ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii1/24

Some Peculiarities of Case 1 Waters Optical Properties

in the Northwestern Mediterranean Sea

(“BOUSSOLE” site, 43°22 ’N; 7°54 ’E)David ANTOINE

André MOREL, Hervé CLAUSTRE

Laboratoire d’Océanographie de Villefranche, 06238 Villefranche sur mer, France

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii2/24

The “problem” we aim at

Bricaud et al. (1998) Morel and Maritorena (2001)

Natural variability of Case 1 waters optical properties is known

Can we explain this variability in terms of the AOPs versus IOPs relationships ?

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Our approach to understand the natural variability of the

AOPs and IOPs

Combination of

- A time series with ~monthly resolution (ship operations)

- A high-frequency (i.e., 15 min.) permanent sampling near the surface with a new type of optical mooring

- Collection of a full set of IOPs, needed to understand the AOPs and their “anomalies” with respect to standard, global, models (--> closure)

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The site where we collect data :

“BOUSSOLE” site & program“Buoy for the acquisition of a long-term (bio)optical

series”

Monthly cruises (started July 2001) + a new type of optical buoy (since Sept. 2003)

Marine optics, Bio-optics, Ocean color calibration / validation program (MERIS, SeaWiFS, POLDER)

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Site characteristics(oligotrophic to eutrophic)

Winter, maximum of the water mixingChl up to ~2-3 mg m-3

mixed layer down to 200 meters

Spring, establishment of the deep chlorophyll maximum around 50 metersChl ~ 0.3 mg m-3

Summer, maximum of the stratification. DCM is maximum, with surface Chl ~ 0.05 mg m-3 (up to 1 in the DCM)

Fall, erosion of the thermocline, the DCM progressively disappearsChl ~ 0.5 mg m-3

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SeaWiFS chlorophyll 2001-2004

Feb March Apr May June Jul Sept Oct Nov Dec

2001

2002

2003

2004

SeaWiFS/SIMBIOS diagnostic data sets(http://seawifs.gsfc.nasa.gov/cgi/seawifs_region_extracts.pl?TYP=ocean)

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii7/24

“Anomalies” we already know

in situ TchlaRegional algorithm(Bricaud et al., 2001)

SeaWiFS OC4v4, rep. #4

Chlorophyll time series at the DYFAMED site

Start of the optics time series

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SeaWiFS OC4v4, rep. #4

Regional algorithm(Bricaud et al., 2001)

May 1999 SeaWiFS composite

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii9/24

Already identifiedanomalous Blue-to-green

ratios Claustre et al., 2002 (Geophys. Res. Letters)(PROSOPE cruise)

And others :(Gitelson et al., 1996 D’Ortenzio et al., 2001, 2002 Corsini et al., 2002)

Possible cause : deposition of small dust particles coming from Sahara, reinforcing absorption in the blue and scattering in the green

Morel & Maritorena (2001).

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii10/24

Which AOPs/IOPs we are looking at

“Blue-to-green reflectance ratios” R(1)/R(2)

Irradiance reflectances at the “0-” level, and normalized for a sun at zenith :

R() = Eu(1)/Ed(2)

Diffuse attenuation coefficients for the downwelling irradiance :

Kd() = -d[ln(Ed()] / dZ

AOPs (using a Satlantic 13-wavelengths “SPMR” radiometer)

IOPs (Wetlabs’ AC9 & particulate absorption on filtered samples)

Total absorption, scattering, attenuation

Particulate absorption (total, phytoplankton, detritus)

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii11/24

Time series of the blue-to-green ratio

R(443)/R(560)Model Data

in situ Chl

“reconstructed” Chl

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Time series of the blue-to-green ratio

R(490)/R(560)

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Time series of the blue-to-green ratio

R(510)/R(560)

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Time series of the Kd’s

443 nm

490 nm

412 nm

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Time series of the Kd’s (continued)510 nm 560 nm

670 nm

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Time series of the reflectances443 nm

490 nm

412 nm

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Time series of the reflectances (cont’d)560 nm510 nm

670 nm

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Time series of particulate absorption coefficients

Summer : absorption by detritus is at least equal to, and actually greater

than, phytoplankton absorption

Winter : phytoplankton absorption dominates

Year 2003 only

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Particulate absorption spectra

ap is decomposed into a and ad following Bricaud and Stramski, 1990

ap

a

ad

February

JulyMay

March

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii20/24

Spectra of the total (minus water)

absorption and scattering coefficient

April

JulyMay

Februaryc()

b()

a()

ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii21/24

Particulate scattering coefficient at 550 nm as a function of Chl

0,01

0,10

1,00

10,00

0,010 0,100 1,000 10,000

Chl (mg m-3)

b(5

50

) (m

-1)

from AC9 measurements

(+/- a factor of 2)

« Fresh bloom » ?A lot of detritusin summer

Loisel & Morel (1998)

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In short...

- “Lower-than-expected” reflectances in the blue are due to high absorption : several causes are possibly intermingled, such as Saharan dust, detritus, CDOM...- “Greater-than-expected” reflectances in the green : Saharan dust, detritus, others (coccolithophorids) ?

The “summer anomaly”

The “winter anomaly”- “Greater-than-expected” reflectances in the blue might be due to a lower absorption : “fresh phytoplankton bloom” with a lower proportion of detritus

- “Lower-than-expected” reflectances in the green : could be due to large proportion of big cells

- It is not a permanent feature

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General conclusions, perspectives- The preliminary analysis of the AOPs and IOPs time series has

confirmed some trends already observed in the Med. Sea (although not permanent), and revealed others

- Understanding of the causes requires further analysis of the data, and may require as well some additional parameters (in particular CDOM absorption, backscattering coefficient, AOPs in the UV), as well as inversion of the AOPs into IOPs (e.g., Loisel and Stramski 2001)

- Exploitation of the buoy time series (AOPs, c(660), bb(443 & 550)) should help in this respect

- Anomalies in the AOPs can be explained in terms of the IOPs, yet fundamental causes remain to be ascertained.

- Any index in the reflectance spectra that may help in a better interpretation of the remotely-sensed observations ?

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AcknowledgementsAlec SCOTT, Chief engineer for the project,

monthly cruises, AOPs collection, data processing

Bernard GENTILI, Data processing codesDavy MERIEN

Joséphine RAS, HPLC and ap measurements

Dominique TAILLIEZ, CTD + IOPs, monthly cruises

R/V Téthys-II Captains & crews

Thank you for your attention