The Living Element Life first made its appearance in the oceans and it was there that it evolved into the main taxonomic categories. That is why all the higher taxonomic categories are present in the marine environment, even though there are far fewer marine species than terrestrial ones. On land, the conditions of heterogeneity in space and time caused a higher speciation rate (or biodiversity) and that is why today the vast majority of living species is terrestrial. The oceans and the seas are nevertheless inhabited by many species, differing greatly in appearance but differing even more in function. The aim of this section is to give a brief overview of this variability and to describe the adaptations which marine organisms have developed in order to cope with different situations (and their intrinsic problems) in the marine environment. Plankton The term plankton is used to denote those organisms which live suspended in the water column, and are unable to move against water currents. Planktonic organisms are somewhat tentatively divided into two categories: phytoplankton (organisms that acquire energy through photosynthesis) and zooplankton (the animal component). In Figure 12 a systematic list of the main plankton groups is presented. Phytoplankton Phytoplankton (phyto = plant, plankton =wanderer) organisms are usually very small unicellular organisms that, in some species, form filaments or undifferentiated sets of cells (thallus). Phytoplankton is the main food source and as such, provides energy to the entire marine ecosystem. Phytoplankton grows by means of nutrients such as inorganic salts, nitrates and phosphates, solar energy and carbon dioxide (CO2). Photosynthesis can only take place in the depth layer that has enough light to run this process. This zone is called the "euphotic zone". In temperate areas, the optimal doses of the first two of the these factors occur in spring and autumn, and there then frequently results a population explosion (sometimes termed bloom) i.e., a rapid increase of the amounts of phytoplankton organisms in sea water. This bloom is very important for a wide range of organisms which try to synchronize their egg-hatching period to it so that their newly-hatched offspring (larvae) will have enough food for the first stages of growth. However, when extremely high concentrations of phytoplankton occur in conjunction with other factors, for instance limited water circulation, there is a possibility of an excessive production of secondary metabolites (toxins) which will lead to a subsequent poisoning of a wide range of marine organisms over an extensive area. These large temporary concentrations dominate the community and this can result in discoloured water which has given rise to the term "red tides".". Zooplankton In this category are included many animal species, usually very small in size, some of which remain planktonic organisms (holoplankton) throughout their entire life cycle; some pass through as young larval individuals which belong to larger species. After the hatching-out stage, the latter spend one stage of the life cycle as planktonic organisms (meroplankton), and after metamorphosis, they adopt a completely different life mode and are incorporated into nekton or benthos. Zooplankton organisms feed on phytoplankton or detritus. Because of their limited mobility, both phyto- and zooplankton organisms have to deal with the problem of sinking. It is therefore necessary for these to have a specific body weight as close as possible to that of sea water. Since cytoplasm (the fluid in the interior of cells) is a just a little heavier than sea water, it must be counterbalanced by the formation of oil (or fat) droplets. This is also why they have developed long thin appendices along with a very small body so that the ratio body surface/body volume becomes maximal since friction which slows down sinking is proportional to that ratio. Other relevant adaptations that have taken place are the minimization of the weight of the skeletal parts, and the excretion of gas retained in various compartments which act like floats. Nekton Organisms whose swimming capacity permits them to move actively in the water column and to move against the direction of the currents form the group known under the collective name of Nekton (Figure 14). In this category are included all the fish,marine mammals (whales, dolphins, seals), sea turtles, some bird species likepenguins, as well as some invertebrate species like Cephalopods (cuttlefish, squids), and shrimps etc. The majority of the Nekton species consists of carnivorous organisms which find their prey over a wide area. Herbivorous species like the anchovy (Engraulis encrasicolus), however, play a major role in upwelling areas of the sea where enormous masses of fish can be harvested. Since swimming speed is an important feature of their life, their whole body structure is characterized by hydrodynamic shape (so that it presents minimal resistance during swimming), well developed muscles and special adaptations which ensure flexibility and easy navigation (fins, tail). Two important adaptations of the most developed fish are the swimbladder and the lateral line. The former is an internal sac filled with air which ensures neutral buoyancy while the latter is a formation growth on both sides of the body (see Figures 15 a & b) where special neural receptors enable the fish to feel the relative movement of the water and consequently the relative speed and direction of neighbouring fish. This latter adaptation is of great importance, especially for fish that form schools where, when they sense danger, all react simultaneously and in the same manner. Many fish species, in order to increase the number of their offspring, have adopted a particular migratory pattern (Figure 16): the mature adult individuals form schools and move together in areas called spawning grounds where they mate and lay their eggs. After hatching the juvenile individuals, usually aided by water currents, move on to areas of increased productivity (nursery grounds). There they spend a certain period of their life until they grow big enough to migrate to the areas where the adults live (feeding grounds). Nursery grounds are usually shallow areas more or less protected from wave action. These areas are of great importance since any disturbance, for instance by a pollution event, could have a severe impact on a large fish population.

Most of these bacteria are aerobic, i.e., they need oxygen for the oxidation process, while a few are anaerobic, that is, they are capable of living without oxygen.

Different decomposing bacteria are involved in different parts of the chemical element cycles in the marine environment. For instance the bacterial species Nitrosomonas oxidizes ammonia into nitrite sand then Nitrobacter another bacterium, oxidizes nitrites into nitrates, Beggiatoa oxidizes H₂S in elemental sulphur while Thiobacillus transforms elemental sulphur in sulphates.

Mineralization is an important aspect of bacterial activity in the marine environment. Various essential elements are bound in organic compounds contained in "dead material". Bacterial activity transforms organic compounds into inorganic nutrients (nitrates, phosphates etc.) necessary for the development of phytoplankton.
Many marine organisms feed on the bacterial biomass including their mucus, i.e., on live bacterial cells of higher nutritional quality than some forms of dead organic material.

Temperature, pressure and radiation are important factors affecting bacterial activity. At very low temperatures bacterial decomposition is slow, it accelerates with rising temperature, while at very high temperatures (higher than 40^oC) a few bacterial species (thermophilic) can carry out the full cycle of their activities. High pressure is also an inhibiting factor and that is why at great depths only certain types of bacteria (barophilic) are encountered. Lastly, ultra-violet radiation has a lethal impact on bacteria, but this effect concerns only the water surface layer since as has already been mentioned, U.V. radiation is absorbed there.

Birds

Seabirds play a growing role in the investigation of marine ecosystems and food chain dynamics. Especially their interaction with marine fishes and the fishery are regarded of growing economic importance."Seabird" is a rather loose term traditionally used to cover those birds which obtain at least part of their food from the sea by travelling some distance over or under its surface. They typically breed on offshore or coastal areas like cliffs, dunes, skerries or remote islands. Some 274 species belong to seabirds, comprising mainly penguins, albatrosses, fulmars, petrels, shearwaters, pelicans, cormorants, skuas, ducks, terns and auks. Seabirds are characterized by longevity (high adults survival rate), high age at first breeding, slow reproductive rate and intense care for the offspring.

Seabirds are highly visible wide-ranging upper trophic level consumers that can indicate marine productivity and biotic interaction. Compared with fish, marine mammals and other marine animals that live primarily or exclusively underwater, seabirds are easy to survey, census and study. They are of high public interest and play a major role in raising public attention to environmental questions. Ë2Câþ@&Àl