Recent observations of mangrove expansion into historical saltmarsh ecosystems on the Atlantic and Gulf Coasts of Florida suggest climate change, in the form of reduced freezing events, is allowing a shift in foundation species in coastal wetlands. The change of dominant vegetation from marsh cordgrass (Spartina alterniflora) to woody, canopy forming black (Avicennia germinans) and red mangroves (Rhizophora mangle) can have significant effects on ecosystem services provided. While much attention has been focused on biological and ecological response, little is known about the concomitant physical changes to these ecosystems that major shifts in vegetation community can impart. The motivating questions for this study include: 1) will the shift from cordgrass to shrub/forest dominated ecosystem affect hydrologic characteristics of the marsh platform? and 2) if so, to what degree might those factors affect recognized ecosystem services?
To address these questions, we measured several key variables (stem height, diameter, density, flexure, leaf counts and structure) used to estimate plant resistance to flow and calculated potential differences between the two competing species within the Matanzas River basin in St. Augustine, FL. Flow through flume experiments were conducted to determine the contribution of vegetation to water movement and ultimately tidal exchange.
Results/Conclusions
Results of flow resistance calculations and flume trials suggest that mangrove expansion is having a significant effect on hydrology of the marsh platforms in the Matanzas River estuary. Increased stem rigidity, diameter, and altered densities resulted in lower velocities of inflow and outflow water. The main impact is realized in sediment transport and deposition within the estuary. Based upon these findings, we argue that potential alteration to ecosystem services includes an increased resistance to coastal flooding and storm surge, but a reduced ability of the marsh platform to keep up with sea level rise, thus reducing blue carbon storage and overall long-term resiliency of the estuary to disturbance events.