Seawater is a complex chemical solution that acts as a global reservoir for almost every naturally occurring element found on Earth. Approximately 96.5% of seawater is pure water, while the remaining 3.5% consists of dissolved materials and suspended components. The measure of these dissolved solids is known as salinity, which averages about 35 parts per thousand across the world’s oceans. This mixture of dissolved ions, gases, and organic matter drives ocean circulation, influences weather patterns, and sustains marine ecosystems.
The Primary Component: Dissolved Salts
The characteristic “saltiness” of the ocean is due to a small number of major dissolved ions that account for the bulk of seawater’s salinity. These dissolved solids are ions, which are atoms or molecules that carry an electric charge. Six specific ions make up over 99% of the total dissolved mass in the ocean.
Chloride and sodium are the two most abundant ions, together forming common table salt, or sodium chloride. Chloride is the most prevalent, representing roughly 55% of the total dissolved solids, while sodium contributes about 30%. The remaining major ions are sulfate, magnesium, calcium, and potassium.
A fundamental concept in ocean chemistry is the Principle of Constant Proportions. This principle states that while the overall salinity of ocean water can vary from one location to another, the relative ratio of these six major ions remains nearly constant throughout the open ocean. For example, the amount of sodium relative to the amount of chloride will be the same whether the total salt content is high or low. This constancy allows scientists to calculate the concentration of all major ions by measuring just one, typically chloride.
Essential Elements: Dissolved Gases and Nutrients
Beyond the major salts, seawater contains dissolved atmospheric gases that are fundamental to life in the ocean. Gases like oxygen, nitrogen, and carbon dioxide enter the water primarily through air-sea exchange at the surface. The amount of gas that can dissolve is affected by temperature and salinity, with colder, less-salty water generally holding more gas.
Dissolved oxygen is crucial for the respiration of marine animals and plants, with its concentration highest near the surface where it is exchanged with the atmosphere and produced by photosynthesis. Carbon dioxide is also highly soluble and plays a significant role in photosynthesis by marine algae and in regulating the ocean’s pH. When carbon dioxide dissolves, it forms bicarbonate and carbonate ions, which act as a buffer against large pH changes.
Inorganic nutrients are essential for primary production, which forms the base of the marine food web. The most notable nutrients are compounds containing nitrogen (like nitrate), phosphorus (like phosphate), and silicon (like silicate). Phytoplankton require these elements to grow, often in a specific ratio. These nutrients are depleted in surface waters due to biological uptake but are regenerated in deeper waters as organic matter sinks and decays.
Suspended Matter and Organic Materials
Ocean water also holds non-dissolved material and complex organic compounds. This suspended matter includes both inorganic particles, such as fine mineral grains and dust carried by wind or river runoff, and organic particles. Particulate inorganic matter includes clay particles and sediments that affect water clarity.
Organic materials are broadly categorized into dissolved organic matter (DOM) and particulate organic matter (POM). DOM consists of complex molecules like carbohydrates and amino acids derived from the waste and decomposition of marine organisms. POM includes larger fragments, such as dead plankton, fecal pellets, and cellular debris, which often sink and carry carbon to the deep ocean. The water also contains living components like microbial life, including phytoplankton and bacteria, and microscopic fragments of human-introduced debris like microplastics.