TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

Appendix A-1: Lake Nutrient, Chl. a, and Clarity Distributional 

Statistics by Lake Class 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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Appendix A-1. Lake Nutrient, Chl. a, and Clarity Distributional Statistics by 

Lake Class 

One of the principal approaches for deriving candidate nutrient criteria is the exploration of 

distributional statistics associated with different waterbody classes. Indeed, original U.S. 

Environmental Protection Agency (U.S. EPA) nutrient guidance was based upon such an 

approach, whereby the 25th percentile of all site values within a class or the 75th percentile of 

reference site values within a class was recommended for deriving criteria, although other 

percentiles could be used by states/tribes as long as defensible reasons were used (USEPA 

2000a, b, c, d; USEPA 2001; USEPA 2007). Florida Department of Environmental Protection 

(FDEP) has decided to pursue such an approach for their stream criteria using the 90th percentile 

of reference or “benchmark” sites, but similar analysis has not been available for lakes. We 

conducted an analysis of distributional statistics for all lakes and reference lakes within the two 

lake color classes identified by FDEP, namely clear lakes (color < 40 platinum-cobalt units 

(PCU)) and colored lakes (color > 40 PCU). This approach is often referred to as the “all sites” 

approach, and usually involves establishing criteria using a lower distribution (e.g., 25th

percentile) of a pool of sites of undetermined ecological quality, as long as the pool is 

sufficiently large enough to represent all waters and can be presumed to reasonably reflect the 

full range of ambient conditions with a disturbance gradient from least to most impacted. Due to 

the intended scale of application and the availability of data at the time of publication, U.S. EPA 

used this method to develop and publish under the Clean Water Act (CWA) section 304(a) 

recommended nutrient criteria for streams and lakes in nutrient ecoregions IX, XII, and XIII. 

While an “all sites data distribution approach” does not directly link nutrient concentrations to 

valued ecological attributes (healthy, well-balanced populations of flora or fauna), it does make 

the presumption that the 25th percentile will closely approximate a reference condition for that 

geographic location. Several examples with data from actual reference sites have validated this 

assumption. 

Methods 

Impaired Waters Rule (IWR) data were received in MS Access 2003® format, and were 

maintained in that format for data selection. Separate lakes data sets were created in Access® 

(using VBA code) and exported as text for statistical analysis. These files were all lakes (annual 

average by station), and subsets for each Weaver nutrient region (Bone Valley, North Central, 

Northeast, Panhandle, and Peninsula for phosphorus; Panhandle and all else for nitrogen). 

Selection Criteria 

Any lake station was removed if it: 

1)Occurred in southern Florida (based on Weaver Regions and determined via geographic 

information system (GIS) analysis) 

2)Had a maximum measured salinity >=0.5 ppt 

3)Had a maximum conductivity >= 3,000 μS/cm 

4)Did not originate from any of the following organizations (exactly as listed in the 

Organizations table): 
TSD for U.S. EPA’s Proposed Numeric Nutrient Criteria for FL Inland Surface Fresh Waters

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a. Unnamed agency (21FLCOT) 

b. Unnamed agency (21FLWQSP) 

c. Unnamed agency (21FLLEON) 

d. Unnamed agency (21FLBRA) 

e. Alachua County (21FLACEP) 

f. Choctawhatchee Basin Alliance (21FLCBA) 

g. City of Lakeland, Florida (LAKELAND) 

h. City of Orlando, Florida (21FLORL) 

i. Collier County Pollution Control, Florida (21FLCOLL) 

j. Environmental Research and Design, Inc (21FLERDI) 

k. FDEP Water Quality Assessment Section (21FLWQA) 

l. FDEP Central District (21FLCEN) 

m. FDEP Charlotte Harbor Aquatic/Buffer Preserves (21FLCHAR) 

n. FDEP Northwest District (21FLPNS) 

o. FDEP Rookery Bay National Estuarine Reserve (21FLROOK) 

p. FDEP South District (21FLFTM) 

q. FDEP Southeast District (21FLWPB) 

r. FDEP Southwest District (21FLTPA) 

s. FDEP/FDEP Northeast District (21FLA) 

t. Florida Game and Freshwater Fish Commission (21FLGFWF) 

u. FDEP - OCEC (21FLCMP) 

v. FDEP (21FLGW) 

w. Hillsboro County (21FLHILL) 

x. Lake County Water Resource Management (21FLLCPC) 

y. Northwest Florida Water Management District (21FLNWFD) 

z. Orange County Environmental Protection (21FLORAN) 

aa. Palm Beach County Department of Environmental Resource Management 

(21FLPBCH) 

bb. Pinellas County Department of Environmental Management (21FLPDEM) 

cc. Polk County Natural Resources Division (21FLPOLK) 

dd. Seminole County Public Works Roads/Stormwater (21FLSEM) 

ee. South Florida Water Management District (21FLSFWM) 

ff. Southwest Florida Water Management District (21FLSWFD) 

gg. St. Johns River Water Management District (21FLSJWM) 

hh. FDEP (21FLDEP) 

ii. Suwannee River Water Management District (21FLSUW) 

jj. Volusia County Environmental Health Lab (21VEMD) 

Lakes were identified as any station with a value of “Lake” for the BODY field, PLUS any 

stations verified as a reference lake (see below). In some cases, the station “BODY” type 

conflicted with elements of the station name. For example, some stations with the “BODY” type 

“stream” had names that suggested that they were lakes or intake or discharge canals. The lake 

data set was likewise limited by excluding stations with names like “borrow pit,” “canal,” or 

“ditch.” However, not all stations with conflicting names and BODY types could be identified 

without reviewing every station name individually (and even that would not guarantee 100% 
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success). Therefore, despite the application of these selection criteria, it is possible that the lake 

data sets contain incorrect data types. 

Reference Lakes 

Reference reaches were mapped to individual stations, which contained both reach IDs and 

station IDs. These reference stations were annotated as reference in the complete lakes data set 

(as described above). Stations that were not marked as references in the combined excel files 

were marked as not reference in the lakes data set. 

Outliers and Invalid Data 

Additionally, individual samples were evaluated for potentially erroneous data. Individual 

samples were removed if they met any of the following criteria: 

1) Salinity < 0 ppt 

2) Conductivity <20 μS/cm 

3) pH <3 or >10 

4) Dissolved Oxygen (DO) >15 mg/L 

5) Nitrate plus Nitrite (NO3+NO2) > 75 mg/L 

6) Total Nitrogen (TN) > 20 mg/L 

7) Total Phosphorus (TP) > 15 mg/L 

8) Chl. a > 1,000 μg/L 

9) Color > 1,000 PCU 

10) Secchi depth ≥ 50 meters 

Zero values were eliminated for all nitrogen and phosphorus parameters. 

Detection Limits 

Detection limits were not reliably reported, and in some cases, reported results fell below 

detection limits. Therefore, we did not adjust reported “below detection limits” results. 

Additionally, we did not alter or eliminate any data based on remark codes. Descriptions were 

not available for all remark codes, and therefore they could not reliably be interpreted. 

Consequently, some data that should be excluded (e.g., results flagged for samples being 

analyzed outside holding time limits) could not be identified and may appear in the data set. 

Station Averages and Data Transformations 

Chlorophyll a (chl. a), color (COLOR), conductivity (COND), ammonia (NH4), nitrate plus 

nitrite (NO3+ NO2), total nitrogen (TN), and total phosphorus (TP) values were natural-log 

transformed prior to proceeding. Transformed values were used to create scatter plots, and native 

values were used to generate descriptive statistics. 

Annual averages were determined for individual stations, using all available data for that station 

in a given year. The final data sets therefore contain one record for each year that a station was 

sampled, with the parameter results equal to the arithmetic mean (average) for non-transformed 

data (DO, pH, salinity, Secchi depth, and temperature) or the geometric mean for ln-transformed 
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data (chl. a, color, conductivity, and nutrients) over all samples available for that station in that 

year. 

Color Groups 

Color groups were assigned to stations based on the geometric mean color value. Therefore, a 

station could be assigned to different color groups in different years, based on the average color 

observed in those years. Lakes with average annual color values less than or equal to 40 PCU are 

considered clear lakes and values greater than 40 PCU considered colored lakes, consistent with 

FDEP definitions. 

Final Data Sets 

Annotating the lakes stations by reference class and color group yielded the possible datasets 

shown in Table A1-1 for further analysis. 

Analyses 

Descriptive statistics (minimum, mean, maximum, standard deviation, and selected percentiles) 

were calculated for each parameter using arithmetic means, standard deviations, and standard 

errors for each data set in the table above (when sufficient data were available). We also 

calculated natural log means and standard deviations which were used to estimate theoretical 

percentiles as well, consistent with FDEP approaches for the stream analysis. 

Results 

Distributional statistics for clear and colored lake classes varied between the classes and there 

were marked differences between the reference site and entire site population within each class 

(Table A1-2). For clear and colored lakes, the 25th percentile of all lake sites was lower for every 

constituent (and higher for Secchi depth) than either the 75th or 90th percentile of reference lakes 

whether the empirical or theoretical percentiles are used. 

Considering both reference and all sites and empirical and theoretical percentiles, the ranges of 

variables can be reported. For clear lakes, the chl. a values range from 4-18 g/L and Secchi 

depth from 0.2-2.9 m. Ammonium (NH3

+

), NO3/NO2, and TN ranged from 10-33 g/L, 7-32 

g/L and 0.6-1.0 mg/L respectively. Finally, TP ranged from 13-52 g/L. 

For colored lakes, the ranges were larger, driven mostly by higher reference site 90th percentile 

values based on both empirical and theoretical calculations. The range of chl. a was 6-30 g/L 

and 0.2-1.3 meters for Secchi depth. NH3

+

, NO3/NO2, and TN ranged from 11-61 g/L, 

7-109 g/L, and 0.9-2.0 mg/L respectively. Lastly, TP ranged from 32-162 g/L. 
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Table A1-1. Possible Datasets for Further Analysis 

Reference 

Class Weaver Region Color Group 

Set Ref 

NonRef Bone 

Valley 

North 

Central Northeast Panhandle Peninsula Low Med. High

All X X X X X X X X X X 

AllRef X X X X X X X X X 

AllClear X X X X X X X X 

RefClear X X X X X X X 

AllColor X X X X X X X X 

RefColor X X X X X X X 
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Table A1-2. Distributional Statistics for Clear lakes (< 40 PCU) and Colored Lakes (>40 PCU) based on ReferenReference and Non-reference Sites Combined within Each Class 

Dataset Variable N Min Mean Max SD SE 25th 75th 90th CV Ln Mean Ln SD The

10t

ClearRef Chl. a 21 1 8 22 6 1 4 12 17 70 1.85 0.82 2 ClearRef NH4 43 8 17 40 9 1 10 21 33 57 -4.22 0.49 8 ClearRef NO3/NO2 45 4 13 50 12 2 6 14 32 90 -4.63 0.75 4 ClearRef TN 55 0.1 0.5 1.4 0.3 0.0 0.3 0.7 0.8 54 -0.83 0.64 0.2 ClearRef TP 52 4 24 120 21 3 10 31 50 89 -4.07 0.87 6 ClearRef Secchi 41 0.3 1.6 5.2 1.0 0.2 0.8 2.1 2.4 67 1.55 1.03 0.2 

Clear Chl. 

a 2437 1 25 846 39 1 4 30 65 154 2.45 1.29 2 

Clear NH4 3008 1 30 10913 232 4 10 23 42 772 -4.10 0.77 6 Clear NO3/NO2 2817 1 36 7925 206 4 7 20 46 576 -4.34 1.01 4 Clear TN 4107 0.0 1.1 12.3 0.9 0.0 0.6 1.2 2.1 85 -0.17 0.68 0.4 Clear TP 3889 1 46 8935 160 3 13 45 82 349 -3.67 0.96 7 Clear Secchi 3676 0.0 1.4 10.3 1.1 0.0 0.6 2.0 2.9 75 1.44 1.07 0.

ColoredRef Chl. 

a 137 1 17 846 72 6 3 11 30 429 1.90 1.07 2 

ColoredRef NH4 262 6 33 361 44 3 12 34 60 132 -3.79 0.78 8 ColoredRef NO3/NO2 267 4 41 432 51 3 7 50 109 124 -3.83 1.17 5 ColoredRef TN 274 0.3 1.2 5.4 0.6 0.0 0.7 1.5 1.9 50 0.06 0.49 0.6 ColoredRef TP 275 8 79 705 78 5 33 106 141 98 -2.87 0.82 20 ColoredRef Secchi 233 0.1 0.6 1.7 0.3 0.0 0.5 0.8 1.0 45 0.60 0.30 0.2 

Colored Chl. 

a 1424 1 38 846 63 2 6 36 110 168 2.76 1.28 3 

Colored NH4 2696 1 38 3254 131 3 11 32 60 342 -3.86 0.83 7 Colored NO3/NO2 2921 1 44 7389 184 3 7 33 93 423 -4.08 1.18 4 Colored TN 3300 0.1 1.6 13.3 1.2 0.0 0.9 2.0 3.3 73 0.30 0.61 0.6 Colored TP 3202 2 115 14880 310 6 32 120 262 268 -2.74 1.02 17 Colored Secchi 2896 0.0 0.7 3.9 0.4 0.0 0.4 0.9 1.3 64 0.70 0.40 0.2 

Note: Most statistics are based on untransformed values except the natural log-transformed (Ln) mean and standard deviation (SD) used theoretical percentiles (Theor). All units are in μg/L, except TN in mg/L and Secchi depth in m. SE = standard error, CV = coefficient of v