156
J . E . G . R A Y M O N T
are small diatoms or dinoflagellates, but others belong to the Chrysophyceae, Chlorophyceae, and Cryptophyceae. Various methods have
been used to try to estimate nanoplankton densities; for example, the
fixation of samples of sea water, followed by sedimentatiQn and counting with reversed microscope; centrifugation; filtration through
hardened filter papers or through membrane or sintered glass filters. All
such methods have disadvantages. Many of the delicate cells are
destroyed by fixation and by centrifuging, and cells cannot be adequately counted if they are first filtered. Counting of unconcentrated
samples is equally unsatisfactory. Although, therefore, we have no
single method which estimates nanoplankton adequately, it is abundantly clear that the use of fine silk nets to estimate the phytoplankton
as a whole is entirely misleading as a quantitative method. Contributors
to a recent symposium on primary production have indeed stated that
net samples alone should no longer be employed for quantitative estimation (Braarud, 1958). The chlorophyll extraction method probably
gives a fair indication of the standing crop of phytoplankton under
most conditions, since provided a suitably fine filter is employed, both
nanoplankton and larger phytoplankton will be captured. But the imperfections of the chlorophyll method (see page 124) must still be taken
into account.
A comparison of the density of the nanoplankton and of the remainder
of the phytoplankton, often termed the “net” phytoplankton, shows
that nanoplankton may be sometimes important in certain regions. In
more inshore waters at temperate latitudes the net phytoplankton,
especially diatoms, is frequently dominant over much of the year, but
Lohmann (1908; 1911) showed that the nanoplankton could be very
important at certain months. Harvey (1950) for the English Channel
found a very much larger crop by filtration estimates, which included
nanoplankton flagellates, as against fine net samples which would
normally retain only the larger phytoplankton. Gross et al. (1947, 1950)
found that the nanoplankton could form a very appreciable part of the
crop in shallow temperate waters. Yentsch and Ryther (1959) have also
called attention to the importance of the nanoplankton fraction in the
waters off Woods Hole. Small flagellates have been recorded in considerable numbers in the North Sea (Gramtved, 1952) and in the Baltic
area (Steemann Nielsen, 2951). The same author (1935) has recorded
nanoplankton flagellates in cold seas, as did Braarud (1935) around
Iceland, and Halldal (1953) found appreciable numbers of coccolithophores and flagellates in the Norwegian Sea (cf. also Bursa, 1963). More
generally at high latitudes it is the larger diatoms which tend to
dominate the phytoplankton. Although much of the earlier work at
high latitudes did not specifically estimate nanoplankton but relied on
J . E . G . R A Y M O N T
are small diatoms or dinoflagellates, but others belong to the Chrysophyceae, Chlorophyceae, and Cryptophyceae. Various methods have
been used to try to estimate nanoplankton densities; for example, the
fixation of samples of sea water, followed by sedimentatiQn and counting with reversed microscope; centrifugation; filtration through
hardened filter papers or through membrane or sintered glass filters. All
such methods have disadvantages. Many of the delicate cells are
destroyed by fixation and by centrifuging, and cells cannot be adequately counted if they are first filtered. Counting of unconcentrated
samples is equally unsatisfactory. Although, therefore, we have no
single method which estimates nanoplankton adequately, it is abundantly clear that the use of fine silk nets to estimate the phytoplankton
as a whole is entirely misleading as a quantitative method. Contributors
to a recent symposium on primary production have indeed stated that
net samples alone should no longer be employed for quantitative estimation (Braarud, 1958). The chlorophyll extraction method probably
gives a fair indication of the standing crop of phytoplankton under
most conditions, since provided a suitably fine filter is employed, both
nanoplankton and larger phytoplankton will be captured. But the imperfections of the chlorophyll method (see page 124) must still be taken
into account.
A comparison of the density of the nanoplankton and of the remainder
of the phytoplankton, often termed the “net” phytoplankton, shows
that nanoplankton may be sometimes important in certain regions. In
more inshore waters at temperate latitudes the net phytoplankton,
especially diatoms, is frequently dominant over much of the year, but
Lohmann (1908; 1911) showed that the nanoplankton could be very
important at certain months. Harvey (1950) for the English Channel
found a very much larger crop by filtration estimates, which included
nanoplankton flagellates, as against fine net samples which would
normally retain only the larger phytoplankton. Gross et al. (1947, 1950)
found that the nanoplankton could form a very appreciable part of the
crop in shallow temperate waters. Yentsch and Ryther (1959) have also
called attention to the importance of the nanoplankton fraction in the
waters off Woods Hole. Small flagellates have been recorded in considerable numbers in the North Sea (Gramtved, 1952) and in the Baltic
area (Steemann Nielsen, 2951). The same author (1935) has recorded
nanoplankton flagellates in cold seas, as did Braarud (1935) around
Iceland, and Halldal (1953) found appreciable numbers of coccolithophores and flagellates in the Norwegian Sea (cf. also Bursa, 1963). More
generally at high latitudes it is the larger diatoms which tend to
dominate the phytoplankton. Although much of the earlier work at
high latitudes did not specifically estimate nanoplankton but relied on