Salt marshes are crucially important ecosystems at the boundary between the land and the sea, that are experiencing significant losses worldwide mainly dictated by the erosion of their margins. Improving our understanding of the mechanisms controlling marsh edge erosion is a key step to address conservation issues and salt‐marsh response to changes in the environmental forcing. Here we have employed a complete, coupled Wind‐Wave Tidal Model (WWTM) to analyse the temporal evolution of the wave field, and in particular of the mean wave‐power density, in the Venice Lagoon over the past four centuries (from 1611 to 2012). We have then related wave‐field changes to the observed erosion patterns determined by comparing recent aerial photographs (1978–2010) and historical bathymetric data. The results of our analyses from the Venice Lagoon show that, while wave‐fields did not significantly change from 1611 to 1901, a rapid increase in wave‐power densities occurred in the last century. This is suggested to depend on the positive feedback between relevant morphological evolutions and changes in the wave field, both influenced by natural forcing and anthropogenic pressures. We also emphasize the existence of a strong positive linear relationship between the volumetric marsh erosion rate and mean wave‐power density. We thus suggest that relating salt‐marsh lateral erosion rates to properly computed mean wave‐power densities provides a valuable tool to address long‐term tidal morphodynamics. © 2019 John Wiley & Sons, Ltd.

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Tidal point bars are generally described as laterally accreting bodies, generated by lateral shift of meander bends, in which the point-bar brink (i.e. the break between bar top and bar slope) and the channel thalweg (i.e. the deepest part of the channel) shift horizontally toward the outer bank. The present study applies the concept of trajectory analysis at the point bar scale, focusing on the trajectories of point-bar brink and channel thalweg, in order to understand how vertical aggradation can interact with lateral migration to shape geometries of tidal point bars developed in a microtidal and highly aggrading salt marsh setting. We selected eight study-case meander bends, located in the Venice Lagoon and characterized by different pointbar morphologies, whose widths and depths range from 2 to 11 m and from 0.5 to 1.6 m, respectively. All the point bars were investigated through a high resolution facies-analysis carried out on closely-spaced sediment cores, collected along the bar axis. Location of bar brink and channel thalweg at different times defined specific trajectories, which were classified either as ascending or descending, and linear or non-linear. All brink trajectories are ascending, and show evidence of lateral shift of the bar brink under aggradational conditions of surrounding marshes. Development of non-linear brink trajectories is linked with changes in the ratio between vertical and lateral shift rates of the brink, which is in turn dictated by changes in local base level due to substrate compaction. Conversely, the thalweg trajectories can be either ascending or descending, reflecting an interaction between rates of lateral shift and aggradation/degradation of the channel floor. The brink and thalweg can either shift consistently (e.g., both trajectories are ascending) or incongruously (e.g., ascending brink vs. descending thalweg trajectory), reflecting different attitudes of the channel to maintain or increase its cross-sectional area.

The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1Á5 m over subtidal platforms and a mean tidal range of about 1Á0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1Á0 to 5Á0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point-bar beds do not show sigmoidal geometries, but consist of horizontally-bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in-channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point-bar deposits point out that the curvature-induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the 'classical' fining-upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally-bedded platform sediments is interpreted here as due to bar top wave-winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar 'apronlike' accumulations, where sand accumulated through avalanching processes and mud settled down from suspension.

The planform evolution of tidal meanders is driven by interactions between channel morphology and periodically reversing tidal flows, which feed back into the development of erosional and depositional patterns. However, the paucity of quantitative data has so far undermined detailed analyses about the geomorphic effects of tidal flows within tidal meanders. Here we aim to bond the morphodynamic evolution of tidal meanders with the structure of three‐dimensional flow that shapes them. By means of an acoustic Doppler current profiler, we have surveyed the flow fields over three different tidal meandering channels in the salt marshes at San Felice (Venice lagoon, Italy), each characterized by distinct planform morphology and evolutionary dynamics. Mutually evasive paths followed by the maximum ebb and flood streamwise velocities determine periodic changes in both the position and the orientation of curvature‐induced cross‐stream flows. These secondary flows can be locally disrupted by patches of submerged vegetation, especially in meanders of small size, with direct implications for the morphodynamic evolution of meander bends. The latter is further affected by flow separation in sharp bends. Flow separation effectively reduces channel width, enhancing bank erosion due to increasing flow velocities. Moreover, it creates low‐velocity zones of recirculating flows at both the inner and outer banks, thereby promoting the formation of point bars and concave‐bank benches, respectively. By relating three‐dimensional flow structure to patterns of channel change, our results provide a first step to unravel the relation between flows and forms within tidal meanders, whose planform characteristics may differ greatly from their fluvial counterparts.

Many salt-marsh systems worldwide are currently threatened by drowning and lateral erosion that are not counteracted by sufficient sediment supply. Here we analyze the response of a salt-marsh system to changes in sediment availability and show that, contrary to what would have been expected, marsh dynamics in the vertical plane can be insensitive to large sediment supply. We integrate sedimentological, geochronological, paleoecological, geophysical, and chemical analyses of salt-marsh sediments accumulated over the past six centuries in the Southern Venice Lagoon (Italy), and suggest that a time lag exists between enhanced river-fed clastic sediment input and its signature in the salt-marsh succession. This time lag is likely caused by the stocking of the sediment along the margins of pre-existing marshes, which started to significantly expand horizontally – rather than accrete vertically – when sediment input increased. When sediment input drastically decreased, wind waves re-suspended the river-fed deposits and distributed them over the marsh platform, eventually allowing for vertical accretion. Understanding the response of salt-marsh systems to changes in sediment supply has important implications for the management of tidal landscapes and the prediction of their evolution under the effects of natural and anthropogenic forcings. Our results highlight that the study of ultra-recent sedimentary successions needs to be carried out on the basis of a deep understanding of specific depositional dynamics.

Tidal salt marshes are widespread along the World's coasts, and are ecologically and economically important as they provide several valuable ecosystem services. In particular, their significant primary production, coupled with sustained vertical accretion rates, enables marshes to sequester and store large amounts of organic carbon and makes them one of the most carbon-rich ecosystems on Earth. Organic carbon accumulation results from the balance between inputs, i.e. organic matter produced by local plants or imported, and outputs through decomposition and erosion. Additionally, organic matter deposition actively contributes to marsh vertical accretion, thus critically affecting the resilience of marsh ecosystems to rising relative sea levels. A better understanding of organic-matter dynamics in salt marshes is key to address salt-marsh conservation issues and to elucidate marsh importance within the global carbon cycle. Toward this goal, we empirically derived rates of organic matter decomposition by burying 712 commercially available tea bags at different marshes in the microtidal Venice Lagoon (Italy), and by analyzing them following the Tea Bag Index protocol. We find values of the decomposition rate (k) and stabilization factor (S) equal to 0.012±0.003 day -1 and 0.15±0.063, respectively. Water temperature critically affects organic matter decomposition, enhancing decomposition rates by 8% per °C on average. We argue that, at least in the short term, the amount of undecomposed organic matter that actively contributes to carbon sequestration and marsh vertical accretion strongly depends on the initial organic matter quality, which is a function of marsh and vegetation characteristics.

<p>Salt-marsh evolution importantly depends on complex feedbacks between hydrodynamic, morphological, and biological processes. These crucial ecogeomorphic structures support a diverse range of ecosystem services, including coastal protection and biodiversity increase. In addition, they are among the most carbon‐rich ecosystems on Earth, as their high primary production coupled with rapid surface accretion results into the ability to sequester atmospheric carbon at high rates. However, salt-marsh future is at risk today, due to the effects of climate changes and local anthropogenic disturbances, in particular sea-level rise and reduced fluvial sediment delivery to the coasts. The organic matter captured and stored by salt marshes results from the balance between inputs and outputs and may contribute to marsh surface accretion, which determines their ability to keep pace with sea-level rise. Therefore, a better understanding of the processes regulating organic matter dynamics on salt marshes is a critical step to elucidate their carbon sink potential and to address salt-marsh management and conservation issues. Toward this goal, we analysed organic matter decomposition processes within salt-marsh ecosystems by burying 712 commercially available tea bags within different marshes in the Venice Lagoon (Italy), following the Tea Bag Index protocol. The process provides the values of two key parameters: the decomposition rate (k) and litter stabilisation factor (S). Based on standardized litter bag experiments, the Tea Bag Index focuses on the effects of abiotic conditions, neglecting litter-quality influences. The mean values of the decomposition metrics from our analyses are in general consistent with previous results and indicate a quite fast decomposition of the organic matter with a remaining mass of about 34% of the initial labile mass after 90 days. We next explore the possible dependence of k and S on environmental drivers. Temperature showed the most significant relationship with decomposition processes, suggesting an organic-matter decay acceleration with warming temperature, in line with previous literature. Moreover, the statistical analysis indicated some significant trends of the decomposition rate also with surface elevation and distance from the marsh edge. This suggests that, at the marsh scale, higher and probably less frequently flooded sites are exposed to faster decomposition, likely due to greater oxygen availability enhancing microbial respiration. In conclusion, the organic matter decay we observed is rapid enough to consume all the labile material before it can be buried and stabilized, hence increased global temperatures may not have a significant effect in increasing organic matter decomposition in coastal marshes. Therefore, we argue that, at least in the short term, the remaining mass of the organic matter contributing to carbon sequestration and marsh accretion, strongly depends on the initial litter quality, recalcitrant or labile, which may differ considerably between different species and plant parts and may be affected by climate change effects.</p>