The Coastal Critical Zone: Processes that transform landscapes and fluxes between land and sea

About the Coastal Cluster:

Ghost forests and abandoned farms are stark indicators of ecological change along world coastlines (Williams et al., 1999; Desantis et al., 2007; Anderson and Thani, 2016; Kirwan and Gedan, 2019) caused by sea level rise (SLR). These changes adversely affect land use and economies by reducing agricultural production, damaging surface and subsurface infrastructure, and degrading water quality (Khan et al., 2016). However, this change is also a positive indicator that our coastal marshes, and their crucial ecosystem services, may survive the effects of rising sea level (Kiran et al., 2016a). Less apparent, but perhaps even more important, are the concurrent changes in function, mass and energy flux, and feedbacks within the coastal critical zone (CZ).

The coastal CZ is a hotspot of biogeochemical activity, where large quantities of nutrients and Blue Carbon are cycled and stored. High rates of photosynthesis remove carbon dioxide from the atmosphere, sequestering it in marsh sediments, where slow decomposition rates allow it to accumulate (Chmura et al., 2013). Salt marsh soils globally bury 5-87 Tg C yr-1 at rates ranging from 18-1713 g C m-2 yr-1 (Chmura et al., 2003; Duarte et al., 2005), much greater than the 10 g C m-2 yr-1sequestered in temperate forests (McLeod et al., 2011). Nutrients and anthropogenic pollutants are trapped and attenuated, reducing fluxes to the coastal ocean (Childers et al., 2002; Mitsch and Gosselink, 2008; Valiela & Cole, 2002). Thus, changes to coastal marshes and transformation of coastal landscapes have important implications not only for the global economy, but also for global elemental fluxes.

The primary driver of changes along the marsh-upland transition is gradual relative SLR, which pushes salt and inundation fronts inland via storms and tides (fast processes) while gradually elevating the water table (slow process). Secondary drivers of change include positive and negative feedbacks that occur as a result. The complexity of the system, with its strong hydrologic transience (e.g. tides, storms), tightly coupled ecosystem mosaics, biogeochemical inputs from both land and sea, and human influences make functioning and response at the marsh-upland transition difficult to understand and predict. While marsh ecosystems have been well studied, the transition from marsh to upland has only more recently come into focus. Our overarching hypothesis is that sea level rise is converting coastal forests and agricultural fields to salt marshes through two driving mechanisms: flooding and salinity increase. Both mechanisms involve feedbacks among coupled HEGB processes. The occurrence of these mechanisms and the nature of their feedbacks, which differ between forested and agricultural land, determine the rate and extent of landscape transformation and the associated changes in elemental stores and fluxes in the coastal CZ. Our project examines coupled hydrological, ecological, geomorphological, and biogeochemical processes and feedbacks that are unique to the coastal CZ, critical for understanding alterations to global elemental cycles and fluxes, driven by tidal to millennial timescales of change, and not currently considered by existing observatories.

Our Vision

To quantify the coupled processes and feedbacks that govern the HEGB transformations in the coastal CZ to understand how shifts in the transition zone will translate to changes in cycling, fluxes, and storage of critical elements at the land-sea margin.

Our Mission

Develop a fully collaborative, multi-disciplinary study at a network of sites to understand and predict current and future changes in coupled hydrological, ecological, geomorphological, and biogeochemical (HEGB) system.