170
J . E. 0. RAYMONT
laborious and time consuming, and the taxonomic identification of the
great variety of zooplankton species is difficult. Even if this is done,
numbers of various zooplankton animals may not be a satisfactory
index to the crop, the size and biomass of species varying enormously.
Wiborg (1954) and others have estimated the relative volumes of the
major zooplankton representatives, and thus computed the volume of
the crop. Another widely used method is to measure the total volume of
the zooplankton haul, usually by a displacement technique or estimate
of settled volume. This is open to error if the catch contains a considerable proportion of the more gelatinous zooplankton. Many investigators attempt to avoid this error by first removing such animals
as salps, medusae and siphonophores. A pump-and-hose method is used
by some research workers to avoid the errors associated with nets.
While useful for quantitative work in shallow water, it is inapplicable
to oceanic depths.
For the measurement of zooplankton biomass, volume is obviously
imperfect; a better estimate would be wet weight of crop or dry organic
weight. One of the few observations employing these methods is that of
Curl (1962b) who studied the biomass of plankton south of New York.
Curl includes data for converting wet and dry weights, and carbon,
nitrogen and phosphorus content for various species of zooplankton. He
emphasizes that organic weight is the best unit for biomass, but for
general assessments of zooplankton crop it would be difficult to use
organic weight methods. I n any event, the problem of selection of net
mesh size is still a major difficulty. The symposium held on zooplankton
estimation (Rapports et Proces Verbaux, 1962) emphasized the di%%
culties of agreeing on standardization. Wickstead (1963) has recently
suggested that two nets of different mesh size should be vertically
hauled together. The recent discovery of an abundant population of
zooplankton animals living right at the surface in oceanic regions poses
again problems for the correct sampling for this particular layer.
B. REGIONAL CROP ASSESSMENTS O F ZOOPLANKTON
Despite the difficulty of estimating zooplankton, the crop tends to be
richest where primary production and the size of standing crop of
phytoplankton tend to be high (Steemann Nielsen, 1958b; 196213). The
work of Hentschel (1933-36) in the southern Atlantic Ocean demonstrated the remarkable agreement between the quantities of phytoplankton and the crop of zooplankton, despite seasonal and other
fluctuations. A recent survey by Reid (1962) for the Pacific Ocean also
indicates good correspondence between the phosphate concentration of
the upper waters, suggesting high primary production and the volumes
of zooplankton. There are several broad oceanic surveys which demon-
J . E. 0. RAYMONT
laborious and time consuming, and the taxonomic identification of the
great variety of zooplankton species is difficult. Even if this is done,
numbers of various zooplankton animals may not be a satisfactory
index to the crop, the size and biomass of species varying enormously.
Wiborg (1954) and others have estimated the relative volumes of the
major zooplankton representatives, and thus computed the volume of
the crop. Another widely used method is to measure the total volume of
the zooplankton haul, usually by a displacement technique or estimate
of settled volume. This is open to error if the catch contains a considerable proportion of the more gelatinous zooplankton. Many investigators attempt to avoid this error by first removing such animals
as salps, medusae and siphonophores. A pump-and-hose method is used
by some research workers to avoid the errors associated with nets.
While useful for quantitative work in shallow water, it is inapplicable
to oceanic depths.
For the measurement of zooplankton biomass, volume is obviously
imperfect; a better estimate would be wet weight of crop or dry organic
weight. One of the few observations employing these methods is that of
Curl (1962b) who studied the biomass of plankton south of New York.
Curl includes data for converting wet and dry weights, and carbon,
nitrogen and phosphorus content for various species of zooplankton. He
emphasizes that organic weight is the best unit for biomass, but for
general assessments of zooplankton crop it would be difficult to use
organic weight methods. I n any event, the problem of selection of net
mesh size is still a major difficulty. The symposium held on zooplankton
estimation (Rapports et Proces Verbaux, 1962) emphasized the di%%
culties of agreeing on standardization. Wickstead (1963) has recently
suggested that two nets of different mesh size should be vertically
hauled together. The recent discovery of an abundant population of
zooplankton animals living right at the surface in oceanic regions poses
again problems for the correct sampling for this particular layer.
B. REGIONAL CROP ASSESSMENTS O F ZOOPLANKTON
Despite the difficulty of estimating zooplankton, the crop tends to be
richest where primary production and the size of standing crop of
phytoplankton tend to be high (Steemann Nielsen, 1958b; 196213). The
work of Hentschel (1933-36) in the southern Atlantic Ocean demonstrated the remarkable agreement between the quantities of phytoplankton and the crop of zooplankton, despite seasonal and other
fluctuations. A recent survey by Reid (1962) for the Pacific Ocean also
indicates good correspondence between the phosphate concentration of
the upper waters, suggesting high primary production and the volumes
of zooplankton. There are several broad oceanic surveys which demon-