Northern Venice Lagoon Research Articles

Conservation actions for restoring the coastal lagoon habitats: Strategy and multidisciplinary approach of LIFE Lagoon Refresh

The Habitat Directive of European Union lists Costal Lagoons (habitat code 1150*) among priority habitats because they are in danger of disappearance. Natural ecosystems may recover from anthropogenic perturbations; however, the recovery can follow natural restoration or it can be redirected through ecological restoration by anthropogenic intervention. Accordingly, by collecting the available theoretical indications for restoration of estuarine and coastal areas, a methodological approach was detailed andit can be summarised into five issues: (i) Environmental context from which it began; (ii) Desired state to be achieved; (iii) Policies and socio-economic context; (iv) Typology of recovery and/or improvement of habitats and ecosystems; and (v) Methods for monitoring the impact of the project. The project strategy, management and measures of LIFE Lagoon Refresh were also presented and discussed, as a case study for the implementation of the multidisciplinary approach for restoration ecology in transitional waters. The project takes place in the northern Venice Lagoon (Italy), started in 2017 and it lasts 5 years. In the Venice Lagoon, since the 20th century, strong reductions of the typical salinity gradient of buffer areas between lagoon and mainland, and of reedbed extensions have occurred due to historic human interventions, with negative consequences on coastal lagoon habitats. To improve the conservation status of habitats and biodiversity of the area, the LIFE Lagoon Refresh project included several conservative actions, which are (i) the diversion of a freshwater flow from the Sile River into the lagoon; (ii) the restoration of intertidal morphology, through biodegradable structures; (iii) the reed and aquatic angiosperm transplantations with the involvement of local fishermen and hunters, and (iv) the reduction of hunting and fishing pressures in the intervention area. To achieve the restoration of the lagoon environment, the strategy of the project covered a combination of different aspects and tools, such as planning activities, through the involvement of local Institutions and communities; stakeholder’s involvement to increase awareness of environment conservation and socioeconomic value improvement; an ecological engineering approach; numerical models as supporting tool for planning and managing of conservation actions; environmental monitoring performed before and after the conservation actions.

Changes in Macrozoobenthos Community after Aquatic Plant Restoration in the Northern Venice Lagoon (IT).

Responses of the macrozoobenthic community to an ecological restoration activity in the northern Venice lagoon were studied, within the scope of the project LIFE SEagrass RESTOration aimed at recreating aquatic phanerogam meadows largely reduced in recent decades. Transplants were successful in almost all project areas. Macrozoobenthos was sampled in eight stations before (2014) and after (2015, 2016, 2017) transplanting activities. An increase in abundance and fluctuations in richness and univariate ecological indices (Shannon’s, Margalef’s, Pielou’s indices) resulted during the years. Comparing non-vegetated and vegetated samples in 2017, every index except Pielou’s increased in the latter. Multivariate analysis (hierarchical cluster analysis, MDS, PERMDISP, SIMPER) grouped samples by localization rather than years, with differences between stations due to the abundance of common species. In 2017, results were also grouped by the presence or absence of aquatic plants, with differences in the abundance of grazer and filter-feeding species. Results of ecological index M-AMBI depicted conditions from moderate to good ecological status (sensu Dir.2000/60/EC) with similar fluctuations, as presented by univariate indices from 2014 to 2017. Responses of the macrozoobenthic community were more evident when comparing vegetated and non-vegetated samples, with the vegetated areas sustaining communities with greater abundance and diversity than non-vegetated samples, thus demonstrating the supporting function of aquatic plants to benthic communities.

Assessment of State Transition Dynamics of Coastal Wetlands in Northern Venice Lagoon, Italy

Coastal wetlands represent particularly valuable natural resources, characterized by the interaction between their geomorphological and biological components. Their adaptation to the changing conditions depends on the rate and extent of spatial and temporal processes and their response is still not fully understood. This work aims at detecting and improving the understanding of the transition dynamics on eco-geomorphological structures in a coastal wetland ecosystem. The approach could support sustainable habitat management improving the detection and optimizing the offer of Earth Observation (EO) products for coastal system monitoring. Such course of action will strengthen evidence-based policy making, surface biophysical data sovereignty and the Space Data downstream sector through remote sensing techniques thanks to the capability of investigating larger scale and short-to-long-term dynamics. The selected case study is the Lido basin (Venice Lagoon, Italy). Our methodology offers a support in the framework of nature-based solutions, allowing the identification of ecosystem-level indicators of the surface biophysical properties influencing stability and evolution of intertidal flats on which a conceptual model is implemented. Landsat satellite imagery is used to delineate the spatial and temporal variability of the main vegetation and sediment typologies in 1990–2011. Within this period, specific anthropic activities were carried out for morphological restoration and flood protection interventions. Specifically, the lower saltmarsh shows its more fragmented part in the Baccan islet, a residual sandy spit in front of the Lido inlet. The area covered by Sarcocornia-Limonium, that triggers sediment deposition, has fluctuated yearly, from a minimum coverage of 13% to a maximum of 50%. The second decade (2001–2009) is identified as the period with major changes of halophytic and Algae-Biofilm cover typologies distribution. The power law and related thresholds, representing the patch size frequency distribution, is an indicator of the ecosystem state transition dynamics. The approach, based on multi-temporal and spatial EO analysis, is scalable elsewhere, from regional to local-to-global scale, considering the variability of climate data and anthropogenic activities. The present research also supports sustainable habitat management, improving the detection, and optimizing the offer of EO products for coastal system monitoring.

Piracy-controlled geometry of tide-dominated point bars: Combined evidence from ancient sedimentary successions and modern channel networks

Establishing critical comparisons between fluvial- and tidal-channel morphodynamics is a major goal in the study of coastal landscapes. Freely migrating meandering rivers are known to produce laterally extensive, sand-prone point-bar bodies which commonly exhibit width:thickness ratios up to 250. Meandering channels are widespread in tidal channel networks draining intertidal plains, where they exhibit planform dynamics similar to their fluvial analogues. However, tidal networks are characterized by a high channel density that likely hinders tidal meanders from migrating laterally for long distances without interacting with other channels. In order to better understand how the interaction between adjacent meandering channels controls the development of tide-dominated point bars, two point-bar bodies from the Eocene Castigaleu Formation (Spain) are investigated and compared with the geometry of a modern tidal point bar of the northern Venice Lagoon (Italy). The Eocene bars are characterized by a low width:thickness ratio (<30), interpreted as the result of premature deactivation of their parent-channel migration. Such deactivations were likely related to avulsive piracy operated by adjoining tide-dominated channels, and likely prevented the parent channel bend from depositing a laterally extensive, tabular sand body along its inner bank. This analysis is corroborated by direct time-lapse observations of similar dynamics in modern tidal networks, and from morphodynamic and sedimentological evidence from recent deposits of the Venice Lagoon. We conclude that, in densely drained tidal networks, meander bends often interact with adjacent channels, thereby triggering piracy events and premature channel abandonments that generate point-bar architectures distinct from those produced in fluvial meandering rivers.

Expected Shifts in Nekton Community Following Salinity Reduction: Insights into Restoration and Management of Transitional Water Habitats

A restoration project is planned to take place in the northern Venice lagoon (northern Adriatic Sea, Italy), aiming at introducing freshwater into a confined shallow water lagoon area and recreating transitional water habitats. This work describes the shifts in the nekton (fish and decapods) community structure to be expected following the future salinity decrease in the restoration area. Nekton was sampled at a series of natural shallow water sites located along salinity gradients in the Venice lagoon. A multivariate GLM approach was followed in order to predict species biomass under the salinity and environmental conditions expected after restoration. Biomass of commercially important species, as well as species of conservation interest, is predicted to increase following salinity reduction and habitat changes. From a functional perspective, an increase in biomass of hyperbenthivores-zooplanctivores, hyperbenthivores-piscivores and detritivores is also expected. This study emphasises the efficacy of a predictive approach for both ecological restoration and ecosystem management in transitional waters. By providing scenarios of community structure, the outcomes of this work could be employed in future evaluations of restoration success in the Venice lagoon, as well as to develop management tools to forecast the effects of alterations of salinity regimes in coastal lagoons due to climate change.

Predicting the response of nekton assemblages to seagrass transplantations in the Venice lagoon: An approach to assess ecological restoration

Abstract One of the major challenges to ensure effective restoration of estuarine habitats is to establish success criteria to determine whether the goals of restoration are met. The aim of this work is to propose and test an approach to identify reference conditions and assess the recovery of nekton (fish, decapods and cephalopods) assemblages at seagrass restoration sites. Nekton sampling took place from 2014 to 2017 in the northern Venice lagoon (northern Adriatic Sea, Italy) during spring at eight sites subjected to seagrass (Zostera marina and Zostera noltei) transplantation. In spring 2016, five natural seagrass sites in the same area were additionally sampled, and physico‐chemical water parameters and habitat structure were also recorded. A multivariate generalized linear model approach was adopted in order to disentangle the relative effect of water quality and seagrass habitat structure on nekton assemblages of natural habitats. Models were subsequently employed to predict species composition of nekton fauna at each transplantation site, to identify the reference assemblage expected under site‐specific abiotic and habitat characteristics. The average distance of the observed assemblage from reference conditions was used to track temporal trajectories of nekton colonization at transplantation sites, and to ultimately evaluate the recovery rate towards restoration goals. This study highlights how a predictive approach could serve management purposes in ecological restoration, providing a concise tool to assess the functionality of restored habitats for associated fauna.

Anthropogenic Modifications Can Significantly Influence the Local Mean Sea Level and Affect the Survival of Salt Marshes in Shallow Tidal Systems

AbstractIn a shallow dissipative tidal system, interventions that modify the local morphology in one location of the basin may quickly affect the surrounding areas, promoting strong changes in their morphology. If the localized modification persists over time, it may produce far‐field morphological modifications at delayed times. These modifications may have an impact on the local mean sea level and on tidal range, affecting the fate of salt marshes with implications for their survival. In this study, we investigate the effect of two anthropic interventions performed between the end of the nineteenth century and the beginning of the twentieth century in the northern Venice lagoon (Italy): the construction of jetties at one inlet and the removal of reed barriers protecting a fish farm in the inner lagoon. Using a 2‐D numerical model to reproduce the hydrodynamics of different historical lagoon configurations and a zero dimensional model of marsh vertical accretion, we investigate the effect of these interventions on the salt marshes of the northern lagoon basin. Interestingly, our results show that the increased depth at the inlet induced by the jetties lowered the local mean sea level of nearby areas and increased the tidal range, producing a temporary positive feedback on the stability of the marshes in proximity of the inlet. On the contrary, in the inner lagoon areas characterized by extremely low marshes, the removal of reed barriers delimiting a fish farm may have reduced the sediment fluxes thus contributing to the drowning of large marsh surfaces.

Morphodynamic evolution and stratal architecture of translating tidal point bars: Inferences from the northern Venice Lagoon (Italy)

AbstractThe widespread distribution of tidal creeks and channels that undertake meandering behaviour in modern coasts contrasts with their limited documentation in the fossil record, where point‐bar elements arising from the interaction between a mix of both fluvial and tidal currents are mainly documented. The sedimentary products of tidal channel‐bend evolution are relatively poorly known, and few studies have focused previously on specific facies models for tidal point bars present in modern settings. This study improves understanding of tidal channel meander bends through a multi‐disciplinary approach that combines analyses of historical aerial photographs, measurements of in‐channel flow velocity, high‐resolution facies analyses of sedimentary cores and three‐dimensional architectural modelling. The studied channel bend (12 to 15 m wide and 2 to 3 m deep) drains a salt marsh area located in the north‐eastern sector of the microtidal Venice Lagoon, Italy. Historical photographs show that, during the past 77 years, the bend has translated seaward ca 15 m. Results show that the channel bend formed on a non‐vegetated mud flat that was progressively colonized by vegetation. Seaward translation occurred under aggradational conditions, with an overall migration rate of 0·2 to 0·3 m year−1, and was promoted by the occurrence of cohesive, poorly erodible outer bank deposits. Ebb currents are dominant, and translation of the channel bend promotes erosion and deposition along the landward and seaward side of the bar, respectively. Tidal currents show a clear asymmetry in terms of velocity distribution, and their offset pattern provides a peculiar grain‐size distribution within the bar. During the flood stage, sand sedimentation occurs in the upper part of the bar, where the maximum flow velocity occurs. During the ebb stage, the bar experiences the secondary helical flow that accumulates sand at the toe of the bar. Lateral stacking of flood and ebb deposits has caused the formation of localized coarsening‐upward and fining‐upward sedimentary packages, respectively.

Aggradation and lateral migration shaping geometry of a tidal point bar: An example from salt marshes of the Northern Venice Lagoon (Italy)

Although meanders are ubiquitous features of the tidal landscape, the architectural geometries of tidal point bar deposits are relatively unexplored and commonly investigated on the basis of facies models developed for their fluvial counterparts. The present study aims at improving current understanding of tidal point bar deposits developed in salt marsh settings, through a high-resolution investigation of an abandoned intertidal meander loop, located in the northern part of the Venice Lagoon (Italy). The study channel is 6m wide and was active until the 1950s, when it was deactivated as consequence of a neck cut-off. A total of 150 cores was recovered from the associated point bar. The bar erosionally overlies a subtidal platform consisting of sand and mud and is covered by both channel fill and salt marsh mud. The bar, floored by a shell-rich sandy lag, consists of stratified fine sand, grading upward into sandy mud. The outer bank of the bend is characterized by well-developed, sand-rich levee deposits and absence of crevasse splays, which represent a distinctive feature of alluvial sedimentation. Sediment grain size distributions suggesting that seaward and landward sides of the point bar experienced comparable changes of bed shear stress due to alternation between flood and ebb currents, highlighting a remarkable difference with the classical downstream-fining characterizing fluvial point bars. Spatial interpolation between key stratal surfaces shows an overall thickening of the bar from 1.2 to 1.7m in the direction of channel migration, associated with both lowering of the bar base and rising of its brink, which occurs in parallel with an increase in channel cross-sectional area, to progressively accommodate the increasing tidal prism. The bar top surface is characterized by a spoon-shaped geometry stemming out from a combination between lateral migration (8–10cm/yr) and vertical aggradation (2.5–3.0mm/yr) of the inner bank. In salt marsh settings, vertical aggradation plays, therefore, a major effect on point bar sedimentation, generating peculiar bar top geometries that are not common in fluvial meanders, where the high rate of lateral migration causes the cutoff to be reached before substantially thick deposits are accumulated on the bar top.

Groundwater connections under a barrier beach: A case study in the Venice Lagoon

The flow of groundwater beneath barrier islands has been cited as a possible pathway for salt water and chemical exchange between a protected embayment and the open sea. Evidence is presented that identifies an exchange of groundwater through a highly permeable paleoinlet along the barrier beach of Cavallino, which separates the northern Venice Lagoon from the Adriatic Sea. We utilized both point measurements of submarine groundwater discharge (SGD) and a geophysical investigation of the subsurface resistivity to analyze the movement of saline groundwater. Discharge of groundwater and associated nutrients, was higher at the site of a former inlet than at a similar site along the barrier and modulated by the difference in tidal water level between the lagoon and Adriatic Sea. If the measured conditions are typical, storm surge barriers could potentially result in a saline groundwater flow of up to 1.5×10 6 m 3 d −1 into the lagoon.

Estimating submarine groundwater discharge around Isola La Cura, northern Venice Lagoon (Italy), by using the radium quartet

The four naturally-occurring radium isotopes ( 223Ra, 224Ra, 226Ra and 228Ra) were used to estimate the submarine groundwater discharge (SGD) in the Isola La Cura marsh area in the northern Venice Lagoon (Italy). By determining the radium contributors to the study area (river, coastal ocean and sediments) the radium excess in the lagoon water was quantified through a mass balance model. This radium excess is attributed to a submarine groundwater discharge source and represents the most important input of radium. Possible endmembers were considered from analysis of groundwater samples (subtidal and marsh piezometers, marsh wells and seepage meters) that were enriched in Ra by one to two orders of magnitude relative to surface waters. In particular, a permeable layer at 80 cm depth in the surrounding marsh is considered to be representative of the most likely SGD source, although similar radium activities were measured in other subtidal porewater samples collected in the Isola La Cura area. The estimated SGD flux to the study area ranged from 1 · 10 9 to 6 · 10 9 L·d − 1 , the same order of magnitude as the overall riverine input to the lagoon (3 · 10 9 L·d − 1 ). A major fraction of this SGD flux is likely recirculated seawater, as evidenced by the endmember salinity. The water residence time of 2 days was estimated by both using the shortest-lived radium isotope and estimating the volume of water exchanged between the lagoon and the open sea during a tidal cycle (tidal prism approach). This SGD flux could be used to estimate the input of other chemical species (metals, nutrients, etc.) via SGD which might affect the Venice Lagoon ecosystem.

Palaeoenvironment reconstruction in the Lagoon of Venice through wide-area acoustic surveys and core sampling

Since 1999, a joint project called ECHOS has started to explore extremely shallow zones of the lagoon of Venice with the help of acoustic techniques. The main aim of ECHOS is to reconstruct the ancient lagoon environment and investigate its evolution and anthropogenic changes. Although this topic has been the subject of many studies, the use of acoustic methods can solve the problem of correlating the levels intersected by precision drilling, and it can reveal buried geological and archaeological structures. After the first in situ trials, during the last three years, an extensive acoustic survey was carried out on geo-archaeologically interesting areas in the central and northern Venice Lagoon. For this purpose, a traditional echosounder was modified for use in very shallow waters. A direct comparison of the acoustic and geological data shows the potential of acoustic methods for archaeological and geological research aimed at the reconstruction of the ancient morphology of the Venice Lagoon. As an example of the encouraging preliminary results that were obtained, we present the discovery of a buried palaeocanal.

A model of sand transport in Treporti channel: northern Venice lagoon

The origin of the sands in the Venice lagoon has been the subject of an extensive field survey in parallel with numerical modelling. Four transects along Treporti and Burano canals were conducted from which 33 bottom sediment samples were collected. These samples were analysed for grain size and sorting to examine any trends in the granulometry of these sediments that might shed light on transport paths. The modelling study consists of three parts: the sediment transport model sedtrans96 was used with a finite-element hydrodynamic model (Shyfem) and an empirical wave model (US Army Corps of Engineering) to simulate sand transport in the Treporti canal. A type of link box model was created where finite elements of the hydrodynamic model have been combined to macro-boxes on which the water and sediment flux over the sections, and a mass balance has been computed. Several grain size classes were simulated; the distributions before and after the simulation were examined. Idealised wind and tidal values were initially used to force 12 h simulations to test the sediment transport sensitivity. Finally, a full-year simulation (1987) has been carried out using measured tidal and wind data. Only a part of Venice lagoon was covered by the simulation: a major channel (Treporti) running from Lido inlet towards the northern lagoon. The total sand transport through all of the sections was computed for 1 year. Sediment mass balance was determined, and the resulting trends of erosion and deposition were computed. There were no trends in the median grain diameter and sorting of bottom samples from the Treporti canal; all sands were fine (120 μm, one outlier of 300 μm was removed). The absence of a trend in grain size suggests that there is no significant import of sand to the lagoon through the Lido inlet. The results from the simulations seem therefore to confirm the hypothesis of reworking of sand within the lagoon. The computed erosion is some centimeters per year diagnostic of channel scouring and enlargement with time. The Treporti canal is subject to strong current velocities of around 1 m/s, which hold fine sand in suspension and thus prevent sedimentation.

Tidal networks: 1. Automatic network extraction and preliminary scaling features from digital terrain maps

We propose a method of automatic extraction of the tidal channel network from topographic data of marsh and tidal flat lands that uses a combination of a threshold elevation and threshold curvature. Not only the location but also the area of the channel bed is identified. This method differs substantially from that used to identify terrestrial channel networks, and it successfully predicts all of the main channels and nearly all of the smaller tributaries of the channel networks derived from SPOT imagery of the northern Venice Lagoon. Channel network maps of Venice and other sites (Petaluma Marsh in the San Francisco Bay and Barnstable Marsh in Massachusetts) were examined for scaling properties. Because of the large width of the channels relative to a characteristic length of their drainage area, we had to develop procedures for automatically delineating channel width and then for identifying the skeleton of the channel network (the pattern connecting the loci of the channel centerlines) for box‐counting analysis. Box‐counting dimensions of the network skeleton proved site‐dependent and showed finite‐size effects. Because of the large widths we also performed a scaling analysis based on the proportions of the total channel bed area occupied by the tidal networks (i.e., a “fat” fractal analysis). This analysis showed a strong break in scaling between large and small channels. These analyses suggest that tidal channels differ significantly in their scaling relationships from terrestrial systems. In subsequent papers [Rinaldo et al., this issue (a), (b)] we pursue this point much further.

Lead-210 as a tracer of atmospheric input of heavy metals in the northern Venice Lagoon

A chronology for the accumulation of a salt marsh core taken in the Venice Lagoon has been obtained from the activity-depth profile of excess 210 Pb , a naturally occurring radionuclide supplied to the marsh from the atmosphere. Over the past century, the accretion rate of the marsh increased from 0.07–0.08 cm yr −1 to a maximum (0.2 cm yr −1 ) in ∼1970, followed by a decrease to ∼0.18 cm yr −1 . The increase and recent constancy of accretion of the marsh follow the historical pattern of sea level change due to natural and human-induced (groundwater extraction) causes. The 210 Pb chronology is used to determine depositional histories of anthropogenic trace metals, defined as metal concentration in excess of the background value determined at depth in the core. Several patterns are evident: fluxes of excess Ag and Cd increase to the present, Ni and Zn show increasing fluxes to ∼1970 with factor-of-two decreases in recently deposited material. Lead shows high and variable fluxes between 1940 and 1984, with recent decreases. Copper fluxes display a broad peak around 1960. Recent (∼1990) metal fluxes recorded in the marsh core agree within factors of 1.5 to 4 with directly measured (1993–1994) atmospheric fluxes in wet and dry deposition at a site in Venice. The data show that Cu and Pb inputs to the marsh can be accounted for by atmospheric deposition and significant fractions (27–42%) of Cd, Ni and Zn can be supplied by this pathway. Excess 210 Pb inventories in both marsh and subtidal sediments from the Venice Lagoon are consistent with atmospheric supply of this radionuclide. If we assume that atmospheric supply of trace metals is the dominant source of heavy metal contaminants to the marsh, integrated deposition of excess metals in the marsh core, normalized to that of excess 210 Pb , can be used to determine upper limits on the atmospheric contribution of trace metals to lagoon sediments. The results demonstrate that point source inputs of Cu, Ni, Pb and Zn are evident in sediments near the mainland, especially near Porto Marghera, but atmospheric inputs of Pb (and possibly Zn) tend to dominate in the northern and eastern portions of the lagoon.

Metal fluxes to the sediments of the northern Venice Lagoon

Abstract Eighteen short cores were analyzed for major and trace metals (Al, Fe, Ca, Mg, Mn, Si, K, Ti, Pb, Zn, Cu, Ni, Cr), 210Pb, 137Cs, and other sediment characteristics, so as to describe the chronology of pollution and calculate metal concentration factors and fluxes. Substantial evidence was found that trace metal profiles are influenced by anthropogenic sources and by changes in sediment composition. Only Zn presents concentrations (up to 13.1 μmol g−) and concentration factors (1.3 to 13.2) that can be attributed to heavy contamination. Pb, Cu and Ni, in this order, are less significant. The areal distribution of concentrations and inventories reflects the importance of direct sources, in particular the industrial area of Porto Marghera and the Dese river. The inventories of excess metals, above pre-industrial levels, were determined for each core and the three different parts of the study area, the amounts of Zn accumulated in sediments are 11.0 Mmol, 5.1 Mmol and 0.37 Mmol in the Campalto, S. Erasmo, and Palude di Cona areas, respectively. Ruxes were also calculated and compared with those suggested for the atmospheric delivery by Cochran et al. [(1995)b. Atmospheric fluxes of heavy metal contaminants to the Venice Lagoon, Rapp. Comm. Int. Mer Medit., 34, 136.], the atmospheric contribution is predominant or significant in many cases, especially at sites far from the major local inputs. Concentrations and fluxes show a significant increase in the anthropogenic metal supply starting from the second decade of this century, with maximum inputs in the period between the (1930)s and the (1970)s. At some stations a decrease in heavy metal contamination of surficial sediments was found and this could be ascribed to a reduced input of pollutants in recent years.