Determination of sediment and TOC, TN, TP accretion rates in restored wetlands

Abstract

Sediment accretion rate plays a vital role in wetland performance to improve water quality and to reduce excess contaminant loads into adjacent water. However, a simple and reliable method to measure the sediment accretion rate remains limited. In this study, sediment profiles in the restored wetlands in the Yellow River Delta (YRD) were collected, and a change point technique (CPT) was proposed to determine the recently accreted sediment (RAS). The variables including TOC, TN, TP, and δ¹³C, which were related to the accumulation of soil organic matter, were more suitable for calculating the depth of the RAS in the restored wetlands. The sediment accretion rate was calculated with the RAS in conjunction with the operational years of the wetland. The results showed that the average sediment, TOC, TN, and TP accretion rates were 0.49 ± 0.08 cm a⁻¹, 138 ± 24 g m⁻² a⁻¹, 15.8 ± 2.6 g m⁻² a⁻¹, and 2.8 ± 0.4 g m⁻² a⁻¹, respectively, for the restored wetlands since 2006 (RW2006). For the restored wetlands since 2002 (RW2002), the sediment, TOC, TN, and TP accretion rates were 0.68 ± 0.05 cm a⁻¹, 268 ± 69 g m⁻² a⁻¹, 25.3 ± 5.7 g m⁻² a⁻¹, and 5.5 ± 0.8 g m⁻² a⁻¹, respectively. These data indicated a significantly higher sediment and nutrient accretion rates in the older restored wetlands. This fact maybe caused by the longer wetland restoration significantly increased sediment organic matter content and sediment accumulation through plant litter decomposition and root stabilization. In conclusion, the CPT technique is a data-driven and high-resolution method, eliminating reliance on stable depositional environments or physical tracers. Therefore, the CPT can be accurately used to determine the sediment accretion rate in restored or constructed wetlands. In future, the restored wetlands will be important for the sediment and nutrient storage in YRD.