Patterns and Controls of Nutrient Concentrations in a Southeastern United States Tidal Creek

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INTRODUCTION
More than a bdlum people live within 100 km << the "Cean.and that number is constantly increasing (Small and Xicholls.2(Xt3), Many ol these individuals rely on coastal and marine resources tor their livelihoods and/or sustenance.
At the same time, human pupulatmns increase nitmgen and pliosph<n as inputs to the coastal ocean through sewage elllueiit, agricultural lunoif, and industrial activities (I lowarth, 20i)S).Ihese activities culminate in over-fertilization oi coastal waters, which harmscoastal resources such as fisheries through species shifts, oxygen depletion, and harmtul algal bl<mms (Nixon, ['•)

'■>'■ •).
Processing and removal of nutrients at the land-ocean intertace (Nixon et al..  Bryson, 1933;Imbergeret al., 1983).Therefore) marsh influence on Duplm chemistry should be much higher on spring than on neap tides.Marshes require deposition ol particulates to keep pace with sea level rise (Stumpi, 1983).and marshes in the Duplin watershed nuked act as a sink ol particulates (Chalmersci al.. 1985), consistent with our model analysis.
Tidal marshes vary widely in theii source/sink behavior with respect to OOMiChildeis.19^4).DOM concentrations in the upper I Hiplm were higher on spring than on neap tides, particularly for DOC", and the watershed was a net produce! o| these materials.I hese patterns weie not the result of marsh inundation, however, as 1 Hiplin marshes are not net exporters of DOC over tidal time scales (Chafanersel al., 1985).' 1 his suggests that benthic primary production supplies DO(' to I Hiplin surface waters (Porubsky, 2IHI8) as diatoms .irescoured Irom the creek bed by higher amplitude, higher energy spring tides Clhorensen, 2<H1-1).

SYNTHESIS
Exploration ot tidal creek-estuai me exchange in and around the Duplin watershed gave rise to the "Outwelling Hypothesis" (Odum, 1968).which states that the coastal ocean is supplied with nutrients and organic material through outwelling from marsh/estuarine systems.Since that time, extensive work has been done to determine how salt marsh systems process inorganic nutrients, organic matter, and particulate materials { ("hilders, 1994).Our model results demonstrate that within-watershed processes are important in structuring surface water chemistry and export to the adjacent estuary.I hese processes included groundwater discharge, exchange ot creek water with the marsh platform, and benthic primary production (Figure 3).Fach process influenced each group of materials in a dillerent way, and their relative strengths varied over seasonal and tidal time scales.
Diatoms growing atop creek-bank sediment produced organic matter that they exported to the water column.

A High Tide
l^Xi).including m s,ilt marshes (Levin et aL, 2(H)|), has the potential to proloundly inlluence the health ol marine ecosystems.When inundated bv tides.marshes filter dissi lived and pai ticulate materials Iran the sui lace watei (Stumpl.I983).Ihechemical signature ol tidal a creek is indicative ot the processes taking place in the surrounding marsh, the creek bed sediment, and the water column (Nelson and Zavaleta, 2012).One ot t he primary missions ot the Georgia Coastal Ecosystems (CX'K) Long Term Ecological Research project is to identity and explain patterns in nutrient concentrations in the coastal zone and how these patterns influence the ecology ol the region.I lere, we present a synthesis ol nearly a decade "l muludisciplmary GCE data from a large tidal creek draining some Hi knr ol salt marsh located in coastal < ieorgia, USA.We describe the temporal and spatial patterns in creek water chemistry and aim to assess the relative importance ol the various processes that drive these patterns.We first discuss the roleol mixing between riverine Ireshwater and ocean water and then estimate the importance oi within-watenhed processes, including creek water microbtal metabolism, groundwater discharge, and marsh inundation to establish a conceptual model "1 the tidal creek system.THE DUPLIN WATERSHED The I Hiplm is a large tidal creek, approximately 12.? km long, located on the west side of Sapelo Island, Georgia, within the GCE domain and the Sapelo Island National listuarine Research Reserve.It lies approximately In km north "I where the mam channel oi the Altamaha River discharges into the Atlantic Ocean The Duplin is typical of marsh-dominated, tidal creek watersheds In the southeastern United States (Wiegert and Freeman, 1990).Its 14 knr watershed is composed of tidal marsh complexes with upland influence and back barrier islands (Figure lA see also I)i lorio and (^astelao, 2013, in this issue).Semidiurnal tides with an amplitude of 2 to 3 m largely drive circulation in the Duplin (Ragotzkieand Bryson, 1955; Boon, 1975), with some residual circulation due to precipitation and groundwater input.The lower 5 km of the Duplm exchange readily with Doboy Sound, while the middle and upper reaches of the I Hiplin are isolated by its sinuous channel, which dissipates tidal energy and mixing (Ragotzkie and

Figure 2 .
Figure 1 IB) iiKTcisai trom.iminiimiin ,it the mouth ol the Uuplin toa in.ixinuun in the middle Duplin (IT) liiv;h tide).Ammonium concentrations also exhibited a general increase from spring to fall (not shown), and maximum concentrations in the middle Duplin were higher on spring than on neap tides.Dissolved organic matter (DOM, represented hereby DON' in Figure IB) concentrati'ins increased from the mouth of the Duplin to the upper Duplm and from winter in summer (not shown).DOM concentrations m the upper Duplin (UD low tide) were higher on spring than on neap tides.The general spatial pattern observed for the dissolved materials was reversed for particulates (represented byTSS in I : igure IB), which had its minimum concentration in the upper Duplin.TSS concentrations in the upper I >uplin were higher on spring than on neap tides.

Figure 3
Figure 3 Internal prociesses within the Duplin watershed on high (A)ancl low (B) tide alter the chemical compoution of aeek water Diatorm growing in the creek bed export dissolved matter (DOM)and are represented by green lines and arrows, respectively Particulates in die water column (brown dots) are Giptured by die marsh on high odes :! i Groundwater flow and discharge are displayf d as oram *■ arrows.(C)Coiiipaiison of groundwater and surface water nutrient con ten nan on s DIN = Dissolved inorganic nitrogen DIP = Dissolved inorganic phosphorus