Extrapolation-Based Regionalized Re-evaluation of the Global Estuarine Surface Area

Abstract At the interface between the continental and oceanic domains, estuaries are essential components of the land–ocean aquatic continuum. These coastal ecosystems play a significant role in biogeochemical cycles, as they transform and export large amounts of terrigenous carbon and nutrients from rivers to marine 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 global quantification of the estuarine spatial extent available for gas exchange 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 geographical information systems) with a novel extrapolation method to provide type-specific regional estimates for 45 regions. The three estuarine types considered are ‘tidal systems and deltas’, ‘lagoons’, and ‘fjords’. 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 km 2 (mean ± 2 σ) is 31% lower than the previous global assessment. It also 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 km 2 ), lagoons (179,946 ± 12,056 km 2 ), and fjords (259,899 ± 22,328 km 2 ). This decrease of the global estuarine surface area is related to the novel method used in this study and does not reflect a temporal trend.