240
N. A . IIOLME
of metIiods uro aviilabk, I)ut Lcforc attempting such analyses it may
bo a,s well to consider well what i t is required t o measurc. It may
not bc possiblc to follow the methods described by Morgans, which
were designed for shallow-water work within easy reach of a laboratory,
and furthrr research is needed to find the most suitable method of
storing and arialysis of samples collected where facilities for immediate
analysis are iiot available.
XII. ESTIMATION OF STANDING CROP
When attempting to make an estimate of the " productivity " of
the bcnthos, thc first step is an estimation of the quantity of living
material, or standing crop. This may be expressed either as: (a) fresh
weight, also hiown as wet weight, rough weight, or biomass. This is
the weight of fresh or living specimens after excess surface moisture
has been removed. The weight of mollusc shells is usually included.
( b ) The dry weight, after evaporation (at 110" or at a rather lower
temperature) to constant weight. The dry weight does not include
mollusc shells, nor other calcareous skeletal materials, and gut contents
should also be excluded.
It is generally agreed that results should ultimately be expressed
in terms of dry weight (but see Longhurst, 1959a), but i t is not usually
possible nor desirable, t o determine this directly for all the animals
collected. Factors are available for converting fresh weight to dry
weight (Pctersen and Boysen Jensen, 1911 ; Thorson, 1957b ; see also
Vinogradov, 1953 ; Nicol, 1 g60), but frequently the specimens will
have been preservcd, so an additioiial correction has t o be made.
It is probably best t o establish conversion factors anew for each
investigation, as thcse will then take into account any local differences
in the animals and in preservation technique. Dry weight determinations require some care, and the more refined techniques applicable t o
plankton are discussed by Lovegrove (1962).
When preserved in 70% methylated Hpirit quite a high proportion of t h c
total dry weight goes into solution in the alcohol. Holme (1953) found
only about 62% of the total dry weight in the dried prcHerved animal, the
remainder being in solution. Petersen and Boysen Jenfien (1!)11) give u
table relating dry weight to alcohol weight, but to the figure so obtained the
weight of' residue after evaporating the preserving fluid must be added. On
the other hand, Holme (1953) gives a table relating dry weight to alcohol
weight directly, allowance being made for that dis~olved in the alcohol.
There seems to be some confusion betwcen the t w o methods (Thorson, 1957b,
p. 493), partly due to uncertainty over the manner in which Petersen arid
Boysen Junsen made their calculations. Yiippove an animal weighs 100 g
fresh, and that the dry weight is 20 g. After preservation in alcohol, the
N. A . IIOLME
of metIiods uro aviilabk, I)ut Lcforc attempting such analyses it may
bo a,s well to consider well what i t is required t o measurc. It may
not bc possiblc to follow the methods described by Morgans, which
were designed for shallow-water work within easy reach of a laboratory,
and furthrr research is needed to find the most suitable method of
storing and arialysis of samples collected where facilities for immediate
analysis are iiot available.
XII. ESTIMATION OF STANDING CROP
When attempting to make an estimate of the " productivity " of
the bcnthos, thc first step is an estimation of the quantity of living
material, or standing crop. This may be expressed either as: (a) fresh
weight, also hiown as wet weight, rough weight, or biomass. This is
the weight of fresh or living specimens after excess surface moisture
has been removed. The weight of mollusc shells is usually included.
( b ) The dry weight, after evaporation (at 110" or at a rather lower
temperature) to constant weight. The dry weight does not include
mollusc shells, nor other calcareous skeletal materials, and gut contents
should also be excluded.
It is generally agreed that results should ultimately be expressed
in terms of dry weight (but see Longhurst, 1959a), but i t is not usually
possible nor desirable, t o determine this directly for all the animals
collected. Factors are available for converting fresh weight to dry
weight (Pctersen and Boysen Jensen, 1911 ; Thorson, 1957b ; see also
Vinogradov, 1953 ; Nicol, 1 g60), but frequently the specimens will
have been preservcd, so an additioiial correction has t o be made.
It is probably best t o establish conversion factors anew for each
investigation, as thcse will then take into account any local differences
in the animals and in preservation technique. Dry weight determinations require some care, and the more refined techniques applicable t o
plankton are discussed by Lovegrove (1962).
When preserved in 70% methylated Hpirit quite a high proportion of t h c
total dry weight goes into solution in the alcohol. Holme (1953) found
only about 62% of the total dry weight in the dried prcHerved animal, the
remainder being in solution. Petersen and Boysen Jenfien (1!)11) give u
table relating dry weight to alcohol weight, but to the figure so obtained the
weight of' residue after evaporating the preserving fluid must be added. On
the other hand, Holme (1953) gives a table relating dry weight to alcohol
weight directly, allowance being made for that dis~olved in the alcohol.
There seems to be some confusion betwcen the t w o methods (Thorson, 1957b,
p. 493), partly due to uncertainty over the manner in which Petersen arid
Boysen Junsen made their calculations. Yiippove an animal weighs 100 g
fresh, and that the dry weight is 20 g. After preservation in alcohol, the