Energetics of a marine ecosystem

Living organisms, unlike machines, cease to exist once they stop working. All biological activity depends upon continual transfers and transformations of energy, without which any natural living system almost immediately disintegrates irreversibly. We will now draw together some of the information from preceding pages in an elementary consideration of certain energy relationships of marine life, with particular reference to shelf seas around the British Isles.

Directly or indirectly the source of all energy for life is the sun, which continually emits radiant energy into space. A tiny fraction of this radiation reaches the earth, where a considerable part is lost by reflection from the earth's atmosphere, clouds and surface. Probably a global average of about 40 per cent of the incoming radiation is reflected. The remainder is absorbed by the atmosphere and the land and ocean surfaces, where its main effect is to cause the heating which generates the movements of atmosphere and ocean. However, despite the continuous absorption of solar radiation, the climate does not appear in the long term to become hotter. This indicates that there is overall an output of radiant energy from the earth equal to that received, and the total heat content of atmosphere, surface and oceans remains virtually constant except for minor fluctuations due to the elliptical form of the earth's orbit round the sun and to changes in solar activity (for example solar flares or sunspots). The incoming energy is received largely at wavelengths within the visible spectrum. The balancing emission from the earth is low-frequency heat radiation which passes out in all directions of space.

The routes through which light energy can flow between penetrating the earth's atmosphere and re-radiation into space as heat are numerous and complex. A small amount, probably only about 1-2 per cent of the light energy reaching the earth's surface, enters pathways beginning with the absorption of sunlight by plants in photosynthesis. In this process radiant energy is transformed to chemical energy by an energy-fixing reduction of carbon dioxide. For instance, the synthesis of 1 mole of glucose from carbon dioxide and water involves the intake of 673 kcal (2826.6 kJ) of light energy.


(Light energy)

This energy is then available in biological processes, for when 1 mole of glucose is oxidized in respiration, 673 kcal (2826.6 kJ) of energy is released. It is by means of transfers and transformations of the energy of chemical compounds formed initially by photosynthesis that power is provided for the activity of living organisms. The movements of materials involved in nutrition occur almost entirely as means of effecting energy transfers. The global total of energy fixation by photosynthesis determines the total amount of biological activity which the earth can support. The intake of radiant energy into the living system by photosynthesis is balanced by a corresponding outflow of energy as heat through pathways of respiration and movement.

We have insufficient knowledge of the energy relationships of marine organisms to be able to trace with much certainty the passage of energy through marine ecosystems, but until we can do this our understanding of the food webs of the sea must remain at an elementary stage. As an indication of some of the processes involved we will attempt in this chapter a simple analysis of the energetics of production and feeding in the shelf waters of the north-east Atlantic, exemplified by the English Channel. No reliance must be placed on the figures given, which should only be regarded as reasoned guesswork based on a modicum of firm information. The exercise is illustrative rather than factual. Obviously, for simplicity, innumerable interactions within the food web have been ignored, and by averaging out all values over a period of a year no account has been taken of the highly fluctuating nature of the system. Nevertheless, something may be learned from critical examination of the figures and from comparison with various sources of data relating to these considerations. The student is recommended also to read the examination of food webs in the North Sea given by Steele (1974) in his book The Structure of Marine Ecosystems. Excellent explanations of the concepts contained in this chapter can also be found in Odum (1971, 1983).

The analysis is summarized diagrammatically in Figure 7.1. Energy has been quantified in calories (kcal) because this system has hitherto been most widely used in marine biological literature. The relationship between the various units of energy measurement used in the literature is as follows:

1 Calorie = 1000 calories (or 1 kcal)

1 Calorie = 4186 joules (1 calorie = 4.2 joules)

A calorie is the amount of energy needed to raise the temperature of 1 g of water by 1°C. Note that the thousand-unit Calorie is spelt with a capital C. Here the notation kcal is used to avoid possible confusion.

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