Investigating ecosystem structure and function in restored coastal saltmarshes: a macro to micro-scale approach

<p dir="ltr">Coastal saltmarsh ecosystems are increasingly degraded by anthropogenic activities. Efforts to restore these important ecosystems are growing, but our knowledge about the potential effectiveness of restoration is limited. Several restoration approaches have been undertaken in different saltmarshes worldwide, some of which include tidal control and revegetation. My PhD thesis examines saltmarshes in Venice lagoon (Italy) that have been restored using sediment remodelling and edge protection measures, saltmarshes in Hunter River estuary (Australia) that have been restored using passive and active tidal restoration approaches and those in Georges River estuary (Australia) that have been restored using revegetation and sediment remodelling. Such anthropogenic management and restoration of tidal saltmarshes can potentially change the ecosystem structure and function, but this has not been explored much at a microbial scale. </p><p dir="ltr">Hence, the overarching aim of this thesis was to investigate if restored saltmarshes can provide ecosystem structure and function similar to those observed in natural saltmarshes. To do this, I applied a range of molecular and biogeochemical techniques to assess four main ecosystem structure and function indicators (macro-benthic structural and functional composition, sediment microbial community diversity and composition, micro-scale biogeochemical processes, and decomposition) in restored and natural saltmarshes in Italy and Australia. Macro-benthic organisms were taxonomically identified to their lowest level and functionally classified using a trait-based approach. Microbial communities were analysed by 16S and ITS sequencing of the V4 and V5 hypervariable regions, and the ITS region of the ribosomal RNA subunit, respectively. Biogeochemical processes were analysed using microbial-scale biogeochemical fluxes of N₂O, H₂S and O₂, and decomposition measures were analysed within the sediment by means of an in-situ field experiment with microsensors and a six-month decomposition study using tea bags, respectively. </p><p dir="ltr">Overall, macro- and micro-scale ecosystem structure and functioning in restored saltmarshes were similar to that in natural saltmarshes, regardless of restoration approach. Both, macro-benthic structure, and function, as well as microbial community diversity, were comparable between natural and restored saltmarshes in Italy and Australia. However, microbial community composition varied between saltmarshes, indicating the presence of distinct microbial groups in each saltmarsh. Nevertheless, microbially mediated biogeochemical processes and decomposition measures – assessed using microscale sediment fluxes of N₂O, H₂S and O₂ and tea bag decomposition respectively – were similar between restored and natural marshes in both Italy and Australia, suggesting that these marshes share similar microbial functions. A nuanced view of microbial functioning was observed in these studies, with the comparable microbial-scale functions likely reflecting regulation by environmental factors or functional redundancy of microbial groups across different saltmarsh types over an extended period. Presence of vegetation strongly influenced sediment microbial communities; however, while the vegetated sediments were more microbially rich and diverse, both vegetation states were made up of similar microbial putative functional groups. </p><p dir="ltr">My results confirm that a range of restoration approaches including sediment remodelling, edge protection, revegetation, and tidal control can support restoration of several macro and micro-scale functions. Understanding how different restoration approaches support ecosystem functioning in saltmarsh ecosystems is crucial, and this thesis provides novel information to support restoration of function. This understanding provides further evidence and demonstrates the value of restoration, which can lead to improved investment in such approaches at a large scale and for a longer time frame.</p>