Sediment dynamic equilibrium, a key for assessing a coastal anthropogenic disturbance using geochemical tracers: Application to the eastern part of the Bay of Seine

Abstract

Geochemical studies are becoming more and more frequent in the context of the increasing pressure of human activities on marine coastal ecosystems and represent an appropriate tool to assess anthropogenic disturbances. Moreover, it is difficult to find discriminant markers. The eastern part of the Bay of Seine (English Channel) is highly impacted by the presence of harbour activities, fishing and sediment extraction. Dredged sediment from the Grand Port Maritime du Havre (GPMH) are deposited at the subtidal Octeville site, in the north-eastern part of the mouth of the Seine estuary (mixed sediment area). To understand natural and anthropogenic sedimentary mechanisms in this area, a geochemical and sedimentological study was conducted at the beginning of 2016. A dense sampling campaign including 179 stations was carried out between Cap de La Hève and Cap d'Antifer. For comparison, sampling was carried out in the harbour (13 samples in basins strongly or very weakly dredged), in the dredged grab itself and in the Seine estuary sediment (one station in the brackish zone and two stations in the river freshwater zone). Elemental compositions were determined by X-Ray Fluorescence spectrometry and infrared spectrometry. Using PCA (Principal Component Analysis), seven constituents were selected (Si, As, S, Pb, Rb, organic Br and TOC) to determine the area influenced by dumped spoil and the sediment transport directions. Sediment areas in dynamic equilibrium display a TOC gradient perpendicular to the coastline, linked to granulometric variations due to a combination of the swell and tidal currents. In the study area, dredged sediment are finer grained and have undergone changes due to the influence of diagenetic processes characteristic of the harbour environment. As a result, these sediment are enriched in sulphides, Pb, TOC and Rb, which allows us to highlight the in-situ deposited sediment spoil. Dumped sediment and the area subject to their influence are clearly identified since they locally disrupt the natural dynamic equilibrium state.

Granulometric Analysis of Shallow Vents Sediments at Banderas Bay (Mexico)

“…According to our results, it would be very fine sands, while the mounds present, in addition to sands, very fine gravels due to the alteration of the basalt by hydrothermal discharges. The findings of this study possibly represent a dynamical equilibrium condition in the system between particle transport, erosion, and deposition, as proposed by Baux et al [27], but that relation should be studied in greater detail.…”

Long-term spatio-temporal changes of the muddy fine sand benthic community of the Bay of Seine (eastern English Channel)

“…variability of densities based on CV, -diversity). Whilst the eastern Bay of Seine is exposed to multiple anthropogenic stressors that have persisted over time, the main changes during the study period have been the evolution of sediment dynamics, resulting in repercussions at different scales: widespread increased siltation in the eastern Bay of Seine since 2006 as evidenced by our results and Lesourd et al (2016) at the regional scale, and large volumes of dredged and dumped sediments at the local scale [4.5 million m .y -1 at the Kannik dumping site (Marmin et al, 2016) and 2-2.5 million m 3 .y -1 at the Octeville dumping site (Baux et al, 2019)].…”

“…More recently, following several years of low Seine river flows, a decrease in muddy area was reported in parallel to an increase in sandy mud and muddy sand mostly in front of the estuary, which has spread over the bay in the form of fine-grained sediment and been incorporated within the sandy fraction (Lesourd et al, 2016). The sediment dynamics within the bay is also influenced by the dumping, in the Northern Channel of the Seine estuary (Kannik) and North of the Cap de la Hève (Octeville), of 6-7 million m 3 per year (cumulated from two major harbours) of dredged material from Seine river and harbour basins (Marmin et al, 2016;Baux et al, 2019). In addition to the disturbances resulting from man-made modifications, the Seine estuary is placed among the most contaminated estuaries with high levels of heavy metals, particularly cadmium and lead, and organic contaminants (e.g.…”

“…However, the first observation of Melinna palmata in 2002 (Dauvin et al, 2007), its ranking within the ten dominant species since 2011 and the increase in density of the polychaete Ampharete baltica, which are both associated with mud and muddy sand habitats (Oyenekan, 1988;Zettler et al, 2013), reflect major effects of the recent siltation of the eastern part of the Bay of Seine. The siltation resulted from both alteration of the morpho-sedimentary dynamics of the Seine estuary via river flows and flood intensity, and the increase of dredged sediment deposit following the extension of Le Havre harbour along the Cap de la Hève-Antifer area (Lesourd et al, 2001;Méar et al, 2018;Baux et al, 2019).…”

“…Using the study site of the eastern Bay of Seine, which is highly representative of urbanized coastal areas exposed to numerous anthropogenic threats (Dauvin, 2008;Marmin et al, 2016;Tecchio et al, 2016;Baux et al, 2019), this study investigated the spatio-temporal changes of species abundance, species composition and species diversity at different scales (-diversity, -diversity, -diversity) between 1988 and 2016. Based on the analysis of spatio-temporal data provided by the original long-term monitoring program of the muddy fine sand benthic community implemented in this area, we address two specific questions: (1) how did the benthic community vary through time and space according to the scale of observation (i.e.…”