Sea water is not the same all over the earth. The chemicals it contains vary from place to place. Different plants and animals need different conditions, and the types and numbers of organisms found in a particular part of the sea may be limited by the type of sea water found there. For instance, in many parts of the ocean there is very little nitrate or phosphate, nutrients which all plants need to grow. Where they are in short supply, these nutrients are said to be limiting factors for the growth of phytoplankton .
Sea water is a salty solution containing traces of nearly every element on earth.
In the open ocean, salinity remains quite constant. But Gray's Reef is relatively near the coast. The salinity at the Reef is sometimes lowered by fresh water from the land, particularly in spring when a lot of water comes down the rivers on the coast. The salinity at the Reef is sometimes raised by meanders from the Gulf Stream, which contains warm, salty water. Overall, the salinity at Gray's Reef ranges from about 34 to 36 ppt.