Seawater is a complex solution containing a wide variety of organic and inorganic chemicals. While some of these are frequently discussed by reef aquarists, others are rarely mentioned. Without a working knowledge of what is present in natural seawater, it is often difficult to assess aquarium problems, as well as the claims of manufacturers and other aquarists about what additives and methodologies are desirable in maintaining reef aquaria.

This article is intended to help aquarists better understand the water in their aquaria. It strives to give a better understanding of what happens in seawater than does a simple table of elemental concentrations, although such tables are also provided.

The topics covered are:

• The Water Itself
• Seawater's Physical Properties
• pH
• Elements in Seawater
• The Big Four Ions
• The Other Major Ions
• Minor Ions
• Dissolved Atmospheric Gases
• Trace Elements
• Complexities to Minor and Trace Elements
• Organics
• How do Ions Behave in Seawater?
• How Ions Behave in Seawater: Simple Ions
• How Ions Behave in Seawater: Carbonate and Bicarbonate
• How Ions Behave in Seawater: Calcium, Magnesium and Strontium
• How Ions Behave in Seawater: Sulfate
• How Ions Behave in Seawater: Phosphate
• How Ions Behave in Seawater: Metals
• Nitrogen Compounds in Seawater
• Iodine in Seawater
• An Artificial Seawater Recipe
• Conclusion
• References

In the references section are links to articles about many of the individual ions present in seawater that most interest reef aquarists. This article does not try to describe what commercial artificial seawater and reef aquarium water contain. The following linked articles are more useful for those purposes:

A Chemical Analysis of Select Trace Elements in Synthetic Sea Salts and Natural Seawater

It's (In) the Water

What We Put in the Water

It Is Still in the Water

The Composition Of Several Synthetic Seawater Mixes

The Water Itself

A water molecule is composed of two hydrogen atoms bonded to a single oxygen atom (H2O; Figure 1). Water comprises about 96.5% of the mass of natural seawater. A 100 gallon aquarium contains approximately 12,500,000,000,000,000,000,000,000,000 water molecules. One of water's most important properties is that it is primarily a liquid, rather than a gas, at room temperature. Most other molecules of similar size and weight (e.g., oxygen, O2; nitrogen, N2; ammonia, NH3) are gases at room temperature. The reason that water is a liquid is that it forms strong intermolecular hydrogen bonds in which the hydrogen atom from one water molecule forms a transient chemical bond, called a hydrogen bond, to the oxygen atom in a nearby water molecule. While each of these bonds lasts only a fraction of a second, it rapidly and repeatedly reforms after being broken. This network of hydrogen bonds (Figure 2) holds the water together as a liquid rather than letting it fly apart as a gas.

Water forms hydrogen bonds because the electrons in water molecules are not evenly distributed. Oxygen is more electronegative than hydrogen, so the central oxygen atom draws electrons from the hydrogen atoms toward itself. This movement of electrons leaves the oxygen atom with a partially negative charge and the hydrogen atoms with a partially positive charge; this redistribution of electrons is called a dipole. When one water molecule interacts with another, there can be an interaction between a partially positively charged hydrogen atom and a partially negatively charged oxygen atom, creating a "hydrogen bond."

Figure 1. A space-filling model of a water molecule. The central oxygen atom is shown in red, and the two hydrogen atoms are shown in white.

Additionally, water's dipolar nature allows it to interact strongly with charged ions in solution. Several water molecules, for example, cluster around each ion, and orient themselves to take advantage of these ion and partial ion interactions. For example, water orients with its oxygen atoms pointed toward the positively charged calcium ion (Ca⁺⁺) in solution. This effect is very important for many properties, from solubility to osmotic pressure.

Seawater's Physical Properties

Seawater tends to have a higher density than does freshwater, due to seawater's higher density of dissolved salts. Seawater with a salinity of 35 ppt is about 1.0264 times as dense as freshwater at the same temperature, and so is said to have a specific gravity of 1.0264. This property is the reason that hydrometers are a suitable way to measure salinity.

Seawater also refracts light (bends light passing through it) more than freshwater does. This effect is due to the more refractive nature of the ions in solution compared to freshwater. The refractive index of freshwater is about 1.33300 while that of seawater with a salinity of 35 ppt is about 1.33940. Refractometers take advantage of this property and allow aquarists to measure salinity by refractive index.

The charged ions in seawater can conduct electricity. Not only does this attribute make seawater aquaria dangerous from an electrical safety perspective, it also allows aquarists to measure salinity via conductivity. The more charged ions present, the higher the conductivity, and a device that can appropriately measure conductivity can lead to useful determinations of the salinity. The conductivity of seawater with a salinity of 35 ppt is 53 mS/cm, while for purified freshwater, it is below 0.001 mS/cm.

When seawater evaporates, water enters the atmosphere, but salts generally remain behind. These salts can then become more and more concentrated if the evaporated water is not replaced, or if it is replaced with seawater containing additional salts. When this happens in a salt collection pond, it may be desirable, but if it happens in a closed lagoon or a marine aquarium, the salinity may rise to the point at which marine organisms are stressed or killed.

Seawater, with its many charged ions, has a higher osmotic pressure than does freshwater. In short, water "prefers" to be mixed with the charged ions. That is, it is in a lower energy state when it contains charged ions for the reasons described in the previous section. Consequently, if freshwater and salt water are separated by a membrane that only water can pass through, water will stream from the freshwater into the salt water. If that process is allowed to equilibrate, water will flow until the salt concentrations on each side are the same or, if pressure is allowed to build, it will continue until higher water pressure on the seawater side pushes back against the incoming water to stop it. That pressure is called osmotic pressure. The osmotic pressure between 35 ppt seawater and freshwater is 25.9 bar (25.5 atmospheres) at 25°C.

Because water is attracted to salts in seawater, water vapor's pressure over seawater is lower than over freshwater at the same temperature. It is about 2% lower over seawater, which at 25°C is 23.323 mm Hg, while freshwater has a vapor pressure of 23.756 mm Hg at the same temperature.

Ions and other dissolved chemicals are usually quick to diffuse and otherwise mix through a few feet of water. An aquarium with typical circulation will show no significant differences in chemical properties as a function of depth or across the aquarium, except in the case of things being continually added (such as dripping limewater) that may take time to be fully mixed in. The ocean, where distances are much greater compared to the movement of currents and diffusion in a few days time frame, can show significant variations in chemical composition as a function of depth and location.

Seawater with a salinity of 35 ppt has a freezing point that is 1.9°C (3.4°F) lower than freshwater. This freezing point depression comes about because the ions in the water tend to make the water more stable in its liquid form than as a solid. When seawater freezes, most ions are excluded from the ice, although some, such as sulfate, can be incorporated to some extent. Consequently, the salts in sea ice do not match the seawater's composition.

pH

The pH of seawater is typically stated to be 8.2 ± 0.1, but it can vary as photosynthesis consumes carbon dioxide locally and as respiration produces it. It also varies by latitude and is often lower where there is upwelling. It is also a function of depth for a variety of reasons, including photosynthesis near the surface, decomposition of organics in the mid-depths (dropping pH to as low as 7.5 by 1000 meters), and dissolution of calcium carbonate in very deep water (raising the pH back up to around 8). In closed lagoons, the pH can cycle from day to night just as in a reef aquarium, rising several tenths of a pH unit during the day. In special circumstances, seawater can be much lower in pH. Seawater in mangroves where highly reducing sediments are present can reduce the pH to below 7.0. In the open ocean, where there is a much larger volume of water containing buffers, the pH fluctuates little. As humans have added carbon dioxide to the atmosphere, more carbon dioxide has also been added to the oceans, with a consequent drop in pH. This is one of the impacts humans have had on the oceans that concerns ecologists in terms of its impact on calcifying organisms, especially on coral reefs but also on other systems involving such organisms as foraminiferans, which have calcareous skeletons and which are important links in many marine food webs.

The alkalinity of natural seawater is primarily a measure of bicarbonate plus two times the carbonate concentration. In the ocean, it varies by location and depth. In surface waters, it usually varies between about 2.25 and 2.45 meq/L (6.3 to 6.9 dKH), and often varies with changes in salinity. In deep water and upwelling water, it may be higher due to dissolution of calcium carbonate that is driven by pressure.

Elements in Seawater

Nearly every element known to man has been found in seawater (Table 1). Some are present at very high concentrations, and some are vanishingly rare. This linked website shows a periodic table of elements that can be pointed at with the cursor to see the concentration of each in seawater, as well as a host of other properties of the element. The sections that follow in this article detail the concentrations and other interesting aspects of many of the elements of most interest to reef aquarists.

The Big Four Ions

Most of seawater's constituents are inorganic ions. Figures 3 and 4 (below) show the primary ions present by weight and number. Sodium and chloride (the two ions in table salt) are the two primary ions in seawater. At 19,000 ppm for chloride and 10,500 ppm for sodium, they comprise 54% and 30% of the total weight of ions in seawater, respectively. The next two most common ions, magnesium (at 1280 ppm) and sulfate (at 2700 ppm) comprise 3.7% and 7.7% of the weight of seawater ions, respectively. Together, these four ions comprise almost 96% of the weight of ions present.

While these facts may seem unimportant to aquarists, they have significant implications. For example, the salinity of seawater, whether measured with a hydrometer, a refractometer or a conductivity meter, is dominated by these four ions. Deviations in the concentration of any other ion, even if significant for other reasons, will not significantly alter such measurements. For example, whether the calcium is 300 ppm or 500 ppm will not be noticeable in a typical salinity determination. That difference represents only a 0.6% change in the total weight of salts present, changing the salinity from 35 ppt to 34.8 ppt. Likewise, whether the alkalinity is 5 meq/L (14 dKH) or 2 meq/L (5.6 dKH), the change in salinity is only about 0.5%.

Another important implication of the high concentration of these other ions is that they move around only very slowly when perturbed by additives and foods. For example, adding calcium chloride boosts chloride more than it does calcium, but since there is already a background of 19,000 ppm of chloride, such additions do not rapidly disturb the relative ratios of the various ions in seawater.

Interesting (well, at least to chemists) is the fact that since a sulfate ion (SO4^--) weighs four times as much as a magnesium ion (Mg⁺⁺), it is actually present in smaller numbers than magnesium ions (Figure 4) even though it is present at a higher weight-based concentration (Figure 3).One other comment on magnesium concentrations in seawater - - seawater's magnesium content, along with that of other ions, has not been constant since the oceans formed. Specifically, it has often been lower, as in the late Cretaceous period. The amount of magnesium incorporated into the calcium carbonate skeletons of organisms such as corals is a function of how much magnesium is in the water. Consequently, the magnesium content of ancient sediments can be significantly lower than more modern ones from similar organisms. In addition to being an interesting fact, this result may also play a role in the suitability of certain limestone deposits in maintaining magnesium in aquaria. For example, such limestone is sometimes used in CaCO3/CO2 reactors or as the raw material for making calcium hydroxide (lime). If it is low in magnesium, additional supplements may be necessary to maintain modern seawater magnesium concentrations.

The Other Major Ions

The seawater's major components are usually defined as those ions present at greater than 1 part per million (ppm) or 1 milligram per liter (mg/L) (Table 2). A different definition of major ions based on the numbers of ions present, rather than the weight of those ions, has a slightly different list, with lithium (0.17 ppm) being added. Together, these ions account for 99.9% of seawater's solutes.

One important point about these concentrations: they are correct for only typical seawater, which contains about 35 parts of salt by weight per thousand parts of seawater (35 ppt). This seawater has a specific gravity of around 1.0264 which may be higher than is maintained in many marine aquaria. As the salinity of seawater varies, these concentrations typically move up and down together. Consequently, if an aquarium contains water with a specific gravity of 1.023, the salinity is about 30.5 ppt and all of the concentrations in Table 1 are reduced by about 13 percent.

All of these major ions are essentially unchanged in concentration at different locations in the ocean, except as salinity changes move them all up or down together. Ions that do not change concentration from place to place are referred to as "conservative type" ions, a description that also applies to some of the minor and trace elements that are discussed below.

The major ions include many that are critical to aquarists, such as calcium and bicarbonate, and others that are rarely considered, such as potassium and fluoride. Many of these have been discussed in previous articles that are linked in the references section at the end of the article.

Organic molecules may also meet the definition of being a major component of seawater (Table 2), but they are traditionally not considered a major specie in seawater. The nature of these organic compounds is discussed later in the article.

Minor Ions

There are various definitions of which ions in seawater constitute the "minor ions." By some definitions, the list of constituents is rather long. Table 3 shows just a few of the constituents of seawater that are often labeled as minor ions. The more abundant of these are sometimes lumped with the major ions (such as lithium), while the least abundant (such as iron) are often lumped in with trace elements. Ions in this category often vary significantly with location in the ocean. That is primarily because many of them are tightly linked to biological activity. These ions can be locally depleted if biological activity is high enough. Ions that vary in this fashion are referred to as "nutrient type" ions, because they are consumed by one or more types of organism.

Dissolved Atmospheric Gases

Any gas present in the atmosphere will be present in seawater. Many of these are unimportant to reef aquarists, but two are of critical importance: oxygen and carbon dioxide. Aside from carbon dioxide, all of the gases have lower solubility in seawater as the temperature and salinity are raised. Table 4 shows the concentration of the most common gases in seawater at 25°C.

Oxygen is generally most highly concentrated near the ocean's surface. In the top 50 meters or so, oxygen's concentration is controlled largely by exchange with the atmosphere, and is usually close to equilibrium with the air. Between 50 and 100 meters, the O2 level often rises due to photosynthesis. Below about 100 meters in the open ocean the oxygen level drops steadily for the next 1000 meters or so due to biological processes that consume it. It then sometimes rises again in the deeper oceans as oxygen there is replenished by sinking cold ocean water that is rich in oxygen. The importance of dissolved oxygen in seawater and reef aquaria has been discussed in a series of previous articles:

The Need to Breathe in Reef Tanks: Is it a Given Right?
http://www.reefkeeping.com/issues/2005-06/eb/index.php

The Need to Breathe, Part 2: Experimental Tanks
http://www.reefkeeping.com/issues/2005-07/eb/index.php

The Need to Breathe, Part 3: Real Tanks and Real Importance
http://reefkeeping.com/issues/2005-08/eb/index.php

Carbon dioxide is a special case. It hydrates on contact with water to form carbonic acid, which can then ionize (break apart) to from hydrogen ions, bicarbonate and carbonate, as shown below.

CO₂  +  H₂O  ßà H₂CO₃  ßà  H⁺  +  HCO₃^-  ßà  2H⁺  +  CO₃^--

For this reason, carbon dioxide is much more soluble in seawater than is any other atmospheric gas. It is more soluble than all the other gases combined, in fact, with a total solubility of about 100 ppm of carbon dioxide. An interesting question to ask is, "What happens to carbon dioxide that is mixed into the ocean?" After 1000 years, it is thought that it ends up in the forms shown in Table 5.

Additional discussion of carbonate and bicarbonate in seawater is provided in subsequent sections of this article.

Many other gases are dissolved in seawater, but it is beyond the scope of this article to describe all of them. Many have biological significance, including hydrogen sulfide (H2S), methane (CH4) and other organic gases, carbon monoxide (CO), hydrogen (H2) and nitrous oxide (N2O).

Trace Elements

There is much discussion about trace elements in marine aquaria, and for good reason. Most chemicals dissolved in seawater are classifi