Invasion mechanisms of Spartina anglica in salt marshes of the Bay of Arcachon and consequences for native vegetation species
Spartina anglica is a hybrid exotic cordgrass that inhabits coastal salt marshes. This species arrived in the Bay of Arcachon in the 1980s and since has importantly colonized the salt marshes and tidal flats formerly only occupied by the native Spartina maritima and Zostera noltei, respectively. This work aims at understanding, with an interdisciplinary perspective, the invasion mechanisms of this exotic cordgrass and the outcoming changes of its introduction in the Bay, both to the physical environment and to the native vegetation. Different approaches were considered in order to assess the niche occupancy by the exotic Spartina and its interactions with the native intertidal species, Spartina maritima and Zostera noltei.The analysis of aerial and satellite images has shown that, in about 30 years after the invasion, within a zone densely populated by the native Spartina, the global high marsh zone did not suffer significant changes with the arrival of the invasive species. Spartina anglica did not replace the existent marsh vegetation, it occupied empty niches along the intertidal area instead. Additionally, experimental works of cross transplantation and biomass measurements have corroborated that the native Spartina maritima offers resistance to the colonization by the exotic Spartina. It was also shown that the invasive occupies the same intertidal niche along the elevation and anoxic gradient than the native. The successful extension of Spartina anglica into the mudflat towards the inner Bay was related to its likely ability to tolerate physical disturbances, its strong growth plasticity in nutrient- and oxygen- rich patches and its self-facilitator behaviour. This latter trait is related to its strong ecosystem-engineering ability due to its prominent root system and consequent ability to ameliorate the oxygenation of highly anoxic soils.The main effect of the exotic Spartina species on the seagrass is related to its stronger ecosystem-engineering ability, favouring bed accretion up to levels that are not favourable to Z. noltei through enhancement of desiccation stress. However, hydrodynamic and altimetry measurements have shown that the process of bed accretion is slow and, due to the cordgrass’ specific preferential biomass allocation to roots, its efficiency is more linked to its resistance to erosion rather than sediment trapping.The results of this study provide relevant information for the definition of appropriate action and conservation strategies of marsh zones in the Bay of Arcachon, and in particular the importance of limiting physical disturbance and nutrient pollution that could disrupt the biotic resistance of the native cord grass. They also suggest a potentially important role of the exotic species in facing increasing Sea Level Rise.
(05/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Aquatic Organic Matter in the Seine Basin: Sources, Spatio-Temporal Variability, Impact of Urban Discharges and Influence on Micro-pollutant Speciation
This research has been conducted over the last 10 years to characterise the spatio-temporal variability of aquatic organic matter (OM) composition in the Seine River watershed upstream and downstream of Paris Megacity and its effect on micro-pollutants. For this purpose, a large number of samples were collected under different hydrological conditions, and, over 1 year, three representative sites were monitored monthly. Furthermore, the evolution of the OM composition along an urbanisation gradient, from upstream to downstream of the Paris agglomeration, was characterised, highlighting the very strong impact of urban discharges, especially during low-water periods. Substantial differences in the chemical composition are emphasised relative to the urban or natural origin of the organic matter. Dissolved organic matter (DOM) interactions with metallic and organic micro-pollutants were studied, allowing us to (1) identify the key role of DOM on their speciation and bioavailability in aquatic systems and (2) demonstrate that these interactions depend on DOM composition and origin. The essential role of urban DOM on the speciation of trace metals in the Seine River downstream of the Paris agglomeration is also shown.
(. vol. 90, pp. 217-242, 03/06/2019)
LEESU, ENPC, UPEC UP12, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
CLIMATE CONTROL ON WINTER WAVE ACTIVITY IN THE NORTHEAST ATLANTIC AND IMPACTS ON BEACH EROSION AND RECOVERY ALONG THE WEST COAST OF EUROPE
The west coast of Europe is exposed to high-energy winter waves generated in the North Atlantic Ocean by low pressure systems traveling eastward. The present study builds on recent research on atmospheric and ocean wave climate in the North East Atlantic over the last 70 years using numerical weather and wave hindcast, in-situ sea-level pressure measurements, and beach response from a unique dataset of decadal beach morphological changes along the west coast of Europe. Interannual to decadal variability of winter wave activity in the North East Atlantic is strongly affected by the North Atlantic Oscillation (NAO) and the West Europe Pressure Anomaly (WEPA). It is found that extreme winter-mean wave heights become more frequent and variability increases, resulting in extreme winters such as that of 2013/14. We show that the recovery signature is site-specific and multi-annual, and that interannual variability of both erosion and recovery is well correlated with WEPA and NAO.
(01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LETG - Brest, LETG, UNICAEN, NU, UA, EPHE, PSL, UBO EPE, UR2, CNRS, IGARUN, UN
SHORECASTS: A BLIND-TEST OF SHORELINE MODELS
Predictions of shoreline change are of great societal importance, but models tend to be tested and tuned for the specific site of interest. To overcome this issue and test the ability of numerical models to simulate shoreline change over the medium scale (order of years) we have organized a non-competitive competition where participants were given data to train their model (1999-2014) and data to predict seasonal to inter-annual future changes (2014-2017). Participants were shown the observed shoreline changes only after submission of their modelling results. Overall, 19 numerical models were tested, the vast majority falling in the broad categories of "hybrid models" or "machine learning". Models were able to reproduce the mean characteristics of shoreline change but often failed to reproduce the observed rapid changes induced by storms.
(01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BRGM
ATMOSPHERIC CLIMATE CONTROL OF DIRECTIONAL WAVES IN THE UNITED KINGDOM AND IRELAND
Understanding multi-annual to decadal atmospheric climate controls on winter-wave climate is critical for coastal vulnerability assessment and future development of 'season ahead' forecasting of coastal risk. We examine the relationships between winter-average climate indices (NAO and WEPA) and directional wave power at 63 inshore locations throughout the United Kingdom and Ireland (UK&I). Analysis of hindcast wave data between 1980-2017 illustrate the extent of directional bi-modality, with 67% of inshore sites displaying directionally multimodal wave climates. Analysis of directional modes as a function of climatic indices illustrated the control exerted by NAO/WEPA on directional balance of inshore winter wave climate. Along Channel and southern North Sea coasts +WEPA significantly explains winter-averaged wave power for southwesterly wave directional modes (r = 0.58-0.77) and-NAO significantly explains variability in all easterly wave modes (r = 0.6-0.76), providing a mechanism for which 'season ahead' inshore wave climate forecasting and rotational beach response can be based.
(01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, LETG - Brest, LETG, UNICAEN, NU, UA, EPHE, PSL, UBO EPE, UR2, CNRS, IGARUN, UN
CROSS-SHORE SANDBARS RESPONSE TO AN ARTIFICIAL REEF: AN INTERSITE COMPARISON
While a clear improvement concerning aesthetic considerations using soft submerged breakwater is undeniable, their design has often focused on wave energy decrease in their lee, overlooking their impact on the dynamics of the nearby nearshore sandbar(s). At the beach of Sète (southeast France), the submerged structure clearly affects the natural net offshore migration cycle (NOM) of the former double barred beach. On the contrary, at Narrowneck (Queensland, Australia), the deployment of a multi-functional submerged structure does not affect the cross-shore sandbar processes. These contrasting behaviors are addressed using high frequency video monitoring. After discussing observations at both field sites, a process-based morphodynamic model provides insight into the morphological sandbars response to artificial reefs.
(01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
OBSERVATIONS AND NUMERICAL MODEL RESULTS OF MORPHODYNAMIC FEEDBACK OWING TO WAVE-CURRENT INTERACTION
Predicting the evolution of mound formations in the nearshore, both natural (e.g. bars) and man-made (e.g. nourishments) requires an understanding of mixed wave-current flow on sediment transport. MODEX (Morphological Diffusivity EXperiment) used a wave-current-sediment flume to observe in detail the evolution of a sand mound in shallow water. Imposed flow included waves-alone, currents-alone, and combined wave-currents. MODEX observations show the impact of currents on waves around the mound and the feedback with morphology. Here, a wave-resolving model SWASH is used to simulate flow patterns around the mound as it diffuses, showing the impact of wave-current interaction on mound evolution. Further, simulations initialized with a diffused mound show the feedback between 3D bed deformation and wave-current flow structures. The results provide a framework for understanding and implementing intra-wave sediment transport in wave-resolving models. Extensions of the results to field conditions with mixed wave-current energy will be explored.
(01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
MODELING REGIONAL COASTAL EVOLUTION IN THE BIGHT OF BENIN, GULF OF GUINEA, WEST AFRICA
The Bight of Benin coast is marked by the presence of three deepwater harbours which have affected the stability of the shoreline. In addition, several studies pointed out the overall diminution of sand supply due to the dams on Volta river channel and climate change effects. The combination of all these factors leads to a mixture of natural and artificial components affecting the coastline evolution in regional and long term scales. Here, we modeled the shoreline in the Bight of Benin, using the CASCADE model. The results show that the overall shape is well maintained and shoreline changes pretty well reconstructed. But, unresolved detailed information did not allowed to consider cross-shore sediment exchange and local deviations can be observed. However, the CASCADE model can be used to investigate regional and long term solutions for decisions-makers in the concerned countries.
(01/06/2019)
LEGOS, IRD, UT3, Comue de Toulouse, INSU - CNRS, CNES, CNRS, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CEREGE, IRD, INRA, AMU, CdF (institution), INSU - CNRS, CNRS
MODEX: LABORATORY EXPERIMENT EXPLORING SEDIMENT SPREADING OF A MOUND UNDER WAVES AND CURRENTS
The dispersal of sand from submerged mounds in the nearshore is driven by the interplay of processes such as converging and recirculating flows, changing roughness, bed slope effects and wave focusing/refraction. This morphological diffusivity is key to understanding sand bars in shallow seas, tidal inlets, estuaries, and the nearshore response to human interventions such as nourishments and dredging. Most of the work on the evolution of submerged mounds has been based on fluvial studies, focusing on flow without waves. In these cases, circular mounds tend to deform to crescentic (barchan) shapes. In contrast, observations of sandbars and berms in the nearshore subjected to waves show much more complex translation and deformation behavior. This contribution introduces the laboratory MOrphological Diffusivity Experiment (MODEX) aimed at examining morphological diffusivity under different forcing conditions. The experiment particularly addresses the linkages between small scale (local) effects (e.g. bed slope, bedforms) on the adjustment of sandy mounds.
(01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, AGBAR, UB
Gene expression patterns and related enzymatic activities of detoxification and oxidative stress systems in zebrafish larvae exposed to the 2,4-dichlorophenoxyacetic acid herbicide.
The present study aims to assess the effects of 2,4-D herbicide on biotransformation and oxidative stress status of zebrafish larvae. Animals were exposed to a range of sublethal concentrations (0.02-0.8 mg/L) and biomarkers at transcriptomic level and biochemical level were assessed. Chemical analysis with showed that the bioaccumulation of 2,4-D in 96 hpf zebrafish larvae were increased in a concentration-dependent manner. This herbicide induced significant effects at both gene expression and enzymatic activities levels after at 96 hpf. Results of mRNA expression showed a differential transcription regulation with all target genes depending on the tested concentrations. The mRNA level of gsr and cyp1a were up regulated at the highest dose of herbicide (0.8 mg/L). The gene expression of gstp1 showed an up regulation at lower dose (0.02 mg/L) and a down regulation at the highest dose (0.8 mg/L) of 2,4-D. A significant induction of EROD activity and inhibition of GST activity were noted in groups exposed to 0.8 mg/L of 2,4-D. Considering the antioxidant defenses, the activity of CAT was increased in larvae exposed to 0.8 mg/L of herbicide and GPx activity was induced at lower doses of 2,4-D (0.02 and 0.051 mg/L). Moreover, peroxidative damage, assessed as MDA content, was markedly increased in larvae exposed to high 2,4-D concentration. Overall, the present study data indicate that bioaccumulation of 2,4-D in 96 hpf zebrafish larvae and alterations in detoxification and oxidative stress related parameters, likely associated with ROS production, which may endanger the embryo-larval stages development of fish.
(Chemosphere. vol. 224, n° 0045-6535, pp. 289-297, 01/06/2019)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS