Spatial Distributions of Surface Sedimentary Organics and Sediment Profile Image Characteristics in a High-Energy Temperate Marine RiOMar: The West Gironde Mud Patch

Abstract

The spatial distributions of (1) surface sediment characteristics (D0.5, Sediment Surface Area (SSA), Particulate Organic Carbon (POC), Chlorophyll-a (Chl-a), Phaeophytin-a (Phaeo-a), Total and Enzymatically Hydrolyzable Amino Acids (THAA, EHAA), δ13C) and (2) sediment profile image (apparent Redox Potential Discontinuity (aRPD), numbers and depths of biological traces) characteristics were quantified based on the sampling of 32 stations located within the West Gironde Mud Patch (Bay of Biscay, NE Atlantic) in view of (1) assessing the spatial structuration of a temperate river-dominated ocean margin located in a high-energy area, (2) disentangling the impacts of hydrodynamics and bottom trawling on this structuration, and (3) comparing the West Gironde Mud Patch with the Rhône River Prodelta (located in a low-energy area). Results support the subdivision of the West Gironde Mud Patch in a proximal and a distal part and show (1) the existence of depth gradients in surface sedimentary organics characteristics and bioturbation within the distal part; (2) no evidence for a significant effect of bottom trawling, as opposed to Bottom Shear Stress, on the West Gironde Mud Patch spatial structuration; and (3) major discrepancies between spatial structuration in the West Gironde Mud Patch and the Rhône River Prodelta, which were attributed to differences in tidal regimes, sedimentation processes, and local hydrodynamics, which is in agreement with current river-dominated ocean margin typologies.

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

“…Growth of microphytobenthos on ES intertidal flats may provide enough input of fresh OM to maintain a high reactivity score while fueling increased levels of carbon degradation. This pattern of more reactive OM in shallow vs deep marine habitats has also been found on continental shelf sediments surrounding the United Kingdom and France (Lamarque et al, 2021;Smeaton and Austin, 2022). Reduced DSi concentrations found in intertidal (vs. subtidal) porewater samples (Fig.…”

“…The ratio chl a/(chl a + phaeophytin) was used as a proxy for organic matter reactivity as assessed with sediment pigments (Bonifácio et al, 2014;Lamarque et al, 2021;Pastor et al, 2011). For solid-P and solid-Fe extraction, a ~500 mg subsample of sediment was obtained within an anaerobic glove box.…”

First observations of seasonal bottom water deoxygenation off the Gironde estuary (Bay of Biscay, North East Atlantic)

“…Dissolved oxygen measurements highlighted substantial deoxygenation of the bottom waters of the WGMP, down to a DO saturation of 45%, in July 2019, October 2016 and 2021. This was very surprising for such an environment subjected to strong hydrodynamics that control its sedimentological and ecological structuration (Lesueur et al, 2002;Dubosq et al, 2021;Lamarque et al, 2021). These DO depletions are related to the establishment of a thermal stratification from spring to autumn (Figure 3).…”

“…Among them, the West Gironde Mud Patch (WGMP) is a welldefined sedimentary area located about 25 km off the Gironde and comprised between 30 and 75 m depth (Figure 1B). The WGMP is of particular interest due to its economic importance for common sole (Solea solea) and Norway lobster (Nephrops norvegicus) fishing but also for its biological activity (Lamarque et al, 2021) and its capability to store carbon (Dubosq et al, 2021). Its sedimentary functioning is studied since the 1990's and its benthic ecological structuration was recently defined (Lesueur et al, 1991;Lesueur et al, 2002;Dubosq et al, 2021;Lamarque et al, 2021;Fontanier et al, 2022).…”

“…The WGMP is of particular interest due to its economic importance for common sole (Solea solea) and Norway lobster (Nephrops norvegicus) fishing but also for its biological activity (Lamarque et al, 2021) and its capability to store carbon (Dubosq et al, 2021). Its sedimentary functioning is studied since the 1990's and its benthic ecological structuration was recently defined (Lesueur et al, 1991;Lesueur et al, 2002;Dubosq et al, 2021;Lamarque et al, 2021;Fontanier et al, 2022). However, there was still no information on DO in the WGMP water column.…”