Acropora at low tide

[greenbean36191]

In most areas where this happens, tides are only that low once or twice a year.

Twice a month when the Sun and Moon line up you get spring tides, which are the lowest low tides. One of those spring tides is usually a little lower than the other. Then on top of that monthly cycle, there's another cycle of the moon's orbit that brings it slightly closer and slightly farther from Earth. When it's close the tides are more extreme. About every 7 months the two cycles coincide to give the most extreme tides of the year. Wind and atmospheric pressure can complicate that a bit more too, especially in enclosed areas.

Also, regardless of how big the magnitude of change is, the duration of single tidal cycle is pretty much fixed. As a result, the more extreme the low tide, the shorter the duration near the minimum. In other words, when the tide gets really low, it drops fast and comes back up fast.

 

[JustinReef]

<a href=showthread.php?s=&postid=11904395#post11904395 target=_blank>Originally posted</a> by greenbean36191
In most areas where this happens, tides are only that low once or twice a year.

Twice a month when the Sun and Moon line up you get spring tides, which are the lowest low tides. One of those spring tides is usually a little lower than the other. Then on top of that monthly cycle, there's another cycle of the moon's orbit that brings it slightly closer and slightly farther from Earth. When it's close the tides are more extreme. About every 7 months the two cycles coincide to give the most extreme tides of the year. Wind and atmospheric pressure can complicate that a bit more too, especially in enclosed areas.

Also, regardless of how big the magnitude of change is, the duration of single tidal cycle is pretty much fixed. As a result, the more extreme the low tide, the shorter the duration near the minimum. In other words, when the tide gets really low, it drops fast and comes back up fast.

Great explanation! The tides we saw in Australia that exposed a significant amoutn of the reef lasted no more than an hour and usually much less. One of the days when the tide was quite low, we went swimming and it was amazing. Kind of weird to be swimming and see the reef above you and partly out of water.

The tide pools were incredible too. My brother and I found a HUGE Puffer in a very small tide pool. He couldn't even turn around in it. The water was extremely hot and the puffer was really gasping so we both picked it up and carried it a much larger pool. Not too sure what ever happened to it but it was a cool experience to see all the fish and inverts in these tide pools.

 

[ReefTECK]

Essentials of Oceanography by Harold V. Thurman explains the gravitational force and alignment of sun and moon that create spring and neap tides, very interesting physical characteristics!

Great summary!

 

[JustinReef]

I found a few articles about corals that are exposed during low tide and all basically said that although they can withstand some exposure, it certainly is not good for them and bleaching and death are quite common. Bascially this is not something you would want to do on purpose in your tank Here is part of one of the articles:

"Extreme tidal events are one of the most predictable natural disturbances in marine benthic habitats and are important determinants of zonation patterns in intertidal benthic communities. On coral reefs, spring low tides are recurrent disturbances, but are rarely reported to cause mass mortality. However, in years when extremely low tides coincide with high noon irradiances, they have the potential to cause widespread damage. Here, we report on such an event on a fringing coral reef in the central Great Barrier Reef (Australia) in September 2005. Visual surveys of colony mortality and bleaching status of more than 13,000 corals at 14 reef sites indicated that most coral taxa at wave-protected sites were severely affected by the event. Between 40 and 75% of colonies in the major coral taxa (Acropora, Porites, Faviidae, Mussidae and Pocilloporidae) were either bleached or suffered partial mortality. In contrast, corals at wave-exposed sites were largely unaffected (<1% of the corals were bleached), as periodic washing by waves prevented desiccation. Surveys along a 1â€"œ9 m depth gradient indicated that high coral mortality was confined to the tidal zone. However, 20â€"œ30% of faviid colonies were bleached throughout the depth range, suggesting that the increase in benthic irradiances during extreme low tides caused light stress in deeper water. Analyses of an 8-year dataset of tidal records for the area indicated that the combination of extended periods of aerial exposure and high irradiances occurs during Mayâ€"œSeptember in most years, but that the event in September 2005 was the most severe. We argue that extreme low-tide, high-irradiance events are important structuring forces of intertidal coral reef communities, and can be as damaging as thermal stress events. Importantly, they occur at a time of year when risks from thermal stress, cyclones and monsoon-associated river run-off are minimal."