Increase in seagrass distribution at Bourgneuf Bay (France) detected by spatial remote sensing

Abstract

The changes in spatial distribution of intertidal Zostera noltii seagrass beds were studied with multispectral visible-infrared remote sensing in Bourgneuf Bay (France) over a 14-year period, between 1991 and 2005. Six SPOT satellite images acquired at low tide were calibrated using in situ spectroradiometric data and processed with the Normalized Difference Vegetation Index (NDVI). A steady and linear increase in meadow areas was observed between 1991 and 2005 with total surfaces colonized by Z. noltii increasing from 208 to 586 ha, respectively. A greater increase in the densest part of the meadow (NDVI > 0.4) was also observed: it represented only 15% of total meadow surfaces in 1991 vs. 35% in 2005. The seagrass expansion took place mainly towards the lower part of the intertidal zone, while in the upper intertidal zone the meadow appeared strictly limited by the +4 m (Lowest Astronomical Tide) bathymetric level. The influence of Z. noltii above-ground biomass variations on spectral reflectance was analyzed experimentally by spectrometry. Z. noltii displays a characteristic steep slope from 700 to 900 nm, increasing with increasing biomass. A quantitative relationship obtained experimentally between NDVI and the dry weight of leaves was used to produce a biomass distribution map. The distribution of Bourgneuf Bay intertidal seagrass beds is certainly constrained by the water turbidity and we suggest that tidal flat accretion could be a significant variable explaining the observed expansion downwards. With very limited spatial interactions, oyster aquaculture cannot be considered as a threat, while a recent increase in recreational hand fishing of Manila clams within the beds could become problematic.