Advective pore water input of nutrients to the Satilla River Estuary ...

Advective pore water input of nutrients to theSatilla River Estuary, Georgia, USA

R.A. Jahnkea,*, C.R. Alexandera, J.E. Kostkab

aSkidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA 31411, USAbDepartment of Oceanography, Florida State University, Tallahassee, FL 32306, USA

Received in revised form 1 February 2002; accepted 1 February 2002

Abstract

In situ benthic flux measurements, pore water nutrient profiles, water column nutrient distributions, sediment grain size distri-butions and side-scan sonar observations suggest that advective transport of pore waters may be a major input pathway of nutrients

into the Satilla River Estuary (coastal Georgia, USA). In situ benthic chamber incubations demonstrate the occurrence of highlyvariable, but occasionally very large sea floor fluxes of silicate, phosphate, and ammonium. Locally occurring benthic microbial min-eralization of organic matter, as estimated by S35-sulphate reduction rate measurements, is insufficient to support these large fluxes.

We hypothesize that the observed interlayering of permeable, sandy sediments with fine-grained, organic-rich sediments in theestuary provides conduits for advective transport of pore water constituents out of the sediments. Because permeable layers mayextend significant distances beneath the salt marsh, the large fluxes observed may be supported by remineralization occurring over

large areas adjacent to the estuary. Advective transport may be induced by pressure gradients generated by a variety of processes,including landward recharge by meteoric or rain waters if sand layers extend far enough into the maritime coastal lands. Alterna-tively, tidal variations across the salt marsh sediment surface may hydraulically pump water through the sediment system. Becausethese fluxes appear to be concentrated into small layers, this source may be a significant input of nutrients to the estuary even if

permeable, sandy layers comprise a very small proportion of the seabed.� 2003 Elsevier Science B.V. All rights reserved.

Keywords: benthic; pore waters; nutrients; estuaries; sediment–water interface; sediments; Georgia coast

1. Introduction

Processes controlling nutrient levels and distributionsin estuarine environments must be understood to assessthe impacts of human activities and global climatechange on the chemical cycling and ecology of thesecoastal ecosystems. Such an assessment is a fundamentalfirst step to the development of effective managementstrategies.

The coupling between benthic and pelagic systems isa critical component of the nutrient cycling within es-tuarine systems (Nixon, 1981). In general, this couplingis thought to consist of (1) water column biological

production and subsequent deposition of organic materi-als at the seabed, (2) remineralization of the organic ma-terials with the release of inorganic nutrients to the porewaters and (3) transport of dissolved nutrients back intothe overlying water column. Molecular diffusion pro-cesses augmented locally by irrigation by benthic organ-isms are generally thought to be the primary pore watersolute transport mechanisms. Because the nutrients flux-ing back into the water column are derived primarilyfrom fresh particulate materials deposited at the sedi-ment surface, this flux is closely coupled to remineraliza-tion occurring in the upper few tens of centimetres of thesediment column. Thus, there is a close, local couplingbetween deposition, remineralization and benthic flux.

There is growing evidence for advective input ofnutrients to coastal systems from primary ground waterefflux or sea water that is advectively circulated over large

* Corresponding author.

E-mail address: [email protected] (R.A. Jahnke).

Estuarine, Coastal and Shelf Science 56 (2003) 641–653

0272-7714/03/$ - see front matter � 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0272-7714(02)00216-0

distances through the bottom sediments (Moore, 1996;Simmons, 1992). Primary ground water inputs may betied to shallow aquifers that may be recharged throughatmospheric and surface water inputs. Such systemsmay transport materials over great distances (hundredsof kilometres) and provide a direct conduit betweencoastal systems and terrestrial sources (Gallagher, Die-trich, Reay, Hayes, & Simmons, 1996; McClelland &Valiela, 1998). Advective pore water transport may alsobe provided by sea water that is recirculated through saltmarsh and other permeable coastal sediments. Theserecirculation processes may transport fluids over cen-timetre distances that occur between sea floor sand rip-ples to kilometre distances given the right conditionsand sedimentological formations (Jahnke, Nelson,Marinelli, & Eckman, 2000; Simmons, 1992; Whiting &Childers, 1989). Such transports increase the role ofsea floor processes in controlling nutrient levels inestuarine and coastal water columns. Regardless ofthe characteristics and controlling processes, advectivetransport may significantly enhance the magnitude ofsolute and particulate exchange between the seabedand overlying water column. Mixing of salt and fresh-waters within the sediment matrix may also enhancediagenetic reactions (Moore, 1999). Because reactionproducts can be transported large distances, advectivepore water inputs are relatively decoupled from thelocal deposition and remineralization processes and can

channel the products of diagenetic reactions throughrelatively narrow pathways.

Here we present in situ benthic flux chamber, porewater, sediment incubation and side-scan sonar resultsfrom the Satilla River Estuary. These results indicatethe occurrence of advective pore water nutrient inputsto the estuary that may be an important component con-tributing to nutrient levels within the estuarine waters.Because these inputs are controlled by advective trans-port, they may be influenced by processes and humanactivities occurring at significant distances from theestuary itself.

1.1. Study location

The Satilla River Estuary is located on the Georgiacoast in the southeastern US (Fig. 1). The estuaryreaches through extensive salt marsh and maritimeforest from the head of tide mark about 80 km upriverto St Andrews Sound, while the typical extent of saltwater intrusion is just upstream from our deploymentno. 10. Salinities across this region range from nearzero to the west of station 10 to approximately 32 inSt Andrews Sound. No significant freshwater input isprovided by White Oak Creek or other dead end tidalcreeks. Thus, tributaries within the Satilla Estuary arenot significant point sources for nutrients to the main

Fig. 1. Study locations within the Satilla River Estuary.

642 R.A. Jahnke et al. / Estuarine, Coastal and Shelf Science 56 (2003) 641–653

channel. The surrounding areas are relatively pristinewith no significant population centres or industrial com-plexes within the estuary. Like other coastal plain rivers,the Satilla has a relatively low flow rate, averaging85m3 s�1, which is lowest in February and March andmaximum in October.

2. Methods

Expeditions were conducted in September 1998 andMarch 1999. River water samples were obtained primar-ily by a shipboard pumping system that also simultane-ously measured temperature, conductivity, fluorescenceand recorded time and location information. Oncecollected, individual samples were immediately filteredthrough 0.4 lm pore diameter polysulfone filters andstored in the dark at 4 �C. One transect from thefreshwater endmember to the marine endmember wasperformed in September and two transects were per-formed in March. Transit and sampling required severalhours for each transect, so a range of tidal stages wereencountered during each sampling transect.

Sediment cores were recovered with two types of boxcorers, depending upon whether sandy or muddy sedi-ments were encountered. At most locations, muddy sedi-ments were recovered with a traditional, single-spade20� 30 cm box corer. Subcores of various diameterswere removed from the box corer with intact overlyingwaters and processed as described later. At stationswhere the bottom sediments were composed primarilyof sand, we used a smaller box corer fitted with a7 cm-diameter core barrel that was equipped with a spe-cial ball-type check valve to minimize pore water wash-out. This corer was described in detail by Marinelli,Jahnke, Craven, Nelson, and Eckman (1998).

Pore waters were extracted from the muddy sites bycentrifugation. The sediment cores were sectioned into1 or 2 cm intervals in an N2 atmosphere in a glove bagat in situ temperature and packed into 50 cm3 centrifugetubes. The tubes were centrifuged for approximately5min at 11 000 rpm in an Eppendorf� 5904 centrifuge.The sealed rotor was placed back in the glove bag.The centrifuge tubes were then opened, pore watersremoved into plastic syringes, filtered through 0.4 lmpore diameter filters into polypropylene vials and storedin the dark at 4 �C until analysis.

At sandy sites, cores were recovered using the corerand methods described by Marinelli et al. (1998). Thecore barrels that were used on this corer have septae in-stalled at centimetre intervals in the barrel walls. Oncethe sediment core was recovered, syringes with 18 gaugeneedles attached were used to penetrate the septae andremove the pore water samples. Sample withdrawal be-gan at the shallowest depth and proceeded sequentiallyto the bottom of the core. Sample volume was typically

�3ml cm�1 depth. Downward percolation of porewaters to replace the volume withdrawn would displacepore water gradients by approximately 1.5mm, a rela-tively insignificant distance considering the relativelycoarse (1 cm) resolution of our pore water samples. Be-cause samples were withdrawn sequentially from the topto the bottom of the core, this displacement due to per-colation is not cumulative. Collected samples were fil-tered through 0.4 lm pore diameter filters and storedin the dark at 4 �C in polypropylene vials. While thesemethods work well for each type of sediment, they areexclusive of each other and sampling cores that con-tained both muds and sands in discrete layers poseda special problem. In these cases, only small amountsof pore waters were recovered and only selected analysescould be performed.

Sulphate reduction rates were determined on dupli-cate intact sediment cores utilizing the 35S–SO4

2� in-cubation method (Jorgensen, 1978). Intact sedimentcores were injected with the radiotracer at centimetre in-tervals and incubated at in situ temperature. The incu-bation was terminated by sectioning the cores andfixing the sediments with 20% zinc acetate. In the labo-ratory, the reduced 35S was recovered by distillationwith boiling acidic Cr2+ solution (Canfield, Raiswell,Westrich, Reaves, & Berner, 1986; Fossing & Jorgensen,1989; Zhabina & Volkov, 1978). Sulphate concentra-tions were determined on pore waters by the methodof Tabatabi (1974).

In situ benthic flux chamber incubations were per-formed with an automated system as described byJahnke and Jahnke (2000) at the locations, times andwater depths listed in Table 1. This system, originallydesigned for deployment by manned submersible, con-sists of a single, 30 cm diameter PVC chamber. Mountedon the instrument frame around the chamber are twopressure cases that house the controlling electronic cir-cuit boards and battery, an impeller pump used to stirthe chamber waters, a pressurized hydraulic reservoirused to close the chamber lid, a rack containing eightspring-loaded sampling syringes and one spring-loadedinjection syringe for injecting an inert tracer at thebeginning of the incubation. Details of the sampler de-sign are provided by Jahnke and Christiansen (1989).The impeller pump was connected to two nozzles whichgently directed the expelled waters across the centre ofthe chamber and counterclockwise within the chamber.This provided gentle mixing while minimizing pressuregradients that would enhance pore water exchange. Thisis discussed further in the interpretation of the Br�

tracer results.The instrument was slowly lowered to the river bot-

tom from a small boat on a line that was secured to asurface float. After a preset time interval, the controllingelectronics actuated the hydraulic system, closing thechamber lid and beginning the incubation. The first

643R.A. Jahnke et al. / Estuarine, Coastal and Shelf Science 56 (2003) 641–653

operation was to inject the tracer (0.3M NaBr) into thechamber waters so that the chamber volume could becalculated and the chamber lid seal could be verified.Eight samples were then sequentially withdrawn at apreset time interval. At the end of the incubation period,the instrument was recovered and the samples removedfor processing.

After the chamber was back aboard, the individualsample syringes were removed and the contents wereimmediately filtered through a 0.4 lm pore diameterpolysulfone filter and stored at 4 �C in the dark inpolypropylene vials. In the laboratory, the chambersamples were analyzed for pH (glass electrode), NH4

+

(modified from Koroleff, 1976), NO3�/NO2

� (using anAlpkem RFA 300 analyser and published methods),PO4

3�, Si(OH)4 (Strickland & Parsons, 1972), titrationalkalinity (Gran titration), Cl� (AgNO3 colourmetrictitration using a LabConco digital chloridometer), andBr� (Morris & Riley, 1966). Pore waters were analyzedfor all constituents named above except NO3

�/NO2�

and Br�. Pore waters were acidified with HCl to a pHof approximately two prior to analysis for PO4

3�.The lower 45 km of the river’s course were imaged

using side-scan sonar to identify sedimentary environ-ments based on acoustic character. Profiles were col-lected using a Klein system 2000 digital towfish anddata were stored/processed using a Triton-Elics ISISsonar processing unit. Side-scan data were typicallycollected using a 150-m range scale, yielding an effectiveswath width of 300m, which was adequate to image thewhole channel in one or two passes. Individual lineswere mosaiced using DelphMap software to produce acomposite image of the riverbed.

3. Results

Grain size analyses and side-scan sonar data show avariety of sediment types and sedimentary environmentsalong the river course (Figs. 2 and 3). Bottom surveys ofsediment grain size indicate that the river bottom is veryheterogeneous, with surface sediments ranging fromfine-grained muds to medium sands. In the upperreaches of the river (Crow Harbor Reach and up-stream), channels exhibit sandy bedforms migrating overmuddy sediments or muddy sediments exposed at theriverbed (Fig. 3). Below Crow Harbor Reach, channelsediments become more sand-dominated until sand dom-inates in St Andrews Sound. All along the river’scourse, its banks (i.e. the salt marsh/channel interface)

Table 1

In situ benthic flux chamber deployment and sediment core summary

Date Station ID BECI Latitude N Longitude W Deploy time Tide Salinity Water depth (m)

29 September 1998 1 30�59.6a 81�37.8a 22:00 Low 23:30 2 6

30 September 1998 2 30�59.6a 81�37.8a 13:00 Low 11:15 2 6

2 March 1999 3 31�00.02 81�26.79 09:03 High 09:00 27 8

2 March 1999 4 31�00.00 81�26.72 13:20 Low 15:00 22 5

2 March 1999 5 30�58.06 81�30.76 17:46 High 21:00 2 7

3 March 1999 6 30�58.13 81�30.15 11:16 High 10:00 26 6

3 March 1999 7 30�59.64 81�37.77 16:00 Low 16:30 2–5 4

4 March 1999 8 30�59.62 81�37.78 08:29 High 10:30 2 5

4 March 1999 9 30�59.59 81�37.74 17:15 Low 17:30 <2 3

5 March 1999 10 30�59.13 81�38.77 12:13 High 13:00 <2 7

Box core Recovery time

29 September 1998 SAS1 30�59.0a 81�28.0a 12:00 Low 10:45 b b

29 September 1998 SAS2 30�59.0a 31�28.0a 13:45 High 16:15

29 September 1998 CHR1 30�59.7a 31�38.0a 17:00 High 16:15

30 September 1998 CHR2 30�59.7a 31�38.0a 14:00 Low 11:30

2 March 1999 SAS3 30�58.14 51�30.18 09:40 High 08:32

2 March 1999 SAS4 31�00.00 51�26.87 10:50 High 08:32

4 March 1999 CHR3 30�59.66 51�36.70 09:05 High 10:18

5 March 1999 CHR4 30�59.67 81�36.72 08:45 High 10:55

a Accuracy of locational equipment employed was less for September 1998 than for March 1999.b Bottom water salinity and exact water depth not determined for box core collections.

Fig. 2. Grain size distribution of Satilla River Estuary sediments.


are characterized by broad, gently dipping ramps orby steep erosional scarp faces. In both cases, numeroussubaqueous, bank-parallel lineations are commonly ob-served. A comprehensive description of the Satilla Riversedimentary environment is outside the scope of thispaper; complete details will appear in a later publica-tion (Alexander, in preparation).

Estuarine water nutrient concentrations from boththe September and March expeditions are plottedagainst salinity (Fig. 4). The results display the complexbiogeochemical cycling of nutrients that occurs withinthis active estuary. Note that sampling and transit forthe 20+ stations in each transect required several hoursand therefore covered a range of tidal stages. Since thewater locations of a particular salinity vary with thetidal cycle, nutrients are plotted against salinity, notlocation. In September, (Fig. 4 upper panels) NH4

+ dis-plays a dramatic removal zone in the salinity range of5–15 with a pronounced production zone at the highersalinities, roughly 20–30. Although not as dramatic, thesame features are observed in the PO4

3� results with evi-dence for removal at low salinities and release at highersalinities.

A very different pattern is observed for Si(OH)4 andNO3

�/NO2�. Silicate displays a consistent decrease

from the high concentration river endmember to lowconcentration sea water endmember. There is also evi-dence of NO3

� and NO2� release throughout the mixing

zone. Nitrate exhibits a large maximum of over 20 lMcentred at a salinity of approximately 10. Nitrite exhibitsa broad maximum with a highest value of approximately

5 lM at a salinity of nearly 25. This very large and broadmaximum in NO3

� suggests that there is a strong sourceof NO3

� throughout the estuary. The oxidation ofNH4

+ that is already in the water column cannot bethe sole source of this NO3

� because NO3� concentra-

tions greatly exceed the NH4+ concentrations. More

likely, the NO3� production is supported by the oxi-

dation of NH4+ that is ultimately derived from the

sediment pore waters.March estuarine transects (Fig. 4 lower panels) dis-

play similar patterns but for some nutrients, the ab-solute variations are less, most likely due to reducedmetabolic rates because of lower water temperatures(September: 27–29 �C, March: 15 �C). During March,PO4

3� and NH4+ distributions tend to be devoid of

prominent features and concentrations are generallylow throughout the estuary. Silicate concentrations againshow a dramatic decrease from river water values ofnear 150 lM to less than 50 lM at a salinity of 25.Nitrate again displays non-conservative behaviour witha marked production region at salinities less than 10.However, overall NO3

� concentrations in March aresignificantly reduced from the warmer, September val-ues. Note that NO2

� was not individually measuredon the March samples and values represent the sumof NO3

� plus NO2�.

In addition to the concentration trends observedwithin the estuarine mixing zone, it is important to notethe absolute values of the concentrations as well.Ammonium and PO4

3� concentrations are always lessthan 4 and 1.5 lM, respectively. Silicate concentrations

Fig. 3. Side-scan sonar trace of Crow Harbor Reach river bottom.


range from approximately 150 lM at the river endmem-ber to 10–20 lM in St Andrews Sound. Nitrate displaysthe greatest seasonal variation—ranging from approxi-mately 22 to 0 lM in September and only 4–0 lM inMarch.

Pore water results from Crow Harbor Reach andSt Andrews Sound are provided in Fig. 5. The CrowHarbor Reach sampling location is near the fresh waterendmember of the sampling area; water column salin-ities of 1–3 can be observed at low tide. Pore watersalinities vary with season and presumably river dis-charge. During the September expedition, pore watersalinities ranged from 5 to nearly 15 while in March,values ranged from approximately 2 to 6.

Nutrient concentrations increase dramatically in thepore waters at sediment depths below 10–20 cm. ForPO4

3� and NH4+, these concentrations are 2–3 orders

of magnitude greater than observed in the water columnthroughout the estuary. For Si(OH)4, the maximumpore water concentration ranges from approximatelyfive times the fresh water endmember concentration tonearly 100 times the sea water concentration.

The shapes of the nutrient profiles suggest that theupper 10 cm of the pore waters exchange more rapidlywith the overlying estuarine waters than do deeper porewaters. This is apparent in the PO4

3� profiles (most pro-nounced at CHR1 and CHR3) which exhibit low valuesin the upper 7 cm. It has been previously noted thatPO4

3� is a particularly sensitive indicator of enhancedexchange between pore waters and oxygenated overlyingwaters. This is because the injection of oxygenated over-lying waters not only dilutes the pore water PO4

3�, butalso tends to stimulate the production of iron oxy-hydroxides which have a strong affinity of PO4

3� andreduce pore water concentrations further by adsorptiveremoval. Phosphate for CHR2 pore waters is complex.The core was comprised of discrete sand and mudlayers, and was collected as for muddy sediments andsampled by sectioning. While the layers that were domi-nated by sands could not be sampled, the intermixedsandy/muddy layers immediately adjacent to the sandylayers display very low PO4

3� concentrations (Fig. 5a).Ammonium and Si(OH)4 pore water concentrations

for CHR are also lower in the upper few centimetres,consistent with enhanced pore water exchange. Theeffect is not as dramatic here as for PO4

3� because theeffect is primarily dilution, with little if any loss tothe solid phase. In the case of Si(OH)4, the low tide,low salinity water column concentration approachesthe surface pore water concentration, minimizing soluteexchange.

Pore water nutrient concentrations from the muddysediments from St Andrews Sound (SAS 1 and 3, Fig.5a and b) also exhibit dramatic increases with sedi-ment depth. In contrast, significantly lower pore waternutrient concentrations are measured at the sandy SASsites (SAS 2 and 4). In addition to overall lower concen-trations, the profiles from the sands are characterizedby an upward curvature. Such curvature may be the re-sult of uptake within the upper 10 cm or, more likely,enhanced transport and exchange with bottom waters.The latter could result from irrigation activities bymacrobenthic organisms but because these are sandy,permeable sediments, it is more likely that bottomcurrent-induced advective exchange is responsible(Huttel & Gust, 1992; Marinelli et al., 1998). Note thatSAS4 is interlayered. It was collected as a sand core in abarrel with septae, which does not allow for samplingfrom muddy sediment interlayers. Below 15 cm, the corewas entirely mud.

Sulphate reduction rate profiles for St Andrews Soundand Crow Harbor Reach locations are displayed in Fig.6. In St Andrews Sound, values increase with sediment

Fig. 4. Nutrient distributions vs. salinity within the estuary. For

September, nitrate, open squares; nitrite, filled squares. For March,

open and filled symbols represent results from two separate sampling

transects for the indicated nutrient. Nitrate and nitrite were not mea-

sured separately for March samples.


depth from approximately 200 nmol cm�3 d�1 to a dis-tinct subsurface maximum of approximately 600 nmolcm�3 d�1 at 7 cm. Below this depth, values decrease rap-idly to very low rates at 20 cm. Pore water SO4

�2 con-centrations range from approximately 24 to 18mMindicating that SO4

�2 availability never limits SO42�

reduction rates at this site. The lower values observednear the surface may be the result of significant oxicremineralization processes in the near surface sediments.

In general, SO42� reduction rates are lower at the

Crow Harbor Reach site. Measured rates in the upper5 cm range from approximately 50 to 150 nmol cm�3 d�1.

Variations among the individual points are sufficientthat identification of a subsurface maximum is equivo-cal. Below 7 cm, rates again decrease to very low levelsat 20 cm. Measured pore water SO4

2� concentrationsrange from 8 to 11mM. This range is consistent withthe salinities displayed in Fig. 5.

Examples of in situ benthic flux chamber results areprovided in Figs. 7 and 8. In general, the results can bedivided into two groups; group 1 deployments are thosein which concentrations increased dramatically, con-sistent with enhanced rates of solute exchange (Fig. 7).In the group 2 deployments, concentrations changed

Fig. 5. Pore water distributions of selected nutrients and salinity. (a) Pore water results for September 1998 for muddy (CHR1) and interlayered

(CHR2) Crow Harbor Reach sediment cores and muddy (SAS1) and sandy (SAS2) St Andrews Sound cores. (b) Pore water results for March 1999

for muddy (CHR3, 4) Crow Harbor Reach sediment cores and muddy (SAS3) and interlayered (SAS4) St Andrews Sound sediments.


only a little, if at all, which is consistent with moleculardiffusive solute exchange rates (Fig. 8) for the durationof these deployments.

The results in Fig. 7 display the dramatic changes inchamber water concentrations that occurred for thegroup 1 deployments during the relatively short l.7 hincubation time. Ammonium concentrations increasefrom near zero to over 500 lM; Si(OH)4 concentrationchanges from water column values of approximately100 to over 500 lM and PO4

3� increases from near zeroto between 50 and 150 lM. Coinciding with these dra-matic changes were more subtle changes in NO3

�, sa-linity and Br� tracer concentration. Nitrate is observedto decrease from approximately 12 to 4 lM, salinity in-creases from approximately three to eight while the ex-cess concentration (concentration above that expected

for sea water at the measured salinity) of the Br� tracerdecreases from around 3.5mM to less than one. All ofthese results are from the Crow Harbor Reach region.

In contrast, the group 2 deployments display little orno significant concentration change in the chamberwaters during the incubation (Fig. 8). In St AndrewsSound (BECI 3), consistent increases in PO4

3�, NH4+

and Si(OH)4 are observed during the incubation. How-ever, instead of increasing by hundreds of micromoles,observed increases are limited to a few micromoles forPO4

3� and 20–25 lmol for NH4+ and Si(OH)4. For

BECI 6, which was deployed at the entrance of themain estuarine channel and BECI 8, which was de-ployed in Crow Harbor Reach, no significant concentra-tion change is observed for any constituent over the 2 hincubation period.

Fig. 5. (continued )


It is important to note that the Br� concentrationchanges observed in the first portion of the incubationsare related to the strength of chamber mixing and thebuoyancy of the spike injected. That is, while every ef-fort was made to match the density of the spike solutionto that of the surrounding bottom waters, tidal changescontinually acted to shift the salinity field within the es-tuary, making it difficult to match density exactly at thestart of the incubation. The density of the spike at BECI3 was slightly less than the surrounding waters. Thiscaused the spike to initially �float� at the top of the cham-ber until it was fully mixed into the chamber waters.Since the samples are removed through a tube extend-ing into the upper portion of the chamber, the first fewsamples are, therefore, elevated in Br� concentration.At BECI 6 the densities were reasonably matched, fullmixing was quickly achieved and a relatively constantconcentration is observed. At BECI 8, the spike washeavier than the chamber waters and rested on thesediment surface at the bottom of the chamber untilmixing finally homogenized the chamber waters.

Two important points can be drawn from the tracerresults. First, mixing of the chamber waters is sufficientto homogenize the chamber waters as indicated by thenearly constant chamber concentrations measured atleast by the end of each incubation. Second, a very gen-tle mixing process was achieved; small variations in den-sity delayed the full mixing of the chamber waters. Asdisplayed in Fig. 5, significant gradients in salinity andhence density, were measured in the sedimentary porewaters. Given the tracer results, it is very unlikely that

the chamber stirring was vigorous enough to signifi-cantly enhance pore water exchange even at sites com-prised of highly permeable, sandy sediments.

What is important about the group 2 results is thatdespite the unsteady initial values, there is no evidenceto suggest rapid loss of the spike as was observed withthe first group of results (Fig. 7). Furthermore, salinitiesare constant throughout the deployment, which is alsoin contrast to the increase in salinity observed in thegroup 1 results.

4. Discussion

We hypothesize that the individual results presentedabove can be explained by localized, advective transportof deep pore waters into the estuarine channel and thatthis process exerts a significant influence on estuarinenutrient distributions. We will start by focusing on thegroup 1 chamber results. In these incubations, chamberwater nutrient concentrations increased from low riverwater values to >100 lM while the Br� tracer decreasedto less than 1/4 of the starting concentration. Some lossof the tracer is expected due to diffusion of the tracerinto the sediment pore waters and dilution of tracer byreplacement waters that are drawn into the chamber aseach sample is removed. These processes should causethe tracer concentration to decrease by about 6% duringthese 2 h incubations.

Normally, a dramatic decrease in the chamber watertracer like that observed here would indicate that the

Fig. 6. Sulphate reduction rate measurements (closed circles) and sulphate concentrations (open circles) from St Andrews Sound and Crow Harbor

Reach in March 1999. Error bars are �1 SD for two replicate core incubations.


chamber lid had not sealed and that the tracer wasexchanging with the surrounding waters. This explana-tion is not consistent with the nutrient results, however.River water PO4

3�and NH4+ concentrations are gen-

erally less than 2 lM. If the loss of the tracer was ex-plained by enhanced exchange between chamber watersand river waters, nutrient increases in the chamberwould be retarded by this process. To cause large posi-tive changes in nutrient concentrations in the presenceof significant leakage between the river and chamberwaters would require a sea floor flux much larger thanimplied by the concentration increase. As we will dem-onstrate later, measured remineralization rates in thesurface sediments cannot support the required fluxesand this possible explanation is rejected.

Another possible explanation for these results wouldbe greatly accelerated exchange between the pore watersand chamber waters. This would increase the flux ofnutrients into the chamber and dilute the tracer byexchange with the pore waters. However, the magnitudeof the exchange required strongly suggests that this isalso not the explanation for the observed results. Giventhe chamber heights (8.9 and 12.1 cm for deployment1 and 2, respectively), to dilute the tracer by a factor

of 4 would require complete mixing between pore watersand chamber waters for the upper 39–51 cm of the sedi-ment column assuming a sediment porosity of 0.7. Thishomogenization would need to be completed within the2 h deployment time. Pore water exchange this rapid isextremely unlikely.

In addition, the above explanation would require thatthe nutrients released to the overlying chamber watersbe replaced by remineralization occurring locally, withinthese surface sediments. When compared to the mea-sured sulphate reduction rates, this is unlikely. For ex-ample, NH4

+ fluxes estimated for group 1 range from0.32 to 0.87molm�2 d�1. If this flux is supported locallyby the remineralization of Redfield ratio organic materi-als (C :N¼ 106 : 16), this flux would imply a reminerali-zation rate of 2.1–5.8molCm�2 d�1. The integratedSO4

2� reduction for the upper 20 cm from the resultsdisplayed in Fig. 4(b) is 0.0073mol SO4

2�m�2 d�1. As-suming a ratio of SO4

2� reduced to C oxidized of 1 : 2,this integrated rate suggests that 0.015molCm�2 d�1

of carbon is oxidized by SO42� reducers. This is more

than two orders of magnitude less than that estimatedto be consistent with the rate of NH4

+ release. Althoughother oxidants, such as O2, MnO2 and FeOOH, may

Fig. 7. Benthic flux chamber results from deployments 1 and 2 in Crow Harbor Reach from September 1998. Nitrate or nitrite graphs: �, nitrite;

d, nitrate.


also contribute to the total organic matter remineraliza-tion rate in these sediments, SO4

2� reduction is expectedto dominate and it is extremely unlikely that the occur-rence of other oxidation pathways can explain theseresults. Note that the disparity of the results increasesif the C :N ratio is greater than Redfield as might beexpected if terrestrial and/or salt marsh vegetation con-tributes significantly to sediment remineralization. Thus,it is clear that the group 1 chamber incubation resultsare not consistent with leaky chambers or enhanced porewater transport supported by local diagenetic processes.

The most straightforward explanation for the ob-served results is the outward, advective transport of porewaters and solutes. Assuming the chambers remain wellmixed throughout the incubation, an outward flow of18–24 cm3 cm�2 h�1 of pore water could simultaneouslyexplain the loss of tracer and the dramatic build-up ofPO4

3�, NH4+ and Si(OH)4 within the chambers. An

influx of pore waters at this rate would also be consis-tent with the decrease in NO3

� and increase in salinityobserved.

Our working hypothesis is that permeable, sandysediment layers underlie the fine-grained, organic-richsediments that characterize the surface of the salt marshas schematically shown in Fig. 9. Pressure gradientsdevelop that induce the flow of water along these per-meable layers out into the river. A variety of processescould contribute to these pressure gradients. The sandlayers may extend far enough into the maritime coastallands to be recharged with rain waters or meteoricwaters. Because sea salt may be entrained along thispath, the escaping fluids may be essentially of sea water

salinity while the forcing pressure may still be caused byfresh water inputs.

Alternatively, tidal variations across the salt marshsediment surface may actively pump water through thesediment system. That is, at high tide, water fills allof the sediment pore spaces and organism burrows.As the water level drops with the ebbing tide, water isretained in these cavities, eventually becoming severalmetres above the water level at low tide in the estuarychannel. This difference in water level height producesa hydraulic pressure gradient. If the sand layers extendover significant distances, even a very small migrationof fluids through the relatively impermeable salt marshsurface sediments would result in a significant flow.

Bottom surveys of sediment grain size (Fig. 2) indicatethat the river bottom is very heterogeneous, with surfacesediments ranging from fine-grained silts and muds tocoarse-grained sands. Further evidence of layering with-in the salt marsh deposits is provided by side-scan imagesof the river bottom (Fig. 3). On these images, sandy bedmaterials show up as lighter-coloured areas and the steepbanks of the channel, cut into the salt marsh deposits,show a black character, indicative of a strong return ofacoustic energy. The steep banks of the river channels ex-hibit white and black banding, which represents a ledge-like structure to the banks themselves and indicateseither sandy–muddy interbeds within the marsh orpartings between depositional units in the muddy saltmarsh deposits. As portrayed on this image from CrowHarbor Reach, these subaqueous ledges are ubiquitousthroughout the study area. Assuming that this layeredstructure extends from the river banks back into/under

Fig. 8. Benthic flux chamber results from deployments 3, 6 and 8 from March 1999. No nitrate flux was observed.


the marsh, the side-scan sonar imagery provides evidenceof a conduit for groundwater inflow.

This process may be extremely important in control-ling the nutrient levels of the estuary because the flux outat the river bottom may represent the integrated re-mineralization occurring over a wide area of the saltmarsh. This may explain why the estimated fluxesgreatly exceed those that could be supported locally asestimated from the measured SO4

2� reduction rate.Furthermore, this hypothesis may provide one ex-

planation for why evidence for large advective flows isnot observed in all of the benthic flux chamber measure-ments. The flow is expected to be restricted to relativelysmall areas where permeable, sandy layers outcrop intothe river channel. The thickness of these layers is un-known but they may only be a few centimetres to afew tens of centimetres thick. It would be very difficultto routinely deploy a chamber directly on such a layer.

Without knowing the distribution of the advectivelayers within the river channel, it is not possible to di-rectly estimate the importance of this process in control-ling nutrient concentrations in the estuarine waters. Toset rough limits, we have developed a simple steady-stateadvection–diffusion–reaction model of the estuary. Themodel does not include the effects of stratification or in-undation of the salt marsh at high tide and is presentedfor illustrative purposes only. The average discharge ofthe river is reported to be 85m3 s�1. At an approximateaverage depth of 10m and channel width of 600m(at least for the critical Crow Harbor Reach region),this transport corresponds to a flow velocity of roughly50mh�1. Using this advection velocity, the best fit ofthe salinity field was achieved with an eddy diffusivityof 5� 105m2 h�1.

The mean fixed N flux for the group 1 deployments is0.5molm�2 d�1. If we assume that advective inputs arerestricted to the region between 0 and 12 salinity andthat there are no other inputs or losses of fixed N, theresulting distribution can be estimated as a function ofseabed area that exhibits the measured fluxes (Fig. 10).The results demonstrate the potential importance of ad-vective nutrient inputs. If only 0.05% (500m2 km�2) ofthe estuary sea floor exhibits fluxes of the magnitudemeasured in the group 1 stations, a broad nitratemaximum of the magnitude displayed in Fig. 2(a) iscalculated. This calculation demonstrates that this pro-cess has the potential to significantly control nutrient

Fig. 9. Schematic of hydrostatic advective pore water transport model.

Fig. 10. Numerical simulation of nitrate vs. salinity in the Satilla Es-

tuary. In addition to advective and diffusive processes that would con-

servatively mix the riverine and sea water endmembers, fixed nitrogen

is supplied from the seabed in the 0–12 salinity region at the rate mea-

sured for the group 1 stations. Different model runs demonstrate

the sensitivity of the simulation to variations in the proportion of the

seabed exhibiting these fluxes. Legend:�, 0.10%;d, 0.05%; D, 0.01%.


concentrations in this estuary. Additionally, because theflux may be derived from such a small portion of thetotal seabed, this calculation qualitatively supports ourconclusion that subsequent deployments may havesimply missed the locations of advective transport.

In summary, the combination of benthic flux mea-surements, pore water nutrient profiles, water columntransect nutrient distributions, sediment grain size distri-butions and side-scan sonar observations support thepremise that advective transport of pore waters throughpermeable sand layers may be an important source ofnutrients to the Satilla River Estuary. Locally occur-ring benthic microbial mineralization measured viaS35–SO4

�2 reduction rate incubations is insufficient togenerate the nutrient fluxes observed in 20% of thedeployments reported here. A simple steady-state ad-vection–diffusion model based on the measured benthicammonium fluxes and water column salinity distribu-tions suggests that the observed water column NO3

�

concentrations could be maintained if only 0.05% ofthe estuary seabed consists of permeable sands whichexhibit advective exchanges of the magnitude observedfor the group 1 deployments.

Acknowledgements

We thank the captain and crew of the RV Blue Fin forlogistical support on the Satilla expeditions. D. Dalton,G. Nickles, D. Jahnke and S. Gentzler assisted in ship-board and/or laboratory analyses. Research was sup-ported in part by NOAA/Georgia SeaGrant and inpart by NSF grant OCE-9906897 (RAY). Grain sizeanalysis results are provided courtesy of LMER NSFDEB-9412089 (CRA). The comments of two anony-mous reviewers were also helpful.

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