Introduction.

Estuaries and deltas are amongst the most heavily populated areas of the world (about 60 per cent of the world's population live along estuaries and the coast) making them the most perturbed parts of the world ocean (Kennish, 2002; Small and Cohen, 2004). Of the 32 largest cities in the world, 22 are located beside estuaries. They are adversely affected by invasive species, sedimentation (from soil erosion caused by deforestation, overgrazing, and other poor farming practices), overfishing, drainage and filling of wetlands, eutrophication due to excessive nutrients from fertilizer, sewage and animal (including aquaculture) wastes, pollutants including heavy metals (see Chapter 20), polychlorinated biphenyls, radionuclides and hydrocarbons from sewage inputs and diking or damming for flood control or water diversion. Estuaries and deltas provide protected harbours used as ports that are associated with introduced marine pests. They are foci of human attention, attracting potentially incompatible uses by society such as heavy industry, urbanization and recreation; they are affected by global sea-level rise and climate change (Crossland et al., 2005). Estuaries and deltas “form a major transition zone with steep gradients in energy and physicochemical properties at the interface between land and sea” (Jennerjahn and Mitchell, 2013).

More than 50 per cent of large river systems are affected by dams, based on a global synthesis on river fragmentation and flow regulation (Nilsson et al., 2005), with obvious consequences for the estuaries and deltas at their coastal termini. The mean age of river water at river mouths has increased from about two weeks to over one month on a global scale and to more than one year in extreme cases (Vorosmarty et al., 2003). Over the last few centuries, the global annual sediment flux into the coastal zone has increased by 2.3 x 109 tons due to human-induced soil erosion and decreased by 3.7 x 109 tons due to retention in reservoirs, the net effect being a reduction of sediment input by 1.4 x 109 tons (Syvitski et al., 2005). A major environmental consequence of river sediment starvation is erosion of the coast and attendant loss of habitat.