1

CHAPTER 12:CHAPTER 12:Remote Sensing ofRemote Sensing of

WaterWaterWaterWaterREFERENCE: Remote Sensing REFERENCE: Remote Sensing of the Environment of the Environment John R. Jensen (2007)John R. Jensen (2007)Second EditionSecond EditionPearson Prentice HallPearson Prentice Hall

Why we study Why we study y yy ythe waterthe water

with remote with remote sensing?sensing?

2

THE BLUE PLANETTHE BLUE PLANET74% of the Earth’s surface is water

WATER RESERVOIRSWATER RESERVOIRS

3

PROCESSES PROCESSES AFECTING THE AFECTING THE

REMOTE REMOTE SIGNALSIGNAL

4

DIFFERENT LAYERSDIFFERENT LAYERS

Total radiance, (Lt) recorded by a remote sensing system over water is a function of the electromagnetic energythe electromagnetic energy received from:

Lp = atmospheric pathradiance

Ls = free-surface layer reflectancereflectance

Lv = subsurface volumetric reflectance

Lb = bottom reflectance

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The total radiance, (Lt) recorded by a remote sensing system over a waterbody is a function of the electromagnetic energy from four sources:

Lt = Lp + Ls + Lv + Lb

L i h h di d d b l i f h d lli l ( ) d

Water Surface, Subsurface Volumetric, and Bottom Radiance

• Lp is the the radiance recorded by a sensor resulting from the downwelling solar (Esun) and sky (Esky) radiation. This is unwanted path radiance that never reaches the water.

• Ls is the radiance that reaches the air-water interface (free-surface layer or boundary layer) but only penetrates it a millimeter or so and is then reflected from the water surface. This reflected energy contains spectral information about the near-surface characteristics of the water.

L i th di th t t t th i t i t f i t t ith th i /i i• Lv is the radiance that penetrates the air-water interface, interacts with the organic/inorganic constituents in the water, and then exits the water column without encountering the bottom. It is called subsurface volumetric radiance and provides information about the internal bulk characteristics of the water column.

• Lb is the radiance that reaches the bottom of the waterbody, is reflected from it and propagates back through the water column, and then exits the water column. This radiance is of value if we want information about the bottom (e.g., depth, color).

BRIGHTNESSBRIGHTNESSIS CHANGEDIS CHANGEDTO TO LLt t AND AND RRrsrs

6

WATER SURFACE CONDITIONSWATER SURFACE CONDITIONSTHAT AFFECT THAT AFFECT LLSS

Examples of Absorption of Near-Infrared Radiant Flux by Water and Sunglint

Black and white infrared photograph of water

bodies in Florida

Black and white infrared photograph with

sunglint

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CHANGESCHANGESIN DEPTHIN DEPTHAFFECTAFFECTAFFECTAFFECT

LLS S AND AND LLVV

BATHYMETRYBATHYMETRY

8

Th b t l th i f di i i ti

Monitoring the Surface Extent of Water Bodies

The best wavelength region for discriminatingland from pure water is in the near-infrared andmiddle-infrared from740 - 2,500 nm.

In the near- and middle-infrared regions, waterbodies appear very dark even black becausebodies appear very dark, even black, becausethey absorb almost all of the incident radiantflux, especially when the water is deep and pureand contains little suspended sediment ororganic matter.

Water Penetration

Cozumel IslandCozumel Island

SPOT Band 1 (0.5 - 0.59 mm) green

SPOT Band 2 (0.61 - 0.68 mm) red

SPOT Band 3 (0.79 - 0.89 mm) NIR

PalancarPalancar ReefReef Caribbean SeaCaribbean Sea

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What we Measure

SEPARATING THE SEPARATING THE REMOTE SIGNALREMOTE SIGNAL

Water Column Reflected RadianceWater Column Reflected Radiance

Reflected Bottom RadianceReflected Bottom Radiance

FromNEMO OverviewNemo.nrl.navy.gov

• Inherent Optical Properties• Bottom Reflectance (Albedo)

PROPERTIES PROPERTIES PROPERTIES PROPERTIES AFFECTING THE AFFECTING THE WATER LEAVING WATER LEAVING RADIANCE (LRADIANCE (L ))RADIANCE (LRADIANCE (LWW))

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When conducting water-quality studies using remotely sensed data we are

Spectral Response of Water as a Function of Organic and Inorganic Constituents - Monitoring Suspended Minerals (Turbidity), Chlorophyll, and Dissolved Organic Matter

When conducting water-quality studies using remotely sensed data, we areusually most interested in measuring the subsurface volumetric radiance, Lvexiting the water column toward the sensor. The characteristics of this radiantenergy are a function of the concentration of pure water (w), inorganic suspendedminerals (SM), organic chlorophyll a (Chl), dissolved organic material (DOM),and the total amount of absorption and scattering attenuation that takes place inthe water column due to each of these constituents, c(λ):

Lv = f [Wc(λ), SMc(λ), Chlc(λ), DOMc(λ) ]

It is useful to look at the effect that each of these constituents has on the spectralreflectance characteristics of a water column.

Absorptionin

Pure Water

Molecular water absorption dominates in the ultraviolet (<400 nm) and in the yellow through the near-infrared portion of the spectrum (>580 nm). Almost all of the incident near-infrared and middle-infrared (740 -middle-infrared (740 -2500 nm) radiant flux entering a pure water body is absorbed with negligible scattering taking place.

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Scatteringin

Pure Water

S i i hScattering in the water column is important in the violet, dark blue, and light blue portions of the spectrum (400 - 500 nm). This is the reason water appears blue to our eyes. The graph truncates theThe graph truncates the absorption data in the ultraviolet and in the yellow through near-infrared regions because the attenuation is so great.

Minerals such as silicon, aluminum, and iron oxides are found in

Spectral Response of Water as a Function of Inorganic Constituents

suspension in most natural water bodies.The particles range from fine clay particles ( 3 - 4 μm indiameter), to silt (5 - 40 μm), to fine-grain sand (41 - 130 μm),and coarse grain sand (131 - 1250 μm).Sediment comes from a variety of sources including agricultureerosion, weathering of mountainous terrain, shore erosioncaused by waves or boat traffic and volcanic eruptions (ash)caused by waves or boat traffic, and volcanic eruptions (ash).Most suspended mineral sediment is concentrated in the inlandand nearshore water bodies.Clear, deep ocean far from shore rarely contains suspendedminerals greater than 1 μm in diameter.

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AMOUNT OF TURBIDITYAMOUNT OF TURBIDITY

In situ Spectroradiometer Measurement of Clear

Water with Various Levels of Clayey and Silty Soil

Suspended Sediment

clay

1 5

2

2.5

3

3.5

4

4.5

5

Per

cent

Ref

lect

ance

50

100

150 200

250

clear water

300

1,000 mg/l

Clayey soil

Suspended Sediment Concentrations

silt

400 450 500 55 0 600 65 0 700 75 0 800 85 0 9000

0.5

1

1.5

Wavelength (nm)

clear water

8

10

12

14

ctan

ce

1,000 mg/l

600

150

200 250 300 350 400

450 500 550

a.

Silty soil Reflectance peak shifts toward longer wavelengths as more suspended sediment is added

Lodhi et al., 1997; Jensen, 2000400 450 500 55 0 600 65 0 700 75 0 800 85 0 900

0

Wavelength (nm)

2

4

6

8

Per

cent

Ref

lec

clear water

50

100

b.

13

Space Shuttle Photograph of the Suspended

Sediment Plume at the M th f thMouth of the

Mississippi River near New Orleans,

Louisiana

Mississippi River Plume-TM

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Mayaguez Bay-AOCI

Añasco River Plume-ATLAS

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Añasco River Plume-IKONOS

Culebrinas River Plume-IKONOS

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Plankton is the generic term used to describe all the living organisms (plant andanimal) present in a waterbody that cannot resist the current (unlike fish).

Spectral Response of Water as a Function Spectral Response of Water as a Function of Organic Constituents of Organic Constituents -- PlanktonPlankton

animal) present in a waterbody that cannot resist the current (unlike fish).Plankton may be subdivided further into algal plant organisms (phytoplankton),animal organisms (zoolankton), bacteria (bacterio-plankton), and lower plantforms such as algal fungi.Phytoplankton are small single-celled plants smaller than the size of a pinhead.Phytoplankton, like plants on land, are composed of substances that containcarbon.Phytoplankton sink to the ocean or water-body floor when they die. All

h l k i b di i h h h i ll i iphytoplankton in water bodies contain the photosynthetically active pigmentchlorpohyll a.There are two other phytoplankton photosynthesizing agents: carotenoids andphycobilins.Bukata et al (1995) suggest, however, that chlorphyll a is a reasonablesurrogate for the organic component of optically complex natural waters.

PHYTOPLANKTONPHYTOPLANKTONPhotosynthesisPhotosynthesis

Ocean ColorOcean Color

chloroplast material

cell wall

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INHERENT OPTICAL PROPERTIES

Pure Seawater Phytoplankton

b w10

-2m

-1

b w10

-2m

-1

bw, Morel (1974)aw, Pope and Fry (1997)

bchl,Loisel and Morel (1998)achl, Sathyendranath et al. (2001)

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Different pigments absorb at different wavelengths

1

1.5

2

2.5

3

3.5

4

Per

cent

Ref

lect

ance

clear water algae-laden

water

Percent reflectance of clear and algae-laden water based on in situ spectroradiometer measurement. Note the strong chlorophyll a absorption of blue

400 500 600 700 800 9000

0.5

1

Wavelength (nm)

20

25

nce

a.

500 mg/l

A lgae-Laden Water with Various Suspended Sediment Concentrations

Percent reflectance of algae-

light between 400 and 500 nm and strong chlorophyll aabsorption of red light at approximately 675 nm

5

10

15

Per

cent

Ref

lect

an

400 500 600 700 800 9000

Wavelength (nm)b.

0 mg/l

e ce t e ecta ce o a gaeladen water with various concentrations of suspended sediment ranging from 0 - 500 mg/l

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• Sunlight penetrates into the water column a certain photic depth (the verticaldistance from the water surface to the 1 percent subsurface irradiance level)

Spectral Response of Water as a Function of Dissolved Organic Constituents

distance from the water surface to the 1 percent subsurface irradiance level).• Phytoplankton within the photic depth of the water column consume nutrients

and convert them into organic matter via photosynthesis. This is calledprimary production.

• Zooplankton eat the phytoplankton and create organic matter.• Bacterioplankton decompose this organic matter.• All this conversion introduces dissolved organic matter (DOM) into oceanic,

nearshore, and inland water bodies.• In certain instances, there may be sufficient dissolved organic matter in the

water to reduce the penetration of light in the water column (Bukata et al.,1995).

• The decomposition of phytoplankton cells yields carbon dioxide, inorganicnitrogen, sulfur, and phosphorous compounds.

• The more productive the phytoplankton, the greater the release of dissolvedorganic matter In addition humic substances may be produced

Spectral Response of Water as a Function of Dissolved Organic Constituents

organic matter. In addition, humic substances may be produced.• These often have a yellow appearance and represent an important colorant

agent in the water column, which may need to be taken into consideration.• These dissolved humic substances are called yellow substance or Gelbstoffe

and can1) impact the absorption and scattering of light in the water column, and2) change the color of the water.

• There are sources of dissolved organic matter other than phytoplankton.• For example, the brownish-yellow color of the water in many rivers in the

northern United States is due to the high concentrations of tannin from theeastern hemlock (Tsuga canadensis) and various other species of trees andplants grown in bogs in these areas (Hoffer, 1978).

• These tannins can create problems when remote sensing inland waterbodies.

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Absorption Coefficient of CDOM at Different Stations in the Mayagüez Bay

0 8

0.9

1.0

Stat ion 1Stat ion 4Stat ion 5Stat ion 7

0.3

0.4

0.5

0.6

0.7

0.8

ag (

m-1

)

Stat ion 9Stat ion 11Stat ion 13Stat ion 15Stat ion 17Stat ion 19Stat ion 21Stat ion 23

0.0

0.1

0.2

0.3

350 400 450 500 550 600 650 700

Wavelength (nm)

Salinity vs. CDOM Absorption Coefficient (Ag 300 nm-1) Correlation During the Wet Season

5r = 0.72n = 32

2

3

4

ptio

n Co

effic

ient

at 3

00 n

m

n = 32

33.0 33.5 34.0 34.5 35.0 35.5 36.00

1Abso

rp

Salinity (ppt)

21

Salinity vs. CDOM Absorption Coefficient (Ag 300 nm-1) Correlation During the Dry Season

1.1 r = 0.2130

0.6

0.7

0.8

0.9

1.0pt

ion

Coef

ficie

nt a

t 300

nm

n = 30

34.6 34.8 35.0 35.2 35.4 35.6 35.8 36.00.3

0.4

0.5

Salinity (ppt)

Abso

rp

MAIN COMPONENTS ABSORVING MAIN COMPONENTS ABSORVING LIGHT IN THE WATER COLUMNLIGHT IN THE WATER COLUMN

22

TYPES OF WATERSTYPES OF WATERSBASED ON OPTICAL PROPERTIESBASED ON OPTICAL PROPERTIES

Oceanic Waters

Coastal Waters

MEASURING MEASURING THE WATER THE WATER THE WATER THE WATER

QUALITY WITH QUALITY WITH REMOTE REMOTE REMOTE REMOTE SENSORSSENSORS

23

Secchi Disk

Used to measure the clarity (related withclarity (related with suspended particles) in water bodies

BIOBIO--OPTICAL PACKAGEOPTICAL PACKAGE

PumpPump

DataDataLoggerLogger

CTDCTD

ACAC--99HSHS--66

OCROCR--200200(Ed)(Ed)

pp

FluorometerFluorometer OCROCR--200200(Lu)(Lu)

Battery PackBattery Pack

24

WATER COLUMN VARIABILITYWATER COLUMN VARIABILITY

SURFACE SPATIAL VARIABILITYSURFACE SPATIAL VARIABILITY

Salinity Fluorescence

Backscattering@589

Absorption@412

25

OCEANOGRAPHIC BUOYSOCEANOGRAPHIC BUOYS

ONDULATING UNDERWATER VEHICLES

26

AUTONOMOUS UNDERWATER VEHICLES

REMOTE SENSING REFLECTANCEREMOTE SENSING REFLECTANCE

GER

E

SUN

LoLoE d

L water

L sky

AbsorptionScattering

AbsorptionScattering L o

Therefore,

R = L0− fL

s

LsLs

Where, f=Fresnel Number (Percent of radiation reflected back into the atmosphere). At 45o angle is 0.028.

Rrs =

Ed

EdEd

GERGER--15001500

27

Three typical spectral shapes of remote sensing reflectance curves found in Mayagüez Bay.

Remote Sensing Reflectance (Rrs) Remote Sensing Reflectance (Rrs) during the Dry and Rainy Seasonsduring the Dry and Rainy Seasons

Low Sediment input by Rivers High Sediment input by RiversLow Sediment input by Rivers High Sediment input by RiversRemote Sensing Reflectance (Rrs) for April 06

0.006

0.008

0.01

0.012

0.014

0.016

0.018

Rrs

(sr-1

)

A1

A2

AAA1

AAA2

Y1

High Chl-a Signal

Remote Sensing Reflectance (Rrs) During August 05

0.03

0.04

0.05

0.06

0.07

Rrs

(sr

-1)

A1

A2

AAA1

AAA2

Y1

Y2

High Sediment load

0

0.002

0.004

400 500 600 700

Waveleght (nm)

Y2

G2

0

0.01

0.02

400 500 600 700Waveleght (nm)

Y2

G1

G2

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OCEAN COLOROCEAN COLORWITHWITH

REMOTE REMOTE SENSINGSENSING

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Chlorophyll in Ocean Water

A remote estimate of near-surface chlorophyll concentration generallyconstitutes an estimate of near-surface biomass (or primary productivity) fordeep ocean (Case 1) water where there is little danger of CDOM andp ( ) gsuspended sediment contamination.

Numerous studies have documented a relationship between selected spectralbands and ocean chlorophyll (Chl) concentration using the equation:

Chl = x [L(λ1)/L(λ2)]y

Wh L(λ ) d L(λ ) th lli di t l t d l thWhere L(λ1) and L(λ2) are the upwelling radiances at selected wavelengthsrecorded by the remote sensing system and x and y are empirically derivedconstants.

For example, the most important SeaWiFS algorithms involve the use of bandratios of 443/355 nm and 490/555 nm.

Global Chlorophyll a (g/m3) Derived from SeaWiFS Imagery Obtained from September 3, 1997 through December 31, 1997

30

True-color SeaWiFS image of the Eastern U.S. on September 30, 1997

Chlorophyll a distribution on September 30, 1997

derived from SeaWiFS data

31

1. Coastal Zone Color Scanner (CZCS)

SPACEBORNEOCEAN COLOR INSTRUMENTS

2. Modular Optoelectronic Scanner (MOS)

3. Ocean Color and Temperature Scanner (OCTS)

4. Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

5. Ocean Color Imager (OCI)

6 Moderate Resolution Imaging Spectroradiometer6. Moderate Resolution Imaging Spectroradiometer

(MODIS)

7. Global Imager (GLI)

8. Medium Resolution Imaging Spectrometer (MERIS)

32

Instrument SatelliteDates of

OperationSpatial

Resolution Swath Width

CZCS Nimbus-7 10/24/78-6/22/86 825 m 1556 km

OCEAN COLOR INSTRUMENTS

MOS IRS P3 3/21/96-Present 520 m 200 km

MOS Priroda 4/23/96-Present 650 m 85 km

OCTS ADEOS 8/17/96-7/1/97 700 m 1400 km

SeaWiFS Orbview-2 8/1/97-Present 1100 m 2800 km

OCI ROCSAT-1 1/99-Present 800 m 690 km

MODIS Terra/Aqua 12/18/99-Present 1000 m 2330 km

GLI ADEOS-2 scheduled 1000 m 1600 km

MERIS ENVISAT-1 scheduled 1200 m 1450 km

Comparison of Wavelength & Bandwidthfor Spaceborne Ocean Color Instruments

33

COASTAL ZONE COLOR SCANNER (CZCS)COASTAL ZONE COLOR SCANNER (CZCS)

34

SCANNING GEOMETRY OF THE CZCSSCANNING GEOMETRY OF THE CZCS

35

CZCS BANDSCZCS BANDS

36

37

38

PROCESSING ALGORITHMSBased on Gordon et al. (1980) and Gordon et al. (1983)

The algorithm used for estimating the pigments content of the ocean from CZCS measurements involves the use of radiance ratios. The general form of the equation isthe equation is

log(C) = a + b*log[Lw(1)/Lw(2)]Where

C is the pigment concentration (mg/m^3) a,b are regression coefficients Lw(1),Lw(2) are the atmospherically corrected radiances for a pair of CZCS h lCZCS channels

For CZCS pigments processing, these channel pairs are

(443, 550 nm), for C < 1.5 mg/m^3 (520, 550 nm), for C > 1.5 mg/m^3

Monthly Composite of CZCS During September 1979Monthly Composite of CZCS During September 1979

39

40

SeaSea--viewing Wide Fieldviewing Wide Field--ofof--view Sensorview Sensor

(SeaWiFS)(SeaWiFS)Band Wavelength (nm)

1 4121 4122 4433 4904 5105 5556 6707 7658 865

Phytoplankton ChlPhytoplankton Chl--aa

CZCS BANDS

41

SeaWiFS ALGORITHMSSeaWiFS ALGORITHMS

42

GLOBAL ESTIMATION OF PHYTOPLANKTONGLOBAL ESTIMATION OF PHYTOPLANKTONCHLOROPHYLLCHLOROPHYLL--A USING SEAWIFS DATAA USING SEAWIFS DATA

Orbview 2RECEIVING CAPABILITIESRECEIVING CAPABILITIESOF SeaWiFS AT UPRMOF SeaWiFS AT UPRM

LL--BAND ANTENNABAND ANTENNA

43

COASTAL UPWELLING IN THECOASTAL UPWELLING IN THECARIBBEAN SEACARIBBEAN SEA

AVHRRAVHRRSea Surface TemperatureSea Surface Temperature

SeaWiFSSeaWiFSChlorophyllChlorophyll--aa

44

Launched on May 4, 2002Launched on December 18, 1999

45

Orbit: 705 km, 10:30 a.m. descending node (Terra) or 1:30 p.m. ascending node (Aqua), sun-synchronous, near-polar, circular

Scan Rate: 20.3 rpm, cross track

Swath 2330 km (cross track) by 10 km (along track at nadir)

MODIS Technical Specifications

Swath Dimensions:

2330 km (cross track) by 10 km (along track at nadir)

Telescope: 17.78 cm diam. off-axis, afocal (collimated), with intermediate field stop

Size: 1.0 x 1.6 x 1.0 m

Weight: 228.7 kg

Power: 162.5 W (single orbit average)

Data Rate: 10.6 Mbps (peak daytime); 6.1 Mbps (orbital average)

Quantization: 12 bits

Spatial Resolution:

250 m (bands 1-2)500 m (bands 3-7)1000 m (bands 8-36)

Design Life: 6 years

Primary Use Band Bandwidth1 SpectralRadiance2

RequiredSNR3

Land/Cloud/AerosolsBoundaries

1 620 - 670 21.8 128

2 841 - 876 24.7 201

Land/Cloud/AerosolsProperties

3 459 - 479 35.3 243

4 545 - 565 29.0 228

MODIS BANDSMODIS BANDS

5 1230 - 1250 5.4 74

6 1628 - 1652 7.3 275

7 2105 - 2155 1.0 110

Ocean Color/Phytoplankton/Biogeochemistry

8 405 - 420 44.9 880

9 438 - 448 41.9 838

10 483 - 493 32.1 802

11 526 - 536 27.9 754

12 546 - 556 21.0 750

13 662 - 672 9.5 910

14 673 - 683 8.7 1087

15 743 - 753 10.2 586

16 862 - 877 6.2 516

AtmosphericWater Vapor

17 890 - 920 10.0 167

18 931 - 941 3.6 57

19 915 - 965 15.0 250

46

Primary Use Band Bandwidth1 SpectralRadiance2

RequiredNE[delta]T(K)4

Surface/CloudTemperature

20 3.660 - 3.840 0.45(300K) 0.05

21 3.929 - 3.989 2.38(335K) 2.00

22 3.929 - 3.989 0.67(300K) 0.07

MODIS BANDSMODIS BANDS

( )

23 4.020 - 4.080 0.79(300K) 0.07

AtmosphericTemperature

24 4.433 - 4.498 0.17(250K) 0.25

25 4.482 - 4.549 0.59(275K) 0.25

Cirrus CloudsWater Vapor

26 1.360 - 1.390 6.00 150(SNR)

27 6.535 - 6.895 1.16(240K) 0.25

28 7.175 - 7.475 2.18(250K) 0.25

Cloud Properties 29 8.400 - 8.700 9.58(300K) 0.05

Ozone 30 9.580 - 9.880 3.69(250K) 0.25

Surface/Cloud 31 10.780 - 11.280 9.55(300K) 0.05Surface/CloudTemperature

31 10.780 11.280 9.55(300K) 0.05

32 11.770 - 12.270 8.94(300K) 0.05

Cloud TopAltitude

33 13.185 - 13.485 4.52(260K) 0.25

34 13.485 - 13.785 3.76(250K) 0.25

35 13.785 - 14.085 3.11(240K) 0.25

36 14.085 - 14.385 2.08(220K) 0.35

Standard MODIS AlgorithmStandard MODIS AlgorithmOC3M MODIS OC3M MODIS ChlorChlor--aa

)403165904571753228300( 432 RRRR

)550490R

550443(RlogR

)403.1659.0457.1753.22830.0(10

103M

43

33

233

>=

−++−=

where

RRRRC MMMM

47

Standard MODIS ChlorophyllStandard MODIS Chlorophyll

SeaSeaSurfaceSurface

TemperatureTemperature(Celsius Degree)(Celsius Degree)

PhytoplanktonPhytoplanktonChlorophyll-a

(mg m^3)

48

Weekly Ocean Net Primary ProductivityWeekly Ocean Net Primary Productivity

PHYTOPLANKTON ROLE INPHYTOPLANKTON ROLE INTHE CARBON CYCLE?THE CARBON CYCLE?