Temperature

Microbial (bacterial) activity increases with increase in temperature resulting in microbial breakdown of organic matter, a process that uses dissolved oxygen.

The metabolic rate of aquatic animals is also related to temperature. In warm water the respiration rate of aquatic organisms increases leading to higher oxygen consumption. With increasing water temperature this combination of factors can lead to a natural depletion of dissolved oxygen in the water body. The solubility of oxygen in water decreases as the water temperature rises. For example at 760mm atmospheric pressure fully saturated low conductivity fresh water at 4 degrees Celcius contains approximately 13 milligrams oxygen per litre of water whereas the same water at 23 degrees Celcius is fully saturated when it contains only 8.6 milligrams of oxygen.

Cardiff Harbour Authority measure conductivity in order to calculate the salinity of the water.  It is a requirement that the barrage shall be managed such that there is no saline intrusion into the freshwaters of the Bay.

Dissolved oxygen as the name implies is the quantity of oxygen that is contained in water. It is often expressed as milligrams of oxygen per litre of water (mg/L). Oxygen enters water in a variety of ways. Diffusion from the atmosphere at the air/water interface and plant (including algae and diatoms) photosynthesis being the main routes. The amount of oxygen that can be dissolved in water depends on the water temperature, salinity, and gas pressure. Oxygen solubility decreases with increasing temperature and salinity.

Once dissolved, oxygen is dispersed through a water body by internal currents. Flowing water usually has higher dissolved oxygen levels than still water because currents within flowing water cause the water at the air/water interface to be mixed with lower depth water. This results in oxygen-rich water at the surface being replaced by water containing less oxygen which in turn is re-oxygenated by diffusion from the atmosphere. Still water has less mixing therefore the upper layer of oxygen-rich water remains at the surface.  This results in lower dissolved oxygen levels throughout the water column. This is the basis for the installation of the air mixing system in the Bay designed primarily to establish circulation currents within the otherwise still water and providing mixing of the water throughout the depth of the Bay.

Dissolved oxygen is consumed by respiration of animals and at night also by respiration of plants and algae. Oxygen is also consumed by aerobic micro-organisms as they decompose organic matter. The rate of respiration increases with increasing water temperature.  It is therefore important to ensure good mixing of the Bay water during summer months.

A good level of oxygen in the water is essential to sustain fish life and general aquatic diversity.  A water quality standard of 5mg Oxygen per litre of water has to be maintained at all places and at all times within the Bay.

pH is defined as the negative log-base 10 of the hydrogen ion concentration:

The numerical pH scale is to log-base 10  and indicates the acidity of a solution on a scale of 0 to 14. The pH of neutral (neither acidic nor alkaline) water, is 7. Alkaline solutions have a pH greater than 7 and acidic solutions have a pH less than 7. 

Addition of acid to a water system may alter the pH. Two main sources of acid entering river waters in Wales are acid mine drainage and acid rain.
 

As well as being toxic in its own right acidic water can cause the mobilisation of metals by desorption from sediments. These metals are then available for uptake by aquatic life and may prove toxic. For example, aluminium is present in most soils. An increase in acidity may mobilise aluminium which at concentrations of 0.1 - 0.3 mg/l will retard growth, increase mortality and damage the egg production of fish.

Free ammonia (unionised) is present in water in equilibrium with ammonium ions (NH4+) and hydroxylions (OH-).  The equilibrium shifts with changes in pH and temperature of the water.

The free ammonia (unionised) concentration in freshwater increases with increasing pH and temperature. At low pH and temperature, ammonia combines with water to produce an ammonium ion (NH4+) and a hydroxide ion (OH-). The ammonium ion is non-toxic whereas unionised ammonia is highly toxic. In water above a pH of 9 unionized ammonia is the predominant species.

Suspended solids can cause changes in composition of an aquatic community. High concentrations of suspended sediment will reduce light penetration and reduce the photosynthetic activity of plants and algae. This can have repercussions through the food chain since if the primary food producers (algae) are limited there will be less food to support invertebrates and a decline in fish numbers feeding on invertebrates may result.

Trace amounts of the heavy metals copper, cobalt, iron, manganese, molybdenum, vanadium, strontium, and zinc are required to sustain life. However excessive concentrations of these essential metals can be detrimental. Other heavy metals such as cadmium, chromium, mercury, lead, arsenic, and antimony are harmful to aquatic life.

Heavy metals are removed from water by adsorption onto sediment particles and by formation of complexes with organic material. The adsorbed metal can be returned to the water column by desorption from sediment particles to become available for bioassimilation by aquatic life. The rate of adsorption and desorption of metals is driven by the overall water chemistry. Metals may be desorbed from sediment if the water experiences increases in salinity or decreases in pH.

The effect of heavy metals on aquatic organisms are varied ranging from sublethal changes in morphology and physiology (which may in turn lead to changes in behavioural patterns, reproduction or growth rates), to, at worst, death.