Nutrients, on the other hand, come from the sea bottom. If dissolved nutrients were constantly supplied at all depths, phytoplankton would flourish in the water column to some distance below the surface, depending on the clarity of the water and the intensity of light striking the surface. Below that, populations would decrease with decreasing light. In fact, dissolved nutrients are not available at all depths in the open water of the Gulf of Mexico. Rather, dissolved nutrients are usually undetectable in the surface because phytoplankton have incorporated the dissolved nitrate and phosphate into their cells.
Nutrient concentrations are low where phyto-plankton abundance is high.
In the surface layer of water, temperature and salinity are uniform because the water is mixed, primarily by wind. The thickness of this layer varies with wind strength and solar heating. Below this mixed layer, temperature decreases rapidly over a short distance. The interval where temperature changes is called a thermocline. As water temperature decreases, density increases.