Biological Oceanography By Prof. A. Balasubramanian Objectives After attending this lesson, the user should be able to know the importance of biological oceanography as a major branch of oceanography. The concepts of marine life, their ecological conditions, distribution controls and the various interactive mechanisms of marine life will be understood in this lesson. Introduction to Marine environment Seas and Oceans occupy about 71% of the Earth’s surface. These are called as marine environments. The total volume of water under this marine environment is 1370 million cubic km. The space available for marine life is 300 times more than the space available for other aquatic or terrestrial life living on the land. It is also believed that the earliest organisms were originated in saline waters of the ancient oceans, many million years before. Oceanography deals with the physico-chemical characteristics of oceanic waters, their interactions with the atmospheric air, temperature, dynamic movements like tides, waves and currents, habitat for marine flora and fauna found at various zones of seas and oceans. Oceans play a major role in controlling global climate, offering plenty of natural resources and providing food to the global population. The biological interactions among water, air and life are studied under the branch of biological oceanography. Biological oceanography Biological oceanography is a major scientific discipline dealing with all aspects of marine life under different zones of the oceanic environments. The interest to study biology by humans started as early as fourth century BC when Aristotle described about 180 species of marine animals. The geographical knowledge of oceans got improved after several great sea expeditions conducted by the people from 15th to 16th centuries. Through Ocean explorations people conducted detailed underwater surveys and mapped the ocean floors with respect to their physical features, chemistry and biological conditions. The First Texts of Oceanography The first texts of Oceanography were published by Charles Wyville Thomson in 1873, entitled as the “The Depths of the sea”. Through the Challenger Expeditions of 1872, people traveled for 110, 900 km and visited all the major oceans of the world , except Arctic. The Challenger expedition also attempted to integrate the geology, biology, chemistry and the physico-chemical phenomena of the oceans. It was carried out for about 19 years by a group of 76 scientists. The first seafloor map was produced with details of life existing at greater depths. About 715 new genera and 4417 new species of marine organisms were described by the great German Biologist Ernst Haeckel. Since 1872, different countries have been engaged in conducting many major biological oceanographic expeditions. The study of marine life has offered more and more data and scientific facts about the dynamics of oceanic environments. Historical development Edward Forbes, a British Naturalist (1815-1854) is the founding father of Oceanography. He has systematically studied the marine biota and benthic marine animals. It was he, who specified that different biological species occupy different depth zones of the seas and oceans. His Nephew, James Ross, during 1839-43, collected the samples of benthic animals as deep as 730 m, and gave a lot of Page 1 of 9 information to others about the existence of marine life at deeper levels. When facilities were very limited to carry out any underwater explorations, this was done. Ecology of marine life was understood and it become a major part in the study of oceanography and biology. Component of marine environment Marine ecosystems are characterized by both biotic and abiotic components. The dominant biotic components are organisms and their species, predators, parasites, competitors and Mates. The dominant abiotic components (ie., the physical and chemical components) are temperature, concentration of nutrients, penetration of sunlight, turbulence and turbidity, salinity and density of water masses, climatic parameters including the action of wind. Marine ecosystems are typical environments ranging from a small tidal inlet to the deep water ocean basins. Depth of water column is a major factor in addition to its movement and circulation. Solar and atmospheric interactions are very important aspects in the biological oceanography. Conditions of marine environment The marine environments show altogether a different kind of physical condition for their life to survive. The major factors influencing the oceanic waters are their temperature, transparency, salinity and density which vary with reference to space, time and depth. The ocean water temperature decreases and light penetration diminishes with reference to depth in oceans. the hydrostatic pressure of water increases with depth. The nutrients become more concentrated when the depth increases. Conditions of Marine Life Water , which is a fundamental constituent of all living organisms, is available in plenty , within the marine environments. Marine life species are buoyed up by moving water and need not have to store a large amount of energy in their skeletal material. Majority of the marine plants are also floating species. Size-wise, they are microscopic. Many of the marine animals are invertebrates. They do not possess massive skeletons. For the purpose of floating and swimming, the marine animals require very little energy. Temperature and light penetration Temperature of oceanic waters is an important parameter which do not vary as drastically as seen on the land masses or as seen in the atmosphere. However, there are certain properties which are less favourable for life to survive in the seas and oceans. The growth of plants in the sea is limited by the availability of sun light. It is a fact that 50% of the total solar radiation which penetrates into the sea surface gets disappeared rapidly with reference to depth. Much of the marine environment is under perpetual darkness. Under such circumstances, the entire marine life depends on the availability of essential nutrients only. It also adds to the release of decayed organic matter, in huge volume, within the seas and oceans. Much of the decaying matter sinks inside the seas and gets mixed up or deposited. The zone just below the sea surface experiences the maximum environmental fluctuation. This is the zone where more air-water interactions exist. The same surface zone experiences much variations in temperature, salinity and turbulence of water from the winds. All the environmental parameters also show distinct vertical variations within the oceanic water masses. Because of these variations, it is necessary to classify the marine zones for any detailed analysis. Classification of marine zones Page 2 of 9 Marine environments can be subdivided into two major divisions as Pelagic and Benthic environments. The word “Pelagic” means ‘Open sea’ and the word “benthic” means ‘bottom’. The Pelagic condition refers to the ocean water column starting from the surface of the oceans and ending at the greatest depths. The Benthic condition refers to the conditions of the deep ocean floors. In the open sea, the zone extending from the high water and low water coastal belt, upto a depth of 200 m inside the sea, is called as the Neritic zone. Pelagic zone The Pelagic zone is further subdivided into five major layers, as epipelagic, mesopelagic, bathypelagic, abyssopelagic and hadal zones. The Epipelagic zone is the zone of water column that is existing upto 200m down from the ocean surface. The Mesopelagic zone is the zone that is existing upto 1000 m down below the epipelagic zone. The Bathypelagic zone is the underwater zone that is existing from 1200 m to 4000 m down below the ocean surface. The Abyssopelagic zone is the zone that is existing beyond 5200 m depth and upto 6000m from the oceanic surfaces. The Hadal Pelagic zone is the zone of deep water extending beyond 6000m and may go upto 10000 m below the ocean surface. Biological oceanography deals with the physic-chemical and biological conditions of marine life of all these zones. Benthic zone The Benthic environments are classified into five zones as supra littoral, littoral, sub-littoral, bathyal, abyssal and hadal types. The Supra littoral zone is the zone existing on the beach with the high tide water line. The Littoral zone is the high water to low water tide line region. The Sub littoral zone refers to the zone of low tide water line to 200m depth on the continental shelf region inside the sea. The Bathyal zone is the zone ranging from 200 m to 3000 m depth of water column inside the sea. The Abyssal zone is ranging from 2000m to 6000m depth inside the sea and it exists mostly on the continental slope regions. The Hadal zone is the life zone existing beyond 6000 m of water depth. This zone may go upto the end of 10000+ m depth of water column in the deep ocean basins. This is the last and dark zone of the oceans. Biological oceanography deals with all the living conditions of marine life and their biological processes prevailing in all these zones. Ecology of seas The Pelagic environment supports the life of all Planktons and Nektons. It is seen that the epipelagic zone is almost a sunlit zone. Enough amount of sun light penetrates into the top layer of oceans. This situation helps all floating plants to carry on their photosynthesis. The mesopelagic zone is known as the twilight zone. This is a dim zone where there is very little light penetration. This condition is not suitable for many plants to grow. The bathypelagic zone is known as the midnight zone. This is the layer between 1000m and 4000m depth below the sea surface , where there is no penetration of light. The abyssal zone is the pitch-black bottom layer of the oceans. The oceanic water masses present in this zone are also at freezing temperature and with great pressure. The Hadal zone is the last deepest zone.