Almost every element on the periodic table is present in seawater. Most elements are present in very small amounts. For example, chlorine is the single most abundant element in seawater. It has a concentration of 19.35 parts per thousand (at salinity = 35). In comparison, iron has an average concentration of 0.56 parts per trillion and gold has an average concentration of 9.9 parts per quadrillion (quadrillion is 1,000,000,000,000,000 or 1 followed by 15 zeros).
Gasses are also dissolved in seawater. The major gasses in the atmosphere, nitrogen, oxygen, argon, and carbon dioxide, are all present in seawater.
Other substances dissolved in seawater include dissolved organic material (DOM) from the creatures in the ocean and nutrients, such as dissolved nitrate and phosphate.
Where do you think all of these substances come from?