Future Marsh Evolution Due To Tidal Changes Induced by Human Adaptation to Sea Level Rise

With sea level rise threatening coastal development, decision-makers are beginning to act by modifying shorelines. Previous research has shown that hardening or softening shorelines may change the tidal range under future sea level rise. Tidal range can also be changed by natural factors. Coastal marshes, which humans increasingly depend on for shoreline protection, are ecologically sensitive to tidal range. Therefore, it is critical to examine how changes in tidal range could influence marsh processes. A marsh accretion model was used to investigate the ecological response of a San Francisco Bay, California, USA marsh to multiple tidal range scenarios and sea level rise from 2010 to 2100. The scenarios include a baseline scenario with no shoreline modifications in the estuary, a shoreline hardening scenario that amplifies the tidal range, and 14 tidal range scenarios as a sensitivity analysis that span tidal amplification and reduction of the baseline scenario. The modeling results expose key tradeoffs to consider when planning for sea level rise. Compared to the baseline, the hardening scenario shows minor differences. However, further tidal amplification prolongs marsh survival but decreases Sarcocornia pacifica cover, an important species for certain threatened wildlife and an effective attenuator of wave energy. Conversely, tidal reduction precipitates marsh drowning but shows gains in Sarcocornia pacifica cover. These mixed impacts of tidal amplification and reduction shown by the model indicate potential tradeoffs in relation to marsh survival, habitat characteristics, and shoreline protection. This study suggests the need for a cross-sectoral, regional approach to sea level rise adaptation. Human adaptation to sea level rise can involve physically modifying shorelines, such as building sea walls (shoreline hardening) or removing them (shoreline softening). Research has shown that these actions can sometimes change the tidal range across a region, which can also be changed by natural factors. Coastal marshes are sensitive to tidal range because their ecology is driven by tidal flooding. We investigated how changes in tidal range and sea level rise could impact a marsh located in San Francisco Bay, California, USA by modeling multiple scenarios from 2010 to 2100. Our results show that increasing the tidal range allows the marsh to survive longer but decreases the amount of Sarcocornia pacifica. This plant is vital for certain threatened wildlife and absorbs wave energy well. Tidal reduction shows the opposite results. The marsh drowns earlier but the amount of Sarcocornia pacifica increases. These results suggest that changes in tidal range could create mixed impacts and tradeoffs. Adaptation planning in urbanized coasts should account for how changes in tidal range due to shoreline modifications and/or natural factors could have regional ecological impacts. The tidal range in coastal regions could change in the future as a result of human shoreline modifications and/or natural factorsA process-based ecological model was used to evaluate the impact of changing tides on a coastal marshChanges in tidal range could have mixed ecological impacts on tidal marshes that occur at different time scales