3.6 Tidal rivers and estuaries
Most of the major cities and harbours in the world are located on estuaries. The estuarine ecosystem is a unique intermediate between the sea, the land and fresh water.
A rather precise definition of an estuary is 'a semi-enclosed coastal body of water, which has a free connection with the open sea, and within which sea water is measurably diluted with fresh water derived from land drainage'. This excludes large bays with little or no freshwater flow, and large brackish seas and inland saline lakes which derive their fresh water from rain only.
This definition is based on salinity; as we have seen in Section 2.4 this is the measure of the total concentration of salts. The salinity of fresh water is less than 0.5 parts per 1000 (0.05%); in the sea it is between 33 and 37 parts per 1000. The salts are composed principally of sodium and chloride ions, but also included are smaller concentrations of K, Mg, Ca and SO4 and others in minute or trace amounts. Many types of salinity distributions occur in different types of estuaries. They are dependent upon freshwater inflow, evaporation, topography, currents and tides.
The deposition of any sediment is controlled by the speed of the estuarine currents and the particle size. In general, as a tidal current enters an estuary and loses speed it will first deposit gravel, then sand and finally silt which accumulates as mud (Figure 17a). Similarly, a river current will first deposit sand and coarse particles, then silt when its direct flow is finally halted by meeting inflowing salt water.
Stratification also occurs (Figure 17b), but this time it is not necessarily thermal as in lakes but depends on salinity. Fresh water is lighter than sea water and can form a layer on top. Saline stratification can vary between estuaries: for example, the Tees estuary is highly stratified whereas the Thames estuary is not. Figure 17(b) shows that the Tees gives nearly horizontal isohalines (lines which show points of equal salinity), whereas the isohalines of the Thames are nearly vertical.
As in rivers, the oxygen concentration is of prime importance. Several factors influence the quantity present, including mud, salinity and temperature. Mud is rich in food material in the form of organic debris, and it may also be rich in bacteria and other micro-organisms which may put a heavy demand on the oxygen present. The available oxygen may soon be used up and then the nitrates present will be used as an oxygen source, being reduced to nitrogen gas in the process. Finally, sulphate-reducing bacteria may consume oxygen from the sulphates present – this process generates foul-smelling hydrogen sulphide.
We have already learnt that an increase in concentration of dissolved salts lessens the saturation concentration of oxygen in water, therefore sea water (of high salinity) holds less oxygen than fresh water at the same temperature and pressure. There may consequently be an added problem of low oxygen content before any pollution occurs.
As in rivers, temperature also affects the oxygen content. In estuaries, temperature conditions are normally very variable because of the shallowness, and because of the different bodies of water mixing together.
Organisms that naturally inhabit estuaries must be adapted to such changing conditions as salinity, temperature and sediments. Examining Figure 18 we find that the number of such organisms is relatively small, even in a clean estuary such as the Tay. In fact, if we compare the Tay estuary with the less clean Tees estuary, we find that there is little difference in the abundance of species. Although there is a marked dip in dissolved oxygen in the Tees estuary, the distribution of organisms is generally comparable to that of the Tay.