• Energy Research

Monitoring photosynthesis is a great challenge to improve our knowledge of plant productivity at the ecosystem level, which may be achieved using remote-sensing techniques with synoptic abilities. The main objective of the current study is to take up this challenge for microphytobenthos (MPB) primary production in intertidal mudflats. This was achieved by coupling hyperspectral radiometry (reflectance, ρ and second derivative, δδ) and PAM-fluorometry (non-sequential light curve, NSLC) measurements. The later allowed the estimation of the primary production via the light use efficiency (LUE) and the electron transport rate (ETR) whereas ρ allowed to estimate pigment composition and optical absorption cross-section (a*). Five MPB species representative of the main growth forms: epipelic (benthic motile), epipsammic (benthic motile and non motile) and thycoplanktonic (temporarily resuspended in the water column) were lighted at increasing light intensity from dark to 1950 µmol photons.m-2.s-1. After spectral measurements, a* was retrieved using a radiative transfer model and several radiometric indices were tested for their capacity to predict LUE and ETR. The spectral estimation of these two photosynthetic variables was subsequently compared to the values estimated by PAM-fluorometry. Results showed that different responses related to the xanthophyll cycle (de-epoxydation state) were observed for the three growth-forms with increasing light levels. However, a single relationship with radiometric index was not affected by species/growth-forms, i.e. δδ496/508, called the MPBLUE index to predict LUE and ETR. This index has the potential to be applied to air borne hyperspectral imagery for large-scale assessment of MPB production.

Monitoring photosynthesis is a great challenge to improve our knowledge of plant productivity at the ecosystem level, which may be achieved using remote-sensing techniques with synoptic abilities. The main objective of the current study is to take up this challenge for microphytobenthos (MPB) primary production in intertidal mudflats. This was achieved by coupling hyperspectral radiometry (reflectance, ρ and second derivative, δδ) and PAM-fluorometry (non-sequential light curve, NSLC) measurements. The later allowed the estimation of the primary production via the light use efficiency (LUE) and the electron transport rate (ETR) whereas ρ allowed to estimate pigment composition and optical absorption cross-section (a*). Five MPB species representative of the main growth forms: epipelic (benthic motile), epipsammic (benthic motile and non motile) and thycoplanktonic (temporarily resuspended in the water column) were lighted at increasing light intensity from dark to 1950 µmol photons.m-2.s-1. After spectral measurements, a* was retrieved using a radiative transfer model and several radiometric indices were tested for their capacity to predict LUE and ETR. The spectral estimation of these two photosynthetic variables was subsequently compared to the values estimated by PAM-fluorometry. Results showed that different responses related to the xanthophyll cycle (de-epoxydation state) were observed for the three growth-forms with increasing light levels. However, a single relationship with radiometric index was not affected by species/growth-forms, i.e. δδ496/508, called the MPBLUE index to predict LUE and ETR. This index has the potential to be applied to air borne hyperspectral imagery for large-scale assessment of MPB production.

The Pacific oyster, Crassostrea gigas (Thunberg), is the main bivalve species cultivated in the world. With global warming enabling its reproduction and larval survival at higher latitudes, this species is now recognized as invasive and creates wild oyster reefs globally. In this study, the spatial distribution of photosynthetic assemblages colonizing the shells of wild C. gigas was investigated on both a large scale (two contrasting types of reefs found in mudflats and rocky areas) and a small scale (within individual shells) using a hyperspectral imager. The microspatial distribution of all phototrophs was obtained by mapping the Normalized Difference Vegetation Index (NDVI). Second derivative (δδ) analyses of hyperspectral images at 462, 524, 571 and 647 nm were subsequently applied to map diatoms, cyanobacteria, rhodophytes and chlorophytes, respectively. A concomitant pigment analysis was carried out by high performance liquid chromatography and completed by taxonomic observations. This study showed that there was high microalgal diversity associated with wild oyster shells and that there were differences in the structure of the phototropic assemblages depending on the type of reef. Namely, vertically-growing oysters in mudflat areas had a higher biomass of epizoic diatoms (hyperspectral proxy at δδ462 nm) and were mainly colonized by species of the genera Navicula, Nitzschia and Hippodonta, which are epipelic or motile epipsammic. The assemblages on the horizontal oysters contained more tychoplanktonic diatoms (e.g. Thalassiosira pseudonana, T. proschkinae and Plagiogrammopsis vanheurckii). Three species of boring cyanobacteria were observed for both types of reef: Mastigocoleus testarum, Leptolyngbya terrebrans, and Hyella caespistosa, but the second derivative analysis at 524 nm showed a significantly higher biomass for the horizontally-growing oysters. There was no biomass difference for the boring chlorophyte assemblages (δδ647 nm), with two species: Eugomontia testarum and Ostreobium quekettii observed for both types of reef. This study shows that oyster shells are an idiosyncratic but ubiquitous habitat for phototrophic assemblages. The contribution of these assemblages in terms of biomass and production to the functioning of coastal areas, and particularly to shellfish ecosystems, remains to be evaluated.

The Pacific oyster, Crassostrea gigas (Thunberg), is the main bivalve species cultivated in the world. With global warming enabling its reproduction and larval survival at higher latitudes, this species is now recognized as invasive and creates wild oyster reefs globally. In this study, the spatial distribution of photosynthetic assemblages colonizing the shells of wild C. gigas was investigated on both a large scale (two contrasting types of reefs found in mudflats and rocky areas) and a small scale (within individual shells) using a hyperspectral imager. The microspatial distribution of all phototrophs was obtained by mapping the Normalized Difference Vegetation Index (NDVI). Second derivative (δδ) analyses of hyperspectral images at 462, 524, 571 and 647 nm were subsequently applied to map diatoms, cyanobacteria, rhodophytes and chlorophytes, respectively. A concomitant pigment analysis was carried out by high performance liquid chromatography and completed by taxonomic observations. This study showed that there was high microalgal diversity associated with wild oyster shells and that there were differences in the structure of the phototropic assemblages depending on the type of reef. Namely, vertically-growing oysters in mudflat areas had a higher biomass of epizoic diatoms (hyperspectral proxy at δδ462 nm) and were mainly colonized by species of the genera Navicula, Nitzschia and Hippodonta, which are epipelic or motile epipsammic. The assemblages on the horizontal oysters contained more tychoplanktonic diatoms (e.g. Thalassiosira pseudonana, T. proschkinae and Plagiogrammopsis vanheurckii). Three species of boring cyanobacteria were observed for both types of reef: Mastigocoleus testarum, Leptolyngbya terrebrans, and Hyella caespistosa, but the second derivative analysis at 524 nm showed a significantly higher biomass for the horizontally-growing oysters. There was no biomass difference for the boring chlorophyte assemblages (δδ647 nm), with two species: Eugomontia testarum and Ostreobium quekettii observed for both types of reef. This study shows that oyster shells are an idiosyncratic but ubiquitous habitat for phototrophic assemblages. The contribution of these assemblages in terms of biomass and production to the functioning of coastal areas, and particularly to shellfish ecosystems, remains to be evaluated.