Modified sediments and subsurface hydrology in natural and recreated salt marshes and implications for delivery of ecosystem services

Abstract

Abstract:Recreation or restoration of salt marsh through the deliberate removal of flood defences (managed realignment or deembankment) is a common practice across Europe and the USA, with potential to enhance delivery of ecosystem services. However, recent research suggests that physical, chemical and ecological processes may be impaired in recreated sites as a result of the modified morphology, sediment structure and hydrology associated with both the restoration process and historic land use. This paper compares physical sediment properties and subsurface water levels recorded in paired natural and de-embanked (recreated) salt marshes in SE England. Using a combination of statistical and time-series modelling, significant differences between the natural and recreated marshes are identified. Sediment properties (bulk density, moisture content and organic content) within each marsh were statistically different and imply that de-embanked sediments are compacted, which may affect subsurface water movement. Analysis of hydrological time series reveals that the de-embanked salt marsh is characterized by a damped response to tidal flooding with elevated and less variable water levels. This, combined with analysis of hydrographs and hysteresis patterns over individual tidal cycles, suggests that fast, horizontal near-surface flows enhanced by the relict land surface may play a greater role in drainage of the de-embanked salt marsh. The importance of hydrological functioning in governing many important physical and biogeochemical processes in salt marshes suggests any modifications would have significant implications for the delivery of ecosystem services.

The evolution of embryonic creek systems in a recently inundated large open coast managed realignment site

“…Analysis of the sediment in the creek beds, sampled in July 2016 and analysed using the same procedure as the surface samples, indicates they developed on top of a less organic layer (loss on ignition, 2.80%) with a higher concentration of fine-grained sediment (d 50 = 22.50 μm; mud content, 54.59%) compared to the surrounding bank. This lower sediment unit is probably the former terrestrial surface, matching the observations of Tempest et al (2015) from the Orplands Farm Managed Realignment Site (Essex, UK).…”

“…This may be due to poor sub-surface hydrological connectivity and differences in drainage pathways within MR sites (Tempest et al 2015), caused by disturbances and former land use practices impacting on the sediment structure (Spencer et al 2017). Even though it has been recognised that poor sediment drainage, and anoxia caused by stagnant water, may be the cause of poor species diversity in MR sites (Mossman et al 2012), there remains a lack of understanding of the geotechnical, morphological, and sedimentary processes within MR schemes (Esteves 2013).…”

Sedimentation rhythms and hydrodynamics in two engineered environments in an open coast managed realignment site

“…Poor drainage could, in turn, hinder the physical, chemical and ecological development of the site and have negative implications for the ecosystem services that it may eventually provide (e.g. Tempest et al, 2015).…”

“…The international importance of intertidal saltmarshes and mudflats has only been realised in recent decades (Rotman et al, 2008). These environments provide a range of ecosystem, economic and cultural services such as wildlife habitat, carbon sequestration, immobilisation of pollutants, water quality improvements, social and recreation opportunities and protection from coastal flooding (Costanza et al, 1997;King and Lester, 1995;Moller et al, 2014;Moller et al, 1999Moller et al, , 2001Tempest et al, 2015). Furthermore, these environments also protect (and reduce the cost of maintaining) engineered defences such as sea walls (e.g.…”

“…MR allows tidal inundation of the formerly defended low-lying coastal hinterland (e.g. Tempest et al, 2015) which usually consists of land that had previously been reclaimed for agricultural use. Many coastal managers advocate MR as a preferred management strategy (French, 2006) as it provides new areas of intertidal habitat and improves the standard of coastal flood defence.…”