Evaluation of Stormwater-Induced Nutrient Gradient in Biscayne Bay Greg Graves, Dana Fike and Carrie Kelly Water Quality Section, Watershed Management Southeast District, Florida Depart ment 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 headquarter s (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 km 2 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 m outh of C-103. The S-21A control structure is located about 690 m upstream of its mout h at the Bay. The canal drains a 66 km 2 watershed composed primarily of agriculture (60%) and urban (25%) land uses. SFWMD Canal C-2 (Snapper Creek Canal) is located in eastern Mi ami-Dade County in the cities of Sweetwaterand Kendall (Figure 1) and discharges to Biscayne Bay through salinity control structure S22. This control structure is located approxi mately 500 m upstream of Biscayne Bay. The canal drains a 120 km 2 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 widesp read contaminant of groundwater in the world (Hallberg and Keeney, 1993). Studies have doc umented the relationship between elevated groundwater nitrate concentrations and the pe rcentage 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 Florid a’s coastal waters are the only place where hermatypic (reef-forming) coral communitie s 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 abundan ces of characteristic reef fish in the Florida Reef Tract has been attributed to cumulativ e land-based nutrient discharges from upland watersheds (LaPointe et al., 2002). It is gene rally understood that periodic loading of carbon, nitrogen, and phosphorus from land runoff coupl ed with a general shallow geomorphology drives and limits estuarine productivity. Change s in primary production induced by increased nutrient loading can result in changes in sp ecies abundance, distribution, and diversity, leading to alteration of the estuarine community structure (Livingston, 2001). Nutrient enrichment can adversely affect seagrass be ds by stimulating growth of epiphytic andplanktonic algae that inhibit transm ittance 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 i nhabitant within the estuarine ecosystem

. In recent years several studies have been initia ted to assess perceived im pacts of canal and groundwater discharges to southern Biscayne Bay (Szm ant, 1987; Byrne and Meeder, 1999; Lietz, 1999; Langevin, 2000). Findings suggest nitrogen-enriched groundwater enters Biscayne Bay through canals and underwater spri ngs. A Biscayne Bay National Park research
m a nagem e nt report docum ented a change in seagrass com m unity com position near the m outh of C-102 and C-103 and found lower salinitie s and higher water colum n nutrient concentrations in the affected areas (Szm ant, 1987). It was speculated that seagrass
com m unities were im pacted by C-102 and C-103 Canal discharges. Despite the input of large am ounts of nitrogen from freshwater discharges, the Bay has low water colum n chlorophyll concentrations (Bra nd, 2002). Typically, nitrogen is the principle lim iting nutrient to prim ary production in estu aries (Kennish, 1999); however, in Biscayne Bay, as well as in m a ny other tropical carbonate shallow waters, phosphorus is chem ically scavenged from the water by calcium carbona te and becom e s the lim iting 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 flushi ng 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 fluc tuations in nitrogen nutrient concentration at long-term water quality m onitoring 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 tim efram e of this analysis covered th e decadal period from 1992 to 2001. Daily m e an discharge data and inorganic nitrogen data (i.e ., nitrate-nitrite nitrogen and am m onia nitrogen) from C-102 and C-103 was extract ed from South Florida W a ter Managem e nt District’s “DBHydro” water quality database. Inorganic n itrogen data from the long-term water quality
m onitoring sites (i.e., sites BB36, BB38, BB41, and BB44; see Figure 1) adm i nistered by Miam i-Dade County Environm ental Resource Ma nagem e nt (DERM) was sim ilarly extracted from the SFW MD database. Canal inorganic n itrogen data, canal flow data and inorganic nitrogen data f r om the f our water quality m onitoring stations was f o lded into one ordered Excel spreadsheet such that all sam p ling dates correctly corresponded.

Since canal flow data was available on a daily basis and canal inorganic nitrogen data was available on a m onthly basis, estim ates of daily inorganic nitrogen concentration was determ ined by the standard practice of em ploying linear regression from observation to subsequent observation. These interpolated daily concentrations were then used to calculate estim ated daily loads from each canal, and the daily load estim ates were added together to

2obtain an estim ated total daily load delivered to the Bay from C-102 and C-103. The resultant spreadsheet was used as the m a ster input dataset for subsequent analysis.

Figure 1. Study area. A software utility was created to query the m a st er input dataset, described in the foregoing paragraph, to extract loads delivered to the Bay a specified num ber of days and specified duration of days prior to each observation at each of the four long-term water quality
m onitoring 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 m a de in the Bay. For exam ple a window could be specified as being the total lo ad delivered to the Bay during the tim efram e of 10 days duration com m e ncing 10 days prior to the observation being evaluated at one of the long-term water quality m onitoring sites. The su m of the load within the specified window was calculated for every observation m a de at each of the 4 long-term water quality
m onitoring sites over the 10 year period being exam ined. The software was thus used to create a series of output files, each differi ng by the specified window in which load was calculated, and these were subsequently em ployed for statistical analysis. Since an infinite
num ber of “load windows” is possible, only a subset of these are discussed.

3Measures of association between the observations of nutrient concentration at the long-term water quality m onitoring sites in the Bay and the corresponding antecedent load delivered to the Bay were evaluated using Minitab ® 14 statistical software’s nonparam e tric tests for correlation, nam e ly the Spearm a n rank correla tion. A p-value is returned by Minitab ® which tests if there is sufficient evidence that the co rrelation 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. Spearm a n’s correlation is si m p ly the Pearson product m o m e nt correlation
com puted on the ranks of the paired values, in this case antecedent load and observed within Bay concentration. Minitab ® Inc. graciously provided a custom m acro to f acilitate this endeavor.



### RESULTS The following tables sum m a rize the statistical significance of correlations between observed within Bay nutrient concentrations at the 4 within-Bay long-term water quality m onitoring sites and com b ined loads from C-102 and C-103 calculated as indicated by the “tim efram e ” 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. Tim e fram e: load calculated based on 1 day interval on the sam e day of within Bay observation. W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 0 2 8 0

. 9 9 8 0

. 9 5 6 B B 3 8 0

. 0 5 4 0

. 7 0 6 0

. 8 5 2 B B 4 1 < 0

. 0 0 1 0

. 5 6 1 0

. 0 1 4 B B 4 4 0

. 0 5 4 0

. 8 5 2 0

. 5 7 2
Table 2. Tim e fram e: load calculated based on 3 day interval prior to day of within Bay observation (day of observation plus previous 2 days). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 0 1 6 0

. 9 9 3 0

. 9 3 1 B B 3 8 0

. 0 3 4 0

. 7 1 5 0

. 7 5 7 B B 4 1 < 0

. 0 0 1 0

. 0 1 4 0

. 0 8 2 B B 4 4 0

. 0 3 1 0

. 9 1 5 0

. 4 7 1
Table 3. Tim e fram e: load calculated based on 5 day interval prior to day of within Bay observation (day of observation plus previous 4 days). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 0 0 8 0

. 7 5 1 0

. 8 1 8 B B 3 8 0

. 0 3 3 0

. 6 4 6 0

. 6 7 4 B B 4 1 < 0

. 0 0 1 0

. 4 6 8 0

. 0 0 5 B B 4 4 0

. 0 0 5 0

. 6 8 9 0

. 6 7 6 Table 4. Tim e fram e: load calculated based on 10 day interval prior to day of within Bay observation (day of observation plus previous 9 days). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen

4B B 3 6 0

. 0 1 2 0

. 5 2 6 0

. 5 3 7 B B 3 8 0

. 0 1 4 0

. 5 7 1 0

. 7 6 8 B B 4 1 < 0

. 0 0 1 0

. 2 5 1 0

. 0 0 1 B B 4 4 0

. 0 0 3 0

. 3 6 7 0

. 8 4 5
Table 5. Tim e fram e: load calculated based on 15 day interval prior to day of within Bay observation (day of observation plus previous 14 days). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 0 1 6 0

. 4 7 1 0

. 4 0 8 B B 3 8 0

. 0 2 7 0

. 6 9 5 0

. 6 1 5 B B 4 1 < 0

. 0 0 1 0

. 2 4 1 < 0

. 0 0 1 B B 4 4 0

. 0 0 4 0

. 3 7 1 0

. 7 2 1
Table 6. Tim e fram e: load calculated based on 10 day interval beginning 10 days prior to day of within Bay observation (day –10 to day -19). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 0 1 0 0

. 8 7 6 0

. 9 3 0 B B 3 8 0

. 0 8 8 0

. 6 3 4 0

. 3 6 4 B B 4 1 < 0

. 0 0 1 0

. 4 2 0 0

. 0 0 3 B B 4 4 0

. 0 0 2 0

. 6 0 3 0

. 9 4 0
Table 7. Tim e fram e: load calculated based on 10 day interval beginning 20 days prior to day of within Bay observation (day –20 to day -29). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 0 7 5 0

. 7 3 2 0

. 6 4 8 B B 3 8 0

. 1 3 0 0

. 5 5 6 0

. 0 8 1 B B 4 1 < 0

. 0 0 1 0

. 6 9 0 0

. 0 0 4 B B 4 4 0

. 0 0 3 0

. 8 0 8 0

. 3 7 6
Table 8. Tim e fram e: load calculated based on 10 day interval beginning 30 days prior to day of within Bay observation (day –30 to day -39). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 5 7 2 0

. 7 7 8 0

. 7 0 1 B B 3 8 0

. 1 6 1 0

. 6 3 5 0

. 4 4 1 B B 4 1 0

. 0 0 1 0

. 8 2 9 0

. 3 3 9 B B 4 4 0

. 0 4 5 0

. 9 5 8 0

. 8 2 8
Table 9. Tim e fram e: load calculated based on 10 day interval beginning 60 days prior to day of within Bay observation (day –60 to day -69). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 9 4 1 0

. 1 0 4 0

. 3 6 0 B B 3 8 0

. 2 9 5 0

. 1 3 5 0

. 0 3 4 B B 4 1 0

. 0 0 4 0

. 2 6 0 0

. 0 6 7 B B 4 4 0

. 0 6 3 0

. 1 4 3 0

. 3 4 8

5Table 10. Tim e fram e: load calculated based on 10 day interval beginning 120 days prior to day of within Bay observation (day –120 to day -129). W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 8 1 2 0

. 2 8 7 0

. 1 3 9 B B 3 8 0

. 3 0 5 0

. 0 9 9 0

. 0 7 5 B B 4 1 0

. 8 2 6 0

. 1 9 4 0

. 2 7 1 B B 4 4 0

. 4 6 9 0

. 0 1 2 0

. 0 3 4
Table 11. Tim e fram e: 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. W a ter Quality Site Nitrate Nitrogen Am m onia Nitrogen Inorganic Nitrogen B B 3 6 0

. 5 9 2 0

. 5 5 3 0

. 6 9 8 B B 3 8 0

. 7 3 9 0

. 8 5 2 0

. 7 1 3 B B 4 1 0

. 4 0 6 0

. 7 9 8 0

. 9 7 8 B B 4 4 [ 0

. 0 1 5 ] 0

. 8 1 0 0

. 7 5 8



### 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 a nd the nitrate concentration observed within the Bay at the long-term water quality m onitori ng sites. In contrast, correlations between
am m onia nitrogen and inorganic nitrogen are less pronounced and less am enable to interpretation. Since over 98% of the inorganic nitrogen discharged fromthe 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 c oncentrations observed m i ght be expected to exist f o r nitrate-nitrite nitrogen.
Table 12 . Ten-year (1992-2001) m ean concentrati on of water discharged fromC-103 and C102. S t a t i o n A m m o n i a (m g/L) Nitrate-Nitrite (m g/L) % Inorganic Npresent as NitrateNitrite C103 @ S20F (MW 04) 0

. 0 3 2

. 1 1 9 8

. 6 % C102 @ S21A (PR03) 0

. 0 5 3

. 8 9 9 8

. 7 %

Tables 1 through 5 indicate that for the m o st part , the nitrate concentration at all four water quality stations are correlated to nitrate loads di scharged into the Bay f r om the subject canals. Tables 2 and 3 evidence the strongest co rrelation between canal discharge and Bay

6concentration values, which m a y indicate that to som e extent the sum of the nitrate load delivered to the Bay over the last 3 to 5 days m o st directly im pacts the f our water quality
m onitoring stations taken as a group. Table 6 shows that significant correlation between nitrate concentrations at Site BB36 and loads di scharged when the loads are calculated based on a 10 day tim e period 10 days antecedent to n itrate concentration observations taken at BB36; however, significant correlation is lost when loads are calculated based on a 10 day tim e period 20 days antecedent to nitrate concentration observations (Table 7). Other than on the one-day interval (Table 1), observed nitrat e concentrations at BB38 were significantly correlated to load calculated based on loads disc harged up to the 15 day interval prior to the observation (Table 5) but lost significance (Tab le 6) when the period of load discharged becam e m o re distant in tim e from the observati on. This suggests that water quality conditions at BB38 are m o re influenced by recent discharges although that relationship m a y 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 tim e period 30 days antecedent of observations in the Bay (Table 8). This suggests that nitrate loads are retained in the southernm o st part of the Bay which is m o re enclosed in com p arison to BB36 or BB38 which are m o re “open” (see Figure 1). Nitrate c oncentrations at BB41, the site closest to the
m ouths of the canals (Table 13), rem a in signi ficantly correlated in Tables 1 through 9, which indicates that the nitrate regim e at BB41 is dom inated by canal discharges across a wide tim e fram e 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 som e m easurable degree for around 2 m onths at BB41. Alternatively, the longer tim efram e correlation between load discharged and observed concentration at both BB44 and BB41 m a y be the result of seasonality, i.e., during the wet season m o re load is discharged and the nitrate regim e in the Bay is elevated as a result. Although seasonality is readily apparent (Figure 2), correlations between m onthly load and m onthly concentration are not significant (P BB44 =0.064 and P BB41 =0.071); however, offsetting the load by one m onth prior results in significant correlations (P BB44 =0.012 and P BB41 <0.001).

Mo n t hMeanNitrateLoad,kg

12 10 8 6 4 2 0 900 0
800 0
700 0
600 0
500 0
400 0
300 0
200 0
100 0

0 M e a n N i t r at e L o ad vs M o n t h o f Y e a r

Mo n t hNitrate+Nitrite,mg/l

12 11 10 9 8 7 6 5 4 3 2 1 0. 1 2

0. 1 0

0. 0 8

0. 0 6

0. 0 4

0. 0 2

0. 0 0 N i tr a t e C o n c e n tr a t i o n a t S i te B B 4 4 v s Mon t h

Mo n t hNitrate +Nitrite,mg/l

12 11 10 9 8 7 6 5 4 3 2 1 0. 2 0

0. 1 5

0. 1 0

0. 0 5

0. 0 0 N i tr a t e C o n c e n tr a t i o n a t S i te B B 4 1 v s Mon t h

Figure 2. Mean m onthly load discharged from C-102 and C-103 com b ined (left hand panel)
versus m ean m onthly concentration at BB44 (center panel) and BB41 (right panel). Once the ten day tim e period is extended to 120 da ys (Table 10), correlations between nitrate load and concentration are not apparent at any of the water quality m onitoring sites; this indicates that correlations between loads and concentration have a tim e lim it beyond which no relationship exists, as m i ght be expected. The loss of significance of correlation as the

7tim efram e is extended also dem onstrates that signif i cant correlations evident within shorter tim efram e s are not spurious artifacts of the statistical process laid out in this paper.

Table 13. Distance from an on-shore m i dpoint between the m ouths of the C-102 and C-103 canal and the long-term water quality m onitoring stations. W a ter Quality Site Distance, m e ters B B 3 6 1 5 , 5 0 0 B B 3 8 1 1 , 7 0 0 B B 4 1 6 , 7 0 0 B B 4 4 1 3 , 6 0 0



### 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 m e ters from th e point of discharge could be attributed to loads em anating from C-102 and C-103. Nitrate concentrations closer to the point of discharge, i.e., at least to a distance of 6,700 m e ters as indicated by m onitoring 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 im pact to f l ora a nd f a una within a 70 m illion square m e ter zone of the Bay (assum i ng the zone of im pact is a sem i -circle with a radius of 6,700 m e ters), and substantiates previous research by Graves et al. (2004) which docum ented nitrate-related im pacts on the m acroinvertebrate com m unity with in this 6,700 m e ter zone. Im pacts outside of this zone are likely as indicated by the statistically signif i cant correlations between loads discharged and water quality m onitoring sites at greater distances.
The m a them atically sophisticated three-dim e ns ional Biscayne Bay water quality m odel, now being refined and calibrated by DERM and th e Arm y Corps of Engineers, should be
em ployed to conf irm the results of this anal ysis of load and water quality correlations; however, in the absence of ability to em ploy this m odel, this analysis m u st suf f i ce. Source files, output files and the custom software application are available on request.


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