Abstract
A prominent form of salt marsh loss is interior conversion to open water, driven by sea level rise in interaction with human activity and other stressors. Persistent inundation drowns vegetation and contributes to open water conversion in salt marsh interiors. Runnels are shallow channels originally developed in Australia to control mosquitoes by draining standing water, but recently used to restore marsh vegetation in the USA. Documentation on runnel efficacy is not widely available; yet over the past 10 years dozens of coastal adaptation projects in the northeastern USA have incorporated runnels. To better understand the efficacy of runnels used for restoration, we organized a workshop of 70 experts and stakeholders in coastal resource management. Through the workshop we developed a collective understanding of how runnels might be used to slow or reverse open water conversion, and identified unresolved questions. In this paper we present a synthesis of workshop discussions and results from a promising case study in which vegetation was restored at a degraded marsh within a few years of runnel construction. Despite case study outcomes, key questions remain on long-term runnel efficacy in marshes differing in elevation, tidal range, and management history. Runnel construction is unlikely to improve long-term marsh resilience alone, as it cannot address underlying causes of open water conversion. As a part of holistic climate planning that includes other management interventions, runnels may “buy time” for salt marshes to respond to management action, or adapt to sea level rise.
Highlights
While for centuries salt marsh loss was driven by direct human alterations (Gedan et al 2009), sea level rise (SLR) poses one of the chief threats to salt marshes globally (FitzGerald and Hughes 2019; Bindoff et al in press)
Three focal questions will be discussed here: (Q1) What problem do managers use runnels to address? (Q2) What is a runnel and how does it work? and (Q3) How effective are runnels? We focus on runnel use to mitigate marsh loss caused by interior shallow water expansion, describe runnel mechanics and present a case study, and discuss lessons learned on efficacy and remaining knowledge gaps
Platform elevations along transect T1 were conducive to high-marsh species growth prior to runnel creation; as a result, draining the shallow water areas allowed bare peat to recolonize with high-marsh species quickly (Fig. 3)
Summary
While for centuries salt marsh loss was driven by direct human alterations (Gedan et al 2009), sea level rise (SLR) poses one of the chief threats to salt marshes globally (FitzGerald and Hughes 2019; Bindoff et al in press). While pannes and pools were not classically considered to contribute to net marsh loss (Ewanchuk and Bertness 2004; Adamowicz and Roman 2005), observed increases in the number and size of unvegetated features suggest that marsh hydrology is not in equilibrium—and a trend toward net conversion of vegetated marsh to bare and shallow water areas (Table 1, and references therein). As a result, drowning and pool collapse are two mechanisms by which interior vegetated marsh areas convert to shallow water and contribute to marsh loss.
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