A nutrient loading budget for Biscayne Bay, Florida

Valentina G. Caccia, Joseph N. Boyer *

Southeast Environmental Research Center, OE-148, Florida International University, Miami, FL 33199, USA

Abstract

The water quality in Biscayne Bay has been significantly affected by past and continuing coastal and watershed development. The

nutrient concentrations in the Bay have been dramatically changed by the conversion of natural creeks and sheet flow freshwater inputs

to rapid and episodic canal inputs from the large and rapidly expanding Miami metropolitan area. This study is an evaluation of nutrient

loadings to Biscayne Bay for 1994–2002 from canal, atmospheric, and groundwater sources. Dissolved inorganic nitrogen (DIN, as

nitrate, nitrite, and ammonium) and total phosphorus (TP) loadings by the canals were influenced by their geographic locations relative

to discharge amount, watershed land use, stormwater runoff, and proximity to landfills. Annual budgets showed that canals contributed

the bulk of N loading to the bay as 1687.2 metric ton N yr1 (88% total load). Direct atmospheric DIN load for Biscayne Bay was only

231.7 ton N yr1

, based on surface area. Of the canal DIN load, nitrate+nitrite ðNO

x Þ loading (1294.5 ton N yr1

) made up a much

greater proportion than that of ammonium (NHþ

4 , 392.6 ton N yr1

). In the urbanized north and central Bay, canal DIN load was evenly

split between NO

x and NHþ

4 . However, in the south, 95% of the DIN load was in the form of NO

x , reflecting the more agricultural land

use. Contrary to N, canals contributed the only 66% of P load to the bay (27.5 ton P yr1

). Atmospheric TP load was 14 ton P yr1. In

the north, canal P load dominated the budget while in the south, atmospheric load was almost double canal load. Groundwater inputs,

estimated only for the south Bay, represented an important source of N and P in this zone. Groundwater input of N (141 ton N yr1

) was

about equal to atmospheric load, while P load (5.9 ton P yr1

) was about equal to canal load.

2007 Elsevier Ltd. All rights reserved.

Keywords: Biscayne Bay; Nutrients; Nutrient loading; Estuary; Atmospheric; Land use

1. Introduction

During the past century, human activities have had a

tremendous impact on the global cycling of nutrients in

coastal systems. The export of P to the oceans has

increased threefold compared to pre-industrial and preagricultural levels while the export of N has increased even

more dramatically (Caraco, 1995). In many estuaries,

anthropogenic sources of N and P now exceed natural

inputs of these elements. Population growth and the

accompanying increase in per capita resource use has significantly changed the N and P cycle on local, regional,

and global scales. Fertilizer production has increased tremendously, from 3 Tg N yr1 in 1950 to 80 Tg N yr1 in

2000 (Galloway, 1998). Over the period 1950–1997, the

amount of N emitted into the atmosphere from fossil fuel

production increased from 8 to 30 Tg N. In the 1950s to

1960s, N mobilization by fossil fuel combustion was more

important than the production of commercial fertilizer.

However, in the 1990s, the total amount of N fixed by

human activities was about 150 Tg N, of which 80% was

due to food production (fertilizer) and 20% was due to

energy production (Valigura, 2001).

One of the most rapidly growing sources of N loading is

atmospheric deposition. In North America, coastal atmospheric N deposition ranged from 400 to over

1000 mg N m2 yr1

, while in the highly urbanized, industrialized, and intensively farmed regions of Western Europe

deposition commonly exceeds 1000 mg N m2 yr1 (Prospero et al., 1996; Holland et al., 1999). It has been estimated that 1 – >40% of new N inputs into coastal

0025-326X/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpolbul.2007.02.009

* Corresponding author. Tel.: +1 305 3484076; fax: +1 305 3484096.

E-mail address: boyerj@fiu.edu (J.N. Boyer).

www.elsevier.com/locate/marpolbul

Marine Pollution Bulletin 54 (2007) 994–1008
waters are now of atmospheric origin, for example: 10–60%

for the Mediterranean Sea (Martin et al., 1989), 40–70%

for the North Pacific Ocean (Prospero and Savoie, 1989),

38% in the New York Bight (Hinga et al., 1991), 26% for

Sarasota Bay (Sarasota Bay NEP, 1996), and 28% for

Tampa Bay (Tampa Bay NEP, 1996).

The relative contribution of atmospheric deposition of

N to total external N loading of an estuary depends on

land use, watershed area, airshed size, and hydrological

and morphological characteristics of receiving waters

(Paerl et al., 2001). Concurrently, increases in nutrient

inputs have also been linked to increases in population density within coastal watersheds, high rates of fertilizer applications, conversion of forest and prairies to agricultural

land, sewage inputs and atmospheric deposition (Turner

and Rabalais, 1991; Prado-Fiedler, 1990; Paerl, 1995).

Globally, coastal watersheds receive 103 Tg yr1 of N from

the combination of synthetic fertilizer (73.6 Tg yr1

), atmospheric deposition (22.5 Tg yr1

), and human sewage

(9.1 Tg yr1

). Approximately 20% of the dissolved inorganic nitrogen (DIN) exported by world rivers to the

coastal zone (21 Tg N yr1

) is attributed to atmospheric

deposition (Seitzinger and Kroeze, 1998), underscoring

the importance of this type of deposition to coastal N loading and eutrophication.

Biscayne Bay, a shallow estuary adjacent to the Miami

metropolitan area (Fig. 1), has been significantly affected

due to a century of extensive regional population growth

that accelerated coastal and watershed development (Harlem, 1979; Alleman et al., 1995). Miami began to grow at

the beginning of the twentieth century, and Biscayne Bay

became the site of one of the most important population

centers in Florida. Large changes in the Bay (nutrient

enrichment, hypersalinity, algal blooms, seagrass die-offs,

toxic pollution (metals, PAHs, PCBs), among others) have

resulted from land use changes and the conversion of

creeks and sheet flow freshwater inputs to managed canal

inputs in the watershed. Water quality of Biscayne Bay

has been significantly affected by changes in the quantity,

quality, timing, and distribution of freshwater releases

from canals, which drain agricultural and urban areas.

This study is a continuation of our previous research on

water quality in Biscayne Bay (Caccia and Boyer, 2005),

and is a response to the EPA National Strategy for the

development of Regional Nutrient Criteria for expanding

efforts to reduce nutrient enrichment of US waters. Two

states already have plans in place, while the other 48 states

are working on developing numeric nutrient criteria for

adoption in 2010. Many States including Florida are currently developing specific nutrient criteria for streams,

lakes, rivers and canals, estuaries, and coastal zones.

Therefore, we hope that the information from this study

will prove to be valuable in future restoration and management of Biscayne Bay.

Our previous research focused on characterizing the spatial distribution of the water quality parameters and their

seasonal trends in the Bay. We observed that water quality

in the Bay is highly dependent of the land use and influences from the watershed, and we also suggested some possible sources. In this study we want to corroborate the

influences from the watershed and the sources of nutrients

to existing water quality conditions of the Bay, by quantifying the nutrient loading from canals, atmosphere, and

groundwater to Biscayne Bay. We also assess the areas of

the Bay most affected by these inputs, using historical subdivisions (Corcoran et al., 1984) based on three regions separated by both natural and man-made structures: North,

Central and South Bay (Fig. 1).

2. Methods

2.1. Site discription

Biscayne Bay is a subtropical, lagoonal estuary located

in Miami-Dade and Monroe Counties, on the southeastern

-80.45 -80.35 -80.25 -80.15

25.30

25.40

25.50

25.60

25.70

25.80

Arch Creek

Snake Creek

Biscayne Canal

Little River

Miami River

Coral Gable

Sna

s W

pper Creek

Cuttler Drain

Black Creek

Princeton Canal

Military Canal

Mowry Canal Agricultural Area

Wetlands

Urban Area

Wetland

SOUTH BAY

CENTRAL BAYNORTH BAY

FLORIDA

U. S. A.

ATLANTIC OCEAN

Fig. 1. Map of Biscayne Bay showing location of canals and corresponding land use.

V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008 995
coast of Florida (Fig. 1). Biscayne Bay extends approximately 55 miles in a southwesterly direction from Dumfoundling Bay in the north to Barnes Sound in the south.

The bay varies from 1 to 10 miles in width and is divided

into three major regions: north, central and south. Depths

are generally in the range of 0.5–3.0 m with an overall surface area of 700 km2

. Most of Biscayne Bay is bounded by

the city of Miami, a sprawing urban region with a population approaching 2 million people. The juxtaposition of

such a sensitive resource next to the most densely populated area of the state distinguishes Biscayne Bay from

other Florida estuaries.

Major development of the area began in the early 1900s

with the construction of artificial navigation inlets. As a

result, circulation and salinity patterns in the Bay were

altered due to the enhanced exchange of water between

the Bay and the Atlantic Ocean. Construction of the major

canals through the Everglades and channelization of natural tributaries and transverse glades that carried fresh water

to Biscayne Bay lowered the regional and coastal water

table, decreased groundwater flow to the bay, and reduced

the surface water sheetflow to the bay. This severely

reduced the nearshore salinity gradient, which had provided critical estuarine habitat for estuarine species. In

addition, delivery of fresh water into the bay changed to

being a pulsed, point-source discharge, causing damaging,

rapid salinity fluctuations near canal mouths. The unnatural timing, the increased quantity, and the point source nature of these canal discharges disrupt and prevent

establishment of viable biological communities. As a result,

managers are planning to re-establish estuarine habitat

through the Biscayne Bay Coastal Wetlands subcomponent

of the Comprehensive Everglades Restoration Project (see

http://www.evergladesplan.org/).

For the purposes of this report, the northern portion of

Biscayne Bay (about 10% of the total bay area) is defined

as that portion of the bay that extends from the Broward/Miami-Dade County line south to the Rickenbacker

Causeway. Major tributaries to North Bay include Snake

Creek, Arch Creek, Biscayne Canal, Little River, and

Miami River. The Miami River is the largest river flowing

to Biscayne Bay, and suffers from a history of contamination with industrial runoff and untreated sewage effluent.

Stormwater runoff from urbanized uplands has been identified as a source of contamination in discharges from

Snake Creek, Arch Creek, Biscayne Canal, Little River,

and of course the Miami River.

The central region of Biscayne Bay, the 17 mile section

south of Rickenbacker Causeway to Featherbed Banks

and Black Point, represents a transition zone between the

North and South Bays and includes the Coral Gables

Waterway, Snapper Creek and Cutler Drain canals. Tidal

exchange with the Atlantic Ocean occurs through a wide

series of shoals called the Safety Valve and directly through

Bear Cut. Water from Coral Gables Waterway and Snapper Creek is primarily residential stormwater. The portion

of central Biscayne Bay that is included in Biscayne

National Park generally has the best water quality. Seagrasses and their associated communities along with small

areas of soft corals and sponges are prominent in this area.

Southern Biscayne Bay includes the area south of Featherbed Banks through Card and Barnes Sounds. Almost all

of this area is incorporated into Biscayne National Park.

Southern Biscayne Bay benthic communities include lush

seagrass beds, large areas of hard bottom, some soft and

hard corals, and other marine life. Ocean exchange occurs

though Angelfish, Broad and Ceasar’s Creeks. The main

canals draining into this area are Black Creek, Princeton,

Military, and Mowry canals. Black Creek drains the South

Dade Landfill, and the Old South Dade Dump, as well as

the Miami/Dade Sewage Treatment Facility. Military and

Mowry Canals drain the former Homestead Air Force

Base and the South Dade agricultural lands, respectively.

2.2. Canal freshwater flows

The names and locations of the canals entering Biscayne

Bay along with their corresponding land use are shown in

Fig. 1. Daily freshwater flow in ft3 s

1 (CFS) at individual

canal flow structures were measured by the South Florida

Water Management District (SFWMD) and retrieved from

their DB-HYDRO database (www.sfwmd.gov/org/ema/

dbhydro/). Daily flows were converted to ft3 d1 and

summed for the month (ft3 mo1

) and for the year

(ft3 yr1

). Total monthly flows were back calculated to

CFS reported as the monthly mean. Annual flows were

back calculated to CFS reported as the annual mean.

The period of record for this analysis was from January

1994 to December 2002.

2.3. Canal nutrient loads

Canal nutrient data, consisting of monthly grab

samples, were gleaned from a database supplied by

Miami-Dade Department of Environmental Resource

Management (DERM). Water samples were analyzed for

NO

3 þ NO2 ðNOx Þ, ammonium ðNHþ

4 Þ and total phosphorus (TP), using EPA methods: 353.2, 350.1 and 365.1,

respectively. Dissolved inorganic nitrogen (DIN) was calculated as the sum of NO

x þ NHþ

4 . All nutrient concentrations were reported in mg L1 as N or P. Missing nutrient

data were rare, but when this occurred we used the mean

nutrient concentration for all years in that specific canal

for that month. The year 1997 was excluded from our analysis because of the lack of nutrient data for the entire year.

Monthly nutrient loads for each canal were calculated

using the following formula using NHþ

4 as an example.

Monthly NHþ

4 load ðkg mo1

Þ 1⁄4 monthly mean flow ðCFSÞ

 7:438  107 L mo1 CFS1  1⁄2NHþ

4 

Monthly loading estimates (kg mo1

) were summed to

get annual loads in metric tons of N or P per year

(ton yr1

). Some canals or creeks (Arch Creek, Coral

996 V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008
Gables W., Cutler Drain) had negative freshwater flow

some months. For these cases, we considered monthly

loading to be zero.

2.4. Atmospheric loads

Atmospheric wet deposition of NO

3 and NHþ

4 in Biscayne Bay (kg N ha1 yr1

) was estimated using monthly

data (1994–2002) from the National Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edu). This

data were collected from one station located in Everglades

National Park (FL11, 25.39 Latitude North and 80.68

Longitude West) as it was the nearest site to Biscayne

Bay. Precipitation data were obtained from the same

source (NADP). Atmospheric TP loading was calculated

using the mean wet deposition rate from 1978–1996 of

20 mg P m2 yr1 (0.2 kg P ha1 yr1

; Fitz and Sklar,

1999). Annual deposition average of each parameter was

multiplied by estuary surface area (700 km2 or 70,000 ha)

to get annual loads in metric tons per year (ton yr1

).

2.5. Groundwater loads

Data for groundwater flows and nutrient loads were

kindly provided by Danielle Mir-Gonzalez as part of her

thesis work (unpublished). Groundwater input was measured using seepage meters placed at four sites along four

transects of four sites from 50–300 m offshore in South Biscayne Bay. Transects were distributed between Black Point

and Turkey Point (Fig. 1). Nutrient concentrations in the

groundwater were measured bimonthly during 2003 and

loads calculated as g m2 day1

. Annual nearshore loads

(out to 300 m) were calculated by extrapolating both along

and between transects and reported in ton yr1

.

2.6. Ocean exchange

According to Wang et al. (2003), the bulk of exchange

between Biscayne Bay and the Atlantic Ocean occurs

through the Safety Valve. Other exchanges at Haulover

Cut in the North and Snake and Angelfish Creeks in the

South are not significant to the budget. As concentrations

of N and P in Biscayne Bay are equal to or greater than

ocean levels (Caccia and Boyer, 2005), we expected

Biscayne Bay to consistently export both N and P to the

ocean. This being the case, we expected that tidal exchange

between the two water masses would result in there being

no net input of N or P from the ocean, only export. This

model is not concerned with sinks in the nutrient budget,

only external loads. Therefore, as the ocean is only a sink,

we did not include it in the analysis.

3. Results

3.1. Canal freshwater flows

From 1994 to 2002, annual freshwater inputs from

canals varied from 1608 to 2551 CFS (4.5–72.2 m3 s

1

),

averaging 2086 CFS (59.1 m3 s

1

) over the study period

(Table 1). The Miami River presented the highest average

flow at 535.5 CFS, and Arch Creek showed the lowest average flow of 1.5 CFS. Arch Creek presented negative flows

in 1998 and 2000, meaning that there were no contributions

of freshwater to the Bay during those years, and its lowest

positive flow occurred in 2001 (0.2 CFS). Cutler Drain also

exhibited negative flow in 1995. In both canals, the negative

flow was too small (1.6 CFS) to be significant.

North Bay receives freshwater from five canals (Snake

Creek, Arch Creek, Biscayne Canal, Little River and

Miami River), South Bay receives freshwater from four

canals (Black Creek, Princeton, Military and Mowry

canals) and Central Bay has influence from three canals

(Coral Gables Waterway, Snapper Creek and Cutler

Drain). The North Bay showed the greatest average of

freshwater flow (1238 CFS) for all the studied years, followed by the South Bay (599 CFS) and the Central Bay

(249 CFS). The Miami River, Snake Creek and Little River

delivered 52.6% of the total freshwater contributions into

Biscayne Bay. The lowest freshwater flow contribution into

the Bay was from Arch Creek (0.1%), while sometimes

exhibited negative flows.

Table 1

Annual mean freshwater flow (CFS) at each canal discharge in Biscayne Bay

Year Snake

Creek

Arch

Creek

Biscayne

Canal

Little

River

Miami

River

Coral

Gables

Snapper

Creek

Cutler

Drain

Black

Creek

Princeton

Canal

Military

Canal

Mowry

Canal

Total

flow

1994 409.1 4.0 161.1 166.8 463.4 11.2 229.1 22.3 253.8 129.5 30.4 240.9 2121.6

1995 362.4 3.4 161.5 213.1 570.3 29.7 345.9 1.5 346.0 162.4 22.9 334.5 2550.7

1996 257.6 1.1 90.8 176.5 469.0 3.1 135.7 18.7 163.1 90.1 14.3 188.3 1608.2

1998 375.4 1.6 100.2 242.6 621.1 15.0 172.6 100.4 227.5 95.5 14.9 204.1 2167.7

1999 342.3 2.9 154.1 249.0 679.2 29.5 235.1 83.5 210.7 136.4 19.1 225.2 2366.8

2000 311.0 1.6 145.9 202.0 515.2 15.0 89.4 54.5 125.4 128.1 16.9 206.4 1808.3

2001 343.2 0.2 154.2 255.2 392.5 4.7 140.0 47.2 158.3 133.0 34.1 203.7 1866.4

2002 323.1 3.4 142.9 270.2 573.4 4.3 163.4 46.2 260.6 146.8 34.9 231.1 2200.3

Mean 340.5 1.5 138.9 221.9 535.5 14.1 188.9 46.4 218.2 127.7 23.4 229.3 2086.2

Percent

of total

16.3 0.1 6.7 10.6 25.7 0.7 9.1 2.2 10.5 6.1 1.1 11.0

V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008 997
3.2. Canal nutrient loads

3.2.1. Nitrogen

NO

x was the most abundant form of DIN present in the

canals (Table 2). Annual NO

x loading from Mowry

(502.14 ton yr1

) and Princeton (455.45 ton yr1) canals,

situated in the South Bay, accounted for 74% of the total

NO

x load to the Bay (Fig. 2). The proportional contribution of NO

x loading from each canal did not change significantly during the period of record. The Miami River

(86.79 ton yr1

), Snake Creek (72.36 ton yr1) and Little

River (49.97 ton yr1

) located in the North Bay accounted

for another 16% of the total load. Black Creek has a similar

NO

x loading (48.16 ton yr1

) as Little River. Arch Creek

exhibited the lowest NO

x load (0.36 ton yr1

) and in some

years was negligible due to negative freshwater flow. Coral

Gables W. (4.82 ton yr1

) and Cutler Drain (8.92 ton yr1)

both located in the Central Bay also produced low NO

x

loadings to the Bay.

Unlike the South Bay, NHþ

4 was the major form of DIN

loading to the North Bay (Table 2). The Miami River

(145.1 ton yr1

), Little River (93.7 ton yr1) and Snake

Creek (52.8 ton yr1) contributed 74% of the total NHþ

4

load to the Bay (Fig. 3). The relative proportion of annual

NHþ

4 loading from each canal varied little over time. The

lowest NHþ

4 average loadings were found in Arch Creek

(0.8 ton yr1

) and Military Canal (1.08 ton yr1). Arch

Creek, Military Canal, Mowry Canal, Princeton Canal,

Cutler Drain and Coral Gables Waterway showed lower

range (<10 ton yr1

). The highest DIN loading occurred

in 1995 (2185 ton yr1

) while 1996 (1253 ton yr1) was

the lowest (Table 2).

3.2.2. Phosphorus

The Miami River (8.37 ton yr1

), Little River

(5.1 ton yr1

) and Snake Creek (3.03 ton yr1) canals

located in the North Bay showed the highest annual TP

loadings (Fig. 4). These three inputs accounted for 60%

of the total TP load to the Bay. Black Point (2.27 ton yr1

)

in the South Bay contributed another 8%. The lowest TP

Table 2

Annual canal loads of NO

x , NHþ

4 , DIN, TP (ton yr1

) and DIN:TP

(molar) to Biscayne Bay

Year Total canal load (ton yr1

)

NO

x NHþ

4 DIN TP DIN:TP

1994 1290.2 474.1 1764.3 22.4 174.4

1995 1670.8 514.6 2185.4 27.4 176.6

1996 967.7 285.7 1253.5 15.0 185.2

1997

1998 1136.7 382.2 1519.0 26.1 128.9

1999 1353.6 444.2 1797.8 26.0 153.2

2000 1177.9 270.8 1448.8 38.6 83.1

2001 1308.0 393.4 1701.5 27.9 135.2

2002 1451.3 376.0 1827.2 36.3 111.5

Mean 1294.5 392.6 1687.2 27.5 143.5

North Bay 235 312 547 18.6 65.1

Central Bay 39 28 67 3.4 43.6

South Bay 1021 52 1073 5.4 440.0

0

200

400

600

800

1000

1200

1400

1600

1800

1994 1995 1996 1997 1998 1999 2000 2001 2002

Year

Ton yr-1

Mowry

Military

Princeton

Black Point

Cutler

Snapper

Coral Gables

Miami River

Little River

Biscayne

Arch

Snake

Fig. 2. Contribution of individual canals to annual NO

x loading (ton yr1

) to Biscayne Bay over the 9-year period of record.

998 V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008
loads were delivered by Arch Creek (0.11 ton yr1

),

Military Canal (0.19 ton yr1

) and Cutler Drain

(0.36 ton yr1

). Highest TP loading occurred in both 2000

(38.6 ton yr1

) and 2002 (36.3 ton yr1), while 1996

(15 ton yr1

) and 1994 (22.4 ton yr1) were lowest

(Table 2).

0

100

200

300

400

500

600

1994 1995 1996 1997 1998 1999 2000 2001 2002

Mowry

Military

Princeton

Black Point

Cutler

Snapper

Coral Gables

Miami River

Little River

Biscayne

Arch

Snake

Year

Ton yr-1 Fig. 3. Contribution of individual canals to annual NHþ

4 loading (ton yr1

) to Biscayne Bay over the 9-year period of record.

0

5

10

15

20

25

30

35

40

45

1994 1995 1996 1997 1998 1999 2000 2001 2002

Mowry

Military

Princeton

Black Point

Cutler

Snapper

Coral Gables

Miami River

Little River

Biscayne

Arch

Snake

Year

Ton yr-1

Fig. 4. Contribution of individual canals to annual TP loading (ton yr1) to Biscayne Bay over the 9-year period of record.

V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008 999
3.3. Atmospheric loads

3.3.1. Nitrogen

Both NO

3 and NHþ

4 contributed relatively equal to the

inorganic N input from direct precipitation to the Bay

(Table 3). NO

3 input ranged from 1.48 to 2.18

kg ha1 yr1

, averaging 1.92 kg ha1 yr1 (58% of total).

On average, the atmospheric NHþ

4 input was

1.39 kg ha1 yr1

. The highest atmospheric N input was

observed in 1998 (4.12 kg N ha1 yr1

) and the lowest in

1996 (2.43 kg N ha1 yr1

).

3.3.2. Phosphorus

TP inputs were calculated using a 0.2 kg P ha1 yr1 as

a mean wet deposition rate obtained by Fitz and Sklar

(1999). This resulted in a total of 14 ton yr1 of direct

deposition to Biscayne Bay (700 km2

). By dividing the

Bay into sections we calculated that North Bay (140 km2

)

received 2.8 ton yr1

, Central Bay (155 km2) 3.1 ton yr1,

and South Bay (405 km2

) 8.1 ton yr1.

3.4. Groundwater loads

Groundwater nutrient loading estimates were the most

incomplete (portion of South Bay only) and are used here

only for comparative purposes. Groundwater DIN inputs

to South Biscayne Bay between Black Point and Turkey

Point was 141 ton yr1

. Of that, 109 ton yr1 was in the

form of NHþ

4 . TP load from groundwater was estimated

to be 5.9 ton yr1

.

4. Discussion

Nutrient loading to Biscayne Bay was budgeted using

only three compartments: canal freshwater inputs, atmospheric wet precipitation, and groundwater. Other internal

sources and sinks, such as N2 fixation, denitrification, and

burial were not included, mostly because very little is

known about any of these processes in Biscayne Bay.

Capone and Taylor (1980) measured N2 fixation associated

with Thalassia sp. roots and found that ‘‘annual rates of N2

fixation were 10–50 kg N ha1, taking into account seasonal variations and activities to a depth of 20 cm.’’ If

we assume that these rates apply all seagrass areas

(40% coverage), we might expect N2 fixation to add

280–1400 ton N yr1

. However, it generally accepted that

acetylene reduction rates seriously overestimate N2 fixation

estimates and we would, therefore, need to revise these estimates down.

Also not included in the budget was the exchange at the

ocean boundary; the reason being that the tidal inputs were

of low nutrient concentration and probably overwhelmed

by tidal output concentrations. The major point of the

study was to determine the relative contributions of the different nutrient sources in an effort to propose future management strategies and predict their effectiveness in

reducing such loads.

4.1. Canal freshwater flows

Annual variability in flows was a result of both climate

and management activities. Although SFWMD has the

ability to modify water delivery schedules, much of the

interannual variability was due to climactic conditions.

Annual precipitation accounted for only half of the variability in annual flow (Fig. 5). Highest precipitation

occurred during El Nin ̃o-Southern Oscillation (ENSO)

years: 1994–1995, 1997–1998, and 2002. Flows were not

so highly correlated with ENSO because of another climatic factor – hurricanes. Although 1999 was a La Nin ̃a

year (typically dry), the passage of Hurricane Irene

dropped 25–65 cm of rain during its passage in August.

Summer water management strategy is to routinely open

the canal gates to draw down the water table in preparation

to hurricane season. These large releases may have significant impacts on nearshore salinity and nutrient concentrations (Caccia and Boyer, 2005).

Monthly precipitation for the region (Fig. 6a) is temporally decoupled from monthly flows (Fig. 6b). Wet season

rainfall occurs in May but canal flows held back until June.

Conversely, precipitation drops off rapidly by November

Table 3

Atmospheric deposition (kg N ha1

) in the Everglades National Park

Research Center from the National Atmospheric Deposition Program

(NADP)

Year NHþ

4 NO

3 DIN

1994 1.73 2.23 3.97

1995 1.45 1.98 3.42

1996 0.95 1.48 2.43

1997 1.43 2.31 3.74

1998 2.18 1.94 4.12

1999 1.13 1.72 2.84

2000 1.02 1.73 2.75

2001 1.23 1.96 3.19

2002 1.39 1.93 3.32

Mean 1.39 1.92 3.31

y = 14.1x - 115.6

r

2

 = 0.515

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

120 130 140 150 160 170 180

Precipitation (cm yr-1)

Annual Flow (CFS)

1995

1996

A

Fig. 5. Regression of annual precipitation (cm yr1) with annual average

canal flow (CFS).

1000 V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008
but flows remain elevated through December and even into

January. The net effect is a management strategy, which

allows for early spring build up of hypersaline conditions

with the extension of freshwater inputs long into the dry

season.

4.2. Canal nutrient loads

Mean DIN loading over the 9-year period was

1687 ton yr1 (Table 2). Overall, canal loading of NO

x

was triple that of NHþ

4 . This relationship was relatively

consistent across the years of the study and is due in part

to the sources of freshwater as the canals drain both urban

and agricultural areas where NO

3 is prevalent. There is

also a significant lag between input to the canal system

and export to the Bay which may allow for significant nitrification to occur (Graves et al., 2004).

TP load was much lower, averaging 27.5 ton yr1

. This

resulted in an average DIN:TP ratio of 143.5. Relating

DIN to TP is not normally done as it may not be assumed

that all the TP is bioavailable. However, comparison of

these variables allows us to show the great potential for

P limitation within this ecosystem, even with overestimated

P availability. It is clear from this analysis that South Bay

is potentially more P (440) limited than the Central or

North (43.6 and 65.1, respectively).

4.2.1. Temporal trends

Highest DIN loading occurred in 1995 (2185 ton yr1

),

while 1996 was the lowest (1253 ton yr1

). These extremes

corresponded to the wettest (El Nin ̃o) and driest years in

the period of record (Fig. 5). No consistent trend in DIN

load was observed during the period of record. Doering

and Chamberlain (2004) in their study in Caloosahatchee

Estuary (Florida) also found the highest TN load in 1995

(5800 ton yr1

) and one of the lowest loads occurred in

1996 (1500 ton yr1

). Annual DIN loading was strongly

dependent of the freshwater flow (Fig. 7). Over 77% of

the variability in DIN load was explained by flow, tying

load, to a certain extent, to climatic conditions.

Although the overall DIN load and the freshwater flow

are correlated, this relationship does not hold for all canals.

For example, Princeton and Mowry canals do not represent the highest freshwater flow; however, they had the

highest the NO

x loadings, meaning they have the highest

flow-weighted mean concentrations. These canals drain

the agricultural area of Dade County where fertilizer use

is prevalent. The relative portions of the different forms

of N vary widely based on proximity of sources to receptors, receiving waters, and atmospheric transformations

as well as on land surface and transfer through the

watershed via surface runoff (Paerl, 1997). In the case of

NHþ

4 loading, the major contributors were the Miami

River, Little River, and Snake Creek, that also showed

the highest freshwater flow. This means that the form of

DIN may be influenced by the urban land use around the

canals.

Contrary to DIN, TP load was not significantly related

to flow, implying that factors other than precipitation-driven input from the watershed were responsible for differences in loading among years. Obviously, there must be

other factors, potentially related to management activity,

which account for the variability in TP loading. The fractionated nature of the watershed and large potential for

variations in flow distribution across the watershed (canalization) increases the variability of TP concentrations and

weakens the load/flow relationship.

0

5

10

15

20

25

30

35

40

45

50

Precipitation (cm)

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

Month of Year

0

100

200

300

400

500

600

Total Canal Flow (CFS)

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

Month of Year

a

b

Fig. 6. Monthly precipitation (cm) for region during 1994–2002 (a) and

mean monthly canal flows (CFS) into Biscayne Bay (b).

y = 0.80x + 10.8

r

2

 = 0.773

1000

1200

1400

1600

1800

2000

2200

2400

1500 1700 1900 2100 2300 2500 2700

Annual Flow (CFS)

Annual Canal DIN Load (Ton yr-1)

1995

1996

B

Fig. 7. Regression of annual average canal flow (CFS) vs. annual DIN

loading (ton yr1

).

V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008 1001
This assertion is supported by the observed interannual

variations in DIN:TP ratios of loads (Table 2). We used the

DIN:TP ratio because the SFWMD did not measure TN.

Although unconventional, the DIN:TP ratio gave us usable

information on loading sources. However, unlike other

ecosystems where NO

3 is the main component in the TN

pool (Dodds, 2006), only 8% of the TN pool in the bay

was in the form of DIN (Caccia and Boyer, 2005). Regression analysis of 12 years of monthly surveys showed that

DIN did not predict TN with any certainty (r

2 = 0.36).

DIN:TP ratios tend to underestimate TN:TP by almost

two orders of magnitude which leads to the conclusion that

the bay is much more N limited than it really is. These data

suggest that the general condition of consistent P limitation

in the bay is some way driven by the unbalance DIN:TP

stoichiometry of the nutrient load itself.

4.2.2. Spatial trends

South Bay received the largest amount of NO

x loading

(1021 ton yr1

) from the canals (Table 2 and Fig. 8). Principal NO

x sources were the South Dade agricultural

basins, the Black Point Landfill, and possibly, the Sewage

Treatment Plant (Fig. 1). In the 1980s these agricultural

areas produced 50% of the nation’s winter vegetables, covering about 80,000 acres (Howie, 1986). Most of Dade

county crops are grown on Rockland soils (oolitic limestone) characterized by low organic content, rapid internal

drainage, alkaline pH, and low nutrient content. The lack

of sufficient nutrients requires frequent and intensive fertilization (5.6–8.3 kg N ha1 and 5.6–16 kg ha1 of P2O5;

DERM, 1978). Nitrate pollution is by far, the most conspicuous impact of agricultural impacts to water quality.

Previously, we reported that the highest water column

NO

3 concentrations found in Biscayne Bay also occurred

in the South Bay, specifically at those stations located near

the Black Point, Princeton, and Mowry Canals (Caccia and

Boyer, 2005). Therefore, these loading estimates confirm

that the low salinity and elevated concentrations of DIN

in the nearshore sites of South Bay are the direct result

of canal inputs.

The highest loading contribution of NHþ

4 to the Bay

(312 ton yr1

) occurred in North Bay and came primarily

from the Miami River (145 ton yr1

). NHþ

4 loadings were

highest in the North Bay because: (1) land use was predominately urban, (2) frequency of chronic sewage contamination, (3) low DO concentration of incoming waters, and (4)

the presence of landfills – Munisport in the North and

Black Point in the South (Fig. 1). The North Bay is heavily

urbanized, 40% has been dredge or filled, and this area also

incorporates industrial complexes, the port of Miami and

Miami River. Sewage contamination has been a problem

since early growth of the Miami urban area (DERM,

1981). Large volumes of raw sewage were discharged

directly into Miami River and Little River from 1920 to

1955 (McNulty, 1970; Wanless, 1976), both have been identified as a conveyance of sewage contamination (McNulty,

1970). This hase since been remediated. Snake Creek also

carries freshwater flow from urban stormwater management systems. DO is chronically low (<4 mg L1

) in some

of the northern canals. Caccia and Boyer (2005) observed

the lowest DO at stations located in the North Bay. The

Dade county standard for unionized ammonia (NH3) is

exceeded frequently in several locations of this region (Alleman et al., 1995).

TP load was also the highest in the North Bay (Table 2

and Fig. 9), being in agreement with the results found in

the update of the surface water improvement and management (SWIM) plan for Biscayne Bay (Alleman et al., 1995).

This mostly a function of the high TP concentrations found

in the canals entering North Bay. Caccia and Boyer (2005)

also reported the highest water column TP concentrations

in the North Bay. High TP concentrations are related to

sewage discharge and the discharge of treatment plants

and landfills (Munisport and Black Point). The accumulation of P in urban regions is mainly a surface problem

(Forsberg, 1995), and there is an urgent need to improve

-80.45 -80.35 -80.25 -80.15

25.30

25.40

25.50

25.60

25.70

25.80

25.90

26.00

CENTRAL

NORTH

SOUTH

ATM =46 DIN

ATM =51 DIN

CANAL =547 DIN

CANAL =67 DIN

ATM =134 DIN

CANAL =1073 DIN

GW =141 DIN

Fig. 8. Annual average DIN loading budget showing canal, atmospheric,

and groundwater loads by region of the Bay (ton yr1

).

1002 V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008
the recycling of nutrients via new methods for more specific

recovery of P in municipal sewage treatment plants (Eggers

et al., 1991). The leaching of P from landfills is something

that we should not undervalue, because many types of

food (butter, margarine, soups, sauces, cheese, jam, fish

products, soft drinks) use phosphorus (ortho-, poly-,

starch-phosphate and phosphoric acid) as food additives,

increasing the concentration of P that may leach from the

landfills. For example, sewage effluent from a treatment

plant with dephosphatation (working effectively) contains

<0.5 mg P L1 while and the concentration of P found in

soft drinks (cola) is 360 times greater (180 mg L1

, Forsberg, 1995). Munisport Landfill has had a history of problems with stormwater runoff into the Bay (Fig. 1). This site

was not an approved landfill and did not have a proper

stormwater system or normal landfill lining precautions.

Although this site has been inactive for years, it is suspected of impacting surrounding mangrove wetlands

within Oleta River State Recreation Area located near to

Snake Creek (Alleman et al., 1995).

Lietz (1999) made the first attempt to estimate nutrient

loads to Biscayne Bay by measuring nutrient concentrations (including organic N) and freshwater discharges at

each canal for 1996 and 1997. He observed the maximum

concentration of NO

x at Princeton Canal, and the maximum NHþ

4 and TP at Arch Creek, being in agreement with

our results. Although there were some variations (due to

short period of study and analytical problems) both studies

arrived at similar conclusions: ‘‘median concentrations of

NO

x were higher in agricultural areas (South Bay), while

NHþ

4 and TP were higher in urban areas (North and Central Bay)’’.

4.3. Atmospheric loads

The highest N atmospheric input was observed in 1998

(4.12 kg N ha1 yr1

) and the lowest in 1996 (2.43 kg N

ha1 yr1

) (Table 3). Coincidently, in 1996 was also

observed the lowest DIN canals loading. In Biscayne Bay

as well as other Florida estuaries, the NO

3 contribution

was also higher than NHþ

4 , and the DIN loads for Biscayne

Bay were similar than those reported for Meyers et al.

(2001) in various estuaries of Florida (Table 4). In general,

Florida has a low atmospheric input of DIN in comparison

with estuaries located in the northeast coast of United

States, such as Chesapeake Bay and Delaware Bay (Meyers

et al., 2001).

Given that Biscayne Bay has a surface area of 70 000 ha

and the average of wet atmospheric deposition is 3.31 kg N

ha1

, then the annual atmospheric input for the Bay is

231.7 ton N yr1

. This value represents 12% of the total

DIN input to Biscayne Bay (Fig. 8). South Bay had the

highest atmospheric DIN load due to its large surface area.

Castro et al. (2003) found that the average percent of N

from atmospheric input for six estuaries from the southeast

coast of United States (South Carolina, Georgia, and Florida) was 29%. The lowest atmospheric input (8%) was

found in the Indian River, FL (East Coast). In the same

study, four estuaries of the West coast of Florida ranged

from 7% to 23% of the N atmospheric input (Castro

-80.45 -80.35 -80.25 -80.15

25.30

25.40

25.50

25.60

25.70

25.80

25.90

26.00

CENTRAL

NORTH

SOUTH

CANAL =18.6 TP

ATM =2.8 TP

ATM =3.1 TP

CANAL =3.4 TP

ATM =8.1 TP

GW =5.9 TP

CANAL =5.4 TP

Fig. 9. Annual average TP loading budget showing canal, atmospheric,

and groundwater loads by region of the Bay (ton yr1

).

Table 4

Atmospheric loads of NO

3 , NHþ

4 , and DIN (kg N ha1 yr1

) in some US

estuaries

ESTUARY NO

3 NHþ

4 DIN

Biscayne Bay, FL 1.92 1.39 3.31

Indian River, FLa 1.96 1.05 3.01

Charlotte Harbor, FLa 2.15 1.24 3.39

Tampa Bay, FLa 1.85 1.08 2.93

Chesapeake Bay, MD-VAa 4.03 2.3 6.33

Delaware Bay, DEa 4.12 1.99 6.11

Hudson River, NYa 3.89 1.87 5.76

Narragansett Bay, MAa 3.3 1.46 4.76

a From Meyers et al. (2001).

V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008 1003
et al., 2003). Therefore, Biscayne Bay is in the expected

range of atmospheric input of N found for Florida.

Atmospheric deposition is a significant and growing

source of biologically available N (NO

x , NHþ

4 and dissolved organic N (DON)) entering nitrogen-limited estuarine and coastal waters (Paerl et al., 2001). Several

studies argue that long range atmospheric transport of N

from sources located in the mid-continental regions is a significant source of N deposition to surface waters on the

East coast of the United States. The processes by which

these atmospheric depositions of N sources are generated

and enter coastal waters are changing both temporally

and spatially. For example, recent urbanization in Southwest Florida: Tampa, St. Petersburg and Sarasota, has

been accompanied by rapidly increasing N emissions from

power plants and automobiles. In Tampa Bay and Sarasota region, oxidized N deposition associated with these

activities accounts for approximately 30% of the new N

input into adjacent near shore waters (Greening et al.,

1997). Sources and sinks of N pollutants operate on spatially explicit scales; all of which are essential for understanding how changing human activities and land-use

affect water quality.

Atmospheric TP loading was significantly lower than

DIN due to the low concentrations of P in rainwater. Data

on atmospheric TP were difficult to obtain, especially for

the period of record of our study. Although the atmospheric deposition of P is an issue in Florida, few studies

have been done and most of them were only for a very

short period. The results of the revised studies presented

much variability: Hendry et al. (1981) found TP input

(wet + dry) to South Florida between 30 and

40 mg m2 yr1 (1-year study); Ahn and James (1999)

reported a mean and standard deviation of P load

33 ± 40 mg m2 yr1

, respectively, at one station in the

Everglades (1992–1996); Grimshaw and Dolske (2002)

found the mean rate of wet atmospheric P deposition

across Florida was 25 ± 5 lgPm2 wk1 (1992–1993);

Graham and Duce (1979) suggested a continent-wide average of 30 mg m2 yr1

. We selected the mean wet deposition rate obtained by Fitz and Sklar (1999) of

20 mg P m2 yr1 (1978–1996) because it had the longest

period of record and it represented a medium value of all

revised studies. In external panel report to the Florida

Department of Environmental Protection ‘‘Overview and

Evaluation of Everglades Nutrient Threshold Research’’

(1996–1997) found a large range of previous estimates of

TP in rainfall in South Florida 13–96 mg P m2 yr1

. The

differences in sampling method and locations have resulted

in a conflicting literature concerning regionally representative concentrations of P in Florida rainfall (Grimshaw and

Dolske, 2002).

Hendry et al. (1981) found spatial and temporal variability in the atmospheric deposition of nutrients across

Florida, also found that P bulk depositions rates were highest in agricultural areas (due to smoke and ash from burning of sugar cane foliage), moderate in urban areas, and

lowest in coastal and non-agricultural rural areas. Grimshaw and Dolske (2002) found no significant variation

among six stations located across all Florida, where the

average rates of P deposition ranged from 20 to

31.5 lgPm2 wk1

. One of their stations was located at

the Everglades National Park (the nearest station to Biscayne Bay), and although the P deposition was in the middle of the range (26.5 lgPm2 wk1

), the volumeweighted mean P concentration was the highest in this station (1.7 lgPL1

). Although it was not possible to identify

the sources of P, they concluded that their regionally rate

of wet atmospheric P deposition to the Florida Peninsula

agreed closely with a calculated estimate of wet atmospheric P deposition to Miami from Saharan dust.

On an area basis, South Bay received the highest TP

atmospheric input (8.1 ton yr1

) representing 42% of the

total TP load. This was greater than the canal (28%) and

groundwater (30%) loads (Fig. 9). North Bay showed the

lowest TP load (2.8 ton yr1

), and in perspective, the TP

input from canal sources (87%) was much higher than

the atmospheric input (13%). In Central Bay, the atmospheric and canal TP loads were similar in proportion.

Although the area of the region leads our results, according

with Hendry et al. (1981) findings agricultural areas had

greater TP loads than urban areas, being the case of South

Bay that is mainly an agricultural area.

4.4. Groundwater

DIN load from groundwater source accounted for

about 10% of the total loading in South Bay (Fig. 8), while

TP load from groundwater was almost equal than canal

loading (30%) (Fig. 9). We believe that groundwater represents an important unregulated and poorly gauged source

of TP to the South Bay. The probable sources of DIN

and TP inputs to South Bay are infiltration from inland

agricultural activities, leaching from the Black Point Landfill, and leakage from a treatment plant where treated

wastewater is injected into the non-potable aquifer. Agricultural land use in the Biscayne Bay watershed is concentrated in southern Dade County, and is located above the

highly porous Biscayne Aquifer. Many studies have found

high NO

3 concentrations in the South Dade groundwater

around the area of intensive agricultural activity (Britt,

1994; Church et al., 1980; DERM, 1978). A large source

of NHþ

4 contamination in South Bay is associated with

Black Point landfill. Meeder and Boyer (2001) found elevated concentrations of nutrients (especially NHþ

4 ) and

very low DO in the canals adjacent to Black Point landfill

and the levels of NHþ

4 in the sediments were an order of

magnitude higher than in the water column. Landfills create a high potential for stormwater to impact surrounding

water bodies. The high water table and the high transmissivity of the Florida aquifer mean that stormwater can easily mix with groundwater and move compounds effectively

into surface waters (Alleman et al., 1995). From a model

estimations (1989–1998) groundwater discharge was 151.2

1004 V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008
CFS, which represented 10% of the surface freshwater

discharge for the same period, and during dry season

groundwater can exceed the surface freshwater discharge

(Langevin, 2003). We hope that future studies focused on

measuring groundwater flows and loads will eventually

provide more information as to this important nutrient

source.

4.5. Nutrient loading comparisons with other estuaries

In an overview study of N and P budgets of the North

Atlantic Ocean including fourteen regions in North America, South America, Europe and Africa, the largest N fluxes

(per area basis >1000 kg N km2 yr1

) were found in the

highly disturbed watersheds around the North Sea (NW

Europe) and NE coast of the USA (Howarth et al.,

1996). The North Sea showed the most perturbed situation

with a largely open N cycle. They also found that in US

coast the NE region had more individuals per km2 and

higher water discharge per area than the SE region, as well

as TN and TP export in rivers were decreasing from the

North to the South Atlantic coast of the USA. The N

and P inputs to the North Atlantic Ocean from terrestrial

sources were higher at the NE coast of the USA

(TN = 0.51 Tg yr1

; TP = 0.067 Tg yr1) than those from

the SE coast (TN = 0.24 Tg yr1

; TP = 0.011 Tg yr1),

with fertilizer accounting for roughly two-thirds of the

total N inputs (Howarth et al., 1996).

In Table 5 is presented a nutrient loading comparison

with several estuaries along the East coast of the USA.

TN and TP data in this table include all measured sources

(atmospheric, rivers, canals, and sewage). Unfortunately,

we could not find data of DIN, TN and TP for all estuaries

presented. Total nitrogen loads are the sum of inorganic

and organic N (which was not routinely measured in Biscayne Bay). Lietz (1999) found that 70% of the TN in the

canals was in the organic form, while Caccia and Boyer

(2005) reported that 90% of the TN in Biscayne Bay was

in the organic form. Assuming that 70% of TN is organic

N and a DIN loading of 24 kg N ha1 yr1

, then the TN

load from canals would be 80 kg N ha1 yr1

. Adding

DIN from atmospheric deposition (3.31 kg N ha1 yr1

)

and groundwater (only to South Bay, 3.48 kg N ha1 yr1

)

without considering the TON of these sources because the

proportions are not known, the overall TN load to the Bay

would be 86.8 kg N ha1 yr1

. The total TP for the Bay

was 47.4 ton yr1 and dividing it for the area was

0.68 kg P 1 yr1

. Biscayne Bay loads were in the middle

range for TN and the low range for TP (Table 5). Estuaries

from the NE coast showed higher TN and TP loads per

area that the estuaries from the SE coast and Florida, with

an exception of Ochlockonee Bay, FL. Boston Harbor,

MA and Ochlockonee Bay, FL had the highest TN values

per hectare, but their short residence times (<1/2 month)

allow them to export more than 85% of TN. In contrast,

the Chesapeake Bay had less TN input per hectare but

longer residence time (9 months) and exported only

25% of TN (Nixon et al., 1996). Therefore, Chesapeake

and Potomac estuaries retained more N (60–70%) than

Boston and Ochlockonee estuaries (15%) (Nixon et al.,

1996). The annual TN and TP budgets for the estuaries

in the North Atlantic revealed that the net fractional transport of these nutrients through estuaries to the continental

shelf was inversely related to the log mean residence time of

water in the system (Nixon et al., 1996).

Biscayne Bay is very different from other major estuaries

of the South Atlantic coast of North America in its geology, geomorphology, and hydrology. These differences

are reflected in its nutrient loading aspects and water quality status (Caccia and Boyer, 2005). Because this present

study is based mostly on DIN data from canal and river

measurements, DIN data in Table 5 only included data

from these two sources. DIN per area basis in Savannah

River and Cape Fear River (>300 kg ha1 yr1

) was more

than the double of all Florida estuaries (140 kg

ha1 yr1

) and at least one order of magnitude higher than

the DIN of Biscayne Bay (24 kg ha1 yr1

). It should be

noticed that Savannah and Cape Fear estuaries are seven

times smaller than Biscayne Bay. The greatest DIN inputs

per tons were found for Apalachicola Bay, Savannah River

and Cape Fear River. Their freshwater flows are two to

three times greater than those of Biscayne Bay. NHþ

4 loading was higher for Cape Fear (700 ton yr1

) than in North

Biscayne Bay (312 ton yr1

), and the NO

x loading were

higher for Savannah (3500 ton yr1

) than those found in

the South Biscayne Bay (1021 ton yr1

). Dame et al.

(2000) found the highest concentrations of NO

x and

NHþ

4 in Cape Fear. Interestingly, these concentrations were

lower than those found in some of the canals entering Biscayne Bay. Median NHþ

4 concentration in the northern

canals of Biscayne Bay (0.3 mg L1

), was twice than that

of the Cape Fear (0.15 mg L1

), while median NO

x concentrations in the South Bay canals (1.9 mg L1

) was triple

of the Cape Fear (0.65 mg L1

).

The majority of the Florida estuaries included in this

study are on the west coast of Florida, except St. Lucie

and Biscayne Bay, and their watersheds were dominated

by agricultural N sources. Ochlockonee Bay showed the

highest DIN loadings per area basis and Tampa Bay had

the lowest. Biscayne Bay was in the middle range of Florida estuaries. The total DIN loading from canals in the Bay

represented 88% (agriculture + sewage) and DIN atmospheric inputs accounted for 12%. We should point out

that depending of the region and land use, the percentages

were different (Fig. 6). In North Bay 92% of the DIN input

(primarily NHþ

4 from sewage) came from canals while only

8% was from atmospheric sources, and in South Bay 80%

was DIN load (primarily NO

x from agriculture) from

canals while DIN loads from atmospheric and groundwater sources accounted for about 10% each, of the total load.

Caraco (1995) found a direct relationship between population development, fertilizer applications, and riverine N

and P fluxes. Florida has the highest average growth rate

(45%) compared with Georgia and the Carolinas (15%)

V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008 1005
(Dame et al., 2000). In Florida, coastal counties occupy

57% of the land, but contain 78% of the population

(Montague and Odum, 1997). With a human population

growth rate of 600 individuals per day, Florida is already

exhibiting environmental stress in many areas. At this rate

of human population growth, it is projected that Florida

will lose the ability to sustain its estuarine environments

within the next 20 years (Montague and Odum, 1997.

Between agriculture, high human population density,

growth rate, urbanization, and in general the habitat

destruction, Miami and Biscayne Bay presents the scenario

of potential environmental collapse. We hope that the

information presented in this study will help in improving

the management strategies to reduce the nutrient loadings

in Biscayne Bay.

Acknowledgments

We thank Stephen Blair and Susan Kemp (Miami-Dade

Department of Environmental Resource Management) for

supplying nutrient data from the canals, Richard Alleman

(SFWMD) for kindly providing us the canal freshwater

flow data, and Joseph Prospero (RSMAS, UM) for his

appreciable help with the atmospheric data and calculus.

We also thank Jack Meeder and Danielle Mir for furnishing us with groundwater flow and nutrient data, respectively. The manuscript was much improved by comments

of an anonymous reviewer. We acknowledge Biscayne

National Park (PMIS Project #94291) and South Florida

Water Management District (C-15397) for continued funding. This is contribution #346 of the Southeast EnvironTable 5

Nutrient loading comparisons with other US estuaries along the Atlantic coast

Area

(km2

)

Depth

(m)

DIN

(ton N yr1)

TN

(ton N yr1)

DIN

(kg ha1 yr1)

TN

(kg ha1 yr1)

TP

(kg ha1 yr1)

Florida estuaries

Entire Biscayne Baya 700 2.4 1687 6076 24.1 86.8 0.68

North Biscayne Bay

(5 canals)a

140 547 243 39 17.4 1.53

Central Biscayne Bay

(3 canals)a

155 67 298 4.3 19.2 0.42

South Biscayne Bay

(4 canals)a

405 1073 2034 26.5 50.2 0.48

Apalachicola Bayb 554 7422 134.0

Caloosahatchee C-43c 62 470 2400 75.8 387

St. Lucie Estuary

(C44, C23, C24)b

16.6 216 130.1

Ochlockonee Bayd 24 1 403 2014 167.9 839.3

Charlotte Harborf 699 2724 39

Tampa Bayf 940 780 3451 8.7 36.7

Southeast USA

Cape Fear River (NC)g 100 3.4 3700 370

Savannah River (SC, GA)g 121 4.6 4000 330.6

Edisto River (SC)g 203 4 600 29.5

St. Marys River (GA)g 64 6.1 180 28

Northeast USA

Pleasant Bay (MA)h 27.5 67 24.4

Little Narragansett Bay (RI)i 9.6 100.8 446 105 465 45.2

Providence Estuary (RI)j 80 1862 4144 235.7 518 56.5

Narragansett Bay (RI)d 328 8.3 9000 274.4 35.6

Boston Harbor (MA)d 108 5.8 13 752 1273.3 204.4

Potomac Estuary (MD-VA)e 1210 5.9 35 489 293.3 13.6

Chesapeake Bay (MD-VA)e 11 542 6 151 546 131.3 12.7

Delaware Bay (DE)d 1989 9.7 52 907 266 49

Greenwich Bay (RI)k 88.8 20.8

Empty space: data not available.

a Present study. b Graves et al. (2004). c Graves et al. (2004) (DIN) and Doering and Chamberlain, 2004) (TN). d Nixon et al. (1996). e Nixon et al. (1996) and Boynton et al. (1995). f Stanley (2001). g Dame et al. (2000). h Carmichael et al. (2004). i Fulweiler and Nixon (2005). j Nixon et al. (2005). k Granger et al. (2000).

1006 V.G. Caccia, J.N. Boyer / Marine Pollution Bulletin 54 (2007) 994–1008
mental Research Center at Florida International

University.

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