Mechanisms of Pond Expansion in a Rapidly Submerging Marsh

Joshua Himmelstein,Matthew L. Kirwan,Orencio Duran Vinent,Stijn Temmerman

doi:10.3389/fmars.2021.704768

Abstract

The development and expansion of ponds within otherwise vegetated coastal marshes is a primary driver of marsh loss throughout the world. Previous studies propose that large ponds expand through a wind wave-driven positive feedback, where pond edge erosion rates increase with pond size, whereas biochemical processes control the formation and expansion of smaller ponds. However, it remains unclear which mechanisms dominate at a given scale, and thus how, and how fast, ponds increase their size. Here, we use historical photographs and field measurements in a rapidly submerging microtidal marsh to quantify pond development and identify the processes involved. We find that as small ponds emerge on the marsh platform, they quickly coalesce and merge, increasing the number of larger ponds. Pond expansion rates are maximized for intermediate size ponds and decrease for larger ponds, where the contribution of wave-driven erosion is negligible. Vegetation biomass, soil shear strength, and porewater biogeochemical indices of marsh health are higher in marshes adjacent to stable ponds than in those adjacent to unstable ponds, suggesting that pond growth rates are negatively related to the health of the surrounding marsh. We find that the model of Vinent et al. (2021) correctly predicts measured pond growth rates and size distribution, which suggest the different mechanisms driving pond growth are a result of marsh drowning due to sea level rise (SLR) and can be estimated by simplified physical models. Finally, we show that all relevant processes increasing pond size can be summarized by an empirical power-law equation for pond growth which predicts the temporal change of the maximum pond size as a lower bound for the total pond area in the system. This gives a timescale for the growth of ponds by merging and thus the critical time window for interventions to prevent the irreversible pond expansion associated with large scale pond merging.

Highlights

Marshes buffer coasts from storms, sequester carbon, improve water quality, and provide rich habitats for fish and wildlife (Shriver et al, 2004; Barbier et al, 2011; Möller et al, 2014; Nahlik and Fennessy, 2016; Zhu et al, 2020)
Our observations from a rapidly submerging marsh in the Chesapeake Bay, United States suggest that the mechanisms and rates of pond expansion change with pond size
Historical photographs indicate that linear growth rates are maximized for intermediate size ponds (103–104 m2), where wave driven erosion is negligible

Summary

Introduction

Marshes buffer coasts from storms, sequester carbon, improve water quality, and provide rich habitats for fish and wildlife (Shriver et al, 2004; Barbier et al, 2011; Möller et al, 2014; Nahlik and Fennessy, 2016; Zhu et al, 2020). While marshes have some ability to survive SLR by migrating into adjacent uplands and accreting soil vertically (Pethick, 1992; Temmerman et al, 2003; Kirwan et al, 2016; Fagherazzi et al, 2019), in many regions of the world they are eroding laterally, accreting less than the local rate of SLR, and converting to open water through the development of unvegetated ponds in the marsh interior (Stevenson et al, 1985; Day et al, 2000; Carniello et al, 2009). Marshes with large proportions of unvegetated area tend to export more sediment than those with large proportions of vegetated area (Ganju et al, 2013; Ganju et al, 2020), suggesting a positive feedback where pond development leads to sediment export, and potentially runaway marsh loss

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