All the seas are interconnected by currents, dominated by waves, influenced by tides and characterized by saline waters. Not only the seashore and banks which are the homes of many organisms but the open ocean, many hundreds of kilometers away from land, supports plant and animal communities of great diversity and complexity.
Physico-Chemical Aspects of Marine Environment:
In the marine environment, the most important physical factors which influence marine life are light, temperature, pressure, salinity, tides and currents.
Light is a very significant factor in regulating the pattern of distribution of marine animals and it contributes significantly to organic production. The autotrophic primary producers exploit the light energy in the photosynthetic production of food for primary macro consumers of marine ecosystem.
The amount of light exploited in photosynthesis depends on intensity of light and turbidity of water. For example, a ten meters deep turbid coastal zone receives an equal amount of light energy as a hundred meters deep clear oceanic zone, but the maximum intensity at a fen meters coastal water is greater.
Light-determines diurnal migrations of marine organize and it also regulates color pattern of marine animals. The deep sea fauna which lives in total darkness exhibit either colorlessness or uniform coloration. It is also somehow related with the development of visual sense organs as shown by absence of functional eyes in deep-sea animals.
The range of temperature in sea is far less than that on land. Arctic waters at 27°F are much colder than tropical waters at 81°F, and currents are warmer or colder than the waters through which they flow. Seasonal and daily temperature changes are larger in coastal waters than in the open sea. The surface of coastal water is the coolest at dawn and the warmest at dusk.
In general, sea water is never more than 2° to 3°F below the freezing point of fresh-water or higher than 81°F. At any given place the temperature of deep water is almost constant cold, below the freezing point of fresh-water. Unlike fresh-water, sea water does not have a density maximum at 4cC (39°F); rather it becomes continuously denser as it gets colder.
Pressure in the ocean varies from 1 atm at the surface to 1,000 atm at the greatest depth. Pressure changes air many times greater n the sea than in terrestrial environments and have a pronounced effect on the distribution of life. Certain organisms are restricted to surface waters when the pressure is not so great; where as other organisms are adapted to life at great depths. Some marine organisms, such as the sperm-whale and certain seals, can dive to great depths and return to the surface without difficulty.
Zonation of marine environment:
Just as Lake Exhibit stratification and zonat on, so do the seas. The ocean itself is divided into two main d visions, the pelagic or whole body of water, and the benthic, or bottom region. The pelagic region is further divided into two provinces; the neritic, water that overlies the continental shelf, and the oceanic provinces. The continental shelf is the underwater extension of the continent and it generally extends to a depth of roughly 125-200 m.
The edge of the continental shelf may be within a few kilometers of the shore, or it may be several hundred kilometers from shore. From the edge of the continental shelf there is a more rapid descent, the continental slope; to the broad flat abyssal plain that underlies most of the ocean at a depth of 4,000—5,000 m.
On the abyssal plain, there are extensive mountain ranges, or midoceanic ridges, some of which have tips projecting above sea level. In addition there are very deep troughs, which drop down below 11,000 m. below sea level. Life exists from the very top of sea to the bottom in virtually all areas, although its abundance is exceedingly variable.
The benthic life zones are defined in terms of these physical subdivisions. The littoral, or intertidal zone, is the zone between high tide and low tide levels. The snblittoral extends from the low tide mark to the edge of the continental shelf, with the bathyal zone comprising the continental slope. The abyssal zone includes the abyssal plains, and the hadal zone includes any life in the deep trenches below 5,000 m.
The pelagic zone is divided into three vertical layers or zones: from the surface to about 200 m is the photic zone, in which there are sharp gradients of illumination, temperature, and salinity. The region below this zone is called aphotic zone. From 200 to 1,000 m where very little light penetrates and where the temperature gradient is more and gradual and without much seasonal variation, is zone of aphotic zone.
It contains an oxygen-minimum layer and often maximum concentrations of nitrate and phosphate. Below mesopelagic is the bathypelagic zones, where darkness is virtually complete except for bioluminescence, and where temperature is low and pressure is great.
Stratification of marine environment:
The upper layers ocean water exhibit a stratification of temperature and salinity. Below 300 m usually thermally stable. In high and latitudes tempera fires remain fairly constant throughout the in middle latitudes temperatures vary with the season, assorted with climatic changes. In summer the surface waters become armer and lighter, forming a temporary seasonal thermo cline.
In subtropical regions the surface waters are constantly heated, developing a marked permanent thermo cline. Between 500 and 1.500 m a permanent but relatively slight thermo cline exists.
Associated with a temperature gradient is a salinity gradient or halocline, especially at the higher latitudes. There the abundant precipitation reduces surface salinity and causes a marked change in salinity with depth. Thus in the middle latitudes in particular, the two produce a marked gradient in density.
Water masses form density layers with increasing depth. Because density of seawater does increase in depth and does not reach its greatest density at 4°C as with freshwater, there is no seasonal overturn.
This results in a normally stable stratification of density known as the pycnocline. Because there are marked changes in temperatures with depth in the open ocean and thus with density, the pycnocline often coincides with the thermo cline. Below the pycnocline is the deep zone which is oxygen rich, cold and comprises 80 per cent of the ocean.
The marine animal life has specific osmoregulatory adaptation for high saline sea waters. The absence of many animal species in marine environment has been related to their inability to tolerate the high salt contents of sea water. Except few insects such as Halobales and shore collembolans like Isotoma and Sminthurus, most insects do not occur in marine ecosystems.
The salinity of marine water fluctuates from place to place and it is caused due to the dissolved salts such as chlorides of sodium, potassium calcium and magnesium, and also the sulphates of calcium and magnesium.
Currents and tides:
Sea water is never static and waves, currents, and tides are the regular features of sea water. All these phenomena are controlled by winds, cosmic forces and varying water densities. Most world-wide water currents occur at the surface and at great depths of sea water.
These currents determine the interchange between the surgical and deep water masses, as well as horizontal movements. Both the horizontal and vertical movements of ocean water are significant, but for different reason. Within a given current, such as warm Gulf Stream, for instance, the water mass retains its identity for great distances and the warm- Sea community ranges far north of what have anticipated as its northernmost limits.
In addition, the climate of terrestrial eco- systems is strongly affected by the nature of adjacent water masses. The prevailing winds blow across the ocean and are heated or cooled by the waters they traverse.
In marine ecosystems, however, the oceanic current system controls the distribution of productivity of marine communities through vertical mixing of water masses, because most of the ocean is permanently stratified, any essential “nutrient that settles out of the upper layers, either as an inorganic mineral or as organic detritus, is not returned to the upper layers at the same location.
This had led to such an impoverishment of, sacrificial waters of the oceans that productivity in most parts of the open oceans is about the same as that of desert, because of the very low concentrations of nutrients, especially phosphorus. At the same time, the lowest waters of the ocean are nutrient-rich.
There are certain regions in the world, notably at the west coasts of continents and in sub polar latitudes in both hemispheres, where deep currents rise to the surface. These are called zones of upwelling, and even though their waters are very cold, they are sites of intense production because of their relatively high concentration of nutrients. Nutrients are then cycled back to the rest of the ocean via surface currents.
Tides are water movements which are caused by some astronomical factors. They represent a rhythmic rise and fall of water and often waves of long wave lengths characterize the process.
The chief astronomic force behind the tidal rhythm is the attraction of the moon and sun which depends on the movement of earth in relation to the moon and the sun. Tides influence the sea-shore fauna variously, as low tides expose the shore and high tides flood the substratum.