Power station cooling water systems have a range of potential impacts on receiving environments including:

• the abstraction of large volumes of water,
• entrainment of organisms on intake screens,
• entrainment and passage of organisms through the cooling system,
• the addition of biocides to the cooling water to control biofouling, and
• the discharge and dispersal of the heated effluent.

These impacts are usefully reviewed by Langford (1990) and in relation to a specific power station in Milford Haven by Langford et al (1998). The information in this profile mainly covers the temperature effects associated with the discharge of the heated effluent.

The primary source of thermal discharges to the marine/estuarine environment is power station cooling water discharges, although cooling-water from other industrial processes could be responsible for more localised temperature changes.

The ultimate temperature of any cooling-water discharge varies with a number of factors, including power station operating load, volume of cooling water used, design criteria for the station and the inlet water temperature. The optimum temperature rise for efficient power station operation is between 10 and 15 °C but rises of up to 30 °C have been recorded. The normal increase from inlet to outlet (°T) for British fossil fuelled power stations is 10-12 °C, although discharge temperatures at nuclear power stations can be up to 15°C higher than inlet temperatures (Langford et al 1998).

The resulting temperature change in the receiving marine environment is very site specific and depends on many factors, including the hydrodynamics of the receiving system and the design and location of the discharge.

In the UK, 12 of the 155 estuaries included in the Estuaries Review (Davidson et al 1991) received thermal discharges from power stations facilities in 1989. Some estuaries received thermal effluents from a number of power stations located either directly on the estuary or upstream on the river system feeding the estuary. For example, the Humber has 3 power stations situated on the estuary (with a further 4 either under construction or planned) and 6 on river systems feeding into the estuary (Barne et al 1995).

The heat in a cooling-water discharge will dissipate in the marine environment as the plume mixes with the water column. Some energy may be lost to the atmosphere if the plume is buoyant. Similarly, some energy will be transferred to the sediments if the discharge passes over intertidal sediments at low tide or is entrained in lower layers of water. Continuous thermal discharges to semi-enclosed bodies of water such as estuaries can result in a net increase in temperature of the water column.

The rate of mixing of the discharge plume with the water column will determine the rate at which heat is dissipated. Discharges to estuaries are most likely to have reduced potential for complete mixing with heated effluent concentrated in a body of water that moves up and down the estuary with the ebb and flow of the tide. This can be exacerbated by stratification where heated effluents can be entrained in distinct layers in the water column. The heated effluent may reinforce stratification as the heated buoyant effluent is entrained in surface layers, increasing the temperature differential between the layers above and below the thermocline. In some situations, cooling-water discharges are of greater salinity than the receiving environment and become entrained in the lower layers of the water column.

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

The direct effects of thermal discharges on the marine environment include:

• change to the temperature regime of the water column, and perhaps the sediment, of the receiving environment;
• lethal and sub-lethal responses of marine organisms to the change in temperature regime;
• stimulation in productivity in a range of organisms;
• reduction in the dissolved oxygen saturation.

Bamber (1995a cited in Langford et al 1998) identified three aspects in which changes to the temperature regime were important to the ecology of the receiving environment:

Examples of the upper temperature limits of seaweeds are shown in the table below, with species, such as Fucus and Ascophyllum spp. declining in abundance where thermal discharges have resulted in temperature increases of 5-7° C above ambient (Langford 1990).

Some introduced invertebrates, including non-native oysters, may be able to reproduce and thrive in artificially heated regimes (Langford et al 1998) or affected populations may exhibit characteristics of more southerly populations, such as breeding earlier in the year (e.g. Bamber and Spencer 1984).

Temperature conditions before, during and after the spawning period appear to be important for the long-term variability (settlement and size) of Tellina tenuis (Barnett and Watson 1986). Likewise, high summer temperatures have been associated with dense settlements of the barnacle Chthamalus and the role of temperature on the seaweed-dwelling amphipod Hyale nilsonni has been demonstrated by Moore (1983). There are numerous other examples where increased temperatures have affected the growth and/or reproduction of invertebrates, such as the amphipods Urothoe brevicornis and Corophium ascherusicum, the harpacticoid crustacean Asellopsid intermedia, the isopod Cyathura carinata and the immigrant barnacles Balanus amphrite and Elminius modestus (see Langford et al 1998).

Behavioural effects are rarely reported in field studies, but the amphipod Corophium volutator has been reported to leave its burrow and enter the water column at temperatures over 25°C. Similar behaviour is shown by the burrowing bivalve Donax serra which leaves its burrow and lies on the sediment (sand) surface as temperature increases.

Ultimately, a long-term thermal discharge is likely to lead to a changed and thermally adapted community, more typical of that found in otherwise more southerly and warmer climates.

There have been a number of suggestions of potential effects on fish and macro-crustaceans ranging from a temperature related water quality barrier to the migration of salmon (NRA 1993) and the elimination of certain species on the boundaries of their geographic distribution to localised behavioural responses to individual discharges.

The evidence for temperature related water quality barriers is necessarily equivocal given the wide number of factors affecting salmon migration. Localised behavioural responses have been observed on a number of occasions where species, such as bass and mullet, have been attracted to these effluents at certain times of year. It is possible that they are exploiting additional food supplies but at the same time they can be exposed to additional predation from birds and anglers. Similarly, fish not suited to the localised increase in temperature/reduction in dissolved oxygen can avoid discharge plumes. Such behaviour has been termed behavioural thermoregulation.

Given the largely localised impacts of thermal pollution, it would appear that local changes in fish populations may be observed and idiosyncratic fisheries may be created. The overall impact on an estuarine fish community is likely to be limited.

In the Thames estuary, a set of quality objectives have been derived to ensure water quality is appropriate for the passage of migratory fish and to support fisheries consistent with the physical characteristics of the estuary. The objectives include a non-statutory temperature standard of a maximum temperature of 28 °C which is the EIFAC standard for cyprinid fish under EU Freshwater Fish Directive 78/659/EEC. The standard for migratory fish is 21.5 °C but it is recognised that this temperature would be exceeded under natural conditions in most summers. However, since 1989, all new discharges of cooling water are subject to a condition whereby when river/estuary temperatures exceed 21.5 °C, they have to switch to alternative methods of cooling (TEMP 1996).

Microbially-mediated processes, such as nitrification, denitrification and manganese oxidation, are all affected by thermal discharges, since every 8-10°C increase in temperature equates to a doubling of microbial activity. The same thermal relationship applies to phytoplankton productivity (providing no other factors, such as light and nutrient availability, are limiting). Thermal discharges are unlikely to have a substantial effect on planktonic populations where the residence time of water within the thermal plume is less than one week, although benthic diatoms are reported to be moribund in the near-field surrounding thermal discharges (Langford et al 1998). This observation is consistent with in with the 5-15°C optimal temperature range reported for a number of planktonic diatoms (freshwater and marine).

Changes to dissolved oxygen saturation potentially arise as a result of the reduction in solubility of oxygen in sea water with increasing temperature and as a consequence of the increased productivity of microbial communities in particular. The consequences of reduced dissolved oxygen are discussed elsewhere.

The indirect effects of thermal discharges on the marine environment include:

• changes in the distribution and composition of communities of marine organisms comprising European marine sites (particularly estuaries);
• localised changes in bird distributions usually in response to increased macroinvertebrate or fish food supplies close to thermal discharges.

Potential effects include:

• change to the temperature regime of the water column, and perhaps the sediment, of the receiving environment;
• lethal and sub-lethal responses of marine organisms to the change in temperature regime;
• stimulation in productivity in a range of organisms;
• reduction in the dissolved oxygen saturation;
• changes in the distribution and composition of communities of marine organisms comprising European marine sites (particularly estuaries);
• localised changes in bird distributions usually in response to increased macroinvertebrate or fish food supplies close to thermal discharges.

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