Macha Sets Sail

What is an oceanographer to do when it rains... (which is quite often, here in Costa Rica!)? When there is that much water pouring out of the sky, it's fun to see how much the ocean's saltiness is getting diluted by the downpour. So out comes the salinity meter (affectionately known as "the YSI" by thousands of Fullerton College students), which lets one measure both the salinity and temperature of seawater. Since salt is heavier (denser) than water, the fresh rain water floats on top of the salty sea water, at least until waves, wind, tides, boats or even energetic fish mix up the layers. So to catch that fleeting moment of layering, it helps to do some measuring during the rain. Fortunately, here in Costa Rica it feels refreshingly cool to get rained on!

So what did I find with the YSI in the rain today? A definite layer of less-salty water, reaching from the surface down to 1.5 meters (about 5 feet). It was also warmer at the surface, due to the tropical sun. (Not as warm as usual, though, since the rain is actually a bit chilly!) The combination of salinity and temperature is what determines the density of seawater, and this has huge implications for whether or not nutrients get mixed from the deep ocean up to surface waters where many critters need them. In fact, in the tropics, all this rain and warm water puts the brakes on the vertical mixing, and that is one of the reasons the tropics have such clear, blue water! (Less mixing=less food for plankton, which in large numbers turn the water green.) Back in California, there is much more mixing, thus much more plankton, fish, and other critters in the water. And maybe you thought the water in Hawaii, Costa Rica, etc. was just bluer because it was cleaner?! Not necessarily so...!

Speaking of plankton and clear water, a handy tool for measuring water clarity is the Secchi Disk. It's a black and white disk that you can lower down into the water on a measured rope, watching to see when it disappears from view. When you can't see it any more, you have an idea of how deep the light is reaching into the ocean in your location on that particular day. So when there is not a lot of mixing, and not a lot of plankton "food", then there aren't a lot of plankton to cloud the water. One day off Guatemala we dropped the Secchi Disk to the end of its 60-foot rope, and it looked like it was just a few feet deep! The water was amazingly clear. Pity the poor plankton! ~(;-) But today, after so much rain and muddy runoff from the nearby shore, the Secchi Disk disappeared after only 15 feet! Tomorrow, it should be clear again, at least until the next downpour.

If by some chance you don't think Secchi Disks sound all that interesting, you might want to check out the photo gallery. You'll find a picture of a beautiful fish that was justifiably fascinated by it! It (along with 5 friends) followed the disk up and down in the water several times. I don't know whether they were hoping it might be edible, were just curious, or perhaps are budding oceanographers!

By the way, another thing oceanographers (and deckhands) can do during the rain is scrub the decks! See the photo gallery for a look at that!