How the Effects of Small and Flexible Seagrass (Zostera Noltii) on Flow Modify Erosion and Deposition Processes

Source: ICHE Conference Archive - https://mdi-de.baw.de/icheArchiv

Effects of bioturbation on the erodibility of cohesive versus non-cohesive sediments along a current-velocity gradient: A case study on cockles

Soft-bottom bioturbators are ecosystem engineers in the sense that they can have considerable effects on sediment erodibility and resuspension. The common cockle Cerastoderma edule is a bioturbating filter feeder that is widespread along the European Atlantic coastline. Its presence and activity can decrease sediment erosion thresholds in cohesive sediments but little is known about its effect on non-cohesive sediments. Using controlled annular flume experiments, we investigated the relative effects of different cockle densities on sediment re suspension in cohesive vs. non-cohesive sediments by assessing the following: (i) the mud and sand burrowing behavior of cockles, (ii) critical erosion thresholds, (iii) the mass of eroded sediment and (iv) erosion rates. Our results show that cackles were more active in non-cohesive sediment compared with cohesive sediment. Despite their lower activity, the presence of cockles in cohesive sediment increased sediment erodibility by reducing the critical erosion threshold (U-crit) and increasing both the mass of eroded sediment and erosion rate. In contrast, cockles had no effect on erodibility in non-cohesive sediment, especially on the eroded sediment mass and erosion rate. The mass eroded was not significantly different between cohesive and non-cohesive sediments when cockles were present. Our experiments show that the increased erodibility of cohesive sediment due to the. bioturbation by cockles is density dependent: higher cockle density results in stronger effects on erodibility. Moreover, this increase in cohesive sediment erosion due to cockle bioturbation was positively correlated with current velocity. In contrast, the erosion of non-cohesive sediment only depended on the current stress and was unaffected by cockle density. Considering the high abundance of C. edule, its widespread distribution and its extensive activities, the results of this study could be widely applicable to intertidal mud flats around the world.</p

Effects of large dams on the aquatic food web along a coastal stream with high sediment loads

The contribution of two basal energy sources – detrital organic matter and primary producers – as part of aquatic food webs varies typically along river continua. A host of barriers to river flow increase the water residence time and sediment and nutrient retention in reservoirs worldwide, and potentially alter the balance between detritus-based and algae-based energy pathways in the downstream food webs. We explored this issue on the Sélune River (Normandy, France), a small coastal stream that drains an agricultural catchment with high sediment runoff. Seasonal measurements of the following parameters were compared upstream and downstream of the reservoirs of two large dams (16 m and 36 m high): sediment fluxes, nutrient and chlorophyll a concentrations, algal communities in the epilithic biofilm (taxonomic composition, biomass and growth), and benthic invertebrate communities (abundance and trophic guild structure). As anticipated, annual sediment fluxes were much lower downstream of the reservoirs, where significant decreases in water turbidity, phosphate and silicate concentrations were recorded. A higher chlorophyll a concentration in water and a higher contribution of pelagic algae taxa to the photosynthetic biofilm suggested drifting and deposition of reservoir-borne phytoplankton downriver. Photosynthetic biofilm growth was higher downstream of the reservoirs in spring and fall, and so was the abundance of herbivores in the invertebrate community, notably scrapers and algae eaters. Energy pathways within riverine food webs were traced using stable isotope analyses of carbon (C) and nitrogen in the tissues of aquatic consumers (invertebrates and fish). Mixing models revealed a discontinuity in the origin of the C entering the food webs along the river continuum, confirming a greater contribution of algal C to aquatic consumers downstream of the reservoirs. These results illustrate mechanisms whereby large reservoirs can modulate C flow in food webs along a small coastal river with high sediment loads, and make it possible to anticipate the effects of dam removal on the future river ecosystem

Effet des épisodes extrêmes en milieu méditerranéen stressé. Phase 1 : étude bibliographique

Future climate predictions place the Mediterranean region and its lagoons at the heart of the social and environmental discussion on climate change. The projected increase in water temperature, sea level rise, and the intensification of extreme climate events (storms, precipitations, heat waves) are identified today, by the IPCC, as major pressures on lagoon ecosystems. Through a systematic review followed by a meta-analysis based on articles published over the period 1975-2020 and indexed in "Web of Science", this work focused on understanding the effects of climate change and extreme climate events on lagoon environements. The question of these effects was first approached at a global scale (lagoon environments), then centered on the four species of marine angiosperms living in French poly-euhaline Mediterranean lagoons, namely Zostera marina, Zostera noltei, Cymodocea nodosa and Ruppia cirrhosa. Only 62 articles deal with the question of the effects of climate change on lagoon environments on a global scale. These articles are mainly based on modeling work and laboratory experiments. In situ observations are not common and there are no studies based on in situ experimentation. According to our results, increasing temperature is the most studied climate change pressure. This would favor phyto-plankton communities, the most studied biotic compartment to date. At the level of Mediterranean lagoons, no scientific article deals with the question of the effect of climate change on seagrass beds. By extending the research to the impact of climate change on Zostera marina, Zostera noltei and Cymodocea nodosa meadows in coastal environments, 36, 14 and 15 articles were respectively analyzed. However, no studies are available for Ruppia cirrhosa. Based mainly on laboratory and mesocosm experiments, the increase in temperature would have an overall negative effect on Zostera marina and Zostera noltei but positive for Cymodocea nodosa. Heat waves would have a negative effect on Zostera marina meadows, but the effect of this pressure could differ depending on the origin of the plant. Although translating these responses to seagrass meadows in Mediterranean lagoon environments seems complex, this could induce the potential replacement in Mediterranean lagoon environment of the species Zostera marina by Cymodocea nodosa in the future. On the basis of this bibliographic work, the state of current knowledge makes it difficult to predict what the impact of climate change will be and how this could modify the ecological state of lagoons and their meadows, on the one hand, and their ecological and socio-economic role, on the other hand. Among the perspectives of this work, work on early warning signals of changes should be initiated to understand and to interpret the future dynamics of these naturally pressurized coastal systems.Les prédictions sur le climat futur placent la région méditerranéenne et ses lagunes au coeur de la problématique sociale et environnementale du changement climatique. L’augmentation de la température de l’eau, l’élévation du niveau de la mer, et l’intensification des événements climatique extrêmes (tempête, précipitations, chaleur) sont identifiées aujourd’hui, par le GIEC, comme des pressions majeures sur les écosystèmes lagunaires . A travers une revue systématique puis d’une méta-analyse basées sur les articles publiés sur la période 1975-2020 et indexé dans «Web of Science », ce travail s’est focalisé sur la compréhension des effets du changement climatique et des événements climatique extrêmes sur les milieux lagunaires. La question de ces effets a été abordée de manière générale (milieux lagunaires), puis centrée sur les quatre espèces d’angiospermes marines inféodées aux lagunes méditerranéennes françaises poly-euhalines, à savoir Zostera marina, Zostera noltei, Cymodocea nodosa et Ruppia cirrhosa. Seuls 62 articles traitent de la question des effets du changement climatique sur les milieux lagunaires à l’échelle mondiale. Ces articles se basent principalement sur des travaux de modélisation et d’ expérimentations en laboratoire. L’observation in situ est très en retrait et aucune étude n’est basée sur de l’expérimentation in situ. D’après nos résultats, l’augmentation de la température est la pression la plus étudiée. Celle-ci favoriserait les communautés phytoplanctoniques, compartiment biotique le plus étudié à ce jour. À l’échelle des lagunes méditerranéennes, aucun article scientifique ne traite de la question de l’effet du changement climatique sur les herbiers. En élargissant la recherche à l’impact du changement climatique sur les herbiers de Zostera marina, Zostera noltei et Cymodocea nodosa en milieu côtier, 36, 14 et 15 articles ont pu respectivement être analysés. Aucune étude n’est en revanche disponible pour Ruppia cirrhosa. Sur la base majoritairement d’expériences au laboratoire ou en mésocosme, l’augmentation de la température aurait un effet globalement négatif pour Zostera marina et Zostera noltei mais positif pour Cymodocea nodosa. Les vagues de chaleur induiraient un effet négatif sur les herbiers de Zostera marina mais l’effet de cette pression pourrait différer en fonction de l’origine de la plante. Même si la transposition de ces réponses aux herbiers des milieux lagunaires méditerranéens semble complexe, ceci pourrait ainsi induire le remplacement potentiel en milieu lagunaire méditerranéen de l’espèce Zostera marina par Cymodocea nodosa dans le futur. Sur la base de ce travail bibliographique, l’état des connaissances actuelles ne permet que difficilement de prédire quel sera l’impact du changement climatique et comment cela pourrait modifier l’état écologique des lagunes et de leurs herbiers, d’une part, et leur rôle écologique et socio-économique, d’autre part. Parmi les perspectives de ce travail, des travaux sur les signes précurseurs de basculement devront être lancés pour comprendre et interpréter la dynamique future de ces systèmes côtiers naturellement sous pression

Effects of short flexible seagrass Zostera noltei on flow, erosion and deposition processes determined using flume experiments

Innovative flume experiments were conducted in a recirculating straight flume. Zostera noltei meadows were sampled in their natural bed sediments in the field at contrasting stages of their seasonal growth. The aims of this study were: (i) to quantify the combined effects of leaf flexibility and development characteristics of Zostera noltei canopies on their interaction with hydrodynamics; and (ii) to quantify the role of Zostera noltei meadows in suspended sediment trapping and bed sediment resuspension related with changes in hydrodynamic forcing caused by the seasonal development of seagrasses. Velocity within the canopy was significantly damped. The attenuation in velocity ranged from 34 to 87% compared with bare sediments and was associated with a density threshold resulting from the flow-induced canopy reconfiguration. The reduction in flow was higher in dense canopies at higher velocities than in less dense canopies, in which the reduction in flow was greater at low velocities. These contrasted results can be explained by competition between a rough-wall boundary layer caused by the bed and a shear layer caused by the canopy. The velocity attenuation was associated with a two to three-fold increase in bottom shear stress compared with unvegetated sediment. Despite the increase in near-bed turbulence, protection of the sediment against erosion increased under a fully developed meadow, while sediment properties were found to be the main factor controlling erosion in a less developed meadow. Deposition fluxes were higher on the vegetated bed than on bare sediments, and these fluxes increased with leaf density. Fewer freshly deposited sediments were resuspended in vegetated beds, resulting in an increase in net sediment deposition with meadow growth. However, in the case of a very high leaf area index, sediment was mostly deposited on leaves, which facilitated subsequent resuspension and resulted in less efficient sediment trapping than in the less developed meadow

Effects of bioturbation on the erodibility of cohesive versus non-cohesive sediments along a current-velocity gradient:A case study on cockles

Soft-bottom bioturbators are ecosystem engineers in the sense that they can have considerable effects on sediment erodibility and resuspension. The common cockle Cerastoderma edule is a bioturbating filter feeder that is widespread along the European Atlantic coastline. Its presence and activity can decrease sediment erosion thresholds in cohesive sediments but little is known about its effect on non-cohesive sediments. Using controlled annular flume experiments, we investigated the relative effects of different cockle densities on sediment re suspension in cohesive vs. non-cohesive sediments by assessing the following: (i) the mud and sand burrowing behavior of cockles, (ii) critical erosion thresholds, (iii) the mass of eroded sediment and (iv) erosion rates. Our results show that cackles were more active in non-cohesive sediment compared with cohesive sediment. Despite their lower activity, the presence of cockles in cohesive sediment increased sediment erodibility by reducing the critical erosion threshold (U-crit) and increasing both the mass of eroded sediment and erosion rate. In contrast, cockles had no effect on erodibility in non-cohesive sediment, especially on the eroded sediment mass and erosion rate. The mass eroded was not significantly different between cohesive and non-cohesive sediments when cockles were present. Our experiments show that the increased erodibility of cohesive sediment due to the. bioturbation by cockles is density dependent: higher cockle density results in stronger effects on erodibility. Moreover, this increase in cohesive sediment erosion due to cockle bioturbation was positively correlated with current velocity. In contrast, the erosion of non-cohesive sediment only depended on the current stress and was unaffected by cockle density. Considering the high abundance of C. edule, its widespread distribution and its extensive activities, the results of this study could be widely applicable to intertidal mud flats around the world

Effects of key species mud snail Bullacta exarata (Gastropoda) on oxygen and nutrient fluxes at the sediment-water interface in the Huanghe River Delta, China

Since the mud snail Bullacta exarata was introduced for economic aquaculture in the Huanghe River (Yellow River) Delta in 2001, its quick population growth and expanded distribution make it a key-species in the intertidal zone of this area. This significantly contributed to the economic income of the local people, but its potential ecological impact on the benthic ecosystem remains unknown. A mesocosm study was conducted to test whether its bioturbation activities affect the microphytobenthos (MPBs; i.e., sedimentary microbes and unicellular algae) productivity and the nutrient exchange between the sediment-water interface. Our results show that the mud snail significantly impacted the dissolved oxygen (DO) flux across the sediment-water interface on the condition of normal sediment and light treatment, and significantly increased the ammonium efflux during recovery period in the defaunated sediment and dark treatment. The presence of micro- and meiofauna significantly increased the NH_4-N flux in dark treatment. Whereas, in light treatment, these small animals had less effects on the DO and NH_4-N flux between sediment-water interface. Our results provide better insight into the effect of the mud snail B. exarata on the ecosystem functioning via benthic fluxes