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Chlorophyll variability in the

northeastern Gulf of MexicoBisman Nababan

a , Frank E. Muller-Karger

b , Chuanmin Hu

b &

Douglas C. Biggs c

a Department of Marine Science and Technology, Bogor

Agricultural University, Bogor, Indonesiab College of Marine Science, University of South Florida, St

Petersburg, FL, USAc Department of Oceanography, Texas A&M University, College

Station, TX, USA

Available online: 15 Aug 2011

To cite this article: Bisman Nababan, Frank E. Muller-Karger, Chuanmin Hu & Douglas C. Biggs

(2011): Chlorophyll variability in the northeastern Gulf of Mexico, International Journal of Remote

Sensing, DOI:10.1080/01431161.2010.542192

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International Journal of Remote Sensing

iFirst, 2011, 1–19

Chlorophyll variability in the northeastern Gulf of Mexico

BISMAN NABABAN∗†, FRANK E. MULLER-KARGER‡, CHUANMIN HU‡

and DOUGLAS C. BIGGS§

†Department of Marine Science and Technology, Bogor Agricultural University, Bogor,Indonesia

‡College of Marine Science, University of South Florida, St Petersburg, FL, USA§Department of Oceanography, Texas A&M University, College Station, TX, USA

(Received 19 March 2009; in final form 22 July 2009)

Changes in chlorophyll concentration distribution in surface waters of the north-eastern Gulf of Mexico (NEGOM) were examined using satellite and in situ datacollected between November 1997 and August 2000. The patterns of chlorophylldistribution derived from in situ data consistently matched the satellite obser-vations, even though the satellite-derived concentrations in coastal and offshorewaters influenced by rivers were overestimated by the standard satellite data pro-cessing algorithms. River discharge and wind-driven upwelling were the majorfactors influencing surface chlorophyll-a variability for inshore regions. High in situ

chlorophyll-a concentrations (≥1 mg m−3) occurred inshore and particularly nearmajor river mouths during the summer seasons of 1998, 1999 and 2000. Plumes ofMississippi River water extended offshore to the southeast of the delta over dis-tances >500 km from the river delta for maximum periods of 14 weeks betweenMay and September every year and could reach the Florida Keys in certain years.The offshore transport of the plume was initiated by eastward or southeastwardwinds and then by separate anticyclonic eddies located southeast of the Mississippidelta and nearby shelf every year. Chlorophyll concentrations during the win-ter to spring transition in 1998 off Escambia, Choctawhatchee, Apalachicola andSuwannee Rivers and off Tampa Bay were up to 4 times higher than during thesame periods in 1999 and 2000. This was related to higher freshwater discharge dur-ing the 1997–1998 winter–spring transition, coinciding with an El Niño–SouthernOscillation event, and to the unusually strong upwelling observed along the coastin spring 1998.

1. Introduction

The northeastern Gulf of Mexico (NEGOM) is an area that experiences significantbiogeochemical variability due to a variety of oceanographic processes acting on shelfand coastal waters. Upwelling, cold- and warm-core rings, river discharge and windsforce significant biogeochemical variability within this region. Several studies havefocused on the effect of river outflow on the chemistry and biology of estuarine andcontinental shelf zones within the NEGOM (Lohrenz et al. 1990, Hu et al. 2003,Morey et al. 2003, Qian et al. 2003), but synoptic patterns and factors that couple

*Corresponding author. Email: [email protected]

International Journal of Remote SensingISSN 0143-1161 print/ISSN 1366-5901 online © 2011 Taylor & Francis

http://www.tandf.co.uk/journalsDOI: 10.1080/01431161.2010.542192

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shelf-slope differences in chlorophyll-a distribution and seasonal variability are onlynow becoming better understood.

The Gulf of Mexico had in the past been described as an oligotrophic system(Ortner et al. 1984). However, satellite data and in situ observations (Muller-Kargeret al. 1991, Gilbert et al. 1996, Gilbes et al. 1996, 2002, Muller-Karger 2000, Biggsand Ressler 2001, Belabbassi et al. 2005, Biggs et al. 2008) helped demonstrate thatthe Gulf of Mexico experiences intermediate to high phytoplankton concentrationsboth over the shelf and in certain offshore areas, and that some of these patterns areseasonal. Some changes in inshore areas are related to wind-driven coastal upwelling(Chuang et al. 1982, Schroeder et al. 1987, Yang and Weisberg 1999, Muller-Karger2000, Weisberg et al. 2000) and river plumes (Gilbes et al. 1996, Walker 1996, DelCastillo et al. 2000, Hu et al. 2003). In deeper water, variation in chlorophyll-ais affected by seasonal convective mixing, divergence- and convergence-associatedcyclonic and anticyclonic eddies (Biggs and Muller-Karger 1994) and the off-marginentraintment dispersal of riverine outflows (Biggs et al. 2008).

Using Coastal Zone Color Scanner (CZCS) images, Muller-Karger et al. (1991)found that, to first order, variability in pigment concentration seaward of the shelf wassynchronous throughout the Gulf of Mexico, with the highest values from Decemberto February and the lowest from May to July. Also using CZCS images, Gilbeset al. (1996) observed an episodic plume with high pigment concentration developedeach spring extending along the West Florida Shelf from an origin region within theNEGOM. This plume persisted for 1–6 weeks in a pattern that extended >250 kmsouthward along the shelf. After the plume dissipates, low pigment concentrationsare generally observed during the summer along the outer West Florida Shelf, whilecoastal concentrations increase as a result of higher river discharge by Florida rivers(Gilbes et al. 1996). But in deep water off the edge of the West Florida Shelf, in somesummers, surface waters from the Mississippi River can be entrained into the easternedge of the Loop Current or by slope eddies and so extend for hundreds of kilometresto the southeast of the bird’s foot delta (Hu et al. 2003, 2005).

The NEGOM receives freshwater inputs from the Mississippi, Mobile, Escambia,Choctawhatchee, Apalachicola and Suwannee Rivers as well as from many othersmaller rivers. River discharge varies seasonally, with a maximum typically duringspring and a minimum during summer (Gilbes et al. 1996). The seasonal dischargeand insolation changes lead to strong seasonal buoyancy variation on the shelf andvariation in nutrient and chlorophyll concentrations (Gilbes et al. 1996, Del Castilloet al. 2000). The Mississippi River provides over half of the freshwater input intothe Gulf of Mexico (Deegan et al. 1986) and is the dominant source of terrestrialnutrient input to the basin (Twilley et al. 1999). Walsh et al. (1989) suggested that theMississippi plume area is the most productive within the Gulf of Mexico. Walsh (1983)also mentioned that secondary production due to sinking herbivores and ungrazedprey, specifically from menhaden fishery activities, was an important factor in the car-bon cycle in the Gulf of Mexico. Chlorophyll-a concentrations in the Mississippi Riverplume are highly variable in space and time, ranging between 1.1 and 14.4 mg m−3 inthe outer plume during four cruises in 1990–1992 (Redalje et al. 1994), but as high as40–80 mg m−3 in the near-coastal waters during the late spring in 1993 (Nelson andDortch 1996).

While buoyancy and wind forcing largely control the circulation of shelf waters(Weisberg et al. 1996, 2000, Yang and Weisberg 1999, He and Weisberg 2002, 2003,Weisberg and He 2003), it is also affected by northward intrusions of the Loop Current

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Chlorophyll variability in NEGOM 3

and the associated eddies (Huh et al. 1981, Vukovich 1988, Muller-Karger 2000, Heand Weisberg 2002, 2003, Weisberg and He 2003). The Loop Current rarely penetratesnorth of 28◦N in the eastern Gulf of Mexico (Vukovich et al. 1979); therefore, its directinfluence in the NEGOM region is considered occasional. However, impingement ofthe Loop Current on the shelf break in the southeastern Gulf of Mexico can lead tostrong shelf circulation effects in the NEGOM (Hetland et al. 1999, He and Weisberg2003, Weisberg and He 2003).

Although satellite data were used to study spatial distributions of chlorophyll andtheir temporal variations (Muller-Karger et al. 1991), due to sensor and algorithmartefacts, most previous studies could only focus on general seasonal patterns andtreated large areas (e.g. 200 km × 200 km) as a whole. Here, using field observationsof hydrographic and biological data from nine cruises conducted between 1997 and2000 and satellite data from the Sea-viewing Wide-Field-of-View Sensor (SeaWiFS)and Ocean Topography Experiment (TOPEX/Poseidon), we examined the spatial andtemporal variability of chlorophyll-a in the NEGOM region. We also sought to deter-mine the extent, duration and forcing factors of offshore river plumes in the NEGOMduring the three consecutive years of 1998, 1999 and 2000.

2. Methodology

2.1 Study area

The study was conducted in the region extending from the Mississippi River delta tothe West Florida Shelf off Tampa Bay (figure 1). Nine oceanographic cruises focusedon the region bounded inshore by the 10 m isobath and offshore by the 1000 m isobath.Six major river-impacted regions, namely, the Mississippi (R1), Mobile (R2), Escambia(R3), Choctawhatchee (R4), Apalachicola (R5) and Suwannee (R6), plus one coastal

Figure 1. The northeastern Gulf of Mexico (NEGOM) region, encompassing the area between27.3–30.7◦N and 82.6–89.6◦W. The map shows the 10, 20, 100, 200, 500 and 1000 m bathymet-ric contours. The line connecting closed triangles shows a typical cross-shelf cruise tracks andstations. Numbered open squares show regions at which time series of chlorophyll-a concentra-tion were analysed. Closed triangles are conductivity, temperature, depth (CTD) stations wherewater samples were taken for chlorophyll-a concentration analyses. Inset: the Gulf of Mexico –the area under study.

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region near Tampa Bay (R7) and three offshore regions (R8a, R8b and R8c) were chosento characterize the NEGOM.

2.2 In situ data collection, processing and analysis

Nine 2-week cruises were conducted in three different seasons between 1997 and 2000onboard the Texas Agricultural and Mechanical University (TAMU) Research Vessel(R/V) Gyre (table 1). Each cruise surveyed 11 cross-margin transects from the 10 tothe 1000 m isobath (figure 1).

A flow-through system was used to measure the fluorescence of surfacechlorophyll-a and coloured dissolved organic matter (CDOM). The procedure to col-lect and calibrate the fluorescence data into chlorophyll-a concentration and CDOMabsorption coefficient (ag443 per metre) using discrete water samples was detailed inHu et al. (2003).

Continuous subsurface (10–250 m deep) currents were measured using a ship-board narrowband and broadband 150 kHz acoustic doppler current profiler (ADCP;Teledyne RDI, Poway, CA, USA). To produce circulation maps, the ADCP data weregridded by TAMU graduate student Ou Wang, employing 40 km as the smoothingradius (Daley 1991). Features with scales less than 40 km are therefore likely smoothedout, but this allowed filtering out noise features and keeping the basic large-scale pat-terns. Details of these measurements and data processing can be found in Wang et al.

(2002). To allow readers to visualize how strongly subsurface currents in a 10–14 mdepth bin are in geostrophic balance with sea surface height (SSH) field, in this articlethe ADCP-gridded currents have been overlaid with the SSH fields rendered for themid-date of each of the nine cruises (see §2.4).

2.3 Wind field and river discharge

Gridded, average surface wind field data for the NEGOM region for the period ofeach of the nine cruises were obtained from the National Centers for EnvironmentalPrediction (NCEP). We also obtained the time series of surface wind field data fromthe National Oceanic and Atmospheric Administration’s (NOAA) National DataBuoy Center (NDBC) meteorological buoys in the region.

The river discharge data for the period of October 1997 to December 2000were compiled from six major rivers entering the NEGOM region, namely, the

Table 1. Cruise identifiers, dates and seasons.

Cruise no. Start date End date Cruise ID Cruise season

N1 16 November 1997 26 November 1997 Au-97 Autumn 1997N2 4 May 1998 15 May 1998 Sp-98 Spring 1998N3 25 July 1998 6 August 1998 Su-98 Summer 1998N4 13 November 1998 24 November 1998 Au-98 Autumn 1998N5 15 May 1999 28 May 1999 Sp-99 Spring 1999N6 15 August 1999 28 August 1999 Su-99 Summer 1999N7 13 November 1999 23 November 1999 Au-99 Autumn 1999N8 15 April 2000 26 April 2000 Sp-00 Spring 2000N9 28 July 2000 8 August 2000 Su-00 Summer 2000

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Mississippi, Mobile, Escambia, Choctawhatchee, Apalachicola and Suwannee Rivers.The data were obtained from the US Geological Survey Water Data Report. TheMississippi, Escambia, Choctawhatchee, Apalachicola and Suwannee Rivers’ dis-charge was recorded at Tarbert Landing (30.96◦N, 91.66◦W), near Century (30.96◦N,87.23◦W), near Bruce (30.45◦N, 85.89◦W), Sumatra (29.95◦N, 85.02◦W) and Branford(29.96◦N, 82.93◦W), respectively. The Alabama River discharge recorded at Claiborne(31.54◦N, 87.51◦W) and the Tombigbee River discharge at Coffeeville (31.75◦N,88.12◦W) were used to estimate the Mobile River discharge. Both Alabama andTombigbee Rivers join to form the Mobile River. Weekly mean time series were builtfrom these rivers’ discharge data.

2.4 Satellite data collection, processing and analysis

The satellite data used in this study are 1 km resolution chlorophyll-a concentrationestimates derived from SeaWiFS ocean colour measurements and SSH estimates fromTOPEX/Poseidon and the Second European Remote Sensing Satellite (ERS-2) obser-vations. The SeaWiFS data were collected using a high-resolution picture transmitter(HRPT) antenna located at University of South Florida (USF), St Petersburg, FL,USA.

SeaWiFS data (October 1997 to December 2000) were processed using the soft-ware package of the SeaWiFS Data Analysis System (SeaDAS) version 4 developedby the US National Aeronautics and Space Administration (NASA). For time seriesand statistical analyses, chlorophyll-a concentration was estimated using Carder semi-analytical (Moderate Resolution Imaging Spectroradiometer (MODIS)) algorithm(Carder et al. 1999), which yielded better estimates than the band-ratio Ocean Color4 version 4 (OC4v4) algorithm (O’Reilly et al. 2000) for most of the waters, espe-cially within regions of high concentration of CDOM (Hu et al. 2003, Nababan 2005,2008). However, the Carder algorithm switches to a band-ratio algorithm for somecoastal waters, resulting in discontinuity in spatial patterns. Therefore, for spatialdistributions, chlorophyll-a concentration was estimated with the band-ratio OC4v4algorithm and served as a colour index for pattern analysis.

SSH fields were obtained from the University of Colorado (courtesy of Dr.Robert Leben). This was a blended product of TOPEX/Poseidon and ERS-2 satel-lite altimeter data. SSH fields were produced by temporal and spatial smoothingusing decorrelation scales of 12 days and 100 km. Therefore, features may appearweaker than they actually were, and smaller-scale features may not be represented.To estimate the total dynamic topography, the residual mean in the SSH was removedbefore adding a model mean to produce the synthetic height estimate. The resultingtime series of SSH fields were interpolated to obtain one SSH field per day. Satellitealtimeter data are usually considered to be of limited value over continental shelvesbecause of reflectance of radar diffractive side-lobes from adjacent land areas, a poorknowledge of the geoid and short time and space scales that may not be resolvedby the sampling frequency of the present-day altimeters. More details about dataprocessing and analysis can be found in Lillibridge et al. (1997) and Leben et al.

(2002). For comparison with ADCP-gridded currents, the interpolated SSH field atthe midpoint in time of each cruise period was selected. For weekly time series, SSHfields from the middle date of the week were overlaid with the weekly composite ofSeaWiFS.

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2.5 Statistical analyses

Programs based on Interactive Data Language (IDL) software by Research Systems,Inc. (Boulder, CO, USA) were developed to process and analyse data as well asfor descriptive statistical analysis. A one-way analysis of variance (ANOVA) usingStatistix8 Software by Analytical Software (Tallahassee, FL, USA) was used to testfor the significance of differences in chlorophyll-a concentration among regions andseasons.

3. Results and discussions

3.1 Spatial and temporal distribution of in situ chlorophyll-a concentrations

The spatial and temporal near-surface distribution of in situ chlorophyll-a concentra-tion for the different sampling periods is shown in figure 2. The results clearly showan influence of freshwater discharge on the distribution of chlorophyll-a concentrationand the importance of the Mississippi plume as a source of nutrients to offshore watersof the Gulf of Mexico. Relatively high chlorophyll-a concentrations (≥1 mg m−3) andag443 (≥0.1 m−1) were generally found only inshore, particularly near the major rivermouths, or offshore in the Mississippi River plume during summer sampling periods(Su-98, Su-99 and Su-00).

To better understand the transport of the high chlorophyll-a concentration of theMississippi River water to the southeast along the West Florida Shelf during summers,we examined the ship-mounted ADCP and the satellite-derived SSH observations.Figure 3 shows that to the west of 85◦W (Cape San Blas) there were marked dif-ferences in SSH fields from season to season. To the east of 85◦W, a region of low(negative) SSH was frequently observed near the coast. Based on drifter path data,Golubev and Hsueh (2002) found that currents on the NEGOM shelf east of 85◦W

Figure 2. Spatial and temporal distributions of near-surface chlorophyll-a concentration in thenortheastern Gulf of Mexico (NEGOM) region based on in situ data. Thin white lines show thecruise track where continuous chlorophyll fluorescence observations were collected. See table 1for cruise identification.

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Figure 3. Ocean circulation patterns in the northeastern Gulf of Mexico (NEGOM). Sea sur-face height (SSH) patterns are shown in colours with ship-board Acoustic Doppler CurrentProfiler (ADCP)-derived currents at 10–14 m depth (arrows).Note: There were no ADCP data from the fall of 1998 east of 87◦W because of mechanicaldamage to the ADCP equipment. SSH data were obtained from the University of Colorado(courtesy of Robert Leben) and ADCP data were obtained from Texas Agricultural andMechanical University (TAMU) (courtesy of Ou Wang).

were well correlated with SSH anomalies in the Florida Straits. This implied thatan offshore pressure forcing drives a barotropic flow on the West Florida Shelf ona nearly annual basis. Other field observations (e.g. Cragg et al. 1983, Weisberg et al.1996) demonstrated that the West Florida Shelf circulation and sea level variationsalong the coast are highly correlated with wind stress. Numerical models also showthat the West Florida Shelf circulation is driven by local winds and surface heat flux,as well as by remote effects of the Loop Current (Yang and Weisberg 1999, He andWeisberg 2002, 2003, Weisberg and He 2003). West of 85◦W, alongshore winds drivethe circulation on the shelf and lead to up- or downwelling (see Muller-Karger 2000).Schroeder et al. (1987) and Chuang et al. (1982) also reported that the inshore circu-lation off eastern Mississippi and off Mobile was strongly wind dependent, in spite ofthe potential for westward geostrophic flow generated by coastal freshwater input.

The SSH field maps constructed for each summer cruise indicated that either smallor large anticyclonic (warm) eddies were present near the slope off the Mississippidelta. SSH ranged from +11 cm for a mesoscale (∼100 km diameter) eddy in thesummer of 2000 (Su-00) to +35 cm for a large offshore eddy (centred around 88.5◦Wand 26.5◦N) in the summer of 1999 (Su-99; eddy largely outside the area shown infigure 3). These anticyclonic eddies entrained and transported low salinity with high-nutrient surface water from the delta area offshore (see also Qian et al. 2003). TheADCP currents at 10–14 m depth documented northeastward and eastward (Su-00),or eastward and southeastward (Su-98, Su-99) flows along the northern edge of theanticyclonic eddies, with speeds of up to 0.6 m s−1. These vectors help explain thechlorophyll-a concentration patterns observed.

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Based on a numerical circulation model and a long time series of in situ observationsat several locations, He and Weisberg (2002) and Weisberg and He (2003) also founda strong southeastward current at mid-shelf of NEGOM by the baroclinic response tocombined wind and buoyancy forcing that leads to annually occurring cold and low-salinity tongues. Eastward dispersal of the Mississippi plume was also recorded duringthe summer of 1993 (Walker et al. 1994), summer of 1997 (Biggs et al. 2000) and inmany other instances (Muller-Karger et al. 1991, Gilbes et al. 1996, Del Castillo et al.2000, Muller-Karger 2000, 2002, Hu et al. 2003, 2005).

Some cyclonic features appeared along the coast west of 85◦W during summer(figure 3). It is unclear, though, whether these features are cyclonic shelf eddies orwhether they are simply local areas of lower than average SSH.

In fall 1997 and 1998 (Fa-97, Fa-98), SSHs of +8 and +13 cm were seen off theMississippi delta (figure 3). Although subsurface currents indicated eastward flow,there was no indication of eastward dispersal of the Mississippi River water. This evi-dence can be seen from the relatively low chlorophyll-a concentration near the slopeoff the Mississippi delta (see figure 2). Relatively high westward (easterly) wind speedcomponents during fall season (at speeds up to ∼10.5 m s−1) may have helped keep theMississippi water inshore and to the west of the delta (figure 4; Jochens et al. 2002).

Based on weekly river discharge and SeaWiFS chlorophyll-a concentration, pos-itive correlation between freshwater discharge and chlorophyll-a concentration wasfound in the above regions as further discussed in §3.3, indicating that the variability ofchlorophyll-a concentration seemed to relate to the variability of freshwater discharge

Figure 4. Average surface wind field in the northeastern Gulf of Mexico (NEGOM) during theperiod of each of the nine cruises. Data were obtained from Texas Agricultural and MechanicalUniversity (TAMU) (courtesy of Joe Yip). Diamonds and their identification indicate stationswhere wind speed and direction were collected. Several offshore National Data Buoy Center(NDBC) stations used to generate the gridded averages are not shown because they were locatedsouth of 27◦N.

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of each major rivers. While the freshwater discharge of Mobile and Escambia Riverswas lower during the summer of 1999 than during the spring of 1999 (figure 5),the higher chlorophyll-a concentration observed during the summer of 1999 alongthe Mobile and Escambia regions may be attributed to the eastward dispersal ofMississippi River water that contains high nutrients (see figure 2) and due to eastward(westerly) upwelling-favourable winds.

For the Apalachicola region, the chlorophyll-a concentrations during fall sea-sons were higher than during spring and summer. However, the discharge of theApalachicola River during fall was less than in spring (figure 5). If the river dischargewas considered as the only nutrient source for this region, opposite chlorophyll-a

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Figure 5. Freshwater discharge of six major rivers in the northeastern Gulf of Mexico(NEGOM) region during the period of the nine cruises. y-Axis is the daily average riverdischarge × 108 (m3 day−1). Notice the y-axis is shown in different scales.

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patterns would be observed. Therefore, there must be other nutrient sources. Strongupwelling-favourable wind observed several weeks prior to fall cruises provided nutri-ents from deeper water (Weisberg and He 2003). He and Weisberg (2003) found thatLoop Current in fall 1998 reinforced the mid-shelf currents and increased the across-shelf transports in the bottom Ekman layers, therefore accentuating the shorewardtransport of cold, nutrient-rich water of deep-ocean origin. Cold frontal systems andstronger winds during fall and winter may also increase the vertical mixing of thephotic layer, bringing nutrients from the deeper water.

For the central offshore NEGOM region (R8a, R8b and R8c), a seasonal pat-tern of chlorophyll-a concentration was observed with maxima occurring duringsummer and minima during spring. Using salinity as an independent variable, vari-ability in chlorophyll-a concentration can be explained up to 93% (R8c), indicatingfreshwater with high nutrients from the Mississippi River plays a major role forthe phytoplankton bloom in these regions during the three consecutive summerseasons.

If all locations are considered, an ANOVA showed that there are significant differ-ences in the mean of chlorophyll-a concentrations among seasons for each location(p < 0.01). The mean chlorophyll-a concentrations are also different (p < 0.01) amonglocations within each season (cruise).

3.2 Spatial and temporal distribution of satellite-retrieved chlorophyll-a

The spatial distributions of chlorophyll-a concentrations from the satellite data(figure 6) matched that inferred from in situ measurements (see figure 2), but theirabsolute values may differ significantly due to algorithm artefacts. SeaWiFS satel-lite data revealed relatively high chlorophyll-a concentration (≥1 mg m−3) inshore,particularly near the major river mouths, and offshore in the Mississippi Riverplume during summer cruises (Su-98, Su-99, Su-00; figure 6). A plume of elevatedchlorophyll-a concentration extended east-southeastward (ESE) from the MississippiRiver delta in each summer of 1998, 1999 and 2000. The plume was more appar-ent in SeaWiFS imagery than in the in situ data maps because the former had moresynoptic coverage in shorter time frame. The plume usually formed towards the endof May and lasted until about the first week of September. In June 1998, the plumeextended up to 330 km to the ESE, with highest concentrations near the NEGOMand lowest apparent concentrations (about 1 mg m−3) down the plume. The plumereached a maximum extent of over 550 km in late July (figure 7) and became smallerafterwards. The plume was effectively absent from the eastern Gulf of Mexico bymid-August.

In 1999, a spatially coherent plume of ∼280 km was observed in the offshoreNEGOM region extending ESE from the Mississippi delta in the last week of May.The plume reached its maximum extent in late July (∼550 km; figure 7). This plumelasted about 14 weeks, through the first week of September.

In 2000, the Mississippi plume was first observed in early July, extending some 220km ESE off the Mississippi delta. The plume reached its maximum extent in lateJuly and early August (about 530 km; figure 7). The plume lasted about 9 weeks anddisappeared during the first week of September.

SeaWiFS imagery covering the entire eastern Gulf of Mexico (not shown here) sug-gests that the summer Mississippi plumes in the NEGOM between 1998 and 2000reached the Florida Straits, consistent with the earlier findings (e.g. Hu et al. 2005).

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Figure 6. Spatial and temporal distribution of near-surface chlorophyll-a concentration in thenortheastern Gulf of Mexico (NEGOM) region based on SeaWiFS data during the nine sepa-rate NEGOM cruise periods. White and purple contours are positive and negative sea surfaceheight, respectively.

The offshore advection and dispersal of the coastal NEGOM plume observed insummers of these three consecutive years (1998, 1999, 2000) appeared to be related tothe wind direction and speed, the surface heat flux, the Mississippi River discharge,the sea surface dynamic height off the Mississippi delta and the impact of the LoopCurrent on the shelf-slope circulation. Weekly average wind data measured south andeast of the Mississippi delta (Buoys BURL1, 28.9◦N, 89.4◦W; and NDBC 42040,29.2◦N, 88.2◦W) recorded eastward (westerly) wind from the end of April to the endof September, with average speeds of 0.5–6.10 m s−1. The satellite data suggest thata plume was advected south or southeastward of the Mississippi delta with the east-ward (westerly) winds and then around an anticyclonic, warm-core ring (high SSH)that drifted into the vicinity of the Mississippi delta (figure 7). SSH ranged from +7cm for a mesoscale (∼100 km diameter) eddy in the summer of 2000 (7–13 July 2000)to +20 cm for a large offshore eddy in the summer of 1998 (22–28 July 1998) when theplume reached maximum extent offshore (figure 7).

Indeed, the dispersal patterns of the Mississippi River plume to the southeastnearly repeated every year since SeaWiFS measurements became available in 1997.Depending on the driving forcings, the plume’s extent, intensity and duration mayvary from year to year. For example, between July and October 2004, about 23% of thetotal Mississippi discharge was estimated to enter the Florida Straits (Hu et al. 2005).Clearly, the Mississippi discharge plays a major role in influencing the bio-opticalproperties in the eastern Gulf of Mexico.

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Figure 7. Initial and maximum position of the Mississippi River plume during the summerof 1998, 1999 and 2000. White and purple contour lines show positive and negative sea surfaceheight. White arrow indicates the general direction of dispersal and extent of the plume.

3.3 Factors affecting chlorophyll-a concentration variability

Figure 8 shows the monthly mean chlorophyll-a concentrations derived from SeaWiFSdata employing the Carder semi-analytical algorithm (Carder et al. 1999) for eachregion defined in figure 1 and for each season (cruise). Mean chlorophyll-a concentra-tion ranged from ∼0.1 mg m−3 in the central NEGOM (R8c) in June 2000 to 23±11mg m−3 off the Mississippi (R1) in April 1999. While specific regions showed season-ality in chlorophyll-a concentrations, there was no consistent or synchronous seasonalpattern for the entire NEGOM, consistent with the findings using CZCS data byMuller-Karger et al. (1991).

Figure 8 also shows the interannual variability of chlorophyll-a concentration inmost regions, particularly during winter and spring. In 1998, chlorophyll-a concen-trations were higher than in 1999 or 2000 by a factor of up to 4 off the Escambia(R3), Choctawhatchee (R4), Apalachicola (R5), Suwannee (R6) and Tampa Bay

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Figure 8. Time series of near-surface monthly mean chlorophyll-a concentration based onSeaWiFS satellite data employing the Carder semi-analytical algorithm (Carder et al. 1999) for10 selected regions. Notice the y-axis is shown in different scales.

(R7) regions, particularly between January and May. The discharges of the Mobile,Escambia, Choctawhatchee, Apalachicola and Suwannee Rivers during the spring of1998 were up to 5 times the discharge during the spring of 1999 or 2000 (see alsofigure 5). The anomalous chlorophyll-a concentration during winter and spring of1998 appeared to be related to the 1997–1998 El Niño–Southern Oscillation event,when there was also anomalously high river discharge (Schmidt et al. 2000). In latespring and summer of 1998, upwelling was also observed in these regions (Muller-Karger 2000, He and Weisberg 2002, Weisberg and He 2003, Jolliff 2004), which alsostimulated phytoplankton production.

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For non-El Niño years (1999 and 2000), what appears to be a seasonal pattern inchlorophyll-a concentration was found off the Mississippi (R1), central NEGOM (R8),Apalachicola (R5), Suwannee (R6) and Tampa Bay (R7) regions. Off the Mississippiand central NEGOM regions, the maxima occurred in summer and the minimain fall. Off Apalachicola, the maxima occurred during winter (December, January)and the minima in spring (March, May). Off the Suwannee (R6) region, the max-ima occurred during summer months (August, September) and the minima in wintermonths (December, February). Off Tampa Bay (R7) region, the maxima occurred infall (October) and the minima in spring (March, May) (figure 8). These patterns werealso consistent with the in situ observations of chlorophyll-a concentration.

To study the effect of river discharge on chlorophyll-a concentration in the NEGOMregion, cross-correlation analyses of weekly satellite chlorophyll-a concentration ver-sus river discharge were conducted for the period of October 1997 to December2000. Similar analysis was used to study the influence of offshore plume of theAmazon River on the bio-optical properties in the tropic Atlantic Ocean (Hu et al.2004) and to study riverine and wind influences on suspended sediments in thenorthern Gulf of Mexico (Salisbury et al. 2004). River discharge has a strong influ-ence on chlorophyll-a concentration variability in the Mississippi, Mobile, Escambia,Choctawhatchee, Apalachicola and Suwannee regions. The cross-correlation coeffi-cients for the six regions were as follows: Mississippi (r = 0.50, lag = 5 weeks); Mobile(r = 0.70, lag = 1 week); Escambia (r = 0.50, lag = 1 week); Choctawhatchee(r = 0.55, lag = 9 week); Apalachicola (r = 0.55, lag = 0 week); and Suwannee (r =

0.65, lag = 1 week) (figure 9). Changes in the river discharge preceded changes inthe chlorophyll-a concentration by a few days to several weeks for the above regions.The relatively low cross-correlation coefficient (r) values and sometimes multiple cor-relation peaks suggest that unmeasured sources of freshwater from local runoff andinterference from other river sources may have played a role in nutrient and CDOMdelivery as well.

Off the Mobile, Escambia and Choctawhatchee regions, upwelling-favourablewinds were observed intermittently between early spring and late summer. Muller-Karger (2000) reported strong intermittent upwelling in May–June 1998 in theinshore areas, generally within 15 km of the coast along the Florida Panhandle.Upwelling-favourable winds also occur occasionally during winter. SeaWiFS-derivedchlorophyll-a concentrations appeared to be elevated during these periods ofupwelling-favourable winds. However, overall, variability in the freshwater dischargeappears to have a stronger influence on the variability of chlorophyll-a concentrationthan the upwelling process. The strength of the upwelling signal was also probablymasked partly by the large size of the sampling boxes along the coast (figure 1) becausethat to the west of Cape San Blas upwelling was largely confined to a narrow stripalong the coast.

Off the Apalachicola, in 1999 and 2000, chlorophyll-a concentrations rose duringfall seasons (see figure 8, R5). The variability in freshwater discharge had an influenceon the variability of chlorophyll-a concentration (r = 0.55, lag = 0 week) (figure 9). Buthigher chlorophyll-a concentration during fall, when the Apalachicola River dischargeis lower than in spring, suggests that upwelling and strong wind-induced vertical mix-ing also play an important role. He and Weisberg (2003) show that the Loop Currentcan also help reinforce mid-shelf currents and a shoreward cross-shelf transport in thebottom Ekman layer that brings cold, nutrient-rich water of deep-ocean origin to theApalachicola region.

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Figure 9. Lagged cross-correlation analyses for weekly mean river discharge versuschlorophyll-a concentration within specific northeastern Gulf of Mexico (NEGOM) regions(figure 1), with lags measured in weeks. Satellite chlorophyll-a concentration was derived fromSeaWiFS data employing the Carder semi-analytical algorithm (Carder et al. 1999). Notice they-axis is shown in different scales.

A chlorophyll plume that extends from Cape San Blas to the south parallel tothe shelf break and as far south as the Florida Keys has been observed in thisregion for many years, each lasting 1–6 weeks during spring (Gilbes et al. 1996).Our observations confirmed the regular occurrence of this plume. Gilbes et al. (1996)speculated that the plume might be associated with one or a combination of the fol-lowing processes: (1) discharge from small, local rivers along the northwest Floridacoast; (2) seasonal changes in steric height differences between the shelf and deepGulf of Mexico waters; (3) circulation of water associated with the Loop Current andupwelling in the DeSoto Canyon; and (4) discharge from the Mississippi and MobileRivers. Gilbes et al. (2002) confirmed that the Apalachicola River plays a role in theformation of offshore blooms. A cold tongue of water has been detected in AdvancedVery High Resolution Radiometer (AVHRR) images in the same area as this chloro-phyll plume. This is independent of the Loop Current position and is a result ofcirculation driven by buoyancy and heat-flux forcing, plus the remote influence of theLoop Current (He and Weisberg 2002, Weisberg and He 2003). Off the Suwannee

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River, variability in chlorophyll concentration was more strongly related to variabilityin the Suwannee discharge (r = 0.65, lag = 1 week). Presumably, wind-driven upwellingprocesses play a smaller role in this coastal area.

In the central NEGOM (R8a, R8b, R8c) region off the West Florida Shelf, the higherapparent chlorophyll-a concentrations seen in summers of 1998, 1999 and 2000 in fig-ure 8 were due to the southeastward transport of Mississippi River water, as explainedabove.

4. Conclusions

In situ and satellite-derived surface chlorophyll-a concentrations collected during nine2-week cruises spanning fall 1997 to summer 2000 showed relatively high chlorophyll-aconcentrations (≥1 mg m−3) inshore, particularly near the rivers of the region. TheMississippi plume extended well off the shelf to the south and southeast during thesummers of 1998, 1999 and 2000. It extended offshore as early as late May and usu-ally until the first week of September, reaching lengths >500 km southeast of theMississippi delta. In August 1998, the plume clearly reached the Florida Keys. Thetransport of the plume was initiated by eastward or southeastward wind and trans-ported further offshore by separate anticyclonic eddies that were located southeastof the Mississippi delta each year. Ship-board observations confirmed the offshoretransport of the Mississippi River, which led to higher chlorophyll-a concentrations,lower salinities and higher CDOM absorption than in purely oceanic Gulf of Mexicowaters.

Variability in river discharge appeared to be the dominant factor causing variabil-ity in the chlorophyll-a concentration in the immediate vicinity off the Mississippi,Mobile, Escambia, Choctawhatchee, Apalachicola and Suwannee Rivers. Strong inter-annual variability of chlorophyll-a concentration was observed, with fourfold higherconcentrations of chlorophyll-a seen off the Escambia (R3), Choctawhatchee (R4),Apalachicola (R5), Suwannee (R6) and Tampa Bay (R7) in the winter and springof 1998 compared to the winter and spring of 1999 and 2000. These higher concen-trations in early 1998 were related to anomalously high fall/winter river dischargescoincident with the 1997–1998 El Niño–Southern Oscillation event and strong inter-mittent coastal upwelling observed due to the combined action of a large anticyclonebringing isotherms close to the surface east of the Mississippi delta and intermittent,upwelling-favourable winds.

Acknowledgements

This research was supported by the US Department of Interior, MineralsManagement Service grants 1335-01-97-CA-30857 (Dr F.E. Muller-Karger, PI) and1435-01-97-CT30851 (Dr W.D. Nowlin, PI) for the NEGOM Chemical Oceanographyand Hydrography (NEGOM-COH) Study. Dr N.L. Guinasso was the Co-ChiefScientist for the NEGOM-COH field work on the R/V Gyre. Special thanks go tohim and to the NEGOM Science team and the ship crew for providing facilities andhelp in collecting in situ bio-optical data.

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