Re: Why is carribean water so blue

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!