REMOTE ESTIMATION OF CHLOROPHYLL-A IN COASTAL WATERS USING RED AND NEAR INFRARED SPECTRAL REGIONS

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1 REMOTE ESTIMATION OF CHLOROPHYLL-A IN COASTAL WATERS USING RED AND NEAR INFRARED SPECTRAL REGIONS Introduction A. Gilerson 1*, A. Gitelson 2, J. Zhou 1, I. Ionnou 1, nd S. Ahmed 1 1 City College of the City University of New York, 1 Convent Ave., New York, NY, 31, USA 2 Center for Advnced Lnd Mngement Informtion Technologies (CALMIT), University of Nersk-Lincoln, USA *gilerson@ccny.cuny.edu Recent dvnces in the development of the tmospheric correction models mde the retrievl of surfce reflectnce spectr of costl wters from the top of tmosphere signls more ccurte nd inspired the further development of the costl retrievl lgorithms. This includes lgorithms which employ the red nd NIR nds nd which re less sensitive to the sorption of the colored dissolved orgnic mtter (CDOM) nd scttering of minerl prticles thn trditionl lue-green rtio lgorithms. Simple Red-NIR nd rtio lgorithms re known for long time [1,2] nd successfully used for the estimtion of chlorophyll [Chl] concentrtions from reflectnce spectr in costl nd inlnd wters [3], s [Chl] is ove 5 mg/m 3 when pek round 7 nm is ecoming pronounced. Generlly this pek includes two components: i) elstic one which is due to the locl minimum of sorption coefficient s result of the confluence of the phytoplnkton nd wter sorption spectr, nd ii) chlorophyll fluorescence with the mximum ner 685 nm [2,4,5]. For [Chl]<5mg/m 3 elstic component is smll nd the mgnitude of the pek nd [Chl] re mostly relted to the fluorescence signl. For [Chl]>5mg/m 3 oth components contriute to the signl with the pek of elstic component nd comined reflectnce moving to the red with incresing [Chl]. Advnced version of Red-NIR lgorithms includes 3 nds insted of 2 [6], which ssumes etter seprtion of the phytoplnkton sorption relted to [Chl] from other sorption nd scttering components in the red nd NIR prt of the spectrum. Both 2 nd 3 nds lgorithms hve een tested in the multiple wter environments [3,7,8] ut the origins of the uncertinties were difficult to trce ecuse multiple unknown prmeters involved were not mesured directly in the experiments. Optimiztion of the nd positions ws done on the synthetic dtsets simulted using semi-nlyticl model [9], however, sensitivity of the retrievls to the CDOM sorption, concentrtion of minerls, phytoplnkton sorption spectr were not considered. In this work we tested 2 nd 3 nds lgorithms with the nds ville on MODIS nd MERIS stellite sensors using comprehensive synthetic dtsets of reflectnce spectr nd inherent opticl properties (IOP) relted to wide rnge of wter prmeters s well s new field dtset. Description of dtsets reflectnce spectr with nd without chlorophyll fluorescence were simulted using HYDROLIGHT [1] with 1 nm resolution for conditions typicl of costl wters: [Chl] = 1 mg/m 3, CDOM sorption t nm y () = 5 m -1, 5 different specific chlorophyll sorption shpes nd concentrtions of non-lgl prticles in the rnge C NAP = 1 (5 runs) nd C NAP = 1-1 mg/l (5 runs). All detils nd ssumptions used for the simultion of the wter prmeters re given in [11]. They were sed on the findings of mny uthors for IOP chrcteristics, nd were similr to the ssumptions used in the construction of the Lee IOCCG dtsets [12]. Solr input ws simulted with cloud-free sky. A set of specific sorptions ws tken ccording to Ciotti et l. [13] s sum of specific sorptions of microplnkton nd picoplnkton with different weights S f where S f =.1.5 ccording to the expression (1) * ph ( λ) = S f * pico ( λ) + (1 S These spectr re shown in Fig. 1. The chlorophyll sorption ws considered proportionl to [Chl]. These specific sorption shpes do not cover full typicl rnge of the mximum t 675 nm which cn e up to.3 m 2 /mg [7,14], so four other shpes were lso considered s shown in Fig. 1 in other 3 runs with the f ) * micro ( λ) (1) Proceedings of V Interntionl Conference Current Prolems in Optics of Nturl Wters (ONW). St Petersurg, Russi, 8-11 Septemer 9, pp

2 sme rnge of [Chl] nd CDOM sorption nd C NAP = -1 mg/l. These shpes include stndrd Cse 1 wter (1) [1], Cryptophyt H (2), Ditoms (3) nd Green lge (4) [15]. * Chl, m 2 (mg Chl ) Sf =.4 Sf =.3 Sf =.2 Sf =.1 *ph, m2(mg [Chl]) Wvelength, nm 5 7 Wvelength Fig. 1. Specific chlorophyll sorption spectr used in simultions. Fluorescence quntum yield (together with the resorption coefficient) in originl simultions ws η =.5% ut the fluorescence contriution ws clculted s the difference etween the totl nd elstic reflectnces nd ws ssumed to chnge proportionlly to η which permitted to ssess the impct of the fluorescence component. All reflectnces were simulted for the sun zenith ngle θ i = 3 nd ndir viewing. Field dtset consisted of dt collected y the CALMIT group t 85 sttions in Fremont Stte Lkes, NE USA with [Chl] = 2 mg/m3, y () =.9 3 m -1 nd C NAP elow 3mg/l [16]. Comprison of the synthetic nd field dt 2 nd NIR lgorithm R rs (78)/R rs (665), which corresponds to MERIS nds ws chosen for the comprison etween synthetic nd field dtsets. There ws no visile impct of C NAP on [Chl] estimtes using field dt, so the dt for ll sttions re shown in Fig. 2. Reflectnce rtios retrieved from simulted dt, s C NAP < 1mg/l nd the high * set (Fig. 1) ws used, mtched well the field dt for [Chl] < 3- mg/m 3. However, reltionship ws weker for [Chl] > mg/m 3 (not shown). It is oviously due to the fct tht such high * vlues re unrelistic for the rnge of [Chl] concentrtions considered in this study. The pckging effect should e tken into ccount y the grdul chnge of * shpes with incresing [Chl] ut the closest in our dtsets is shpe, dt points for which ly slightly ove the field dt with the minimum vriility for the whole [Chl] rnge. Reflectnces with this specific sorption shpe will e primrily used in the further nlysis. Sensitivity of 2 nd lgorithms to wter prmeters The sme 2-nd lgorithm R rs (78)/R rs (665) ws further nlyzed using synthetic dtsets. The impct of * through Sf fctor is shown in Fig. 3. For high Sf vlues the reltionship is very close. However, s Sf decreses, the slopes of the reltionships increse nd reltionships etween [Chl] nd the rtio ecome much weker. In Fig. 3 for ll Sf vlues, CDOM rnge ws < y ()<5m -1, though lmost no effect of vrition of CDOM concentrtion on the reltionships cn e noticed. Results of simultions for three vlues of fluorescence quntum yield η =.25,.5 nd 1. re shown in Fig. 3.. Smll differences cn e oserved only for [Chl] > mg/m 3. Proceedings of V Interntionl Conference Current Prolems in Optics of Nturl Wters (ONW). St Petersurg, Russi, 8-11 Septemer 9, pp

3 1 field high * Sf=.5 <y()<3 m -1 <min<1 mg/l Rrs(78)/Rrs(665) Fig. 2. Chlorophyll- concentrtion vs. 2-nd Red-NIR lgorithm for the synthetic nd field dt. The lgorithm output is significntly ffected y vrition in concentrtions of minerl prticles C NAP is (Fig. 4). With the increse in C NAP the rtio decreses nd the lgorithm underestimtes [Chl] concentrtion. This effect cn e reduced considerly tking into ccount tht R rs (665) vlues re quite good proxies for minerl concentrtions. By using R rs (78)(1+25 R rs (665))/Rrs(665) insted of the simple rtio the split etween estimtes for vrious C NAP cn e significntly reduced (Fig. 4) Sf=.5 Sf=.4 Sf=.3 Sf=.2 Sf= η =.25 η =.5 η = <y()<5 <min<1 3 1 <y()<5 m -1 <min<1mg/l Rrs(78)/Rrs(665) Rrs(78)/Rrs(665) Fig. 3. Chlorophyll- concentrtion vs. 2-nd Red-NIR lgorithm estimtes: impct of the specific sorption shpe () nd fluorescence quntum yield () Anlysis showed tht the use of NIR nds t 753 nm (MERIS) nd 748 nm (MODIS) in numertor of Red-NIR lgorithm does not provide ccurte [Chl] estimtes especilly for concentrtions elow mg/m 3. These lgorithms re lso less ccurte with the proximl nd especilly stellite dt due to the low vlues of reflectnce in the nds ner 75 nm nd ssocited with tht higher uncertinty in reflectnce mesurements nd tmospheric correction. On the other hnd, there is no choice for Red-NIR lgorithm with MODIS nds. It proly cn e used only for the detection of phytoplnkton looms. Proceedings of V Interntionl Conference Current Prolems in Optics of Nturl Wters (ONW). St Petersurg, Russi, 8-11 Septemer 9, pp

4 <min<1 mg/l 1<min<2 mg/l 2<min<3 mg/l 3<min<5 mg/l 5<min<1 mg/l <y()<5 m <min<1mg/l 1<min<1mg/l <y()<5 m Rrs(78)/Rrs(665) Fig. 4. Chlorophyll- concentrtion vs. () 2-nd Red-NIR lgorithm nd () its modifiction estimtes: impct of C NAP concentrtions Sensitivity of 3 nd lgorithms to wter prmeters Results for 3 nd lgorithm [Rrs 1 (665) Rrs 1 (78)] Rrs(753) were found to e very similr to 2 nd lgorithm Rrs(78)/Rrs(665) in the sensitivity to the specific sorption shpe, fluorescence quntum yield nd minerl concentrtions. However, we were not le to find the simple mechnism similr to the shown ove for 2 nd lgorithm to decrese the sensitivity of this lgorithm to chnge in concentrtion of minerl prticles. Comprison of NIR nd lue-green rtio lgorithms (1+25* Rrs(665))*Rrs(78)/Rrs(665) OC3M rtio OC3M lgorithm 3 25 OC3M rtio, min< 1mg/l OC3M lgorithm OC3M rtio, 1<min< 1mg/l 15 Sf =.1-.5 <y()<5 m -1 <min<1 mg/l Mx[Rrs(443)/Rrs(555), Rrs(489)/Rrs(555)] Mx[Rrs(443)/Rrs(555), Rrs(489)/Rrs(555)] Fig. 5. Tests of OC3M lgorithm. We lso nlyzed the performnce of the lue-green MODIS OC3M lgorithm on the sme dtsets. In Fig. 5 lue points show the known from our dtset [Chl] s function of the OC3M rtio, green points re [Chl] vlues determined using OC3M expression for [Chl] s function of this rtio. Results include ll Sf vlues Sf =.1-.5 nd <C NAP <1 mg/l. The spred is extremely high nd the dt somehow correlte with the expression only for low [Chl] vlues proly elow 1 mg/m 3. Similr results re shown in Fig. 5 ut only for. We lso dded results for 1<C NAP <1 mg/l (red points) to this figure. The spred is lower ut the lgorithm performnce is worse thn even for MODIS Rrs(748)/Rrs(667) lgorithm. Proceedings of V Interntionl Conference Current Prolems in Optics of Nturl Wters (ONW). St Petersurg, Russi, 8-11 Septemer 9, pp

5 Comprison with chlorophyll retrievl from MERIS dt Moses et l. [17] clirted nd vlidted Red-NIR models using MERIS stellite dt nd {Chl] mesured in the field. As result of clirtion following reltionships etween [Chl] nd the models were otined. The two-nd MERIS lgorithm: 1 [Chl] = 61.26( R R78 ) The three-nd MERIS lgorithm: ( R R ) R ] [Chl] = [ (2) (3) We clculted [Chl] vlues using 2- nd 3-nd lgorithms (2) nd (3) from generted reflectnce spectr nd plotted them versus ctully mesured [Chl] vlues. The results re very close (Figs. 6 nd ). Thus, Red- NIR lgorithms clirted using MERIS dt re le to estimte chlorophyll- concentrtions using generted spectr in quite wide rnge of constituent concentrtions. 9 7 Sf =.5 <y()<5 m -1 <min<1mg/l chl= Rrs78/Rrs Rrs(78)/Rrs(665) Fig. 6. Estimtes of chlorophyll- concentrtions from generted reflectnce spectr using clirted Red- NIR MERIS lgorithms: ) 2 nd lgorithm, ) 3 nd lgorithm. Conclusions 1 Sf =.5 <y()<5 m -1 <min<1mg/l Chl = (R(753)(1/R(665)-1/R(78)) [Rrs -1 (665) - Rrs -1 (78)] Rrs(753) 1. Synthetic dtsets for costl wters re compred with very consistent field dtset with [Chl] = 2 - mg/m 3 nd showed good mtch. Further improvement etween dtsets cn e chieved y tking into ccount more ccurtely the chnge of specific chlorophyll sorption with incresing [Chl] due to the pckging effect nd Red-NIR lgorithm R rs (78)/R rs (665), which uses MERIS nds, provides good estimtion of chlorophyll concentrtion for [Chl] > 5 mg/m 3 with wek dependency on vrition in CDOM concentrtions. While ovious sensitivity to minerl concentrtion ws oserved it cn e prtilly pred y the ddition of (1+ 25*Rrs(665)) term to the numertor. Sensitivity to the fluorescence quntum yield in the rnge.25 1% is noticele for [Chl]> mg/m nd lgorithms, which use in numertor MODIS nd MERIS nds round 75 nm, showed worse performnce. Errors cn e even higher on the proximl sensing nd stellite dt ecuse of the low R rs (753) vlues. 4. According to the tests on synthetic dtsets, 3-nd lgorithm [R rs 1 (665) R rs 1 (78)] R rs (753) hd similr dependency on CDOM nd minerl concentrtion nd fluorescence quntum yield s the 2-nd R rs (78)/R rs (665) lgorithm. Inclusion of R rs (753) lso hs potentil of error increse due to the low reflectnces t this nd. We hve not found t this time the mechnism to decrese the sensitivity of the Proceedings of V Interntionl Conference Current Prolems in Optics of Nturl Wters (ONW). St Petersurg, Russi, 8-11 Septemer 9, pp

6 lgorithm to vrition in minerl concentrtion. Advntges of 3-nd lgorithms should e further studied on the synthetic dt with higher [Chl] concentrtions s well s on dtsets which trce more ccurtely the chnge of specific chlorophyll sorption with [Chl] due to the pckging effect. 5. The performnce of MODIS lue-green rtio OC3M lgorithm in costl wters is poor nd it should e replced in turid productive Cse-2 wters y Red-NIR lgorithm. Exct oundries for this replcement should e further studied. 6. Reltionships of [Chl] vs R rs (78)/R rs (665) nd [Chl] vs [R rs 1 (665) R rs 1 (78)] R rs (753) estlished recently from MERIS spectr nd field dt mtch very well synthetic dt which confirms ccurcy of the field nd synthetic dt s well s stility of these reltionships. REFERENCES 1. A. A. Gitelson, G. Keydn, & V. Shishkin. Inlnd wters qulity ssessment from stellite dt in visile rnge of the spectrum. Soviet Remote Sensing, 6, 28-36, A.A. Gitelson. The pek ner 7 nm on reflectnce spectr of lge nd wter: Reltionships of its mgnitude nd position with chlorophyll concentrtion. Interntionl Journl of Remote Sensing, 13, A.A. Gitelson, G. Dll Olmo, W. Moses, D.C. Rundquist, T. Brrow, T.R. Fisher, D. Gurlin, nd J. Holz. A simple semi-nlyticl model for remote estimtion of chlorophyll- in turid wters: Vlidtion. Remote Sensing of Environment, 112: , doi:1.116/j.rse , 8 4. H. R. Gordon. Diffusive reflectnce of the ocen: the theory of its ugmenttion y chlorophyll- fluorescence t 685 nm. Applied Optics, 18, , A. Vsilkov nd O. Kopelevich. Resons for the ppernce of the mximum ner 7 nm in the rdince spectrum emitted y the ocen lyer, Ocenology, 22, , G. Dll'Olmo, A. A. Gitelson, nd D. C. Rundquist. Towrds unified pproch for remote estimtion of chlorophyll- in oth terrestril vegettion nd turid productive wters, Geophys. Res. Lett., 3(18), 1938, doi:1.129/3gl165, G. Dll Olmo nd A. A. Gitelson. Effect of io-opticl prmeter vriility on the remote estimtion of chlorophyll- concentrtion in turid productive wters: experimentl results, Applied Optics, 44, , A.A. Gitelson, J. Schlles, C.M. Hldik. Remote chlorophyll- retrievl in turid productive esturine: Chespeke By cse study. Remote Sensing of Environment, 19: , G. Dll'Olmo nd A. A. Gitelson. Effect of io-opticl prmeter vriility nd uncertinties in reflectnce mesurements on the remote estimtion of chlorophyll- concentrtion in turid productive wters: modeling results, Applied Optics,, , C. D. Moley, Light nd Wter. Rditive Trnsfer in Nturl Wters, Acdemic Press, New York, A. Gilerson, J. Zhou, S. Hling, I. Ionnou, J. Schlles, B. Gross, F. Moshry,S. Ahmed, Fluorescence component in the reflectnce spectr from costl wters. Dependence on wter composition, Opt. Express, 15, , Z. P. Lee, A. M. Ciotti, M. R. Lewis, nd J. J. Cullen, Assessment of the reltionships etween dominnt cell size in nturl phytoplnkton communities nd the spectrl shpe of the sorption coefficient, Limnol. Ocenogr. 47(2), 4 417, A. Gilerson, J. Zhou, S. Hling, I. Ionnou, B. Gross, F. Moshry, S. Ahmed, Improved fluorescence modeling nd retrievl in costl zones, Ocen Optics, XIX, Itly, P. Gege nd A. Alert, A tool for inverse modeling of spectrl mesurements in deep nd shllow wters in Remote Sensing of Aqutic Costl Ecosystem Processes: Science nd Mngement Applictions, L.L. Richrdson nd E.F. LeDrew, eds. Chp. 4, Springer, A.A. Gitelson, D. Gurlin, W. Moses nd T. Brrow. A io-opticl lgorithm for the remote estimtion of the chlorophyll- concentrtion in cse 2 wters. Environ. Res. Lett. (9) (5pp), W.J. Moses, A. A. Gitelson, S. Berdnikov, nd V. Povzhnyy, (9). Stellite Estimtion of Chlorophyll- Concentrtion Using the Red nd NIR Bnds of MERIS The Azov Se Cse Study, IEEE Geoscience nd Remote Sensing Letters, 6, Proceedings of V Interntionl Conference Current Prolems in Optics of Nturl Wters (ONW). St Petersurg, Russi, 8-11 Septemer 9, pp