6.2.3 External terrestrial and marine influences

External terrestrial influences have led to substantial environmental stresses on coastal and nearshore marine habitats (Sahagian, 2000; Saito, 2001; NRC, 2004; Crossland et al., 2005). As a consequence of activities outside the coastal zone, natural ecosystems (particularly within the catchments draining to the coast) have been fragmented and the downstream flow of water, sediment and nutrients has been disrupted (Nilsson et al., 2005; Section 6.4.1.3). Land-use change, particularly deforestation, and hydrological modifications have had downstream impacts, in addition to localised development on the coast. Erosion in the catchment has increased river sediment load; for example, suspended loads in the Huanghe (Yellow) River have increased 2 to 10 times over the past 2000 years (Jiongxin, 2003). In contrast, damming and channelisation have greatly reduced the supply of sediments to the coast on other rivers through retention of sediment in dams (Syvitski et al., 2005). This effect will likely dominate during the 21st century (Section 6.4.1).

Coasts can be affected by external marine influences (Figure 6.1). Waves generated by storms over the oceans reach the coast as swell; there are also more extreme, but infrequent, high-energy swells generated remotely (Vassie et al., 2004). Tsunamis are still rarer, but can be particularly devastating (Bryant, 2001). Ocean currents modify coastal environments through their influence on heat transfer, with both ecological and geomorphological consequences. Sea ice has physical impacts, and its presence or absence influences whether or not waves reach the coast (Jaagus, 2006). Other external influences include atmospheric inputs, such as dust (Shinn et al., 2000), and invasive species.