A M . ZOOLOGIST, 8:107-118 (1968).
Zooplankton—Life in a Nutritionally Dilute Environment
ROBERT J. CONOVER
Fisheries Research Board of Canada, Bedford Institute of Oceanography,
Dartmouth, Nova Scotia
SYNOPSIS. The classic concept of "filtering rate" often used to describe feeding by zooplankton
may be outmoded. The Ivlev (1945) type of fitted curve may describe more precisely the relation between a zooplanktonic organism and its food supply. Once food is captured it is
assimilated with a high degree of efficiency. There does not appear to be any relationship
between the quantity of food eaten and the digestive efficiency. Examination is made of the
relationship between respiratory rate and size in zooplankton. The exponential constant varies
considerably from one organism to another, but may not be affected by physical parameters
such as temperature. A multiple regression equation relating respiration to weight and temperature simultaneously gives a moderately good estimate of respiration for a variety of marine
copepods. The relationship between uptake of oxygen and excretion of nitrogen (O:N ratio)
is a useful indicator of metabolic substrate used in respiration and is related to the chemical
composition of the animals. The "specific dynamic effect" (SDA) of food should be considered
in studies of zooplanktonic metabolism. The literature on growth efficiency for zooplankton is
reviewed and the applicability of recent growth models for fish (Paloheimo and Dickie, 1965,
1966) to zooplanktonic problems is considered.
The pelagic zone of the ocean or a large
lake represents an essentially limitless food
supply for the individual zooplanktonic
animal, but, at the same time, this source
of energy may be so thinly dispersed that
the problem of getting enough to eat is
still formidable. Elsewhere on Earth, matter and energy are generally concentrated
at the air-soil or water-soil interface, and
the organisms occupy a two dimensional
environment most of the time. Recent work
indicates that the air-water interface is also
much enriched; this zone is occupied by a
specialized group of animals, neuston,
which are largely distinct from the plankton. To be sure, the material suspended
in the remaining water is by no means
uniformly distributed, horizontally or vertically, but the animals which feed on it
must still have their own concentrating
mechanisms. I am grateful that these various mechanisms have been described by
J0rgensen (1966) and hence I will not need
to discuss them in detail.
Before proceeding further I should state
the "boundary conditions" for my discussion. I shall here consider nutrition to
mean the balance of physiological processes enabling an animal to survive, grow,
and reproduce. These processes can be described as a simple mathematical relation:
Input (I) = Growth (G) -f- Respiration
(R) _|_ Egestion (E) (Richman, 1958). Secondly, I shall discuss principally work on
planktonic Crustacea because in most environments they are the dominant members of the zooplankton and consequently
have received most attention. Finally, I
shall treat the events by which the radiant
energy fixed by plants is converted into
secondary production, even though it is
becoming increasingly clear that most
plankton are not confined to a single
trophic level.
INPUT
Paniculate organic matter in some form
undoubtedly constitutes the primary energy input for the zooplankton. To be
sure, many organisms can incorporate dissolved organic matter into their tissue (see
Stephens, p. 93, this volume), but it is unlikely that any planktonic animal could
meet its energy requirements in this way.
Here again I refer you to J0rgensen (1966)
for an extensive review of the potential
food sources available to the zooplankton.
We must accept the fact that sufficient particulate food is available to support the
zooplankton and try to find out how it is
acquired and utilized.
Getting materials and energy into an or-
107
108
ROBERT J. CONOVER
ganism breaks down into two processes,
catching the food and assimilating it. I
shall consider them in this order.
Feeding
The animals we are concerned with in
this paper are usually considered to be
filter-feeders, that is, they are believed to
pass a current of water through some kind
of sieve or filter which strains out the particulate matter. If one could measure the
amount of water passed through the filter,
knew the size of the filter apertures, and
the concentration of particles large enough
to be retained by the filter, it should be
easy to determine the energy input for zooplankton. Since the plankton are small
and the feeding currents diffuse, there has
been no successful attempt to measure the
volume of flow through the filter directly.
Alternatively, the assumption is generally
made that an animal, when placed in a suspension of suitable particles, will remove
them at some exponential rate, enabling
us to compute "volume swept clear" per
unit time, F, following the notation of
Gauld (1951), as
tloge
where V is the volume of water per animal,
Co and Ct are the initial and final concentrations of particles, respectively, t, the
length of the experiment and e the base
of the natural logarithm.
Reviews by J0rgensen (1966) and Sushchenya (1963) show how extremely varied
the experimen tally determined filtering
rates for zooplankton are. Moreover, copepods can engage in "active hunting" (Beklemishev, 1954; Conover, 19666; Haq, 1967;
Petipa, 1965) if the food is actively moving. "Encounter-feeding" has been suggested as a third form of feeding by Cushing (1959) and confirmed for copepods by
Petipa (1959) and Conover (19666). In
Calanus hyperboreiis encounter-feeding and
filter-feeding are distinctly different, probably mutually exclusive processes. An individual conditioned to small-sized food,
which it can capture by filtration, ignores
the chance encounter with a lai-ge cell,
while an individual fed only large cells
does not bother to filter at all (Conover,
19666).
If zooplanktonic organisms are behaving
as automatic filtering machines, the rate of
ingestion should be directly proportional
to the concentration of food, and there
should be no change in the rate of filtration, at least until the filter clogs. In recent years, a number of papers have shown
that filtering rate usually decreases with
increasing concentration, both for marine
and freshwater zooplankton.
Two types of relationships between filtering or ingestion and concentration of
food have been observed. For some Cladocera, Artemia, and for the freshwater copepod, Dinptomus orcgonensis, ingestion is
directly proportional to concentration, and
the rate of filtration is constant up to some
critical level, above which it decreases
(Rigler, 1961; Sushchenya, 1962, 1963,
1964; Reeve, 1963a; McMahon and Rigler,
1965; Richman, 1966). Although the critical concentration in numbers of cells varies
tremendously with species of food, if comparison is made in terms of biomass or volume of the food particles, the critical levels
may be nearly the same (Reeve, 1963a),
or they may still differ by an order of magnitude (McMahon and Rigler, 1965). The
last mentioned authors believe that the
maximum volume eaten by Daphnia magna is determined by digestibility, not size
of food cells.
In the second type, rate of filtering declines continuously with increasing concentration of food. At the same time the
amount of food ingested rises to a plateau
and then may actually decrease. This relationship between concentration of food
and rate of feeding has been observed for
marine copepods by Mullin (1963) and
Haq (1967) and for Daphnia rosea by
Burns and Rigler (1967). Curiously, Ryther's (1954) studies on feeding of Daphnia
magna would suggest that it, too, behaves
like marine copepods, in contrast with the
observations by McMahon and Rigler
(1965). Perhaps the feeding response will
vary with different foods. Certainly the
ZOOPLANKTONIC NUTRITION
critical level or maximal level of feeding
is much greater for Metridia feeding on
Artemia than on Dunaliella, and the copepod has a far better chance to meet its
energy requirements with the animal food
than with the plant (Haq, 1967).
From the preceding illustrations it
should be clear that there is no one "filtering rate" that can be of much use to the
ecologist studying trophic dynamics. Our
real concern is with the input of energy
and not with water swept clear.
Some years ago, the Russian physiologist,
Ivlev (1945, 1955), noted that the amount
of food consumed by fish feeding ad libitum approached some maximum level;
that is, the rate of increase of the ration,
dR, with increasing concentration of food,
dp, is proportional to the difference between the maximum ration, Rmax, and the
actual ration, R, or
109
centration of food, p0, no feeding took
place. Adams and Steele (1966) found that
the filtering rate for Calanus from the
North Sea decreased below a chlorophyll
concentration of 2.5 /tg/liter suggesting that
here, too, there may be a food level (po)
necessary to initiate feeding.
Assimilation
Insofar as they have been studied, zooplankton have the expected complement of
enzymes. The work of Bond (1934) and
Hasler (1937) suggests that starches, proteins, and some lipids should be digested.
A more recent investigation using starchgel electrophoretic techniques (Manwell,
et al., 1967) indicated the presence of several esterases, "peptidases," and an amylase. R. F. Vaccaro (personal communication) found that bacteria with enzymes capable of hydrolyzing chitin and alginic acid
were part of the intestinal flora of Calanus
hyperboreus. Nonetheless, there is still
dR
some question regarding the percentage of
ingested food which may be assimilated
On integration
under different conditions.
k
In this discussion, I define assimilation
R = Rmax (1 — e~ P),
(1)
to be that process by which the food inwhere k is a coefficient of proportionality. gested is broken down and transported
This expression would seem to give quite across the gut wall into the animal. Hence,
a reasonable fit to the type of data obtained percentage assimilation is that portion of
by Reeve (1963a), McMahon and Rigler the ingested food which is assimilated.
(1965), and Mullin (1963).
Some years ago Beklemishev (1954) posThe first application known to me of tulated that at times of high food density,
the Ivlev approach to zooplanktonic feed- zooplankton might ingest more food by filing was that made by Sushchenya (1962, tration than they could digest efficiently.
1964) for Artemia salina. The numerical This process he called "superfluous feedsolution to his data from the 1964 paper ing." If superfluous feeding occurred, one
gave values for k of —0.1566 for juveniles would expect to find a decreasing percentand —0.1270 for adults. These are quite age of assimilation with increasing concensimilar to Ivlev's (1955) calculations for tration of food in laboratory feeding excarp, roach, and bleak, —0.1688, —0.2113, periments. My own studies with Calanus
and —0.1917, respectively.
hyperboreus failed to show any significant
The same type of approach has recently decrease in percent assimilated with food
been taken by Parsons, LeBrasseur, and levels ranging from 0.09 to 1.8 pg carbon/
Fulton (1967) in studies on populations of liter (Conover, 1966a).
phytoplankton and zooplankton in nature.
However, there were appreciable differHowever, Parsons, et al. found it neces- ences in the assimilability of different kinds
sary to modify the original Ivlev relation to of phytoplankton and with the same food
cell grown under different conditions. The
percentage assimilated (A') was found to
because they found that below some con- be negatively correlated with the ash con-
110
ROBERT J. CONOVER
tent of the food according to the equation
A' = 87.8 — 0.73X where X is the percentage ash per unit dry weight of food.
On the basis of a number of additional
reports in the literature the zooplankton
would normally assimilate 60-95% of the
ingested food (see also Conover, 1964).
Growth Factors
A few words should be included about
the significance of trace materials such as
vitamins, etc., for zooplankton even though
we are extremely ignorant regarding such
requirements. Our inability to grow these
creatures in defined media is evidence of
our ignorance, yet we must grow them in
culture to learn what the requirements are.
Provasoli, et al. (1959) showed that only
certain algae were entirely adequate foods
for axenic culture of Artemia and Tigriopus. However, the addition of a supplemental vitamin mix or glutathione corrected nutritional deficiencies found with
certain incomplete foods (Shiraishi and
Provasoli, 1959). Fritsch (1953) improved
fecundity and lengthened the life of
Daphnia by adding pantothenic acid to
the culture medium. Rodina (1963) found
"artificial" detritus inadequate as food for
Daphnia in sterile culture unless vitamins
of the B-complex were added. Lewis (1967)
has used a mixture of trace metals and
vitamins to improve survival in Euchaeta
japonica. This is the current state of our
knowledge, but since several species of marine zooplankton and considerably more
freshwater forms have now been successfully cultured, the prospects for more refined nutritional studies are considerably
improved.
RESPIRATION
The amount of oxygen taken up by an
animal, T, is an indicator of its metabolic
rate and is believed to be related to its
size, W, by the equation
T = aWy
(3)
where y is an exponential constant, usually
between 0.66 and 1.0 and o a constant of
proportionality, indicating the level of me-
tabolism. In later discussion I shall refer
to this relation as the T-line. Most of the
size-metabolism data for zooplankton are
from different-sized animals of different
species. The range of size is frequently
quite limited. For the marine zooplankton,
only the study by Petipa (19666) comes
close to including the entire size-range for
a species, which is all the more remarkable
since she was working with the tiny neritic
copepod, Acartia clausi. Values of y in the
literature range from 0.662 in work by
Comita (in press) on Diaptomus spp. to
1.141 by Small, Hebard, and Mclntire
(1966) for Euphausia pacifica (Table I)1.
Petipa's observations with Acartia yielded
0.811, which is in midrange close to Winberg's (1950) value of 0.81 for Crustacea
in general.
In addition to size, a considerable list
of factors is known to affect the metabolism of zooplankton. It would be very nice
if we were certain that all these factors
only affect the value of a, the level of metabolism. In 1960, I showed that the value
of y was significantly different when measured at 5°C (0.67) and 13°C (0.93) for
Artemia salina, but now I am inclined to
discredit these experiments because both
temperatures are really outside the normal
habitat temperature. Three recent studies
by Small, et al. (1966), Paranjape (1967),
and Comita (in press) have shown that y
does not vary with temperature for
Euphausia pacifica and for freshwater diaptomid copepods. Comita has calculated
a multiple regression equation which can
be used to predict respiratory rates for a
Diaptomus of any size at any temperature
in its normal environment. This equation
has the form: log T = 0.0364t — 0.34181og
W -f 0.6182 where T is O2 used in
i Conover (1959) published some lower values for
7 but these are probably not reliable because the
observations were few and the size-range very limited. Petipa (1966a) has calculated values for 7
of 1.62 and 1.64 for Calanus helgolandicus based
on rate of change in fat content of different stages
and the extent of migratory movement of the different stages. She argues that the extra energy
expended in diurnal migration by the later stages
more than counteracts the natural trend toward
higher metabolic rates in smaller organsims.
111
ZOOPLANKTONIC NUTRITION
TABLE 1. Exponential constants from the expression, T = a"WT, relating respiration (R) to weight (W)
for zooplankton.
Weight Range
Temp
MS
°C
7
20
0.856
0.652
0.649
0.67
0.93
0.663
Animals
Neritic copepods
Herbivorous plankton
Carnivorous plankton.
Artemia salina
2-15
4-36803
820-27.3 X 10
90-590
110-610
40-400
Various Crustaeea including
about
10-10'
plankton
about
Euphausia pacifica & Thysanoessa spinifera
2000-14000
Six species of diaptomids
4-18.5
4-18.5
5
13
25
about
10-25
5
10
15
5
3-300
"Herbivorous" plankton
' ' Carnivorous'' plankton
Acartia clausi
Daphnia pulex
Euphausia pacifica
219-4576
21-97403
0.06-6.4
2.7-46.0
976-8018
868-6146
946-15064
2070-7056
in
15
20
25
3-5
3-5
24-26
20
5
10
15
20
mg- 1 hr- 1 , t is degrees C, and W is /ig
dry weight.
I have substituted into Comita's equation data of my own from different marine
copepods (Tables 2 and 3). There is apparently a slight tendency to overestimate
the measured rates, but a simple non-parametric statistical test (Wilcoxon signedrank, matched-pairs test) indicates that the
two sets of data were not quite significantly
different (p = 0.054). For Calanus hyper-
Source
Conover (1959)
• Conover (1960)
• Conover (1960)
Gilchrist (1956)
0.81
0.962 "
0.935
1.141
0.669 :
Winberg (1950)
0.654
0.626
0.622
0.76
0.78
0.811
0.881
0.985 '
1.008
0.992
1.052
Comita (in press)
n
791
V . I u-L
Small, Herbard, and Maelntire (1966)
Conover and Corner (in press)
Petipa (1966b)
Riehman (1958)
Paranjape (1967)
boreus (Table 2) the formula overestimates
metabolism in summer, fall, and winter
when the animals are not so metabolically
active (see Conover, 1962), but gives quite
good agreement in spring during and just
after the spring bloom when the animals
are feeding and growing (IV/29/65 and
V/13/65 in Table 2). Perhaps this expression gives an approximate estimate of active metabolism for zooplankton, but it
cannot be expected to cope with all the
TABLE 2. Comparison of respiratory rates determined by the Winkler technique with those
calculated from the expression, log T = O.OSSit—0.3418 log W -\-0.618S (Comita, in press) for
Calanus hyperboreus.
DryWt.
Species
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Calanus hyperboreus
Stage
V
F
V
F
V
IV
F
V
IV
F
V
Date Expt.
W
VI/30/65
XI/17/65
XI/17/65
IV/14/65
IV/14/65
IV/14/65
IV/29/65
IV/29/65
IV/29/65
V/13/65
V/13/65
2860
3593
2561
953
660
395
910
728
517
3609
2684
Temp.
°C
Measured
Respiration1
J O ' h
Calculated
Respiration
1 0 1 h 1
6.3
3.3
3.3
4.2
4.2
4.2
7.8
7.8
7.8
7.6
7.6
0.345
0.246
0.200
0.434
0.545
0.505
0.990
0.975
0.778
0.550
0.601
0.467
0.333
0.374
0.566
0.641
0.766
0.788
0.840
0.944
0.514
0.504
112
ROBERT J.
CONOVER
TABLE 3. Comparison of respiratory rates determined by the Wirikler technique with those
calculated from the expression, log T = 0.0364t—O.3418log W +0.6182 (Comita, in press) for
other marine copepods.
Species
Date Expt.
DryWt/tg Temp. °C
"W
t
Calanus finmarchicus
"it
XI/17/65
IV/14/65
IV/14/65
it
V/13/65
»t
V/13/65
Metridia longa
XI/17/65
XI/17/65
IV/14/65
**
IV/14/65
V/13/65
V/13/65
Pareuchacta norvegica
XI/17/65
XI/17/65
Centropages typicus
XII/11/65
**
XII/11/65
Euchaeta spp.
XII/13/65
XII/13/65
XII/13/65
Euaetidiiis armatiis
XII/14/65
XII/14/65
Bhincalanus nasutus
XII/13/65
Pleuromamma aodominalis XII/14/65
"
XII/16/65
XII/16/65
XII/16/65
P. xiphias
XII/14/65
XII/14/65
n
XII/16/65
tt
XII/16/65
tt
XII/16/65
a
XII/16/65
tt
XII/16/65
tt
XII/16/65
tt
XII/16/65
other factors affecting metabolic level.
Included in this list are seasonal and
regional variations, which may involve acclimatization to temperature and adjustment of metabolism to varying food levels.
For the past eight years I have been studying the physiology of the large, coldwater
copepod, Calanus hyperboreus, which actively feeds and grows for only a month
or two in the spring, and during the remainder of the year exists on stored fat
at a reduced metabolic level. We know that
different amounts of oxygen are required
to metabolize fat than are required for
protein or carbohydrate, and different
amounts of energy are produced per unit
of substrate oxidized. In order to learn
327
295
259
514
599
3.3
4.2
4.2
7.6
7.6
353
327
216
219
321
307
3.3
3.3
4.2
4.2
7.6
7.6
3672
1711
3.3
3.3
8.6
8.6
24
21
230
143
152
40
43
201
171
153
144
174
362
271
506
357
410
412
407
443
432
22.5
22.5
22.5
23.2
23.2
22.5
23.2
21.7
17.8
12.6
23.2
23.2
21.7
21.7
21.7
17.8
17.8
12.6
12.6
Measured
Calculated
Respiration Bespiration
jJOsmg-'hr- 1 /JOsmg-ihr- 1
0.287
0.934
1.155
0.962
0.853
0.685
0.754
1.692
1.826
1.902
2.088
0.423
0.367
2.951
3.410
3.660
3.310
4.276
3.635
4.128
1.233
3.177
4.145
2.700
1.693
3.594
3.381
2.788
2.925
3.627
2.491
2.138
1.651
1.425
0.757
0.883
0.843
0.924
0.883
0.738
0.757
0.940
0.935
1.091
3.109
0.331
0.430
2.884
3.013
4.26(5
5.012
4.909
8.222
8.017
4.46T
5.000'
4.571
3.373
2.046
3.873
4.276
3.041
3.436
3.273
2.355
2.366
1.486
1.500
more about the seasonal changes in metabolic behavior we have been studying the
ratio of oxygen taken up to ammonia excreted for Calanus hyperboreus and other
zooplankton. (Incidentally, we do not find
appreciable organic nitrogen being excreted.) The expected ratio by atoms would
be 16 or 17 to 1 if the substrate oxidized
consisted of a "normal" mixture of fat,
protein, and carbohydrate. A lower ratio
would indicate a shift to protein metabo'
lism, and a higher one presumably a shift
to fat because carbohydrate is not an im*
portant energy reserve in the planktonic
animals (Raymont and Conover, 1961).
Figure 1, taken from a recent paper (Conover and Corner, 1968), shows the seasonal
ZOOI'LANKTONIC NUTRITION
| g
.0
7
ZD
I
"i
113
found, perhaps not surprisingly, that respiration and excretion of nitrogen were
positively correlated with nitrogen content,
no matter how large the animal, and negatively correlated with fat. But the zooplankton could be divided into two groups,
essentially the same two groups I called
herbivores and carnivores in 1960 (see Fig.
2, Conover, 1960). Strictly speaking this
separation on the basis of trophic level is
not accurate because most zooplankton are,
at times anyway, omnivorous. Actually the
"herbivores" are the fat-storing copepods,
such as Calanus, that make adjustments in
their metabolic level when conditions become unfavorable.
While we were investigating factors affecting the O:N ratio we found that vigorously feeding animals had a markedly
higher respiratory rate than non-feeding
animals no matter what their nutritional
state prior to the experiment. Perhaps this
is due in part to the SDA or "specific dynamic effect" of the food, to use the term
preferred by Kleiber (1961) to explain the
FIG. 1. Seasonal variation in rates o£ respiration
and excretion of nitrogen and in the O:N ratio
(by atoms) for Calanus hyperboreus from the Gulf
of Maine. Points are usually the average of three
or more individual observations, and the vertical
bars represent the range of variability.
shifts in respiration, excretion, and O:N
ratio for C. hyperboreus. The ratio was
high during most of the year but lower in
late winter and spring during active
growth.
Because there is a marked seasonal change
in the chemical composition of many zooplankters (Fig. 2) we wondered to what
extent the metabolic level could be directly
correlated with changes in chemical composition. Accordingly, we ran a multiple
regression analysis with data from ten species of zooplankton to examine the effect
of weight, total nitrogen, and fat, separately and together, on metabolic level. We
FIG. 2. Seasonal changes in dry weight, nitrogen
(% dry wl), and fat (% dry wt), for Calanus hyperboteits from the Gulf of Maine. Points are usually
the average of three or more individual observations, and the vertical bars represent the range of
variability.
114
ROBERT J. CONOVER
energy actually expended to metabolize different foods. In any event an assumption
that respiration during active growth will
be a function of size and temperature only
will probably introduce a significant error
into growth studies.
GROWTH
An organism achieves growth if input
exceeds outgo. Following the terminology
of Paloheimo and Dickie (1965), growth is
described as
AW
= R —T
(4)
At
where the energy equivalent of growth,
AW, over any period of time, At, is the
difference between R, the energy acquired
in feeding, and T, the metabolic loss of
energy measured over some unit of time.
One of the things one would like to
know about growth is how much new organism can be produced from a known
amount of food. Still following Paloheimo
and Dickie, we write the following indices
of growth efficiency
,v
AW
/K .
and K2 =
(5)
RAt
A'RAt
where the only new symbol is A', the fraction of the ingested food which is assimilated. Kj is equivalent to gross growth efficiency, and K2 is equivalent to net growth
efficiency.
There are a number of estimates for the
first and second order growth coefficients
for zooplankton, some of which I list in
Table 4. For Klt they range from < 1 % in
the case of adult Daphnia pulex (Richman,
1958) to more than 60% for juvenile Artemia under optimal food conditions (Reeve,
1963ft). For K2, the values are usually between 20 and 60%, but may be higher for
actively growing young and for copepods
during fat storage (Conover, 1964).
Euphausia pacifica presents an interesting
enigma in that it discards one third of its
net growth in the form of cast exoskeleton
in molting (Lasker, 1966). The effect of
molting on the growth of other zooplankters
has been estimated only for Calanus finv
K, =
AW
marchicus by Corner, et al. (1967) in which
the loss is less than 1% of the total material
assimilated.
Paloheimo and Dickie (1966) have found
a linear relation between efficiency (Kx)
in fish and the amount of food ingested
(R) such that
K1
=
AW
-a-&R
RAt = e
and hence
AW — R g — a — &R
(6)
At
where a and b are constants. They argue
that e~ a decreases by a constant fraction,
e - b , for each unit increase in ration per
unit time, regardless of the size of the fish.
For fish, neither a nor b varied with temperature as a rule, provided that the fish
were permitted an opportunity to adjust
the amount they would eat. Like a in the
size-metabolism relation, a appears to be
a function of metabolic level. It may well
be affected by such factors as salinity or
kind of food, but the slope, b, was unaffected. Studies of the behavior of the K-line
and its constants are still in their infancy
with fishes, but there is no reason why
the worker on zooplankton should not
make a contribution.
Indeed, Daphnia (Richman, 1958) and
Artemia (Reeve, 19636) show decreasing
efficiencies (Kx) with increasing food-consumption. However, the effect of age or
size is difficult to assess. Reeve's Artemia
show a markedly higher growth efficiency
between 15 and 30 days of age than at
younger or older ages. Many planktonic
Crustacea, including Daphnia and copepods, have a finite maximum size. On the
basis of present knowledge, the efficiency
of reproduction would seem to be lower
than that of growth in some cases and not
in others (Table 4). Paloheimo and Dickie
(1965) point out that separate K-lines may
be needed during different periods in the
life history as defined by ranges of body
size or food types in fishes. The same may
well be true of planktonic animals.
Paloheimo and Dickie (1966) also point
115
ZOOPLANKTONIC NUTRITION
out the relationship between the T-line Since we also have an expression for R (2),
and the K-line by use of the balanced we might ultimately be able to discuss megrowth equation AW/At = R — T. Hence tabolism and growth for zooplankton in
by substituting our new expression for terms of food concentration in the sea or
AW/At (6) we can shew that metabolic
R = aW-v
rate
= Rmax (1 - e-tpekpo) (i_ e -»->B) (8)
T = aWv = R (1 — e- « - & R ).
On the basis of recent observations on the
(7)
o
a.
•3CD
CO
3
2
a,
o to
OOl
a
•S
PH
<"^
^
S
O
M
I 60
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IO
M
•*3
I
I M I I ? JL^
co <£ o o
A?
S
"5
A K5 <? "? M . "? co od
/ \ o a i c oo io b- »o T-t
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A
V
h O
J3
60
bo
^3
60
% g -a.
in 3
Is
bi)
OO TP CO 00
d 10 Tji 10
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P
h
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o
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1
o
•a
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bc
to
o
•3
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ft
60
60
ft
bo
bo
,3
111
bo to ft
S
ft ft cj
•«
2
S
to
ft
"3
•2
e
I
(=1
•S -a fe
111
•! |
S
S
e
i
5
e
o
§•
c
.rt
116
ROBERT J. CONOVER
stability of f with temperature for plankton
(Comita, in press; Small, el al., 1966;
Paranjape, 1967) one is tempted to embellish this expression further so that
when conditions become favorable for
plant growth, production is in excess of
immediate utilization. The zooplankton
eat more than they can use immediately,
and so increase their storage capacity by
growing larger and storing fat. The stand(9) ing crop increases as the P/B ratio gets
= Rmax ( l - e where ct is a temperature correction. Awk- smaller. McLaren (1963) has pointed out
ward as this expression appears, the param- that fecundity also increases with larger
eters theoretically can be determined by size.
A small, surface-dwelling, zooplanktonic
regression techniques.
organism in the warmer montiis will take
DISCUSSION
on the characteristics of a tropical form
There are a couple of interesting aspects even in the north; but an animal which
to zooplanktonic food relations which do makes vertical migrations, feeding with
not seem to fit within my general outline, higher efficiency in warm water and then
so I shall bring them up now as a form "resting" in deeper, cooler water gets a
of discussion. One concerns the ratio of bonus of energy which may be put into
production to biomass (P/B, to adopt the greater fecundity and increased storage
Russian formulation) and the other is the capacity (McLaren, 1963). The lengthening
possible adaptive significance of vertical of generation time as a result might offset
migration. The relation between these two the greater fecundity, but a greater storage
capacity is of great advantage to animals
subjects is part of the story.
which breed only at certain seasons and
The T-line relation (3) assures us that have an annual life cycle.
smaller organisms have more rapid metaboThese final thoughts were suggested by
lism; the same is true of warmer organisms.
a
conversation
with C. D. McAllister. Why
Odum, et al. (1963) have demonstrated the
do
zooplanktonic
organisms migrate to the
consequences of small storage capacity in
surface
at
night
instead
of some other time
nanoplankton, but an analogous situation,
of
day?
Possibly
because
by arriving in
with a different time constant, applies in
the case of the zooplankton. Small, tropical the surface water in early evening, they
or neritic zooplankton have limited storage harvest the maximum available energy
capacity and as a consequence, are in quasi- accumulated by the day's photosynthesis,
steady state with their environment. Their while inflicting the least predatory pressure
level of metabolism is probably controlled on the phytoplankton which normally diby temperature and food level. When food vide in the early morning or during the
is abundant they grow quickly and repro- day. And, why do zooplankton frequently
duce; if food is withdrawn, they die sink out of the surface waters considerably
quickly. This form of existence assures before dawn? Perhaps because they are not
that materials and energy are rapidly re- following an isolume in descent but, in one
cycled, and the P/B ratio will be high. sense an Ivlev curve, R.nax having already
Thus, Heinle (1966) has found turnover been achieved.
times of about four days for the neritic
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