AN EXPERIMENTAL FIELD STUDY
OF THE EFFECTS OF INTERSPECIFIC COMPETITION
ON DIAPTOMUS LEPTOPUS (COPEPODA:CALANOIDA)
IN A MONTANE LAKE
by
ROBERTA JILL OLENICK
B.Sc. Honors, University of Manitoba, Winnipeg, 1978
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Zoology)
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA April 1982
0 Roberta Jill Olenick, 1982 In presenting this thesis in partial fulfilment of the
requirements for an advanced degree at the University
of British Columbia, I agree that the Library shall make
it freely available for reference and study. I further
agree that permission for extensive copying of this thesis
for scholarly purposes may be granted by the head of my
department or by his or her representatives. It is
understood that copying or publication of this thesis
for financial gain shall not be allowed without my written
permission.
Zoology Department of
The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5
April, 1982 Date
DE-6 (2/79) David I. MacKenzie, for always caring i i
ABSTRACT
Despite high densities in an upstream lake, the herbivorous calanoid copepod, Diaptomus leptopus, is extremely rare in oligotrophia Eunice Lake in the Coastal Range Mountains near
Vancouver, British Columbia. I_n situ experiments conducted in
1979 and 1980 tested the hypothesis that competition from zooplankton species resident in Eunice Lake prevents immigrant
D. leptopus from colonizing the lake. Polyethylene enclosures, each holding 29,000 1 of lake water, contained all experimental treatments.
Experiments in 1979 exposed a standard density of
D. leptopus to all Eunice Lake zooplankton species at lake densities (control), to all Eunice Lake species at reduced densities (low density), and to all Eunice Lake species except
one of Daphnia rosea (Daphnia-removal), Diaptomus kenai (kenai-
removal), or Diaptomus tyrelli (tyrelli-removal). Improved
performances of D. leptopus in non-control treatments was
evidence for competition in controls. Measures of performance
included density, number of eggs per female, and adult size.
Similarity between D. leptopus performances in control,
Daphnia-removal, and kenai-removal treatments plus relatively
high concentrations of D. leptopus nauplii in the tyrelli-
removal treatment suggested that D. leptopus did not compete
with species other than D. tyrelli. However, similarity
between tyrelli-removal and low density treatments in
concentrations of D. leptopus nauplii confounded competition
between D. leptopus and D. tyrelli with diffuse competition from several Eunice Lake species combined. D. leptopus overlapped in vertical distribution and seasonal cycle more with D. tyrelli than with other species. Differences' among experimental treatments in algal size compositions did not conclusively show whether zooplankton partitioned food resources.
Experiments in 1980, designed to separate D. tyrelli competition from diffuse competition, did not provide any evidence of interspecific interactions. By increasing zooplankton metabolic efficiencies, cool temperatures in 1980 may have virtually eliminated, competition for algal foods. TABLE OF CONTENTS
ABSTRACT ii
LIST OF TABLES vii
LIST OF FIGURES x
ACKNOWLEDGEMENTS xiii
INTRODUCTION 1
THE STUDY SYSTEM . 14
MATERIALS AND METHODS 21
EXPERIMENTAL DESIGN, 1979 -25
RESULTS, 1979 '. . 28
1. Results Confirming Proper Establishment of
Experiments 28
1.1 Enclosure Densities of Abundant Eunice Lake
Zooplankton Species 28
1.2 Enclosure Densities of Rare Eunice Lake
Zooplankton Species 39
1.3 Effects of Enclosure 47
2. Results Describing Effects of Experiments 49
2.1 Treatment Effects on Total Densities of Diaptomus
leptopus 49
2.2 Treatment Effects on Densities of Diaptomus
leptopus Eggs, Nauplii, Copepodites, and Adults .. 57
2.3 Treatment Effects on Adult Size of Diaptomus
leptopus 62
2.4 Seasonal Cycles, Vertical Distribution and Phytoplankton Biomass 64
3. Summary of 1979 Results 86
EXPERIMENTAL DESIGN, 1980 88
RESULTS, 1980 91
1. Results Confirming Proper Establishment of
Experiments 91
1.1 Enclosure Densities of Abundant Eunice Lake
Zooplankton Species 91
1.2 Enclosure Densities of Rare Eunice Lake
Zooplankton Species 103
1.3 Effects of Enclosure 109
2. Results Describing Effects of Experiments 110
2.1 Treatment Effects on Total Densities of Diaptomus
leptopus 110
2.2 Treatment Effects on Densities of Diaptomus
leptopus Eggs, Nauplii, Copepodites, and Adults ..116
2.3 Treatment Effects on Adult Size of Diaptomus
leptopus 116
2.4 Seasonal Cycles, Vertical Distribution and
Phytoplankton Biomass 121
3. Summary of 1980 Results 130
DISCUSSION 132
Separation of Diaptomus tyrelli Competition from Diffuse
Competition 132
Vulnerability of Diaptomus leptopus Nauplii to
• Competition 136
Relevance of Experimental Results to the Size-Efficiency vi
Hypothesis . 137
Relationship Between Temperature and Intensity of
Competition .139
Limitations of Phytoplankton Standing Stock Data 145
SUMMARY AND CONCLUSIONS .' 150
LITERATURE CITED 153 LIST OF TABLES
Table I. Physical, chemical, and biological characteristics
of Gwendoline and Eunice Lakes 17
Table II. Summary of 1979 experimental treatments 26
Table III. T-test statistics comparing mean densities of
major Eunice Lake species in Eunice Lake and in control
enclosures at start of 1979 experiments 29
Table IV. Statistical tests comparing mean densities of
Diaptomus tyrelli across treatments on initial sampling
date of 1979 experiments 30
Table V. Statistical tests comparing mean densities of
,Diaptomus kenai across treatments on initial sampling
date of 1979 experiments 31
Table VI. Statistical tests comparing mean densities of
Daphnia rosea across treatments on initial sampling date
of 1979 experiments 32
Table VII. Statistical tests comparing across treatments
for mean maximum densities of Bosmina longirostris
attained during 1979 experiments 42
Table VIII. Statistical tests comparing across treatments
for mean maximum densities of cyclopoid copepodites plus
adults attained during 1979 experiments 46
Table IX. Summary of statistical tests comparing mean
densities of nauplii, copepodites, adults, and mean
total densities of Diaptomus leptopus across treatments vi i i
for each sampling date of 1979 experiments 50
Table X. Statistical tests comparing across treatments for
mean maximum densities of Diaptomus leptopus attained
during 1979 experiments 55
Table XI. Statistical tests comparing across treatments for
mean persistence times of Diaptomus leptopus in 1979
experiments 56
Table XII. Statistical tests comparing across treatments
for mean number of Diaptomus leptopus eggs produced per
female during 1979 experiments 60
Table XIII. Lengths of adult diaptomid copepods and of
Daphnia rosea in 1979 experiments 63
Table XIV. Summary of 1980 experimental treatments 89
Table XV. T-test statistics comparing mean densities of
major Eunice Lake species in Eunice Lake and in control
enclosures at start of 1980 experiments 92
Table XVI. Statistical tests comparing mean densities of
Diaptomus tyrelli across treatments on initial sampling
date of 1980 experiments 93
Table XVII. Statistical tests comparing mean densities of
Diaptomus kenai across treatments on initial sampling
date of 1980 experiments 98
Table XXVIII. Statistical tests comparing mean densities of
Daphnia rosea across treatments on initial sampling date
of 1980 experiments 99
Table XIX. Statitical tests comparing mean densities of
Holopedium qibberum across treatments on initial sampling date of 1980 experiments 100
Table XX. Statistical tests comparing across treatments for
mean maximum densities of cyclopoid copepods attained
during 1980 experiments 108
Table XXI. Summary of statistical tests comparing mean
densities of nauplii, copepodites, adults, and mean
total densities of Diaptomus leptopus across treatments
for each sampling date of 1980 experiments .111
Table XXII. Analysis of variance comparing across
treatments for mean number of Diaptomus leptopus eggs
produced per female during 1980 experiments 119
Table XXIII. Lengths of adult diaptomid copepods and of
Daphnia rosea in 1980 experiments 120
Table XXIV. Maximum lengths arid maximum summer densities of
calanoid copepods and Daphn ia rosea in Ratherine,
Gwendoline, and Eunice Lakes 134 X
LIST OF FIGURES
Figure 1 •. Map showing location of study site 15
Figure 2. Densities of Diaptomus tyrelli in Eunice Lake and
in 1979 experiments 33
Figure 3. Densities of Diaptomus kenai in Eunice Lake and
in 1979 experiments 35
Figure 4. Densities of Daphnia rosea and Holopedium
gibberurn in Eunice Lake and in 1979 experiments 37
Figure 5. Densities of Bosmina longirostris in Eunice Lake
and in 1979 experiments 40
Figure 6. Densities of cyclopoid copepods in Eunice Lake
and in 1979 experiments 43
Figure 7. Total densities of Diaptomus leptopus in 'Eunice
Lake and in 1979 experiments 53
Figure 8. Densities of nauplii and of copepodites plus
adults of Diaptomus leptopus in Eunice Lake and in 1979
experiments 58
Figure 9. Life cycle of Diaptomus leptopus in Gwendoline
Lake 66
Figure 10. Life cycle of Diaptomus tyrelli in Eunice Lake . 68
Figure 11. Life cycle of Diaptomus kenai in Eunice Lake ... 70
Figure 12. Vertical distributions of zooplankton species in
1979 experiments during July 73
Figure 13. Vertical distributions of zooplankton species in
1979 experiments during September 75 Figure 14. July vertical distributions of Diaptomus
leptopus nauplii in the presence and in the absence of
Diaptomus tyrelli 77
Figure 15. Temperature profile for Eunice Lake during 1979
experiments 79
Figure 16. Phytoplankton standing stocks in Eunice Lake and
in 1979 experiments 83
Figure 17. Densities of Diaptomus tyrelli in Eunice Lake
and in 1980 experiments 94
Figure 18. Densities of Diaptomus kenai in Eunice Lake and
in 1980 experiments 96
Figure 19. Densities of Daphnia rosea and Holopedium
gibberum in Eunice Lake and in 1980 experiments 101
Figure 20. Densities of Bosmina longirostris in Eunice Lake
and in 1980 experiments 104
Figure 21. Densities of cyclopoid copepods in Eunice Lake
and in 1980 experiments 106
Figure 22. Total densities of Diaptomus leptopus in Eunice
Lake and in 1980 experiments 114
Figure 23. Densities of nauplii and of copepodites plus
adults of Diaptomus leptopus in Eunice Lake and in 1980
experiments ..117
Figure 24. Vertical distributions of zooplankton species in
1980 experiments during July 122
Figure 25. Temperature profile for Eunice Lake during 1980
experiments 125
Figure 26. Phytoplankton standing stocks in Eunice Lake and in 1980 experiments ...127
Figure 27. Diaptomid respiration rates as a function of
temperature 140 ACKNOWLEDGEMENTS
I thank my supervisor, Dr. W.E. Neill, for advice, assistance, and encouragement throughout my tenure as his student. In particular, I benefitted from the healthy scepticism and editorial skill with which he approached earlier drafts of the manuscript. I also thank the members of my advisory committee, Drs. C.J. Krebs and J.D. McPhail, for thoughtful input during the planning and writing stages of the thesis.
I acknowledge J. Walters and the staff of the University of
British Columbia Research Forest for permission to conduct my
research there and for general helpfulness. G. Borchert,
S. Brisson, A. Copsey, I. Forster, A. Leitae, D. Marmorek,
W. Neill, A. Peacock, J. Pennant, A. Reddenbach, and C. Wyatt worked long hours in the rain setting up or sampling
experimental enclosures. I am grateful to M.A. Chapman,
J. Green, W. Neill, A. Peacock, and G. Sandercock for helping
with zooplankton identification, to G. Jung and E. Krause for
counting phytoplankton samples, and to E. Krause, D. Marmorek,
W. Neill, and A. Peacock for sharing critical insights gleaned
from their extensive experience with zooplankton systems. I
apologize to the hundreds of thousands of. zooplankters whose
lives I unwillingly, but I hope humanely, sacrificed to science.
I acknowledge P. DeJong, D. Williams, and especially
N. Gilbert for useful statistical consultations. A. Copsey,
S. Harrison, D. Marmorek, T. Tenisci, C. Walters, and D. Zitten
provided computer programs and patiently answered endless xiv
questions about them. I particularly thank D. Marmorek for the computer plot program that saved me eons of drafting by producing most of the figures in the thesis.
Faculty and students of the Institute of Animal Resource
Ecology generated the intellectually stimulating and socially sustaining environment that made graduate studies a multi-
faceted and rewarding education. The unfailing support and genuine concern of L. Gass and C. Krebs eased the trying and discouraging times. As always, and in all ways, I am grateful
to my two friends and mentors, D.I. MacKenzie and A. Peacock.
To my sister, Debby Olenick Hirsch, who made me realize that
contradictory, confusing, and inconclusive results are exciting
results, I owe my own enthusiasm for this project. .To her, and
to my parents, I am indebted for understanding, reassurance, and
love.
During the study, personal support came from a Natural
Sciences and Engineering Research Council Scholarship, a
University of British Columbia Graduate Fellowship, and a
Teaching Assistantship. Research support was provided by an
Operating Grant from the Natural Sciences and Engineering
Research Council of Canada to W.E. Neill. 1
INTRODUCTION
Many ecologists accept that the process of interspecific competition determines patterns of species distribution and abundance, species diversity, and community organization
(Crombie 1947; Miller 1967; Levins 1968; MacArthur 1972; Cody
1974; Pianka 1976; Diamond 1978). Three types of studies laboratory experiments, field observations, and controlled field experiments - have contributed to these convictions.
The earliest competition studies were laboratory experiments in which species were cultured together in simple, artificial environments. Gause's (1934) famous experiments demonstrated that, although Paramec ium caudatum could be successfully cultured alone, when grown with P. aurelia it was invariably outcompeted for limited bacterial food. Which of
Tribolium confusum and T. castaneum displaced the other depended on temperature and humidity conditions (Park 1954). From these and similar experiments developed the Competitive Exclusion
Principle, which states that "complete competitors cannot coexist" (Hardin 1960). The applicability of this tenet to natural populations is unclear because the laboratory systems generating it may overestimate the importance of competition.
Single-resource laboratory environments force species to compete, but in natural environments multiple factors can alleviate competition. Predation and environmental variation can hold. populations of potential competitors below carrying capacity (Paine 1966; Wiens 1977), and resource heterogeneity
can facilitate resource partitioning (Schoener 1974). More 2
elaborate laboratory experiments have exemplified the importance of mediating factors. Parasites altered competitive outcomes between Tribolium species (Park 1948). Crombie (1945) demonstrated prolonged coexistence of two grain beetles provided with two food resources. Paramecium aurelia and P. bursaria coexisted in complex culture media where they could vertically
segregate (Gause 1936). Results from even these sophisticated
laboratory environments are extensible to field situations only with caution because total replication of complex natural
environments in an artificial context is not possible.
Despite these limitations, the first ecologists to study
competition in the field took their lead from laboratory
results. They attempted to validate the Competitive Exclusion
Principle with detailed observations on resource use patterns of
potentially competing species (usually sympatric congeners).
They looked among these patterns for species differences
interpretable as competition-reducing coexistence mechanisms. If
they detected no differences, they assumed that coexistence
mechanisms remained among as yet unmeasured niche dimensions
(Hardin 1960; Peters 1976). Because infinitely dimensioned
niches are never entirely quantifiable (Hutchinson 1957), the
Competitive Exclusion Principle is untestable.
A classic example of a search for differences among
coexisting congenerics is MacArthur's (1958) study of foraging
behaviour of five insectivorous .Dendroica warblers. MacArthur
concluded that disparities in position of feeding, in methods of
prey capture, and in direction and speed of movement were 3
sufficient to allow coexistence. Countless similar studies on various organisms exist (e. g., Rand 1964; Cody 1968; Brown and
Lieberman 1973; reviewed in Schoener 1974). Many of them have summarized observations on resource use with indices of niche overlap (MacArthur and Levins 1967; Levins 1968; Williamson
1971; Brown 1975; Pianka 1975).
All of these field studies share a serious flaw. They have inferred the process of competition from an observed pattern of resource use. In fact, several processes could have produced the same pattern (Dayton 1973). For example, rather than competition, species differences in predator avoidance strategies (Rosenzweig 1973), in phylogenetically-determined preferences, or in physiological tolerances (Dumas 1956) can cause differences in habitat use. The relationship between niche overlap and competition is ambiguous; high overlap in use of particular resources indicates intense competition when those resources are limited, and no competition when they are not
(Colwell and Futuyma 1971; Zaret and Rand 1971; Pianka 1974).
Knowledge of resource abundance and availability, then, is critical to interpreting which processes underlie resource use patterns. Obtaining such- information is notoriously difficult
(e. g., Jaeger 1972; Fraser 1976) because it requires being able to perceive resources as animals themselves do.
In addition to differences in resource use patterns among
sympatric species, ecologists have inferred competition from evidence of niche shifts and character displacement (Brown and
Wilson 1956) among similar species occurring both sympatrically 4
and allopatrically. Greater similarity in allopatry than in sympatry of resource use, or of morphological characters correlated with resource use, presumably reflects competition in the zone of sympatry • (Grant 1972a; Diamond 1978). Lister (1976) noted niche expansion in habitat use by Anolis where congeneric competitors were few. Beak sizes (and therefore probably diets) of Geospiza fuliqinosa and G. fortis are more dissimilar on those islands of Galapagos where they occur sympatrically (Lack
1947; Abbott et al. 1977). These types of observations form natural "experiments" in that zones of sympatry and allopatry correspond to with- and without-competitor "treatments", but they are uncontrolled experiments in that competitor abundance is not the sole difference between zones. Consequently, niche shifts can result from differences between areas of sympatry and allopatry in predator abundance, prey type, abiotic factors, or any number of attributes unrelated to competition intensity.
Grant (1972a) demonstrated that clinal variation in morphological features explained many classical examples of character displacement. Strong et al. (1979) argued that random processes alone were sufficient to account for differences in morphology among bird species sympatric on islands (but see
Grant and Abbott 1980).
The strongest evidence for competition has come not from
laboratory experiments or field observations, but from controlled field experiments. These involve monitoring
performances of species exposed to artificially manipulated
competitor densities under otherwise natural conditions. 5
Measures of performance include indices of fitness such as survival, fecundity, density, abundance, and growth rate. Better performance of one species in the absence than in the presence of a second demonstrates competition between the two (Grant
1972b). Connell's (1961) early field experiment on barnacles showed that Chthamalus stellatus existed in the lower intertidal only when Balanus balanoides, which reduced survival and reproduction of Chthamalus, was removed. Jaeger (1971) experimentally demonstrated that the salamander Plethodon cinereus competitively excluded P. richmondi from optimal moist habitats. Reciprocal increases in densities of granivorous ants and rodents in the absence of each other verified competitive
interactions between taxonomically dissimilar organisms
(Davidson et al. 1980).
Field experiments combine advantages of field observations and laboratory experiments. As in descriptive studies, they
retain complexity of natural systems. As in laboratory studies,
the competitive process is indexed directly as performance,
rather than inferred from a pattern. This permits affirmation of competition without often unobtainable knowledge of resource
abundance and availability. The major limitation in field
experiments is that populations of large or mobile species, such
as large mammals or birds, are not readily manipulable (Connell
1975). Many studies have effected perturbations by confining
populations (e. g., Jaeger 1971; Wilbur 1972; DeBenedictis
1974). These studies must consider confounding of competitive
outcomes with reduced dispersal (Boonstra and Rrebs 1977), 6
predator exclusion, limited nutrient flow (Lehman 1980a and b), and other possible enclosure effects (Connell 1975).
Observational and experimental field studies can be complementary. Observations are useful in recognizing potentially competitive situations. Once experimentation has confirmed that competition is involved, observations on patterns of resource use can assist in identifying limited, competed-for resources. (However, responses of competitors to experimental alterations of resource levels more reliably detect crucial resources [Vance 1972; Brown and Davidson 1977; but see Thomson
1980]). Werner and Hall (1976,1977) and Werner (1977) successfully amalgamated observation and experiment in their work on centrarchid fishes. On the other hand, disagreement between observational and experimental results for two species of Dipodomys emphasized the need to test observations experimentally. High overlap in habitat use indicated intense interspecific competition between the rodents; yet, removal experiments implicated only intraspecific competition (Schroder and Rosenzweig 1975). Since competition may occur only periodically if environmental conditions are variable (Wiens
1977), both observational and experimental studies must be long- term relative to generation times of competitors (Johannes and
Larkin 1-961; Schroder and Rosenzweig 1975; Wiens 1977).
Field experiments are more profitable to the study of competition than either laboratory experiments or purely descriptive field studies because they provide explicit evidence for the process in natural environments. Increasingly, field 7
experiments have been examining competition among all types of organisms, from mammals (Grant 1972b; Krebs 1977; Price 1978), birds (Davis 1973), reptiles (Dunham 1980), amphibians (Wilbur
1972; Jaeger 1974; Hairston 1980), and fish (Werner 1977; Hixon
1979, 1980; Larson"l980), to invertebrates (Paine 1971, 1974;
Underwood 1.978). Full evaluation of the importance of competition to ecological systems requires more field experiments in a wider variety of communties (Connell 1975;
Pianka 1976; MacLean and Magnuson 1977; Wiens 1977).
Among those communities requiring further competition studies are lacustrine herbivorous zooplankton communities
(Lynch 1977a). Current ideas on zooplankton community organization stem from the size-efficiency hypothesis (Brooks and Dodson 1965; Kerfoot and DeMott 1980). This hypothesis
states that, because they feed less efficiently on small particles and consume a narrower size range of particles, small herbivorous zooplankton species are competitively inferior to
large species. Consequently, only under intense size-selective
predation on large species can small ones dominate the community
(Brooks and Dodson 1965; Hall et al. 1976) A variation of the
size-efficiency hypothesis, the complementary feeding niche
hypothesis (Dodson 1970), claims that visual vertebrate
predators enhance the small zooplankton prey of invertebrate
planktivores by selecting competitively superior prey. These and
similar views of zooplankton community organization stress the
role of predation while implicitly assuming the importance of
differential competitive abilities of large and small 8
herbivores.
Numerous studies, some of them contradictory and many of them experimental, have examined contributions, especially of vertebrate predation, to organization of zooplankton communities
(Hall et al. 1970; Lynch 1979; O'Brien 1979; Zaret 1980).
Experimental additions of fish to fishless lakes often (Brooks and Dodson 1965; Anderson 1972), but not always (Northcote et al. 1978), have caused shifts in zooplankton species compositions from larger to smaller organisms. Field experiments have supported the complementary feeding niche hypothesis for salamanders and Chaoborus larvae (Dodson 1970; Giguere 1979).
Several studies have emphasized the impact of invertebrate predation on community structure and population dynamics of herbivorous prey (McQueen 1969; Allan 1973; Dodson 1974; Lewis
1979; Lynch 1979; reviewed in Zaret 1980), though field experiments in oligotrophic lakes have shown resistance of zooplankton populations to variations in densities of Chaoborus larvae and predaceous cyclopoid copepods (Neill and Peacock
1980; Neill 1981a; Peacock 1981).
While predation in zooplankton communities has been amply
investigated, competition has not (Hall et al. 1976; Lynch
1979). Most zooplankton competition studies have been
observational, describing differences among co-occurring species
in seasonal cycle, spatial distribution or diet as coexistence mechanisms (e. g., Tappa 1965; Heip 1973; Lane and McNaught
1970, 1973; Lane 1975; Poulet 1978; Ranta 1979; Seitz 1980).
Like most descriptive studies, these have assumed rather than 9
demonstrated competition. Many laboratory experiments have exhibited competitive exclusion among cladocerans (e. g., Frank
1952, 1957; Parker 1960, 1961; deBernardi 1979; Goulden and
Hornig 1980), but because field tests have shown coexistence in nature of species incompatible in the laboratory (Allan 1973; deBernardi 1979; Kerfoot and DeMott 1980), laboratory data are suspect. The laboratory microcosm work of Neill (1975a and b) demonstrated that, contrary to the size-efficiency hypothesis, small Ceriodaphnia outcompeted larger cladocerans. In an unusual example of allelopathy among copepods, Fol-t and Goldman (1981)
found that Epischura nevadensis in laboratory systems released a chemical causing Diaptomus tyrel1i to reduce its filtering capacity by 60%.
Few field experiments on zooplankton competition exist.
None has been very conclusive, and all have been on cladocerans.
Dodson (1974) concluded that Diaptomus shoshone predation on
small Daphnia minnehaha explained the latter's absence from
ponds containing large Daphnia middendorffiana. Working on the
same system in a previous year, Sprules (1972) concluded that
the larger species competitively excluded the smaller one. In
experimentally enclosed communities, small Cer i odaphn ia
reticulata either coexisted with large Daphn ia pulex or
outcompeted it (Lynch 1978). Similarly, competition between
D. pulex and small Bosmina longirostris produced no consistent
outcome (Kerfoot and DeMott 1980).
Amending the lack of conclusive field experiments on
zooplankton competition could well begin with the calanoid 1 0
copepod genus Diaptomus. Although some lake survey studies have stressed that Diaptomus species do not commonly coexist
(Grainger 1952; Pennak 1957; Patalas 1971), examples of co• occurrences abound in the literature (Cole 1961). Reed (1964) recorded four diaptomid copepods in one north Saskatchewan lake.
Davis (1954, 1961) noted five diaptomids in Lake Erie. Cole
(1961) cited 34 different combinations of 2 or more coexisting
Diaptomus species. Fifty-one of 100 south Ontario lakes sampled contained more than one diaptomid (Rigler and Langford 1967). To date, the many field studies investigating interspecific interactions among Diaptomus species have been purely observational. Together they have provided a notion of diaptomid competition that needs experimental validation.
Descriptive studies on competition in Diaptomus typically have interpreted differences among co-occurring species as coexistence mechanisms. Three main mechanisms have dominated the diaptomid literature.
(1) Dietary differences favour diaptomid coexistence
(Hutchinson 1951; Cole 1961; Sandercock 1967). Coexisting
D. grac i1is and D. lat iceps eat small green algal cells and
the diatom Melosira, respectively (Fryer 1954). Small
D. minutus and larger D. oregonensis coexist in Gull Lake
by partitioning food cells by size (Mellinger 1974).
Hutchinson (1951) suggested diaptomid size disparity as a
coexistence mechanism equivalent to dietary difference.
D. pribilofensis and D. arcticus differ in length by a 11
factor of two, and coexist in Alaskan lakes (Tash and
Armitage 1967). Similarly-sized D. tyrelli and D. lintoni
do not co-occur in high altitude Wyoming lakes (Williams
1976).
(2) Vertical segregation promotes diaptomid coexistence
(Cole 1961; Sandercock 1967). D. minutus coexists with
D. sicili s in lakes in northwestern Ontario (Patalas 1971)
and with D. oregonensis in lakes in southern Ontario
(Rigler and Langford 1967) because it resides higher in the
water column.
(3) Seasonal separation facilitates diaptomid coexistence
(Cole 1961; Davis 1961). Up to four diaptomids can inhabit
one shallow Saskatchewan pond because spring and summer
breeding species occur asynchronously (Sawchyn 1966; Hammer
and Sawchyn 1968; Sawchyn and Hammer 1968). D. sanguineus
and D. birqei similarly segregate in Maryland ponds
(Smrchek 1973).
While studying three diaptomids in Clarke Lake, Ontario,
Sandercock (1965, 1967) found that two of these coexistence mechanisms operated for each species pair. D. minutus and
D. sanguineus differed in size and vertical distribution;
D. oreqonensis and D. sanguineus, in vertical distribution and seasonal cycle; and D. minutus and D. oregonensis, in size, and seasonal cycle. She suggested that in general two mechanisms in combination are needed for diaptomid coexistence. Most Diaptomus 1 2
studies have invoked at least two of these three factors (e. g.,
Cole 1961; Tash and Armitage 1967; Hammer and Sawchyn 1968;
Sprules 1972; Maly and Maly 1974; Maly 1976; Pederson and Litt
1976).
Coexistence mechanisms other than these three common ones have occasionally been invoked. The short duration over which competition operates allows many similar diaptomid species to coexist in temporary ponds (Cole 1966; Anderson and Fabris 1970) and in arctic environments with short growing seasons (Tash and
Armitage 1967). Fugitive species can temporarily evade superior competitors by invading environments not yet colonized by them
(Elton 1929; Hutchinson 1951). Cole (1961) suggested that dissimilarities among Diaptomus species in different subgenera are sufficient to permit coexistence, but Turvey (1968) showed statistically that subgeneric co-occurrences happen as
frequently as predicted by chance. In occasional cases of diaptomid co-occurrence, no coexistence mechanism is apparent
(Davis 1961; Hoffman 1979).
Verification of differences among Diaptomus in diet, vertical distribution, and seasonal cycle as coexistence mechanisms needs experimental demonstration of diaptomid
competition, particularly in non-headwater lakes. Continuous
influx into these lakes from upstream waters can obscure
diaptomid interactions by maintaining populations of immigrants
unable to compete with resident species (Tonolli 1953; Cowell
1967; Turvey 1968). Yet no diaptomid researchers have examined
experimentally whether non-headwater co-occurrences rely on 13
species differences as coexistence mechanisms or on successive immigrations. Few even have indicated the hydrologic status of their study lakes (Turvey 1968).
This thesis experimentally evaluates effects of competition from zooplankton resident in a non-headwater lake on performance of a diaptomid copepod that regularly immigrates from an upstream headwater lake. Hopefully, insights gained will begin to remedy the shortage of conclusive experimental information on competition in zooplankton communities. 1 4
THE STUDY SYSTEM
The study site is a coastal montane lake in the University of British Columbia Research Forest, near Haney, British
Columbia (Fig. 1). Long-term monitoring studies (Efford 1967;
Northcote and Clarotto 1975) and experimental manipulations
(Northcote et al. 1978; Neill 1978, 1981a; Neill and Peacock
1980; Peacock 1981) provide background information on Research
Forest lakes. In general, they are small, oligotrophic, thermally stratified in summer, well oxygenated, and deep (Neill
1978). Planktivorous fish naturally occur in some of them
(Northcote and Clarotto 1975), and recently have been introduced
into others (Northcote et al. 1978). The lakes contain similar crustacean zooplankton species in different relative abundances
(Northcote and Clarotto 1975). Detritus, bacteria, and algae provide food for filter-feeding species. Concentrations of grazable (<30 urn in diameter) particulate organic matter are
low, usually varying between 0.3 and 0.9 mg/1 ash-free dry
weight (Neill 1978), but increasing to nearly 1.6 mg/1 after
heavy rains (Peacock 1981; Neill and Walters unpubl. data).
Table I summarizes abiotic and biotic characteristics of
two connected Research Forest lakes. A small stream joins
Gwendoline, a headwater lake, to Eunice Lake. Except for
Diaptomus tyrell1, which occurs only in Eunice Lake, species
compositions in the two lakes are similar. Except for Diaptomus
leptopus, which is abundant in Gwendoline Lake, densities of
species common to both lakes are higher in Eunice Lake.
D. leptopus occurs at barely detectable levels in Eunice despite 15
Figure 1. Map showing location of study site. From Neill (1978) 16 1 7
Table I. Physical, chemical, and biological characteristics of Gwendoline and Eunice Lakes. Crustacean densities are maximum summer densities in limnetic region, averaged over water column sampled near midday. Adapted from Northcote . and Clarotto (1975)
Characterisitic Gwendoline Eunice
Elevation (m) 522 480
Drainage area (ha) 81 191
Surface area (ha) 13.0 18.2
Maximum depth (m) 27 42
Mean depth (m) 13.4 15.8
Total dissolved solids (mg/1) 18 16
pH 6.6 6.4
Colour (Pt units) 15 15
Transparency (Secchi disc, m) 5-7.2 6-10
Epilimnion depth, late summer (m) 5 4-6
Hypolimnion 02, minimum (mg/1) 4.0 0.6
Crustacean density (#/l00 1) Daphn ia rosea 95 233 Holopedium gibberum 45 124 Diaphanosoma brachyurum 194 253 Polyphemus pediculus 4.5 13 Ceriodaphnia pulcheila rare rare Bosmina longirostris 116 230 Diaptomus kenai 10 70 Diaptomus leptopus 13 0.5 Diaptomus tyrelli -- 800 Cyclopoid copepods 14 27
Years zooplankton sampled 1967, 1969 1967, 1969 1971, 1972 18
a ready means of immigration via the stream from Gwendoline.
This study experimentally tests the hypothesis that competition from one or more resident zooplankton species prevents immigrant D. leptopus from colonizing in Eunice Lake.
The occurrence of D. tyrelli only in Eunice Lake and the higher crustacean densities there than in Gwendoline Lake both support a competitive explanation of D. leptopus rarity in Eunice. Two other possible explanations - abiotic factors and predation - can be disregarded a priori. Considering their connection, the striking physical and chemical similarities between Gwendoline and Eunice Lakes are not surprising (Table I). That some abiotic characteristic unique to Eunice is unfavourable to D. leptopus is thus unlikely.. That some Eunice Lake predator eliminates
D. leptopus is equally improbable. Historically, both Eunice and
Gwendoline Lakes contained larvae of Chaoborus americanus and
C. trivittatus as major predators (Fedorenko and Swift 1972?
Northcote and Clarotto 1975). Cutthroat trout (Salmo clarki)
introduced into Eunice in 1974-76 eliminated Chaoborus, and are now themselves the major predator (Northcote et al. 1978).
Throughout these disparate predation regimes, D. leptopus has
remained rare in Eunice Lake.
Diaptomus leptopus is a herbivorous, filter-feeding calanoid (Schindler and Comita 1966; Anderson 1970). This widespread species occurs across North America from Virginia and
Oregon in the south to Alaska in the north (Wilson 1959). It
survives in alpine and subalpine lakes (Wilson 1959; Anderson
1971), but is more common in montane lakes (Patalas 1964; 1 9
Anderson 1974) and prairie ponds (Reed and Olive 1958; Anderson
1971). Although it occurs in oligotrophic environments (Winner
1970; Patalas 1971; Schindler and Noven 1971), many records are from eutrophic waters (Hazelwood and Parker 1961; Healey 1967;
Smith 1968; Anderson and Fabris 1970) and from temporary (Hammer and Sawchyn 1968; O'Brien et al. 1973) or semi-permanent ponds
(Whittaker and Fairbanks 1958).
In montane Eunice Lake, D. leptopus co-occurs at low density with two other herbivorous (Anderson • 1970) diaptomids.
Both D. tyrelli (Carl 1940; Wilson 1959; Anderson 1971, 1972) and D. kenai (Wilson 1959; Pederson and Litt 1976) are more characteristic of oligotrophic, high altitude lakes than
D. leptopus. D. tyrelli is smaller (length = 1.1 - 1.9 mm), and
D. kenai larger (length = 1.8 - 3.0 mm) than ' D. leptopus
(length = 1.25 - 2.5 mm) (Wilson 1959). Cole (1961) cited co• occurrences of D. leptopus with D. wilsonae and D. minutus. From a survey of 52 Colorado lakes, Patalas (1964) concluded that
D. leptopus tends not to co-occur with congeners. Of seven diaptomids in Saskatchewan ponds, only D. leptopus occurs alone for periods of more than 3 weeks (Hammer and Sawchyn 1968).
Information exists for D. leptopus on responses to abiotic
factors (Hazelwood and Parker 1961, 1963; Winner 1970; O'Brien et al. 1973), on feeding (Schindler and Comita 1966; Levy and
Comita 1971) and respiration rates (Comita 1968), and on
reproductive physiology (Watras 1980; Watras and Haney 1980), general ecology, and life history (Sawchyn and Hammer 1968;
Winner 1970; O'Brien et al. 1973). As is typical of diaptomids, 20
D. leptopus develops from sexually reproduced eggs through six naupliar and six copepodite stages. The final copepodite is the adult (O'Brien et al. 1973). D. leptopus produces from one
(O'Brien et al. 1973) to three generations (Sawchyn and Hammer
1968) per year. Generation time varies from 4.6 weeks at 20-25°C in a temporary pond (O'Brien et al. 1973) to 7.2 weeks at 8-16°C in a bog lake (Winner 1970). Winner (1970) found that adults of fall generations in a bog lake produced overwintering resting eggs. D. leptopus in a temporary pond produced only subitaneous eggs, but not all of these hatched immediately (O'Brien et al. 1973). 21
MATERIALS AND METHODS
Experiments designed to evaluate effects of interspecific competition on Diaptomus leptopus in Eunice Lake involved confining this species with various combinations of Eunice Lake
species. Specific experimental designs and the hypotheses they
test are described with their results. This section details the
general methods of establishing and monitoring all
manipulations.
Experiments were conducted in Eunice Lake from June to
October, 1979 and from May to September, 1980. Large enclosures
similar to those of Neill (1978) contained the experimental
treatments. Tubes of 6 mil (1979) or ultraviolet-resistant 4 mil
(1980) transparent polyethylene formed the enclosures. Wood and
styrofoam frames suspended them from the surface of the lake,
and tied off lower ends prevented contact with the sediments.
Enclosures measured 1.5 m wide by 12-13 m deep. Approximately
29,000 1 of lake water, filtered through a 54 urn net to remove
zooplankton but not grazable seston, filled each one.
Ten vertical hauls, taken from Eunice Lake at random points
with a 54 urn plankton net, determined natural lake densities of
each zooplankton species. Experimental densities were derived
from these determinations (see descriptions of experimental
designs). Animals required for the manipulations were obtained
by townet, and then sorted by sieve and pipette into the
appropriate combinations. To reduce mortality during sorting,
zooplankton were kept out of the lake for less than 24 hr and
were maintained at cool temperatures. Sorted zooplankton were 22
released into the enclosures over a period of approximately 21 days in 1979, and 5 days in 1980. All animals, except
D. leptopus from Gwendoline, came from Eunice Lake.
Zooplankton were sampled from each enclosure at approximately 5-day intervals with a 2.5-cm diameter hose attached to an electric bilge pump capable of pumping 25 1/min.
Samples of 150 1 were filtered through a 54 urn net to concentrate the animals. Equal volumes were filtered at 1-m
intervals from the surface to 11 m (1979) or 12 m (1980). A
similar procedure along a mid-lake transect sampled Eunice Lake.
Peacock (1981) showed that this method provides random and non•
selective samples of the different crustaceans. Collected
zooplankton were preserved in dilute formalin.
Twice in 1979 (July 5-6 and September 18-19), 50 1 samples
were collected from each of 0-2, 2-4, 4-7, and >7 m depth
strata. These depth-stratified samples were collected in both
daylight and darkness to determine vertical and diel
distributions of zooplankton in each enclosure. On July 10,
1980, vertically stratified samples were taken from one
replicate per treatment in daylight only.
Zooplankton samples were examined for species identity,
instar distribution, and reproductive condition (number of
ovigerous females, number of eggs per female, and number of
unattached eggs) of calanoid copepods, for instar distribution
of cyclopoid copepods, and for species identity of cladocerans.
All animals in all samples were counted according to these
categories under a dissecting microscope at 25x magnification 23
for nauplii and at 10X magnification for other organisms.
Immature calanoids were identified to species using taxonomic descriptions developed specifically for the Research Forest lakes (A. Chapman and J. Green pers. comm. and unpubl. data).
Other zooplankton were identified using keys in Edmondson (1959) and Pennak (1978).
Under 25x magnification, lengths of the first 10
individuals of calanoid and major cladoceran species encountered
in a sample were measured for samples collected approximately
10-14 days apart. Copepod measurements extended from anterior
tip of cephalothorax to posterior end of urosome, excluding
setae. Cladocerans were measured from anterior to posterior tip
of carapace, excluding caudal spines, helmets, and other
accessory structures. Only adult calanoids were sized.
Cladocerans were measured regardless of maturity.
Analyses on zooplankton data included Analysis of Variance
(ANOVA), Analysis of Covariance (ANCOVA), and t-test comparisons
(Snedecor and Cochran 1967; Sokal and Rohlf 1969; Neter and
Wasserman 1974) computed by MIDAS statistical packages (Fox and
Guire 1976). To stabilize the variance, species counts were log-
transformed (log{x+l}) prior to analysis (Elliot 1977).
Phytoplankton were sampled from enclosures and from Eunice
Lake every 10-14 days. From a mixture containing 1 1 of water
pumped from each of four depths (1.5, 4.5, 7.5, and 10.5 m), a
250-ml sample was preserved in Lugol's solution. A 100-ml
subsample was allowed to settle for at least 24 h. Then 75 ml of
the supernatant was decanted and the remaining 25 ml allowed to 24
settle for a further 24 h. Using an inverted microscope at 400x magnification, cells were counted until at least 200 cells >2 urn in diameter or two microscope fields were inspected. Counts were converted to biomass assuming a cell specific gravity of 1 g/ml and cell volumes of 4, 42, 286, 497, 1571, and 3252 urn3 for single cells <2, 2-6, 6-10, 10-14, 14-20, and >20 urn in diameter, respectively, and volumes of 300 urn3 for colonies and
filaments.
Water temperatures were recorded for each sampling date at
1-m depth intervals by pumping water from the appropriate depth over a thermometer accurate to ±0.5°C. The subroutine CNTOUR
from the University of British Columbia Computing Centre (Mair
1980) interpolated temperature isoclines over depth and time. 25
EXPERIMENTAL DESIGN, 197 9
1979 experiments attempted to determine which, if any, abundant Eunice Lake zooplankton species competitively prevents
Diaptomus leptopus from colonizing the lake. Similar to most competition experiments, these compared several indices of performance of D. leptopus in the presence and in the absence of potential competitors. Improved performance in the absence of a
species implicated that species as a competitor. Indices of
performance included density, reproduction and adult size.
Table II outlines the experiments and the hypotheses they
test. Polyethylene tubes described in Materials and Methods
enclosed two replicates of each treatment. All enclosures
contained a standard density of 0.14 .D. leptopus per litre
(adults and copepodites combined). This density, although
considerably higher than that in Eunice Lake during initiation
of experiments, was the minimum deemed detectable in zooplankton
samples. In addition to D. leptopus, experimental controls
contained the entire Eunice Lake crustacean assemblage at
natural lake densities. Three treatments, by almost entirely
excluding one of Daphnia rosea, Diaptomus kenai, or Diaptomus
tyrelli, tested competitive effects of these species on
D. leptopus. Like D. leptopus, all three are herbivorous filter-
feeders. Its absence from Gwendoline Lake, where D. leptopus
thrives, makes D. tyrelli a likely candidate for competitor.
D. kenai and D. rosea coexist with D. leptopus in Gwendoline,
but may compete with it in Eunice where they reach higher
densities (Table I). Because zooplankton biomass is lower in Table II. Summary of 1979 experimental treatments = Eunice Lake zooplankton assemblage at 1979 natural lake densities.
Treatment Species composition Hypothesis tested
Control N + D i aptomus 1eptopus Compare performance of D. 1eptopus here with that in all other treatment to test all hypotheses.
Low density 10% N + D. 1 eptopus Diffuse competiti on from several zooplankton speci es prevents D. 1eptopus from co1 on i z i ng Eunice Lake.
Daphnia-removal N - Daphnia rosea Competition with Daphnia rosea + D. 1eptopus prevents D. 1eptopus from colonizing Eunice Lake.
kenai-removal N - Diaptomus kenai Competition with D i aptomus kenai + D. 1eptopus prevents D. 1eptopus from colonizing Eunice Lake.
tyrel1i-removal N - Diaptomus tyrel1i Competition with Diaptomus tyrel1i + D. 1eptopus prevents D. 1eptopus from colonizing Eunice Lake. 27
Gwendoline than in Eunice Lake, a low density treatment tested for effects of diffuse competition on D. leptopus in Eunice.
Lowered biomasses of D. rosea, D. kenai, and D. tyrelli in
removal treatments potentially confounded diffuse competitive effects with effects of competition from specific species.
Therefore, the biomass of species excluded from removal
treatments was replaced with a roughly equivalent biomass of
remaining Eunice Lake species in their lake relative
proport ions.
In addition to treatments outlined in Table . II, two
replicate enclosures contained the entire Eunice Lake assemblage
without D. leptopus. Direct comparison between these and Eunice
Lake would detect enclosure effects on zooplankton population
dynamics. 28
RESULTS, 197 9
j_. Results Confirming Proper Establishment of Experiments
J_.J_ Enclosure Densities of Abundant Eunice Lake Zooplankton
Spec ies
Similarities between the control treatment and Eunice Lake
in starting densities of Diaptomus tyrelli, Diaptomus kenai,
Daphn ia rosea, and Holopedium gibberurn (Table III) indicate
proper stocking of control enclosures with zooplankton. The
experimental design designates densities in remaining treatments
as perturbations of control densities. Statistical analyses
presented in Tables IV, V, and VI show that initial enclosure
concentrations of common Eunice Lake species fulfilled design
requirements. Relative to controls, t y r e11i-, kenai-, and
Daphn ia-removal treatments contained fewer of the excluded
species. Low density enclosures contained fewer of all species.
H. gibberum levels did not differ among treatments (ANOVA:
F = 1.80; p = 0.266). Figs. 2, 3, and 4 reveal that, overall,
these treatment differences persisted throughout the experiment.
Because they were used to replace removed biomass, unexcluded
species were slightly more common in removal than in control
treatments. 29
Table III. T-test statistics comparing mean densities (numbers of animals per 150 1 samples) of major Eunice Lake species in Eunice Lake and in control enclosures at start of 1979 experiments. Tests analyzed log-transformed counts from the first two sampling dates, but means and standard deviations shown are of untransformed data. N=nauplii; C=copepodites; A=adults; J=juveniles.
Location Spec ies Lake Controls df
Diaptomus tyrelli X: 541.5 482, 0.39 4 0.719 (N+C+A) s: 211.4 1 55, n: 2 4
Diaptomus kenai X: 68.5 1 1 1 3 -0.97 4 0.387 (N+C+A) s: 0.7 44 5 n: 2 4
Daphnia rosea X: 21 1 5 1 09.8 1 . 40 4 0.233 (J+A) s: 1 52 0 20.0 n: 2 4
Holopedium qibberum X: 131.0 84 0.46 4 0.671 (J+A) s: 127.3 69 n: 2 4 30
Table IV. Statistical tests comparing mean densities of Diaptomus tyrelli (numbers of nauplii, copepodites, and adults combined per 150 1 sample) across treatments on initial sampling date (June 20) of 1979 experiments. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log-transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 4 16.40 4.10 16.59 0.004 Within 5 1.24 0.25 Total 9 17.64
Pairwise Multiple Comparisons:
Treatment Mean (#/150 1) Std. Dev,
Control: 603.0 114.6 :Low density 56.0 7. 1 22.45 0.005 :Daphnia-removal 485.5 208.6 0.26 0.629 : kenai-removal 593.0 444 . 1 0.12 0.744 : tyrelli-removal 29.0 15.6 38.03 0.002 31
Table V. Statistical tests comparing mean densities of Diaptomus kenai (numbers of nauplii, copepodites, and adults combined per 150 1 sample) across treatments on initial sampling date (June 20) of 1979 experiments. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log-transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 4 18.81 4.70 8.54 0.019 Within 5 2.75 0.55 Total 9 21.56
Pairwise Multiple Comparisons:
Treatment Mean (#/150 1) Std. Dev.
Control: 1 33.5 9.2 :Low density 21.5 23.3 8.58 0.033 :Daphnia-removal 489.5 234. 1 2.77 0. 157 :k_ena_i-removal 11.5 6.4 10.85 0.022 :tyrelli-removal 80.5 51 .6 0.68 0.447 32
Table VI. Statistical tests comparing mean densities of Daphnia rosea (numbers per 150 1 sample) across treatments on initial sampling date (June 20) of 1979 experiments. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log-transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 4 7.91 .1.98 9.48 0.015 Within 5 1.04 0.21 Total 9 8.95
Pairwise Multiple Comparisons:
Treatment Mean (#/l50 1) Std. Dev. F p
Control: 109.0: 26.9: .-Low density :33.0 :26.9 8.69 0.032 :Daphnia-removal :10.5 :3.5 24.65 0.004 :kena_i-removal :86.5 :21 .9 0.25 .0.636 :tyrelli-removal :101.0 :15.6 0.02 0.890 33
Figure 2. Densities of Diaptomus tyrelli (nauplii, copepodites, and adults combined) in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphn ia-removal; E = kenai-removal; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES 35
Figure 3. Densities of Diaptomus kenai (nauplii, copepodites, and adults combined) in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphnia-removal; E = kenai-removal; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES 37
Figure 4. Densities of Daphnia rosea and Holopedium gibberum in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphnia-removal; E = kenai-removal; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES
Bo o o o o o ° o o o o o B cs oo o B 39
J_.2_ Enclosure Densities of Rare Eunice Lake Zooplankton Spec ies
Enclosure densities were not regulated for species rare
(<0.5/l) in Eunice Lake at the start of the experiments.
Nevertheless, maximum densities of Chydorus spp. did not differ
across treatments (ANOVA: F = 0.39; p = 0.810). Ceriodaphnia
pulchella, Scapholeber i s sp., Diaphanosoma brachyurum,
Polyphemus pediculus, and especially larval chironomids and
Chaoborus remained rare in all enclosures throughout the
experiments. Relatively high levels (1/1) in Daphnia-removals of
Ceriodaphnia, Scapholeberis, and Diaphanosoma combined suggest
cladoceran. competition. These three species and Bosmina
longirostris reached higher densities in kena_i-removals than in
controls. Bosmina was most abundant in low density and Daphnia-
removal treatments (Fig. 5; Table VII). In low density
situations, it probably responded to excess food made available
through reduced grazing. High levels in Daphnia-removals imply
competition with D. rosea. In Goulden and Hornig's (1980)
laboratory experiments, Daphn ia qaleata mendotae outcompeted
small B. longirostris because the larger species was more able
to accumulate lipid reserves. However, in Kerfoot and DeMott's
(1980) field experiments, Daphn ia and Bosmina avoided
competition by using different food resources.
Densities of cyclopoid copepods, which included mostly
Diacyclops thomasi and some Tropocyclops prasinus, were
inconsistent between replicates and among treatments (Fig. 6).
Low density and Daphnia-removal treatments each had. one
replicate with high and one with low cyclopoid levels. In 40
Figure 5. Densities of Bosmina lonqirostris in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphnia-removal; E = kenai-removal; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES 42
Table VII. Statistical tests comparing across treatments for mean maximum densities of Bosmina longirostris attained during 1979 experiments. Values analyzed are log- transformed maximum numbers of animals collected from enclosures in a 150 1 sample regardless of sampling date. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between' 4 2.55 0.64 ' 9.55 0.015 Within 5 0.33 ' 0.07 Total 9 2.88
Pairwise Multiple Comparisons:
Treatment Mean (#/l50 1) Std. Dev. F p
Control: 357.5: 55.9: :Low density :1396.0 :463.9 26.84 0.004 :Daphnia-removal :1084.5 :125.2 18.50 0.008 :kenai-removal :1009.5 :303.4 15.58 0.011 rtyrelli-removal :513.5 :150.6 1.79 0.239 gure 6. Densities of cyclopoid copepods (nauplii, copepodites, and adults of Diacyclops thomasi and Tropocyclops prasinus combined) in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphnia-removal; E = kenai-removal; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES 4 5
general, levels were similar in control and tyrelli-removal enclosures (Fig. 6). Keria_i-removals contained relatively many cyclopoids. ANOVAs comparing maximum densities of nauplii
(ANOVA: F = 0.40; p = 0.803) and of copepodites plus adults
(Table VIII) show a significant difference among treatments only for more mature cyclopoids.
Dissimilarities among enclosures in D. thomasi densities potentially modified treatment effects on Diaptomus leptopus through competition from herbivorous nauplii and early copepodites and/or through predation from later stages (Peacock
1981). Enclosure experiments in Gwendoline Lake demonstrated relative immunity of the herbivorous zooplankton community to variations in numbers of predaceous D. thomasi (Neill and
Peacock 1980; Neill 1981a; Peacock 1981). High prey survival coincided with, and counterbalanced effects of, increases in cyclopoid concentrations because herbivorous stages of
D. thomasi experienced the same algal food constraints as the prey of later predaceous stages. That result, and the low proportion of predaceous stages among cyclopoids enclosed in
this study, refute predation effects. ANCOVAs using total cyclopoid density.as the covariate indicate that for no sampling
date did uncontrolled cyclopoids influence treatment effects on densities of Diaptomus leptopus nauplii, copepodites, adults, or
on densities of all three combined. In only 2 of the 72 analyses
(18 sampling dates x 4 variates) is the covariate significant at
p = 0.05. This frequency lies well within the 5% statistically
expected with an insignificant covariate. Similarities between 46
Table VIII. Statistical tests comparing across treatments for mean maximum densities of cyclopoid copepodites plus adults attained during 1979 experiments. Values analyzed are log-transformed maximum numbers of animals collected from enclosures in a 150 1 sample regardless of sampling date. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 4 ' 11.54 2.89 11.20 0.010 Within 5 1.29 0.26 Total 9 12.83
Pairwise Multiple Comparisons:
Treatment Mean (#/150 1) Std. Dev
Control: 5.0 4.2: :Low density 95.0 :56.6 30.87 0.003 :Daphn ia-removal 6.5 :2.1 0.47 0.525 : kenai-removal 48.5 :16.3 19.23 0.007 :tyrelli-removal 22.0 :7.1 8.31 0.035 47
D. leptopus responses to both replicates of low density and of
Daphnia-removal treatments underscore ANCOVA results. In a bog
lake, Winner (1970) found no correlation" between cyclopoid and
D. leptopus densities.
In summary, uncontrolled enclosure differences in densities
of zooplankton uncommon in Eunice Lake do not confound intended
differences in densities of common species. Except for
cyclopoids, most incidental species remained rare in enclosures.
Evidence indicates no cyclopoid modification of treatment
effects on D. leptopus.
J_.3_ Effects of Enclosure
Early collapse of enclosures corralling Eunice Lake species
without D. leptopus prevented proper assessment of containment
effects on zooplankton populations. In enclosures in other
Research Forest lakes, confined populations tracked lake
populations for up to 3 months (Neill 1978, 1981a; Peacock
1981). Dissimilarities increased with duration of enclosure.
Enclosures mimicked lakes in oxygen, temperature and grazable
seston levels.
In this study, differences occurred between uncollapsed
enclosures and Eunice Lake in densities of several zooplankton
species. The presence of planktivorous trout in the lake, and
not in enclosures, may. have caused some of these differences.
Densities of Ceriodaphnia, Scapholeberis, and Diaphanosoma were
higher in enclosures than in the lake. Holopedium (Fig. 4) and 48
Polyphemus rapidly decreased to extinction only in enclosures.
Daphnia rosea (Fig. 4) and cyclopoid copepods (Fig. 6) achieved higher levels in the lake and in enclosures, respectively. Late
in the study, when epiphytic algal food was beginning to develop
on the polyethylene, enclosure densities of Chydorus and chironomid larvae exceeded lake levels. Other enclosure studies
verify these effects on Holopedium (Neill pers. comm.), on
Daphnia and Diacyclops (Smyley 1976), and on Chydorus and
chironomid larvae (Peacock 1981).
These minor differences between the lake and enclosures do
not invalidate the experimental design because comparisons
between control and non-control enclosures, rather than between
enclosures and the lake, . measured effects of treatments on
D. leptopus. 49
2. Results Describing Effects of Experiments
2.J_ Treatment Effects on Total Densities of Diaptomus leptopus
Initial similarities among treatments in combined densities of nauplii, copepodites, and adults (Table IX) provide evidence for proper stocking of each enclosure with 4000 D. leptopus. In controls, D. leptopus increased slightly from stocking densities before rapidly decreasing to near extinction by early August.
Parallel patterns occurred in Daphn ia- and kenai-removals.
D. leptopus reached higher levels in low density and tyrelli- removal treatments than in controls. It persisted until late
August in low density enclosures, and until late September in tyrelli-removals.
These trends are apparent from Fig. 7. Statistical analyses comparing total D. leptopus across treatments for each sampling date substantiate them (Table IX). Significant differences were not consistently evident until after mid-July. Then, D. leptopus densities relative to those in controls were similar in Daphnia- and in k_ena_i-removals, and higher in tyrelli-removal and low density enclosures. Maximum D. leptopus densities exhibited
similar disparities (Table X).
Table XI compares persistence times for D. leptopus in the
different treatments. "Persistence time" is the number of days
elapsed from the first sampling date until D. leptopus remained
at densities of ^10 animals per 150 1 sample. Relative to
controls, persistence times were greater in tyrelli-removal and,
to a lesser extent, in low density treatments. 50
Table IX. Summary of statistical tests comparing mean densities of nauplii, copepodites, adults, and mean total densities of Diaptomus leptopus across treatments for each sampling date of 1979 experiments. Values analyzed are log-transformed numbers of animals per 150 1 sample. For each date, the first entry for each development stage indicates whether ANOVA comparing densities of that stage across treatments is significant at p = 0.05. Entries following significant ANOVAs indicate whether pairwise multiple comparisons between controls and each perturbation and between low density and tyrelli-removal treatments are significant, and give probability levels if p < 0.05. NS = insignificant at p = 0.05. [l] = Control; [2] = Low density; [3] = Daphnia-removal; [4] = kena i-removal; [5] = tyrelli-removal. (+) = higher mean densities of - animals in perturbation than in control treatments or in tyrelli-removal than in low density; (-) = higher mean densities in control than in perturbation treatments or in low density than in tyrelli-removal. Table IX
Naupli i Copepod i tes Adults Total dune 20 NS NS NS NS
June 26 NS NS NS NS duly 1 p<0. 05 NS NS NS [2]-[1] p=0. 01 1 ( + ) [3]-[1] NS [4]-[1] NS 034 [5]-[1] P=o. ( + ) [5]-[2] NS
du 1 y 5 p<0. 05 NS p<0.05 p<0. 05 [2]-[1] p=0. 014 ( + ) p=0.022 ( + )p = 0.00 5 ( + ) [3]-[1] NS p=0.012 ( + ) p=0. 034 ( + ) [4]-[1] NS p=0.026 ( + ) p=0. 018 ( + ) [5]-[1] . p = 0.02 5 ( + ) p=0.007 ( + ) p=0..00 4 ( + ) [5]-[2] NS NS NS
duly 11 p<0..0 5 NS NS NS [2]-[1] p<0. 001 ( + ) [3]-[1] p=0..02 5 ( + ) t4]-[1] p=0.,00 8 ( + ) [5]-[1] p<0..00 1 ( + ) [5]-[2] p=p..01 9 (-) duly 16 NS NS NS NS
duly 21 p<0 .05 NS NS NS [2]-[1] p = 0 .010 •( + ) [3]-[1] NS [4]-[1] NS [5]-[1] NS [5]-[2] NS
duly 26 p<0 .05 NS • NS p<0 .05 [2]-[1] p=0 .007 ( + ) p=0 .014 ( + ) [3]-[1] NS NS [4]-[1] NS NS [5]-[1] p = 0 .037 < + ) p=0 .032 ( + ) [5]-[2] NS NS
August 2 p<0 .05 NS NS p<0 .05 [2]-[1] p<0 .001 ( + ) p=0 .004 ( + .) t3]-[1] NS NS [4]-[ 1] NS NS [5]-[1] p=0 .007 ( + ) p=0 .034 ( + ) [5]-[2] p=0 .008 (-) NS Table IX. continued
Nauplii Copepodites Adults Total
August 7 p<0. 05 NS NS NS [2]-[1] p=0. 002 ( + ) [3]-[1] NS [4]-[1] NS [5]-[1] p=0..02 0 ( + ) [5]-[2] p=0. 045 (-)
August 12 p<0. 05 NS ' NS p<0. 05 [2]-[1] p=0. 030 ( + ) p=0. 005 ( + ) [3]-[1] NS p=0. 029 ( + ) [4]-[1] NS p=0..02 0 ( + ) [5]-[1] p=0. 032 ( + ) p=0. 004 ( + ) [5]-[2] NS NS
August 19 NS NS p<0..0 5 p<0..0 5 [2]-[1] NS NS [3]-[ 1] NS NS [4]-[1] NS NS [5]-[1] p=0..01 5 ( + ) p = 0 .021 ( + ) [5]-[2] NS NS
August 24 NS NS NS NS
August 31 NS NS NS NS
September 10 p<0..0 5 p<0 .05 NS p<0 .05 [2]-[1] NS p=0..00 7 ( + ) p=0 .047 ( + ) [3]"[ 1 ] NS NS NS [4]-[1] p=0 .010 ( + ) NS p=0 .017 ( + ) [5]-[1] NS p<0 .001 ( + ) p=0 .013 ( + ) [5]-[2] NS p=0 .003 ( + ) NS
September 19 NS NS p<0..0 5 p<0 .05 [2]-[ 1] NS NS [3]-[1] NS NS [4]-[ 1 ] NS NS [5]-[1] p = 0 .020 ( + ) p=0 .015 ( + ) [5]-[2] NS NS
September 29 NS NS p<0 .05 p<0 .05 [2]-[1] p=0 .003 ( + ) NS [3]-[1] NS NS [4]-[1] p=0 .01 1 ( + ) NS [5]-[1] p=0 .001 ( + ) p=0 .022 ( + ) [5]-[2] NS NS
October 12 NS NS NS NS 53
Figure 7. Densities of Diaptomus leptopus (nauplii, copepodites, and adults combined) in Eunice Lake and in 1979 experiments. .Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphn i a-removal; E = ke_na_i - removal ; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES 55
Table X. Statistical tests comparing across treatments for mean maximum densities of Diaptomus leptopus (nauplii, copepodites, and adults combined) attained during 1979 experiments. Values analyzed are log-transformed numbers of animals collected from enclosures in a 150 1 sample regardless of sampling date. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 4 . 1.75 0.44 7.83 0.022 Within 5 0.28 0.06 Total 9 2.03
Pairwise Multiple Comparisons:
Treatment Mean (#/150 1) Std. Dev. F p
Control: 52.0: 8.5: :Low density :121.0 :31.1 12.15 0.018 :Da_pjinia-removal :43.0 :14.1 0.76 0.423 rkenai-removal :44.0 :1.4 0.44 0.535 :tyrelli-removal :96.0 :26.9 6.26 0.054 56
Table XI. Statistical tests comparing across treatments for mean persistence times of Diaptomus leptopus in 1979 experiments. Since ANOVA comparing all treatments is almost significant, pairwise multiple comparisons of controls with each perturbation are shown.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 4 3651.0 912.75 3.66 0.094 Within 5 1247.0 249.40 Total 9 4898.0
Pairwise Multiple Comparisons
Treatment Mean (days) Std. Dev.
Control: 46.5 3.5: :Low density 74.5 : 12.0 3 1 4 0. 136 :Daphnia-removal 49.0 :0.0 0 03 0.880 : kenai-removal 66.0 :24.0 1 52 0.272 : tyrelli-removal 99.0 :22.6 1 1 05 0.021 57
2.2 Treatment Effects on Densities of Diaptomus leptopus Eggs,
Naupli i, Copepodites, and Adults
Table IX summarizes statistical analyses comparing naupliar, copepodite, and adult densities across treatments for each sampling date. Insignificant differences in early samples
for all developmental stages imply initially comparable age distributions for all enclosed D. leptopus populations.
Throughout the experiments, copepodites and adults continued to experience no consistent treatment effects. Most copepodites in control, Daphn ia-removal and k_enai_- removal enclosures were among
those originally stocked from Gwendoline Lake. Only in tyrelli-
removal and low density treatments did D. leptopus persist long
enough for nauplii produced within the enclosures to mature.
Treatment differences in naupliar densities (Fig. 8) parallel
those noted in Fig. 7 for total D. leptopus. From early July to mid-August, nauplii were more numerous in tyrelli-removal and
low density than in other treatments. After late August, no
significant differences in their densities occurred because most
had already matured into later stages.
Table XII compares the total number of D. leptopus eggs
encountered in each treatment throughout the experiment divided
by the total number of females. Egg totals include eggs carried
by females and unattached eggs. Loose D. tyrelli and D. leptopus
eggs were not reliably distinguishable. Because most attached
D. tyrelli and D. leptopus clutches contained <5 and >5 eggs,
respectively, unattached egg bunches were similarly assigned to
species. Relative to controls, reproduction increased only in 58
Figure 8. Densities of nauplii and of copepodites plus adults of Diaptomus leptopus in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = Low density; D = Daphnia-removal; E = kenai-removal; F = tyrelli-removal. NUMBER OF ANIMALS PER 150 LITRES 60
Table XII. Statistical tests comparing across treatments for mean number of Diaptomus leptopus eggs produced per female during 1979 experiments. Values analyzed are log-transformed total numbers of loose and attached eggs divided by total numbers of adult females collected from enclosures in 150 1 samples from all sampling dates combined. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df- Sum of Squares Mean Square- F p
Between 4 5.44 1.36 5.76 0.041 Within 5 1.18' 0.24 Total 9 6.62
Pairwise Multiple Comparisons:
Treatment Mean (#/female) Std. Dev
Control: 0.00 0.00: :Low density 8.35 :2.76 20.73 0.006 :Daphn ia-removal 1.65 :1.91 2.88 0.151 : kenai-removal 1.10 :1.13 1.86 0.231 : tyrelli-removal 0.75 :0.64 1.17 0.329 61
low density enclosures, where reduced grazing pressures presumably had enhanced food available to reproductive females.
Together these results have several implications.
(1) Lack of treatment effects of Daphnia- or kena_i-removals
implies no competition between these species and
D. leptopus.
(2) Similar levels of D. leptopus copepodites and adults in
all enclosures, despite high densities of nauplii in
tyrelli-removal and low density perturbations, suggest poor
survival of nauplius to adult in these two treatments.
(3) High naupliar densities relative to controls in
tyrelli-removals, despite relatively unenhanced
reproduction, indicate high survival of egg to nauplius in
the absence of competition from D. tyrelli. The experiments
were not designed to determine which developmental stages
of D. tyrelli compete. Correlations between densities of
D. leptopus nauplii and D. tyrelli of different stages
provide no unambiguous clues.
(4) Reduction of either competition from D. tyrelli and/or
diffuse competition could explain high naupliar
concentrations in low density enclosures. Lowered densities
of all species, including D. tyrelli, confounded effects of
D. tyrelli competition on egg to nauplius survival with 62
effects of diffuse competition on egg production. Absences
of interactions between D. leptopus and either D. kenai or
D. rosea imply that D. tyrelli competition, rather than
diffuse competition, affected D. leptopus. Similarities
between tyrelli-removal and low density treatments in
copepodite, adult, and total D. leptopus densities also
imply this because both treatments contained similar
D. tyrelli concentrations (Fig. 2; pairwise comparisons
between low density and tyrelli-removal treatments in Table
IX). In contrast, the higher proportion of D. leptopus
nauplii in low density than in tyrelli-removal enclosures
(Fig. 8) argues for the importance of diffuse competition.
However, due to inter-replicate variation, pairwise
comparisons between these two treatments yield differences
in naupliar densities at p < 0.05 on only two sampling
dates (Table IX). Because these comparisons were suggested
a poster iori by the data, whether these differences are
significant is uncertain (Snedecor and Cochran 1967).
2.3 Treatment Ef fects on Adult Size of Diaptomus leptopus
Treatment differences in lengths of adult D. leptopus were evident on only two of the seven sampling dates for which
zooplankton were measured (ANOVAs: p < 0.05). These differences may not have been treatment effects; uncontrolled shrinkage due
to formalin .and unrecorded sex differences in size (males are
smaller [Wilson 1959]) also influenced measurements. Table XIII 63
Table XIII. Lengths (mm) of adult diaptomid copepods and of Daphnia rosea in 1979 experiments. Means are based on all animals measured, regardless of treatment or sampling date.
Species N Mean Std. Dev.
Diaptomus kenai 4 1 9 2 . 46 0. 1 3
Diaptomus leptopus 332 1 .70 0. 09
Diaptomus tyrelli 525 1 .29 0. 06
Daphnia rosea 627 2 .07 0. 33 64
displays mean length of D. leptopus and, for comparison, lengths of D. tyrelli, D. kenai, and Daphnia rosea averaged over all treatments and dates. Low standard deviations for all species suggest that seasonal changes in size were small. Inclusion of both juvenile and adult sizes inflated the standard deviation for D. rosea.
2.4 Seasonal Cycles, Vertical Distribution and Phytoplankton
Biomass
Temporal asynchrony, spatial separation, and dietary differences can moderate intensity of competition among diaptomids. Comparisons of seasonal cycles and vertical distributions between D. leptopus and Eunice Lake zooplankton and comparison of phytoplankton biomass among treatments seek evidence of these coexistence mechanisms.
Seasonal Cycles
High variation among small samples and limited duration of experiments prevented resolution of annual cycles for enclosed diaptomids. Presumably their life cycles were similar to those
Chapman and Green (unpubl. data) defined for calanoid copepods from Research Forest lakes. Figs. 10 and 11 schematically present seasonal cycles of D. tyrelli and D. kenai in Eunice
Lake. Because D. leptopus is barely detectable in Eunice Lake, 65
Fig. 9 shows its life history in Gwendoline Lake, instead.
Timing of seasonal changes in densities of different development
stages is similar from year to year, but actual densities attained are variable.
In Gwendoline Lake, overwintering D. leptopus adults die after reproducing in early spring. Their offspring reach adulthood by July or late August and then propagate a fall generation (Fig. 9). In Eunice Lake, even in low density and
tyrelli-removal treatments, no fall generation occurred in 1979
(Fig. 8). D. tyrelli overwinters in Eunice Lake as resting eggs.
Nauplii hatching in spring mature into adults by June or July.
These produce a second generation which in turn produces
overwintering eggs (Fig. 10). Overwintering D. kenai nauplii
achieve maturity.by April or May. Spring adults breed a fall
generation. Fall adults produce either subitaneous or resting
eggs (Fig. 11).
Greater temporal synchrony of D. leptopus with D. tyrelli
than with D. kenai is consistent with improved D. leptopus
performance in tyrelli-removal and not in kena i-removal
treatments. During July and August, when marked treatment
effects on D. leptopus nauplii occurred, all stages of
D. tyrelli were represented in the enclosures. Because it
develops earlier than D. leptopus, no nauplii and few early
copepodites of D. kenai occurred simultaneously with these
stages of D. leptopus.
Temporal overlap between D. leptopus and Daphnia rosea is
difficult to assess because developmental stages of cladocerans 66
Figure 9. Life cycle of Diaptomus leptopus in Gwendoline Lake. Shaded areas schematically represent seasonal changes in abundances of different development stages. Small arrows trace cohorts. Large arrows indicate reproduction. After Chapman (unpubl.) DIAPTOMUS LEPTOPUS
EGGS
sub i tone ous subitaneous
NAUPLII \ COPEPODITES \ \
ADULTS 68
Figure 10. Life cycle of Diaptomus tyrelli in Eunice Lake. Shaded areas schematically represent seasonal changes in abundances of different development stages. Small arrows trace cohorts. Large arrows indicate reproduction. After Chapman (unpubl.) DIAPTOMUS TYRELLI
EGGS 70
Figure 11. Life cycle of Diaptomus kenai in Eunice Lake. Shaded areas schematically represent seasonal changes in abundances of different development stages. Small arrows trace cohorts. Large arrows indicate reproduction. After Chapman (unpubl.) DIAPTOMUS KENAI
EGGS 72
and copepods do not correspond. D. rosea, a facultative parthenogen, breeds asexually throughout spring and summer. As temperature and food conditions decline in the fall, sexual ephippial eggs appear. In the enclosures, reproduction was primarily asexual. Both adult and juvenile D. rosea were consistently present.
Vertical Distributions
Figs. 12 and 13 display July and September distributions of
D. tyrelli, D. kenai, D. leptopus, and Daphnia rosea within the water column. Data from all enclosures were combined because no distinct or consistent treatment differences were evident. No
D. kenai nauplii occurred in either July or September.
Restriction of D. leptopus and D. tyrelli nauplii primarily to the upper 2 m (Figs. 12 and 13) was especially marked for
D. leptopus in the absence of D. tyrelli (Fig. 14). These stages experienced warm (>20°C) surface temperatures (Fig. 15). In most enclosures, copepodites and adults of D. leptopus and D. tyrelli concentrated above 4 and 7 m, respectively (Figs. 12 and 13).
D. kenai existed below the thermocline occurring at 4-5 m
(Fig. 1.5). D. rosea dispersed more deeply in summer than in fall. No species exhibited prominent diel vertical migrations, but D. tyrelli moved slightly up, and D. kenai and D. leptopus moved slightly down diurnally during September. 73
Figure 12. Nocturnal (solid bars) and diurnal (open bars) vertical distributions of Diaptomus leptopus, Diaptomus tyrelli, Diaptomus kenai, and Daphnia rosea in 1979 experiments during July. Observations from all enclosures are combined. LN = D. leptopus nauplii; LC = D. leptopus copepodites; LA = D. leptopus adults; TN = D. tyrelli nauplii; TC = D. tyrelli copepodites; TA = D. tyrelli adults; KC = D. kenai copepodites; RA = D. kenai adults; DA = D. rosea. N, = size of
nocturnal samples; N2 = size of diurnal samples. 0-2
LN 2-4 N, = 274 ] N, = 304 N2>332
4-7
>7
i— 1 1 i—— 1 1 i ' r 1 0 50 100 0 50 100 0 50 100 •/•OCCURRENCE gure 13. Nocturnal (solid bars) and diurnal (open bars) vertical distributions of Diaptomus tyrelli, Diaptomus kenai, Diaptomus leptopus, and Daphnia rosea in 1979 experiments during September. Observations from all enclosures are combined. LN = D. leptopus nauplii; LC = D. leptopus copepodites; LA = D. leptopus adults; TN = D. tyrelli nauplii; TC = D. tyrelli copepodites; TA = 2- tyrelli adults; KC = D. kenai copepodites; KA = D. kenai adults; DA = D. rosea. N, = size of
nocturnal samples; N2 = size of diurnal samples. 0-2 3
LN LC LA 2-4 N,=29
2 N2*88 N=6 N2*S8 4-7 3
I
0-2
TN TC TA 2-4 N, =19 N,=340 N| s|409 X N2=3I N2=3T9 N_=648 I- 0. 4-7 UJ Q 7
0-2 1
DA KC u KA 2-4 N, =670 N,=69 • N,=I36
N2=604 N2=46 N2=88 4-7
.r- —I— I— 0 50 100 50 loo 0 50 "Joo % OCCURRENCE cn 77
Figure 14. July vertical distributions of Diaptomus leptopus nauplii in the presence (solid bars) and in the absence (open bars) of Diaptomus tyrelli. Data from control, Daphn ia-removal, and k_ena_i-removal treatments are combined to give naupliar distributions in the presence of D. tyrelli • Data from tyrelli-removal and low density treatments are combined to give naupliar distributions in the absence of D. tyrelli. Both nocturnal and diurnal data are included. 2-4
With D.tvrellh N = I80
Without D. tvrellh N=6I0 X I- CL 4-7 UJ O
>7
1 50 100 % OCCURRENCE gure 15. Temperature profile for Eunice Lake during 1979 experiments. O CM *T CD CO CM ^" CD 00 (N) Hld30 81
Lack of spatial segregation between surface-dwelling
D. leptopus and D. tyrelli accords with experimental evidence of
D. tyrelli competition. Non-competing D. kenai and D. rosea inhabited deeper strata.
Phytoplankton Biomass
Studies on filter-feeding herbivorous crustaceans suggest a positive correlation between size of animal and size of particle filtered (Burns 1968; Hall et al. 1976; Poulet 1979). In the absence of a particular zooplankton species, phytoplankton of sizes eaten by that species should increase in biomass (Porter
1972; Lynch and Shapiro 1981). From average lengths of Eunice
Lake zooplankton' species (Table XIII), the following treatment differences in phytoplankton standing stocks are predicted: relative to controls, small particles should increase in tyrelli-removals; large particles, in kenai- and Daphnia- removals; and all particles, in low density enclosures.
Predictions could be derived from sizes of adult calanoids, without consideration of population age distributions, because
in the enclosures only late copepodites and adults of D. kenai occurred, and these were larger than all D. tyrelli. If, as
Neill (1975a and b) found in his microcosms, Eunice Lake zooplankton species partition food by size and not by taxonomy, then data supporting these predictions would provide indirect evidence of diet differences among zooplankton.
Results do not support a positive correlation between 82
zooplankton size and food size (Fig. 16). Throughout the experiments, biomass of small phytoplankton (<20 urn in diameter) was approximately equal in all treatments. Relative to controls, large cells (>20 urn) increased substantially in tyrelli-removal and low density treatments, increased slightly in Daphnia-
removals, and decreased in kenai-removals. Presumably, treatment differences in zooplankton grazing caused these changes because they occurred within the first 4 to 6 weeks of the experiment.
Later increases of large particles and colonies occurred in all enclosures after other factors (e. g., seasonal successions of phytoplankton and zooplankton species, recycling of nutrients,
sinking and shading of algae) had had sufficient time to
influence phytoplankton size distributions.
Cells <2 urn were mostly coccoid bacteria. Among those 2-
20 urn were chlorophytes (Oosystis), chrysophytes (Mallomonas,
Ochromonas, Dinobryon), diatoms, and Cryptomonas. Cells >20 urn
and colonies included chlorophytes (Ankistrodesmus,
Scenedesmus), chrysophytes (Dinobryon, Chrysocapsa), and
colonial cyanophytes (Chroococcus, Dactylococcopsi s,
Mer i smopedia). gure 16. Phytoplankton standing stocks in Eunice Lake and in 1979 experiments. Plots for replicate enclosures are side by side. For each enclosure, upper plot shows total biomass; lower plot shows percent size composition of total biomass. A = Eunice Lake; B = Control; C = Low density; D = Daphnia-removal; E = k_ena_i-removal; F = tyrelli-removal. 84
5 A £1.6
TOTAL BICWRSS O
PERCENT 6IDMBSS lr <2 MICRONS 100
JULT | RUG. I SEPT. | OCT. J I JULT | RUG. | SEPT. | OCT. 1979 PERCENT BIOMASS PERCENT BIOMASS PERCENT BIOMASS TOTAL BIOMASS (MG/L) TOTAL BIOMASS 1MG/L) TOTAL BJOMHSS 1MG/LJ o s . s a' 'b )' ' ' o o CD oi I I I A I I I I
\
00 86
3. Summary of 1979 Results
At the start of the experiments, enclosures contained
zooplankton in densities appropriate to the treatments.
Effects of confinement on zooplankton populations did not
invalidate experiments because enclosures containing the
Eunice Lake zooplankton assemblage plus Diaptomus leptopus
served as controls.
Similarities between performances of D. leptopus in
control, Daphnia-removal, and tyrelli-removal treatments
indicated no competition from Daphn ia rosea or Diaptomus
kenai.
D. leptopus persisted longer in tyrelli-removal and low
density enclosures than in controls.
Numbers of eggs per D. leptopus female were highest in low
density enclosures.
Increased D. leptopus naupliar densities in tyrelli-
removals relative to controls indicated competition from
Diaptomus tyrelli.
Increased D. leptopus naupliar densities in low density 87
enclosures relative to controls confounded diffuse with
D. tyrelli competition.
8. Densities of D. leptopus copepodites and adults were
similar in all treatments.
9. Adult size of D. leptopus was similar in all treatments.
10. Vertical and seasonal distributions of D. leptopus and
D. tyrelli overlapped. D. kenai and D. rosea occurred
deeper than D. leptopus, and D. kenai reproduced earlier.
11. Treatment differences in standing stocks of small and large
phytoplankton provided no evidence for differences in
diaptomid diets. 88
EXPERIMENTAL DESIGN, 1980
Experiments in 1980 attempted to verify effects of 1979 tyrelli-removal enclosures, and to separate them from effects of low density enclosures. In 1980, eight enclosures contained four twice-replicated treatments. Each enclosure contained a standard density of 0.64 Diaptomus leptopus per litre (nauplii, copepodites, and adults combined). This density exceeded that from 1979 because initial densities of D. leptopus in Eunice
Lake were higher in 1980 and because numbers of D. leptopus in samples had been low in 1979.
Table XIV describes 1980 treatments and the hypotheses they test. Except that their zooplankton densities reflected lake densities from a different year, 1980 control and tyrelli- removal treatments duplicated those from 1979. Two treatments enclosed reduced biomasses of Eunice Lake zooplankton. Low density contained all species in lake relative proportions, while tyrelli/low density contained mostly Diaptomus tyrelli.
These treatments reduced biomass by 70%, rather than by 90% as in 1979, because D. tyrelli formed a greater majority of Eunice
Lake zooplankton in 1980. The lesser reduction enhanced treatment differences in D. tyrelli concentrations. D. tyrelli was less dense in tyrelli/low density enclosures than in the lake and in controls, and more dense than in low density enclosures. Together, low density and tyrelli/low density treatments aimed to distinguish diffuse and D. tyrelli competition confounded in 1979. High densities of D. leptopus nauplii relative to controls-in both treatments would provide Table XIV. Summary of 1980 experimental treatments. N = Eunice Lake zooplankton assemblage at 1980 natural lake densities; B = N converted to biomass.
Treatment Species composition Hypothesis tested
Contro1 N + D i aptomus 1eptopus Compare performance of D. 1eptopus here with that in all other treatments to test all hypotheses.
tyrel1i-removal N - Diaptomus tyrel1i Competition with Diaptomus tyrel1i + D. 1eptopus prevents D. 1eptopus from colonizing Eunice Lake.
Low density 30% N + D. 1eptopus Diffuse competition from several zooplankton species prevents D. 1eptopus from colonizing Eunice Lake.
tyrel1i/ 30% B as D. tyrel1i Competition with D. tyrel1i low density + D. 1eptopus explains effects of 1979 low density enclosures on D. 1eptopus.
CO 90
evidence for diffuse competition. High naupliar densities in only low density enclosures would provide evidence for
D. tyrelli competition by implying that relatively abundant
D. tyrelli suppressed D. leptopus in tyrelli/low density enclosures. High egg production in both treatments would reflect reduced grazing pressures in both. 91
RESULTS, 1980
J_. Results Confirming Proper Establishment of Experiments
J_.J_ Enclosure Densities of Abundant Eunice Lake Zooplankton
Spec ies
Initial similarities between Eunice Lake and control
enclosures in total densities of Diaptomus tyrelli, D. kenai,
Daphn ia rosea, and Holopedium gibberum demonstrate proper
stocking of controls with these species (Table XV). Remaining
treatments contained variations on control densities. D. tyrelli
occurred in increasingly greater concentrations in tyrelli-
remo.val, low density, tyrell i/low density, and control
treatments (Table XVI; Fig. 17). Relative to controls, D. kenai
densities were low in low density and, especially, in
tyrelli/low density enclosures (Fig. 18). They were higher in
tyrelli-removals (Table XVII). Control and tyrelli-removal
levels of cladocerans were similar (Tables XVIII and XIX;
Fig. 19). Both low density treatments contained equally reduced
D. rosea concentrations (Fig. 19).
These results indicate that, with two exceptions, enclosure
densities of common Eunice Lake zooplankton species conformed to
the experimental design. To.fully conform, D. rosea should have
been less dense in tyrelli/low density than in low density
enclosures; and D. kenai, as dense in tyrelli-removaIs as in
controls. 92
Table XV. T-test statistics comparing mean densities (numbers of animals per 150 1 samples) of major Eunice lake species in Eunice Lake and in control enclosures at start of 1980 experiments. Tests analyzed log-transformed counts from the first two sampling dates, but means and standard deviations shown are of untransformed data. N=nauplii; C=copepodites; A=adults; J=juveniles.
Locat ion Spec ies Lake Controls df
Diaptomus tyrelli X: 464.5 763 .8 -1 .24 4 0.283 (N+C+A) s: 328.8 304.3 n: 2 4
Diaptomus kenai X: 84.0 17.0 3.73 4 0.020 (N+C+A) s: 19.8 11.7 n: 2 4
Daphn ia rosea X: 192.0 65.0 5.70 4 0.005 (J+A) s: 72. 1 7.0 n: 2 4
Holopedium gibberum X: 77.0 32.3 1 .62 4 0.182 (J+A) ~~ s: 59.4 12.0 n: 2 93
Table XVI. Statistical tests comparing mean densities of Diaptomus tyrelli (numbers of nauplii, copepodites, and adults combined per 150 1 sample) across treatments on initial sampling date (May 13) of 1980 experiments. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log-transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 3 9.19 3.06 71.04 0.001 Within 4 0.17 0.04 Total 7 9.36
Pairwise Multiple Comparisons:
Treatment Mean (#/l50 1) Std. Dev.
Control: 1023.0 18.4 :tyrelli-removal 68.5 16, 169.65 <0.001 :Low density 131.0 36, 99.27 0.001 :tyrelli/low density 512.0 97, 11.38 0.028 94
Figure 17. Densities of Diaptomus tyrelli (nauplii, copepodites, and adults combined) in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli-removal; D = Low density; E = tyrelli/low density. NUMBER OF ANIMALS PER 150 LITRES 96
Figure 18. Densities of Diaptomus kenai (nauplii, copepodites, and adults combined! in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli- removal; D = Low density; E = tyrelli/low density. NUMBER OF ANIMALS PER 150 LITRES 98
Table XVII. Statistical tests comparing mean densities of Diaptomus kenai (numbers of nauplii, copepodites, and adults combined per 150 1 sample) across treatments on initial sampling date (May 13) of 1980 experiments. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log-transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p-
Between 3 5.94 1.98 5.82 0.061 Within 4 1.36 0.34 Total 7 7.30
Pairwise Multiple Comparisons:
Treatment Mean (#/l50 1) Std. Dev,
Control: 13.0 2.8 :tyrelli-removal 71.5 24.7 7.76 0.050 :Low density 24.5 23.3 0.34 0.593 :tyrelli/low density 5.5 0.7 1 .70 0.263 99
Table XVIII. Statistical tests comparing mean densities of Daphn ia rosea (number per 150 1 sample) across treatments on initial sampling date (May 13) of 1980 experiments. Since ANOVA comparing all treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log-transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 3 2.77 0.92 5.65 0.064 Within 4 0.65 0.16 Total 7 3.43
Pairwise Multiple Comparisons:
Treatment Mean (#/l50 1) Std. Dev. F p
Control: 68.5: 9.2: :tyrelli-removal :75.0 :46.7 0.001 0.980 :Low density :16.0 :4.2 12.33 0.025 :tyrelli/low density : 57 .0 : 21. 2 0. 27 0 . 629 100
Table XIX. Statistical tests comparing mean densities of Holopedium gibberum (numbers per 150 1 sample) across treatments on initial sampling date (May 13) of 1980 experiments. Since ANOVA comparing treatments is significant, pairwise multiple comparisons of controls with each perturbation are shown. Tests analyzed log- transformed counts, but means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 3 3.83 1.28 12.14 0.018 Within 4 0 . 42 0.. 1 1 Total 7 4.25
Pairwise Multiple Comparisons:
Treatment Mean (#/l50 1) Std. Dev,
Control: 40.5 12.0 :tyrelli-removal 47. 5 9 0.29 0.619 :Low density 14.5 4 9.65 0.036 :tyrelli/low density 8.5 5 21 .07 0.010 101
Figure 19. Densities of Daphn ia rosea and Holopedium gibberurn in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli-removal; D = Low density; E = tyrelli/low density. NUMBER OF ANIMALS PER 150 LITRES
o ro 103
Excessive D. kenai levels occurred in tyrelli-removals because
D. kenai was used to compensate for the large D. tyrelli biomass removed. Only D. tyrelli densities were crucial to the experiment, and these satisfied requirements.
]_.2 Enclosure Densit ies of Rare Eun ice Lake Zooplankton Spec ies
As in 1979, enclosure densities of incidental Eunice Lake zooplankton were uncontrolled. Except for Chydorus spp., Bosmina longirostris, and cyclopoid copepods, all species maintained low densities (<0.3/l) in all enclosures. Chydorus attained higher densities only during the last two weeks of the experiments.
Presumably in response to decreased grazing pressures, Bosmina increased relative to controls in both low density treatments
(Fig. 20). These increases could not have confounded treatment effects because they primarily occurred only after Diaptomus leptopus became extinct in all enclosures. Cyclopoid copepods, mostly naupliar Diacyclops thomasi, were less consistent among treatments and more consistent between replicates than in 1979.
Replicates of only the tyrelli/low density treatment contained notably different cyclopoid densities (Fig. 21). Tyrelli-removal and low density treatments contained fewer cyclopoids than controls (Table XX). As in 1979, ANCOVAs comparing densities of
D. leptopus across treatments for each sampling date yield an insignificant cyclopoid covariate for most dates. 1 04
Figure 20. Densities of Bosmina longirostris in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli-removal; D = Low density; E = tyrelli/low density. NUMBER OF ANIMALS PER 150 LITRES
o 106
Figure 21. Densities of cyclopoid copepods (nauplii, copepodites, and adults of Diacyclops thomasi and Tropocyclops prasinus combined) in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli-remo'val; D = Low density; E = tyrell i/low density.
108
Table XX. Statistical tests comparing across treatments for mean maximum densities of cyclopoid copepods (nauplii, copepodites, and adults combined) attained during 1980 experiments. Values analyzed are log-transformed maximum numbers of animals collected from enclosures in a 150 1 sample regardless of sampling date. Since ANOVA comparing all treatments is signficant, pairwise multiple comparisons of controls with each perturbation are shown. Means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source . df Sum of Squares Mean Square F p
Between 3 5.10 1.70 13.65 0.014 Within 4 0.50 0.12 Total 7 5.60
Pairwise Multiple Comparisons:
Treatment Mean (#/i50 1) Std. Dev.
Control: 688.5 20. 5 :tyrelli-removal 184.0 36.8 14.10 0.020 :Low density 81.0 19.8 36.88 0.004 :tyrelli/low density 405.0 240.4 3.14 0.151 109
J_.3 Effects of Enclosure
Differences between enclosure and Eunice Lake densities of zooplankton in 1980 paralleled 1979 differences. Again,
Holopedium gibberum and Polyphemus pediculus became extinct in enclosures. Again, Daphnia rosea attained higher levels in the lake; Chydorus and larval chironomids, higher levels in enclosures. In contrast to 1979, Eunice Lake densities of cyclopoids approached control concentrations in 1980. 1 1 0
2. Results Describing Effects of Experiments
2.J_ Treatment Effects on Total Densities of Diaptomus leptopus
Insignificant treatment differences in combined densities of nauplii, copepodites, and adults from early sampling dates indicate proper stocking of each enclosure with 19500
D. leptopus (Table XXI). In controls, a peak density of three to five D. leptopus per litre in mid-June preceded a crash to near extinction in early to mid-July (Fig. 22). Equivalent density changes occurred in all other treatments. All ANOVAs comparing treatments for total D. leptopus on each sampling date (Table
XXI) or for maximum D. leptopus from all sampling dates (ANOVA:
F = 0.72; p = 0.591) were insignificant. D. leptopus approached extinction synchronously (mean persistence time ± 1S.D. = 58 ± 9 days) in all treatments (ANOVA:.F = 0.26; p = 0.852).
Peak D. leptopus densities in 1980 were at least 5x greater than densities in 1979 low density and tyrelli-removal treatments. D. leptopus persisted twice as long in all 1980 enclosures as in 1979 controls. 111
Table XXI. Summary of statistical tests comparing mean densities of nauplii, copepodites, adults, and mean total densities of Diaptomus leptopus across treatments for each sampling date of 1980 experiments. Values analyzed are log-transformed numbers of animals per 150 1 sample. For each date, the first entry for each development stage indicates whether ANOVA comparing densities of that stage across treatments is significant at p = 0.05. Entries following significant ANOVAs indicate whether pairwise multiple comparisons of controls with each perturbation are significant, and give probability levels if p < 0.05. NS =' insignificant at p = 0.05. [1] = Control; [2] = tyrelli-removal; [3] = Low density; [4] = tyrelli/low density. (+) = higher mean densities of animals in perturbation than in control treatments; (-) = higher mean densities in control than in perturbation treatments. Table XXI
Naupli i Copepodites Adults Total
May 13 NS NS NS NS
May 18 NS -NS NS NS
May 22 NS NS NS NS
May 26 NS NS NS NS
May 29 NS NS NS NS
June 2 NS p<0.05 NS NS [2]-[1] p=0.026 < + ) [3]-[1] p=0. 018 ( + ) [4]-[1] p = 0. 018 ( + )
June 6 NS NS NS NS
June 9 NS p<0.05 NS NS [2]-[1] p=0.015 (+) [3]-[1] p=0.027 (+) t4]-[1] p=0.024 (+)
June 12 NS NS NS NS
June 16 NS NS NS NS
June 19 NS NS NS NS
June 23 NS p<0.05 NS NS [ 2 ] - [ 1 ] p=0.013 (+) [3]-[1] p=0.014 (+) [4]-[1] p=0.026 (+)
June 27 NS NS p<0.05 p<0.05 [2]-[1] p=0.015 (• p=0.005 [3]-[1] NS NS [4]-[1] NS NS Table XXI. continued
Nauplii Copepodites Adults Total
July 3 NS p<0..0 5 NS NS [2]-[1] p=0..02 0 ( + ) [3]-[1] p=0..00 6 ( + ) C4]-[ 1 ] NS
July 10 NS p<0..0 5 NS NS [2]-[1] p=0. 001 ( + ) [3]-[1] p<0..00 1 ( + ) [4]-[1] NS
July 17 NS NS p<0..0 5 NS [2]-[1] p=0..00 8 ( -) [3]-[1] p=0..0 4 ( -) [4]-[1] p=0..01 6 ( -)
July 24 NS NS NS NS
July 3 1 NS NS NS NS
August 7 NS NS NS NS
August 21 p<0.05 NS NS NS [2]-[1] p=0.011 (+) [3]-[1] NS [4]-[1] NS
September 4 NS NS NS NS 1 14
Figure 22. Densities of Diaptomus leptopus (nauplii, copepodites, and adults combined) in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli-removal; D = Low density; E = tyrelli/low density. 115
13 18222629 2 6 912 16192327 3 10 17 24 31 13 18222629 2 6 912 161923 27 3 10 17 24 31 MOT I JUNE | JULT | MAT | JUNE | JULT I 1980 1 16
2.2 Treatment Effects on Densities of Diaptomus leptopus Eggs,
Nauplii, Copepodites, and Adults
Table XXI summarizes ANOVAs comparing naupliar, copepodite, and adult densities across treatments for each sampling date. On early dates, all treatments contained comparable densities of
D. leptopus in each developmental stage. On later dates, no notable treatment differences occurred for nauplii or adults. On five dates, copepodites were more dense in perturbations than in controls. These differences provide little evidence for treatment effects because sample sizes for copepodites were small (Fig. 23) and because, unlike nauplii and adults,
D. leptopus copepodites were difficult to distinguish from corresponding stages of D. tyrelli. Relative to controls, numbers of eggs per female increased insignificantly in the two low density treatments (Table XXII).
In contrast to 1979, 1980 treatments had no marked effects on D. leptopus of any stage. They neither support the D. tyrelli competition effects from 1979 nor distinguish them from diffuse competition effects.
2.3 Treatment Ef fects on Adult Size of Diaptomus leptopus
As in 1979, lengths of adult D. leptopus did not differ among treatments (ANOVA: p < 0.05). Adult calanoids and Daphnia rosea were smaller in 1980 than in 1979 (Table XXIII). 1 1 7
Figure 23. Densities of nauplii and of copepodites plus adults of Diaptomus leptopus in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. A = Eunice Lake; B = Control; C = tyrelli- removal; D = Low density; E = tyrelli/low density. NUMBER OF ANIMALS PER 150 LITRES 1 19
Table XXII. Analysis of variance comparing across treatments for mean numbers of Diaptomus leptopus eggs produced per female during 1980 experiments. Values analyzed are log-transformed total numbers of loose and attached eggs divided by total numbers of adult females collected from enclosures in 150 1 samples from all sampling dates combined. Means and standard deviations shown are of untransformed data.
One-way Analysis of Variance:
Source df Sum of Squares Mean Square F p
Between 3 0.32 0.11 0.81 0.511 Within 4 0.46 0.12 Total 7 0.78
Descriptive Statistics:
Treatment Mean (#/female) Std. Dev,
Control 1.25 0.49 tyrelli-removal 1.65 0.64 Low density 3.00 1.98 tyrelli/low density 2.20 0.85 120
Table XXIII. Lengths (mm) of adult diaptomid copepods and of Daphnia rosea in 1980 experiments. Means are based on all animals measured, regardless of treatment or sampling date.
Species N Mean Std. Dev.
Diaptomus kenai 466 2 . 1'4 0 . 1 6
Diaptomus'leptopus 353 1 .53 0 .07
Diaptomus tyrelli 592 1 . 1 0 0 .07
Daphnia rosea 718 1 .04 0 .30 121
2.4 Seasonal Cycles, Vertical Distribution and Phytoplankton
Biomass
Seasonal Cycles
A few overwintering nauplii and early copepodites of
D. kenai persisted past the beginning of 1980 experiments. A later start to experiments and higher water temperatures excluded these immature stages from 1979 experiments. In both years, all development stages of D. tyrelli and D. leptopus occurred, but a greater proportion of D. leptopus were nauplii in 1980. As in 1979, both juvenile and adult Daphnia rosea were present throughout 1980 experiments.
Vertical Distributions
D. leptopus was almost extinct when vertically stratified sampling occurred in July, 1980. The few remaining individuals concentrated, as in 1979, in the upper 2 or 4 m of the water column. Vertical distributions of D. tyrelli, D. kenai, and
Daphnia rosea did not differ with developmental stage or experimental treatment. D. tyrelli occurred in the upper 2 m;
D. kenai, at all depths; and D. rosea, in the upper 4 m
(Fig. 24). Relative to July 1979, all three species were 1 22
Figure 24. Diurnal vertical distributions of Diaptomus tyrelli, Diaptomus kenai, and Daphnia rosea in 1980 experiments during July. Observations from all treatments are combined. N-2280
D D. kenoi N-219
Dgphnia N = II39
-i— 0 50 100 % OCCURRENCE 1 24
displaced upward. These higher vertical distributions in 1980 corresponded to cooler temperatures then than in 1979 (Fig. 25).
They caused increased spatial overlap between D. kenai and other diaptomids.
Phytoplankton Biomass
In 1980, few clear differences occurred among treatments in standing stocks of large and small phytoplankton (Fig. 26).
However,' after the first 3-4 weeks of the experiment, large cells contributed more biomass in controls than in tyrelli- removals, where densities of large D. kenai were high. Biomass of large phytoplankton- peaked during June-July in. 1980 experiments, and during September-October in 1979 experiments.
Similar phytoplankton genera occurred in both years. 1 25
Figure 25. Temperature profile for Eunice Lake during 1980 experiments. TEMPERATURE PROFILE (OCJ - 1980 igure 26. Phytoplankton standing stocks in Eunice Lake and in 1980 experiments. Plots for replicate enclosures are side by side. For each enclosure, upper plot shows total biomass; lower plot shows percent size composition of total biomass. A = Eunice Lake; B = Control; C = tyrelli-removal; D = Low density; E = tyrelli/low density. 29 9 19 3 10 24 • 21 18 29 9 19 3 24 21 HAT | JUNE | JULT | AUG. MAT | JUNE | JULT | AUG. 1980
1 30
3_. Summary of 1980 Results
1. At the start of the experiment, enclosures contained
Diaptomus tyrelli in densities appropriate to the
treatments.
2. Deviations from experimental design in densities of
Daphn ia rosea and Diaptomus kena i reflected replacement of
removed D. tyrelli biomass.
3. Performances of Diaptomus leptopus nauplii, copepodites,
and adults did not differ among treatments. Total
D. leptopus densities peaked and declined simultaneously
in all enclosures. These results neither verified effects
of 1979 tyrelli-removal enclosures nor separated them from
effects of low density enclosures from that year.
4. D. leptopus reached higher maximum densities and persisted
longer in enclosures in 1980 than in 1979.
5. A greater proportion of enclosed D. leptopus were nauplii
in 1980 than in 1979. D. kenai nauplii and early
copepodites occurred in enclosures in 1980 only.
6. Relative to 1979, water temperatures were cooler in 1980.
Correspondingly, zooplankton species occurred higher in 131
the water column and were less vertically segregated.
7. The only treatment effect on phytoplankton standing stocks
in 1980 was an increase for large cells in tyrelli-removal
enclosures. Peak biomass occurred earlier in 1980 than in
1979. 1 32
DISCUSSION
Examining why rare species are rare can provide valuable information about organization of ecological communities
(Robinson and Valentine 1979). In particular, those species having continual migratory access to the communities where they are rare deserve investigation. Diaptomus leptopus in Eunice
Lake is one of these species. Although it is abundant in an upstream lake, it does not colonize Eunice Lake. This experimental study has tested the hypothesis that competition from resident zooplankton species prevents densities of immigrant D. leptopus from increasing in Eunice Lake.
Separation of Diaptomus tyrelli Competition from Piffuse
Competition
High concentrations relative to controls of D. leptopus nauplii in 1979 in both tyrelli-removal and low density treatments (Fig. 8) confounded competition from D. tyrelli with diffuse competition from several Eunice Lake species combined.
Experiments in 1980 also failed to distinguish these. However, the evidence outlined below supports D. tyrelli competition.
(1) In 1979, D. leptopus densities were similar in tyrelli-
removal and low density enclosures (Table IX), despite
removal of species besides D. tyrelli from the latter
enclosures. 1 33
(2) Relative to controls, D. leptopus densities increased in tyrelli-removals, but not in Daphnia- and k_ena_i-removals
(Table IX).
(3) D. leptopus coexists with Daphn ia rosea and Diaptomus kenai in Gwendoline Lake, where D. tyrelli is lacking
(Table XXIV).
(4) Among UBC Research Forest lakes, D. leptopus co-occurs at detectable levels with D. tyrelli only in Katherine
Lake, where maximum D. tyrelli densities are a mere 1.3% of maximum densities in Eunice Lake (from data in Table XXIV).
Greater similarity in size between D. leptopus and
D. tyrelli in Eunice than in Katherine Lake (Table XXIV) increases potential in Eunice for competition for similarly-sized foods (Hutchinson 1967; Burns 1968; Poulet
1977).
(5) Hutchinson (1967) suggested that dietary differences between diaptomids differing in length by at least 35% are sufficient to ensure coexistence. In Eunice Lake, D. kenai is 84% and 150% larger than D. leptopus and D. tyrelli, respectively (Table XXIV). D. leptopus and D. tyrelli differ by a marginal 36%. Hammer and Sawchyn (1968) submitted that coexisting diaptomids must differ in length by more than 0.5 mm. D. leptopus and D. tyrelli from Eunice
Lake'differ by only 0.41 mm (Table XXIV). Table XXIV. Maximum lengths and maximum summer densities of calanoid copepods and Daphni a rosea i n Katherine (KA), Gwendoline (GW), and Eunice (EU) Lakes, 1963-1974. From Northcote and Clarotto (1975)
Maximum length (mm) Maximum density (#/100 1) KA GW EU KA GW EU
Diaptomus kenai 2 .9. 7 2 . 70 2 .87 8.9 10 70
Diaptomus leptopus 1 .9. 2 1 .6.8 1 . 56 2 . 5 13 0.5
Diaptomus tyrelli 0..9 9 -- 1 . 15 10 -- 800
Daphnia rosea 2 . 25 2 .61 2 .50 12 95 233
CO 135
(6) Sandercock (1967) proposed that, to coexist, diaptomids
must segregate by at least two of diet, seasonal
occurrence, and vertical distribution. In 1979 in Eunice
Lake, extensive seasonal synchrony (Figs. 9 and 10) and
vertical overlap (Fig. 12) hindered coexistence between
D. leptopus and D. tyrelli. D. kenai bred earlier (Fig. 11)
and distributed deeper (Fig. 12) than either of these two
species, and coexisted with both. D. leptopus and Daphnia
rosea segregated vertically (Fig. 12). They probably also
partitioned food resources because calanoids generally
consume coarser particles than cladocerans (Hutchinson
1967).
Almost all evidence from 1979 indicts. D. tyrelli competition for preventing D. leptopus from increasing in Eunice
Lake. Only increased D. leptopus eggs per female in food-rich
low density treatments (Table XI) implicates diffuse competition. However, diffuse competition cannot explain
D. leptopus rarity in Eunice Lake because enhanced reproduction
failed to generate higher D. leptopus levels in low density enclosures than in tyrelli-removals (Table IX). Lynch (1978)
found a similar situation among cladocerans. In the presence of
Ceriodaphnia and Bosmina, enhanced Daphnia pulex reproduction caused no population increases because of reduced juvenile
survival. By grazing the small foods of juvenile Daphnia,
Ceriodaphnia and Bosmina enhanced large phytoplankton for
reproductive adult Daphn ia. Lynch's result suggests an
explanation for why D. leptopus concentrations were similar' in 136
tyrelli-removal and low density treatments when reproductive potentials were not: Alteration in relative abundances of food resources due to grazing by D. tyrelli may affect D. leptopus more adversely than depression of absolute resource abundances due to grazing by all Eunice Lake zooplankton species combined.
Vulnerability of Diaptomus leptopus Naupli i to Competition
In 1979 experiments, competition from D. tyrelli adversely
affected primarily naupliar stages of D. leptopus (Table IX).
Stage-related energetic differences may explain this, result. If,
as Green (1975) found for Calamoecia lucasi, balance between
assimilated food intake and respiratory output is positive for
all but naupliar D. leptopus, then nauplii should be
particularly vulnerable to food shortages. Both late nauplii,
which filter-feed (Marshall and Orr 1956; Paffenhofer 1971), and
early nauplii, which carry stored food in amounts dependent on
nutritive conditions of reproducing females (Cooley et al. 1978;
Cooney and Gehrs 1980), would be affected. The stronger positive
correlation between metabolic rate and body weight for nauplii
than for copepodites and adults (Epp and Lewis 1979, 1980)
further supports naupliar susceptibility. Metabolism increases
abruptly during the radical morphological and behavioural
transition from N6 to CI (Gehrs and Robertson 1975; Epp and
Lewis 1979, 1980). High mortality related to these increases may
explain low densities of copepodite and adult D. leptopus in
tyrelli-removal and low density enclosures despite high naupliar 1 37
concentrat ions.
1979 experiments were unable to determine which stage(s) of
D. tyrelli outcompeted D. leptopus nauplii. Unequivocal determination would require exposure of D. leptopus to each
D. tyrelli stage separately. Parallels between nauplii of both
species in size, morphology, and behaviour (Epp and Lewis 1979) and in vertical distribution (Fig. 12) implicate immature
D. tyrelli as competitors. Alternatively, copepodites and adults
of small D. tyrelli may have starved nauplii of larger
D. leptopus by using -small food particles more efficiently.
Small cladocerans (Alonella, Cer iodaphn ia) may impose such
competitive bottlenecks on immature stages of larger herbivorous
crustaceans (Diaptomus, Daphn ia) (Neill 1975a; Lynch 1978).
Relevance of Experimental Results to the Size-Efficiency
Hypothesis
The result from 1979 that small D. tyrelli outcompeted
larger D. leptopus reverses predictions of the size-efficiency
hypothesis (Brooks and Dodson 1965). In several other
zooplankton communities, small herbivores are superior
competitors (Dodson 1974; Bogdan and McNaught 1975; Neill 1975a
and b; Kerfoot 1977; Lynch 1977b, 1978, 1979, 1980). Lynch
(1977b) recognized three relevant criticisms of the hypothesis.
(1) The size-efficiency hypothesis falsely assumes
competitive interactions among adults only. As already 138
discussed, adults of small zooplankton species can create competitive bottlenecks for juveniles of large species
(Neill 1975a). If juvenile mortality is sufficient, large adults can coexist with small species even when food is scarce. High survival of D. leptopus nauplii in tyrelli-
removals and lack of treatment effects on adults (Table
XII) suggest that in 1979 D. tyrelli created a competitive bottleneck for D. leptopus. Large copepods that reproduce
before small ones avoid bottlenecks by co-occurring as late copepodites and adults with younger stages of the smaller
species (Williams 1976). In Eunice Lake, early-breeding
D. kenai (Fig. 12) coexists with both D. leptopus and
D. tyrelli
(2) The size-efficiency hypothesis falsely assumes similar
relationships between physiological rates and body size for
all species. Species differences occur at least for
respiration rate (Gauld and Raymont 1953; Comita 1968;
Green 1975; Epp and Lewis 1979). Assimilation rate probably
depends more on food source than on zooplankton size
(Schindler 1971). These and other rates are poorly studied
as functions of body size (Hall et al. 1976). In a
community of marine copepods, Poulet (1978) found that net
energy intake per unit body weight was greater for small
than for large species. Perhaps a greater energetic
efficiency of small D. tyrelli explains its competitive
superiority over D. leptopus. 1 39
(3) The size-efficiency hypothesis falsely assumes equal
adaptation of all species to prevailing environmental
conditions. Small Daphnia pulex forage more efficiently
than large individuals at high concentrations of small
foods and at high temperatures (Lynch 1977b). Its more
oligotrophic distribution (Wilson 1959) suggests that
D. tyrelli is better adapted than D. leptopus to the low
food levels in Eunice Lake.
Relationship Between Temperature and Intensity of Competition•
In 1979 experiments, D. tyrelli outcompeted D. leptopus nauplii, yet 1980 .experiments produced no . evidence of competition between these species (Table XXI). Temperature differences between the two years may reconcile these contrary
results (Figs. 15 and 25). Competition for limited food could
have been severe in warm 1979 and undetectable in cool 1980
because copepod metabolic demands, and therefore food
requirements, increase with temperature (Fig. 27). Temperature-
dependent increases in food intake potentially offset increases
in metabolism, but at the low food concentrations in UBC
Research Forest lakes, zooplankton feeding rates are probably
independent of temperature (Buckingham 1978).
Occurrence of competition between D. leptopus and
D. tyrelli primarily at warm temperatures fits with Wiens'
(1977) proposal that- variation in environmental conditions
regulates intensity of competition by determining whether 1 40
Figure 27. Diaptomid respiration rates as a function of temperature. Because rates for Diaptomus tyrelli unavailable, those for a similar spec ies rDiaptomus oregonensis) are shown. From Comita (19681 OXYGEN UPTAKE (ul'Vhr) 00 OOQQOO op b b b b — — — ro ho ro
001 o>
• i i i i i 1
LH 1 42
resources are effectively limited. Supporting evidence exists for rainfall and temperature variations. Amount of rainfall correlates with food abundance in tropical fish (Zaret and Rand
1971) and desert rodent communities (Schroder and Rosenzweig
1975), and with food availability in terrestrial salamander communities (Jaeger 1972). Experimental removal of one insectivorous lizard species effected improved performance in another during arthropod-poor dry years, and not during food- rich wet years (Dunham 1980). Only at cool temperatures favouring its feeding efficiency can Daphnia pulex outcompete
Ceriodaphnia reticulata (Lynch 1978). Percids, centrarchids, and salmonids coexist only in winter, when metabolic costs correlated with food demands are low (MacLean and Magnuson
1977). Correspondingly, vertical overlap between D. tyrelli and
D. kenai in Eunice Lake increased in cool 1980 (Fig. 24).
In these typical systems, competition is detectable only when stressful environmental conditions increase demands for
resources and/or decrease resource concentrations (Wiens 1977).
If, as experimental evidence suggests, high temperatures
intensify competition between D. leptopus and D. tyrelli, then
D. leptopus densities in Eunice Lake should be relatively high
only during cool periods. Warm temperatures should occur
frequently enough for D. tyrelli to check D. leptopus advances.
The presence of detectable D. leptopus levels in Eunice Lake
during only two periods - after temperatures decreased in fall
1979 (Fig. 7A) and before they increased in spring 1980
(Fig. 22A) - upholds these predictions. Unfortunately, besides 1 43
low temperatures(Figs. 15 and 25), relatively high phytoplankton
biomass (Figs. 16A and 26A) and potentially high D. leptopus emigration from Gwendoline due to heavy rainfall also occurred
during these two periods. For D. leptopus in an acidic bog lake,
larval survival was highest at low temperatures (Winner 1970).
One important result apparently contradicts the hypothesis
that competition between D. leptopus and D. tyrelli is
temperature-dependent. Late in 1980 experiments, as temperatures
increased to 1979 levels, D. leptopus decreased to extinction
simultaneously in all treatments (Fig. 22), regardless of
D. tyrelli concentration. Stronger competition from conspecifics
than from D. tyrelli may have been responsible. Before
temperatures increased, D. leptopus in all 1980 enclosures had
reached extremely high densities. These exceeded by 5x densities
in 1979 tyrelli-removals (Figs. 7 and 22). In addition to
greater initial stocking densities, favourable food and
temperature conditions early in 1980 contributed to high
D. leptopus concentrations. Probably reflecting experimental
fertilization of Gwendoline Lake from May to September 1979 (Hay
1981), the phytoplankton biomass in enclosures during spring
reproduction was larger in 1980 than in 1979 (Figs. 16 and 26).
Cool spring conditions allowed reproducing females to make full
use of the added food by reducing their metabolic demands
(Sawchyn and Hammer 1968; O'Brien et al. 1972; Kamps 1978).
Slowed development rates at low temperatures (McLaren 1963;
Rigler and Cooley 1974; Kamps 1978) caused accumulations of
D. leptopus nauplii in all 1980 enclosures (Fig. 23). Consequent 1 44
concurrence of temperature increases with high concentrations of
D. leptopus at the energetically critical N6 to CI transition
(Epp and Lewis 1980) probably exaggerated effects of
intraspecific competition on enclosed D. leptopus. In Eunice
Lake, where D. leptopus densities are always much less than in
1980 enclosures, interactions with conspecifics are probably negligible compared to competition with D. tyrelli.
Proper assessment of the relationship between temperature and intensity of competition between D. tyrelli and D. leptopus
would require a laboratory experiment similar to Neill's
(unpubl.) on two herbivorous cladocerans. Despite high
abundances in nearby eutrophic lakes, Daphnia pulex does not co-
occur with D. rosea in oligotrophic Research Forest lakes (Neill
1978).. Neill confined both species together in a factorial
experiment with three food concentrations (0.4, 0.7, and
1.0 mg/1 dry weight) and three temperature levels (12, 15, and
21°C) (see Neill 1981b). Regardless of temperature, D. pulex
excluded D. rosea at intermediate and high food concentrations.
At low food concentrations, both species coexisted at the two
lower temperatures. Competitive superiority of D. rosea at the
highest temperature reflected its adaptation to oligotrophy. For
a corresponding experiment on diaptomids, the hypothesized
correlation between temperature and competition predicts
parallels of D. leptopus with D. pulex, and of D. tyrelli with
D. rosea. The more oligotrophic distribution of D. tyrelli
(Wilson 1959) than of D. leptopus (Reed and Olive 1958)
strengthens these predictions. Consistent with them are 1 45
extinction of D. leptopus (Fig. 22A) and survival of D. tyrelli
(Fig. 17A) in Eunice Lake during concurrent temperature
increases (Fig. 25) and phytoplankton decreases (Fig. 26A).
Limitations of Phytoplankton Standing Stock Data
Most of the foregoing discussion implicitly assumes that
D. leptopus and D. tyrelli in Eunice Lake are food limited. Low
zooplankton densities make space limitation and interference competition implausible; low phytoplankton levels make
exploitation competition for food probable. Supporting evidence
includes biomass and density increases of zooplankton in Eunice
(Marmorek 1982) and Gwendoline Lakes (Neill and Peacock 1980;
Peacock 1981; Hay 1981) in response to experimental
fertilizations. Because egg production is a particularly
sensitive indicator of food abundance (Marshall and Orr 1958;
Edmondson et al. 1962; Weglenska 1971; Dagg 1977; Cooney et
al. 1978), elevated D. leptopus and D. tyrelli reproduction in
1979 low density enclosures (Table XII) implies food limitation
in other treatments from that year. Increased D. leptopus per
female egg numbers in all enclosures in cool 1980 (Table XXII)
implies a connection between reduced food limitation and the
absence of experimental evidence for competition. However, this
link is obscured by the inverse correlation between temperature
and diaptomid clutch size (Sawchyn and Hammer 1968; Kamps 1978).
The assumption of food limitation is central to the
hypothesis that temperature governs competition between 1 46
D. leptopus and D. tyrelli. This hypothesis assumes that temperature is positively correlated with zooplankton demands for food resources, but uncorrelated with resource concentrations. Similarities between warm 1979 and cool 1980 in phytoplankton standing stocks (Figs. 16 and 26) do not necessarily confirm independence of temperature and resource levels because standing stocks do not incorporate phytoplankton turnover rates (Cooper 1973). As temperatures increase, turnover
rates may increase, hold, or decrease, depending on whether
increases in phytoplankton production rates (Wetzel 1975) are greater than, equal to, or less than increases in nutrient depletion rates. Unchanged or increased turnover rates support
temperature mediation of competition by implying more intense competition in 1979 despite equally or more favourable food
conditions than in 1980. As an alternative to higher zooplankton
food requirements, decreased turnover rates provide lower food
levels as an explanation for 1979 competition. Which of these
possibilities applied during the experiments is unknown because
turnover rates were not measured.
If zooplankton grazing decreases biomass of edible algae
(Porter 1972), then differences among 1979 treatments in
phytoplankton size composition support an inverse feeding
relationship between zooplankton body size and food particle
size. Relative to controls, standing stocks of large particles
increased in tyrelli-removal and low density enclosures, and
decreased in kenai-removals within the first 6 weeks of the
experiment (Fig. 16). Standing stocks of small cells did not 147
differ across treatments. These results violate predictions derived from zooplankton feeding studies showing a positive correlation between animal and food sizes (Burns 1968). However, standing stocks may improperly estimate relative importance of large and small cells because turnover rates are higher for small cells (Kalff 1972; Gutelmacher 1975; Redfield 1980).
Further limiting the usefulness of standing stock data is the dependence of turnover rates on nutrient regeneration rates
(Titman 1976; Lehman 1980a and b), which are themselves functions of zooplankton excretion rates (Porter 1977; Lehman
1980a), body sizes (Lehman 1980b), and nutritional states
(Schindler 1971).
At least two explanations are capable of aligning phytoplankton results observed in 1979 with those predicted from zooplankton feeding studies.
(1) If grazing by zooplankton increases, rather than
decreases, biomass of grazed algae, then decreases relative
to controls of large phytoplankton in kenai-removals and
lack of increases of small phytoplankton in tyrelli-
removals may affirm a positive correlation between
zooplankton size and food size. Moderate grazing in
controls may have enhanced productivity and/or biomass of
grazed phytoplankton by causing compensatory increases in
their turnover rates (Cooper 1973) and/or increases in
nutrient regeneration rates (Gliwicz 1975). 1 48
(2) Low grazing pressures on small edible algae may explain
why only less edible large cells increased in low density
and tyrelli-removal enclosures. Small phytoplankton cells
with high surface to volume ratios may be competitively
superior to large cells under oligotrophic conditions
(Parsons and Takahashi 1973; Gliwicz 1975; McCauley and
Briand 1979). However, after grazing pressures are reduced,
small algal species may initially increase to
concentrations at which they deplete nutrients not limiting
to large species. Then, both the biomass of small algae and
their competitive advantage over large algae for use of
common resources should decrease, and biomass of large
phytoplankton should increase. This interpretation does not
seem to hold for non-oligotrophic systems; in small
enclosure experiments, Gliwicz (1975) found a direct
relationship between biomass of inedible netplankton and
intensity of grazing by Daphn ia pulex on nannoplankton.
Interactions between large and small cells are complicated
by enhancement effects of viable gut passage on certain
large inedible algae (Porter 1973, 1975, 1976, 1977).
Although these two mechanisms are possible causes of observed treatment differences in algal size compositions, whether they were the actual causes cannot be determined without data on phytoplankton turnover rates. Unfortunately, methods of estimating turnover rates separately for cells of different sizes are complex and unreliable (see Vollenweider 1969; 1 49
reviewed in Knoechel and Kalff 1976).
In addition to unknown turnover rates, inadequate knowledge of copepod diets impedes interpretation of algal size compositions. Bacteria and detritus, rather than algae, may be principal foods for some zooplankton species (Pennak 1946;
Nauwerck in Saunders 1969; Gliwicz 1969; Moore 1977, 1978, 1979;
Neill 1981b). Ability of large zooplankton to filter small particles (Brooks and Dodson 1965) and ability of small zooplankton to raptorially capture large particles (Mullin 1963;
McQueen 1970; Porter 1977) cloud positive correlations between zooplankton size and food size. Both nauplii and adults of
Calanus finmarchicus eat particles of similar size (Marshall and
Orr 1956).
Limited information on diaptomid diets is available.
D. minutus • filters particles <20 urn in diameter (Bogdan and
McNaught 1975); D. oregonensis, particles £30 urn (Richman et al. 1980); and D. coloradensis and D. Shoshone, particles <120-
150 urn (Maly and Maly 1974). Contradictory studies have reported
algae £60 urn (Hazelwood and Parker 1961) and detritus <22 urn
(Winner 1970) as diets of D. leptopus. Even these few data are
questionable because methods used to study zooplankton feeding
have been unreliable (Harbison and McAlister 1980; Roman and
Rublee 1980). 1 50
SUMMARY AND CONCLUSIONS
This study experimentally tested the hypothesis that
interspecific competition from resident zooplankton species prevents immigrant Diaptomus leptopus from colonizing oligotrophic, montane Eunice Lake. Experiments were conducted in the lake in large polyethylene enclosures during spring, summer, and fall of 1979 and 1980. Control treatments in both years confined D. leptopus with all Eunice Lake species at lake densities. Other treatments in 1979 confined D. leptopus with all species at low densities or with all species except one of
Daphnia rosea, Diaptomus kenai, or Diaptomus tyrelli. Treatments
in 1980 exposed D. leptopus to various concentrations of
D. tyrelli. Comparisons between D. leptopus responses to control and non-control treatments yielded the following conclusions:
1. in 1979 experiments, equal and positive responses of
D. leptopus to tyrelli-removal and low density treatments
prevented separation of competition between D. leptopus and
D. tyrelli from diffuse competition. Other results
supported D. tyrelli competition. Experimental evidence
included neutral responses of D. leptopus to Daphnia- and
kenai-removals. Non-experimental evidence included marked
spatial and temporal overlap between D. leptopus .and
D. tyrelli in Eunice Lake and disjunct occurrences of these
two species in lakes other than Eunice.
2. Low biomass of phytoplankton in most enclosures and high 151
numbers of eggs per D. leptopus female in low density enclosures suggested that competition between D. leptopus and D. tyrelli in 1979 was for food. Treatment differences in algal size composition did not indicate whether competitors partitioned limited resources.
Only D. leptopus nauplii were vulnerable to competition from D. tyrelli possibly because nauplii are energetically less efficient than later stages. 1979 experiments could not distinguish which stages of D. tyrelli competed.
In contrast to 1979, D. leptopus responded similarly in
1980 to all treatments, regardless of D. tyrelli concentrations. Low temperatures during most of 1980 probably curtailed competition between D. leptopus and
D. tyrelli by reducing their food requirements. D. leptopus nauplii reached extremely high concentrations in all enclosures because cool conditions promoted reproduction and retarded development. Presumably, competition from conspecifies, rather than from D. tyrelli, caused extinctions of D. leptopus in all enclosures as
temperatures increased late in 1980 experiments.
In Eunice Lake, where D. leptopus densities are always very
low, intraspecific competition undoubtedly is
insignificant. Competition with D. tyrelli probably is
temperature-dependent. At low temperatures during fall 1979 1 52
and spring 1980, densities of D. leptopus in Eunice Lake were relatively high. Undetectable densities occurred when higher temperatures increased potential for competition with D. tyrelli. High temperatures presumably- occur
frequently enough for D. tyrelli to prevent D. leptopus
from colonizing Eunice Lake. 1 53
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