Worldwide typology of nearshore coastal systems: Defining the estuarine filter of river inputs to the oceans

We present a spatially explicit global overview of nearshore coastal types, based on hydrological, lithological and morphological criteria. A total of four main operational types act as active filters of both dissolved and suspended material entering the ocean from land: small deltas (type I), tidal systems (II), lagoons (III) and fjords (IV). Large rivers (V) largely bypass the nearshore filter, while karstic (VI) and arheic coasts (VII) act as inactive filters. This typology provides new insight into the spatial distribution and inherent heterogeneity of estuarine filters worldwide. The relative importance of each type at the global scale is calculated and types I, II, III and IV account for 32%, 22%, 8% and 26% of the global coastline, respectively, while 12% have a very limited nearshore coastal filter. As an application of this typology, the global estuarine surface area is re-estimated to 1.1 × 106 km2 instead of 1.4 × 106 km2 in earlier work.