Benthic Fauna of Lakes
185
SEPARATION OF ANIMALS FROM SEDIMENTS
Separation of benthic organisms from sediments and debris is difficult to accomplish
quantitatively. A number of methods have been developed to assist with this task;
all are tedious and require patience and care. Often initial separation of the organisms
from the sediments is done in the field by screening through sieves that remove some
of the sediment and reduce bulk. When such separation cannot be accomplished
within a few hours, the samples should be preserved by the addition of a formaldehyde
solution (40%) to give a final formalin concentration of about 5% (a 5% formalin
solution = 2% formaldehyde).
When screening with sieves, a decision must be made as to what size of mesh to
use. The smaller the mesh size, the more organisms that are retained but also the
more sediment retained, and so the time required for sieving and sorting increases
greatly. Compromise is needed. In general, as small a mash size as practical should
be used. A mesh size of 0.40 mm will retain only large and older stages of benthic
organisms. The better mesh size is 0.20 mm, which represents the practical lower size
limit for general study of benthic animals. Even with this smaller mesh size, some
small animals will be lost [e.g., see Nalepa and Robertson (1981), Storey and Pinder
(1985), and Strayer (1985)]. In some instances where sediments are large-grained or
contain fibrous organic materials, sieving with even I-mm screens may require
extraordinary amounts of time or prove practically impossible. The objectives of the
studies should be kept in mind. When small stages of benthic organisms are required
for analyses of growth and productivity the smallest mesh size is mandatory. On the
other hand, when measurement of biomass of the composite benthic populations is
the goal the instantaneous biomass will be associated with the larger organisms, and
over 90% of the total biomass often will be retained using a coarser sieve [cf., Reish,
(1959)].
When small core subsamples are taken simultaneously from surficial sediments (to
a depth of 4 to 5 cm), estimates also can be made of the numbers ad biomass of young
stages of the benthic animals from the materials of the macrosample. The sieved
materials should be washed carefully into collection bottles and preserved and the
samples diluted with water to make the task more pleasant. The samples can be
transferred to 90% alcohol, but this treatment will cause much shrinkage and confound
biomass determinations (see the discussion below).
Small amounts of sediment samples can be placed into shallow, white enamel trays
and sorted manually. When much organic debris is present, the addition of a small
amount of eosin or rose bengal dye assists in seeing the organisms, since they tend
to stain more intensely than debris. The organisms are even more conspicuous when
stained with a very small amount of fluorescent dye such as rhodamine B and viewed
under long-wave ultraviolet light (Hamilton, 1969a).
Flotation methods are used widely to assist in the separation of benthic organisms
from sediments. Samples are placed into a salt cr sugar solution of high density
relative to the organisms. The organisms float, and sediment particles, more dense
than the solution, sink. When the samples contain much plant material, the flotation
method does not work well as much plant debris also floats.
Although a number of solutions have been used (NaCl, CaCI 2 , MgS0 4 , and so
on), sugar (sucrose) of a concentration sufficient to yield 1.12 to 1.13 gjml density
after mixing with the sediment sample works very well [see Anderson (1959) and
Kajak et al. (1969)]. Samples are mixed with sugar or salt solution in a volumetric
ratio of about 1:5 or 1: 1 0, stirred vigorously, and the floating organisms removed
185
SEPARATION OF ANIMALS FROM SEDIMENTS
Separation of benthic organisms from sediments and debris is difficult to accomplish
quantitatively. A number of methods have been developed to assist with this task;
all are tedious and require patience and care. Often initial separation of the organisms
from the sediments is done in the field by screening through sieves that remove some
of the sediment and reduce bulk. When such separation cannot be accomplished
within a few hours, the samples should be preserved by the addition of a formaldehyde
solution (40%) to give a final formalin concentration of about 5% (a 5% formalin
solution = 2% formaldehyde).
When screening with sieves, a decision must be made as to what size of mesh to
use. The smaller the mesh size, the more organisms that are retained but also the
more sediment retained, and so the time required for sieving and sorting increases
greatly. Compromise is needed. In general, as small a mash size as practical should
be used. A mesh size of 0.40 mm will retain only large and older stages of benthic
organisms. The better mesh size is 0.20 mm, which represents the practical lower size
limit for general study of benthic animals. Even with this smaller mesh size, some
small animals will be lost [e.g., see Nalepa and Robertson (1981), Storey and Pinder
(1985), and Strayer (1985)]. In some instances where sediments are large-grained or
contain fibrous organic materials, sieving with even I-mm screens may require
extraordinary amounts of time or prove practically impossible. The objectives of the
studies should be kept in mind. When small stages of benthic organisms are required
for analyses of growth and productivity the smallest mesh size is mandatory. On the
other hand, when measurement of biomass of the composite benthic populations is
the goal the instantaneous biomass will be associated with the larger organisms, and
over 90% of the total biomass often will be retained using a coarser sieve [cf., Reish,
(1959)].
When small core subsamples are taken simultaneously from surficial sediments (to
a depth of 4 to 5 cm), estimates also can be made of the numbers ad biomass of young
stages of the benthic animals from the materials of the macrosample. The sieved
materials should be washed carefully into collection bottles and preserved and the
samples diluted with water to make the task more pleasant. The samples can be
transferred to 90% alcohol, but this treatment will cause much shrinkage and confound
biomass determinations (see the discussion below).
Small amounts of sediment samples can be placed into shallow, white enamel trays
and sorted manually. When much organic debris is present, the addition of a small
amount of eosin or rose bengal dye assists in seeing the organisms, since they tend
to stain more intensely than debris. The organisms are even more conspicuous when
stained with a very small amount of fluorescent dye such as rhodamine B and viewed
under long-wave ultraviolet light (Hamilton, 1969a).
Flotation methods are used widely to assist in the separation of benthic organisms
from sediments. Samples are placed into a salt cr sugar solution of high density
relative to the organisms. The organisms float, and sediment particles, more dense
than the solution, sink. When the samples contain much plant material, the flotation
method does not work well as much plant debris also floats.
Although a number of solutions have been used (NaCl, CaCI 2 , MgS0 4 , and so
on), sugar (sucrose) of a concentration sufficient to yield 1.12 to 1.13 gjml density
after mixing with the sediment sample works very well [see Anderson (1959) and
Kajak et al. (1969)]. Samples are mixed with sugar or salt solution in a volumetric
ratio of about 1:5 or 1: 1 0, stirred vigorously, and the floating organisms removed