GIScience & Remote Sensing, 2014 Vol. 51, No. 2, 139–157, http://dx.doi.org/10.1080/15481603.2014.895578
Assessment of chlorophyll variability along the Louisiana coast using multi-satellite data Eurico J. D’Sa* Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, USA (Received 29 September 2013; accepted 7 January 2014) Variability in surface chlorophyll (Chl) concentrations derived from the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) were examined in conjunction with river discharge, QuikSCAT satellite-derived winds, and sea surface height (SSH) anomaly data along the Louisiana coast, USA. Surface Chl distributions exhibited rapid response to strong northerly winds following a frontal passage. A comparison of time series (1998–2010) river discharge and monthly Chl data indicated Chl variability to be well correlated to seasonal river discharge only for locations near the two river deltas, while offshore, enhancements in Chl during fall–winter was likely due to crossshelf transport or mixing associated with strong northerly wind stress. Variance in Chl examined using wavelet analysis applied to nearly 10 years (1998–2007) of SeaWiFS data indicated patterns of significant Chl variability due to combined enhanced wind and river discharge, offshore flows associated with Ekman transport and coastal wind convergence, and the effect of Hurricane Rita in 2005. Instances of significant Chl variance were also observed to occur during years of large hypoxic zone size suggesting potential linkages to hypoxia. SSH anomaly imagery indicated the presence of warm-core eddies that were responsible for the offshore dispersal of elevated Chl observed in the monthly SeaWiFS imagery. Overall, the use of multi-satellite data better described the forcing and patterns of Chl distributions along the river-dominated Louisiana coast and shelf. Keywords: SeaWiFS; QuikSCAT winds; ocean color; chlorophyll; wavelet analysis; hypoxia; Mississippi River; Gulf of Mexico
Introduction The northern Gulf of Mexico is strongly influenced by the Mississippi River which drains more than 40% of the contiguous United States and ranks sixth in the world in terms of fresh water discharge (Milliman and Meade 1983). The river discharges approximately 19,000 m3 s−1 on average, to the Louisiana shelf (Wiseman et al. 1997) with 70% flowing through the birdsfoot delta and the remaining through the Atchafalaya River delta. The highly productive coastal margin has in recent decades experienced increased nutrient loading from the Mississippi River resulting in elevated chlorophyll (Chl) biomass that has been linked to summer-time hypoxia in near bottom shelf waters (Rabalais, Turner, and Wiseman 2001, 2002). Due to the importance of the Mississippi River nutrient loading of shelf waters and its influence on primary productivity and phytoplankton biomass and their linkages to hypoxia, many studies have examined phytoplankton dynamics using field measurements, modeling, and satellite remote sensing (Lohrenz et al. 1997, 2008; Chen, Lohrenz, and Wiesenburg 2000; *Email:
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D’Sa, Miller, and Del Castillo 2006; Green et al. 2008; Walker and Rabalais 2006; DiMarco et al. 2010; Fennel et al. 2011). In addition to fluvial nutrients, physical factors such as winds, tropical storms, and eddies have been shown to influence phytoplankton biomass variability in the northern Gulf of Mexico (Toner et al. 2003; Yuan et al. 2004; Walker et al. 2005; Walker and Rabalais 2006). Although winds are the dominant driver of upper circulation along the Texas–Louisiana coast (Cochrane and Kelly 1986), freshwater influx and loop current (LC) detached eddies can also contribute to the generation of currents over the Louisiana– Texas shelf (Oey 1995; Wiseman et al. 1997). During fall, winter, and spring, wind stressgenerated currents generally flow westward along the Louisiana coast and then southwards along the Texas coast. This inner shelf current or the downcoast current is well developed and is responsible for the distribution of freshwater, sediments, and nutrients on the Louisiana and Texas coast. During fall–winter period, the frequent frontal passages with its strong westerly and northwesterly winds of one to several days duration have a rapid impact on surface circulation and the downcoast current (Wiseman et al. 1997; Walker et al. 2005). During summer, the downcoast current weakens and disappears due to wind shift from easterly to southerly and southwesterly, and an upcoast current develops (Cochrane and Kelly 1986; Nowlin et al. 2005; Jarosz and Murray 2005). The downcoast current is re-established by September when easterly winds resume (Cochrane and Kelly 1986). Over the outer shelf, the eastward-directed outer shelf currents are often influenced by the presence of anticyclonic LC eddies (Oey 1995). Furthermore, near the Mississippi River delta, both wind- and eddy-driven dynamics have been shown to play a major role in the transport and dispersion of the Mississippi River plume waters, indicating the strong role of physical processes in enhancing biological activity in the deeper Gulf waters (Schiller et al. 2011). Although nutrient loading from the Mississippi River is the primary driver of phytoplankton variability in the river-dominated shelf waters, satellite observations in combination with in situ current and wind measurements have been used to show strong linkages between wind forcing, circulation, plume dynamics, and the seawater constituents such as suspended sediments and Chl-a concentrations (Salisbury et al. 2004; Walker et al. 2005; D’Sa and Ko 2008; Schiller et al. 2011). During fall–winter, coastal waters strongly respond to the frequent frontal passages that transport the more productive plume waters into the deeper shelf waters (Walker and Rabalais 2006). Ocean color imagery used in conjunction with satellite-derived sea surface height (SSH) data has also revealed the role of eddies in the offshore dispersal and transport of phytoplankton biomass (Toner et al. 2003; Yuan et al. 2004; Schiller et al. 2011). Most of these studies have examined shorter time-scale events such as hurricanes or frontal passages on phytoplankton biomass variability or used limited time series ocean color data to examine linkages to physical drivers such as winds that were generally obtained from a single field monitoring station (Walker et al. 2006; Green et al. 2008). QuikSCAT-derived wind field obtained at large synoptic scales has revealed spatial variability in the wind speed and direction along the Louisiana–Texas shelf (D’Sa and Ko 2008; Sharma and D’Sa 2008), suggesting its use in conjunction with ocean color data could better explain the spatial and temporal patterns of Chl variability in the study area. The main objective of this study was to therefore examine short-term, seasonal, and interannual shelf-wide distribution and variability of Sea-Viewing Wide Field-of-View Sensor (SeaWiFS)-derived Chl in conjunction with river discharge and satellite-derived wind and SSH data to examine linkages to physical drivers. Furthermore, wavelet analysis of a 10-year record of SeaWiFS Chl was used to examine the dominant temporal scales of Chl variability and its linkages to physical forcing.
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Figure 1. Study area in the northern Gulf of Mexico (Louisiana and part of Texas shelf) with bathymetric contours drawn at 20, 50, 100, 500, and 1000 m depths. Black labeled squares denote areas used for time series analysis of SeaWiFS-derived chlorophyll data and are referred to as (A) delta, (B) intermediate, (C) offshore, and (D) far-field in the text. AR and MR represent the Atchafalaya River and Mississippi River and the arrow points to the Atchafalaya delta region discussed in the text. Color figures are available in the online version of this article.
Data and methods Study area The study area comprises mainly the Louisiana and part of Texas coast, extending between 88.2° and 95.5° W and into the Gulf of Mexico to 27° N latitude (Figure 1). The Mississippi River discharges through the birdsfoot delta which extends into the deeper waters of the Gulf of Mexico while the Atchafalaya River located west of the birdsfoot delta and with about 30% of the Mississippi River flow discharges into the Atchafalaya delta (Figure 1). River discharge data were obtained from the US Army Corps of Engineers from Tarbert Landing Station, MS for the Mississippi River and from Simmersport Station, LA for the Atchafalaya River. A narrow shelf off the birdsfoot delta expands westwards into a wide shelf off the Atchafalaya delta with coastal currents dominating within the inner shelf (~50 m isobath) and LC eddies influencing the outer shelf (Nowlin et al. 2005). With satellite imagery generally showing high Chl values within the 20 m isobath during spring where average salinity contours and elevated particle distributions are close to the coast and narrowly spaced (Dinnel and Wiseman 1986; Salisbury et al. 2004), Chl estimates are likely to be influenced by other seawater constituents such as suspended sediments and colored dissolved organic matter. Therefore, the three study sites A, B, and D from where satellite time series data were obtained for analysis were approximately located between the 20and 50-m isobaths, while site C (Figure 1) was located just beyond the outer shelf where the river influences would be minimal.
SeaWiFS satellite data SeaWiFS satellite data were obtained and processed for the Louisiana–Texas shelf waters in the northern Gulf of Mexico (Figure 1). Level 1A SeaWiFS satellite data at 1-km resolution were downloaded from NASA Goddard Distributed Active Archive Center (DAAC) and processed using the standard atmospheric correction and the standard OC4 Chl algorithm using the SeaDAS software to obtain Level 2 Chl at 2-km spatial
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resolution. For wavelet analysis, 15-day averaged Chl time series data were used only for the period 1998–2007 due to data gaps in the latter years of SeaWiFS operation. Monthly Chl imagery was also derived using the same 2-km high spatial resolution data. Monthly average surface SeaWiFS Chl data at the four locations (A, B, C, and D) on the Louisiana shelf were also downloaded from NASA’s GES-DISC Interactive Online Visualization ANd aNalysis Infrastructure (Giovanni) website for the period 1998–2010. The monthly Chl data at 9-km spatial resolution available for download from the Giovanni website is the most recent reprocessed data derived using the OC4v6 Chl algorithm. SeaWiFS Chl data were acquired over an approximate 20 × 20 km area in the inner shelf near the Mississippi delta (A), an intermediate location B off the Atchafalaya delta, and a far-field location D not directly influenced by the two rivers. The offshore location C was just beyond the outer shelf between 500- and 1000-m isobaths off the Atchafalaya shelf in deeper Gulf oligotrophic waters where river influences are likely to be minimal.
Wavelet analysis Time series composited 15-day means of SeaWiFS-derived Chl between 1998 and 2007 were used in the wavelet analysis. Chl anomalies of 15-day means were extracted along the 50-m isobath near locations B and D for the same period. Wavelet transform, a method that decomposes a signal into time–frequency space allows for the determination of the dominant modes of variability and their modes of variation in time. Wavelet transform was applied to the time series Chl anomaly data using the Morlet wavelet that used a nondimensional frequency of 6 to satisfy the admissibility condition. The Morlet wavelet also known as the ‘Mother wavelet’ consists of a Gaussian-windowed complex sinusoid that is used as a reference function and is designed to have a zero mean (admissibility condition) and be localized in both time and frequency (Morlet 1983; Torrence and Compo 1998). The wavelet transform of a time series is then defined as ð 1 tb f ðt Þdt W ðb; aÞ ¼ pffiffiffi ’ a a
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where φ is the mother wavelet, and the parameters a and b determine the shape and location of the wavelets (Henson and Thomas 2007). Changes in a stretches or compresses the wavelet over different frequencies, and the translation parameter b moves the center of the wavelet in time as the wavelet transform is applied across all the data points in the time series; this operation can also be considered as a convolution of the time series with a scaled and translated version of the wavelet (Torrence and Compo 1998). The 2-D output matrix W in Equation (1) describes the relative amplitude of features at a particular frequency (or wavelet scale a) and time. On applying the wavelet transform using the Morlet wavelet to the Chl time series we obtain the local wavelet power spectrum that depicts variance in Chl (mg m−3)2 and can be interpreted as a ‘map’ of the time variability of dominant frequencies (Machu, Ferret, and Garçon 1999; Henson and Thomas 2007).
QuikSCAT satellite data Satellite wind data were derived from the SeaWinds instrument, a scanning radar Ku-band scatterometer onboard the QuikSCAT satellite that was deployed approximately 2 years
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following the SeaWiFS launch. It measures surface roughness of the ocean affected by the wind magnitude and direction by transmitting microwave pulses (13.4 GHz) and receiving the backscatter from a swath width of 1800 km (Tang, Liu, and Stiles 2004). In this study the 25-km QuikSCAT wind product downloaded from the Jet Propulsion Laboratory (JPL) NASA website (http://podaac-www.jpl.nasa.gov/) was used. Comparisons between the QuikSCAT and buoy winds indicated the QuikSCAT winds to perform reasonably well for the Gulf of Mexico region (Sharma and D’Sa 2008). Root mean square error (RMSE) differences (satellite minus buoy) for moderate winds (3–20 m s−1) were found to be 1.48 m s−1 and 29.5° for wind speed and direction, respectively, for buoys located in the Gulf of Mexico. The 10-m scatterometer winds were converted to wind stress using the drag coefficient algorithm from Large and Pond (1981). Monthly averaged QuikSCAT derived wind stress was then superimposed on the monthly Chl imagery (D’Sa and Korobkin 2009).
SSH data Historical SSH anomaly data were obtained using the Colorado Center for Astrodynamics Research Gulf of Mexico Near Real-Time Altimeter Viewer (Leben, Born, and Engebreth 2002), which are derived from data acquired by satellite altimeters such as Jason-1, Geosat Follow-On, and Envisat. SSH anomaly imagery coincident with the monthly SeaWiFS imagery was obtained for this study.
Results and discussion Short-term variability during frontal passages The northern Gulf of Mexico is frequently impacted by frontal passages (about 30–40 per year) during the fall–winter period. Winds associated with these cold fronts generally undergo patterns of change in surface wind speed and direction, barometric pressure, temperature, and humidity (Mossa and Roberts 1990). These fronts are synoptic scale convergence or boundaries between polar continental and tropical maritime air that impart energy to coastal and shelf waters in the northern Gulf of Mexico (Hu, Rouse, and Walker 1984). Along the Louisiana coast, these frontal passages have been shown to impact coastal and shelf environments through redistribution of sediments, initiate off-shelf transport, and reduce residence times of river waters (Roberts et al. 1987; Walker et al. 2005). The influence of a frontal passage on the suspended particulate matter distribution along the Louisiana–Texas coast using QuikSCAT winds and model simulation demonstrated the spatial evolution of the wind field and the corresponding rapid reversals of the surface currents resulting in the offshore transport of suspended material in the shelf waters (D’Sa and Ko 2008). A similar pattern of wind field during a frontal passage is demonstrated by the QuikSCAT satellite imagery (Figure 2) which reveals strong northerly winds up to 25 m s−1 passing through the western Louisiana and Texas shelf on 6 January, 2006. Winds were also strong south of the Atchafalaya delta and throughout the western part of the Gulf extending into the Yucatan Peninsula that strongly impacted the phytoplankton biomass distribution in surface waters of the northern Gulf (Figure 3). SeaWiFS imagery of 5 January 2006 before the frontal passage shows elevated Chl along the inner shelf from the Mississippi delta into the Texas coast with highest concentrations off the Mississippi and the Atchafalaya River deltas (Figure 3a). Following
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Figure 3. SeaWiFS-derived chlorophyll a (mg m−3) for the study region on (a) 5 January and (b) 7 January 2006. Clouds have been masked in black.
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the frontal passage on 6 January, the SeaWiFS Chl imagery of 7 January revealed a strong response off the Atchafalaya shelf where a plume of elevated Chl is observed extending out into the offshore waters of the shelf (Figure 3b). West of the Atchafalaya delta, plumes of elevated Chl are also seen extending out into the outer shelf waters, suggesting the role of wind forcing during frontal passages as a mechanism for the offshore transport of phytoplankton biomass. In waters around the Mississippi delta, elevated Chl concentrations observed before the frontal passage appeared to have decreased most probably due to mixing. Although previous studies have documented such response using ocean color imagery and wind data from coastal stations (Walker et al. 2005), the use of QuikSCAT satellite imagery has provided a synoptic view of the wind field over a larger area of the Gulf revealing some differential response of the surface Chl distribution to the wind forcing on the shelf.
Seasonal variability In the delta region (e.g., location A) immediately impacted by the discharge of the Mississippi River, phytoplankton biomass as indicated by Chl (January 2004–December 2006) appears to be significantly correlated to the river discharge (Table 1; Figure 4a). Although river discharge generally peaks in spring and drops to its lowest level in late summer, there are variations in the magnitude and timing of the river discharge. Multiple peaks in river discharge were recorded in 2004 that extended up to July with the river discharge eventually falling below 10,000 m3 s−1 in August. Phytoplankton biomass (mean Chl = 3.98 ± 1.9 mg m−3) which varied over the range 1.43–8.45 mg m−3 showed increases or peaks corresponding to increases in river discharge with small but variable lag in time. The peak in July that resulted in lower salinity waters in the Louisiana shelf could be attributed to extensive rainfall (DiMarco et al. 2010), which likely delivered elevated nutrients that resulted in higher than usual phytoplankton biomass. Although in 2005 the peak river discharge was in winter, there was no corresponding peak in Chl most likely due to lower water temperatures and reduced solar radiation limiting phytoplankton growth rates or to reduced residence of river waters due to their off-shelf transport associated with short-term wind reversals from frontal passages (Walker et al. 2005; Fennel et al. 2011). However, a Chl peak occurred latter in spring corresponding to lower river discharge peaks and dropped down to its lowest level during summer. In spring of 2006, Chl peaks followed the multiple river discharge peaks (Figure 4a). River discharge from the Atchafalaya River was similar to the Mississippi River but lower in volume. Chl response at the intermediate location B (mean = 1.81 ± 0.77; min-max = 0.45– 4.08 mg m−3) appeared generally similar to region A, and while lower, was also well correlated to the river discharge (Table 1); however, patterns of Chl concentrations differed Table 1. Pearson correlation coefficients for the Mississippi and Atchafalaya River discharge and the Chlorophyll concentrations at the four locations: A, delta; B, intermediate-B; C, offshore; and D, far-field. Location
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Figure 4. (a) Mississippi River discharge (m3 s−1) obtained from Corps of Engineers gauge station at Tarbert Landing, MS, and the monthly chlorophyll (Chl) concentrations at location A. (b) Atchafalaya River discharge (m3 s−1) from gauge station at Simmersport, LA and monthly Chl at locations B, C, and D between 1 January 2004 and 31 December 2006.
slightly during some months (Figure 4b) likely due to its location which comes under the combined influence of the Mississippi and Atchafalaya Rivers. At the offshore station C off the Atchafalaya shelf, Chl was low (mean = 0.19 ± 0.17; min-max = 0.09–0.36 mg m−3) but exhibited peak values in the winter (Figure 4b). At the far-field region D located between the 20- and 50-m isobath, the Chl values were less than that at location B but showed larger seasonal variability. Both locations B and D showed elevated levels from September to December 2005 that appeared to be due to the two hurricanes (Katrina and Rita) that made landfall along the Louisiana coast. The monthly wind stress (derived from QuikSCAT winds) maps superimposed on SeaWiFS Chl and shown bimonthly (January, March, May, July, September, and November) for the years 2004 and 2006 revealed similar seasonal trends in wind stress
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patterns with some month-to-month variability (Figures 5 and 6). During winter (December 2003 (not shown), January, and February (not shown)) southward wind stress associated with the frequent frontal passages prevailed over the Louisiana–Texas shelf waters with the highest average levels in February (~0.08 N m−2). Low Chl observed beyond the 20-m isobath before winter (e.g., September 2004) became elevated throughout the outer shelf (e.g., Figures 5a and 6a) during winter under the mean southward wind stress. The elevated levels of Chl in the offshore waters were exemplified by winter values at location C (Figure 4b). Following the increase in river discharge in spring (Figure 4), there was a general increase in Chl along the coast; however, the easterly, southeasterly downwelling favorable winds constrained the Louisiana coastal currents to the inner shelf (Figures 5b, 5c, 6b, and 6c). In June 2004 (not shown), southerly winds prevailed, while mean wind stress in July was rather chaotic (Figure 5d). Patterns of surface Chl indicated offshore dispersal of phytoplankton biomass during the summer. Northeasterly/easterly wind stress resumed in September and October 2004 (Figure 5e and f). Similar patterns were observed during 2006, with the offshore Chl levels falling slightly. Generally, similar response to the wind stress was observed during 2006 (Figure 6). It may be noted that although surface Chl distribution patterns appear to respond to the wind stress, a large component of this response is due to surface currents and the associated movement of the productive river plume waters due to wind forcing (Cochrane and Kelly 1986; Morey,
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Figure 5. Bimonthly mean wind stress vectors (N m−2) derived from QuikSCAT satellite superimposed on mean SeaWiFS-derived Chl (mg m−3) for (a) January, (b) March, (c) May, (d) July, (e) September, and (f) November of 2004.
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Zavala-Hidalgo, and O’Brien 2005; Nowlin et al. 2005) that result in the redistribution of Chl on the shelf.
Interannual variability A comparison of the Mississippi River discharge (mean flow = 14,461; min-max = 3907– 41,225 m3 s−1) and the mean monthly SeaWiFS Chl (mean = 4.48; min-max = 0.52– 13.03 mg m−3) for the period 1998–2010 show similar interannual trends in variability with corresponding highs in river discharge and Chl during spring and lows in summer (Figure 7a). Similar patterns were also observed for discharge from the Atchafalaya River (mean = 6213; min-max = 1557–17,725 m3 s−1) and Chl (mean = 1.86; min-max = 0.25– 5.5 mg m−3) at the intermediate region B (Figure 7b). At the offshore region C, peaks in Chl (mean = 0.21; min-max = 0.07–0.61 mg m−3) generally occurred during the winter months. Wind stress associated with the frontal passages transferring fresher, higher nutrient waters offshore or greater mixing of the water column appears to be the main reason for the elevated Chl levels in the offshore waters during winter. At the far-field region D, Chl values were lower than that at B with more complex pattern of variability suggesting both river and other influences in this region (Figure 7b).
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Figure 7. (a) Daily Mississippi River discharge (m3 s−1) and the monthly Chl at location A. (b) Atchafalaya River discharge (m3 s−1) and monthly Chl at locations B, C, and D between 1 January 1998 and 31 December 2010, (c) Climatological mean monthly discharge from the Mississippi and Atchafalaya Rivers (MR/AR) and climatological mean monthly Chl concentrations at locations A, B (inner-right axis), and C (outer-right axis) for the period between January 1998 to December 2010.
The climatological mean monthly river discharge and Chl concentrations at locations A, B, and C for the period 1998–2010 corresponding to the SeaWiFS satellite record provide an overview of the seasonal variability in river discharge and Chl concentrations (Figure 7c). Climatological seasonal peaks in the Mississippi and Atchafalaya River discharge occurred in April (20,847 ± 6923 and 8902 ± 2996 m3 s−1, respectively) while the lowest monthly discharge was in September (7632 ± 2323 and 3268 ± 979 m3 s−1, respectively). Chl concentrations at the delta location A increased with increased river discharge to its maximum value of 6.54 ± 2.14 mg m−3 in April and then decreased to its lowest value of 2.53 ± 0.94 mg m−3 in October. Similar pattern of monthly Chl variability was observed at location B off the Atchafalaya shelf with a mean high of 2.79 ± 1.26 mg m−3 in March and a low of 1.14 ± 0.48 mg m−3 in August, respectively. However, a slight increase in mean Chl concentration in July at location B
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suggests the surface current reversals in summer associated with wind forcing may cause some offshore dispersal of the more productive near shore waters. Similar peaks in Chl concentrations during summer have been previously reported (Walker and Rabalais 2006); the implications on the timing and magnitude of this increase in the shelf productivity on hypoxia could be an interesting study. In contrast, at the offshore location C, Chl was highest (0.30 ± 0.04 mg m−3) in January and lowest (0.12 ± 0.02 mg m−3) in May (Figure 7c). Such patterns of elevated Chl concentrations in the offshore waters off the Louisiana coast have been previously reported (Müller-Karger et al. 1991; Biggs, Hu, and MüllerKarger 2008). Wavelet transform applied to mean Chl anomaly data at 50 m isobath in a section close to locations B and D show maps of time variability of dominant frequencies of Chl variance (mg m−3)2 (Figures 8a and 9a). The normalized scale-averaged time series (Figures 8b and 9b) denotes the mean variance in the 30- to 100-day period band obtained by normalizing the wavelet power by N/2σ2 (where N is the number of data points and σ2 its variance); the thin horizontal line denotes the 95% confidence interval. Off the Atchafalaya shelf, significant peaks in Chl variances (Figure 8b) with dominant periods between approximately 30 and 60 days (Figure 8a) were observed in early spring of 2001, 2003, and 2007 that could be mainly attributed to a combination of anomalously high river discharge (Figure 10) and strong northerly wind stress that likely transported the more productive coastal waters offshore as shown for years 2001 and 2003 (Figure 11a and b). The size of the hypoxic zone in late July during two of the three years (2001 and 2007) was some of the highest reported (Turner, Rabalais, and Justic 2008), indicating that patterns of wind forcing in addition to riverine nutrient input could be important in
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Figure 8. Wavelet power spectra (mg m−2)2 of Chl along the 50-m isobath at location B south of the Atchafalaya delta. (b) Corresponding scale-averaged time series for the period band 30–100 days with the green line showing the 95% confidence interval.
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determining the extent of hypoxia in the northern Gulf of Mexico (Forrest, Hetland, and DiMarco 2011; Feng, DiMarco, and Jackson 2012). The unusually large variance in Chl detected in July/August of 1999 (Figure 8a), however, occurred during a period of negative river discharge anomaly (Figure 10). An examination of the wind field over the region revealed southerly winds approaching the Texas coast turning eastward and the wind stress intensifying over the inner shelf waters
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of the Atchafalaya and Mississippi deltas (Figure 11c and d). In contrast to other years, this reversal in the wind field over the Louisiana shelf waters was sustained for a period of approximately 2 months that resulted in the offshore flow of the more productive coastal waters due to Ekman transport. Thus, in spite of the unusually low river discharge during this period, upwelling and offshore transport of nutrient rich waters likely sustained the elevated Chl in the more offshore shelf waters of the Louisiana coast and the high variance at location B. It has been previously reported that such westerly winds can facilitate hypoxia development (Forrest, Hetland, and DiMarco 2011) which might explain the relatively large extent of the 1999 summer hypoxic zone (Turner, Rabalais, and Justic 2008). In the far-field region west of the Atchafalaya shelf (,50-m isobath section at D), significant peaks in wavelet power were observed in 2002, 2005, and 2007 (Figure 9a and b). Peak Chl variances with periods between 30 and 60 days in June/July 2002 and 2007 appeared to be due to particularly elevated wind stress associated with south winds and the summer reversal of currents from downcoast to upcoast that occurs along the Louisiana–Texas coast (Cochrane and Kelly 1986; Nowlin et al. 2005; Morey, ZavalaHidalgo, and O’Brien 2005). In June 2007, under southeasterly wind stress elevated Chl appeared to be constrained within the 20-m isobath (Figure 12a). Following intensification and the strong convergence of the along-coast wind stress along the Texas coast (Figure 12b), there was a significant increase in the offshore spatial extent of Chl (Figure 12b). Model simulations have linked such wind convergence to cross-shelf transport (Zavala-Hidalgo et al. 2006) and likely contributed to the significant Chl variance detected in the far-field region D. The reported hypoxic zone size during 2002 and 2007 was one of the largest with greater extent west of the Atchafalaya delta (Obenour et al. 2013), suggesting a potentially different mechanism for its formation. The strongest peak in wavelet power with a period greater than 60 days was however
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observed in September 2005 (Figure 9a) and was due to Hurricane Rita which made landfall along the Louisiana–Texas border.
Effects of hurricanes and eddies Use of satellite ocean color imagery has been useful to examine the biological response of the oceanic and coastal waters to hurricane passages in the Gulf of Mexico (Yuan et al. 2004; Gierach and Subrahmanyam 2008). Storm passages generally produce short-term increases in surface Chl (Gierach and Subrahmanyam 2008). Wavelet analysis (Figure 9) suggests that over a 10-year period the various hurricanes that impacted the Louisiana coast generally did not have a long-term impact (>30 days) on the Chl variability. However, Hurricane Rita which made landfall on 24 September 2005 on the Louisiana– Texas border and an earlier Hurricane Katrina which made landfall on 29 August in southeast Louisiana appeared to enhance Chl along the Louisiana shelf waters (Figure 12c). While Hurricane Katrina appeared to enhance Chl east of the Atchafalaya delta during the month of September (Figure 12c), Hurricane Rita, which caused widespread coastal flooding along west Louisiana and Texas coast, appeared to enhance Chl west of the Atchafalaya delta for a longer period (Figure 12d). Discharge of ponded flooded waters with elevated nutrients over time may have contributed to the longer period of elevated Chl in the shelf waters off its landfall location. Wavelet analysis indicated significant variance in Chl that persisted for greater than 2 months in the western Louisiana shelf (Figure 9). The combination of monthly QuikSCAT winds overlaid on SeaWiFS Chl maps and wavelet analysis allowed for a better understanding of the hurricane impact on the Louisiana–Texas coast.
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In addition to the buoyancy-driven flows of river plume waters and wind forcing, LC and LC eddies influence circulation and mixing along the Louisiana–Texas coast (Oey 1995; Ohlmann et al. 2001). The entrainment of the Mississippi plume waters containing high phytoplankton biomass with interacting cyclonic and anticyclonic LC eddies and its transport offshore have been well documented using satellite ocean color imagery (Toner et al. 2003; Yuan et al. 2004; Walker et al. 2005). Using a decade of SeaWiFS-derived mean monthly Chl imagery a number of such entrainment events were observed (e.g., Figure 13a and c). A warm-core eddy with positive SSH anomaly was located south of the Mississippi delta in November 2006 (Figure 13b). Clockwise circulating surface currents associated with this eddy appeared to have entrained the river plume waters resulting in its offshore transport as indicated in Figure 13a. A similarly located warm-core eddy present in May 2008 appeared to have also entrained and transported a large volume of phytoplankton biomass offshore into the oligotrophic waters of the Gulf of Mexico (Figures 13c and d). The clockwise circulating currents could also entrain the river plume waters with elevated fluvial nutrients that could sustain primary production in the offshore waters for a sustained period of time as indicated by the monthly patterns of elevated Chl in the offshore waters. As in previous studies, satellite imagery of Figure 13 demonstrates the important contribution of LC-generated eddies on the redistribution and offshore transport of phytoplankton biomass and river plume waters in the northern Gulf of Mexico.
Figure 13. (a, c) QuikSCAT-derived wind stress (N m−2) superimposed on mean SeaWiFS-derived Chl (mg m−3) for the months November 2006 and May 2008, (b, d) corresponding SSH anomaly (cm) for the same region and periods.
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Conclusion In this study, long-term river discharge, satellite winds, and Chl were used to examine linkages and physical processes influencing Chl variability along the Louisiana coast and shelf waters influenced by the discharge from the Mississippi and Atchafalaya Rivers. QuikSCAT winds indicated the strong influence of a frontal passage on spatial patterns of Chl variability in the shelf waters of the study area. Mapping of the monthly mean wind stress vectors superimposed on the satellite-derived Chl distribution captured the synoptic influences of the wind field on surface Chl distributions. At locations directly influenced by the two rivers, Chl appeared well correlated to the seasonal peaks in river discharge. However, at the offshore locations, northerly wind stress during fall–winter contributed to peaks in Chl likely due to mixing or offshore dispersal of coastal waters. The use of QuikSCAT winds in conjunction with Chl revealed the spatially variable wind forcing such as the cross-shelf transport associated with strong wind convergence that likely contributed to a significant increase in phytoplankton biomass off the Texas coast. Wavelet analysis indicated that the significant interannual variability in Chl variance associated with river discharge and spatially variable wind forcing may be linked to the occurrence of large hypoxic zones in the Gulf. Wavelet analysis further revealed the long-term increase in Chl due to Hurricane Rita off the western Louisiana–Texas coast. Satellite imagery indicated the role of eddies in the transport of plumes of elevated Chl waters into the more oligotrophic waters of the northern Gulf of Mexico. This study demonstrated the utility of integrating multi-satellite data to better understand the physical–biological linkages in highly productive coastal waters.
Acknowledgements The author would like to thank the SeaWiFS Project at NASA GSFC for making available the SeaWiFS data. The latest reprocessed time series Chl data used in this study were acquired using the GES-DISC Interactive Online Visualization ANd aNalysis Infrastructure (Giovanni) as part of the NASA’s Goddard Earth Sciences (GES) Data and Information Services Center (DISC). The comments and suggestions of two anonymous reviewers greatly improved this manuscript.
Funding This work has been partially funded by a NASA grant [NNA07CN12A].
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