Tidal marshes are a globally important intertidal habitat, and are highly reflective of the local environment, exhibiting plant communities that tightly reflect inundation period and salinity. In estuarine ecosystems, sea level rise-associated increases in water levels change both intertidal inundation patterns and salinity distribution. These changes propagate through the ecosystem, altering the complex structure and shifting ecological niches of tidal marshes; consequently affecting habitat provision, nutrient cycling, and shoreline stability. Observable shifts in tidal marsh plant community composition should result from sea level rise and therefore can be useful sentinels of change in estuarine ecosystems.
In our study, a total of 17,658 marsh plant communities from the Chesapeake Bay were surveyed in two distinct survey efforts, representing on average a 32-year time period and an approximately 0.15 m rise in sea level. A subset of the total tidal marshes (263 tidal marshes) had detailed surveys where plants were identified to species level and the proportion of each species in the total marsh plant community was noted. The species matrices resulting from these species level surveys were used to examine granular changes in tidal marsh species composition over time.
Results/Conclusions
In this study we saw widespread shifts in marsh plant communities in an estuary undergoing rapid rates of sea level rise. Patterns of change included relatively systemic community shifts indicative of increased inundation period and community shifts indicative of salinization at the interface between fresh and brackish water. There is a critical need for the ability to observe and predicted changes in estuarine ecosystems. Sea level rise causes non-linear changes in hydrodynamics, leading to changes in sediment transport and ecological processes, which will affect the signal of change in shoreline systems. This non-linearity means that signals of change may be muted until sea level rise acceleration passes a critical threshold. The change in sea level over the period of examination was relatively small, approximately 15-20 cm, but shifts in communities were still evident. The expected rise over the next 30 years is three times higher. The ability of the vegetation to reflect a small shift in sea level suggests that monitoring of vegetation is a useful sentinel of change, allowing for enhanced projections of sea level rise-drive ecosystem shifts.