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Combination of field sampling and multispectral remote sensing to study the development of intertidal macroalgal communities at local scale in Western Brittany

Authors:

Abstract

Macroalgal communities are major components of temperate coastal ecosystems, since they play an essential role in primary production and develop various habitats. Recently, variations in the extent of intertidal seaweed communities were noticed and have been attributed to the global climatic change. To better understand these variations, this work aims at 1- evaluating the ecological state of intertidal communities 2- developing a canopy indicator assessing the development of the community based on remote sensing. Field sampling was conducted between January and July 2021 on 9 sites of Western Brittany (France). Macroalgal communities were monitored by sampling macroalgal species and the sessile fauna. Then, the quality index Ics (Index of community structure), which assess the structural state of macroalgal communities and shows their development, was calculated. Four bathymetric levels were considered on the shore, from the highest to the lowest: 1. Pelvetia canaliculata / Fucus spiralis 2. Ascophyllum nodosum / Fucus vesiculosus 3. Fucus serratus and 4. Himanthalia elongata / Bifurcaria birfurcata / Laminaria digitata. In parallel with biological sampling, a satellite acquisition (Pléiades) was used to estimate the cover of macroalgae and the Normalized Difference Vegetation Index (NDVI) was calculated. Results show a positive correlation between field sampling (Ics) and remote sensing data (NDVI) for the mid bathymetric levels of the shore, i.e 2. and 3., with respectively Pearson’s r coefficients of 0.52 and 0.45. On the contrary, no correlation was evidenced between Ics and NDVI for both highest and lowest levels of the shore. It may be due to the higher seaweed cover in the mid bathymetric level compared to the highest level, and to some more variability in the results for the lowest level because of tide oscillations. Refining this approach using hyperspectral UAV imaging (spatial resolution: 2cm, spectral resolution: 3nm with 200 spectral bands) is in progress.
COMBINATION OF FIELD SAMPLING AND MULTISPECTRAL REMOTE SENSING
TO STUDY THE DEVELOPMENT OF INTERTIDAL MACROALGAL COMMUNITIES AT LOCAL SCALE IN WESTERN BRITTANY
Context of the study
Macroalgal communities are major
components of temperate coastal
ecosystems, since they play a key role in
primary production and provide habitats to
a large biodiversity.
We test here the benefit of combining field
sampling and satellite image acquisition in
order to study the development of intertidal
macroagal communities. By contrast with
field sampling restricted to local scale,
remote sensing based approaches tend to
offer a better solution for a larger scale
monitoring.
Copyright © 2021 Diruit Wendy Wendy.Diruit@univ-brest.fr
Bajjouk Touria, Le Bris Anthony, Richier Sophie
Burel Thomas, Ar Gall Erwan
Results
Ar Gall, E., & Le Duff, M. (2014). Development of a quality index to evaluate the structure of macroalgal communities.
Estuarine, Coastal and Shelf Science, 139: 99-109.
Bajjouk, T., Guillaumont, B., & Populus, J. (1996). Application of airborne imaging spectrometry system data to intertidal
seaweed classification and mapping. Hydrobiologia, 326/327: 463-471.
Burel, T., Schaal, G., … & Ar Gall, E. (2019c). Small-scale effects of hydrodynamics on the structure of intertidal macroalgal
communities: A novel approach. Estuarine, Coastal and Shelf Science, 226: 106290.
Rossiter, T., Furey, T., … & Stengel, D. B. (2020). Application of multiplatform, multispectral remote sensors for mapping
intertidal macroalgae: A comparative approach. Aquatic Conservation: Marine and Freshwater Ecosystems, 30: 1595-1612.
ISWEE-MS-1451
Material and Methods
Field sampling: intra-community analysis
structuration index : Ics
(Index of community structure)
Remote sensing:
intra-community analysis
Pléiades acquisitions :
50 cm spatial resolution
4 spectral bands (RGB-NIR)
vegetation index: NDVI
(Normalized Difference
Vegetation Index)
Higher macroalgal cover on lower shore.
Increase of covers from top to bottom of
the shore, in particular for red algae < 30
cm.
NDVI lower in Pc-Fspi (0.2) and higher in
He-Ld (0.79).
Ics and NDVI are correlated in An and
Fser, but not in Pc-Fspi and He-Ld.
This first approach evidences correlations
between the NDVI index and the Ics indicator
in the mid-bathymetric levels showing less
diversity but a higher cover of brown algae
(less subjected to uncertainty such as
substrates, desiccation, tide...). Further analyses
are in progress in order to improve macroalgal
identification using hyperspectral UAV, offering
a better spectral and spatial resolution.
Discussion
Bathymetric levels determined by macroalgal
dominance:
1. Pelvetia canaliculata Fucus spiralis (Pc-Fspi)
2. Ascophyllum nodosum Fucus vesiculosus (An)
3. Fucus serratus (Fser)
4. Himanthalia elongata Bifurcaria bifurcata
Laminaria digitata (He-Ld)
Green
Algae Red
Algae
Brown
Algae
Crustose < 30 cm 30 cm 1 m > 1 m
Satellite images (Pléiades)
NDVI map with
sampling spots in
Trégunc
r= -0.24
He-Ld
Ics
NDVI
≤ 0
0.2
0.4
0.6
≥ 0.8
9 sites within 100 km, 4 bathymetric levels,
6 spots/level
Covers/group
and stratum
NDVI/Ics
correlations
Pc-Fspi
%
0
20
40
60
An
0
20
40
60
80
100
%
Ics
NDVI
r= 0.15
Pc-Fspi
%
Fser
0
50
100
150
r = 0.52***An
Ics
r= 0.42**
Fser
NDVI
He-Ld
%
0
20
40
60
80
100
120
140
Ics
NDVI
50 m
0 m
Upper shore
Lower shore
40 km
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