Water movements (waves, tides, currents)

Waves

Waves are caused by the action of the wind and depend on its velocity, duration and the distance over which the wind blows.
The distance between two successive wave crests is called wave length (L). At depths less than L/2 from the sea surface,waves cause orbital movements which mix the water and suspend sedimentary particles, while below this depth wave influence becomes negligible ( Figure 6)



As the wave approaches the coast, it "breaks" and releases its kinetic energy through the formation of smaller waves and turbulent water masses which wash on to the beach. The energy of these waves has a major impact on both the formation, the size spectrum and the distribution of the sea shore substrate and the organisms living there. The latter have developed specific adaptations, like holdfasts, special flat shapes etc. ( Figure 7 ) in order to cope with these high energy conditions.
Wave impact on the shore differs according to the coast morphology, the substrate texture and the frequency and direction of the prevailing winds of the area.

Tides

Tides are caused by the combined action of the Earth, Moon and Sun gravity forces. In Figure 8 the extreme water levels occurring during high and low water are presented. When Earth, Sun and Moon are all in one line a spring tide occurs with maximal increase and decrease alternately of water level. When they form a right angle, neap tides occur. The period of spring tides is two weeks and low-high water approximately one day (24 hours and 50 min.).


The influence of tides on the coastal ecosystem is quite important. During the elevation or fall of the water level horizontal movements of water masses (tidal currents) occur which are more pronounced when the coast inclination is small. This movement results in water renewal, in the increase of oxygen levels and in the transport not only of sediments and suspended material but also of plankton and pelagic larvae. It is a process is of great importance, especially for areas protected from wave action (closed gulfs, fjords etc.) where tide is the main factor in water renewal.

Organismic adaptations

Organisms living in coasts of intense tidal variation are obliged to adopt a mode of living to make sure that they survive during low water; this is the time when they have to face and overcome the problems of respiration, desiccation and predation from large predators (i.e. marine birds). That is why they prefer to live in the sediment rather than on its surface. There they may also develop exoskeletons or hard shells (chitinous or calcareous) in order to protect themselves from predation, or hermetically closing shells or mucous covers that retain water. They may also develop the ability to use atmospheric oxygen, using anaerobic metabolic pathways. They may also minimize their energy requirements (and consequently their oxygen demand) during emersion. There are many other adaptations which may occur at the morphological, physiological and behavioural level..

Water currents

The term "water currents" is used to describe water movements of different kinds caused by different events.

• the movement of the earth which causes the development of the Coriolis effect
• the combined effect of wave action and barometric pressure and
• water density differences between tropical areas and polar areas.

Currents play an important role not only through oxygen, nutrients and other elementary substances of sea water transportation but through the ..dispersion.. of organisms (especially low mobility ones) as well.

Upwelling

I The upwelling phenomenon is known from western coasts of the continents and also on a local scale where cold water masses rich in nutrients are slowly rising to the surface. In this way the upper water masses are enriched in nutrients. Upwelling is usually encountered on the western coasts of large continental areas (due to Coriolis forces and Ekmann transport and ensures extremely high productivity (Sardine fishery at the Peruvian coast). Disturbances of this phenomenon due to large scale climatic variablity which occur mainly around Christmas time are called the "El Nino"- phenomenon when the whole current system is shifted and warm water covers the cold and productive deep water. In this years extreme water precipitation in coastal regions are combined with drastic changes in the biotic environment leading to massive crops of some and extreme kills of other species.

Apart from well-known major currents like the Humboldt current or the Labrador Current with a more or less permanent direction, there are many others which are smaller in scale but of great local importance. These may show differences in intensity and/or direction according to the season (Monsoon and/or local meteorological conditions.

Oxygen

We know from chemistry that an atom of oxygen and two atoms of hydrogen form a molecule of water. Consequently, oxygen forms part of the water at all depths in all areas of the world. However, oxygen bound in this manner cannot be used for the respiration of marine organisms. What they need is molecular oxygen (O₂) which is dissolved in sea water, as well as other gases, in much smaller quantities.

Dissolved oxygen concentration is affected by physical, chemical and biological processes. Factors which cause an increase in dissolved oxygen concentration are photosynthesis, diffusion from the sea surface, and mainly the action of the wind and the currents which by causing surface water turbulence, saturate the surface layers with oxygen. Reduction of dissolved oxygen concentration is caused by the respiration of marine organisms and by the oxidation of organic substances either by simple chemical reactions or by bacterial activity. High temperatures and high salinity values lead to a reduction in oxygen solubility.
Nearly all living organisms need oxygen in order to carry out their biological processes. However, the quantity of oxygen demanded differs according to species, mode of life, sex, age as well as environmental factors such as temperature, salinity and the presence of various types of pollutants. Thus, both the increase in and decrease of oxygen in the water have different effects on different species. For instance, animals are much more sensitive than plants are to anoxia; in general high mobility organisms need much more oxygen, since they can migrate away from anoxic areas, while there are organisms which have developed the ability to use glycogen as an alternative source of oxygen over a relatively short time interval. And others are able to use the sulphur from hydrogen sulphide that occurs when all oxygen is used up by the bacteria. Anoxic or hypoxic areas are an increasing problem in coastal waters where increasing eutrophication leads to an increased oxygen demand on the sea bottom and leads to fauna kills of considerable extent. Also in other areas of high organic anthropogenic input such as sewage outlets, paper mills and fish farms there may be locally restricted areas of bottom anoxy devoid of any macroscopic fauna.