Page 1 Ocean Color Remote Sensing from Space Lecture in Remote Sensing at 7 May 2007 Astrid Bracher Room NW1 - U3215 Tel

Page 2 Basic principles of Ocean Color Remote Sensing (Doerffer et al. 2006)

Page 3 Absorption, Scattering and Beam Attenuation

Page 4 Spectral color and wavelength in Nanometer [nm= m -9 ] Attenuation by water and water constituents awas = absorption by water kwas = attenuation by water ksus = attenuation by suspended particles kwas = attenuation by phytoplankton kgelb = attenuation by yellow substance (dissolved organic matter) (Modelled with SIRTRAM by Doerffer 1992)

Page 5 Marine Phytoplankton Falkowski et al. Science, 2004 Global Contribution: Plant biomass 1-2% Primary production ~50% Functional Groups: -Build-up of biominerals (e.g. silicate by diatoms) - Calcifiers (e.g. Emiliania) - Cloud formation (via DMSP: Phaeocystis) - Nitrogen-Fixation (blue algae) - Toxic Algae

Page 6 Climate Change and Phytoplankton Composition: Bering Sea: extraordinarily warm summer 1997– the first time ever bloom of calcifying algae True Color from SeaWiFS (Napp et al. 2001)

Page Spec. phytoplankton absorption [m 2 /mg] MERIS SeaWiFS ---- low chl a, mainly Picoplankton ---- diatom bloom ---- Phaeocystis bloom Bracher & Tilzer wavelength [nm] Phytoplankton Absorb light by pigments (chlorophylls, carotenoids,...) Pigments are excited Excitation energy used in photosynthesis to make O 2 & organic compounds Basis for marine ecosystem and carbon cycle Phytoplankton absorption variable among species and location! photoacclimation and community composit.

Page 8 Downwelling irradiance attenuation coefficient Green: 5 mg/l Total substanc Green: 5 mg/l Total substa m -1 Green: 5 mg/l Total Suspended Matter (TSM), 5 µg/l chl a (phytoplankton), yellow substance ag440= 0.4 m -1 Blue: 0.1 µg/m -3 chl a (Doerffer et al. 2006)

Page 9 Signal depth Coastal waters (= case-2) Blue-green: 5 mg/l TSM, 5 µg/l chl a, ag440= 0.4 m -1 Open Ocean (= case-1) Blue: 0.1 µg/m -3 chl a z90 = 1/k (Doerffer et al. 2006)

Page 10 Absorption spectra in case 1 waters for water, yellow substance and phytoplankton In case-1 waters: attenuation dominated by phytoplankton, ratio of yellow substance conc. to chl a is constant while it is not for case-2 (=coastal) waters Empirical Model for phytoplankton biomass from remote sensing for case-1 waters

Page 11 Comparison of ratio of Reflectances (at 445 nm to 555 nm) to phytoplankton biomass (chl a) measurements Morel & Antoine MERIS ATBD

Page 12 MERIS – Median Resolution Imaging Spectrometer- Ocean Color Sensor Other Ocean Color Sensors: Coastal-Zone-Color-Scanner ( ), SeaWiFS (1997-), Modis (1999- on TERRA, on AQUA) MOS, POLDER, GLI, OCTS

Page 13 MERIS – Median Resolution Imaging Spectrometer- Ocean Color Sensor

Page 14 MERIS true color picture: A large aquamarine- coloured plankton bloom streches across the length of Ireland in the North Atlantic Ocean

Page 15 MERIS global chl a (phytoplankton biomass) distribution from algorithm using Rrs[443] / Rrs[560]

Page 16 Water leaving Radiance Reflectance Spectra of North Sea water with first 10 MERIS spectral bands Chl a from ocean color: Ratio of reflectance at certain wavebands (blue /green) But: Differences in phyto- plankton absorption photoacclimation + species composition Requires higher spectral resolution!

Page 17 Global Models on Marine Primary production Function of fixed organic carbon to biomass (chl a) & light Use data of ocean color satellite sensors (MERIS, MODIS, SeaWIFS,…) on chl a, surface water reflectance and light penetration depth Rarely consider spectral dependency of photosynthesis primary production modeling: Directly affected: light actually absorbed Indirectly: influences chl a retrieval from ocean color data Limited data base on specific phytoplankton absorption (in situ measurements) Phytoplankton absorption and major phytoplankton groups from space using highly spectrally resolved remote sensing data!

Page 18 (Scanning Imaging Absorption Spectrometer for Atmospheric Cartography) UV-VIS-NIR spectrometer on Envisat since 2002 in orbit 8 high resolution and 6 polarization channels measures transmitted, reflected and scattered sunlight wavelength coverage 220 – 2380 nm at nm resolution global information within 6 days, >30 km X >30 km resolution Delivers information on: -distributions of geophysical parameters in atmosphere from km ozone depletion, greenhouse effect, air pollution, climate change - but now on ocean optics: phytoplankton, vibrational raman scattering SCIAMACHY

Page 19 Processing of SCIAMACHY nadir spectra with DOAS DOAS = Differential Optical Absorption Spectroscopy (Perner and Platt, 1979) Uses differential absorption signal of the molecular absorber in the earthshine spectrum wrt. extraterrestrial solar irradiance Ratio Earthshine / Solar irradiance removes instrumental and Fraunhofer features Input: Absorption cross section for each molecular species in spectral interval Least squares fit of DOAS equation based on Beer`s law to observations Separation of high- and low frequency absorption features by low order polynomial Output: Slant column density SCD = number of molecules along average photon path

Page 20 Phytoplankton absorption from hyperspectral sensor SCIAMACHY Differential phytoplankton absorption at high chl a Clear differential signal from phytoplankton pigments! --- reference spectrum from in-situ meas. of mixed population (by Bracher & Tilzer 2001 ) __ DOAS-fit with SCIAMACHY meas. DOAS fit from 430 to 500 nm - included in analysis: O 3, NO 2, H 2 O (both vapor and liquid), Ring and differential phytoplankton absorption spectrum measured in situ

Page 21 DOAS fit of phytoplankton pigment absorption in vivo Phytoplankton Absorption Specific In vivo reference spectra yield much better fits than chl a Clear differential signal from phytoplankton pigments! Chl a Standard Absorption (mixed population, dominated by <20µm) from Bracher and Tilzer 2001 from 430 to 500 nm

Page 22 Global Phytoplankton Absorption Fits from SCIAMACHY Compared to MODIS chl a level-3 product SCIAMACHY DOAS-Fits of phytoplankton absorption S chl (Fit-Factor) Monthly Average: 15.Oct-14.Nov 2005 Strong correlation to ocean color chl a ! S chl = slant column of specific phytoplankton absorption Bracher et al. 2006

Page 23 Vibrational Raman Scattering (VRS) from SCIAMACHY Vountas et al. submitted to Ocean Sciences High sensititvity of VRS fit at low chl a Averages over July model __ SCIA meas. VRS always accompanied by an elastic scattering process Proxy for light penetration depth (δ) ( transformation to λ of phytoplankton absorption fit)

Page 24 Phytoplankton biomass from Ocean color SCIAMACHY chl a conc. c First SCIAMACHY phytoplankton biomass determined with DOAS (whole spectrum fit) shows good visual agreement to MERIS algal-1 chl a product Vountas et al. submitted from DOAS-Fits of phytopl. absorption (mixed community) and VRS: C = S chl / δ

Page 25 Ocean Color Satellite Information