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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 18, Iss. 23 — Nov. 8, 2010
  • pp: 24109–24125
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Algorithms for remote estimation of chlorophyll-a in coastal and inland waters using red and near infrared bands

Alexander A. Gilerson, Anatoly A. Gitelson, Jing Zhou, Daniela Gurlin, Wesley Moses, Ioannis Ioannou, and Samir A. Ahmed  »View Author Affiliations


Optics Express, Vol. 18, Issue 23, pp. 24109-24125 (2010)
http://dx.doi.org/10.1364/OE.18.024109


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Abstract

Remote sensing algorithms that use red and NIR bands for the estimation of chlorophyll-a concentration [Chl] can be more effective in inland and coastal waters than algorithms that use blue and green bands. We tested such two-band and three-band red-NIR algorithms using comprehensive synthetic data sets of reflectance spectra and inherent optical properties related to various water parameters and a very consistent in situ data set from several lakes in Nebraska, USA. The two-band algorithms tested with MERIS bands were Rrs(708)/Rrs(665) and Rrs(753)/Rrs(665). The three-band algorithm with MERIS bands was in the form R3 = [Rrs−1(665) − Rrs−1(708)] × Rrs(753). It is shown that the relationships of both Rrs(708)/Rrs(665) and R3 with [Chl] do not depend much on the absorption by CDOM and non-algal particles, or the backscattering properties of water constituents, and can be defined in terms of water absorption coefficients at the respective bands as well as the phytoplankton specific absorption coefficient at 665 nm. The relationship of the latter with [Chl] was established for [Chl] > 1 mg/m3 and then further used to develop algorithms which showed a very good match with field data and should not require regional tuning.

© 2010 OSA

1. Introduction

Many algorithms have been developed which employ two or more bands in the red-NIR spectral range or use a combination of these bands with bands in the blue-green part of the spectrum (see the comprehensive review in [13

13. J. F. Schalles, “Optical Remote Sensing Techniques to Estimate Phytoplankton Chlorophyll a Concentrations in Coastal Waters with Varying Suspended Matter and CDOM Concentrations,” in Remote Sensing of Aquatic Coastal Ecosystem Processes: Science and Management Applications, L.L. Richardson and E.F. LeDrew, eds. (Springer, 2006), Chap. 3.

] and other references [14

14. G. Dall'Olmo, A. A. Gitelson, and D. C. Rundquist, “Towards a unified approach for remote estimation of chlorophyll-a in both terrestrial vegetation and turbid productive waters,” Geophys. Res. Lett. 30(18), 1938 (2003), doi:. [CrossRef]

16

16. C. Le, Y. Li, Y. Zha, D. Sun, C. Huang, and H. Lu, “A four-band semi-analytical model for estimating chlorophyll a in highly turbid lakes: The case of Taihu Lake, China,” Remote Sens. Environ. 113(6), 1175–1182 (2009). [CrossRef]

]). Some of the algorithms included correction for the backscattering at different bands [17

17. H. J. Gons, M. Rijkeboer, and K. G. Ruddick, “A chlorophyll-retrieval algorithm for satellite imagery (Medium Resolution Imaging Spectrometer) of inland and coastal waters,” J. Plankton Res. 24(9), 947–951 (2002). [CrossRef]

] and the band tuning procedures [18

18. K. G. Ruddick, H. J. Gons, M. Rijkeboer, and G. Tilstone, “Optical remote sensing of chlorophyll a in case 2 waters by use of an adaptive two-band algorithm with optimal error properties,” Appl. Opt. 40(21), 3575–3585 (2001). [CrossRef]

]. In most cases, the algorithms had spectral bands where the contribution of chlorophyll fluorescence cannot be considered negligible. Such algorithms required additional tuning and often did not produce acceptable accuracies in retrieving [Chl], especially for [Chl] values below 10 mg/m3. Moreover, the bands in some of the algorithms are not available on the current ocean color satellites, which limits the applicability of those algorithms. Gower [19

19. J. Gower, S. King, G. Borstad, and L. Brown, “Detection of intense plankton blooms using the 709 nm band of the MERIS imaging spectrometer,” Int. J. Remote Sens. 26(9), 2005–2012 (2005). [CrossRef]

] introduced the Maximum Chlorophyll Index (MCI), which was successfully used for the detection of algal blooms. This index uses bands at 681, 709 and 753 nm, so it is very sensitive to the chlorophyll fluorescence and is used only for qualitative detection of algal blooms.

The advanced version of red-NIR algorithm [10

10. A. A. Gitelson, G. Dall’Olmo, W. Moses, D. C. Rundquist, T. Barrow, T. R. Fisher, D. Gurlin, and J. Holz, “A simple semi-analytical model for remote estimation of chlorophyll-a in turbid waters: Validation,” Remote Sens. Environ. 112(9), 3582–3593 (2008). [CrossRef]

] includes three bands instead of two [14

14. G. Dall'Olmo, A. A. Gitelson, and D. C. Rundquist, “Towards a unified approach for remote estimation of chlorophyll-a in both terrestrial vegetation and turbid productive waters,” Geophys. Res. Lett. 30(18), 1938 (2003), doi:. [CrossRef]

], which enables better separation of the absorption by [Chl] from absorption and scattering by other constituents in water in the red and NIR parts of the spectrum. Both two- and three-band algorithms have been tested in multiple water environments [10

10. A. A. Gitelson, G. Dall’Olmo, W. Moses, D. C. Rundquist, T. Barrow, T. R. Fisher, D. Gurlin, and J. Holz, “A simple semi-analytical model for remote estimation of chlorophyll-a in turbid waters: Validation,” Remote Sens. Environ. 112(9), 3582–3593 (2008). [CrossRef]

,20

20. G. Dall’Olmo and A. A. Gitelson, “Effect of bio-optical parameter variability on the remote estimation of chlorophyll-a concentration in turbid productive waters: experimental results,” Appl. Opt. 44(3), 412–422 (2005). [CrossRef] [PubMed]

,21

21. A. A. Gitelson, J. Schalles, and C. M. Hladik, “Remote chlorophyll-a retrieval in turbid productive estuarine: Chesapeake Bay case study,” Remote Sens. Environ. 109(4), 464–472 (2007). [CrossRef]

]. But the sources of the uncertainties were difficult to trace because multiple parameters involved (absorption and backscattering coefficients of water components) were not directly measured in the experiments. Optimization of the band positions was done using the synthetic data sets simulated with a semi-analytical model [22

22. G. Dall’Olmo and A. A. Gitelson, “Effect of bio-optical parameter variability and uncertainties in reflectance measurements on the remote estimation of chlorophyll-a concentration in turbid productive waters: modeling results,” Appl. Opt. 45(15), 3577–3592 (2006). [CrossRef] [PubMed]

]. However, the sensitivity of the retrievals to CDOM absorption, concentration of minerals, and shape of phytoplankton absorption spectrum were not considered.

In this work we tested two- and three-band algorithms with bands that matched the spectral channels of MERIS (Medium Resolution Imaging Spectrometer) centered at 665, 708 and 753 nm. Comprehensive synthetic data sets of reflectance spectra and inherent optical properties (IOP) for a wide range of water constituents were used together with field data. Specifically, we analyzed relationships between [Chl] and Rrs(708)/Rrs(665), Rrs(753)/Rrs(665), and [Rrs(665)−1 – Rrs(708)−1]*Rrs(753), where Rrs(λ) is the remote sensing reflectance at the wavelengths λ = 665, 708, and 753 nm. The main goal of the work was to analyze the contributions of different components to the estimated [Chl] values and arrive at more robust algorithms which would not require significant regional tuning.

In Section 2, which follows, we describe our methods, which are based on our synthetic and field (in situ) data sets; in Sections 3-6 we report results, specifically, in Section 3 we make a preliminary analysis of the sensitivity of the algorithms to water quality parameters; in Section 4 we analyze the contributions of main components to the relationships between the algorithms and [Chl]; in Section 5 we establish the relationship between the phytoplankton specific absorption coefficient in the red-NIR part of the spectrum and [Chl]; in Section 6 we arrive at more accurate algorithms that relate [Chl] and reflectances; and in Section 7 we analyze the performance of blue-green ratio algorithms on the same data sets and compare them with the performance of the red-NIR algorithms.

2. Description of the data sets

About two thousand reflectance spectra were simulated using Hydrolight [23

23. C. D. Mobley, Light and Water. Radiative Transfer in Natural Waters (Academic Press, New York, 1994).

], with and without taking into account the chlorophyll fluorescence, with 1 nm resolution for a wide range of conditions typical for inland and coastal waters: [Chl] = 1-100 mg/m3, CDOM (yellow substance) absorption coefficient at 400 nm, ay(400) = 0-5 m−1, different shapes of phytoplankton specific absorption coefficient spectra, and concentrations of non-algal particles, CNAP = 0-10 mg/l. All the details and assumptions used for the simulation of water optical properties are given in Gilerson et al. [24

24. A. Gilerson, J. Zhou, S. Hlaing, I. Ioannou, J. Schalles, B. Gross, F. Moshary, and S. Ahmed, “Fluorescence component in the reflectance spectra from coastal waters. Dependence on water composition,” Opt. Express 15(24), 15702–15721 (2007). [CrossRef] [PubMed]

]. They were based on the findings of many authors for IOP characteristics, with similar assumptions to those used in the development of the data sets of the IOCCG working group [25], and included variations of absorption and backscattering spectra in a very wide range [24

24. A. Gilerson, J. Zhou, S. Hlaing, I. Ioannou, J. Schalles, B. Gross, F. Moshary, and S. Ahmed, “Fluorescence component in the reflectance spectra from coastal waters. Dependence on water composition,” Opt. Express 15(24), 15702–15721 (2007). [CrossRef] [PubMed]

]. Thus, for example, the power coefficient in the CDOM absorption model, ay=ay(400)*exp[Sy(λ400)], was in the range Sy = 0.1-0.2 nm−1 and the specific scattering of non-algal particles, bNAP*, was in the range 0.5-1.0m2/g. Solar input was simulated with a cloud-free sky.

To be able to analyze the impact of the phytoplankton specific absorption coefficient, aph*, on the performance of the algorithms, as a first step, two data sets were simulated with the shapes of the specific absorption coefficient spectra taken, according to Eq. (2) from Ciotti et al. [26

26. A. M. Ciotti, M. R. Lewis, and J. J. Cullen, “Assessment of the relationships between dominant cell size in natural phytoplankton communities and the spectral shape of the absorption coefficient,” Limnol. Oceanogr. 47(2), 404–417 (2002). [CrossRef]

], as a sum of the specific absorptions coefficients of microplankton and picoplankton, as shown in Fig. 1
Fig. 1 Phytoplankton specific absorption spectra used in the simulations.
. The weighting factor Sf ranged from 0.1 to 0.3. CNAP ranged from 0 to 1 mg/l in one data set and from 1 to 10 mg/l in the other data set. It should be noted that based on the dependence of aph* (440) on [Chl], as shown in [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

], the range 0-0.3 for Sf is typical for [Chl] > 1mg/m3. But we found that the model [26

26. A. M. Ciotti, M. R. Lewis, and J. J. Cullen, “Assessment of the relationships between dominant cell size in natural phytoplankton communities and the spectral shape of the absorption coefficient,” Limnol. Oceanogr. 47(2), 404–417 (2002). [CrossRef]

] usually underestimates aph*values in the red and NIR parts of the spectrum [see Fig. 8(b)
Fig. 8 Phytoplankton specific absorption coefficient at (a) 440 nm and (b) 675 nm plotted against [Chl] for the field data collected from inland and coastal waters.
below]. For the purpose of evaluating the red–NIR models, we also used the spectral shapes from the model [26

26. A. M. Ciotti, M. R. Lewis, and J. J. Cullen, “Assessment of the relationships between dominant cell size in natural phytoplankton communities and the spectral shape of the absorption coefficient,” Limnol. Oceanogr. 47(2), 404–417 (2002). [CrossRef]

] in the 650-800 nm part of the spectrum with aph*(675) = 0.0142, 0.0156, and 0.02 m2/(mg Chl a) which are also shown in Fig. 1. For the spectral shapes with aph*(675) = 0.0142 and 0.02 m2(mg Chl a)−1 an additional set of IOP spectra and the associated reflectances was generated with CNAP = 0-10 mg/l, 1 < [Chl] < 40 mg/m3 (1 < [Chl] < 20 mg/m3 for aph*(675) = 0.02 m2/(mg Chl a) and CDOM absorption at 400 nm, ay(400) = 0-3 m−1. This range of [Chl], 1 < [Chl] < 40 mg/m3, is more typical for coastal and inland waters (rather than higher [Chl] values). Thus, a very accurate evaluation of the algorithms is required for this [Chl] range. In all simulations the phytoplankton absorption was considered proportional to [Chl]
aph(λ)=[Chl]aph*(λ)
(1)
and

aph*(λ)=Sfapico*(λ)+(1Sf)amicro*(λ)
(2)

Fluorescence quantum yield (together with the chlorophyll reabsorption coefficient) in the original simulations for Sf = 0.1-0.3 was, η = 0.5%. The fluorescence contribution was calculated as the difference between the total and elastic reflectances and was assumed to change proportionally with η, which enabled the assessment of the impact of the fluorescence component. All reflectances were simulated for the sun zenith angle θi = 30° and for nadir viewing.

Field data set consisted of data collected by the CALMIT group at 85 stations in Fremont State Lakes, Nebraska, USA, during the summer of 2008, with [Chl] = 2-100 mg/m3, ay(400) = 0.9-3 m−1, and CNAP = 0-3 mg/l [28

28. A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

]. A standard set of water quality parameters was measured: turbidity, [Chl], total, inorganic, and organic suspended solids. Hyperspectral reflectance measurements were taken from a boat using two intercalibrated Ocean Optics USB2000 spectrometers. More details on the instrumentation and methodology are provided in Gitelson et al [28

28. A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

].

3. Preliminary analysis of the main factors impacting red – NIR algorithms

3.1 Comparison of the synthetic and field data

Moses et al. [29

29. W. J. Moses, A. A. Gitelson, S. Berdnikov, and V. Povazhnyy, “Satellite Estimation of Chlorophyll-a Concentration Using the Red and NIR Bands of MERIS—The Azov Sea Case Study,” IEEE Geosci. Remote Sens. Lett. 6(4), 845–849 (2009). [CrossRef]

] calibrated and validated red-NIR models using MERIS satellite reflectances and [Chl] measured in the field. As a result of the calibration, the following linear relationships between [Chl] and the models were obtained.

For all simulations from the synthetic data, remote sensing reflectances, Rrs, at the band centers were used instead of integrating the water leaving radiance over the entire bandwidth and normalizing it by the irradiance over the same spectral range. This was done because the latter approach did not produce a noticeable change in the performance of the algorithms.

3.2 Sensitivity of the two-band model to water parameters

The two-band model, Rrs(708)/Rrs(665), was further analyzed using synthetic data sets. The impact of aph* on the relationship between the model values and [Chl] is shown in Fig. 3
Fig. 3 Chlorophyll-a concentration vs. two-band red-NIR model calculated using synthetic data, the impact of the shape of the phytoplankton specific absorption coefficient.
. In Fig. 3, aph*(675) is in the range 0.01-0.156 m2/(mg Chl a); CDOM absorption is in the range, 0 < ay(400) < 5m−1; CNAP is in the range, 0 < CNAP < 1mg/l. It can be seen that the relationship between [Chl] and Rrs(708)/Rrs(665) is sensitive to aph* values. However, not all this variability can be seen for the realistic concentrations of water constituents: it is well known that due to the packaging effect [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

] the values of aph* in all parts of the spectrum as well as the variability of aph*decreases with increasing [Chl]. So the actual sensitivity of the model to aph* should be assessed by taking into account the relationship between [Chl] andaph*.

Sensitivity of the Rrs(708)/Rrs (665) model to changes in CDOM absorption is presented in Fig. 4(a)
Fig. 4 Chlorophyll-a concentration vs. two-band red-NIR ratio with variations in (a) CDOM absorption, 0<ay(400)<5m−1, (b) CNAP concentration, 0<CNAP<10 mg/l, and (c) fluorescence quantum yield, η = 0.25, 0.5 and 1.0.
, which shows minimal impact for a very broad range of CDOM absorption coefficient values, 0 < ay(400) < 5m−1. The impact of CNAP variability is shown in Fig. 4(b): for a very wide variation in CNAP values, 0 < CNAP < 10 mg/l, standard deviation was 4.58 mg/m3, thereby showing that variations in CNAP do not dramatically affect the accuracy of [Chl] estimation. Results of simulations for three values of fluorescence quantum yield, η = 0.25, 0.5, and 1.0, showed that small differences can be observed only for [Chl] > 60 mg/m3 [Fig. 4(c)]. The impacts ofaph*, ay(400), CNAP, and η were found to be similar for the three-band model and are not shown here.

Thus, the variability of aph* and the relationship between [Chl] and aph* are the major factors that affect the performance of the Rrs(708)/Rrs(665) model. These factors should be assessed properly in order to make simulations that are relevant to realistic water conditions.

4. Basic relationships for red – NIR models and contributions from its main components

For inland and coastal waters the remote sensing reflectance can be estimated as [30

30. H. R. Gordon, O. B. Brown, R. H. Evans, J. W. Brown, R. C. Smith, K. S. Baker, and D. K. Clark, “A semianalytic radiance model of ocean color,” J. Geophys. Res. 93(D9), 10909–10924 (1988). [CrossRef]

]
Rrs(λ)=0.53fQbb(λ)a(λ)+bb(λ)
(5)
where bb(λ) is the backscattering coefficient, a(λ) is the total absorption coefficient, f is a factor that depends mostly on the angle of the incident light, and Q is the bidirectional reflectance distribution function (BRDF) coefficient. Generally, both fand Q are also functions of the wavelength, but as a first order approximation, we ignore this dependence for the relatively narrow red – NIR spectral range of interest.

We also assume that the total absorption coefficient a(λ) is the sum of absorption coefficients by four components: phytoplankton, CDOM, non-algal particles, and water
a(λ)=aph(λ)+ay(λ)+aNAP(λ)+aw(λ)
(6)
Absorption by non-algal particles, aNAP(λ), has a spectral shape similar to that of the absorption by CDOM, and for the range of parameters considered, its magnitude was on average two times smaller than the CDOM absorption. To simplify the equations, aNAP(λ) is neglected henceforth in this model but this simplification will not in any way affect the analysis and the final results since aNAP(λ)was included in the bio-optical model for the calculation of the IOPs and the reflectances simulated using Hydrolight.

Thus, the ratio of the reflectances, which for two-band model we denote as R2, at two wavelengthsλ1 andλ2can be written as
R2 = Rrs(λ1)/Rrs(λ2)=[bb(λ1)/bb(λ2)]*[aph(λ2)+ay(λ2)+aw(λ2)+bb(λ2)]/[aph(λ1)+ay(λ1)+aw(λ1)+bb(λ1)]
(7)
where, for our study, λ1 = 708 or 753 nm and λ2 = 665nm.

Assuming [bb(λ1)/bb(λ2)]1 (quite an accurate assumption for Rrs(708)/Rrs(665) but less accurate for Rrs(753/Rrs(665)), we can find:
[aph(λ1)+ay(λ1)+aw(λ1)+bb(λ1)]*R2=[aph(λ2)+ay(λ2)+aw(λ2)+bb(λ2)]
(8)
Taking into account (1) we arrive at a relationship similar to that in [17

17. H. J. Gons, M. Rijkeboer, and K. G. Ruddick, “A chlorophyll-retrieval algorithm for satellite imagery (Medium Resolution Imaging Spectrometer) of inland and coastal waters,” J. Plankton Res. 24(9), 947–951 (2002). [CrossRef]

]
[Chl]={[ay(λ1)R2ay(λ2)]+[aw(λ1)R2aw(λ2)]+[bb(λ1)R2bb(λ2)]}/[aph*(λ2)aph*(λ1)R2]
(9)
Absorption coefficients of water at the wavelengths of interest are, aw(665) ≈0.42, aw(708) ≈0.79, and aw(753) ≈2.5 ([31

31. R. C. Smith and K. S. Baker, “Optical properties of the clearest natural waters (200-800 nm),” Appl. Opt. 20(2), 177–184 (1981). [CrossRef] [PubMed]

] with interpolation). The range of R2 (see for example Fig. 2(a) for Rrs(708)/Rrs(665) values) is R2 = 0.6-2.2. The differences between CDOM absorptions (i.e., [ay(λ1)R2ay(λ2)]), and backscatterings (i.e., [bb(λ1)R2bb(λ2)]), with R2 as a multiplier for the respective values at λ1, are in the same form as the difference between water absorptions [aw(λ1)R2aw(λ2)], with high values of aw(λ1) and a significant difference between aw1) and aw(λ2). So, for [Chl] > 5 mg/m3, the contribution of CDOM absorption and backscattering terms are significantly smaller than the contribution from water absorption. At [Chl] < 5mg/m3, when R2 < 0.6 and the water term becomes smaller, the variation in CDOM absorption and backscattering will create additional noise/uncertainties in the estimation of [Chl] values. The contributions of these components to the relationship [Chl] vs. Rrs(708)/Rrs(665) is shown in Fig. 5
Fig. 5 Contributions of the main components of Eq. (9) to the relationship [Chl] vs. Rrs(708)/Rrs(665): The ranges of concentrations considered were, [Chl] = 1-40 mg/m3, ay = 0-3m−1, CNAP = 0-10 mg/l, for (a) a*ph(675) = 0.0142 m2/(mg Chl a) and (b) a*ph(675) = 0.02 m2/(mg Chl a). “chl” is the [Chl] from the synthetic data set, “chl calc” is the [Chl] calculated using the Eq. (9), “acdom” is [ay(λ1)R2ay(λ2)]/aph*(λ2), the CDOM absorption term from Eq. (9), “awater” = [aw(λ1)R2aw(λ2)]/aph*(λ2), the water absorption term from Eq. (9), and “b/scatter” is [bb(λ1)R2bb(λ2)]/aph*(λ2), the backscattering term from Eq. (9).
. Simulations were done for the data set with ay(400) = 0-3 m−1 and CNAP = 0-10 mg/l. In Fig. 5(a), a*ph(675) = 0.0142 m2/(mg Chl a) and [Chl] = 1-40 mg/m3, and in Fig. 5(b), a*ph(675) = 0.02 m2/(mg Chl a) and [Chl] = 1-20 mg/m3. It can be seen that the relationship is governed by the water absorption component with a small contribution from CDOM absorption and backscattering.

There is no significant difference in the relationship between the cases of a*ph(675) = 0.0142 m2/(mg Chl a) [Fig. 5(a)] and a*ph(675) = 0.02 m2/(mg Chl a) [Fig. 5(b)]. aph*(λ1) is very small, about 1% of aph*(λ2), and it changes the slope of the relationship only slightly. There is also no significant scattering of points due to CNAP variations.

For the [Chl] vs. Rrs(753)/Rrs(665) relationship, the effect of different terms in Eq. (9), shown in Fig. 6
Fig. 6 Contributions of the main components of Eq. (9) to the relationship [Chl] vs. Rrs(753)/Rrs(665): a) a*ph(675) = 0.0142 m2/(mg Chl a), b) a*ph(675) = 0.02 m2/(mg Chl a). Notation and the ranges of values for [Chl], ay, and CNAP are the same as for Fig. 5.
, is completely different from those for the relationship [Chl] vs. Rrs(708)/Rrs(665) shown in Fig. 5. The assumption [bb(λ1)/bb(λ2)] ≈1 is not very accurate for the Rrs(753)/Rrs(665) case. Thus, the water absorption term and “chl calc” [Eq. (9)] do not match the respective values simulated by Hydrolight. The contributions of CDOM absorption and backscattering terms from Eq. (9) are also significantly higher than those for the Rrs(708)/Rrs(665) model (Fig. 5), which makes the Rrs(753)/Rrs(665) model unreliable for estimating low to moderate [Chl].

Basic relationships for the three-band model, similar to the ones for the two-band model, can be written as follows: let us denote three-band model as
R3 =[Rrs(λ1)1Rrs(λ2)1]*Rrs(λ3)
(10)
where λ1 = 665nm, λ2 = 708nm and λ3 = 753nm.

Then using Eqs. (5) and (6) we have
R3 ={[aph(λ1)+ay(λ1)+aw(λ1)+bb(λ1)]/bb(λ1)[aph(λ2)+ay(λ2)+aw(λ2)+bb(λ2)]/bb(λ2)}*{bb(λ3)/[aph(λ3)+ay(λ3)+aw(λ3)+bb(λ3)]}
(11)
Assuming bb(λ1)bb(λ2)bb(λ3) and the dominance of aw(λ3)over other terms at λ3
[Chl]={[aw(λ3)R3aw(λ1)+aw(λ2)]+[ay(λ2)ay(λ1)]+[bb(λ2)bb(λ1)]}/[aph*(λ1)aph*(λ2)]
(12)
Despite the assumption about bb(λ), the term [bb(λ2)bb(λ1)] in (12) is left as non-zero in order to estimate possible errors. The contributions of all components to the relationship between [Chl] and R3 is shown in Fig. 7
Fig. 7 Contributions of main components of Eq. (12) to the [Chl] vs R3 relationship: a) a*ph(675) = 0.0142 m2/(mg Chl a), b) a*ph(675) = 0.0142 m2/(mg Chl a). “chl” is the [Chl] from the data set, “chl calc” is the [Chl] from the Eq. (12), “acdom” is [ay(λ2)ay(λ1)]/aph*(λ2), the CDOM absorption term from (12), “awater” is [aw(λ3)R3aw(λ1)+aw(λ2)]/aph*(λ2), the water absorption term from (12) and “b/scatter” is [bb(λ2)bb(λ1)]/aph*(λ2), the backscattering term from (12). The ranges of values for [Chl], ay, and CNAP are the same as for Fig. 5.
. [Chl] vs. R3 relationship is mostly defined by the water absorption term, with minimal effect of other components, which, as expected, is even smaller than that for the Rrs(708)/Rrs(665) model because CDOM absorption term and backscattering term do not have R3 as a multiplier in the equation. The slope of the relationship is determined by the a*ph(665) value in the denominator of Eq. (12).

5. Relationship between [Chl] and phytoplankton specific absorption coefficient

The results derived from the simulated reflectance spectra show that the performance of the red - NIR models is strongly affected by the magnitude and spectral shape of the phytoplankton specific absorption coefficient. As it was already mentioned, aph* decreases with increasing [Chl], especially for [Chl] > 3-5 mg/m3, due to the packaging effect [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

]. This decrease was observed for the mean value as well as the variability of aph* [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

], but the exact range of this decrease was not well studied, especially for [Chl] > 10 mg/m3.

Figure 8 shows the range of values for the peaks of aph* at 440 and 675 nm in the data sets from several field campaigns [21

21. A. A. Gitelson, J. Schalles, and C. M. Hladik, “Remote chlorophyll-a retrieval in turbid productive estuarine: Chesapeake Bay case study,” Remote Sens. Environ. 109(4), 464–472 (2007). [CrossRef]

,28

28. A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

,32

32. J. Zhou, A. Gilerson, I. Ioannou, S. Hlaing, J. Schalles, B. Gross, F. Moshary, and S. Ahmed, “Retrieving quantum yield of sun-induced chlorophyll fluorescence near surface from hyperspectral in-situ measurement in productive water,” Opt. Express 16(22), 17468–17483 (2008). [CrossRef] [PubMed]

] (the data from the Nebraska campaign in 2009 are unpublished as yet). [Chl] values were obtained using standard measurement techniques [21

21. A. A. Gitelson, J. Schalles, and C. M. Hladik, “Remote chlorophyll-a retrieval in turbid productive estuarine: Chesapeake Bay case study,” Remote Sens. Environ. 109(4), 464–472 (2007). [CrossRef]

,28

28. A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

]. Phytoplankton absorption values for Nebraska lakes were obtained using standard filter absorption measurements [28

28. A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

]. For the Chesapeake Bay and the Long Island Sound phytoplankton absorption spectra were retrieved from the total absorption spectra measured by an ac-s instrument (WET Labs) [32

32. J. Zhou, A. Gilerson, I. Ioannou, S. Hlaing, J. Schalles, B. Gross, F. Moshary, and S. Ahmed, “Retrieving quantum yield of sun-induced chlorophyll fluorescence near surface from hyperspectral in-situ measurement in productive water,” Opt. Express 16(22), 17468–17483 (2008). [CrossRef] [PubMed]

]. The variability of both aph*(440)and aph*(675), as shown in Fig. 8, is higher than those in [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

]. Moreover, the mean values do not decrease with [Chl] as rapidly as reported in [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

].

The power fit for aph*(675)vs. [Chl] for all points of Fig. 8(b) is shown in Fig. 9(a)
Fig. 9 a) power fit for all points of Fig. 8(b) for a*ph(675) vs. [Chl]; R2 = 0.2265; the brown and pink lines represent bounds for the 95% confidence interval. b) power fit for a*ph(675) vs. a*ph(665); R2 = 0.965. Inset: a*ph spectra with higher a*ph(675) values – specific absorption coefficient spectra of: Cryptophyta “H” (1), Diatoms (2) and Green algae (3) from Gege et al. [33].
(red solid line). The power equation is:

aph*(675)=0.03[Chl]0.2
(13)

The brown and pink lines in Fig. 9(a) represent the bounds for the 95% confidence interval. Taking into account the known decrease of variability in aph*(675)as [Chl] increases, we also added other bounds (green lines) with a wider range of aph*(675)values at low [Chl] (close to 1 mg/m3) and a narrower range for higher [Chl]. These bounds were, for [Chl] = 1 mg/m3, a*ph (max) (675) = 0.0439 m2/(mg Chl a) and a*ph (min) (675) = 0.016 m2/(mg Chl a) respectively. aph*(665)is always smaller than aph*(675)and it does not change proportionally with aph*(675). The relationship between aph*(665)and aph*(675) is shown in Fig. 9(b) and is approximated as
aph*(665)=0.412aph*(675)]0.8373
(14)
and together with (13)
aph*(665)=0.022[Chl]0.1675
(15)
The points in Fig. 9(b) are taken from the specific absorption coefficient spectra in Fig. 1 as well as from the spectra with higher a*ph values: these are spectra of Cryptophyta “H” (1), Diatoms (2) and Green algae (3) from Gege et al. [33

33. P. Gege, and A. Albert, “A tool for inverse modeling of spectral measurements in deep and shallow waters,” in Remote Sensing of Aquatic Coastal Ecosystem Processes: Science and Management Applications, L.L. Richardson and E.F. LeDrew, eds. Chap. 4, Springer, 2006.

], and are shown in the inset of Fig. 9(b).

Analysis of aph*(675)as a function of [Chl] shows that aph*(675)values in the inland and coastal waters studied are in the range of 0.02 – 0.03 m2/(mg Chl) for [Chl] ≈1mg/m3 and gradually decreases to 0.01 – 0.015 m2/(mg Chl) for [Chl] > 10-20 mg/m3. Spectra in Fig. 1 give lower values of aph*(675)especially for [Chl] < 10-20 mg/m3, which makes simulations less accurate. Additional measurements for various water conditions are necessary for the clarification of this issue.

6. Advanced versions of two- and three-band models and their comparison with the field data

Based on the conclusion in Section 4, water absorption terms are dominant in the equations for the two-band model, Rrs(708)/Rrs(665) [Eq. (9)] and the three-band model, [Rrs(665)−1 – Rrs(708)−1]*Rrs(753) [Eq. (12)]. So Eq. (9) can be simplified as
[Chl]=[aw(λ1)R2aw(λ2)]/aph*(665)
(16)
Then substituting aph*(665)from Eq. (15) after simple modifications, we arrive at
[Chl]={[aw(λ1)R2aw(λ2)]/0.022  ​}1/p
(17.1)
[Chl]=[35.75*R219.30]1.124
(17.2)
Similarly, for the three-band model, Eq. (12) can be simplified as,
[Chl]=[aw(λ3)R3aw(λ1)+aw(λ2)]/aph*(665)
(18)
and after including aph*(665)from Eq. (15),
[Chl]={[aw(λ3)R3aw(λ1)+aw(λ2)]/0.022}(1/p)
(19.1)
[Chl]=[113.36*R3+16.45]1.124
(19.2)
For both Eqs. (17.1) and (19.1), p = 0.8325. In order to better fit the field data, p was adjusted slightly from 0.8325 to 0.89. Comparison of relationships Eqs. (17.1) and (19.1) with the Nebraska field data for all three models is shown in Fig. 10
Fig. 10 Comparison of analytical relationships Eqs. (17.1) and (19.1) with the field data (Nebraska 2008 [28]) and empirical algorithms from MERIS –field data [29]: a) R2: Rrs(708)/Rrs(665), b) R2: Rrs(753)/Rrs(665), c) R3 = [Rrs(665)−1 – Rrs(708)−1]*Rrs(753). Red lines correspond to the Eq. (17.1) and (17.2) for Rrs(708)/Rrs(665) algorithm [Fig. 10(a)], the Eq. (17.1) for Rrs(753)/Rrs(665) algorithm [Fig. 10(b)] and to the Eqs. (19.1) and (19.2) for [Rrs(665)−1 –Rrs(708)−1] *Rrs(753) algorithm [Fig. 10(c)]. Lower and upper a*ph bounds correspond to the bounds in Fig. 9(a), MERIS – field algorithm – cyan lines [Eq. (3)] for Rrs(708)/Rrs(665) algorithm [Fig. 10(a)] and Eq. (4) for 3 bands algorithm [Fig. 10(c)].
. Respective values in the power coefficient for the upper and lower bounds corresponding to the bounds in Fig. 9(a) were also increased accordingly by δ = [0.89 – 0.8325] = 0.0575.

Taking aw(665) = 0.4245 m−1, aw(708) = 0.7864 m−1, and aw(753) = 2.494 m−1 ([31

31. R. C. Smith and K. S. Baker, “Optical properties of the clearest natural waters (200-800 nm),” Appl. Opt. 20(2), 177–184 (1981). [CrossRef] [PubMed]

] with interpolation), Eqs. (17.1) and (19.1) with p = 0.89 can be rewritten as Eqs. (17.2) and (19.2), where R2 = Rrs(708)/Rrs(665) and R3 = (1/Rrs(665) – 1/Rrs(708))*Rrs(753).

Very close relationships between the field data [28

28. A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

] and both the two-band model, Rrs(708)/Rrs(665), and the three-band model, [Rrs(665)−1 – Rrs(708)−1]*Rrs(753) are observed in Fig. 10. Once again, the two-band model, Rrs(753)/Rrs(665), proved to be inaccurate for low to moderate [Chl]. MERIS-derived relationships for the Rrs(708)/Rrs(665) model, and the three-band model [Eqs. (3) and (4) respectively] from Moses et al. [29

29. W. J. Moses, A. A. Gitelson, S. Berdnikov, and V. Povazhnyy, “Satellite Estimation of Chlorophyll-a Concentration Using the Red and NIR Bands of MERIS—The Azov Sea Case Study,” IEEE Geosci. Remote Sens. Lett. 6(4), 845–849 (2009). [CrossRef]

] [cyan lines in Fig. 10(a) and Fig. 10(c)] match very well with the linear parts of the analytically derived algorithms, Eqs. (17.2) and (19.2) [red lines in Fig. 10(a) and 10(c)]. Equations (17.2) and (19.2) match well with the field data even in the non-linear part of the curve for [Chl] < 10mg/m3. Correlations between measured [Chl] from the Nebraska field data set and [Chl] from the analytically derived algorithms for the models Rrs(708)/Rrs(665) [Eq. (17.2) and [Rrs(665)−1 – Rrs(708)−1]* Rrs(753) (Eq. (19.2)] are shown in Fig. 11
Fig. 11 Correlations between [Chl] measured in the Nebraska and [Chl] from the analytically-derived [Chl] from (a) Eq. (17.2): Rrs(708)/Rrs(665), and (b) Eq. (19.2): [Rrs(665)−1 – Rrs(708)−1]*Rrs(753).
. In both cases, the determination coefficient is above 0.95.

It should be also noted that aph*(675)decreases rapidly with [Chl] below 5-10 mg/m3 [Fig. 9(a)], but in this [Chl] range its impact on the algorithms is minimal (see Fig. 3). For higher [Chl] values, the aph*(675)change is much smaller as is its impact on the algorithms. For the whole [Chl] range the impact is also mitigated by the fact that the algorithms use aph*(665) which, according to Eq. (14), has much weaker dependence on [Chl] than aph*(675). As a result, a very broad range of aph*(675)in Fig. 9(a) (upper bound was approximated as 0.0439[Chl]-0.25 and lower bound as 0.016[Chl]-0.1) corresponds to a relatively small range of [Chl] variations between lower and upper boundaries [Fig. 10(a) and 10(c)], which makes both two- and three-bands algorithms less sensitive to changes in the phytoplankton specific absorption.

Additional studies in various water conditions and water environments are necessary for the estimation of the actual sensitivity of the models to a*ph. In such investigations special attention should be given to the accuracy of [Chl] and a*ph measurements.

7. Comparison of the performance of the blue-green MODIS OC3M and red/NIR algorithms

We also analyzed the performance of the blue-green MODIS OC3M algorithm [34

34. J. E. O'Reilly, and 24 Coauthors, “SeaWiFS Postlaunch Calibration and Validation Analyses, Part 3,” in NASA Tech. Memo. 2000–206892, Vol. 11, S. B. Hooker and E. R. Firestone, eds., (NASA Goddard Space Flight Center, Greenbelt, MD, 2000) 49 pp.

] using the same data sets to compare it with the red–NIR algorithms. Higher variability of aph*(440)observed in the field [Fig. 8(a)] corresponds to a broad range of aph*with Sf in the range 0-0.6. Preliminary estimations showed that Sf in the range 0-0.2 can be used for the assessment of OC3M for the whole range of [Chl] = 1-100 mg/m3. Higher Sf values are valid for [Chl] < 10-20 mg/m3 and these results are not presented below.

In Fig. 12
Fig. 12 Performance of OC3M algorithm on synthetic and field data. On the x-axis is the blue-green ratio, which is the primary element of the OC3M algorithm. The red curve represents the [Chl] values estimated by the OC3M algorithm.
we show the relationship between [Chl] and the maximum of two ratios, Rbg = Max[Rrs(443)/Rrs(555), Rrs(489)/Rrs(555)], which is the primary element of the OC3M algorithm. The solid red line shows [Chl] values estimated from the Rbg ratios based on the OC3M algorithm. The field data and synthetic data are both distributed approximately the same way along the OC3M line but the spread is quite high for both data sets.

Correlation between the field measured [Chl] and the blue-green algorithm [Chl] [Fig. 13(a)
Fig. 13 Plots of OC3M-derived [Chl] versus (a) [Chl] from the Nebraska field data set and (b) synthetically derived [Chl]. Concentrations of non-algal particles, CNAP, ranged from 0 to 10 mg/l, CDOM absorption was in the range of 0<ay(400)<3 m−1 .
] and the correlation between the synthetic [Chl] and the blue-green algorithm [Chl] [Fig. 13(b)] are significantly lower (R2 is below 0.4) than those for the red – NIR algorithms. Thus, for estimating [Chl] > 3-5g/m3, the red-NIR algorithms are significantly more accurate than the blue-green algorithm.

8. Conclusions

Several synthetic data sets of reflectances and inherent optical properties simulated with Hydrolight for inland and coastal waters and a very consistent field data set with [Chl] = 2 −100 mg/m3 were utilized to evaluate (a) the performance of red-NIR algorithms for the remote estimation of chlorophyll-a concentration and (b) the sensitivity of these algorithms to the main water parameters that characterize the optical properties of water, namely, absorption and backscattering coefficients. Two-band algorithms under study were Rrs(708)/Rrs(665) and Rrs(753)/Rrs(665). The three-band algorithm was [1/Rrs(665) – 1/Rrs(708)]*Rrs(753). All algorithms use wavebands that match the MERIS spectral channels. The main results and conclusions are the following.

1. The red-NIR algorithms are affected by possible variations in phytoplankton specific absorption coefficient in the red – NIR part of the spectrum. The realistic ranges of a*ph(675) and a*ph(665) and their dependence on [Chl] for various water environments were determined based on the experimental data taken in coastal and inland waters and absorption spectra analysis. It is shown that both mean values and variability of values do decrease with increase in [Chl] but more gradually than presented in [27

27. A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

]. It is also shown that the red-NIR algorithms are not very sensitive to the phytoplankton specific absorption coefficient because the most rapid change in a*ph(665) in response to a change in [Chl] occurs at [Chl] < 5-10 mg/m3, where its impact on the algorithms is minimal.

2. The contributions of CDOM and water absorption and particulate backscattering in the basic relationships for the red – NIR algorithms were evaluated using synthetic data sets. The Rrs(708)/Rrs(665) model and the three-band model are not very sensitive to CDOM absorption and backscattering by non-algal particles and the model equations are mostly controlled by terms that contain water absorption coefficients as well as by phytoplankton specific absorption coefficients. It was also shown that chlorophyll fluorescence does not noticeably affect the performance of these models. On the contrary, the Rrs(753)/Rrs(665) model appeared to be very sensitive to the main water optical properties and cannot be used for estimating low to moderate [Chl].

3. Established aph*(665) vs. [Chl] relationship and the simplified basic equations were used for the analytical development of advanced versions of Rrs(708)/Rrs(665) and [1/Rrs(665) – 1/Rrs(708)]*Rrs(753) algorithms, whose [Chl] estimations matched very well with the field data even for [Chl] < 10mg/m3. The analytically derived algorithms also matched well with red-NIR algorithms that were previously calibrated using MERIS data [29

29. W. J. Moses, A. A. Gitelson, S. Berdnikov, and V. Povazhnyy, “Satellite Estimation of Chlorophyll-a Concentration Using the Red and NIR Bands of MERIS—The Azov Sea Case Study,” IEEE Geosci. Remote Sens. Lett. 6(4), 845–849 (2009). [CrossRef]

]. This suggests that these algorithms, which are based on the known values of the water absorption coefficient at the appropriate bands and generalized aph*(665) vs. [Chl] relationship, do not require regional tuning.

4. It is shown that the performance of the blue-green ratio based algorithm is poor in turbid productive inland and coastal waters and that the red-NIR algorithms are much preferred for these waters.

Future work will include validation of the developed algorithms for water bodies from various regions, with an emphasis on the study of the variation of the aph*(665) vs. [Chl] relationship.

Acknowledgements

References and links

1.

G. Moore, J. Aiken, and J. Lavender, “The atmospheric correction of water colour and the quantitative retrieval of suspended particulate matter in Case II waters: application to MERIS,” Int. J. Remote Sens. 20(9), 1713–1733 (1999). [CrossRef]

2.

J. Aiken and G. Moore, “ATBD case 2 bright pixel atmospheric correction,” Center for Coastal & Marine Sciences, Plymouth Marine Laboratory, U.K., Rep, PO-TN-MEL-GS 4, 0005 (2000).

3.

M. Wang and W. Shi, “Estimation of ocean contribution at the MODIS near-infrared wavelengths along the east coast of the U.S.: Two case studies,” Geophys. Res. Lett. 32(13), L13606 (2005), doi:. [CrossRef]

4.

M. Wang and W. Shi, “The NIR-SWIR combined atmospheric correction approach for MODIS ocean color data processing,” Opt. Express 15(24), 15722–15733 (2007). [CrossRef] [PubMed]

5.

R. Doerffer and H. Schiller, “MERIS regional coastal and lake case 2 water project atmospheric correction ATBD,” Institute for Coastal Research, GKSS Research Center, Geesthacht, Germany, Rep, GKSS-KOF-MERIS-ATB D01, 1 (2008).

6.

P. J. Werdell, S. W. Bailey, B. A. Franz, L. W. Harding Jr, G. C. Feldman, and C. R. McClain, “Regional and seasonal variability of chlorophyll-a in Chesapeake Bay as observed by SeaWiFS and MODIS-Aqua,” Remote Sens. Environ. 113(6), 1319–1330 (2009). [CrossRef]

7.

A. A. Gitelson, G. Keydan, and V. Shishkin, “Inland waters quality assessment from satellite data in visible range of the spectrum,” Sov. Remote Sens. 6, 28–36 (1985).

8.

R. P. Stumpf and M. A. Tyler, “Satellite detection of bloom and pigment distributions in estuaries,” Remote Sens. Environ. 24(3), 385–404 (1988). [CrossRef]

9.

A. A. Gitelson, “The peak near 700 nm on radiance spectra of algae and water: relationships of its magnitude and position with chlorophyll concentration,” Int. J. Remote Sens. 13(17), 3367–3373 (1992). [CrossRef]

10.

A. A. Gitelson, G. Dall’Olmo, W. Moses, D. C. Rundquist, T. Barrow, T. R. Fisher, D. Gurlin, and J. Holz, “A simple semi-analytical model for remote estimation of chlorophyll-a in turbid waters: Validation,” Remote Sens. Environ. 112(9), 3582–3593 (2008). [CrossRef]

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H. R. Gordon, “Diffuse reflectance of the ocean: the theory of its augmentation by chlorophyll a fluorescence at 685 nm,” Appl. Opt. 18(8), 1161–1166 (1979). [CrossRef] [PubMed]

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A. Vasilkov and O. Kopelevich, “Reasons for the appearance of the maximum near 700 nm in the radiance spectrum emitted by the ocean layer,” Oceanology (Mosc.) 22, 697–701 (1982).

13.

J. F. Schalles, “Optical Remote Sensing Techniques to Estimate Phytoplankton Chlorophyll a Concentrations in Coastal Waters with Varying Suspended Matter and CDOM Concentrations,” in Remote Sensing of Aquatic Coastal Ecosystem Processes: Science and Management Applications, L.L. Richardson and E.F. LeDrew, eds. (Springer, 2006), Chap. 3.

14.

G. Dall'Olmo, A. A. Gitelson, and D. C. Rundquist, “Towards a unified approach for remote estimation of chlorophyll-a in both terrestrial vegetation and turbid productive waters,” Geophys. Res. Lett. 30(18), 1938 (2003), doi:. [CrossRef]

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L. Han and D. Rundquist, “Comparison of NIR/Red ratio and first derivative of reflectance in estimating algal-chlorophyll concentration: a case study in a turbid reservoir,” Remote Sens. Environ. 62(3), 253–261 (1997). [CrossRef]

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C. Le, Y. Li, Y. Zha, D. Sun, C. Huang, and H. Lu, “A four-band semi-analytical model for estimating chlorophyll a in highly turbid lakes: The case of Taihu Lake, China,” Remote Sens. Environ. 113(6), 1175–1182 (2009). [CrossRef]

17.

H. J. Gons, M. Rijkeboer, and K. G. Ruddick, “A chlorophyll-retrieval algorithm for satellite imagery (Medium Resolution Imaging Spectrometer) of inland and coastal waters,” J. Plankton Res. 24(9), 947–951 (2002). [CrossRef]

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K. G. Ruddick, H. J. Gons, M. Rijkeboer, and G. Tilstone, “Optical remote sensing of chlorophyll a in case 2 waters by use of an adaptive two-band algorithm with optimal error properties,” Appl. Opt. 40(21), 3575–3585 (2001). [CrossRef]

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J. Gower, S. King, G. Borstad, and L. Brown, “Detection of intense plankton blooms using the 709 nm band of the MERIS imaging spectrometer,” Int. J. Remote Sens. 26(9), 2005–2012 (2005). [CrossRef]

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G. Dall’Olmo and A. A. Gitelson, “Effect of bio-optical parameter variability on the remote estimation of chlorophyll-a concentration in turbid productive waters: experimental results,” Appl. Opt. 44(3), 412–422 (2005). [CrossRef] [PubMed]

21.

A. A. Gitelson, J. Schalles, and C. M. Hladik, “Remote chlorophyll-a retrieval in turbid productive estuarine: Chesapeake Bay case study,” Remote Sens. Environ. 109(4), 464–472 (2007). [CrossRef]

22.

G. Dall’Olmo and A. A. Gitelson, “Effect of bio-optical parameter variability and uncertainties in reflectance measurements on the remote estimation of chlorophyll-a concentration in turbid productive waters: modeling results,” Appl. Opt. 45(15), 3577–3592 (2006). [CrossRef] [PubMed]

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24.

A. Gilerson, J. Zhou, S. Hlaing, I. Ioannou, J. Schalles, B. Gross, F. Moshary, and S. Ahmed, “Fluorescence component in the reflectance spectra from coastal waters. Dependence on water composition,” Opt. Express 15(24), 15702–15721 (2007). [CrossRef] [PubMed]

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Z. P. Lee, http://www.ioccg.org/groups/OCAG_data.html.

26.

A. M. Ciotti, M. R. Lewis, and J. J. Cullen, “Assessment of the relationships between dominant cell size in natural phytoplankton communities and the spectral shape of the absorption coefficient,” Limnol. Oceanogr. 47(2), 404–417 (2002). [CrossRef]

27.

A. Bricaud, M. Babin, A. Morel, and H. Claustre, “Variability in the chlorophyll-specific absorption coefficients of natural phytoplankton: Analysis and parameterization,” J. Geophys. Res. 100(C7), 13321–13332 (1995). [CrossRef]

28.

A. A. Gitelson, D. Gurlin, W. J. Moses, and T. Barrow, “A bio-optical algorithm for the remote estimation of the chlorophyll-a concentration in case 2 waters,” Environ. Res. Lett. 4(4), 045003 (2009), doi:. [CrossRef]

29.

W. J. Moses, A. A. Gitelson, S. Berdnikov, and V. Povazhnyy, “Satellite Estimation of Chlorophyll-a Concentration Using the Red and NIR Bands of MERIS—The Azov Sea Case Study,” IEEE Geosci. Remote Sens. Lett. 6(4), 845–849 (2009). [CrossRef]

30.

H. R. Gordon, O. B. Brown, R. H. Evans, J. W. Brown, R. C. Smith, K. S. Baker, and D. K. Clark, “A semianalytic radiance model of ocean color,” J. Geophys. Res. 93(D9), 10909–10924 (1988). [CrossRef]

31.

R. C. Smith and K. S. Baker, “Optical properties of the clearest natural waters (200-800 nm),” Appl. Opt. 20(2), 177–184 (1981). [CrossRef] [PubMed]

32.

J. Zhou, A. Gilerson, I. Ioannou, S. Hlaing, J. Schalles, B. Gross, F. Moshary, and S. Ahmed, “Retrieving quantum yield of sun-induced chlorophyll fluorescence near surface from hyperspectral in-situ measurement in productive water,” Opt. Express 16(22), 17468–17483 (2008). [CrossRef] [PubMed]

33.

P. Gege, and A. Albert, “A tool for inverse modeling of spectral measurements in deep and shallow waters,” in Remote Sensing of Aquatic Coastal Ecosystem Processes: Science and Management Applications, L.L. Richardson and E.F. LeDrew, eds. Chap. 4, Springer, 2006.

34.

J. E. O'Reilly, and 24 Coauthors, “SeaWiFS Postlaunch Calibration and Validation Analyses, Part 3,” in NASA Tech. Memo. 2000–206892, Vol. 11, S. B. Hooker and E. R. Firestone, eds., (NASA Goddard Space Flight Center, Greenbelt, MD, 2000) 49 pp.

OCIS Codes
(010.4450) Atmospheric and oceanic optics : Oceanic optics
(280.0280) Remote sensing and sensors : Remote sensing and sensors

ToC Category:
Atmospheric and Oceanic Optics

History
Original Manuscript: August 25, 2010
Revised Manuscript: October 25, 2010
Manuscript Accepted: October 26, 2010
Published: November 3, 2010

Virtual Issues
Vol. 6, Iss. 1 Virtual Journal for Biomedical Optics

Citation
Alexander A. Gilerson, Anatoly A. Gitelson, Jing Zhou, Daniela Gurlin, Wesley Moses, Ioannis Ioannou, and Samir A. Ahmed, "Algorithms for remote estimation of chlorophyll-a in coastal and inland waters using red and near infrared bands," Opt. Express 18, 24109-24125 (2010)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-23-24109


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References

  1. G. Moore, J. Aiken, and J. Lavender, “The atmospheric correction of water colour and the quantitative retrieval of suspended particulate matter in Case II waters: application to MERIS,” Int. J. Remote Sens. 20(9), 1713–1733 (1999). [CrossRef]
  2. J. Aiken and G. Moore, “ATBD case 2 bright pixel atmospheric correction,” Center for Coastal & Marine Sciences, Plymouth Marine Laboratory, U.K., Rep, PO-TN-MEL-GS 4, 0005 (2000).
  3. M. Wang and W. Shi, “Estimation of ocean contribution at the MODIS near-infrared wavelengths along the east coast of the U.S.: Two case studies,” Geophys. Res. Lett. 32(13), L13606 (2005), doi:. [CrossRef]
  4. M. Wang and W. Shi, “The NIR-SWIR combined atmospheric correction approach for MODIS ocean color data processing,” Opt. Express 15(24), 15722–15733 (2007). [CrossRef] [PubMed]
  5. R. Doerffer and H. Schiller, “MERIS regional coastal and lake case 2 water project atmospheric correction ATBD,” Institute for Coastal Research, GKSS Research Center, Geesthacht, Germany, Rep, GKSS-KOF-MERIS-ATB D01, 1 (2008).
  6. P. J. Werdell, S. W. Bailey, B. A. Franz, L. W. Harding, G. C. Feldman, and C. R. McClain, “Regional and seasonal variability of chlorophyll-a in Chesapeake Bay as observed by SeaWiFS and MODIS-Aqua,” Remote Sens. Environ. 113(6), 1319–1330 (2009). [CrossRef]
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