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Spatial and temporal variations in SeaWiFS chlorophyll concentration
at Deep Gulf of Mexico Benthos sampling stations
DOUGLAS C. BIGGS,* CHUANMIN HU† and FRANK E. MÜLLER-KARGER†
(contributed 15 September 2005; in revised form _____; accepted _____)
Abstract--We used SeaWiFS ocean color data to make biweekly composite averages
(1998-2000) of the chlorophyll concentration (CHL) in surface waters 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 CHL 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
the three years of SeaWiFS data 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, CHL 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 CHL in
June-August at 9 of the 22 NE Gulf stations, reaching average monthly concentrations
>50% greater than in November-February. Export of particulate organic carbon (POC
flux to the benthos) likely varies directly in proportion with actual CHL concentrations in
near surface waters or apparent CHL concentrations associated with the Mississippi River
plume. Therefore, more POC likely reaches the seabed in the NE than in the NW Gulf,
and at some stations in the NE Gulf this export is likely greater in summer than in winter.
*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 (CHL + phaeopigment) from multiyear
series of Coastal Zone Color Scanner (CZCS) satellite images for the period 1978-1985. 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 Sensor (SeaWiFS) provide
more accurate and frequent observations of the biological state of the Gulf of Mexico waters,
thanks to improvements in technology (sensors with more spectral bands (wavelengths) and
higher sensitivity) and in algorithms (atmospheric correction and bio-optical inversion). Our task
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 CHL, among 44 DGoMB stations, some of
which are located on the < 500 m continental shelf.
As Rowe and Kennicutt have explained in their Introduction to this volume, 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. Since export of particulate organic carbon (POC flux to the benthos) likely varies
directly in proportion with CHL concentrations in near surface waters, less losses from grazing
and/or remineralization in the water column, for this paper we will summarize the west-to-east,
nearshore-to-offshore, seasonal, and interannual variations in CHL at the DGoMB stations.
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2 METHODS
In early September 1997, the SeaWiFS sensor on the SeaStar satellite (Orbimage Corporation)
began collecting ocean color data, providing global coverage every one to two days (see
http://oceancolor.gsfc.nasa.gov/SeaWiFS/). The Institute for Marine Remote Sensing at the
College of Marine Science at the University of South Florida (USF) downlinked and archived the
regional SeaWiFS data covering the Gulf of Mexico, and we derived weekly, biweekly, monthly,
and annual composite averages of the SeaWiFS-derived CHL. The imagery provided an estimate
of the near-surface (one optical depth or about 30-50 m in clear water and shallower in more
turbid water) phytoplankton abundance, and provided an effective means to trace the circulation,
the dispersal of riverine waters, and the location of oceanographic fronts.
In support of DGoMB oceanographic habitat characterization, we used biweekly composites for
the three year period 1998-2000 to compute the annual mean CHL at each DGoMB station, and
its interannual 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 CHL was calculated using the available SeaWiFS ocean color data from
January 1998 through December 1999 (Biggs and Ressler, 2001). As shown in Plate 1 (this
article), we have extended the data through December 2000, and then summarized the three-year
mean CHL distribution in the northern GOM separately for winter (21 December – 20 March)
and summer (21 June – 20 September) seasons.
For Plate 1 and for the additional characterization of DGoMB oceanographic habitat that we
present in this paper, the SeaWiFS data were processed at high resolution (pixel size 1.0 km x
0.7 km) with the SeaWiFS Data Analysis System (SeaDAS version 2). More recent reprocessing
of the SeaWiFS data (versions 3 and 4) are now available using updated and improved
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calibration and data processing algorithms (atmospheric correction and bio-optical). The
reprocessed data show improved CHL 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).
CHL 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 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 CHL 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 CHL 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, prior to doing monthly composite averages for the 22 DGoMB
stations in the NE Gulf. Each of the monthly-composite SeaWiFS scenes for the NE Gulf was
centered on the middle date for the month (Julian Days 15, 46, 74, 105, 135, 166, 196, 227, 258,
288, 319, and 349). Four such scenes, from four different years, are shown as Plate 2.
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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.
3 RESULTS
Table 1 summarizes the annual averages of CHL at NW versus NE Gulf DGoMB stations, as
computed from the biweekly data for the 3-year period 1998-2000. The overall 3-year average
CHL for the 22 stations in the NE Gulf is 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.
Figure 1 illustrates the seasonal cycle of CHL at the nine deepest stations in the NW Gulf (RW6,
AC1, W5, W6, NB5, B1, NB4, B2, and B3). Water depth at each of these nine stations was
greater than 2 km. Panel A in Figure 1 shows the three-year time series of biweekly CHL at each
of the nine stations for the three years 1998-2000. There seemed to be relatively little year-toyear variability in the amplitude or phase of the annual maximum or minimum CHL. At most of
the locations, CHL 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 1 shows the climatological (three-year) CHL
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 CHL is lowest in spring-summer (Julian Days 100-250) and highest Nov-Feb (Julian
Days 330-060).
Additional analyses illustrate that the annual cycle of CHL 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 Rowe and Kennicutt, 2001 and 2002).
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Only at the shallowest stations (RW1, RW2, W1, W2) did biweekly CHL exceed 0.4 mg m-3 in
November-February.
East of 91oW, the "typical" deepwater annual cycle in CHL not only averaged almost three-fold
higher but was frequently punctuated by times when CHL reached greater than average
conditions for a particular season, especially during summertime (Figure 2). Table 2 shows that
9 of the 22 DGoMB stations in the NE Gulf had CHL in June, July, or August that was 50% or
more greater than the average CHL for the period November-February. In particular, high
summertime CHL levels occurred at S35, S36, S37, S41, S42, S43, HiPro, and at MT3 and MT4.
At most of these 9 stations, as well as at some of the other NE Gulf stations, the year-to-year
variability in annual averages of CHL exceeded a factor of two (Table 1).
4 DISCUSSION
The annual cycle of CHL 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 east of 91oW, however, the typical “deepwater” annual
cycle in CHL was punctuated by high summertime CHL.
A combination of remote sensing altimetry data and hydrographic data collected in support of a
companion field program (the Northeastern GOM Chemical Oceanography and Hydrography
program, or NEGOM) has provided the explanation why this occurred. 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; Jochens et al,
2002; Hu et al., 2003, and Belabbassi et al., 2005). 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 CHL “green water” from the
Mississippi River, and to transport this turbid plume seaward in July and August of each of these
years.
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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 CHL standing stocks.
This was important “sea-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 chlorophyll 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 filters) 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 CHL contributes
to the ocean color signal measured by SeaWiFS, we likely over-estimated CHL at some of the
shallowest DGoMB stations in the NE Gulf (Hu et al, 2003). Specifically, CHL > 4 mg m-3
shown in Figure 2 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 CHL which we
have presented in Tables 1 and 2 should be considered only in a relative way for a number of
reasons. Specifically, CHL estimates for deep stations without river plume interference have high
accuracy (Hu et al., 2003), but CHL 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 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.
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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). We hypothesize 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 100-150 mg C m-2
d-1 (Koblenz-Mishke et al, 1970). A few years later, extensive ship surveys of phytoplankton
chlorophyll and primary production that span the period 1964-1971 were summarized by ElSayed (1972) in atlas format, as averages within 2o squares of latitude and longitude. These atlas
maps showed that surface CHL generally ranges 0.06 - 0.32 mg m-3 in deepwater central and
western GOM. There is usually a subsurface "deep chlorophyll maximum" (DCM) within which
concentrations are 2-3 fold higher, and so the atlas reported that CHL in deepwater could reach
21 mg m-2 when integrated from the surface to the base of the photic zone. Most values for CHL,
though, ranged 5-17 mg m-2 where water depth was > 2 km (El-Sayed 1972).
During an algal bloom, the C:CHL ratio for living phytoplankton cells ranges from 23 to 79
(Antia et al., 1963). From mesocosm experiments, Antia et al (1963) reported that vigorously
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growing phytoplankton with measurable nitrate in the water had C:CHL ratios of about 25, while
senescent phytoplankton in nitrate-depleted water had C:CHL ratios of 60 or more. Using a
C:CHL ratio of 25, a CHL standing stock of 0.1 mg m-3 is equivalent to a POC concentration of
about 2.5 mg m-3. From 14C uptake experiments done in the GOM, El-Sayed (1972) has shown
that primary production at "blue water" locations increases the phytoplankton POC pool by on
average 0.25 mg C m-3 h-1, which is equivalent to an increase of about 10% per hour. Such low
values of primary production are typical for surface waters at the majority of the oceanic stations
in the El-Sayed (1972) atlas, equivalent to only about 10 mg C m-2 h-1 when integrated from the
surface to the base of the photic zone. If there are on average 12 hours of sunlight per day, this
rate is equivalent to 120 mg C m-2 d-1 and so is in good agreement with the characterization by
Koblenz-Mishke et al (1970). Allowing for primary production to proceed 365 days a year in the
GOM because of its subtropical climate, this rate of primary productivity is about 44 g C m-2 y--1.
As a consequence the deepwater GOM is usually placed at the low end of the estimated range of
50-160 g C m-2 y--1 that is generally accepted for the annual gross primary production in openocean ecosystems (Smith and Hollibaugh, 1993).
If on average 10% of the primary production sinks out of the photic zone and if 3-10% of this
flux in turn reaches the seabed (Lalli and Parsons, 1997; Muller-Karger et al., 2005), then at a
typical deepwater location just 100-360 mg C m-2 y--1 might reach the benthos. However,
productivity in “hot spots” of locally higher nutrient concentrations over the continental slope
can be more than an order of magnitude higher than the typical 100-150 mg C m-2 d-1 (Biggs and
Ressler, 2001). 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, we hypothesize that POC flux to the seabed is
likely to be significantly greater than the deepwater average of 100-360 mg C m-2 y-1. Using the
grand mean annual averages from Table 1, we hypothesize that in such "hot spots" and at most of
the stations in water depths < 2000 m along central and eastern DGoMB transects C, MT, and S,
POC input rates to the benthos are likely to be at least 0.54/0.19 = 2.8 times higher than average.
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At MT1 and MT2, with annual averages for CHL greater than 2 mg m-3, we hypothesize that
POC input rates to the benthos are likely more than 10 times higher than average.
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
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 CHL. Specifically, Wang’s dissertation illustrates
there is a strong first-order relationship between polychaete density (numbers m-2) and annual
average CHL for the MT and C stations (r2 = 0.82). Station MT1 had the highest ocean color and
the highest density of polychaetes, followed by MT2. Polychaete density decreased linearly with
average surface CHL concentration. 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 CHL there (p < 0.05). Wang then went several steps further. Her
statistical analyses led her to reject other potential proxies for food availability for the infaunal
polychaete community. She 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 CHL was a useful proxy for food availability to the infaunal benthos.
5
CONCLUSION
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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 CHL 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. 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 CHL
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 CHL Mississippi River water off the shelf and into the deep eastern GOM,
leading to irregularities in the seasonal cycle. Export of particulate organic carbon (POC flux
to the benthos) likely varies directly in proportion with actual CHL concentrations in near
surface waters or apparent CHL concentrations associated with the Mississippi River plume.
Therefore, more POC likely reaches the seabed in the NE than in the NW Gulf, and at some
stations in the NE Gulf this export is likely greater in summer than in winter.
Acknowledgments: TAMU support came from MMS contracts 1435-01-97-CT-30851 and 1435-01-99CT-30991. USF support came from NASA contracts NAS5-97128 and NAG5-10738. 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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Biggs, Hu, and Müller-Karger for Deep-Sea Res II topical issue on DGoMB program
Legends for Figures, Color Plate, and Tables
Figure 1: Panel A gives three year climatology of SeaWiFS ocean color data averaged biweekly
1998-2000 at the nine DGoMB stations in water depth > 2000 m in the NW Gulf.
Panel B plots the average annual cycle of chlorophyll 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 2: Three year climatology of SeaWiFS ocean color data averaged biweekly 1998-2000 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 S-transect stations 39-44.
In each panel, the darkest gray is the westernmost station and lightest gray is the
easternmost station.
Plate 1: Average CHL 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 in white.
Plate 2: Four examples of entrainment of locally high CHL 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.
Table 1: Annual averages of satellite-estimated CHL at DGoMB stations in the NW Gulf, versus
NE Gulf.
Table 2: Monthly average satellite-estimated CHL at DGoMB stations in the NE Gulf.
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