Resilience is currently a key theme within salt marsh ecological studies. Understanding the factors that affect salt marsh accretion and elevation gains is of paramount importance if management of these ecosystems is to be successful under increasing synergistic stresses of storm surge, inundation period, and eutrophication. We present the results of salt marsh fucoid algae (ecads) removal experiments on Spartina alterniflora abundance, production and decomposition, and the sedimentary dynamics of two marshes on Cape Cod, Massachusetts. The presence of the thick layer of marsh fucoids had a significant and positive influence on sediment deposition, accretion, and concentration of water column particulates, while it inhibited water flow. Decomposition rates of S. alterniflora in the field were significantly higher under the fucoid macroalgae layer, and, in lab experiments, S. alterniflora seedlings added more leaves when the marsh fucoids were present. In contrast, fucoids caused a significant decrease in S. alterniflora seedlings' survival in the field. We found that marsh fucoids are stable despite not being attached to any substrate, and field surveys revealed a relatively widespread, but not ubiquitous, distribution along outer Cape Cod. Salt marsh fucoid algae directly and substantially contribute to salt marsh sediment elevation gain, yet their potential inhibitory effects on colonizing S. alterniflora may counteract some of their overall contributions to salt marsh persistence and resilience.