Estuarine habitats in the St. Louis Estuary, Lake Superior, Wisconsin: past, present, and future

In the face of climate change, understanding trajectories of change is critical for coastal management, particularly for identifying future restoration and climate adaptation opportunities. The National Estuarine Research Reserves (NERRs), individually and as a system, therefore have an urgent need to identify the extent and spatial patterns of estuarine habitat loss. To meet this challenge, we studied habitat change across 30 U.S. estuaries to document coastal habitat loss, and identify key opportunities for future restoration and enhancement. At the St. Louis River Estuary, elevation-based mapping revealed 5,043 ha currently within the reach of high water levels and appeared to provide a fairly accurate estimate of current estuary extent. Within a focal area for historical mapping, emergent marsh underwent substantial (52%) loss between 1861 and the present. In order for the estuary to realize its full capacity to provide benefits to plants, animals, and humans, it is critical for the estuary to regain habitats that have seen the most losses. Active restoration, carefully planned to enhance future resilience, can help recover these lost habitats. The new information from this habitat change analysis thus helps us envision a more resilient coast as a legacy for future generations

Evaluating Thin-Layer Sediment Placement as a Tool for Enhancing Tidal Marsh Resilience: a Coordinated Experiment Across Eight US National Estuarine Research Reserves

Thin-layer sediment placement (TLP) is a promising management tool for enhancing tidal marsh resilience to rising seas. We conducted a 3-year experiment at eight US National Estuarine Research Reserves using a standardized implementation protocol and subsequent monitoring to evaluate effects of sediment placement on vegetation in low and high marsh, and compared this to control and reference plots. Sediments added to experimental plots were sourced from nearby quarries, were sandier than ambient marsh soils, and had more crab burrowing, but proved effective, suggesting that terrestrial sources can be used for tidal marsh restoration. We found strong differences among sites but detected general trends across the eight contrasting systems. Colonization by marsh plants was generally rapid following sediment addition, such that TLP plot cover was similar to control plots. While we found that 14-cm TLP plots were initially colonized more slowly than 7-cm plots, this difference largely disappeared after three years. In the face of accelerated sea-level rise, we thus recommend adding thicker sediment layers. Despite rapid revegetation, TLP plots did not approximate vegetation characteristics of higher elevation reference plots. Thus, while managers can expect fairly fast revegetation at TLP sites, the ultimate goal of achieving reference marsh conditions may be achieved slowly if at all. Vegetation recovered rapidly in both high and low marsh; thus, TLP can serve as a climate adaptation strategy across the marsh landscape. Our study illustrates the value of conducting experiments across disparate geographies and provides restoration practitioners with guidance for conducting future TLP projects

Constraints on the adjustment of tidal marshes to accelerating sea level rise

Much uncertainty exists about the vulnerability of valuable tidal marsh ecosystems to relative sea level rise. Previous assessments of resilience to sea level rise, to which marshes can adjust by sediment accretion and elevation gain, revealed contrasting results, depending on contemporary or Holocene geological data. By analyzing globally distributed contemporary data, we found that marsh sediment accretion increases in parity with sea level rise, seemingly confirming previously claimed marsh resilience. However, subsidence of the substrate shows a nonlinear increase with accretion. As a result, marsh elevation gain is constrained in relation to sea level rise, and deficits emerge that are consistent with Holocene observations of tidal marsh vulnerability