Dissolved Organic Carbon along the Louisiana coast from MODIS and MERIS satellite data
Nazanin Chaichi Tehrani1, Eurico J. D’Sa 1
1School of the Coast and Environment, Department of Oceanography and Coastal Sciences ,Coastal Studies Institute
Louisiana State University, Baton Rouge, LA 70803, U.S.A. [email protected] Abstract 5.
Results
Colored dissolved organic matter (CDOM), the optically active fraction of dissolved organic
matter can be used as an alternative proxy to trace Dissolved organic carbon (DOC).
Satellite-derived CDOM, and in situ CDOM and DOC concentrations were used to derive
DOC from MODIS and MERIS. To develop a CDOM retrieval algorithm, empirical
relationships between CDOM absorption coefficient at 412 nm (aCDOM(412)) and remote
sensing reflectance ratios (Rrs(488)/Rrs(555)) for MODIS and (Rrs(510)/Rrs(560)) for MERIS
were established. Further, empirical algorithms were developed to estimate DOC
concentration using the relationship between in situ aCDOM(412) and DOC, as well as using
the newly developed CDOM empirical algorithms. Algorithms were evaluated by comparing
MODIS/MERIS-derived DOC with in situ DOC measurements. These preliminary results
indicate that newly developed DOC retrieval algorithms performed reasonably well for the
Louisiana coast. However, further evaluation of these algorithms are ongoing.
1. Introduction
5.1. CDOM-DOC relationship:
To develop an empirical algorithm to derive DOC concentration remotely and using CDOM’s optical
signature as a proxy for DOC, the main condition is conservative behavior between DOC and CDOM. So,
DOC was plotted against CDOM to investigate conservative behavior of DOC. Figure (2-A) and (2-B)
show the relationship between DOC and CDOM in spring-winter and summer, respectively.
5.2. CDOM algorithm development
In order to develop an empirical algorithm to retrieve surface aCDOM(412), The Rrs band ratios Rrs(488)/
Rrs(555) for MODIS-Aqua and Rrs(510)/Rrs(560) for MERIS-Envisat were regressed against coincident in
situ aCDOM(412) (Figure (3-A) and (3-B)). Figure (4-A) and (4-B) show surface CDOM distribution map
from MODIS and MERIS for Feb 6, 2007, respectively. The validation matchup comparison between in
situ and satellite-derived aCDOM(412) illustrates estimation of aCDOM(412) with Bias = 0.093, RMSE= 0.176,
and R2 = 0.4 for MODIS(Figure 4-C), and Bias= 0.089, RMSE= 0.3, R2= 0.42 for MERIS (Figure 4-D).
800
(A)
y = 137.22x+124.20
R2=0.90
400
400
200
200
0
0
1
2
3
4
2.5
(A)
2.0
1.5
1.0
y=0.4729+1.4831*exp(-4.6419*x)
R2=0.679
0.5
0.5
0.0
(D)
1.0
1.5
3.0
0.0
1.0
2.0
2.0
2.5
3.0
May/Jul/Aug 2005
Apr 2007
Apr/Jun 2008
Aug 2009
best-fit line
2.5
(B)
2.0
B
C
aCDOM(412) MODIS
0.472 1.48 4.64 0.67
aCDOM(412) MERIS
510/560
0.617 0.66 2.47 0.62
Products
DOC_MODIS_summer
Bias
2.420
RMSE
26.69
SI
0.15
R2
0.52
Slope
0.61
Intercept
66.18
N
25
DOC_MERIS_summer
5.300
30.02
0.17
0.58
0.39
109.15
19
DOC_MODIS_spring-winter -13.67
32.29
0.22
0.40
0.43
56.96
25
DOC_MERIS_spring-winter
44.22
0.21
0.72
0.99
-2.39
7
-3.500
(A)
(B)
1.5
y= 0.6176+0.6606*exp(-2.477x)
R2=0.624
1.0
0.5
0.0
3.0
0.0
In situ aCDOM (412) (m-1)
(E)
A
1.0
2.0
3.0
In situ aCDOM(412) (m-1)
Figure 3. (A) MODIS-derived Rrs band ratio (488nm and 555 nm) plotted against in situ
surface aCDOM (412) (B) MERIS-derived Rrs band ratio ( 510nm and 560nm) plotted against
in situ surface aCDOM (412).
(A)
(B)
Figure 6. (A) MODIS-derived surface DOC (µmol C L-1) concentration for Feb 6, 2007, (B) MERIS-derived
surface DOC concentration (µmol C L-1) for the same date.
Figure .1 (A). Map of study area in the northern Gulf of Mexico(A). Location of stations in (B)
March, May, July and August 2005, (C) March, April, May, July 2007, (D) Feb, April, June
2008, (E) August 2009.
6. Conclusion
3. Data
A conservative behavior of CDOM and DOC was obtained for both spring-winter and summer seasons. The
high correlation between CDOM and DOC shows that the distribution of DOC was highly influenced by physical
mixing between two-end members. The conservative behavior enables us to relate DOC to remote sensing
reflectance. The seasonal relationships between aCDOM(412) and DOC were combined with the aCDOM(412)-Rrs
ratio to construct DOC seasonal empirical algorithms. Then satellite-derived DOC values were correlated against
in situ DOC values to test algorithms’ performance.
3.1. Satellite remote sensing
3.2. Field Data
2.0
1.0
(C)
(D)
0.8
0.6
0.4
Bias= 0.093
RMSE=0.17
SI=0.45
2
R = 0.4
n=18
0.2
1.5
7. Acknowledgments: This work was supported by a NASA grant NNX09AR7OG. Authors would like to
thank C. Osburn (UNC), T. Bianchi (Texas A&M) and B. Schaeffer (EPA) for the data used in this study
1.0
8. References
Bias=0.089
RMSE=0.3
SI=0.16
R2=0.42
n=17
0.5
0.0
0.0
0.0
0.2
0.4
0.6
0.8
In situ aCDOM(412) (m-1)
Field-measured data containing biogeochemical (CDOM absorption coefficient, DOC
concentration) properties of water were obtained from the study area during 17 oceanographic
cruises in 2005 and 2007 through 2009.
Estimated aCDOM(412) (m-1)
Estimated aCDOM (412) (m-1)
Satellite remote sensing provides repeated and synoptic coverage of coastal and oceanic waters
to monitor and analyze coastal processes. In this study MODIS/Aqua Level 1A LAC (~ 1 km at
nadir, daily temporal resolution) were obtained from the NASA’s Ocean Color website and
processed to Level 2 (L2) to retrieve Rrs bands at 488nm and 555nm using SeaDAS 6.0
software package. In addition, to develop an empirical algorithm to retrieve CDOM and DOC
from MERIS/ENVISAT, Level 1 reduced resolution (RR) data (with a spatial resolution of ~1.2
km and daily temporal resolution) were obtained from European Space Agency (ESA) and
processed to Level 2 using SeaDAS 6.0 software package.
1.0
R2
Rrs Band
Ratio
488/555
Algorithms Evaluation
aCDOM(412) (m-1)
Mar/May/jun/jul2005
Apr/May/Auh 2007
Apr 2008
Aug 2009
best-fit line
Satellite
Figure 5. The procedure chart to develop DOC empirical
algorithm.
Table 2. Summary of error statistics obtained from validation matchup comparisons surface DOC concentration derived from
MODIS and MERIS for the summer and spring-winter seasons.
0.0
Satellite (Rrs510/Rrs560)
(A)
3.0
Product
MODIS Data
(B)
Figure 2. Relationship between (A) aCDOM (412) and DOC in spring-winter,
(B) aCDOM (412) and DOC in summer.
Satellite (Rrs488/Rrs555)
The study area is located in the northern Gulf of Mexico on the Texas-Louisiana shelf,
covering the region from latitude 28.00° to 30.5° N and longitude 88.0° to 93.00° W. It is
reported that the Mississippi River discharges 3.1×10-3 Pg of DOC into the Gulf of Mexico
annually which accounts for 1.2% of the total global input of DOC from rivers to the
ocean. Figure 1 shows our study area, our sampling locations for 2005, 2007, 2008, and
2009.
(C)
(B)
Table 1. Summary of fitted coefficients for the
regional MODIS and MERIS empirical algorithm
for surface DOC concentration retrieval.
MERIS Data
0
5
Seasonal DOC-CDOM
Equations
DOC=127.027 ln [(Rrs ratio-A)/B])/(-C)+77 Spring-Winter DOC=137.22 ln[(Rrs ratio-A)/B])/(-C)+124.20 Summer
600
aCDOM(412) (m-1)
2. Study Area
CDOM-Rrs Equations
for MODIS and MERIS
800
y = 127.027x+77.97
R2=0.90
600
DOC (µM)
Dissolved organic carbon (DOC) as an energy source for heterotrophic bacteria and as a
pool of carbon on the time scale of ocean circulation plays a critical role in the ocean
carbon cycle and in the biological pump. As in situ measurement and analysis of DOC is
time-consuming and expensive ocean color sensors provides an unprecedented tool with
synoptic and repeated coverage. However, DOC cannot be sensed by satellites directly, but
CDOM, the colored fraction of DOC, can be measured remotely and utilized as an
inexpensive intermediary to investigate the carbon cycle and to estimate the standing stock
of DOC in aquatic environments. Therefore a satellite based algorithm is first required to
remotely estimate CDOM; then a conservative relationship between CDOM and DOC is
essential for relating satellite-derive CDOM to DOC.
5.3. DOC Empirical Algorithm
The DOC retrieval algorithms for both MODIS and MERIS were obtained by combining the aCDOM(412)-Rrs
relationships with seasonal aCDOM(412)-DOC relationships, and then the seasonal DOC-Rrs relationships for the springwinter and the summer seasons were constructed , see Table 1.
To evaluate DOC algorithms performance, MODIS/MERIS-derived DOC was compared with in situ measured
surface DOC concentration, see Table 2. Figure (6-A) and (6-B) show surface DOC concentration from MODIS and
MERIS, respectively, for Feb 6, 2007.
0.0
0.5
1.0
1.5
2.0
In situ aCDOM(412) (m-1)
Figure 4. CDOM absorption map (aCDOM(412)) for Feb 6, 2007 using (A) the MODIS algorithm,
(B) the MERIS algorithm for Feb 6, 2007; scatter plots between (C) MODIS-derived aCDOM (412)
and in situ surface aCDOM(412), and (D) MERIS-derived aCDOM(412) and in situ surface aCDOM(412).
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