Effect of emersion/immersion cycles on the elemental and isotopic compositions of the organic matter from surface sediments of an intertidal mud bank (French Guiana): a preliminary study.

RATIONALE: In various marine environments, the combination of elemental ratios and stable isotopic compositions is usually used to trace the sources of organic matter (OM) in sediments. However, in intertidal areas, the sediments might be temporarily exposed to air during a more or less prolonged duration and the impact of this exposure on the latter parameter is unknown. METHODS: The spatial variations of atomic Total Organic Carbon (TOC) and Total Nitrogen (TN) ratios, and δ(13)C and δ(15)N values, were measured on surface sediments of French Guiana, at the beginning and the end of five consecutive days of emersion during equinoctial tides, as well as at the beginning of a new emersion phase after returning to a normal tidal cycle. The concentrations of chlorophyll a and phaeopigments in sediments were also measured, in order to estimate the influence of microphytobenthos (MPB), growing at surface sediments, on these geochemical parameters. RESULTS: The results showed that the emersion/immersion cycles influenced the development of MPB at the surface sediments, which, in turn, significantly controlled the spatio-temporal changes in the atomic elemental ratios and the δ(13)C values. This variability seemed not to be significantly altered by OM degradation. On the contrary, sediments were always (15) N-enriched compared with OM sources, indicating that OM diagenetic processes mainly controlled their spatio-temporal fluctuations. CONCLUSIONS: The [TOC/TN]atomic ratios and the δ(13)C values, indicating the primary signal of OM sources, represented the most reliable geochemical proxies for calculating the relative contribution of OM sources to sediments in environments characterized by variable air exposure duration. The use of δ(15)N values in such environments is limited by OM degradation processes but their variation might enable the nature and the degree of these processes to be identified.

Benthic impact assessment of a dredge sediment disposal in a dynamic resilient environment.

The eastern part of the Bay of Seine (English Channel) is highly impacted by harbour activities and the dumping of dredged sediment by the port authorities of Le Havre (GPMH) and Rouen (GPMR). Sediment dredged by the GPMH (2-2.5 millions of m3 per year) has been disposed at the subtidal Octeville site since 1947. Since the 2000s, mainly fine sediment (80% of fine particles <63 µm) has been disposed using alternate mosaic boxes with limited thickness (0.2 to 0.6 m per box per year), preventing the accumulation of disposed sediment in some parts of the dumping site. During the period August 2016 to September 2017, an experimental study was set up to identify the spatio-temporal changes of the macrobenthos collected at ten stations on six dates: three stations where different volumes of sediments were dumped (from 41,000 to 186,000 m3), two stations located within the Octeville site but without dumping operations during the study and five stations outside the disposal site (northern and southern zones). The Taxonomic Richness, total abundance and abundances of the polychaete Owenia fusiformis show negative correlations with the volumes of deposited sediment, whereas use of the AMBI (AZTI's Marine Biotic Index) and B2OA (Benthic Opportunist Annelids Amphipods) indices does not allow us to distinguish the impacted zone. Our study shows that the impact of dumping remains local and the benthic habitats display a high degree of resilience with rapid recovery of the community after the cessation of disturbance. Recommendations on the future long-term strategy are proposed to improve assessment and minimise the impact of dumping sediment on this benthic habitat.