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T T-1V /T-T Dissertation U 1 V 1 1 Information Service
University Microfilms International A Bell & Howell Information C om pany 300 N. Zeeb Road, Ann Arbor, Michigan 48106
8625197
Chilton, Earl Wallace
MACROINVERTEBRATE COMMUNITIES ASSOCIATED WITH SELECTED MACROPHYTES IN LAKE ONALASKA: EFFECTS OF PLANT TYPE, PREDATION, AND SELECTIVE FEEDING
The Ohio State University Ph.D. 1986
University Microfilms International300 N. Zeeb Road, Ann Arbor, Ml 48106
Copyright 1986 by Chilton, Earl Wallace All Rights Reserved
MACROINVERTEBRATE COMMUNITIES ASSOCIATED WITH
SELECTED MACROPHYTES IN LAKE ONALASKA: EFFECTS OF
PLANT TYPE, PREDATION, AND SELECTIVE FEEDING
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
the Degree Doctor of Philosophy in the
Graduate School of the Ohio State University
By
Earl Wallace Chilton, B.S., M.S.
***
The Ohio State University
1986
Reading Committee: Approved By
Dr. F. J. Margraf
Dr. R. A. Stein
Dr. C. A. Triplehorn Ad^sor
Department of Zoology to Kathryn ACKNOWLEDGEMENTS
I would like to thank my advisor, Professor F. J. Margraf, for all of his help through the latter part of my graduate career. I appreciate both his honest advise, and his help in the field.
I also want to thank Professor R. A. Stein, Professor C. A. Triplehorn, and Professor D. A. Stansbery for their critical comments , help on my committee, and particularly for the speed with which they reviewed early drafts of my paper.
Many fine technicians including Vicki Cole, Kurt Drottar, Laura Lee
Clark, Pat Wagner, Tom Rice, Kevin Callan, Valerie Simich, and Paul Andreas, also deserve my sincere thanks.
Funding for this project was provided by the U.S. Fish and Wildlife
Service through the National Fishery Research Laboratory, LaCrosse,
Wisconsin. The people at the lab provided not only the money, but a healthy working atmosphere. My special thanks to the Director of the lab, Dr. Fred P.
Meyer, for all of his help.
Finally, I would like to acknowledge the support of my parents, Mary and
Earl, my wife, Kathryn (who spent many hours with me in the lab), and my Lord
Jesus. VITA
June 5, 1957 Born - Columbus, Ohio
June, 1979 B. S., Union College,
Schenectady, New York
August, 1982 M. S., Bowling Green
State University
Bowling Green, Ohio
Publications
Chilton, E.W., R.L. Lowe and K.M. Schurr. (In Press) Invertebrate
communities associated with Cladophora qlomerata and Banqia
atropurpurea in western Lake Erie. Journal of Great Lakes
Research.
Presented Papers
Chilton, E.W. 1983. A study of invertebrates associated with Banqia
atropurpurea and Cladophora qlomerata in the western basin of
Lake Erie. Annual meeting of the Ohio Academy of Science. April
1983.
iv TABLE OF CONTENTS
DEDICATION...... ii
ACKNOWLEDGEMENTS...... iii
VITA...... iv
LIST OF TABLES...... vii
LIST OF FIG U R ES...... ix
GENERAL INTRODUCTION...... I
STUDY A R E A ...... 6
CHAPTER I: DESCRIPTION OF INVERTEBRATE COMMUNITIES
ASSOCIATED WITH VALLISNER1A AMERICANA,
MYRIOPHYLLUM SPICATUM, AND CERATOPHYLLUM
DEMERSUM
Introduction ...... 12
Materials and M ethods ...... 16
R e s u lts ...... 21
D iscussion ...... 63
Conclusion ...... 75
CHAPTER 2: EFFECTS OF VERTEBRATE PREDATION ON THE
VALLISNERIA-ASSOCIATED INVERTEBRATE
COMMUNITY
Introduction ...... 76
Materials and M ethods ...... 81
v R e s u lts ...... 86
D iscussion ...... 114
Conclusion ...... 120
CHAPTER 3: CHIRONOMID FEEDING BEHAVIOR
Introduction ...... 121
Materials and M ethods...... 124
R e s u lts ...... 126
D iscussion ...... 131
Conclusion ...... 133
SUMMARY ...... 134
LIST OF REFERENCES...... 136
APPENDICES
A. Information used in Chapter I ...... 154
B. Information used in Chapter II ...... 164
vi LIST OF TABLES Table
1. ANOVA generated probabilities indicating the significance of date, site, and plant type, as well as interactions among these variables, on invertebrate abundances in Lake Onalaska in 1983. Probability values of 0.05 or lessindicate that differences among dates, sites, plants, or interactions were statistically significant. Ns indicates that differences were not significant.
2. Results of Duncan’s Multiple Range Test, indicating the magnitude and significance of date, site, and plant effects on the abundance of macrophytes-associated invertebrate taxa in Lake Onalaska in 1983. Means of log-transformed (base e) data are presented. Means with different letters indicate that statistically significant differences existed among those values (p<0.05).
3. Percentage of total abundance of major (numerically dominant) invertebrate taxonomic groups found in association with Vallis- neria americana, Myriophyllum spicatum, and Ceratophyllum demersum, during summer 1983 in Lake Onalaska.
4. Feeding habits of some important game fishes found in Lake Onalaska, based on the literature. Data are presented as percent volume of various food items found in the stomachs of these fishes. Data are from Price 1963, Seaburg 1964, Keast and Webb 1966, Voightlander and Wissing 1974, Cherry and Guthrie 1975, Keast 1978, Phillips et al. 1982, and Schaeffer and Margraf 1986.
5. T-test comparisons of invertebrate abundance (number per gram dry weight of plant) among 1984 enclosure/exclosure experiment treatments. Data are presented by date, for all major (numerically dominant) invertebrate taxa, and for total invertebrate abundance. Calculations were performed on log-transformed data ln(n+l). Con = open control treatments, Cage = cage-effect control treatments, Ex = exclosure treatments, En = enclosure treatments.
6. ANOVA performed on 1984 enclosure/exclosure experiment data in Lake Onalaska. Probability values are presented which indicate the probability that the effects of date, treatment, and the date-by- treatment interaction on invertebrate abundance was due to chance. Data are presented for all major (numerically dominant) invertebrate taxa, and for total invertebrate abundance. The test was peformed on log-transformed data ln(n+I) . Values indicate the probability that differences in invertebrate abundance among dates, treatments, or the interaction occurred by chance. Values of 0.05 or less indicate statistical significance. Ns indicates no statistical significance.
vii 7. Results of stocked bluegill stomach analysis. Data are presented as the percentage of the total number of invertebrates found in bluegill digestive tracts. Nothing was found in the stomach of the madtom that was collected. Analysis of smaller, unstocked bluegills, found in cages at the conclusion of the experiment is also presented.
8. Composition of diatom flora associated with Myriophyllum spicatum samples, collected September 14, 1983, at sites I and 2 in Lake Onalaska, Wisconsin. The total number counted of each taxa are presented along with the percentage of the total. S-l and S-2 indicate sites I and 2, respectively.
9. Chesson's Preference Index (Chesson 1983), calculated for diatom flora found in chironomid digestive tracts, compared to those found epiphytically on the plants (Myriophyllum spicatum samples) from which chironomids were collected. The index ranges from a value of -I (indicating strong negative preference) to +1 (indicating strong positive preference). Zero indicates no preference. C. placent = C. placentula, N. amphib = N. amphibia, N. conferv = N. confervacea, Melosir = Melosira spp., Cyclote = Cyclotella spp., and Cymbell = Cymbella spp.
10. Changes in the abundance (natural log of the number of invertebrates per gram dry weight of plant) of invertebrate communities associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented
11. Changes in invertebrate biomass (grams invertebrate dry weight per gram dry weight of plant) associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented.
12. Changes in abundance (natural log of the number of invertebrates per gram dry weight of plant) of Vallisneria-associated invertebrates during summer 1984 endosure/excIosure experiments in Lake Onalaska. Standard error of the mean is presented.
13. Changes in the biomass (invertebrate dry weight per gram dry weight of plant) of the Vallisneria-associated invertebrate community during summer 1984 enclosure/exclosure experiments in Lake Onalaska. Standard error of the mean is presented. LIST OF FIGURES
Figure
1. Map of Lake Onalaska, Wisconsin, showing the Black River to the east, and the Mississippi River to the west of the lake. Collection sites for summer 1983 (I through 5) and for summer 1984 (I and 6) are indicated by numbered stars. Site 2 is positioned just inside Sommer's chute.
2. Sampler used for manual aquatic macrophyte collections during summers 1983 and 1984. The 20.3-cm diameter opening at the top of the sampler was lowered over plants. The 500-micron mesh bag was 1.5 m long, and constructed of nitex.
3. Changes in the abundance (natural log of the number of invertebrates per gram dry weight of plant) of invertebrate communities associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
4. Changes in the abundance (natural log of the number of invertebrates per gram dry weight of plant) of invertebrate communities associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites 1, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in'Table 10.
5. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Trichoptera associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
6. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Chironomidae associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
7. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Hemiptera associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
8. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Odonata associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in
ix Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
9. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Amphipoda (Hyallela azteca) associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
10. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Gastropoda associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 10.
I I. Changes in invertebrate biomass (grams invertebrate dry weight per gram dry weight of plant) associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Sample sizes are given in Table 11
12. Current velocity (meters per second) readings at sites I, 2, and 5, during July through September, 1983 in Lake Onalaska.
13. Surface area (cm ) to dry weight (g) regressions for Vallisneria samples (n = 59, r = 0.81), and Myriophyllum samples (n = 18, r = 0.69), collected in Lake Onalaska during summer 1983, from sites I, 2, and 5.
14. Design for enclosure, exclosure, and cage-effect control cages used during summer 1984 enclosure/exclosure experiments. Enclosure and exclosure treatment cages were completely enclosed by 0.8 cm mesh aluminum wire screening. One half of each side of cage-effect controls was left open.
15. Changes in abundance (natural log of the number of invertebrates per gram dry weight of plant) of Vallisneria-associated invertebrates during summer 1984 enclosure/exclosure experiments. Control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
16. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Hemiptera during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
x 17. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Gastropoda during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatm ents are represented by dotted lines, exclosure treatments are represented by
dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
18. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Amphipoda (Hyallela azteca) during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
19. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Odonata during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
20. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Trichoptera during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatm ents are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
21. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Chironomidae during summer 1984 enclosure/exclosure experiments. Open control treatm ents are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
22. Changes in the biomass (invertebrate dry weight per gram dry weight of plant) of the Vallisneria-associated invertebrate community during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by
xi GENERAL INTRODUCTION
Aquatic vascular plants greatly influence benthic invertebrate communities. Increasing plant biomass increases habitat complexity and is well correlated with increasing size and diversity of aquatic invertebrate associations (Heck and Wetstone 1977; Stoner 1980; Wiley et al 1984; Bell and
Westoby 1986). This correlation may be the result of various primary influences acting singly or in concert with each other. First, as habitat complexity increases, efficiency of predators decreases ( Heck and Thomas
1981; Savino and Stein 1982; Stoner 1982; Savino 1985). Thus, aquatic macrophyte beds function as refugia for various invertebrate prey. In this scenario increasing the complexity of a macrophyte bed serves to increase its utility as a refuge. A corresponding increase in prey density and diversity may result.
Second, perhaps not only biomass of a macrophyte bed is important, but also the plant species in the bed. Various workers have found that the size and composition of macrophyte-associated invertebrate communites is influenced by plant type (Krecker 1939; Mackie and Quadri 1971; Gerrish and Bristow
1979; Rooke 1986). Brown (1984) and others (Krecker 1939; Andrews and Hosier
1943) have suggested that these differences are due to morphologial dissimilarities among plants. Their data indicate that increased leaf complexity correlates with increasing size of associated invertebrate communities. Plants with highly dissected leaves supported greater numbers of invertebrates than plants with simple leaves. Therefore, plants with highly
dissected leaves may offer a more complex habitat. Greater habitat
complexity may correlate well with increased invertebrate standing crop for a
variety of reasons, including protection from predators, protection from
current, easier attachment, and increased food availability because of detritus
trapped by finely dissected leaves.
Third, invertebrate communties may differ among plants because of
differences in food availability among plants, i.e., different plants may support
differing epifloral assemblages. Epiflora associated with macroscopic
filamentous algae may differ among algal species (Rosen 1981). These
differences may result in differences among associated invertebrate communities (Chilton et al. 1986). Similar differences may exist among
epifloral assemblages associated with aquatic vascular plants. If this is the case, herbivorous invertebrates may choose specific plants as habitat because they support preferred epifloral assemblages.
To determine whether plants support different macroinvertebrate assemblages, and to explore possible reasons for differences should they exist, I performed three studies. Because plant type often influences invertebrate community structure (Rosine 1955; Mackie and Quadri 1971; Scheffer et al.
1984), Chapter I examines the invertebrate communties associated with three aquatic vascular plants, Vallisneria americana, Ceratophyllum demersum, and
Myriophyllum spicatum during summer 1983. Andrews and Hosier (1943) ranked the invertebrate standing crops of several macrophytes in Lake Mendota,
Wisconsin. They found that Ceratophyllum demersum had the highest macroinvertebrate standing crop, followed by Myriophyllum exalbescens and
Vallisneria americana with the smallest standing crop. Krecker (1939) reported similar findings whereas Macki and Quadri (1971) found Myriophyllum supported 3 nearly 7.5 times as many invertebrates as Vallisneria. Data from Montezuma refuge (main pool of the north end of Cayuga Lake, New York), were also consistent with Andrews and Hosier (1943), suggesting that C. demersum maintains a high macroinvertebrate standing crop (Krull 1970). Here, I tested whether or not macrophyte-invertebrate associations followed established abundance patterns from published literature, and if these patterns differred among invertebrate taxonomic groups.
To understand these differences in invertebrate associations I examined effects of vertebrate predation on invertebrate communties associated with
Vallisneria americana. If vertebrate predation can influence the Vallisneria- associated community, then it may be hypothesized that differential predation among plants may be responsible for inter-plant differences. A number of studies have suggested that invertebrate predation (Paine 1966; Menge and
Sutherland 1976; Sutherland and Ortega 1986) and/or competition (Woodin 1974;
Brown 1982; Sutherland and Ortega 1986) can have a significant effect on the structure of invertebrate communities in marine systems. Similar data exist for various freshwater systems (Brown 1982; Walde and Davies 1984). However, data on the effects of vertebrate predation are unclear. Thorp and Bergey
(1981), in a series of experiments that excluded vertebrate predators (fish and turtles) from macrophyte beds, found that gastropods were the only invertebrate group affected. However, these results have come under much criticism. No enclosures were used and the density of predators outside cages was unknown, so that no treatment had a known predator density. In addition, the lack of enclosures and partial cages meant that possible cage effects were not adequately controlled for. Workers in a variety of freshwater systems, both natural and artificial, have found that vertebrate predation does influence invertebrate populations. Hurlbert et al. (1972) reported that mosquitofish (Gambusia affinis) in small pools were able to effectively reduce insect and
large zooplankton populations. Andersson et al. (1978) found that, not only
were fish in lake-water enclosures able to reduce macrobenthos and large zooplankton populations, but also that exclosures from which fish were excluded, experienced a doubling of chironomid and oligochaete populations.
Similarly, in a series of enclosure/exclosure experiments, Fairchild (1982) discovered that largemouth bass (Micropterus salmoides) fry were able to significantly reduce Sida crystalling (Cladocera) populations. Recent data also
indicate that fish are able to effect qualitative as well as quantitative changes
in invertebrate populations. Morin (1984) found that odonate community structure was affected by fish predation, whereas Bennett and Streams (1986) report changes in hemipteran community structure as a result of fish predation.
Walde and Davies (1984) have suggested that experiments designed to test the effect of factors such as predation or competition on a community yield less ambiguous results if enclosures are used in conjunction with exclosures. Therefore, to determine the effects of vertebrate predation on the
Vallisneria-associated invertebrate community, a series of enclosure/exclosure experiments were conducted in Lake Onalaska during summer 1984.
Invertebrates may associate more strongly with certain plants because of the distribution of preferred food types growing epiphytically on those plants. However, for this to be true invertebrates must be selective feeders.
Although work has been done on chironomid feeding behavior, it is primarily of a descriptive nature, categorizing on the basis of food collection techniques
(Berg 1950; Cummins 1973; Coffman 1984). Chironomid diets are made up of algae, detritus, and animals. However, very little information is reported on specific animal or plant food organisms. In this study I compared epiphytic diatoms associated with Myriophyllum spicatum with diatoms found in the digestive tract of chironomids collected on the same plant. I used these data to
examine selective herbivory by chironomids.
This study seeks to demonstrate the effect of aquatic vascular plant
species composition on the invertebrate communities found associated with
macrophyte beds. In addition, manipulative enclosure/exclosure experiments
and invertebrate feeding studies provide explanations for these patterns of
invertebrate abundance. Therefore, this study will provide a better understanding of the relationship between aquatic vascular plants and their associated invertebrate communities, as well as the mechanisms that may structure these communties. STUDY AREA
All field work was conducted in Lake Onalaska, Wisconsin. Lake
Onalaska is an artificial lake located in southwestern Wisconsin. It is shallow, with an average depth of 1.5-2.0 m under normal water conditions (McHenry et al. 1984) without summer stratification (J.A. Holzer, Wisconsin Department of
Natural Resources, LaCrosse, Wisconsin, personal communication). It was flooded in 1937 after the completion of a lock and dam on the Mississippi River, at Dresback, Minnesota. The lock and dam at Dresback were part of a series of
26 locks and dams built by the Army Corps of Engineers which have modified the original character of the upper Mississippi River in the last 50 years. Pools created by these locks and dams regulate water flow and permit the passage of barge traffic. The upper portion of each pool closely approximates the lotic character of the river before impoundment whereas the lower portion takes on the character of a lentic environment (Holland and Sylvester 1983; Fremling and Claflin 1984).
Lake Onalaska is the lower nearly lentic portion of Pool 7 (Fig. I). Pool
7 extends 19 km from lock and dam 7 at Dresbank, Minnesota, north to lock and dam 6 at Trempealeau, Wisconsin. Estimates of the lake's surface area vary, because confusion exists regarding a strict definition of lake boundaries.
Jackson et al. (1982) reported a surface area of 2,020 ha, whereas Howard
(1983) reported 2,190 ha. However, the more recent calculation (Carl
Korschgen, Northern Prairie Wildlife Research Center- LaCrosse Field Station, Figure I. Map of Lake Onalaska, Wisconsin, showing the Black River to the east, and the Mississippi River to the west of the lake. Collection sites for summer 1983 (I through 5) and for summer 1984 (I and 6) are indicated by numbered stars.
7 8
Lock and & Pam 6
1 MN J
\ 1A .BLACK RIVER \ IL 1
WISCONSIN
a chute MNNESOTA LAKE ONALASKA
Lock & Dam 7 <
N T 9
LaCrosse, Wisconsin, personal communication) of 3,060 ha. agrees closely with
Finke's (1964) figure of 3,050 ha Using Finke's (1964) value, Lake Onalaska
comprises about 52% of Pool 7.
Any sampling regime for aquatic macrophytes should take into account
the fact that water enters the lake from three main sources, the Mississippi
River, The Black River, and Halfway Creek. Site-specific water quality
differences could result in biotic differences among sites. Although Lake
Onalaska is bounded on the west by a series of islands which effectively
separate it from the main channel, 20 - 30% of the Mississippi River's main
channel flow passes through the lake (Howard 1983). These waters enter the
lake primarily through Sommer's chute. About 66% of the Mississippi River
water entering the lake enters through this chute; the remainder enters through
a series of smaller chutes (Gerry Jackson, Columbia National Research
Fisheries Laboratory, Yankton, South Dakota, personal communication).
A lesser amount of water, probably less than 25% of the Mississippi
River inflow, enters the lake from the Black River. However, conductivity
measurements indicate that over 50% of the water in Lake Onalaska is Black
River water (Dexter et al. 1978). Hence, the Black River has a much greater
effect on the lake than inflow contribution would indicate (Dexter et al.
1978). The contribution of Halfway Creek is insignificant at normal runoff
rates (Claflin and Rada 1976).
Before flooding, the area was primarily meadowland, with interspersed
stands of mature flood-plain forest, i.e., cottonwood and willow (Claflin and
Rada 1976). As a result stumpfields, the remants of trees hewn down before
flooding, occur over large portions of the lake. The lake is also characterized by large aquatic vascular plant beds, particulary in summer when peak biomass occurs. The combination of shallow water, and abundant vegetation have made 10
the lake a favorable habitat for the production and survival of bluegi I Is
(Lepomis macrochirus) (Rach and Meyer 1982). Hence, Lake Onalaska has
become the most important fishing area in Pool 7 (Finke 1964). Pool 7 has a
higher overall angling catch than Pools 8 or 9 (Merz and Wright 1974), and the
number of fish caught per angling hour is higher than any of the seven pools
censused by the Upper Mississippi River Conservation Commission, including
Pools 4, 5, 7, II, 13, 18, and 26 (Fink 1964; Kline and Golden 1977a). The most
abundant sport fish in Poo! 7 is the bluegill (Rasmussen 1979; Held 1983). The
bluegill fishery is one of the major reasons for high overall angling yields.
Pool 7 also supports an important commerical fishery, ranking 9th in
harvest among the pools commercially fished, 3 through 26B (Kline and Golden
1979b). The most important commercial fish is the common carp (Cyprinus
carpio), followed by buffalo (Ictiobus spp.), catfish (Ictalurus spp.), and
freshwater drum (Aplodinotus qrunniens) (Kline and Golden 1979b).
The most abundant fishes in Pool 7 are bullhead minnows (Pimephalus viqilax), gizzard shad (Dorosoma cepedianum), emerald shiners (Notropis atherinoides), and river shiners (Notopis blennius) (Rasmussen 1979). Of these, gizzard shad are most abundant (J.A. Holzer, Wisconsin Department of Natural
Resources, LaCrosse, Wisconsin, personal communication).
Many of these characteristics make Lake Onalaska a very good study area for macrophyte-invertebrate research. The lake is small and shallow, macrophytes are abundant everywhere, and visibility is usually good down to the lake's sandy bottom. These factors greatly reduce potential problems with study site selection. Because the lake is relatively small a number of study sites can be visited in a single day, thus minimizing temporal variation among samples collected at different sites, i.e., it is unlikely that major seasonal community shifts will occur in a single day. The shallow nature of the lake makes it an advantageous study area for several reasons. All study sites are
easily accessible. Hand sampling is possible without the use of SCUBA gear,
usually in less than 2 m of water. This, coupled with the fact that submergent vegetation is easily identified in shallow, clear water, makes selective sampling
of particular macrophyte species possible. In addition, enclosure/exclosure cages are easily placed and sampled in shallow water. This combination of
factors makes Lake Onalaska an ideal sites for macrophyte sampling as well as enclosure/exclosure experiments. 12
Chapter I
Description of invertebrate communities associated with
Vallisneria americana, Myriophyllum spicatum,
and Ceratophyllum demersum
Introduction
Aquatic vascular plants serve as habitat for a wide variety of invertebrate species (Muttkowski l9l8;Soszka 1975, Biltgen 1981). Often large percentages of the total invertebrate biomass of a system are found associated with macrophyte beds. Watkins et al. (1983) determined that the number of benthic organisms found in the vegetated areas of Orange Lake, Florida, was almost three times the number found in unvegetated areas. About 60% of all arthropods were found in associations with vegetation. Wiley et al. (1984) obtained similar results from 10 Illinois ponds, finding that presence of macrophytes may increase invertebrate productivity by as much as 90%. The same pattern seems to be expressed in lotic habitats as well. Iversen et al.
(1985) found that 95% of the invertebrate community of the River Susa,
Denmark was found in vegetation. In addition to the simple presence or absence of macrophytes, plant density, biomass, and type may be important in understanding the relationships between invertebrates and aquatic vascular plants.
Plant density and/or biomass may regulate invertebrate communities.
12 13
Heck and Wetstone (1977), working along the coast of Panama, found that both species richness and abundance of associated vertebrates were significantly correlated with the above substrate biomass of tropical seagrass
(Thalassia testudinum). Similar results were obtained for seagrass meadows in
Apalachee Bay, Florida (Stoner 1980).
Various macrophytes may be used by invertebrates differentially (Dvorak and Best 1982; Keast 1984; Rooke 1984). Krecker (1939) suggested that the reason for such differences may be explained in terms of morphological differences among plants. Plants such as Myriophyllum spp., whose dissected leaves offer a more protected space and easier attachment, have the highest density of invertebrates, whereas plants with long, flat, "hard-to-cling-to" leaves such as Vallisneria have lower invertebrate densities.
To examine differences among macroinvertebrate communities associated with three submergent aquatic vascular plants, I described quantitatively and qualitatively invertebrate communities found in association with Vallisneria americana, Ceratophyllum demersum, and Myriophyllum spicatum. Vallisneria was choosen because it is one of the most dominant macrophytes in Lake Onalaska. It is a rooted submergent whose long leaves often extend to the surface in shallow water. It is dioecious, stoloniferous, and perennial (Cook et al. 1974). Plant growth begins from winter buds (tubers or turions) in spring when water temperatures rise to I0°-I4°C (Zamuda 1976a).
Leaf growth is primarily in the basal portion of the leaves with less than 9% elongation occurring beyond that point (Zamuda 1976b). Leaves are part of an upright vegetative axis, arranged in a rosette. Prostate stolons also are produced that may give rise to new rosettes (Wilder 1974). During the growing season, plants increase the number of rosettes as well as biomass. During middle and late summer, fruit is produced on a peduncle. The fruit may disintegrate on the peduncle or break off. Seeds then lodge in the substrate
(Kaul 1978). Near the end of the growing season, production of rosettes stops and some develop one or more winter buds (Titus and Stephens 1983). In fall, leaves and interconnecting stolons die off and disintegrate. Winter buds (or turions) then comprise 100% of the plant biomass (Donnermeyer 1982). In spring each surviving winter bud begins as a single plant.
Vallisneria is generally found in shallow water less than 3.4 m (Hunt
1963; Titus 1983), though it is able to survive at low light levels. Stuckey
(1978) found it had the lowest compensation point of any macrophyte in Put-in-
Bay, Lake Erie. Meyer et al. (1943) determined that at low light levels
Vallisneria could maintain over twice the apparent rate of photosynthesis of the next highest plant. It generally grows best in silty sand bottoms, substrates that predominate in Lake Onalaska (Hunt 1963; Dexter et al. 1978).
Ceratophyllum, like Vallisneria, is a submergent. It is monoecious and bears small flowers in the axil of one of the leaves in a whorl (Sculthorpe
1967). The plant is entirely without roots and free-floating (Fassett I960;
Sculthorpe 1967; Cook 1974). However, it may sometimes anchor itself in the substrate or become entangled with rooted aquatics so that it often appears to be rooted (Sculthorpe 1967). Ceratophyllum overwinters by means of densely crowded stem apices or as an intact vegetative plant body (Sculthorpe 1967;
Stuckey et al 1978; Kunii and Maeda 1982).
Myriophyllum spicatum, a Eurasian plant species first introduced into
North America in the late nineteenth century (Reed 1977), has since gained a reputation as a nuisance plant (Nichols and Shaw 1986). M. spicatum is quite similar morphologically to the indignous species M. exalbescens and, confusion exists over their morphological characteristics (Stuckey et al 1978). Typically,
M. spicatum produces leaves with 5-24 pairs of leaflets, while M. exalbescens 15 produces leaves with 4-14 pairs of leaflets (Aiken and McNeill 1980). About
70% accuracy can be obtained by characterizing everything with 14 or more pairs of leaflets as M. spicatum (Nichols 1975). This method was used to identify samples in the field, but dried and pressed specimens were later sent to
R. Stuckey, The Ohio State University, Columbus, Ohio, to confirm identifications. M. spicatum has two flowering periods and two growth peaks.
The plant flowers in mid-June and late summer. Shortly after each flowering, a biomass peak is reached (Nichols 1975). Myriophyllum in Lake Wingra,
Wisconsin has one of the highest rates of productivity of any macrophyte reported in the literature (Howard-Williams 1978). In addition to flowering, the plant may reproduce asexual ly by means of vegetative buds and stem fragments
(Nichols 1975). M. spicatum may overwinter as an entire plant, as a mass of roots with short healthy shoots, or by means of turions or winter buds (Stuckey et al 1978; Titus and Adams 1979). Vallisneria americana and Ceratophyllum demersum have been classed as "tolerant" species which are usually dominant or subdominant in disturbed systems, whereas Myriophyllum spicatum is classed as an "extremely tolerant introduced species" (Davis and Brinson 1980).
Ceratophyllum and Myriophyllum were chosen for the present study because upon preliminary examination of Lake Onalaska, it was obvious that they, like Vallisneria, were extremely common. The objective of the study was to determine whether or not these three macrophytes supported distinct invertebrate communities. ! posed two specific hypotheses: (I) that the invertebrate communities supported by each plant would vary among plants numerically and (2) that the invertebrate communities supported by each plant would vary among plants qualitatively (in terms of species composition). 16
Materials and methods
During summer 1983 samples of Vallisneria americana, Myriophyllum spicatum, and Ceratophyllum demersum were taken at five sites in Lake
Onalaska (Fig. I). Water depth at all sites was less than 1.7 m. Sites I, 2, and
5 were sampled about once every 2 weeks during late June through mid
September. However, because of a massive growth of filametous algae, sites 3 and 4 could only be sampled intermittantly and were eventually dropped. Sites
2 and 4 were selected because they were heavily influenced by Mississippi River water. Sites I, 3, and 5 were chosen because they were dominated by Black
River water. I was able to determine which water regime was dominant at the sites on the basis of conductivity measurements. Sites were chosen in this way because I wanted to test whether or not community changes during June through September were general for the lake or specific to a particular water regime.
A number of methods and devices have been developed for sampling chironomids and other invertebrates associated with aquatic macrophytes
(Gillespie and Brown 1966; Stark 1980). However, for this study I judged hand sampling to be the best. Hand sampling allowed me to select a single macrophyte species for collection. To sample macrophytes, I lowered a sampler (Fig. 2) over leaves of the desired plant sampled to a depth of about 0.8 m. Enclosed plants were then broken off about 30 cm above the substrate
(except for Ceratophyllum which was often found unattached to the substrate), and the mouth of the sampler inverted and brought to the surface. In thick beds, leaves from more than one plant were sometimes collected. Benthic organisms were not collected as the sampler never made contact with the bottom substrate. At each site the nearest plant of the appropriate species was Figure 2. Sampler used for manual aquatic macrophyte collections during summers 1983 and 1984. The 20.3-cm diameter opening at the top of the sampler was lowered over plants. The 500-micron mesh bag was 1.5 m long, and constructed of nitex.
17 18
Plastic ring to prevent collapse
20.3 cm
micron mesh bag
MIHMIt SSliiO ii Ring clamp
Removable sieve with 500 micron 19 sampled.
Samples were taken to the laboratory fresh and kept refrigerated until counted. If samples could not be counted immediately, they were preserved in
70-80% ETOH. Preservation should not have greatly altered the weight of
invertebrates in the samples (Howmiller 1972; Wiederholm and Eriksson 1977).
Although a number of options were explored for removing invertebrates from plant samples, including sugar floatation (Anderson 1959; Fast 1970) and bubbling (Kingsbury and Beveridge 1977), I simply washed organisms from samples in a water bath. Water containing the detached invertebrates was then passed through a No. 35 sieve. Organisms caught in the sieve were counted and identified under a dissection microscope. Attached organisms remaining on plants were removed by hand under a dissection microscope.
After invertebrates were removed from plants, leaf surface area was measured using a digital surface area meter. This device is less time consuming, more precise, and just as accurate as a variety of other tecniques
(Brown and Manny 1984). Plants were then dried for 48 h at 80°C and weighted.
All non-planktonic invertebrates found in the samples were identified and counted. Oligochaeta, Ectoprocta, Nematoda, and Turbellaria were not identified to lower levels. Other taxa were identified to genus or species if possible. Because mounted chironomids cannot be dry weighted, most were identified only to family. However, a species list was determined. After identification and counting, invertebrates were dried at I05°C for 24 h, cooled in a dessicator, and weighted.
1 calculated organisms Per gram dry plant weight of for each sample.
Variance among samples for a given date, site, and plant was greater than the mean. Variance also increased with the mean. Therefore, raw data were log 20 transformed (In (n + I)) to stabilize the variance (Sokal and Rohlf 1969; Greeson et al. 1977; Watkins et al 1983; Gilinsky 1984). Tranformed data were then used to calculate how many samples would be required to produce a sample mean within 20% of the actual mean (Elliot 1971; Greeson et al. 1977). The number of samples required was about 6. Therefore, 6-8 samples were always collected.
Surface temperature readings were taken whenever samples were collected, using a thermister. Current readings were also taken whenever samples were collected, using an electronic current meter. The probe was placed at a depth about 60% of the distance from the surface to the bottom.
This yielded a value roughly equal to the mean velocity (Hynes 1970).
For statistical, analysis taxa were lumped into 14 major groups:
Chironomidae, Trichoptera, Hemiptera, Odonata, Megaloptera, Lepidoptera,
Coleoptera, Diptera (including all non-chironomid dipterans), Ephemeroptera,
Gastropoda, Oligochaeta, Hirudinea, Other (including all taxa of minor numeric importance, i.e., taxa from which five or less individuals were found in all), and
Amphipoda (Hyallela azteca). Means for the log of the total number of organisms, and well as for each group were calculated for each plant, at each site, on every date. An analysis of variance (ANOVA) tested for significant differences in the invertebrate populations found associated with each of the macrophytes, differences among sites, and differences among dates. Duncans
Multiple Range Test determined the magnitude and direction of these differences. 21
Results
During June through September 120 macrophyte-associated invertebrate
taxa were identified. Distribution of taxa was non-random and influenced by
plant type. Vallisneria samples contained 21 taxa found only on that plant, 18
taxa were found only on Myriophyllum, and 11 were found only on
Ceratophyllum. There were 36 taxa never found in association with Vallisneria,
33 never associated with Myriophyllum, and 45 never associated with
Ceratophyllum.
Plant type also strongly affected total abundance. Analysis of variance
(ANOVA) indicated that differences in the overall mean number of
invertebrates (all sites and dates June through September) among plants were
significant (p< 0.0001) (Table I). Ceratophyllum nearly always supported the
greatest number of invertebrates per gram dry weight of plant, followed by
Myriophyllum and Vallisneria (Fig. 3). Duncan's Multiple Range Test indicated
that these differences among means were significant (p< 0.05) (Table 2).
Temporal fluctuations were evident at all sites (Fig. 3). At sites I and 2
temporal patterns of abundance were similar. Graphically, numbers declined
from moderately high values in June to seasonal minima in mid-July or early
August. Then abundances generally rose through the end of the study period.
These trends were continuous across plant types (Fig. 4). The same pattern was exhibited at site 5; however, it was displaced somewhat so that the seasonal minima was in late August or early September rather than July.
ANOVA revealed that the effects of date were significant (p< .0001) (Table
I). Duncan's Multiple Range Test showed that generally, values were lower during the first half of the summer than during the second half (Table 2).
Site was also a relevant factor in determining abundance levels. 22
TABLE I. ANOVA-generated probabilities indicating the significance of date, site, and plant type, as well as interactions among these variables, on invertebrate abundances in Lake Onalaska in 1983. Probability values of 0.05 or less indicate that differences among dates, sites, plants, or interactions were statistically significant. NS indicates that differences were not significant.
Date Site Plant Date x Site
T richoptera 0.0001 0.0001 0.0001 0.0001
Gastropoda 0.0001 0.0001 0.0001 0.0001
Odonata 0.0001 0.0001 0.0001 0.0001
Lepidoptera 0.0001 0.0001 0.0001 0.0001
Ephemeroptera 0.0001 0.0001 0.0001 0.0001
Hirudinea 0.0001 0.0004 0.0001 0.0002
Hemiptera 0.0001 0.0001 0.0001 0.0001
Diptera 0.0006 NS 0.0001 0.0005
Coleoptera NS NS NS NS
Megaloptera 0.0001 NS NS NS
Oligocheata 0.0001 0.0007 0.0001 0.0001
Other 0.0001 NS 0.002 0.056
Chironomidae 0.0001 0.0001 0.0001 0.0001
Amphipoda 0.0001 0.0001 0.0001 0.001
Total 0.0001 0.04 0.0001 0.036 23
TABLE I. Continued
Date x Plant Site x Plant All
Trichoptera 0.0001 0.0001 0.0001
Gastropoda 0.0001 0.0001 0.0003
Odonata NS 0.007 NS
Lepidoptera 0.05 0.001 NS
Ephemeroptera 0.01 0.0001 0.0001
Hirudinea NS NS NS
Hemiptera 0.026 0.0005 NS
Diptera NS 0.0003 0.0001
Coleoptera NS NS NS
Megaloptera NS NS NS
Oligochaeta 0.0001 0.0002 NS
Other 0.0002 NS NS
Chironomidae 0.0001 0.0001 0.0004
Amphipoda 0.008 0.0001 NS
Total 0.008 NS NS Figure 3. Changes in the abundance (natural log of the number of invertebrates
per gram dry weight of plant) of invertebrate communities associated with
Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in
Lake Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
24 25
StteS
Vallisneria .C O)
*
$ Site 2 Ceratophyllum
Myriophyllum
*3> ♦ Vallisneria £ Ul Oz < Q Z 3 < Site 1 -J 2 o 6 ■ 4
3
2
1
8/i!1 7/10 7/30 8/19 0/8 9/28 DATE 26
TABLE 2. Results of Duncan's Multiple Range Test, indicating the magnitude and significance of date, site, and plant effects on the abundance of macrophyte-associated invertebrate taxa. Means of log transformed (base e) data are presented. Means with different letters indicate that statistically significant differences existed among those values (p<0.05). Results are from summer 1983 collections in Lake Onalaska.
Taxa Means (By Plant)
Ceratophyllum Myriophyllum Vallisneria
Trichoptera 2.45 (a) 2.58 (a) 1.75 (b)
Gastropoda 1.74 (b) 2.29 (a) 1.85 (b)
Odonata 1.81 (c) 1.05 (b) 0.45 (a)
Lepidoptera 1.22 (c) 0.72 (b) 0.38 (a)
Ephemeroptera 0.89 (b) 1.07 (c) 0.04 (a)
Hirudinea 0.52 (b) 0.08 (a) 0.16 (a)
Hemiptera 1.81 (b) 1.32 (a) 1.09 (a)
Diptera 0.13 (a) 0.10 (a) 0.61 (b)
Coleoptera 0.10 (b) 0.11 (b) 0.02 (a)
Megaloptera 0.01 (a) 0.00 (a) 0.00 (a)
Oligochaeta 0.29 (a) 0.40 (a) 0.30 (a)
Other 1.22 (b) 0.74 (a) 0.58 (a)
Chironomidae 3.07 (a) 3.35 (b) 3.71 (c)
Amphipoda 5.40 (c) 3.32 (b) 1.64 (a)
Total 5.37 (c) 3.44 (b) 2.49 (a) 27
TABLE 2. Continued
Taxa Means (By Site)
Site 1 Site 2 Site 5
Trichoptera 2.40 (b) 2.23 (b) 1.73 (a)
Gastropoda 2.65 (c) 2.01 (b) 0.80 (a)
Odonata 0.81 (a) 1.05 (b) 1.01 (b)
Lepidoptera 0.78 (b) 0.69 (b) 0.51 (a)
Ephemeroptera 1.00 (c) 0.73 (b) 0.29 (a)
Hirudinea 0.25 (b) 0.08 (a) 0.39 (c)
Hemiptera 0.90 (a) 1.84 (c) 1.18 (b)
Diptera 0.33 (a,b) 0.42 (b) 0.24 (a)
Coleoptera 0.1 1 (a) 0.04 (a) 0.06 (a)
Megaloptera 0.00 (a) 0.01 (a) 0.00 (a)
Oligochaeta 0.53 (c) 0.15 (a) 0.32 (b)
Other 0.85 (a) 0.74 (a) 0.74 (a)
Chironomidae 2.52 (a) 4.20 (c) 3.66 (b)
Amphipoda 2.86 (a) 3.16 (b) 3.10 (a,b)
Total 3.07 (a) 3.60 (b) 3.81 (b) 28
TABLE 2. Continued
T axa Means (By Date)
6/21 6/24 7/8
Trichop tera 3.45 (a) 2.13 (d,e) 1.05 (f)
Gastropoda 2.82 (a,b) 3.06 (a) 3.19 (a)
Odonata 0.30 (d,e) 0.12 (e) 0.37 (d,e)
Lepidoptera 0.00 (e) 0.00 (e) 0.47 (d)
Ephemeroptera 0.95 (a,b) 0.63 (b,c) 0.76 (a,b)
Hirudinea 0.16 (b,c) 0.28 (b,c) 0.18 (b,c)
Hemiptera 0.22 (f) 0.68 (d,e,f) 1.16 (c,d,e)
Diptera 0.17 (b) 0.20 (b) 0.45 (a,b)
Coleoptera 0.07 (a) 0.02 (a) 0.05 (a)
Megaloptera 0.00 (b) 0.00 (b) 0.00 (b)
Oligochaeta 0.35 (c) 0.38 (c) 0.05 (c)
Other 1.10 (b,c) 1.06 (b,c) 0.36 (d)
Chironomidae 4.02 (b,c) 2.31 (e,f) 1.69 ((f)
Amphipoda 3.55 (c) 1.76 (f) 2.52 (d,e)
Total 3.51 (b) 2.47 (c) 2.62 (b,c) 29
TABLE 2. Continued
T axa Means (By Date)
7/12 7/14 7/25
Trichop tera 2.96 (a,b) 2.73 (b,c,d) 2.25 (c,d,e)
Gastropoda 0.46 (f) 1.21 (e) 2.32 (b,c)
Odonata 0.05 (e) 0.53 (c,d,e) 0.91 (c)
Lepidoptera 0.30 (d,e) 0.40 (d,e) 1.36 (a,b)
Ephemeroptera 1.22 (a) 0.98 (a,b) 0.91 (a,b)
Hirudinea 0.00 (c) 0.25 (b,c) 0.03 (c)
Hemiptera 3.42 (a) 1.64 (c) 0.52 (e,f)
Diptera 0.31 (a,b) 0.18 (b) 0.00 (b)
Coleoptera 0.00 (a) 0.07 (a) 0.19 (a)
Megaloptera 0.19 (a) 0.00 (b) 0.00 (b)
Oligochaeta 0.00 (c) 0.07 (c) 0.17 (c)
Other 0.39 (d) 1.30 (b) 0.59 (c,d)
Chironomidae 2.03 (f) 3.37 (c,d) 4.13 (b)
Amphipoda 2.88 (c,d) 5.51 (a) 4.60 (b)
Total 2.59 (b,c) 5.51 (a) 4.59 (a) 30
TABLE 2. Continued
Taxa Means (By Date)
7/26 8/9 8/29
Trichop tera 2.23 (c,d,e) 1.97 (a,b) 3.28 (a,b)
Gastropoda 1.21 (e) 1.36 (d,e) 1.85 (c,d)
Odonata 0.61 (c,d) 0.42 (d,e) 0.62 (c,d)
Lepidoptera 0.13 (d,e) 0.42 (d,e) 1.16 (bjc)
Ephemeroptera 0.86 (a,b) 0.29 (c,d) 0.13 (d)
Hirudinea 0.04 (c) 0.05 (c) 0.17 (b,c)
Hemiptera 0.73 (d,e,f) 0.78 (d,e,f) 1.67 (c)
Diptera 0.67 (a) 0.40 (a,b) 0.70 (a)
Coleoptera 0.04 (a) 0.04 (a) 0.00 (a)
Megaloptera 0.00 (b) 0.00 (b) 0.00 (b)
Oligochaeta 0.00 (c) 0.00 (c) 0.04 (c)
Other 0.37 (d) 0.38 (d) 0.25 (d)
Chironomidae 4.08 (b) 2.99 (d,e) 5.11 (a)
Amphipoda 1.86 (e,f) 1.64 (f) 0.62 (g)
Total 2.75 (b,c) 2.36 (c) 2.60 (b,c) 31
TABLE 2. Continued
T axa Means (By Date)
8/30 9/1 9/13
Trichop tera 1.84 (e) 2.03 (d,e) 1.87 (e)
Gastropoda 1.84 (c,d) 2.33 (b,c) 1.88 (c,d)
Odonata 0.98 (c) 2.83 (a) 2.53 (a)
Lepidoptera 0.91 (c) 1.30 (a,b,c) 1.24 (a,b,c)
Ephemeroptera 0.82 (a,b) 0.94 (a,b) 0.62 (b,c)
Hirudinea 0.00 (c) 0.69 (a) 0.59 (a)
Hemiptera 1.25 (c,d) 1.23 (cjd) 2.59 (b)
Diptera 0.11 (b) 0.00 (b) 0.28 (a,b)
Coleoptera 0.13 (a) 0.13 (a) 0.02 (a)
Megaloptera 0.00 (b) 0.00 (b) 0.00 (b)
Oligochaeta 0.00 (c) 0.25 (c) 0.77 (b)
Other 0.21 (d) 1.25 (b) 1.09 (b,c)
Chironomidae 4.46 (b) 2.87 (d,e) 4.03 (b,c)
Amphipoda 2.81 (d) 5.38 (a) 4.83 (a,b)
Total 3.11 (b,c) 5.38 (a) 4.81 (a) TABLE 2. Continued
T axa Means (By Date)
9/14
Trichoptera 2.90 (a,b,c)
Gastropoda 1.76 (c,d,e)
Odonata 1.82 (b)
Lepidoptera 1.65 (a)
Ephemeroptera 1.15 (a)
Hirudinea 0.40 (a,b)
Hemiptera 3.11 (a,b)
Diptera 0.35 (a,b)
Coleoptera 0.18 (a)
Megaloptera 0.00 (b)
Oligochaeta 2.07 (a)
Other 2.26 (a)
Chironomidae 4.28 (b)
Amphipoda 4.58 (b)
Total 4.81 (a) Figure 4. Changes in the abundance (natural log of the number of invertebrates per gram dry weight of plant) of invertebrate communities associated with
Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in
Lake Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
33 34 Ceratophyllum 7
e Site 2 Site 1
5 S tteS 4 a 3 o 2
$ 1
E Myriophyllum 0)k. O) * 4 c c ? UJ 3 O Site 1 z Site 6 < 2 Q Z 3 1 CO < _ l Vallisneria £ 5 o 4 S iteS
3
2
1 Site 1
6/21 7/10 7/30 8/19 9/6 9/28 DATE 35
Abundance levels for all three plants were highest at site 2 for most dates (Fig.
4). ANOVA indicated significant site effects at p* 0.05 (Table I). However, the overall means at sites 2 and 5 did not differ whereas site I was distinct from both (Table 2).
Of the interactions among the three main effects tested, date-by-site and site-by-plant were the only ones significant (ANOVA, p<0.05; Table I).
This indicated that the relationship among sites was not always consistent across dates, and that the relationship among plants was not consistent across sites.
The most abundant taxonomic groups were Trichoptera, Hemiptera,
Chironomidae, Odanata, Amphipoda (Hyallela azteca), and Gastropoda. In nearly every case these groups comprised over 90% of the invertebrate community (Table 3). Groups that were of occasional numerical importance were Lepidoptera, Oligochaeta, Emphemeroptera, and Diptera. Hirudinea,
Coleoptera, Megaloptera, and Other were of relatively minor numerical importance.
Temporal changes in abundance were evident in all major groups.
ANOVA revealed that date significantly influenced abundance in all six major groups (Table I). The exact nature of this influence was often difficult to determine. Although significant differences among dates occurred for both
Trichoptera and Chironomidae (Table I), little overall pattern was evident
(Figs. 5, 6; Table 2). However, overall patterns of temporal abundance were exhibited by other groups tested. Generally, for the Hemiptera, Odonata, and
Amphipoda (Hyallela azteca) abundances tended to be lower during the first part of the summer than during the latter part (Figs. 7, 8, 9 ; Table 2).
Hemiptera demonstrated this trend most dramatically. At site I the hemipteran standing crop associated with Myriophyllum underwent a 74 fold 36
TABLE 3. Percentage of total abundance of major (numerically dominant) invertebrate taxonomic groups found in association with Vallisneria americana, Myriophyllum spicatum, and Cerato- phyllum demersum, during summer 1983 in Lake Onalaska. Data are presented by date. Myrio = Myriophyllum, Val = Vallisneria, and Cerat = Ceratophyllum. S-l = Site I, S-2 = Site 2, and S-5 = Site 5.
Taxa 6/21 S-l Myrio 6/21 S-2 Myrio 6/24 S-l Val
Trichoptera 17% 10% 5%
Gastropoda 47% 1% 75%
Chironomidae 15% 73% 4%
H. azteca 18% 15% 5%
Sum = 97% 99% 89%
Taxa 6/24 S-2 Val 7/8 S-l Val 7/8 S-l Myrio
T richoptera 23% 1% 5%
Gastropoda 6% 87% 53%
Chironomidae 54% 2% 5%
H. azteca 9% 5% 34%
Sum = 92% 95% 97% 37
TABLE 3. Continued
Taxa 7/8 S-2 Val 7/8 S-2 Myrio 7/12 S-2 Cerat
T richoptera 5% 1% 16%
Gastropoda 10% 4% !%
Chironomidae 25% 22% 15%
H. azteca 16% 64% 25%
Hemiptera 37% 7% 42%
Sum = 93% 98% 99%
Taxa 7/14 S-2 Cerat 7/14 S-5 Cerat 7/25 S-2 Cerat
Trichoptera 6% 6% 1%
Gastropoda 3% 1% 13%
Chironomidae 11% 9% 41%
H. azteca 69% 84% 40%
Hemiptera 7% 1% 1%
Odonata 1% 1% 1%
Lepidoptera 1% 1% 2%
Sum = 96% 99% 97%
Taxa 7/25 S-5 Cerat 7/26 S-l Val 7/26 S-l Myrio
Trichoptera 23% 9% 26%
Gastropoda 0% 2% 22%
Chironomidae 29% 83% 3%
H. azteca 47% 5% 36%
Hemiptera 0% 0% 5%
Odonata 0% I % 5%
Sum = 99% 99% 99% 38
TABLE 3. Continued
Taxa 7/26 S-2 Val 7/26 S-2 Myrio 7/26 S-5 Val
Trichoptera 13% 18% 1%
Gastropoda 1% 6% 1%
Chironomidae 77% 45% 98%
H. azteca 1% 19% 1%
Hemiptera 3% 9% 0%
Odonata 1% 1% 1%
Diptera 6% 1% 1%
Sum = 99% 97% 99%
Taxa 8/9 S-l Val 8/9 S-l Myrio 8/9 S-2 Val
Trichoptera 13% 17% 14%
Gastropoda 38% 11% 2%
Chironomidae 6% 3% 75%
H. azteca 29% 54% 1%
Hemiptera 0% 3% 3%
Odonata 4% 5% 1%
Sum = 90% 93% 95% 39
TABLE 3. Continued
Taxa 8/9 S-2 Myrio 8/9 S-5 Val 8/9 S-5 Myrio
T richoptera 2% 6% 39%
Gastropoda 18% 1% 1%
Chironomidae 52% 84% 54%
H. azteca 12% 7% 4%
Hemiptera 13% 0% 0%
Sum = 97% 97% 97%
Taxa 8/29 S-l Val 8/29 S-2 Val 8/30 S-l Myrio
T richoptera 44% 2% 14%
Gastropoda 2% 3% 4%
Chironomidae 50% 81% 46%
H. azteca 1% 1% 27%
Hemiptera 1% 11% 1%
Lepidoptera 3% 1% 4%
Sum = 99% 97% 96%
Taxa 8/30 S-2 Myrio 8/30 S-5 Val 9/1 S-l Cerat
Trichoptera 2% 2% 5%
Gastropoda 6% 6% 2%
Chironomidae 67% 84% 8%
H. azteca 20% 4% 81%
Hemiptera 5% 2% 1%
Odonata 1% 1% 2%
Sum = 99% 99% 98% 40
TABLE 3. Continued
Taxa 9/1 S-2 Cerat 9/1 S-5 Cerat 9/13 S-2 Val
Trichoptera 1% 1% 2%
Gastropoda 12% 4% 3%
Chironomidae 19% 3% 73%
H. azteca 65% 72% 9%
Hemiptera 1% 2% 2%
Odonata 2% 14% 8%
Sum = 99% 96% 97%
Taxa 9/13 S-2 Cerat 9/13 S-5 Val 9/13 S-5 Cerat
Trichoptera 2% 1% 2%
Gastropoda 2% 3% 1%
Chironomidae 15% 58% 1%
H. azteca 78% 14% 77%
Hemiptera 1% 14% 16%
Odonata 3% 1% 3%
Oligochaeta I % 6% 1 %
Sum = 99% 97% 98% 41
TABLE 3. Continued
Taxa 9/14 S-l Val 9/14 S-l Myrio 9/14 S-l Cerat
Trichoptera 9% 15% 3%
Gastropoda 5% 1% 1%
Chironomidae 57% 27% 1%
H. azteca 6 % 13% 78%
Hemiptera 19% 26% 8%
Odonata 1% 1% 1%
Oligochaeta 2% 14% 1%
Sum = 99% 97% 93%
Taxa 9/14 S-2 Myrio
Trichoptera 1%
Gastropoda I %
Chironomidae 28%
H. azteca 55%
Hemiptera 1%
Odonata 8%
Oligochaeta I %
Sum = 93% Figure 5. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Trichoptera associated with Vallisneria,
Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake
Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
42 A3
Myriophyllum Site 5 O) Ceratophyllum © £ £ Vallisneria ia
Myriophyllum Site 2 b
Vallisneria
Vallisneria
Ceratophyllum
8/21 7/10 7/30 3/19 9/8 9/28 DATE Figure 6. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Chironomidae associated with Vallisneria,
Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake
Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
44 45
6 S iteS Vallisneria
5
4 £ O) Myriophyllum ■55 3
£ ■O 2 itophyllum
1
6 O) J Myriophyllum . Ceratophyllum
* 5 c
LLI 4 Oz < 3 Vallisneria Q Z 2 CD < 1 oz o 5 Site 1 DC X Vallisneria Myriophyllum o 4
3
2
1
6/21 7/10 7/30 8/19 9/8 9/28 DATE Figure 7. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Hemiptera associated with Vallisneria,
Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake
Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
46 kl
Site 5
£ Ceratophyllum O) ‘55 £ £ TJ
Vallisneria
D) Site 2 * [Myric Hum c
UJ Vallisneria zo < Q z tophyllum 3 m < Site 1 Myriophyllum z < oc £ 0 . 2 Ui X Ceratophyllum Vallisneria
e/21 7/10 7/30 8/19 9/0 9/28 DATE Figure 8. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Odonata associated with Vallisneria,
Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake
Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
48 O)
TJ / Myriophyllum
a 'allisnerla
Site 2
Ceratophyllum s
Myriophyllum
ralli8nerla
Site 1 Ceratophyllum
6/21 7/10 7/30 8/10 9/8 9/28 DATE Figure 9. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Amphipoda (Hyallela azteca) associated with
Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in
Lake Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
50 5 1 Site 5 ;
Ceratophyllum
Vallisneria
1 Myriophyllum
_ J[ Site 2 Ceratophyllum 6
Myriophyllum 5
4
3
2
1 Vallisneria
0 Site 1 Ceratophyllum
5 Myriophyllum 4
3
2
1 Vallisneria
7/10 7/30 8/10 9/8 9/28 DATE 52
increase during September. Hemipterans associated with Ceratophyllum and
Vallisneria at site I increased 18 and 19 fold respectively. Gastropods
associated with Myriophyllum and Ceratophyllum tended to reach peaks in early
or mid-summer, and then drop, while those associated with Vallisneria tended
to rebound from mid-summer minima (Fig. 10).
ANOVA revealed that site also significantly affected abundances of all
major groups (P-^.OOOI). Odonata, Hemiptera, Chironomidae and Amphipoda
(Hyallela azteca), all showed similar abundance patterns with repect to site.
Site 2 was highest followed by site 5 and site I. In all cases differences between sites I and 2 were significant (P<-0.05). However, odonate abundances at sites 2 and 5 did not differ, and amphipod abundances at site 5 were not significantly different from either site I or 2.
Trichoptera and Gastropoda exhibited different abundance patterns with respect to site. In both, the highest values were found at site I and the lowest at site 5, with site 2 intermediate. For the Gastropoda all sites were significantly different (p<0.05). However, for the Trichoptera, though sites I and 5 were different (P-'-O.OS), whereas sites I and 2 were not (Table 2).
In general all but two of the major groups followed similar abundance patterns with respect to plant type. Trichoptera, Odonata, Hemiptera, and
Amphipoda (Hyallela azteca) abundances were all lowest in Vallisneria samples. The highest standing crop of all of these taxa, with the exception of
Trichoptera, were found in association with Ceratophyllum. Trichoptera were most numerous in Myriophyllum samples. In all four taxa significant differences existed between Vallisneria and Ceratophyllum (P< 0.05) (Table 2).
This pattern was most dramaticallly illustrated by the Amphipoda (Hyallela azteca) (Fig. 9). This was reversed in the Chironomidae. That group maintained the highest standing crop in Vallisneria samples, with Myriophyllum Figure 10. Changes in the abundance (natural log of the number of individuals per gram dry weight of plant) of Gastropoda associated with Vallisneria,
Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake
Onalaska during summer 1983. Standard error of the mean is presented.
Sample sizes are given in Table 10.
53 54
Site 5
O) 'qJ $ Ceratophyllum — Vallisneria T>
Mvrloohvllum
Site 2 Ceratophyllum b> •—
LU oz < Q 1 Myriophyllum z Vallisneria ID CD < Q Site 1 O CL O Myriophyllum e co < o
Vallisneria
6/21 7/10 7/30 8/19 9/8 9/28 DATE 55
intermediate, and Ceratophyllum lowest. Means for all three plants were significantly different (Table 2). Gastropoda standing crops were highest in association with Myriophyllum. Vallisneria and Ceratophyllum gastropod standing crops were not significantly different; however, both were significantly lower than that of Myriophyllum (Table 2).
All of the minor groups, with the exception of Coleoptera and
Megaloptera, exhibited significant changes in abundance due to plant effects
(Table I). In all cases (except Diptera which followed an abundance pattern similar to that of the Chironomidae) Ceratophyllum or Myriophyllum maintained the highest standing crops (Table 2). Temporal fluctuations (date effects) were significant in all but the Coleoptera (Table I). Although overall trends were difficult to distinguish for most groups, the Oligochaeta were significantly more abundant in September than earlier in the summer (Table 2).
Biomass of the total number of invertebrates was quite variable during
June through September (Fig II). At site I invertebrate biomass associated with Myriophyllum declined from June maxima to a seasonal minima in late
August and then began to rebound. Biomass associated with Vallisneria fluctuated wildly. However, by the end of the study period, the means for all three plants were quite similar. At site 2 the trend was the same for total invertebrate biomas associated with Myriophyllum. However, Vallisneria samples showed a rapid rise in biomass until a peak in late August. After this maximum occurred biomass quickly dropped. Ceratophyllum samples exhibited a similar pattern, but the peak occurred more rapidly, and maintained itself until late August (Fig. II). At site 5 biomass minima were in late July, values then increased through September (Fig. 11). ANOVA resulted in significant date (p<0.000l), date by site (p<0.0007), date by plant (0.007), site by plant (p<
0.004), and date by site by plant (p<0.009) effects. Results of Duncan's Figure I I. Changes in invertebrate biomass (grams invertebrate dry weight per gram dry weight of plant) associated with Vallisneria, Myriophyllum, and
Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer
1983. Standard error of the mean is presented. Sample sizes are given in Table
II.
56 57 Ceratophyllum
0.05 VaManeria s z o> © Site 5 $ £ TJ
0.15 a
O)
.2* 0.10 © £ £ *o © 0.05 •Q © t : © > VaRisnerla c Site 2
F<0 0.15 o> , Myriophyllum CO CO < 2 0.10 O CD
Ceratophyllum 0.05
’aflisnerla site 1
e/21 7/10 7/30 8/19 9/8 9/28 DATE 58
Multiple Range Test revealed that overall, sites I and 2 maintained similar invertebrate biomasses, 0.012 and 0.012 grams invertebrate dry weight per gram plant dry weight, respectively. However, biomass at site 5 , 0.007 grams invertebrate dry weight per gram plant dry weight, was significantly less (p<
0.05). Among plants, Myriophyllum maintained the greatest biomass, 0.015 grams invertebrate dry weight per gram plant dry weight. Vallisneria and
Ceratophyllum maintained invertebrate biomasses of 0.009 and 0.008 grams invertebrate dry weight per gram plant dry weight, respectively. Whereas biomasses associated with Vallisneria and Ceratophyllum were not significantly different from each other, both were were significantly less than the biomass associated with Myriophyllum (p<0.05).
In general current velocities were similar at sites 2 and 5. Velocities at site I were usually less (Fig. 12).
Finally, there was a strong correlation between surface area and dry wt. for both Vallisneria and Myriophyllum. Vallisneria offered greater surface area per unit dry weight than did Myriophyllum (Fig. 13). Analysis of covariance
(ANCOVA) showed that Vallisneria offered significantly (P<0.0l) more surface area per gram dry weight than Myriophyllum. Figure 12. Current velocity (meters per second) readings at sites I, 2, and 5, during July through September, 1983 in Lake Onalaska.
59 CURRENT VELOCITY (m eters/second) 0.50 7a .7 0 0.60 0.30 0.40 0.10 0.20 6/21 7/10 • • / / / / / / DATE v . 7/30 / / r • A ie 1 Site 8/19 w W \ / \ • 4 Site 5 Site Site 2 Site 9/8 60 Figure 13. Surface area (cm^) to dry weight (g) regressions for Vallisneria samples (n = 59, r^ = 0.81), and Myriophyllum samples (n = 18, r^ = 0.69), collected in Lake Onalaska during summer 1983, from sites 1, 2, and 5.
61 SURFACE AREA (cnf) 2000 3000 1000 10001 \ D R YW E I G H T(g r a m s ) 1.0 Vallisnerla Myriophyllum 2.0 62 Discussion
The results of this study agreed closely with earlier published work in this area. In every study where invertebrate communities associated with
Ceratophyllum, Myriophyllum, and Vallisneria were examined, the relative abundances of macroinvertebrates associated with each plant were similar to my findings. The method of measuring abundance did not seem to change the results significantly. Andrews and Hosier (1943) measured macroinvertebrate standing crops of plants in Lake Mendota, Wisconsin, in terms of animals per kilogram of plant dry weight. The relationships of macroinvertebrate communities among plants were identical to my results. Ceratophyllum demersum supported the largest standing crop, followed by Myriophyllum exalbescens (very similar morphologically to M. spicatum) and Vallisneria americana. Krecker (1939) also examined macroinvertebrate communities associated with Vallisneria and Myriophyllum (as well as other plants not relevant to my study). Even though he measured abundance as the number of animals per 10 linear feet of plant, results were essentially the same.
Myriophyllum supported a much larger standing crop of invertebrates than
Vallisneria. Mackie and Quadri (1971) measured abundance as the number of animals per 100 cc of plant volume, again the same results were obtained. The number of organisms associated with Myriophyllum was 7.4-11.1 times the number associated with Vallisneria. Finally, Gerrish and Bristow (1979) reported similar results. Despite the fact that they calculated abundance as the number of organisms per m plant surface area, they also found that the
Myriophyllum-associated invertebrate community was much larger than that associated with Vallisneria. The results of these early studies, combined with my more recent data, clearly suggest that quantitative differences do exist 64 among invertebrate communities associated with Ceratophyllum demersum,
Myriophyllum spicatum, and Vallisneria americana.
The qualitative differences among invertebrate communities associated with different plants was also consistant with the findings of other workers.
My data, which suggest that Gastropoda often comprised a larger percentage of the Vallisneria community than it did of the Ceratophyllum or Myriophyllum communities, is consistant with data reported by Mackie and Quadri (1971),
Keast (1984), and Gerrish and Bristow (1979). The fact that Chironomidae larvae often comprised a larger percentage of the Vallisneria community than they did other assemblages was consistant with Krecker (1939), Andrews and
Hosier (1943), and Keast (1984). My data also suggest that Hyalella azteca comprises a larger portion of the Ceratophyllum community than it does of the other two assemblages. Hyalella contributes the least to the Vallisneria community. These findings agree with Krecker (1939), Andrews and Hosier
(1943), Mackie and Quadri (1971), and Gerrish and Bristow (1979). These data, then, suggest that differences exist in the relative importance of various taxonomic groups among plants. Data reported by Biggs and Mathews (1982) support this conclusion. They examined the macroinvertebrate communities of six macrophytes from the backwaters and lakes of the Upper Clutha Valley,
New Zealand. Although a single invertebrate taxa, Potamopyqus antipodarum
(Gastropoda), was dominant, subdominant taxa varied among plants.
Plant type may affect not only the relative contribution of a specific taxon to an invertebrate community, but in some cases may completely limit the distribution of certain species. Dvorak and Best (1982) studied macroinvertebrate communities associated with various macrophytes growing in Lake Vechten, Netherlands. Although Ceratophyllum spp., Myriophyllum spicatum, Elodea canadensis, Phraqmites spp., Polygonum spp., and Typha spp. 65 were studied, they discovered that the gastropod Potamopyqus jenkins, and the
trichopteran Timodes waeneri were found only on Phraqmites, Polygonum, and
Typha. One lepidopteran (Noctuidae) was found only on Phraqmites. The fact
that I found 21 taxa completely limited to Vallisneria samples, 18 limited to
Myriophyllum, and I I limited to Ceratophyllum, is an example of the same kind of plant specific invertebrate distribution pattern reported by Dvorak and Best
(1982). These data suggest that one or several factors associated with various plant species can differentially affect colonization patterns of invertebrates by species.
Trichoptera probably deviated from the general abundance pattern primarily because of complicated emergence patterns, and species affinities
(similar to those reported by Dvorak and Best 1982). For example, early in the summer Myriophyllum supported high trichopteran numbers primarily because of one species, Leptocerus americanus, which although found on other plants was primarily associated with Myriophyllum. When this species emerged, it primarily affected Myriophyllum-associated trichopteran abundance. At least four trichopteran emergences affected macrophyte-associated abundances differentially at sites I and 2. These data are in agreement with Keast (1984) who reported erratic abundance patterns for trichopterans associated with
Vallisneria americana and Potamoqeton robbinsii.
Similar affinities of particular species for a certain plant contributed to the deviations exhibited by the Gastropoda at site I and the Hemiptera at site
2. The abundance of Gastropoda was primarily linked to one species in the middle of the summer, Gyraulus spp., which although found on other plants, was found predominantly in association with Myriophyllum samples. Hence, because of its numerical dominance, changes in Gyraulus abundance strongly affected the overall gastropod abundance pattern. Corixidae, primarily associated with 66
Myriophyllum, were major contributors to hemipteran deviations from the general pattern at site 2.
My data are consistant with previous studies and clearly suggest that macrophyte-associated invertebrate communities are affected not only quantitatively, but qualitatively by plant type. The relative numerical importance of several major taxonomic groups varied among plants. Plant species completely limited the distribution of certain taxa. Therefore, 1 am unable to reject the original hypothesis that invertebrate communities would vary quantitatively and qualitatively among plants.
Data obtained during the present study do not support the suggestion that these differences among plants are due to surface area differences, i.e., that plants with the most surface area should harbor the most individuals. I found that Vallisneria offered more surface area per gram dry weight than did
Myriophyllum. These data are consistant with the findings of C. Brown (Great
Lakes Fishery Laboratory, Ann Arbor, Michigan, personal communication). Yet
Myriophyllum supported more organisms per gram than Vallisneria. There may be several reasons for this surprising outcome. First, it is possible that
Vallisneria exudes some chemical defense substance that inhibits extreme colonization by invertebrates. It has been reported that macrophytes inhibit the production of algae (Hosier and Jones 1949), as well as various invertebrates such as rotifers (Hosier and Jones 1949), insect (mosquito) larvae
(Gonzalves and Vaidya 1963), and Daphnia (Pennak 1973). Inhibitory properties associated with various plants may be the result of toxic substances secreted by those plants (Pennak 1973). Recent work suggests that aquatic plant tissue contains a number of toxic, alkaloid, substances (Ostrofsky and Zettler 1986).
It may be significant that their work indicates that Vallisneria contains a higher alkaloid content than either Myriophyllum or Ceratophyllum. Myriophyllum is 67 intermediate, and Ceratophyllum has the lowest alkaloid content of the three plants. These results may help explain why, in my study, Vallisneria supported the smallest macroinvertebrate community, whereas Ceratophyllum supported the largest. Alkaloids may inhibit colonization by invertebrates. However, although this explanation may be useful in explaining the distribution patterns of some invertebrates, it may not explain fully the distributions of the majority of macroinvertebrates which do not feed on macrophyte tissue and only use the plants as a convenient habitat. Second, perhaps surface area is unimportant.
Some other factor such as the differential distribution of epiphytic algae among plants, or the nutritional quality of the respective plants, may be primarily responsible for differences in abundance (Chapter 3). Third, my measure of surface area may be misleading. I measured the surface area of entire plant samples. Entire samples were also dry weighed. Because Vallisneria has no above substrate stem, the entire sample consisted of leaves whereas
Myriophyllum samples (due to its growth form) contained both stems and leaves. Based upon my own observations leaves are the primary sites for invertebrate colonization in both plants, hence, it may be more appropriate to use only leaves for the calculation of available surface area. Thus
Myriophyllum stems, which contribute much dry weight, but little surface area would be eliminated. This may be a more direct way to determine usable surface area per gram. However, problems with this procedure may arise because minimal colonization does occur on Myriophyllum stems. Finally, the number of Myriophyllum samples for which I was able to obtain surface area readings was minimal. It may be that more data points would have shown
Myriophyllum to have a greater surface area to dry weight ratio than my present results indicate. However, this seems unlikely in light of the high n- of the present regression for Myriophyllum. 68
Although invertebrate abundance differences may be related to available surface area, 1 propose that these differences are most likely due to other morphological dissimilarities among plants. Growth form may be more important than differences in available surface area. The growth form of
Ceratophyllum, which supported the greatest number of organisms, is very dense and close packed. This may make it difficult for foragers to crop the invertebrate community. During summers 1983, 1984, and 1985, although fish were observed in close proximity to Ceratophyllum, only one (nearly dead) catfish was observed actually among its leaves. If predation pressure is low among its closely packed leaves, this may help explain why invertebrate abundances were greatest in Ceratophyllum patches. This plant may provide the best refuge. The growth form of Vallisneria is very loose by comparison.
Individual leaves emerge from a rosette and stand erect. Often an underwater observer can see several feet into a Vallisneria patch whereas this is impossible in a Ceratophyllum patch of comparable size. There is ample room in
Vallisneria beds, even very dense Vallisneria beds, for voracious predators such as a variety of centrarchids to forage. During summers 1983, 1984, and 1985, I often observed bluegills swimming freely through Vallisneria beds. In fact, the deep, laterally compressed body form of centrarchids seemed to be ideal for maneuvering in Vallisneria beds. Hence, Vallisneria seems to be a minimal refuge from vertebrate predators. Even though Vallisneria provides more surface area per gram than some other plants, its invertebrate standing crop is minimal. This may be due to constant grazing, possibly by centrarchids such as bluegill. Myriophyllum whose growth form is less dense than that of
Ceratophyllum, but more so than Vallisneria supported an invertebrate standing crop which was greater than that of Vallisneria, but less than that of
Ceratphyllum. Perhaps this was because invertebrates associated with 69
Myriophyllum were subject to grazing pressures intermediate between the two
other plants.
Differences in predation pressure among plants may have been
instrumental in structuring chironomid communities. At sites I and 5
chironomid abundance patterns differed from those of the other major
invertebrate taxa and from the total invertebrate community. Ceratophyllum
samples had the least number of chironomids, Myriophyllum samples were
intermediate, and Vallisneria had the most. This may have been caused by a combination of two factors. Chironomids are prey for a large variety of predators. The increased number of invertebrate predators (Hemiptera and
Odonata) in Ceratophyllum may have caused a reduction in the chironomid
standing crop. Fish predation on Vallisneria-associated invertebrate predators may have released chironomids from the effects of intense invertebrate predation, allowing their numbers to increase. Invertebrate predators associated with Myriophyllum may have been exposed to intermediate levels of predation by fish, so they were able to inflict intermediate predation pressure on chironomids. At site 2 the abundance of corixids, one of the most numerous hemipteran predators in the lake, dropped off sharply in Ceratophyllum samples. This was accompanied by an increase in chironomid abundance.
1 suggest then that even though my results indicate that Vallisneria supported the smallest invertebrate standing crop, Vallisneria beds may in fact be very important in terms of providing available fish food. Because invertebrates may be more accessible to vertebrate predators among
Vallisneria leaves than among the leaves of other plants, these beds could be extremely important feeding areas for grazing fish. The other two plants may offer larger, but largely inaccessible invertebrate standing crops. Differential rates of vertebrate predation among plants may serve to structure these 70
invertebrate communities.
A number of studies have suggested that predation may play a role in
increasing diversity by relieving the competition experienced by species which
would otherwise be eliminated by competitively dominant species (Paine 1966;
Menge and Sutherland 1976; Menge et al. 1985; Bennett and Streams 1986). If
primary prey organisms are competitively dominant, intermediate levels of
predation probably allow for the greatest prey species diversity (Lubchenco
1978; Menge et al. 1985). If Vallisneria-associated invertebrate communities
are experiencing more intense predation than Ceratophyllum or Myriophyllum,
this may be expressed by differences in the number and type of species found
among plants. My data are mixed, but generally supportive of this hypothesis.
Vallisneria harbored 21 taxa associated with no other plant, Ceratophyllum harbored only I I exclusive taxa, while Myriophyllum was intermediate with
18. The total number of taxa associated with Vallisneria, Myriophyllum, and
Ceratophyllum were 84, 87, and 75 respectively. Decreased predation pressure may have caused a reduction in the diversity of the Ceratophyllum community whereas increased predation may have reduced competitive pressure on a number of species in the Myriophyllum and Vallisneria communities. This may have allowed the survival of competitively inferior rare species. However, a formal testing of this hypothesis would require the collection of data regarding the relative competitive abilities of many of the species involved, and collection of such data was beyond the scope of this study.
Although other factors may be involved, if vertebrate predation is mediating invertebrate abundance, it may be possible to roughly predict the relative abundance of macrophyte-associated invertebrate fauna and their relative species richness, using some measure of accessibility to vertebrate predation. Abundance of invertebrate fauna should be inversely proportional to 71 accessibility, i.e., as predator accessibility (and hence predation pressure) increases, invertebrate abundance should decrease. Species richness should be directly proportional to accessibility, up to certain high levels, above which predation pressure would begin to cause extinction, i.e., as predation pressure, brought on by increased accessibility, increases, so should species richness up to a certain level, at which point species richness would begin to decline. Some measure of the space between leaves or branches may be of use in assessing accessibility, such as available surface area per mJ of water. However, before such predictive measures are undertaken it will be necessary to determine if vertebrate predation can structure macrophyte-associated invertebrate communities. Enclosure/exlosure experiments are probably the best way to determine the effects of such predation (Chapter 2).
Not surprisingly, macrophyte-associated invertebrate communities are subject to seasonal variations. Typically, abundances fall to minima in mid summer and rise again through fall. The reason for this pattern may be better understood if we recall that my abundance data are presented as numbers of organisms per gram of plant dry weight, not numbers per rrr surface area of the lake. Mid-July through late August is the time when macrophyte biomass reaches its seasonal maximum (Hargrave 1970; Voights 1976). Invertebrate abundance maxima per unit area are often linked with seasonal peaks in macrophyte standing crops (Hargrave 1970; Barber and Kevern 1973; mason and
Bryant 1975; Pip and Stewart 1976; Ali and Baggs 1982). My data which indicates a mid-summer minima in invertebrate abundance may possibly reflect an inability of invertebrates to produce biomass as quickly as macrophytes.
Macrophytes may gain biomass so quickly in mid-summer that a measure of invertebrate abundance per gram of plant masks an absolute increase in invertebrate abundance. However, this problem is unavoidable if one wishes to 72 compare the abundance of invertebrates associated with different plant species. Even the upswing of abundance in mid-September may have been accentuated by this phenomena. Although it could have represented a true increase in the abundance of one or several species, with plant biomass decreasing as water temperatures declined, and the annual period of senescence beginning, a relatively unchanged number of invertebrates (or even a slightly reduced number) may have been left to occupy a greatly reduced plant community. Hence, my measure of abundance increased.
However, this artifact of my abundance measure did not affect the main conclusions. My purpose was to compare the abundances of invertebrates associated with three plants. Because samples of all three plants were usually collected simultaneously during June through mid-September, temporal variation among plants was not a factor in community differences between plants, i.e., temporal variation was not the cause of differences among plant samples. Although the date-by-plant interaction was significant, small variation in the relative abundance of invertebrates among plants by date was not enough to disrupt overall abundance patterns. Hence, the possibility that temporal variation was the cause of differences among macrophyte-associated invertebrate communities was not a concern in my decision not to reject the hypothesis that differences exist among these communities.
My data suggest that site as well as plant type was a significant factor affecting invertebrate distribution. ANOVA indicated that not only was site a significant factor but the date-by-site interaction was also significant. This means that not only did sites differ, but the number of organisms differed among sites for most dates. Hence, differences were real and continuous. This conclusion was also supported by Duncan's Multiple Range Test, which indicated that invertebrate abundance at sites 2 and 5 were similar, whereas both were 73 significantly higher than the value for site I. Because conductivity readings suggest that sites I and 5 are primarily influenced by Black River water (low conductivities), while site 2 receives its w ater from the Mississippi River (high conductivities), these data suggest that differences between sites were not due primarily to water quality or source. Current readings, however, indicate that sites 2 and 5 were more similar to each other than either was to site I. Hence, current velocity was probably a more important determining factor for among site abundance differences than was water source.
Biomass data were highly variable, probably owing to the fact that invertebrate dry weights were usually very low. Often invertebrate dry weights were close to the limits of reliability of the Mettler and Torbal balances used to measure them. However, in general, total biomass data followed patterns similar to those of the numerical abundance data. Because of the weight of their shells, Gastropoda seem to have had more influence than any other taxa on total biomass trends. Site 5 maintained significantly lower total invertebrate biomass than any other site, because of its lack of gastropods.
Gastropods may have been reduced at site 5 because of increased current velocities at that site. Although sites 2 and 5 both maintained greater current velocities than site I, the current at site 5 was often somewhat greater than that even at site 2. Gyraulus spp., Amnicola spp., and Physa spp. dominated the gastropod communities. These snails are found primarily in quiet waters.
Increased water velocities at site 5 may have been enough to reduce their abundances. It is unlikely that water quality differences were the cause, since site I (which maintained a much larger snail community) received water from the Black River, as did site 5.
Continued work in this area is needed to determine the relative importance of a number of factors, including predation in structuring 74
invertebrate community abundance patterns. Enclosure/exclosure experiments
to determine the effects of fish predation on invertebrate predators (Chapter 2) and the effects of invertebrate predators on invertebrate prey species will be necessary. 75
Conclusion
Results of this study indicate that differences exist among the
invertebrate communities associated with Vallisneria americana, Myriophyllum spicatum, and Ceratophyllum demersum. These differences are consistant with
the findings of previous studies. They are probably not due to available surface area differences among plants, since Vallisneria, which had a high surface area
to dry weight ratio maintained the smallest invertebrate community. I hypothesize that differences in the invertebrate communities among plants may be due to differential levels of some naturally produced toxin, such as alkaloids, or to differential vertebrate predation associated with different plants as a
result of variable growth forms. Vertebrate predation may be a key factor determining the structure of these communities, because plants, that seem to provide easiest access to fish maintain lower invertebrate standing crops.
Vertebrate predation may mediate invertebrate predator abundance levels, which then in turn mediate abundance levels of invertebrate prey animals. 76
Chapter II
Effects of vertebrate predation on the
Vallisneria-associated invertebrate community
Introduction
Many fish species in Lake Onalaska depend on the macroinvertebrates
found in aquatic vegetation during some stage in their life history. Bluegill
(Lepomis macrochirus), one of the most common sport fish in the lake, use
aquatic insects to a large degree in their diet (Keast and Webb 1966; Etnier
1971; Cherry and Guthrie l975;Sarker 1977; Keast l978;Suleet al. l98l;Krska
and Applegate 1984). Chironomid (midge) larvae can comprise as much as 50%
of the bluegill's diet (Krska and Applegate 1984). Trichoptera larvae as well as
Odonata and Ephemeroptera nymphs also comprise a substantial portion of their
diet (Keast and Webb 1966; Cherry and Guthrie 1975; Keast 1978).
Aquatic invertebrates comprise a substantial portion of the diet of other
important fish in Lake Onalaska. Chaoborus (Diptera) alone can comprise as
much as 70% of the black crappie (Pomoxis nigricans) diet. Chironomidae
larvae, fish fry and Ephemeroptera nymphs are also important. Although white
crappie (Pomoxis annularis) is present, it is only about one-sixth as abundant as
black crappie, which is one of the most common species in Pool 7 (Sylvester and
Broughton 1983). Aquatic insects comprise as much as 80% of the crappie diet
during the early stages of its life cycle, before it becomes piscivorous (Forbes
76 77
and Richardson 1920).
Table 4 contains a list of the common game fish in Pool 7 and a summary
of work done on their feeding habits. Generally, these data confirm that many
of these fish feed heavily (at least during some stage in their life history) on
invertebrates, particularly on aquatic insects and especially on chironomid
larvae. Even piscivorous fish such as the largemouth bass may feed heavily on
insects while in the fingerling or juvenile stages (Bennett 1948; Mraz et al.
197 I). Other species that are common in Pool 7 are the common carp (Cyprinus
carpio), silver redhorse (Moxostoma anisurum), shorthead redhorse (Moxostoma
macrolepidatum), and northern pike (Esox lucius). Carp and redhorse are
bottom feeders that at times feed heavily on invertebrates. Northern pike are
piscivorus, but their young may feed on invertebrates.
Fish may effect invertebrate communities in ways not directly related
to predation, such as habitat destruction. Grass carp (Ctenopharynqodon
idella), because of its voracious feeding habits, often destroy aquatic
macrophyte beds (Anderson 1950; Colie et al. 1978; Lembi et al. 1978; Mitzner
1978; Stanley et al. 1978; Crivelli 1983). However, here I am primarily
concerned with predatory effects.
Because fish feed upon macrophyte-associated invertebrates, they may
play a role in structuring macrophyte-associated invertebrate communities. If
they are browsing differentially among macrophyte species, then invertebrate
communities associated with various plants may be affected differently across
plant species. However, the first step in understanding these interactions is to
determine whether or not fish predation can affect macrophyte associated-
invertebrate communities. Here 1 determine the effects of fish predation on
the invertebrate community associated with Vallisneria americana in Lake
Onalaska, Wisconsin. I chose to conduct a series of enclosure/exclosure 78
TABLE 4. Feeding habits of some important game fishes found in Lake Onalaska, based on the literature. Data are presented as percent volume of various food items found in the stomachs of these fishes. Data are from Price 1963, Seaburg 1964, Keast and Webb 1966, Voightlander and Wissing 1974, Cherry and Guthrie 1975, Keast 1978, Phillips et al. 1982, and Schaeffer and Margraf 1986.
Fishes Percent volume found in Stomach
Diptera Ephemeroptera
Largemouth bass (Micropterus salmoides) up to 20% up to 60%
Walleye (Stizostedion vitreum) --
Northern Pike (Esox lucius) --
Channel catfish (Ictalurus punctatus) up to 26% up to 19%
Brown bullhead (Ictalurus nebulosus) up to 50% -
Freshwater drum (Aplodinotus qrunniens) up to 30% -
White bass (Morone chrysops) up to 16% -
Yellow perch (perca flavescens) up to 39% -
Bluegill (Lepomis macrochirus) up to 50% up to 10% 79
TABLE 4. Continued
Fishes Percent volume found in stomach
Odonata Fish
Largemouth bass (Micropterus salmoides) up to 20% up to 82%
Waileye (Stizostedion vitreum) up to 99%
Northern pike (Esox lucius) up to 100%
Channel catfish (Ictalurus punctatus)
Brown bullhead (Ictalurus nebulosus) variable
Freshwater drum (Aplodinotus qrunniens) up to 24%
White bass (Morone chrysops) up to 94%
Yellow perch (Perea flavescens) up to 49%
Bluegill (Lepomis macrochirus) up to 20% 80 experments to test the specific hypothesis that fish predation will have no effect on the Vallisneria-associated invertebrate community. Vallisneria was chosen because its large flat leaves may provide easiest access for vertebrate predators. Hence, differences between treatments with and without predators,
may be more readily observed than if another plant was used which may have offered a more complete refuge from vertebrate predators. 81
Materials and Methods
During summer 1984 three sets of enclosure/exclosure experiments were conducted in Lake Onalaska, Wisconsin. Structures were placed in the most homogeneous beds of Vallisneria available to reduce variability introduced by the presence of other macrophytes in the cages. Eight enclosures, eight exclosures, and eight partially enclosed frames (cage-effect controls) were used y (Fig. 14). Each cage was about 0.37m . After cages were placed I waited about 3 weeks. All fully enclosed frames (enclosures and exclosures) were checked when the cages were placed to be certain that no fish were inside at the start of the experiment, none were found. Each enclosure was then stocked with one bluegill, about 60-80 mm total length and 0.05-0.1 g, representing a density of 130-260 kg/ha, similar to Coriander's (1977) figures of I 12-450 kg/ha y for natural populations in non-forested Illinois ponds. One fish per 0.37 nrr is y also well within the natural density range of 0.1-5.0 fish per m (for 70 mm bluegi 1 Is) reported by Savino and Stein (1982).
Eight initial samples of Vallisneria were taken from the viscinity of cages (open controls) when the structures were emplaced. Eight was chosen as the sample number because an analysis of 1983 data indicated that six samples should yield a 95% certainty of no more than a 20% error in the average number of invertebrates per sample site; eight provided a two-replicate buffer. Sampling procedure for macrophytes was essentially the same as that used during summer 1983 (Chapter I). Cages were approached slowly so that no unusual water currents were created that might have disturbed invertebrates. I removed the top of each cage as it was sampled. After the top was removed 1 leaned over the cage and lowered the sampling device (Fig. 2) over Vallisneria leaves to a depth about 30 cm above the substrate, the plant was then broken Figure 14. Design for enclosure, exclosure, and cage-effect control cages used during summer 1984 enclosure/exclosure experiments. Enclosure and exclosure treatment cages were completely enclosed by 0.8 cm mesh aluminum wire screening. One half of each side of cage-effect controls was left open.
82 83
cover
6.1 x 6.1 cm wood b e a m 8
cv
c- .61 m Wire reinforcement
1.3 cm diameter Iron rod 84 off and the sampler was inverted and brought to the surface. Samples were
taken to the laboratory fresh and refrigerated until the invertebrates could be counted. Procedures for removal of invertebrates from plant samples,
identification of invertebrates, and for determining the dry weight of plants and invertebrates were the same as those used during summer 1983 (Chapter
I). Ten days later, approximately halfway through the experiment, macrophyte
samples were collected from all cages, and an additional eight open controls were taken as well. Final samples from all cages, as well as eight open controls were collected at the end of 3 weeks for the first two sets of experiments.
Unfortunately, a severe storm forced the premature end of the third set of experiments shortly after 12 day samples had been collected. At the end of the second set bluegi I Is were removed from the cages, weighed, measured, and stomach contents examined. No fish were recaptured at the end of the third set, because a storm uprooted all vegetation, blew away the tops of the cages, and submerged nearly all cages, so that enclosed bluegills escaped.
The first set of experiments took place at site I (see Fig. I). No enclosures were used. The second set also took place at site I; however, during this set, enclosures were added. The third set took place at site 6 (see Fig.I); all treatments were used. However, the cage-effect control treatment had to be dropped. Cage-effect control cages either became so clogged with free- floating macrophytes of various species that reliable samples were impossible to obtain, or they were colonized by muskrats.
On September 27, just after the beginning of the third set of experiments, macrophyte samples were collected from sites I and 6 to determine the relative stem density of Vallisneria at the sites. A 0.5 m aluminum frame was thrown randomly from a boat and a diver collected all plants within the frame. Plant samples were then transported to the laboratory 85 and refrigerated until Vallisneria stems could be counted. Five samples were collected at site 6 and seven were collected at site I. An average was calculated for each site, and a t-test was performed on the data to determine if stem densities at the sites differed from one another. 86
Results
During the first set of experiments, the abundance (number per gram plant dry weight) of Vallisneria-associated invertebrates differed among treatments after 10 days. Although the cage-effect controls and the exclosures did not differ, both had significantly higher invertebrate abundance means than open controls, p* 0.005 and p<0.05 respectively. However, after 20 days invertebrate abundances among treatments had converged and no differences were observed.
During the next two sets of experiments a somewhat different pattern emerged. In the second set of experiments no significant differences among treatments was found after 10 days. However, after 3 weeks, although no differences occurred among cage-effect controls, exclosures, and enclosures, all had significantly larger invertebrate communities than open controls, pc
0.05, p<0.00l, and p<0.00l respectively (Fig. 15; Table 5). In the third set of experiments, after 12 days mean invertebrate abundances tended to be somewhat higher for enclosures and exclosures than for controls (Fig. 15); however, these differences were not significant (p<0.20). ANOVA revealed that over all three experiments, differences among treatments were significant
(p<0.0002). The date-by-treatment interaction was also significant (p< 0.0002), indicating that the significance of the treatment effect was not the result of one or two unusual sampling dates, but that the pattern was consistant by date
(Table 6).
The most abundant taxa found were the same as those collected during
1983 (Chapter 2). ANOVA revealed that across experiments, of the six major groups, four, the Gastropoda, Odonata, Hemiptera, and Amphipoda (Hyallela azteca) showed significant variation in abundance among treatments. Figure 15. Changes in abundance (natural log of the number of invertebrates per gram dry weight of plant) of Vallisneria-associated invertebrates during summer 1984 enclosure/exclosure experiments. Control treatments are represented by solid lines, cage-effect control treatm ents are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
87 TOTAL ABUNDANCE (In */gram plant dry weight) - 6 8 /07/30 7/10 DATE 8/19 9/8 .: P 11
9/28 88 89
TABLE 5. T-test comparisons of invertebrate abundance (number per gram dry weight of plant) among 1984 enclosure/exclosure experiment treatments. Data are presented by date, for all major (numerically dominant) invertebrate taxa, and for total invertebrate abundance. Calculations were performed on log-transformed data, In ( n + I ). Con = open control treatments, Cage = cage-effect control treatments, Ex = exclosure treatments, En = enclosure treatments.
Total invertebrate abundance
Date Con x Cage Cage x Ex Ex x En
7/23/84 0.005 NS -
8/2/84 NS NS -
9/7/84 NS NS NS
9/19/84 0.05 NS NS
10/8/84 _ _ NS
T richoptera
Date Con x Cage Cage x Ex Ex x En
7/23/84 0.05 NS -
8/2/84 NS NS -
9/7/84 NS NS NS (0.10)
9/19/84 NS NS NS
10/8/84 NS 90
TABLE 5. Continued
Odonata
Date Con x Cage Cage x Ex Ex x En
7/23/84 NS NS
8/2/84 NS NS
9/7/84 NS NS NS
9/19/84 0.02 0.05 NS
10/8/84 NS
Hemiptera
Date Con x Cage Cgge x Ex Ex x En
7/23/84 NS NS
8/2/84 NS NS
9/7/84 NS NS NS
9/19/84 0.01 NS NS
10/8/84 NS
Chironomidae
Date Con x Cage Cgge x Ex Ex x En
7/23/84 0.02 NS
8/2/84 0.02 NS
9/7/84 NS NS NS
9/19/84 0.01 NS NS
10/8/84 NS 91
TABLE 5. Continued
Gastropoda
Date Con x Cage Cage x Ex Ex x En
7/23/84 NS NS
8/2/84 NS NS
9/7/84 NS NS 0.01
9/19/84 0.05 NS NS
10/8/84 - - NS
H. azteca
Date Con x Cage Cage x Ex Ex x En
7/23/84 0.005 NS
8/2/84 NS NS
9/7/84 NS NS NS
9/19/84 0.05 NS NS
10/8/84 - - NS 92
TABLE 5. Continued
Total invertebrate abundance
Date Con x En Con x Ex Cage x En
7/23/84 - 0.05
8/2/84 - NS
9/7/84 NS NS NS
9/19/84 0.001 0.001 NS
10/8/84 NS NS
Trichoptera
Dgte Con x En Con x Ex Cage x En
7/23/84 - 0.02
8/2/84 - NS
9/7/84 0.05 NS 0.02
9/19/84 NS NS NS
10/8/84 NS NS
Odonata
Date Con x En Con x Ex Cgge x En
7/23/84 - NS
8/2/84 - NS
9/7/84 NS NS NS
9/19/84 0.01 0.01 NS
10/8/84 0.02 NS 93
TABLE 5. Continued
Hemiptero
Date Con x En Con x Ex Cage x En
7/23/84 - NS
8/2/84 - NS
9/7/84 NS NS NS
9/19/84 0.01 0.001 NS
10/8/84 0.001 0.001
Chironomidae
Date Con xEn Con x Ex Cage x En
7/23/84 - 0.01
8/2/84 - 0.05
9/7/84 NS NS NS
9/19/84 NS NS NS
10/8/84 NS NS
Gastropoda
Date Con x En Con x Ex Cage x En
7/23/84 - NS
8/2/84 - NS
9/7/84 0.01 NS 0.05
9/19/84 0.001 0.001 NS
10/8/84 NS NS 94
TABLE 5. Continued
H. azteca
Date Con x En Con x Ex Cage x En
7/23/84 - 0.05
8/2/84 - NS
9/7/84 NS NS NS
9/19/84 0.001 0.001 NS
10/8/84 NS NS 95
TABLE 6. Results of ANOVA performed on 1984 enclosure/exclosure experiment data. Data are presented for all major (numerically dominant) invertebrate taxa, and for total invertebrate abundance. The test was performed on log-transformed data, In ( n + I ). Values indicate the probability that differences in invertebrate abundance among dates, treatments, or the interaction of these variables, occurred by chance. Values of 0.05 or less indicate statistical significance. NS indicates no statistical significance.
Taxon Date Treatment Date x Treatment
Trichoptera 0.04 NS NS
Gastropoda 0.0001 0.02 NS (0.09)
Odonata 0.0001 0.0001 NS
Lepidoptera 0.0001 NS 0.002
Ephemeroptera NS (0.06) NS 0.001
Hirudinea NS NS NS
Hemiptera 0.0001 0.0001 0.0003
Diptera 0.0001 NS NS
Coleoptera 0.004 NS NS
Megaloptera NS NS NS
Oligochaeta NS (0.08) NS NS
Other 0.0002 0.02 NS (0.08)
Chironomidae 0.002 NS NS
H. azteca 0.0001 0.0002 0.006
Total 0.0001 0.0002 0.0002 96
Treatment effects were insignificant for the Trichoptera and Chironomidae
(Table 6).
All major groups with the exception of Trichoptera and Chironomidae
tended to follow the same abundance pattern. Control samples usually
contained the least number of invertebrates, enclosure and exclosure samples
contained the most, and partial control samples were usually intermediate, but
variable, with respect to invertebrate abundances (Figs. 16, 17, 18, 19). All
statistically significant differences among treatments followed this pattern
(Table 5). Duncan's Multiple Range Test, which tested means across
experiments and dates, confirmed these results. All major groups, with the
exception of Trichoptera and Chironomidae, exhibited the greatest abundance
in the enclosure treatment. This test revealed that for the Gastropoda,
Odonata, and Amphipoda (Hyallela azteca) indicated that this trend was
significant (p<0.05). The treatment with the second highest overall
invertebrate abundance was usually the exclosure treatment. This was the case
for the Amphipoda (Hyallela azteca), Gastropoda, and Hemiptera.
Trichopteran samples were highly variable, exhibiting little if any overall pattern (Fig. 20). However, Chironomidae results were consistant, and distinct from all others. Their abundance pattern was almost the opposite of
that described for the Gastropoda, Odonata, Hemiptera, and Amphipoda (Fig.
21). During the first two sets of experiments the abundance of chironomids was greatest in control samples. Samples from enclosures, exclosures, and partial controls contained substantially fewer chironomids. These differences were usually significant (Table 5). Chironomid abundance during the final set of experiments was low. Differences among treatments were not statistically significant. Overall, Duncan's Multiple Range Test showed that enclosure and exclosure treatments harbored significantly fewer chironomids than other Figure 16. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Hemiptera during summer
1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatm ents are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
97 HEMIPTERAN ABUNDANCE On #/gram plant dry weight) 1 2 4 3 5 7/10 7/30 DATE 9/88/19 9/28 98 Figure 17. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Gastropoda during summer
1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatm ents are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12.
99 o o ->i co ^ cn o> jo -* -* GASTROPOD ABUNDANCE (In #/gram plant dry weight) a CO ro - o DATE Figure 18. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Amphipoda (Hyallela azteca) during summer 1984 enclosure/exclosure experiments. Open control treatm ents are represented by solid lines, cage-effect control treatm ents are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12. 101 AMPHIPOD ABUNDANCE (In #/gram plant dry weight) 6 8 7/10 7/30 DATE 8/19 9/8 9/28 102 Figure 19. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Odonata during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12. 103 ODONATE ABUNDANCE (In * t gram plant dry weight) Figure 20. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Trichoptera during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatm ents are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12. 105 TRICHOPTERAN ABUNDANCE (In #/gram plant dry weight) 2 3 1 5 4 6 7/10 7/30 DATE 8/19 9/8 9/28 106 Figure 21. Changes in abundance (natural log of the number of individuals per gram dry weight of plant) of Vallisneria-associated Chironomidae during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatm ents are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 12. 107 o 00 -*> fO GO ^ CXI O) ->4 CHIRONOMID ABUNDANCE CHIRONOMID (In */gram plant dry weight) /ft y ■>* IO 00 CO 00 CO CO 00 ■M - x § O DATE 109 treatments (p<0.05). Mean total biomass followed the same pattern as that seen for abundances. In general, the mean invertebrate biomass found associated with control samples was the lowest. Biomass of enclosures and exclosures was substantially higher whereas samples from cage effect controls were usually at some intermediate value (Fig. 22). Duncan's Multiple Range Test revealed the same pattern. ValIisneria-associated invertebrate biomass was significantly higher in enclosure and exclosure treatments than in cage-effect controls or open controls. It was only possible to analyze the stomach contents of six of the eight bluegills stocked at the beginning of the second set of experiments. One had died (invertebrate data from that cage were not used), and one escaped during recapture. In addition to the stocked bluegills that were collected, five smaller bluegills (28-35 mm) and one madtom (34 mm), were also collected from enclosures. These were small enough to have entered the cages through the wire mesh. Gut analysis of these smaller specimens was also carried out. Generally, the larger, stocked fish, were cropping relatively large invertebrates such as Hyalella azteca, Trichoptera, and Hemiptera, whereas smaller fish stomachs contained no trichopterans, no hemipterans, and many copepods (Table 7). However, large numbers of the tiny aquatic macrophyte Wolffia, which was not included in the analysis of invertebrate prey, were found in nearly all specimens. Total invertebrate abundance differed between sites. The average number for all treatments at site 6 (1,861) was over three times that at site I (600). This difference was significant at p<0.05. The average density of Vallisneria stems at site I, on September 27, was 2 6.6 stems per m , whereas the average density at site 6 was 68.8 stems per Figure 22. Changes in the biomass (invertebrate dry weight per gram dry weight of plant) of the Vallisneria-associated invertebrate community during summer 1984 enclosure/exclosure experiments. Open control treatments are represented by solid lines, cage-effect control treatments are represented by dotted lines, exclosure treatments are represented by dashed lines, and enclosure treatments are represented by dotted and dashed lines. Standard error of the mean is presented. Sample sizes are given in Table 13. BIOMASS (grams invertebrate dry weight/gram plant dry weight) TABLE 7. Results of bluegill stomach analysis. Data are presented as the percentage of the total number of invertebrates found in bluegill stomachs. Nothing was found in the stomach of the madtom that was collected. Analysis of smaller, unstocked bluegills, found in cages at the conclusion of the experiment is also presented. Invertebrate taxa Stocked blueqi 1 Is Unstocked blueqills (67-76 mm) (28-35 mm) H. azteca 24% 22% Chironomidae 5% 10% Trichoptera 26% 0% Hemiptera 19% 0% Physa spp. 7% 0% Gyraulus spp. 10% 0% Cladocera 10% 2% Copepoda 0% 66% 113 m . A t-test showed that the difference in stem density between the two sites was statistically significant (p 0.001). Densities at site I ranged from 0-26 stems per m . Densities at site 6 ranged from 18-154 stems per m . 114 Discussion The interactions among invertebrates, aquatic macrophytes, and fish may be quite complex. It has been shown that increased surface area for the attachment of aquatic invertebrates can cause increases in bluegill productivity (Pardue 1973). Increased macrophyte densities may provide increased surface area for the attachm ent of invertebrates, and so enhance bluegill (and possibly other fishes as well) production. Juvenile bluegi I Is are to be found primarily at of depths of 0.5-1.Om, just above the aquatic vegetation (Werner et al. 1977). However, data suggest that low macrophyte densities, as well as very high densities may inhibit the bluegills (and again possibly other fishes) ability to use the resources within a particular habitat. Intermediate densities appear to be optimal for bluegill growth (Crowder and Cooper 1982). Food availability may only be one reason for the close association of bluegills with macrophytes. There is evidence to suggest that aquatic plants also serve as a refuge from predators. Largemouth bass, for example, are less able to capture bluegills in the presence of dense macrophytes (Savino and Stein 1982; Savino 1985). Optimal habitat for bluegills, therefore, may be in aquatic vegetation that is dense enough to confuse predators, but not so dense that it inhibits the bluegi I I's feeding activities. Crowder and Cooper (1982) used 36 ± 5 stems per rrr as a low density, I I I ± 7 stems per m as an intermediate density, and 177 ± 10 stems per m'1 as a high density. However, because these counts were made from a heterogeneous macrophyte community their numbers may be somewhat misleading. The volume occupied by a "stem", and the number of leaves per stem, may vary widely depending on which plant species is being observed. Savino (1985) eliminated this problem by using artificial "stems", made of polypropylene line, which, when suspended in water, closely approximate the 115 vertical growth form of Vallisneria. She defined low density as 50 stems per 2 2 m , medium density as 250 stems per m , and high density as 1000 stems per 2 2 m . The average Vallisneria stem density of Lake Onalaska, 96.4 stems per m (Carl Korschgen, Northern Prairie field station, LaCrosse, Wisconsin, personal communication), is roughly low to medium. Vallisneria stem density at site I was low, whereas the density at site 6 was low-medium when compared to other studies. Therefore, during this study stem density probably did not inhibit the bluegills ability to forage effectively. As in most aquatic systems, the distribution of macrophytes in Lake Onalaska is patchy. Although various workers have obtained mixed results in laboratory studies (Devries 1985; Marschall 1984); in theory, bluegills should move from patch to patch. They should stay in a given patch until their marginal capture rate in the patch drops to the average capture for the habitat (Charnov 1976). One expects to see them spending the most time in patches of high food availabilty (ie., high marginal capture rates). Marginal capture rates should decline over time in the presence of a predator, only if predators are significantly reducing prey population densities and thus food availability. My results suggest that vertebrate predators do have a significant impact on the invertebrate faunal assemblages associated with aquatic macrophytes. Because control samples generally had fewer invertebrates than cage effect controls, exclosure, or enclosure samples, I suggest that cages were excluding at least one, and possibly more than one, vertebrate predator (all known invertebrate predators were found in the cages). Because cage-effect control samples were intermediate between control samples and enclosure/exclosure samples (though differences between cage effect controls and enclosure/exclosure samples was rarely statistically significant in terms of numerical abundance, their means were lower, and differences between enclosure and exclosure treatments, and I 16 cage-effect control treatments were significant in terms of total biomass), it seems that the presence of the caging structures alone was not totally responsible for invertebrate abundance increases. Although a few invertebrates were found in the stomachs of enclosed bluegills, it is plain that they were not substantialy cropping invertebrates. Invertebrate abundances were often higher in enclosures than other treatments. It is unlikely that the enclosed bluegills were reducing predation pressure on prey organisms by cropping invertebrate predators. The abundances of invertebrate predators such as Odonata and Hemiptera were also often greater in enclosures than other treatments. Bluegills seemed to feed heavily on Woffia (a tiny vascular plant), which was conspicuous in their diet. Perhaps this was due to the size of the bluegills or to the abundance of Woffia. Juvenile bluegills collected from Lake Onalaska tend to feed heavily on Cladocera and Copepoda until they reach a total body length of about 30 mm. At that point they switch to somewhat larger prey such as Amphipoda and Chironomidae (Leslie Holland, National Fishery Research Laboratory, LaCrosse, Wisconsin, personal communication). My data were consistant with those findings. It is possible that bluegills were unable to distinguish the tiny floating macrophyte from prey organisms such as Amphipoda, Cladocera, or Copepoda. Because invertebrate abundance patterns were often similar for enclosures and exclosures these two treatments were effecting invertebrates similarly. Because bluegills do not seem to have exerted significant predation pressure on the Vallisneria-associated invertebrate community, perhaps some other predator was excluded, one responsible for cropping invertebrates. This is supported by the fact that both enclosures and exclosures maintained greater invertebrate abundances than controls, and generally higher means than cage effect controls. Perhaps the cages served as a refuge from larger, adult, 117 bluegills, or any number of other insectivorous fishes found in the lake. Trichoptera samples were highly variable exhibiting little if any overall pattern. I attribute this to the wide variety of trichopteran body forms ( Ross 1944; Wiggins 1977; Coffman 1984), and hence their wide variety in susceptibility to predation. Trichopteran case construction and size are so variable it is difficult to make a clear statement about trichopteran vulnerability. Also a variety of emergence dates complicated these data. Fortunately, this was not as much a problem for the other major groups. Amphipods and gastropods do not emerge. Hemipteran adults live in the same environment as the juveniles, so that loss of individuals, caused by emergence, does not occur. The odonate community was dominated by a single species, Enallaqma sp., again eliminating emergence pattern differences among species as a complicating factor. Therefore, of the major groups examined only Trichoptera data were confused by several species undergoing emergence at different times. Data for Chironomidae were unique among the major groups. Although data for the last set of experiments are unclear, the first two sets of experiments both show chironomid abundance substantially reduced in the caged treatments (both enclosures and exclosures). Caging was thus not responsible for reducing predatory pressure on chironomids. On the contrary caging seems to have exposed chironomids to some inhibitory pressure. I suggest that caging reduced predation on larger, carnivorous invertebrates such as Hemiptera and Odonata, which optimally foraging fish would selectively crop. If this was the case one would expect to see increases in the abundances of these carnivorous forms. My data support this idea because caged treatments often exhibited greater abundances than uncaged treatments for both the hemipterans and the odonates. All statistically significant results 118 were in this direction. With these groups increasing in abundance, the predatory pressure may have increased on chironomids. These data are consistant with data from Chapter I which suggested that chironomids associated with plants which offer a minimal refuge from vertebrate predators, such as Vallisneria, are often found in greater abundance than chironomids associated with plants which may offer a more complete refuge. This may be because, to some degree, vertebrate predators crop large numbers of invertebrate predators, thus releasing chironomids from invertebrate predation. These data suggest that vertebrate predation may be important in determining macrophyte-associated invertebrate community abundance and composition. It is possible that the collection of initial samples outside of cages may have introduced some uncertainty into the results. Taking initial samples from within cages may have given me a more accurate measure of invertebrate abundance within each cage at the start of the experiment. In this way individual differences among cages could have been assessed. However, it seems unlikely that these differences had any profound influence on the main results and conclusions of the study, because, in general, the same trends were evident across sets of experiments, thus reducing the probability that chance differences among cages significantly altered results. It is improbable that the same chance conditions would have occured over three experimental sets. Continued effort in this area is necessary to determine whether or not the effects of vertebrate predation, suggested by this study, are differential among plants. If the effects of vertebrate predation are, as seems likely, differential among plants, then abundance patterns of macrophyte-associated invertebrates could be explained in terms of the quality of refuge offered to the invertebrate fauna of a given plant. Some quantifiable measure of refuge quality would then be useful as a predictive index of relative invertebrate abundances among plants. As in the assessment of accessibility (Chapter I), measure of the space between leaves or branches, such as available surface 2 area per m of water, may be a useful index. Conclusion Results of this study suggest that vertebrate predation may be an important factor in the structuring of macrophyte-associated invertebrate communities. Vertebrate predation affects these communities in both a qualitative and a quantitative way. By preying upon invertebrate predators, one or several vertebrate species may act as a keystone predator, secondarily affecting predation rates on organisms such as chironomids. My data support these suggestions, because under circumstances of low vertebrate predation pressure (caged treatments), the abundances of invertebrate predators such as Hemiptera and Odonata were high whereas the abundance of invertebrate prey organisms such as chironomids was low. These results were reversed in control samples which were presumably subject to predation from a wide variety of vertebrate predators. 121 Chapter III Chironomid feeding behavior Introduction Non-morphological factors may be important in any comprehensive explanation of the differences in invertebrate standing crops among macrophytes (Chapter 2). Lowe et al. (1982) and Rosen et al. (1981) found that different macroscopic algae may have different epifloral assemblages. The same could be true of aquatic macrophytes. If so, it is possible that different epifloral assemblages on plants may affect invertebrate distribution. However, this could only be true if invertebrates were selective feeders. If they are selective feeders they may also selectively colonize plants whose epiflora offers the greatest abundance of preferred food items. Although work has been done on chironomid feeding behavior, it is primarily of a general nature. Chironomids, like many other aquatic organisms, are presented with a wide variety of possible food items. Consequently, diet and food gathering methods vary widely within the family. Some, primarily members of the subfamily Tanypodinae and a few genera in the Chironominae and Diamesinae, are predators (Leathers 1922; Oliver 1971; Monakov 1972; Cummins 1973; Loden 1974; Coffman 1984). However, the majority feed on algae, detritus, or algal detritus. Diatoms are a major food item for many members of the subfamilies Orthocladiinae, Diamesinae, and Chironominae. All Aphroteniinae and Podonominae feed on diatoms (Oliver 1971). 121 122 Food gathering mechanisms are as varied as food items. Berg (1950) discovered that chironomids which were found in association with the aquatic vascular plant, Potamoqeton spp., could be placed into a number of catagories, according to feeding habits. Other workers have found much the same thing. Larvae may mine algae or vascular macrophytes (Berg 1950; Walshe 1951; Brock I960; Cummins 1973; Coffman 1984). They may scrape food from the substrate (Monakov 1972; Cummins 1973; Coffman 1984). They have been found actively foraging through detritus or on the surface of a substrate (Leathers 1922; Cummins 1973; Stoffer 1978; Coffman 1984), and they have often been observed filter feeding with silken nets of their own construction (Lieux and Mulrennan 1956; Monakov 1972; Stoffer 1978; Wallace and Merritt 1980; Coffman 1984). However, although chironomid diets consist of algae, detritus, and animals, the specific composition of organisms caught in nets or cropped from macrophyte leaves is rarely reported. Although many members of the subfamily Tanypodinae and a few genera in the Chironominae and Diamesinae are facultative carnivores (Oliver 1971; Loden 1974), most chironomid larvae are microphagous. The majority feed on algae and algal detritus. Although most of the algae eaten is in the form of diatoms (Oliver 197 I), little is reported concerning which species are consumed, or whether certain species are preferred over others. The purpose of this study was to determine if selective feeding takes place among Chironomidae larve. If selective feeding occurs then perhaps food distribution determines chironomid (and possibly other invertebrate taxa as well) abundance patterns. Chironomid larvae are relatively easy to identify (to genus) and are found in abundance among macrophyte beds in Lake Onalaska. In addition there is ample evidence that macrophyte-associated Chironomidae communities vary by plant (Soszka 1975; Urban 1975; Chapter I). 123 The specific hypothesis in this study was that diatom assemblages found within chironomid digestive tracts would be substantially different from assemblages found epiphytically on the plants from which the chironomids were collected, i.e., chironomids are selective feeders. 124 Materials and Methods Samples of the aquatic macrophyte Myriophyllum spicatum were collected on September 14, 1983. Sampling procedures were the same as those described in Chapter I. Four plants were collected, 2 at site I, and 2 at site 2. Chironomidae larvae were removed from Myriophyllum by hand. After removal larvae were heated in 10% KOH for clearing, washed in 95% ETOH (Simpson and Bode 1980), and mounted in balsam (with xylene solvent) medium (Johannson 1969; Pennak 1978). This method dissolved muscle tissue, making it possible to view the delicate hard parts used in identification. As a by-product of the clearing process diatoms in chironomid digestive tracts also were cleared and visible through the body wall of the midges. The body wall of cleared chironomids is transparent enough so that diatoms can often be identified to the genus or species level. When 100 or fewer diatoms were found in chironomid guts all were counted and identified to the genus or species level. When diatoms in the gut were extremely numerous, 100-200 were counted and identified. 1 began the examination of the contents of the digestive tract at the mouth and proceeded posteriorly until at least 100 diatoms had been counted. Epiphytic diatoms found on plants from which chironomids were collected were mounted. All plant samples were placed in separate beakers and boiled in concentrated nitric acid until all organic matter had been dissolved. Diatoms were allowed 12 h to settle to the bottom of beakers after the solution was cooled. Nitric acid was then siphoned away, leaving diatoms in the bottom of beakers undisturbed. Distilled water was added, and again diatoms were given 12 hours to settle. After 12 hours the water was siphoned away, again leaving diatoms in the bottom of beakers undisturbed. Rinsing diatom samples in water removed residual nitric acid, which may have reacted to mounting media, and washed away impurities. Diatom samples were then diluted in distilled water. After thorough mixing, drops of each sample were placed on coverslips and allowed to dry. Drops of permount mounting medium were placed on glass slides. Each dried coverslip was inverted and placed on a glass slide on top of a drop of mounting medium. All slides were then heated so that the mounting medium solvent, xylene, was boiled off. Five slides were made of diatoms from each Myriophyllum sample. After cooling ten fields of view were counted on each slide at lOOx. Data for each plant sample were then pooled. Diatoms were then compared to those in chironomid guts. Using a measure of preference developed by Chesson (1978, 1983), I determined whether or not midge larvae were feeding selectively on certain diatoms. Although forage ratio, Ivlev's electivity index (I960), and the preference coefficient of Rapport and Turner (1970) have been commonly used in the past, these measures depend on the relative abundances of food organisms found in the environment (Jacobs 1974), and thus may change if depletion or species specific blooms occur. Therefore, Chesson's index, which is independent of the density of food items in the environment, was used. 126 Results By far the most abundant epiphytic diatom collected was Cocconeis placentula. This was true at both collection sites. It comprised over 80% of all diatoms. The next most abundant was Navicula confervacea which accounted for 3% of the total number identified. Nitzschia amphibia was third, accounting for only 2.5%. Diatom community composition was similar at both sites (Table 8). Seven species of chironomid were found. The two most numerous species were Endochironomus nigricans and Rheotanytarsus exiquus group. Although the genus Cricotopus is abundant in the lake (personal observation) very few were found on Myriophyllum. All chironomids selected against Cocconeis placentula in their diets. Chesson's index of preference ranged from -.20 to -.99 (The index ranges from -I to +1). Diatoms for which chironomids showed the most preference were Navicula confervacea (average preference =.51) and Melosira spp. (average preference = .69) (Table 9). Preference for certain diatoms varied with chironomid species in some cases. Endochironomus nigricans (n= 11) preferred Cyclotella spp. (average preference = .54). However, the Rheotanytarsus exiguus group exhibited virtually no preference for Cyclotella (average preference = .02). 127 TABLE 8. Composition of diatom flora associated with Myriophyllum spicatum samples, collected September 14, 1983, at sites I and 2 in Lake Onalaska, Wisconsin. The total number counted of each taxa are presented along with the percentage of the total. S-l and S-2 indicate sites I and 2, respectively. Site I Site 2 Taxa Number % Number % Cocconeis placentula 712 80.0% 1079 89.4% Navicula confervacea 51 5.7% 21 1.7% Navicula minima 13 1.5% 20 1.7% Navicula spp. 19 2.1% 6 0.5% Gomphonema spp. II 1.2% 11 0.9% Gomphonema parvulum 9 1.0% - - Gomphonema constrictum 1 0. 1 % - - Cymbella spp 7 0.8% 1 0.1% Melosira spp. 9 1.0% 13 1.1% Synedra spp. 14 1.6% 4 0.3% Nitzschia amphibia 13 1.5% 40 3.3% Nitzschia palea 4 0.4% 2 0.2% Achnanthes lanceolata 19 2.1% 3 0.2% Achnanthes spp. - - 1 0.1% Cyclotella spp. 6 0.7% 5 0.4% Surirella spp. 1 o.i% 1 0.1% Eunotia spp. 1 0.1% -- Total 894 1207 TABLE 9. Chesson's Preference Index (Chesson 1983), calculated for diatom flora found in chironomid digestive tracts, compared to those found epiphytically on the plants (Myriophyllum spicatum samples) from which chironomids were collected. The index ranges from a value of -I (indicating strong negative preference) to +1 (indicating strong positive preference). Zero indicates no preference. C. placent = C. placentula. N. amphib = N. amphibia. N. conferv = N. confervacea, Melosir = Melosira spp., Cyclote = Cyclotella spp.. and Cymbell = Cvmbella spp. Chironomid C. placent N. amphib N. conferv Melosir Cyclote Cymbell Site 1 R. exiquus qroup -0.51 0.23 -0.09 - _ -0.34 R. exiquus qroup -0.72 0.86 - -- - R. exiquus qroup -0.72 -0.34 0.81 -0.34 - - R. exiquus qroup -0.72 0.86 -0.20 - 0.77 - R. exiquus qroup -0.72 0.28 0.48 0.72 -0.20 0.28 R. exiquus qroup -0.72 0.01 0.70 0.68 - -0.09 R. exiquus qroup -0.72 0.45 0.56 0.51 -0.51 0.37 R. exiquus qroup -0.72 0.37 0.77 0.23 -0.20 0.16 C. sylvestris qroup -0.99 -0.72 0.58 0.97 -0.72 - C. sylvestris qroup -0.72 0.45 0.82 -0.20 - -0.09 TABLE 9. Continued C. bicinctus -0.72 0.71 0.45 0.53 0.09 Paratanytarsus spp. -0.72 -0.09 0.67 0.28 Site 2 E. niqricans -0.72 0.64 0.58 0.89 E. niqricans -0.72 0.23 0.7! 0.85 E. niqricans -0.99 - 0.94 0.81 E. niqricans -0.51 0.71 0.71 0.16 E. niqricans -0.34 -0.72 0.78 0.37 E. niqricans -0.88 - 0.93 0.83 E. niqricans -0.20 0.28 0.82 0.87 E. niqricans - - 1.00 E. niqricans -0.34 0.33 0.98 E. niqricans -0.72 -0.34 0.95 0.51 E. niqricans -0.72 -0.5! 0.77 0.93 N. distinctus -0.72 0.45 0.98 N. distinctus -0.997 - 0.96 0.70 VO TABLE 9. Continued Rheotanytarsus spp. -0.72 -0.20 0.77 0.87 0.41 R. exiquus group -0.51 0.48 0.37 0.87 0.23 u> o 131 Discussion The idea that animals forage optimally (maximize food intake and minimize time) may be useful in terms of predicting preference of an organism for a specific habitat or food item (Estabrook and Dunham 1976; Mittelbach 1981; Anderson 1984; Stein et al. 1984). If chironomids feed optimally, I would expect a preference for three general types of diatoms. Large diatoms may be preferred because they optimize the amount of food received per collection (intake is high per unit time). Optimally foraging chironomids may also select colonial diatoms. Long chains provide a known source of biomass with little foraging requirements. Once a chain is discovered it can be continually processed. No more searching is necessary until it is completely processed. Foraging time per cell is probably reduced. Of course it is possible that difficulties in handling may make up for the energy saved from decreased foraging times. Finally, optimally foraging chironomids may simply wait for their food by filter feeding. However, the cost of spinning a net and maintaining a current through it is often high (Lieux and Mulrennan 1956). Because of this, and because gape size of the larvae may set an upper limit on the size of diatoms that can be processed, I would expect the most preferred group to be colonial forms. I would also anticipate that diatoms that are present as numerous small cells, which must be individually processed, to be selected against. Cells such as these would tend to decrease caloric intake per capture (per unit time), because processing time must be spent on each cell. My results agreed closely with these a priori predictions. Colonial diatoms such as Melosira spp. and Navicula confervacea were heavily favored. Large forms such as Cymbella spp. and planktonic forms such as Cyclotella spp. were also favored, but not to the same degree. These results may of course 132 have been due to differences in feeding behavior among species. However, that is unlikely because both E. niqricans and R. exiquus qroup are capable of filter feeding (Stoffer 1978), and both showed greater preference for Melosira spp. and N. confervacea than they did for Cyclotella spp. (a planktonic form) or Cymbella spp. Finally, in addition to finding positive preferences for Melosira spp., N. confervacea, Cyclotella spp. and Cymbella spp., Chesson's index revealed a conspicuous 100% negative selection against C. placentula, the most abundant species found on Myriophyllum. C. placentula is not colonial, it is small in comparison to Cymbella, and it is not planktonic, growing strictly epiphytically. In addition to these factors, C. placentula grows relatively flat and close to its substrate. This, when contrasted to the growth form of Cymbella which is often stalked, may help explain why C. placentula was selected against, whereas Cymbella was selected. Its growth form probably makes C. placentula harder to crop because it must be pried loose from the substrate, whereas a stalked alga such as Cymbella may be more easily removed. Because of the conspicuous 100% negative selection against C. placentula, and the marked positive preference for several other species, I conclude that selective feeding was taking place among the chironomids that I investigated. If chironomids are preferential feeders as these data suggest it may be possible to determine if preference has any effect on chironomid distribution among plants. Thus, if certain plants support an abundance of preferred diatom species it is likely that the distribution of chironomids would be skewed toward those plant species. 133 Conclusion The results of this study suggest that several species of chironomidae are preferential feeders. Feeding preferences may be linked to chironomid distribution. This could have far ranging consequences because the same may be true of other macrophyte-associated invertebrates, however, further study in the area is needed in order to determine the full impact of preference as it relates to invertebrate distribution patterns. 134 Summary Investigation of the invertebrate communities associated with Vallisneria americana, Myriophyllum spicatum, and Ceratophyllum demersum in Lake Onalaska, Wisconsin, revealed that invertebrates were not distributed randomly. Differences existed among these communities, both qualitative and quantitative. In nearly every case Vallisneria supported the least number of invertebrates, whereas Ceratophyllum harbored the most. Though hypothesized that such differences are due to available surface area differences among plants, this was not substantiated by my results. The plant with the most surface area per gram dry weight, Vallisneria americana, supported the smallest invertebrate community. It may be that toxic substances found in the tissue of aquatic plants, such as alkaloids, which are found in differing concentrations among plants contribute to invertebrate community differences among plants. It may be significant that the plant with the highest concentration of alkaloids in its tissue, Vallisneria, was here found to support the fewest number of invertebrates. It is also possible that invertebrate community differences are due to differences in growth form among plants. Vallisneria leaves are broad and widely separated in comparison to those of Ceratophyllum, while Myriophyllum is intermediate. Therefore, Vallisneria may provide easier access to vertebrate predators, such as fish. If this is the case, perhaps the increased predation associated with its growth form is a factor contributing to Vallisneria's low invertebrate community. 134 135 This last hypothesis could only be true if fish were capable of affecting changes in macrophyte-associated invertebrate communities through predation. Therefore, the effects of fish predation on the Vallisneria- associated invertebrate community in Lake Onalaska was investigated through a series on enclosure/exclosure experiments. Results of these experiments suggested that fish were capable of affecting changes in macrophyte-associated invertebrate communities. In the Vallisneria community, when predators were excluded, increases in invertebrate abundances were observed. But, whereas these changes in abundance were conspicuously manifested among invertebrate predators, such as Hemiptera and Odonata, other organisms, such as chironomids, which are typically prey organisms for invertebrate predators, decreased in abundance. These data suggest that fish predation may structure aquatic macrophyte invertebrate communities by mediating the effects of invertebrate predation. Non-morphological factors may also be important in structuring macrophyte-associated invertebrate communities. Different plants may offer herbivorous invertebrates differential assemblages of periphyton to graze; however, such differences could only affect invertebrate abundance patterns if invertebrates feed selectively. Therefore, feeding preferences of chironomids were investigated. Results indicate that at least several species of chironomidae are preferential feeders. This study has provided valuable information about the factors which structure aquatic macrophyte-associated invertebrate communities. It has increased our understanding of the effects of fish predation on these communities. 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Date Site Vallisneria Myriophyllum Ceratophyllum 6/21/83 1 - 2.99 ± 0.28 — (N=8) 6/21/83 2 - 4.19 ± 0.29 - (N=6) 6/24/83 1 2.19 ± 0.43 -- (N= 10) 6/24/83 2 2.75 ± 0.35 -- (N= 10) 7/8/83 1 1.52 ± 0.28 3.59 ± 0.32 - (N= 10) (N=8) 7/8/83 2 1.80 ± 0.21 4.76 ± 0.24 - (N=9) (N=5) 7/12/83 2 - - 2.59 ± 1.15 (N=4) 7/14/83 2 - - 5.55 ±0.12 (N=7) 7/14/83 5 - - 5.46 ±0.10 (N=7) 7/25/83 2 - - 5.35 ± 0.21 (N=6) 7/25/83 5 - - 3.66 ± 0.31 (N=5) TABLE 10. Continued 7/26/83 I 2.62 ±0.41 2.64 ± 0.58 (N=8) (N=8) 7/26/83 2 1.53 ± 0.81 2.54 ± 0.52 (N=7) (N=7) 7/26/83 5 4.46 ± 1.05 (N=7) 8/9/83 I 1.48 ± 0.32 2.29 ± 0.52 (N=8) (N=7) 8/9/83 2 2.44 ± 0.70 3.27 ± 0.32 (N=8) (N=7) 8/9/83 5 2.48 ± 0.29 2.31 ± 0.40 (N=8) (N=8) 8/29/83 I 2.03 ± 0.66 (N=6) 8/29/83 2 3.10 ± 1.01 (N=7) 8/30/83 I 3.81 ± 0.33 (N=6) 8/30/83 2 3.65 ± 0.21 (N=6) 8/30/83 5 2.18 ± 0.37 (N=8) 9/1/83 I 5.91 ± 0.22 (N=7) 9/1/83 2 6.53 ±0.16 (N=2) 9/1/83 5 4.62 ± 0.39 (N=8) 9/13/83 2 2.98 ± 0.27 6.76 ± 0.09 (N=6) (N=6) 9/13/83 5 3.38 ±0.14 6.32 ±0.18 (N=8) (N=7) 157 TABLE 10. Continued 9/14/83 I 3.59 ± 0.45 4.65 ±0.17 5.98 ± 0.28 (N=7) (N=6) (N=5) 9/14/83 2 - 5.41 ±0.09 (N=6) 158 TABLE I I. Changes in invertebrate biomass (grams invertebrate dry weight per gram dry weight of plant) associated with Vallisneria, Myriophyllum, and Ceratophyllum, over time, at sites I, 2, and 5, in Lake Onalaska during summer 1983. Standard error of the mean is presented. Date Site Vallisneria Myriophyllum Ceratophyllum 6/21/83 1 “ 0.22 ± 0.07 - (N=8) 6/21/83 2 - 0.18 ± 0.03 - (N=6) 6/24/83 1 0.02 ± 0.01 -- (N= 10) 6/24/83 2 0.005 ± 0.001 - - (N= 10) 7/8/83 1 0.09 ± 0.03 0.14 ± 0.03 - (N= 10) (N=9) 7/8/83 2 0.02 ± 0.01 0.13 ± 0.01 - (N=9) (N=5) 7/12/83 2 0.03 ± 0.01 -- (N=6) 7/14/83 2 -- 0.13 ± 0.02 (N=7) 7/14/83 5 -- 0.05 ± 0.01 (N=7) 7/25/83 2 - - 0.02 ± 0.05 (N=6) 7/25/83 5 - - 0.004 ± 0.001 (N=5) 7/26/83 1 0.004 ± 0.001 0.06 ± 0.02 - (N=7) (N=7) 7/26/83 2 0.01 ± 0.002 0.12 ± 0.08 - (N=7) (N=6) 7/26/83 5 0 _ 159 TABLE 11 (Continued) 8/9/83 1 0.06 ± 0.02 0.06 ± 0.04 - (N=8) (N=8) 8/9/83 2 0.09 ± 0.02 0.14 ± 0.03 - (N=8) (N=7) 8/9/83 5 0.04 ± 0.01 0.05 ± 0.02 - (N=6) (N=8) 8/29/83 1 0.04 ± 0.02 - - (N=6) 8/29/83 2 0.13 ± 0.02 - - (N=6) 8/30/83 1 - 0.03 ± 0.01 - (N=6) 8/30/83 2 - 0.04 ± 0.01 - (N=6) 8/30/83 5 0.03 ± 0.01 - - (N=6) 9/1/83 1 - - 0.05 ± 0.01 (N=8) 9/1/83 2 - - 0.13 ± 0.07 (N=3) 9/1/83 5 - - 0.04 ± 0.02 (N=7) 9/13/83 2 0.06 ± 0.02 - 0.06 ± 0.02 (N=6) (N=7) 9/13/83 5 0.03 ± 0.01 - 0.05 ± 0.02 (N=8) (N=6) 9/14/83 1 0.06 ± 0.01 0.07 ± 0.01 0.06 ± 0.01 (N=7) (N=6) (N=5) 9/14/83 2 _ 0.06 ± 0.01 .. (N=6) Figure 23. Temperature (°C) readings at sites I, 2, and 5, from July through Septem ber, 1983 in Lake Onalaska. 160 TEMPERATURE (centigrade) 301 25 26J 0 2 24 29" 23r 28 17 18 2 2 19 27 1 2 6/21 /07/30 7/10 DATE ie 5 Site ie 1 Site ie ### Z Site 8/19 9/8 e e e e e e e * 161 Figure 24. Conductivity (umhos/centimeter) readings at sites I, 2, and 5, from July through September, 1983 in Lake Onalaska. 162 CONDUCTIVITY (um hos/centim eter) 200 300 400 600 0 0 7 100^ 500 6/21 7/10 • V •• x A DATE 7/30 8/19 ie 5 Site ie 2 Site 9/8 1 Site V 163 APPENDIX B. INFORMATION USED IN CHAPTER II 164 165 TABLE 12. Changes in abundance (natural log of the number of invertebrates per gram dry weight of plant) of Vallisneria-associated invertebrates during summer 1984 enclosure/exclosure experiments. Standard error of the mean is presented. Date Open Caqe-effect Exclosure Enclosure Control Control 7/12/84 4.44 ± 0.29 “ -- (N=6) 7/23/84 5.79 ±0.12 6.53 ±0.16 - - (N=8) (N=8) 7/24/84 -- 6.16 ± 0.11 - (N=8) 8/2/84 5.93 ±0.17 5.77 ± 0.39 5.56 ± 0.38 - (N=5) (N=6) (N=6) 8/28/84 6.25 ± 0.20 - - - (N=8) 9/7/84 6.49 ± 0.45 6.85 ± 0.49 6.15 ± 0.33 6.61 ± 0.12 (N=8) (N=5) (N=6) (N=7) 9/19/84 5.99 ± 0.21 7.64 ± 0.64 8.42 ±0.17 8.52 ± 0.34 (N=8) (N=7) (N=6) (N=7) 9/26/84 6.88 ± 0.21 --- (N=8) 10/8/84 7.16 ± 0.52 - 8.16 ± 0.06 8.26 ± 0.24 (N=7) (N=5) (N=6) 166 TABLE 13. Changes in the biomass (invertebrate dry weight per gram dry weight of plant) of the Vallisneria-associated invertebrate community during summer 1984 enclosure/exclosure experiments. Standard error of the mean is presented. Date Open Caqe-effect Exclosure Enclosure Control Control 7/12/84 1.14 ± 0.65 - -- (N=6) 7/23/84 0.27 ± 0.06 0.66 ± 0.40 - - (N=8) (N=8) 7/24/84 - - 0.38 ± 0.08 - (N=8) 8/2/84 0.15 ± 0.03 0.39 ±0.18 0.45 ± 0.25 - (N=5) (N=6) (N=6) 8/28/84 0.55 ±0.12 - -- (N=8) 9/7/84 0.40 ±0.16 0.46 ±0.10 1.22 ± 0.68 1.29 ±0.40 (N=8) (N=5) (N=6) (N=7) 9/19/84 0.42 ±0.15 1.49 ± 0.22 5.38 ± 2.37 2.92 ± 0.53 (N=8) (N=7) (N=6) (N=7) 9/26/84 0.31 ± 0.20 - - - (N=8) 10/8/84 1.65 ± 0.47 - 2.90 ± 0.63 2.38 ± 0.36 (N=7) (N=5) (N=6)