Seas and oceans occupy ¾ of the Earth, covering approximately 70% (seventy percent) of the surface of our Planet. The average depth is 4 km, but the “Fenda das Marianas”, in the Pacific Ocean, is about 11 km deep.
The main abiotic factors in marine environment are: light, temperature, salinity and hydrostatic pressure.
When we compare the marine environment with the terrestrial we find a minor variation regarding these factors. Temperature and salinity can be identified as the two main abiotic factors in the oceans.
The surface temperature of the water varies according to the seasons of the year and the latitude; however, this variation is smaller than the one observed in terrestrial environments, because the oceans retain greater amounts of heat, which is released more slowly than on Earth.
The average salinity of the sea is 3.5% (35 parts in a thousand). In coastal areas it is usually less and tends to present a greater variation, since it also suffers from the influence of rivers and rain water from the coast.
We can classify the marine environment as to:
Penetration of light:
– Euphotic Zone: well lit and therefore rich in algae (autotroph beings) and animals.
– Disphotic Zone: dimly lit; the “loss” of light occurs due to faster absorption of the long wavelengths (red and yellow) and slower absorption of short wavelengths (green and blue).
– Aphotic Zone: no light at all and no living being capable of photosynthesis, only heterotrophic beings are found.
– Coastal Zone or Zone between tides: area affected by the tides, sometimes emerged, sometimes submerged; it is well lit, oxygenated, rich in nutrients; there are plenty of organisms fixed onto rocks, such as barnacles and mussels, highly adapted to this way of life.
– Neritic Zone: is the region over the continental platform; its depth increases gradually, reaching 200 meters; it is the area of greatest economic importance, because of the immense richness of plankton and nekton, mainly large shoals of fish.
– Oceanic Zone or Bathyal Zone: corresponds to the steep declivity that occurs after the continental platform, 200 to 2,000 meters deep, occupying the so called continental slope; due to the absence of light there are no algae and animals are reduced.
– Abyssal Zone: an area with still water and evenly cold without any light. It spreads from 2,000 meters to the deepest areas with some species extremely adapted to high pressures, complete absence of light and consequent shortage of food; organic remains fall slowly from above towards the bottom providing the use of organic matter by animals of lower layers, as well as serving as food to saprophytic animals and decomposing bacteria; the animals of this region exhibit bioluminescence, have a very sensitive eyesight, able to react to small light stimuli, and very bizarre forms, like large mouths and teeth, to facilitate the capture of prey, and expandable stomach.
Ecologically the marine organisms can be divided into:
Plankton – organisms that live in suspension in water and are swept away by waves and currents; they are divided into phytoplankton represented by organisms capable of photosynthesis and therefore are producers of the food chain and responsible for the production of more than 90% of oxygen released on the planet (algae) and zooplankton represented by heterotrophic organisms (larvae of many invertebrates, of fish, small crustaceans etc), primary and secondary consumers in the food chain and that serve as food for other animals.
Benthos – organisms that live attached (sessile) to the substrate or depend on it for survival, is represented by countless seaweed and by a wide range of invertebrates (sea sponges, sea anemones, corals, worms, various mollusks, crabs, starfish, sea urchins, ascidia etc ) and fish (neon-goby, maria-da-toca etc).
Nekton – formed by free-living organisms that manage to beat the currents; represented primarily by octopus and squid (cephalopods), fish, marine mammals (whales, dolphins, seals etc) and seabirds.
We can divide the marine environment into systems:
– Benthic: is the habitat of the submarine substrates, whether natural, biological, artificial or non-living; the organisms that colonize these substrates, both during its entire existence or only in one phase of the life cycle, are collectively named benthos; host the largest part of the biological diversity of the oceans, as well as being the final destination of the pollutants that are released in the seas by man.
– Pelagic: comprises the field of water masses, colonized by planktonic and nektonic organisms, or simply plankton and nekton; it houses the main primary producers in the ocean and most of the fish caught by man.