13. EUPHAUSIIDS IN THE MARINE ECONOMY
379
the East Wind-Weddell surface stream as a whole. Marr admits that
he takes no account of populations of E . superba present in the water
column below the uppermost 5 m layer. Consequently, his value of
2.5 g per square metre of the water column is undoubtedly low,
especially as he assumed that the average weight of one E. superba in
a swarm is 0.06 g, the weight of an individual of about 20 mm total
length. Consequently, his higher estimate of biomass, using 0.72 g as
the body weight of an individual, is probably nearer the truth, being
29-28 g/m2. This estimation is probably still low because it takes no
account of the unknown density of E . superba in the deeper regions of
the water column.
All these figures on biomass of euphausiids are extremely tentative
but what does emerge is that euphausiids do constitute a major fraction
of the total biomass of the plankton and that the greatest biomass
occurs in higher latitudes, that is in the distributional areas of the eight
or nine species of primary importance in the economy of the seas.
What specific contributions do euphausiids in these higher latitudes
make to the general economy of the areas? In these regions, the rate of
phytoplankton production shows a marked increase in the spring and
summer followed by increases in the rate of secondary production in the
zooplankton. Euphausia superba (Marr, 1962), Thysanoessa raschii
(Ponomareva, 1955), and possibly Euphausia frigida (Hart, 1942) feed
to a notable extent on phytoplankton but no other species of euphausiids are known to be herbivorous to any great degree, even during
periods of high phytoplankton production. Hart (1942) constructed a
diagram (Fig. 135) to illustrate what he considered to be the more
important food relationships in the Antarctic Ocean. I n this diagram, he
assumes that the phytoplankton is the only truly important source of
nourishment for E. superba but this is unlikely, especially if his data on
phytoplankton production in the Antarctic are examined (Fig. 136).
There is a marked seasonal change in the density of phytoplankton
available and in the winter the levels of production are low, probably
too low to support the population of E . superba which, as individuals,
are growing in size (Fig. 116) and whose gonads, in the 1-year-old
animals, are maturing. Consequently, other sources of nourishment
must be utilized, sources such as the smaller zooplankton. Seasonal
changes in the diets of euphausiids were discussed in Chapter 6 and the
conclusion reached was that most species of euphausiids will eat the
food that is available and abundant in the environment in which they
live and that they can adapt their mode of feeding to utilize a wide
variety of foods ; it is probable that E. superba is no exception to this
general rule. Consequently, an arrow should probably connect Copepoda
379
the East Wind-Weddell surface stream as a whole. Marr admits that
he takes no account of populations of E . superba present in the water
column below the uppermost 5 m layer. Consequently, his value of
2.5 g per square metre of the water column is undoubtedly low,
especially as he assumed that the average weight of one E. superba in
a swarm is 0.06 g, the weight of an individual of about 20 mm total
length. Consequently, his higher estimate of biomass, using 0.72 g as
the body weight of an individual, is probably nearer the truth, being
29-28 g/m2. This estimation is probably still low because it takes no
account of the unknown density of E . superba in the deeper regions of
the water column.
All these figures on biomass of euphausiids are extremely tentative
but what does emerge is that euphausiids do constitute a major fraction
of the total biomass of the plankton and that the greatest biomass
occurs in higher latitudes, that is in the distributional areas of the eight
or nine species of primary importance in the economy of the seas.
What specific contributions do euphausiids in these higher latitudes
make to the general economy of the areas? In these regions, the rate of
phytoplankton production shows a marked increase in the spring and
summer followed by increases in the rate of secondary production in the
zooplankton. Euphausia superba (Marr, 1962), Thysanoessa raschii
(Ponomareva, 1955), and possibly Euphausia frigida (Hart, 1942) feed
to a notable extent on phytoplankton but no other species of euphausiids are known to be herbivorous to any great degree, even during
periods of high phytoplankton production. Hart (1942) constructed a
diagram (Fig. 135) to illustrate what he considered to be the more
important food relationships in the Antarctic Ocean. I n this diagram, he
assumes that the phytoplankton is the only truly important source of
nourishment for E. superba but this is unlikely, especially if his data on
phytoplankton production in the Antarctic are examined (Fig. 136).
There is a marked seasonal change in the density of phytoplankton
available and in the winter the levels of production are low, probably
too low to support the population of E . superba which, as individuals,
are growing in size (Fig. 116) and whose gonads, in the 1-year-old
animals, are maturing. Consequently, other sources of nourishment
must be utilized, sources such as the smaller zooplankton. Seasonal
changes in the diets of euphausiids were discussed in Chapter 6 and the
conclusion reached was that most species of euphausiids will eat the
food that is available and abundant in the environment in which they
live and that they can adapt their mode of feeding to utilize a wide
variety of foods ; it is probable that E. superba is no exception to this
general rule. Consequently, an arrow should probably connect Copepoda