Tides are caused by the gravitational effects of the Moon and Sun, which ideally produce a cycle of two high tides and two low tides per day. However, the amplitude and frequency of the tides are altered by the phases of the Moon, the Earth's orbit and declination, latitude, and the configurations of the shoreline and the seafloor. The tidal range tends to be smaller toward the equator and can vary from several meters in high latitudes to less than tens of centimeters near the equator. Configuration of the coast and the ocean basin can cause harmonic resonances and create tides that vary dramatically in amplitude and frequency. In extreme cases, the reinforcing and canceling effects can produce a single high and low tide per day or almost no change over the course of a day.
The timing of low tides can have a profound effect by exposing organisms to extreme conditions. For example, the lowest tides in the Gulf of Maine, USA tend to occur near dusk or dawn, and so organisms are rarely exposed to mid-day sun in the summer but are often exposed to below freezing temperatures on winter mornings. In contrast, the lowest summer tides in southeastern Australia occur mid-day and expose organisms to extraordinarily high temperatures.
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