Density determines the relative bouyancy of objects or fluids. Out of the oil, water, and corn syrup, oil is the least dense so it floats on the water. The corn syrup is more dense so the water floats on top of it. Warm water is less dense than cold water - as the temperature increases, the motion of the tiny water molecules increases and they spread out more, which renders the liquid less dense as a whole. On the other hand, cold water is more dense because the cold water molecules move more slowly and are much closer together.
Winds blow the surface water.
As the deep cold water flows in the oceans, nutrients accumulate in it partly from the sea floor, but mainly from decaying animals and their fecal pellets that sink down through the water column. Nutrients stimulate growth of phytoplankton and zooplankton, which in turn provide food for fish, resulting eventually in greater fish production. The depth at which the cold water and warm water meet in the oceans is called the thermocline.