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Ocean color climatology of chlorophyll at DGoMB stations in 1998, 1999, and 2000
and its use as a proxy for POC flux to the seabed
Douglas C. Biggs1, Andrew W. Remsen2, Chuanmin Hu2, and Frank E. Müller-Karger2
1
Department of Oceanography, Texas A&M University, College Station TX, 77843
College of Marine Science, University of South Florida, St Petersburg FL 33701
2
The amount of particulate organic carbon (POC) reaching the seabed is determined by the annual
primary production in near surface waters, less losses from grazing and/or remineralization in the
water column. Because primary production covaries as the time rate of change in phytoplankton
biomass in near surface waters, we compiled time series of variations in chlorophyll (CHL) at
each of the DGoMB stations from SeaWiFS ocean color data. These data were generated with the
SeaWiFS Data Analysis System (SeaDAS) software version 2. Comparison with field
measurements in the NEGOM for 1998 and 1999 shows that for deep waters the uncertainties at
any individual location are generally within 35% unless significant river plume intrusion is
found, and for larger areas the average uncertainties are much smaller (Hu et al., 2003).
During DGoMB Year Two, all available SeaWiFS imagery for the Gulf of Mexico for 1998 and
1999 was composited biweekly to produce a 2 year time series of 26 biweekly means per year x 2
years = 52 biweekly averages. The mean CHL in the first optical depth at each of the 43
DGoMB stations was then computed for each biweekly interval, as the mean of a 5 pixel x 5
pixel grid centered on the pixel closest to the specified Lat+Lon location of each DGoMB
station. The choice of multi-pixel average rather than single-pixel value is to remove residual
digitization-noise errors (Hu et al., 2001). This biweekly averaging was first done with SeaWiFS
data with pixel resolution of 4.1 km 2.8 km, making the effective area around each DGoMB
station that was being averaged at each biweekly interval about 287 km2. The results are
reported in a DGOMB Interim Report (section 5.3.2 in: Rowe and Kennicutt, in press). But for
this ITM presentation, the biweekly averaging has now been done a second time, using not just
two but three years of ocean color data 1998, 1999, and 2000, and with the high-resolution
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SeaWiFS data set which has about 1 km x 1 km pixel resolution. For the averages we will
present today, the area around each DGoMB station has been focused to about 25 km2.
CHL in the first optical depth in the deepwater GOM undergoes a well-defined seasonal cycle
which is generally synchronous throughout the region. Müller-Karger et al. (1991) reviewed
monthly climatologies of near-surface phytoplankton pigment concentration from multiyear
series of coastal zone color scanner (CZCS) images for the period 1978-1985. They reported that
highest near-surface pigment concentration (CHL+phaeopigment) occurs between December and
February and lowest values occur between May and July. For deep waters the CZCS pigment is
dominated by CHL, not by phaeopigment. There is only about 3-fold variation between the
lowest (~0.06 mg m-3) and highest (0.2 mg m-3) deepwater near-surface pigment, however.
Model simulations show that the single most important factor controlling the seasonal cycle in
near-surface CHL concentration is the depth of the mixed layer (Walsh et al, 1989). MüllerKarger 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.
Subsequent study of CZCS imagery by Melo-Gonzalez et al (2000), who looked at three-month
averages, has reinforced this conclusion.
Not surprisingly, then, the annual cycle of CHL at the deepest stations (water depth > 2 km)
shows the high-in-winter, low-in-summer pattern previously reported from analysis of the CZCS
archives by Müller-Karger et al (1991). The annual cycle of CHL at the Western Stations (W1W6) and at the Lousiana Slope Stations (the cluster of 9 stations between 93oW and 91oW) also
shows this “deepwater” pattern. Moreover, the annual cycle of CHL at the Far Western Stations
(RW1-RW6) generally follows the “deepwater” pattern, but there are several periods of the year
in which CHL at the shallower stations (RW1 and RW2) exceeds 0.5 mg m-3. Not all of this high
CHL signal occurs November-February.
East of 91oW, the typical “deepwater” annual cycle in CHL is swamped by unusually high
summertime CHL. In summers 1998, 1999, and 2000, warm slope eddies (WSEs) that were
centered over DeSoto Canyon acted to entrain low salinity, high CHL “green water” from the
Mississippi River and transport this plume seaward into deepwater. As a result, high surface
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CHL in summertime was evident at all DGoMB stations on the Mississippi Trough Transect
(MT1-MT6), and at the 3 stations farthest upslope along the Central Transect (C1, C7, C4), and
at the 3 stations farthest upslope along theDeSoto Canyon Transect (S35, S36, S37). High
summertime CHL was evident, as well, at 3 of the stations along the Eastern Transect (S44, S43,
S42).
How much of the primary production from surface waters reaches the seabed? In the central and
western deepwater GOM, the standing stocks and biological productivity of the plant and animal
communities living in the upper part of the water column are also 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 surveys of phytoplankton chlorophyll
and primary production that span the period 1964-1971 were summarized by El-Sayed (1972) in
atlas format, as averages within 2o squares of latitude and longitude. These atlas maps show 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, though, ranged 5-17 mg
m-2 where water depth was > 2 km (El-Sayed 1972). Low values of primary production (< 0.25
mg C m-3 h-1) are typical for surface waters at the majority of the oceanic stations in this atlas,
equivalent to < 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 300 days a year in the GOM because of its subtropical climate,
this rate of primary productivity is < 36 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 open-ocean ecosystems (Smith and
Hollibaugh, 1993).
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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, 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 DeSoto Canyon during
summertime, we suggest that POC flux to the seabed at these locations is likely to be
significantly greater than the deepwater average of 100-360 mg C m-2 y-1. In fact, most of the
stations along central and eastern DGoMB transects C, MT, and S probably have substantially
higher-than-average POC input rates.
References
Biggs, D.C., and P.H. Ressler. 2001. Distribution and abundance of phytoplankton,
zooplankton, ichthyoplankton, and micronekton in the deepwater Gulf of Mexico. Gulf
Mex. Sci. 19: 7-35.
El-Sayed, S.Z. 1972. Primary productivity and standing crop of phytoplankton. pp 8-13, in: V.C.
Bushnell (Editor), Chemistry, Primary Productivity, and Benthic Algae of the Gulf of Mexico.
American Geographical Society, New York.
Gonzalez-Rodas, G.E. 1999. Physical Forcing of Primary Productivity in the Northwestern Gulf
of Mexico. PhD dissertation, Department of Oceanography, Texas A&M University, College
Station, TX. 149 pp.
Hu, C., K. L. Carder, and F. E. Muller-Karger, F. 2001. How precise are SeaWiFS ocean color
estimates? Implications of digitization-noise errors. Remote Sensing of Environment, 76:239249.
Hu, C., F. E. Muller-Karger, D. C. Biggs, K. L. Carder, B. Nababan, D. Nadeau, and J.
Vanderbloemen. 2003. Comparison of ship and satellite bio-optical measurements on the
continental margin of the NE Gulf of Mexico. Int. J. Remote. Sens. 24:2597-2612.
Koblenz-Mishke, O.J., V.K. Volkovinsky, and J.C. Kabanova. 1970. Plankton primary production
of the world ocean. In: W.W. Wooster (Editor), Scientific Exploration of the South Pacific,
National Academy of Science, Washington, D.C., pp. 183-193.
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Melo-Gonzalez, N., F.E. Müller-Karger, S. Cerdeira-Estrada, R. Perez de los Reyes, I Victoria del
Rio, P. Cardenas Perez, and I. Mitrani-Arenal. 2000. Near-surface phytoplankton distribution in
the western Intra-Americas Sea: The influence of El Nino and weather events. J. Geophys. Res.
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Müller-Karger, F.E., J.J. Walsh, R.H. Evans, and M.B. Meyers. 1991. On the seasonal
phytoplankton concentration and sea surface temperature cycles of the Gulf of Mexico as
determined by satellites. J. Geophys. Res. 96:12,645-12,665.
Rowe, G.T., and M.C. Kennicutt II (Editors), in press. Deepwater Program: Northern Gulf of Mexico
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Walsh, J.J., D.A. Dieterle, M.B. Meyers, and F.E. Müller-Karger, Nitrogen exchange at the
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1989.
Supporting Figure
Figure 1: Three year climatology of SeaWiFS ocean color data shows that annual
maxima in CHL concentration at continental margin stations along DGoMB Eastern
Transect (S stations, top panel) reach higher concentrations and are often out of
phase with those in the deepwater western Gulf of Mexico (bottom panel).
Authors
Douglas Biggs is Professor and Chair of the 6-person Biological Oceanography faculty at
TAMU. In addition to his service as a co-PI on DGoMB, Biggs was a co-PI on MMS-sponsored
NEGOM-COH and GulfCet-II projects and he is presently working to define the oceanographic
habitat of cetaceans in the NE Gulf of Mexico as part of an interagency MMS-NOAA-ONR
study known as the Sperm Whales and Acoustic Monitoring Program (SWAMP).
Andrew Remsen is a PhD candidate at the College of Marine Science, University of South
Florida. His research includes the development and application of novel optical instrumentation
to study zooplankton processes, investigation of mesoscale processes such as Mississippi River
discharge and eddies on biological productivity in the Gulf of Mexico and the seasonal dynamics
of zooplankton grazing on the West Florida Shelf.
Chuanmin Hu is a research assistant professor and Executive Director of the Institute for Marine
Remote Sensing (IMaRS) at the College of Marine Science University of South Florida. He has
participated in the NASA sponsored SIMBIOS and MMS-sponsored NEGOM projects. He is
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working on ocean color remote sensing in coastal waters and is principle investigator on several
NASA and USGS funded projects to study river plumes and estuaries.
Frank Muller-Karger is Professor and Director of IMaRS at the College of Marine Science
University of South Florida. Muller-Karger's research focuses on primary production in the sea,
and he uses remote sensing (ocean color, SST, SSH) to study the importance of continental
margins, including areas of upwelling and river discharge in the global carbon budget.