Texas Coastal Hypoxia Linked to Brazos River Discharge as ...

Aquat GeochemFig. 2 Station map for BR: June (TX-06-07), SEAMAP: June (CR0276), BR: July (TX-07-07), BRRR(TX-08-07), and BR: September (TX-09-07) cruises of coastal Texas in summer 2007. Bathymetry contoursshown are 10, 20, 30, and 40 m isobathsof these cruises is to assess fisheries stocks and health in relation to DO and otherhydrographic (salinity and temperature) observations. Trawl stations are determined on arandom grid with areal coverage of the continental shelf from about the 5-m isobath to theshelf break (about 200 m depth). Dissolved oxygen, salinity, and temperature data werecollected during the central coastal Texas leg of cruise CR0276 on June 26–28, 2007. Thedata were collected using a Seabird CTD and Seabird SB43 dissolved oxygen probe. Allinstruments and sensors are factory-calibrated once per year. All data are processed andquality controlled by NOAA. Locations of the SEAMAP data are shown in Fig. 2; Table 3presents the SEAMAP hydrographic and DO data used in this study.2.3 Brazos River (BR) Hydrographic and DO Data SetsShort surveys close to the Brazos River mouth were conducted by D.I.V.E., LLC, on June14 (TX-06-07), July 14 (TX-07-07), and September 18, (TX-09-07) 2007. This group alsoparticipated in the August 8, 2007 cruise. DO and water temperature were measured usinga YSI-55 DO meter, and conductivity was measured using a Pinpoint Salinity Monitor.Comparisons of measurements made using TAMU’s YSI-650 and D.I.V.E.’s YSI-55 andPinpoint Salinity Monitor were within acceptable tolerances for coastal ocean observations(i.e., *1 PSU). Locations of the BR stations are shown in Fig. 2; Table 4 presents thehydrographic and DO data used in this study.2.4 River Discharge DataRiver discharge time-series data for the Brazos River were obtained from the U.S. GeologicalSurvey (http://waterdata.usgs.gov/tx). Daily discharge values from the gage near123

Aquat GeochemTable 2 Summary of summer 2007 hydrographic and DO data from BRRR cruise (TX-08-07)Station Latitude (°N) Longitude (°W) Totaldepth (m)Observ.depthDO(mL L -1 )Temperature(°C)SalinityPSS-781A 28° 56.549 0 95° 12.689 0 12.19 Bottom 2.05 27.39 –Surface – – 32.721B 28° 53.549 0 95° 10.868 0 16.00 Bottom 1.92 27.25 35.57Surface – – 31.982A’ 28° 50.223 0 95° 22.056 0 9.75 Bottom 1.97 28.35 –Surface – 29.22 6.952A 28° 50.071 0 95° 22.285 0 13.08 Bottom 1.97 28.31 34.73Surface – – 32.872B 28° 47.581 0 95° 20.388 0 18.99 Bottom 2.07 27.56 35.50Surface – – 31.552C 28° 44.502 0 95° 18.462 0 17.50 Bottom 2.70 27.38 34.36Surface – – 31.242D 28° 42.061 0 95° 16.402 0 21.18 Bottom 3.56 27.61 35.76Surface – – 31.473A 28° 45.019 0 95° 32.020 0 9.45 Bottom 3.76 28.59 35.44Surface – 30.39 32.313B 28° 42.857 0 95° 29.914 0 13.72 Bottom 3.96 27.98 35.56Surface – 30.15 31.533C 28° 39.565 0 95° 27.883 0 17.07 Bottom 3.89 27.89 35.68Surface – 30.67 31.163D 28° 37.464 0 95° 25.939 0 18.29 Bottom 3.96 27.62 35.72Surface – 30.33 32.104A 28° 41.549 0 95° 40.539 0 10.06 Bottom 2.24 28.14 35.32Surface – 30.57 32.604B 28° 38.522 0 95° 38.380 0 13.62 Bottom 2.59 27.77 35.37Surface – 30.38 30.564C 28° 36.104 0 95° 36.581 0 15.09 Bottom 3.86 27.90 35.68Surface – 30.63 31.634D 28° 33.527 0 95° 35.011 0 19.51 Bottom 4.17 27.92 35.74Surface – 32.20 32.435A 28° 38.513 0 95° 49.005 0 8.53 Bottom 1.91 28.10 –Surface – 30.41 33.675B 28° 35.505 0 95° 47.155 0 12.68 Bottom 2.41 27.97 35.30Surface – 30.53 32.815C 28° 33.122 0 95° 44.756 0 15.70 Bottom 4.10 27.90 35.56Surface – 30.71 32.835D 28° 30.542 0 95° 43.728 0 19.26 Bottom 2.44 27.41 35.70Surface – 30.71 32.516A 28° 34.518 0 95° 57.979 0 8.29 Bottom 2.44 28.05 35.77Surface – 30.76 30.676D 28° 25.507 0 95° 52.431 0 20.42 Bottom 3.21 27.63 35.77Surface – 30.79 32.91123

Aquat GeochemTable 3 Summary of 26–28 June 2007 hydrographic and DO data from SEAMAP cruise (CR0276)Station Latitude (°N) Longitude (°W) Depth (m) DO(mL L -1 )Temperature(°C)SalinityPSS-78116 28° 48.44 0 94° 44.20 0 3 3.72 28.28 33.3628° 48.44 0 94° 44.20 0 21 2.01 26.52 35.70124 28° 40.63 0 95° 06.51 0 2 4.19 28.69 31.8928° 40.63 0 95° 06.51 0 26 0.50 26.26 35.25125 29° 10.31 0 94° 52.31 0 2 4.45 29.20 28.9929° 10.31 0 94° 52.31 0 13 3.56 28.66 28.67126 28° 57.61 0 94° 59.65 0 3 4.49 29.08 30.9728° 57.61 0 94° 59.65 0 19 0.66 27.37 33.31127 28° 44.93 0 95° 27.67 0 2 3.76 28.88 31.1728° 44.93 0 95° 27.67 0 12 3.71 28.62 31.49128 28° 30.65 0 95° 46.74 0 3 4.18 28.77 32.2928° 30.65 0 95° 46.74 0 17 1.27 27.68 33.13129 28° 29.88 0 95° 30.08 0 3 4.27 28.85 32.0328° 29.88 0 95° 30.08 0 26 0.73 25.88 35.84130 28° 37.22 0 95° 24.35 0 2 4.22 28.77 32.0828° 37.22 0 95° 24.35 0 22 0.40 26.41 34.83131 28° 39.09 0 95° 18.58 0 1 4.21 28.73 31.9728° 39.09 0 95° 18.58 0 22 0.68 26.42 35.15132 28° 46.24 0 95° 27.89 0 3 3.84 28.91 31.0628° 46.24 0 95° 27.89 0 10 3.69 28.85 31.21133 28° 43.85 0 95° 29.97 0 1 4.05 28.82 31.3328° 43.85 0 95° 29.97 0 13 3.47 28.62 31.78134 28° 36.62 0 95° 45.89 0 3 4.36 28.91 31.5028° 36.62 0 95° 45.89 0 14 3.31 28.75 31.35135 28° 31.56 0 95° 45.74 0 2 4.40 29.22 31.8128° 31.56 0 95° 45.74 0 17 1.21 27.30 33.97136 28° 27.68 0 95° 37.63 0 3 4.28 29.11 32.0228° 27.68 0 95° 37.63 0 24 0.74 25.90 35.84137 28° 26.94 0 95° 58.75 0 2 4.58 29.59 31.5528° 26.94 0 95° 58.75 0 17 1.53 27.87 32.97Rosharon, TX (USGS 08116650), between the years 1967 and 2011 were analyzed. A dailyclimatology of the river discharge was calculated by averaging daily values for the44 years considered. Variability about the daily mean values was calculated by determiningthe standard deviation of the log-transformed daily values (which have a reasonablynormal distribution in the log-transformed space). In addition, daily discharge data forthe Brazos River near Hempstead, TX (USGS 08111500), and near Highbank, TX (USGS08098290), were used for comparison with Brazos River d 18 O data.2.5 Oxygen Isotopic MeasurementsTo trace freshwater sources over the Texas shelf, 16 surface samples were collected fromTexas shelf waters during the TX-08-07 survey cruise, and five samples were collected on123

Aquat GeochemTable 4 Summary of summer 2007 hydrographic and DO data from BR cruises (TX-06-07,TX-07-07,TX-09-07)Station Latitude (°N) Longitude (°W) Totaldepth (m)TX-06-07 14-June TX-07-07 16-Jul TX-09-07 18-SepDO(mL L -1 )Temp.(°C)SalinityPSS-78DO(mL L -1 )Temp.(°C)SalinityPSS-78DO(mL L -1 )Temp.(°C)SalinityPSS-78B2001A-T 28° 54.787 0 95° 18.492 0 5 1.67 26.2 31 4.15 28.7 23.89B2001A-B 0.25 25.7 35 3.09 29.8 28.66B2001-T 28° 53.552 0 95° 20.361 0 5 4.04 27.0 25.30 1.83 26.8 27 4.45 28.7 25.17B2001-B 1.49 26.4 33.03 0.28 25.8 35 4.10 28.9 25.70B2002-T 28° 52.081 0 95° 22.124 0 5 3.82 27.6 2.13 3.37 28.2 4 4.62 28.7 6.90B2002-B 1.29 25.8 33.03 0.27 25.9 35 3.95 28.7 23.62B2003-T 28° 50.327 0 95° 24.440 0 5 4.49 29.5 30.34 2.38 26.5 31 4.81 28.8 10.19B2003-B 2.78 27.4 32.02 0.71 25.8 35 3.68 29.2 25.84B4001A-T 28° 53.365 0 95° 17.217 0 10 2.79 27.8 21 4.74 29.1 25.30B4001A-B 0.29 25.7 36 3.51 29.3 29.40B4001-T 28° 51.837 0 95° 19.084 0 10 3.42 28.8 22.82 2.98 27.9 12 4.66 28.3 25.64B4001-B 0.62 25.5 34.24 0.62 25.9 35 3.35 29.4 30.00B4002-T 28° 50.114 0 95° 20.350 0 10 4.49 29.8 30.34 4.01 27.4 25 4.55 29.1 25.91B4002-B 1.80 25.9 34.57 0.64 25.9 35 3.39 29.4 30.47B4003-T 28° 48.360 0 95° 22.510 0 10 3.76 27.4 34 4.48 29.2 25.91B4003-B 0.93 25.8 36 3.40 29.4 30.54B5501A-T 28° 50.327 0 95° 16.600 0 20 3.67 27.4 36B5501A-B 1.45 25.7 35B5501-T 28° 49.702 0 95° 17.153 0 20 4.73 29.0 22.61 3.89 27.6 36B5501-B 1.62 25.4 34.91 0.90 25.8 35B5502-T 28° 48.360 0 95° 18.492 0 20 4.44 28 34B5502-B 1.23 25.9 35123

Aquat GeochemTable 4 continuedStation Latitude (°N) Longitude (°W) Totaldepth (m)TX-06-07 14-June TX-07-07 16-Jul TX-09-07 18-SepDO(mL L -1 )Temp.(°C)SalinityPSS-78DO(mL L -1 )Temp.(°C)SalinityPSS-78DO(mL L -1 )Temp.(°C)B5503-T 28° 46.400 0 95° 20.381 0 20 4.57 28.7 35B5503-B 2.16 26.3 35T/B in station name indicates measurement at the top 1 m or bottom 1 m of water column, respectivelySalinityPSS-78123

Aquat GeochemMay 25, 2008 (TX-05-08). These data are compared with d 18 O and salinity data forLouisiana shelf water collected during July 1994 (Zerai 2001), May and August 2005(Wagner and Slowey 2011), and April (LA-04-08) and July (LA-07-08) 2008 (Strauss2010). The Louisiana shelf data characterize mixing where Mississippi and Atchafalayadischarge is the dominant freshwater source.Water samples were refrigerated and analyzed for 18 O/ 16 O at TAMU’s Stable IsotopeGeosciences Facility (TAMU; http://stableisotope.tamu.edu). Oxygen isotope analyseswere carried out using an equilibration method, where 250-lL aliquots of water wereinjected into an airtight vial containing an atmosphere of 99.7% He and 0.3% CO 2 . Waterswere equilibrated for two days at *22°C. Following equilibration, the headspace CO 2 wasanalyzed using a Thermo Finnigan DeltaPlusXP Isotope Ratio Mass Spectrometer attachedto a GasBench II automated gas preparation and delivery system. All data are reported ind-notation where d 18 O is defined as:d 18 O ¼ðR sample =R standard 1Þ1000 ð1Þand R is the 18 O/ 16 O ratio. Oxygen isotope measurements were standardized using ViennaStandard Mean Ocean Water (VSMOW; d 18 O = 0%); precision for standard analyses was±0.06%.3 Results3.1 Development, Breakdown, Dispersal of Near-Bottom HypoxiaNear-bottom observations of DO concentration in June 2007 showed along-shelf and crossshelfvariability in coastal waters near the BR mouth (Fig. 3a). The stations visited duringthe short survey (TX-06-07) in mid-June were hypoxic. Wind conditions during this cruiseand the week prior were predominately directed up-coast (from southwest to the northeast)and upwelling favorable (based on NDBC observations). This pattern likely resulted in thefreshwater BR plume being driven up-coast toward Galveston, Texas. MODIS true-colorsatellite imagery on June 10, 2007 shows the plume as brownish suspended particulatematerial extending shoreward and up-coast, consistent with a coastal oceanic response of ariver plume to an upwelling favorable wind stress (Yankovsky and Chapman 1997).Surface salinities were in the low 20s near the BR mouth at the same locations (Table 4).Subsequent nearshore observations from the SEAMAP cruise (CR0276) on 26–28 Junealong the Texas coast showed persistently low DO (Fig. 3a) and stratified conditions(Table 3). The lowest DO waters were seen at the 20 m isobath and between 95° and96°W.Near-bottom DO concentrations on July 14, 2007 were lowest nearshore and up-coast ofthe BR mouth and showed a cross-shore gradient to higher DO concentrations of about2mLL -1 at about 10 km from shore (Fig. 3b). Surface salinity was about four at the rivermouth and increased to more than 30 at the offshore stations (Table 4).The hypoxia encountered in mid-July is coincident with the peak river discharge of theBrazos River in summer 2007 (Fig. 4). The discharge rates of July 2007 reached1,900 m 3 s -1 , an order of magnitude larger than typical July rates, and are also thehighest ever recorded at Brazos River gauging station USGS 8116650. The high dischargerate supplied freshwater to the shelf, presumably contributing to the stratifiedwater column.123

Aquat GeochemFig. 3 a Near-bottom dissolved oxygen concentrations near BR mouth near Freeport, Texas, on June 14,2007. Dissolved oxygen concentrations of less than 1.4 mL L -1 is considered hypoxic. b Near-bottomdissolved oxygen concentrations near BR mouth, near Freeport, Texas, on July 14, 2007. c Near-bottomdissolved oxygen concentrations near BR mouth, near Freeport, Texas on August 7, 2007 BRRR cruise.d Near-bottom dissolved oxygen concentrations near BR mouth, near Freeport, Texas on September 18,2007The August 2007 survey observed low DO concentrations near the BR mouth coincidentwith low-salinity surface water (Fig. 3c). The DO pattern was not spatially uniformand showed alongshore and cross-shore variability. Surface salinities near the BR mouthwere about seven and increased with distance from the mouth to about 30–32. Near-bottomsalinities were above 35. The distribution and variability of DO and surface salinity suggestthat the reduced stratification decreased water column stability and promoted ventilation ofnear-bottom waters, thereby increasing oxygen concentration. The unfavorable seasencountered, with 1.0–1.5 m significant wave height caused by relatively strong winds(National Data Buoy Center observations; http://ndbc.noaa.gov), also contributed to ventilationby reducing stratification through mixing of the freshwater plume.On September 12, 2007, Hurricane Humberto formed rapidly off the Texas coast southof Freeport, Texas. The hurricane moved quickly to the northeast and made landfall on 13September near High Island, Texas (94.5°W). Just prior to landfall, Hurricane Humbertoreached Category One status, with wind speeds in excess of 110 km h -1 . The windscreated surface waves that would have broken down the stratification and ventilated thebottom waters. Observations on 18 September show bottom salinities ranging from 23.6123

Aquat GeochemFig. 4 Hydrograph of river discharge (m 3 /s) of the BR. Daily mean discharge for years 1967–2010 isshown as heavy blue line; red line is 2007 hydrograph and green line is 2008 hydrograph. Blue dash linesrepresent plus and minus one standard deviation of the log-transformed distribution from the mean value(discharge data from USGS)near the BR mouth to about 30 offshore and less than the 32–35 salinity range observedearlier. Near-bottom dissolved oxygen concentrations are above 3.5 mL L -1 in the Texashypoxic region, thus indicating the end of the summer 2007 Texas coastal hypoxia event(Fig. 3d).The time history of dissolved oxygen concentration on the Texas shelf is shown in thewhisker plot of Fig. 5. Average DO concentration decreased from *1.6 mL L -1 in June to0.6 mL L -1 in July. In August, the mean DO value increased to *3.0 mL L -1 . After theSeptember hurricane, the mean DO concentration is 3.5 mL L -1 . An analysis of varianceFig. 5 Box-and-whisker plot of dissolved oxygen concentration (mL L -1 ) data from four cruises duringsummer 2007. The center horizontal line of the box represents the mean value for the cruise; the upper andlower edges of the box represent the 75th and 25th percentiles of the data, respectfully. The whiskers extendto the most extreme data points that are not considered outliers. Outliers are plotted individually as ‘‘?’’.Two medians are considered to be different at the 5% significance level if the notched regions of the boxesdo not overlap123

Aquat GeochemFig. 6 Bottom salinity minus surface salinity, DS B-T , versus near-bottom oxygen concentration (blue dots)for SEAMAP (CR0276) stations near the Brazos River mouth on June 26–28, 2007. Regression line is solid,95% confidence interval (dashed line) bracket the regression line. The coefficient of determination is equalto 0.79 with a p-value less than 0.0001(ANOVA) shows that the difference in mean DO concentration between cruises is significant(F-statistic is 17.4; degrees of freedom is 54; p-value\10 -8 ). A comparison of theindividual mean values shows that the means for the June and July cruises (TX-06-07,SEAMAP-CR0276, TX-07-07) differ at the 95% confidence level from the means for theAugust (TX-08-07) and September (TX-09-07) cruises.The salinity difference between bottom and top (DS) is used as a proxy for stabilityfrequency (obtained from vertically differentiating the water column density) because notall cruises recorded full water column profiles. While not perfect, the salinity differencedoes offer a reasonable representation of water stability and therefore a measure of verticalmixing and the ease of ventilation of the lower layers. A linear regression between DS andbottom DO shows a significant negative correlation for the June SEAMAP cruise,r 2 = 0.79 and p-value \0.0001 (Fig. 6). The July (TX-07-09) data also show significantnegative correlation (r 2 = 0.34) with a p-value of 0.01. None of the other cruises, however,yield a significant correlation between DS and bottom DO.3.2 Oxygen Isotopic Compositions of Texas–Louisiana Shelf WatersThe relationships between oxygen isotope and salinity data of Texas shelf surface watersare illustrated in Fig. 7 and listed in Table 5. Cruise TX-08-07 surface water d 18 O averaged0.6% and ranged from 0.4 to 0.8%, with salinity ranging from 30.6 to 33.7 andaveraging 32.1. The TX-05-08 surface water d 18 O averaged -0.2% with a smaller range of-0.3 to -0.1%. The corresponding salinities were 28.7–29.5 with an average of 29.2. Asexpected, d 18 O measurements of Texas shelf waters correlate with salinity. The salinity–d 18 O regression for each set of surface water samples is as follows:andTX-08 07 d 18 O ¼ 0:122 0:022 S 3:46ð0:70Þ r 2 ¼ 0:581 ð2Þ123

Aquat GeochemFig. 7 Plot of d 18 O versus salinity of surface waters collected from the Texas shelf in 2007 and 2008 andLouisiana shelf in 2008. Solid lines represent mixing lines between the d 18 O of spring river discharge andGulf of Mexico seawater (salinity = 36.5, d 18 O = 1.1%). Dotted lines represent mixing zones (±1r of theaverage). The 2007 Texas shelf values identify BR discharge as the dominant source of freshwater. In 2008,concurrent with reduced rainfall over eastern Texas, values identify Mississippi–Atchafalaya discharge to bethe dominant freshwater source at the Texas locality. The strong Brazos influence on the Texas shelfwitnessed in 2007 may be only associated with high discharge events, or potentially reduced transport ofMississippi–Atchafalaya dischargeTX-05 08 d 18 O ¼ 0:186 0:017 S 5:64ð0:48Þ r 2 ¼ 0:976 ð3ÞAssuming two-component mixing between freshwater and seawater, these regressionsimply that the d 18 O of the freshwater source on the Texas shelf during August 2007 was-3.5 ± 0.7%, whereas the d 18 O of freshwater source on the Texas shelf during May 2007was much lower at -5.6 ± 0.5%.4 Discussion4.1 Circulation and Water Column Stability Affecting Hypoxia on the Texas ShelfThe series of observations in June, July, August, and September 2007 captured the formation,breakup, and dissipation of a coastal hypoxic event that formed off the BR mouth.The event was coincident with uncharacteristically large BR discharge (Fig. 4), whichprovided freshwater that increased water column stability and inhibited vertical mixing andventilation of the lower layers of the water column.The spatial distribution of near-bottom DO concentrations across the shelf providebroad spatial context to the Texas coastal observations. Observations from the full SEA-MAP cruise in 2007 show a plume of hypoxic near-bottom DO concentrations extendingfrom the BR mouth southwestward along the Texas coast to Matagorda Bay (Fig. 1). Here,as reported in Pokryfki and Randall (1987), ‘‘the hypoxic area is not homogeneous,’’showing a break between the hypoxic areas of Louisiana and Texas. In 2007, the breakoccurred just south of Galveston, TX.123

Aquat GeochemTable 5 Summary of d 18 O data from 2007 and 2008 Texas shelf cruisesDate Station Stationdepth (m)Latitude Longitude Salinity d 18 O(%)Cruise TX-08-078/8/2007 3D 18.3 28° 37.46 0 N 95° 25.94 0 W 32.10 0.488/8/2007 4D 19.5 28° 33.53 0 N 95° 35.01 0 W 32.43 0.538/8/2007 5D 19.3 28° 30.54 0 N 95° 43.73 0 W 32.51 0.698/8/2007 5A 8.5 28° 38.51 0 N 95° 49.01 0 W 33.67 0.758/9/2007 4A 10.1 28° 41.55 0 N 95° 40.54 0 W 32.60 0.648/8/2007 3B 13.7 28° 42.86 0 N 95° 29.92 0 W 31.53 0.568/8/2007 4B 13.6 28° 38.52 0 N 95° 38.38 0 W 30.56 0.518/8/2007 5B 12.7 28° 0 35.51 0 N 95° 47.15 0 W 32.81 0.648/8/2007 5C 15.7 28° 33.12 0 N 95° 44.75 0 W 32.83 0.658/8/2007 3C 17.1 28° 39.56 0 N 95° 27.88 0 W 31.16 0.548/8/2007 2A 13.1 28° 50.07 0 N 95° 22.28 0 W 32.87 0.748/8/2007 2B 19.0 28° 47.58 0 N 95° 20.39 0 W 31.55 0.448/8/2007 2C 17.5 28° 44.50 0 N 95° 18.46 0 W 31.24 0.418/8/2007 2D 21.2 28° 42.06 0 N 95° 16.40 0 W 31.47 0.598/8/2007 2E 16.0 28° 53.55 0 N 95° 10.87 0 W 31.98 0.628/8/2007 2F 12.2 28° 56.55 0 N 95° 12.69 0 W 32.27 0.65Cruise TX-05-085/25/2008 2 18.1 28° 42.36 0 N 95° 24.19 0 W 28.70 -0.305/25/2008 3 18.3 28° 45.44 0 N 95° 19.64 0 W 28.91 -0.245/25/2008 4 12.1 28° 49.88 0 N 95° 20.49 0 W 29.30 -0.195/25/2008 5 10.1 28° 50.77 0 N 95° 21.27 0 W 29.37 -0.165/25/2008 6 7.8 28° 51.63 0 N 95° 21.48 0 W 29.54 -0.14The results discussed in Sect. 3.1 suggest that in late June and July, during peak riverdischarge, the more stratified the waters, the lower the bottom DO values. The lack ofcorrelation for June and September is due to a distribution in DO and salinity values thatare too narrow to provide a sufficient spread for the regression. The lack of correlation inthe August cruise suggests that the relationship between water column stability and bottomDO concentration was more complicated than a simple linear relationship and may beattributable to the competing influences of multiple forcing processes.The spatial heterogeneity of the salinity and DO fields on the Texas shelf encounteredduring the August 2007 cruise was a result of energetic wind activity, peak windsexceeding 7.5 m/s and directionally varying (based on observations of NDBC Buoy42035), on the shelf that advected and dispersed the surface freshwater plume. Thereduction in stratification, therefore, would lead to the ventilation of the bottom waters.The amount of ventilation depends upon the strength of stratification. The strength ofstratification is spatially and temporally varying as the freshwater plume responds tosurface wind driving. Under such non-steady conditions, the relationship between watercolumn stability and near-bottom DO concentration is non-linear (DiMarco et al. 2010;Kiselkova 2008). Significant correlation on the Louisiana shelf between water columnstability and near-bottom DO was observed during July of years 1992–1994 (Belabbassi2006). The summer of 1993, which had the highest correlation, was also a period of123

Aquat GeochemFig. 8 Oxygen isotopic compositions of waters from the Atchafalaya, Brazos, and Mississippi Rivers as afunction of day of the year. Where more than 1 year’s samples were collected, data are connected by linesand labeled. Note that the Brazos River was experiencing flooding during the June 1987 sampling, hence theunusually low summer values. Upper inset shows relationship between discharge and oxygen isotopiccomposition of Brazos River water (1987–1988, 2001–2002, and 2008)unusually high flooding of the Mississippi–Atchafalaya Rivers and a spatially homogeneoussurface salinity field.The distribution of freshwater and the variability of water column stability on theTexas–Louisiana shelf can be driven by multiple forcing processes including: freshwaterdischarge variability and surface salinity (Nowlin et al. 2005) and solar insolation andatmospheric forcing variability (Cho et al. 1998; DiMarco et al. 2000; Zhang et al. 2009).However, the general pattern is that water column stability varies seasonally. Bianchi et al.(2010) show a 25-year composite of water column stability versus bottom oxygen, representedin their figure as oxygen deficit, from observations for all seasons. Here, the basicpattern is seasonal with winter having a low stability and relatively high bottom DO, andspring–summer with relatively higher stability and low DO values. Within seasons, there isconsiderable variability of the stability/DO relationship, suggesting that periods of linearityand non-linearity likely occur during a given season (Kiselkova 2008). The occurrence ofperiods of non-linear and linear stability/DO relationships in a single season is supportedby the observations on the Texas shelf in 2007.4.2 Oxygen Isotopic Characterization of Freshwater SourcesApplication of oxygen isotopes as a tracer for freshwater sources requires an understandingof river discharge and its oxygen isotopic composition. Summer 2007 was an unusual timeof frequent flooding in the BR basin of central Texas, with more than 30 cm of precipitationin areas of the BR watershed in June alone (National Climate Data Center;http://www.dcdc.noaa.gov). Figure 4 shows mean seasonal variation of BR discharge for1967–2010 along with 2007 and 2008 discharge. Daily mean discharge peaks in May123

Aquat GeochemTable 6 Summary of oxygen isotope data for relevant river watersLocality Date Day of year d 18 O ReferenceMississippi River at St. Francisville, LA 5/10/2000 131 -6.56 aMississippi River at St. Francisville, LA 5/23/2000 144 -6.08 aMississippi River at St. Francisville, LA 6/6/2000 158 -5.02 aMississippi River at St. Francisville, LA 6/23/2000 175 -6.03 aMississippi River at St. Francisville, LA 6/18/2000 170 -5.60 aMississippi River at St. Francisville, LA 8/25/2000 238 -5.19 aMississippi River at St. Francisville, LA 9/21/2000 265 -5.64 aMississippi River at St. Francisville, LA 11/28/2000 333 -5.91 aMississippi River at St. Francisville, LA 12/14/2000 349 -6.77 aMississippi River at St. Francisville, LA 1/24/2001 24 -6.74 aMississippi River at St. Francisville, LA 2/27/2001 58 -6.51 aMississippi River at St. Francisville, LA 3/14/2001 73 -6.32 aMississippi River at St. Francisville, LA 4/2/2001 92 -7.50 aAtchafalaya River at Melville, LA 5/11/2000 132 -4.80 aAtchafalaya River at Melville, LA 5/24/2000 145 -3.79 aAtchafalaya River at Melville, LA 6/7/2000 159 -5.54 aAtchafalaya River at Melville, LA 6/23/2000 175 -4.65 aAtchafalaya River at Melville, LA 7/19/2000 201 -5.12 aAtchafalaya River at Melville, LA 8/25/2000 238 -4.84 aAtchafalaya River at Melville, LA 9/19/2000 263 -5.13 aAtchafalaya River at Melville, LA 11/30/2000 335 -4.15 aAtchafalaya River at Melville, LA 12/12/2000 347 -4.73 aAtchafalaya River at Melville, LA 1/30/2001 30 -5.86 aAtchafalaya River at Melville, LA 2/28/2001 59 -5.68 aAtchafalaya River at Melville, LA 3/15/2001 74 -5.17 aAtchafalaya River at Melville, LA 4/3/2001 93 -5.68 aMississippi River at St. Francisville, LA 2/11/2008 42 -6.15 bMississippi River at St. Francisville, LA 3/10/2008 70 -6.26 bMississippi River at St. Francisville, LA 4/7/2008 98 -6.98 bMississippi River at St. Francisville, LA 4/21/2008 112 -6.13 bMississippi River at St. Francisville, LA 5/5/2008 126 -5.94 bMississippi River at St. Francisville, LA 5/27/2008 148 -6.26 bMississippi River at St. Francisville, LA 6/9/2008 161 -5.86 bMississippi River at St. Francisville, LA 6/23/2008 175 -6.05 bMississippi River at St. Francisville, LA 7/8/2008 190 -5.92 bMississippi River at St. Francisville, LA 8/18/2008 231 -4.85 bMississippi River at St. Francisville, LA 10/20/2008 294 -5.97 bMississippi River at St. Francisville, LA 11/17/2008 322 -5.73 bAtchafalaya River at Melville, LA 1/17/2008 17 -6.00 bAtchafalaya River at Melville, LA 2/12/2008 43 -5.73 bAtchafalaya River at Melville, LA 3/11/2008 71 -5.35 bAtchafalaya River at Melville, LA 3/25/2008 85 -6.17 bAtchafalaya River at Melville, LA 4/8/2008 99 -6.21 b123

Aquat GeochemTable 6 continuedLocality Date Day of year d 18 O ReferenceAtchafalaya River at Melville, LA 4/22/2008 113 -5.47 bAtchafalaya River at Melville, LA 5/6/2008 127 -5.05 bAtchafalaya River at Melville, LA 5/28/2008 149 -5.55 bAtchafalaya River at Melville, LA 6/10/2008 162 -5.54 bAtchafalaya River at Melville, LA 10/21/2008 295 -5.55 bAtchafalaya River at Melville, LA 11/13/2008 318 -5.55 bBrazos River at SH 21 near Bryan TX 6/9/1987 160 -3.99 cBrazos River at SH 21 near Bryan TX 7/25/1987 206 -3.29 cBrazos River at SH 21 near Bryan TX 8/27/1987 239 -2.80 cBrazos River at SH 21 near Bryan TX 9/20/1987 263 -2.66 cBrazos River at SH 21 near Bryan TX 11/24/1987 328 -3.01 cBrazos River at SH 21 near Bryan TX 1/7/1988 7 -3.04 cBrazos River at SH 21 near Bryan TX 3/12/1988 72 -2.27 cBrazos River at SH 21 near Bryan TX 4/1/1988 92 -2.32 cBrazos River at SH 21 near Bryan TX 4/23/1988 114 -1.65 cBrazos River at SH 21 near Bryan TX 5/21/1988 142 -1.88 cBrazos River (@ Hempstead) 4/14/2008 105 -1.63 dBrazos River (@ Waco) 7/17/2008 198 -0.94 dBrazos River near Brazoria (BR-1) 12/19/2001 353 -3.30 eBrazos River near Brazoria (BR-1) 1/28/2002 28 -3.30 eBrazos River near Brazoria (BR-1) 3/4/2002 63 -2.70 ea Lee and Veizer (2003)Landwehr and Coplen (unpublished)Coffman and Grossman (unpublished)dStrauss (2010)e Hyeong and Lawrence (2003)f Coplen and Kendall (2000)(*400 m 3 s -1 ) and is at a minimum in the summer months (*100 m 3 s -1 ), particularlyJuly through October. In contrast, discharge for summer 2007 exceeds 1,800 m 3 s -1 priorto the August 8 sampling trip.The oxygen isotopic compositions of river waters discharging into the Gulf of Mexicodepend upon the latitude and altitude of the watershed, time of year, and amount ofprecipitation (Craig 1961; Kendall and Coplen 2001). Unfortunately, time-series d 18 O datafor river waters are sparse. Figure 8 and Table 6 show a compilation of available d 18 O datafor the Brazos, Atchafalaya, and Mississippi Rivers plotted as a function of day of the year.Brazos River waters collected sporadically in 1987, 1988, 2001, 2002, and 2008 rangefrom -4.0 to 1.6% (B. K. Coffman and E. L. Grossman, unpublished data; Hyeong andLawrence 2003; this study), depending on season and especially discharge (Fig. 8). Mississippiand Atchafalaya River waters collected in 2000, 2001, and 2008 also depend onseason and discharge and range from -7.5 to -4.9% and -6.2 to -3.8%, respectively(Lee and Veizer 2003; Wagner and Slowey 2011; T. Coplen and J. Landwehr, unpublisheddata, 2010).123

Aquat GeochemMississippi River water d 18 O values are low due to the higher average elevation andlatitude of its basin and sub-basins, including the Missouri, Ohio, and Arkansas Riverbasins. The higher d 18 O of Atchafalaya River water is mainly caused by the contribution of18 O-enriched waters of the Red River (Coplen and Kendall 2000), which flows along theTexas–Oklahoma border before joining the Atchafalaya River in Louisiana. TheAtchafalaya also receives 30% of the Mississippi River flow at the Old River ControlStructure, which contributes 18 O-depleted water. The d 18 O of Mississippi and AtchafalayaRiver waters is generally lowest during winter and spring and highest during the summerand fall (Fig. 8). The low values during the spring are caused by the influence of snowmelt,which also results in a pulse of high discharge. The sparse oxygen isotope data for BrazosRiver waters indicate increasing d 18 O values from late spring through summer 1987, aslight decrease in winter 1987–1988, and an increase through spring 1988. The low June1987 value was caused by excessive rainfall and flooding (http://waterdata.usgs.gov/nwis/uv?08098290), which disrupted the usual summer increase in d 18 O. Hyeong and Lawrence(2003) report a low average value of -3.8% following heavy rains in December of 2001.Even when its d 18 O values are lowest, Brazos River water still exhibits significantly higherd 18 O than Mississippi River water. There is an overlap of 0.2% between the range of d 18 Ovalues for the Brazos and Atchafalaya Rivers; however, this trend reflects the combinationof extremes: flooding on the Brazos during 1987 and unusually low Atchafalaya dischargeduring the summer of 2000 (http://www.mvn.usace.army.mil/cgi-bin/watercontrol.pl?03045). The low-salinity waters advecting westward to the Texas shelf will contain a mixtureof Mississippi and Atchafalaya discharge, and thus Mississippi–Atchafalaya and BrazosRiver waters will be isotopically distinct during both high and low flow regimes of theBrazos River.4.3 Oxygen Isotopic Tracing of Texas–Louisiana Shelf WatersAs discussed earlier, the d 18 O measurements for Texas shelf surface waters collected inAugust 2007 (TX-08-07) and May 2008 (TX-05-08) follow linear trends with salinity andsuggest two-component mixing with freshwater sources having distinctly different d 18 Ovalues (Fig. 7). The y-intercept of Texas shelf surface waters collected during TX-08-07(-3.5 ± 0.7%) is lower than the average d 18 O of Brazos River water (-2.2 ± 1.3%), butmatches d 18 O values for Brazos River water during high-discharge conditions in June andJuly 1987 (-4.0 and -3.3%, respectively; Table 5), an analog for August 2007 conditions.This supports a Brazos River source of freshwater and suggests that Brazos River dischargecreated stratification and consequently hypoxia off the Texas coast. TX-05-08 samplesfrom the Texas shelf, however, yield an intercept of -5.6 ± 0.5%, between the means ofMississippi and Atchafalaya River waters, -6.1 ± 0.6 and -5.3 ± 0.6%, respectively(Lee and Veizer 2003; Wagner and Slowey 2011; T. Coplen and J. Landwehr, unpublisheddata, 2010), and suggesting downcoast (i.e., to the west) transport of water. These twocontrasting years show both the Mississippi–Atchafalaya and Brazos Rivers having adominant influence on the Texas shelf locality. Further surveys are needed to determine theminimum Brazos River discharge required to trigger hypoxia on the adjacent Texas shelf.Previous analyses (Belabbassi 2006; Bianchi et al. 2010; Dale et al. 2010) have shown thata minimum vertical stratification, that is, stability frequency of about 40 cycles h -1 isneeded for bottom oxygen levels on the Texas–Louisiana shelf to fall to hypoxic levels.As with Brazos River water, annual averages may not fully represent the d 18 OofMississippi and Atchafalaya River waters that entered the Louisiana shelf during spring orsummer, prior to westward advection, and to collection at the Texas sample sites. With123

Aquat Geochemminor exception, Fig. 8 shows that summer and fall precipitation tends to be enriched in18 O relative to winter and spring precipitation. This effect is seen in the salinity–d 18 Orelationships for Louisiana shelf waters, which track the influx of Mississippi andAtchafalaya freshwater. The freshwater endmember for May (-6.0%) is 1.7% lower thanthat for August (-4.3%; Wagner and Slowey 2011). Wagner and Slowey (2011) interpretedthis as a reflection of the dominant influence of the Atchafalaya River water duringthe summer, when coastal circulation promotes eastward advection of Mississippi Riverdischarge (Dinnel and Wiseman 1986; Nowlin et al. 2005).4.4 Mixing ModelWe have used a three-component mixing mass balance model to estimate the relativepercentages of Mississippi–Atchafalaya and Brazos River freshwater fluxed to the Texasshelf. Because the discharge of the Mississippi and the Atchafalaya Rivers are mixed upontransport to the Texas shelf, the model treats discharge from both rivers as a singlecomponent. Estimates show that roughly half of Mississippi River discharge is transportedto the west and is then mixed with the flow of the Atchafalaya (Dinnel and Wiseman 1986;Etter et al. 2004).The mass balance uses salinity as a measure of river input and d 18 O as a tracer offreshwater source, either the Brazos or Mississippi–Atchafalaya:F Br þ F M A þ F GoM ¼ 1 ð4ÞS m ¼ F Br S Br þ F M A S M A þ F GoM S GoM ð5Þwhere S = salinity, F = fraction, m = measured, Br = Brazos, M–A = Mississippi andAtchafalaya, and GoM = Gulf of Mexico. We can assume that S Br & S M–A & 0 so thatS T = F GoM S GoM . As with salinity, d 18 O can be calculated using simple mass balance:d 18 O m ¼ d 18 O Br F Br þ d 18 O M A F M A þ d 18 O GoM F GoM ð6ÞCombining Eqs. 4, 5, and 6 and solving for F Br yields the equation:F Br ¼ d18 O m d 18 O M A þ S mS GoMðd 18 O M A d 18 O GoM Þd 18 O Br d 18 ð7ÞO M AFollowing Wagner and Slowey (2011), a value of 36.1 is used for S GoM . Measurements ofd 18 O at the Louisiana shelf break show d 18 O GoM to average 1.1% (Wagner and Slowey2011; Strauss 2010). For the combined Mississippi–Atchafalaya flux, late-spring d 18 Oriver water values best represent the discharge that would advect to the Texas shelf by theearly summer, nearly a month-long journey (Nowlin et al. 2005). To adjust for theroughly 1% difference between Mississippi and Atchafalaya discharge, the value ford 18 O M–A is calculated based on a 1:1 mixture of Mississippi and Atchafalaya discharge.For Mississippi River water, we have assigned a value of -6.2 ± 0.6%, and forAtchafalaya River water, a value of -5.3 ± 0.6%, which are the averages of spring 2008measurements (Lee and Veizer 2003; T. Coplen and J. Landwehr, unpublished data,2010). Thus, the estimated d 18 O of Mississippi and Atchafalaya River water expected toadvect westward under spring circulation conditions is -5.8 ± 0.6%. For Brazos Riverdata, we use an average of June and July 1987 values (-3.6%), representing a highdischarge summer conditions.123

Aquat GeochemResults from the mixing model, using the above mentioned river water values, indicatethat during 2007, the fraction of Brazos River water on the Texas shelf (F Br ) averaged11.6 ± 0.5% (±1r) and the Mississippi–Atchafalaya fraction (F M–A ) was 0 ± 2.9%,whereas in May of 2008, F Br averaged 0.8 ± 0.4% and F M–A was 18.4 ± 1.0%. Thus, the2007 Texas shelf waters were mainly diluted with BR discharge, while 2008 Texas shelfwaters were diluted with Mississippi–Atchafalaya discharge. Unfortunately, the absence ofriver water d 18 O data corresponding to the 2007 event precludes the results from beingabsolute. However, the calculation error associated with uncertainty in river d 18 O valuesdoes not invalidate the model. The sensitivities of F Br and F Ma to adjustments of d 18 O M–Awithin its standard deviation (±0.6%) are both less than 1% for 2007. Furthermore, for2007, F M–A is greater than F Br only when the d 18 O Br is raised to -1%, a value greater thanBR observations during periods of high and low discharge (Table 6; Fig. 8).The mixing model clearly shows that Brazos River was the main source of fresh water tothe study area on the inner Texas shelf during the 2007 hypoxic event. In contrast, Mississippi–Atchafalayadischarge was the dominant source of fresh water to the May 2008Texas shelf. Additionally, the dilution of Texas shelf waters by Mississippi–Atchafalayadischarge in 2008, yielding an average salinity of 29.3, was roughly twice that caused by thedramatically high discharge of the Brazos River in 2007, where salinity averaged 32.1.5 ConclusionsObservations of salinity and dissolved oxygen concentration on the Texas coast near theBrazos River mouth in summer 2007 show vertically stratified conditions between Juneand August resulting in low near-bottom oxygen values. The lowest values occurred in Julyand coincide with the peak of discharge rates of the Brazos River. In September, thepassage of a short-lived hurricane resulted in a breakdown of water column stratificationand the ventilation of near-bottom waters.Oxygen isotopic compositions of shelf waters of the northwestern Gulf of Mexico wereused to identify the freshwater sources that stratify shelf waters and create conditionsfavorable for hypoxia formation. Although the Mississippi–Atchafalaya River Systemaccounts for[90% of freshwater discharge into the northern Gulf of Mexico, hydrographicand oxygen isotope data indicate that high discharge of the Brazos River in summer 2007led to hypoxic-favorable conditions. Isotopic mass balance calculations suggest that duringsummer 2007, the Brazos was the main source of fresh water to the inner Texas shelf.Although these calculations are limited by a lack of synchronous data for river waters, theyare consistent with trends found in historical data. Isotope data from 2008 indicate thatMississippi–Atchafalaya waters were the dominant freshwater source on the Texas coast.Thus, in some years, there is considerable capability of Mississippi–Atchafalaya dischargeto stratify Texas shelf waters and, as a consequence, cause Texas shelf hypoxia. Moreprecise identification of freshwater sources will require detailed time-series studies of d 18 Ovariation in major Texas and Louisiana rivers and wetlands.The observations during the high flow event of 2007 support that Texas coastal hypoxiacan be driven by local forcing and can persist for several weeks. We conclude from thesefindings that despite the large and significant influence of the Mississippi–AtchafalayaRiver System, it is not the sole cause of hypoxia in the northern Gulf of Mexico. Identifyingthe relative influence of local sources and the frequency of occurrence of Texascoastal hypoxia during low and normal river flow conditions is the subject of futureinvestigation.123

Aquat GeochemAcknowledgments This research was supported by a Rapid Response Award by Texas Sea Grant CollegeProgram (No. 404538). Partial funding was through a grant to S. DiMarco (NOAA-CSCORNA06NOS4780198), contribution number NGOMEX-132, and the TAMU Department of Oceanography.Support for the stable isotope analyses was provided by a grant from Texas’ Norman Hackerman AdvancedResearch Program (No. 010366-0053-2007). The authors thank the dedication of Drs. Matthew Howard,Timothy Dellapena, GERG Marine Technicians, high school students, Scott Hall, Nathaniel Weidner, JoshAndrews, and the cohort of TAMU and TAMUG graduate students for their fortitude and dedication duringthe August 7, 2007 small boat armada to collect the ocean samples. The authors also thank T. S. Bianchi(TAMU), N. Walker (LSU), A. Quigg (TAMUG), R. Smith (Yale), and M. Fisher (Texas Parks and WildlifeDivision) for useful discussions related to this work. The thoughtful comments of four reviewers greatlyhelped to focus this manuscript. This work is dedicated to the memory of our friend and colleague ProfessorJohn W. Morse (Mackenzie et al. 2011).ReferencesBelabbassi L (2006) Examination of the relationship of river water to occurrences of bottom water withreduced oxygen concentration in the northern Gulf of Mexico. Ph.D. dissertation, Texas A&M University,College StationBianchi TS, DiMarco SF, Cowan JH Jr, Hetland RD, Chapman P, Day JW, Allison MA (2010) The scienceof hypoxia in the northern Gulf of Mexico: a review. Sci Total Environ 408(7):1471–1484Boesch DF (2002) Challenges and opportunities for science in reducing nutrient over-enrichment of coastalecosystems. Estuaries 25(4B):886–900Cho K, Reid RO, Nowlin WD Jr (1998) Objectively mapped stream function fields on the Texas-Louisianashelf based on 32 months of moored current meter data. J Geophys Res 103(C5):10377–10390Conley DL, Paerl HW, Howarth RW, Boesch DF, Seitzinger SP, Havens KE, Lancelot C, Likens GE (2009)Controlling eutrophication: nitrogen and phosphorus. Science 323(5917):1014–1015Cooper LW, Benner R, McClelland JW, Peterson BJ, Holmes RM, Raymond PA, Hansell DA, GrebmeierJM, Grebmeier JM, Codispoti LA (2005) Linkages among runoff, dissolved organic carbon, and thestable oxygen isotope composition of seawater and other water mass indicators in the Arctic Ocean.J Geophys Res 110(G2):GO2013. doi:10.1029/2005JG000031Coplen TB, Kendall C (2000) Stable hydrogen and oxygen isotope ratios for selected sites of the U.S.Geological Survey’s NASQAN and benchmark surface-water networks. Open-file report, USGSCraig H (1961) Isotopic variations in precipitation. Science 133:1702–1703Dale VH et al (2010) Hypoxia in the Northern Gulf of Mexico. Springer series on environmental management.Springer, New YorkDansgaard W (1964) Stable isotopes in precipitation. Tellus 16(436):468Diaz RJ, Rosenberg R (1995) Marine benthic hypoxia: a review of its ecological effects and the behavioralresponses of benthic macrofauna. Ocean Mar Biol Ann Rev 33:245–303Diaz RJ, Rosenberg R (2008) Spreading dead zones and consequences for marine ecosystems. Science321:926–929DiMarco SF, Howard MK, Reid RO (2000) Seasonal variation of wind-driven diurnal cycling on the Texas-Louisiana continental shelf. Geophys Res Lett 21(7):1017–1020DiMarco SF, Chapman P, Hetland RD, Walker ND (2010) Does local topography control hypoxia on theLouisiana shelf? J Mar Sci 80(1–2):25–35Dinnel SP, Wiseman WJ Jr (1986) Fresh water on the Louisiana and Texas Shelf. Cont Shelf Res6(6):765–784Etter PC, Howard MK, Cochrane JD (2004) Heat and freshwater budgets of the Texas-Louisiana Shelf.J Geophys Res. doi:10.1029/2003JC001820Harper DE Jr, McKinney LD, Nance JM, Salzer RR (1991) Recovery responses of two benthic assemblagesfollowing an acute hypoxic event on the Texas continental shelf, northwestern Gulf of Mexico. FromTyson RV, Pearson TH (eds) Modern and ancient continental shelf anoxia. Geological Society,London, Special Publication no. 58, pp 49–64Harper DE, Guillen GJ (1989) Occurrence of a dinoflagellate bloom associated with low salinity water offGalveston, Texas and coincident mortalities of demersal fish and benthic invertebrates. Contrib MarSci 31:147–161Harper DE Jr, Salzer RR, Case RJ (1981) The occurrence of hypoxic bottom water off the upper Texas coastand its effect on the benthic biota. Contrib Mar Sci 24:53–79123

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