|MadSci Network: Chemistry|
The main reason for the difference in color is the greater abundance of phytoplankton in the water off North Carolina. Just like plant pigments give them a green color, phytoplankton pigments do the same thing and give the water its greenish hue in temperate latitudes. This of course begs the follow-up question: why are phytoplankton so much more abundant in those waters? At first this does seem counterintuitive, because phytoplankton, like land plants, do grow faster in warmer conditions, so one would expect that they would be more abundant in the warmer waters of the Carribean. Phytoplankton aren't as abundant in these warmer waters because they lack nutrients. Nutrients (primarily nitrogen and phosphorous compounds) are much more abundant in deeper waters, and in the waters off North Carolina these deep waters are periodically mixed to the surface, whereas in tropical waters they are not. Oceans in warm areas have a surface layer of warmer water and a deeper layer of colder water. There is not much mixing across the boundary between the two layers, which is called the thermocline. Even temperate waters like those off North Carolina have a thermocline in the summer, and there is not as much phytoplankton production there during that season. However, colder temperatures and greater storm activity during the winter do mix nutrients to the surface waters where phytoplankton can use them. The study of the best conditions for phytoplankton growth has been going on for over a century and continues to be an area of intense research. The field was kicked off laregely because people wanted to know why some waters were so much more green at certain times of year!
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