Spatial and Temporal Variability of Sediment Organic Matter Recycling in Two Temperate Eutrophicated Estuaries

Abstract

International audienceThis paper deals with the spatial and seasonal recycling of organic matter in sediments of two temperate small estuaries (Elorn and Aulne, France). The spatio-temporal distribution of oxygen, nutrient and metal concentrations as well as the organic carbon and nitrogen contents in surficial sediments were determined and diffusive oxygen fluxes were calculated. In order to assess the source of organic carbon (OC) in the two estuaries, the isotopic composition of carbon (δ 13C) was also measured. The temporal variation of organic matter recycling was studied during four seasons in order to understand the driving forces of sediment mineralization and storage in these temperate estuaries. Low spatial variability of vertical profiles of oxygen, nutrient, and metal concentrations and diffusive oxygen fluxes were monitored at the station scale (within meters of the exact location) and cross-section scale. We observed diffusive oxygen fluxes around 15 mmol m−2 day−1 in the Elorn estuary and 10 mmol m−2 day−1 in the Aulne estuary. The outer (marine) stations of the two estuaries displayed similar diffusive O2 fluxes. Suboxic and anoxic mineralization was large in the sediments from the two estuaries as shown by the rapid removal of very high bottom water concentrations of NO x − (>200 μM) and the large NH4 + increase at depth at all stations. OC contents and C/N ratios were high in upstream sediments (11-15 % d.w. and 4-6, respectively) and decreased downstream to values around 2 % d.w. and C/N ≤ 10. δ 13C values show that the organic matter has different origins in the two watersheds as exemplified by lower δ 13C values in the Aulne watershed. A high increase of δ 13C and C/N values was visible in the two estuaries from upstream to downstream indicating a progressive mixing of terrestrial with marine organic matter. The Elorn estuary is influenced by human activities in its watershed (urban area, animal farming) which suggest the input of labile organic matter, whereas the Aulne estuary displays larger river primary production which can be either mineralized in the water column or transferred to the lower estuary, thus leaving a lower mineralization in Aulne than Elorn estuary. This study highlights that (1) meter scale heterogeneity of benthic biogeochemical properties can be low in small and linear macrotidal estuaries, (2) two estuaries that are geographically close can show different pattern of organic matter origin and recycling related to human activities on watersheds, (3) small estuaries can have an important role in recycling and retention of organic matter

Phosphorus speciation, transformation, and preservation in the coastal area of Rushan Bay

“…The Tot-P contents of the cores are relatively higher than the Tot-P contents measured in the Yellow Sea (Liu et al, 2004) and other continental shelves of Eastern China (Table 3). However, the Tot-P contents of the cores are lower than the Tot-P contents of samples from inner Rushan Bay (Wang et al, 2008) and some other coastal areas around the world (Mort et al, 2010;Raimonet et al, 2013;Khalil et al, 2013;Zaaboub et al, 2014), where the sediment organic matter and water column DRP contents are relatively higher than contents in the study.…”

“…f Andrieux-Loyer et al (2008). g Raimonet et al (2013) and Khalil et al (2013). h Zaaboub et al (2014).…”

Long-term changes in ecosystem functioning of a coastal bay expected from a shifting balance between intertidal and subtidal habitats

“…Coastal sediments are well known for their high primary production and importance in global biogeochemical cycling (Woodward, 2007) These habitats receive OM and nutrients from autochthonous (e.g., local algal blooms) and allochthonous sources (e.g., terrestrial including sewage and fertilizers; Gilbert et al, 2007) which enter the sediment by tidal action, sedimentation, and sediment reworking by benthic organisms (Arndt et al, 2013;Kang et al, 2015). Other essential ecosystem functions mediated by coastal sediments include the mineralization of organic matter (OM), bentho-pelagic nutrient exchange, as well as the cycling, and the export and/or removal of inorganic nutrients (Khalil et al, 2013;Lessin et al, 2018;Magalhães et al, 2002).…”

Comparison of benthic oxygen exchange measured by aquatic Eddy Covariance and Benthic Chambers in two contrasting coastal biotopes (Bay of Brest, France)

“…Indeed, key processes (biological activities vs. chemical redox reactions) typically occur at different spatial and temporal scales, which cannot be simultaneously fully captured using classical methodologies. For instance, total benthic O2 exchange is typically quantified using incubation techniques either in situ or in sediment cores incubated at the laboratory (McGlathery et al, 2001, Berg et al, 2003, Martin et al, 2005, Huettel et al, 2007, Khalil et al, 2013. Although fundamental insights into the role of benthic fauna on biogeochemical processes have been gained using both methodologies, benthic chambers (BC) have significant limitations.…”