Environmental gradients and complexity in coastal landscape response to sea level rise

Relative sea-level rise (SLR) raises geomorphic base levels, displaces salt water and tidal or backwater effects inland, and changes the hydrology of aquatic and upland environments. On an all-other-things-being equal basis, we can predict some transitions associated with SLR. However, in real coastal landscapes, all other things are not equal. Factors other than sea-level influence geomorphic, hydrological, and ecological processes and controls, environmental interactions often complicate or obscure process-response relationships, and local disturbances may interrupt or overprint them. In this study relationships among coastal environments in North Carolina, USA were investigated as they respond to changes in multiple environmental gradients driven by relative sea level rise, to determine the extent to which the spatial complexity of landscape response can be explained by environmental gradients. Spatial adjacency graphs reflecting observed patterns of contiguity were derived empirically, and five key environmental gradients related to relative sea-level were identified (elevation, hydroperiod, salinity, vegetation, and process regime). The spectral radius of the spatial adjacency graph indicates a complex system that on the landscape level cannot be described or modeled based on linear gradients or successional relationships. Yet, spectral graph theory measures show that the complexity of the system can be fully explained, in the aggregate, by the five identified gradients, despite some redundancy of information therein. This indicates that coastal responses to SLR should be assessed based on multiscalar, nested environmental gradients rather than a single advancing front of change or linear sequence.