Zooplankton Production
An approach to the problems of organic production of the sea is possible through a study of zooplankton production, since the animals of this group are in general the chief grazers and must fluctuate with the supply of their food (cf. p. 899). They are vastly more numerous and regularly distributed than the predatory animals and any fluctuations in population numbers are not influenced by fishing.
That the production of zooplankton is indeed large may be seen from Redfield's analysis (1941) of the standing calanoid population of the Gulf of Maine. The average monthly catch for all sections of the Gulf was about 40 ml of water-free plankton per square meter of surface. The highest monthly value, 90 ml, was obtained in September and the lowest value, about 10 ml, was obtained in May. Water-free or dry plankton volumes were obtained by filtering the water from the animals. This allows them to become more compact than when measuring the volume by precipitation in fluid. The latter method gave volumes usually about 4.9 times greater.
On the basis of the above water-free values the standing crop for the whole Gulf would be some four million tons. The crop increased by about 80 ml of water-free plankton per square meter between May and September, which is a net gain of about eight million tons. In the English Channel the maximum standing crop for zooplankton in 1934 was calculated (Harvey et al, 1935) at about 40 mm3 per 100 liters. Considering the depth at 100 m, this would be about 40 ml/m2 of surface, or the average annual value found for the Gulf of Maine. If the zooplankton production is to be considered a measure of the phytoplankton production, many factors tending to vary the volume of zooplankton in relation to the plants have to be taken into account. We enumerate the four following:
(1) Much of the zooplankton is consumed by predatory plankton feeders that live regularly in the area of production or that enter it as invaders from less productive areas. The extent of such consumption is not known, but we know, for instance, that rich herring hauls are correlated with abundant Calanus, and that plankton-feeding whales also seek out productive waters such as those of the Antarctic and of northern coastal banks that are rich in their favorite food (p. 905). It has been estimated that on an area 8,000,000 km2 there may be as many as 300,000 whales. The food required by these animals alone must be enormous. They may grow to a length of nearly 28 m and a weight of 80 tons in about two years. Their resting metabolism requires 38 l of oxygen per minute (p. 907). The energy required for this rapid growth and katabolic processes must make notable inroads on the animal plankton.
(2) Many of the plankton animals eat more plants than they can use in their metabolic processes and thus pass through the digestive tract much undigested material during periods of high plant production. This waste makes it impossible to assign a fixed value to the amount of food that animals of a given weight should consume per day to grow and repair breaking-down processes within certain temperature ranges. If a figure for consumption is to be obtained for filter-feeding forms it would
(3) Few plankton animals are strictly plant eaters, but instead have a mixed diet of plants and animals or of detritus arising from both, while yet others are wholly carnivorous. The direct relationship of the zooplankton production to the phytoplankton as a source of energy is in this way obscured and separated in time.
(4) Finally, much of the phytoplankton production is not consumed by planktonic animals but instead sinks to the bottom to serve as food for benthic animals or dies and becomes food directly for the bacteria without the intervention of animals at all.