Seasonal Hydrodynamics along the Louisiana

Journal of Coastal Research

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0

000–000

West Palm Beach, Florida

Month 0000

Seasonal Hydrodynamics along the Louisiana Coast: Implications for Hypoxia Spreading Mohammad Nabi Allahdadi, Felix Jose*, and Cecily Patin Coastal Studies Institute Department of Oceanography and Coastal Science Howe–Russell Geoscience Complex Louisiana State University Baton Rouge, LA 70803, U.S.A. [email protected]

ABSTRACT Allahdadi, M.N.; Jose, F., and Patin, C., 0000. Seasonal hydrodynamics along the Louisiana coast: implications for hypoxia spreading. Journal of Coastal Research, 00(0), 000–000. West Palm Beach (Florida), ISSN 0749-0208. Summer and fall inner-shelf flow characteristics, including the vertical current structure, obtained from three WAVCIS (WAVE-Current-surge Information System) stations off the Louisiana coast were analyzed to delineate the hydrodynamic conditions that contribute to the formation of seasonal hypoxia at each station location. Two of the WAVCIS data stations used for analysis are located west of the Mississippi bird-foot delta, whereas the third is located east of the delta. Relatively small vertical gradients in the horizontal velocity current (i.e., u and v velocity components) were observed during summer (2009) for both CSI-6 and CSI-9 stations, which are located well inside the hypoxia-prone zone west of the delta. In contrast, the summertime vertical gradient of horizontal current at CSI-16 (located east of the delta) was significantly higher than that of western stations. Significant differences in the vertical gradient of flow velocities along with contrast in water column density gradient during the summer culminated in a strong stratification, which is considered the main physical requirement for the formation of hypoxia. A computed criteria based on the Richardson number also pointed to higher potential for stratification at both stations west of the delta, whereas the water column east of the delta still remained subject to vertical mixing. Furthermore, summertime current fields at CSI-6 and CSI-9 were significantly less compared with CSI-16, suggesting less reoxygenation driven by advection from surrounding waters. These conditions may exacerbate hypoxia.

ADDITIONAL INDEX WORDS:

Louisiana shelf, shelf currents, stratification, advection flux, depth gradient,

reoxygenation.

INTRODUCTION The northern Gulf of Mexico is exposed to the second largest seasonal hypoxic zone (less than 2 ppm dissolved oxygen) in the world (Rabalais, Turner, and Wiseman, 2002). A host of factors contribute to the formation of hypoxia in this region, including seasonal discharge of enormous volumes of fresh water from the Mississippi and Atchafalaya rivers and excessive nutrient loading of the shelf, particularly nitrogen and phosphorous, sourced from upstream runoff of fertilizers, urban sewage, erosion, and deforestation (Rabalais, Turner, and Wiseman, 2002). The situation is further exacerbated by the seasonal reversal of wind and, thereby, the coastal current (Cochrane and Kelly, 1986). The microtidal Louisiana coast, having a maximum tidal range of ~40 cm (Wright, Sherwood, and Sternberg, 1997) when exposed to stagnant wind and wave DOI: 10.2112/JCOASTRES-D-11-00122.1 received 10 July 2011; accepted in revision February 11, 2012. * Present address: Department of Marine and Ecological Sciences, Florida Gulf Coast University, Fort Myers, FL 33965-6565, U.S.A. Published Pre-print online 16 July 2012. Ó Coastal Education & Research Foundation 2012

forcing during summer, becomes an isolated waterbody with limited mixing and exchange with the surroundings (Wiseman et al., 1997). The factors contributing to the seasonal formation of hypoxia can be divided into physical and biochemical components (Justic et al., 1993; Rabalais, Turner, and Scavia, 2002; Rabalais, Turner, and Wiseman, 2002; Wiseman et al., 1997, 2004). Wiseman et al. (1997) noticed that Mississippi River flooding in early spring coincides with a perceptible decline in atmospheric activity, causing water column stratification. Increased solar heating and sunshine, together with the high nutrient loading from the Mississippi River, provide favorable conditions for phytoplankton bloom during early spring. This seasonally fixed organic carbon sinks in a short period of time, eventually leading to the depletion of bottomwater oxygen during summer months as it is oxidized (Wiseman et al., 1997). In fact, biochemical factors cause eutrophication, and physical factors cause water column stratification, and the interaction between these two results in hypoxia (Belabbassi, 2006). For some upwelling systems, the strength of physical forces (mostly wind) causes severe hypoxic events. For instance, along the New Jersey shelf, persistent summertime upwelling due to southwesterly winds produces hypoxia, as upwelling delivers

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Allahdadi, Jose, and Patin

nutrients to the surface, depleting almost 75% of the bottom oxygen (Glenn et al., 2004). The same mechanism also develops severe hypoxia along the Oregon coast (Grantham et al., 2004). Nevertheless, the Louisiana shelf has never been identified as an upwelling zone in the northern Gulf of Mexico (Walker et al., 2003). Prevalence of sluggish wind, waves, and currents during summer, along with heavy discharge of nutrient-rich fresh water into the shelf during spring, facilitates water column stratification and, thereby, hypoxia. Because human control of physical conditions is not possible for the sake of hypoxia mitigation, all efforts have been concentrated on reducing biochemical effects. Recent efforts that focus on biochemical factors envisage plans for reducing hypoxia by reducing river nutrient load (Scavia, Justic, and Bierman, 2004). However, it is critical to quantify the relative effect of eutrophication and stratification to design viable hypoxia management plans (Stow, Qian, and Craig, 2005), in that stratification induced by physical factors is the necessary condition for hypoxia formation (Walker and Rabalais, 2006). The effect of biochemical factors in formation of Gulf hypoxia has been studied thoroughly (e.g., Justic, Rabalais, and Turner, 1996, 2003; Rabalais et al., 1996, 2004; Turner and Rabalais, 1994; Turner, Rabalais, and Justic, 2006). Some studies have focused on the role of physical factors on the formation of hypoxic zones in the northern Gulf of Mexico. However, studies in this direction mostly synthesized the role of stratification based on salinity and temperature profiles and the seasonal vertical mixing events induced by wind (Rabalais, Wiseman, and Turner, 1994; Walker and Rabalais, 2006; Wang and Justic, 2009; Wiseman et al., 1997), whereas the role of hydrodynamics was seldom studied in relation to hypoxia formation. Wiseman et al. (2004) studied the current characteristics at a point located west of the bird-foot delta, based on Acoustic Doppler Current Profiler (ADCP) data for the period March–November 2002. The study revealed a weak vertical velocity shear for the majority of the measurement duration. Simultaneous measurement of density across the water column demonstrated the effect of shear on water column mixing (Wiseman et al., 2004). It is observed that the effect of seasonal currents along the Louisiana shelf on the formation of shelf stratification and, thereby, hypoxia has not been addressed well. Moreover, the hypoxic conditions at either side of the birdfoot delta have not been studied in relation to shelf hydrodynamics, which itself has been significantly influenced by the Mississippi River and its deltaic system (Rego et al., 2008). The objective of this study is to analyze seasonal flow structure and its spatiotemporal variability from east and west of the birdfoot delta, the dominant morphological feature along the Louisiana coast. The time series of 3D velocity profiles, measured using ADCPs, at three WAVCIS (WAVE-Currentsurge Information System) stations, along with wind data, were analyzed for summer and fall seasons. The spatially varying nature of currents east and west of the delta might have contributed to the spread/dissipation of oxygen depletion zones on the west and east side of the delta. An expanding seasonal hypoxic zone has been detected west of the bird-foot delta (from the bird-foot delta to the Texas– Louisiana border), which is being mapped annually from a 5-d cruise in midsummer, usually between mid-July and mid-

August (Rabalais, Turner, and Wiseman, 2002). In contrast, the hypoxia occurrence east of the bird-foot delta is ‘‘isolated’’ (Rabalais, Turner, and Wiseman, 2002). Three specific areas of more frequent hypoxic events have been detected from the Mississippi bight, including the barrier islands adjacent to the Mississippi–Alabama coasts, the region just east of the Mississippi River delta and its tributaries, and areas close to the outer shelf (Brunner et al., 2006). Also, hypoxia was reported from various regions around the Chandeleur Islands (Jochens, Nowlin, and DiMarco, 2000; Lopez, Baker, and Boyd, 2010). Data from some sampling locations close to CSI-16 (e.g.,10 or 15 km southeast of the station from Southeast Area Monitoring and Assessment Program or SEAMAP) showed relatively high or nonhypoxic oxygen concentration (Brunner et al., 2006). Comparing and contrasting current data from stations located either side of the bird-foot delta and relating them to the physical conditions favoring hypoxia formation could unravel the role of shelf current characteristics (vertical profiles, velocities, and current directions) in the formation and sustainment of seasonal hypoxia along this coast and shed more light on hypoxic conditions at the CSI-16 location (see Figure 1 for station locations). This was implemented through a detailed study of seasonal current characteristics, which might be contributing to hypoxia formation at each specific location.

DATA AND METHODS Wind and current data from WAVCIS stations were analyzed for their seasonal and spatial variability along the Louisiana coast. WAVCIS was launched back in 1998 (Stone et al. 2009; Zhang, 2003), and it continuously monitors a wide array of metocean data from fixed offshore platforms scattered along the Louisiana coast. Observations along the Mississippi and west Florida coast were also conducted earlier, and the archived data are available online (www.wavcis.lsu.edu). Hourly observations of directional waves, vertical current profiles, tide, wind speed and direction, sea surface temperature, pressure, etc. from individual stations are communicated via cellular communication to the WAVCIS Laboratory at Coastal Studies Institute, Louisiana State University. Teledyne RDIt ADCPs are used for wave and current measurements with a vertical bin interval of 35–50 cm. More details on the offshore instrumentation, data processing, and communication protocol implemented for WAVCIS can be found in Zhang (2003). Figure 1 shows the locations of various measurement stations along the Louisiana coast. For this study, current and wind data collected during 2009 for stations CSI-6 and CSI-9, located west of the bird-foot delta and well inside the recognized hypoxic zone, and CSI-16, located east of the delta, were analyzed. Although archived data from WAVCIS stations have been available since 1998, stations like CSI-16 were installed within the last couple of years (August 2008 in the case of CSI-16). Considering some significant data gaps, the most complete simultaneous dataset, representing conditions for both summer and fall for all three stations, was from the 2009 measurements. Regarding the seasonal pattern of currents from the study area, the selected data record from one specific year could be a proper representation of current regime at each station. Mean water

Journal of Coastal Research, Vol. 00, No. 0, 2012

Seasonal Hydrodynamics and Hypoxia

Figure 1.

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Study area and the locations of CSI-6, CSI-9, and CSI-16.

depths at CSI-6, CSI-9, and CSI-16 stations are 21, 19, and 13 m, respectively. Archived current data profiles were extracted from each station and further analyzed for the seasonal current structure across the entire water column and their plausible relation to hypoxia. Precautions were taken to eliminate data that were contaminated by the bed effect and the bins exposed above the water surface. Also, profile range was kept uniform by extrapolating to the nonvalue vertical gaps; hence, a smaller depth range was applied for extraction of current data for each station. The measurement period in 2009 was divided into summer (June–August) and fall (September–November). Other nonsummer months were not included in the study because no data were available for CSI-6 from January to mid-June and also for CSI-16 from January to March. Data from 2008, although it is available, were also not considered in this study because the Louisiana shelf was significantly affected by two strong hurricanes (Gustav and Ike), and hydrodynamic conditions were less representative of average forcing over the study area. Wind data were extracted for the same duration in 2009 as for the currents from all the three stations. The data were utilized to calculate wind stress, the critical forcing that contributes to generating the currents and, consequently, influences the formation of hypoxia.

RESULTS Summer and Fall Current Pattern Considering the seasonal variations in current pattern in the study area and that hypoxia occurs mostly in summer, the current field for all stations (see Figure 1 for station locations) were studied for both summer and fall (as a nonsummer period) seasons. For each station, time series of current speed and direction were analyzed further to understand their variability during both summer and fall. Accordingly, current vectors from the near surface are provided in Figure 2. Surface currents at CSI-6 during the summer (Figure 2a) showed a disorganized pattern that is in agreement with previous studies (e.g., Crout, Wiseman, and Chuang, 1984). Current direction changed

Figure 2. Time series measurements of surface currents from three WAVCIS stations, corresponding to summer and fall 2009; (a and b) CSI-6, (c and d) CSI-9, (e and f) CSI-16.

within a short period of time. In fact, during June (during the 10 d investigated) and July, current direction was not consistent. During these 2 mo, current direction changed frequently in response to changing wind direction from south/ southeast to west, whereas from mid-August onward, persistent northerly winds resulted in dominance of southward currents. Current vectors during fall months at CSI-6 (Figure 2b) were more organized and were consistent with the patterns presented by Crout, Wiseman, and Chuang (1984). During this period, especially from mid-October, the area was affected by frequent passage of cold fronts, generating southward currents (directed from southeast to southwest); hence, relatively regular changes in current direction and consistent currents were observed. Currents at CSI-9, located off the mouth of Barataria Bay (see Figure 1 for location), were obviously sluggish compared with CSI-6 (Figure 2c for summer and Figure 2d for fall current patterns at CSI-9). Similar to CSI-6, current pattern during summer was disorganized at CSI-9, whereas fall current pattern was more aligned. Although the station location was affected by frontal passages during fall months, the predominant current direction was northeastward, as the station is located inside a seasonal gyre, which is


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Figure 3. Variations of wind stress based on wind speed measurements at CSI-6 during the study period. Events 1 and 2 are discussed in the text.

formed just west of the bird-foot delta (Rouse, 1998; Rouse and Coleman, 1976; Walker et al., 2005; Wiseman et al., 1976). The disorganized pattern of summer currents was less pronounced at CSI-16 than was observed at CSI-6 and CSI-9. Regular occurrences of northeastward and southwestward currents were observed at CSI-16 and were more persistent during summer (Figure 2e), whereas frequent shifts in current directions to the southwest occurred during the fall passage of cold fronts associated with high wind speeds (Figure 2f). To demonstrate the effect of wind on the generation of seasonal currents in the study area (especially at the location of CSI-6 and CSI-9), wind stress time series, computed using wind data from CSI-6, is presented in Figure 3. Wind stress estimated for CSI-9 and CSI-16 stations was similar to that of CSI-6 and, hence, were not presented. Although there were discrete events associated with high wind stress in June– August, in general, computed summer wind stress values were conspicuously smaller than that of fall. A significant increase in wind stress, mostly from the passage of cold fronts, was observed during fall months (September–November). Two discrete events, one during summertime and the other during fall, were picked for a more detailed examination of the response of current profiles at the three stations during this time. A number of other wind events were available, especially during fall, whose corresponding current profile could also be investigated. However, the main purpose of the study was to address the effect of wind on surface and subsurface current generation, which could be quantified on the basis of these two selected events during summer and fall. Wind condition at three stations during these events is summarized in Table 1. During event 1, the average wind speed at three stations was 7 m/s, whereas wind direction for both CSI-6 and CSI-16 was southwesterly, and for CSI-9, it turned to the northwest. Event 2 was a severe winter storm with wind speeds reaching 11 m/s at CSI-6 and 24 m/s at CSI-16. Wind direction was northwest-

Table 1.

1 2

Current Profiles Because the vertical gradient of horizontal current is one of the critical factors affecting water column mixing (Turner, 1973), vertical profiles of both u and v velocity components for all three stations were examined during the study period (Figures 5 and 6). The dominant feature identified from these plots was the small velocity shear of both the u and v component for CSI-6 and CSI-9 stations, compared with CSI16, especially during summertime. Velocity profiles of the u component at station CSI-6 exhibited low velocity values, as well as insignificant velocity differences between different depths during late June (data are available from June 20) and up to mid-August, corresponding to the maximum hypoxic events. Velocity values along with velocity shear increased from mid-August to mid-September as a result of energetic northern storms breaking down summertime stratification. During November, storms were more frequent and strong enough to produce high velocities, affecting the entire water column. The behavior of the v component of current at CSI-6 was more or less the same. Velocity component u from CSI-9

Wind characteristics measured at each station corresponding to events 1 and 2. CSI-6

Event

erly for both CSI-6 and CSI-9 and north-to-northeasterly for CSI-16. Variations of current velocity components (u and v components corresponding to east–west and north–south directions) within the water column during events 1 and 2 are presented in Figure 4. During event 1, dominant southwesterly wind generated an eastward component of surface current at CSI6, but currents associated with deeper depths were still directed westward. The velocity component v showed a southward orientation at the surface, but current directions in deeper levels of the water column were northward. Vertical profiles of horizontal current components at this time showed a small gradient, which was also the case for CSI-9. At both stations on the western side of the bird-foot delta, u and v velocity components were small (0.1 m/s or less except for the u component of the surface current at CSI-6). In contrast to these two stations, during event 1, CSI-16 showed a high vertical gradient for both u and v velocity components when current velocities reached 0.4 m/s. During event 2, which corresponds to the passage of a severe northern front in fall, measurements showed high current speeds for all stations. Northwesterly winds produced southwestward current speeds of more than 0.5 m/s at CSI-6, but still smaller vertical gradients for the velocity components were observed. At CSI-9, currents were directed to the northeast, exhibiting a high vertical gradient. Current velocities at CSI-16 occasionally reached 1.5 m/s, and significant depth gradients were associated with the v component.

CSI-9

CSI-16

Speed (m/s)

Direction (degrees)

Speed (m/s)

Direction (degrees)

Speed (m/s)

Direction (degrees)

7.5 11

264 341

6.5 15

322 344

6.5 24

274 10



5

Figure 4. Vertical profiles of u and v velocity components for the three stations corresponding to events 1 and 2; (a and b) CSI-6 for events 1 and 2, respectively, (c and d) CSI-9, (e and f) CSI-16.

even experienced smaller values and less vertical gradient during summertime. The energetic atmospheric events of midAugust to mid-September, as well as during November, were less translated as velocities increased across the water column. The same characteristics were also observed with the velocity profile of the v component at CSI-9. Velocity profiles at CSI-16 exhibited an entirely different response. July and August profiles of the u velocity component exhibited significant differences between velocities across the water column, whereas more uniformity was observed from late August to the end of November. Summertime velocity shear was more pronounced in velocity profiles of the v component. In fact, a high-velocity core was detected within the 6–9-m depth range above the seabed. This high-velocity core caused significant mixing to both the upper and lower water columns. A similar core was also detected in the u component profiles, but it was not as obvious as in the case of the v component. Velocity shear across the water column was persistent during the entire summer study period, especially for the u component, whereas the rare velocity shear events at CSI-6 and CSI-9 were ephemeral. In addition to significant differences in velocity profiles of CSI-6 and CSI-9 with that of CSI-16, there were major

Figure 5. Vertical profile of u velocity component for three stations during the study period (upper: CSI-6, middle: CSI-9, lower: CSI-16).

differences in actual velocity values. As inferred from Figures 5 and 6, velocity values (both u and v components) were generally larger at CSI-16 compared with both CSI-6 and CSI9. All the aforementioned contrasting aspects of current fields from stations east and west of the bird foot delta have been summarized in Figure 7, illustrating time-averaged u and v velocity profiles during both summer and fall for all three stations. Maximum absolute time-averaged velocities of CSI-16 were larger than velocity values from CSI-6 and CSI-9 during both summer and fall, but it was most pronounced during summertime, especially for the v component. Maximum summertime absolute value of the v component of velocity at CSI-16 across the water column was two times larger than that of CSI-6 and five times larger than that of CSI-9.


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Figure 7. Seasonal time-averaged u and v velocities from three stations; (a and c) u for summer and fall, respectively, (b and d) v for summer and fall, respectively.

hypoxia studies published from the region (e.g., Rabalais, Wiseman, and Turner, 1994).

Water Column Stratification

Figure 6. Vertical profile of v velocity component for three stations during the study period (upper: CSI-6, middle: CSI-9, lower: CSI-16).

HYDRODYNAMICS AND HYPOXIA Various aspects of flow regime could contribute to the formation of shelf hypoxia. Depth variations of current velocity, current speed and direction, and current persistency were discussed in the previous section. The interconnection between each of these aspects and hypoxia can be established by relating them to dissolved oxygen concentration from the measurement locations. These data, however, were not available for the present study. Hence, no quantitative comparison between shelf currents and oxygen concentration, a prime indicator for hypoxia, was attempted. Instead, plausible interpretations were established by relating the hydrodynamics from the observation stations and detailed

Water column stratification during summer is identified to be the main contributor in forming seasonal hypoxia in the Gulf of Mexico and elsewhere (Hagy and Murrell, 2007). A stratified water column associated with a specific amount of surface-tobottom density difference ( at least 6 kg/m3 in the case of the northern Gulf of Mexico) is designated as the necessary condition for a hypoxic event to be formed (Walker and Rabalais, 2006). Formation of water column stratification is influenced by buoyancy as well as turbulence effects from wind, waves, and ambient currents. It is usually expressed in terms of the Richardson number (Lyons, Panofsky, and Wollaston, 1964): Ri ¼

N2 2 2 ; ]u ]v þ ]x ]y

N2 ¼

g ]q ; q ]z

¨ or Where Ri is the Richardson number, N is the Brunt–Väisäla, buoyancy, frequency (in s1), q is the density of water, and g is acceleration due to gravity. Small Richardson number values show the predominant effect of velocity shear across the water



Table 2. Mean values of denominator in Richardson number relationship [(]u/]x)2 þ (]v/]y)2], based on measured currents from each station. Station

Summer

Fall

CSI-9 CSI-6 CSI-16

0.000220 0.0011 0.0097

0.000708 0.0015 0.0036

column and the turbulence forces preventing stratification. Increasing the value of the Richardson number from a threshold (0.2–0.25) suppresses turbulence forces. Buoyancy driven by a vertical density gradient dominates, so for Ri . 1, stratification is assumed stable (Lyons, Panofsky, and Wollaston, 1964; Turner, 1973; Galperin, Sukoriansky, and Anderson, 2007). The denominator of the Richardson number equation accounts for the effect of vertical shear mixing induced by currents. Hence, the larger the vertical gradient of the current component, the larger and the smaller are the denominator and the resulting Richardson number, respectively; implying a well-mixed water column. Considering the nature of current profiles discussed in the previous section, a smaller shear effect was expected for both stations west of the bird-foot delta (CSI-6 and CSI-9) compared with CSI-16 east of the bird-foot delta. To be more quantitative, water column stratification for stations across the bird-foot delta were compared and contrasted with regard to this parameter. Table 2 presents the average values of the Richardson number denominator for all three stations during both summer and fall. A much larger denominator at CSI-16 compared with CSI-6 and CSI-9, especially during summertime, was obtained. The denominator in this station during the summer was even higher than during the fall. In the summertime, it was one order of magnitude larger than that of CSI-6 and two orders of magnitude larger than the denominator at CSI-9. Even during the fall, it was more than two times larger than CSI-6 and seven times larger than CSI-9. This means that even if the buoyancy effect (Richardson number numerator) was the same for stations east and west of the birdfoot delta, a still smaller Richardson number was obtained for CSI-16, accounting for less stratification. However, it has been reported that the Mississippi River discharge to the eastern side of the bird-foot delta is 15% less than that of the western side (Rego et al., 2008); therefore, less of a buoyancy effect would be expected for the eastern side. Hence, less stratification reduces the hypoxia potential for the CSI-16 region.

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advection rate (Kasai, Yamada, and Takeda, 2007). It is more likely for a hypoxic zone to exhibit sluggish current velocities and become isolated from the surrounding environments that have higher current velocities (Hagy and Murrell, 2007). The effect of advection on hypoxia over the study area can be primarily examined by comparing and contrasting current speeds at each of the three stations. As discussed earlier, velocity values from CSI-16 were significantly larger than velocities at both CSI-6 and CSI-9. The average summertime resultant current speed (vertically and temporally averaged) at CSI-16 was more than two times larger than the same quantity at CSI-6 and CSI-9, whereas for the maximum time-averaged depth speed, it was three -to- four times larger (see Figure 8). A high-velocity core (within the depth range 6–9 m from the bottom) was identified as a distinct configuration of vertical current structure at CSI-16. This core is easily distinguished in the evolution of current speed profiles for this station (see Figure 8). In addition to the high current regime at station CSI16, this high-speed core might have a crucial effect on transporting water and dissolved oxygen from surrounding areas to the location of this station. Examining the corresponding time-averaged profiles of current speed for the other two stations west of the delta revealed completely different structures. They are characterized by a smaller range of current speed, decreasing from the surface of the water to the bottom. Hence, considering the effect of currents alone, lateral advection to the CSI-16 location would be approximately two times larger than to the stations on the western side. Besides, the oxygen concentration of the probable sources contributing to lateral reoxygenation might be higher for the eastern station because the advection process likely starts from a less hypoxic area farther east or south. Summertime current directions for both CSI-6 and CSI-9, as well as their physical locations being bounded within the well-demarcated summer hypoxic area

Effect of Advection on Hypoxia Although stratification is the dominant factor in hypoxia formation, other hydrodynamic aspects are still critical. Transport of oxygen-rich water to the oxygen-depleted shelf waters, which in fact is regulated by the advection process, plays an important role in reoxygenation of hypoxic areas (Rabalais, Wiseman, and Turner, 1994). Reduced advection to the hypoxic area can decrease the transport of dissolved oxygen from the surroundings and would increase susceptibility to hypoxia (Hagy and Murrell, 2007). Velocity range within a specific area is the main hydrodynamic aspect controlling the

Figure 8. Mean profiles of depth current speeds from the three stations during summer and fall.


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west of the bird-foot delta (based on Rabalais, Turner, and Wiseman, 2002), imply that advection flux to these stations would be mostly from the already hypoxic areas, which would have no significant contribution for reoxygenation. Currents might even transport hypoxic waters to nonhypoxic areas, causing ‘‘jubilees’’ in which fish and crabs move to nonhypoxic shallower areas (Hagy and Murrell, 2007). Reoxygenation by advection requires a consistent current direction, producing a continued flux of oxygen to the hypoxic area. As mentioned earlier, the current pattern at CSI-6, and to some extent at CSI-9, was feeble and irregular during summertime, i.e., there was no consistent flow direction for prolonged periods of time (Figure 2). These frequent changes in current direction prevent a steady influx of oxygen-rich water to the dead zone west of the delta. Reoxygenation of the water column at a marginal point of the hypoxic zone west of the birdfoot delta because of tidal advection has been reported by Rabalais, Wiseman, and Turner (1994). However, this condition might not be strong enough at locations CSI-6 and CSI-9 because either vector representation of currents (Figure 2) or time series of flow did not demonstrate harmonic variations. Two distinct dominant current directions were detected for summertime currents at CSI-16 station: southwestwards and northeastwards. In fact, the currents were persistent enough to exhibit a dominant direction. This can cause a steady influx of oxygen-rich water to this station’s location.

SUMMARY AND CONCLUSIONS Summer and fall flow characteristics from locations west and east of the Mississippi bird-foot delta, in terms of their effect on the northern Gulf of Mexico seasonal hypoxia, were studied on the basis of current and wind data from three WAVCIS stations off the Louisiana coast, viz., CSI-6, CSI-9, and CSI-16. Current profiles from station CSI-16 (east of the delta) exhibited larger vertical gradients during the summer compared with the other two stations located on the western side. This higher velocity gradient translated to smaller Richardson numbers, showing a higher degree of mixing for the eastern station. At the same time, advection could come into action and might consistently reoxygenate the CSI-16 location and, therefore, reduce the hypoxia potential, considering the high current velocities observed from there. Smaller current speeds observed at the stations west of the delta, particularly during summertime, were less likely to cause significant reoxygenation to the water column below the pycnocline. The lack of reoxygenation by advective processes at stations west of the bird-foot delta was further reduced by the onset of a disorganized current pattern during summertime, suppressing any consistent flux of dissolved oxygen to this area. The contrasting hydrodynamic data from different locations east and west of the bird-foot delta highlights the significant role of physical processes in the formation and sustenance of hypoxia in the northern Gulf of Mexico. Both stratification and advection hypotheses can be developed further by using more available current data, especially over the western shelf. By taking advantage of long-term data measured by other WAVCIS stations, such as CSI-5 and CSI-15, along with data from CSI-6 and CSI-9, a more precise spatial depiction of shelf stratification might be

determined. Furthermore, the idea of oxygen advection across the shelf can be more quantitatively, and thus accurately, measured.

ACKNOWLEDGMENTS The authors are grateful to Dr. Gregory W. Stone (deceased) for his constant encouragement and support for this work. Also, we acknowledge colleagues at the WAVCIS Lab for data support, as well as for insightful discussions during the preparation of this manuscript.

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