Effects of tropical cyclones on river chemistry: A case study of the lower Pearl River during Hurricanes Gustav and Ike

Estuarine, Coastal and Shelf Science 129 (2013) 180e188

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Effects of tropical cyclones on river chemistry: A case study of the lower Pearl River during Hurricanes Gustav and Ike Yihua Cai a, b, *, Laodong Guo b, c, Xuri Wang b, Steven E. Lohrenz b, d, Allison K. Mojzis b a

State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen University, 182 Daxue Rd, Xiamen 361005, China Department of Marine Science, The University of Southern Mississippi, Stennis Space Center, MS 39529, USA c School of Freshwater Sciences, University of Wisconsin-Milwaukee, Milwaukee, WI 53204, USA d School for Marine Science and Technology, University of Massachusetts Dartmouth, North Dartmouth, MA 02747, USA b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 January 2013 Accepted 17 May 2013 Available online 3 June 2013

To investigate the effects of tropical cyclones on the water chemistry of Gulf of Mexico coastal rivers, time series samples from the lower Pearl River at Stennis Space Center, Mississippi, were collected on August and September, 2008, during Hurricanes Gustav and Ike. Hurricane Gustav, which landed near the sampling site, caused intensive storm surge and strong seawater intrusion, resulting in an elevated salinity of 7.5 in the lower Pearl River and subsequent flooding induced by heavy rainfall. Hurricane Ike, which passed further away from the sampling site, caused only a mild seawater intrusion with a salinity of 1.2 at the sampling site. The river showed distinct variations in water chemistry corresponding to different hydrographic disturbance of hurricanes. Abrupt increase of suspended particulate matter and associated organic carbon and nitrogen concentrations coincided with the intensive storm surge due to coastal sediment resuspension. A remarkable drop in the concentrations of phosphate and dissolved organic matter was also observed during the intense seawater intrusion, a result of both dilution by seawater and resultant flocculation of dissolved organic matter. During hurricane-induced flooding, the river showed a mild increase in the concentrations of organic matter, reflecting a dominant contribution of terrestrial inputs from the watershed by surface runoffs while the concentrations of inorganic nutrient species in the river water decreased. In contrast, water chemistry in the Pearl River underwent little change in most carbon and nutrient species under the mild seawater intrusion. Overall, tropical cyclones could induce unique variations in coastal river water chemistry and variable material export which would further alter the coastal water quality. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: hurricanes USA, Mississippi, lower Pearl River USA, Mississippi, Gulf of Mexico organic matter nutrients

1. Introduction Tropical cyclones have been well-documented as major factors of dramatic coastal environmental changes. For example, previous studies have shown that Hurricane Katrina, an upper-end Category 3 hurricane when made its landfall on the northern Gulf of Mexico, caused significant land loss to the Mississippi and Alabama Barrier Islands, extensive sediment resuspension and terrestrial material export, and the deposition of a storm layer along the river mouth marsh (Fritz et al., 2007; Lohrenz et al., 2008; Reese et al., 2008). Correspondingly, the biogeochemical effects of hurricanes on coastal ecosystems have been intensively investigated, such as organic matter reworking and redistribution (Kang and Trefry, * Corresponding author. State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen University, 182 Daxue Rd, Xiamen 361005, China. E-mail address: [email protected] (Y. Cai). 0272-7714/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ecss.2013.05.019

2003; Allison et al., 2005, 2010; Goñi et al., 2006), post-hurricane phytoplankton bloom (Yuan et al., 2004; Roman et al., 2005), and a decrease in water quality (Walker, 2001; Burkholder et al., 2004; Lohrenz et al., 2008; Zhang et al., 2009). River water is a major source of sediment, organic matter and nutrients to coastal ecosystems (e.g. Spitzy and Ittekkot, 1991; Turner et al., 2007; Milliman and Farnsworth, 2010). Riverine material influx is often responsible for eutrophication and other environmental issues in the coastal waters of the northern Gulf of Mexico (e. g. Lohrenz et al., 1997; Turner et al., 2006). While many studies have examined the impacts of hurricanes on coastal ecosystems, little attention has been paid to the hurricane-related effects on river chemistry and simultaneous riverine chemical export. To date, related investigations either focused on limited chemical parameters, such as dissolved oxygen, pH, turbidity, and dissolved organic carbon (DOC) or could not sample the time period immediately following the landfall of a hurricane (Mallin et al., 1999; Avery et al., 2004; Tomasko et al., 2006; Bonvillain et al., 2011;

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Shiller et al., 2012). The storm surge incurred by direct influence of hurricanes would entrain massive coastal seawater into lower coastal rivers and result in the elevated salinity. Unlike hydrological parameters (salinity and temperature, etc.) that could be monitored by in situ instruments, the acquisition of water chemical data were largely hindered by the collection of discrete water samples under extreme weather and rough river conditions. Therefore, the obtained datasets are often insufficient to completely understand the mechanisms of hurricane effects on the coastal river water chemistry, riverine solutes export, and the interaction between lower coastal rivers and the coastal ocean. Previous investigations suggested hurricane-induced riverine material export caused significant variation of the coastal environment in the northern Gulf of Mexico (Yuan et al., 2004; Lohrenz et al., 2008), but few measurements of water chemistry or solute export from the coastal river were quoted in these investigations. During the 2008 hurricane season, the northern Gulf of Mexico had been struck consecutively by Hurricanes Gustav and Ike, which made landfall at Cocodrie, Louisiana, and Galveston, Texas, respectively. Both hurricanes caused intensive seawater intrusion in the coastal water channels of Lake Portchartrain (Li et al., 2009, 2010). However, simultaneous variation of river water chemistry and riverine material export during the intensive seawater intrusion and subsequent freshwater flooding were rarely reported (Bonvillain et al., 2011). In this study, we sampled the lower Pearl River at Stennis Space Center, Mississippi, during the period from pre-Gustav to post-Ike to investigate the hurricane impacts on water chemistry and material export from the Pearl River. Indeed, the results would help to support the observation of coastal environmental variation in other studies. 2. Materials and methods 2.1. Hurricanes Gustav and Ike Hurricane Gustav, an upper end Category 2 hurricane, made its final landfall near Cocodrie, Louisiana, on September 1, 2008 with a maximum sustained wind speed of near 160 km h 1 at landfall (Fig. 1a). It caused widespread storm surge along the coasts of northern Gulf of Mexico as well as heavy rainfall over the states of Louisiana and Mississippi, leading to moderate freshwater flooding in many coastal rivers. Specifically, storm surges of w1.5 m and w3 m were observed in Lake Pontchartrain and the Bay of St. Louis, respectively (Beven and Kimberlain, 2009). There was about 170 mm rainfall over the lower Pearl River basin (Beven and Kimberlain, 2009). Our river sampling site at Stennis Space Center, Mississippi, is about 120 km northeast of Cocodrie and was under the direct influence of the rain bands of Hurricane Gustav. Hurricane Ike made landfall in Galveston, Texas, on September 13, 2008 (Fig. 1a) as a high-end Category 2 hurricane with a maximum sustained wind speed of near 177 km h 1 at landfall. Hurricane Ike was vast in size with tropical storm force winds extending more than 800 km on September 12, 2008. It caused 1.7 m storm surge and 2.2 m storm tide in the Bay of St. Louis, and 1.7 m storm surge and 1.8 m storm tide in Lake Pontchartrain (Berg, 2009). However, there was little rainfall in the lower Pearl River basin and therefore no freshwater flooding occurred in the lower Pearl River during and immediately after Hurricane Ike’s passage (Berg, 2009). 2.2. Site description The Pearl River originates in the eastecentral portion of the state of Mississippi and discharges into the Mississippi Sound and Mississippi Bight in the northern Gulf of Mexico between the Bay

181

of St. Louis and Lake Pontchartrain (Fig. 1b). Located in southeastern Louisiana and southwestern Mississippi, the Pearl River is a forested blackwater river, with a length of 780 km and a drainage basin of 22,690 km2 (Duan et al., 2007a, b). The Pearl River basin is dominated by forested and agricultural land with 65% and 30% coverage, respectively (http://www.deq.state.ms.us). Moreover, areas of swamp and salt marsh are widespread along the riparian zone and river delta (Duan et al., 2007a, b). The Pearl River diverges between Bogalusa, Louisiana, and Picayune, Mississippi, into the (East) Pearl River and the West Pearl River. During low flow seasons, most of the upper Pearl River discharge flows into the West Pearl River, while the East Pearl River flow receives input largely from the Hobolochitto Creek, Mike River, and other minor tributaries. However, during the flooding season, the East Pearl River is connected to and receives water flow from the main river channel. The sampling site on the lower (East) Pearl River is located at Stennis Space Center (30.349 N, 89.642 W), Mississippi, about 25 km upstream of the Mississippi Sound coastline (Fig. 1). 2.3. Sample collection and analysis Upstream surface water samples were collected from the shoreline at the site every 2 or 3 days from August to September 2008, using a pre-cleaned 1 L high density polyethylene bottle bound to a pole sampler. Aliquots of water samples were filtered through 0.4 mm polycarbonate filters (Millipore) and precombusted 0.7 mm GF/F filters (Whatman). Filtrates passing through the polycarbonate filters were collected for inorganic nutrient analyses, including dissolved inorganic nitrogen (DIN, consisting of NO3, NO2 and NH4), phosphate (DIP), and silicate (Si(OH)4), while filtrates passing through the GF/F filters were analyzed for total dissolved nitrogen (TDN), chromophoric dissolved organic matter (CDOM), DOC, dissolved inorganic carbon (DIC) and stable carbon isotopic composition of DOC and DIC. After filtration, the polycarbonate filter samples were freeze-dried and weighed for suspended particulate matter (SPM) concentrations. The GF/F filter samples were freeze-dried and acid-fumed to determine concentrations and stable isotopic composition of particulate organic carbon (POC) and particulate nitrogen (PN). Concentrations of DIN (including nitrate, nitrite and ammonium) were measured using fluorometric methods and concentrations of DIP and Si(OH)4 were analyzed with spectrophotometric methods on an Astoria Pacific A2 nutrient autoanalyzer (Methods #A179, A027, A205, and A221; Astoria-Pacific International, Oregon USA). Concentrations of DOC, DIC and TDN were measured on a Shimadzu TOC analyzer (TOC-V) interfaced with a nitrogen detector (TNM-1; Cai et al., 2008). The total DOC blank (including Milli-Q water, acid for sample acidification, and the instrument blank) was generally less than 6 mM. Precision was better than 2% and accuracy was within 1% based on the analysis of DOC standards (Cai et al., 2008). Dissolved organic nitrogen (DON) concentrations were calculated from the difference between TDN and DIN concentrations. d13C of DOC (d13C-DOC) and DIC (d13C-DIC) were measured on a PDZ Europa 20e20 isotope ratio mass spectrometer (Sercon Ltd.) interfacing with an O.I. Analytical Model 1010 TOC Analyzer (OI Analytical) at the University of California-Davis Stable Isotope Facility. Concentrations of POC and PN as well as their d13C (d13C-POC) and d15N (d15N-PN) values were also measured at the University of California-Davis Stable Isotope Facility on a PDZ Europa ANCA-GSL elemental analyzer interfaced to a PDZ Europa 20e20 isotope ratio mass spectrometer (Sercon Ltd.). The absorption coefficient of CDOM was measured on a UVe Visible spectrophotometer (Cary 300) with 1 cm quartz cuvettes

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Fig. 1. (a) Map of the northern Gulf of Mexico, showing the sampling location at the Pearl River and the landfall locations of Hurricane Gustav at Cocodrie, Louisiana, on September 1, 2008, and Hurricane Ike at Galveston, Texas, on September 13, 2008 (upper panel); (b) Locations of time series samples collected from the lower (East) Pearl River (EPR) at Stennis Space Center (SSC), Mississippi, from August 12 to September 26, 2008. WPR denotes West Pearl River.

using Milli-Q water as a blank reference. The absorption coefficient at 370 nm (a370) was reported as CDOM concentration for consistency with the reference wavelength of the spectral slope parameter. The spectral slope parameter (S370) was obtained using nonlinear fitting of the absorption coefficient over the range from 300 to 700 nm, with 370 nm as a reference wavelength (Belzile and Guo, 2006). Specific ultraviolet absorbance (SUVA) at 254 nm, an index of aromaticity of dissolved organic matter (DOM), was calculated as the ratio of absorbance at 254 nm to DOC concentration.

3. Results 3.1. Hydrology Pearl River discharge was obtained from the closest US Geological Survey hydrological station near Bogalusa, Louisiana (http://waterdata.usgs.gov/nwis/uv?02489500). Discharge was less than 250 m3 s 1 during most of the sampling period except for the flooding period between September 3e12, in response to the rainfall brought by Hurricane Gustav (Table S1, Fig. 2a).

Y. Cai et al. / Estuarine, Coastal and Shelf Science 129 (2013) 180e188

14 10 8

DIN PO4

6 4

SiO3 DIC

2

1

09/01

09/11

c

DOC DON a

5 1000

20

500

40

30

0 08/12 50

500

60 200 100

20 0 08/12

0 08/22

09/01 09/11 Date

30 20

0

Fig. 2. Variations of hydrological parameters and water chemistry in the lower Pearl River during Hurricanes Gustav and Ike. (a) Discharge (Q), salinity (S), water temperature (T) and pH value; (b) Concentrations of nutrients: dissolved inorganic nitrogen (DIN), phosphate (DIP), dissolved silicate (Si(OH)4), and DIC; (c) Concentrations of DOC, DON and CDOM (a370); (d) Concentrations of suspended particulate matter (SPM), POC and PN. Stages 1, 2 and 3 denote Hurricane Gustav induced intense seawater intrusion, Hurricane Gustav rainfall induced freshwater flooding and Hurricane Ike induced mild seawater intrusion, respectively.

Salinity at the sampling site remained close to 0 but jumped to 7.5 on September 2 when Hurricane Gustav made landfall. The river water salinity then dropped to near 0 again in the two days post-landfall and increased slightly to 1.2 on September 12 when the outer rainbands of Hurricane Ike swept over coastal Louisiana and Mississippi (Table S1, Fig. 2a). The variation of pH in the river is quite similar to that of salinity, with two peak values on September 2 and 12, respectively (Table S1, Fig. 2a). The water temperature generally decreased from 30  C to 24  C during the sampling period, in accordance with the seasonal variation. Furthermore, a sudden drop (4  C) of the river water temperature was observed during the Hurricane Gustav induced intensive seawater intrusion and river flooding events (Table S1, Fig. 2a).

δ13C-DIC 08/22

09/01

-16 09/11

δ C-DOC DOC/DON 13

09/21

b

40

-27.5 -28.0 -28.5

35 -29.0

30

20 08/12

10

09/21

-14

N/P

-29.5

25

-30.0 08/22

09/01

09/11

0.18 -1

300

40

40

400

-12

09/21

c

0.020

0.16

0.018

0.14

0.016

-1

d

SUVA (m μM )

POC PN SPM

20

09/21

0.12 0.10 08/12

0.014

SUVA S370

0.012 08/22

09/01

09/11

20

POC/PN

09/11

PN (µM)

80

09/01

POC (µM)

SPM (mg/L)

100

0 08/22

DOC/DON

45 10 08/12 120

-10

15 10

50

-6 -8

20

09/21 1500

a

25

0 08/22

2 3

30

13

12

δ C-DIC (‰)

b

δ13C-DOC (‰)

4.5

09/21

09/21

d

5

15 0 -25 10

5 08/12

S370

a370 (m )

30

5.0

10

13

PO C 15 P N POC/PN

08/22

15

300

5.5

15

13

09/11

6.0

Concentrations of DIN, DIP and Si(OH)4 varied between 3.0 and 13.0 mM, 0.33e1.57 mM and 12.3e142 mM, respectively, during the sampling period (Table S1). Concentrations of all nutrient species decreased to half to one-tenth of their pre-hurricane values during September 2e10, when the salinity maximum and peak discharge were observed (Fig. 2b). In contrast, little variation was observed on September 12, when salinity in the Pearl River increased to 1.2. The N/P ratio (3e28) of the inorganic nutrient pool was usually lower than the Redfield Ratio (16) for all samples other than that collected on September 2, which had an N/P ratio of 28 (Table S1, Fig. 3a). DIC

δ C-POC or δ N-PN (‰)

09/01

600

0 08/12 40

-1

08/22

6.5 pH

25 20

0 08/12 900 DIC or Si(OH)4 (μM)

0

7.0

4 2

100

30

N/P

200

7.5

DON (µM)

300

35

T (ºC)

400

a

Q S T pH

DOC (µM)

6 Salinity

Q (m3/s)

500

3.2. Nutrients and dissolved inorganic carbon

2 3

8

DIN or DIP (µM)

1 600

183

-30 09/01 09/11 Date

09/21

Fig. 3. Variations in elemental and isotopic compositions of inorganic and organic nutrients, DIC, DOM, and POM in the lower Pearl River during Hurricanes Gustav and Ike. (a) N/P ratio of the inorganic nutrient pool and d13C of DIC; (b) C/N ratio and d13C of DOM; (c) Specific ultraviolet absorbance (SUVA) and spectral slope parameter (S370) of CDOM; (c) C/N ratio, d13C and d15N of POM. Stages 1, 2 and 3 denote Hurricane Gustav induced intense seawater intrusion, Hurricane Gustav rainfall induced freshwater flooding and Hurricane Ike induced mild seawater intrusion, respectively.

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concentrations ranged from 439 to 811 mM and its d13C value was 14.5& to 6.45& (Table S1). Both DIC concentration and its carbon isotopic composition reached the maximum values (811 mM and 6.45&, respectively) on September 2. The DIC concentration also increased to more than 700 mM around September 12, though d13C-DIC remained close to pre-hurricane data (Fig. 3a). The DIC concentration decreased to less than 500 mM when the freshwater level rose significantly in the river, but d13C-DIC did not vary much from the pre-hurricane data (Figs. 2b and 3a).

4. Discussion 4.1. Hydrology

Concentrations of DOC, DON and CDOM showed a similar pattern during the sampling period. Minimum values were observed (655 mM, 22.6 mM and 11.4 m 1 for DOC, DON and CDOM, respectively) on September 2 when the salinity dramatically increased to the 7.5. DOC, DON and CDOM reached their peak values at 1408 mM, 40.6 mM and 35.9 m 1, respectively, during September 4e10 when rainfall-induced flooding occurred in the river. Concentrations of DOC, DON and CDOM also showed a slight decrease on September 12, corresponding to a seawater intrusion event as shown by the elevated salinity (1.2; Table S2, Fig. 2c). Ratios of DOC/DON and SUVA values also varied with time, indicating changes in organic matter composition and sources. The DOC/DON ratio increased during September 2e8 and reached a maximum of 36 at the ascending flank of the freshwater flooding (Table 2, Fig. 3b), while the SUVA ratio decreased sharply to 0.118 m 1 mM 1 on September 2 (Table S2, Fig. 3c). d13C-DOC values ranged from 29.1& to 28.0& during the hurricane periods, with an indistinguishable difference from pre- or posthurricane seasons (p > 0.05; Table S2, Fig. 3b). In contrast to d13C-DOC, the spectral slope value, S370, increased from 0.014 nm 1 prior to hurricanes to 0.017 nm 1 under the hurricane impact and returned to 0.014 nm 1 in the post-hurricane period (Table S2, Fig. 3c).

Li et al. (2009, 2010) observed that Hurricane Gustav caused more severe seawater intrusion in the Lake Pontchartrain, an estuary adjacent to the Pearl River, than Hurricane Ike. Salinity at our sampling site was 6 times higher during Hurricane Gustav (September 2) than Hurricane Ike (September 12) (Fig. 2a). This was very similar to the record in Lake Pontchartrain (Li et al., 2009, 2010) and was coincident with storm surge propagation along the Louisiana-Mississippi Gulf Coast (Berg, 2009; Beven and Kimberlain, 2009). In addition to the seawater intrusion during hurricane-induced storm surge, freshwater flooding was observed in the lower Pearl River during September 5e10 after the landfall of Hurricane Gustav. This freshwater flooding was mainly caused by rainfall of the Hurricane Gustav, which had been reported as much as 50 cm over the states of Louisiana and Mississippi (Beven and Kimberlain, 2009). On the contrary, Hurricane Ike only brought 13 cm rainfall in eastern Louisiana and minimal rainfall (0.3 cm in the city of Pascagoula) in Mississippi (Berg, 2009), and thus no freshwater flooding was observed in the Pearl River following storm surge on September 12. Based on the variations in hydrographic parameters, three distinctive hydrological events can be identified: (1) intense seawater intrusion induced by Hurricane Gustav’s strong storm surge; (2) river flood induced by Hurricane Gustav’s rainfall; and (3) mild seawater intrusion and rising water level due to Hurricane Ike’s storm surge (Fig. 2). Given a salinity of 30 for northern Gulf of Mexico coastal seawater (Wang et al., 2010), the fraction of seawater in the lower Pearl River could be estimated as 25% during the intense seawater intrusion and 4% during mild seawater intrusion, respectively. However, the seawater fraction may be underestimated due to the possible stratification at the sampling site, especially during the mild seawater intrusion when the fairly calm river water surface was observed.

3.4. Particulate organic matter

4.2. Hurricane Gustav induced seawater intrusion

The SPM concentration was in the range of 18.5e105 mg L 1 during the sampling period, with maximum concentrations (105 and 45.5 mg L 1) occurring on September 2 and 8, respectively (Table S1, Fig. 2d). The variation of both POC and PN concentrations generally followed the trend of SPM concentration. Under the influence of Hurricane Gustav, concentrations of SPM, POC and PN rapidly increased by 3e5 times above their pre-hurricane levels on September 2. Concentrations decreased after the landfall of Hurricane Gustav, but still remained at a relatively high level during the freshwater flooding period. The concentrations of POC and PN increased on September 12 while the SPM concentrations showed no difference from pre-hurricane data (Table S1, Table S2, Fig. 2d). Abundances of 13C, 15N and C/N ratio of particulate organic matter (POM) are presented in Table S2 and Fig. 3d. d13C-POC ranged from 29.0& to 28.3& during both pre- and posthurricane periods, which was almost identical to d13C-DOC (p > 0.05). However, d13C-POC increased to 25.7& on September 2 and remained above 28.0& until September 12, higher than d13C-DOC during hurricane-impacted periods (Table S2, Fig. 3d). d15N-PN varied between 1.3 and 4.5 without clear correspondence to hurricane events. The POC/PN ratio was in the range of 9.2e14.9, lower than the DOC/DON ratio for the same samples but higher than the Redfield Ratio (6.6). The POC/PN ratio of the sample collected on September 2 was slightly less than those collected during August 28 to September 8, while the minimum value of 9.2 occurred on September 15.

During the intense seawater intrusion caused by the strong storm surge of Hurricane Gustav, river water chemistry at the sampling site was dramatically altered. To distinguish if there were any processes other than conservative mixing between seawater from the northern Gulf of Mexico and freshwater from the Pearl River to dominate the variation of water chemistry, a conservative (isotopic) mixing model was applied to predict the concentrations and compositions of each individual constituent for comparison with the observed values (Table 1). Both DIC and DOC and their stable isotopic compositions showed differences less than 10% between observed results and the conservative mixing estimation (Table 1). This indicated that water mixing was a major factor controlling the variation of DIC and bulk DOC concentrations and compositions in river water during the intense seawater intrusion. Unlike DOC which showed conservative mixing behavior, CDOM concentrations were 43% below the model predicted results during the intense seawater intrusion (Table 1), suggesting the preferential removal of CDOM during mixing of seawater and river water. The observation of preferential removal of CDOM during the seawater intrusion was consistent with numerous studies that showed the preferential removal of high molecular weight DOM as well as CDOM during estuarine mixing (e.g. Guo and Santschi, 1997; Chen et al., 2007; Wang et al., 2010). Incidentally, the Pearl River is a blackwater river enriched in high molecular weight DOM and humic substances (Duan et al., 2007a, b; Cai and Guo, 2009). These high molecular weight DOM and

3.3. Dissolved organic matter

Y. Cai et al. / Estuarine, Coastal and Shelf Science 129 (2013) 180e188

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Table 1 Model estimation of chemical parameters based on the conservative mixing of Gulf of Mexico water and Pearl River effluent during normal weather conditions, and their differences with observations during intense seawater intrusion induced by Hurricane Gustav (Stage 1) and mild seawater intrusion induced by Hurricane Ike (Stage 3). Considering the uncertainties in both end-members, 20% is tentatively chosen as the criterion to indicate the significant difference between model-predicted and observed results. Chemical compositions of the Gulf of Mexico are from Cai and Guo (2009), Cai et al. (2012), Gruber et al. (1999), Lohrenz and Cai (2006), Montoya et al. (1992), Sackett (1991) and Cai Yihua (unpublished CDOM data). Species

DIN (mM) PO4 (mM)

Gulf of Mexico 0.2 PR at normal 8.8 condition PR at stage 1 9.3 Model result at 6.7 stage 1 Difference 40% PR at stage 2 3.9 PR at stage 3 4.3 Model result at 8.5 stage 3 Difference L49%

0.05 1.1

SiO3 (mM) DIC (mM) DI13C (&) DOC (mM) DON (mM) a370 (m

1 )

DO13C (&) POC (mM) PN (mM) PO13C (&) P15N (&)

5.0 127

1900 550

1.0 14

100 900

6.0 34

2.3 25.9

20.0 28.5

100 125

7.0 10.5

20.0 28.9

6.0 2.2

84 97

811 888

6.4 6.0

655 700

22.6 27

11.4 20.0

28.6 28.2

439 119

35.8 9.6

25.7 27.1

3.5 2.9

L64% 0.9 0.7 1.1

13% 44.0 134 122

9% 530 714 604

7% 13.6 13.6 12.1

6% 1256 983 868

16% 40 37 33

L43% 34.6 27.9 25.0

1% 28.5 28.2 28.5

270% 217 108 124

272% 16.4 9.6 10.4

5% 27.5 27.7 28.8

21% 2.5 2.7 2.3

L34%

10%

18%

13

13%

12%

4%

17%

0.3 0.8

12%

humic substances could be efficiently removed when mixing with brackish waters (Sholkovitz et al., 1978). The variations in both S370 and SUVA values further supported the above hypothesis of the preferential removal of terrestrial high molecular weight DOM during seawater intrusion induced by Hurricane Gustav. The different behaviors between CDOM and bulk DOC could have resulted in the decreased SUVA value (Table S2, Fig. 3c). The decreased SUVA value suggested the less aromatic nature of DOM during seawater intrusion and was consistent with the decrease in the abundance of high molecular weight DOM (Weishaar et al., 2003). The variation in DOM molecular weights and quality was also suggested by the increasing S370 value of the CDOM sample collected on September 2 (Table S2, Fig. 3c), corresponding to an increased fraction of low molecular weight DOM (Stedmon and Markager, 2001; Weishaar et al., 2003; Zhou et al., 2013). DIN and phosphate concentrations showed contrasting behaviors during the intense seawater intrusion induced by Hurricane Gustav and deviated from the model-predicted results by 40% and 51%, respectively (Table 1). This indicated the simultaneous occurrence of both addition of DIN and removal of phosphate during the seawater-river water mixing, causing a three-fold increase in the N/P ratio (Fig. 3a). The extra DIN was likely derived from the release of benthic interstitial water and the dissociation of suspended particles. This is consistent with previous observations of significant DIN flux across the sedimentewater interface in the northern Gulf of Mexico estuaries under normal weather conditions (Gardner et al., 2006; Bianchi, 2007). It was reasonable to expect that the hurricane-induced activities might enhance the DIN flux released by surface sediment while the sediment layers were heavily disturbed. However, phosphate concentration in the river water did not increase as expected through the release of regenerated nutrients from the sediment. This was likely the result of high SPM concentrations which increased by around 5 times during the intense seawater intrusion (Fig. 2d; Table 1). The elevated SPM concentration would cause the re-partitioning of P between dissolved and particulate phases due to its high particle reactivity and small variation of apparent distribution coefficient during estuarine mixing, and thus scavenged the phosphate from the solution (Froelich, 1988; Santschi, 1995). Nevertheless, a better understanding of the non-conservative behavior of nutrients during the intense seawater intrusion requires future field and laboratory investigations of benthic nutrient cycles at the riveresea interface. Hurricanes have long been recognized as natural modifiers of coastal geomorphology through intensive coastal erosion and sediment reworking in the northern Gulf of Mexico (Fritz et al., 2007; Reese et al., 2008; Ravens et al., 2009; Chen et al., 2009;

1%

13%

7%

Allison et al., 2010). Both processes will not only increase the concentrations of SPM and associated POM in the hurricaneimpacted coastal and estuarine environments, but also alter the composition of POM. Comparing the results of the conservative mixing model, the observed concentrations of POM were found to be 2.7 times higher (Table 1), indicating the dominance of additional sources of particles and associated POM under the influence of Hurricane Gustav. Meanwhile, POM was also characterized with higher d13C and d15N values than the isotopic conservative mixing results (Table 1), indicating the alteration of riverine POM composition by the additional POM sources. Our previous investigation in the Bay of St. Louis, a small estuary about 20 km to the north of the sampling site, showed that POM in the water column of the bay mouth had an average isotopic and elemental composition of 25.5&, 3.13& and 11.4 for d13C, d15N and C/N ratio, respectively, due to the resuspension of coastal sediments (Cai et al., 2012). The POM compositions measured in the river during the intense seawater intrusion were similar to those found at the mouth of the Bay of St. Louis (Table S2; Fig. 3d). This might indicate that coastal sediments, carried by the entrained seawater, were one of the major sources of SPM and POM in the Pearl River during seawater intrusion. This was also consistent with the previous satellite observation that hurricanes would cause intense sediment resuspension and disturbance in the same area (Lohrenz et al., 2008). 4.3. Hurricane Gustav induced freshwater flooding Hurricane-induced precipitation caused the freshwater flood in the lower Pearl River shortly after the landfall of Hurricane Gustav (Table S1, Fig. 2a). During the flood, nutrient concentrations (N, P, and Si) in the Pearl River dropped sharply and only accounted for half to one-tenth of the pre-hurricane concentration (Table 1). Zhang et al. (2009) observed increasing nutrient levels in Biscayne Bay, Florida, after Hurricane Katrina due to the agricultural effluent brought by the hurricane-induced runoff. In contrary, our results indicated that there was no major nutrient source from the upper Pearl River watershed during the heavy rainfall events. This was in accordance with the dominant forest coverage (65%) over agricultural land (30%) in the Pearl River basin. The DIN/DIP ratio in the river during freshwater flooding was similar to the pre-hurricane samples (Table S1, Fig. 3a), further implying identical nutrient sources between pre-hurricane and freshwater flooding samples. In contrast to inorganic nutrient concentrations, DOM concentrations (including DOC, DON and CDOM) in the river increased from the minimum value during the hurricane-induced intensive seawater intrusion to its peak value during freshwater flooding

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(Table S2, Fig. 2c). The hydrological control of DOM concentrations in the Pearl River had been reported in previous investigations (Duan et al., 2007a, b). Based on elemental, isotopic, and biomarker compositions, Duan et al. (2007a, b) suggested that riverine DOM during flooding events were mostly from surface soil and plant litters, while deep soil layers supplied DOM during dry seasons. In this study, d13C values of bulk DOM during river flooding did not change compared to the pre-hurricane values, indicating the same terrestrial C3 plant source for DOM. However, the C/N ratio of bulk DOM did increase from 22 at base flow to 36 (Table S2, Fig. 3b), coincident with the lower SUVA and higher S370 during freshwater flooding than those of pre-hurricane levels (Table S2, Fig. 3c). The variation of C/N ratio and optical characteristics further confirmed the input of freshwater high molecular weight DOM to the river during flood seasons. Concentrations of SPM and POM increased more than 50% during river flooding compared to the pre-hurricane period, though they were less than half of those during the intensive seawater intrusion (Table 1, Fig. 2d). Meanwhile, the riverine POM during river flooding was more enriched in 13C than pre-hurricane samples (Table S2, Fig. 3d), implying a different source of POM. One of the likely sources for such POM could be the degradation products of swamp marsh plants (widespread in the lower Pearl River), since riverine POM derived from marsh plants had slightly higher d13C and d15N than those of typical terrestrial vascular plants (Fry and Sherr, 1984). The high river flow would connect the swamps, which were usually isolated during dry seasons, to the main river channel and flush out the swamp materials deposited during dry seasons. The enriched 13C of POM might also reflect the lower contribution of autochthonous primary production during high flow events as suggested by the composition of particulate pigments and amino acids (Duan and Bianchi, 2006, 2007).

4.5. Potential impacts on coastal waters The average concentrations of chemical parameters in the lower Pearl River are listed in the Table 1 for different hydrological stages: pre- and post-hurricane, Hurricane Gustav induced intense seawater intrusion, Hurricane Gustav induced freshwater flooding, and Hurricane Ike induced mild seawater intrusion. Mild seawater intrusion from Hurricane Ike resulted in slight variations of water chemistry in the river except in DIN and phosphate concentrations. Thus, the export of riverine materials to the coastal waters under weak impacts from Hurricane Ike would not be noticeably altered. In contrast to Hurricane Ike, Hurricane Gustav had a great impact on river chemistry through the intense seawater intrusion and subsequent freshwater flooding in the river. During the intense seawater intrusion, which decreased the DOM and nutrient concentrations, the concentrations of SPM and associated POM in the river increased by 3e5 times compared with pre- and posthurricane periods at the sampling station (Table 1). Such high amounts of particulate substrate would be delivered into estuarine and coastal waters during the retreat of storm surge waters. It might be one of the important processes that created the highly turbid water shortly after the passage of the hurricanes, as well as the coastal sediment redistribution and the formation of a storm sedimentation layer in the Pearl River estuary and the Mississippi Sound as observed after Hurricane Katrina (Fritz et al., 2007; Lohrenz et al., 2008; Reese et al., 2008). The freshwater flooding following the intense seawater intrusion also increased the DOC and POM concentrations in the river by 1.5e2 times through the flushing of terrestrial materials into the lower river (Table 1). To better understand the role of hurricaneinduced freshwater flooding on the transport of terrestrial material to the coastal waters, riverine exports of individual constituents from the Pearl River during the freshwater flooding were estimated with the flow-weighted mean concentrations method (e.g. Warnken and Santschi, 2004; Guo et al., 2012). The annual exports of these chemicals (DIC, DOC, POC and nutrients) during 2007e 2008 were also calculated with USGS LOADEST program and flowweighted mean concentrations, respectively (Guo et al., 2012). As seen in Table 2, the Pearl River discharged 4107 m3 of freshwater during a 9 days flooding period, accounting for 7% of the annual runoff. Consistent with the disproportional river discharge, the Pearl River exported about 15% and 24% of annual DIC and DOC loads, respectively, during the freshwater flooding period (Table 2). Meanwhile, up to 6% of the annual phosphate load was transported during the hurricane-induced freshwater flooding, which was higher than the transported portions of DIN (1.7%; Table 2). The sudden export of riverine organic matter and nutrients after the passage of hurricanes was consistent with previous observations in other coastal rivers and estuaries (Avery Jr. et al., 2004; Burkholder et al., 2004; Zhang et al., 2009). Our results in the Pearl River provide fluvial evidences to support previous observations of hurricane impacts on the optical characteristics and organic geochemistry in the river-dominated ocean margin along the northern Gulf of Mexico. Yuan et al. (2004) proposed the nutrient-rich Mississippi River plume water

4.4. Hurricane Ike During Hurricane Ike, only a slight increase in salinity (1.2) in the lower Pearl River was observed (Table S1, Fig. 2a). A salinity of 1.2 corresponded to the presence of about 4% coastal seawater at the sampling site. Therefore, seawater intrusion to the lower Pearl River during Hurricane Ike was much weaker than during Hurricane Gustav, which resulted in 25% coastal seawater intrusion at the same sampling site. Nevertheless, the 4% of coastal seawater entrained in the river during Hurricane Ike could cause a mixing effect on solutes overwhelmed by the natural variation of water chemistry in the Pearl River. This was supported by the less than 20% difference between field observations during mild seawater intrusion and conservative mixing model predictions for concentrations and compositions of most solutes in the river (Table 1, Fig. 2, Fig. 3). However, DIN and phosphate concentrations were 34%e49% less than values calculated from the conservative mixing model (Table 1), implying the removal of both nutrient species during the mild seawater intrusion. Overall, the effects of Hurricane Ike on the water chemistry in the sampling site were much weaker than Hurricane Gustav due to the long distance between the landfall site and sample site.

Table 2 Freshwater discharge (m3) and fluxes of carbon and nutrients (106 mol) from the Pearl River.

Flux during Hurricane Gustav Annual flux Percentage of flux during Hurricane Gustav in annual flux

Days

Discharge

DIC

DOC

POC

DIN

9 365

4107 58985

191 1259

443 1852

75.2 2665

1.41 82.6

7%

15%

24%

2.5%

2.8%

1.7%

PO4 0.32 5.74 6%

SiO3

Data source

17.2 378

This study Yihua Cai and Alan Shiller (unpublished data)

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would be injected into the open Gulf of Mexico by hurricaneinduced eddies, thus causing a phytoplankton bloom. For a Plimited marine ecosystem in the Gulf of Mexico (Cai and Guo, 2009), our observation of the higher portion of riverine phosphate export flux during the hurricane was especially important to promote and maintain the high primary production. Lohrenz et al. (2008) observed the discolored seawater persisted in the Mississippi Sound after the dissipation of highly turbid waters following the landfall of Hurricane Katrina. This was consistent with our observation in the river that only up to 3% of the annual POC flux was transported into coastal waters during hurricane induced freshwater flooding, while 24% of the annual DOC flux was transported (Table 2). Together with the high aromatic characteristics of DOM in the Pearl River, the longer residence time of riverine DOM compared to POM might account for the persistence of discolored water in the Mississippi Sound after the landfall of Hurricane Katrina (Lohrenz et al., 2008). Acknowledgment This work was supported in part by Northern Gulf Institute/ NOAA (Projects 09-NGI-13 and 09-NGI-04), National Natural Science Foundation of China (40906040), Fundamental Research Funds for the Central Universities of China, and Natural Science Foundation of Fujian Province of China (2011J01277). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.ecss.2013.05.019. References Allison, M.A., Sheremet, A., Goñi, M.A., Stone, G.W., 2005. Storm layer deposition on the Mississippi-Atchafalaya subaqueous delta generated by Hurrican Lili in 2002. Continental Shelf Research 25, 2213e2232. Allison, M.A., Dellapenna, T.M., Gordon, E.S., Mitra, S., Petsch, S.T., 2010. Impact of Hurricane Katrina (2005) on shelf organic carbon burial and deltaic evolution. Geophysical Research Letter 37, L21605. http://dx.doi.org/10.1029/2010GL044547. Avery Jr., G.B., Kieber, R.J., Willey, J.D., 2004. Impact of hurricanes on the flux of rainwater and Cape Fear River water dissolved organic carbon to Long Bay, southeastern United States. Global Biogeochemical Cycles 18, GB3015. http:// dx.doi.org/10.1029/2004GB002229. Belzile, C., Guo, L.D., 2006. Optical properties of low molecular weight and colloidal organic matter: application of the ultrafiltration permeation model to DOM absorption and fluorescence. Marine Chemistry 98, 183e196. Berg, R., 2009. Tropical Cyclone Report, Hurricane Ike, 1e14 September 2008. NOAA National Hurricane Center. unpublished. Beven II, J.L., Kimberlain, T.B., 2009. Tropical Cyclone Report, Hurricane Gustav, 25 August-4 September 2008. NOAA National Hurricane Center. unpublished. Bianchi, T.S., 2007. Biogeochemistry of Estuaries. Oxford University Press, New York, p. 706. Bonvillain, C.P., Halloran, B.T., Boswell, K.M., Kelso, W.E., Harlan, A.R., Rutherford, D.A., 2011. Acute physiochemical effects in a large river-floodplain system associated with the passage of Hurricane Gustav. Wetlands 31, 979e987. Burkholder, J., Eggleston, D., Glasgow, H., Brownie, C., Reed, R., Janowitz, G., Posey, M., Melia, G., Kinder, C., Corbett, R., Toms, D., Alphin, T., Deamer, N., Springer, J., 2004. Comparative impacts of two major hurricane seasons on the Neuse River and western Pamlico Sound ecosystems. Proceedings of the National Academy of Sciences of the United States of America 101, 9291e9296. Cai, Y., Guo, L., 2009. Abundances and variations of colloidal organic phosphorus in river, estuarine and coastal waters in the northern Gulf of Mexico. Limnology and Oceanography 54, 1393e1402. Cai, Y., Guo, L., Douglas, T.A., 2008. Temporal variation in organic carbon species and fluxes from the Chena River, Alaska. Limnology and Oceanography 53, 1408e 1489. Cai, Y., Guo, L., Wang, X., Mojzis, A.K., Redalje, D.G., 2012. Sources and distribution of dissolved and particulate organic matter in the Bay of St. Louis in the northern Gulf of Mexico. Estuarine, Coastal and Shelf Science 96, 96e104. Chen, S., Huang, W., Wang, H., Li, D., 2009. Remote sensing assessment of sediment re-suspension during Hurricane Frances in Apalachicola Bay, USA. Remote Sensing of Environment 113, 2670e2681. Chen, Z., Hu, C., Conmy, R.N., Muller-Karger, F., Swarzenski, P., 2007. Colored dissolved organic matter in Tampa Bay, Florida. Marine Chemistry 104, 98e109.

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