The detritus chain

From the preceding sections we have derived several figures for energy content of various contributions to the organic detritus reaching the sea bottom. To these must be added a small amount to represent the bodies of dead pelagic predators sinking to the sea floor, which we will take to be equivalent to 20 per cent of tertiary pelagic production, giving a further 0.2 X 17 = 3.4 kcal-m~2-yr~\ These energy sources in detrital form can be summarized as follows:

This gives us a figure of 378.4 kcal-m_2-yr_1 for food energy reaching the shallow sea bottom from several trophic levels in the water above.

Examination of the composition of shallow-water sediments often reveals a significant additional amount of organic matter recognizably derived from the seashore or the land, mainly fragments of large algae, wood or leaf. Also, where sufficient light reaches the bottom there will be primary production by plants, the compensation depth for benthic diatoms and other algae being generally lower than for phytoplankton. There will also be a slight contribution from chemosynthetic autotrophs, and also from saprophytes utilizing dissolved organic matter which may have been produced by the phytoplankton but does not register

From primary production

From secondary production From tertiary production sinking, uneaten phytoplankton phytoplankton eaten but egested dead, uneaten zooplankton zooplankton eaten but egested dead pelagic predators in the 14C estimate of primary production. We must therefore make some addition to the energy available for biological processes on the sea bottom to allow for these sources. On the other hand we must make some deduction for organic matter lost from the living system by permanent inclusion within the sediment or by oxidations not effected by living organisms. There is as yet insufficient evidence to permit any strict quantification of these energy transfers. We will assume that 2 kcal-m-2-yr-1 becomes permanently incorporated in the sediment, and that a round figure of 400 kcal-m-2-yr-1 is the food energy utilized by benthic organisms.

Some of this energy is directly available to the benthic fauna by digestion and assimilation of detritus, but many detritic materials reaching the bottom cannot be digested by animals. These materials are acted upon by bacteria, which utilize these energy sources to multiply rapidly. This production of bacterial protoplasm contributes a significant proportion of the food of benthic animals. We have insufficient knowledge of the feeding metabolism of the benthos to make a reasoned generalization about the proportions of energy derived from detritus directly by digestion and assimilation of organic debris or indirectly via consumption of bacterial protoplasm, but some experimental work suggests that bacteria contribute much the greater part. We shall assume that 25 per cent of the energy intake of the benthic herbivores comes directly from detritus and 75 per cent from the consumption of bacteria.

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