Modelling of embayed beach equilibrium planform and rotation signal
9 Embayed beaches are highly attractive sandy beaches bounded laterally by rigid boundaries, which 10 deeply affect equilibrium beach planform and shoreline dynamics. We use LX-Shore, a state-of-the-art 11 shoreline change model coupled with a spectral wave model to address embayed beach shoreline 12 dynamics driven by longshore sediment transport processes. The model is applied to different 13 idealized embayed beach configurations including variations in headland lengths. The model simulates 14 a large range of equilibrium embayed beach planforms and associated spatial and temporal modes of 15 shoreline variability. For short headlands enabling occasional headland sand bypassing, both embayed 16 beach curvature and maximum erosion at the upwave side of the embayment increases with increasing 17 headland length. Beach curvature also increases with increasing headland length for headlands long 18 enough to prevent any headland sand bypassing. In contrast, at the same time, embayed beach 19 becomes increasingly curved and symmetric, with maximum localised erosion within the embayment 20 decreasing in intensity. When there is no headland sand bypassing, rotation signal decreases in 21 amplitude and becomes increasingly symmetric with increasing headland length. The modal (time-22 invariant) directional spreading of incident waves is critical to embayed beach behaviour, with the 23 envelope and variance of cross-shore shoreline change and time-averaged shoreline curvature all 24 increasing with decreasing modal directional spreading. Embayed beach rotation characteristic 25 timescale increases with increasing embayed beach length, while the narrower the embayment the 26 smaller the cross-shore amplitude of shoreline variability. Our simulations provide new insight into the 27 influence of embayment characteristics and incident wave conditions on equilibrium planform and 28 shoreline dynamics of embayed beaches. This work also implies that the degree of potential headland 29 sand bypassing should be taken into account for modelling of beach rotational dynamics and embayed 30 beach dynamic planform configuration. 31 Highlights 32 • Embayed beach shoreline response is simulated with a hybrid shoreline model 33 • Headland length and headland sediment bypassing control shoreline response 34 • Wave directional spreading is critical to both mean shoreline and rotation signal 35 • Embayment beach length controls rotation characteristic timescale 36 37 Keywords: embayed beach ; hybrid shoreline model ; headland length ; rotation ; equilibrium 38 beach planform; headland sand bypassing 39 40
(Geomorphology. vol. 369, n° 0169-555X, pp. 107367, 01/11/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BRGM
The 3rd workshop on sediment dynamics of muddy coasts and estuaries: An introduction and synthesis
The workshop series on sediment dynamics of muddy coasts and estuaries focuses on international frontier issues and major social needs, such as land-sea interactions, estuarine and coastal monitoring, numerical simulation, sediment transport and its biogeochemical effects. The Third Workshop was held in Qingdao, China, in November 2018, hosted by the Institute of Estuarine and Coastal Studies, Ocean University of China. As a result of the Third Workshop, this special issue contains 18 papers with case studies of muddy coasts in the Bohai, Yellow and East China Seas in China and other regions worldwide. These papers represent the most recent advances in Chinese and international estuarine and coastal sediment research in the topics including 1) In-situ observations of sediment dynamics in muddy coasts and estuaries and satellite remote sensing; 2) Modelling of sediment transport and associated sedimentary processes; 3) Fluid mud transport and process in bottom boundary layer, and 4) Blue bay remediation action plan and coastal restoration.
(Estuarine, Coastal and Shelf Science. vol. 245, n° 0272-7714, pp. 106994, 30/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Parasite Assemblages in a Bivalve Host Associated with Changes in Hydrodynamics
Studies on marine community structure and their modulation by horizontal and vertical abiotic gradients have focused on free-living species, neglecting parasites despite their important role in ecosystems. Field observations, a laboratory flume experiment and modelling were used to examine parasite infection patterns related to host tidal and spatial position along a Portuguese coastal lagoon (Ria de Aveiro). Cockles (Cerastoderma edule) and trematodes were used as a host-parasite model to test the hypotheses: (1) higher infection levels in subtidal cockles, positioned downstream and (2) higher current velocity in downstream areas with consequent higher cockle infection. Cockles were collected from two spatial and tidal positions in two seasons. The relationship between current velocity and cockle infection was examined experimentally by exposing cockles to cercariae of Himasthla elongata at two current velocities. Intertidal cockles displayed higher infection, highlighting vertical position as an important driver of infection possibly due to first host proximity and/or trematode life cycle facilitation (higher cockle exposure to final host predation). The hydrodynamic model showed that the Ria de Aveiro was abiotically heterogenous with cockles displaying higher infection levels in the downstream area. Multiple regression analysis demonstrated a positive correlation between salinity, dissolved oxygen, current velocity and trematode abundance. The influence of current velocity on infection success was corroborated experimentally. The present study demonstrated the importance of host vertical and horizontal position on trematode infection, highlighting the positive influence of current velocity by supporting settlement and promoting favourable abiotic conditions.
(Estuaries and Coasts, n° 1559-2723, 25/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Influence of Seasonal River Discharge on Tidal Propagation in the Ganges‐Brahmaputra‐Meghna Delta, Bangladesh
Significant research efforts have been devoted to understanding river-tide interactions in estuaries. However, studies on the impact of monsoon-driven fluctuations of river discharge are limited. Here, the role of varying river discharge on the tidal propagation and tidal limit along the Ganges-Brahmaputra-Meghna Delta (GBMD), a macrotidal estuary subject to seasonal and annual river discharge variations, is investigated. The Delft3D hydrodynamic model is validated and applied to an average flood year condition and nine idealized scenarios covering the typical hydrological conditions. Results reveal that the upper limit of the tidal propagation shifts 75 km upstream during the dry season. The residual water level slope and tidal damping rate increase with river discharge beyond 100 km from the estuary mouth. The balance between the generation and dissipation of quarterdiurnal tides shifts spatially as a result of changes in channel convergence and friction and temporally as a function of river discharge, which controls the total friction in the upper tidal river. The balance between tidal dissipation and generation depends on the residual velocity generated by river discharge and the velocity of the principal tides. The maximal generation of quarterdiurnal tides in the upper GBMD depends on the friction generated from the river-tide interaction. Critical river discharge thresholds produce an optimal condition of dissipation of semidiurnal tides and generation of quarterdiurnal tides through friction at the upper and middle estuary. River discharge above the critical river discharge amount more rapidly dissipates both semidiurnal and quarterdiurnal tides than generates quarterdiurnal tides from nonlinear interactions.
(Journal of Geophysical Research. Oceans. vol. 125, n° 2169-9275, pp. e2020JC016417, 12/10/2020)
UNSW, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Sandy beaches can survive sea-level rise
(Nature Climate Change, n° 1758-678X, 08/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, CEREGE, IRD, AMU, CdF (institution), INSU - CNRS, CNRS, INRAE
A Regional Assessment of Changes in Plant–Plant Interactions Along Topography Gradients in Tunisian Sebkhas
Facilitation among plants in dry ecosystems is crucial for diversity and ecosystem functioning and stability. However, the importance of facilitation in extremely stressful conditions is highly debated. We attempt to separate the effects of direct non-resource and resource stress factors on plant–plant interactions by assessing changes in facilitation and competition along salinity gradients at two contrasting levels of aridity. We selected eight saline depressions (hereafter sebkhas) in the wet and dry Mediterranean arid climate of North Africa, from central Tunisia to the Libyan border 500 km south-eastward. In each sebkha, we transplanted at four positions along the salinity gradient induced by topography, both in open areas and below dominant shrubs, three target species with contrasting tolerances to salinity stress. Target plant survival, soil electrical conductivity and moisture were recorded before and after the dry summer season in all treatments. Shrubs decreased salinity and drought stresses in all treatments, and facilitation was the dominant interaction. However, we found a strong collapse of facilitation along the salinity gradient, due to a dramatic mortality of the three target species both with and without neighbours above their threshold of salinity tolerance. Increasing aridity induced an earlier collapse of facilitation along the gradient. The three target species had contrasting responses to neighbours, with the least stress-tolerant species being facilitated and the two most stress-tolerant ones negatively affected by neighbours. Our study shows that disentangling resource and non-resource stresses along gradients and controlling for target species effects help understanding variation in plant–plant interactions under highly stressful conditions. © 2020, Springer Science+Business Media, LLC, part of Springer Nature.
(Ecosystems. vol. 24, n° 1432-9840, pp. 1024-1037, 06/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
How costly are metacercarial infections in a bivalve host? Effects of two trematode species on biochemical performance of cockles
Bivalve stocks have been decreasing in the last decades largely due to emergent diseases and consequent mass mortality episodes. Cerastoderma edule (the edible cockle) is one of the most exploited bivalves in Europe and is among the most common hosts for trematodes, the most prevalent macroparasites in coastal waters but yet poorly studied. Therefore, in the present study, this bivalve species was used as host model to determine if trematode infection exerts a negative effect on bivalve energy metabolism and balance and if the tissues targeted by different trematodes influence the metabolic cost, with physiological and biochemical consequences. Cockles were experimentally infected with two trematode species, Himasthla elongata and Renicola roscovitus, that infect the foot and palps, respectively. Trematode infection exerted a negative effect on the metabolism of C. edule, the second intermediate host, by reduction of oxygen consumption. A different host biochemical response was found depending on trematode species, especially in regard to the level of oxygen consumption decrease and the preferential accumulation of lipids and glycogen. This study represents a step towards the understanding of host-trematode relationships that can be used to better predict potential conservation threats to bivalve populations and to maximize the success of stock and disease management.
(Journal of Invertebrate Pathology, n° 0022-2011, pp. 107479, 01/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS
Dietary bioaccumulation of persistent organic pollutants in the common sole Solea solea in the context of global change. Part 1: Revisiting parameterisation and calibration of a DEB model to consider inter-individual variability in experimental and natural conditions
Studying adverse effects of chemical pressure on aquatic ecosystems needs a comprehensive knowledge of bioaccumulation mechanisms of pollutants in biota to predict internal concentrations, especially for Persistent Organic Pollutants (POPs). However, the large variability of responses in measured POP concentrations requires explicit consideration of both individual variability and environmental influences. Dynamic Energy Budget (DEB) theory provides a rigorous and generic conceptual framework for tackling these questions in a relevant mechanistic way. In the present study, parameterisation and calibration of previous DEB models for Solea solea were revisited in order to accurately represent the full life cycle with an original emphasis on larval stage, metamorphosis, reproduction rules and sexual differences. We first improved calibration thanks to the use of the estimation procedure developed by the DEB network coupled with a broad compilation of data from literature. Then, we validated this set of parameter estimates on independent datasets of i) individual monitoring of larval growth in controlled food conditions from a novel experiment, and ii) juvenile and adult growth, and female fecundity, from a natural population. Finally, we combined the DEB model developed in the present paper with we used a simple toxicokinetic (TK) model from literature. This TK model was also combined to a previous DEB model and was used to reproduce the mean trajectories of a growth and contamination dataset. We applied the same TK model with our DEB model considering inter-individual variability in food availability. This application highlighted the need to accurately consider inter-individual variability in ingestion to correctly estimate growth and contamination variability. The present work is the first step in the development of a mechanistic TK model that will be used in a companion paper for investigations of juvenile sole sensitivity to warming, nursery quality and prey contamination, in highly fluctuating estuarine environments.
(Ecological Modelling. vol. 433, n° 0304-3800, pp. 109224, 01/10/2020)
UR EABX, INRAE, LEMAR, IRD, IFREMER, UBO EPE, CNRS, IRD, EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, BE, IFREMER, IFREMER
Climate and land-use as the main drivers of recent environmental change in a mid-altitude mountain lake, Romanian Carpathians
Recent decades have been marked by unprecendented environmental changes which threaten the integrity of freshwater systems and their ecological value. Although most of these changes can be attributed to human activities, disentagling natural and anthropogenic drivers remains a challenge. In this study, surface sediments from Lake Ighiel, a mid-altitude site in the Carpathian Mts (Romania) were investigated following high-resolution sedimentological, geochemical, environmental magnetic and diatom analyses supported by historical cartographic and documentary evidence. Our results suggest that between 1920 and 1960 the study area experienced no significant anthropogenic impact. An excellent correspondence is observed between lake proxy responses (e.g., growth of submerged macrophytes, high detrital input, shifts in diatom assemblages) and parameters tracking natural hydroclimate variability (e.g., temperature, NAO). This highlights a dominant natural hydroclimatic control on the lacustrine system. From 1960 however, the depositional regime shifted markedly from laminated to homogenous clays; since then geochemical and magnetic data document a trend of significant (and on-going) subsurface erosion across the catchment. This is paralleled by a shift in lake ecosystem conditions denoting a strong response to an intensified anthropogenic impact, mainly through forestry. An increase in detrital input and marked changes in the diatom community are observed over the last three decades, alongside accelerated sedimentation rates following enhanced grazing and deforestation in the catchment. Recent shifts in diatom assemblages may also reflect forcing from atmospheric nitrogen (N) deposition, a key recent drive of diatom community turnover in mountain lakes. In general, enhanced human pressure alongside intermittent hydroclimate forcing drastically altered the landscape around Lake Ighiel and thus, the sedimentation regime and the ecosystem’s health. However, paleoenvironmental signals tracking natural hydroclimate variability are also clearly discernible in the proxy data. Our work illustrates the complex link between the drivers of catchment-scale impacts on one hand, and lake proxy responses on the other, highlighting the importance of an integrated historical and palaeolimnological approach to better assess lake system changes.
(PLoS ONE. vol. 15, n° 1932-6203, pp. e0239209, 01/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, UK, ICUB, UniBuc, UK
High‐Energy Surf Zone Currents and Headland Rips at a Geologically Constrained Mesotidal Beach
We analyze Eulerian and Lagrangian measurements of wave-induced circulation collected during a 3-week field experiment at a high-energy mesotidal barred beach with the presence of a 500-m headland and a submerged reef. Small changes in wave and tide conditions were found to largely impact circulation patterns. Three main regimes were identified depending on offshore wave obliquity: (1) Under shore-normal configuration, the flow was dominated by cross-shore motions, except for moderate waves at low tide, with the presence of a quasi-steady circulation cell on the reef. (2) Under shadowed configuration, an onshore-directed current flowing away from the headland and a weak oscillating eddy were present outside and inside the shadow region, respectively. (3) Under deflection configuration, a deflection rip flowing against the headland and extending well beyond the surf zone was present, with activity maximized around low tide for moderate waves. Under 4-m oblique waves, the deflection rip was active regardless of the tide with mean depth-averaged velocities up to 0.7 m/s 800-m offshore in 12-m depth, with energetic low-frequency fluctuations. Our results emphasize the ability of deflection rips to transport materials far offshore, suggesting that such rips can transport sediment beyond the depth of closure. This study indicates that a wide variety of wave-driven circulation patterns can occur and sometimes coexist on beaches with prominent geological settings. Changes in the dominant driving mechanism can occur as a result of small changes in wave and tide conditions, resulting in more spatially and temporally variable circulation than along open sandy beaches. Plain Language Summary Most field experiments about wave-induced circulation patterns have been conducted along open sandy beaches, while experiments in geologically constrained environments are scarce. We performed intensive field measurements at a high-energy beach with the presence of a 500-m headland and a submerged natural reef. Three main circulation patterns were identified depending on the offshore wave obliquity. For shore-normal waves, cross-shore motions dominated the nearshore region, while oblique wave configurations resulted in more complex horizontal circulation. In particular, under intense headland-directed longshore current, the flow was deflected seaward against the headland. This deflection resulted in an intense seaward flowing jet (deflection rip) extending well beyond the surf zone edge, particularly during storm conditions. Such findings highlight the ability of these deflection rips to dominate water and sediment exchanges between the nearshore and the inner shelf region. Our study further outlines the more spatially and temporally variable circulation patterns occurring along geologically constrained beaches compared to open sandy beaches, ranging from small recirculating cells across the reef to a large deflection rip extending hundreds of meters beyond the surf zone.
(Journal of Geophysical Research. Oceans. vol. 125, n° 2169-9275, 01/10/2020)
EPOC, EPHE, PSL, UB, INSU - CNRS, CNRS, SIAME, UPPA