Biggs, Hu, and Müller-Karger, revised for Deep-Sea Res II topical issue on DGoMB program
Remotely sensed sea surface chlorophyll and POC flux at Deep Gulf of Mexico Benthos
sampling stations
DOUGLAS C. BIGGS,* CHUANMIN HU† and FRANK E. MÜLLER-KARGER†
(contributed 15 September 2005; revised 9 Nov 2006)
Table 1: longitude should be negative
Fig 4 caption: should be “January 1998 to December 2000” instead of “December 1998 to
January 2000”
Fig 4 caption typo: “East of 91” instead of “ast of 91
Abstract--SeaWiFS ocean color data were used to make biweekly composite averages of
the standing stock of sea surface chlorophyll (SSC) at 44 stations along the continental
slope and rise that were the focus of May-June 2000 benthic sampling by the Deep Gulf
of Mexico Benthos (DGoMB) program. At the 22 DGoMB sites north of 26oN and west
of 91oW in the NW Gulf, the annual average remotely sensed SSC was 0.19 mg m-3,
ranging at most locations from annual highs of about 0.3 mg m-3 in November-February
to annual lows of about 0.1 mg m-3 in May-August. Comparison of three years of
SeaWiFS data between Jan 1998 and Dec 2000 showed little inter-annual variation in this
pattern at these NW Gulf stations. In contrast, at the 22 NE Gulf sites north of 26oN and
east of 91oW, SSC was more inter-annually variable and averaged 2.8 times higher than
in the NW Gulf. Maxima in the eastern region occurred in November-February and also
during summers, when eddies in the NE Gulf entrained and transported Mississippi River
water east- and southward off the margin. This summertime cross-margin transport led to
apparent increases in SSC in June-August at 9 of the 22 NE Gulf stations, reaching
average monthly concentrations >50% greater than in November-February. Based on a
primary productivity model and a vertical flux model, the calculated export of particulate
organic carbon (POC flux reaching the seafloor) averaged 18 mg C m-2 day-1 at the 22 NE
Gulf stations, and 9 mg C m-2 day-1 at the 22 NW Gulf stations. These estimates are
comparable to those measured by benthic lander.
*Department of Oceanography, Texas A&M University, College Station, TX 77843, U.S.A.
†College of Marine Science, University of South Florida, St Petersburg, FL 33701, U.S.A.
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1
INTRODUCTION
Phytoplankton pigment concentration in surface waters of the Gulf of Mexico (GOM) undergoes
a well-defined seasonal cycle which is generally synchronous off the shelf, where the bottom is
deeper than 200 m. Müller-Karger et al. (1991) reviewed monthly climatologies of remotelysensed near-surface phytoplankton pigment concentration (chlorophyll + phaeopigment) from
multiyear series of Coastal Zone Color Scanner (CZCS) satellite images for the period 19781985. Using two representative 200x200 km2 sampling areas located in the southern GOM (one
centered at 24°N, 86°W; the other at 25°N, 93°W), they reported that highest near-surface
pigment concentration in deep waters occurs between December and February and lowest values
occur between May and July. In these deep GOM waters there was only about a 3-fold variation
between lowest (~0.06 mg m-3) and highest (0.2 mg m-3) near-surface pigment concentration.
Modern ocean color sensors such as the Sea-viewing Wide-Field-of-View Sensor (SeaWiFS)
provide more accurate and frequent observations of the biological state of the Gulf of Mexico
waters (McClain et al., 2004), thanks to improvements in technology (sensors with more spectral
bands (wavelengths) and higher sensitivity) and in algorithms (atmospheric correction and biooptical inversion). Our objective for the Deep Gulf of Mexico Benthos (DGoMB) program was
to provide a remote-sensing perspective of the potential food for the benthos, so we used
SeaWiFS data to revisit the seasonal variations, and to look at detail in spatial variations in nearsurface phytoplankton pigment concentrations among 44 DGoMB stations located north of 26oN
and ranging in water depth from 213 m to 3146 m.
As Rowe et al (this volume) explain in their Introduction to this special issue, among the null
hypotheses that DGoMB fieldwork sought to test were a) there are west-to-east differences, and
b) there are nearshore-to-offshore differences, in benthic community standing stocks and species
diversity. So for this paper we have summarized the west-to-east, nearshore-to-offshore,
seasonal, and inter-annual variations in remotely-sensed phytoplankton pigment concentration at
the DGoMB stations. We have also calculated the remotely sensed primary production (mg C m-2
day-1) and particulate organic carbon export (POC flux) that reaches the seabed (mg C m-2 day-1)
using the methods of Behrenfeld and Falkowski (1977) and Pace et al (1987), respectively.
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2 METHODS
In early September 1997, SeaWiFS on the SeaStar satellite (Orbimage Corporation) began
collecting ocean color data, providing global coverage every one to two days. The Institute for
Marine Remote Sensing at the College of Marine Science at the University of South Florida
(USF) downlinked, processed, and archived the regional SeaWiFS data covering the Gulf of
Mexico. The SeaWiFS data/imagery provided an estimate of the surface (to one optical depth or
about 30-50 m in clear water and shallower in more turbid water) chlorophyll concentration
(SSC), and provided an effective means to trace the circulation, the dispersal of riverine waters,
and the location of oceanographic fronts. Based on the daily observations, we derived weekly,
biweekly, monthly, and annual composite averages.
In support of DGoMB oceanographic habitat characterization, we used biweekly composites for
the three year period between January 1998 and December 2000 to estimate the annual mean
SSC at each DGoMB station, and its inter-annual variability. Table 1 gives location and water
depth at each of the 44 DGoMB stations north of 26oN at which benthic sampling was carried out
in May-June 2000, and shows how we have divided these stations into 22 in the NW Gulf (north
of 26oN and west of 91oW) and 22 in the NE Gulf ((north of 26oN and east of 91oW).
Prior to beginning DGoMB fieldwork in May 2000, the mean Gulf of Mexico basin-wide
distribution of surface SSC was calculated using the available SeaWiFS ocean color data from
January 1998 through December 1999 (Biggs and Ressler, 2001). As shown in Figure 1 (color
plate in this article), we have extended the data through December 2000, and then summarized
the three-year mean SSC distribution in the northern GOM separately for winter (21 December –
20 March) and summer (21 June – 20 September) seasons.
For Figure 1 and for the additional characterization of DGoMB oceanographic habitat that we
present in this paper, the SeaWiFS data were processed at high resolution (~ 1 km pixel size)
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with the SeaWiFS Data Analysis System (SeaDAS version 2). More recent reprocessing of the
SeaWiFS data (versions 3 to 5) are now available using updated and improved calibration and
data processing algorithms (atmospheric correction and bio-optical). The reprocessed data show
improved SSC retrievals in coastal waters, but not in the deep waters such as those studied in the
DGoMB program. Comparisons of SeaWiFS version 2 data with field measurements in 1998
and 1999 show that over the continental slope, uncertainties were less than 35%, except in river
plume waters (Hu et al., 2003).
SSC around each DGoMB location was obtained by averaging over a box of variable size to help
remove residual digitization-noise errors (Hu et al., 2001). All invalid pixels (as defined by the
various SeaWiFS flags, such as cloud and stray light, large solar or viewing angle, etc.) were
discarded. The choice of the size of the averaging box, either 3 x 3, 5 x 5, or 9 x 9, was a
compromise between the number of valid pixels and the standard deviation found within the box.
Even over a two-week averaging period, a 3 x 3 pixel grid centered on the location of each
DGoMB station often contained too few valid pixels due to cloud cover and cloud adjacency
contamination effects. Therefore, a 5 x 5 pixel grid (n = 25) was used to derive the biweekly
mean SSC concentration time series. The coefficient of variation (CV = standard deviation to
mean ratio) was about 2-fold higher for NE Gulf stations than for NW Gulf stations. The
average CV was about 9% (0.087) for the group of 22 DGoMB stations west of 91oW (stations
along RW and W transects and AC, WC, B, and NB sites over the Louisiana continental margin),
and 18% (0.185) for the 22 stations east of 91oW (stations along C, S, and MT transects,
including Hi-Pro station).
Because the year-to-year variability in annual averages of SSC at DGoMB stations in the NE
Gulf was higher than at those in the NW Gulf, the 1998-2000 time series was extended by adding
data for the subsequent year 2001 to produce the summary of month-by-month variability for the
22 DGoMB stations in the NE Gulf that is reported in Table 2. Each of the monthly-composite
means for the NE Gulf was centered on the middle date for the month (Julian Days 15, 46, 74,
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105, 135, 166, 196, 227, 258, 288, 319, and 349). Four such monthly composite means, from
four different years, are shown as Figure 2 (color plate).
Weekly and biweekly composites of SeaWiFS data for the entire Gulf of Mexico are archived as
png-format files that cover the 7-year period October 1997 – September 2004, on a CD-ROM
appendix to a recent paper by Biggs et al (2005). Copies may be obtained from the American
Geophysical Union, Geophysical Monographs division.
We followed the methodology outlined by Müller-Karger et al (2005) to compute monthly mean
net primary production (NPP) using the Vertically Generalized Production Model (VGPM)
[Behrenfeld and Falkowski, 1997]. The model estimates depth-integrated NPP (mg C m-2 day-1)
based on remotely sensed SSC (mg m-3), surface PAR (obtained from SeaWiFS data from the
NASA Goddard Space Flight Center), and sea surface temperature (SST, obtained from the
advanced very high resolution radiometer or AVHRR). Sinking POC flux was in turn computed
from NPP using an exponential decay model (flux(Z) = 3.523*NPP*Z-0.734) [Pace et al, 1987].
3 RESULTS
Table 1 summarizes the annual averages of SSC at NW versus NE Gulf DGoMB stations, as
computed from the biweekly composite data for the 3-year period 1998-2000. The overall 3-year
average SSC for the 22 stations in the NE Gulf was 0.54 mg m-3, or nearly three times greater
than that for the 22 stations in the NW Gulf (0.19 mg m-3). The standard deviation about the
annual mean at stations in the NE Gulf is also higher than at those in the NW Gulf. In both
regions, however, the locally highest SSC concentrations were generally found at stations on the
upper slope (water depths < 500 m) and locally lowest concentrations were generally found in
deepest water. A negative exponential fit to the 3-year average SSC versus depth data removed
54% of the variance in SSC versus water depth at NW Gulf stations, and 37% of the variance at
NE Gulf stations.
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Figure 3 illustrates the seasonal cycle of SSC at the nine deepest stations in the NW Gulf (AC1,
RW6, W5, W6, NB4, B2, B3, B1, and NB5). Water depth at each of these nine stations was
greater than 2 km. Panel A in Figure 3 shows the three-year time series of biweekly SSC at each
of the nine stations for the three years 1998-2000. There was relatively little year-to-year
variability in the amplitude or phase of the annual maximum or minimum SSC. At most of the
locations, SSC ranged from about 0.3 mg m-3 in November-February to minima of about 0.1 mg
m-3 in May-August. Panel B of Figure 3 shows the climatological (three-year) SSC concentration
(26 biweekly periods) at each of the 9 stations. These results agree with the pattern previously
reported from CZCS data (Müller-Karger et al., 1991; Melo-Gonzalez et al., 2000): Deepwater
SSC is lowest in spring-summer (Julian Days 100-250) and highest Nov-Feb (Julian Days 330060).
Additional analyses not shown here confirmed that the annual cycle of SSC at most of the Far
Western stations (RW1-RW6), Western Stations (W1-W6), and Louisiana Slope Stations was
similar to the cycle at the nine NW Gulf deepwater stations shown above (see the figures in
DGoMB interim technical reports edited by Rowe and Kennicutt, 2001 and 2002). Only at the
shallowest stations (water depths < 1 km) did biweekly SSC exceed 0.4 mg m-3 in NovemberFebruary.
East of 91oW, the "typical" deepwater annual cycle in SSC not only averaged more than two-fold
higher but was frequently punctuated by times when SSC reached greater than average conditions
for a particular season, especially during summertime (Figure 4). Table 2 shows that 9 of the 22
DGoMB stations in the NE Gulf had SSC in June, July, or August that was 50% or more greater
than the average SSC for the period November-February. In particular, high summertime SSC
levels occurred at MT3, MT4, HiPro, S37, S36, S35, S41, S42, and S43.
Table 3 summarizes the calculated remotely-sensed primary production and POC flux to the
seabed. At NW GOM stations west of 91oW, calculated primary production averaged 0.4 g C m2
day-1 and calculated POC flux to the seabed averaged 9 mg C m-2 day-1. At NEGOM stations
east of 91oW, calculated primary production averaged 0.7 g C m-2 day-1 and calculated POC flux
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to the seabed averaged 18 mg C m-2 day-1. Because that the annual mean SST averaged 26oC in
both regions, the average two-fold differences between NW GOM and NE GOM stations are not
due to Q10 effects. Rather, the two-fold higher productivity and POC flux in the eastern region
appears to be driven by greater off-margin export of high SSC coastal water to the eastern region,
especially during summers when eddies in the NE Gulf entrained and transported Mississippi
River water east- and southward, off the margin. At five stations in very blue water where
DGoMB cruises did some limited box-coring and trawling in the Mexican EEZ south of 26oN,
the calculated primary production averaged only about 50% that in the NE Gulf, and POC flux to
the deep abyssal plain there averaged only 3 mg C m-2 day-1 (Table 3).
4 DISCUSSION
Temporal and spatial variations in SSC
The annual cycle of SSC at DGoMB stations in the NW Gulf and at the deepest stations (water
depth > 2 km) in the NE Gulf showed a maximum in winter, and a minimum in summer, as
previously reported (Müller-Karger et al., 1991; Melo-Gonzalez et al., 2000). At DGoMB
stations in water depths of 300-1800 m and east of 91oW, however, the typical “deepwater”
annual cycle in SSC was punctuated by high summertime SSC.
Remote-sensing altimetry data and at sea-truth physical oceanographic data collected in support
of the DGoMB program (Jochens and DiMarco, this volume) and in support a companion field
program (the Northeastern GOM Chemical Oceanography and Hydrography program, or
NEGOM) explain why there was high summertime SSC in the NE Gulf. The sea surface height
(SSH) altimetry data showed that in summers 1998, 1999, and 2000, warm slope eddies (WSEs)
were centered over the deepwater of the DeSoto Canyon (Muller-Karger, 2000; Hu et al., 2003;
Belabbassi et al., 2005; Jochens and DiMarco, this volume). A large WSE that was centered
south of 28oN in summer 1999, and similar but smaller WSEs located farther north on the slope
in summers 1998 and 2000, each acted to entrain low salinity, high SSC “green water” from the
Mississippi River, and to transport this turbid plume seaward in July and August of each of these
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years. A more recent study (Hu et al., 2005) showed that such entrained low-salinity water could
be transported to the Atlantic through the through the Florida Current and Gulf Stream.
High performance liquid chromatography (HPLC) analyses of the phytoplankton pigments
collected from ship in these low salinity plumes during the NEGOM cruises (Qian et al., 2003)
confirmed that the low salinity Mississippi plume water had locally high SSC standing stocks.
These were important ground-truth data needed to understand the optical signal detected by
satellite over the Mississippi River plume, since waters of various river plumes frequently absorb
strongly in the blue wavelengths due to high concentrations of colored dissolved organic matter
(CDOM).
CDOM fluorescence and light absorption coefficient were also measured along with surface
salinity and SSC from ship during the NEGOM cruises. Strong correlations between the CDOM
absorption coefficient at 443 nm (ag443, m-1) and CDOM fluorescence (330 nm excitation and 450
nm emission) were consistently observed (Hu et al., 2003; Nababan, 2005). Relatively high ag443
(>0.1 m-1) was generally observed over the inner shelf, near the major river mouths. The ag443
signal decreased rapidly with distance from shore, except when riverine waters were entrained
and transported offshore by WSEs. Because CDOM as well as SSC contributes to the ocean
color signal measured by SeaWiFS, we likely over-estimated SSC at some of the shallowest
DGoMB stations in the NE Gulf (Hu et al, 2003). Specifically, SSC > 4 mg m-3 shown in Figure
4 for stations MT1 and MT2 close off the Mississippi River delta and at HighPro station may be
overestimated, by perhaps 50% - 100%. However, the relative synoptic patterns as well as the
relative temporal variation patterns should remain valid.
The statistics (standard deviations, CVs) on the seasonal cycle and variability in SSC (Tables 1
and 2) should be considered only in a relative way for a number of reasons. Specifically, SSC
estimates for deep stations without river plume interference have high accuracy (Hu et al., 2003),
but SSC in shallow water coastal stations (depth < 30 m) and in river plumes will be
overestimated. Statistics were derived from two-week and one month composites, not from the
original daily data which often has significant cloud cover. For a pixel with 14 valid data values
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from the 2-week period, only the average value is used in the composite data, and its weight in
the statistics is one. i.e., the same as another pixel with only 1 valid data value from the 2-week
period.
Temporal and spatial variations in primary production and POC flux reaching the seabed
Slope eddies contribute biological and physical heterogeneity along the continental margin of the
northern GOM. Temporal and spatial variations in the geometry of the eddy field along the
middle slope determine whether low salinity green water flows off-margin, or if high salinity
blue water flows on-margin (Biggs et al, 2005; Jochens and DiMarco, this volume). Our
calculations of POC flux to the seabed in Table 3 indicate that more particulate organic carbon
(POC) sinks out from green water than from blue water regions. We also hypothesize that green
water features that persist for weeks to months in time likely are the most important in the export
of POC from surface waters to the benthos.
Model simulations suggest that the most important factor controlling the seasonal cycle of new
production in the Gulf is the depth of the mixed layer (Walsh et al, 1989). Müller-Karger et al
(1991) concluded that because of this dependence, annual cycles of algal biomass are one or
more months out of phase relative to the seasonal cycle of sea surface temperature.
In the western and central deepwater GOM, the standing stocks and biological productivity of the
plant and animal communities living in the upper part of the water column are in general those
that might be expected in a nutrient-limited ecosystem. In the late 1960s, as part of a review of
plankton productivity of the world ocean, Soviet scientists characterized the deepwater GOM as
very low in standing plankton biomass, with mean primary productivity of just 0.10-0.15 g C m-2
d-1 (Koblenz-Mishke et al, 1970). Because the 14C uptake experiments carried out in the 1960s
were not done under trace-metal free conditions, however, these rates are now generally believed
to be under-estimates. Lohrenz et al (1999) reported a range of values of 0.1 – 0.5 g C m-2 d-1
including observations from both the eastern and western GOM. That report put the GOM in a
comparable range for daily primary production to that generally accepted for other open ocean
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Biggs, Hu, and Müller-Karger, revised for Deep-Sea Res II topical issue on DGoMB program
ecosystems (Smith and Hollibaugh, 1993). Moreover, the primary production data in the
Lohrenz et al review paper agree well with the remotely-sensed calculations of primary
production shown in Table 3 in the present study; the calculations summarized in Table 3
average about 0.3 g C m-2 d-1 in surface waters south of 26oN in the blue-water, central Gulf, and
0.4 g C m-2 d-1 in surface waters of the NW Gulf.
Primary productivity in “hot spots” of locally higher nutrient concentrations over the continental
slope can be more than an order of magnitude higher. For example, Gonzalez-Rodas (1999)
documented 14C uptake of > 2 g C m-2 d-1 in the northern margins of two deepwater eddies
interacting with the continental slope of the central Gulf of Mexico. When/where such eddy
interactions with the slope are common, as they appear to be in the NE Gulf during summertime,
the calculated POC flux to the seabed is 6-fold higher than the blue-water, central Gulf average
of 3 mg C m-2 d-1, and double the deepwater average of 9 mg C m-2 y-1 for the NW Gulf. At
stations MT1 and MT2, where calculated primary production averaged 2 g C m-2 d-1, the
calculated POC input rates to the benthos are 78 and 52 mg C m-2 d-1. These calculated rates
agree to within a factor of two with measured rates of sediment community oxygen consumption
(SCOC) in DGoMB benthic lander experiments (Rowe et al, this volume). Rowe et al report that
SCOC averaged 36 + 14 mg C m-2 d-1 from 7 lander measurements of oxygen consumption at
stations MT1 and MT3, and 31 + 7 mg C m-2 d-1 from 4 lander measurements at S42 and S36.
Five lander measurements of SCOC at three stations south of 26oN in water depths of 3.4 – 3.65
km averaged 3.9 + 2.1 mg C m-2 d-1, compared to a calculated average of 2.9 mg C m-2 d-1 by the
method of Pace et al (1987) that we’ve summarized in Table 3.
Relationships between SSC and benthic biomass in the GOM
Watts et al (1992), who examined the relationship between large-scale variability in deep-sea
benthic community structure in the temperate North Atlantic Ocean and mesoscale surface
pigment biomass estimated by CZCS, reported that benthic biomass and density were
significantly and positively correlated with surface pigment biomass. However, when they
statistically removed the effect of depth by partial correlation analysis, measures of benthic
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community structure became either uncorrelated or only weakly correlated with surface pigment
biomass. Because of this, they concluded that surface and benthic communities are largely
decoupled by depth-related processes in the water column, or within bottom communities.
Working with the DGoMB data, Wang (2004) likewise found a strong positive correlation
between polychaete numerical abundance and SSC. Specifically, Wang’s dissertation illustrates
there is a strong first-order relationship between infaunal polychaete density (numbers m-2) and
annual average SSC for the MT and C stations (r2 = 0.82). However, in contrast to the results of
Watts et al (1992), when Wang statistically removed the effect of depth by partial correlation
analysis, polychaete numerical abundance at MT and C transect stations remained robustly
correlated with the annual averages of satellite-estimated SSC there (p < 0.05). The statistical
analyses led to reject other potential proxies for food availability for the infaunal polychaete
community. Wang found that meiofaunal biomass (g C m-2) was not significantly correlated with
polychaete numerical abundance, and she noted that initial correlations between polychaete
abundance and sediment POC and C:N had to be discarded when the effect of depth was
statistically removed. Wang therefore concluded that, at least for MT transect and C transect
stations, satellite-estimated SSC was a useful proxy for food availability to the infaunal benthos.
Wei et al (this volume) offer an overview synopsis of how GOM macrofaunal zonation and
community structure are correlated with water depth, and how these correlations differ between
NW stations and NE stations. That paper, and a review of organic matter in deep-water
sediments by Morse and Beazley (this volume), offer independent perspectives on the
interrelationships between SSC, POC, and macrofaunal biomass at DGoMB sampling stations.
5
CONCLUSION
The 1998-2000 ocean color data collected using the Sea-viewing Wide Field-of-view Sensor
over the continental slope and rise in the NW Gulf (west of 91oW and north of 26oN), show
that the annual average remotely-sensed SSC was 0.19 mg m-3, ranging at most locations
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from annual highs of about 0.3 mg m-3 in November-February to annual lows of about 0.1 mg
m-3 in May-August. However, over the continental slope in the NE Gulf (east of 91oW, north
of 26oN), there were large biweekly variations in both magnitude and phase of the mean SSC
concentration. These variations were associated with mesoscale eddies along or near the
continental slope in the northeastern GOM. In summer, these eddies entrained low salinity,
high apparent SSC Mississippi River water off the shelf and into the deep eastern GOM,
leading to irregularities in the seasonal cycle. Calculated export of particulate organic carbon
(POC flux to the benthos) varied in proportion with actual SSC concentrations in near surface
waters or apparent SSC concentrations associated with the Mississippi River plume, and
there was generally good agreement between the POC flux calculated from the remotesensing data and the SCOC measurements made by benthic lander.
Acknowledgments: Comments by two anonymous reviewers encouraged us to add calculations of
remotely sensed NPP and POC flux, which were done by Remy Luerssen (USF). TAMU support came
from MMS contracts 1435-01-97-CT-30851 and 1435-01-99-CT-30991. USF support came from NASA
grants NNG04GG04G and NN804AB59G. SeaWiFS data are property of Orbimage Corporation and
data use here is in accordance with the SeaWiFS Research Data Use Terms and Conditions Agreement of
the SeaWiFS project.
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Legends for Figures and Tables
Figure 1 (color plate): Average SSC at DGoMB stations for the three-year period 1 Jan 1998 31 Dec 2000, for A) winter (21 December - 20 March); B) summer (21 June - 20
September). Contour lines for 500 m, 1000 m, 2000 m, and 3000 m bathymetry are
shown as white curved lines.
Figure 2 (color plate): Four examples of entrainment of locally high SSC Mississippi River
water by warm slope eddies in the NE Gulf, as monthly composites of SeaWiFS data
for A) July 1998; B) August 1999; C) July 2000; D) August 2001. DGoMB stations
are overlaid as white-color, open squares. SSC color bar same as for Figure 1.
Figure 3 (B/W figure): Panel A: Three year climatology of SSC averaged biweekly from January
1998 to December 2000 at the nine DGoMB stations in water depth > 2000 m in the
NW Gulf. Panel B: Average annual cycle of SSC at these NW Gulf deepwater
stations, illustrating the typical range from summertime lows of about 0.1 mg m-3 to
November-February highs of about 0.3 mg m-3. In each panel, the darkest gray is the
westernmost station and lightest gray is the easternmost station.
Figure 4 (B/W figure): Three year climatology of SSC averaged biweekly from December 1998
to January 2000 at the DGoMB stations east of 91oW in the NE Gulf. Panel A
summarizes the five Central (C transect) stations; Panel B the six Mississippi Trough
(MT transect) stations; Panel C the HiPro and S-transect stations 35-38; Panel D the Stransect stations 39-44. In each panel, the darkest gray is the westernmost station and
lightest gray is the easternmost station.
Table 1: Inter-annual variability of satellite-estimated SSC at the 22 DGoMB stations in the NW
Gulf, versus those in the NE Gulf.
Table 2: Monthly average satellite-estimated SSC at the 22 DGoMB stations in the NE Gulf, in
comparison with monthly grand means for all stations in NW Gulf and in deepwater
south of 26oN.
Table 3: Calculated daily NPP and POC flux to the seabed at DGoMB stations in the NW Gulf,
NE Gulf, and in deepwater south of 26oN.
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