The size and distribution of tidal creeks affects salt marsh restoration

Highlights

• Depth of a tidal creek had a strong positive correlation with P. communis.

• Cross-sectional area of a tidal creek had a significant positive impact on S. glauca.

• Both S. glauca and P. communis had obvious correlations with the distance from tidal creeks.

• Areas located in the subtidal zone near a tidal creek was more conducive to S. glauca.

• Relevant features in the intertidal and subtidal zones are useful for P. communis.

Abstract

Salt marshes are changeable and important ecosystems that currently face various threats, including global climate change and human activities. The influence of these factors can result in the degradation of salt marshes. Tidal creeks, which are an important source of nutrients and other substances for salt marsh vegetation, play an important role in the health of salt marshes. In this study, the morphological characteristics of tidal creeks and the characteristics of two typical plants, Suaeda glauca (SG) and Phragmites communis (PC), in the Yellow River Delta, China were investigated to determine the effect of tidal creeks on these plants. Aerial photography and field measurements of tidal creeks were carried out from May to July 2018 in the study area. At the same time, nine line-intercepts were set in the vertical direction of tidal creeks to investigate plants. The results showed that different grades of tidal creek exerted no significant influence on the growth of either S. glauca or P. communis. However, unlike grade, the size of a creek and the distance from it had marked effects on these plants. The cross-sectional area of a tidal creek had a significant positive impact on the density of S. glauca (r = 0.39, p = 0.02). For P. communis, the depth of a tidal creek had a strong correlation with this species’ density (r = 0.51, p = 0.04) and height (r = 0.63, p = 0.01). Meanwhile, there was a negative relationship between the distance from tidal creeks and the height of S. glauca (r = −0.52, p = 0.02). Conversely, the height (r = 0 0.90, p = 0.00) and density (r = 0.62, p = 0.01) of P. communis were positively affected by its vertical distance from tidal creeks. We found that the subtidal zone near a tidal creek was more conducive to the recovery and growth of S. glauca, and that areas further away from a tidal creek, located in the intertidal and subtidal zones, were more conducive to the recovery and growth of P. communis. The parameters associated with tidal creeks in the subtidal zone (cross sectional area 4.55 m², distance 0–10 m) were beneficial for the growth of S. glauca. For P. communis, relevant features in the intertidal and subtidal zones (depth 0.40–0.45 m, distance 20–60 m) are useful. Our results suggest that attention should be paid to the effects of size and distribution of tidal creeks during the process of salt marsh restoration. This work also provides practical guidance for the restoration of native salt marshes in China.

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.