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

Appendix B-8: Extrapolating Natural Background Nutrient 

Concentrations 

(Chapter 12 from USEPA. 2009. Proposed Methods and Approaches for Developing Numeric 

Nutrient Criteria for Florida’s Inland Waters. United States Environmental Protection Agency, 

Office of Water, Office of Science and Technology, Health and Ecological Criteria Division, 

Washington, DC.) 
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Appendix B-8. Extrapolating Natural Background Nutrient Concentrations 

Dodds and Oakes (2004) proposed an approach to predict natural background, or reference 

nutrient concentrations in streams. A multiple regression model is developed to determine the 

relationship between nutrient concentrations and human land uses in the watershed. Reference 

nutrient concentrations in the stream water column are extrapolated from the intercept of 

predicted nutrient concentrations, where human land use is zero. Other similar applications have 

used additional natural predictors and allowed for some acceptable disturbance (Soranno et al. 

2008). Dodds and Oakes (2004) original approach required development of a multiple regression 

model using all human land uses as explanatory variables, and extrapolating those land uses to 

zero altered land cover and 100% natural land cover. For this analysis, many of the sites only 

have Florida Landscape Development Intensity Index (LDI) scores available, so simple 

regressions could not be performed. 

Natural land cover of Florida includes extensive wetlands. Southeastern wetlands and 

bottomland forests contribute large amounts of dissolved organic compounds to surface water 

and shallow groundwater. The organic compounds also include organic nitrogen compounds, and 

southeastern wetland waters often have high organic nitrogen. 

Human-altered land uses contribute more inorganic nitrogen (ammonium (NH4

+

), nitrate (NO3), 

and nitrite (NO2)) to runoff from domestic sewage, animal water, and fertilizers applied to crop 

and lawns. Inorganic nitrogen, NO3 in particular, is also transported long distance in ground 

water. These complications lead us to predict that the percent of altered land cover will be 

positively correlated with dissolved inorganic nitrogen (DIN), but that a significant correlation 

between land use and total nitrogen (TN) was less likely. 

Methods Description 

Two separate data sets were used to model the relationships between land use and nutrient 

concentrations in streams. One data set was provided by the Florida Department of 

Environmental Protection (FDEP), the Macroinvertebrate metrics and chemistry datasheet – 

Stream file2. This file included LDI and instantaneous measurement of nutrient concentrations in 

stream water. LDI score was calculated based on land uses in a catchment or corridor, and each 

land use category was weighted by a coefficient and the total scores were summarized as an 

index. The minimum coefficient for all land use categories was one so the smallest LDI expected 

where no human disturbance was introduced in the stream watershed was one. The sites 

comprised the entire gradient of human disturbance from LDI score range of 1.0 (completely 

undisturbed) to 8.82 (heavily altered). However, the LDI values were based on 100 m stream 

corridor calculation on each side of stream (not the watershed). Site delineation for 100 

benchmark stations was performed. Land use/land cover and LDI of near-field and drainage area 

based on 1995 and 2004 land use layers were calculated. Nutrient measurements for these 

stations were extracted from a data set provided by FDEP and were averaged (geometric mean) 

for each station/year. Statistical analysis was performed using open source software - R. 
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Stream Corridor LDI vs. Nutrients 

FDEP developed stream corridor LDI scores for streams in their gradient study sites (388 sites). 

The relationships between these LDI and instantaneous measurement of nutrient concentrations 

in these streams were examined first. The results (Figure B8-1) showed that although a 

significant relationship existed between total phosphorus (TP) and LDI and between total 

inorganic nitrogen (TIN) and LDI, the models explained little of the variance in nutrient 

concentrations (R2=0.07 and 0.08, respectively, outliers TP > 1mg/L and TIN > 2mg/L were 

excluded). Due to the variability, the predicted/extrapolated natural nutrient concentrations had 

very wide prediction intervals (Figure B8-1). 

As expected, the relationship between TN and LDI was not significant. Relations between 

nutrients and LDI scores in each nutrient region were plotted in separated graphs (Figure B8-2). 

Although most of the observed relationships were significant, due to variability, it was hard to 

extrapolate nutrient endpoints from these weak patterns. 

Near-field and Drainage Area Land Uses vs. Nutrients 

The LDI scores between 1995 and 2004 data were strongly correlated (R2=0.84). However, the 

slope of the regression between 1995 and 2004 was 1.09, indicating that LDI has increased by 

9% over the ten year period (Figure B8-3). It is unknown if this reflects a 9% increase in 

landscape development or if land cover measurement methods may have changed between 1995 

and 2004. 

As shown in Table B8-1, LDIs calculated from both near-field and drainage area were only 

weakly correlated with TN and TIN concentrations, similar to those observed in corridor land 

use relations shown in Figure B8-1. No matter which land use layers and delineation methods are 

used, organic nitrogen concentrations were always positively correlated with wetland percent, 

and negatively correlated with forest percent. In the meantime, both TN and TP concentrations 

were negatively correlated with percent forest. NO3 and TIN appeared positively correlated with 

urban land use and negatively correlated with wetlands. 

Selected nutrient variables were plotted verse 2004 drainage area percent land cover (Figure B8-

4). Based on the patterns observed, it seems TN concentrations in Florida streams were 

confounded by high organic nitrogen input from wetland areas, as expected. TIN and NO3

concentrations were correlated with urban land use but this was characterized by a generally nonlinear response. 

Candidate nutrient criteria could not be reliably derived using the Dodds and Oakes (2004) 

methods as applied here with the available data. 
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Table B8-1. Correlations between Nutrient Variables and Percent Land Use and LDI 
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Figure B8-1. TN and TIN concentrations vs. LDI in all streams. The 80% confidence (inner 

dashed lines) and prediction (outer dashed lines) limits were plotted. 
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Figure B8-2. TP and TIN concentrations vs. LDI in different nutrient regions. The 80% 

confidence (inner dashed lines) and prediction (outer dashed lines) intervals were plotted as well. 
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Figure B8-3. Relationships between near-field and drainage area LDI in 1995 vs. those in 2004. 

Solid lines are regression limits and dashed lines are 1:1 lines. 
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Figure B8-4. Nutrient variables vs. percent 2004 drainage areas land cover. LOESS regression 

lines and their confidence intervals were also plotted.