Some of the food ingested by the planktonic herbivores is not fully digested and absorbed but passes through the gut and is egested, contributing to the fall of organic debris to the sea floor. We will assume that 10 per cent of the energy content of the food consumed goes in this way to detritus. Of the assimilated food, most is used for respiration and the remainder forms new animal tissue. Herbivorous planktonts are relatively efficient converters of food to new tissue, with some early larval stages apparently using about 50 per cent of food intake for growth. The general level of efficiency is certainly lower than this (Paffenhofer, 1976; Reeve, 1969) and we will assume that 70 per cent of food intake is used for respiration and 20 per cent for secondary production. We will call this a gross conversion efficiency (GCE) of 0.2, where calorific value of new tissue formed calorific value of ingested food
We can summarize the energetics as follows:
Energy inflow by grazing 10% egested unassimilated
960 kcal-m^-yr"1 96 kcal-m^-yr"1 to detritus
20% utilized for secondary production
70% utilized for respiration and movement
672 kcal-m 2-yr 1 lost from the system 192 kcal-m-2-yr-1 available for predators
Calorific values for copepod and euphausid species occurring around the British Isles are generally about 5.0 kcal g-1 dry wt. Using this figure, secondary production amounts to 192/5 = 38.4 g dry wt-yr \ or (if 1 g dry wt = 0.44 gC) 38.4 X 0.44 = 16.9 gC-m-2-yr-1.
General observation of plankton samples taken around the British Isles suggests that a mean value for biomass of standing stock of herbivores must often be appreciably greater than the standing stock of phytoplankton, and some quantitative investigations indicate this. However, taking our value from Harvey's figures (Table 7.1), the mean annual biomass of zooplankton throughout the year in the English Channel is given as 1.5 g dry wt-m-2. At this apparently low value, secondary production amounts to about 25 times the weight of standing crop. This much lower rate of turnover compared with phytoplankton corresponds with the slower rates of growth and reproduction of zooplankton.
Some herbivorous planktonts die and reach the bottom uneaten by pelagic predators, but probably the great majority are consumed by carnivorous zooplanktonts or pelagic fish such as herring and mackerel. Assuming that dead pelagic herbivores sinking to the bottom amount to a little over 10 per cent of secondary pelagic production, this adds approximately 22 kcal-m-2-yr-1 to sinking detritus, leaving 170 kcal-m-2-yr-1 for consumption by pelagic carnivores.
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