Spatial variation of salt-marsh organic and inorganic deposition and organic carbon accumulation: Inferences from the Venice lagoon, Italy
Introduction
Coastal salt marshes represent a transition zone between submerged and emerged environments, occupying the upper margins of the intertidal landscape. Because of their unique position in the tidal frame, salt marshes represent a crucially important ecosystem. They offer valuable services, by providing a buffer against wave and storm surges [e.g., [1], [2], [3], [4]], nursery areas for coastal biota [e.g., [5], [6]] and filtering of nutrients and pollutants [e.g., [7], [8]]. In the last few decades a number of authors also highlighted the importance of salt marshes serving as an organic carbon sink due to their great ability to sequester atmospheric carbon [e.g., [9], [10], [11], [12], [13]]. The future of these valuable coastal landforms and ecosystems is today at risk, exposed as they are to possibly irreversible transformations due to the effects of climate changes and human interferences [e.g., [11], [14], [15], [16], [17], [18], [19]]. The extent of these coastal features, in fact, has dramatically decreased worldwide in the last century [10], [15], [20], [21], [22], [23]. For example, salt-marsh areas in the Venice lagoon decreased from about 180 km2 in 1811 to about 50 km2 in 2002, a reduction of more than 70% [15], [22], [24]. Losses of vast amounts of marsh areas have also been documented for San Francisco Bay (about 200 km2, [23]) over the last century, while vast amounts of wetlands disappeared in the Mississippi Delta plain (about 4800 km2 [16], [20]).
The rising sea level and the lack of available sediments are key factors in determining the drowning and disappearance of salt marshes worldwide [15], [17], [20], [23], [25], [26], [27], [28], [29]. In the horizontal plane the prevalence of wind-wave induced lateral erosion over marsh progradation is responsible for the retreat of salt marsh edges [22], [30], [31], [32]. Once the marsh drowns or is laterally eroded and converted to tidal flat or to a subtidal platform, unless the environmental conditions change (i.e. the sediment concentration increases) the re-growth of the marsh platforms is unlikely because the system is characterized by a hysteretic behavior [e.g., [33], [34]].
Salt marshes are populated by halophytic vegetation species, adapted to saline environments. The spatial distribution of halophytes over salt marshes is organized in characteristic patches, a phenomenon known as zonation [35], [36], [37], [38], [39], [40]. It has been recently shown that zonation patterns are not just the result of ecological and physiological processes and that their emergence is the consequence of the feedback on soil accretion of organic soil production by plants, which act as a landscape engineer [41], [42]. The development of vegetation over salt marshes is mainly determined by the frequency and duration of marsh flooding [e.g., [25], [37], [43]], which, in turn, depend on elevation, position and local topography of the marshes. Halophytic vegetation species which populate marsh platforms control sediment trapping efficiency, by enhancing particle settling via reduction of turbulence kinetic energy [e.g., [44], [45]], by directly capturing sediment particles [e.g., [46], [47]], and determine vertical organic accretion, by directly depositing organic matter due to root growth and litter deposition [48], [49], [50]. Halophytes interact with inorganic sedimentation and the rate of Relative Sea Level Rise (RSLR, sea level variations plus local subsidence), their combined effects controlling marsh surface elevation. Surface elevation, in turn, affects the vegetation productivity [25] which influences inorganic sediment deposition and control organic accretion, thus closing the bio-geomorphic feedback. In this framework it is clear that the interaction between physical and biological processes acting within salt marsh systems plays a fundamental role in salt-marsh survival or disappearance. Although in the last few decades a number of studies have analysed salt-marsh biomorphological evolution [e.g., [15], [18], [25], [33], [41], [50], [51], [52], [53], [54], [55]], a predictive understanding of the two-way feedbacks between physical and biological processes still appears to be elusive. Moreover, analyses of bio-geomorphic feedbacks based on data collected in the field at the marsh scale are rare [14], [25], [48], [49], [56], [57], [58], [59]. The evolution in time of marsh elevation z (x, t) (referenced to Mean Sea Level – hereinafter MSL) at a given site x and at time t, is governed by the sediment continuity equation (where erosion is neglected because of the stabilizing presence of vegetation, see [34], [41]): where Qi(x, t) and Qo(x, t) are the local rates of inorganic and organic deposition, respectively, and R is the rate of RSLR. In equilibrium conditions, the marsh elevation referenced to MSL is constant over time and therefore the left-hand side term in the above equation vanishes. The sediment balance equation then reads:
Hence, in a stable marsh, if Qi increases, Qo needs to decrease by the same magnitude, and vice versa, such that the forcing rate of RSLR is matched everywhere in the marsh. Field observations and numerical models [e.g., [52], [60], [61], [62]] suggest that marsh inorganic accretion rates, and the related platform elevations, decrease with distance from the main channels. Therefore, the organic accretion should be expected to gradually increase as the distance from the main channel increases for the equilibrium assumption to hold. However, a direct and detailed characterization of spatial variations in the accretion rates, and, in particular, in the organic soil production, is still lacking, and is the focus of the present work.
The paper is organized as follows. In the next section we provide a brief description of our study sites within the Venice lagoon. We then fully describe the methods used to determine the organic and inorganic sediment content, grain size distribution, above-ground biomass. The subsequent Results and Discussion sections analyze the role of physical and biological factors shaping the tidal landscape, and how these factors influence salt-marsh biogeomorphic patterns.
Section snippets
Study area
The Venice lagoon (Fig. 1a) is part of a foreland basin located between the NE-verging northern Appenninic chain and the SSE-verging eastern South-Alpine chain (Italy). Located in the northwestern Adriatic Sea, the Venice lagoon is the largest lagoon in the Mediterranean, with an area of about 550 km2, a mean water depth of 1.5 m, and a semi-diurnal micro-tidal regime (maximum water excursion at the inlets of ± 70 cm around MSL). The Lagoon is connected with the Adriatic Sea via three inlets:
Materials and methods
We collected 33 undisturbed cubic sediment samples with side of 5 cm, delimited at the top by the present-day depositional interface, to study the spatial variations of the soil inorganic and organic matter content, for the determination of the inorganic (Qi) and the organic (Qo) accretion rates, respectively. Each sample consists of massive brownish to blackish silt and contains abundant stems and roots. Some of the samples also contained scattered mm-size bivalve and gastropod shells and/or
Elevation
GPS field surveys show that marsh surface elevations (Figs. 3a, 4a, 5a) are highest near the marsh edge close to the San Felice channel levees (maximum elevation along the transects was equal to 32.1 cm above MSL). Toward the inner marsh, platform elevations decrease, reaching minimum values at about 30 m from the edge, where stagnant pools exist (minimum elevation along the transects was equal to 16.9 cm above MSL). Soil elevations at the end of the transects are slightly higher than in the
Discussion
The marsh elevations along the three transects considered show that the San Felice and Rigà salt marshes display a weakly concave-up profile, as commonly observed elsewhere [e.g., [82], [83], [84]]. Field observations and model results show that sedimentation patterns are characterized by a decrease in inorganic deposition with increasing distance from the channels [52], [60], [85], [86], [87]. This behavior emerges as a consequence of the progressive sediment settling as water moves towards
Conclusions
We analyzed variations in organic and inorganic soil production processes at the marsh scale, a critical issue to improve our understanding of the mutual feedbacks among processes of physical and biological nature in salt-marsh landscapes and of the responses of salt marshes to a changing climate.
Our results emphasize that surface elevations, inorganic and organic sediment content, and grain size distribution along marsh transects are tightly related. In particular, our results show that
Acknowledgments
This work was supported by the CARIPARO Project titled “Reading signatures of the past to predict the future: 1000 years of stratigraphic record as a key for the future of the Venice Lagoon”, and by the “Fondazione Ing. Aldo Gini”, that are gratefully acknowledged.
This manuscript benefits of the constructive comments from Andrea Rinaldo (Editor), Brad Murray and two anonymous Reviewers.
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