Spatially integrative metrics reveal hidden vulnerability of microtidal salt marshes

Neil K Ganju,Andrea D’Alpaos,Luca Carniello,Sergio Fagherazzi,Zafer Defne,Matthew L Kirwan

doi:10.1038/ncomms14156

Abstract

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.

Highlights

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape
We show that the sediment budgets of all sites scale with the unvegetatedvegetated marsh ratio (UVVR) and that the sediment-based lifespan of these marshes can be predicted via the unvegetated/vegetated marsh ratios (UVVR)
The sediment budgets of the marsh complexes consistently scale with the UVVR, an independent measure of marsh health (Fig. 2a)

Summary

Introduction

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability. We present a synthesis of marsh tidal channel flux studies from eight microtidal sites along the Atlantic and Pacific coasts of the United States (Fig. 1; Table 1) These sites span a spectrum of vegetation type, climatic forcing, tidal range, geomorphic setting and watershed land use. We identify a strong relationship between the flood-ebb SSC differential and the sediment budget, allowing for simplification of sediment transport monitoring We find that these sediment transport metrics and the UVVR are useful indicators of marsh trajectory, allowing for widespread mapping of marsh vulnerability using in-situ measurements and remote sensing

Methods

Results

Conclusion