Monitoring the Spatial and Interannual Dynamic of Zostera noltei.

Seagrass is a vital structural and functional element of the marine environment worldwide and is highly valued for its ecological benefits. Monitoring the evolution of the seagrass habitat is essential to understand how this coastal ecosystem changes, and to develop good environmental management practices. For the present study, two remote sensing methods were used to map and monitor Zostera noltei Hornemann, 1832 (Z. noltei), in the Merja Zerga lagoon from 2010 to 2020. These methods which are the random forest algorithm and the object-oriented classification, were convenient to provide significant results. The first approach employed Sentinel-2 images from 2018 to 2020, which were used to extract information on changes in Z. noltei (commonly called dwarf eelgrass) distribution and aboveground biomass estimation. The second involved three orthophotography (orthophoto) mosaics from the years 2010, 2016, and 2018, which were analyzed to map the distribution of the species. It was revealed that Z. noltei coverage has increased by 212 ha since 2010, with most of the growth occurring in the center and upstream part of the lagoon. The mean aboveground biomass of dwarf eelgrass in the lagoon was 78.5 DW/m² in 2018, 92.6 DW/m² in 2019, and 115.2 g DW/m² in 2020. The approach used in this study has provided important insights into the dynamic and mean biomass of Z. noltei in the Merja Zerga lagoon. It is therefore a valuable, non-destructive method that uses freely-available Sentinel-2 satellite data.

Marine heatwaves drive recurrent mass mortalities in the Mediterranean Sea.

Climate change is causing an increase in the frequency and intensity of marine heatwaves (MHWs) and mass mortality events (MMEs) of marine organisms are one of their main ecological impacts. Here, we show that during the 2015-2019 period, the Mediterranean Sea has experienced exceptional thermal conditions resulting in the onset of five consecutive years of widespread MMEs across the basin. These MMEs affected thousands of kilometers of coastline from the surface to 45 m, across a range of marine habitats and taxa (50 taxa across 8 phyla). Significant relationships were found between the incidence of MMEs and the heat exposure associated with MHWs observed both at the surface and across depths. Our findings reveal that the Mediterranean Sea is experiencing an acceleration of the ecological impacts of MHWs which poses an unprecedented threat to its ecosystems' health and functioning. Overall, we show that increasing the resolution of empirical observation is critical to enhancing our ability to more effectively understand and manage the consequences of climate change.

Trace element bioaccumulation in the seagrass Cymodocea nodosa from a polluted coastal lagoon: Biomonitoring implications.

This is the first investigation of the potential for using Cymodocea nodosa to biomonitor trace element (TE) contamination in Marchica lagoon (Morocco), a Mediterranean pollution hotspot. We measured concentrations of seven TEs in seagrass tissues (leaf-rhizome-root) and sediments. Single and multi-element indices confirmed that sediments near illegal discharges were heavily polluted and we predicted risks of frequent adverse biological effects in these areas. Four of the TEs increased concentrations in C. nodosa leaf and root along sediment pollution gradient. Leaves and roots were both good indicators of Cu and Cd contamination in sediment, whereas leaves were the best indicator of Zn and roots for Pb. This seagrass was not a bioindicator of Al, Cr and Ni contamination. These results show the bioaccumulation patterns of TEs in C. nodosa, and can be used to design biomonitoring programs.

Bacciger bacciger (Trematoda: Fellodistomidae) infection effects on wedge clam Donax trunculus condition.

Wedge clams Donax trunculus inhabit high-energy environments along sandy coasts of the northeastern Atlantic Ocean and the Mediterranean Sea. Two sites were sampled monthly, one in Morocco (Mehdia), where the density was normal, and one in France (Biscarosse), where the density was very low. We tested the hypothesis that the difference in density between the sites was related to infection by the trematode parasite Bacciger bacciger. Identity of both the parasite and the host were verified using anatomical and molecular criteria. Parasite prevalence (i.e. the percentage of parasitized clams) was almost 3 times higher at Biscarosse. At this site, overall prevalence reached 32% in July and was correlated with the migration of several individuals (with a prevalence of 88%) to the sediment surface. After this peak, prevalence decreased rapidly, suggesting death of parasitized clams. The deleterious effect of B. bacciger on wedge clams was also supported by our calculations indicating that the weight of the parasite made up to 56% of the total weight of the parasitized clams. However, condition indices of trematode-free clams were also lower in Biscarosse than in Mehdia or other sites, suggesting that other factors such as pollutants or microparasites (Microcytos sp.) may alter wedge clam population fitness in Biscarosse.

[Structure and bioassessment of benthic communities of a lagoonal ecosystem of the Atlantic Moroccan coast]. / Structure et bioévaluation de l'état écologique des communautés benthiques d'un écosystème lagunaire de la côte atlantique marocaine.

The Merja Zerga lagoon is a semi-enclosed marine ecosystem in which various types of human activities have been developed. This paper characterizes the biosedimentary units of the lagoon and defines a reference status of the quality and health of the macrozoobenthic communities that can be used as bioindicators of the quality of the global marine environment. Specific and functional diversity were high: 147 taxa were identified; they were distributed within seven main trophic groups. Trophic structure is dominated by the suspension-feeding bivalve Cerastoderma edule and the deposit-feeding bivalve Scrobicularia plana, while micrograzers and macroherbivores remain low. Biotic index values indicated that the site is moderately perturbed and that the benthic communities are unbalanced. Nevertheless, the communities showed a seasonal stability of abundances and a high specific richness all through the year.