The regions of the earth where the land and sea interact form a dynamic and interconnected boundary, or ecotone, referred to as the coastal zone. Over geological time, sea-level changes have meant that much of the landscape at the coastal zone that we see today has been at various stages of submergence and emergence. The geomorphology of the coastline itself reflects the exposure to the sea at a range of spatial and temporal scales. In a profound way, too, the biophysical characteristics of the sea are inextricably linked to the land because the very mineral composition of the sea, particularly the salinity, is in part derived from riverborne materials and runoff from land as a result of millions of years of land erosion. In effect, the water cycle, combined with riverine erosive forces on land, drives many biogeochemical cycles in the ocean through land-sourced nutrients. As such, the connectivity of land to sea has been important for maintaining the productivity of marine and estuarine habitats and supporting the evolution of human civilization.

More recently in our history, coastal waters globally have experienced a substantial net loss of biogenic ecosystem structure–eg, seagrasses (Waycott et al. 2009), coral reefs (Fabricius 2005, 2011), salt marshes (Deegan et al. 2012)–which is connected to large localized increases in terrestrial material entering coastal waters. These changes have occurred primarily through suspended organic and inorganic material flowing to the sea through river discharge, nutrients and other contaminants delivered through groundwater, surface water flow (ie, runoff) across the land and engineered point discharges such as sewage pipes and storm water drainage. At broader spatial and temporal scales, the changing climate has increased rainfall in some areas, which has contributed to greater suspended sediment load in riverine discharges and in other regions it has reduced rainfall, decreasing sediment load in places like the Yellow River in China (Walling & Fang 2003).