Map of macrofauna sampling points of the Rewrite Project, in Ria de Aveiro Lagoon.
RS sampling of Aveiro Lagoon
Methods
Macrofauna sampling was conducted during the field campaign in the Ria de Aveiro Lagoon, which took place from July 14th to 18th, 2025. Sampling effort was distributed across two sites: “Rewilded” and “Natural.” In this study, the sampling points will be compared with metrics derived from Sentinel-2 imagery. The Sentinel-2 image used was acquired on July 15th, 2025, and corresponds to a Level-2A product, which has been atmospherically corrected and orthorectified by the data provider using Sen2Cor.
Images processing
All image processing was performed in R, using the terra, sf and the tidyverse packages.
NDVI
The NDVI as been computed using the following equation:
\[ NDVI = (B8-B4)/(B8+B4) \]
Seagrass Percent Cover
The methodology applied in this study is adapted from Zoffoli et al. (2021).
Seagrass Percentage Cover (SPC) was derived from the NDVI using the equation established by Zoffoli et al. (2020):
\[ SPC = 172.06 * NDVI - 22.18 \] ### SHSI
This methodology is currently under review but a draft of the submitted paper can be found here.
Based on the observed spectral changes in seagrasses exposed to HWs, we developed a new radiometric index to better detect the radiometric change caused by the HW. The browning of the leaves was characterized by substantial radiometric changes in both the green and red-edge spectral regions. The Seagrass Heat Shock Index (SHSI) was introduced as the reflectance line height at 740 nm, compared to the 560 - 842 nm baseline. Namely the SHSI subtracts the reflectance observed at 740 nm to the linearly interpolated reflectance between 560 and 842 nm, so that the index is positive in the case of brown, HW-impacted seagrass leaves, and negative in the case of green, non-impacted leaves. Compared to the submited version of the index, the SHSI used in this study has been normalized to constrain its values strictly between -1 and 1:
\[ \text{SHSI} = (I_{SHSI} - R(740))/(I_{SHSI} + R(740)) \]
where :
\[ I_{SHSI} = R(560) + \tau [R(842) - R(560)] \] and :
\[ \tau = \frac{740 - 560}{842 - 560} \]
where \(R(560)\), \(R(740)\), and \(R(842)\) represent the reflectance at 560, 740, and 842 nm, respectively, and \(\tau\) is equal to a contant of 0.64. These wavelengths were selected to align with the spectral resolution of satellites missions such as Sentinel-2, for broader remote sensing application.
Results
Figure 1 shows the spatial distribution of the three satellite-derived metrics considered for the rewilded study site in the Ria de Aveiro Lagoon. The average NDVI within the extent of the macrofauna sampling stations was only 0.25, with a maximum of 0.48 observed in the central part of the meadow. When converted to Seagrass Percent Cover (SPC), this site exhibited a low average SPC of 23.12%, just above the 20% threshold commonly used to define a seagrass meadow (Zoffoli et al., 2020). In the densest part of the meadow, SPC reached up to 60.55%. Regarding the SHSI, which serves as an indicator of heat-induced color change in seagrass, the average value was -0.11, suggesting that the meadow was not affected by a heatwave at the time of observation.
Figure 1 shows the spatial distribution of the three satellite-derived metrics considered for the natural study site in the Ria de Aveiro Lagoon. The average NDVI within the extent of the macrofauna sampling stations was 0.34, with a maximum of 0.55 observed in the central part of the meadow. When converted to Seagrass Percent Cover (SPC), this site exhibited a higher average SPC compared to the rewilded site, with an average of 38.28%. In the densest part of the meadow, SPC reached up to 71.82%. Regarding the SHSI, which serves as an indicator of heat-induced color change in seagrass, the average value was -0.09, suggesting that the meadow was not affected by a heatwave at the time of observation. However, some pixel of that study sites are showing SHSI values close to 0 and sometime positive, maybe suggesting heat-induce discoloration over some part of the meadow.
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