Mapping microphytobenthos in the intertidal zone of Northern ...

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Figure 8. Chlorophyll a surface concentration maps derived from ROSIS data: a – “false color composite” (red: 752.5 nm; green: 672.5 nm; blue: 632.5 nm); b – simple band ratio; c – normalized ratio; d – scaled band area Table 2 . Comparison between Chl a concentrations (mg.m -2 ) derived from field samples and from ROSIS data – NR: Normalized ratio; SBA: Scaled Band Area. SITES SAMPLES RATIO NR SBA I 474.39 12.90 10.23 27.44 J 199.05 13.99 13.4 23.53 K 360.45 8.38 6.92 22.37 L 91.76 10.56 6.78 19.98 M 517.88 47.36 48.37 69.04 N 204.72 11.20 10.17 25.23 O 98.86 24.55 20.86 39.80 P 5.12 42.43 35.70 53.60 Q 108.14 52.62 49.90 71.07 R 149.04 11.34 13.42 31.25 S 17.93 6.09 7.37 15.53 T 3.39 2.19 1.12 4.72 U 12.28 4.73 7.23 14.38 406
4. CONCLUSIONS Preliminary results regarding the simple approaches developed here indicate that all three methods are rather robust (R 2 > 0.8) when applied to field spectra. There seems to be very little difference between the three, indicating either that there is very little influence of measurement conditions in the spectral range considered or that the field sampling technique averages out this impact. In addition, the instrument used in the field is fairly stable which explains why integrating over the absorption feature does not dramatically improve the estimation. When applied to airborne CASI and ROSIS data, all three techniques appear also equally robust when looking at the new regression laws derived from field spectra convolved to instrument bandpasses. Better results were obtained for ROSIS data which was to be expected as ROSIS has a higher spectral resolution than CASI and contiguous bands. Inversion of the resulting functions on the images produced Chl a concentration maps that appeared qualitatively correct. Unfortunately, CASI maps could not be validated through comparison with field samples but appeared overestimated based on our knowledge of the area. On the other hand, ROSIS Chl a maps could be validated and show an underestimation of Chl a concentration when compared to reference sites. These discrepancies could mainly be explained by the scaling factor between field spectral measurements and airborne instrument pixel size. It is well known that the spatial distribution of the biofilm is generally fairly patchy; therefore field sampling and spectral measurements might not be representative of a 2 x 2 m pixel. In addition, both data sets present different characteristics for the same spatial resolution. CASI data have a coarser spectral resolution than field spectra and were acquired under poor weather conditions, preventing for an accurate atmospheric correction. ROSIS data have a higher spectral resolution and contiguous bands but are fairly noisy which partly explains why the scaled band area approach gives better results as it “smoothes” the noise. In both cases, instrument calibration might also be an important factor. These preliminary results clearly show that further studies are required to better understand the influence of the sampling strategy (particularly estimating Chl a concentration within the first centimeter of sediment) and sediment optical properties on the spectral signature at various scales. Field measurements have to be extended to other sites with different sediment characteristics. Sampling strategies have to be compared and validated in order to extrapolate to airborne instrument pixel size. We are also investigating the possibility of using this approach to identify and quantify other pigments, leading to an estimation of biodiversity as pigments can be used as fingerprints for different groups of microalgae. ACKNOWLEDGEMENTS This work was supported by the French “Programme National d’Environnement Côtier”. CASI data were acquired thanks to special funding from Université des Sciences et Techniques de Lille. ROSIS data were acquired as part of the 2001 HySens campaign. The author would particularly like to thank the DLR HySens Team for providing atmospherically and geometrically corrected data and for their great flexibility in acquiring data under such strong constraints. Samuel Degézelle and Nicolas Spilmont helped with the Chlorophyll a extraction and Aurélie Robin characterized the Bay d’Authie sediment samples. REFERENCES [1] BLANCHARD, G.F., GUARINI, J.M., RICHARD, P., GROS, P., AND MORNET, F., 1996: Quantifying the short-term temperature effect of light-saturated photosynthesis of intertidal microphytobenthos. Mar. Ecol. Prog. Ser. 134:309-313. [2] BLANCHARD, G.F., GUARINI, J.M., GROS, P., AND RICHARD, P., 1997: Seasonal effect on the relationship between the photosynthetic capacity of intertidal microphytobenthos and temperature. J. Phycol. 33:723-728. [3] JONGE DE, V.N., 1980: Fluctuations in the organic carbon to chlorophyll a ratio for estuarine benthic diatom populations. Mar Ecol Prog Ser 2:345-353. [4] MCINTYRE, H.L., GEIDER, R.J., AND MILLER, D.C., 1996: Microphytobenthos: the ecological role of the ‘secret garden’ of unvegetated, shallow-water marine habitats. Distribution, abundance and primary production. Estuaries 19:186-201. 407
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