168
J. E. G. R A Y M O N T
thickness of the mixed layer; and on the rate of change of the zooplankton population. Cushing believes that grazing in this area of the
North Sea was so intense that practically the whole of the production
was grazed down over the spring outburst so that the final standing crop
was extremely small. Other models have been developed recently by
Steele and Baird (1962), but it is agreed generally that until we have
greater knowledge of the precise physiology of algae, and especially of
the factors affecting grazing and the rates of change of the zooplankton
population, we cannot obtain much more accurate pictures of the
seasonal changes in phytoplankton.
VI. ZOOPLANKTON
Although the zooplankton grazes upon the phytoplankton and
therefore temporally and spatially the two populations may show some
alternation, over the broad oceans, areas of rich zooplankton correspond to productive phytoplankton regions (Steemann Nielsen, 1958b,
1962b). At any latitude there is also a marked tendency for shallow
coastal regions and for submarine banks to have a richer phytoplankton
and a correspondingly higher zooplankton crop than deep oceanic
regions. Such observations on zooplankton abundance are founded
upon biomass and not on a rate of production of zooplankt,on. Even
biomass of zooplankton, however, is open to varied interpretation
since the methods for sampling zooplankton are by no means accurate
or uniform (cf. Zooplankton Symposium, 1962).
A. METHODS FOR ESTIMATING THE STANDING CROP O F ZOOPLANKTON
Essentially almost all methods for estimating zooplankton crop rely
upon the catching power of fine mesh nets, usually of nylon or silk.
Mesh size is, therefore, of the utmost significance and poses problems of
selection. Whilst a few species of zooplankton may measure several
centimetres in length, few plankton animals exceed a few millimetres in
size. A whole range exists however from those of several miilimetres,
the macroplankton, to the very small-sized protozoans, especially small
ciliates and flagellates, measuring a few p in diameter. The finest silk
nets (200 mesh per inch) will capture the very young, small, stages and
eggs of planktonic animals, as well as rneroplanktonic larvae, the smallest
copepods, appendicularians, an'd similar small animals, providing
quantitative estimates. Such fine nets filter too slowly and too small a
volume of water t o sample quantitatively the larger, more active, zooplankton - euphausids, the larger copepods, chaetognaths, mysids and
especially large pelagic decapods. Relatively coarse nets can be used for
the macroplankton, but between these two extremes every mesh size
selects a particular range of zooplankton. Even the finest silk nets do
J. E. G. R A Y M O N T
thickness of the mixed layer; and on the rate of change of the zooplankton population. Cushing believes that grazing in this area of the
North Sea was so intense that practically the whole of the production
was grazed down over the spring outburst so that the final standing crop
was extremely small. Other models have been developed recently by
Steele and Baird (1962), but it is agreed generally that until we have
greater knowledge of the precise physiology of algae, and especially of
the factors affecting grazing and the rates of change of the zooplankton
population, we cannot obtain much more accurate pictures of the
seasonal changes in phytoplankton.
VI. ZOOPLANKTON
Although the zooplankton grazes upon the phytoplankton and
therefore temporally and spatially the two populations may show some
alternation, over the broad oceans, areas of rich zooplankton correspond to productive phytoplankton regions (Steemann Nielsen, 1958b,
1962b). At any latitude there is also a marked tendency for shallow
coastal regions and for submarine banks to have a richer phytoplankton
and a correspondingly higher zooplankton crop than deep oceanic
regions. Such observations on zooplankton abundance are founded
upon biomass and not on a rate of production of zooplankt,on. Even
biomass of zooplankton, however, is open to varied interpretation
since the methods for sampling zooplankton are by no means accurate
or uniform (cf. Zooplankton Symposium, 1962).
A. METHODS FOR ESTIMATING THE STANDING CROP O F ZOOPLANKTON
Essentially almost all methods for estimating zooplankton crop rely
upon the catching power of fine mesh nets, usually of nylon or silk.
Mesh size is, therefore, of the utmost significance and poses problems of
selection. Whilst a few species of zooplankton may measure several
centimetres in length, few plankton animals exceed a few millimetres in
size. A whole range exists however from those of several miilimetres,
the macroplankton, to the very small-sized protozoans, especially small
ciliates and flagellates, measuring a few p in diameter. The finest silk
nets (200 mesh per inch) will capture the very young, small, stages and
eggs of planktonic animals, as well as rneroplanktonic larvae, the smallest
copepods, appendicularians, an'd similar small animals, providing
quantitative estimates. Such fine nets filter too slowly and too small a
volume of water t o sample quantitatively the larger, more active, zooplankton - euphausids, the larger copepods, chaetognaths, mysids and
especially large pelagic decapods. Relatively coarse nets can be used for
the macroplankton, but between these two extremes every mesh size
selects a particular range of zooplankton. Even the finest silk nets do