biological oceanography

biological oceanography is the study of all aspects of the biology of the oceans particularly in the context of their physical and chemical environments, so that it overlaps with marine biology in many respects. The range of living organisms extends from the smallest living things, like viruses and bacteria, to the largest animal ever to have lived on earth, the blue whale (Balaenoptera musculus). This vast size range also reflects a wide range of life cycles—a bacterium's lifetime may be just a few hours during which it experiences the conditions in just a few millilitres of water, whereas a whale, which may live 50 years, will have repeatedly migrated halfway round the world. Techniques used to study bacteria and whales are totally different, yet the aim is to blend information for all organisms into an overall conceptual understanding of life in the oceans.

Life occurs at all depths, and there is 180 times more living space in the oceans than in terrestrial habitats. On land one can stand on a hilltop and survey the landscape, whereas oceans are remote, out of sight, and difficult to study. The classical questions investigated by biologists are: what species live where and when? How big are the populations? How are these species organized into communities? What are the dynamics of these populations and how do they cope with their environment's challenges? How are they responding to mankind's activities? Such questions cannot be studied in isolation from the other disciplines in oceanography. Biological processes not only respond to physical and chemical processes but also play a central role in many biogeochemical cycles, especially the fate of pollutants and the dynamics of the carbon cycle (see environmental issues). It is important to understand how marine communities are responding to fluctuations in ocean climate, how changes in the morphology of ocean basins have been reflected in the zoogeographical distribution of species today, and what the micro-fossils in ocean sediments can tell us about the climate changes of past eras.

Whether a study focuses on a single species, like a whale or a species of fish, or takes the broader approach of focusing on communities and how they change throughout the year, the organisms need to be sampled and quantified. Sampling the tiniest organisms (viruses, bacteria, and marine plants like phytoplankton) involves collecting samples of water and filtering them. The extracts are either examined by microscopy or plated out to grow cultures. The volume of water processed may be less than a litre, so studies of these micro-organisms are akin to studying the whole of the Sahara Desert by examining a few grains of sand. Living cells have to be distinguished from the abundant inanimate particles and aggregates if they are to be counted. The surfaces of bacteria suspended in the water are highly active chemically. The total surface area of bacteria, suspended in the water or attached to particles, greatly exceeds that of the inanimate particles, so bacteria play a major role in many aspects of water chemistry.

The tiniest animals, protozoans and their relatives, are equally hard to sample, identify, and quantify, and yet their activities are the key to understanding how the sun's energy fixed by the phytoplankton gets passed through to all parts of the oceanic ecosystem. Plankton is sampled with various types of nets. Small fine-meshed nets are used to catch animals about a millimetre in length, and much larger coarser-meshed nets to catch the faster-swimming Crustacea like shrimps, and small fishes a few centimetres long. Larger animals can swim faster and so can dodge slowly towed nets. Even so, many of the abundant species are gelatinous and fragile, and when caught disintegrate and so are essentially ‘invisible’ to us. They have to be collected individually by underwater vehicles or photographed in situ. In contrast, the larger animals, game fishes like tuna and marlin, sharks and whales, are too large and too fast to be caught in conventional nets.

Studying the animals that live on the seabed (the benthos) is equally challenging. A few millilitres of surface sediment from the bed of the ocean at a depth of 4,000 metres (13,000 ft) may contain 100 species of both nematodes (round worms) and foraminifers (protozoans), but huge trawls need to be towed many kilometres along the seabed to catch the bigger animals. But even the largest trawls cannot catch large sharks like the six-gilled shark (Hexanthus) and the mysterious megamouth (Megachasma pelagios). All these organisms occur irregularly in space and in time, so how representative is each sample? How can data from a few litres or even thousands of cubic metres of sea water be related to what is happening in the vast volumes of the oceans? It is clear that the oceanic communities in the water and on the seabed are zoned by depth, and they vary with latitude and within different parts of ocean currents. In addition, communities inhabiting coastal seas are very different from those living in and over very deep water.

Large-scale coverage of gross biological characteristics can be achieved by remote sensing. At finer scales sonars of different frequencies can discriminate variations in the concentrations of particles in the water, many of which are living organisms. Particle counters that continuously count abundances of animals and marine plants within prescribed size ranges can be towed or deployed on moorings. These techniques neither identify the species nor discriminate between live and inanimate particles. However, they can be used to survey the variations in the gross distributions of the organisms and how they are influenced by currents and eddies, and vertically by the thermocline. They can be used to estimate the living mass of organisms in the water, how fast they are growing, and how energy is flowing through the ecosystem. But these techniques are akin to working out what a large painting looks like from a few dozen pinpoint samples of the colour and chemistry of paint. Computer models based on theories of how the ecosystems function are now being used to fill in the unavoidable gaps in the sampling to give some idea of what the big picture may look like. The strengths and weaknesses of the models are evaluated, so they can be tuned to postdict known events and patterns. Ideally these tuned models will be effective in other scenarios, but this ideal is seldom if ever achieved. These approaches need the largest and fastest computers as well as some of the most sophisticated instrumentation to provide the real data needed to validate or to modify the theories.

Conducting biological surveys of the seabed is even more difficult, although enormous strides have been made with the development of sophisticated underwater vehicles, which can be used remotely to survey and conduct experiments on the seabed, or can even carry the scientist down to observe, sample, or set up experiments there. This task is made all the more demanding because there are twice as many of the basic types of animal (phyla) living on the seabed as in terrestrial environments, moreover the numbers of species found in tiny sediment samples is staggeringly large. Estimates of how many species may inhabit the seabed of the deep ocean range from a million to over a billion; if the latter estimate is correct then the deep ocean must be far richer in species than are all terrestrial habitats. A biologist may spend a lifetime naming a few hundred novel species and yet make almost no impression on the immense task of identifying the total inventory of fauna and flora. Yet the potential value to man of some of these organisms, particularly some of the bacteria, which are unique to the deep ocean, as sources of new marine pharmaceuticals is enormous.

Perhaps the most unexpected discovery made using underwater vehicles has been the large communities of animals that inhabit hydrothermal vents, which emit superheated, sulphide-laden waters. These communities are exceptional in that they are based on chemical synthesis to produce their organic material. Worms and shellfish exploit internal gardens of bacteria to provide their energy, which is derived from oxidizing sulphide ions or methane contained in the vent waters. The bacteria involved are of very ancient lineages, and may have been linked to the origins of life on earth.

Another important discovery has come from using time-lapse cameras deployed on the seabed for several months. The sequences of pictures have revealed the rapidity with which plant material produced at the surface reaches the seabed. Phytoplankton cells are microscopic, and so they sink very slowly if at all. A cell that escapes being eaten will take many months to free-fall the 4–5 kilometres (2.5–3 mls.) to the seabed. However, sticky mucilage produced by various animals clumps them together into aggregates called marine snow, and the flakes of marine snow get progressively heavier and sink faster and faster and can reach the seabed within a very few days. Hence life on the seabed is dynamic and seasonal.

Bibliography

Herring, P. , The Biology of the Deep Ocean (2002).
Lalli, C., and and Parsons T. , Biological Oceanography: An Introduction (1997).

M. V. Angel