Physical drivers of seagrass spatial configuration: the role of thresholds

Abstract

Seagrass landscapes vary substantially in extent and pattern, resulting from depth zonation and hydrodynamic stress gradients and may exhibit threshold behavior in response to changes in physical drivers. Seagrass landscapes persist in a delicate balance between processes of disturbance and recovery and therefore may exhibit behavior typical of classic critical systems. Examine how hydrodynamic drivers and physical setting influence seagrass landscape composition and configuration. Determine if seagrass patch size distributions typify patterns observed for critical systems. We used landscape metrics to quantify the spatial configuration of seagrass and then modeled the response of these metrics to wave energy, tidal current speed, and water depth at 62 estuarine sites in North Carolina, USA. Seagrass landscapes were representative of cover types observed in the estuary generated by wave energy. Percent cover, patch size, and number of patches all declined with increasing wave energy. Threshold behavior occurred at wave energy change points between 675–774 J m−1. Seagrass landscapes differed in spatial configuration and physical setting, above and below change points. There was moderate support for a power law relationship for patch size distribution across a wide range of seagrass landscape cover and wave energy. With weather extremes on the rise, much of this estuarine seagrass will be exposed to increased wave energy. Where seagrass exists just below the wave energy change points, increases in wave energy could tip those habitats into a new stable state of lower cover resulting in less cover overall in the estuary.