Small-scale sediment transport and deposition patterns within a salt-marsh basin, Paulinaschor, Western Scheldt

During inundation of tidal marshes, fine-grained suspended matter is transported to and partly deposited on the marsh surface. In this research the complex spatial patterns of sediment transport and deposition are studied at the temporal scale of individual inundations and spatial scale of a small tidal creek basin (ca. 6 ha) within the salt Paulina marsh, Western Scheldt. Field measurements are used for the implementation and validation of 2-dimensional numerical models for tidal marsh sedimentation. Near the mouth of the creek system, the incoming and outgoing sediment mass is estimated, by way of water level, flow velocity and suspended sediment concentration (SSC) measurements. Spatial variations in SSC, at the moment of marsh inundation, are measured at about 35 locations within the creek system and above the marsh surface, using siphon samplers. Finally the sediment that is deposited on the marsh surface is sampled with sediment traps on 50 sites, both during 4 individual inundations (about 4-5 hours) and 2 spring-neap tidal cycles (15 days). First, it is investigated how the 2-dimensional pattern of SSC and sedimentation can be described by statistical models, incorporating detailed topographic information on the creek network, surface elevation and vegetation pattern. Secondly, the application of physically-based hydrodynamic models, coupled with sediment transport models, is explored and evaluated against the field data. Once validated, these models may be useful to simulate flooding and sedimentation patterns in other tidal marshes and controlled inundation areas in the Scheldt estuary

Emergence and Modular Evolution of a Novel Motility Machinery in Bacteria

Bacteria glide across solid surfaces by mechanisms that have remained largely mysterious despite decades of research. In the deltaproteobacterium Myxococcus xanthus, this locomotion allows the formation stress-resistant fruiting bodies where sporulation takes place. However, despite the large number of genes identified as important for gliding, no specific machinery has been identified so far, hampering in-depth investigations. Based on the premise that components of the gliding machinery must have co-evolved and encode both envelope-spanning proteins and a molecular motor, we re-annotated known gliding motility genes and examined their taxonomic distribution, genomic localization, and phylogeny. We successfully delineated three functionally related genetic clusters, which we proved experimentally carry genes encoding the basal gliding machinery in M. xanthus, using genetic and localization techniques. For the first time, this study identifies structural gliding motility genes in the Myxobacteria and opens new perspectives to study the motility mechanism. Furthermore, phylogenomics provide insight into how this machinery emerged from an ancestral conserved core of genes of unknown function that evolved to gliding by the recruitment of functional modules in Myxococcales. Surprisingly, this motility machinery appears to be highly related to a sporulation system, underscoring unsuspected common mechanisms in these apparently distinct morphogenic phenomena

Ras GTPase-like protein MglA, a controller of bacterial social-motility in Myxobacteria, has evolved to control bacterial predation by Bdellovibrio

Bdellovibrio bacteriovorus invade Gram-negative bacteria in a predatory process requiring Type IV pili (T4P) at a single invasive pole, and also glide on surfaces to locate prey. Ras-like G-protein MglA, working with MglB and RomR in the deltaproteobacterium Myxococcus xanthus, regulates adventurous gliding and T4P-mediated social motility at both M. xanthus cell poles. Our bioinformatic analyses suggested that the GTPase activating protein (GAP)-encoding gene mglB was lost in Bdellovibrio, but critical residues for MglABd GTP-binding are conserved. Deletion of mglABd abolished prey-invasion, but not gliding, and reduced T4P formation. MglABd interacted with a previously uncharacterised tetratricopeptide repeat (TPR) domain protein Bd2492, which we show localises at the single invasive pole and is required for predation. Bd2492 and RomR also interacted with cyclic-di-GMP-binding receptor CdgA, required for rapid prey-invasion. Bd2492, RomRBd and CdgA localize to the invasive pole and may facilitate MglA-docking. Bd2492 was encoded from an operon encoding a TamAB-like secretion system. The TamA protein and RomR were found, by gene deletion tests, to be essential for viability in both predatory and non-predatory modes. Control proteins, which regulate bipolar T4P-mediated social motility in swarming groups of deltaproteobacteria, have adapted in evolution to regulate the anti-social process of unipolar prey-invasion in the “lone-hunter” Bdellovibrio. Thus GTP-binding proteins and cyclic-di-GMP inputs combine at a regulatory hub, turning on prey-invasion and allowing invasion and killing of bacterial pathogens and consequent predatory growth of Bdellovibrio

Sediment dynamics and geomorphic changes in tidal marshes

Tidal marshes act as net sinks of sediment, which leads, in the long-term, to geomorphic and ecological changes of marshes and estuaries. Tidal marsh sedimentation is studied in the Scheldt estuary on different spatial and temporal scales. On the small-scale (10-100 m and 1 tide to 1 year), field measurements show that temporal variations are controlled by a positive linear relationship between incoming suspended sediment concentration (SSC), at the beginning of marsh flooding, and maximum inundation height, at high tide. The spatial sedimentation pattern is determined by three parameters: elevation of the marsh surface, distance to the nearest tidal creek, and distance, along this creek, to the marsh edge. The long-term (10-100 years) implications of these sediment dynamics were investigated using a physically-based numerical model, which takes the observed increase of incoming SSC with maximum inundation height into account. The modelling results, which are in good agreement with observed long-term accumulation rates, show how young low marshes accumulate much faster than old high marshes and how both tend to the same equilibrium elevation. This explains the generally flat topography of tidal marshes. The model also simulates the fast formation of natural levees along tidal creeks, and also here a geomorphic equilibrium exists: once levees grow 20 to 30 cm higher than inner marsh basins, which are located farther away from tidal creeks, the influence of distance to the tidal creek is compensated by the influence of surface elevation, so that levees and lower marsh basins accumulate at the same rate

Modelling long-term tidal marsh growth under changing tidal conditions and suspended sediment concentrations, Scheldt estuary, Belgium

Existing numerical models simulating the vertical growth of tidal marshes have only, to a very limited degree, been validated using observed data. In this study, we describe a refined zero-dimensional time-stepping model, which is based on the mass balance approach of Krone [in: Coastal Sediments '87, 1987, pp. 316-323], Allen [Mar. Geol. 95 (1990) 77-96] and French [Earth Surf. Process. Landforms 18 (1993) 63-81]. The model is applied and evaluated, using field data on suspended sediment and tidal regime as input and the historical growth of a specific minerogenic tidal marsh in the Scheldt estuary (Belgium) as independent data for model testing. First, the historical rise of the marsh surface during the past 55 years is reconstructed based on land use and vegetation cover changes, which are dated using aerial photographs and which are recognised in sediment cores. After marsh formation, the marsh surface builds up very quickly and asymptotically to an equilibrium level relative to the tidal frame. Second, temporal variations in suspended sediment concentration (SSC) were measured above the actual marsh surface during a 1-year period. These measurements show that the SSC, in the water that floods the marsh surface at the beginning of an inundation, increases linearly with maximum inundation height. The application of existing models, which assume a constant incoming SSC, leads to an underestimation of the observed historical growth and to biased predictions under scenarios of future sea-level rise. However, after incorporation of the relationship between SSC and inundation height, the observed vertical growth is successfully simulated. This leads to the conclusion that not only the decrease in tidal inundation, but also the decrease in SSC with decreasing marsh inundation height, is of great importance to fully explain and successfully simulate the long-term vertical morphodynamics of tidal marshes

Simulating the long-term development of levee-basin topography on tidal marshes

Although natural levees and lower basins are typical geomorphic features along tidal marsh creeks, long-term sedimentation and elevation changes in tidal marshes were traditionally studied using 0-dimensional point models without considering spatial variations. In this study, the long-term evolution of the surface elevation of tidal marsh levees and adjacent basins was studied by applying a 0-dimensional, time-stepping model (MARSED) using spatially differentiated model parameter values for levees and basins. Firstly, the model was calibrated using field data on short-term (<1 year) spatio-temporal variations in sedimentation rates measured along four levee-basin transects within tidal marshes along the Scheldt estuary (SW Netherlands-Belgium). Secondly, the long-term (10-100 years) elevation change of the levees and basins was simulated, starting from a historically known marsh elevation. Predicted elevations were successfully validated against the present-day observed topography along each of the studied levee-basin transects. The model simulations show that the elevation difference between levees and basins tends to an equilibrium. Once levees grow 20 to 30 cm higher than the adjacent basins, the positive influence of the proximity of a tidal creek on the sedimentation rate on the levees is compensated by the negative influence on the sedimentation rate of the higher surface elevation on the levees. Once this sedimentological, geomorphic equilibrium condition is attained, both levees and basins accumulate at the same rate, which is in equilibrium with the rate of mean high water level (MHWL) rise. Finally, additional simulations show that the equilibrium elevation difference between levees and basins is mainly determined by the rate of mean sea-level rise and the incoming sediment concentration. A faster sea-level rise will result not only in a lower equilibrium elevation of the marsh surface relative to MHWL but also in a more pronounced elevation difference between levees and adjacent basins. On the other hand, higher incoming sediment concentrations will result in higher equilibrium elevations. Significantly larger elevation differences between levees and basins are only obtained for increased differences in incoming sediment concentrations between levees and basins. This study demonstrates that the long-term response of tidal marsh surfaces to different scenarios of changing sea-level and incoming sediment concentrations is not uniform in space, but that spatial variability in tidal marsh morphodynamics is important and that natural levees and inner basins will react in different ways

Spatial and temporal factors controlling short-term sedimentation in a salt and freshwater tidal marsh, Scheldt estuary, Belgium, SW Netherlands

During a one-year period temporal and spatial variations in suspended sediment concentration (SSC) and deposition were studied on a salt and freshwater tidal marsh in the Scheldt estuary (Belgium, SW Netherlands) using automatic water sampling stations and sediment traps. Temporal variations were found to be controlled by tidal inundation. The initial SSC, measured above the marsh surface at the beginning of inundation events, increases linearly with inundation height at high tide. In accordance with this an exponential relationship is observed between inundation time and sedimentation rates, measured over 25 spring-neap cycles. In addition both SSC and sedimentation rates are higher during winter than during summer for the same inundation height or time. Although spatial differences in vegetation characteristics are large between and within the studied salt and freshwater marsh, they do not affect the spatial sedimentation pattern. Sedimentation rates however strongly decrease with increasing (1) surface elevation, (2) distance from the nearest creek or marsh edge and (3) distance from the marsh edge measured along the nearest creek. Based on these three morphometric parameters, the spatio-temporal sedimentation pattern can be modelled very well using a single multiple regression model for both the salt and freshwater marsh. A method is presented to compute two-dimensional sedimentation patterns, based on spatial implementation of this regression model