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.

The metazoan parasite community of Cerastoderma edule was studied in the southern geographical range of the host (the coastal lagoon Merja Zerga, Morocco). A total of 11 metazoan species was found in cockles. Nine of these were trematodes using cockles as either first intermediate host (three species) or second intermediate host (six species). In addition, two other endo-metazoan species (Pinnotheres pisum and Paravortex cardii) were recorded from cockles in the studied lagoon. All the observed metazoans in cockles from Merja Zerga have previously been recorded at sites north of Africa.Up to 10% of the cockles in the studied size-groups were first intermediate hosts to castrating parasites (Gymnophallus choledochus, Labratrema minimus and Monorchis parvus). Among trematodes having metacercariae in cockles (second intermediate host) Meiogymnophallus minutus was the most widespread as it was observed in all cockles from all the examined habitats in the lagoon and it occurred in record high intensities. Different sub-communities of the trematode fauna using cockles as second intermediate host could be identified (subtidal vs intertidal associations).The richness and species composition of the macroparasite community in cockles from Morocco are discussed in relation to patterns seen in cockles from other sites along their geographical range. Migratory fish and waterbirds (final hosts) are generally responsible for the large scale spread (latitudinal spread) of trematodes. However, the distributional patterns of involved intermediate hosts in the life-cycles of the different trematode species in cockles are determining the richness and species composition patterns seen in cockles at shallow water sites along the east Atlantic shoreline.

Merja Zerga lagoon, located on the Moroccan Atlantic coast, is a site of international value (Ramsar Site) in terms of its ornithological diversity. However, the lagoon is heavily exploited for its clams and fishes. In an effort to further understanding of lagoon ecosystems, and thus to facilitate the management and conservation of their resources, an ecological survey of its benthic component was carried out. Benthos is a valuable food source for birds, fishes and humans. This work involved identifying the macrozoobenthic communities in the lagoon and assessing their spatial distribution. The study was based on monthly sampling of the intertidal zone and seasonal sampling in subtidal zone, over a one-year period. In the intertidal zone, salinity and median diameter and silt content of the sediment exhibited a gradient extending from the entrance to the inner lagoon, according to tidal flow. Sediment grain size characteristics reflected a gradual decrease of tidal currents from the lagoon entrance towards the inner parts of the lagoon, i.e. silt content increased with distance from the entrance. In the subtidal zone, the tidal currents were fairly strong throughout the lagoon, leading to the presence of coarser sediments than in the intertidal zone. Based on these physical, chemical and substratum characteristics, three communities were identified: (i) Cerastoderma edule and (ii) Scrobicularia plana communities were located in both the intertidal and subtidal zones; and (iii) a Tapes decussata community that was only found in the subtidal zone. The assemblages in the subtidal zone were more diverse and the mean abundances of the constituent species were higher than in the intertidal zone, which is an interesting feature for a lagoon environment. Communities were distributed along an ecological gradient, without showing a discontinuity or ecotone. The lagoon functioned like an estuary in which the community structure was controlled by edaphic factors in the intertidal zone, and by the hydrological factors in the subtidal zone.

This study analyses the seasonal distribution in the superficial sediments of the Marchica watershed of trace and major elements, grain size, and total organic carbon. Samples were taken during the wet and dry seasons of 2018 in the mainstream valleys carrying the discharges to the Marchica Lagoon. Ecotoxicological indices (Enrichment Factor, Contamination Factor, Pollution-Load Index) and sedimen-quality guidelines were used to evaluate the ecotoxicological risk. The results revealed during both wet and dry seasons contamination by lead in all stream valleys. The northwest stream valley, characterized by some industrial activities, and the stream valley crossing the abandoned mine were the most polluted by zinc and copper. Minimal pollution for Cr, Co, and Ba was found during both the wet and dry seasons. Despite the restoration actions achieved around the Ramsar site of Marchica lagoon during the last decades, the watershed continues to have a negative impact on this ecosystem. Therefore, an environmental management strategy isnecessary, taking into account the rehabilitation of the abandoned mining sites, the rational use of pesticides and fertilizers in agriculture, and the environmental responsibility of industrial companies, especially those installed before the implementation of the national law on environmental impact studies.

The Marchica lagoon is the single lagoon on the Mediterranean coast of Morocco. Beside its ecological and socio-economic values, the lagoon is under pressure of a complex mixture of human-mediated stressors (urbanization, pollution, overfishing, tourism, etc.). Nowadays, many institutional efforts have been undertaken to establish a sustainable management plan of the lagoon in the context of an Integrated Coastal Zone Management (ICZM) approach. Thus, understanding how these stressors impact upon ecological status and ecosystem services is a sound step for any efficient integral management. Based on sampling performed on June 2014, the present work aimed at assessing the ecological quality status (ES) of the northern part of the lagoon using Cymodocea nodosa (Ucria) Ascherson meadows and benthic macrofauna. 16 stations (11 with C. nodosa and 5 on bare sediment) were sampled using a Van Veen grab. Preliminary results showed differences between stations. Shoots biomass fluctuated between 8.02 and 61.2 g dw m -2 while the roots-rhizomes biomass oscillated between 10.8 and 235.5 g dw m -2 . The root-rhizome and leaf biomass ratio revealed high values (up to 10.8) in the central sector suggesting a potential nutrient enrichment. Benthic macrofauna revealed the presence of 90 species belonging to six zoological groups and dominated by crustaceans, molluscs and polychaetes. The assessment of the ecological quality status, based on benthic macrofauna, was evaluated using Biotic Indices (AMBI, M-AMBI, Benthix, BITS). The results showed partial agreement between single index-derived ES and underlined the dependency of these biotic indices on the habitat characteristics, on the one hand, and the necessity to define specific reference conditions to the Marchica lagoon.PeerJ PrePrints | https://dx.doi.org/10.7287/peerj.preprints.1082v1 | CC-BY 4.0 Open Access |