Microphytobenthos interannual variations in a north-European estuary (Loire estuary, France) detected by visible-infrared multispectral remote sensing

Estuarine intertidal sediments are colonized by photosynthetic microorganisms grouped under the generic term of microphytobenthos (MPB). These microbial assemblages form transient biofilms at the sediment surface and have important ecosystem functions. MPB biofilms are well known to exhibit high microscale patchiness whereas meso- and macroscale spatio-temporal structures are little known. In this work, satellite remote sensing was used to map MPB assemblages at such scales. MPB interannual distribution was investigated in the poly- and mesohaline domain of the north-European estuary (Loire estuary), using a multispectral SPOT image time series (1991–2009). The Normalized Difference Vegetation Index (NDVI) was calculated from two SPOT channels, XS2 and XS3, (red and near-infrared wavelengths, respectively). MPB biofilms were identified by NDVI values between 0 and 0.3. At the scale of the whole intertidal area, the results showed that MPB biofilms in the Loire estuary exhibited perennial structures in both the polyhaline and mesohaline sectors, occupying nearly 90% of the mudflat surfaces. MPB biofilm density was closely associated with intertidal position, with thicker biofilms developing mostly in the upper and middle shore, and formed kilometric longitudinal structures parallel to the shoreline. Mean NDVI values showed a strong positive correlation with mean seasonal air temperature (τ = 0.714, p < 0.05 in the polyhaline domain and τ = 0.810, p < 0.05 in the mesohaline domain), with a strong correlation in the upper intertidal mudflat (between +3 and 4 m isobaths). Negative wind effect was mainly detected in the upper intertidal areas, particularly between the +3 and 4 m isobaths (τ = −0.810, p < 0.05 in the polyhaline domain and τ = −0.910 in the mesohaline).