Over the last two centuries, the sediment cover of the mouth of the Seine estuary has changed significantly, from predominantly sandy facies (between 1913 and 1967) to predominantly muddy facies (end of the 20th century). This trend is linked to the natural evolution of the estuary, which tends to fill up, but also to the industrial and port developments which have modified its hydrodynamics, favouring first the trapping of finer sediments, then the displacement of this sedimentation towards the mouth [downstream migration of the turbidity maximum (Lesourd et al., 2016)]. Since 2006, we have reported a homogenisation of siltation in the eastern part of the bay, due to a 'spreading' of fine particles onto the adjacent sandier facies, particularly along the coast of the Pays de Caux. This sedimentation dynamics is reflected, at the scale of the study area, by a reduction in silted areas in favour of the development of areas of sandy mud and silted sand.

The analysis of the data on the benthic macrofauna showed the following main results

1. The installation of new species that are associated with silted sectors of the bay (Melinna palmata, Asthenognathus atlanticus) or the American knife Ensis leei. Several observations of Melinna palmata (polychaete annelid) and Ensis leei (bivalve mollusc) have been made in the eastern part of the Bay of Seine since the late 1990s, indicating a recent change in the benthic fauna of this region (Dauvin et al., 2007). M. palmata had never been recorded in the Bay of Seine, although populations are thriving in the sandy mud habitats of costal embayments of the western Channel along the French coasts (e.g. in the nearby Rade de Cherbourg), and in all the coastal bays in southern England (e.g. the nearest in the Solent, Southampton). It was first collected in 2002 and its community-wide installation was described following the 2006 PECTOW campaign. Recently, based in part on data from recent Pectow surveys, Baffreau et al (2017) suggested the presence of a Melinna palmata facies in the Abra alba community. The sampling in 2006 of an abundant population of E. leei, a non-indigenous species, confirms its establishment, which was first observed in 1998. The introduction of M. palmata is probably linked to the recent silting up of the eastern part of the bay, while that of E. leei is related to the south-western spreading (from the Scheldt estuary to the Seine estuary) of this invasive species. For these two species with a complex life cycle, the colonisation of the Bay of Seine may result from a natural larval supply from neighbouring populations or from an accidental introduction due to man (i.e. ballast water).

Another more recent result is the discovery, during the PECTOW 2011 campaign and other previous campaigns carried out in other projects, of the presence of the small decapod crustacean of Atlantic origin Asthenognathus atlanticus Monod, 1932 (Jourde et al. 2012). This is the first record of this species in the eastern English Channel basin. The geographical range of A. atlanticus previously extended along the eastern coasts of the Atlantic Ocean, from Angola to the western part of the English Channel, which was its northern limit. The installation of this new muddy species confirms the impact of recent changes in the superficial sediment composition of the Eastern Bay of Seine on the benthic communities. Several hypotheses have been put forward to explain the arrival of this species: (1) a natural colonisation by the dispersal of larvae originating from populations in the western Channel and, (2) the possibility of a more distant origin, the introduction of the individuals collected being linked to human activities. In the current state of knowledge, neither hypothesis can be excluded.

2. Contrasting spatial distribution of the dominant species of the Abra alba community in three patches of high abundance that are stable over time, probably in response to structuring environmental gradients on a regional scale (e.g. sedimentary gradient). These three abundance patches are found each year in three main areas: (1) in the north-eastern part of the bay, between Cap de la Hève and the Port d'Antifer, (2) in the downstream part of the mouth estuary, in the extension of the North Trench, and (3) between Cabourg and Ouistreham along the Calvados coast. These abundance patches, whatever their exact position, are spread along a coastal strip between Ouistreham and Antifer. This particular distribution of macrofauna abundance and biomass is therefore an important element in terms of trophic transfer from macrofauna to fish and contribute to the feeding grounds role of the bay. This spatial organization of the community was also observed when considering the images obtained in 2008-2009 during the COLMATAGE campaigns carried out in September, which gives the system a high degree of long-term stability in its spatial structuring in abundance patches, despite the strong anthropic pressures that the benthic communities in this sector are subject to.

3. Strong spatial structure of the community into different species assemblages that differ both in terms of richness and the relative densities of the most dominant species: the Lagis koreni facies in front of the Seine estuary and off Deauville, the Owenia fusiformis - Acrocnida brachiata facies between Antifer and Ouistreham, and the Phaxas pellucidus facies at the periphery of the study area. As Thiébaut et al. (1997) have already mentioned with regard to the persistence of certain faunal assemblages or populations, this spatial structure suggests the existence of sufficiently robust and stable environmental factors over the long term to perpetuate this aspect of faunal organisation in patches of high abundance that are relatively homogeneous in terms of faunal composition. 

4. A relative spatio-temporal stability of this spatial structure despite the strong inter-annual variability of the recruitment of certain species. There is indeed inter-annual variability in the organisation of populations and faunal assemblages which does not systematically concern all three abundance patches in a synchronous manner. Thus, only a few stations at the mouth of the estuary, and between Cabourg and Ouistreham, are characterised by very high values each year. In contrast, in the north-eastern part of the Bay of Seine, abundances were much lower in 1996. The latest analyses, which include data from 1988 to 2016, nevertheless suggest a trend towards homogenisation of the different assemblages in relation with the sediment homogenisation mentioned above (Bacouillard et al., 2020). 

5. The perception of the stability of the community must be qualified according to the spatial scale of observation. At the scale of the entire study area, neither the species richness nor the diversity indices show any significant temporal trends. At the scale of a group of stations or "multi-stations", the community structure persists over time around the three assemblages mentioned above. On the scale of a station, the temporal variations in densities are much more complex and respond to a strong variability in recruitment. The most variable stations are located in front of the Seine estuary and off Deauville, in response to the complex morpho-sedimentary dynamics in these sectors. The notion of stability-variability in the organisation of the community therefore appear to be clearly dependent on the scale of observation and thus justify the sampling strategy adopted during the Pectow campaigns.