Bourgneuf Bay

Bourgneuf Bay is a broad, shallow coastal bay situated on the French Atlantic coast, between the island of Noirmoutier and the mainland. Bordered by salt marshes, oyster farms, and mudflats, it is characterized by its extensive intertidal zones. The bay hosts large seagrass meadows, mainly composed of Zostera noltei, which cover wide areas of the seabed. These meadows are especially visible at low tide and stretch across the southern and western parts of the bay. As shown in Figure 1, the area considered in this study spans more than 23 km².

Figure 1: Intertidal Mask of Bourgneuf Bay

For this site, a time series combining images from the Landsat, SPOT, and Sentinel-2 missions has been used to monitor the seagrass meadow since 1985 (Table 1). This work was published by Zoffoli et al. (2021). The present work updates the time series from Zoffoli et al. (2021) by incorporating newly available images and computing additional metrics (see Methodology).

Table 1

Date	Capteur	Date	Capteur	Date	Capteur
septembre 13, 1985	Landsat	septembre 24, 2002	SPOT	septembre 30, 2015	Sentinel-2
octobre 02, 1986	Landsat	septembre 26, 2003	SPOT	août 22, 2016	Sentinel-2
juillet 26, 1987	Landsat	septembre 17, 2004	Landsat	octobre 06, 2017	Sentinel-2
octobre 15, 1988	SPOT	septembre 18, 2005	SPOT	septembre 14, 2018	Sentinel-2
septembre 04, 1990	Landsat	septembre 23, 2006	Landsat	septembre 16, 2019	Sentinel-2
septembre 28, 1991	SPOT	septembre 28, 2008	Landsat	septembre 05, 2020	Sentinel-2
septembre 20, 1993	SPOT	septembre 08, 2009	SPOT	septembre 23, 2021	Sentinel-2
octobre 11, 1995	Landsat	septembre 12, 2010	SPOT	août 11, 2022	Sentinel-2
septembre 07, 1997	SPOT	octobre 02, 2011	SPOT	septembre 03, 2023	Sentinel-2
septembre 21, 1998	SPOT	septembre 28, 2012	SPOT	septembre 17, 2024	Sentinel-2
septembre 02, 2001	Landsat	octobre 05, 2013	SPOT

List of Satellite images used to follow the meadows in Bourngeuf Bay

Seagrass Percent Cover time series

Figure 2: Seagrass Cover in Percent over the years of the meadow of Bourgneuf Bay. Only pixels with an SPC above 20 % are displayed.

Figure 2 is show the evolution of the Seagrass Cover over time, since 1985. The minimum density was recorded in 1988 with 28% of average cover, while the maximum density has been recorded in 2022 with 59% of average cover.

Historical seagrass patches of the meadow

Figure 3: Consecutive years with ≥50% seagrass cover since 2024. The value X for each pixel indicates that the meadow has maintained a Seagrass Percentage Cover (SPC) above 50% every year for the past X years, counting backward from 2024.

Figure 3 shows the number of consecutive years, starting in 2024, during which each pixel maintained a Seagrass Percentage Cover (SPC) above 50% every single year. One area of the meadow, covering 3100 m², has sustained an SPC above 50% for the past 29 years. Within this area, 200 m² have maintained an SPC above 50% for over 34 consecutive years without interruption. This patch is centered around the coordinates y= 46.9593738 and x= -2.1710815.

Year of the maximum of cover

Figure 4: Map of the year of maximum cover of the meadow

Figure 5: Area of the meadow reaching its maximum cover value ever for each year.

Figure 4 shows, for each pixel, the year when it reached its maximum Seagrass Percentage Cover (SPC). This same information is presented in Figure 5 as a histogram, where the area of meadow reaching its all-time maximum SPC is displayed for each year. Three key periods stand out: 1985–1986, 2002, and 2022. In 1985–1986, more than 15% of the meadow reached its highest SPC values in the time series. These are shown as red pixels in Figure 4, typically located along the edges of the meadow. In 2002, 28% of the meadow reached its historical maximum, corresponding to yellow areas in Figure 4, where the meadow expanded eastward and westward. Finally, in 2022, 52% of the meadow reached its highest recorded SPC, represented by blue areas in Figure 4, mainly located in the central part of the meadow.

Rate of decline since the date of maximum

Figure 6: Map of the rate of decline (in Percent Cover per Year) since the date of maximum shown in Figure 4

Figure 6 shows the rate of decline in seagrass cover, expressed as percent cover lost per year, since the year of maximum SPC indicated in Figure 4. On average, the meadow has been losing 1.78% of its seagrass cover per year since reaching its maximum. However, certain areas located in the central part of the meadow—likely where fishermen regularly carry out clam fishing activities—are experiencing much sharper declines, with rates reaching up to 25% per year.

Amount of time above 50% since the start of the time serie

Figure 7: Map of the amount of time each pixel had SPC values above 50% since the start of the time serie

Figure 7 shows the proportion of time each pixel maintained an SPC value above 50%. Most of the meadow displays SPC values above this threshold between 50% and 60% of the time. More stable areas, where SPC exceeded 50% more than 80–90% of the time, are found in restricted zones within the main meadow (around 46.960679, -2.172888). In the northwestern part of the meadow, near the seaport of Noirmoutier-en-l’Île, no pixel exceeds the 60% frequency. Finally, the southernmost meadow, located near the Gois, shows low frequencies, with SPC values above 50% reached only about 30% of the time.