Estimating diffuse attenuation coefficient from Landsat imagery in the
Florida Keys: Implications for water quality assessment of coral reef
ecosystems
Palandro, D., Hu, C., Andréfouët, S. and Muller-Karger, F. E.
College of Marine Science, University of South Florida
140 7th Avenue South, Saint Petersburg, FL 33701, USA
Tel: 727-553-1186, Fax: 727-553-1103
E-mail: palandro@seas.marine.usf.edu
Submitted as a Research Note to the Special ICCB7 Proceedings Issue of:
Hydrobiologia
Keywords: remote sensing, satellite, water clarity, classification
This paper has not been submitted elsewhere in identical or similar form, nor will it be
during the first three months after its submission to Hydrobiologia.
ABSTRACT
The diffuse attenuation coefficient, K (m-1), describes the effective attenuation of light in
the water column, therefore representing water clarity. The water column correction of
remotely sensed imagery requires K in the mapping of shallow benthic habitats.
Traditionally, K is derived from imagery using known locations of the same bottom type
located at different known depths, assuming that light attenuates exponentially with
depth. This technique is applied to three Landsat 7 satellite images (February, May and
July 2000) for the Florida Keys Reef Tract between Key Largo to Key West.
Interpolated depth, initially from NOAA vector data, with uncertainties of ±0.5m, was
draped over the 30m spatial resolution Landsat data. K was derived for 27 sites where
bright sand could be observed at varying depths between 3m and 20m. Results show that
the blue (band 1) and green (band 2) bands provided the best information and were
relatively consistent with time and location. The red (band 3) band provided more
ambiguous results with several negative K values, attributed primarily to two combined
factors: use of a simple atmospheric correction and the rapid attenuation of the red band
in water. Despite the fact that these observations are a snapshot in time, trends were
observed for the Upper, Middle and Lower Keys possibly due to the varying influence of
Florida Bay waters. However, the validity of these trends is yet to be verified. Combined
with bathymetry data, the K values derived at the discrete sampling sites may be
extrapolated to nearby pixels to provide an explicit water column correction for benthic
classification. [is there a limit for word #? It's better to put the actual K values (even a
range) in the abstract – many people read abstract only]
INTRODUCTION
Water quality is an important aspect of the reef ecosystems of the Florida Keys (LaPointe
& Matzie, 1996; Szmant & Forrester, 1996). Declining water quality has been discussed
as the major cause for the increase in disease visible in the reefs of the Florida Keys
(Lipp et al, 2002; Patterson et al. 2002) as well as the phase shift from a coral to algal
communities (LaPointe, 1997). Efforts are underway to study water quality parameters in
the Florida Keys. These include seasonal in situ data collection via the Environmental
Protection Agency’s (EPA) Water Quality Monitoring Project (Boyer et al., 2000) and
the SEAKEYS program buoy data (Ogden et al., 1994). Water quality is highly variable
both seasonally and spatially within the three major geologic zones of the Florida Keys
(Upper, Middle and Lower) (Smith, 1994; Szmant & Forrester, 1996; Smith, 1998), with
the highest nutrient concentrations located in the Middle Keys during the summer months
(LaPointe & Clark, 1992). Yet to date there is no systematic, synoptic estimate of water
quality for the Florida Keys at fine resolution. [this gives you one justification why you
do this]
The diffuse attenuation coefficient, K (m-1), an apparent optical property, describes the
effective attenuation of light in the water column, and hence water clarity. To classify
shallow benthic habitats using remotely sensed imagery and correct for water column
effects, K is required, but has seldom been measured concurrently with satellite
overpasses. Therefore, K, or the ratio of K in different spectral bands, has been
traditionally derived directly from satellite imagery using the same bottom type (e.g.,
sand) located at different depths, assuming that light attenuates exponentially with depth
(Maritorena, 1996). For this study, as a first attempt to map K and study it's spatial and
temporal variations at a large scale throughout the entire Florida Keys Reef Tract at fine
fine resolution [this explains what's new], we applied this technique to bands 1 (blue –
centered at 485nm), 2 (green – 560nm) and 3 (red – 660nm) of the Landsat 7 Enhanced
Thematic Mapper Plus (ETM+) satellite images. Three images acquired 5 February, 27
May and 14 July of 2000 for the Florida Keys Reef Tract between Key Largo and Key
West were used. Specifically, this study addresses the variation of K throughout the
Florida Keys’ three major geologic zones: Upper, Middle and Lower Keys, to determine
if there is a discernable variation between the three zones and among the three zones for
the winter (February), spring (May) and summer (July).
METHODS
The Landsat 7 data represented here utilizes bands 1-3 of the ETM+ sensor (the visible
bands). The Landsat images were atmospherically corrected by subtracting the deep
water pixel value obtained over open water, where the albedo is assumed to be zero, thus
providing the equivalent water-leaving radiance (Lw*. Note that Lw* is a product of Lw and
the diffuse transmittance from water to the sensor, t) . The bathymetry data, originally
from NOAA vector data, at 30m spatial resolution and 1m or 5m depth intervals, were
interpolated so that all depth intervals were at 1m. The depth data was then registered to
and draped over the 30m Landsat data. For each of the three Landsat bands, several
surface Lw* values were taken over sand for 27 different sites throughout the image
(Figure 1). Each of these sites provided sand patches at various depths (z1, z2, …) so that
an estimation of K(equation 1) was possible. K was computed with the assumption that
the natural log of the surface Lw* values decline linearly with depth due to Lw* being
dominated by the bottom signal. K is therefore equal to the regression slope of the ln(Lw*)
versus depth line (Lyzenga, 1981; Maritorena, 1996).
K = ln [Lw*(z1)/ Lw* (z2)] / [2*(z2 –z1)]
(1)
A factor of ‘2’ is used to approximate the attenuation of light in both the upward and
downward directions (Maritorena et al., 1994).
RESULTS and DISCUSSION
First, the K values for the red band were often negative (thus invalid). The invalid data
can be attributed primarily to two combined factors: the use of a simple atmospheric
correction, which tends to overcorrect in the red band, and the rapid attenuation of the red
light in water, which causes the water column signal (not the bottom signal) to be
dominate. The results obtained for the blue and green band were far more consistent. The
geographic variation data show that the average K values are lowest (clearest water) for
the Lower Keys (Kb1 = 0.0289 m-1) followed by the Middle Keys (Kb1 = 0.0498 m-1, with
the exception of the May image) followed by the Upper Keys (Kb1 = 0.0625 m-1) (Figure
2). This seems to be in contradiction to the common belief that the highest turbidity levels
would be in the Middle Keys due to its waters being directly influenced by the adjacent
nutrient-rich Florida Bay waters (Porter et al., 2002).
The exception is the May image, which agrees with Porter et al. (2002) and exhibits K
values four times (band 1) and two times (band 2) the mean of the other images. This
high K anomaly was mirrored in ocean color data from the Sea-viewing Wide Field-ofView Sensor (SeaWiFS, McClain et al., 1998), which showed that the color-rich patterns
in the Middle Keys also occurred from May to June in 2001, 2002 and 2003. If the
SeaWiFS color is dominated by chlorophyll a, the result is then slightly different in
timing from that of LaPointe and Clark (1992), who found the highest chlorophyll a
levels during the summer for the same geographic region. If the May image average K
values are used to derive a new average K value for the Middle Keys region, then indeed
the averaged overall K values fro this area would be greater than that of the Upper Keys
(Kb1 = 0.0943 m-1). Seasonal variation, as a whole, between the images showed little
variation in K, with each of the values within one standard deviation, except for the May
Middle Keys image.
For benthic classification, instantaneous measurements of K are required to optimize the
corrections (Mumby et al., 1998). Palandro et al. (submitted) show an excellent fit
between benthic classification results (from Landsat) for the five reefs in comparison
with in situ data taken by the Coral Reef Monitoring Project (CRMP) (Porter et al.,
2002). Overall annual declines [?] computed for the reef sites were 35.81% (CRMP) and
37.21% (Landsat). Note that the comparison is between percent coral cover (in situ
CRMP) and percent ‘coral habitat’ pixels (Landsat), thus at two different scales. The
similarity in trends can be attributed to the fact that the changes taking place at the in situ
scale are also taking place at the larger habitat scale. Also, these values were greatly
enhanced by the input of the K value as a correction to the Landsat imagery. [No.
Logically you can't say this. The agreement can be due to many reasons, and your
conclusion (K is correct because of this) is a circular argument for several reasons].
Thus, the K values derived here seems [note the word "seems"] to be reasonable in this
regard, yet field data are needed for direct validation. [Dave: it is interesting to see how
Landsat and CRMP compare with each other WITHOUT the explicit K correction, i.e.,
how much improvement does K correction improve the classification? The reviewers
may ask this question, so be prepared]
CONCLUSION
An attempt has been made to derive the diffuse attenuation coefficient in water, K (m-1),
in the Florida Keys from Landsat imagery. Results from the two visible bands (centered
at 485 and 560 nm, respectively) show generally consistent behavior for the three areas
(Lower, Middle, and Upper Keys) from the three Landsat images (Feb., May, and July
2000). Observations of large-scale variations of K, in blue and green bands, at three
different times suggest that snapshots in water quality somewhat reflect the expected
geographical trends in water quality variations within the Florida Keys seascape.
However, as these are only three discreet snapshots in time, more images will be required
to confirm this. In particular, field data will provide direct validation of the results. The
derivation of K and subsequent use of the K value in the bathymetric correction made the
comparison between the two datasets possible and contributed greatly to their similarity
in percent change value [I would remove this sentence unless you have classification
results without K correction]. The derivation of K provides a useful tool for both the
inference of water quality and is indeed necessary for the bathymetric correction of
satellite images.
ACKNOWLEDGEMEMTS
This work was supported by NASA Headquarters under the Earth System Science
Fellowship Grant NGT5-30414 to David Palandro.
REFERENCES
Boyer, J.N., P. Sterling & R.D. Jones, 2000, Maximizing information from a water
quality network through visualization techniques. Estuarine, Coastal and Shelf Science,
50: 39-48.
LaPointe, B.E., 1997, Nutrient thresholds for bottum-up control of macroalgal blooms on
coral reefs in Jamaica and southeast Florida. Limnology and Oceanography 24: 11191131.
LaPointe, B.E. & M.W. Clark, 1992, Nutrient inouts from the watershed and coastal
eutrophication in the Florida Keys. Estuaries 15: 465-476.
LaPointe, B.E. & W.R. Matzie, 1996, Effects of stormwater nutrient discharges on
eutrophication processes in nearshore waters of the Florida Keys. Estuaries 19: 422-435.
Lipp, E.K. et al., 2002, Preliminary evidence for human fecal contamination in corals of
the Florida Keys, USA. Marine Pollution Bulletin 44: 666-670.
Lyzenga, D.R., 1981, Remote sensing of bottom reflectance and water attenuation
parameters in shallow water using aircraft and Landsat data. International Journal of
Remote Sensing 1: 71-82.
Maritorena, S., 1996, Remote sensing of the water attenuation in coral reefs: a case study
in French Polynesia. International Journal of Remote Sensing 17: 155-166.
Maritorena, S., A. Morel & B. Gentili, 1994, Diffuse reflectance of oceanic shallow
waters; influence of the water depth and bottom albedo. Limnology and Oceanography,
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Mumby, P.J., C.D. Clark, E.P. Green & A.J. Edwards, 1998, Benefits of water column
correction and centextual editing for mapping coral reefs. International Journal of Remote
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Ogden, J.C. et al., 1994, A long-term interdisciplinary study of the Florida Keys
Seascape. Bulletin of Marine Science, 54: 1059-1071.
Palandro, D., M. Lybolt, C. Hu, S. Andrefouet & F.E. Muller-Karger, (submitted),
Changes in Benthic Coral Communities in the Florida Keys: A Comparison Between In
Situ and Remote Sensing Datasets (1996-2002). Coral Reefs submitted.
Pattterson, K.L. et al., 2002, The etiology of white pox, a lethal disease of the Caribbean
elkhorn coral, Acropora palmata. Proceedings of the National Academy of Sciences of
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Porter, J.W. et al., 2002, Detection of coral reef change by the Florida Keys Coral Reef
Monitoring Project. In: J.W. and K.G. Porter (Eds.), The Everglades, Florida Bay, and
Coral Reefs of the Florida Keys. CRC Press, Boca Raton (FL): 749-769.
Smith, N.P., 1998, Tidal and long-term exchanges through channels in the middle and
upper Florida Keys. Bulletin of Marine Science, 62: 199-211.
Smith, N.P., 1994, Long-term Gulf-to-Atlantic transport through tidal channels in the
Florida Keys. Bulletin of Marine Science, 54: 602-609.
Szmant, A.M. and Forrester, A., 1996. Water column and sediment nitrogen and
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FIGURES
Figure 1: Landsat image of the Florida Keys displaying the location of the 27 sites
(circles) characterized for the diffuse attenuation coefficient (K).
Figure 2: Graphs displaying K-values, by location, derived from Landsat bands one
(blue) and two (green) for each of the three images studied.
81° 40’ W
81° 20’ W
81° W
82° 40’ W
82° 20’ W
N
25° N
24° 40’ N
0
20km
Figure 1.
[Annotate Key Largo, Keywest (not shown?) and whatever places mentioned in the text]
0.3
February K-values
0.25
0.2
0.15
K 0.1
0.05
0
-0.05
-0.1
1
3
5
7
9
11
13
15
17
19
21
0.3
23
25
27
May K-values
0.25
0.2
0.15
K
0.1
0.05
0
-0.05
-0.1
1
3
5
7
9
11
13
15
17
19
21
0.3
0.25
0.2
0.15
K
0.1
0.05
0
-0.05
-0.1
23
25
27
July K-values
1
3
5
7
9
11
Upper
Landsat band 1
Figure 2.
13
15
17
Middle
LOCATION
19
21
23
Lower
Landsat band 2
25
27