Nutrient Dynamics in Vegetated and Unvegetated Areas of a Southern Everglades Mangrove Creek

Estuarine, Coastal and Shelf Science (2001) 52, 753–768 doi:10.1006/ecss.2000.0755, available online at http://www.idealibrary.com on

Nutrient Dynamics in Vegetated and Unvegetated Areas of a Southern Everglades Mangrove Creek S. E. Davis IIIa,e, D. L. Childersb, J. W. Day, Jrc, D. T. Rudnickd and F. H. Sklard a

Southeast Environmental Research Center, Florida International University, University Park Campus, Miami, FL 33199, U.S.A. b Department of Biological Sciences/Southeast Environmental Research Center, Florida International University, University Park Campus, Miami, FL 3319, U.S.A. c Coastal Ecology Institute, Center for Coastal, Energy, and Environmental Resources, Louisiana State University, Baton Rouge, LA 70803, U.S.A. d Everglades Systems Research Division, South Florida Water Management District, 3301 Gun Club Rd., West Palm Beach, FL 33416, U.S.A. Received 19 September 2000, and accepted in revised form 16 November 2000 Flow-through flumes were used to quantify net areal fluxes of nutrients in the fringe mangrove zone of lower Taylor River in the southern Everglades National Park. We also quantified net areal fluxes along the open water portion of the channel to determine the relative importance of either zone (vegetated vs. unvegetated) in the regulation of nutrient exchange in this system. Taylor River’s hydrology is driven mainly by precipitation and wind, as there is little influence of tide. Therefore, quarterly samplings of the vegetated and unvegetated flumes were slated to include typical wet season and dry season periods, as well as between seasons, over a duration of two years. Concentrations of dissolved and total organic carbon (DOC and TOC) were highest during the wet season and similar to one another throughout the study, reflecting the low particulate loads in this creek. Dissolved inorganic nitrogen (nitrate+nitrite+ammonium) was 10–15% of the total nitrogen (TN) content, with NOx
and NH4+ showing similar concentration ranges over the 2-year study. Soluble reactive phosphorus (SRP) was usually <0·05
M, while total phosphorus (TP) was typically an order of magnitude higher. Net areal fluxes were calculated from nutrient concentration change over the length of the flumes. Most flux occurred in the vegetated zone. Dissolved inorganic nitrogen and DOC were usually taken up from the water column; however, we saw no seasonal pattern for any constituent over the course of this study. Total nutrients (TOC, TN, and TP) showed little net exchange and, like SRP, had fluxes that shifted irregularly throughout the study. Despite the lack of a clear seasonal pattern, there was a great deal of consistency between vegetated flumes, especially for NOx
and NH4+ , and fluxes in the vegetated flumes were generally in the same direction (import, export, or no net flux) during a given sampling. These findings suggest that the fringe mangrove zone is of considerable importance in regulating nutrient dynamics in lower Taylor River. Furthermore, the influence of this zone may at times extend into northeast Florida Bay, as the bay is the primary recipient of water and nutrients during the wet season.  2001 Academic Press Keywords: net areal flux; organic carbon; nitrogen; phosphorus; flume; Rhizophora mangle; Everglades; Florida Bay

Introduction Estuarine ecologists have been speculating on and seeking to quantify the exchanges of materials between coastal wetlands and nearshore waters for some 40 years now (Nixon, 1980; Childers et al., 2000). Much of this work was inspired by the Outwelling Hypothesis that was formulated in the 1960s (Teal, 1962; Odum & de la Cruz, 1967). Although studies testing this concept have not actually proven its universality, they have led to a better understanding of patterns of wetland– estuarine and estuarine–nearshore exchanges of e Author for correspondence. Current address: Department of Wildlife and Fisheries Sciences, 210 Nagle Hall, Texas A&M University, College Station, Texas, 77843, U.S.A.

0272–7714/01/060753+16 $35.00/0

materials. Another outcome of such studies has been the advancement of a number of laboratory and field methods for determining wetland–water column nutrient fluxes. Among these, the flume technique has proven effective in a number of temperate estuarine wetland systems (Childers, 1994). Regardless of whether water samples are collected at one end, from an array of points, or at both ends of the flume, this rather simple in situ technique allows for the direct quantification of the effect of wetland soils and vegetation on water column nutrient concentrations. While flumes have been frequently employed to help quantify net areal fluxes of nutrients in temperate estuaries over much of the past two decades, this technique has  2001 Academic Press

754 S. E. Davis, III et al.

been utilized in only a few tropical mangrove settings (Twilley, 1985; Rivera-Monroy et al., 1995). The number of nutrient flux studies in mangrove wetlands has increased dramatically over the past 15 years. This trend has stemmed from recent deterioration of water quality in many tropical and subtropical coastal areas as a result of deforestation, coastal development, oil spills and freshwater diversion (Twilley, 1998). By and large, the focus of these studies has been on the exchange of organic matter between the mangrove and nearshore environment (Boto & Bunt, 1981; Twilley, 1985; Woodroffe, 1985; FloresVerdugo et al., 1987; Robertson, 1986; Lee, 1990). However, there have been a few studies that have quantified the exchanges of inorganic nutrients (i.e. nitrogen and phosphorus) within mangrove wetlands or between mangroves and nearshore systems (Boto & Wellington, 1988; Kristensen et al., 1988; Nedwell et al., 1994; Rivera-Monroy et al., 1995; Davis, 1999). Although it is generally held that mangroves export organic matter in relation to tidal energy (Odum et al., 1979; Twilley, 1985; Lee, 1995), the fate of inorganic nutrients in estuarine mangrove systems is still poorly understood. Furthermore, little is known about the exchanges and transformations of either organic or inorganic nutrients in mangrove systems with little or no tidal influence. Along the southern tip of Florida, there are a number of microtidal mangrove waterways linking the freshwater marshes of the Everglades to the Florida Bay Estuary. These are primarily wind and rainfall-driven systems that transport large quantities of fresh water and nutrients to Florida Bay during the wet season. During the dry season, water and nutrients from the bay infiltrate these creeks, raising salinity and changing nutrient profiles (Davis, 1999; Childers, unpubl. data). While this is the general hydrological pattern for mangrove creeks in this region, water management had led to an overall decrease in the volume of fresh water flowing through these creeks into Florida Bay since the early to mid 1900s (McIvor et al., 1994). Furthermore, decreased freshwater inputs in this region have been linked to extended periods of hypersalinity in the eastern and central regions of the bay as well as numerous deleterious effects on biotic communities (McIvor et al., 1994; Fourqurean & Robblee, 1999). In an attempt to offset these negative impacts and restore the system to ‘ pre-management ’ conditions, the Everglades Forever Act mandated an increase in overland freshwater flow to Florida Bay. It is possible that this increased flow will affect not only the annual salinity and nutrient concentration patterns in these creeks, but also fluxes of nutrients within

the creeks and, ultimately, exchanges with Florida Bay. The primary goals of our study were three-fold. First, we sought to quantify concentrations and net areal fluxes of carbon, nitrogen, and phosphorus in the fringe mangrove zone of a mangrove creek linking the southern Everglades marshes to eastern Florida Bay. Second, we wanted to understand the importance of the submersed/creekside mangrove vegetation in terms of regulating these fluxes. In order to do this, we developed an in-channel, flow-through flume system that isolated the open water (unvegetated) portion of the creek from the banks containing submersed mangrove vegetation (vegetated). In mangrove creeks of the southern Everglades, the vegetated zone is typically dominated by the fringe form of Rhizophora mangle L. and is characterized by an abundance of submersed prop roots that are typically ‘ fouled ’ with autotrophic and heterotrophic organisms interlaced with adventitious mangrove root hairs. Recent work has already demonstrated substantial uptake and exchange of water and nutrients between fouled prop roots and the adjacent water column (Elison et al., 1996; Childers, unpubl. data). As water and nutrients in the open water portion of the channel are temporarily isolated from the vegetated zone, nutrient dynamics in the unvegetated zone are limited to transformation processes occurring in the water column or between the water column and benthos. Consequently, we expected nutrient fluxes to be more appreciable in the vegetated zone of the creek. Finally, we wanted to know if the net areal flux of these materials in lower Taylor River varied according to season. In mangrove creeks of the southern Everglades, season is important in driving both salinity and nutrient concentrations (Childers, unpubl. data) and, thus, may have a substantial effect on flux patterns. Therefore, we performed flux studies during typical wet and dry season months as well as months that often mark the transition between seasons. Site description This study was conducted in Taylor River, a mangrove creek draining Taylor Slough in southern Everglades National Park (ENP), Florida, U.S.A. (Figure 1). Taylor Slough is the largest natural drainage for fresh water in the southern Everglades, second only to Shark River Slough in all of ENP. It is located in the south-east corner of ENP and feeds a number of mangrove creeks that discharge into the north-eastern Florida Bay. One of the most important of these channels in terms of freshwater flow is Taylor River (Figure 1).

Nutrient dynamics in vegetated and unvegetated areas 755 25.40 Everglades National Park Florida

25.33 Taylor Slough 25.26

25.18

25.10

25.03 10 km

Florida Bay 24.95 Atlantic Ocean 24.88

81.10

81.00

80.90

80.80

80.70

80.60

80.50

24.80 80.30

80.40

Approximate location of flume study Pond 1

Little Madeira Bay Buttonwood Ridge ≈500 m

Florida Bay

Taylor River

Little Madeira Bay

F 1. Map of southern Everglades National Park, Florida Bay, and upper Florida Keys with Taylor Slough boundaries delineated. The enlarged areal photo depicts a portion of lower Taylor Slough and all of Little Madeira Bay. The lower stretch of Taylor River begins at a shallow pond (Pond 1), dissects the Buttonwood Ridge, and empties into Little Madeira Bay as shown in the illustration to the lower right. The approximate location of the flume study is also indicated.

Taylor River is a fairly small channel (]10 m wide; 1–2 m deep) that links a number of small, shallow mangrove ponds along the north–south gradient of the

southern Everglades salinity transition zone. Like many creeks in this region, Taylor River is not significantly affected by tides. In fact, the direction and

756 S. E. Davis, III et al. Wet

Dry

Wet

Dry

Wet

35

30

Salinity

25

20

15

10

5

0

May

Aug 1996

Nov

Jan

May Aug 1997

Nov

Jan

May 1998

Aug

F 2. Plot of daily averages of surface water salinity at the mouth of Taylor River from May 1996–October 1998. Typical timeframes for south Florida seasons are indicated.

velocity of flow in this system is driven mostly by the interactions of local precipitation, wind, and upland runoff. These factors are all related to climatic seasons in south Florida and combine to produce a characteristic annual pattern of salinity at the mouth of Taylor River (Figure 2). Generally speaking, the fresh-water Everglades marshes (upstream) are the major sources of water and materials to Taylor River during the wet season, while Florida Bay is typically the major source during the dry season. In addition, the wet season–dry season transition period (usually from November– March) is characterized by frontal passages that bring both precipitation and winds, resulting in rapid shifts in the salinity and flow patterns. The lower stretch of Taylor River begins at a shallow mangrove pond (Pond 1) and dissects the Buttonwood Ridge before emptying into Little Madeira Bay (Figure 1). The Buttonwood Ridge is actually a depositional feature of carbonate and marl sediment approximately 1 m above mean sea-level, and is chiefly inhabited by a basin mangrove community dominated by the black (Avicennia germinans L.), white (Laguncularia racemosa L. Gaertn.), and button mangroves (Conocarpus erectus L.). The stretch of Taylor River that cuts through the Buttonwood Ridge consists of high, steep sloping banks and a flat limestone bottom. These banks support a red mangrove-

dominated (Rhizophora mangle L.), fringe forest, characterized by an abundance of epibiont covered prop roots that extend out into the channel. The prop root/epibiont matrix not only impedes flow but also may be an active zone of nutrient uptake and exchange (Childers, unpubl. data). Due to the morphometry of lower Taylor River and the density of submersed prop roots, it is probable that all water passing through this zone comes into contact with the submersed mangrove zone at some point, and thus has the potential for exchange with the prop root/ epibiont matrix. Materials and methods A flume system composed of three side-by-side flumes (each 14 m long) was constructed parallel to the direction of flow, in lower Taylor River for the purpose of isolating the submersed mangrove vegetation along the banks (east and west flumes) from the open water portion or channel (Figure 3). This design utilized the high, steep sloping banks of Taylor River as the lateral (outer) walls of the flumes along with fabricated medial (inner) walls (Figure 3). Medial walls were constructed of clear, plastic film attached to aluminium fence posts spaced 2 m apart. Each fabricated wall had a galvanized chain installed along

Nutrient dynamics in vegetated and unvegetated areas 757 ve zone

d mangro

Submerse

Open

We flu st me

r wate ve

Ch

ann

ed

ers

el

m Sub

ro ang

e

zon

m

Ea flu st me Bu

tto

nw

ood

Rid

ge

F 3. Illustration of in-channel flumes utilized in lower Taylor River. Steep banks on either side of the channel and fabricated in-channel (medial) walls isolated the submersed mangrove zone (vegetated) from the open water (unvegetated) portion of the channel.

the lower margin to hold the wall to the bottom of the channel. Since the lower stretch of Taylor River has a flat, rocky bottom, these posts were set into blocks of concrete to hold them in an upright position. Medial flume walls were constructed parallel to one another (2·9 m apart), preventing any lateral exchange of water between the vegetated and unvegetated flumes (Figure 3). In order to minimize sediment and prop root disturbance that might affect the flux of materials through the flumes, these medial walls were installed approximately 24 h prior to each sampling. Field and laboratory methods Both vegetated flumes and the unvegetated flume were sampled quarterly over a period of two years to investigate intra-annual variability in nutrient fluxes (Table 1). We also repeated samplings on consecutive days to address short-term variability, for a total of 16 samplings (Table 1). On each sampling day, six pairs of upstream/downstream water samples were collected at regular intervals (usually 1 h) from the flumes as long as flow was measurable (>0·01 m s
1) and in a consistent direction. Using a protocol similar to that of Childers and Day (1988) and RiveraMonroy et al. (1995), we collected water from single, fixed points at both ends of each flume and channel with a hand pump and side-arm flask apparatus. Prior to each use, this apparatus was triple-rinsed with water from the station being sampled. Simultaneous current velocity readings were taken at each upstream

and downstream sampling station with a MarshMcBirney (Model 201D) current meter, and water level was recorded at every sampling interval from a fixed point in the channel. Measurements of salinity and temperature were also taken during every sampling interval with a calibrated, analogue S-C-T meter (YSI Model 33). Water samples collected from each station were temporarily stored in 1-l, acid-rinsed, collapsible cubitainers and portions of each were immediately filtered (Whatman GF/F). Filtered samples were stored frozen and unfiltered samples were kept at 4 C until analysed for nutrient content. At the conclusion of each two-day sampling event, the medial flume walls were removed to prevent shading or isolation effects. All nutrient analyses on water samples were performed at the Southeast Environmental Research Center’s (SERC) analytical laboratory at Florida International University. Unfiltered water samples were analysed for total phosphorus (TP) using a modified dry-ashing, acid-hydrolysis technique (Solorzano & Sharp, 1980), total nitrogen (TN) using an Antec 7000N total nitrogen analyser, and total organic carbon (TOC) using a hot platinum catalyst, direct injection analyser (Shimadzu model TOC5000). Filtered water samples were analysed for soluble reactive phosphorus (SRP), ammonium (NH4+ ), and nitrate+nitrite (NOx
) on a four-channel autoanalyser (Alpkem model RFA 300), and dissolved organic carbon (DOC) using the same method as for TOC.

758 S. E. Davis, III et al. T 1. List of sampling dates and times of flumes (vegetated=VEG; unvegetated=NVEG) in lower Taylor River. Corresponding season (wet season=WET; dry season=DRY; transitional=TR) and hydrologic data (salinity=SAL; water level=WL; current velocity=CV) for each sampling are also given. SAL is presented as a range (min. to max.) and WL represents the change in water level over the course of a given sampling Direction of flow

SAL

WL (cm)

Mean CV in VEG (cm s
1)

Mean CV in NVEG (cm s
1)

TRWET

north north

8–10·5 8–100

1·5 1·5

8·2 8·3

16·7 21·8

10:00–15:00 11:00–16:00

TRDRY

north south

12–13·5 11 5–12

1·5 0

10·9 14·6

26·5 38

17 May 97 18 May 97

13:00–16:30 11:30–14:30

DRY

north south

27·5–29 28

0·5 0·5

3·2 2·9

4·6 4

6 Aug 97 7 Aug 97

12:30–17:30 13:00–18:00

WET

south south

2 2

1 1

5·3 7·9

7·5 13·2

15 Nov 97 16 Nov 97

11:00–16:30 10:30–15:30

TRWET

north south

11·5–13 7–8·5

2 2

5·9 14·8

15·3 38·8

17 Jan 98 18 Jan 98

12:00 17:30 11:00–18:00

TRDRY

south south

0 0

2 1·5

8·7 12·4

24·8 29·8

23 May 98 24 May 98

11:00–15:30 11:00–15:30

DRY

north north

15–17 16·5–18

2 1

5·2 3·8

14·9 8

7 Aug 98 8 Aug 98

11:30–15:00 12:00–15:00

WET

south south

5–8·5 4

1 1

5·3 5·7

15 17

Sampling date

Sample time

Season

19 Nov 96 20 Nov 96

11:00–18:00 08:00–16:00

17 Jan 97 18 Jan 97

Net areal flux calculation and calibration Given the weak tidal character of lower Taylor River (see Table 1), we used a cross-sectional, velocity-area approach to calculate instantaneous water flux rather than a hypsometric method based on water level or flume volume change. This approach required an understanding of the variability in cross sectional discharge at either end of the flumes. Discharge in the unvegetated area was uniform, as there were no impediments to flow in this zone. However, the abundance of prop roots in the vegetated flumes often produced non-uniformity in cross sectional discharge. In order to account for this, we performed a series of calibration samplings over the course of this study that included scenarios of high and low flows. The specific purpose of these samplings was to generate relationships that could be used to predict cross sectional discharge at the downstream ends of each vegetated flume based on current velocity measurements at our fixed sampling points [Figure 4(a)]. During each of the calibration samplings, we took current velocity readings from an array of points over the cross sectional profile of the south end of the vegetated flumes [Figure 4(a)]. These current velocity

measurements were then used to calculate instantaneous water flux (m3s
1) according to a method similar to that of Kjerfve et al. (1981). For each flume, instantaneous water fluxes (a.k.a. cross sectional discharges) were regressed with the current velocities from our fixed sampling stations [Figure 4(b)]. Utilizing the equations generated from these flume-specific regressions, we were able to estimate instantaneous water fluxes from a single measure of current velocity in each vegetated flume. For the unvegetated flume, instantaneous water fluxes (m3 s
1) were estimated by simply multiplying current velocity (m s
1) at the fixed sampling point by the cross sectional area (m2). Since it was assumed that the flux of water into each of three flumes equalled the outward flux (inflow= outflow), all instantaneous water flux estimates were made from the current velocity readings taken at the south end of the flumes, regardless of the direction of flow. Instantaneous (
moles s
1) and total fluxes (
moles sampling
1) of nutrients were calculated according to the equations set forth in Childers and Day (1988). However, we applied a velocity-area estimate of instantaneous water flux to calculate instantaneous nutrient flux in this study. We then

Nutrient dynamics in vegetated and unvegetated areas 759 (a) West vegetated flume

Unvegetated flume

East vegetated flume

Mean high water

X

X ≈2 m

≈12 m (b) 0.6

0.8

West vegetated flume Water flux (m3 s–1)

Water flux (m3 s–1)

0.4 0.3 0.2 0.1 0

East vegetated flume

0.7

0.5

0.6 0.5 0.4 0.3 0.2 0.1

0.05

0.1

0.15 0.2 0.25 Velocity (m s–1)

0.3

0.35

0.4

Y = –0.037 + 1.604*X; R2 = 0.993

0

0.05

0.1

0.15 0.2 0.25 Velocity (m s–1)

0.3

0.35

0.4

Y = –0.032 + 2.002*X; R2 = 0.997

F 4(a) Illustration of downstream cross section of Taylor River flumes that were used to separate vegetated areas from unvegetated areas. Fixed water sampling stations are indicated by ‘ X ’ and calibration sampling points by ‘ ’. On eight occasions, representing a range of flow characteristics, an array of current velocity measurements was taken along the downstream (southern) cross sections of both vegetated flumes. The East Flume was slightly wider than the West; hence more calibration sampling points on the eastside. Each set of measurements was then used to calculate an instantaneous water flux. (b) Linear regression analysis was used to model the relationship between current velocity measurements at the fixed sampling point with the instantaneous water fluxes across each flume cross section. The regression equation and R2 for each flume are shown.

calculated net areal fluxes (
moles m
2 h
1) according to the modified Childers and Day (1988) equation used in Rivera-Monroy et al. (1995), with ‘ flume area ’ as the total bottom area of the flume. In order to obtain mean fluxes for the fringe mangrove zone, we averaged the net areal fluxes of nutrients in the vegetated flumes from each two-day sampling event. In doing so, we assigned a value of zero to all non-significant fluxes. Finally, we combined fluxes measured in the unvegetated flume with these values, yielding averages for the entire cross section of the creek (vegetated+unvegetated zones). Statistical methods and data analysis and interpretation All statistical procedures were performed using StatView 5 for the Power PC (SAS Institute, Inc.,

Cary, NC). Concentrations of nutrients in the flumes and channel were pooled by location (upstream or downstream) and sampling date (month/year). Box and whisker plots of ambient nutrient concentrations were constructed for each constituent to illustrate the variability in concentration distributions between samplings. Only upstream data were used in these plots, as they were considered ‘ pre-flume treatment ’. Next, we employed a single-factor analysis of variance to test for an effect of sampling date on ambient nutrient concentrations. Fisher’s Pairwise Least Significant Differences (PLSD) post hoc tests were used in each significant ANOVA to identify differences between the means from individual samplings. To determine if fluxes of nutrients in the vegetated or unvegetated zone were significant during a given

760 S. E. Davis, III et al.

sampling, we used a paired t-test to test for a significant difference (P<0·05) between upstream and downstream concentrations (
M) for each flume. We calculated a net areal flux only when a significant difference was observed. Higher concentrations at the upstream end indicated a nutrient uptake by the wetland, while lower concentrations at the upstream end indicated a nutrient release. We interpreted no difference in concentration between upstream and downstream ends as no net flux (net areal flux= 0
moles X m
2 h
1). All flux data (including zero values) for each constituent were subjected to a onefactor ANOVA to determine the effect of season on net nutrient exchange in this system. Fisher’s PLSD was used to identify significant differences between wet, dry, and transitional season flux means. Results Hydrologic and nutrient data from Lower Taylor River A pattern of high salinity and northerly flow during the dry season, low salinity and southerly flow during the wet season, and variable salinity and flow during the transitional season samplings were anticipated. A pattern resembling this existed for much of the study; however, the combination of north winds and an extended wet season in 1997 (June 1997– March 1998) produced consistently low salinity and strong southerly flow in Taylor River during the January 1998 transitional sampling (Figure 2; Table 1). In general, salinity at the fringe mangrove site was highest during the dry season (May), followed by the transition from the wet season to the dry season (November and January), and lowest during the wet season (August; Table 1). The direction of flow followed a similar pattern, for the most part, with southerly flow recorded during the wet season samplings and northerly flow during most of the dry season samplings (Table 1). Water level fluctuation during most samplings was d1 cm, yet appeared to be slightly higher during the transition from the wet season to the dry season (1·5–2 cm; Table 1). Similarly, current velocity was higher in both vegetated and unvegetated areas of lower Taylor River during the transitional samplings (Table 1). Hydraulic residence times within the flumes reflected changes in current velocity, ranging from 0·72 to 2·4 min in the unvegetated flume and 1·1 to 3·5 min in the vegetated flumes. Current velocities measured in the vegetated flumes always fell within the range measured during the calibration samplings (Table 1; Figure 4). This permitted use of the flume-specific relationships between cross sectional discharge and velocity to

calculate net areal flux when concentration changed was observed [Figure 4(b)]. Concentrations of most constituents varied noticeably over the course of the study. However, dissolved and total organic carbon concentrations were nearly identical to one another in every sampling [Figure 5(a and b)]. In fact, roughly 95% of the TOC in Taylor River was in the dissolved form, reflecting the low suspended particulate load common to this creek. Concentrations of organic carbon (OC) were highest in August, and ranged from 800–1700
M over the course of the study [Figure 5(a and b)]. Nitrate+nitrite (NOx
) concentrations ranged from a high of 5·75
M in November 1996 to a low of 0·2
M in August 1998 [Figure 5(c)]. For the most part, concentrations of NOx
declined throughout the calendar year [Figure 5(c)]. Ammonium displayed a similar overall range of concentration to NOx
(0·1– 6·3
M), with the lowest concentrations and least amount of variability generally occurring during the wet season [Figure 5(d)]. Total nitrogen fluctuations throughout this study paralleled changes in OC, ranging from a low of 41
M in November 1996 to 89
M in August 1997 [Figure 5(e)]. Concentrations of soluble reactive phosphorus (SRP) were typically less than 0·1
M and, in many instances, were at or below the limits of detection [<0·01
M; Figure 5(f)]. However, SRP concentrations were significantly higher during the May 1998 sampling (0·13–0·24
M), making up roughly 45–50% of total phosphorus fraction (TP) at that time [Figure 5(f and g)]. Throughout our study, Taylor River SRP was typically 5–25% of the TP that varied from 0·18–0·67
M [Figure 5(g)].

Flux results The submersed, fringe mangrove zone was found to be the active site of nutrient exchange in lower Taylor River when compared to the unvegetated channel. Significant changes in nutrient concentrations along the unvegetated flumes were infrequently observed and rarely corresponded to those measured in the vegetated flumes. Moreover, there was no evidence of a seasonal pattern in either the direction or magnitude of nutrient fluxes in lower Taylor River. This was mainly due to a number of constituents showing little or no net areal flux in either zone over the course of the study. Among these, the ‘ total ’ nutrients (TOC, TN, and TP) were not as dynamic as DOC or the dissolved inorganic constituents we measured. Although the bulk of OC in lower Taylor River was in the dissolved form [Figure 5(a and b)], TOC and

Nutrient dynamics in vegetated and unvegetated areas 761 1800

1800

(a)

F

1600 E

1200 1000

D

D B

B

C A

800

600

6

Jan May Aug ' 98

7

(c) F

2

BC

1

µM TN

µM NH4+

D

D

Nov ' 96

C

AB

Jan May Aug Nov ' 97

A

Jan May Aug Nov ' 97

Jan May Aug ' 98 E

(d) F

4

C

D

3

B

B

A

A

1 0

Jan May Aug ' 98

0.25

(e)

Nov ' 96

Jan May Aug Nov ' 97

Jan May Aug ' 98

(f)

C

0.2 F

A

D

G E

D

C B

0.15

B

B

0.1 0.05

A

A A

A

A

0 Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

(g) E

F

0.6 0.5

D C

0.4 AB 0.3

Nov ' 96

2

µM SRP

µM NOX–

3

0.7

A

5 E

4

90 85 80 75 70 65 60 55 50 45 40

C B

6

5

0

D

B

800

400

7

µM TP

1000

400

Jan May Aug Nov ' 97

E

1200

600 Nov ' 96

G F

1400 µM TOC

µM DOC

1400

(b)

1600

BC

D

A

0.2 Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

F 5. Box-and-whisker plots of water column nutrient concentration distributions (
M; (a) dissolved organic carbon [DOC]; (b) total organic carbon [TOC]; (c) nitrate and nitrite [NOx
]; (d) ammonium [NH4+ ]; (e) total nitrogen [TN]; (f) soluble reactive phosphorus [SRP]; and (g) total phosphorus [TP]) at the Taylor River fringe mangrove site from November 1996––August 1998 (N=36 for each plot). The middle line is the median of each distribution. The notches represent the 95% confidence interval about the median. The bottom and top lines of each box are the 25th and 75th percentiles and the bottom and top lines of the whiskers indicate the 10th and 90th percentiles, respectively. Different letters represent significant differences in concentration distributions between samplings (ANOVA; P<0·05).

762 S. E. Davis, III et al.

DOC behaved quite differently. In fact, with one exception (January 1997, west flume, day 2), there was no overlap in the fluxes of these two constituents. Dissolved organic carbon was taken up in the vegetated zone during November 1996, January 1997 and May 1998 (62·26 to 726 38
moles m
2 h
1) and exported in November 1997 and January 1998 [
385·65 to
454·74
moles m
2 h
1; Figure 6(a)]. Total organic carbon, on the other hand, was exported in one of the vegetated flumes on the first sampling day of May 1997 (
234·09 to
434·17
moles m
2 h
1) and taken up in both on the next day [135·24 to 434·17
moles m
2 h
1; Figure 6(b)]. A small uptake of TOC also occurred in August 1998 [1·39
moles m
2 h
1; Figure 6(b)]. Flux of organic carbon in the unvegetated flume was measured on a single occasion during this study [May 1998 sampling; Figure 6(a)]. Most of the net areal fluxes we measured in the vegetated zone of lower Taylor River were associated with nitrogen [Figure 6(c–e)]. The direction and magnitude of nitrogen fluxes, especially inorganic nitrogen, were consistent between vegetated flumes. When significant fluxes were measured on a given day, there was only a single case for nitrogen in which the vegetated flumes did not correspond [Figure 6(c)]. We observed relatively large uptakes of NOx
(1·55 to 5·61
moles NOx
m
2 h
1) in the mangrove zone during both November 1996 and January 1997 [Figure 6(c)]. One of these NOx
uptakes coincided with uptake of TN in the same flume during November 1996 [Figure 6(c and e)]. Much smaller quantities of NOx
(
0·12 to
1·13
moles NOx
m
2 h
1) were exported from the mangrove zone during the dry and wet season samplings of 1997 [Figure 6(c)]. This was followed by more net uptake of NO x
in November 1997 and an export in May 1998 [Figure 6(c)]. While NO x
was consistently imported during the January 1997 sampling in the submersed mangrove zone, NH4+ flux shifted from an uptake (0·94 to 1·27
moles NH4+ m
2 h
1) on the first day of sampling, to export (
0 48 to
0·81
moles NH4+ m
2 h
1) on the second day [Figure 6(d)]. Export of NH4+ in the flumes occurred again in August 1997 followed by a large uptake in January 1998 [Figure 6(d)]. Fluxes of TN were most often exports, occurring between November 1996 and May 1998 and ranging from 4–11
moles TN m
2 h
1 [Figure 6(e)]. Phosphorus exhibited the fewest instances of net areal flux compared to the other constituents, the majority of which were attributed to SRP [Figure 6(f and g)]. We observed net imports of SRP by the mangrove wetland in November 1996 and 1997 and

again in January 1998 (0·11 to 0·27
moles SRP m
2 h
1) and net exports in January and August 1997 and May 1998 [
0·04 to
0·57
moles SRP m
2 h
1; Figure 6(f)]. A detectable flux of SRP occurred once in the unvegetated zone, almost equalling the largest flux measured in the vegetated flumes [Figure 6(f)]. Similarly, open water fluxes of TP were large compared to fluxes measured in the vegetated zone [Figure 6(g)]. Total phosphorus was imported by the submersed mangrove wetland in January 1997 and exported in January 1998 [0·23 and
0·66
moles TP m
2 h
1, respectively; Figure 6(g)]. A considerably smaller export TP occurred in May 1997 [0·01 and
0·01
moles TP m
2 h 1; Figure 6(g)]. Although fluxes in all three flumes were variable from one sampling to the next (Figure 6), averaged net areal fluxes for each sampling revealed trends in the vegetated zone of lower Taylor River. Our data indicated a net import for most constituents during the first two samplings (November 1996 and January 1997; Table 2) and zero net flux or a net export for all constituents during the next two samplings (May 1997 and August 1997, Table 2). Vegetated zone averages from May 1998 also showed a net export for all constituents except DOC, which was imported, and TP, which showed no net flux (Table 2). Conversely, data from the wet season sampling of 1998 showed no net flux for most constituents and net import of both TOC and NOx
(Table 2). The remaining samplings (November 1997 and January 1998) were characterized by export of both DOC and TN in the vegetated zone with net uptake or zero net flux for all other constituents (Table 2). When unvegetated zone fluxes were included in these averages to yield across-channel fluxes, many of these patterns were not as evident (Table 2). Discussion Physical factors and water chemistry The interaction of wind, precipitation, and upland runoff played an important role in determining the hydrodynamics and chemistry of lower Taylor River during the 2-year period of study. Salinity at our site was generally lowest during the wet season and highest during the dry season. The transitional period from the wet season to the dry season exhibited the highest current velocities and the greatest range in salinity and water level. These samplings were characterized by frequent (i.e. hourly to daily), wind-driven shifts in the direction of flow. This general pattern was illustrated by daily discharge data from the mouth, where Sutula (1999) showed positive net flux of water

Nutrient dynamics in vegetated and unvegetated areas 763 800

(a) –1

600

0 –200 –400

6

–2

µmoles NH4 m

+

–2 –4

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98 (d)

4

Nov ' 96

Jan May Aug Nov ' 97

–4

h

–1

0.2 –2

20 10 0 –10

0

–2

0.3

(e)

30

0.5

0

–8

Jan May Aug ' 98

40

–20

2

–6

µmoles SRP m

–1

h –2

µmoles TN m

–200

h

h –2 –

µmoles NOX m

0

50

–1

0

6

(c)

2

60

h

200

–600

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

4

–8

–2

400

–400

–6

µmoles TP m

(b)

600

h µmoles TOC m

200

–600

–1

–2

400

–1

–

µmoles DOCX m

–2

h

–1

800

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98 (f)

0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

–0.6

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

(g)

–0.5 –1 –1.5 –2 –2.5 –3 –3.5 –4

Nov Jan May Aug Nov Jan May Aug ' 96 ' 97 ' 98

F 6. Bar charts showing significant net areal fluxes of nutrients (
moles m
2 h
1; (a) DOC; (b) TOC; (c) NOx
; (d) NH4+ ; (e) TN; (f) SRP; (g) TP) in the flumes (West Flume: black bars; East Flume: white bars) and channel (gray bars) on consecutive days over the course of this study. Positive values indicate nutrient removal from the water column and negative values signify nutrient release into the water column. No bars=zero net areal flux.

764 S. E. Davis, III et al. T 2. Averaged net areal fluxes of nutrients per sampling (
moles m
2 h
1) in the vegetated zones of lower Taylor River. Across-channel averages of net areal flux for each sampling, including unvegetated channel fluxes, are shown in parentheses. Positive values indicate nutrient removal from the water column and negative values signify nutrient release into the water column Sampling Nov 96 Jan 97 May 97 Aug 97 Nov 97 Jan 98 May 98 Aug 98

DOC

TOC

NOx+

NH+ 4

TN

SRP

TP

183·19 (122·13) 38·2 (25·47) 0 (
69·1) 0 (0)
58·29 (
38·86)
113·69 (
75·79) 270·68 (180·51) 0 (0)

0 (0) 12·69 (8·46) 0 (0) 0 (0) 0 (0) 146·21 (
97·47)
6·18 (
4·12) 0·35 (0·23)

1·32 (0·88) 3·25 (2·17)
0·07 (
0·05)
0·36 (
0·24) 0·19 (0·53) 0 (0)
0·3 (
0·2) 0·25 (0·17)

0 (
1·16) 0·69 (0·15) 0 (0)
0·76 (
0·51) 0 (0) 2·11 (1·72)
0·34 (0·49) 0 (0)

12·59 (14·04) 0 (0) 0 (0) 0 (0)
5·49 (
3·66)
2·61 (
1·74)
2·17 (
1·44) 0 (0)

0·07 (0·05)
0·14 (
0·1) 0 (0)
0·05 (
0·04) 0·03 (0·02) 0·03 (0·02)
0·05 (
0·13) 0 (0)

0 (0) 0·06 (0·04) 0 (0) (0) (
0·1) 0 (0)
0·17 (
0·11) 0 (
0·32) 0 (
0·64)

from Taylor River to Little Madeira Bay during most days of the wet season. However, during the transition period from the wet to dry season, the daily exchange of water and materials was more balanced (Sutula, 1999). The short-term effects of these shifts on the water chemistry in Taylor River appeared to be limited to the lower, fringe mangrove zone. Davis et al. (in press) showed evidence of the localized influence of these frequent shifts, as they measured consistently lower salinity (by 10–15) at a dwarf mangrove site approximately 1 km upstream during the same transitional samplings. The concentrations of many nutrients in the fringe zone also appeared to be influenced by these shifts in flow. Our data revealed that concentrations of TOC and DOC in lower Taylor River were highest during the wet season and variable (700–1200
M) during the transitional and dry season samplings as a result of the unique hydrology of this zone. This was not the same pattern observed in the dwarf mangrove zone, where there was a consistent, early wet season-late dry season decrease in concentrations of TOC and DOC (Davis et al., in press). Overall, total and dissolved organic carbon in lower Taylor River ranged from about 700–1800
M, showing a strong upland source signal. By comparison, DOC concentrations in Coral Creek (Australia), a tidal creek with no significant upland influence, were about an order of magnitude lower (Table 3; Boto & Wellington, 1988). In a basin mangrove forest near Rookery Bay, Florida, Twilley (1985) measured a range of TOC concentrations

similar to ours (Table 3). However, DOC was only 70–76% of the TOC measured in that tidal system (Twilley, 1985), whereas TOC in lower Taylor River was 90–95% in the dissolved form. Total nitrogen concentrations in lower Taylor River were also significantly higher during the wet season samplings, sometimes by more than 40
M. RiveraMonroy et al. (1995) found a similar seasonal trend for DON in a fringe mangrove wetland of Estero Pargo (Mexico), another tidal mangrove system with little upland influence. Still, their highest wet season value for total nitrogen (approximately 65
M; estimated by summing reported concentrations for DON, PN, NH4+ , NO2
, and NO3
; Rivera-Monroy et al., 1995) was lower than our wet season average (77
M). Total nitrogen concentrations in the Taylor River system appear to be more comparable to those found along the Sangga River (Malaysia), a tidal mangrove river with a strong upland influence (Table 3; Nixon et al., 1984). However, concentrations of TP in the Sangga system were more than twofold higher than the values we measured (Table 3), with molar ratios of TN:TP ranging from 20–40 (Nixon et al., 1984). In Taylor River, TP was usually <0·5
M and TN:TP often exceeded 100, reflecting the oligotrophic and P-limited status of this region. Such low concentrations of surface water TP are not limited to the southern Everglades mangrove transition zone. They are typical of both the freshwater southern Everglades (Childers, unpubl. data) and eastern Florida Bay estuary (Boyer et al., 1999), as well as

Nutrient dynamics in vegetated and unvegetated areas 765 T 3. List of concentration ranges for various constituents measured in Taylor River and other mangrove systems around the world. Some ranges were estimated by summing values of other parameters given in the literature and are indicated by an asterisk (*). All concentrations are in units of
M Constituent TOC DOC TN TP NH4+ NOx
SRP

Rookery Bay, U.S.A.a

Estero Pargo, Mexicob

783–1750 583–1667

Coral Creek, Australiac

Sangga River, Malaysiad

Sepetiba Bay, Brazile

Gazi Bay, Kenyaf

Taylor River, U.S.A.g

0·3–3 0·2–8 0 5–3·9

727–1821 657–1691 41–95 0·2–0·7 0·1–6·3 0 2–5·75 <0·01–0·26

92–125 30–65* 1–51 0·2–5

0·3–0·95* 0·1–0·8 0·1–0·6 0·05–0·45

40–90 1·5–3 3–24 <0·5 0·05–1

0·9–7 0·8–5 0·5–1·8

Sources of data: aTwilley, 1985; bRivera-Monroy et al., 1995; cBoto and Wellington, 1988; dNixon et al., 1984; eOvalle et al., 1990; fOhowa et al., 1997; gthis study.

other carbonate-dominated mangrove settings, such as Coral Creek (TP estimated by summing DOP and PO3
4 ; Table 3; Boto & Wellington, 1988). Concentrations of dissolved inorganic phosphorus and nitrogen in Taylor River were similar to other mangrove systems (Table 3). Mangrove waters, in general, have relatively low stocks of dissolved inorganic phosphorus (DIP or SRP) and nitrogen (NH4+ +NOx
; Alongi et al., 1992). In some cases, the degree of human impact seems to control inorganic nutrient profiles (Nedwell, 1975; Nixon et al., 1984), while in others the degree of upland influence and the hydrology of the system appear to be of greater importance (Boto & Wellington, 1988; Ovalle et al., 1990). In south Florida, where systems tend to be oligotrophic and limited by the availability of phosphorus, SRP concentrations are extremely low. Our SRP concentrations were typically 0·01–0·05
M, and sometimes below the limits of analytical detection (<0·01
M). These concentrations are much lower, in some cases more than two orders of magnitude lower, than SRP values from other mangrove systems (Table 3; also see Alongi et al., 1992). Alternatively, our NH4+ and NOx
numbers were comparable to other systems, showing as much variability through time in lower Taylor River as between other mangrove systems (Table 3). Despite the similarity in DIN concentration ranges between these systems, molar ratios of DIN:DIP in Taylor River were much higher (sometimes exceeding 300) than the others, reflecting the low availability of inorganic P. Synthesis of flux data and comparison with other studies At present, there are few studies of inorganic and organic nutrient exchange from other mangrove systems. Of these, Boto and Wellington (1988) and

Rivera-Monroy et al. (1995) are among the few to quantify in situ, system-level measurements of nutrient exchange in mangroves. Both of these studies were quite different from ours in that they were done in tidal systems that lacked a significant terrestrial influence. However, all three studies were similar in that they were concerned with exchanges of nutrients in mangrove creek systems. Furthermore, our work bridged a gap between these works in that it addressed aspects of both. Boto and Wellington (1988) applied a ‘ Eulerian ’ approach to quantify the flux of nutrients in Coral Creek over several tidal cycles. Although this approach allowed them to estimate annual exchanges between the mangrove wetland and Missionary Bay, it was not explicit enough to separate the effect of the mangrove wetland from the water column on these exchanges. On the other hand, Rivera-Monroy et al. (1995) utilized a flume to determine the direct effect of the mangrove wetland on the nitrogen content of floodwater from Estero Pargo. Using the flume design presented here, we were able to determine the fluxes of nutrients along a mangrove creek as well as differentiate the contribution of the wetland from water column on these fluxes. The net areal fluxes of nutrients we measured were quite variable both within and between samplings, yielding no apparent seasonal effect for any constituent. Furthermore, over the course of this study, we observed no significant changes in the density or nature of the epibenthic or epibiotic communities that may have contributed to this variability. Nevertheless, most fluxes occurred in the vegetated zone, emphasizing the importance of submersed, creekside mangrove vegetation in regulating nutrient loads in lower Taylor River. This was especially true for nitrogen, where almost 90% of measurable N flux occurred in the vegetated zone of the creek. Nitrate+nitrite dynamics

766 S. E. Davis, III et al.

in the fringe mangrove zone of Taylor River were characterized by small exports that were overshadowed by large imports, while net NH4+ exchange was balanced between export and import. Boto and Wellington (1988) described a similar balance with DIN exchange in Coral Creek. However, another study found a consistent uptake of both NOx
and NH4+ by a fringe mangrove in Mexico (RiveraMonroy et al., 1995). These same investigators also found that dissolved organic nitrogen (DON) and particulate nitrogen (PN) were consistently exported (Rivera-Monroy et al., 1995). In Taylor River, the majority of the TN fraction is dissolved organic (DON) while the remainder is composed mainly of DIN and PN. Although there were only a few instances of significant TN flux, they indicated an export of TN from the fringe mangrove zone. Overall, the fringe mangrove zone was a small sink for nitrogen (0·015 g N m
2 yr
1), while the entire cross section of Taylor River (open water included) was a larger sink (2·19 g N m
2 yr
1). By comparison, estimates of annual N exchange showed a net uptake by the fringe mangrove wetland in Estero Pargo (0·06 g N m
2 yr
1; Rivera-Monroy et al., 1995) and a net export from Coral Creek (0·58 g N m
2 yr
1; Boto & Bunt, 1981; Boto & Wellington, 1988). Unlike nitrogen, phosphorus dynamics have been neglected in the handful of mangrove nutrient flux studies that have been conducted. Of those that have considered this nutrient, Boto and Wellington (1988) concluded there were insufficient data to suggest any constancy in the direction or magnitude of SRP flux in Coral Creek, however they did measure consistent uptake of total dissolved phosphorus (TDP) in their system (Boto & Wellington, 1988). Data collected from a dwarf mangrove site in Taylor River also showed inconsistencies in the direction and magnitude of SRP flux throughout the year (Davis et al., in press). These patterns were neither associated with season or with the availability (i.e. concentration) of SRP in the water column (Davis, 1999). Given the P-limited nature of this region, we expected a considerable uptake of phosphorus in the fringe mangrove zone of Taylor River. This was not the case, as SRP flux seemed balanced between imports and exports and TP flux was negligible over the course of the study. Our findings for phosphorus were likely confounded by the low ambient concentrations of P in the Taylor River system and the sensitivity of the flux calculations to minute changes in P concentrations. In addition, fluxes of P (especially TP) in the unvegetated flume were further amplified by large water fluxes relative to the vegetated flumes. Regardless, we found that both the fringe mangrove zone

(0·12 g P m
2 yr
1) and the entire cross section of lower Taylor River (1·03 g P m
2 yr
1) were net sources of phosphorus to the water column. Boto and Wellington (1988) found just the opposite, as about 0·5 g P m
2 yr
1 was imported by the mangrove system adjacent to Coral Creek. However, it is worth mentioning that their analyses did not account for particulate forms of phosphorus. Much of the outwelling and materials exchange work in mangrove systems has been devoted to organic matter (including POC, TOC and DOC). In a recent review of organic matter dynamics, Lee (1995) showed that mangrove wetlands, in general, are sources of detritus (i.e. organic matter) to nearshore environments. Among the works included in this review was an investigation in which a flume was employed to quantify organic carbon exchange between a basin mangrove and Rookery Bay. In this study, Twilley (1985) found large, seasonal and tidally influenced exports of TOC (64 g m
2 yr
1) and DOC (75% of TOC export) from the mangrove wetland. Unfortunately, few other mangrove studies have included TOC flux in their analyses. In Taylor River, TOC showed little net flux, shifting between import and export when measurable. Contrary to this, the fringe mangrove zone consistently imported DOC, ranging from 62–726
moles m
2 h
1. Dissolved organic carbon fluxes reported for Coral Creek were similar, with uptakes averaging 69·4
moles m
2 h
1 (Boto & Wellington, 1988). However, our estimated annual uptake of DOC (67·3 g DOC m
2 yr
1) was an order of magnitude higher than for Coral Creek (7·3 g DOC m
2 yr
1). Coral Creek has a tidal range of 2–3 m, whereas Taylor River is essentially nontidal (tidal range <0·05 m). Given the disparity in tidal influence between these two carbonate systems and the evidence of increasing organic export with tidal range, one might have expected considerably greater export of DOC in Coral Creek. However, in terms of organic carbon dynamics, the difference in tidal range between these two systems may be overshadowed by the difference in upland influence. Coral Creek has no significant upland influence with considerably lower ambient concentrations of DOC compared to Taylor River, which is directly linked to the freshwater marshes of the southern Everglades. Analysis of our approach We successfully adapted a technique developed in tidal salt marsh systems for use in a microtidal mangrove creek. It is our belief that this or a similar adaptation may be useful in quantifying the exchanges of organic and inorganic materials between a creek

Nutrient dynamics in vegetated and unvegetated areas 767

and submersed creekside vegetation in mangroves and, possibly, other riparian wetland systems. Prior to this study, only two other studies have used flumes to quantify materials exchange in mangrove wetlands, both of which were in tidal forests (Twilley, 1985; Rivera-Monroy et al., 1995). In general, flumes employed in tidal systems are used as a tool in quantifying exchanges of materials between the wetland and water column of the flooding or ebbing tide (Wolaver et al., 1985; Childers & Day, 1988; Rivera-Monroy et al., 1995). In Taylor River, we used flumes to isolate vegetated areas from unvegetated areas for the purpose of quantifying exchanges and transformations of nutrients in each zone along a specified length of this microtidal creek. Nutrient fluxes in the flumes were likely associated with dynamics at the prop root–water column and benthos–water column interfaces. Whereas, channel fluxes were limited to transformations occurring within the water column or exchanges between the water column and the benthos, as there was no vegetation in the channel. Although we did not measure each of these exchanges individually, our design allowed us to determine the relative contribution of either zone to the flux of nutrients across this southern Everglades mangrove creek. Little is presently known about the exchanges of materials (i.e. nutrients and organic matter) in Everglades’ mangrove wetlands, despite their large areal coverage. Our study fills a gap not only in the regional understanding of nutrient dynamics in these systems but in the general understanding of microtidal, seasonally driven mangrove systems. Based on our flux results, nitrogen, phosphorus, and carbon appear to be fairy well balanced in terms of annual imports and exports in lower Taylor River, a likely result of the unique hydrology of this system. Nevertheless, the submersed mangrove zone appears to be an active site of nutrient exchange, as was evidenced by the discrepancy in the number of significant fluxes measured in the flumes relative to the channel. This is an indication of the potential importance of this zone in regulating the availability of nutrients in both Taylor River and Little Madeira Bay, especially during peak wet season. These findings also support the primacy of wetland dynamics over water column processes in regulating mangrove nutrient fluxes. Given the large number of similar mangrove creeks along the southern tip of Florida, the additive effect of the fringe mangrove zone may be large enough to influence the availability of nutrients in northeastern Florida Bay for part of the year. This may have important ramifications for the restoration of the Everglades, as freshwater flow is being increased to meet historical, pre-drainage levels.

Acknowledgements Our thanks also go to numerous people for providing field and laboratory support throughout this study, including: Nick Oehm, Jenny Davis, Matt Ostentoski, Chris Buzzelli, Frank Parker, Ruth Parker, Luz Romero, Jennifer Skislak and, especially, Steve Kelly and Mike Korvela. We would also like to thank Ron Jones, Elaine Kotler, and Peter Lorenzo at the Southeast Environmental Research Center at Florida International University (FIU) for their analytical expertise. The Wetland Ecosystems Ecology Lab at FIU provided helpful comments on an earlier version of this manuscript. Finally, we would like to acknowledge the South Florida Water Management District for funding all of this work. This manuscript is SERC contribution # 150 and contribution # 36 from the Tropical Biology Program at Florida International University. References Alongi, D. M., Boto, K. G. & Robertson, A. I. 1992 Nitrogen and phosphorus cycles. In Tropical Mangrove Systems (Robertson, A. I. & Alongi, D. M., eds), American Geophysical Union, Washington, D.C., pp. 251–292. Boto, K. G. & Bunt, J. S. 1981 Tidal export of particulate organic matter from a northern Australian mangrove forest. Estuarine, Coastal and Shelf Science 13, 247–255. Boto, K. G., & Wellington, J. T. 1988 Seasonal variations in concentration and fluxes of dissolved organic and inorganic materials in a tropical, tidally-dominated, mangrove waterway. Marine Ecology Progress Series 50, 151–160. Boyer, J. N., Fourqurean, J. W. & Jones, R. D., 1999 Seasonal and long-term trends in the water quality of Florida Bay (1989– 1997). Estuaries 22, 417–430. Childers, D. L. 1994 Fifteen years of marsh flumes: a review of marsh-water column interactions in southeastern USA estuaries. In Global Wetlands: Old World and New (Mitsch, W. J., ed.). Elsevier, Amsterdam, pp. 277–293. Childers, D. L. & Day, J. W. 1988 A flow-through flume technique for quantifying nutrient and materials fluxes in microtidal estuaries. Estuarine, Coastal and Shelf Science 27, 483–494. Childers, D. L., Day, J. W. & McKellar, H. N. 2000 Twenty more years of marsh and estuarine flux studies: revisiting Nixon (1980). In Concepts and Controversies in Tidal Marsh Ecology (Weinstein, M. P. & Kreeger, D. Q., eds). Kluwer Academic Press, Nordrecht, Netherlands, 864 pp. Davis, S. E. 1999 The exchange of carbon, nitrogen, and phosphorus in dwarf and fringe mangroves of the oligotrophc southern Everglades. Ph.D. Dissertation, Department of Biological Sciences, Florida International University, Miami, Florida. Davis, S. E., Childers, D. L., Day, J. W., Rudnick, D. T. & Sklar, F. H. Carbon, nitrogen, and phosphorus dynamics in a microtidal dwarf mangrove wetland of the southern Everglades, USA. Estuaries (in press). Elison, A. E., Farnsworth, E. J. & Twilley, R. R. 1996 Facultative mutualism between mangroves and root-fouling sponges in Belizean mangal. Ecology 77, 2431–2444. Flores-Verdugo, F. J., Day, J. W. & Briseno-Duenas, R. 1987 Structure, litter fall, decomposition, and detritus dynamics of mangroves in a Mexican coastal lagoon with an ephemeral inlet. Marine Ecology Progress Series 35, 83–90. Fourqurean, J. W. & Robblee, M. B. 1999 Florida Bay: a history of recent ecological changes. Estuaries 22, 345–357.

768 S. E. Davis, III et al. Kjerfve, B., Stevenson, H., Proehl, J. A., Chrzanowski, T. H. & Kitchens, W. M. 1981 Estimation of material fluxes in an estuarine cross section: a critical analysis of spatial measurement density and errors. Limnology and Oceanography 26, 325–335. Kristensen, E., Andersen, F. O. & Kofoed, L. H. 1988 Preliminary assessment of benthic community metabolism in a south-east Asian mangrove swamp. Marine Ecology Progress Series 48, 137– 145. Lee, S. Y. 1990 Primary productivity and particulate organic matter flow in an estuarine mangrove-wetland in Hong Kong. Marine Biology 106, 453–463. Lee, S. Y. 1995 Mangrove outwelling: a review. Hydrobiologia 295, 203–212. McIvor, C. C., Ley, J. A. & Bjork, R. D. 1994 Changes in freshwater inflow from the Everglades to Florida Bay including effects on biota and biotic processes: a review. In Everglades: The Ecosystem and its Restoration (Davis, S. M. & Ogden, J. C., eds). St Lucie Press, Delray Beach, FL, pp. 117–146. Nedwell, D. B., Blackburn, T. H. & Wiebe, W. J. 1994 Dynamic nature of the turnover of organic carbon, nitrogen and sulfur in the sediments of a Jamaican mangrove forest. Marine Ecology Progress Series 110, 223–231. Nixon, S. W. 1980 Between coastal marshes and coastal waters—a review of twenty years of speculation and research on the role of salt marshes in estuarine productivity and water chemistry. In Estuarine and Wetland Processes (Hamilton, P. & McDonald, K. B., eds). Plenum Press, New York, pp. 437–525. Nixon, S. W., Furnas, B. N., Lee, V., Marshall, N., Ong, J. E., Wong, C. H., Gong, W. K. & Sasekumar, A. 1984 The role of mangroves in the carbon and nutrient dynamics of Malaysia estuaries. In Proceedings of the Asian Symposium on Mangrove Environment: research and management (Soepadmo, E., Rao, A. N. & Macintosh, D. J., eds). Univ. of Malaya, Kuala Lumpur, pp. 535–544. Odum, E. P. & de la Cruz, A. 1967 Particulate organic detritus in a Georgia salt marsh-ecosystem. American Association for the Advancement of Science Publication 83, 383–388. Odum, W. E., Fisher, J. S. & Pickral, J. C. 1979 Factors controlling the flux of particulate organic carbon from estuarine wetlands. In Ecological Processes in Coastal and Marine Systems (Livingstone, R. J., ed.). Marine Science Series Plenum Press, New York, pp. 69–80.

Ohowa, B. O., Mwashote, B. M. & Shimbora, W. S. 1997 Dissolved inorganic nutrient fluxes from two seasonal rivers into Gazi Bay, Kenya. Estuarine, Coastal and Shelf Science 45, 189– 195. Ovalle, A. R. C., Rezende, C. E., Lacerda, L. D. & Silva, C. A. R. 1990 Factors affecting the hydrochemistry of a mangrove tidal creek, Sepetiba Bay, Brazil. Estuarine, Coastal and Shelf Science 31, 639–650. Rivera-Monroy, V. H., Day, J. W., Twilley, R. R., Vera-Herrera, F. & Coronado-Molina, C. 1995 Flux of nitrogen and sediment in a fringe mangrove forest in Terminos Lagoon, Mexico. Estuarine, Coastal and Shelf Science 40, 139–160. Robertson, A. I. 1986 Leaf-burying crabs: their influence on energy flow and export from mixed mangrove forests (Rhizophora spp.) in northeastern Australia. Journal of Experimental Marine Biology and Ecology 102, 237–248. Solorzano, L. & Sharp, J. H. 1980 Determination of total phosphorus and particulate phosphorus in natural waters. Limnology and Oceanography 25, 754–758. Sutula, M. 1999 Processes controlling nutrient transport in the southeastern Everglades wetlands, Florida, USA. Ph.D Dissertation. Louisiana State University, Department of Oceanography and Coastal Sciences, Baton Rouge, Louisiana. Teal, J. M. 1962 Energy flow in the salt marsh ecosystem of Georgia. Ecology 43, 614–624. Twilley, R. R. 1985 The exchange of organic carbon in basin mangrove forests in a southwest Florida estuary. Estuarine, Coastal and Shelf Science 20, 543–558. Twilley, R. R. 1998 Mangrove Wetlands. In Southern Forested Wetlands: Ecology and Management (Messina, M. G. & Conner, W. H., eds). Lewis Publishers, Boca Raton, Florida, pp. 445– 473. Wolaver, T., Whiting, G., Kjerfve, B., Spurrier, J., McKellar, H., Dame, R., Chrzanowski, T., Zingmark, R. & Williams, T. 1985 The flume design-Methodology for evaluating material fluxes between a vegetated salt marsh and the adjacent tidal creek. Journal of Experimental Marine Biology and Ecology 91, 281– 291. Woodroffe, C. D. 1985 Studies of a mangrove basin, Tuff Crater, New Zealand: III. The flux of organic and inorganic particulate matter. Estuarine, Coastal and Shelf Science 20, 447–461.