The Scottish Government

1 - Water Quality

Indicator

Water body

More information

Main pressures

Dissolved oxygen

Rivers, lochs, transitional, coastal waters

Low levels of dissolved oxygen can result in death of many species.

Discharges of organic material, e.g. from sewage treatment works, storm overflows, agricultural slurry, silage liquor.

Enrichment by nutrients results in lower oxygen levels through eutrophication (see below) .

Biochemical Oxygen Demand ( BOD)

Rivers only

BOD is a measure of the amount of dissolved oxygen used up in a sample of water or effluent over a set period of time.
Oxygen is used up when micro-organisms break down organic material in the water.
BOD can reflect pollution by organic compounds - high BOD discharges cause a lot of oxygen to be removed from water.

Ammonia

Rivers only

Ammonia is toxic to many forms of aquatic life.
Ammonia is formed during the decay of organic wastes containing nitrogen, including vegetable and animal wastes.

Discharges of organic material, e.g. from sewage treatment works, storm overflows, agricultural slurry, silage liquor.

Acid Conditions

Rivers (pH)

Freshwater lochs ( ANC)

Acidic water can be toxic to many forms of aquatic life.
pH is a measure of the concentration of hydrogen ions (H ⁺) in the water, formed when an acid dissolves. Acidic water has a low pH (lot of H ⁺ ions). pH 7 is defined as neutral.
Acid Neutralising Capacity ( ANC) is an indicator of acidity that also considers a loch's sensitivity to acidity (based on the geology) and its ability to cope with additions of acid (due to the presence of organic acids that can soak up some of the H ⁺ ions).

Acidic rainfall, caused by air pollution.

Salinity

Freshwater lochs only

Many freshwater plants and animals are sensitive to increased salt levels.

It is unlikely that significant pressures, risking failure of the standard, will occur in the foreseeable future.

Nutrients -

Phosphorus

Rivers only

High nutrient concentrations affect the biological balance in water, and can contribute to eutrophication - very high plant and algal production.

Eutrophication causes a number of problems including:

• 'Blooms' of blue-green algae which can be toxic.

• Major reductions in dissolved oxygen, harming other aquatic life.

Nitrogen and phosphorus are the main nutrients needed by plants. In freshwater, the natural ratio of phosphorus to nitrogen is generally low, so plant growth is limited by the amount of phosphorus available. Any increase in phosphorus concentrations causes rapid plant growth.

Point source and diffuse discharges containing phosphorus, including:

agricultural and forestry runoff, particularly including fertilisers;

waste water treatment plants;

aquaculture.

2 - Water Resources

Minimum water levels are needed for aquatic life all year round. Additional protection is particularly important at key times - spring and early summer for aquatic plants; late summer and early autumn for invertebrates and fish.

Where abstraction takes place from a loch, the ecological impact in the outflow watercourse (downstream) is often larger than in the loch itself.

Abstractions, e.g. for public water supply, industry, energy generation and agriculture.

Impoundment of water behind dams and weirs that reduce downstream flows.

% change from natural conditions

Rivers only

Ecology can be affected significantly by changes in the physical structure or shape of a water body. Engineering and development works can affect water bodies directly, e.g. by destruction of habitats on river banks or beds, and more indirectly by altering flow patterns and water levels.

Engineering and construction works, e.g. dredging, strengthening of banks, straightening channels, building structures (such as bridges and piers) with in-channel supports.