the detection limit of
0.0001 mg/l. The only City lakes which exceeded State Class III water
quality standards for this metal was Buck Lake and Rock Lake, which had
cadmium concentrations of 0.0007 mg/l and 0.0002 mg/l respectively. The
State Class III water quality standard for this metal is based on the
hardness of the water and requires a calculation to determine.
Chromium concentrations did not exceed the hardness-based Class
III water quality standard in any City lakes.
All chromium concentrations were below the detection limit of 0.005
mg/l.
Copper was present above the detection limit of 0.0020 mg/l in
25 of the City lakes. The
maximum copper concentrations reported were 0.0706 mg/l, which occurred in
Lake Monterey. Five City
lakes had copper concentrations above the hardness based State Class III
standard for this metal. Copper
is sometimes used in lakes as a herbicide to control algae blooms and is
also common in stormwater runoff.
Iron was present above the detection limit of 0.0300 mg/l in
54 of the 86 lakes surveyed. The
range for iron concentrations was <0.0300 mg/l in many lakes to 1.342
mg/l in Lake Pat. All City
lakes were below the State Class III water quality standards of 1.0 mg/l
except Lake Pat. Iron
is a naturally abundant metal with low toxicity and plays an important role in controlling phosphorus
concentrations in the water column of lakes.
Lead
results indicated concentrations above the detection limit of 0.0030 mg/l
in 4 lakes. Lake Davis
had the highest concentration of lead at 0.0116 mg/l. Three of the lakes
exceeded the hardness based State Class III water quality standard for
this metal.
Mercury concentrations
were below the detection limit of 0.00020 mg/l in all lakes.
The State Class III water quality standard for mercury is 0.000012
mg/l, which is lower than commonly used analytical technology can detect.
Mercury concentrations in Florida lakes are of special concern
because of high toxicity and the ability to accumulate in the food chain.
Nickel
concentrations were below the hardness based State
Class III water quality standard in all City lakes. All lakes were below the detection limit of 0.0300 mg/l for
nickel.
Selenium concentrations
were below the detection limit of 0.0050 mg/l in all lakes surveyed except
middle lobe of Lake Ivanhoe and Lake Concord, which had concentrations of
0.0055 and 0.0051 mg/l respectively.
The State Class III standard for this metal is 0.005 mg/l.
Silver was
below the detection limit of 0.00007 mg/l in all City lakes except Lake
Davis, which had a concentration of 0.0009 mg/l.
The Class III water quality standard for silver is 0.0007 mg/l. PHYSICAL
CHARACTERISTICS 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, 41.5% of Orlando's
lakes exhibited thermal stratification during at least one quarterly
sampling event in 2000 and 25.5% 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 70.7% of the lakes had average temperature differences
of <20C between top and bottom.
The range for average temperature difference between top and bottom
was 0.10C to 7.1 0C and the median value was 1.00C
(Table 5). Only a few of the deeper lakes develop stable stratification.
Average temperature differences >50C between top and
bottom were present in 7.67% of the lakes in 2000.
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, 1995,
1996, and 2000 stratification was observed most frequently during the
months of February. During
1997, 1998, and 1999 stratification was observed most frequently during
the May - July time period. This indicates that a large number of City
lakes can stratify during summer or late winter months depending on
weather conditions.
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 65.3% of the time. Despite the fact that the majority of
the lakes did not exhibit stable thermal stratification, low dissolved
oxygen was common in the lower water columns.
Dissolved oxygen values of <3.0 mg/l occurred in 67.0% of the
lakes at an average depth of 4.7 m. Anoxic conditions on the bottom of
lakes were common with bottom dissolved oxygen values of <1.0 mg/l
occurring in 59.6% 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
0.0 mg/l to 11.7 mg/l and 3.9 mg/l respectively, as opposed to surface
values ranging from 0.1 mg/l to 13.9 mg/l with a median value of 8.5 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, 68.0% had average temperature
differences between top and bottom of
<10C as opposed to 40.0% for non-aerated lakes.
This indicates that aeration tends to reduce stratification.
Surface temperatures in City lakes ranged from 14.30C
(57.70F) to 32.30C (90.1 0F) with a
median value of 23.80C (75.00F).
Maximum
water temperatures occurred in June, and minimum water temperatures
occurred in January. WATER
QUALITY TRENDS
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 had at least 11 years of data with a minimum of 4 sample
events per year. Regression
analysis indicated that 12 (12.8%) of 92 City lakes have improving water
quality trends and 8 (8.5%) have degrading water quality trends (Table
6).
The remaining 74 lakes did not show any significant change in water
quality over time. Lakes Beauty, George, Lawsona, and Sandy had the
largest improvements in water quality.
Lakes Pamela, Fairview, Weldona and Lancaster 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 of City lakes are maintaining their
water quality or improving. Further
research should be done on the lakes that are showing negative trends, to
determine the factors responsible for the changes, so that corrective
actions can be taken.
During
2000, the majority (67.0%) 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 24.5%, which is commonly used to indicate eutrophic
conditions and fair water quality. Hypereutrophic or poor water quality
conditions were found in 8.5% of the lakes.
The five lakes with the best water quality as measured by low
Trophic State Index values were George, Spring SW, Nona, Fredrica and
Porter. Except for Lake
Porter, these lakes were also ranked in the top five in 1998 and 1999.
The five lakes with the highest Trophic State Values were Weldona,
Walker, Kozart, Richmond, and Davis.
The majority (58.5%) of City lakes are nutrient balanced, so
reductions in phosphorus or nitrogen should reduce productivity.
Phosphorus limited lakes comprised 30.9% of the lakes sampled and 10.6%
were nitrogen limited.
The results of the monitoring program revealed that the majority of
City lakes are hardwater lakes with 97.9% of the lakes having average pH
values above 7.0 and 55.3% 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 7.4%
of City lakes are highly susceptible to acidification, 6.4% are moderately
susceptible and 86.2% have a low susceptibility to acidification.
Dissolved solids levels were typical of Central Florida lakes with
a median value of 130 mg/l. Total
suspended solids were generally low with a median value of 5.0 mg/l.
The suspended solids in all but one lake consisted of predominately
organic material such as algal cells.
Results of metal analysis on City lakes indicated that metal
concentrations were generally below State Class III standards.
Metals that exceeded standards in a small percent of the lakes were
beryllium, cadmium, copper, lead and selenium.
Temperature data from 2000 indicated that a substantial percent
(41.5%) of the lakes developed thermal stratification.
Most of these lakes could be considered polymitic, which means the
lake stratifies and mixes multiple times during the year.
Only a small percent (7.6%) of the lakes developed stable thermal
stratification identified by average annual temperature differences >50C
between top and bottom and the presence of a well-defined metalimnion.
During 2000, stratification occurred most frequently during the
winter months. During other years, stratification was observed most
frequently during summer months. This
data indicates that a large number of City lakes can stratify during
summer or winter months depending upon weather conditions.
Low dissolved oxygen and anoxic conditions on the lake bottoms were
more prevalent in lakes that exhibited thermal stratification, but also
occurred in non-stratified lakes. Dissolved
oxygen values of <3.0 mg/l occurred in 67.0% of the lakes at an average
depth of 4.7 m.
Regression analysis indicated that 12.8% of the lakes have
improving water quality trends and 8.5% have degrading trends.
The remaining 78.7% of the lakes did not have any significant
changes over the past several years.
LITERATURE
CITED Brezonik,
P.L. 1984. Trophic state indices: rational for multivariate approaches,
Pages 441-445 in Lake and Reservoir Management, October 18-23,
Knoxville Tenn. EPA 440/5-84-001 U.S. E.P.A.
Washington, D.C.
Canfield
and Hoyer. 1988. Regional geology and the chemical and trophic state
characteristics of Florida lakes, Lake and Reservoir Management Soc., 4(1)
21-23 pp.
Hand and Paulic. 1992. Florida Water Quality Assessment 305 (b) Technical Appendix. Department of Environmental Regulation, 21 – 25 pp. Brenner
et al.
1990. Ecosystems of
Florida, University of Central Florida Press, 376 p.
Swihart
et al. 1984.
Water
Resource Atlas of Florida, FL.
Institute of Science and Public Affairs, 77 p.
Wetzel,
R.G. 1983.
Limnology. The Dryden Press, 75 p. |
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