Mapping microphytobenthos in the intertidal zone of Northern ...

At present, mapping sediments or biomass in intertidal areas by conventional methods involves extensive sampling
programs that are often difficult in practice and expensive in time and manpower ([15], [24]). No matter how
extensive such programs are, the accuracy of the resultant maps is limited by the need to extrapolate from sample
sites to the whole area, usually by linking similar sites in a series of contours. Remote sensing may offer a more
efficient method of mapping sediment distribution if sediment types or features are unique in their reflection of
light and infrared radiation, since a much less extensive sampling program with more accurate extrapolation to the
whole would be involved. The potential of remote sensing for mapping and monitoring intertidal areas has been
realized and successfully applied by workers using both aerial photography and satellite imagery ([25], [26]).
Detailed mapping of intertidal seaweeds has been carried out in Canada by Zacharias et al. [27] and in France by
Bajjouk et al. [28] using Compact Airborne Spectrographic Imager (CASI). Remote sensing at higher spectral and
spatial resolutions using instrument such as CASI offers the prospect of extremely detailed land cover mapping and
the potential to model erosion and accretion rates of intertidal surfaces. Developments in hyperspectral remote
sensing have opened up the possibility of quantifying individual photosynthetic pigments within vegetation.
However, most published papers present empirical relationships between pigment concentration and various indices
([29], for example). Very few rely on the understanding of the physics of the reflectance signal to quantitatively
estimate pigment concentrations.
The objective of this paper is to present a comparison between three simple, physically based, techniques developed
to quantitatively estimate Chl a surface concentration in the intertidal zone from high spectral resolution field
spectrometer reflectance data. To test the robustness of these approaches, regression equations developed were used
to predict the concentrations in newly acquired field data sets.. These algorithms were then applied to airborne
CASI and ROSIS data. Finally, validation was performed, when possible, by using reference sites to compare Chl a
estimation from the images to field samples concentrations.
2. METHODOLOGY
2.1. Study sites
Selected study sites for field spectral measurements are located along the French coast of the Eastern English
Channel (Figure 1). To allow a wide range of concentrations and variable species, several campaigns took place at
various times of year. Measurements were performed at low tide, in 4 different test sites, reflecting different
conditions. From North to South:
- Site 1, Wimereux: this site is typical of an hydrodynamically exposed sandy beach habitat that can be
encountered on this shore. Measurements were performed on an area located in the upper intertidal zone, rather flat
and without sharp topographical features such as ripple marks, high pinnacles or deep surge channels. The substrate
was homogeneous medium size sand (200-250 µm, modal size), typical of the surrounding sandy habitat. Due to
the highly dynamic environment, microphytobenthos is resuspended and surface concentrations at low tide are low.
- Site 2, Baie d’Authie: here, several sites were measured in this macrotidal estuary where sandy to mixed
sandy flats prevail. The substrate composition varied from fine to medium grain sand mainly, with some coarse and
very fine grain mixed sandy sites. Microphytobenthos Chl a surface concentration varies as a function of sediment
grain size, estuary dynamics and seasonal blooms.
- Site 3, Baie de Somme: the Baie de Somme is the second ranked estuarine system, after the Seine
estuary, and the largest sandy-muddy (72 km 2 ) intertidal area on the French coasts of the Eastern English Channel.
This site is similar to the Baie d’Authie but the estuary being larger, sediment spatial variation occurs at a larger
scale, meaning that the measurements performed covered less diverse sediment types. The substrate grain size
varied mainly between 125 and 250 µm where most measurements were performed.
- Site 4, Baie de Seine: this site is very different from the previous ones as mudflats dominate where
measurements were performed. Particle size < 63 µm corresponded to more than 50% of the sediment composition.
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