186
Exercise 12
quickly with forceps or a small strainer. Stirring is repeated several times. The
remaining sediment is then sorted by hand because many organisms will not float,
e.g., molluscs and caddisfly larvae in cases. Small larvae, such as those of chironomids,
are very difficult to sort from sediments, and tedious efforts are required to do this
quantitatively.
Many invertebrates can be separated from substrata by elutriation techniques, in
which a flow of water is passed rapidly from below through the sediments. The
invertebrates are less dense than sand and gravel and are separated from the sediments
and pass with the flow to collecting sieves. Light-bodied organisms (e.g., copepods,
chironomids, nematodes, odonates, and emphemeropterans) are separated most
effectively [e.g., Whitman et al. (1983)]. Dense animals, such as molluscs, are recovered
very poorly.
ESTIMATION OF BIOMASS
Biomass estimations of benthic organisms usually are made on the basis of dry weight
and of organic weight (ash-free dry weight). Because of the large amount of inorganic
materials that is part of many benthic organisms, e.g., shells of snails and clams, it
is important that dry weight take these components into account. Dry weight is
determined at 105°C. A small percentage of organic materials will be lost at this
temperature; hence, for very exact work, lyophilization (freeze-drying) is necessary
[cf., Downing (1984) and Leuven et al. (1985)]. The samples then are heated to 550"C
in tared crucibles for 4 h to remove organic matter, cooled under desiccation, and
reweighed.
When organisms have been preserved in alcohol, as much as 25% of the biomass
(dry weight) will be lost by leaching (Holme, 1964; Stanford, 1973; Leuven, et aI.,
1985). A slight increase in weight occurs in some organisms when preserved in formalin.
In both cases, evaluation of the changes is necessary, and appropriate correction
factors must be applied.
Shrinkage may also occur upon preservation. The extent of shrinkage must be
evaluated when making size measurements for the determination of growth and
ins tar calculations.
Estimates of biomass can be extremely time-consuming. Rather than weighing all
animals from field samples, it is common practice to develop independent
length-weight relationships, so that the length of individuals can be measured from
samples and converted into biomass. The following relationship is used:
Dry wt = (a)· (length b ) or log dry wt = log a + h log length
where b is usually ~ 3 [cf., Smock (1980)].
PRODUCTIVITY OF BENTHIC FAUNAL POPULATIONS
Estimates ofrates of production of benthic animal populations have been made using
four basic approaches [cf., Waters (1977), Rigler and Downing (1984), and Benke
(1984)]. The first three methods often are referred to as "cohort" methods, in which
changes in a population cohort the followed by sampling through time. The fourth,
size-frequency method evaluates changes in average, pooled populations and their
population frequencies in different size categories.
Exercise 12
quickly with forceps or a small strainer. Stirring is repeated several times. The
remaining sediment is then sorted by hand because many organisms will not float,
e.g., molluscs and caddisfly larvae in cases. Small larvae, such as those of chironomids,
are very difficult to sort from sediments, and tedious efforts are required to do this
quantitatively.
Many invertebrates can be separated from substrata by elutriation techniques, in
which a flow of water is passed rapidly from below through the sediments. The
invertebrates are less dense than sand and gravel and are separated from the sediments
and pass with the flow to collecting sieves. Light-bodied organisms (e.g., copepods,
chironomids, nematodes, odonates, and emphemeropterans) are separated most
effectively [e.g., Whitman et al. (1983)]. Dense animals, such as molluscs, are recovered
very poorly.
ESTIMATION OF BIOMASS
Biomass estimations of benthic organisms usually are made on the basis of dry weight
and of organic weight (ash-free dry weight). Because of the large amount of inorganic
materials that is part of many benthic organisms, e.g., shells of snails and clams, it
is important that dry weight take these components into account. Dry weight is
determined at 105°C. A small percentage of organic materials will be lost at this
temperature; hence, for very exact work, lyophilization (freeze-drying) is necessary
[cf., Downing (1984) and Leuven et al. (1985)]. The samples then are heated to 550"C
in tared crucibles for 4 h to remove organic matter, cooled under desiccation, and
reweighed.
When organisms have been preserved in alcohol, as much as 25% of the biomass
(dry weight) will be lost by leaching (Holme, 1964; Stanford, 1973; Leuven, et aI.,
1985). A slight increase in weight occurs in some organisms when preserved in formalin.
In both cases, evaluation of the changes is necessary, and appropriate correction
factors must be applied.
Shrinkage may also occur upon preservation. The extent of shrinkage must be
evaluated when making size measurements for the determination of growth and
ins tar calculations.
Estimates of biomass can be extremely time-consuming. Rather than weighing all
animals from field samples, it is common practice to develop independent
length-weight relationships, so that the length of individuals can be measured from
samples and converted into biomass. The following relationship is used:
Dry wt = (a)· (length b ) or log dry wt = log a + h log length
where b is usually ~ 3 [cf., Smock (1980)].
PRODUCTIVITY OF BENTHIC FAUNAL POPULATIONS
Estimates ofrates of production of benthic animal populations have been made using
four basic approaches [cf., Waters (1977), Rigler and Downing (1984), and Benke
(1984)]. The first three methods often are referred to as "cohort" methods, in which
changes in a population cohort the followed by sampling through time. The fourth,
size-frequency method evaluates changes in average, pooled populations and their
population frequencies in different size categories.