Effluents containing acids and alkalis are discharged into the marine environment but generally the high buffering capacity of saline waters ensures that pH levels are returned to the normal range. Any variations arising from the discharge are likely to be local to the discharge point.

pH also varies naturally in the water column with maximum values occurring at or slightly after the time of maximum illumination with high values in the summer months. This is related to photosynthetic activity with bicarbonate used as a source of carbon dioxide. The degree of photosynthesis is affected by the supply of nutrients, organic carbon and turbidity.

Unlike freshwater, saline water has a high buffering capacity for pH, so any pH changes in marine waters tend to be small and localised around the source. The pH of oceanic water is fairly constant at about 8.2 pH units, but variations in seawater pH of a few tenths of a unit are possible, for example, on a diurnal scale associated with algal photosynthesis. As might be expected, buffering in estuarine waters is greater than that in freshwaters, but weaker than that in fully marine waters. Typical pH levels in UK estuaries range from 7.0 to 8.3 (see table below).

The effects of changes in pH on the marine environment can be sub-divided into direct effects (those organisms directly affected by changes in pH) and secondary effects (those arising in the ecosystem as a result of the changes in the organisms directly affected).

The direct effects of a change in pH in the marine environment include:

• the potential for the release of C0₂ following the rapid release of acids;
• influence on the speciation and toxicity of substances, such as ammonia, silicate, phosphate, borate, some metals and some phenolic organic compounds;
• lethal and sub-lethal effects on marine organisms.

At pH 8, bicarbonate is the predominate carbonate species, but below pH 6, CO₂ predominates, so that the rapid discharge of acids to tidal waters may be able to liberate sufficient CO₂ to be lethal to aquatic life. pH affects the equilibrium position of other systems, such as that for silicate, phosphate and borate in a similar way.

pH affects the equilibrium for ammonia. At high pH the proportion of the toxic unionised form of ammonia increases and may cause water quality problems (see Section B1).

Low pH can increase the solubility of toxic metals, such as cadmium, copper, lead, aluminium, mercury and zinc but the degree of mobilisation in high alkalinity saline waters is less than that in freshwaters.

The quality of available data for the effects of pH on marine fish is questionable, since the effects of high CO₂ and low pH were not separated from each other until Brownell=s (1980) study. However, LC50 values have been reported below 5.4, and above 9.0. Feeding of fish larvae appears to be affected at pHs below 6.0 and above 8.4. Some adult fish are reported to be to be unaffected at pH values above 9.0, but for the larval stage, a more appropriate standard appears to be 8.5 (Wolff et al 1988).

Data for bacteria appear to be sparse and difficult to interpret, particularly since different species have different media requirements for laboratory studies.

A wide range of tolerance exists for different marine algal species, with different optima for different physiological or reproductive processes, so that no overall trends or conclusions can be drawn. At low pH, the increased free CO₂/bicarbonate ratio may favour some species, but hinder growth/reproduction in others. Several species show reduced calcification as the pH is reduced towards 6.0 (Borowitzka and Larkum 1976, Paasche 1963, Smith and Roth 1979), and toxicity of copper may increase as pH is reduced (Sunda and Guillard 1976).

For molluscs, adverse effects are seen at pHs greater than 8.5 and less than 7.0, including shell dissolution at lower pH values (see Wolff et al 1988).

Some crustaceans (e.g. Crangon crangon) survive at well below pH 6.0, but others have LC50 values in the range pH 5.5-6.7 (e.g. Pseudocalanus sp., Arcatia tonsa, Temora longicornis; see Wolff et al 1988).

Wolff et al (1988) proposed an EQS for the protection of saltwater fish to be pH 6.0 - 8.5 as an annual average and for shellfish to be pH 7.0 to 8.5 as a 75 percentile.

The indirect effects of a change in pH in the marine environment are likely to be limited because the scale of the direct effects is limited by the buffering capacity of seawater. However, a precautionary approach should be adopted around known discharges of acids and alkalis to ensure that the direct effects are indeed minimal.

Potential effects include:

• the potential for the release of C0₂ following the rapid release of acids which may be sufficient to be lethal to aquatic organisms;
• influence on the speciation and toxicity of substances, such as ammonia;
• lethal and sub-lethal effects on marine organisms and, in particular, to fish outside the EQS range of 6.0 - 8.5 (annual average) and to shellfish outside the EQS range of 7.0 -8.5 (75 percentile). No standard was proposed for the protection of all saltwater life.

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