wide range of living conditions in the seas

Within the large living spaces of the oceans, there is a wide range of living conditions available:

a. Salinity--dilute estuaries and bays and areas in the polar regions
with the runoff from the glaciers-- to more than 37 o/oo in the Red Sea and the Mediterranean due to the excess of evaporation over condensation in the climates of the latter seas.

b. Light intensity--brilliant sunlight to absolute, perpetual darkness at the deepest depths. Sunlight itself is a balanced range of wavelengths that includes the whole color spectrum. This is seen as "white light".

When sunlight strikes the surface of the oceans, the various wavelengths are selectively absorbed. Longer wavelengths of light are blocked more readily than the shorter ones. Thus, red wavelengths have been largely removed within the upper 10 meters (33 feet) of the sea's surface, whereas the longer blue, green, and violet wavelengths penetrate much farther. It is this blue light, reflected back to the surface in clear water, that gives the ocean its blue color.

This phenomenon of differential light absorption explains why coral reefs look so spectacular at shallow depths of less than 30 feet but appear to become more drab as a scuba diver descends to 100 feet. The red and orange wavelengths are filtered by the water in the shallower depths.

At 1000 meters of depth (3280 feet), there is little light remaining from that which penetrated the sea's surface. The residual light is mostly of blue and green wavelengths.

At this depth, what color would a red prawn appear to be?

Without any red wavelengths in the ambient light, the prawn will appear "dark" since there are no red wavelengths of light to be reflected from it. Thus, being red in the deep oceans is an useful camouflage strategy because it cannot be seen readily against a background of dim blue-green light. Hence, we find that there are many examples of animals living at great depths that are spectacularly red in color when brought to the surface and examined.

Since the sunlit surface waters reaches only down to 1000 meters, and since phytoplankton (floating one-celled plants) need to be in the shallow upper well-lit waters in which to grow (the phytoplankton forming the base for any food chain), most of the marine life is found in this narrow band of the oceans.

Still, this relatively great vertical range for a liquid (ca. 200 meters) available for photosynthesis by microscopic floating plants multiplied by the great expanse of the world's oceans yields the high productivity of organic matter in the surface waters of the oceans.

Light gradient (both in quantity and quality) with depth allows for adjustment of animals to seek optimum conditions, which reults in the diurnal migrations (up and down within a 24 hour period) noted in many of the deeper communities of animals.

Temperature--30 degrees Centigrade in surface tropical waters to nearly freezing waters at depth.

It is the heating effect of sunlight, largely by wavelengths within the infrared range, which drives the ocean currents and earth's weather systems.
There is a narrow band of surface waters that reacts directly and virtually immediately to the introduction of heat by sunlight. Thus, the oceanographer does not take meaningful, long-term ocean temperatures from the upper 100 meters of depth, since these are readily influenced by local conditions.
That zone in the oceans where the temperature changes steadily with depth is called the thermocline. In science, this zone is reserved for that area where the temperature changes at least one degree Centigrade per 10 meters of depth.
The fairly uniform temperatures of the water certainly "coddles" organisms that require a certain temperature range in which they can live and grow and reproduce; it also profoundly affects "buoyancy," in that warmer waters are less dense--hence causing greater "sinking" due to less of an upward force (remember Archimedes' Principle that the forces exerted on an object in a liquid is equal to the weight/density of the liquid that is displaced). This explains why one sees adaptations of greater surface areas in related organisms living in the warmer waters than their counterparts in colder waters.

Sound: sound is transmitted faster and farther in water than it is in air. In sea water, it travels at roughly 4750 feet per second, more than 4 times faster than it does in air. The implication of this fact is that it is difficult to pinpoint the origin of a sound while underwater for the scuba diver (the sound reaches both ears virtually simultaneously instead of being differentially received by each ear, which gives us the direction of the origin of a sound in air).
Sound travels great distances within an "envelope" (above the thermocline or below the thermocline but not through it), scientists at the prestigious Scripps Institution of Oceanography were able to send acoustic signals in 1991 from the southern Indian Ocean that were picked up as far as the east and west coasts of North America (nearly 10000 miles away). This technique is envisioned as a means of monitoring wholesale temperature changes in the open ocean, and to date has produced much of the data that speaks to the "warming of the oceans". However, this "experiment" provoked a strong controversy over whether the sound generated "harms" the whales and there was a delay of more than 3 years in implementing this research.

Pressure--1 Atmosphere (remember, this is 14.7 pounds per square inch) at the surface to more than 1000 Atm in the deepest parts of the ocean ocean.
By itself, pressure does not exclude life from the abyssal regions as water is relativel incompressible. This means that an equilibrium/balance between the external water medium and the internal watery fluids of animals is fairly easy to maintain.

However, pressure may limit the vertical range of motile forms, although some eurybathic forms (ability to tolerate a wide range of depths/pressures) make daily wanderings of more than 400 meters in range, undergoing pressure changes of more than 40 Atm.

But, pressure differentials do have some implications for those animals that bear any gas/air spaces within their body structures, as we know from our study of the deep diving marine mammals. When we discuss the parameters/theories of scuba diving, we will discuss this factor in much greater detail.

However impressive this range of living conditions appear, very uniform conditions prevail over extensive areas in the oceans. Many organisms are adapted for living in these unvarying conditions; they are sensitive to minor changes in the conditions. Thus, faunal/floral areas characterized by specific forms can be recognized.

In those specific areas, like in tidepools, there can be a great change in conditions over a relatively short period of time. Those tolerant forms that reside in these environments are adapted so that they are able to be abundant and productive within these fluctuating environments.