Goulven Gildas Laruelle , Judith A. Rosentreter, Pierre Regnier

At the interface between the continental and oceanic domains, estuaries are essential components of the land-ocean aquatic continuum that play a significant role in biogeochemical cycles, as they transform and export large amounts of carbon and nutrients from rivers to coastal waters. Because of this intense biogeochemical processing, they are significant ecosystems in terms of greenhouse gas exchange with the atmosphere. However, in spite of recent advances in remote sensing and the need for accurate estimates to calculate regional and global estuarine budgets, the quantification of their global surface area has not been updated in over a decade and remains poorly constrained. This is due to the lack of a global extensive database, the diversity of estuaries, and the controversial definition of their boundaries. To address these challenges, a hybrid approach was developed that combines the surface areas of over 700 estuaries worldwide (extracted from the literature or calculated using geographic information systems) with a novel extrapolation method to provide type-specific regional estimates for 45 regions. The upscaling formula applied is determined and calibrated using data from several regions where an extensive survey of total estuarine surface areas was available. The new global estimate of 733,801 ± 39,892 km2 is 31% lower than the previous global assessment and provides quantitative uncertainty estimates for regional and global estuarine surface areas as well as a breakdown between tidal systems and deltas (294,956 ± 30,780 km2), lagoons (179,946 ± 12,056 km2), and fjords (259,899 ± 22,328 km2).