Coastal marshes and their valuable ecosystem services are feared to be lost by sea level rise, yet the mechanisms of marsh degradation into ponds and potential recovery are poorly understood. We quantified and analyzed elevations of marsh surfaces and pond bottoms along a marsh loss gradient (Blackwater River, Maryland, USA). Our analyses show that ponds deepen with increasing tidal channel width connecting the ponds to the river, indicating a new feedback mechanism where channels lead to enhanced tidal export of pond bottom material. Pond elevations also decrease with increasing pond size, consistent with previous work identifying a positive feedback between wind wave erosion and pond size. These two positive feedbacks, combined with bimodal elevation distributions and sharp topographic boundaries between interior ponds and the marsh platform, indicate alternative elevation states and imply that marsh loss by pond formation is nearly irreversible once pond deepening exceeds a critical level.
Bibliographical noteFunding Information:
This project was financed by an UA-BOF DOCPRO grant (to L.?S. and S.?T.), the Research Foundation Flanders (FWO, PhD grant to L.?S. [11S9614N]; travel grants to L.?S. [V428214N] and S.?T. [K217414N]). G.?G. and P.?B. acknowledge support from the U.S. Geological Survey Land Change Science Climate R&D Program and U.S. Geological Survey Ecosystems Mission Area. M.?K. acknowledges support by NSF GLD 1529245, NSF SEES 1426981, and NSF LTER 1237733. We would like to thank the managers and biologists of the Blackwater National Wildlife Refuge for their assistance and valuable comments. We thank Erik Fransen and Joanna Horemans (University of Antwerp) for their feedback and help with the statistical analysis. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The Spartina plants in Figure?4 are by Tracey Saxby, Integration and Application Network, University of Maryland Center for Environmental Science (ian.umces.edu/imagelibrary/).
- marsh degradation
- pond expansion
- sea level rise