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. 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. 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.).
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. 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. 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. 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. |