LIMNOLOGY
October 1968
\‘OLUhlE
AND
SUMBER
OCEANOGRAPHY
A SIMPLE METHOD
PRODUCTION
XIII
4
OF ASSESSING THE ANNUAL
OF STREAM BENTHOS
H. B. N. Hynes and Mary J. Coleman
Department
of Biology,
University
of Waterloo,
Waterloo,
Ontario
ABSTRACT
It is possible to calculate the annual production
of stream benthic animals from data
obtained from a series of good quantitative
samples collected at intervals during the year.
The method is explained and its limitations
and shortcomings are discussed. This seems at
present to be the only simple and direct method of estimating production.
and equating increasing size with increasing age, the same method can be applied
to other animals, as long as their life
histories are thoroughly understood. This
has been done for example for Baetis
uagans by Waters ( 1966)) but to attempt
to do it for a whole fauna, much of which
is not completely identifiable, would be an
impossibly
lengthy
task. Moreover,
as
Macan ( 1958) and Hynes ( 1961) have
pointed out, the fact that many streamdwelling insects continue to hatch from
eggs over a long period constantly lowers
the mean size of the population
during
that period and thus introduces complications.
An alternative method to direct study
of the benthos itself is back-calculation
of
the amount of food which must have been
eaten by fishes to sustain their production,
making due allowance for that proportion
of their diet that is not of benthic origin.
This was first done by Allen ( 1951) and
later by Horton ( 1961), and both found
that the fishes had apparently eaten many
’ Part of the work on which this paper is based
was supported by National
Research Council of
times the standing biomass of the benthic
Canada Grant No. A 1975 for which we are grateinvertebrates.
The discrepancy
was so
ful. We also express our gratitude to Mr. J. Bishop
large,
since
many,
probably
most,
of the
of the University
of Waterloo
for information
on
invertebrates involved are univoltine, that
drift rates in the Speed River,
569
INTRODUCTION
In view of the increasing emphasis on
production studies in ecology, and particularly because of the importance that stream
studies will shortly have in the International Biological Program, it is necessary
to devise a fairly simple method of estimating the amount of biomass produced
by benthic invertebrates.
We have devoted much thought to this problem, and
we offer this paper as providing at least
some part of a solution.
It is possible to use the instantaneous
growth rate method to calculate production in those animals of known age and
where one can ascertain both the density
and the composition of the population.
This is, of course, standard practice in fisheries work ( e.g., Allen 1951)) and it has
also been applied successfully to Unionidae, of which the age, like that of fishes,
can be determined by annular structures
( Negus 1966). By extension of this idea,
570
IL
B.
N.
HYNES
AND
something is clearly amiss. Gerking (1962)
has pointed out that there were some
faulty assumptions in Allen’s calculations
of production
of biomass of fish, and
Hynes ( 1961) has stressed that his benthic
samples did not contain small animals
because of the mesh size of the sampler
used. Horton was aware of the mesh-size
factor, but even so the amount that her
fishes consumed greatly exceeded the
standing biomass as determined by her samples. Tier results give turnover ratios (production/standing
biomass ) ranging from 8
to 26. These values are unexpectedly high,
especially when it is recalled that backcalculation
is concerned only with that
part of the production which was eaten
by one species of fish. The direct studies
mentioned above give turnover ratios of
about 3 per generation for BaetZs and only
about 0.X3-0.20 per year for Unionidae.
Probably the fault in the back-calculations lies with the sampling techniques for
the benthos, and the fish are better samplers than the biologists.
We shall not
here, however, consider the difficult question of the sampling of stream benthos,
which has been very well discussed by
Albrecht
( 1959). Angelier
( 1953) and
Schwoerbel (1961) have both shown that
stream insects occur in considerable numbers down to at least 30 cm in the gravel,
and we ourselves have data from a stream
in Ontario indicating that only about 20%
of the fauna occurring down to a depth
of 30 cm in the gravel is present in the
topmost 7.5 cm. It seems likely that this
occurrence of large numbers of animals
deep in the substratum is characteristic of
many streams where the bed is not solid
rock or very densely compacted material.
Quantitative sampling techniques therefore
stand in great need of modification
and
improvement.
But, assuming that a good sampling
technique is available and that one can
collect adequate samples at intervals during the year, how can the information so
obtained be used to make even a rough
direct estimate of the production of benthic
biomass? Hynes ( 1961) attempted to do
MARY
J.
COLEMAN
this for a fauna in which he was able to
identify a large proportion of the species.
He summed the losses of specimens of
identifiable
species in each size group
throughout the year and then applied a
correction factor to encompass the whole
fauna. His estimate of production
was
2.995 g m-2 year-l from a standing biomass
(calculated from data given in his paper)
of 4.76 g. This gives a turnover ratio of
0.629, which is obviously too low. However, closer consideration of his method
reveals a conceptual error, as it was based
only on the production of the smallest size
group as it grew throughout the course of
one year, and the production of the other
size ranges was not taken into account.
The details of the calculation
and the
assumptions it entailed are explained in
the original paper. Here we shall rework
the data as we believe they should have
been calculated, together with a. similar
set of data from the same stream to illustrate the use of a different unit of measurement, and from this exercise suggest
a method for roughly estimating production of the benthic fauna in the rhithron
reaches of running water (Illies and Botosaneanu 1963).
METHOD
The second column of Table 1 shows the
number of specimens in each size group
(to the nearest millimeter)
of the 10 identifiable common species taken in 13 sets
of samples from a total area of 1,800 cm2
during a year. These represented about
70% (752/1,063)
of the total fauna, although the way this percentage was dctermined, and its validity, do not concern us
here. The second column therefore represents 13 ideal samples of these univoltine
species from 1,800 cm2; it is, however, co,nvenient to divide by 13 at a later stage to
avoid working with fractions. During the
year therefore, one such sample will go
through its full development, and on the
way specimens will be lost, representing
production in the sense of Ivlev ( 1966).
Thus, 6,278 - 5,726 = 552 is the loss of
l-mm specimens, from 13 such samples,
5,726 - 2,031 = 3,695 is the loss of 2-mm
PI~ODUCITON
OF
STREAM
571
BENTHOS
TABLE
1. Calculation
of totals of l-mm units of standing biomass and production from identified insects in 13 sets of samples from 1,800 cm8 spaced over the year November
1955-1956 from the Afon
Hirnant,
Wales
Number
conversion
factor
X
Loss x
conversion
factor
Sizegroup
(mm)
No. of
specimens
Loss
at each
stage
Conversion
factor
to
l-mm
units
1
2
3
4
5
6,278
5,726
2,031
805
348
552
3,695
1,226
457
245
1
8
27
64
125
6,278
45,808
54,837
51,520
43,500
552
29,560
33,102
29,248
30,625
;
8
1:
103
23
5
23
80
18
2
21
216
343,
512
729
1,000
22,248
7,889
2,560
2,000
2,187
17,280,
6,174
1,024
2,000
729
Total
units
Standing
specimens and so on. However, all the
animals in a single ideal sample should
complete their growth within
the 12month period, and as they grow they pass
from one size category to the next with
appropriate loss in numbers at each stage.
In Table 1 there are 10 size-groups and the
l-mm group passed through 10 categories
before growth was complete, the 2-mm
group through
9, the 4-mm nymphs
through 7, and so on. Thus, the real production is the loss xl, x2, ~4 and so on
up to 10 as shown in the penultimate column of the table.
We have then only to convert the numbers into biomass and we have an estimate
of production.
Ideally this would be done
by weighing or by calorie-equivalent
determinations, but as that information
is not
available the conversion is made here to
l-mm units, based on the fact that the
insects concerned do not change shape
much as they grow. Thus a 2-mm specimen is 23 the volume of a l-mm specimen,
a 3-mm one 33, and so on, as shown by the
conversion
factor in the table,
Then
8 (total number of specimens x conversion
factor) gives us the standing biomass, and
S (loss X factor X number of times loss occurs) gives us production, both in terms of
l-mm units, but still ~13, the number of
sets of samples. If we assume, as did
Hynes, that the insects are approximately
cylinders five times as long as wide, with
1
5
552
59,120
99,306
116,992
153,125
7
8
109
103,680
43,218
8,192
20,000,
6,561
2
:
Production
biomass = 238,827
a specific
weighs:
gravity
Jkuluction
(procluct
of
last 2 col.)
No. of
times loss
occurs
= 610,746~
of 1.05, a l-mm
unit
7r x 0.12 x 1 x 1.05 g
1,000
and the final calculations,
tions for the area sampled
number of samples ( 13),
tion factor for the total
then become :
applying correc( 1,800 cm2), the
and the correcfauna 1,063/752
no. of units X 7TX 0.12 X 1
x 1.05 x 10,060 x 1,063
1,000 x 1,800 x 13 x 752 g’m2’
Thcsc give us 12.17 g/m2 of production
from 4.76 g/m2 of standing biomass.
Table 2 shows a similar set of figures
from the same stream in another year
during which severe floods reduced the
fauna and when 11 sets of samples were
taken from 6,300 cm2. Here, however, the
system of measurement was different (O-l
mm, 1-2 mm, etc. ), and it is necessary to
work in 0.5-mm units with corresponding
changes in conversion factor and volume
of the unit (0.5 x rr X 0.052), The calculations here give us 3.81 g/m2 of production
from 1.30 g/m2 of standing biomass. The
figures are much lower because of the
floods, but the turnover ratio 2.94 is not
very different from the previous one of
2.56.
572
II.
B.
N.
IIYNES
AND
MARY
J.
COLEMAN
2. Calculation
of totals of OS-mm units of standing biomass and production
from identified
insects in 11 sets of samples from 6,300 cm’ spaced ouer the year November 1959-1960 from the Afon
Hirnant,
Wales
TABLE
Size
group
(Inm)
o-1
l-2
2-3
3-4
4-5
5-6
7:;
8-9
9-10
10-11
11-12
Total
No. of
spccimens
Loss at
each
stngc
6,069
4,848
2,158
946
584
193
28
w
1,221
2,690
1,212
362
391
165
21
4
1
1
0
1
3
2
1
1
units
Conversion
factor
to
0.5-mm
unit9
Standing
1
27
125
343
729,
1,331
2,197
3,375
4,913
6,859,
9,261
12,167
Number
X
conversion
factor
Loss x
conversion
factor
No. of
times loss
occurs
6,069
130,896
269,750
324,478
425,736
256,883
61,516
23,625
14,739
13,718
0
12,167
1,221
72,630
151,500
124,166
285,039
219,615
46,137
13,500
4,913
6,859
0
12,167
1
2
3
biomass = 1,539,577
DISCUSSION
The ratios calculated above for populations composed of largely univoltine
insects are more or less in line with the
ratio/generation
found by Waters from direct study of Baetis, and this suggests they
are of the right order of magnitude. Even
though the Afon Hirnant is a soft-water
unproductive
stream, the production figures are undoubtedly too low because of
inadequate sampling in depth, but this
should not affect the ratios, We have by
this method calculated a production
of
620.2 g/m2 from 195.5 g/m2 (turnover
ratio 3.17) from the much richer hardwater Speed River, Ontario, in which our
sampling method was better; this rate of
production
appears able to supply the
measured rate of drift from the streambed
and the probable needs of the fishes.
We would suggest therefore
that a
rough estimate of the production of stream
benthos can be obtained quite simply by
taking adequate, thorough samples at intervals throughout the year and placing all
the individual
animals into l-mm size
classes. The total number in each size
class divided by the number of sampling
dates taken then represents the product of
an ideal set of samples from the area sampled on each occasion. From this it is
possible to calculate, as outlined above,
2
7
8
9
10
11
12
Production
Production
(product
of
last 2 col.)
1,221
145,260
454,500
496,664
1,425,195
1,317,690
322,959
108,000
44,217
68,590’
0
146,004
= 4,530,300
the production in terms of l-mm or O.5-mm
size units and this in turn can be converted
to biomass either by calculations based on
assumptions about density and dimensions, as is done above, or, preferably, by
direct weighing of representatives of the
fauna or determination
of their calorific
equivalents.
This method is crude in that it assumes
that all the species are univoltine, which
they are not, and it will be an undcrestimate in that specimens hatching from eggs
after one sampling date and perishing
before the next will not be counted. On
the other hand, bivoltine or multivoltine
species will be to’ some extent countcracted by those large ones, for example,
some Plecoptera, Odonata, and Megaloptera, that grow for more than a year; and
the assumption made that all species grow
to the size of the largest ones, which they
most certainly do not, will compensate,
perhaps even over-compensate, for losses
of newly hatched individuals.
Indeed
where a fauna has been well studied and
it is known that certain species are bivoltine and univoltine and that others grow
for two years or more, it would be possible
to handle each class of organism separately
and so increase the precision. Similarly the
inaccuracy of the assumption that all species, except obviously very large ones,
PRODUCTION
OF
grow to about the same size could be
reduced by classifying the spccics into
large, medium, and small, or into even
finer grades, and treating each grade separately. It is also possible to divide the year
into seasons and compare, say, summer
production with winter production.
Although the method is crude, and so
will be frowned upon by many ecologists,
it has the merit of simplicity and should
produce results of the right order of magnitude. Even in its simplest form it can
be applied to the entire benthic invertcbratc fauna, with the possible exception of
crayfish and crabs and some particularly
large insects that could be handled separately; and it would serve for the International Biological Program, much of which
will be done in remote areas and by workers with no special expertise in benthic
fauna.
We know of no other &rect
method that could be used under these
circumstances.
Moreover, if this method were to be
applied to a great number of streams, we
should, even if the sampling method were
not very good, begin to accumulate data
on the true values o,f the turnover ratio,
which probably
varies greatly between
stream types. This would then give us a
means of comparing streams directly on
the basis of standing biomass without at
least some of the doubts which that
process at prcscnt engenders,
SUMMARY
The problems involved in estimating the
productivity
of the rhithron reaches of running water arc discussed. The error present in a previous paper is explained. A
modification
of the method described in
the previous paper is put forward.
The
data from the Rfon Hirnant, Wales, used
in the earlier paper, are reworked together
with similar data from the same stream
for the year 1959-1960. The standing biomass, production, and turnover ratio are
calculated for both years, and the differenccs between the values are discussed.
The figures obtained from the Afon Hir-
STiREAM
573
DENTIIOS
nant, an unproductive stream, are compared
with those for the richer, hard-water,
Speed River, Ontario, and with the data
obtained by Waters for Baetis vagans using
the instantaneous growth rate method for
estimating
produ&on.
The sources of
error involved in this method are discussed
and improvements are suggested. Because
of the simplicity of this method it is suggested that it could be widely applicable
in obtaining approximate estimates of the
turnover ratios of different types of stream.
REFERENCES
ALBRECIIT, M.
L.
1959.
Die
quantitative
Untersuchung
der Bodenfauna
fliessender
Gewssser ( Untersuchungsmethodcn
und Arbeitsergebnissc ) , Z. Fischerei,
8: 481-550.
ALLEN, K. R. 1951. The Horokiwi
stream. A
study of a trout population.
Fishery Bull.
New Zealand, 10: 231 p.
ANGELIER, l3. 1953. Recherches &ologiqucs
et
biogbographiques
sur la faune des sables subArch. Zool. Exp. Gen., 90: 37-162.
merg&.
GERKING, S. D.
1962.
Production
and foocl
utilization
in a population
of blue gill sunfish.
Ecol. Monographs,
32: 31-78.
HORTON, P. A. 1961. The bionomics of brown
trout in a Dartmoor stream.
J. Animal Ecol.,
30: 3:11-338.
HYNES, H. B. N. 1961. The invertebrate
fauna
of a Welsh mountain stream.
Arch. Hydrobiol., 57: 344-388.
ILLIH, J., AND L. BOTOSANEANU. 1963. Probl&mcs et methodcs de la classification
ct dc
la zonation
Bcologiquc
des eaux courantes,
considorkes surtout du point de vue faunistique.
Mitt.
Intern.
Ver. Theoret.
Angew.
Limnol.,
12: 57 p,
IVLEV, V. !<. 1966. The biological productivity
of waters.
[Fisheries
Res. Board
Can.
Transl.
Ser., No. 394.1 J. Fisheries
Res.
Board Can., 23: 1727-1759.
MACAN, T. T.
1958.
Causes and cffccts
of
short emcrgcnce
periods
in insects.
Verhandl. Intern. Ver. Limnol., 13: 845-849.
NEGUS, C. L.
1966.
A quantitative
study of
growth and production
of unionid mussels in
the River Thames at Reading.
J. Animal
Ecol., 35: 513-532.
SCHWOERBEL, J. 1961.
Ober die Lebensbedingungen
und die Besiedlung
des hyporheischen
Lebensraumcs.
Arch. Hydrobiol.
Suppl., 25: 182-214.
WATERS, T. F. 1966. Production rate, populntion density, and drift of a stream invertebrate.
Ecology, 47: 595-604.