THE PRODUCTION O F MARINE I’LANKTON
155
this is not necessarily a true estimate of productivity. He suggests using
an index which averages the crop to a depth of 30m, and takes into
account the duration of the phytoplankton increase in months, relative
to the whole year. The index is:
mean crop (0-30m) x duration of flowering (months)
12
Using this index he shows that for the Arctic basin the biomass may
be only one-twentieth that of the northern I3arents Sea. The factors
responsible for this low production are fairly obvious. Although in seas
north of Siberia the low surface salinity may impede circulation and
reduce productivity, in the Arctic Ocean proper it is essentially the
thick ice cover, which does not permit phytoplankton growth for some
10 months of the year, which is mainly responsible. Even the northern
Barents Sea is far less productive on this annual basis than a warm
temperate area. Zenkevitch includes an index based on this same formula for the Sea of Azov, a warm temperate area which being largely
enclosed, has a high productivity. But the rich ;$owth of phytoplankton
also ccntinues for between 9 and 10 months of the year, so that the
index of production for the Sea of Azov is some 15 times that of the
northern Barents area.
~
_ _ ~ ~ _ _ _ _ _ _ _ _
111. THE STANDINQ CROP OF PHYTOPLANKTON
These estimates of Zenkevitch really represent an average biomass
of plankton over the year; they are therefore estimates of standing crop
rather than production. Reference has already been made to the difficulty of estimating standing crop; one widely used method is chlorophyll concentration although we have already noted some of the errors
in this technique. Earlier methods for estimating standing crop were
almost all based on counts of phytoplankton cells. These were collected
mainly with the finest silk nets, which though rihowing some variability
in porosity, have an average pore size of 40p x 5Op. But apart from
variations in mesh size due to manufacture, to the degree of wetting, to
age and strain, such nets will clog to varying degrees depending on the
concentration of phytoplankton and detritus They will therefore in
effect vary greatly in catching power. but in any event a large number
of the finest photosynthetic organisms (below 50p) will normally pass
through them. The term nanoplarikton has been given to these very
small photosynthetic organisms which normally escape the finest nets,
and it has become obvious, especially in the last 20 or 30 years, that the
nanoplankton may make a significant contribu1;ion to the standing crop
and indeed to production.
The nnnoplanktoii plants belong to a variety of algal groups; some
155
this is not necessarily a true estimate of productivity. He suggests using
an index which averages the crop to a depth of 30m, and takes into
account the duration of the phytoplankton increase in months, relative
to the whole year. The index is:
mean crop (0-30m) x duration of flowering (months)
12
Using this index he shows that for the Arctic basin the biomass may
be only one-twentieth that of the northern I3arents Sea. The factors
responsible for this low production are fairly obvious. Although in seas
north of Siberia the low surface salinity may impede circulation and
reduce productivity, in the Arctic Ocean proper it is essentially the
thick ice cover, which does not permit phytoplankton growth for some
10 months of the year, which is mainly responsible. Even the northern
Barents Sea is far less productive on this annual basis than a warm
temperate area. Zenkevitch includes an index based on this same formula for the Sea of Azov, a warm temperate area which being largely
enclosed, has a high productivity. But the rich ;$owth of phytoplankton
also ccntinues for between 9 and 10 months of the year, so that the
index of production for the Sea of Azov is some 15 times that of the
northern Barents area.
~
_ _ ~ ~ _ _ _ _ _ _ _ _
111. THE STANDINQ CROP OF PHYTOPLANKTON
These estimates of Zenkevitch really represent an average biomass
of plankton over the year; they are therefore estimates of standing crop
rather than production. Reference has already been made to the difficulty of estimating standing crop; one widely used method is chlorophyll concentration although we have already noted some of the errors
in this technique. Earlier methods for estimating standing crop were
almost all based on counts of phytoplankton cells. These were collected
mainly with the finest silk nets, which though rihowing some variability
in porosity, have an average pore size of 40p x 5Op. But apart from
variations in mesh size due to manufacture, to the degree of wetting, to
age and strain, such nets will clog to varying degrees depending on the
concentration of phytoplankton and detritus They will therefore in
effect vary greatly in catching power. but in any event a large number
of the finest photosynthetic organisms (below 50p) will normally pass
through them. The term nanoplarikton has been given to these very
small photosynthetic organisms which normally escape the finest nets,
and it has become obvious, especially in the last 20 or 30 years, that the
nanoplankton may make a significant contribu1;ion to the standing crop
and indeed to production.
The nnnoplanktoii plants belong to a variety of algal groups; some