Abstract
Abstract. The presence of bare patches within otherwise vegetated coastal marshes is sometimes considered to be a symptom of marsh dieback and the subsequent loss of important ecosystem services. Here we studied the topographical conditions determining the presence and revegetation of bare patches in three marsh sites with contrasting tidal range, sediment supply, and plant species: the Scheldt estuary (the Netherlands), Venice lagoon (Italy), and Blackwater marshes (Maryland, USA). Based on GIS (geographic information system) analyses of aerial photos and lidar imagery of high resolution (≤2×2 m pixels), we analyzed the topographic conditions under which bare patches occur, including their surface elevation, size, distance from channels, and whether they are connected or not to channels. Our results demonstrate that, for the different marsh sites, bare patches can be connected or unconnected to the channel network and that there is a positive relationship between the width of the connecting channels and the size of the bare patches, in each of the three marsh sites. Further, pixels located in bare patches connected to channels occur most frequently at the lowest elevations and farthest distance from the channels. Pixels in bare patches disconnected from channels occur most frequently at intermediate elevations and distances from channels, and vegetated marshes dominate at highest elevations and shortest distances from channels. In line with previous studies, revegetation in bare patches is observed in only one site with the highest tidal range and highest sediment availability, and it preferentially occurs from the edges of small unconnected bare patches at intermediate elevations and intermediate distances from channels. Although our study is only for three different marsh sites with large variations in local conditions, such as tidal range, sediment availability, and plant species, it suggests that similar topographic conditions determine the occurrence of bare patches. Such insights may inform decision makers on coastal marsh management on where to focus monitoring of early signatures of marsh degradation.
Highlights
Tidal marshes are coastal ecosystems that provide many valuable ecosystem services such as fishery production (Barbier et al, 2011), sequestration of CO2 (McLeod et al, 2011), protection against shoreline erosion, and mitigation of flood risks during storm surges (Barbier et al, 2008; Wamsley et al, 2010; Gedan et al, 2011; Temmerman et al, 2013; Temmerman and Kirwan, 2015)
For three marsh sites with different tidal ranges, sediment input, and plant species, we showed that (1) bare patches connected to channels occur most frequently at the lowest surface elevations and farthest distances from creeks; unconnected bare patches most frequently occupy intermediate elevations and distances from creeks, and they are smaller in size and larger in number; and vegetated marshes dominate at the highest surface elevations and closest to creeks
We studied the topographical conditions for presence and revegetation of bare patches within three coastal marsh sites that are largely different in tidal range, sediment supply, and plant species
Summary
Tidal marshes are coastal ecosystems that provide many valuable ecosystem services such as fishery production (Barbier et al, 2011), sequestration of CO2 (McLeod et al, 2011), protection against shoreline erosion, and mitigation of flood risks during storm surges (Barbier et al, 2008; Wamsley et al, 2010; Gedan et al, 2011; Temmerman et al, 2013; Temmerman and Kirwan, 2015). Large-scale tidal marsh loss by conversion of marshes into bare tidal flats, open water, or bare patches within marshes has been reported from different locations around the world (Baumann et al, 1984; Day et al, 2000; Kearney et al, 2002; Carniello et al, 2009; Kirwan and Megonigal, 2013). Bare patches within marshes, which are often covered by standing water and referred to as pools, ponds (Stevenson et al, 1985), or marsh basins (Mariotti and Fagherazzi, 2013), are a common feature in salt marshes around the world. Bare patches do not revegetate and are causing permanent marsh loss on a large scale (e.g., Mississippi Delta; Penland et al, 2000; Morton et al, 2003)
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