Evaluation of Stormwater-Induced Nutrient Gradient in Biscayne Bay

Greg Graves, Dana Fike and Carrie Kelly 

Water Quality Section, Watershed Management 

Southeast District, Florida Department of Environmental Protection 

Port Saint Lucie 

2005 

INTRODUCTION 

South Florida Water Management District (SFWMD) Canal C-103 (Mowry Canal) discharges 

directly into Biscayne National Park through SFWMD control structure S-20F, located 900 m 

north of the Biscayne National Park headquarters (Figure 1). The S-20F structure is located 

about 650 m upstream from the mouth of C-103 at the Bay. Major land uses within the 105 

km2 watershed are agriculture (50 %) and urban (39%) (1995 SFWMD landuse cover). 

SFWMD Canal C-102 (Princeton Canal) discharges into the Bay through control structure S21A about four kilometers north of the mouth of C-103. The S-21A control structure is 

located about 690 m upstream of its mouth at the Bay. The canal drains a 66 km2 watershed 

composed primarily of agriculture (60%) and urban (25%) land uses. SFWMD Canal C-2 

(Snapper Creek Canal) is located in eastern Miami-Dade County in the cities of Sweetwater 

and Kendall (Figure 1) and discharges to Biscayne Bay through salinity control structure S22. This control structure is located approximately 500 m upstream of Biscayne Bay. The 

canal drains a 120 km2 basin composed primarily of urban development (80%) with a small 

proportion being agriculture (5%). 

Predominant soil types in the C-103 and C-102 have high permeability and low water holding 

capacity. These soil conditions result in low use-efficiency of fertilizers and potential leaching 

of nutrients into groundwater (Li, 2001; Li and Zhang, 2002). Nitrate nitrogen is highly 

soluble, mobile and perhaps the most widespread contaminant of groundwater in the world 

(Hallberg and Keeney, 1993). Studies have documented the relationship between elevated 

groundwater nitrate concentrations and the percentage of land area in fertilized crops 

(Embleton et al., 1986). Nitrate in groundwater can also originate in urban areas where overwatering and over-fertilization may occur (Morton et al., 1988). 

In the continental United States, South Florida’s coastal waters are the only place where 

hermatypic (reef-forming) coral communities are found (LaPointe et al., 2002). These 

communities are highly diverse and thrive in clear, warm, nutrient poor tropical water. A 

widespread decline in coral cover and in abundances of characteristic reef fish in the Florida 

Reef Tract has been attributed to cumulative land-based nutrient discharges from upland 

watersheds (LaPointe et al., 2002). It is generally understood that periodic loading of carbon, 

nitrogen, and phosphorus from land runoff coupled with a general shallow geomorphology 

drives and limits estuarine productivity. Changes in primary production induced by increased 

nutrient loading can result in changes in species abundance, distribution, and diversity, 

leading to alteration of the estuarine community structure (Livingston, 2001). Nutrient 

enrichment can adversely affect seagrass beds by stimulating growth of epiphytic and 
planktonic algae that inhibit transmittance of light (Duarte, 1995; Valiela et al., 1997). 

Changes in phytoplankton and epiphytic algal growth rates also affect the type and 

availability of food for each level of trophic inhabitant within the estuarine ecosystem. 

In recent years several studies have been initiated to assess perceived impacts of canal and 

groundwater discharges to southern Biscayne Bay (Szmant, 1987; Byrne and Meeder, 1999; 

Lietz, 1999; Langevin, 2000). Findings suggest nitrogen-enriched groundwater enters 

Biscayne Bay through canals and underwater springs. A Biscayne Bay National Park research 

management report documented a change in seagrass community composition near the mouth 

of C-102 and C-103 and found lower salinities and higher water column nutrient 

concentrations in the affected areas (Szmant, 1987). It was speculated that seagrass 

communities were impacted by C-102 and C-103 Canal discharges. 

Despite the input of large amounts of nitrogen from freshwater discharges, the Bay has low 

water column chlorophyll concentrations (Brand, 2002). Typically, nitrogen is the principle 

limiting nutrient to primary production in estuaries (Kennish, 1999); however, in Biscayne 

Bay, as well as in many other tropical carbonate shallow waters, phosphorus is chemically 

scavenged from the water by calcium carbonate and becomes the limiting nutrient (Brand, 

1988). Relatively elevated chlorophyll concentrations in southern Biscayne Bay have been 

related to discharges from canals (including C-103 and C-102) in association with dry-season 

phosphorus buildup in soils and groundwater flushing into the Bay at the onset of the rainy 

season (Brand, 1988; Brand et al., 1991; Brand, 2002). 

The objective of this study was to evaluate fluctuations in nitrogen nutrient concentration at 

long-term water quality monitoring stations located within southern Biscayne Bay within the 

context of nitrogen nutrient loads discharged into the Bay from C-102 and C-103. 

METHODS 

The timeframe of this analysis covered the decadal period from 1992 to 2001. Daily mean 

discharge data and inorganic nitrogen data (i.e., nitrate-nitrite nitrogen and ammonia nitrogen) 

from C-102 and C-103 was extracted from South Florida Water Management District’s 

“DBHydro” water quality database. Inorganic nitrogen data from the long-term water quality 

monitoring sites (i.e., sites BB36, BB38, BB41, and BB44; see Figure 1) administered by 

Miami-Dade County Environmental Resource Management (DERM) was similarly extracted 

from the SFWMD database. Canal inorganic nitrogen data, canal flow data and inorganic 

nitrogen data from the four water quality monitoring stations was folded into one ordered 

Excel spreadsheet such that all sampling dates correctly corresponded. 

Since canal flow data was available on a daily basis and canal inorganic nitrogen data was 

available on a monthly basis, estimates of daily inorganic nitrogen concentration was 

determined by the standard practice of employing linear regression from observation to 

subsequent observation. These interpolated daily concentrations were then used to calculate 

estimated daily loads from each canal, and the daily load estimates were added together to 

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obtain an estimated total daily load delivered to the Bay from C-102 and C-103. The resultant 

spreadsheet was used as the master input dataset for subsequent analysis. 

Figure 1. Study area. 

A software utility was created to query the master input dataset, described in the foregoing 

paragraph, to extract loads delivered to the Bay a specified number of days and specified 

duration of days prior to each observation at each of the four long-term water quality 

monitoring sites, i.e., the software would extract and calculate the sum of load delivered to the 

Bay in a specified window of days for each observation made in the Bay. For example a 

window could be specified as being the total load delivered to the Bay during the timeframe

of 10 days duration commencing 10 days prior to the observation being evaluated at one of 

the 

long-term water quality monitoring sites. The sum of the load within the specified window 

was calculated for every observation made at each of the 4 long-term water quality 

monitoring sites over the 10 year period being examined. The software was thus used to 

create a series of output files, each differing by the specified window in which load was 

calculated, and these were subsequently employed for statistical analysis. Since an infinite 

number of “load windows” is possible, only a subset of these are discussed. 

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Measures of association between the observations of nutrient concentration at the long-term

water quality monitoring sites in the Bay and the corresponding antecedent load delivered to 

the Bay were evaluated using Minitab® 14 statistical software’s nonparametric tests for 

correlation, namely the Spearman rank correlation. A p-value is returned by Minitab® which 

tests if there is sufficient evidence that the correlation coefficient is not zero, such that a pvalue ≤ 0.050 indicates that the null hypothesis is false and a relationship between the pairs of 

values exists. Spearman’s correlation is simply the Pearson product moment correlation 

computed on the ranks of the paired values, in this case antecedent load and observed within 

Bay concentration. Minitab® Inc. graciously provided a custom macro to facilitate this 

endeavor. 

RESULTS

The following tables summarize the statistical significance of correlations between observed 

within Bay nutrient concentrations at the 4 within-Bay long-term water quality monitoring 

sites and combined loads from C-102 and C-103 calculated as indicated by the “timeframe” 

indicated on each table. P-values less than 0.05 indicate significance, i.e., that a relationship 

between observed concentration within the Bay and loads from C-102/C-103 exist. 

Table 1. Timeframe: load calculated based on 1 day interval on the same day of within Bay 

observation. 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.028 0.998 0.956

BB38 0.054 0.706 0.852

BB41 <0.001 0.561 0.014

BB44 0.054 0.852 0.572

Table 2. Timeframe: load calculated based on 3 day interval prior to day of within Bay 

observation (day of observation plus previous 2 days). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.016 0.993 0.931

BB38 0.034 0.715 0.757

BB41 <0.001 0.014 0.082

BB44 0.031 0.915 0.471

 

Table 3. Timeframe: load calculated based on 5 day interval prior to day of within Bay 

observation (day of observation plus previous 4 days). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.008 0.751 0.818

BB38 0.033 0.646 0.674

BB41 <0.001 0.468 0.005

BB44 0.005 0.689 0.676

Table 4. Timeframe: load calculated based on 10 day interval prior to day of within Bay 

observation (day of observation plus previous 9 days). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

4
BB36 0.012 0.526 0.537

BB38 0.014 0.571 0.768

BB41 <0.001 0.251 0.001

BB44 0.003 0.367 0.845

Table 5. Timeframe: load calculated based on 15 day interval prior to day of within Bay 

observation (day of observation plus previous 14 days). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.016 0.471 0.408

BB38 0.027 0.695 0.615

BB41 <0.001 0.241 <0.001

BB44 0.004 0.371 0.721

Table 6. Timeframe: load calculated based on 10 day interval beginning 10 days prior to day 

of within Bay observation (day –10 to day -19). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.010 0.876 0.930

BB38 0.088 0.634 0.364

BB41 <0.001 0.420 0.003

BB44 0.002 0.603 0.940

Table 7. Timeframe: load calculated based on 10 day interval beginning 20 days prior to day 

of within Bay observation (day –20 to day -29). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.075 0.732 0.648

BB38 0.130 0.556 0.081

BB41 <0.001 0.690 0.004

BB44 0.003 0.808 0.376

Table 8. Timeframe: load calculated based on 10 day interval beginning 30 days prior to day 

of within Bay observation (day –30 to day -39). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.572 0.778 0.701

BB38 0.161 0.635 0.441

BB41 0.001 0.829 0.339

BB44 0.045 0.958 0.828

Table 9. Timeframe: load calculated based on 10 day interval beginning 60 days prior to day 

of within Bay observation (day –60 to day -69). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.941 0.104 0.360

BB38 0.295 0.135 0.034

BB41 0.004 0.260 0.067

BB44 0.063 0.143 0.348

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Table 10. Timeframe: load calculated based on 10 day interval beginning 120 days prior to 

day of within Bay observation (day –120 to day -129). 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.812 0.287 0.139

BB38 0.305 0.099 0.075

BB41 0.826 0.194 0.271

BB44 0.469 0.012 0.034

Table 11. Timeframe: load calculated based on 10 day interval beginning 240 days prior to 

day of within Bay observation (day –240 to day -249). P-values less than 0.05 in brackets 

indicate a significant but inverse correlation. 

Water Quality Site Nitrate Nitrogen Ammonia Nitrogen Inorganic Nitrogen 

BB36 0.592 0.553 0.698

BB38 0.739 0.852 0.713

BB41 0.406 0.798 0.978

BB44 [0.015] 0.810 0.758

DISCUSSION 

A review of Tables 1 through 10 indicates a relationship exists between nitrate loads 

discharged into the Bay from C-102 and C-103 and the nitrate concentration observed within 

the Bay at the long-term water quality monitoring sites. In contrast, correlations between 

ammonia nitrogen and inorganic nitrogen are less pronounced and less amenable to 

interpretation. Since over 98% of the inorganic nitrogen discharged from the C-102 and C103 Canals is in the nitrate form (Table 12), it follows that the greatest likelihood of observing 

relationships between loads discharged and concentrations observed might be expected to 

exist for nitrate-nitrite nitrogen. 

Table 12. Ten-year (1992-2001) mean concentration of water discharged from C-103 and C102. 

Station Ammonia

(mg/L) 

Nitrate-Nitrite

(mg/L) 

% Inorganic N 

present as NitrateNitrite 

C103 @ S20F 

(MW04) 

0.03 2.11 98.6%

C102 @ S21A 

(PR03) 

0.05 3.89 98.7%

Tables 1 through 5 indicate that for the most part, the nitrate concentration at all four water 

quality stations are correlated to nitrate loads discharged into the Bay from the subject canals. 

Tables 2 and 3 evidence the strongest correlation between canal discharge and Bay 

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concentration values, which may indicate that to some extent the sum of the nitrate load 

delivered to the Bay over the last 3 to 5 days most directly impacts the four water quality 

monitoring stations taken as a group. Table 6 shows that significant correlation between 

nitrate concentrations at Site BB36 and loads discharged when the loads are calculated based 

on a 10 day time period 10 days antecedent to nitrate concentration observations taken at 

BB36; however, significant correlation is lost when loads are calculated based on a 10 day 

time period 20 days antecedent to nitrate concentration observations (Table 7). Other than on 

the one-day interval (Table 1), observed nitrate concentrations at BB38 were significantly 

correlated to load calculated based on loads discharged up to the 15 day interval prior to the 

observation (Table 5) but lost significance (Table 6) when the period of load discharged 

became more distant in time from the observation. This suggests that water quality conditions 

at BB38 are more influenced by recent discharges although that relationship may take a day to 

develop. 

Other than on the one-day interval (Table 1), significant correlation at BB44 between nitrate 

load and concentration persists until loads are based on a 10 day time period 30 days 

antecedent of observations in the Bay (Table 8). This suggests that nitrate loads are retained 

in the southernmost part of the Bay which is more enclosed in comparison to BB36 or BB38 

which are more “open” (see Figure 1). Nitrate concentrations at BB41, the site closest to the 

mouths of the canals (Table 13), remain significantly correlated in Tables 1 through 9, which 

indicates that the nitrate regime at BB41 is dominated by canal discharges across a wide time 

frame between discharge and observation, i.e., that the effect of the discharge of the nitrate 

loads from the C-102 and C-103 Canals appears to persist to some measurable degree for 

around 2 months at BB41. Alternatively, the longer timeframe correlation between load 

discharged and observed concentration at both BB44 and BB41 may be the result of 

seasonality, i.e., during the wet season more load is discharged and the nitrate regime in the 

Bay is elevated as a result. Although seasonality is readily apparent (Figure 2), correlations 

between monthly load and monthly concentration are not significant (PBB44=0.064 and 

PBB41=0.071); however, offsetting the load by one month prior results in significant 

correlations (PBB44=0.012 and PBB41<0.001). 

Month

Mean Nitrate Load, kg

0 2 4 6 8 10 12

9000

8000

7000

6000

5000

4000

3000

2000

1000

0

Mean Nitrate Load vs Month of Year

Month

Nitrate + Nitrite, mg/l

1 2 3 4 5 6 7 8 9 10 11 12

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Nitrate Concentration at Site BB44 vs Month

Month

Nitrate + Nitrite, mg/l

1 2 3 4 5 6 7 8 9 10 11 12

0.20

0.15

0.10

0.05

0.00

Nitrate Concentration at Site BB41 vs Month

Figure 2. Mean monthly load discharged from C-102 and C-103 combined (left hand panel) 

versus mean monthly concentration at BB44 (center panel) and BB41 (right panel). 

Once the ten day time period is extended to 120 days (Table 10), correlations between nitrate 

load and concentration are not apparent at any of the water quality monitoring sites; this 

indicates that correlations between loads and concentration have a time limit beyond which no 

relationship exists, as might be expected. The loss of significance of correlation as the 

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timeframe is extended also demonstrates that significant correlations evident within shorter 

timeframes are not spurious artifacts of the statistical process laid out in this paper. 

Table 13. Distance from an on-shore midpoint between the mouths of the C-102 and C-103 

canal and the long-term water quality monitoring stations. 

Water Quality Site Distance, meters 

BB36 15,500

BB38 11,700

BB41 6,700

BB44 13,600

CONCLUSIONS 

This study has shown that the areal extent which discharge of nitrate-laden water has affected 

water quality conditions in Biscayne Bay is surprisingly large. Increases in nitrate 

concentration as far away as 15,500 meters from the point of discharge could be attributed to 

loads emanating from C-102 and C-103. Nitrate concentrations closer to the point of 

discharge, i.e., at least to a distance of 6,700 meters as indicated by monitoring site BB41, are 

correlated to discharges from C-102 and C-103 for up to 60 days. This suggests a high 

probability of nutrient-related impact to flora and fauna within a 70 million square meter zone 

of the Bay (assuming the zone of impact is a semi-circle with a radius of 6,700 meters), and 

substantiates previous research by Graves et al. (2004) which documented nitrate-related 

impacts on the macroinvertebrate community within this 6,700 meter zone. Impacts outside of 

this zone are likely as indicated by the statistically significant correlations between loads 

discharged and water quality monitoring sites at greater distances. 

The mathematically sophisticated three-dimensional Biscayne Bay water quality model, now 

being refined and calibrated by DERM and the Army Corps of Engineers, should be 

employed to confirm the results of this analysis of load and water quality correlations; 

however, in the absence of ability to employ this model, this analysis must suffice. Source 

files, output files and the custom software application are available on request. 

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