Introduction
In many tropical and subtropical regions, mangrove forests are a typical
feature surrounding estuaries (Twilley et al., 1992; Bouillon et al., 2008a).
Mangroves are thought to play an important role in tropical and subtropical
coastal biogeochemical cycling and the global coastal carbon budget, due to
their high productivity and rapid cycling of organic and inorganic carbon
(Twilley et al., 1992; Jennerjahn and Ittekkot, 2002; Dittmar et al., 2006).
However, there remain uncertainties regarding the fate of mangrove-fixed
carbon and the amount of carbon exported to the coastal waters from these
ecosystems (Bouillon et al., 2008a, b; Kristensen et al., 2008).
Bouillon et al. (2008a) showed that over 50 % of the carbon fixed by
mangroves through photosynthesis could not be accounted for by growth in
biomass, accumulation in soils, and export of organic carbon, and suggested
that a large fraction of this missing organic carbon may be mineralized to
dissolved inorganic carbon (DIC) and either lost to the atmosphere or
exported to the surrounding waters. In fact, several studies have shown that
the lateral advective transport of interstitial waters through tidal pumping
represents a major carbon export pathway from mangroves into adjacent waters,
both for DIC (Koné and Borges, 2008; Miyajima et al., 2009; Maher et al.,
2013) and dissolved organic carbon (DOC) (Dittmar and Lara, 2001; Bouillon et
al., 2007c). However, to date, lateral mangrove-derived aquatic carbon fluxes
(as a proportion of overall forest carbon mass balance) have only been
estimated for short time periods and over limited spatial (e.g., plot) scales
(e.g., Troxler et al., 2015). These studies also typically do not determine
the fate of mangrove-derived carbon once it is exported from the forest
through tidal pumping and drainage. Additional measurements of the magnitude
and fate of mangrove carbon export at the basin scale are needed to help
quantify connections between inter-tidal, estuarine, and coastal ocean carbon
cycles.
Rates of lateral dissolved carbon export from tidal mangrove forest are
heterogeneous over space and time due to variability in inundation patterns,
forest structure, topography, and soil hydraulic properties. Direct,
plot-scale measurements of dissolved carbon export therefore may not
represent rates quantified at the basin scale. However, mangrove-derived
dissolved carbon fluxes may be estimated in some systems using information
on the spatial distribution of carbon-related measurements in adjacent
waters. For example, the carbon balance of tidal riverine systems adjacent
to mangrove forests should integrate the spatial and temporal variability of
these lateral fluxes.
The objective of this study is to quantify dissolved carbon source–sink
dynamics in a subtropical estuary dominated by two tidal rivers, the Shark
and Harney rivers in Everglades National Park, Florida, USA. These rivers
are centrally located within the largest contiguous mangrove forest in North
America and they discharge to the Gulf of Mexico. The total dissolved carbon
inventories and fluxes in these rivers are determined using a series of
discrete and continuous measurements of carbon-related parameters along a
salinity gradient, and the mangrove contribution separated using
measurements of stable isotopic composition of dissolved organic and
inorganic carbon. The results are then scaled by the area of mangrove forest
that surrounds these rivers to express dissolved carbon fluxes on an aerial
basis for comparison to independent measurements of dissolved carbon fluxes
from this forest.
Map of the study area near the southern tip of Florida,
USA, showing locations of Shark River, Harney River, and Tarpon Bay. The
blue circles indicate the locations where discrete samples were taken, and
the black stars denote the USGS gaging stations on both rivers. The green
areas in the inset are part of the largest contiguous mangrove forest in
North America. Indicated in the inset are the boundaries of Everglades
National Park.
Methods
Study site
The tidal-dominated Shark and Harney rivers (river and estuary are used
interchangeably in this contribution) are surrounded by mangrove forests and
located on the southwest coast of Florida (Fig. 1), within Everglades
National Park. The subtropical climate in southern Florida is characterized
by a May to October wet season, when approximately 60 % of the annual
precipitation occurs (Southeast Regional Climate Center,
http://www.sercc.com). The Shark and Harney rivers together discharge
approximately 50 % of the flow from the Shark River Slough (SRS), the
primary drainage feature of Everglades National Park, to the Gulf of Mexico
(GOM) (Levesque, 2004). Seasonal variation of the water discharge from SRS
mostly follows the precipitation patterns (Saha et al., 2012), and influences
the transport of nutrients to the mangrove ecotone (Rivera-Monroy et al.,
2011). The Shark and Harney rivers are each approximately 15 km long, and
connect in Tarpon Bay (Fig. 1). The mean depths of Tarpon Bay, Shark River,
and Harney River at mid-tide are 1.4 ± 0.3, 2.8 ± 0.4, and
2.6 ± 0.4 m (Ho et al., 2014), respectively, and the surface areas are
1.48 × 106, 2.54 × 106, and
2.75 × 106 m2, respectively. The inter-tidal zones
bordering the Shark and Harney rivers are dominated by Rhizophora mangle (red mangrove), Avicennia germinans (black mangrove),
Laguncularia racemose (white mangrove), and Conocarpus erectus (buttonwood). Semi-diurnal tides in this region inundate the forest
as often as twice a day. River discharge to the GOM is primarily influenced
by tides, wind, and freshwater inflow from SRS (Levesque, 2004).
Discharges are determined by the US Geological Survey at stations near the
midpoints of Shark River (USGS 252230081021300 Shark River) and Harney River
(USGS 252551081050900 Harney River) (Fig. 1). Discharges are generally lower
during March–May than the rest of the year. Hourly mean residual discharge
values (i.e., filtered for tides) from March to May of the 5-year period from
2007 to 2011 ranged from -21.9 to 24.1 m3 s-1, with a mean of
0 m3 s-1 for Shark River, and ranged from -28.9 to
38.5 m3 s-1, with a mean of 4.4 m3 s-1 for Harney
River. Positive values indicate flow towards the GOM. For the rest of the
year (i.e., June to February), these values ranged from -46.2 to
89.2 m3 s-1, with a mean of 8.8 m3 s-1 for Shark
River, and -41.6 to 75.0 m3 s-1, with a mean of
11.3 m3 s-1for Harney River.
Shark River Tracer Release Experiments
Two field studies were conducted as part of the Shark River Tracer Release
Experiment (SharkTREx 1: 20 to 25 November 2010; SharkTREx 2: 10 to
15 November 2011; Ho et al., 2014). The mean residual discharges for Shark
River were 6.9 (hourly range: -2 to 19.9) and 4.9 (hourly range: -18.9 to
34.8) m3 s-1, during SharkTREx 1 and 2, respectively, and those
for Harney River were 6.0 (hourly range: -1.6 to 22.8) and 1.9 (hourly
range: -17.3 to 30.6) m3 s-1, during SharkTREx 1 and 2,
respectively (U.S. Geological Survey, 2016).
During both campaigns, an inert tracer (sulfur hexafluoride; SF6) was
injected in the river near the point where the rivers diverge just downstream
of Tarpon Bay (25.4092, -81.0083) to determine the rates of longitudinal
dispersion, and the water residence time. Each day, longitudinal surveys were
made along the Shark and Harney rivers from Tarpon Bay to the GOM, and
included continuous underway measurements of temperature, salinity, SF6,
dissolved O2 (DO; µmol kg-1), and partial pressure of
CO2 (pCO2; µatm), and discrete measurements of total
alkalinity (TAlk; µmol kg-1), DIC
(µmol kg-1), DOC
(µmol kg-1), and stable carbon isotopic composition of DIC and DOC
(δ13CDIC and δ13CDOC,
respectively; ‰).
Discrete measurements
During SharkTREx 1, three to five surface water samples were collected daily
in the Shark River with a 5 L Niskin bottle at ∼ 0.5 m below
the surface for the analysis of TAlk, DOC, δ13CDIC, and
δ13CDOC. At each sampling site, vertical profiles of
temperature, salinity, and DO were recorded using a conductivity,
temperature, and depth sonde (Sea-Bird SBE 19plus V2) equipped with a Clark
type polarographic O2 sensor (SBE 43). These profiles showed that the
water column was vertically well mixed. No discrete samples were collected in
the Harney River during SharkTREx 1. During SharkTREx 2, discrete samples for
DIC, TAlk, DOC, δ13CDIC, and
δ13CDOC were collected daily at 20 stations distributed
within the Shark and Harney rivers (Fig. 1).
Total alkalinity and dissolved inorganic carbon
During SharkTREx 1, samples for TAlk were collected in 250 mL HDPE bottles
after passing through a 0.45 µm filter. They were stored on ice for
transport to the laboratory at Florida International University (FIU), where TAlk was determined at room
temperature using an automated titrator (Brinkman Titrino 751) with 0.1 N
HCl to a pH of 2. TAlk was calculated from the volume of acid added at the
inflection point closest to a pH of 4, and reported as
µmol L-1 HCO3- since the original pH of the water
samples was near neutral. The precision of the measurements was ±2 %
from replicate analysis (n=5) with an accuracy of ±2 % as
determined by analysis of certified reference material (Dickson, 2010). DIC
and pH were computed from TAlk and pCO2 using the dissociation
constants of Cai and Wang (1998) for estuarine waters.
During SharkTREx 2, samples for TAlk and DIC were collected in 550 mL
borosilicate glass bottles, poisoned with HgCl2, and sealed with
hydrocarbon grease (Apiezon M). The samples were stored at room temperature
in the dark for travel to the laboratory at NOAA/AOML. Samples for TAlk were
measured in an open thermostated cell (25 ∘C) with an automated
titrator (Metrohm 765 Dosimat) connected to a pH glass-reference electrode
system (Orion), using 0.2 M HCl as a titrant, and determined from the
equivalence point of the titration curve using a nonlinear least-squares
fit. For DIC analysis, water samples were first acidified to convert all the
carbonate species to CO2 in a DIC analyzer (Apollo SciTech), and then
measured with a non-dispersive infrared absorption (NDIR) detector (LI-COR LI-7000). Calibrations for DIC and TAlk
were performed using certified reference material (Dickson, 2010). The
analytical uncertainty of the DIC and TAlk measurements based on replicate
samples are 0.1 and 0.2 %, respectively.
The measured TAlk and pCO2 from SharkTREx 2 were used to calculate DIC
using CO2SYS (Pierrot et al., 2006) and the dissociation constants of Cai and
Wang (1998), and the results were 1.3 ± 1.1 %
(mean ± SD; n=77; range: -2.4 to +4.4 %) higher than
the measured DIC, possibly indicating a slight contribution (ca. 1 %) to
TAlk from organic or particulate material, as the samples were not filtered.
Dissolved organic carbon
The samples analyzed for DOC were filtered with pre-combusted
0.7 µm GF/F filters and collected in pre-cleaned, acid-washed,
brown high-density polyethylene bottles (HDPE; Nalgene). Containers were
rinsed 3 times before sample collection, transported on ice to the FIU
SERC Nutrient Analysis Lab, and stored in a refrigerator until analyses
within 3 weeks of collection. DOC was measured using the high-temperature
catalytic combustion method on a total organic carbon analyzer (Shimadzu
TOC-V), and standardized using 10 and 50 ppm of potassium hydrogen
phthalate (KHP), with reagent water as a blank. The analytical precision
based on replicates of KHP is ca. ±25 µmol kg-1.
Stable carbon isotopic composition
Samples for δ13CDIC were collected in 40 mL glass
bottles after passing the sample through a GF/F filter, and then poisoning
with HgCl2. In the laboratory at the Rosenstiel School of Marine and Atmospheric Sciences, vials with 0.5 mL 103 %
H3PO4 were flushed for 60 s with He. Approximately 2 mL of sample
were then injected into the vial, and after sonification the accumulated
CO2 was analyzed by a gas chromatograph (GC) coupled to an isotope ratio
mass spectrometer (GC-IRMS; Thermo Delta V). The δ13C was
calibrated using two standards of NHCO3 with differing δ13C
values dissolved in H2O, whose isotopic compositions had been previously
calibrated relative to NBS-19 using conventional dual inlet mass spectrometry
(Finnigan-MAT 251). The δ13C values are reported relative to the
Vienna Pee Dee Belemnite (VPDB) standard, and has a reproducibility of
±0.2 ‰ as determined by repeated analysis of internal DIC
standards.
Samples for δ13CDOC were collected in 60 mL brown HDPE
bottles and stored on ice until returned to the lab at FIU. δ13CDOC samples were filtered with GF/F (0.7 µm)
filter, and then stored in pre-cleaned 40 mL bottles until analyses.
Measurements for δ13CDOC were made using a total
organic carbon (TOC) analyzer (Aurora 1030W, OI Analytical) coupled to a
cavity ring-down spectroscopy system (CRDS; G1111-i, Picarro) following the
approach of Ya et al. (2015). DIC was removed by adding H3PO4 and
sparging with N2. 1.5 mL of sample was chemically oxidized to CO2
at a temperature of 98 ∘C in the presence of sodium persulfate
(Na2S2O8). The CO2 generated was detected by
NDIR for determination of DOC. The
CO2 was collected in a gas-tight bag and then pulsed into the CRDS for
the δ13C measurement. In order to measure the different isotopic
ranges within the collected samples, an isotopic calibration was based on two
external standards of potassium hydrogen phthalate (KHP -29.8 ‰,
OI-Analytical) and glutamine (-11.45 ‰, Fisher) with a
concentration range of 0–2080 µmol kg-1. These standards were prepared in synthetic
seawater to match the salinity of the sample matrix. The isotope values of
these two standards were determined by using an elemental analyzer isotope
ratio mass spectrometer (EA-IRMS). Analytical precision based on replicated
standards ranged from ±0.15 to ±1.52 ‰ for this study.
Underway measurements
Surface water was continuously pumped from an intake located near the bow of
the boat at a water depth of approximately 1 m during tracer recovery
operations. Water temperature and salinity were continuously recorded using
a thermosalinograph (SBE 45 MicroTSG). During SharkTREx 1, DO was measured
underway with a membrane covered galvanic sensor (WTW Cellox 325) calibrated
with saturated air. During SharkTREx 2, DO was measured using an oxygen
optode (Aanderaa 3835) calibrated against Winkler titration.
Underway measurements of atmospheric and waterside pCO2 were made.
Waterside pCO2 were obtained with a showerhead type equilibrator
coupled to a NDIR analyzer (LI-COR 840A).
Measurements of underway SF6 were made with an automated SF6
analysis system (Ho et al., 2002), which is comprised of gas extraction
(membrane contactor), separation (molecular sieve 5A), and detection units
(gas chromatograph equipped with an electron capture detector). Both the
underway pCO2 and SF6 measurements are described in greater
detail in Ho et al. (2014)
Inventories of DIC, DOC, and DO
The inventories of DIC, DOC, and DO were calculated in the same way that
SF6 inventories were determined in Ho et al. (2014). The river was
divided into 100 m longitudinal sections, and the measured concentrations,
corrected for tidal movement to slack before ebb for each day, were assigned
to each section i and then summed over the entire length of the river. For
example, to calculate the inventory of DIC, denoted ∑DICobserved (mol):
∑DICobserved=∑i=1nDICi×Vi,
where DICi is the mean concentration
(mol L-1) in section i, Vi is the volume of the river (L) in
section i at mid-tide, and n is the number of sections in each river (n=273 for Shark River and Tarpon Bay; n=152 for Harney River). DOC and
DO inventories were also calculated using Eq. (1), by substituting
[DOC]i or [DO]i for [DIC]i accordingly. The inventories of DIC
and DOC were separated into contributions from estuarine and non-estuarine
sources, first by determining inventories for DIC assuming conservative
mixing between the freshwater and marine end-members and then subtracting
these inventories from the total observed inventories while correcting for
air–water gas exchange. The estuarine DIC inventory, ∑[DIC]estuary, representing the DIC from all estuarine
sources, was calculated as follows:
∑[DIC]estuary=∑[DIC]observed-∑DICconserv+∑[DIC]gasex,
where ∑[DIC]conserv is the inventory of DIC
assuming conservative mixing between freshwater and marine end-members (i.e.,
from non-estuarine sources), and ∑[DIC]gasex is
the inventory of DIC lost to air–water gas exchange from the estuary, due to
pCO2 in the water being above solubility equilibrium with the
atmosphere (see Sect. 2.6). The freshwater and marine end-members were
assigned to the values measured at the lowest (Tarpon Bay) and highest
salinities, respectively.
The total O2 deficit in Shark River during the experiments was
determined by examining the difference in O2 inventories for
conservative mixing and actual measurements, correcting for O2 influx
due to gas exchange using a formulation similar to Eq. (2) above (i.e., ∑[DO]deficit=∑DOconserv-∑DOobserved+∑[DO]gasex).
Air–water O2 and CO2 fluxes
To enable comparison between different gases and different aquatic
environments, it is customary to normalize gas transfer velocities to a
Schmidt number (Sc; kinematic viscosity of water divided by
diffusion coefficient of gas in water) of 600, k(600), corresponding to
that of CO2 in freshwater at 20 ∘C. k(600) for SharkTREx 1
and 2, determined from the wind speed and current velocity parameterization
proposed in Ho et al. (2016), were 3.5 ± 1.0 and
4.2 ± 1.8 cm h-1, respectively. To determine k for O2 and
CO2 at the temperature and salinity measured in the rivers, the
following equation was used, assuming a Sc-1/2 scaling
(Jähne et al., 1987):
kO2=k(600)ScO2600-1/2,
where k and Sc of CO2 could be substituted in Eq. (3) for
O2, and Sc for O2 and CO2 were calculated as a
function of temperature and salinity using data compiled by
Wanninkhof (2014).
Air–water O2 fluxes (FO2; mmol m-2 d-1)
were calculated as follows:
FO2=kO2O2equil-O2,
where kO2 (cm h-1) is the gas transfer velocity for
O2, O2equil (mmol m-3) is the equilibrium
concentration of O2 in the water at a given temperature and salinity
(Garcia and Gordon, 1992), and O2 is the measured oxygen concentration
in the water.
Similarly, air–water CO2 fluxes (FCO2;
mmol m-2 d-1), which were used to determine changes in DIC due to
gas exchange, were calculated as follows:
FCO2=kCO2K0ΔpCO2,
where kCO2 (cm h-1) is the gas transfer velocity for
CO2, K0 (mol atm-1 m-3) is the aqueous-phase solubility
of CO2 (Weiss, 1974), and ΔpCO2 (µatm) is the
difference between the measured pCO2 in air equilibrated with water
and atmospheric pCO2.
As with the inventories, FCO2 were separated into
estuarine and non-estuarine contributions. Because of the nonlinearity in
the relationship between pCO2 and other carbonate system parameters,
the pCO2 in the river expected from conservative mixing was calculated
by assuming conservative mixing for DIC and TAlk, and then calculating
pCO2 using CO2SYS (Pierrot et al., 2006), with the dissociation
constants of Cai and Wang (1998). Then, the non-estuarine
FCO2 was calculated as above with Eq. (5), and the
FCO2 attributed to estuarine sources was determined as the
difference between total and non-estuarine FCO2.
Estuarine and mangrove contributions to DIC
DIC in the Shark and Harney rivers may originate from several sources in
addition to input from the freshwater marsh upstream and the coastal ocean,
including (1) mangrove root respiration, (2) organic matter mineralization
in sediments or in river water, (3) dissolution of CaCO3 in sediments or
in river water, and (4) groundwater discharge. Groundwater in this region is
likely to contain DIC from CaCO3 dissolution that occurs when saltwater
intrudes into the karst aquifer that underlies this region (Price et al.,
2006), as well as DIC from sediment organic matter mineralization. In this
setting, the combination of no. 1 and no. 2 represents the mangrove source of
DIC ([DIC]mangrove), and the combination of no. 3 and no. 4
represents the CaCO3 dissolution source ([DIC]dissolution)
to estuarine [DIC]:
DICestuary=DICobserved-DICconserv+DICgasex=DICmangrove+DICdissolution,
where [DIC]observed is the observed DIC concentration,
[DIC]conserv is the DIC concentration expected by conservative
mixing of the two end-members, and [DIC]gasex is the correction
for change in [DIC]observed due to loss through air–water gas
exchange as the water transits through the estuary. [DIC]gasex
was determined from FCO2 and the residence time of water
during each experiment (Ho et al., 2016).
Measurements of δ13CDIC and estuarine DIC / TAlk
ratios were used to determine the mangrove sources to estuarine DIC. Fixation
of CO2 through photosynthesis is neglected in both models as these
rivers are characterized by low chlorophyll a concentration and low
phytoplankton biomass (Boyer et al., 1997). During SharkTREx 1 and 2, there
was a negligible difference between pCO2 measured during the day and
night (ca. 3 %).
Determining mangrove contribution from
δ13CDIC
Processes 1 through 4 listed above influence δ13CDIC in
the estuary differently due to the differences in the δ13C values
originating from respiration of mangrove-derived organic matter, and
CaCO3 dissolution. The isotopic fractionation during respiration of
organic matter is small, and the δ13CDIC values
produced via this pathway should be approximately equivalent to the
δ13C of the organic matter respired (DeNiro and Epstein, 1978). The
isotopic fractionation during dissolution/re-precipitation of CaCO3 is
also considered to be negligible (Salomons and Mook, 1986).
The expected δ13C values of DIC in the rivers as a result of
conservative mixing (δ13Cconserv) of the marine and
freshwater end-members of the Shark and Harney rivers were calculated as
follows (Mook and Tan, 1991):
δ13Cconserv=SDICFδ13CF-DICMδ13CM+SFDICMδ13CM-SMDICFδ13CFSDICF-DICM+SFDICM-SMDICF,
where [DIC] is the observed DIC concentration, S is the measured salinity,
and M and F subscripts refer to the marine and freshwater end-members,
respectively.
An estimate of the maximum contribution of [DIC]mangrove and
[DIC]dissolution to [DIC]estuary can be obtained by
solving Eq. (6) and the following:
δ13CDIC×DICobserved=δ13Cconserv×DICconserv+δ13Cmangrove×DICmangrove+δ13Cdissolution×DICdissolution-δ13CDIC-ε13×DICgasex,
where the δ13Cconserv value is the DIC isotopic
composition expected for conservative mixing (Mook and Tan, 1991), δ13Cmangrove is the isotopic composition for mangrove-derived
material (-30 ‰; Mancera-Pineda et al., 2009), the
δ13Cdissolution value is the δ13C composition of
calcite (∼ 1 ‰), and ε13 is the equilibrium isotope fractionation
between DIC and CO2 gas (∼ 8 ‰; Zhang et al., 1995).
Determining the mangrove contribution from TAlk / DIC
An independent approach to separate the mangrove contribution from CaCO3
dissolution is to use the covariation of [DIC]estuary and
[TAlk]estuary as an indicator of the biogeochemical processes
affecting DIC dynamics (Borges et al., 2003; Bouillon et al., 2007c), as
these processes have different effects on DIC and TAlk. Assuming that
[TAlk]estuary is mainly produced by the dissolution of
CaCO3, [DIC]dissolution can be determined as
0.5 × [TAlk]estuary, and then [DIC]mangrove
can be calculated from Eq. (6). However, since sulfate reduction, a primary
mineralization pathway in mangrove sediments, may also contribute to TAlk
(Alongi, 1998; Alongi et al., 2005) this calculation represents an upper
bound estimate for [DIC]dissolution and a lower bound estimate
for [DIC]mangrove.
Determining the mangrove contribution to DOC
In the Shark and Harney rivers, dissolved organic matter may be derived from
upstream freshwater wetland species such as periphyton and sawgrass, from
seagrass communities and marine phytoplankton, or from mangrove vegetation
inside the estuary (Jaffe et al., 2001). The estuarine contributions to DOC
([DOC]estuary) in the rivers was determined in the same way as
for DIC above using Eq. (6), by substituting DOC for DIC accordingly, without
the correction for gas exchange:
DOCestuary=DOCobserved-DOCconserv,
where [DOC]observed is the observed DOC concentration, and
[DOC]conserv is the DOC concentration expected from conservative
mixing of the two end-members.
Then, measurements of δ13CDOC were made to ascertain
the mangrove source of DOC in the river, in order to determine the proportion
of [DOC]estuary that is of mangrove origin. The expected δ13C values of DOC as a result of conservative mixing
(δ13Cconserv) were calculated using Eq. (7),
substituting DOC for DIC. Assuming that [DOC]estuary was entirely
mangrove derived, [DOC]mangrove should equal
DOCmangrove=DOCobservedδ13CDOC+DOCconservδ13Cconservδ13Cmangrove,
where δ13Cmangrove is the isotopic composition for
mangrove-derived material (-30 ‰).
Longitudinal dispersion
The longitudinal SF6 distribution was corrected for tidal movement to
slack water before ebb for each day using a method described in Ho et
al. (2002). The absolute magnitudes of the average daily corrections were 2.0
and 2.7 km for SharkTREx 1 and 2, respectively, with a range for individual
measurements of 0 to 5.8 km and 0 to 7.3 km for SharkTREx 1 and 2,
respectively. Longitudinal dispersion coefficient Kx (m2 s-1)
was calculated from the change of moment of the longitudinal SF6
distribution over time as follows (Fischer et al., 1979; Rutherford, 1994):
Kx=12dσx2dt,
where σx2 is the second moment of the longitudinal SF6
distribution for each day.
Distributions of pCO2 (a–d) and dissolved O2 (e–h)
along the salinity gradient in the Shark and Harney rivers during the
2010 (SharkTREx 1) and 2011 (SharkTREx 2) campaigns. Different symbols
represent measurements made on different days.
Longitudinal fluxes to the Gulf of Mexico
The longitudinal fluxes of DIC and DOC from Shark and Harney rivers to the
GOM were calculated using the averaged DIC or DOC inventories, and
the residence time of water (τ; d), which was determined from the
decrease in the inventory of SF6 after correcting for air–water gas
exchange (Ho et al., 2016). For example, the longitudinal DIC flux
(FDIC; mol d-1) can be calculated as follows (e.g.,
Dettmann, 2001):
FDIC=∑DICobservedτ.
Equation (12) can be used to calculate the fluxes of any other dissolved or
suspended substance in the river by substituting its inventory in place of
DIC. In addition, using the estuarine and non-estuarine fractions of the
inventories in Eq. (12) allowed the estuarine and non-estuarine proportions
of the longitudinal carbon fluxes to be quantified.
The advantage of this method to calculate longitudinal flux in a tidal river
over a method that uses net discharge and constituent concentration is that
the effect of tidal flushing is implicitly accounted for by the residence
time, and therefore there is not a need to explicitly define the fraction of
river water in the return flow during each flood tide.
Distribution of TAlk (a–c), DIC (d–f), pH (g–i), and
δ13CDIC (j–l) along the salinity gradient in the Shark
and Harney rivers during the 2010 (SharkTREx 1) and 2011(SharkTREx 2)
campaigns. During SharkTREx 1, TAlk and pH were measured at FIU, and DIC was
calculated using CO2SYS (Pierrot et al., 2006). During
SharkTREx 2, DIC and TAlk were measured at NOAA/AOML, and pH was calculated
using CO2SYS. The dashed lines indicate the distribution expected for
conservative mixing.
Distribution of DOC and δ13CDOC
along the salinity gradient in the Shark and Harney rivers in samples
collected during SharkTREx 1 and 2. The dashed lines indicate the
distribution expected for conservative mixing.
Results and discussion
Distribution patterns and carbon inventories
During SharkTREx 1, the salinity along the longitudinal transects ranged from
1.2 to 27.1, and the mean (±SD) water temperature was
23.4 ± 0.2 ∘C (n=3767). During SharkTREx 2, salinity
ranged from 0.6 to 27.1, and water temperatures averaged
22.7 ± 0.9 ∘C (n=3818).
Both pCO2 and DO showed large spatial variability within the Shark and
Harney rivers during SharkTREx 1 and 2 (Fig. 2). Measured pCO2 values
were well above atmospheric equilibrium along the entire salinity range, with
values ranging from ca. 1000 to 6200 µatm. Maximum pCO2
values were observed at intermediate salinities, decreasing towards both
end-members, while DO showed the opposite pattern, with saturations ranging
from 36 to 113 %.
The patterns of TAlk and DIC along the salinity gradient followed the same
trend as pCO2 and were clearly non-conservative (Fig. 3a–f). TAlk
varied between ca. 3400 and 5000 µmol kg-1 during SharkTREx 1
and between ca. 3000 and 3900 µmol kg-1 during SharkTREx 2.
DIC ranged from ca. 3400 to 5100 µmol kg-1 during
SharkTREx 1, and ca. 2800 to 4000 µmol kg-1 during
SharkTREx 2. δ13CDIC values ranged from -10.3 to
-6.6 ‰ and from -11.4 to -5.8 ‰ during SharkTREx 1
and 2, respectively. Higher DIC, TAlk, and pCO2 coincided with lower
O2 saturation, lower δ13CDIC, and lower pH
values (Fig. 3g–i), indicative of mineralization of mangrove-derived organic
matter within the estuary.
During SharkTREx 1, the DOC concentrations in the freshwater end-member were
higher than SharkTREx 2 (Fig. 4). For both experiments, DOC concentrations
followed a non-conservative pattern (see also Cawley et al., 2013), but this
trend was less apparent during SharkTREx 1 compared to SharkTREx 2 (Fig. 4).
Diagrams showing the main DIC fluxes (in 105 mol d-1) entering and exiting the Shark and Harney rivers during SharkTREx
1 and 2. Fluxes from the freshwater marsh were assumed to be fluxes
estimated from the conservative DIC curves.
The inventories of DIC, DOC, DO, TAlk, and pCO2 were relatively
constant in the Shark and Harney rivers, indicating quasi-steady-state
conditions during SharkTREx 1 and 2. Under these conditions, carbon inputs
and exports are balanced, and fluxes and concentrations may be examined
interchangeably. Kx during the experiments (16.4 ± 4.7 and
77.3 ± 6.5 m2 s-1 for Shark River during SharkTREx 1 and 2,
respectively, and 136.1 ± 16.5 m2 s-1 for Harney River
during SharkTREx 2) were relatively large, and suggest that any perturbations
(such as export of DIC from mangroves) would be quickly mixed thoroughly in
the estuary.
Inventories of DIC and DOC in Shark and Harney rivers, as well as
contributions from estuarine and non-estuarine sources.
SharkTREx 1
SharkTREx 2
Shark River
Shark River
Harney River
Inventorya
Percentage
Proportion
Proportion
Inventorya
Percentage
Proportion
Proportion of
Inventorya
Percentage
Proportion
Proportion
(× 106 mol)
of total
of mangrove
of total
(× 106 mol)
of total
of mangrove
total
(× 106 mol)
of total
of mangrove
of total
inventoryb
carbonc
carbond
inventoryb
carbonc
carbond
inventoryb
carbonc
carbond
DIC
Observed
19.5 ± 0.9
–
94 %
82 %
15.2 ± 1.3
–
93 %
79 %
7.4 ± 0.4
–
93 %
79 %
Gas exchange
2.5 ± 0.2
11 %
3.2 ± 0.1
17 %
1.5 ± 0.3
17 %
Non-estuarine
17.6 ± 0.7
80 %
13.6 ± 1.2
74 %
6.5 ± 0.4
72 %
Estuarine
4.4 ± 1.1
20 %
4.8 ± 1.8
26 %
2.5 ± 0.6
28 %
Mangrove
2.9 ± 0.8
13 %
3.1 ± 1.2
17 %
1.7 ± 0.4
19 %
DOC
Observed
4.4 ± 0.1
–
6 %
18 %
4.1 ± 0.4
–
7 %
21 %
1.9 ± 0.1
–
7 %
21 %
Non-estuarine
4.2 ± 0.1
96 %
3.9 ± 0.4
94 %
1.8 ± 0.1
93 %
Estuarinee
0.2 ± 0.1
4 %
0.2 ± 0.6
6 %
0.1 ± 0.2
7 %
a The uncertainty in the observed and non-estuarine inventories are
the standard deviations of the inventories for all the days of the
experiment. The estuarine contribution is calculated from the observed and
non-estuarine contribution, and the uncertainty is from propagating the
errors of the two. The uncertainty in contribution from gas exchange is from
propagating the uncertainty in CO2 flux and the residence time. The
uncertainty in mangrove contribution is calculated from propagating the
error from the estuarine contribution.
b The DIC inventory is relative to the total DIC (i.e., ∑[DIC]observed+∑[DIC]gasex).
c Proportion of each form of carbon (i.e., DIC, DOC) relative to the
total mangrove-derived carbon pool.
d Proportion of each form of carbon (i.e., DIC, DOC) relative to the
total carbon pool.
e Estuarine DOC is assumed to be entirely of mangrove origin.
Longitudinal DIC and DOC fluxes, and air–water CO2
fluxes for the Shark and Harney rivers during SharkTREx 1 and 2.
SharkTREx 1
SharkTREx 2
Shark River
Harney River
Shark River
Harney River
Longitudinal DIC fluxes (× 105 mol d-1)a
Total
33.6 ± 1.6
N/A
18.8 ± 1.6
15.8 ± 0.9
Non-estuarine contribution
30.3 ± 1.1
N/A
16.8 ± 1.5
13.7 ± 0.8
Estuarine contribution
3.3 ± 1.9
N/A
2.0 ± 2.2
2.1 ± 1.3
Mangrove contribution
2.2 ± 1.3
N/A
1.3 ± 1.5
1.4 ± 0.8
Air–water CO2 fluxes (× 105 mol d-1)a
Total
4.2 ± 0.4
4.1 ± 0.2
4.0 ± 0.2
3.1 ± 0.6
Non-estuarine contribution
2.1 ± 0.2
2.0 ± 0.1
1.9 ± 0.1
1.1 ± 0.2
Estuarine contribution
2.1 ± 0.4
2.1 ± 0.3
2.1 ± 0.2
2.0 ± 0.6
Mangrove contribution
1.4 ± 0.3
1.4 ± 0.2
1.4 ± 0.1
1.3 ± 0.4
Longitudinal DOC fluxes (× 105 mol d-1)a,b
Total
7.5 ± 0.2
3.3 ± 0.4
5.1 ± 0.5
4.2 ± 0.2
Non-estuarine contribution
7.2 ± 0.1
2.6 ± 0.3
4.8 ± 0.5
3.9 ± 0.2
Estuarine contributionc
0.3 ± 0.2
0.6 ± 0.6
0.3 ± 0.7
0.3 ± 0.3
a Uncertainty in total and non-estuarine fluxes are from propagating
the error in total inventory and the residence time. The uncertainty in the
estuarine fluxes are from propagating the errors in total and non-estuarine
fluxes. The uncertainty in mangrove contribution is from propagating the
errors in the estuarine contribution.
b Data for DOC concentration in Harney River during SharkTREx 1 taken
from Cawley et al. (2013).
c Estuarine contribution to DOC is assumed to be entirely of mangrove
origin. N/A = not applicable.
In the following, for brevity, fluxes and inventories are summarized as
ranges, which cover the two rivers and two experiments, so they reflect both
temporal and spatial variability. The individual values are given in Tables 1
and 2.
DIC was the dominant form of dissolved carbon in both rivers and accounted
for 79 to 82 % of the total dissolved carbon in the rivers. The
contribution of DOC to the total carbon pool varied between 18 and 21 %
(Table 1).
Air–water CO2 fluxes
As shown by Ho et al. (2014), pCO2 observed during SharkTREx 1 and 2
fall in the upper range of those reported in other estuarine (Borges, 2005)
and mangrove-dominated systems (Bouillon et al., 2003, 2007a, b; Koné and
Borges, 2008; Call et al., 2015). The mean air–water CO2 fluxes in Shark
River for SharkTREx 1 and 2 were 105 ± 9 and
99 ± 6 mmol m-2 d-1 (Ho et al., 2016). The analysis is
taken further here by including data from Harney River. The mean air–water
CO2 fluxes in Harney River were 150 ± 8 and
114 ± 21 mmol m-2 d-1 for SharkTREx 1 and 2, respectively.
Borges et al. (2003) summarized all available pCO2 data from mangrove
surrounding waters, and calculated CO2 fluxes to the atmosphere that
averaged 50 mmol m-2 d-1 (with a range of 4.6 to
113.5 mmol m-2 d-1), and Bouillon et al. (2008a) estimated a
global CO2 flux from mangroves of ca.
60 ± 45 mmol m-2 d-1. One reason that the fluxes from
SharkTREx 1 and 2 are on the upper end of those estimates may be that the
Shark and Harney rivers receive a large input of DIC from the freshwater
marsh upstream (Table 1), causing higher pCO2 in the estuary compared
to the global average.
Scaling the air–water CO2 fluxes by the area of open water in the Shark
and Harney rivers, where Tarpon Bay is included with Shark River, suggests
that the total carbon emissions to the atmosphere through air–water gas
exchange in Shark River was 4.2 ± 0.4 × 105 and
4.0 ± 0.2 × 105 mol d-1 during SharkTREx 1 and
2, respectively, and were 4.1 ± 0.2 × 105 and
3.1 ± 0.6 × 105 mol d-1 from the Harney River
during SharkTREx 1 and 2, respectively (Fig. 5), which is remarkably
consistent, both spatially and temporally.
These fluxes were incorporated into the DIC mass balance of the Shark and
Harney rivers (Eq. 2) by calculating the total CO2 degassed over the
residence time of water in the rivers. Given the mean air–water CO2
fluxes (Table 2), the total CO2 degassed in the Shark River represents
approximately 13 and 21 % of ∑[DIC]observed
during SharkTREx 1 and 2, respectively, and the CO2 degassed from the
Harney River during SharkTREx 2 represents 20 % of ∑[DIC]observed, indicating that air–water CO2 exchange removes a
non-negligible fraction of the inorganic carbon in these rivers. Exclusion of
∑[DIC]gasex from the mass balance in Eq. (2) would
lead to an underestimation of ∑[DIC]estuary of
between 33 and 44 %.
Mangrove contribution to DIC inventory
The highest DIC concentrations were correlated with low DO (Fig. 2) and
characterized by 13C depletion (Fig. 3j, k, l). Observations of elevated
DIC and pCO2 in the middle of the estuary, coupled with
δ13CDIC and O2 depletion may indicate the
importance, noted by other authors, of lateral transport of pore water from
the peat-based mangrove forest into the river via tidal pumping (Bouillon et
al., 2008a; Maher et al., 2013). However, as demonstrated below, the observed
DIC and δ13CDIC distributions in these rivers cannot be
explained solely by mineralization of mangrove-derived organic carbon.
Evidence from δ13CDIC
The distributions of DIC and δ13CDIC cannot be
explained solely by the addition of mangrove-derived DIC and air–water gas
exchange. Solving Eq. (8) for δ13CDIC, assuming that
[DIC]dissolution is negligible and that the only source of DIC in
the rivers is of mangrove origin, would result in δ13C values
significantly lower than those observed. The low pH in interstitial waters of
mangrove sediments due to organic matter mineralization processes may be
favorable to CaCO3 dissolution in mangrove sediments, and this process
could have an effect on estuarine δ13CDIC. Groundwater
discharge could also influence DIC and δ13CDIC. Inputs
of DIC derived from CaCO3 dissolution from either of these sources may
explain the differences in observed δ13CDIC and those
expected if [DIC]estuary was entirely of mangrove origin.
Solving Eqs. (6) and (8), the mineralization of mangrove-derived organic
matter is estimated to account for ca. 60 ± 6 % of ∑[DIC]estuary (Table 3), with the remainder originating
from the dissolution of CaCO3. This estimate is sensitive to the end-member
value chosen for δ13Cmangroves and δ13Cdissolution. For instance, if δ13Cmangroves
were -29 ‰ instead of -30 ‰, the mangrove contribution
would increase to 62 %.
Mangrove contribution to ∑[DIC]estuary determined from δ13CDIC mass balance
and TAlk / DIC ratios.
River
Experiment
Methods
δ13CDIC
TAlk / DIC
Shark River
SharkTREx 1
60 ± 6 %
70 ± 3 %
SharkTREx 2
61 ± 6 %
70 ± 3 %
Harney River
SharkTREx 1
–
–
SharkTREx 2
61 ± 6 %
70 ± 2 %
Evidence from DIC and TAlk
In the Shark and Harney rivers, the high correlation (r2=0.99; Fig. 6)
between [DIC]estuary and [TAlk]estuary indicates the
same processes control the inputs of DIC and TAlk to these rivers. By
examining the covariation of [DIC]estuary and
[TAlk]estuary, mangroves were found to contribute a minimum of
70 ± 3 % of ∑[DIC]estuary (Table 3),
with the remainder due to the dissolution of CaCO3. These estimates are
in reasonable agreement with those based on the carbon isotopic mass balance.
(a) Covariation of DICestuary and TAlkestuary.
Black squares are samples from the Shark River during SharkTREx 1, and black
and gray circles are from the Shark and Harney rivers, respectively, during
SharkTREx 2. Dotted lines represent the theoretical covariation of DIC and
TAlk for different biogeochemical processes: (1) aerobic respiration;
(2) CO2 emission, (3) sulfate reduction, (4) CaCO3 dissolution, (5) manganese reduction, and (6) iron reduction.
The [TAlk]estuary vs. [DIC]estuary ratios were 0.84 and 0.92 for
Shark River during SharkTREx 1 and 2, and 0.90 for the Harney River during
SharkTREx 2 (Fig. 6). The TAlk to DIC ratios for CaCO3 dissolution,
sulfate reduction, and aerobic respiration are -0.2, 0.99, and 2,
respectively. Hence, in order to achieve the observed ratios, and given the
estimated contribution of CaCO3 dissolution to ∑[DIC]estuary of ca. 30 %, sulfate reduction and aerobic respiration
were estimated to contribute 32 to 39 and 31 to 38 %, respectively.
Evidence from DO
The deficit of O2 in Shark River was found to be
2.7 ± 0.7 × 106 and
3.7 ± 0.3 × 106 mol during SharkTREx 1 and 2,
respectively. Assuming a stoichiometric ratio of ca. 1.1 for O2 to
CO2 during degradation/remineralization of terrestrial organic matter
(Severinghaus, 1995; Keeling and Manning, 2014), the maximum contribution of
aerobic respiration to the DIC added to the estuary was estimated to be 57 to
69 %. However, O2 may also be consumed during oxidation of reduced
products from anaerobic metabolism, such as H2S, Mn2+ or Fe2+,
with similar O2 to CO2 stoichiometry as aerobic respiration. Hence,
the numbers derived above represent an upper limit for aerobic respiration,
and if there were complete re-oxidation of metabolites from anaerobic
respiration, the O2 deficit would represent total mineralization of
terrestrial organic matter instead of just aerobic respiration. The mangrove
contributions estimated from δ13CDIC (Sect. 3.3.1) and
TAlk / DIC (Sect. 3.3.2) are consistent with this analysis of the O2
deficit, which indicates that a minimum of 57–69 % of ∑[DIC]estuary derived from the mineralization of organic
matter.
Mangrove contributions to DOC inventory
During both experiments, the δ13CDOC was highly
depleted, indicative of contribution from higher plants, including mangroves.
During SharkTREx 1, the lowest observed δ13CDOC value
(-31.6 ‰) was in the mid-estuary (i.e., from salinity of ca. 10 to
20) (Fig. 4d). Previous studies of DOC from mangrove-dominated systems have
reported values as low as -30.4 ‰ (Dittmar et al., 2006), and some
of the more depleted samples from SharkTREx 1 might have DOC sourced from
algae associated with mangrove roots, which can have relatively depleted
values (Kieckbusch et al., 2004). The overall δ13CDOC
depletion was less during SharkTREx 2, and the overall distribution was
indicative of a stronger marine influence and/or mixing (Fig. 4e, f). The
marine end-member had a more enriched δ13CDOC,
indicating a greater contribution of seagrass and/or marine phytoplankton-derived
organic matter to the marine DOC pool (Anderson and Fourqurean,
2003). These observations are consistent with the greater longitudinal
dispersion observed during SharkTREx 2 compared to SharkTREx 1.
The calculations of mangrove contribution using δ13CDOC
mass balances (Eq. 10) also suggest that the majority of [DOC]estuary,
but only a small percentage of the total DOC inventory, was derived from
mangroves (7 and 5 % in the Shark River during SharkTREx 1 and 2, and
7 % in the Harney River during SharkTREx 2).
Distribution of total and mangrove fluxes of DIC
and DOC for Shark and Harney rivers during SharkTREx 1 and 2.
SharkTREx
Estuarine
Percent of total
Percent of total
experiment no.
contributiona
export fluxb
mangroves fluxc
Longitudinal DIC flux
Shark River
1
10 %
74 %
57 %
2
11 %
67 %
45 %
Harney River
1
–
–
–
2
13 %
68 %
48 %
Air–water CO2 flux
Shark River
1
49 %
9 %
35 %
2
52 %
14 %
45 %
Harney River
1
51 %
–
–
2
63 %
14 %
43 %
All DIC fluxes
Shark River
1
–
83 %
92 %
2
–
82 %
90 %
Harney River
1
–
–
–
2
–
82 %
91 %
Longitudinal DOC flux
Shark River
1
4 %
17 %
8 %
2
6 %
18 %
10 %
Harney River
1
19 %
–
–
2
7 %
18 %
9 %
a Estuarine contribution to the individual fluxes in each river during
each experiment.
b Flux as a percentage of the total dissolved carbon flux (i.e.,
longitudinal DIC, DOC, and air–water CO2 fluxes).
c Flux as a percentage of the total mangrove-derived dissolved carbon
flux (i.e., longitudinal DIC, DOC, and air–water CO2 fluxes).
Longitudinal fluxes to the Gulf of Mexico and comparison with
previous studies
Residence times of Shark River (including Tarpon Bay) for SharkTREx 1 and 2
were, 5.8 ± 0.4 and 8.1 ± 1.1 days, respectively (Ho et al.,
2016), and that of Harney River was 4.7 ± 0.7 days for SharkTREx 2. The
resulting longitudinal DIC fluxes to the Gulf of Mexico (15.8 to
33.6 × 105 mol d-1) were significantly larger than the
longitudinal DOC fluxes (3.3 to 7.5 × 105 mol d-1) at
salinity of ca. 27 (Fig. 5; Table 2).
There are no previously published DIC inventories or fluxes for the Shark and
Harney rivers, so comparison with previous studies is focused on the DOC
results. The DOC flux from the Shark River to the coastal ocean in SharkTREx
1 (7.5 ± 0.2 × 105 mol d-1) is in very good
agreement to that estimated by Bergamaschi et al. (2011) in an experiment
conducted in the Shark River from 20 to 30 September 2010
(7.6 ± 0.5 × 105 mol d-1). However, the net
discharge during the Bergamaschi et al. (2011) study was higher than
SharkTREx 1 (mean ± SD: 9.1 ± 7.1 vs.
6.9 ± 5.3 m3 s-1), which would lead to a shorter residence
time of 4.6 days using a relationship presented in Ho et al. (2016). Using
the DOC concentration data presented in Bergamaschi et al. (2011) yields an
inventory that is ca. 3 % higher than the DOC inventory in Shark River
during SharkTREx 1. Calculations using the shorter residence time and higher
DOC inventory yields a DOC flux of
9.7 ± 0.2 × 105 mol d-1, which is ca. 30 %
higher than the estimates of Bergamaschi et al. (2011).
The longitudinal flux of mangrove-derived DOC from Shark River during
SharkTREx 1 (0.3 ± 0.2 × 105 mol d-1; Table 2) is
in rough agreement with the estimate of Cawley et al. (2013) during the same
period (0.2 × 105 mol d-1), but the value for Harney
River (0.6 ± 0.6 × 105 mol d-1) is lower than
their estimate (1.6 × 105 mol d-1).
Mangroves contributed 4 to 6 % of the total longitudinal DOC flux in the
Shark River and 7 % in the Harney River during SharkTREx 2 (Tables 1 and
4). Cawley et al. (2013), estimated a mangrove contribution to DOC flux of
3 ± 10 % for Shark River and 21 ± 8 % for the Harney
River during November 2010, the same time period as SharkTREx 1. DOC
measurements were not made in Harney River as part of SharkTREx 1. However,
using the November 2010 DOC data from Harney River collected by Cawley et
al. (2013) for inventory calculations, along with residence time derived from
the tracers, a mangrove contribution of 19 % to the total DOC
longitudinal flux to the Gulf of Mexico was obtained.
Distribution of carbon fluxes
During SharkTREx 1 and 2, ∑[DIC]estuary made up
20–28 % of the total DIC in the rivers, and ∑[DOC]estuary made up only 4 to 7 % of the total DOC in the rivers.
Mangroves are estimated to contribute 13 to 19 % to the total DIC
inventory. In all cases, the mangrove contribution to the DIC inventory is a
factor of 3 greater than the mangrove contribution to the DOC inventory
(Table 1).
During SharkTREx 1 and 2, the inventory of mangrove-derived DIC exceeded that
of DOC by a factor of 15 to 17, which supports the idea that a large fraction
of the carbon exported by mangroves to surrounding water is as DIC (Bouillon
et al., 2008a), but is considerably larger than the estimates of ca. 3 to 10
compiled by Bouillon et al. (2008a) for mangroves at five sites in Asia and
Africa.
The total dissolved carbon fluxes from all sources (i.e., freshwater wetland,
mangrove, carbonate dissolution, and marine input) out of the Shark and
Harney rivers during SharkTREx 1 and 2 are dominated by inorganic carbon
(82–83 %; see Tables 2 and 4), either via air–water CO2 exchange or
longitudinal flux of DIC to the coastal ocean (Fig. 5). The remaining
17–18 % of the export is as DOC. This proportioning is remarkably
similar between SharkTREx 1 and 2, and between the Shark and Harney rivers
(Table 1). The estuarine contribution to these fluxes is relatively small
(generally < 15 %), with the exception of air–water CO2 flux,
where the estuary contribution was 49 to 63 % (Table 4).
In this study, the particulate organic carbon (POC) flux was not examined.
However, He et al. (2014) estimated the mangrove-derived POC flux in Shark
River by taking the total volume discharge from the five major rivers along
the southwest coast of Everglades National Park from 2004 to 2008, and
assuming that Shark River contributed 14 % to the mean annual discharge.
They then multiplied this discharge by the average POM concentration
(5.20 ± 0.614 mg L-1) in the middle of the estuary to yield an
annual POM flux from Shark River. Based on analysis of organic matter
biomarkers, He et al. (2014) estimated that mangrove-derived POM was
70–90 % of the total POM pool in the Shark River. Using this
contribution and further assuming that 58 % of POM weight is POC (Howard,
1965), they estimated a POC flux of 1.0 to
2.2 × 104 mol d-1. Because this estimate was based on
biomarker and POM data from the mid-estuary, where the POM concentration and
the mangrove contribution to POM are both likely to be much higher than
either toward the freshwater end-member or the marine end-member, it is
likely an overestimate of the mangrove-derived POC flux. Nevertheless, the
mangrove-derived POC flux determined by He et al. (2014) is still only a
small fraction (3 to 7 %) of the mangrove-derived dissolved carbon fluxes
in Shark River during SharkTREx 1 and 2.
Mangrove contributing area and estuary carbon balance
One of the challenges of relating the results reported here to other studies
is to scale the results to a mangrove contributing area, and thereby relate
the findings to mangrove forest carbon balance, typically expressed on an
aerial basis. Estimates of forest carbon export derived here are compared
with other investigations in this estuary. The entire area of mangroves
surrounding the Shark and Harney rivers region is ca. 111 km2, and the
water area is ca. 17.5 km2 (Ho et al., 2014). Scaling the forest area
by the water area of Shark River (2.5 km2) yields an associated forest
area of 15.9 km2. The forest area associated with Harney River
(2.8 km2) is 17.4 km2.
Using the total forest area associated with Shark River to scale estimates of
total export of mangrove-derived carbon (the combination of longitudinal
fluxes and air–water gas exchange) suggests an average dissolved carbon
lateral export rate from the forest of 18.9 to
24.5 mmol m-2 d-1, including both DIC and DOC. However, since it
is unknown what fraction of the total forest area associated with these
rivers exported dissolved carbon through tidal pumping (a function of tidal
height and duration), this is considered to be a minimum estimate. Average
water levels at high tide during SharkTREx 1 and 2 at the USGS Shark River
station were 88 and 95 % of maximum wet season water levels reported at
this site over the period from November 2007 to December 2012 (U.S.
Geological Survey, 2016), and 12 inundation events occurred during both
SharkTREx 1 and 2. Water levels in the main river channel at the USGS Shark
River station were above an estimate of the average minimum ground surface
elevation derived from nearby groundwater monitoring wells in the estuary
(sites SH3 and SH4; http://sofia.usgs.gov/eden/stationlist.php) for 21
and 28 % of the time during the SharkTREx 1 and 2 experimental periods,
respectively. These values indicate the export of dissolved carbon from
flooded portions of the forest during the discontinuous inundation periods
should be significantly greater than the dissolved carbon lateral export rate
derived above in order to produce the observed inventories of
mangrove-derived dissolved carbon in the main channel.
Bergamaschi et al. (2011) proposed an annual total DOC export from the forest
surrounding Shark River of 15.1 ± 1.1 mol m-2 yr-1 and
describe their method of calculating contributing area using a model based on
the relationship between discharge volume and changes in water levels during
tidal cycles. They do not provide a contributing area, but this can be
calculated from their results. They determined longitudinal DOC fluxes of
7.6 ± 0.5 × 105 and
1.3 ± 0.02 × 105 mol d-1 for the wet and dry
seasons, respectively, and assumed that they are entirely of mangrove origin.
Given the lengths of the wet and dry seasons, this would yield a mean annual
DOC flux of 3.9 ± 0.2 × 105 mol d-1, and
9.4 ± 0.7 km2 of mangrove forest contributing to carbon fluxes
through tidal flushing in this segment of Shark River. However, data from
SharkTREx 1 and 2 indicate that ca. 5 % of the total longitudinal DOC
fluxes were of mangrove origin, with an average mangrove-derived DIC to DOC
flux ratio of 10.5. Using this information, the Bergamaschi et al. (2011)
results were recalculated to yield a wet season dissolved carbon lateral
export rate of 46.5 ± 4.4 mmol m-2 d-1 (as DIC and DOC)
from the forest.
Another method of estimating forest lateral carbon export utilizes the
difference between measurements of net ecosystem–atmosphere CO2 exchange
(NEE) above the mangrove forest surrounding Shark
River (267 ± 15 mmol m-2 yr-1 in 2004; (Barr et al., 2012)
and corresponding measures of net ecosystem carbon balance (NECB;
227 ± 14 mmol m-2 d-1). NECB in 2004 can be estimated as
the sum of carbon in litter fall (104 ± 8 mmol m-2 d-1),
wood production (44 ± 3 mmol m-2 d-1) (Castañeda-Moya
et al., 2013), root growth (47 ± 11 mmol m-2 d-1)
(Castañeda-Moya et al., 2011), and soil carbon accumulation
(31.7 mmol m-2 d-1) (Breithaupt et al., 2014) measured at the
same location (FCE LTER site SRS6) in this forest. The difference between NEE
and NECB (40 ± 17 mmol m-2 d-1) provides an estimate of
the annual rate of forest carbon export to Shark River on a daily basis
(Chapin et al., 2006).
The rate of mangrove-derived carbon exported to estuarine waters is likely to
vary over space and time, as a result of factors that include tidal cycles,
phenology, and forest and soil structural characteristics. For example,
Bergamaschi et al. (2011) found that DOC fluxes were 6 times higher during
the wet season (September) than the dry season (April), whereas Cawley et
al. (2013) found that the DOC fluxes were 4 and 10 times higher during the
wet vs. dry season (November vs. March) in the Shark and Harney rivers,
respectively. Barr et al. (2013) showed that forest respiration rates derived
from NEE data are greater during the wet than dry seasons. Higher respiration
rates combined with increased inundation during the wet compared to dry
seasons suggest that wet season DIC export will also be greater than dry
season values. For these reasons, the annual carbon export rates derived from
the difference between NECB and NEE are expect to underestimate wet season
values. If annual lateral carbon export rates are considered as equivalent to
a time-weighted sum of dry season (7 months) and wet season (5 months) values
(after Bergamaschi et al., 2011), and wet season export is assumed to be, for
example, 5 times greater than dry season values, the seasonal export rates
(15 and 75 mmol m-2 d-1 for dry and wet seasons, respectively)
that correspond with the difference between annual NECB and NEE can be
calculated.
The discrepancies between the estimates of carbon export rates derived here,
and those derived from Bergamaschi et al. (2011) and the difference between
NEE and NECB point out the need for additional studies to reduce the
uncertainty in the relationships between riverine carbon fluxes, forest
carbon export, and estimates of contributing areas. For example, Bergamaschi
et al. (2011) conducted an Eulerian study at a single location in the middle
of the estuary, where the mangrove influence might be higher than the
Lagrangian study conducted during SharkTREx 1 and 2, which covered the entire
estuary. Also, the estimate of forest carbon export based on the difference
between NEE and NECB is from a single location along Shark River (at FCE LTER
site SRS6), and may not be representative of the entire forest. Furthermore,
forest lateral carbon export rates and contributing areas should be
considered dynamic, varying over semi-diurnal timescales with the extent and
duration of inundation during individual tidal cycles. The correct
interpretation of a single, static value for contributing area such as
derived above is therefore uncertain, since the tracer-based results
represent an integration of carbon sources and sinks calculated over the
water residence time and expressed on daily timescales. To improve
understanding of how mangrove forest carbon balance and export influence
riverine carbon inventories and fluxes to the Gulf of Mexico in this system,
wet and dry season measurements over multiple years, information on the
relationships between forest structure, productivity and lateral carbon
export rates, and independent estimates of forest inundation area in relation
to tidal height are needed.
Conclusions
The SharkTREx 1 and 2 studies are the first to provide estimates of
longitudinal DIC export, air–water CO2 fluxes, and mangrove-derived DIC
inputs for the Shark and Harney rivers. The results show that air–water
CO2 exchange and longitudinal DIC fluxes account for ca. 90 % of the
mangrove-derived dissolved carbon export out of the Shark and Harney rivers,
with the remainder being exported as dissolved organic carbon.
The mangrove contribution to the total longitudinal flux was 6.5 to 8.9 %
for DIC and 4 to 18 % for DOC. A lower bound estimate of the dissolved
carbon export (DIC and DOC) from the forest surrounding Shark River during
the wet season was 18.9 to 24.5 mmol m-2 d-1 with 15.9 km2
of mangrove contributing area. This basin-scale estimate is somewhat lower by
comparison than other independent estimates of lateral carbon export from
this mangrove forest. However, mangrove forest carbon export rates on an
aerial basis are expected to vary with the spatial and temporal scales over
which they are calculated, and depend on factors such as tidal inundation
frequency, distance from the riverbank and the coast, and forest and soil
characteristics.
Future experiments should investigate the contribution of DIC from
groundwater to the rivers, by making measurements of δ13CDIC of groundwater, Sr and Ca concentrations in the river
to quantify CaCO3 dissolution and to separate carbonate alkalinity from
TAlk, radon to quantify groundwater discharge, and 14CDIC to
separate input of DIC from remineralization of organic matter from
dissolution of CaCO3. Experiments should also examine the seasonal
variability in the carbon dynamics and export, by conducting process-based
studies like SharkTREx during both wet and dry seasons. Also, time series
measurement of current velocities, wind speeds, pCO2 and pH (to
calculate DIC), DO, chromophoric dissolved organic matter (CDOM, as a proxy
for DOC), and radon will also allow for the temporal variability of the sources
and sinks of DIC in these rivers to be examined.