Ocean color satellites show extensive surface lines of floating vegetation in the Gulf of
Mexico
Jim Gower, Chuanmin Hu, …
Abstract
We present satellite imagery showing extensive lines of floating sargassum biomass in
the western Gulf of Mexico in the summer of 2005. Satellite observations were made
with MERIS which provides a positive signal in the MCI (Maximum Chlorophyll Index)
used to detect surface plankton blooms, and with MODIS which provides a negative
fluorescence signal where the reflected radiance from the plants exceeds that from
chlorophyll fluorescence. Observations suggest sargassum biomass of about 20 g/m2,
equivalent to about 20 mg/m2 of chlorophyll a over large areas of the western Gulf of
Mexico during the summer of 2005, showing that sargassum is a significant component
of oceanic productivity in this area and period. MERIS observations show lower
concentrations in 2003, and have also detected similar lines in Gulf Stream water off the
east coast of the US, and as far north as 42 N.
Introduction
The area of the North Atlantic from 20 to 35 N and 30 to 70 W has been called the
Sargasso Sea after the floating weed (Sargassum natans or Sargassum fluitans)
commonly encountered there. However, the area where this weed is found extends into
the Gulf of Mexico as well as to other parts of the world. Early reports of distribution
and abundance of the weed in the Sargasso Sea were summarized by Winge (1923), and
visual observations of the lines in which it is commonly observed were used by Langmuir
(1938) in his description of Langmuir circulation. This circulation results in surface
convergence of floating weed and other material into lines oriented along the wind
direction, with typical separations of 20 to 50 m (Faller and Woodcock, 1964). The
present satellite observations relate to larger-area patterns where lines on the order of 100
to 1000 m wide are separated by tens of kilometers. Surface convergence on this larger
scale are due to mesoscale surface flow patterns. In several cases the satellite images also
show evidence of the mesoscale features that result in these flows.
The authors are not aware of previous reports of sargassum in satellite images. Attempts
have been made in the past to estimate the total biomass of pelagic sargassum from
surface net tows, and satellite images should provide useful new data. Stoner (1983)
summarized surface data collected by Parr (1939) between 1933 and 1935 and compared
it with his own measurements from 1977 to 1981. He reported a major decrease in
biomass in both the Sargasso Sea and the Gulf of Mexico over this time interval, and
suggested oil pollution as a possible reason. This work does not appear to have been
followed up. More recently, a possible long-term increase in UV radiation due to a
decrease in stratospheric ozone has been proposed as a significant additional source of
stress for sargassum (Franklin and Forster, 1997). Total biomass estimates are important
for defining ocean productivity and carbon flux (Muraoka, 2004). Press reports from the
southern US suggest that 2005 was an extremely “high sargassum” year in the Gulf of
Mexico with impacts on local fishing that were both positive (good fishing beneath weed
patches) and negative (tangled gear and propellers).
The satellite images used here have spatial resolution down to 300 m (MERIS Full
Resolution mode), with most images (MERIS Reduced Resolution and MODIS) having
resolution of 1 to 2 km. Weed aggregations therefore have to be dense enough over a
large enough area to affect the average colour (visible surface reflectance) of pixels of
this area. Vegetation is characterized by the abrupt change in reflectance (chlorophyll red
edge) from low (<0.2) in the visible, to high (>0.5) at wavelengths longer than about 700
nm. This gives Sargassum good contrast against the dark water background for sensors
such as MERIS and MODIS which have the appropriate spectral capabilities.
Satellite image data
MERIS image data for June 2 2005 show an extensive field of curvilinear surface slicks
covering the western half of the Gulf of Mexico (Figure 1). The formation and pattern of
the slicks are clearly influenced by the mesoscale circulation, as outlined by the surface
chlorophyll image. The true colour image shows strong sun glint on the right side if the
image, with scattered cloud over the western Gulf. The centre Maximum Chlorophyll
Index (MCI) image shows the slicks. We assume they are composed of the floating
sargassum weed that has been reported as unusually abundant in the area this summer,
concentrated into narrow lines by the convergence and shear of the mesoscale current
field. The right image is processed to show surface chlorophyll concentration, with the
low-chlorophyll water of the loop current contrasting with higher-chlorophyll coastal
water.
These images have the 1200 m resolution of MERIS Reduced Resolution (RR) mode.
The MCI signal is computed to show the magnitude of a peak in the water-leaving
radiance spectra near 705 nm. This peak indicates very high surface concentrations of
chlorophyll-a or some other form of floating vegetation (Gower, 2005). It is mapped by
MERIS using a band centred at 709 nm, computed as the radiance at this wavelength,
above a baseline radiance linearly interpolated from measurements at 681 and 753 nm.
Figure 2 shows spectra and difference spectra from pairs of points inside and outside the
slicks at the points shown. Difference spectra for the slicks show the “red edge”
characteristic of land vegetation, suggesting dense mats of floating phytoplankton, similar
to those observed in cyanobacteria blooms in the Baltic (Kahru, 2004). Spectra are also
shown from coastal lagoons Atchafalaya Bay and Laguna Madre where water shows the
peak near 705 nm more clearly. In water off Atchafalaya Bay, south of the Mississippi
delta, the peak is the dominant feature of the water-leaving radiance spectrum, indicating
relatively low backscatter, and chlorophyll concentrations on the order of 100 mg.m-3
(Gower, 2005).
Figure 1 shows slicks in the Gulf west of about 92 W, affecting about 1 to 2% of the
surface area of the Gulf at the 1.2 km resolution of this MERIS image. Figure 3 shows a
full resolution (FR, 300 m) MERIS image for 23 May 2005 in which the structure of the
slicks is better defined, compared to the equivalent reduced resolution (RR) image. The
more intense lines are only a single pixel wide at the higher resolution. Fractional cover
can be computed for each pixel using an estimated radiance for full vegetation cover
based on adjacent land. Fractional cover of a 200 km square area of the image gives
.0006 for the high resolution (FR) data and .0004 for the RR data.
The slicks were first noted on MERIS MCI images which show a positive signal for the
sargassum lines. The time period over which slicks could be seen was then better refined
using MODIS images, for which a larger number of images are available. Both sensors
lack the capability to tilt the sensor away from the sun, so that sun glint makes detection
impossible over significant areas of every image since the sun is nearly exactly overhead
at local noon. For MODIS, The University of South Florida makes a wide variety of
products available on the web in near real time. The most suitable product for detecting
the lines is the fluorescence product which shows them as a negative signal.
A comparison of MERIS and MODIS images of the slicks is shown in Figure 4 for 2
June 2005. The MODIS Image is affected by residual striping, but its wider swath allows
it to cover more area and give images that are less affected by sunglint.
The earliest sighting of the slick field in the Gulf of Mexico in 2005 was in March.
Examples increase in extent and contrast into July and then become fainter and smaller in
area into September. The USF processed data are available only since September 2004.
They show small areas of lines lines on two days in that month. Limited data are
available from MERIS for 2003. These show significantly fewer slicks compared to
2005. MERIS images also show examples of similar lines off Florida and the east coast
of the USA, closer to the area known as the Sargasso Sea. One image on 27 August 2005
shows what appears to be lines of sargassum in Gulf Stream water at 42 N, 60 W,
significantly north of the accepted northern limit of the Sargasso Sea (35 N).
Discussion
The statistics of the MERIS full-resolution image of 23 May 2005 show that of the
430,000 “clear-sky” pixels (300 by 300 m) in a test area measuring about 200 km square,
45 show an MCI signal greater than 5 mW/(m2.nm.sr.) and 3500 show a signal greater
than 0.5 mW/(m2.nm.sr.). We estimate that for the solar illumination conditions on that
day, a 5 mW/(m2.nm.sr.) MCI signal corresponds to about 16 % coverage of the pixel by
exposed biomass. Maximum observed MCI signal in the test area was 12.7
mW/(m2.nm.sr.), corresponding to 40% coverage of the pixel by exposed biomass.
Computation from the histogram of the MCI values in the FR image of Figure 4 gave an
average coverage fraction for the whole area of 0.0006, equivalent to 1 in 1700 pixels
being completely covered in sargassum.
Estimating the average thickness of the sargassum layer as 3 cm then gives a biomass
volume of 20 cc or mass of 20 g per square meter, approximately equivalent to a dry
weight of 2 g, a carbon weight of 1 g and a chlorophyll pigment concentration of 20 mg
per square meter. This is a significant biomass for an oceanic area. The slicks were
observed to cover a large fraction of the western Gulf of Mexico during the summer of
2005. The least precise number in this estimate is the average layer thickness. In situ
observations may refine the 3 cm estimate, though the actual thickness would be hard to
determine in situ and highly variable with position.
Parr (1939) estimated an average biomass of sargassum as approximately 1 g/m2 in the
1930s. Stoner reported a significant drop in this concentration to less than 100 mg/m2
during his sampling period (1977 to 1981). The present satellite observations suggest
concentrations of 20 g/m2, an order of magnitude or more greater than those of Parr,
suggesting that sargassum concentrations have recovered to above their 1930 values, at
least in the Gulf of Mexico. Carpenter and Cox (1974) use Parr’s biomass estimate to
conclude that sargassum productivity is only 0.5% of that for phytoplankton in the
Sargasso Sea. The present observations suggest that in some cases, sargassum
productivity can represent a higher fraction, making a significant contribution, near 10%,
to total productivity.
Conclusions
Detection of sargassum slicks by MERIS and MODIS indicate a useful new tool for
monitoring sargassum biomass. Satellite observations should better define its geographic
extent and any interannual variability. The concentrations in the western Gulf observed
in 2005 show that sargassum can represent a significant fraction of the productivity.
Figure 4 suggests that a spatial resolution of about 1 km is adequate, but that higher
resolution is needed to resolve the lines. Spectral bands need to define the chlorophyll
“red edge” which provides contrast with cloud (including contrails from aircraft) and sun
glint. Observation of the lines in sun glint patterns show that they sometimes reduce
surface roughness, and should therefore be detectable as dark lines in SAR images. This
would allow monitoring through cloud and at night. It is interesting to note that data on
these slicks are not available from NASA’s Ocean Color web site, since they do not show
up in standard “Ocean Colour” products.
(Possible note to be added:
Any floating phytoplankton will form similar lines, and similar patterns of
Trichodesmium covering smaller areas have been reported (Sarangi et al., 2005) Sarangi,
R.K., P. Chauhan, S.R. Nayak, and U. Shreedhar, 2005, “Remote sensing of
trichodesmium blooms in the coastal waters off Gujarat, India using IRS-P4 OCM,
International Journal of Remote Sensing, 26, 1777-1780. However, the lines reported
here are almost certainly sargassum, based on several in-situ observations in the western
Gulf in the summer of 2005.
…wording TBD)
References
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Figure 1. Sargassum lines in the western Gulf of Mexico, observed with the MERIS
imager in reduced resolution (1200 m) mode on 2 June 2005. The left image shows a
true colour image with a coastline added in red. High sunglint covers the right side if the
image, with scattered cloud over the Western Gulf. The centre MCI image shows the
slicks. The right image is processed to show surface chlorophyll concentration.
Figure 2. The Figure 1 image, showing the spectra of the radiance increases in sargassum
lines and coastal blooms compared to nearby “clear water” areas. Sargassum lines show
the characteristic “red edge” spectrum of vegetation. Coastal bloom spectra show the
peak near 705 nm, which can dominate the spectrum.
Figure 3. Sargassum lines in the western Gulf of Mexico, observed with the MERIS
imager in full resolution mode (300 m, left) on 23 May 2005, and compared with the
equivalent reduced resolution image (right).
Figure 4. Comparison of slick images derived from MERIS MCI data (left) and MODIS
fluorescence signal (right).