Tolar by MMaxi

Coral Bleaching

Coral bleaching is the whitening of coral colonies due to the loss of symbiotic zooxanthellae from the tissues of polyps. This loss exposes the white calcium carbonate skeletons of the coral colony. Corals naturally lose less than 0.1% of their zooxanthellae during processes of regulation and replacement (Brown and Ogden, 1993). However, adverse changes in a coral's environment can cause an increase in the number of zooxanthellae lost. There are a number of stresses or environmental changes that may cause bleaching including disease, excess shade, increased levels of ultraviolet radiation, sedimentation, pollution, salinity changes, and increased temperatures.

Corals tolerate a narrow temperature range between 25 degrees Celsius and 29 degrees Celsius depending on location. Corals bleach in response to prolonged temperature change and not due to rapidly fluctuating temperatures. Lab experiments show that corals bleach when water reaches a constant 32 degrees Celsius (Brown and Ogden, 1993).

Experiments have shown that an increase in UV light causes coral bleaching. UV light experienced by corals can increase in calm waters. The amount of mycosporine-like amino acids in a coral's tissues helps to determine how much UV it can withstand without bleaching (Gleason, 1993).

The exact mechanism by which corals bleach or the trigger that induces bleaching is unknown. However, there are a number of hypotheses that attempt to answer this question. Laboratory experiments have shown that the zooxanthellae are released into the gut of the polyp and then are expelled from the polyp through the mouth (Brown and Ogden, 1993). However, this has not been observed in nature. Another hypothesis is that stressed corals give algae fewer nutrients and thus the algae leave the polyp. Algae may produce oxide toxicity under stress, and these toxins may affect the polyps.

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P. MMaxi

Why is the Sky Blue?

If you were to travel 20 miles or so above the Earth's surface, the sky would appear black. What happens during light's descent to Earth that makes the sky take on a wonderful azure hue?
"White" sunlight passes through our atmosphere, and molecules in the air, primarily nitrogen, are just the right size to scatter light from the blue end of the visible spectrum. The other colors travel to the ground with little interference.The blue light is scattered from molecule to molecule in the sky, until the light seems to be coming from every direction.


And Clouds are White Because...?
...the water droplets that make up clouds are much larger than the molecules that scatter blue light. The clouds scatter and reflect all the visible colors of light that strike them. Hence, we have white clouds.


But if the cloud is thick enough, light does not penetrate completely through the cloud, resulting in dark, heavy-looking cloud bottoms.


Why do clouds form?

Clouds are nothing more than water vapor that condenses and accretes into a visible form. The usual mechanism is for moisture-laden air near the Earth's surface to be raised higher into the atmosphere either by an encroaching air mass or the heat of the sun. As the air is lifted, the pressure drops and the air is subsequently cooled. The combination of the two causes water vapor to condense.

photo MMaxi on an Aiplane contributing maybe to more pollution " Mea Culpa "

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