Interior marsh pond formation has been commonly observed in tidal marshes affected by high rates of relative sea level rise (RSLR). However, it is difficult to conclude whether an accretion deficit (accretion which does not keep pace with RSLR) or natural ice and wrack disturbance has driven pond formation. We propose that marsh deterioration caused by accretion deficit can be differentiated from that caused by other disturbances based upon temporal vegetation changes and the spatial configuration of vegetation zones relative to tidal creeks and the marsh platform. We tested this hypothesis in six newly ponded sites within RSLR‐affected marshes in Deal Island, Chesapeake Bay. At each site, we used field surveys and remote sensing to study spatiotemporal dynamics of marsh vegetation, marsh topography, and tidal creek incision. We found flood tolerant plants displaced flood intolerant species over time in the landward direction, or upslope, of ponds. A reverse species transition was observed seaward of ponds because tidal creek incision alleviated interior marsh inundation. The landscape‐scale biogeographic pattern we have recognized sheds light on how plants adapt to chronically reshaped geomorphological configurations of the marsh platform, which differentiates ponding caused by accretion deficit from ponding caused by natural and artificial disturbances. Furthermore, our results point to vegetation patterns that can be used as early warning signals of interior marsh loss to ponding. As ponding has been a major driver of tidal marsh habitat loss in microtidal marshes around the world, early indicators of decline are sorely needed to direct conservation activities.
Salt marsh ponds expand and deepen over time, potentially reducing ecosystem carbon storage and resilience. The water filled volumes of ponds represent missing carbon due to prevented soil accumulation and removal by erosion and decomposition. Removal mechanisms have different implications as eroded carbon can be redistributed while decomposition results in loss. We constrained ponding effects on carbon dynamics in a New England marsh and determined whether expansion and deepening impact nearby soils by conducting geochemical characterizations of cores from three ponds and surrounding high marshes and models of wind‐driven erosion. Radioisotope profiles demonstrate that ponds are not depositional environments and that contemporaneous marsh accretion represents prevented accumulation accounting for 32%–42% of the missing carbon. Erosion accounted for 0%–38% and was bracketed using radioisotope inventories and wind‐driven resuspension models. Decomposition, calculated by difference, removes 22%–68%, and when normalized over pond lifespans, produces rates that agree with previous metabolism measurements. Pond surface soils contain new contributions from submerged primary producers and evidence of microbial alteration of underlying peat, as higher levels of detrital biomarkers and thermal stability indices, compared to the marsh. Below pond surface horizons, soil properties and organic matter composition were similar to the marsh, indicating that ponding effects are shallow. Soil bulk density, elemental content, and accretion rates were similar between marsh sites but different from ponds, suggesting that lateral effects are spatially confined. Consequently, ponds negatively impact ecosystem carbon storage but at current densities are not causing pervasive degradation of marshes in this system.
more » « less- NSF-PAR ID:
- 10396819
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Biogeosciences
- Volume:
- 128
- Issue:
- 2
- ISSN:
- 2169-8953
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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