The synthesis of organic compounds from the inorganic constituents of seawater by the activity of organisms is termed production. It is effected almost entirely by the photosynthetic activity of marine plants. The raw materials are water (H2O), carbon dioxide (CO2) and various other substances known as nutrients. The latter are mainly inorganic ions, principally nitrate and phosphate. Chlorophyll-containing plants, by making use of light energy, are able to combine these simple substances to synthesize complex organic molecules. This is termed gross primary production. The chief products are the three major categories of food materials, namely carbohydrates, proteins and fats (Steeman Nielsen, 1975). Oxygen, derived from the water, is produced as a byproduct. The process involves a number of steps but can be summarized by the following very general equation:
Carbon Water Carbohydrate Oxygen dioxide Respiration
Some of the organic material manufactured by plants is broken down again by an oxidative reaction, to an inorganic state by the plants themselves in the course of their respiration. Hence the equation is written as a reversible one. The remainder is referred to as net primary production and much of this becomes new plant tissue. This is of major importance as the source of food for herbivorous animals. The animal population of the sea therefore depends, directly or indirectly, upon the net primary production.
By far the greater part of primary production in the sea is performed by the phytoplankton (Raymont, 1963, 1966). Under favourable conditions this is capable of remarkably rapid growth, sometimes producing its own weight of new organic material within 24 hours, a rate greater than that achievable by land plants. The large marine algae (seaweeds) growing on the sea bottom in shallow water make only a relatively small contribution to the total production in the sea
because they are of very restricted distribution. There is also some primary production by bacterial chemosynthesis (see Section 5.1.4).
According to the simplest concept of feeding relationships, consumption of plants by herbivorous animals leads to the formation of animal tissue. This is secondary production. Herbivores provide food for the first rank of carnivorous animals i.e. tertiary production. These may in turn fall prey to other carnivores, and so on (Figure 5.1). Each of these successive stages of production of living tissue is a link in a food chain, each link being termed a trophic level. Because many animals take food from several trophic levels, food chains become interconnected to form intricate food webs.
Obviously there are large losses of organic material between each trophic level, caused in several ways. For instance, a proportion of the organisms at each trophic level are not eaten by animals but simply die and decompose by autolysis and bacterial action. Some of the food that animals consume is egested unassimilated and most of their assimilated food is broken down by respiration, leaving only a small proportion to form new tissue. The efficiency of transfer of organic matter from one trophic level to the next varies with the types of organisms, herbivores generally doing rather better than carnivores. In broad terms about 100 g of food are consumed for every 10 g of animal tissue formed, i.e. a gross conversion efficiency of 10 per cent (see page 248). Herbivorous zooplanktonts sometimes exhibit efficiencies of about 30 per cent and certain larval stages do somewhat better, but even allowing for these higher efficiencies only a small part of the original plant production becomes incorporated in animal tissue. Thus, primary production can be regarded as the broad base of a food pyramid, the successive smaller trophic levels being a series of steps towards the apex of final rank carnivores.
Eventually, as a result of respiration and excretion, death and decomposition, organic materials become broken down and returned to the water as simpler substances which plants can utilize in primary production. In this way, matter is continually cycled from inorganic to organic forms and back to inorganic state. The initial synthesis of organic material involves the intake of energy to the system, and this is supplied by sunlight. The transference of organic matter from one trophic level to the next is part of the energy flow of the cycle (Figure 5.1), energy being continually lost from the system and in due course becoming dissipated as heat. This is discussed further in Chapter 7.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.