Marine Biology

Salinity

The salinity of the open ocean at about 300 metres depth is about 3.5%. There is a slight variation in salinity in some seas, as in the Mediterranean where it is 3.9% while in the red sea it is 4.6%. The salinity of the sea is due to the two elements sodium and chlorine which account for 80% of the salts of the sea. The composition of chemicals contributing to seawater salinity is given in the following table.

In the sea water, cations and anions are not balanced against each other. As a result, sea water is weakly alkaline (pH 8 to 8.3) and strongly buffered. This factor possesses much biological importance. The various salts of the sea are indispensable to the marine life. Animals absorb and utilize many substances like Ca, Na, K, Mg, S, C l etc. They also use many inorganic materials like Na, Mg, Ca, and silicic acid to build their bodies. A few animals even use and store rare elements. Strontium sulphate is utilized by some radiolarians. Bromine and iodine is stored by horny corals and vanadium is used by ascidians.

An increase or decrease of salinity brings about changes in the specific gravity of the sea water. All marine animals are affected by changes in specific gravity. Only some animals like the teleostean fishes have the swim bladders which are used for hydrostatic control. Animals with hard skeletal materials of calcium and silicon face the problem of sinking. These animals however have various adaptations developed to keep themselves afloat, which include reduction of calcium contents by having perforations in the shell of foramnifers and thin shell in radiolarians. Some pelagic molluscs have thin and uncalcified shells, which aids in floatation.

Osmotic properties of the seawater present another problem to some of the marine animals. Most of the marine animals are isotonic with seawater and when they come across any change in salinity, they are put to much difficulty. The stenohaline animals have a restricted distribution. These animals are usually found in the open oceans far away from estuaries and below the level of tidal variation and only a few metres below the surface.

Euryhaline animals include the coastal forms found between tide levels. Arenicola, Mytilus, Sagitta and Oikopleura are some good examples. Some animals like the shore crab, Carcinus, can tolerate lower salinity at higher temperature. The younger organisms have lesser tolerance for lower salinity than their adults.

Salinity has a profound effect on the respiratory activities of marine animals. The respiratory rates increase with a reduction in salinity. The animals spend much of their energy in osmoregulation, when salinity falls and this leads to a higher rate of oxygen consumption and higher respiratory rates. The highest respiratory rates are found in estuarine forms like Hydrobia, Carophium and Pygospio.

More calcium carbonate is deposited in the skeleton of molluscs, crustaceans and other animals living in the water with a high salinity content. The molluscs found living in lower salinity have thinner shells. Animals can tolerate lower salinity when the temperature is high.