9  Chemistry of the oceans

Normally in any course or book introducing marine science/oceanography the physics is described first, then the chemistry and lastly the biology. However, in this short introduction the focus will be on the chemistry that directly influences the biology, and so it was important to introduce some of the key groups of organisms in the previous chapter. Marine organisms are greatly influenced by the chemical composition of the medium surrounding them and, in turn, can modify it. Some can tolerate a wide range of chemical conditions, whereas others only have a narrow tolerance to changes in the chemical composition of the surrounding environment. Microbial activity drives many of the chemical processes routinely monitored by marine chemists, and this realisation has led to increasingly closer union between biologists and chemists in their study of the oceans, the dynamics of biology and the elemental flow between the living and non-living reservoirs – so much so that it is popular to talk about the study of ocean biogeochemistry (although the term is at least 80 years old). The ‘bio-’ and ‘-chemistry’ parts of this word are clear, but the ‘-geo-’ bit in the middle is there because the processes being studied are important for the cycling of elements over geological timescales. They also produce chemical signatures that are important for interpreting the geological record in rocks and ocean sediments, or even in long-lived organisms, such as corals.

9.1 Salty water

As said in Chapter 1, ocean waters are salty and the concentration of salts in the water is measured as salinity. The average oceanic salinity is about 35, although in coastal waters it can be closer to 0, and in some regions (e.g. the Red Sea) give up to 40. However, there are parts of the ocean where the dissolved salts become very concentrated, such as in brines trapped in sea ice with salinity exceeding 100, which are produced when seawater freezes, and in hypersaline bodies of water (salinity > 250) which have been discovered in deep-water basins in the eastern Mediterranean Sea and are thought to be tens of thousands of years old, and are also characterised by having no oxygen (anoxic). Despite the large range in salinity in oceanic waters, the ratios between the major constituents of seawater (Table 9.1) remain constant, and they are said to behave conservatively. Most of the naturally occurring elements (around 90) have been measured in seawater, but the largest fraction of the ions in seawater is made up of fewer than ten elements (Table 9.1).

Table 9.1: Major constituents of seawater at a salinity of 35.

Ion	% by weight
Chloride (Cl–)	55.03
Sodium (Na+)	30.59
Sulphate (SO42–)	7.68
Magnesium (Mg2+)	3.68
Calcium (Ca2+)	1.18
Potassium (K+)	1.11
Bicarbonate (HCO3–)	0.41
Bromide (Br–)	0.19
Borate (BH2O3–)	0.08
Strontium (Sr2+)	0.04
Everything else	< 0.01

Figure 9.1 (A) Global view of temperature (red warm, blue cold waters).

Figure 9.1 (B) Global view of salinity (red high, blue low) at the surface of the oceans.

The conservative relationship between the constituents of seawater and salinity provide a useful tool for looking at the behaviour of material entering coastal water from rivers through estuaries. Progressing from a river mouth towards the sea normally results in the salinity gradually increasing from 0 to the salinity of the coastal water. If the concentration of a particular ion is greater in river water than in seawater and no other processes occur in the intervening estuary than simple physical mixing of the riverine water and the seawater masses, the concentration of the ion will decrease with increasing salinity in the estuary (dilution line, Figure 9.2A). Conversely, if the concentration of a particular ion is greater in seawater than in the river water, there will be an increasing concentration with increasing salinity in the estuary when physical mixing of the two water masses is the only process there (Figure 9.2B).

Figure 9.2 The straight lines show the conservative mixing plot of the concentration of a substance vs. salinity. (A) Concentration of a particular ion or substance is greater in river water than in seawater (B) Concentration of a particular ion is greater in seawater than in the river water.

However, if the values in the estuary plot below the dilution line (physical mixing), this is an indication that some additional process occurs in the estuary that removes the ion from the water. Similarly, if the data is above the dilution line, it indicates that some additional process has added more of the ion to the estuarine water.

9.2 What does ‘dissolved’ mean?

Talking about ions dissolved in water is rather straightforward, but on the broader scale, seawater is a complex salty chemical soup, also comprising the products from the death and decay, excretion and defecation of all the organisms in the ocean. The ions of elements in seawater and the compounds they can form are termed ‘inorganic’, whereas the biology and its products are organic (shells, etc. are inorganic). In terrestrial systems most dead animal and plant material, and waste products are incorporated into the soils and are broken down there. In contrast, in the oceans many of the equivalent processes are taking place in the water column. So there are carbohydrates, lipids, proteins, amino acids, and complex compounds of many molecular sizes in seawater.

In normal oceanographic practice it is common to use filters with pore sizes between 0.2 and 0.45 µm to separate between dissolved and particulate fractions (that passing through the filter is dissolved, and that retained on the filter is particulate). Commonly used terms for the particulate and dissolved organic phases are dissolved and particulate organic matter (DOM and POM). However, the dissolved fraction through such size cut-offs must contain some bacteria and viruses, since as we saw in the last chapter, the smallest bacteria and viruses are in the 0.02 to 0.2 µm size range. In practice routinely filtering seawater through filters with such tiny pore sizes is impractical, since they become clogged very quickly.

Some phytoplankton and bacteria secrete complex sugar molecules (polysaccharides) for protection against changes in external conditions. This material is called extracellular polymeric substances (EPS) and can be present in seawater in considerable amounts. The EPS, visible only when stained with special dyes, are gels which change the viscosity of the water, especially important for the smaller size classes of plankton. They are of undefined shape and size, and span the continuum between the dissolved and particulate phases (they can range from micrometres to centimetres). These gels also provide surfaces on which bacteria grow, so that they can become hotspots of bacterial activity, since many more bacteria are concentrated into a smaller volume than is normal in the open water.

When looking at the carbon in the waters of the oceans (not the sediments) the dissolved organic carbon (DOC) pool is by far the largest pool of organic carbon. At first this is rather surprising, since the particulate organic carbon pool (POC) includes all organic particles and organisms from viruses through to blue whales. But there are about 700 gigatonnes (giga = x10⁹) of carbon as DOC and only 30 gigatonnes of POC. Of the POC only around 3 gigatonnes consist of living organisms.

Figure 9.3 EPS gels produced by Arctic diatoms in sea ice are stained blue.

Figure 9.4 Foam washed up onto a beach is not a nasty chemical spill, but rather whisked up dissolved organic matter (DOM).

Sometimes there is so much DOM in the water that when it is sufficiently agitated, as in during a storm, the DOM is whisked up to form a foam that washes up onto beaches, frequently to the alarm of the public (Figure 9.4). There is, of course, nothing to worry about – it is just the organic compound soup being churned up and the proteins and carbohydrates frothing up, just like when whisking milk to make a cappuccino coffee.

9.3 Gases in seawater

As temperature and salinity increase, the solubility of gases in seawater decreases. As pressure increases (with increasing depth) gases become more soluble. A good illustration of these principles happens when a bottle of carbonated (fizzy) drink is opened. In the factory the liquid is injected with carbon dioxide under high pressure, and when the bottle was sealed in the factory the pressure inside the bottle was very high. Upon opening the bottle top the pressure is released, which causes a lot of gas to come out of solution (in bubbles) since the pressure is lowered. If the bottle were warmed up, more bubbles of gas would be released, and throw in some salt, even more so. So following these principles, polar oceans should have higher concentrations of dissolved gases than those at the tropics, deeper waters higher concentrations, and more saline waters lower gas concentrations than those in fresh waters. As we will see, this is not always the case.

The four main gases in air are nitrogen (N₂ – 78.1% by volume), oxygen (O₂ – 20.9%), argon (Ar – 0.9%) and carbon dioxide (CO₂ – 0.03%), although here we will only deal with O₂ and CO₂. Typically when in contact with the air the gas content of a liquid changes so that it reaches an equilibrium with the concentration, or rather, partial pressure (the pressure the gas would have if it occupied the volume alone), of the gas in the air.

However, in the case of CO₂ it is not so straightforward, since in the ocean the CO₂ reacts with water to form carbonic acid:

CO₂ + H₂O ⇔ H₂CO₃

Carbonic acid in turn dissociates rapidly to form the bicarbonate ion:

H₂CO₃ ⇔ H⁺ + HCO₃^–

Furthermore, the bicarbonate ion dissociates further to form the carbonate ion:

HCO₃^– _{⇔ H⁺ + CO3^2–}

So in seawater dissolved inorganic carbon occurs as dissolved CO₂ gas, carbonic acid (H₂CO₃), and bicarbonate (HCO₃^–), and carbonate (more CO₃^2–) ions. The proportions of these in seawater are in an equilibrium (see equation above) that is primarily governed by the pH (but also salinity and temperature) of the water:

CO₂ + H₂O ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^– ⇔ H⁺ + CO₃^2–

In seawater of a salinity of 35 and a ‘typical’ pH of around 8, around 90% of the inorganic carbon occurs as HCO₃^–, and only about 0.5% in the form of CO₂ gas (see Figure 9.5). In more acidic waters (lowering of pH) the shift of this equilibrium is to the left (more CO₂ gas) and in more alkaline waters (increase of pH) the shift is towards the right (more CO₃^2–). So if you drop some concentrated acid into some seawater, shifting the pH to below 2, the whole equilibrium (9.4) shifts to the left.

Figure 9.5 The form of CO₂ in the water is strongly dependent on pH.

However, in normal ranges of oceanic pH if CO₂ gas is removed from the water the disturbed equilibrium will shift to the left in 9.4, resulting in the other ions changing concentration until more CO₂ is produced and equilibrium is re-established. NB the total amount of carbon will still have gone down because of the lost CO₂.

The case for O₂ is not as complex as that of CO₂. The oxygen simply dissolves in seawater, although often its concentration in the surface of the ocean is in excess of that expected from the concentration in the seawater being in equilibrium with the air. This is because waves on the ocean surface cause bubbles of gas to form and be carried down into the water column, and because of the increased pressure the gases dissolve in the water, causing them to be present in supersaturated concentrations. The other reason for the supersaturated levels of O₂ is a result of photosynthesis by phytoplankton that occurs in the photic layer. Photosynthesis uses CO₂ and produces O₂ (Chapter 10), and when there is a lot of photosynthesis (only in the light) this further increases the O₂ inputs into the photic layer.

Respiration, which is carried out, day and night, by all marine organisms from bacteria to whales, utilises O₂ and produces CO₂ (again see Chapter 10 for more details). In particular, when bacteria break down decaying organic matter (DOM and POM) as it falls through the ocean, considerable O₂ is consumed and CO₂ is produced.

All of these processes combine to produce commonly found O₂ profiles with depth in open ocean waters: high concentrations (often supersaturated) in the upper waters (top 100 m or so) and then decreasing to an oxygen minimum between 200 and 1000 m, due to O₂ being utilised by respiring organisms (there is also a corresponding maximum in CO₂ concentration). However, the concentrations do not continue to fall, as would be expected since bacteria and animals continue to respire right down to the sea floor. This is because much of the deep water of the oceans was formed in the polar oceans, and is therefore very cold and contains high concentrations of oxygen (see chapter 1). The low oxygen zones in the water profile are called the oxygen minimum zone (OMZ). The OMZ regions of the world are thought to be a lower boundary that restricts the depth to which pelagic fish with high oxygen demands (e.g. tuna) are able to dive. At low oxygen concentrations, typical of OMZs, the fish may become physiologically stressed and so have to avoid these areas of the water column. Outside of OMZs, the maximum diving depths of electronically tagged blue marlin (Makaira nigricans) are greater than in waters with a pronounced OMZ.

Figure 9.6 Oxygen concentrations (ml/litre) in the surface waters of the oceans (red high concentrations and blue lower); cf. Figure 9.1.

Sometimes, especially if there is a lot of organic matter being broken down by bacteria, such as after a dense algal bloom, the oxygen mimina can be quite low (suboxic). However, in shallower coastal waters, especially those with little exchange of water from other regions, if there is strong stratification, the respiration below the photic zone may be so great that it actually utilises all the available oxygen, and the system becomes anoxic. When this happens it is a disaster, since it leads to the death of all those organisms requiring oxygen to respire – from algae and bacteria through to fish. NB marine mammals are not killed because they have to come to the water surface to breathe.

There is yet another twist to the complexity of the carbonate dynamics in seawater. In the last chapter, coccolithophores were introduced as being important bloom-forming phytoplankton. They are covered in coccoliths made of calcium carbonate, and when calcium carbonate structures form, one of the products is CO₂. So although the coccolithophores are taking up CO₂ through photosynthesis, they are producing it when they respire and as they produce their distinctive outer coverings:

Ca²⁺ + 2HCO₃^– ⇒ CaCO₃ + CO₂ + H₂O

9.4 Nutrients in seawater

Among the ‘everything else’ in the major constituents of seawater in Table 9.1 are the main nutrients that phytoplankton require for growth (see Chapter 10). This alone indicates that they are not there in unlimited quantities, and in fact in many parts of the oceans they can be present in such low amounts that phytoplankton cannot grow. Conversely, especially in coastal regions with a lot of run-off from agricultural land, they can be present in such quantities that the growth of phytoplankton they support is a problem (Eutrophication, see Chapter 13).

Although many nutrients are important for phytoplankton growth, the most commonly limiting major ones are nitrogen and phosphorus, exactly as on land, and the reason why farmers add nitrogen and phosphorus fertilisers to their arable crops. However, the land–ocean comparison is a weak one because even in the waters containing the highest concentrations of nutrients, their concentrations are a tiny fraction of the concentrations in most unfertilised soils.

Nitrogen is present in seawater as dissolved N₂ gas, ammonium (NH₄⁺), nitrite (NO₂^–), nitrate (NO₃^–), and in a range of organic molecules. N₂ gas cannot be used except by a few cyanobacteria that are able to fix the nitrogen so that it can be incorporated into amino acids and eventually protein. The main nitrogen source used by phytoplankton is NO₃^– and to a lesser extent NH₄⁺. Phosphorus occurs in several inorganic forms in seawater, although at pH 8 HPO₄^2– accounts for most of the free phosphate ions, and it is the form of inorganic phosphorus that is most bio-available for cell uptake and metabolism.

Clearly since the nitrogen and phosphorus are not sufficiently abundant, and the inputs from rivers are not enough to sustain continued phytoplankton growth over years, there must be another source of these nutrients. This comes from the excretion of urea and NH₄⁺ by zooplankton and nekton, and from the breakdown of dead organisms and faecal material by bacteria in the water column, as well as the sediments on the sea floor. The bacteria break down the proteins and amino acids to produce NH₄⁺ which is subsequently converted to NO₃^– by a group of bacteria called nitrifying bacteria. This whole process of converting organic matter into inorganic nutrients is referred to as nutrient regeneration or remineralisation. Phosphate is also released back into the water through bacterial activity breaking down organic matter.

In the deep oceans the surface waters have generally low concentrations of NO₃^– and HPO₄^2–. Although often associated with the oxygen minima between 500 and 100 m, there are peaks in these nutrients due to the high amounts of bacterial activity going on that have led to the oxygen depletion. Generally waters below surface mixed layers have higher concentrations of nutrients, and it is when mixing events occur, bringing deep waters to the surface, that nutrients are injected into the photic zone. In shallow shelf seas where seasonal thermoclines break down in autumn and winter, it is then that the surface waters become replete with nutrients that are of key importance for the following spring bloom (Chapter 10).

Where waters are low in nutrients they are said to be oligotrophic. Where there are high concentrations of nutrients the waters are said to be eutrophic, and in between the two they are mesotrophic.

9.5 Interconnected oceans

An enormous movement of water, which is called the global ocean conveyor belt or the thermohaline circulation (Chapter 1), interconnects the oceans. As millions of square kilometres of seawater freeze in the Arctic and Southern (Antarctic) Oceans (Chapter 12), cold, highly saline brines are expelled from the growing ice sheets, increasing the density of the water and causing it to sink.

In the conveyor belt circulation, warm surface and intermediate waters (0 to 1000 m) are transported towards the Arctic in the north Atlantic, where they are cooled and sink to form North Atlantic Deep Water, which flows southwards. In the Southern Ocean ice formation also produces cold high-density water that sinks to form Antarctic Bottom Water. These deep-water masses move into the South Indian and Pacific Oceans, where they rise towards the surface. The return leg of the transport begins with surface waters from the north-eastern Pacific Ocean flowing into the Indian Ocean and then into the Atlantic. This is not a fast process since, if it were possible to tag a molecule of water at the start in the north Atlantic, it would be several thousand years before it got back to the start again.

It is not just the temperature and salinity of the deep water formation in the polar regions that is crucial to the ocean circulation. As said above, the oxygen-rich waters from the poles are essential for ensuring that the deep waters remain well oxygenated despite all the bacterial activity going on to break down organic matter. But also, the longer the water masses are away from the surface of the oceans, the longer time there is for the bacteria to drive these remineralisation processes. As a consequence of this, the waters coming towards the surface in the north-eastern Pacific have much higher concentrations of NO₃^– and HPO₄^2–, (and lower O₂ concentrations) than north Atlantic waters.

Figure 9.7 Sketch of the global thermohaline circulation showing the regions where deep bottom water is produced.

Figure 9.8 Nitrate concentrations (µmol per litre) in the surface waters of the oceans (red high concentrations and blue lower).

Figure 9.9 Phosphate concentrations (µmol per litre) in the surface waters of the oceans (red high concentrations and blue lower).