The source and distribution of dissolved and particulate organic matter in the Bay of St. Louis, northern Gulf of Mexico

Estuarine, Coastal and Shelf Science 96 (2012) 96e104

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The source and distribution of dissolved and particulate organic matter in the Bay of St. Louis, northern Gulf of Mexico Yihua Cai a, b, *, Laodong Guo a,1, Xuri Wang a, Allison K. Mojzis a, Donald G. Redalje a a b

Department of Marine Science, The University of Southern Mississippi, Stennis Space Center, MS 39529, USA State Key Lab of Marine Environmental Science, Xiamen University, Xiamen 361005, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 January 2011 Accepted 18 October 2011 Available online 6 November 2011

Dissolved (DOM) and particulate organic matter (POM) samples were collected from the Bay of St. Louis (BSL) in the northern Gulf of Mexico during MarcheSeptember, 2007 for chemical and isotopic characterization to examine the distribution and sources of organic matter species in the estuarine environment. Similar to the variations in hydrographic parameters and nutrients, concentrations of organic C, N, and P and stable isotopic composition show large spatial and seasonal variations during the sampling period. Concentrations of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) decreased with increasing salinity while d13C-DOC increased with salinity from
30.23& to
19.04&, suggesting a shift of DOC sources from terrestrial- to marine-dominated inputs during estuarine mixing. In contrast to both DOC and DON, the concentration of dissolved organic phosphorus (DOP) increased with increasing salinity, indicating additional DOP sources at higher salinity stations. Concentrations of particulate organic carbon (POC) and nitrogen (PN) decreased with increasing salinity, showing a negative correlation with the concentration of suspended particulate mater. Both POC/PN ratio (8.6e19.6) and d13C-POC (
28.51& to
23.79&) increased in general with increasing salinity, indicating the predominance of terrestrially derived organic matter in the upper bay and increasing diagenetically altered marine POM component in the lower bay. Fresh microalgae might account for about one third of POM in the BSL as estimated from POC/Chl a ratio. Overall, the terrestrial inputs, in situ primary production and diagenetically altered marine POM mostly from sediment resuspension are the major sources of POM in the BSL. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: dissolved organic matter nutrients, POC estuary biogeochemistry Bay of St. Louis Gulf of Mexico

1. Introduction Rivers are major conduits for the transport of terrestrial organic matter to the coastal marine environment (Meybeck, 1982; Spitzy and Ittekkot, 1991). However, only a small fraction of terrestrial organic matter reaches the open ocean based on biomarker and isotopic evidence (Hedges et al., 1997; Benner, 2004). Therefore, the degradation, transformation, and burial of terrestrial organic matter through physicochemical, microbial, and photochemical processes in estuaries are important (e.g., Benner and Opsahl, 2001; Goñi et al., 2005; Dagg et al., 2008). Indeed, the biogeochemical cycling of organic matter in estuaries at the landeocean interface has received increasing attention (e.g., Meybeck et al., 1988; Guo

* Corresponding author. State Key Lab of Marine Environmental Science, Xiamen University, 182 Daxue Rd, Xiamen 361005, China. E-mail address: [email protected] (Y. Cai). 1 Present address: School of Freshwater Sciences, University of Wisconsin-Milwaukee, 600 E Greenfield Ave., Milwaukee, WI 53204, USA. 0272-7714/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2011.10.017

and Santschi, 1997; Raymond and Bauer, 2001a; Dagg et al., 2007; Spencer et al., 2007; Sleighter and Hatcher, 2008; Wang et al., 2010). The bulk concentration, elemental ratio, stable carbon and nitrogen isotopic compositions, and biomarkers of organic matter along the estuarine salinity gradient have also been investigated to determine if the organic matter follows conservative or nonconservative mixing behavior in the estuaries (Bianchi et al., 1997; Guo et al., 1999; Benner and Opsahl, 2001; Raymond and Bauer, 2001a; Amon and Meon, 2004; Guo et al., 2009; Wang et al., 2010). In accordance with the dynamic hydrographic features of estuaries, the mixing behavior of organic matter is complicated by the significant variation of terrestrial inputs, in situ autochthonous production, sedimentewater processes and distinct biogeochemical cycling pathways for different functional groups of organic matter. Widespread occurrence of hypoxia has been observed in the northern Gulf of Mexico (Rabalais et al., 2002; Brunner et al., 2006), mainly due to riverine nutrient and terrestrial organic matter influx through the Mississippi River plume and other estuaries including

Y. Cai et al. / Estuarine, Coastal and Shelf Science 96 (2012) 96e104

the Bay of St. Louis (BSL). Though nutrient enrichment could be the major factor in the development of hypoxia in the northern Gulf of Mexico (Rabalais et al., 2002; Boesch et al., 2009), terrestrial organic matter had been proposed as a possible contributing factor in the northern Gulf (Dagg et al., 2007; Bianchi et al., 2008). However, details of how and to what extent terrestrial organic matter contributes to the development of hypoxia are still unclear and have been subject to debate (Boesch et al., 2009). Given the large amount of terrestrial organic matter derived from the Mississippi River and marshland erosion in the Gulf coast region (Barras et al., 2003; Bianchi et al., 2004, 2007), the fate and role of terrestrial organic matter in coastal environments warrant further studies. Water and environmental quality in the BSL and the Mississippi Sound and Bight have received increasing attention especially after Hurricane Katrina (e.g., Phelps, 1999; Brunner et al., 2006; Sawant, 2009; Mojzis, 2010; Wang et al., 2010). However, the sources, distribution and biogeochemical cycling of organic matter in the BSL remain largely unknown. Wang et al. (2010) investigated the mixing behavior of carbohydrates in the BSL and revealed the preferential removal of carbohydrates in the bulk dissolved organic matter (DOM) pool. The mechanisms responsible for such removal are not fully understood though the physical mixing and microbial respiration accounted for 30e40% of the removal. To better elucidate the biogeochemistry of organic matter species in the BSL and the role of estuaries in the marine organic matter cycling and the coastal environmental quality, further studies of organic biogeochemistry in the BSL are needed. In this study, water samples were collected from the BSL during MarcheSeptember 2007, and measured for the elemental and isotopic compositions of DOM and POM in order to provide baseline data of organic matter and water chemistry and to examine the sources, distribution, and mixing behavior of nutrients and organic matter species in the BSL. 2. Materials and methods 2.1. Site description The BSL is a shallow (average depth of 1.5 m), semi-enclosed estuary, which is connected to the Mississippi Sound through a narrow passage (Fig. 1). The BSL has multiple sources of

Fig. 1. Sampling stations in the Bay of St. Louis in the northern Gulf of Mexico during MarcheSeptember 2007.

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freshwater and associated organic matter and nutrients. Two small blackwater rivers, the Jourdan and Wolf Rivers, discharge into the BSL from the northwest and northeast of the bay, respectively. There are numerous bayous around the BSL, of which the most dominant bayou, Portage Bayou, joins the BSL from the east. The BSL also receives organics and nutrients from anthropogenic point sources, including sewage treatment outfalls from Hancock County, Harrison County via Bayou Portage, the Diamondhead community and a wastewater outfall of the Dupont titanium dioxide plant on the upper north side of the BSL. Non-point sources, such as leaking septic tanks, recreational and agricultural activities within the watersheds of the Jourdan and Wolf Rivers also contribute organic matter and nutrients to the bay (Phelps, 1999). 2.2. Sample collection and analysis Nine stations were sampled on each trip during this study in the BSL (Fig. 1). Station 1, located at the mouth of the Jourdan River, represents inputs of riverine discharge and sewage effluent from Hancock County and the Diamondhead community. Station 2, located near the Dupont titanium dioxide plant, represents another point source. Station 4 is located at the mouth of the Wolf River representing the other river endmember input. Station 5 is located at the mouth of Bayou Portage with additional sewage outfall input from Harrison County. Other stations (Stations 3, 6e9) form one transect from the inner bay to the mouth of the bay in the Mississippi Sound. The BSL is shallow (average depth of 1.5 m) and well mixed vertically as evidenced by the profiles of salinity and temperature (data not shown) which were monitored simultaneously during the sampling. Thus, only surface water samples were collected. Monthly samples were collected using acid cleaned high-density polyethylene bottles at each station from March to September 2007 during an outgoing tide. Due to rough sea conditions, station 9 was not sampled during the cruises in March and May 2007. Furthermore, water samples from October to February could not be obtained because of weather conditions. Aliquots of water samples were filtered through pre-combusted GF/F filters and the filtrates were sampled for measurement of total dissolved nitrogen (TDN), DOC with acidification, total dissolved carbon (TDC) without acidification, and d13C in the DOC. Filter samples were stored frozen for the determination of particulate organic carbon (POC), particulate nitrogen (PN), and their stable isotopic compositions (d13C and d15N) or stored in liquid nitrogen until fluorometric analysis for chlorophyll a (Chl a; Cai et al., 2008a). An aliquot of sample was also filtered through a pre-weighed 0.45 mm polycarbonate filter to collect filtrate for nutrient analysis while the filters were used to determine the concentration of suspended particulate matter (SPM) after rinsed 3 times with 15 mL Milli-Q water each to remove salts (Cai et al., 2008b). Salinity was obtained with a YSI 30 handheld conductivity instrument. Water temperature was monitored in situ with a YSI 6000UPG multi-parameter monitor during MarcheJuly and with an In Situ Troll 9500 monitor during August and September. Chl a concentration was determined using a Turner Designs Model 10AU fluorometer (Parsons et al., 1984). Concentrations of TDC, DOC and TDN were measured on a high temperature combustion TOC analyzer (Shimadzu TOC-V) interfaced with a nitrogen detector (TNM-1; Guo and Macdonald, 2006). The total DOC blank (including Milli-Q water, acid for sample acidification, and the instrument blank) was usually less than 2e8 mM. Precision was better than 2% and accuracy was within 1%, based on DOC standards (Cai et al., 2008a). Concentrations of dissolved inorganic carbon (DIC) were calculated from the difference between TDC and DOC. Dissolved inorganic nitrogen (DIN, including NO3, NO2, and

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NH4) was analyzed using colorimetry on an Astoria 2 nutrient autoanalyzer (Astoria-Pacific). Concentrations of dissolved organic nitrogen (DON) were calculated from the difference between TDN and DIN concentrations. Total dissolved phosphorus (TDP) concentrations were measured by autoclave-assisted persulfate oxidation of the DOP at boiling temperature, followed by the standard phosphomolybdenum blue method on a Cary 300 Bio UVeVisible dual-beam spectrophotometer, while dissolved phosphate (DIP) concentrations were measured without persulfate oxidation (Cai et al., 2008b). Concentrations of DOP were calculated from the difference between TDP and DIP. Values of d13C-DOC were measured on a PDZ Europa 20e20 isotope ratio mass spectrometer (Sercon Ltd.) interfaced with an O.I. Analytical Model 1010 TOC Analyzer (OI Analytical) at the University of California Davis Stable Isotope Facility (St. Jean, 2003). Concentrations of POC and PN as well as their d13C and d15N values were measured on a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20e20 isotope ratio mass spectrometer (Sercon Ltd.). Data on the freshwater discharge from the Wolf River were obtained from the U.S. Geological Survey Hydrological Station 02481510 near Landon, Mississippi (http://waterdata.usgs.gov/ms/ nwis/uv/?site_no¼02481510). The annual mean discharge in the Wolf River during 1972e2010 shows a relative high discharge (w30 m3 s
1) from December to April and relative low discharge (w10 m3 s
1) from May to November. Comparing with the annual mean discharge, the Wolf River was dry for most of time during 2007 with a discharge less than 10 m3 s
1. However, the river discharge during January and November reached 80 and 170 m3 s
1, likely prompted by the strong precipitation events. Discharge data for the Jourdan River are not available. Since the Jourdan and Wolf Rivers both have small drainage basins that adjoin each other and have similar meteorologic settings (Sawant, 2009), it is likely that they have similar variation and trends in discharge. Thus, the Wolf River discharge was used for the relationship analysis between the total river discharges and the environmental variations in the BSL. For each sampling trip, the average of the discharges of the sampling day and four previous days was

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calculated to represent the discharge corresponding to that sampling day. 3. Results and discussion 3.1. Variations in hydrographic parameters and nutrient concentrations For the sampling period, the river discharge of the Wolf River was the highest in July (13.1 m3 s
1) and the lowest in August and September (2.3 and 2.1 m3 s
1, respectively). The freshwater replacement time in the BSL was estimated as 0.3e32.7 days, with an average of 4.1 days based on estimates of freshwater replacement from both of the rivers (Sawant, 2009). However, it must be recognized that the winds and diurnal tidal cycles will have a pronounced effect on water residence times in BSL. This is among the shortest in the Gulf of Mexico estuaries (Solis and Powell, 1999). The BSL had a wide range of salinity during this study, varying from 4.4 at station 1 in July to 23.2 at station 9 in September (Table S1, Fig. 2). Spatially, salinity gradually increased from the inner bay with freshwater inputs to the estuarine mouth in the Mississippi Sound. The average salinity was relatively low in July and high in August and September during the sampling period, coincident with variations in the river discharge. The Mississippi Sound also receives freshwater discharge from various rivers and bayous along the LouisianaeMississippieAlabama coasts other than the BSL, in which the Pearl River and the Pascagoula River may account for 90% of the freshwater input. Another two rivers, the Mobile River and the Mississippi River through Bonnet Carre Spillway/Lake Pontchartrain, may also have a significant influence on the Mississippi Sound during the high flow season from the east and west sides, respectively (Chigbu et al., 2004). The freshwater input may determine the variation of salinity and other hydrological parameters in the Mississippi Sound and further influence the hydrographic parameters and organic matter dynamics inside the BSL through the entrance of seawater from the Mississippi Sound during the incoming tide. The average water temperature increased

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3.2. Distributions and sources of DOM The concentration of DOC in the BSL varied from 273 to 783 mM during this sampling period (Table S2, Fig. 4a) and was well between the concentrations of riverine and seawater endmembers: 1000e1478 mM for the Jourdan River and 60e100 mM for the Mississippi Bight and the Gulf of Mexico (Cai and Guo, 2009; Wang et al., 2010). For individual sampling trip, the DOC concentration was the highest in the river mouth of the Jourdan River and decreased to the lowest in the Mississippi Sound (Table S2, Fig. 5). Average DOC concentration in July (592 mM) was significantly higher than those in other months (344e407 mM; Table S2), coincident with the high river discharge in July and indicating that river inputs have a significant influence on the distribution of DOC and the composition of DOM in the BSL. Concentrations of DON in the BSL varied between 13 and 28 mM (Table S2, Fig. 4b), similar to DON values of northeastern U.S. rivers (about 10e30 mM; Pellerin et al., 2004), the Pearl and Mississippi Rivers (10e40 mM; Duan et al., 2007), and Apalachicola Bay, another coastal bay in the northern Gulf of Mexico (10e22 mM; Mortazavi et al., 2001). However, it is about 2e6 times of the value of the open Gulf of Mexico (about 4e6 mM; López-Veneroni and Cifuentes, 1994). Meanwhile, DON concentrations in the BSL are one to two orders of magnitude

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from 20.1  C in March to 33.2  C in August and then decreased to 28.5  C in September with little spatial variation in the BSL for individual sampling event, similar to the air temperature variation (Table S1, Fig. 2a). SPM concentration was highly variable in the BSL, ranging from 1.6 mg L
1 at Station 3 in April to 50.4 mg L
1 at Station 4 in September (Table S1, Fig. 2b). Statistically, SPM concentration increased with increasing salinity, implying additional source of SPM from the lower estuary. DIC concentration was low in the inner bay and increased with increasing salinity toward the Mississippi Sound (Table S1). DIC concentration showed a significant positive correlation with salinity (Fig. 2c), suggesting conservative mixing of DIC between the river waters and seawater. Chl a concentration was in the range of 4e21 mg L
1, except one sample with a Chl a concentration of 40 mg L
1 (Table S1, Fig. 2d). Concentration of DIN was usually lower than 1 mM (Table S1, Fig. 3a). It exhibited large spatial variations in the BSL, consistent with its characteristics of variable sources and complex species transformation. Phosphate concentration was in the range of 0.03e1.41 mM and increased with increasing salinity (Table S1, Fig. 3b), implying addition of phosphate in the water column in the lower estuary. Sources for additional phosphate may include the desorption of phosphate from suspended sediments (Lin et al., 2012), anthropogenic inputs from the Bayou Portage and in the lower bay, and the tidal exchange with the phosphate-rich seawater from the Mississippi Sound that derived from anthropogenic sources upcurrent from the mouth of the bay (Sawant, 2009). Within the nutrient pool, the inorganic N/P ratios were, in general, significantly lower than the Redfield ratio in the water column (Table S1). Together with the low DIN concentration, the BSL is thus a nitrogen deficient estuarine environment with potential N-limitation to primary production. The dissolved silicate concentration in the BSL ranged from 4.2 to 98 mM and generally decreased with increasing salinity (Table S1, Fig. 3c), showing a strong source from river waters. The variability of dissolved silicate concentration of both rivers and, especially, seawater in the Mississippi Sound might be responsible for the dynamic distribution of dissolved silicate in the BSL. Since the Mississippi Sound receives a number of coastal riverine inputs, the relative contribution from these coastal rivers may partially affect the silicate concentration in the Mississippi Sound and therefore in the BSL.

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S Fig. 3. Variations in nutrient concentration with salinity in the Bay of St. Louis: (a) dissolved inorganic nitrogen, (b) phosphate, and (c) dissolved silicate.

higher than DIN concentration for the same samples (Table S1, Fig. 3a). The DON concentration generally decreased with increasing salinity for individual cruise (p < 0.05) other than the March cruise though DON had no clear correlation with salinity for the whole dataset (p > 0.05, Fig. 4b). DOP concentrations in the BSL ranged from 0.10 to 0.64 mM (Table S2, Fig. 4c). Average DOP concentrations for each sampling campaign was high in June and September (0.40 and 0.47 mM, respectively) but low in May (0.19 mM). Similar to the DIP, the concentration of DOP also increased with increasing salinity, indicating additional DOP sources at higher salinity stations. The mixing behavior of solutes in estuaries has often been examined with the application of properties-salinity plots. Though the DOC concentration only showed a general decrease with salinity when all data were plotted (Fig. 4a), the DOC concentration and salinity for individual sampling trip fell well on the conservative mixing line of two endmembers (Fig. 5), indicating a conservative mixing behavior within the BSL. Seasonal variations of riverine DOC concentration with discharge had been frequently

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Fig. 4. Variations in concentration and stable carbon isotopic composition of dissolved organic matter with salinity in the Bay St. Louis: (a) dissolved organic carbon (DOC), (b) dissolved organic nitrogen (DON), (c) dissolved organic phosphorus (DOP), and (d) d13C-DOC.

observed for Gulf of Mexico rivers (e.g., Warnken and Santschi, 2004; Duan et al., 2007). This may also give rise to the seasonal variation of DOC concentration in estuaries. Unlike the DOC concentration, the stable carbon isotopic composition (d13C) of DOC shows little variation and thus the carbon isotopic mixing model can better provide information on the DOC sources and biogeochemical cycling in estuaries (Raymond and Bauer, 2001a,b; Cai et al., 2008a; Guo et al., 2009). River endmember samples from the Jourdan River, Rotten Bayou (one of main tributaries of the Jourdan River) and Wolf River were sampled on July 16th, 2007, and their values of d13C-DOC were
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28.32&, respectively. They are very similar to the 2 years time series dataset in the lower Pearl River at Stennis Space Center, Mississippi, from August 2006 to September 2008. The latter showed a d13C-DOC value ranging from
28.89& to
26.73&, with an average of
28.23  0.44& (Y. Cai, unpublished data). Apparently, the

Fig. 5. Seasonal variations in DOC concentrations in the Bay of St. Louis.

terrestrial DOM input from the Jourdan and Wolf Rivers had a typical C3 plant signal (Goñi et al., 2005). As shown in Fig. 4d, most of the values of d13C-DOC fall on or close to the isotopic mixing curve, further elucidating the conservative mixing behavior of DOC in the BSL. Nevertheless, some measured d13C-DOC values seem to deviate slightly from model curve. The variability of the stable isotopic composition of DOM from both riverine inputs and seawater endmember, which was influenced by the LouisianaeMississippieAlabama coastal freshwater inputs, might partly account for such a discrepancy. The distribution of DOC in the BSL seems mainly controlled by the mixing of terrestrial and marine-dominated sources. At the low salinity zone, DOM had a lighter stable carbon isotopic composition, with a d13C value of about
29& (Table S2; Fig. 4d), and a C/N ratio of as high as 27 (Table S2; Fig. 6a). Both values are within the range of those reported for rivers in the Gulf of Mexico, such as the Mississippi River, the Pearl River, and the Trinity River (Guo et al., 1999; Goñi et al., 2005; Bianchi, 2007; Duan et al., 2007), suggesting the dominant terrestrial DOC sources at the upper reach of the BSL. In contrast, DOM in the lower BSL exhibited lower DOC concentrations with heavier d13C values of about
20& and lower DOC/DON ratios of about 14 when the salinity was higher than 20 (Table S2; Figs. 4d, 6a). These values are within those reported for DOC in the coastal waters (Guo et al., 1995) and marine phytoplankton d13C value (Sackett, 1991) in the Gulf of Mexico. Wang et al. (2010) reported that DOC and carbohydrates could be significantly removed during the seaward transport from the BSL to the Mississippi Sound. They also pointed out that the apparent removal of DOC could be largely the result of a third endmember water mass outside the bay. Indeed, the mixing line shown in Wang et al. (2010) could be divided into two conservative mixing segments: one within the BSL, as we sampled in this study, and the other in the Mississippi Sound/Bight. Therefore, DOC seems to behave conservatively within two mixing segments considering the intrusion of the third endmember of coastal waters outside the BSL (Wang et al., 2010).

Y. Cai et al. / Estuarine, Coastal and Shelf Science 96 (2012) 96e104

a

sources of DOP in the lower BSL. Laboratory mixing experiments conducted by Lin et al. (2012) suggested that desorption of phosphorus from the suspended particles would induce the elevated DOP concentration at higher salinity stations in the BSL. Furthermore, coastal waters might support part of additional DOP through the release of freshly produced marine organic matter. This is supported by the d13C-DOM value in the lower bay which is within phytoplankton value (Table S2, Fig. 4d) and the size fractionation results of DOP which showed higher fractions of high molecular weight DOP in high salinity coastal waters than in the lower bay (Cai and Guo, 2009; Lin et al., 2012). Further measurements of DOM quantity and quality are needed to quantify sources and cycling pathways of DOP in the BSL.

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Differences existed in the property-salinity relationship among DOC, DON and DOP in the BSL. Unlike DOC concentration, which generally decreased with the increasing salinity, DON concentration showed significant variability with salinity (Fig. 4b). Similar distributions and variations in DOC and DON had been observed in the Mississippi River plume and the Apalachicola Bay, a bar-built estuary in the northern Gulf of Mexico (López-Veneroni and Cifuentes, 1994; Mortazavi et al., 2001). Highly variable DON concentration in the BSL could result from the strong seasonal riverine DON signatures (Duan et al., 2007). In addition, the autochthonous production of DON by primary and secondary producers and bacteria and the addition of benthic DON might intensify such variability. As summarized in Bronk (2002), the average released DON could account for one fourth of the gross N uptake in the Chesapeake Bay and the Choptank River estuary. Contrary to DOC and DON, the DOP concentration increased with increasing salinity in the BSL, resulting in a decrease in organic C/P and N/P ratios with increasing salinity (Figs. 4, 6). The increase in DOP concentration with increasing salinity may imply additional

The concentration of POC in the BSL was in the range of 67e282 mM, accounting for about one quarter of the total organic carbon pool (DOC þ POC), while PN concentration was in the range of 5.4e26.3 mM, about one third of the total organic N pool (DON þ PN) (Table S3, Fig. 7a, b). Both POC and PN concentrations generally decreased with increasing salinity (Fig. 7a, b). The POC/ SPM and PN/SPM ratios varied simultaneously and generally decreased with increasing salinity as well. This likely resulted from the decreasing POC and PN concentrations and increasing SPM concentration with increasing salinity. The high concentrations of POC and PN at low salinity stations point to the importance of riverine inputs at the upper bay. However, the decrease in POC/SPM and PN/SPM ratios with increasing salinity is somewhat surprising since one would expect to see higher POC/SPM in lower estuaries due to the decrease of terrestrial POM and increasing autochthonous production in the water column. Therefore, sources of SPM in the lower bay were likely from sediment resuspension and inputs of coastal waters transported along the coastal line in the Mississippi Sound. POC/PN molar ratio in the BSL varied from 8.6 to 19.6 (Table S3, Fig. 7c). In contrast to the C/N ratio of DOM, which decreased with increasing salinity, the POC/PN ratio in the BSL increased with increasing salinity (Fig. 7c). In the lower BSL (including Stations 7e9), the POC/PN ratio was about 12, almost twice of the Redfield ratio. The increase in POC/PN is consistent with the decrease in POC/SPM ratio along the salinity gradient, indicating increased diagenetically altered POM components in lower estuary region, likely from sediment resuspension and/or coastal water inputs. Thus, additional POM source other than freshly produced autochthonous material seemed to contribute significantly to POM in the lower BSL. d13C-POC in the BSL ranged from
28.51& to
23.79&, showing a general increase with increasing salinity although the nitrogen isotopic composition of POM had no significant relationship with salinity, suggesting the source of POM shifting from terrestrial input to the marine-dominated source (Table S3, Fig. 7d, e). The dominant terrestrial POM inputs to the low salinity stations in the upper bay are supported by the carbon isotopic composition of POM which is in the range of typical C3 plant (Goñi et al., 2005) and is close to river endmember values in the Jourdan and Wolf Rivers (
27.69& and
27.85&, respectively). As the salinity increased from the upper bay to the lower bay, POM was transported offshore, the terrestrial POM gradually degraded and the contribution of terrestrially derived POM decreased and phytoplankton-derived POM components increased in the POM pool in the BSL. This should result in the increase in the d13C-POC value in the lower BSL and make the carbon isotopic composition approaching that of marine primary production (
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Fig. 7. Variations in particulate organic matter content and isotopic composition with salinity in the Bay of St. Louis: (a) particulate organic carbon (POC), (b) particulate nitrogen (PN), (c) POC/PN ratio, (d) d13C-POC, (e) d15N-PN, and (f) POC/Chl a ratio.

marine-derived POM source by the high POC/Chl a ratio, high POC/ PN ratio, and low organic carbon content of the SPM in the lower BSL. The average POC/Chl a ratio in the BSL was 153  50 (Table S3, Fig. 7f), about three times of that of phytoplankton (Sun et al., 1993). This suggests that a relatively small fraction of POM in the BSL was contributed by fresh marine microalgae. Combining with the high POC/PN ratio (about twice of the Redfield ratio), we thus hypothesize that diagenetically altered organic matter was an important source of the POM in the lower BSL. However, the provenance of such diagenetically altered POM is still unknown although reworked sediments may be one of the possible sources. POC/PN ratio in the sediment of the lower BSL were about 16, comparable to 12 in suspended particles (Phelps, 1999). Thus, it is likely that the local sediment with higher C/N ratio could be one of the major sources of SPM and diagenetically altered POM in the water column in the lower BSL. Indeed, previous studies had suggested reworked sediments may contribute significantly to the POM and DOM in estuarine and coastal marine environments (Bauer et al., 2001; Guo et al., 2009). Overall, terrestrial inputs, fresh microalgae and diagenetically altered organic matter are the major sources of POM in the BSL, and further studies are needed to validate the role of sedimentewater processes or coastal water influences on the organic matter cycling in the BSL.

4. Conclusions To examine the distribution and sources of organic matter in the BSL and to provide baseline data for further investigation, water samples were collected monthly during MarcheSeptember 2007 and were analyzed for chemical and isotopic composition of DOM and POM. Concentrations of DOC, DON and DOP in the BSL were in the range of 273e783 mM, 13.9e29.2 mM and 0.10e0.64 mM, respectively, while d13C-DOC ranged from
30.23& to
19.04&. The relationship between the above chemical and isotopic compositions and salinity revealed the mixing of riverborne terrestrial inputs and the autochthonous marine organic matter. While DOC demonstrated a conservative mixing in the estuary, the concentration of DOP increased with increasing salinity with DOP sources largely from the desorption of POM and in situ production in the lower BSL and Mississippi Sound. Concentrations of POC and PN in the BSL varied between 67e282 mM and 5.4e26.3 mM, respectively, resulting in a C/N ratio of 8.6e19.6. The d13C-POC increased with salinity from
28.51& to
23.79& in the BSL, indicating the shift of the POM source from terrestrial C3 plant in the upper bay to marine-dominant biomass in the lower bay. However, the high POC/Chl a ratio (w153) and high POC/PN ratio (w12) excluded the predominant contribution from freshly

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photosynthesized marine organic matter in the lower BSL. Instead, diagenetically altered marine organic matter was an important source. Thus, terrestrial organic matter, in situ primary production, and diagenetically altered marine POM are the main sources of POM in the BSL. Further studies on the sediment geochemistry and organic composition are warranted for fully understanding the influence of sedimentewater processes on organic biogeochemistry in the overlying water column in the BSL. Acknowledgments We thank Merritt Tuel, Kevin Martin and Zhengzhen Zhou for their assistance during sample collections and processing, and Diana Lovejoy for critical reading of the manuscript. We also thank two anonymous reviewers for their constructive comments. This work was supported in part by the Northern Gulf Institute/NOAA (09-NGI-13 and 09-NGI-04), National Science Foundation (OCE#0850957 to LG), National Natural Science Foundation of China (#40906040 to YC) and the Fundamental Research Funds for the Central Universities of China (#2010121027 to YC). Appendix. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ecss.2011.10.017. References Amon, R.M.W., Meon, B., 2004. The biogeochemistry of dissolved organic matter and nutrients in two large Arctic estuaries and potential implications for our understanding of the Arctic Ocean system. Marine Chemistry 92, 311e330. Barras, J., Beville, S., Britsch, D., Hartley, S., Hawes, S., Johnston, J., Kemp, P., Kinler, Q., Martucci, A., Porthouse, J., Reed, D., Roy, K., Sapkota, S., Suhayda, J., 2003. Historical and projected coastal Louisiana land changes: 1978e2050. USGS Open File Report 03-334, 39 pp. Bauer, J., Druffel, E.R.M., Wolgast, D.M., Griffin, S., 2001. Sources and cycling of dissolved and particulate organic radiocarbon in the northwest Atlantic continental margin. Global Biogeochemical Cycles 15, 615e636. Benner, R., Opsahl, S., 2001. Molecular indicators of the sources and transformations of dissolved organic matter in the Mississippi River plume. Organic Geochemistry 32, 597e611. Benner, R., 2004. What happens to terrestrial organic matter in the ocean? Marine Chemistry 92, 307e310. Bianchi, T.S., Lambert, C.D., Santschi, P.H., Guo, L., 1997. Sources and transport of land-derived particulate and dissolved organic matter in the Gulf of Mexico (Texas shelf/slope): the use of lignin-phenols and loliolides as biomarkers. Organic Geochemistry 27, 65e78. Bianchi, T.S., Filley, T., Dria, K., Hatcher, P.G., 2004. Temporal variability in sources of dissolved organic carbon in the lower Mississippi River. Geochimica et Cosmochimica Acta 68, 959e967. Bianchi, T.S., 2007. Biogeochemistry of Estuaries. Oxford University Press, New York, 706 pp. Bianchi, T.S., Galler, J.J., Allison, M.A., 2007. Hydrodynamic sorting and transport of terrestrially derived organic carbon in sediments of the Mississippi and Atchafalaya Rivers. Estuarine Coastal and Shelf Science 73, 211e222. Bianchi, T.S., DiMarco, S.F., Allison, M.A., Chapman, P., Cowan Jr., J.H., Hetland, R.D., Morse, J.W., Rowe, G., 2008. Controlling hypoxia on the U.S. Louisiana shelf: beyond the nutrient-centric view. EOS 89, 236e237. Boesch, D.F., Boynton, W.R., Crowder, L.B., Diaz, R.J., Howarth, R.W., Mee, L.D., Nixon, S.W., Rabalais, N.N., Rosenberg, R., Sanders, J.G., Scavia, D., Turner, R.E., 2009. Nutrient enrichment drives Gulf of Mexico hypoxia. EOS 90, 117e118. Bronk, D.A., 2002. Dynamics of dissolved organic nitrogen. In: Hansell, D.A., Carlson, C.A. (Eds.), Biogeochemistry of Marine Dissolved Organic Matter. Academic Press, San Diego, pp. 153e247. Brunner, C.A., Beall, J.M., Bentley, S.J., Furukawa, Y., 2006. Hypoxia hotspots in the Mississippi Bight. Journal of Foraminiferal Research 36, 95e107. Cai, Y., Guo, L., 2009. Abundance and variation of colloidal organic phosphorus in riverine, estuarine and coastal waters in the northern Gulf of Mexico. Limnology and Oceanography 54, 1393e1402. Cai, Y., Guo, L., Douglas, T.A., 2008a. Temporary variations in organic carbon species and fluxes from the Chena River, Alaska. Limnology and Oceanography 53, 1408e1419. Cai, Y., Guo, L., Douglas, T.A., Whitledge, T., 2008b. Seasonal variations in nutrient concentrations and speciation in the Chena River, Alaska. Journal of Geophysical Research 113, G030035. doi:10.1029/2008JG000733.

103

Chigbu, P., Gordon, S., Strange, T., 2004. Influence of inter-annual variations in climatic factors on fecal coliform levels in Mississippi Sound. Water Research 38, 4341e4352. Dagg, M.J., Ammerman, J.W., Amon, R.M.W., Gardner, W.S., Green, R.E., Lohrenz, S.E., 2007. A review of water column processes influencing hypoxia in the Northern Gulf of Mexico. Estuaries and Coasts 30, 735e752. Dagg, M.J., Bianchi, T., McKee, B., Powell, R., 2008. Fates of dissolved and particulate materials from the Mississippi river immediately after discharge into the northern Gulf of Mexico, USA, during a period of low wind stress. Continental Shelf Research 28, 1443e1450. Duan, S., Bianchi, T.S., Shiller, A.M., Dria, K., Hatcher, P.G., Carman, K.R., 2007. Variability in the bulk composition and abundance of dissolved organic matter in the lower Mississippi and Pearl rivers. Journal of Geophysical Research 112, G02024. doi:10.1029/2006JG000206. Goñi, M.A., Yunkerb, M.B., Macdonald, R.W., Eglinton, T.I., 2005. The supply and preservation of ancient and modern components of organic carbon in the Canadian Beaufort Shelf of the Arctic Ocean. Marine Chemistry 93, 53e73. Guo, L., Macdonald, R.W., 2006. Source and transport of terrigenous organic matter in the upper Yukon River: evidence from isotope (13C, 14C and 15N) composition of dissolved, colloidal and particulate phases. Global Biogeochemical Cycles 20, GB2011. doi:10.1029/2005GB002593. Guo, L., Santschi, P.H., 1997. Isotopic and elemental characterization of colloidal organic matter from the Chesapeake Bay and Galveston Bay. Marine Chemistry 59, 1e15. Guo, L., Santschi, P.H., Bianchi, T.S., 1999. Dissolved organic matter in estuaries of the Gulf of Mexico. In: Bianchi, T.S., Pennock, J.R., Twilley, R.R. (Eds.), Biogeochemistry of Gulf of Mexico Estuaries. John Wiley and Sons, New York, pp. 269e299. Guo, L., Santschi, P.H., Warnken, K.W., 1995. Dynamics of dissolved organic carbon in oceanic environments. Limnology and Oceanography 40, 1392e1403. Guo, L., White, D.M., Xu, C., Santschi, P.H., 2009. Chemical and isotopic composition of high-molecular-weight dissolved organic matter from the Mississippi River plume. Marine Chemistry 114, 63e71. Hedges, J.I., Keil, R.G., Benner, R., 1997. What happens to terrestrial organic matter in the ocean? Organic Geochemistry 27, 195e212. Lin, P., Chen, M., Guo, L., 2012. Speciation and transformation of phosphorus and its mixing behavior in the Bay of Saint Louis estuary in the northern Gulf of Mexico. Geochimica et Cosmochimica Acta, in press. López-Veneroni, D., Cifuentes, L.A., 1994. Transport of dissolved organic nitrogen in Mississippi River Plume and TexaseLouisiana continental shelf near-surface waters. Estuaries 17, 796e808. Meybeck, M., Cauwet, G., Dessery, S., Somville, M., Gouleau, D., Billen, G., 1988. Nutrients (Organic C, P, N, Si) in the eutrophic river Loire (France) and its estuary. Estuarine Coastal and Shelf Science 27, 595e624. Meybeck, M., 1982. Carbon, nitrogen, and phosphorus transport by world rivers. American Journal of Science 282, 401e450. Mortazavi, B., Iverson, R.L., Huang, W., 2001. Dissolved organic nitrogen and nitrate in Apalachicola Bay, Florida: spatial distributions and monthly budgets. Marine Ecology Progress Series 214, 79e91. Mojzis, A.K., 2010. Explaining the variability of free-living and attached Bacterioplankton abundances in the Bay of St. Louis, Mississippi. M.S. thesis, The Univ. of Southern Mississippi. Parsons, T.R., Maita, Y., Lalli, C.M., 1984. A Manual of Chemical and Biological Methods for Seawater Analysis. Pergamon Press, Oxford, 173 pp. Pellerin, B.A., Wollheim, W.M., Hopkinson, C.S., McDowell, W.H., Williams, M.R., Vörösmarty, C.J., Daley, M.L., 2004. Role of wetlands and developed land use on dissolved organic nitrogen concentrations and DON/TDN in northeastern U.S. rivers and streams. Limnology and Oceanography 49, 910e918. Phelps, E.I., 1999. Environmental quality of Saint Louis Bay, Mississippi. M.S. thesis, The Univ. of Southern Mississippi. Rabalais, N.N., Turner, R.E., Dortch, Q., Justic, D., Bierman Jr., V.J., Wiseman Jr., W.J., 2002. Nutrient-enhanced productivity in the northern Gulf of Mexico: past, present and future. Hydrobiologia 475/476, 39e63. Raymond, P.A., Bauer, J.E., 2001a. Use of 14C and 13C natural abundance for evaluating riverine, estuarine, and coastal DOC and POC sources and cycling: a review and synthesis. Organic Geochemistry 32, 469e485. Raymond, P.A., Bauer, J.M., 2001b. DOC cycling in a temperate estuary: a mass balance approach using 14C and 13C isotopes. Limnology and Oceanography 46, 655e667. Sackett, W.M., 1991. A history of the d13C composition of oceanic plankton. Marine Chemistry 34, 153e156. Sawant, P.A., 2009. Factors influencing the environmental quality of the Bay of Saint Louis, Mississippi and implications for evolving coastal management policies. Ph.D. thesis, The Univ. of Southern Mississippi. Sleighter, R.L., Hatcher, P.G., 2008. Molecular characterization of dissolved organic matter (DOM) along a river to ocean transect of the lower Chesapeake Bay by ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. Marine Chemistry 110, 140e152. Solis, R.S., Powell, G.L., 1999. Hydrography, mixing characteristics, and residence times of Gulf of Mexico estuaries. In: Bianchi, T.S., Pennock, J.R., Twilley, R.R. (Eds.), Biogeochemistry of Gulf of Mexico Estuaries. John Wiley and Sons, New York, pp. 269e299. Spencer, R.G.M., Ahad, J.M.E., Baker, A., Cowie, G.L., Ganeshram, R., UpstillGoddard, R.C., Uher, G., 2007. The estuarine mixing behaviour of peatland derived dissolved organic carbon and its relationship to chromophoric

104

Y. Cai et al. / Estuarine, Coastal and Shelf Science 96 (2012) 96e104

dissolved organic matter in two North Sea estuaries (U.K.). Estuarine Coastal and Shelf Science 74, 131e144. Spitzy, A., Ittekkot, V., 1991. Dissolved and particulate organic matter in rivers. In: Mantoura, R.F.C., Martin, J.M., Wollast, R. (Eds.), Ocean Margin Processes in Global Change. John Wiley and Sons, New York, pp. 5e17. St. Jean, G., 2003. Automated quantitative and isotopic (13C) analysis of dissolved inorganic carbon and dissolved organic carbon in continuous flow using a total organic carbon analyzer. Rapid Communications in Mass Spectrometry 17, 419e428.

Sun, M.Y., Aller, R.C., Lee, C., 1993. Spatial and temporal distributions of sedimentary chloropigments as indicators of benthic processes in Long Island Sound. Journal of Marine Research 52, 149e176. Wang, X., Cai, Y., Guo, L., 2010. Preferential removal of dissolved carbohydrates during estuarine mixing in the Bay of St Louis in the northern Gulf of Mexico. Marine Chemistry 119, 130e138. Warnken, K.W., Santschi, P.H., 2004. Biogeochemical behavior of organic carbon in the Trinity River downstream of a large reservoir lake in Texas, USA. Science of the Total Environment 329, 131e144.