Molly Mitchell

Nonlinear Change in Sea Level Observed at North American Tide Stations

ABSTRACT Boon, J.D. and Mitchell, M., 2015. Nonlinear change in sea level observed at North American tide stations. The rate at which coastal sea level is expected to rise or fall is of considerable interest to coastal residents and managers who view changes on the time scale of a 30-year mortgage. Analysis of historical records at North American tide stations provides evidence of recent nonlinear sea-level change at this scale using relative mean sea-level (RMSL) observations. RMSL tracks local inundation risk directly without the need to correct an accepted worldwide geocentric measure—e.g., global mean sea-level rise—with locally estimated vertical rate adjustments. Published RMSL linear trends provide essential information but are routinely compared between tide stations with widely varying record lengths, thereby obfuscating nonlinear change (acceleration or deceleration) over a specific period of time. Here monthly averaged RMSL data from 45 U.S. tide stations and one Canadian tide station are analyzed from 1969 through 2014, extending a definitive period of acceleration previously noted along the U.S. NE Coast. Using a Bayesian approach to determine the joint probability of paired regression parameters for RMSL quadratic trends, probabilities for forward projections to the year 2050 based on these trends suggest continued sea-level rise will be aided by acceleration presently on the order of 0.1 to 0.2 mm/y2 in the U.S. NE and Gulf Coast regions. Deceleration ranging from −0.1 to −0.4 mm/y2 is likely to reinforce falling sea levels at specific locations on the U.S. West Coast in the near term.

The Role of Living Shorelines as Estuarine Habitat Conservation Strategies

ABSTRACT Globally, shoreline protection approaches are evolving towards the incorporation of natural and nature-based features (living shorelines henceforth) as a preferred alternative to shoreline armoring. Emerging research suggests that living shorelines may be a viable approach to conserving coastal habitats (marshes, beaches, shallows, seagrasses) along eroding shorelines. Living shorelines typically involve the use of coastal habitats, such as wetlands, that have a natural capacity to stabilize the shore, restore or conserve habitat, and maintain coastal processes. They provide stability while still being dynamic components of the ecosystem, but due to their dynamic nature, careful designs and some maintenance will be required if habitat conservation is a goal. Living shorelines may represent a singular opportunity for habitat conservation in urban and developing estuaries because of their value to society as a shoreline protection approach and resilience to sea level rise. However, enhanced public acceptance and coordination among regulatory and advisory authorities will be essential to expand their use. To fully understand their significance as habitat conservation strategies, systematic and standardized monitoring at both regional and national scales is vital to evaluate the evolution, persistence, and maximum achievable functionality (e.g., ecosystem service provision) of living shoreline habitats.

Transitional Wetland Faunal Community Characterization and Response to Precipitation-Driven Salinity Fluctuations

Transitional wetlands occur along an estuarine salinity continuum bracketed by salt and nontidal freshwater wetlands. Long-term disturbances, such as sea level rise, may shift physical characteristics and hydrologic features of estuarine systems leading to transitional wetland habitat alterations, with unknown affects to associated biological communities. To begin to assess the ecological significance of a change in transitional wetland character, physicochemical parameters, and fish and invertebrate communities were surveyed seasonally in two extensive meso-oligohaline marshes, currently undergoing changes due to sea level rise. Faunal communities were similar between marshes, and included economically important species and forage fish. Temporal shifts in communities occurred seasonally and annually with high variability in species abundance among years. Drastic annual variability in salinity regimes due to extremes in precipitation occurred during the survey period driving shifts in biotic community composition.

Marsh persistence under sea-level rise is controlled by multiple, geologically variable stressors

ABSTRACT Introduction: Marshes contribute to habitat and water quality in estuaries and coastal bays. Their importance to continued ecosystem functioning has led to concerns about their persistence. Outcomes: Concurrent with sea-level rise, marshes are eroding and appear to be disappearing through ponding in their interior; in addition, in many places, they are being replaced with shoreline stabilization structures. We examined the changes in marsh extent over the past 40 years within a subestuary of Chesapeake Bay, the largest estuary in the United States, to better understand the effects of sea-level rise and human pressure on marsh coverage. Discussion: Approximately 30 years ago, an inventory of York River estuary marshes documented the historic extent of marshes. Marshes were resurveyed in 2010 to examine shifts in tidal marsh extent and distribution. Marsh change varied spatially along the estuary, with watershed changes between a 32% loss and an 11% gain in marsh area. Loss of marsh was apparent in high energy sections of the estuary while there was marsh gain in the upper/riverine section of the estuary and where forested hummocks on marsh islands have become inundated. Marshes showed little change in the small tributary creeks, except in the creeks dominated by fringing marshes and high shoreline development. Conclusions: Differential resilience to sea-level rise and spatial variations in erosion, sediment supply, and human development have resulted in spatially variable changes in specific marsh extents and are predicted to lead to a redistribution of marshes along the estuarine gradient, with consequences for their unique communities.