5.6: Dissolved Chemicals in Seawater

The chemicals dissolved in seawater include most elements, a variety of naturally occurring and human-made radionuclides (radioisotopes), and numerous organic compounds. Elements in solution are generally ionized. Many of these ions are compound ions, such as nitrate (NO^3- and phosphate (PO₄^3-), in which atoms of one element are combined with atoms of other elements. Hence, although we talk about dissolved nitrogen or phosphorus, we generally use the term “dissolved constituents,” not “dissolved elements.” Dissolved constituents are designated major, minor, or trace according to their concentration. Trace constituents include organic compounds, radionuclides, and trace elements. The term “trace elements” is widely used, although it is not accurate, because dissolved trace elements are present in seawater as simple or compound ions. Dissolved gases are generally considered separately.

Concentrations of dissolved constituents of seawater are usually measured in parts per million (mg•kg^–1) or parts per billion (µg•kg^–1). One part per million is roughly equivalent to one teaspoonful mixed into 5000 liters of water, or enough water to fill more than 14,000 cans of soda. One part per billion is roughly equivalent to mixing one teaspoonful into 5,000,000 liters of water, enough water to fill about five Olympic-size swimming pools. Some dissolved trace metal and organic constituents of seawater occur at concentrations in the parts per trillion range. One part per trillion is equivalent to one cent in 10 million dollars or one second in 31,700 years. Oceanographers now routinely express concentrations in molality units, discussed in Chapter 1. 

Major Constituents

The major dissolved constituents have concentrations greater than 100 parts per million by weight. The six major constituents (Table 5-5) are chlorine, sodium, magnesium, sulfur (as sulfate), calcium, and potassium. They occur as the ions identified in Table 5-5. Together, these six ions constitute 99.28% of all dissolved salts in the oceans (Fig. 5-6), and sodium and chloride (the constituents of table salt) alone constitute more than 85%.

Figure 5-6. The dissolved elements in 1 kg of seawater. The total mass of dissolved elements is less than the total mass of dissolved salts (salinity) because the mass of dissolved salts includes the H⁺ and OH^– ions associated with the dissolved species of some elements in seawater (see Table 5-5).

Table 5-5. Concentrations and Speciation of the Elements in Seawater with a Salinity of 35

Constituent Elements	Chemical Symbol	Concentration (mg•kg^–1)	Some Probable Dissolved Species
Chlorine	Cl	1.936•10^4	Cl^–
Sodium	Na	1.078•10^4	Na^+
Magnesium	Mg	1.28•10^3	Mg^2^+
Sulfur	S	8.98•10^2	SO4^2^+, NaSO4^-
Calcium	Ca	4.12•10^2	Ca^2^+
Potassium	K	3.99•10^2	K^+
Bromine	Br	67	Br^–
Carbon	C	27	HCO^3^–, CO3^2^–, CO2
Nitrogen	N	8.3	N2 gas, NO^3^– , NH4
Strontium	Sr	7.8	Sr^2^+
Boron	B	4.5	B(OH)3, B(OH)4^–, H2BO3
Oxygen	O	2.8	O2 gas
Silicon	Si	2.8	Si(OH)4
Fluorine	F	1.3	F^–, MgF^+
Argon	Ar	0.62	Ar gas
Lithium	Li	0.18	Li^+
Rubidium	Rb	0.12	Rb^+
Phosphorus	P	6.2•10^–2	HPO4^2^–, PO4^3^–, H2PO4
Iodine	I	5.8•10^–2	IO^3^–, I2
Barium	Ba	1.5•10^–2	Ba^2^+
Molybdenum	Mo	1•10^–2	MoO4^2^–
Uranium	U	3.2•10–3	UO2(CO3)2^4–
Vanadium	V	2.0•10^–3	H2VO4^–, HVO4^2^–
Arsenic	As	1.2•10^–3	HAsO4^2^–, H2AsO4^–
Nickel	Ni	4.8•10^–4	Ni^2^+
Zinc	Zn	3.5•10^–4	Zn(OH)2, Zn^2^+, ZnCO3
Krypton	Kr	3.1•10^–4	Kr gas
Cesium	Cs	3.1•10^–4	Cs^+
Chromium	Cr	2.1•10^–4	Cr(OH)3, CrO4^2^–
Antimony	Sb	2.0•10^–4	Sb(OH)^6–
Neon	Ne	1.6•10^–4	Ne gas
Copper	Cu	1.5•10^–4	CuCO3, CuOH^+
Selenium	Se	1.55•10^–4	SeO3^2^–
Cadmium	Cd	7•10^–5	CdCl2
Xenon	Xe	6.6•10^–5	Xe gas
Aluminium	Al	3•10^–5	Al(OH)4^–
Iron	Fe	3•10^–5	Fe(OH)2^+, Fe(OH)4
Manganese	Mn	2•10^–5	Mn^2^+, MnCl^+
Yttrium	Y	1.7•10^–5	Y(OH)3
Zirconium	Zr	1.5•10^–5	Zr(OH)4
Thallium	Tl	1.3•10^–5	Tl^+
Tungsten	W	1•10^–5	WO4^2^–
Niobium	Nb	5•10^–6	Not known
Rhenium	Re	7.8•10^–6	ReO4^–
Helium	He	7.6•10^–6	He gas
Titanium	Ti	6.5•10^–6	Ti(OH)4
Lanthanum	La	5.6•10^–6	La(OH)3
Germanium	Ge	5.5•10^–6	Ge(OH)4
Hafnium	Hf	3.4•10^–6	Not known
Neodymium	Nd	3.3•10^–6	Nd(OH)3
Lead	Pb	2.7•10^–6	PbCO3, Pb(CO3)2^2–
Tantalum	Ta	<2.5•10^–6	Not known
Silver	Ag	2•10^–6	AgCl2^–
Cobalt	Co	1.2•10^–6	Co^2^+
Gallium	Ga	1.2•10^–6	Ga(OH)4
Erbium	Er	1.2•10^–6	Er(OH)3
Ytterbium	Yb	1.2•10^–6	Yb(OH)3
Dysprosium	Dy	1.1•10^–6	Dy(OH)3
Gadolinium	Gd	9•10^–7	Gd(OH)3
Praseodymium	Pr	7•10^–7	Pr(OH)3
Scandium	Sc	7•10^–7	Sc(OH)3
Cerium	Ce	7•10^–7	Ce(OH)3
Samarium	Sm	5.7•10^–7	Sm(OH)3
Tin	Sn	5•10^–7	SnO(OH)3^–
Holmium	Ho	3.6•10^–7	Ho(OH)3
Lutetium	Lu	2.3•10^–7	Lu(OH)3
Beryllium	Be	2.1•10^–7	BeOH^+
Thulium	Tm	2•10^–7	Tm(OH)3
Europium	Eu	1.7•10^–7	Eu(OH)3
Terbium	Tb	1.7•10^–7	Tb(OH)3
Mercury	Hg	1.4•10^–7	HgCl4^2^–, HgCl2
Indium	In	1•10^–7	In(OH)^2+
Rhodium	Rh	8•10^–8	Not known
Palladium	Pd	6•10^–8	Not known
Platinum	Pt	5•10^–8	Not known
Tellurium	Te	7•10^–8	Te(OH)3
Bismuth	Bi	3•10^–8	BiO^+, Bi(OH)^2+
Thorium	Th	2•10^–8	Th(OH)4
Gold	Au	9.9•10^–9	AuCl2^–
Ruthenium	Ru	5•10^–9	Not known
Osmium	Os	2•10^–9	Not known
Radium	Ra	1.3•10^–10	Ra^2^+
Iridium	Ir	1.3•10^–10	Not known
Protactinium	Pa	5•10^–11	Not known
Radon	Rn	6•10^–16	Rn gas

*Note: Even for the more abundant constituents, concentration may vary slightly. For the rarer elements, the listed concentrations are uncertain and may be revised as analytical methods improve.

With the exception of calcium, which is used by many marine organisms to build calcium carbonate hard parts of their bodies (Chap. 6), the major constituents are not utilized significantly in biological processes and do not interact readily with inorganic particles. Therefore, the primary process by which they are removed from the oceans is by the precipitation of salt deposits. This process occurs in shallow, partially enclosed marginal seas or embayments where the rate of removal of water by evaporation far exceeds its replacement by precipitation, river flow, and mixing with the open ocean (Chap. 6).

Inputs of major constituents from rivers and other sources are very small in comparison with the quantities in the oceans. Because concentrations of these constituents are not affected significantly by inputs or removal processes, their residence time is very long and the relative concentrations of these major constituents do not vary significantly. This principle of constant proportions is a cornerstone of chemical oceanography. The ratios of major constituent concentrations in seawater only vary significantly in enclosed seas, where evaporation leads to salt precipitation (Chap. 6), and in estuaries, where the ocean water is mixed with substantial quantities of river water. River water has a much more variable composition of the constituents.

Minor Constituents

The minor constituents of seawater include bromine, carbon, strontium, boron, silicon, and fluorine. They have concentrations between 1 part per million and 100 parts per million (Table 5-5, Fig. 5-6). Nitrogen and oxygen are not considered to be among the minor constituents, because they are present in seawater primarily as dissolved molecular oxygen (O₂) and nitrogen (N₂) gases. Dissolved gases are considered separately later in the chapter. Together, the six major and six minor constituents constitute more than 99.6% of all the dissolved solids. Several minor constituents, notably carbon and silicon, are utilized extensively in biological processes. Therefore, the concentrations of these constituents in ocean waters are variable geographically and with time, changing in response to uptake by marine organisms and release during decay of organic matter. Such variations are discussed in Chapter 12.

Trace Elements

Other than the 12 major and minor constituents and the dissolved gases, all the elements listed in Table 5-5 have concentrations in seawater of less than 1 part per million. Together, these trace elements constitute only about 0.4% of the total dissolved solids in seawater. Most trace elements are used extensively in biological processes or are absorbed by or adsorb easily to particles that remove them from seawater. Many are introduced in significant quantities by hydrothermal vents, undersea volcanoes, decomposition of organic matter, atmospheric sources such as volcanic gases, river outflows, and release from seafloor sediment when it is disturbed. Concentrations of the various trace constituents vary substantially in different parts of the ocean in response to variations in the input and removal processes.

Many trace elements are essential minerals for marine life, others are toxic, and many are essential for marine life at low concentrations but toxic at higher concentrations (CC18). Measurements of the concentrations of trace metals and observations of their spatial and temporal variations in the oceans are important to marine biological and pollution studies.

Many trace elements that are essential (e.g., iron, zinc, and copper) or toxic (e.g., lead and mercury) have seawater concentrations of about one part per billion (10⁹) down to less than one part in 10¹³ (Table 5-5). Marine chemists have great difficulty measuring such extremely small concentrations. For many elements, the amount that dissolves into the water sample from surfaces of samplers and sample bottles or is contributed by dust in laboratory air can be greater than the quantity in the original water sample itself. This necessitates water sampling bottles designed to minimize or eliminate such contamination for the specific element being analyzed and clean laboratories to avoid contamination during sample analysis.

Radionuclides

A variety of naturally occurring and human-made radionuclides are present in seawater at extremely low concentrations (CC7). Radioactivity emanating from individual isotopes can be measured at exceedingly low levels, and sample contamination is generally a less critical problem than it is for trace metals. Therefore, determining even extremely small concentrations of radionuclides in seawater is relatively easy. In addition, many radionuclides are introduced to the oceans at known locations, and thus radionuclides can be used to trace the movements of ocean water (Chap. 8).

Radionuclides behave in biogeochemical cycles in ways that are virtually identical to those of the stable (nonradioactive) isotopes of their elements. Therefore, radionuclide distributions in seawater, sediments, and marine organisms, and movements of radionuclides among them, are used to infer movements of stable isotopes through the marine biogeosphere.

Organic Compounds

Thousands of different dissolved organic compounds are present in seawater. They include naturally occurring compounds such as proteins, carbohydrates, lipids, amino acids, vitamins, and petroleum hydrocarbons, as well as synthetic contaminants, including DDT and PCBs (polychlorinated biphenyls). The number of organic compounds is so large that probably less than 1% of them have been identified. Most organic compounds are very difficult to study because they are present in the parts per trillion concentration range (one teaspoonful in 5000 Olympic-size swimming pools). However, we believe that marine algae and perhaps some animals cannot grow successfully without dissolved compounds, such as vitamins (Chaps. 12, 13). We also know that certain organic compounds are highly toxic, even at concentrations in the parts per trillion range.

Most organic compounds in seawater are naturally occurring compounds. Many are produced by marine organisms and released to seawater, either through excretions (similar to urine) or by being dissolved after the death and decomposition of the organism. Organic compounds are also transported from land to the ocean in rivers and through the atmosphere. In some areas, especially where runoff from mangrove swamps or salt marshes carries large quantities of organic matter, the concentrations of colored dissolved organic compounds (sometimes called “gelbstoff”) are high enough to make the water appear brownish yellow.

Some dissolved organic compounds are removed from solution by attachment to particles that sink to the seafloor. However, most such compounds are taken up by marine organisms or decomposed to their inorganic constituents in the water column, primarily by bacteria and archaea (Chap. 12).

Dissolved Gases

Gases are free to move between the atmosphere and the oceans at the ocean surface. The net direction of the exchange is determined by the saturation solubility and concentration of the gas in seawater. The saturation solubility is the maximum amount of the gas that can be dissolved in water at a specific temperature, salinity, and pressure. If seawater is undersaturated, a net transfer of gas molecules into the water occurs. If seawater is oversaturated, the net transfer is from the water into the atmosphere.

The atmosphere is composed primarily of nitrogen (78%) and oxygen (21%) and contains several other minor gases. Carbon dioxide constitutes about 0.037% of all atmospheric gases. The distribution of gases dissolved in ocean waters is very different (Table 5-6). The oceans have proportionally more oxygen and less nitrogen than the atmosphere because of differences in the saturation solubility of these gases. In addition, the ratios of carbon dioxide concentration to oxygen and nitrogen concentrations are much higher in seawater than in the atmosphere because carbon dioxide reacts with water in a complicated way to produce highly soluble carbonate (CO₃^2–) and bicarbonate (HCO^₃–) anions.

Table 5-6. Distribution of Gases by Volume in the Atmosphere and Dissolved in Seawater

Gas	Atmosphere (%)	Surface Ocean (%)	Total Ocean (%)
Nitrogen (N2)	78	48	11
Oxygen (O2)	21	36	6
Carbon Dioxide (CO2)*	0.037	15	83

*CO₂ in the atmosphere, CO₂ plus HCO^3– plus CO₃^2– in the oceans.

Oxygen and Carbon Dioxide

Gases can be exchanged between the ocean and the atmosphere only at the ocean surface. In the water column beneath the ocean surface, the proportions of dissolved gases are changed primarily by biochemical processes. In the shallow photic zone where light penetrates, carbon dioxide is consumed, and oxygen released during photosynthesis. However, in the much larger volume of deep-ocean waters where no photosynthesis occurs, the dominant process affecting dissolved gas concentrations is the consumption of oxygen through respiration and decomposition. The excess carbon dioxide produced by respiration and decomposition can escape to the atmosphere only at the ocean surface, and there is no mechanism for resupplying dissolved oxygen to the water below the photic zone. Therefore, deep-ocean water is depleted of oxygen and stores large quantities of carbon dioxide (Table 5-6).

The total quantity of carbon dioxide dissolved in the oceans is about 70 times as large as the total in the atmosphere. The processes that control oxygen and carbon dioxide concentrations at different depths and locations within the oceans are discussed in more detail in Chapter 12. The role of the oceans in absorbing and storing carbon dioxide is critical to the fate of carbon dioxide that has been, or will be, released by industrialized civilization, and to the severity of global climate change due to the enhanced greenhouse effect (CC9).

The saturation solubility of gases varies with pressure, temperature, and salinity. It increases with increasing pressure and decreasing temperature. Therefore, seawater at depth in the oceans can dissolve much higher concentrations of gases than surface seawater can. There is continuous movement of water from ocean surface layers to the deep layers and eventually back to the surface (Chap. 8). As water moves through the ocean depths, concentrations of dissolved gases are changed by biochemical processes and by mixing with other water masses that have different concentrations of the gases. Geological processes, including the release of gases from undersea volcanoes and from decaying organic matter in sediments, can also change the concentrations of some gases, but those processes are generally of minor significance.

When water that is saturated with carbon dioxide sinks below the surface layer, gases can no longer be exchanged with the atmosphere. However, carbon dioxide is released into deep-ocean waters by respiration and by decay of organic matter. The added gas remains dissolved because the saturation solubility is increased by the higher pressures.

The increase in solubility caused by increased pressure is what keeps carbon dioxide dissolved in carbonated sodas. The carbon dioxide is dissolved in the soda under increased pressure at the bottling plant and then sealed in its container at the higher pressure. When the container is opened, the internal pressure is released, and the carbon dioxide bubbles out because the concentration exceeds the saturation solubility at the lower pressure.

The saturation solubility of gases generally increases as temperature decreases and is generally lower at ocean water salinities than in pure water (Fig. 5-7). Therefore, oxygen concentrations are higher in cold surface waters near the polar regions than in tropical waters. In tropical waters, the oxygen concentration is low enough that, under certain circumstances, it can be inadequate for the respiration needs of some marine species. For the same reason, tropical waters are more vulnerable to marine pollution by oxygen-consuming waste materials, such as sewage (Chap. 16).

Figure 5-7. Solubility of (a) oxygen and (b) nitrogen in pure water and seawater. Note that the solubility of each gas is reduced as temperature increases and is lower at seawater salinity than in freshwater.

Other Gases

In addition to the major atmospheric gases, several other gases are present in seawater (Table 5-7). With the exception of sulfur dioxide, these gases are produced primarily by marine organisms, so surface waters are oversaturated and net movement of the gases is into the atmosphere. The quantities of such gases supplied to the atmosphere by the oceans are relatively small in comparison with those from other sources. However, the ocean concentrations of methane, for example, must be taken into account in global climate change studies because atmospheric methane contributes significantly to the greenhouse effect (CC9). Atmospheric sulfur dioxide comes primarily from fossil fuel burning, industrial processes, and volcanoes. Sulfur dioxide can be converted to sulfuric acid, the principal component of acid rain. The oceans act as a sink for this air contaminant by absorbing sulfur dioxide and the sulfate ions present in the runoff from acid rain.

Table 5-7. Estimated Flux of Gases between Oceans and Atmosphere

Gas	Total Transfer (g•yr^–1)	Direction of Net Transfer
Sulfur dioxide (SO2)	1.5•10^14	Atmosphere to ocean
Nitrous oxide (N2O)	1.2•10^14	Ocean to atmosphere
Carbon monoxide (CO)	4.3•10^13	Ocean to atmosphere
Methane (CH4)	3.2•10^12	Ocean to atmosphere
Methyl iodide (CH3I)	2.7•10^11	Ocean to atmosphere
Dimethyl sulfide (CH3)2S	4.0•10^13	Ocean to atmosphere

pH and Buffering

The acidic or alkaline property of water is expressed as pH, which is a measure of the concentration of hydrogen ions (H⁺). The pH increases as the hydrogen ion concentration decreases, and it is measured on a logarithmic scale of 0 to 14: pH 0 is the most acidic, pH 14 is the most alkaline, and pH 7 is neutral. Pure water (free of dissolved carbon dioxide) is neutral, pH 7. Seawater normally is about pH 8, mildly alkaline. The near-neutral pH of natural waters is very important to aquatic biology. For example, persistent acid rain, which can be about pH 5, can severely damage or destroy aquatic life in lakes by reducing the natural pH of the lake water. In addition, the numerous marine species with calcium carbonate hard body parts cannot construct these parts if the water is even mildly acidic.

Seawater pH is buffered (maintained within a narrow range of pH 7.5 to 8.1) through the reactions of dissolved carbon dioxide. Dissolved carbon dioxide combines with water to form carbonic acid (H₂CO₃). The carbonic acid partially dissociates (separates into ions) to form a hydrogen ion and a bicarbonate ion (HCO₃^–), or two hydrogen ions and a carbonate ion (CO₃^2–). Carbon dioxide, carbonic acid, bicarbonate, carbonate, and hydrogen ions coexist in equilibrium in seawater:

CO₂ (gas) + H₂O  ⇆  H₂CO₃  ⇆  H+ + HCO₃^- + 2H⁺ + CO₃^2-

The addition of acid to seawater increases the number of hydrogen ions, which reduces the pH of the seawater and forces the equilibrium to shift so that less carbonate and bicarbonate are present. This shift reduces the number of hydrogen ions present, which offsets the reduction of pH. The opposite shift occurs if alkali is added to seawater. Such buffering capacity partially protects the ocean waters from pH changes that otherwise might result from acid rain or from acidic or alkaline industrial effluents. However, the amount of carbon dioxide released by humans is so large that ocean pH is indeed changing slowly. Some of the possible consequences of this change were discussed in Chapter 1.