Tidal regimes of temperate coasts and their influences on aerial exposure for intertidal organisms

Thermal stress and desiccation are the main causes of the upper limit to the distribution of organisms on rocky shores. The timing of low tides plays a key role in determining the exposure of intertidal organisms to potentially stressful desiccation and thermal conditions, and has recently been suggested as pivotal in predicting the effects of global warming. Using data generated from tidal harmonics for evenly spaced stations of temperate latitude (23 to 50 degrees), we compare the amount of time exposed to low tide during summer middays along 7 different coastlines (East Australia, east and west coasts of North and South America, and the west coasts of southern Africa and northern Africa-western Europe) for 3 standardized tidal heights. Eastern Australia and western South America consistently showed the greatest amount of time exposed to low tides during summer middays at all 3 tidal heights. Some of the highest exposures were observed in western North America in the higher intertidal, but were among the lowest in the mid- and lower intertidal levels. The east coast of North America showed intermediate exposure in the high intertidal, but exposure was among the highest at mid- and lower tidal levels. Among the lowest exposure times were observed at the remaining 3 coasts at all 3 tidal levels. Therefore, predictions about the consequences of climate change on coastal organisms will vary across continents.