dissolved organic carbon ( doc ) export 
 from mangrove swamps accounts 
 for 10% of the global terrestrial flux of doc to coastal oceans . given recent reports of high concentrations of mercury ( hg ) in mangrove 
 tissues and mangrove marsh waters , and the known high affinity 
 of hg species for doc in surface waters , there is concern 
 that a large hg flux may accompany the doc flux from mangroves . in 
 other aquatic environments ,
transport of hg species is intimately 
 linked to the transport of doc because of strong complexation , particularly in tidal wetlands .
elevated mercury levels have long been reported in 
 coastal aquatic 
 food webs from areas near the extensive mangrove swamps of southwest 
 florida . in recent 
 years ,
hg concentrations in some fish and wading birds in the everglades 
 have declined , but concomitant declines have not been observed in 
 florida bay species .
these decoupled trends suggest that mitigation efforts 
 may have diminished the terrestrial accumulation of hg but have not 
 similarly benefited coastal food webs , perhaps because the hg source 
 to coastal food webs is different than that of the uplands .
methylmercury 
 ( mehg ) , the form of hg that bioaccumulates , has several potential 
 sources in coastal waters : it may be produced in low - oxygen environments 
 in terrestrial systems and then exported to coastal 
 waters ; it may be produced in coastal wetlands such as mangroves and 
 then tidally pumped into surrounding waters ; or it may be produced within estuarine and 
 coastal sediments . although elevated 
 tissue concentrations of hg and mehg are known to occur in mangroves and high concentrations of aqueous hg and mehg 
 have been reported in mangrove - dominated environments , remarkably little is known about the magnitudes of hg and mehg export 
 from mangrove swamps into coastal and estuarine waters .
this 
 study sought to characterize the export of doc and hg from 
 the mesohaline estuarine mangrove swamps and fresher interior sawgrass 
 marshes of the shark river estuary ( everglades national park , florida , 
 usa ) to nearshore open coastal waters to test the hypothesis that 
 mangroves can be significant sources of doc , hg , and mehg to coastal 
 waters . estimating mass fluxes of carbon and mercury species in estuarine 
 environments
is challenging because of bidirectional flows , rapidly 
 changing concentrations , and the multiple factors that affect tidal 
 interactions with wetlands .
we capitalized on the known strong association 
 between doc and hg and applied a recently developed strategy that 
 uses in situ continuous measurements of fluorescent dissolved organic 
 material ( fdom ) as a proxy .
we first established the relationship of fdom to 
 doc , total hg and mehg concentrations by measuring fdom in situ while collecting discrete water samples throughout the estuary 
 over a range of salinities and constituent concentrations during both 
 wet and dry seasons .
we also measured fdom continuously 
 at a fixed site in the middle of the estuary for approximately two 
 weeks of each season .
regression models developed from the estuary - wide 
 data were then applied to convert the seasonal midestuary fdom time series to time series of doc , total hg , and mehg 
 concentrations , which were in turn used to calculate fluxes and yields . 

the premise of this study was that fluxes past the midestuary site 
 would integrate processes operating over the range of nontidal to 
 tidal domains , thereby providing insight into the production and transport 
 of doc and mercury species .
the concentration and flux time series 
 data improve our understanding of processes controlling production 
 and transport of doc , total hg ( thg ) , and mehg from mangrove - dominated 
 areas , assessments previously unattainable in dynamic estuarine systems .
the southwest florida coast is a region 
 of mangrove - dominated tidal inlets and rivers .
the shark river ( si figure s1 ) is one such river , located entirely 
 within everglades national park .
shark river slough drains an area 
 of predominantly freshwater sawgrass marsh in its headwaters , and 
 expansive mangrove swamp characterizes the estuary s lower 
 reaches .
tides are semidiurnal with a mean amplitude of approximately 
 1 m. the everglades experiences two distinct precipitation seasons : 
 a may  october rainy season and a november  april dry 
 season .
regional average annual rainfall is approximately 1.4 m. our midestuary sampling site was near gunboat 
 island ( si figure s1 ) , approximately 10 
 km inland from the coast and adjacent to a u.s .
water depth at the site is 9 m. 
 hg deposition and cycling in the region are strongly coupled to the 
 pattern of precipitation .
soil hg concentrations 
 in the lower shark river are relatively high ( 0.15 to 0.30 mg kg ) . during a 2005 reconnaissance 
 study ,
water samples were collected from a number of southwest florida 
 coastal rivers ( si table s1 ) to identify 
 areas of high hg . for the follow - on flux study ,
additional samples 
 were collected from the shark river estuary during september 2122 , 
 2010 ( wet season ) and april 56 , 2011 ( dry season ) ( si figure s1 , si table 
 s2 ) .
surface water samples were collected by boat directly into 1 
 l polycarbonate containers . during the september sampling period , 
 water
was also collected at gunboat island from 0.5 m below the air  sea 
 interface , at 2 m depth , and at 1 m above the bottom .
these samples 
 were collected using a peristaltic pump , a 20 cm length of c - flex 
 tubing , and three teflon tubes ( 1/4 in .
samples collected for doc analysis were filtered 
 within 2 h of collection through prerinsed gelman gelcap filters ( nominal 
 cutoff of 0.45 m ) into precleaned , precombusted 1 l or 125 
 ml amber glass bottles .
these samples were refrigerated and express - shipped 
 on ice to the usgs laboratory in boulder , co. samples were analyzed 
 for doc content by wet chemical oxidation and for uv absorbance by 
 diode array spectrophotometry ( see the supporting 
 information for details ) .
specific uv absorbance ( suva254 ) was calculated as the uv absorbance at 254 nm divided 
 by the doc concentration .
samples collected specifically for 
 total mercury ( thg ) and methylmercury 
 ( mehg ) analyses were drawn directly into acid - washed 1 l pet bottles 
 and then immediately filtered using an all - teflon filtration tower , 
 fired quartz fiber filters ( 0.7 m pore size ) , and teflon sample 
 bottles .
all field sampling followed mercury - clean procedures and 
 included the collection of numerous field blanks to monitor for possible 
 contamination .
filtered waters were immediately 
 acidified to 1% ( v / v ) with concentrated hcl and were stored at ambient 
 temperature out of direct sunlight .
all samples were express - shipped 
 to the usgs mercury research laboratory in madison wi .
epa method 1631 and 1630 , 
 respectively , as implemented by the usgs mercury research laboratory 
 ( see the supporting information for details ) . concurrent with collection 
 of each discrete water sample , fluorescent dissolved organic matter 
 ( fdom ) fluorescence and other parameters were measured 
 in situ with an open - faced optical sensor ( wet labs eco ) attached to a water quality sonde ( ysi 6920 ) . for continuous measurements , 
 in situ instruments
were deployed at the gunboat island site during 
 september 2030 , 2010 , and april 419 , 2011 .
a flow - through fdom fluorometer ( wet labs wetstar ; excitation wavelength 
 370 nm , emission wavelength 450 nm ) fitted with a pump ( sea - bird electronics 
 5 t ) and a water quality sonde ( ysi 6920 ) was mounted approximately 
 1.5 m above the seabed to collect data at 15 min intervals .
the fluorometer 
 was purged for one minute , followed by 60 s of data collection .
temperature , 
 salinity , dissolved oxygen , ph , and turbidity were also measured during 
 the same 60 s interval .
the data were binned by averaging over the 
 last 30 s of each data - collection period .
midway through each field 
 deployment , the instruments were briefly removed and cleaned ; the 
 resulting data gaps were filled by interpolation .
fluorescence is reported 
 here as quinine sulfate equivalents ( qse ; the concentration of quinine 
 sulfate dehydrate in ppb that results in an equivalent instrument 
 response ) .
clean - water offsets , determined prior and subsequent to 
 field deployment , were less than 1% of the measured in situ sample 
 fluorescence values .
water discharge data were obtained from the usgs gunboat island 
 stream gage , which was equipped with a side - looking acoustic doppler 
 velocimeter that measured cross - sectional velocities every 15 min .
discharge measurements were calibrated using 
 a moving boat equipped with a downward - looking acoustic doppler current 
 profiler , and discharge was calculated according to published methods .
concentrations 
 of doc , filtered total mercury ( fthg ) , and filtered methylmercury 
 ( fmehg ) at gunboat island were derived from the continuous in situ fdom measurements .
discrete - sample data were used to develop 
 season - specific type i linear regression models to relate doc , fthg , 
 and fmehg concentrations to fdom as described in supporting information section s2 ; no simple 
 relationships were found to relate particle - associated hg concentrations 
 to fdom or other parameters measured continuously 
 in situ .
the regression models ( one set applicable where freshwater 
 and midestuary waters mixed and another set applicable where midestuary 
 and saline waters mixed ) were applied to the continuous fdom data to obtain continuous time series estimates of doc , fthg , and 
 fmehg concentrations . the freshwater 
estuary submodels were 
 applied where fdom > 125 qse ; the estuary  seawater 
 submodels , where fdom  125 qse , corresponding 
 to inflection points in salinity , suva254 , fdom , and constituent values ( figure 1 ) .
measured 
 dissolved organic carbon ( doc ) and mercury species as 
 a function of salinity ( in practical salinity units , psu ) and dissolved 
 organic matter fluorescence ( fdom ; in quinine sulfate 
 units , qse ) in discrete samples of the shark river estuary , sept . 

2122 , 2010 , and april 56 , 2011 : doc ( top row ) , specific 
 uv absorbance at 254 nm ( suva ; second row ) , filter - passing total mercury 
 ( fthg ; third row ) , and filter - passing methylmercury ( fmehg ; bottom 
 row ) .
the small gray circles in parts a  h show the modeled constituent 
 values at gunboat island for comparison .
the large black circles in 
 parts i  p indicate fdom < 125 qse , while 
 the large gray circles in these figures indicate fdom  125 qse , corresponding to the two mixing models described 
 in the text .
constituent fluxes were calculated for each 15-min 
 interval as 
 the product of the discharge and the estimated constituent concentration . 

flux is here defined as positive seaward and negative landward ; thus 
 a positive flux represents net export out of the estuary toward the 
 gulf of mexico .
yields ( fluxes per unit area ) were estimated 
 by dividing constituent 
 fluxes by the area likely contributing doc and mercury to the midestuary 
 water column . the contributing area in relatively flat tidal systems 
 can not be defined in terms of watershed area , as is typically done 
 for estimates of terrestrial yields .
instead , it was estimated for 
 the shark river site as the planar area covered by the water volume 
 of each deployment s tide of maximum exchange .
for the tide 
 of greatest ebb  flood water - height difference at gunboat island , 
 the volume of the flooding waters was divided by the accompanying 
 change in water height .
this initial estimate of area was then refined 
 by adjusting for the effects of soil porosity and water retention .
the doc measurement 
 error was 0.2 mg l , for a median error of 
 1% of the reported values .
the laboratory error in fthg measurements 
 was constrained by laboratory protocols to be less than 10% of the 
 measured value .
based on the regression models ( si figure s2 ) , the root - mean - square errors of 
 prediction ( rmseps ) for the wet - season models were 2% for the doc 
 model , 11% for fthg , and 5% for fmehg .
the dry - season rmseps were 
 1% for the doc model , 6% for fthg , and 4% for fmehg .
cumulative 
 error was estimated as the square root of the sum of 
 the squared error from the component measurements in the flux calculations .
, the average errors were estimated to be 
 12% of the calculated doc flux value , 16% of the calculated fthg value , 
 and 13% of the calculated fmehg value .
for the april study period , 
 the average errors were estimated to be 12% of the calculated doc 
 flux value , 14% of the calculated fthg value , and 13% of the calculated 
 fmehg value .
the 2005 reconnaissance 
 water sampling of nearshore areas , tidal rivers , and mangrove swamps 
 ( si table s1 ) revealed some of the highest 
 reported levels of mehg for south florida ( > 26 ng l ) , including the everglades .
concentrations of fthg and fmehg in 
 samples collected near mangroves were higher than in samples collected 
 from nearby channel waters , suggesting that mangrove areas were contributing 
 large amounts of doc and mercury to estuary and coastal waters .
the 
 20102011 shark river flux study was designed to assess the 
 magnitude of this contribution .
water samples and in situ measurements 
 were collected during portions of both the wet and dry seasons to 
 help gauge the range of fluxes that may occur and to provide insights 
 into underlying and perhaps contrasting processes . during the september 
 wet - season sampling period ( figure 2a ) ,
the 
 gunboat island site experienced much greater precipitation than during 
 the april dry - season period ( figure 2d ) .
the 
 midestuary median salinity was 9 psu in september , with a range of 
 over 21 psu ; the median salinity in april was much higher ( 31 psu ) , 
 with a much smaller range ( 9 psu ) .
median water height during the 
 september period ( 0.1 ft ) was higher than during april ( 0.7 
 ft ) .
as is discussed below , the salinity gradient , precipitation , 
 and regional water height all have important implications for the 
 estuarine export of doc , thg , and mehg .
times series of tidal - stream 
 gage data and modeled constituent 
 concentrations and fluxes in shark river estuary at gunboat island 
 during sept .
2122 , 2010 , and april 56 , 2011 : salinity , 
 water height , and precipitation ( top row ) ; modeled dissolved organic 
 carbon ( doc ) , filtered total mercury ( fthg ) , and filtered methylmercury 
 ( fmehg ) concentrations ( middle row ) , and modeled cumulative doc , fthg , 
 and fmehg fluxes ( bottom row ) .
sampling during both seasons encompassed freshwater 
 sawgrass slough , 
 mangrove - dominated coastal swampland , and the higher salinity open 
 coastal waters of ponce de leon bay ( si figure s1 ) .
doc concentrations were highest near the most inland , 
 low - salinity sampling sites ; concentrations were lowest offshore in 
 ponce de leon bay ( figure 1a , si table s2 ) . the doc :
salinity concentration gradient was 
 steeper during the april dry - season sampling period than during september 
 ( figure 1a , e ) .
similar to doc , the highest fdom values during 
 both seasons were from inland fresh waters , while the lowest values 
 were found in outer ponce de leon bay ( si table s2 ) . the down - estuary fdom gradient was steeper 
 in the dry season . during both seasons ,
fdom and 
 doc concentrations were strongly positively correlated ( figure 1i , m ; r = 0.98 in september ; r = 0.99 in april ) .
in contrast , suva254 , a compositional property of doc 
 related to doc aromaticity and source , exhibited maximum values at midrange salinities during both sampling 
 periods ( figure 1b , f ) .
fall suva254 values peaked at 5 psu salinity , when the gunboat island median 
 salinity was 9 psu ( figure 2a ) .
spring suva254 values peaked at 26 psu , when the median salinity at gunboat 
 island was 31 psu ( figure 2d ) .
the spatial 
 distribution of fthg was similar to that of suva254 in 
 both seasons ( figure 1 ) , with 
 both quantities peaking near the region of 5 psu salinity in september 
 ( close to salinities at gunboat island ) and near 20 psu salinity in 
 april ( also close to salinities at gunboat island ) .
likewise , september 
 fmehg values peaked near 5 psu salinity , in the mangrove zone , with 
 the lowest concentrations found at offshore salinities ( figure 1d ) . in april , however , the spatial distribution 
 of fmehg more closely resembled that of doc , with the highest concentrations 
 at inland sites and progressively lower values offshore ( figure 1h ) .
even so , the april relationship between fdom and fmehg showed an inflection point ( figure 1p ) near 25 psu salinity ( figure 1h ) , indicating a change in doc composition from that previously 
 associated with inland fmehg .
interestingly , the spatial distributions 
 of particulate thg ( pthg ) 
 and particulate mehg ( pmehg ) also resembled those of suva254 ( si table s2 ) , suggesting a possible 
 coupling between the particulate and dissolved hg phases .
these values for the distribution of mercury between particulate and 
 filtered phases  with the majority occurring in the filter - passing 
 fractions  are similar to those previously reported for the 
 everglades .
the concurrent change in fthg , fmehg , fdom , and 
 suva254 values at the salinities found near gunboat island 
 in both seasons ( figure 1 ) strongly suggests 
 that the primary zone of production of doc , thg , and mehg is geographically 
 fixed within the mangrove portion of the estuary , in the general vicinity 
 of gunboat island .
if doc and hg production and hg methylation occurred 
 within a specific salinity zone of the estuary , the peak would shift 
 seasonally away from gunboat island salinities .
if the large majority 
 of doc and hg were produced in the fresh waters of the sawgrass slough 
 or in the outer estuary and this was where the majority of hg methylation 
 occurred , there would not have been a midestuary maximum .
further 
 evidence that the mangroves supply the majority of doc , hg , and mehg 
 is that midestuary high suva254 values can not be explained 
 by invoking an appreciable contribution of freshwater - derived or outer 
 estuary doc ( figure 1 ) .
that the mangrove zone 
 is the primary source of doc and hg supply to the coast is consistent 
 with the findings of romigh et al . , who 
 reported high doc fluxes from shark river mangroves .
they also agree 
 with rumbold et al . , who found little 
 hg contribution from uplands and the highest thg and mehg concentrations 
 in the transition zone from sawgrass to mangrove , and with the high 
 thg and mehg concentrations found in our 2005 southwest florida reconnaissance 
 study ( si table s1 ) .
one implication of 
 this finding is that , since little of the hg and mehg is derived from 
 the sawgrass portion of the estuary , reducing hg loads to sawgrass 
 marsh areas may have little effect on coastal hg fluxes from mangroves .
modeled constituent 
 concentrations in the shark river estuary at gunboat island in september 
 and april generally tracked the water - height variations that marked 
 the flood and ebb of the semidiurnal tides and the longer - period spring 
the highest modeled 
 doc , fthg , and fmehg concentrations for each time series ( figure 2b , e ) most often occurred just after the lowest 
 water of the ebb tide , near the lowest salinity portion of the tidal 
 cycle .
lower concentrations , the result of mixing with coastal waters , 
 were seen coincident with the higher relative salinities that accompanied 
 the flood tides . this difference in concentration between the ebbing 
 and flooding waters drove the majority of flux in shark river estuary .
material export from estuaries is driven by both net flow ( the 
 residual water flow after tidal exchange ) and tidal pumping ( net transport 
 resulting from different constituent concentrations in ebbing and 
 flooding waters ) .
for the periods measured , net water discharge from 
 the estuary accounted for less than 5% 
 of the estimated constituent flux in september and less than 1% in 
 april .
most shark river doc , fthg , and fmehg export was driven by 
 tidal pumping .
daily fluxes were always positive ( seaward ) , 
 except for a few periods 
 in the middle of the april deployment ( figure 2c , f )
. for september , estimated daily fluxes from the shark river 
 estuary averaged 9.1 ( 0.6 )  10 mg d for doc , 1.5 ( 0.2 )  10 ng d for fthg , and 1.7 ( 0.2 )  10 ng d for fmehg . in april , the average estimated total daily fluxes were 
 much smaller : 1.5 ( 0.02 )  10 mg d for doc , 1.7 ( 0.1 )  10 ng d for fthg , and 1.1 ( 0.04 )  10 ng d for fmehg . despite finding that all constituents were related to fdom , estimated flux through time was different for each 
 constituent ( figure 2 )
for example , between 
 september 25 and 27 , average daily fluxes were lower than those seen 
 during the first three days of the measurement period to a degree 
 that varied depending on the constituent : approximately 70% lower 
 for doc , 60% lower for fthg , and 35% lower for fmehg ( figure 2c ) .
although tidal pumping is the engine that 
 drives shark river doc 
 and hg exports , it appears that flux magnitudes are controlled by 
 hydrologic and geomorphic properties that affect material exchanges 
 between marsh / swamp surfaces and tidal channel waters .
tidal exchange volumes were similar 
 during the september and april deployments , but mercury fluxes were 
 significantly larger in the fall than in the spring ( 9 times higher 
 for fthg ; 17 times higher for fmehg ) .
water levels were also elevated 
 in september : median water height was 0.6 ft higher and median high 
 water was approximately 0.2 ft higher ( figure 2a , c ) .
the greater water heights in fall would have inundated more 
 mangrove - swamp surface area , thus making available more mangrove - derived 
 material for ebb transport to channel waters .
the effect of this enhanced 
 supply can be seen in the contrast between the modeled concentration 
 time series in september and april ( figure 2b , e ) . in april ,
after the tide reversed , a smooth decline 
 in concentration accompanied the incoming flood . in september ,
however , 
 modeled constituent values did not decline smoothly with the incoming 
 tide .
instead , high concentrations often persisted into the subsequent 
 flood stage , likely due to greater inundation during this period of 
 higher water height and slow draining , the result of ebb 
it appears from these 
 observations that even modest increases in regional sea level due 
 to storms , wind , or rising global sea level have the potential to 
 significantly alter constituent fluxes from mangroves .
following the heavy rains of september 27 ( figure 2a ) , for example , estimated total daily fluxes doubled 
 for doc and fthg and increased by 75% for fmehg ( figure 2c ) .
note that elevated modeled constituent concentrations 
 ( figure 2b ) did not occur until a full day 
 after the precipitation fell .
the heavy rains may have connected a 
 supra - tidal mangrove plain ( normally isolated from the shark river ) 
 to interior intertidal zones , supplying them with doc and hg .
the modeled concentrations and fluxes of all constituents remained 
 high ( figure 2c ) .
a similar lag effect in flux 
 response to precipitation is seen following the rainfall event of 
 april 4 ( figure 2d , f ) .
we sought 
 to compare our flux estimates to previously published values and to 
 assess whether exports from mangroves might represent a significant 
 flux of thg and mehg to coastal waters . to do this , it was necessary 
 to convert our flux values to yields ( fluxes per unit area ) and make 
 a series of assumptions to generate annual values .
we first converted 
 the doc , fthg , and fmehg daily fluxes to annual ones by assuming that 
 our deployment periods were representative of their respective seasons 
 ( wet and dry ) .
we then converted these to yields by normalizing the 
 fluxes to the area of influence ( see methods ) .
this calculation provided 
 an estimated annual doc yield for the shark river estuary of 180 ( 12.6 ) 
 g c m yr .
this value is somewhat 
 higher than the estimated annual doc yield reported by romigh et al .
( 56 g c m yr ) for a small mangrove subdrainage in the shark river estuary but 
 is well within the 44381 g c m yr range reported for mangrove swamps worldwide in a 
 review by bouillon et al .
the similarity 
 between our results and other published values indicates that our fdom - based methods provided doc yield estimates of a reasonable 
 magnitude .
the estimated annual yield for fthg was 28 ( 4.5 ) 
 g m yr : eight times 
 the highest previously reported wetlands value and 20 to 100 times 
 more than is generally reported for wetlands .
the estimated 
 yield for fmehg was 3.1 ( 0.4 ) g m yr : 10 to 100 times values previously reported 
 from terrestrial wetlands for fmehg and 5 to 100 times previously 
 published values from terrestrial wetlands for unfiltered mehg .
the estimated shark river fmehg 
 yields are more than five times those observed in the st .
mary s 
 river , a north florida blackwater river with a high doc concentration 
 and flux .
however , the mehg yields found 
 here are similar to those measured in a tule - dominated tidal marsh 
 in san francisco bay ( 2.5 mg m yr ) .
our estimates , in combination with 
 other recent studies of tidal wetlands , indicate that tidal 
 flushing of marshes and swamps represents a potentially large and 
 previously unrecognized source of thg and mehg to estuarine and coastal 
 aquatic ecosystems .
to assess whether fthg and fmehg exports 
 from mangroves to the 
 coastal ocean might be significant in comparison to other potential 
 sources , we multiplied the shark river yields derived above by the 
 1400 km of fringing mangrove swamps in southwest florida . 

this exploratory calculation resulted in estimated annual exports 
 in the range of 250 mt doc , 39 kg fthg , and 4.3 kg fmehg . to include 
 particulate fractions , we increased the mercury fluxes according to 
 the average ratios of particulate to filter - passing hg species found 
 in this ( si table s2 ) and other studies 
 for the everglades .
the resulting total 
 ( combined particulate and filtered ) hg exports were thereby found 
 to be approximately 55 kg for thg and 6.5 kg for mehg .
we caution 
 that these calculations are wholly heuristic in nature , made only 
 to scale up the magnitudes determined in this study for the purpose 
 of understanding the potential implications of our shark river findings 
 within the context of what is currently known about coastal mercury 
 supply in general .
longer time series  preferably multiyear 
 time series  and studies at multiple locations are needed to 
 adequately constrain fluxes at larger scales .
do fluxes of this 
 magnitude represent a significant source to the 
 coastal environment in comparison to other potential sources ? for 
 thg ,
the primary source to southwest florida coastal waters is currently 
 thought to be atmospheric deposition , with freshwater runoff accounting 
 for only a small fraction . flux from mangroves 

comparing the potential mangrove 
 flux to estimates of the combined wet and dry deposition of thg to 
 the 5000 km of adjacent coastal waters suggests that mangroves 
 could account for a loading equivalent to approximately 40% of atmospheric 
 thg deposition .
. found hg in waters exported from a mangrove marsh 
 to be more reactive with respect to methylation than was hg in ambient 
 estuary water .
for mehg , the dominant source to coastal waters 
 in southwest florida 
 is presently thought to be local production in coastal sediments and 
 diffusion into overlying waters ; the amount 
 discharged from uplands and that attributed to wet and dry atmospheric 
 deposition is small .
benthic flux 
 magnitudes of mehg from florida coastal sediments have not been reported , 
 but if magnitudes are similar to fluxes observed elsewhere ( 1 pmol 
 m d in chesapeake bay ; 30 pmol m d in san francisco bay ) , then potential 
 mehg loading from mangroves to the 5000 km coastal - ocean 
 area would be the equivalent of 2.5 to 20 times the flux from sediments , 
 more than 80% of the total flux .
this assessment , though preliminary 
 in nature , clearly establishes 
 that tidal mangrove systems have the potential to be a significant 
 source of thg and mehg to tropical coastal waters , in addition to 
 their previously demonstrated contributions of doc .
our findings , in conjunction with previously published 
 reports from tidal herbaceous wetlands , bolster the suggestion that 
 tidal pumping of doc and hg from tidal wetlands may represent an important 
 source to coastal environments .
additional studies are needed to quantify 
 fluxes from mangroves and other tidal systems and to resolve the fate 
 of the material once it enters the nearshore environment .
if 
 the large shark river fluxes found in this study are generally 
 representative of tidal mangrove systems , there may be global implications . 

recently reported particularly 
 high concentrations of hg in tissues and litter fall of mangroves , 
 suggesting that mangroves are particularly efficient at canopy capture 
 of atmospheric hg .
high rates of canopy capture in an environment 
 conducive to methylation combined with the high fluxes found here 
 would result in a system particularly efficient at extracting hg from 
 the atmosphere and transporting it to the coastal oceans as mehg , 
 potentially generating large coastal fluxes of mehg wherever mangroves 
 are found .
continuous in situ measurements of fdom can serve 
 as a powerful tool for investigating these doc , thg , and mehg mass 
 fluxes in many tidal systems .
such measurements are critically needed 
 to improve our understanding of hg cycling , advance predictive models 
 and budgets of carbon and hg in coastal areas , and inform hg total 
 maximum daily load regulation development .
further , high - resolution 
 time series of concentrations and fluxes provide valuable insights 
 into the myriad biological , physical , hydrologic , and geomorphic processes 
 affecting tidal exchange  insights that should in turn improve 
 estimates of carbon and mercury fluxes into coastal oceans .
 the flux of dissolved organic carbon ( doc ) from mangrove 
 swamps 
 accounts for 10% of the global terrestrial flux of doc to coastal 
 oceans . 
 recent findings of high concentrations of mercury ( hg ) and 
 methylmercury ( mehg ) in mangroves , in conjunction with the common 
 co - occurrence of doc and hg species , have raised concerns that mercury 
 fluxes may also be large . 
 we used a novel approach to estimate export 
 of doc , hg , and mehg to coastal waters from a mangrove - dominated estuary 
 in everglades national park ( florida , usa ) . using in situ measurements 
 of fluorescent dissolved organic matter as a proxy for doc , filtered 
 total hg , and filtered mehg , we estimated the doc yield to be 180 
 ( 12.6 ) g c m2 yr1 , which 
 is in the range of previously reported values . although hg and mehg 
 yields from tidal mangrove swamps have not been previously measured , 
 our estimated yields of hg species ( 28  4.5 g total hg 
 m2 yr1 and 3.1  0.4 g 
 methyl hg m2 yr1 ) were five 
 times greater than is typically reported for terrestrial wetlands . 
 
 these results indicate that in addition to the well documented contributions 
 of doc , tidally driven export from mangroves represents a significant 
 potential source of hg and mehg to nearby coastal waters . 
Introduction
Experimental Section
Results and Discussion