Understanding coastal ecosystem responses to complex, large-scale disturbances such as sea-level rise and extreme events requires a diverse set of biogeochemical, hydrological, and ecological measures. Restoration efforts that re-connect ocean tides to coastal systems can provide useful analogues for sea-level rise and extreme flooding. Since January 2018, we have conducted an interdisciplinary and multi-scale study of one such system, a small creek (Beaver Creek, WA, USA) where tidal inundation (tidal range=2m) was recently re-introduced in 2014 through removal of a culvert. This has provided the opportunity for a cause-and-effect study on the impact of sea-level rise on a system that has been predominately freshwater for a century.
This interdisciplinary work has assessed how hydrology, tree physiology/mortality, soil organic matter composition, microbial activity, and greenhouse gas emissions are responding to the culvert removal through numerical modeling, in-situ monitoring, and field sampling. We have additionally explored its impact on the understory vegetation in the floodplain across the watershed. Four and six years after culvert removal, we compared vegetation cover at three floodplain sites along the river (one downstream of the culvert removal and two upstream) to elevation, floodwater inundation (based on hydrological modeling and in-situ sensors), and groundwater salinity (high-resolution, in-situ sensors).
Results/Conclusions
Components of the ecosystem have responded to the re-introduced tidal intrusion at different timescales. Monthly flooding is increasing soil salinity, and predicted to take ~20 years to reach dynamic equilibrium. Tree growth declined within one year of culvert removal; however, tree mortality has lagged, with 27% of trees still surviving after four years. Similarly, soil organic matter composition—but not microbial community composition—has already shifted with the increased soil salinity.
Across the watershed, elevation ranges from 2.4—3.5 m, equating to inundation of 371.1 hrs/year at 2.4 m to only 1.1 hrs/year at 3.4 m. Groundwater salinity in the marsh averaged 12.14 PSU downstream of the culvert removal and 7.56 PSU at the marsh farthest upstream. Vegetation composition similarly shifted across the watershed. For instance, the dominant species (Agrostis sp.) represented 44% of groundcover downstream of the culvert removal, increasing to 65.3% just upstream and declining to 20% farther upstream. Plot-scale analysis of vegetation compared to physical gradients and an additional vegetation survey in summer 2020 will further elucidate how marsh vegetation has responded to tidal intrusion and provide an analogue for how coastal systems will adapt to long-term and periodic disturbances.