Research articleThe size and distribution of tidal creeks affects salt marsh restoration
Introduction
Coastal wetlands are one of the most vulnerable ecosystems worldwide because of their unique location (Gabler et al., 2017, Kirwan and Megonigal, 2013); they are also highly sensitive to global climate change (Gabler et al., 2017). It has been predicted that 20%–45% of salt marshes will be lost this century as a result of rising sea-levels (Craft et al., 2009, Kirwan and Megonigal, 2013). Tidal creeks are the route via which water, nutrients, and sediment can be exchanged the between marine and inland environments (Kearney and Fagherazzi, 2016). Tidal creeks should be considered as important factors that influence salt marsh vegetation growing inland of coasts.
Several previous studies have shown that the distribution and growth of salt marshes is influenced by their elevation, salinity, and amount of tidal flooding (Callaway et al., 1990, Kim et al., 2013, Snow and Susan, 1984, Zedler et al., 1999). Plant zonal distribution in salt marshes arises due to the combined effects of these factors (Cui et al., 2011, Huckle et al., 2010, Silvestri et al., 2005). However, some of these earlier descriptions might be overly generalized and simplified (Davy et al., 2011, French and Tom, 1993; Zedler et al., 1999). Tidal creeks are features common to most salt marsh ecosystems (Sanderson et al., 2000), and can cut off the elevation gradient from coastal to inland (Allen, 2000, Reed et al., 1999, Wheeler, 1991). Tidal creeks can also shape topographic sequences at a small scale on marsh platforms (Kim et al., 2013). Sediment types of different tidal creek morphologies, such as cut banks and point bars, are different, which in turn can cause soil zonation near tidal creeks; this will also have an effect on the plants in small marsh areas (Kim et al., 2010, Kim et al., 2013). In addition, tidal creeks and even small channels can influence the pattern of canopy water content (Sanderson et al., 1998). Taken together, these studies show that tidal creeks can affect vegetation by influencing soil nutrient levels, soil water content, topography, and other conditions. Additionally, the characteristics of a tidal creek itself can also have an impact on vegetation. For example, the size and location of tidal creeks can affect species assemblage, plant species distribution, species richness, and the sequence of species (Sanderson et al., 1998, Sanderson et al., 2000, Wheeler, 1991). However, these studies focused on local parts of tidal creeks. There is a need to investigate the complete tidal creek system.
The unique nature and the importance of salt marsh ecosystems has led to many restoration activities being performed to protect salt marshes worldwide (Barrett and William, 2010, Boumans and John, 1994, Burdick et al., 1996, Smith et al., 2009). There are some important factors that should be considered if restoration is to be successful, including historical ecosystem types, hydrology and topography, creeks and channels, salinity, sediment organic content, colonizer presence, and proximity (Raposa et al., 2018, Teal and Michael, 2002, Weinstein et al., 2001). There has also been a variety of approaches taken, including excavation and filling of marsh surfaces, dike breaching, herbicide application, prescribed burns, hydrologic reconnection, and more to restore salt marsh vegetation (Raposa et al., 2018, Weinstein et al., 2001, Zedler et al., 1999). The excavation of tidal creek as an important approach for the restoration of salt marsh vegetation should be designed to ensure they would function “properly” which means the marsh could flood and drain to aerate surface sediments (Dacey and Howes, 1984, Teal and Michael, 2002). The reintroduction of tidal flooding through the creation or reestablishment of tidal creeks can improve tidal exchange and help to recover the structure and function of salt marsh ecosystems (Burdick and Roman, 2012, Burdick et al., 1996). However, some researchers have pointed out that vegetation communities show different responses to tidal reintroduction (Barrett and William, 2010, Burdick et al., 1996, Smith et al., 2009). Thus, accurate measurement of tidal creeks and vegetation sites is needed. On the basis of this, practical guidance could then be provided to salt marsh restoration projects.
Our research focused on the following aspects, because of current research and protection needs: (a) to investigate the morphological characteristics of a complete tidal channel system and the basic growth characteristics of plants in the Yellow River Delta, China; (b) to identify the influence of grade and size of tidal creek on vegetation; (c) to measure how vertical distance from a tidal creek affects vegetation; and (d) to propose suitable parameters for tidal creeks when restoring salt marshes.
Section snippets
Material and methods
The Yellow River Delta is located in the city of Dongying, Shandong province, China (He et al., 2012). It is an alluvial plain formed by the deposition of a large amount of sediment in the Bohai. Tidal gullies are widespread in the Yellow River Delta (He et al., 2007, Zong et al., 2009). The wetland ecosystem of the Yellow River Delta is a typical new coastal wetland ecosystem that is the youngest, the least damaged, and has the largest area in China. The region's biodiversity is also one of
Characteristics of tidal creeks
Fig. 1 shows the distribution of tidal creeks in our study area. Tidal flats included the subtidal, intertidal, and supratidal zones. The morphological characteristics of tidal creeks in these three regions are shown in Table 1. The tidal creeks were relatively wide and straight in the subtidal zone, which had a total length of tidal creeks of 0.76 km. Tidal creeks in the intertidal and supratidal zones contained more branching and meandering. The total lengths of tidal creeks in the intertidal
Effects on vegetation of the size of tidal creeks
The degree of branching and grades of tidal creeks gradually increased moving inland from the sea. Our study showed that S. glauca and P. communis were distributed throughout these three tidal creek grades. Thus, the spatial distribution of these two species was widespread; they were both dominant species in the Yellow River Delta. As salt marsh species they must combine tolerance to salinity with a propensity to withstand flooding (Pétillon et al., 2004). However, there were some differences
Conclusion
The purpose of this paper was to provide practical guidance for salt marsh restoration through the investigation of the morphological characteristics of tidal channels and the characteristics of two typical plants, S. glauca and P. communis, in the Yellow River Delta of China. Our results showed that although the growth characteristics of these species were different dependent on the grade of tidal creeks, the effect of tidal-creek grade was not significant. Only tidal creek depth had a strong
Author contributions
Yanan Wu:Investigation, Data curation, Writing-Original draft, Writing-Reviewing and Editing, Jiakai Liu: Investigation, Software, Guoxin Yan: Investigation, Data curation, Jiexiu Zhai: Software, Ling Cong: Data curation, Liyi Dai: Investigation, Software, Zhenming Zhang: Conceptualization, Methodology, Writing-Reviewing and Editing, Mingxiang Zhang: Supervision, Project administration.
Acknowledgements
This research was supported by the National Key R&D Program of China (2017YFC0505903).
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