Why is water in the tropics so blue and clear?

as compared to some which is dark…

i suspect it has to do with the cleanliness and temp, but im not sure

Chief’s Domain - http://www.seas.ucla.edu/~ravi

Mostly its because so little lives in the tropics. Colder waters hold much more oxygen and CO2 per volume than warmer waters. The dissolved gasses allow much more phytoplankton and zooplankton to live. Its the phytoplankters that make water green and cloudy (mostly) and where they are absent the water appears blue and clear.

With a broader base for the food chain, colder waters also support more larger animals than warm water. There are no great fisheries in the tropics to compare with George’s Bank or the richness of the Wendell Sea.

Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
“You cannot reason a man out of a position he did not reach through reason.”

The blue, clear water you’re used to seeing in the Bahamas et al ads is always shallow. The purity of the water is no doubt due to a location remote from the polluting effects of large population centers, and the effect is enhanced by the layer of white sand closely underlying the surface. Maybe the declination angle of the tropical sun has something to do with it too, I dunno.

The deep ocean offshore is the same color just about everywhere.

Damn typos. That should read “Weddell Sea” of course.

The marine biologists here at the aquarium also note that surface waters of the tropics hold far fewer nutrients (including the plankton Dr. F noted earlier). It’s a combination of gravity and chemistry. When an animal (of any size – microscopic to levithan) dies, it sinks, and carries all the nutrients down to the bottom. Scavengers eat some; decomposers release the rest. So deep water holds a lot of life-sustaining nutrients.

Deep water is also cold, being denied the sun. In the topics, the upper layer of the water is heated. And because warm water rises just as cold air does, it rarely mixes with the cold water below. So all those nutrients stay trapped down there.

In colder climes, the temperature difference between the upper and lower layers is far less pronounced, which allows for “upwelling,” or currents that rise vertically from the bottom, brining all that rich nutrient matter to the surface.

Of course, it is possible for life to harness the meager resources of the tropical seas. Take coral. It sits in one place, eating plankton as it floats by. It has algae living under its skin (zooxanthellae, for the Greeks among us) which photosynthesizes sunlight. Both of these activities require it to live relatively near the surface. Yet it is so efficeint, it produces massive coral reefs, which are among the most productive and diverse habitats on Earth.

Ain’t science fun?

Im a surfer & I have seen the oceans in lots of places & it never gets like those blue water ads or commercials. You ever hear of retouching or polarizing filters? Makes the water look like that by taking the reflection off the top.

Whatever surf spot I have been in one thing remains weird, during the day the water is black sometimes in every spot I have surfed.

You’ve obviously not surfed near the coral reefs in Cozumel, Mexico. The water there is blue and so clear that you can almost see to the bottom. It’s a beautiful and amazing place.

Cozumel. Brings back memories of the years before we bred.

Did you dive Palancar? Awesome cliff.

Dr. Fidelius, Charlatan
Associate Curator Anomalous Paleontology, Miskatonic University
“You cannot reason a man out of a position he did not reach through reason.”

Mynd you, møøse bites Kan be pretty nasti…

The windward side of Oahu has some nice clear blue water.

Handy, don’t tell me you haven’t surfed Pipeline? You can see right into the caves below when you’re on the wave. Downright spooky. It’s even spookier when it’s +10 feet, but I’m relatively safe on the beach.

[[The deep ocean offshore is the same color just about everywhere]]

I’ve been SCUBA diving in places in the Caribbean where I could see sharks swimming at least 60 feet down from the boat. In some places the visibility is 150 feet.

No, I just snorkled, but what an amazing experience! I often think back to those reefs and long to return. Hopefully one day I will.

Handy: I know you live close to here and I’m surprised you dont think the shallower waters of the bay can look quite blue and clear (compared to the deeper waters more offshore).

Anyway, I remember theres a kind of secluded beach in one of the nature reserves here (Pt. Lobos, near Carmel, CA) where the sands are white, and the water near the cove (called China Cove) is clear and mostly plant free, and on sunny days can look almost like a tropical beach. I remember being amazed at the clarity of the waters at that spot since usually, the waters in the area are dark.

However Monterey Bay is usually so full of plankton that its almost always dark, since it gets lots of nutrients from upwelling.

Sorry to take this post somewhat off topic, but IIRC, my chem teacher told our class last year that she was a part of a scientific group studying how iron would affect tropical seas. She said when they dispersed the iron into the waters, there was in a short time a bloom of plankton that attracted great numbers of fish along with their predators. She said it didn’t last long as the iron was used up and the plankton died and sank to deeper waters.

Shallower, cleaner water probably has something to do with it, but also the angle of the sun, closer to the equator, will affect water color. If you scoop a bucket of water out of the Caribbean, it will be the same color as a bucket of water scooped out of the North Atlantic - clear! Water doesn’t have a color, it’s the (bear with me, is it the refraction?) of light as the sun passes through it which produces color.

Same way the sky appears blue during the day while the sun is overhead, but will show the other colors of the spectrum (orange, purple, yellow) when the sun is low in the sky during sunrise and especially sunset.

Same again as the prism. Light appears clear and colorless, the colors only show when (again, refracted?) through glass or water. Hence, the rainbow.

You can’t save your face and your ass at the same time.

      • I stayed at a lake in the Mt Rainier area (can’t recall the name, can’t find an atlas) and it had 35+ foot visibility and was still warm enough to swim in. The water looked brown beyond that distance. It was clearer than some swimming pools I’ve been in. I never did see any fish! - MC

Finally! A question that is right up my alley, and I don’t get to it until numerous other smart dopers get a chance to give intelligent and correct answers. :slight_smile:

Warm subtropical/tropical seas are known as ‘biological deserts’ because of comparatively low levels of biomass. This is why the world’s greatest fishery areas are in cold-water seas, or in areas of coastal upwelling where nutrients are brought up from deeper waters.

I’ve done a lot af research diving in the Caribbean, and sometimes the visibility is easily over 100 feet. Low levels of nutrients—>low levels of phytoplankton—>low levels of zooplankton—>not as many fishes, right on up the food chain. (The paucity of nutrients and plankton is the major contribution to clear waters).

Most tropical productivity is concentrated around coral reefs and, to a lesser extent, other shallow areas right off the coasts. Factors:

  1. more light contributes to what photosynthetic productivity there is

  2. nutrients can’t sink out of the photic layer

  3. two reasons coral reefs are so productive (even in very nutrient-poor waters):
    a) tightly-linked system/efficient use
    b) shear effect on water as it passes over reef areas, making nutrients and plankton more available for use by filter-feeding organisms.

Huh. All this time, I thought the tourist agencies jiust put lots of blue food coloring into the water.

Seems to me if the sand was white, water was clear and the sky blue, the sand would just reflect the blue sky so the water would look blue?

Hong Kong is in the tropics, and the water in the harbor bears a lovely opaque, olive drab hue.

Must have something to do with the 6,000,000 people living nearby.

And people *fish</> there!

I never believed the water could really look that bright blue either, always thinking it was a “camera trick” or something. Then I went to the Bahamas with my in-laws. I spent probably an hour just staring at the water, saying “Look at that water! It’s so blue!” I took pictures and hung them up on the wall of my little corporate veal pen (aka cubicle) for inspiration.


I am Chaos, I am alive, and I tell you that you are free.

Around the Apostle Islands the water is crystal clear and you can see everything on the bottom around the islands. The water is a beautiful bright blue. The rest of Lake Michigan is brown and turgid.

Here’s the best picture I can send and keep the file managable.

<img src=“http://members.theglobe.com/Sonic62/sd/blue_water_opt.jpg” alt=“Madeline Island”>