AUFw'UCHS AND BENTHIC Hl\CROINVERTEBRATE COMMUNITY
STRUCTURE ASSOCIATED h'ITH THREE SPECIES OF ROOTED
AQUATIC K~CROPHYTES IN LAKE ONALASK~, 1976-1977
A Thesis
Submitted to the Faculty
of
University of Wisconsin - La Crosse
La Crosse, Wisconsin 54601
by
Bruce Arnold Biltgen
In Partial Fulfillment of the
Requirements for the Degree
of
Master of Science in Biology
May 1981 UNIVERSITY OF WISCONSIN - LA CROSSE
La Crosse, Wisconsin 54601
COLLEGE OF ARTS, LETTERS, AND SCIENCES
Candidate: Bruce Arnold Biltaen ----~------We recommend acceptance of this thesis to the College of Arts, Letters, and Sciences in partial fulfillment of this candidate's requirements for the degree Master of Science in Biology. The candidate has completed his oral defense of the thesis. , J
~~-~~,.~ /~l---
/f'p;,-/ ._:)"")v, / / 9~/ Date
.ora; ----~~~zvn~n~-~.1~~?.~vz4~2~~~------/ I.:_. Thesis Corrm1ittee Member Date Department of Botany Iowa State University, Ames '1fo-~::,~ ----dihesis Cominittee lfember
ca~ .3~ Date
This thesis is approved for the College of Arts, Letters, and Sciences. ii
ABSTRACT
A survey of the Aunwuc.lt6 and benthic macroinvertebrate corrununities associated with vegetated and non-vegetated areas was conducted in Lake Onalaska, Navigation Pool No. 7 of the Upper Hississippi River, between 1976 and 1977. The structure and dynamics of the invertebrate community were investigated. Four sampling sites were studied; a monotypic stand of SpMgmU.u.m e.Wlyc.a!Lpwn, a monotypic stand of SagLt:ta.!Ua fa;t.[6o.ua, a stand of Nymplw..e.a tube)w,sa, and a non-vegetated control site to determine the influence of macrophytes on invertebrate populations. Samples \vere collected at weekly intervals from June until autumnal senescence of vegetation; after that time, samples were collected at two week intervals. On each sampling date, one ponar dredge sample was collected and one macrophyte in the water column \vas removed. The macroinvertebrates were subse quently harvested. Five such samples along a transect were pooled to represent one composite sample for each macrophyte stand on each sampling date. Physical and chemical variables were measured at every other s~~ple station on the transect. Taxa, numbers, and biomasses were determined for the Au6wuc.l1.-6 and benthic macroinvertebrates. Time series analyses \.rere conducted for the dominant invertebrate groups. Simple linear correlations among physical-chemical variables and biological variables were also calculated. The macroinvertebrate col".:nunity l·ms dominated (90%-93/~) by eight taxonomic groups including Oligochaeta, Hirudinea, Isopcda, Amphipoda, Gastropoda, Diptera, and Lepidoptera. Increases in benthic inverte brates in vegetated sites '"ere observed, corresponding to autumnal macrophytes senescence and in non-vegetated sites, corresponding to winte-r ice cover. Areal benthic production \-las greatest in non-vegetated sites and exceeded Au&wudlh areal production. Water temperature, pH, dissolved oxygen, and alkalinity did not effect the distribution of macroinvertebrates. Fev7 significant correlations 1;-1ere observed bet•.veen numbers of macroinvertebrates/m2 and mc::crophyte surf<1ce area or benveen the Au.t)wu.clt& and benthic cmlliuunities. ACKNOHLEDGEMENTS
The author would like to acknowledge committee members who have been instrumental in the completion of this thesis. I am forever
indebted to Dr. Thomas Claflin for serving as my major advisor and
initiating this research. His devotion to and knowledge of the
Upper Mississippi Ri'Jer are inestimable. I thank Dr. Ronald Rada
for his dedication and professional criticism in revie\oJing this manuscript and for his sincerity and thoughtfullness. Thanks are
also extended to Dr. James Peck for his critical comments on all phases of my work, Dr. Thomas Weeks, whose ceaseless energy will always be a spiritual inspiration, and Dr. John Scheidt for serving
on my committee.
My appreciation and gratitude are expressed to Hs. Beverly
Erickson, who typed the manuscript, and Ms. Gloria Wiener for typing many of the tables. Their patience and persistence have greatly
expedited the completion of this thesis.
Finally, I thank my parents for giving me encouragement and
understanding in my career and education these many years. TABLE OF CONTENTS
PAGE
LIST OF TABLES . vii
LIST OF FIGURES ix
LIST OF APPENDICES xi
INTRODUCTION . . . . 1
LITERATUP~ REVIEW 2
DESCRIPTION OF THE STUDY AREA 14
The Upper Mississippi River 14
Navigation Pool No. 7 14
Lake Onalaska 15
Hydrography 18
Water Chemistry . 19
Sedimentation . 20
Aquatic Macrophytes . . . . 22
Specific Site Descriptions . 23
23
25
~ite III - Nymphae.a tube.Jto-t.a 27
Site IV - Control 27
METHODS AND MATERIALS 29
Field Methods 30
Aquatic Macrophytes 30 v
Benthic Invertebrates . . • 30
Physical-~hemical Variables 30
Laboratory Hethods . 31
Aquatic Hacrophytes 31
Benthic Invertebrates 32
.£.!!. and Alkalinity 32
Sediments 33
RESULTS 34
Site I - SpaJLganiwn e.Myc.aJLpwn 3Lf
Site II - Sagitt~a latino!ia . . . . . 39 Site III - Nymphaea ;tube.Jto.oa 43
Site IV - Control 47
Oligo chaeta 52
Hirudinea 53 Isopoda . . . . 53 Amphipoda 53
Lepidoptera 55
Diptera 55
Gastropoda 55
Pclecypoda 58
DISCUSSION 60
Oligochaeta 61
Hirudinea 65
Isopoda 69
llmphi_poda 70
Lepidoptera 75 vi
Diptera 77
Gastropoda . 84
Pelecypoda • 87
SUM}UillY AND CONCLUSIONS 100
LITERATURE CITED 102
APPENDIX 113
Values of physical, chemical, and biological I. 2 variables and number of invertebrate organisms/m of taxonomic groups in the SpaJtgan{wn eUJtljCiVrpum stand, 1976-1977 ..•.•••.••..•. 113
II. Values of physical, chemical, and biological 2 variables and number of invertebrate organisms/m of taxonomic groups in the Sag~a f~inotia stand, 19 7 6-19 7 7 . • . • • . • • • • . • • • 117
III. Values of physical, chemical, and biological · 2 variables and number of invertebrate organisms/m of taxonomic groups in the Nymphaea t.u.befLoha stand, 1976-1977 ..•••.•...••.• 121
IV. Values of physical, chemical, and biological 2 variables and nlli~ber of invertebrate organisms/m of taxonomic groups in the control site, 1976-1977 125
V. Taxonomic list of Au.nwu.c.h.o and benthic macroinvertebrates collected from all sites, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 • • . • . • • 128 vii
LIST OF TABLES
TABLE PAGE
1. Mean concentrations and ranges of selected physical and chemical variables of sample sites, Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River, 1976-1977 . . • . . . . • . . . . . • •.••..•••••. 26
2. Mean standing crop (individuals/m2) and biomass (g/m2) of macroinvertebrates for Au£wttc.ho and benthos, Site I, SpaJLgavu.wn euJtyc.Mpwn, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River ...•..•..•...••••. 37
3. Relative abundance, frequency of occurrence, rank, and total numbers of each taxonomic group sampled in the SpCVLgavu.wn euJtyc.Mpum stand (Site I), Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River, 1976-1977 . • • • • . • . . . . • • . . • • • • . • . . 38
4. Mean standing crop (individuals/m2) and biomass (g/m2) of macroinvertebrates for Aut)wudu and benthos, Site II, Sag--lt.tcvUa faX{_£oua, Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River ...•...•. • . • . • 41
5. Relative abundance, frequency of occurrence, rank, and total numbers of each taxonomic group sampled .in the Sagaxa!L)_a tali£olia stand (Site II), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 ...... • . • . • • • . • . • • • • 42 2 6. Mean standing crop (individuals/m ) and biomass (g/m2) of macroinvertebrates for Aut)vJLtc.h-6 a~d benthos, Site III, Nymrhaea tube)WlJa, Lake Onalaska, Navigation -Pool No. 7, Upper Mississippi River. . . • . • • . . • • • • • . • • • 44
7. Relative abundance, frequency of occurrence, rank, and total numbers of each taxonomic group sampled in the Ntjmphac_a tubetwoa stand (Site III), Lake Onalaska, Navigation Pool No. 7, Upper Nississippi River, 1976-1977 ...... • • • . • . . . • . . • • . • • 46
8. Mean standing crop (individuals/m2) and biomass of macroinvertebrates, Site IV, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River. • • • • • 48 viii
9. Relative abundance, frequency of occurrence, rank, and total numbers of each taxonomic group sampled in the control site (Site IV), Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River, 1976-1977 ••••• 50
10. Number of taxa represented by AufiWuQh0 and benthic macroinvertebrates for 5 ranked taxonomic groups at each site, Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River, 1976-1977. . . •••• 51
11. Taxonomic composition of Hirudinea at each site, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. . . . • •••• 54
12. Taxonomic composition of Diptera at each site, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 ..•••..• 56
13. Taxonomic composition of Gastropoda at each site Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 •..••..• 57
14. Taxonomic composition of Pelecypoda at each site, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 ...•.•.• 59 ix
LIST OF FIGURES
FIGURE PAGE
1. Navigation Pool No. 7 of the Upper Mississippi River. Lake Onalaska lies adjacent to and east of the navigation channel . . . • . . • • • . . • . • • • • • • 16
2. Lake Onalaska showing physiographic details. Stars indicate the locations of feeder channels 1 and 2. Enclosed area between French and Bell Islands represents the approximate location of study sites • ...... • . • • . • • • . • . . . • 17
3. Location of specific study sites (Sites I - IV) between French and Bell Islands: Site I - Sp~ganium e.WLyc.~pwn, Site II - Sagi:tta..lr..Za .ta.Xi£oua, Site III - Nymphae.a tub~~o~a, Site IV - Control (non-vegetated site). Broken lines indicate the approximate macrophyte stand boundaries during 1976 ...... 24
4. Change in mean surface area of macrophytes based on planimetric determinations. Means were determined from a composite of three samples for each macrophyte stand . • . • . • . • • • . . . • . . • • . • • • • . • • 36 2 5. Number of Oligochaeta/m sediment at Spa.Jtganium site (I) and Sag~a site (II) between 1976 and 19 7 7 . . . . • . • . • . . . • • • • . • . • • 62 2 6. Number of Oligochaeta/m sediment at Nymphae.a site (III) and non-vegetated site (IV) between 1976 and 1977 . . . . . • . • . . • • • • • • • 63 2 7. Number of Oligochaeta/m macrophyte (Au£wuc.fu) at the three vegetated sites between 1976 and 1977 . • • . . • • . 64 2 8. Number of Hirudinea/m sediment at the vegetated and non-vegetated sites between 1976 and 1977 ...... 66 2 9. Number of Hirudinea/m macrophyte (Au£wuc.~) at the three vegetated sites bet\veen 1976 and 1977 .....••. 67 2 10. Number of Isopoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977 ...... •.. 71 X
2 11. Number of Isopoda/m macrophyte (Au6wueho) at the three vegetated sites between 1976 and 1977 • 72 2 12. Number of Amphipoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977 • 73 2 13. Number of Amphipoda/m macrophyte (Au6wueh6) at the three vegetated sites between 1976 and 1977 74 2 14. Number of Lepidoptera/m sediment at the three vegetated sites and number/m2 macrophyte (Aunwueho) at the Nymphae_a site (III) • . . • . • • • . • . 76 2 15. Number of Diptera/m sediment at Spanganiwn site (I) and Sag~~a site (II) between 1976 and 19 7 7 . • . • • • . . • • . . . • . • • 79 2 16. Number of Diptera/m sediment (Aunwuefu) at the Nymphae_a site (III) and the non-vegetated site site (IV) between 1976 and 1977 . • . . . . . • • • • • • . • • 80 ? 17. Number of Diptera/m- macrophyte (Au6wue{~) at the three vegetated sites between 1976 and 1977 81 2 18. Number of Gastropoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977 85 2 19. Number of Gastropoda/m macrophyte (Au&wueho) at the three vegetated sites between 1976 and 1977 • 86 2 20. Number of Pelecypoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977 .. 89 2 21. Areal production (total number of individuals/m ) from the associated sediment underlying Spanganium e_unyc.anpwn beh;reen 1976 and 1977 •...•.•. 91 2 22. Areal production (total number of individuals/m ) from the associated sediment underlying Sag~ttania latinolia between 1976 and 1977 .•....••• 92 ? 23. Areal production (total number of individuals/m-) from the associated sediment underlying Nymphae_a tube_no~a between 1976 and 1977 .•.•..•• 93 2 24. Areal production (total number of individuals/m ) from the sediment at the non-vegetated site between 1976 and 1977 ..•..•.•.••••• 94 2 25. Areal production (total number of individuals/m ) from the macrophytes at the three vegetated sites between 1976 and 1977 •..•••.••..••• 97 LIST OF APPENDICES
APPENDIX PAGE
I-A. Values of selected physical, chemical, and biological variables in the Sp~ganiwn tLmy~anpwn stand (Site I), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 ...... •.•.. 114 2 I-B. Number of invertebrate organisms/m from composite samples of macrophytes (Au.J)wudu) and associated sediments (benthos) in the Spanganiwn euJtyc.~pwn stand (Site I), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 •...... •.... 115
1-C. Number of invertebrate organisms/m2 from composite samples of macrophytes (Au6wu~h6) and associated sediments (benthos) in the Spanganiwn euny~a~pwn stand (Stte I), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. . ...•..• 116
II-A. Values of selected physical, chemical, and biological variables in the Sag~cuua tatil)oiia stand (Site II), Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River, 1976-1977...... •... 118 2 II-B. Number of invertebrate organisms/m from composite samples of macrophytes (Au6wudu) and associated sediments (benthos) in the Sag~a;Ua fatil)oiia stand (Site II), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. . . . • ...... 119 2 II-C. Number of invertebrate organisms/m from composite samples of macrophytes (Au6Wtt~ho) and associated sediments (benthos) in the Sag~a;Ua lcz..til)oua stand (Site II), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. . ..•.•.. 120
III-A. Values of selected physical, chemical, and biological variables in the Nympltaea tube~o.oa stand (Site III), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 .....•...••. 122 xii
Number of invertebrate organisms/m2 from composite samples of macrophytes (Au.&wuc.M) and associated sediments (benthos) in the Nymphae.a tubeJLMa stand (Site III), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. • .•••••• 123
Values of selected physical, chemical, and biological variables in the control site (Site IV), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. . • • • • • • 126
IV-B. Number of invertebrate organisms/m2 from composite samples of sediments in the control site (Site IV), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 . • . . • • • • • • .• 127
V. Taxonomic list of Au6wuc.h6 and benthic macroinvertebrates collected from all sites, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 .••.••••• 128 INTRODUCTION
The associations between aquatic invertebrates and macrophytes have been the subject of few quantitative studies. Difficulties in conducting such studies arise from 1) the diversity in form and habit of aquatic plants, 2) non-uniform environmental conditions in which plants must compete, 3) variability of plant stands in size, density, and distribution,
4) variability of macroinvertebrate communities in structure, function, and occurrence, and 5) continuous fluctuations by aquatic invertebrate populations because of natality, mortality, predation, and emergence.
Macrophytes are utilized by invertebrates for shelter, oviposition, maturation, and either directly or indirectly as a nutrient source.
Furthermore, macrophytes ~end to stabilize sediments through binding of roots, preventing excessive erosion of the bottom. They also recycle nutrients assimilated from the water column and sediments, to the ecosystem at senescence.
This study was conducted to collect baseline data on associations between aquatic invertebrates on macrophytes and those in sediments of a littoral area. The objectives were to:
1) survey invertebrate communities associated with three species of rooted aquatic macrophytes,
2) deterrr.ine dynamics of invertebrate populations (taxa, numbers, and biomass) on those macrophytes,
3) determine differences in dynamj_cs among the macrophyte fauna and littoral benthos in the immediate area, and
4) compare the structure and diversity of benthic communities in vegetated and non-vegetated areas. 2
LITERATURE REVIEW
More than 240 years ago, Reamur (1735) wrote an exacting paper on the habits and adaptations of the aquatic larvae of Nymphuia nymphae;ta
(Pyralididae: Nymphylinae). His work may represent one of the earliest investigations of the fauna on aquatic macrophytes. Other early works were of a descriptive and taxonomic nature, with special emphasis on the life histories of the aquatic Lepidoptera. Packard (1884) described a Pyralid moth living under the leaves of pond lilies. Hart (1895) studied the relationships of Nymphuia spp. reared from Po;tamogeton natan6 during a general survey of the insects of the Illinois River.
In Europe, Klapalek and Grunberg (1909) in their studies of the Ephem eroptera, Plecoptera, and Lepidoptera of Germany discussed sixty species of aquatic Lepidoptera from fresh waters. In addition they discovQred
19 lepidopterous insects feeding within PfvtagmUeo spp. Until the early
1900's, information concerning the American and European forms of aquo.tic invertebrates on vegetation was still quite limited.
However, since 1900, numerous significant investigations into the aquatic co~.munities associated with aquatic plants were undertaken.
Moore (1913) studied phytophagous invertebrates on Po:tamogeton. Welch
(1914a, 1914b, 1916, 1924) studied insects on the yellow waterlily in the Douglas Lake area of northern Nichigan. Patch (1915) studied the plum aphid (Phapafo-6-i.phwn nympiwea) on aquatic plants. He made migra tion tests and proved this species spends >vinter and early summer on fruit trees and migrates to water in mid or late summer. Here it passes 3
of its life cycle on several different aquatic plants including
Nuph~ spp. and Nympha~a spp. Wilson (1928) also reported this aphid as a pest of cultivated water lilies in England. Claassen (1921) in a detailed investigation of the ecological relationships of Typha insects, related 20 phytophagous and 5 parasitic species to this plant. Weiss and West (1920) and Scott (1924) studied the effects of Gatcnu~eita nympha~a~ on yellow water lilies. Frohne (1938, 1939a, 19396, 1939~) investigated the limnological role of reed and rush plants and reported
species of insects living within or upon them. He documented the life histories of several of the insects. Hoffman (1940a, 19406, 1940~) studied the limnological relations of some northern Michigan Donaciini, and found three species associated with water lilies. Scotland (1940) reported 40 species of i.nsects associated with Le.mna mino!t, a number of which depended on L~mna as their source of food. Berg (1949a, 19496,
1950a, 19506) studied insects associated with pondweeds (Polarfloge.:ton) in Michigan waters. Although not discussed in detail in the above re ports, the areas studied, species of plants collected, and general habits and relationships of the insect fauna were briefly recorded.
In Europe, the first detailed study of insects associated with vegetation was carried out by Wundsch (1923) \..1ho stressed that its role vJas becoming more appreciated by ichthyologists. Wunder (1936) was chiefly concerned with the larvae of Chironomidae, and investigated their distribution on various species of aquatic macrophytes.
Studies of animal populations in various aquatic habitats have usually included some referenc2 to the types of animals present on the vegetation (Carpenter 1927, Whitehead 1935, Moon 1936, Jones 1949, 1957), however, few quantitative surveys of the animals on plants have been attempted. Assemblages of animal-plant associations
teristic of specific habitats in the littoral zone have been discussed by Krecker and Lancaster (1933), Pennak (1940), Moffett (1943),
(1948), and Anderson and Hooper (1956).
Pearcy (1953) found no difference in the numbers or biomass of the invertebrate fauna between vegetated and barren shallow water regions of Clear Lake, Im.;ra. This finding contrasted with the work of Hohlschlag
(1950), Rawson (193Lf), Edmondson (1944), Krull (1970), and HcLachlan
(1975) who concluded that the fauna of vegetated areas was greater than that of non-vegetated areas.
Beatty and Hooper (1958) and Schneider (1965) demonstrated that definite associations exist between aquatic vegetation and benthic in- vertebrates. Keiper (1965), in an analysis of the macroscopic bottom fauna, stated that the distribution and abundance of the aquatic vegeta tion determined both the type and concentration of the bottom fauna.
Muttkowski (1918) gathered qualitative and quantitative data on macro scopic fauna of all types of ecological habitats in Lake Mendota,
Wisconsin. His paper primarily addressed the distribution of fauna by depth and bottom type.
The importance of bottom in determining benthic compcsition is well known. Various types of soft bottoms such as gravels and mud have specific faunal characteristics (Morgans 1956). Davis (1925) emphasized the importance of both living and dead organic matter to marine benthos, and Wene (1940) demonstrated that soil texture and organic matter influenced the abundance of chironomid larvae. Gerking (1957) revealed J
most invertebrates common to plant samples and plant-plus-sub strate samples, were many times more abundant within masses of plants
the substrates. The importance of macrophytes as a substrate for benthos when the sediment is unfavorable or precludes further develop ment has been observed in a number of studies. For example, in back waters of the Amazon River oxygen depletion at the sediment surface renders in uninhabitable and the benthic fauna is almost exclusively associated with floating mats of Pa~palidium spp. and other macrophytes
(Marlier 1967, Fittkau 1971, Junk 1973).
The effects of aquatic vegetation in impoundments is well known.
Welch (1952) investigated the broader aspects of the problem, while a number of workers, including Klugh (1926), Moore (1913)· and Irwin (1945) considered the increase in productivity resulting from the presence of aquatic macrophytes. Schiemenz (1927) also indicated the importance of pondweeds as being indicative of productive waters. Baker (1918),
Muttkowski (1918), Moore (1913), Lundbeck (1927), Rawson (1930), and
Ball (1948) all stressed the importance of vegetation in the production of fish food organisms. Reidenhour (1958) observed that in the years when aquatic vegetation at Clear Lake, Iowa, was abundant, the growth of that year class of yello\v bass, Roc.c.u.J.> rn..U,~,Lo~;.ppien6L6, vms usually better. Seasonal trends in the abundance of littoral benthos of shallow eutrophic lakes have been described by Alm (1922), Lundbeck
(1926), Ball (1948), Eggleton (1952), and Hunt and Jones (1972). How ever, it has rarely been possible to interpret fluctuations of the littoral fauna biomass in terms of the population biology of constituent species (Borutzky 1939). 0
The fauna of aquatic vegetation has been studied during the early stages of the man-made lakes Kariba (McLachlan 1969) and Volta (Petr
1 9) as well as natural situations such as the Amazon River basin and
Chad in North Africa (Dejoux and Saint-Jean 1972). These studies indicated that many artificial and natural waters are subject to sub stantial annual fluctuations in water level due to flood control mea sures and the nature of the cliTiate. Consequently, there is an important distinction to be made between emergent and floating plants.
A drop in water level strands the organisms on emergent plants such as drowned trees or Typha stems (McLachlan 1970a, Petr 1970), whereas floating species adapt to these fluctuations. As a result, their habitat remains relatively stable (Fittkau 1971). Stube (1958) found that an increase in water level '"hich eliminated rooted vegetation also eliminated Acarina, Amphipoda, Coleoptera, Ephermeroptera, Neuroptera,
Plecoptera, and most of the Trichoptera. Floating debris from dead plants, both terrestrial and aquatic, is a common habitat in the Amazon
River system and supports a fauna similar to floating aquatic vegetation.
Both McGaha (1952) and Berg (1949a) concluded that some aquatic insects are restricted on one or a few closely related species of plants with floating or emergent parts, whereas insects on submerged macrophytes show almost no specificity.
There have been few quantitative studies of invertebrates on vegetation in streams. Richardson (1921) examined the upper nine inches of aquatic plants from the middle and lower Illinois River and counted invertebrates which were washed off as the plants were shaken in water. Streams surveys by the New York State Conservation Commission 7
the productivity of various streambeds compared to plant
(Needham 1928, 1929, Pate 1932, Nevin and Townes 1935). Although
stands of vegetation were designated, no distinction was made between invertebrates on vegetation and those in the underlying sediment, and populations were not determined on a plant by plant basis. However,
Needham's (1928) study of New York streams showed that plant stands were
times as productive in trout food as bare pool bottoms and 7 times as productive as bare stream beds. Jones (1949) studied invertebrates from the Rheidol River, Wales, and noted differences in the composition of invertebrates living among different species of aquatic plants. He recorded the distribution, habitats, and degree of abundance of various invertebrates.
Most stream studies describe invertebrate associations based on ecological divisions of the streambed. Carpenter (1927) used the designation 'phytophilous association' to refer to three classes of plants as important to invertebrates in running water: Bryophyta, algae, and phanerogams. She noted invertebrates associated with each class based on their nutritional role and physiological adaptations.
Percival and vfuitehead (1929) quantitatively studied several streams in
West Riding of Yorkshire, England. They listed the dominant macro invertebrate groups, food habits, and physical environmental variables which accounted for their occurrer1ce. Whitehead (1935) studied invertebrates of a chalk stream in Great Driffield, Yorkshire. He studied seasonal changes of invertebrates and observed populations decreasing in stoney regions and increasing in vegetation during autumn.
The numbers of insect larvae diminished rapidly in autumn with the 8
ority of insects over-wlntering as very small larvae or as eggs, and non-insect invertebrates increasing rapidly in autumn.
Harrod (1964) quantitatively studied distributions and abundance
invertebrates on four species of aquatic plants in a small chalk stream. Her observations were based on the surface area of plants and 2 she related numbers of invertebrates to 1 m of plant surface area·. She concluded that differences in invertebrate populations among plants were correlated with morphology of the plant and not to environn1ental con- ditions in which they grew. For example, Canex spp. provided a substrate, but no protection and the few organisms present had to withstand direct effects of current. However, CaJ.LU:!Uc..he spp. and VeJtonic.a bec.c..abu.nga gre'" in dense clumps and consequently many invertebrate groups were present.
There is evidence from studies on vegetation of ponds and lakes that specific plants shelter specific animals. Krecker (1939), working on the animal populations of submerged plants in Lake Erie, concluded there were some degree of correlation between the morphological features of a plant and the density of its macroinvertebrate population. Andrews and Hasler (1943) and Entz (1947) arrived at similar conclusions after studying summer populations of macroscopic animals on the aquatic plants in Lalr:e Hen do ta, \\Tis cons in and in Lake Balaton, Hungary, respectively.
Important European literature on comp~rable marine communities was revieHed by Wieser (1951). Edmondson (1940, 1944, 1946) discovered that many species of sessile rotifers were limited in their occurrence and distribution to the surfaces of certain aquatic plants. Of particular interest was his study of populations of sessile rotifers on U.tJL-Lc.ula.!tta 9
vul9~~. He calculated the surface area of plants as a basis for comparison of standing population and correlated rates of reproduction, migration, and growth of U. vulg~ with growth of the plants. Later
Bryden (1952) observed a species of Hydna occurring on some plant surfaces and not on others.
Explanations for the occurrence of invertebrates on aquatic plants usually center on their nutritional role in the aquatic environment.
Rosine (1955) cited the growth of periphyton on aquatic macrophytes as conferring an ecological advantage for invertebrates that utilize them as a substrate. Percival and ~Tbitehead (1929) and Whitehead (1935), in their studies of English streams, concluded that conditions which favored the growth of unicellular and filamentous algae would also enhance the development of an invertebrate fauna. They observed that those organ isms ,.,hich formed the greatest portion of the fauna derived their nutrition chiefly from the algae, especially diatoms and desmids. The number of species existing on other material was small. Patrick (1976) has noted that diatoms and other unicellular green algae are a more valuable food source to invertebrates than some filamentous green algae.
She believed that the shapes of diatoms and ease with which they can be ingested may be one reason for their selectivity b~ invertebrates.
In few cases has periphyton and macroinvertebrates on plants been studied concomitantly. In studies that have been completed, there is an observable relationship between the presence of periphyton and numbers of the invertebrate fauna on macrophytes. Entz (1947), studying the Aunwu.ch-6 of Potamogeton spp., noted 80% of the organisms fed on detritus and 12% fed on diatoms. Smirnov (1958) studied the consumption 10
moss in bogs by invertebrates. He observed that the substrates with the greatest quantities of diatoms and dead plants were one of the main
sources in a reed swamp. Living macrophytes were consumed very
He noted that gastropods, oligochaetes, chironomids and the larvae of other insects consumed these food sources. Harrod (1964) noted the abundant surface film of green algae, diatoms, sessile rbtifers,
ciliates to explain the presence of a diverse invertebrate popula tion of 3 species of plants in a stream. However, she observed little periphyton on C~ex and few invertebrates were found there. Bownik
(1970) studied periphyton on 4 species of submerged macrophytes in
Mikolajskie Lake. He observed that the dominant invertebrate groups of the periphyton were nematodes, chironomids, and oligochaetes. He further noted that algae were the dominant periphyton and that diatoms dominated the algae.
The chemical nature of the plant may also make it a favorable or unfavorable. habitat for organisms. :For example, an apparent biochemical influence of some aquatic plants on the emergence of mosquito larvae was suggested by Abdel-Malek (1948), and Edmondson (1944) thought the sessile rotifer, Cai.fo:thec.a., avoided the plant Ch~a. because of a vola tile compound produced by the ~lga.
A limited amount of data is available on relations of certain vertebrate animals to aquatic flowering plants. McAtee (1939), Martin and Uhler (1939), and Bellrose (1941) have shmm the importance of these plants as food for waterfowl. The importance of aquatic invertebrates as preferred waterfowl foods during the breeding season has been established only recently (Chura 1961, Perret 1962, Collias and Collias 1963, Bartonek 11
Hickey 1969, Dirschl 1969, McKnight and How 1969, Swanson and Nelson
70). Wetland areas with large invertebrate food supplies are used heavily by breeding waterfowl. For example, wood duck (Aix ~ponoa) production in two Mississippi refuges was related to invertebrate pro duction (Arner eta£.. 1970).
Other marsh nesting birds are also affected by the production of invertebrate foods. Orians (1966) found that yellow-headed blackbirds
(Xan:thoc.ephafuo xan:thoc.ephafuo) nesting in marshes in British Columbia had greater nesting success than those in adjacent but less productive marshes. Voigts (1973) observed that diets of both yellow-headed and redwinged blackbirds (Age.£.aiuo phoeJuc.e..LV..) changed from mostly aquatic insects during seasonal emergence peaks of odonates to an exte~sive use of terrestrial insects when odonate emergence rate declined at Rush Lake,
Iowa. In his study of vertebrate populations and vegetative cover in several Imv-a marshes, the greatest numbers of invertebrates occurred where beds of submerged vegetation were interspersed with stands of emergent vegetation. Weller and Spatcher (1965) found that marshes characterized by emergent vegetation interspersed with open water (called hemi-marsh) attracted more species and larger numbers of invertebrates.
In studies of invertebrates inhabiting macrophytes, plants rarely have been substituted by experiemental substrates under natural conditions.
Recently, Macan and Kitching (1972) used plastic experimental substrates to analyze several regularities in the occurrence of invertebrates in the littoral zone. They observed the composition of invertebrates is similar on both substrates and the numbers of invertebrates on experi mental substrates is frequently higher. Soszka (1975) used plastic and 12
experimental substrates to analyze changes in numbers and the biomass of invertebrates in fish ponds with water having various tem peratures. The number of fauna on both experimental substrates exceeded the numbers of invertebrates on Pofygonum spp. growing in the vicinity.
The associations of invertebrates and aquatic plants often result in partial damage or destruction to the plant, particularly among forms which mine within leaves and petioles. According to McGaha (1952), intensive mining of macrophytes by dipteran larva resulted in leaf abscission at the slightest movement within the water. Hesenberg-Lund
(1943) observed ponds and found that Trichoptera larvae almost completely destroy the leaves of Potamog~on natan6 in autumn. Urban (1975), analyzing the effect of mining fauna on submerged plants in Mikolajskie
Lake, Poland, found that the mining of invertebrates damage stems more than they damage leaves. Both Urban and Soszka (1975) conclude, however, that the organisms living on the plant surfaces damage the macrophytes more than the activities of those that mine because of their preferen tial feeding on leaves. Smirnov (1961) stated that daily losses may reach 7% of the total leaf biomass. Soszka (1975) estimated that at the end of a year, 40%-50% of the leaf surface area was lost in Potamog~on spp.
The destruction of macrophytes does not result in an immediate decrease in invertebrate numbers. In fact, an increase has often been observed. This phenomenon has been recorded by McLachlan (197 Oa) and
Petr (1970) in drmmed woodland, by Hartland-Rowe (1958) in dead papyrus stems, and McLachlcm (1975) on dead Typha leaves in Lake Chilwa.
McLachlan (1974) suggested that once dead, the plant is more readily 13
trated by mining organisms, providing for an effective increase in habitable space. In addition, he cited the observation that dead macrophyte material floats, at least initially, thereby conferring the advantages of floating vegetation. 14
DESCRIPTION OF THE STUDY AREA
The Upper Mississippi River
The Upper Mississippi River is defined as the reach of Mississippi
River from its origin in Lake Itasca, Minnesota, to its confluence·
2,198 km downriver with the mouth of the Ohio River at Cairo, Illinois.
Congressional approval of the Rivers and Harbors Act of 1930 authorized the U. S. Army Corps of Engineers to provide for a 9-ft deep, 300-ft wide navigation channel on the Upper Mississippi River. This channeliza tion was achieved during the 1930's by the construction of a system of
locks and dams to regulate flow of the river and by supplemental dredging to maintain the channel. Each reach was developed by open river training works such as contraction dikes, revetments, and levees. With the addition of spillways, dike works, and a combination of roller and tainter gates on dams, it is possible to maintain uniform flow in times of high, normal, and low discharges.
Closure of the dams created a series of impoundments from Minneapolis to the Middle Mississippi River. Consequently, it is possible for 9-ft draft commercial vessels to navigate the river between higher northern impoundments and St. Louis. Each impoundment is termed a navigation pool and is numbered according to v.>hich dam impounds its waters.
Navigation Pool No. 7
Navigation Pool No. 7 is impounded by Lock and Dam No. 7 at
Dresbach, Hinnesota, 702.5 river miles above the mouth of the Ohio River.
It supports an 8-ft reservoir head at flat pool conditions that extends 15
km upstream to Lock and Dam No. 6 at Trempealeau, Wisconsin. Naviga tion Pool No. 7 contains approximately 5,443 ha of open water.
The closure of Lock and Dam No. 7 in 1937 resulted in the formation
three general habitat types within the pool. The northern one-third
the pool contains the tailwaters of Lock and Dam No. 6 (Fig. 1 ).
This area consists of channels that carry water from and parallel to the main channel. This area was least affected hydrologically by dam closure and can be considered to approximate conditions that existed prior to closure. The middle one-third of the pool is a transition zone. Water levels in this area have displaced the original lateral channels. This area consists of numerous small potholes and flooded marshes which are isolated from the main channel and from a fresh water supply. The southern one-third of the pool consists primarily of Lake
Onalaska. The greatest effect of dam closure occurred in this lowest reach ,.;rhere large stands of timber and meadow were completely inundated.
The emergent and submergent stump fields are a result of timbering just prior to dam closure.
Lake Onalaska
Lake Onalaska is bounded on its southern end by an earthern dike extending from Lock and Dam No. 7 across Island 98 and French Slough until it meets high ground on the west side of French Island (Fig. 2 ).
A fixed concrete spillway is located in this section of the dike which supplies water to Round Lake. Another section of dike extends eastward from the east shore of French Island and connects with a submersible dam section with a culvert spillway (Onalaska Dam and Spillway). This structure extends across the main channel of the Black River, three miles Fig. 1. Navigation Pool No. 7 of the Upper Mississippi River. Lake Onalaska lies adjacent to and east of the navigation channel. 16
;! 0 i
...... 0 0 D.. z 0 -~ "-~ z Fig. 2. Lake Onalaska showing physiographic details. Stars-indicate the locations of feeder channels 1 and 2. Enclosed area between French and Bell Islands represents the approximate location of study sites. 17
o·0 ~
. z iii z 0 (.) (/) 0 §;
,,o'-··
g (/) UJ z z :E 18 above its confluence with the Mississippi River.
The eastern side of Lake Onalaska is bordered by the Wisconsin floodplain. The majority of containment on this side is profiled by the 7-9 m (25-30 ft) high Brice Prairie Terrace. The northern portion of Onalaska is limited by the Black River delta and inundated
Mississippi River floodplain. The western side of the lake is bounded on the riverine side by a series of barrier islands that separate it from the main channel (Fig. 2 ).
Hydrography
Water enters Lake Onalaska at nine locations along the barrier islands, primarily through feeder channels 1 and 2 (Fig. 2 ). The lake receives water from the Mississippi River on the western and northern sides, and the Black River on the eastern side.
Yearly average discharge of water from the Mississippi River and 2 3 Black River into Lake Onalaska is approximately 1.4 X 10 to 2.3 X 10 3 -1 m sec Overtopping of channel and barrier islands occur when the discharge rate exceeds the higher figure; this generally happens when 3 3 1 the rate is greater than 2.8 X 10 m sec- • During periods of moderate 3 3 -1 flow (>1 X 10 m sec ), 90% of inflowing water enters through feeder channel 1. During periods of low flow, approximately 20% to 30% of the main channel discharge enters through feeder channels 1 and 2. The Black
River discharge into Lake Onalaska is probably less than 25% of the
Mississippi River inflow.
The lake proper consists of approximately 2,200 ha of open water at normal pool operating level of 639 feet above mean sea level (msl).
The basin shmvs little topographic relief \vith a mean depth (Z) of 1. 8 m. 19
A small, elongated island (Red Oak Ridge) in the lower part of the pool is located midway between French Island, Wisconsin, and the chain of barrier islands that form the western demarcation of the lake. Rosebud
Island is located north of and midway between French Island and the
Brice Prairie Terrace (Fig. 2 ).
Three depressions occur in the lower part of the lake. A 6 m deep borrow-pit adjacent to the southeastern side of Bell Island is the result of dredging for fill material during construction of the La Crosse
Municipal Airport. A broken trough (ca. 4 m) exists from east to west upstream from the dike as a result of material dredging used in construction of the dike works. The third depression at the lower opening of the lake (ca. 12 m) is the result of scour in an area where the majority of lake water returns to the navigation channel.
There is little detectable current in the middle and lower parts of the lake. Claflin (1974) released ill1odamine B into several small feeder channels in the upper portions of the lake. In his study, the
travelled only short distances within the pool. Another study by
CNFRL (1978) obtained comparable results.
Wate~ Chemistry
Only limited water chemistry data are available for Lake Onalaska, but studies by Claflin (1974, 1977) and the CNFRL (1978) provide some information. Nitrogen and phosphate levels increase in the autumn due to the decay of aquatic vegetation (Smart 1977, Strodthoff 1978).
Claflin (1977) estimated there is a net autumnal increase in the lake system of apprmdmately 5,000 kg and 1,300 kg of nitrogen and phosphorus, respectively. The fate of these nutrients is unknmm, however, it can 20 be speculated that they are reincoporated into existing systems within
lake or they are made available to downstream pools. With the long turnover time for the more isolated portions of the pool, it is highly probable that the bulk of these nutrients return to local sediments.
The turbidity of the water is low during the majority of the year, ranging from 0- 18 JTU (Claflin 1977). The Black River derives much of its water from boggy areas of the lower portions of the geological
Lake Wisconsin basin. This area is rich in soluble refractory organic material. These materials are introduced to Lake Onalaska by the
Mississippi and Black Rivers, imparting a characteristic brown color to the water. The average annual color values are approximately 35 PtCo units (Claflin 1977).
Temperatures fluctuate between 0 - 0.4°C in winter to 24 - 30°C in summer. Dissolved oxygen concentrations vary between 0 - 13 mg/L in vegetation stands, but remain high in the open areas of the lake (8.0 -
11.9 mg/L). The pH of the lake remains relatively constant throughout the year with an annual average between 9.42 and 8.1 (CNFRL 1978).
This compares with that observed for the M:ississippi where it varies from 7.5- 7.8 (Claflin 1977). Specific conductance within the lake is similar to that of the main channel. It is also relatively constant throughout the year, with a yearly average of approximately 300 umhos/cm.
The total alkalinity of Lake Onalaska has an annual average of approxi mately 125 mg/L as Caco , compared with that of the navigation channel 3 where it exhibits an average of 175 mg/L (Claflin 1977).
Sedimentation
Lake Onalaska has decreased in depth due to sedimentation since its 21 creation in 1937. Decreases rangefrom 20% in the lower part of the lake to 50% in the upstream part. Decreased current velocity accounts for much of the actual deposition in the lake from natural sedimentation processes. Mississippi and Black River bedloads are transported through active feeder channels. Particulate material deposited in upper regions of the lake remains for long periods of time, whereas in lower regions, particulate matter is resuspended and redeposited by two physical processes:
1) Traveling surface waves generated by the predominantly north west winds tend to resuspend material \vithin the shallo-v1 basin. The general configuration of the lake lies in a northwest to southeast direction, providing a 4-mi fetch. The suspended sediments are transported toward the southeast areas of the lake where deposition occurs.
2) Scour activity occurring during times of peak discharge can transport large amounts of material into the center of the lake.
It has been estimated that approximately 80% of the total suspended sediment load carried by the Mississippi River occurs during peak discharge (USACE 1974). During such times overtopping of the land forms occurs in the upper parts of the lake, transporting large volumes of suspended and bed loads in these areas. The trapping efficiency for the lake is between 55% and 60% of the suspended load and 100% of the bedload
(Claflin 1977).
Maintenance dredging operations have also probably increased sedimentation rates \vi thin Lake Onalaska. Dredge spoil from the navigation channel has been deposited on barrier islartds upstream from feeder channels 1 and 2 (USACE 1974). Feeder channel 1 is one of fe>v actj_ve channels supplying water into the lake. Material deposited at 3 -1 feeder channel 2 averaged approximately 15,000 yd yr from 1948 - 1972 22
1974), and is possibly transported directly into the upper reaches
the lake during high discharges.
Sediment types within the lake range from medium sand to hydrated
organic muck. Sediments in the open pool are dominated by silt and clay, whereas the adjacent navigation channel area consists of fine to medium
sand. Suspended sediment in the Mississippi ranges from 0% to lC~ sand
and with 10% to 20% more silt than clay. The suspended load in Black
River is 4% sand with equal amounts of silt and clay (48% each). During periods of high discharge, the percentage of sand in-the suspended load increases significantly, reaching 50% in the Mississippi River and more than 60% in the Black River.
The majority of the upper and middle parts of the lake have a firm sandy bottom. Clay size particulate material in this area is constantly resuspended by wave action and carried to the wind protected areas of the lake where deposition occurs. Organic matter is also present in the sediments and is usually superimposed over a layer of medium sand. The organic layers range from 1.5 ern to 0.9 rn in depth in the extremely eutrophic portions of the lake.
Aquatic Macrophytes
Formation of the pool created new habitat which facilitated the growth of aquatic vegetation. Stands of rooted aquatic macrophytes within the lake are diverse in species and their biomass exhibits great variation. Generally, the highest standing crops occur where they have been established for the longest period of time. High standing crops within these areas reflect two processes that are continuing in the lake: 23
1) autochthanous production by the plants have added significant inputs of nutrients (Smart 1977, Strodthoff 1978), and
2) sedimentation has aided the encroachment of macrophytes into new areas previously too deep to support growth.
Vegetation stands associated with Rosebud and Bell Islands have been established for at least 20 years. Accumulated organic material in these areas has enchanced the growth of extremely dense growths of macrophytes.
Specific Site Descriptions
The present study was conducted in well established vegetation stands of the Bell Island region (Fig. 3 ). The area supported a diverse littoral aquatic macrophyte community. There was little detectable current, and it was well protected from the mechanical action of wind and waves. The following three criteria were used to establish this area for study:
1) A predetermined sampling procedure could be applied to the various stands of vegetation. Each stand possessed sufficient spatial dimensions to be sampled using parallel transects.
2) Since the area was not used for recreational activities, human impact with negligible. Therefore, prop-wash effects would be minimal on Au6wu.du and benthic communities in the extremely shallm..r water stands. Since most of the available water column was occupied by vegetation, this influence was negligible. Feral animals were also not observed. Muskrats were of primary concern due to their adverse harvesting activities on arrowhead (CurUs 1959).
3) The macrophyte community composition was homogeneous, often monotypic, for the species of interest. The species sele~ted for the study showed minimal intergrading between species, thus facilitating delineation of the sample areas.
Site I was a stand of SpaJLga.nium eu.Jtyc..a..lzpwn, the common burreed. Fig. 3. Location of specific study sites (Sites I - IV) between French and Bell Islands: Site I - SpMgavu.wn eu/tC:fc.Mpwn, Site II - Sag~ fcdinolia, Site III - Nymphaea tubeAo~a. Site IV - Control (non-vegetated site). Broken lines indicate the approximate macrophyte stand boundaries during 1976. 24
IV
'\ ' \ \ \ l I \ I I \ I ) I ( I \ I I I I I I I I I I 25
Although it abutted high ground on French Island on the west, no
riparian plant taxa were observed within this site (Fig. 3 ). Other
shallow water species were prevalent only on the north and south edges
on the site; the eastern border intergraded with Sag~ lati6olia. 2 - Site I had an area of approximately 158m of shallow water (z = 0.11 m).
The sediments were composed of 93% sand (primarily fine), 0.4% silt
(variable), 5% clay, and approximately 2% organic material.
The pH of the water was relatively constant throughout the year with an annual average of 7.22 (Table 1 ). Total alkalinity also remained
stable, averaging 118 mg/L as CaC0 . The water temperature exhibited 3 seasonal fluctuations ranging from 29°C in July to 4°C in November.
Dissolved oxygen concentrations ranged from 1.2 mg/L to 7.8 mg/L.
Site II contained a homogeneous, monotypic stand of Sag~ la.;tt6olia (arrowhead) that bordered Site I on the landHard edge. Nymphae.a
tub~o~a formed a contiguous border along the eastern and southern edges
of this stand and Netwnbo p~ntapctula predominated along the northern 2 side (Fig. 3 ). Site II was slightly larger (260m ) with a mean depth
of 0.29 m. The sediment was 86% sand with fine sand comprising 70% of the
fraction. Clay and silt fractions were 12% and 2%-, respectively, and the
organic content was 8%.
The pH of the water averaged 7.45 on an annual basis (Table 1)
and water temperature varied between 4°C and 28°C. Dissolved oxygen
concentrations ranged from 1.2 mg/L to 8.6 mg/L. Alkalinity remained
constant at about 123 mg/L as CaC0 . 3 26
Table 1. Mean concentrations and ranges of selected physical and chemical variables of sample sites, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977.
Dissolved Alkalinity Temperature Depth oxygen (mg/1 as (OC) (m) pH (mg/1) CaC0 ) Site 3
I Mean 17.43 0.11 7.22 118.11 Range 4-29 .04-.20 6. 77-7.52 1.2-7. 8 58.24-164.32
II Mean 20.48 0.24 7.45 121.61 Range 4-28 .09-.35 6. 71-8.75 1.2-8.0 80.99-154.27
III Mean 20.39 0.51 7.99 122.22 Range 3-29 .20-.75 6.98-9.54 0.8-14.8 96.20-147.24
IV Mean 19.70 0.95 8.80 126.05 Range 2-28 . 59-1.27 8.03-9.26 3.6-14.2 102.96-156.24 27
~II - Nymphaea tub~o~a
Site III was composed primarily of Nymphaea tub~Ma, the white
water lily. This stand began at the landward of Site II and extended
approximately 41 m to vegetation stands ~ssociated with a small island
to the east. The north and south margins of this site were not
determined, but they extended in both directions for several hundred
meters. This site had the deepest water of three vegetation stands
studied, with an average depth of 0.5 m (Table 1 ). Sand dominated the
sediments (74%) with fine sand comprising 56% of the fraction. Silt
and clay composed 9% and 18%, respectively, and the organic content of
the sediment averaged 12%.
Water temperature fluctuated bet>-wen 3°C and 29°C during the
sampling period. Dissolved oxygen levels exhibited wide variations
that ranged from 0.8 mg/L to 14.8 mg/L. The pH remained relatively
constant with a yearly average of 7.99. Total alkalinity was also
constant with a mean concentration of 123 mg/L as Caco . 3
Site IV - Control
The non-vegetated (control) site was located north of the macrophyte
community (Fig. 3 ). This area lacked macrophytes during the entire
sampling period. Water depth averaged 0. 95 m (Table 1 ) . The dominant
sediment type at this site was sand, comprising 53% of the fraction,
and almost 85% of this portion was composed of fine sand. Silt and
clay were about 27% and 16%, respectively. Organic content of the
sediment averaged 0.8%.
Water temperature within thE: control site varied from 2°c in winter to 28°C in spring during the sampling period. The pH was 28
constant at about 8.8 and alkalinity also remained constant with a mean yearly concentration of 126 rng/L as CaC0 . Dissolved oxygen 3 concentrations fluctuated between 4.4 rng/L and 14.4 rng/1. 29
METHODS AND MATERIALS
Sampling was initiated on 16 June 1976, and continued through
9 July 1977 at each site. One transect paralleling the shore was established through each vegetation stand during each sampling visit; a modification of the transect method was used (Cu~~ins 1962). Five sampling stations were located at 10-m intervals along each transect.
The resulting five samples were pooled to represent one composite sample for each of three vegetation sites and one control site. In each case, one floating or emergent plant sample and one sediment sample was collected at each station.
Samples were collected at weekly intervals until autumnal senescence of vegetation. After that time and proceeding through the period of ice cover, the sampling interval was lengthened to two weeks and only bottom samples were collected.
Frost penetrated for a considerable distance into the hydrated sediments due to the shallow depths in the vegetation .stands. Therefore, only samples from the deeper control site were taken. Samples were col lected at 4-week intervals in the control area during winter.
The original weekly sampling schedule was resumed in June 1977 when Spa!Lganiwn, Sagi-ttafua, and Nymphae-a. exhibited aerial emergent or floating parts. 30
Field Methods
~uatic Macrophytes
The following procedure was employed in sampling vegetation. At each station, one plant was randomly selected for removal. The emergent portions of Sp~ganium and Sag~a were first severed at the air water interface with shears, and the portion of plant within the water column was severed at the sediment-water interface. Severed portions were placed in l~quart freezer containers, covered, and taken to the laboratory for further analyses. Samples of Nymphae_a were collected by severing the leaf petiole at the sediment-water interface. The severed petiole and attached leaf were placed in a container and returned to the laboratory.
Benthic Invertebrates
Bottom samples were simultaneously collected within a 5-crn radius of each plant using a 6-in (15.24 ern) Ponar dredge (Wildlife Supply
Company, Saginau, Michigan). Samples were field->vashed through a
U. S. Standard #30 (0.600 w~ openings) seive and placed in 1-qt freezer containers. Samples were preserved in a solution of 70% ethanol and 5% glycerol with a 1:1000 dilution of Rose Bengal (~fuson and Yevich 1967).
Physical-Chemical Variables
Water depth and temperature were measured at every third sampling station along each transect. Two discrete water samples were also collecteJ in acid washed bottles. Prior to collection, each bottle was rinsed with a small aliquot of water from the corresponding sampling 31
station.
The azide modification of the Winkler method (API~ 1976) was
used in the determination of dissolved oxygen content of the water.
Samples were fixed in the field, kept in the dark, and returned to the
laboratory for further analysis.
Water temperature determinations were made using a hand held -10°
to 100 0 C mercury thermometer. All readings were made at the middle of
the water column.
Water depths were determined using a standard meter stick. Values were recorded to the nearest 0.1 m.
An additional sediment sample was collected from each of the
four sampling areas at the onset and at the conclusion of the annual
sampling regime. These samples were frozen until preparation for
sediment particle size analysis.
Laboratory Methods
Aquatic Macrophytes
A tooth brush was used to remove all organisms from severed
portions of plants. Plant portions were then rinsed twice and finally
re-examined carefully with a 2X hand-held lens to assure the harvest of all macroinvertebrates. The organisms were placed in shell vials, and were preserved in a solution of 70% ethanol and 5% glycerol.
Plant surface area measurements were made with a metric ruler.
In each case, solid or plane geometry figures were approximated as
representative of the specific macrophyte portion under consideration.
In the case of Numphae_a, the petiole was assumed to approximate the area of a frustum of a right regular cone, and leaf a circle. With 32
Sag~a, the inside and outside area of the petiole approximated the
area of a frustum of a right regular cone. The inside and outside area
of the leaf petiole of Sp~ganlwn approximated the area of a right
regular prism. Therefore, planimetric determinations of surface area
could be made and the geometric figures calculated.
Benthic Invertebrates
Samples were washed in the laboratory through a U. S. Standard
#30 sieve and hand sorted from the residual detritus. All samples were preserved in a solution of 70% ethanol and 5% glycerol. Identifi- cation and enumeration of all organisms were determined to the generic
level in most cases and specific level when possible. The following keys and publications were used in the identification of macroinverte- brates: Hilsenhoff (1975), Johannsen (1937), Klemm (1972), Mason (1973),
Pennak (1978), Usinger (1956), Ward and \fuipple (1975), and Higgins
(1977). All macroinvertebrates were retained as voucher specimens.
Wet weights were used for biomass determinations for each sample.
Organisms were blotted dry on filter paper, placed on tared weighing paper, allowed to air-dry for two minutes, and then weighed to the -3 nearest 10 g on an analytical balance (Mettler Model H20T). The number of taxa per site, number of organisms per site, and number of organisms per taxon were recorded for each sample .
.£!! and Alkalinity
The pH was determined using a digital pH meter (Orion Model 701) and a combination electrode (Corning Hodel 91-02). Total alkalinity measurements, expressed in me/1, were made by potentiometric titration samples with 0.02 N H (APHA 1973). 2so4
Sediments
Sediment samples were tha~.Jed and dried at 106°C for 24 hr (APHA
1973) and triturated in a porcelain mortar with a rubber pestle. Two
duplicate 50.0 g aliquots of each sample >vere used for particle size
analyses. A sample was placed in a nest of U. S. Standard brass testing
sieves (numbers 18, 35, 60, 120, and 230) and placed on a horizontal
replicating shaker for 30 minutes. Data were reported as percent total
of each particle size.
A hydrometric analysis by the Bouyoucos hydrometer method was also
conducted (Foth, Jacobs, and Wither 1971). A 100-ml aliquot of a 5%
polyphosphate solution (Calgon @) \vas added to duplicate oven-dried
50.0 g samples. This solution was then transferred to a Bouyoucos
cylinder and water was added to bring the final volume to 1 L. Disper-
sion of sediment Has accomplished by mechanical stirring for 2 min.
Corrected hydrometer readings Here then taken at 0.5-, 30-, and 120-
min intervals, and the resulting reading Here used to calculate
percentages of sand, silt, and clay (Day 1965).
Organic content Has determined by a modification of n o oxidation 2 2 process (Jackson 1962). A 2-g sediment sample Has ·placed in a tared
125-ml erlenmeyer flask and placed in a drying oven (110 0 C) for 12 hr.
The flask Has removed, placed in a desiccator to cool to room tempera-
ture, and reHeighed. Reagent grade 30% H o Has added to the sample 2 2 and digestion continued until all peroxide was decomposed. Oxidation was considered complete Hhen further additions of H o did not produce 2 2 rapid frothing of the sample. Final weights were determined after oven
drying for 12 hr. 34
RESULTS
A total of 785 dredge and plant samples were collected for this study. The number of individuals/taxa, total number of taxa/sample, and total wet-weight/sample were recorded. Surface areas of stems and leaves of each macrophyte were also determined. During this investiga tion, 93,800 macroinvertebrates were enumerated and identified from the study area. Platyhelminths, nematodes, annelids, arthropods, and molluscs representing 131 taxa were collected (Appendix V). Their presence as AunWuQho or benthos was also noted.
Macroinvertebrates were subdivided into 17 taxonomic groups, and standing crops were determined for each macrophyte stand on each sampling date (Appendices I-A through IV-B). Identifications were made at different taxonomic levels for the various groups of organisms.
Class level was maintained for the Turbellaria as the only representative of Platyhelminthes. Phyletic level was maintained for nematodes.
Annelids were divided into 2 class levels, Oligochaeta and Hirudinea.
Arthropod groups were divided at ordinal levels: Amphipoda, Hydracarina
(Acarina), and Isopoda. The insects ,.,ere grouped into 8 ordinal levels:
Coleoptera, Diptera, Ephemeroptera, Hemiptera, Lepidoptera, Megaloptera,
Odonata, and Tric.hoptera. The molluscs were divided into the univalve class Gastropoda and the bivalve class Pelec.ypoda (Bivalvia).
Site I - Spa/Lganiwn euhtjQaJtpwn 2 Initial me2n surface area per plant was approximately 164 c.m in 35
June, 1976 (Fig. 4 ). Maximum surface area was noted during September 2 with a mean of 990 em per macrophyte. This represented an increase in surface area of approximately 600% from June to September.
Au6wuc.h6 standing crops were variable but reached the maximum 2 (222 individuals/m ) during early September (Table 2 ); this was coincident with maximum surface area of S. eunyc.anpum. Additional peaks in mean standing crop were observed in mid-June, 1976 (219 2 2 individuals/m ) and mid-August (203 individuals/m ). Low macroinverte 2 brate standing crop (38 individuals/m ) were obtained in late September prior to macrophyte senescence. Mean biomass for Aunwuc.ho also exhibited great variability (Table 2 ) with the maximum biomass observed 2 in late July, 1976 (0.897 g/m ) and the minimum in late June, 1976 2 (0.022 g/m ).
The 2,575 macroinvertebrates removed from stems of S. eunyc.anpum represented 52 taxa. Relative abundance, frequency of occurrence, and rank of the 17 taxonomic groups are presented in Table 3. The most frequently encountered macroinvertebrates were members of the Oligochaeta
(64.5%), and they were also the most abundant (30.5%). Hirudinea ranked second in occurrence (61.3%) and were slightly less abundant (26.4%).
Diptera were the third most frequently encountered group in samples
(47.3%) and the third most abundant (19.1%). Isopods and gastropods were the next most frequently occurring macroinvertebrates (39.8% and
31.2%, respectively). They were, however, less abundant (8.8% and 3.8%, respectively). These 5 taxonomic groups constituted approximately 90% of all invertebrates found on stems of S. e_u/tYC.£Vrpum.
Benthic invertebrate standing crops were higher than those recorded Fig. 4. Change in mean surface area of macrophytes based on planimetric determinations. Neans were determined from a composite of three samples for each macrophyte stand. 5000-r------~ .... ••• •••• Nymphae a tuberosa Sag!ttarla latlfolla - •-•- Sparganium eurycarpum
... ••• •• • • N • • • • • •• . -...: .. ·.·.. .. ·...... ········· .• !:.?.. •• .· •.. ·· .....• · .., •. 1000- •• , . <( • I lJJ .:······ . II :x: •. I SENESCENCE •. < . •. I I u.: . (.) . <:( . ~ ,I u. .. r.. ,--_,.." (!: . ;:) . i .. ./ I ~? 500- :. I I. , ~ • z f : . \ .... ' ' ,,. .., I I u""" 400- \ ' :: I .. i I \ I 300-1 I I \ I \ I \ , II \ I 1/ ' I I I ~ 200- I \1 I I '
w 100-~------~------~------~------~------~------~------~ 0" JUN JUL AUG SEP OCT JUN JUL 1976 1977 37
Table 2. Mean standing crop (individuals/m2) and biomass (g/m2) of macroinvertebrates for AunwucJL.O and benthos, Site I, Sp }facrophytes Sediments Date Standing crop Biomass Standing crop Biomass (individuals/m2) (g/m2) (individuals 1m2) (g/m2) 061676 219 0.327 1951 4.497 062476 107 0.022 1531 0.191 070176 172 0.059 1037 1.693 071676 133 0.034 1512 2.025 072276 176 0.897 2832 2.146 073076 158 0.079 1593 1. 501 080576 96 0.071 944 1.413 081276 203 0.110 1044 2.052 081976 193 0.094 677 2.461 082676 160 0.135 782 2.137 090376 96 0.103 577 1.170 091076 222 0.185 517 0.753 091776 217 0.869 711 1.564 092676 38 0.014 1796 2.574 100176 1238 2.736 101576 773 1.937 102976 799 2.741 111276 1464 5.611 061277 90 0.072 331 1.927 061877 48 0.192 458 2.035 062577 50 0. 712 184 0.497 070277 107 0.855 1123 3.015 070977 109 0.307 808 1.274 a b c S • Table 3. Relative abundance , frequency of occurrence , rank , and total numbers of each taxonomic group sampled in the paAga~ e~~cdApum stand (Site I), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. Relative abundance Frequency of occurrence Rank T:;o.xonom:i.c group Macro!Jhyte Sedinent Macrophyte Sediment Macrophyte Se~iment Amphipoda 5. 7 5.5 29.0 76.4 3 Coleoptf:'ra 2.0 0.4 16. 1 24.5 Diptera 19.1 8.8 47.3 72.7 3 4 Ephemeroptera {), 1 0.1 1.1 10.0 Gastropoda 3.8 1.5 31.2 38.2 5 H~r:i?tera 0.2 0.2 2.2 ll.S ~H ru.j inea 26.4 16.4 61.3 95.5 2 2 d Hydrac:arina 0.3 -- 4.3 4.5 Isopoda 8.8 11.7 39.8 71.8 4 5 L<:>pidopte!"a 0.1 0.1 2.2 9.1 ~·'"'g.a1~)ptcra 0.1 -- 1.1 1.8 S"eo.Jtoda l.S 0. 7 9.7 45.5 Odonar.a 0 -- 0 0.9 Olisochaeta 30.5 49.7 64.5 96.4 Pelecypoda 0.3 4.4 3.2 60.9 Trichoptera 0.2 0.1 1.1 7.3 Tur bell aria 1.0 0.1 12.9 10.9 total 8 ~el~tive abundance: ~ of total number represented by each taxonomic group. 0 frequency of occurrence: 7. of sites at which each taxonomic group was collected; baaed on 93 sites for macrophytes and 110 sites for sediments. w c:.i\.ank: taxonomic groiJps ranked by frequency of occurrence; only 5 most abundant groups were ranked. 00 d--nenotes taxonomic group representing less than 0.1Z of totdl number. 39 in the Au.fiwu.c.M (Table 2 ). High mean standing crop occurred in late 2 , 1976 (2,832 individuals/m ). Additional peaks were noted in 2 mid-June 1976 (1,951 individuals/m ) and late September 1,796 individuals/ m2), coincident with a decrease in Au.fiwu.~M standing crops. Lowest 2 standing crop occurred in June 1977 (331 individuals/m ). Biomass values for benthic macroinvertebrates at this site were variable. The lowest 2 biomass was recorded from samples collected in late July (0.191 g/m ) 2 and highest biomass (5.611 g/m ) was observed from samples taken prior to ice cover. Of the 14,301 macroinvertebrates recorded from the sediment in Site I, 85 taxa were identified. As with the Au.fiwu.~M, the most frequently encountered taxonomic groups were Oligochaeta and Hirudinea (96.4% and 95.5%, respectively). In descending order of frequency of occurrence Amphipoda (76.5%), Diptera (72.7%) and Isopoda (71.8%) were noted. Although these groups were found in most samples, their abundance was variable. Oligochaetes were the most abundant invertebrates sampled in the sediment (49.7%). Hirudinea were the next most abundant (16.4%) and them amphipods (5.5%). Numerically, midges and isopods were approximately twice as abundant as amphipods (8.8% and 11.7%), but they occurred less frequently than amphipods. These 5 taxonomic groups comprised over 92% of all invertebrates from sediment samples. Site II - Sag~ hLf~ooLLa Low initial surface area of S. latifioL[a occurred in June 1976 (252 2 em). Maximum surface area was exhibited during early September (970 2 em), representing an increase of approximately 400% in surface area during the growth season. During September, surface area decreased 40 until senescence in late September. 2 Aufiwuc.M standing crops attained their maximum (656 individuals/m ) one week prior to the attainment of maximum macrophyte surface area, and then decreased until plant senescence (Table 4 ). Low invertebrate 2 standing crops occurred in September (48 individuals/m ) and late 2 June (83 individuals/m ). Biomasses were variable, but reached a 2 maximum at the time of maximum standing crop (0.990 g/m ). Lowest 2 biomasses (0.085 g/m ) were recorded from samples collected in early August. In Site II, 57 taxa \vere identified from the 2, 401 macroinvertebrates removed from stems of S. latir)olia. Relative abundance, frequency of occurrence, and rank of the 17 taxonomic groups are listed in Table 5 The most frequently occurring invertebrates were members of the Hirudinec;1 (83.9%); they represented approximately one-half of all the individuals examined (44.6%). Isopods ranked second in occurrence (59.1%) and were less abundant (17.2%). Midges (Diptera) occurred less frequently (53.8%), but are numerically more abundant (20.6%) than isopods. Amphipods (41.9%) and Gastropods (34.3%) were the next most frequently encountered invertebrates. Amphipods contributed 6. 3% to the total ALLr)Wuc.h.-5 population and gastropods 3.2%. These 5 taxonomic groups constituted greater than 90% of the Aunwuc.M coTih"Uunity on macrophytes in Site II. Benthic standing crops in this area exceeded those of the Aufiwuc.f'l.-6 (Table 4). The maximum benthic standing crop was recorded in late 2 June 1977 (2,270 indiYiduals/m ), and corresponded to a low Au6wuc.h standing crop. Additional peaks in benthic standing crops occurred 2 2 in mid-July (2,158 individuals/m ), early August (l, 221 individuals 1m ) 41 2 2 Table 4. Mean standing crop (individuals/m ) and biomass (g/m ) of macroinvertebrates for Au&wuc.h-6 and benthos, Site II, Sag~ lati6olia, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River. Nacrophytes Sediments Date Standing crop Biomass Standing crop Biomass (individuals 1m2) (g/m2) (individuals 1m2) (g/m2) 061676 246 0.183 1326 3.441 062476 83 0.296 2270 4.568 070176 98 0.336 713 2. 2L~6 071676 115 0.127 2158 3.058 072276 109 0. L~81 2124 3.069 070376 272 0.194 1278 1.676 080576 117 0.085 908 1.509 081276 412 0.461 1221 4.395 081976 257 0.212 1139 1. 581 082676 656 0.991 828 1.366 090376 238 0.255 575 1.363 091076 219 0.276 1304 2.317 091776 463 0.513 1230 2.419 092676 48 0.025 1962 L~.093 100176 722 1.007 101576 934 2.809 102976 949 2.682 111276 1102 3.975 061277 145 0.316 243 1.479 061877 174 0.405 739 3.374 062577 131 0.130 744 1.829 070277 126 0.3ld 994 2.722 070977 210 0.438 1323 6.557 8 Table 5. Relative ahundance , frequency of occurrenceb, rankc, and total numbers of each taxonomic group samples in the Sagit.t~ tati6olid stand (Site II), Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. Relative abundance Frequency of occurrence Rank Total numbers Taxonol!liC grD'Jp Macrophyte Sediment Macrophyte Sediment Macrophyte Sediment Macrophyte Sediment A.-nphipoda 1).3 8.3 41.9 88.2 4 5 !52 1306 Coleoptera 1.0 0.5 10.8 23.6 24 76 Diptera 20.6 9.4 53.8 97.3 3 2 495 1473 Epl1cmeroptera 0 0.1 0 4.5 0 13 Gastropoda 3.2 1.7 34.4 49.1 5 78 264 Hemiptera 0.1 0.3 2.2 9. 1 2 48 Hirudinea 44.6 21.3 83.9 98.2 1071 3339 d H'·dra.•.arina 0.3 -- 3.2 1.8 8 4 Isopoda 17.2 13.4 59.1 90.0 2 4 412 2090 Le?idoptera -- 0.3 1.1 6.4 51 ~egaloptera 0.1 -- 2.2 0.9 3 Ser.:~atoda 1.7 1.2 18.3 43.6 41 184 Odonata 0.1 -- 1.1 2.7 2 2 Ollgochaeta 3.3 40.5 30.1 96.4 3 80 6333 P~lecypo•ju 0.1 2.5 2.2 59.1 3 394 Trichoptera 0.7 0. 2 7.5 9.1 16 36 Turbellaria 0.5 0.3 12.9 20.0 13 40 total 2401 15654 a Relative abundance: Z of total number represPnted by each taxonomic group. b Frequen•cy of occurrence: % of sites at which each taxonomic group was collected; based on 93 sites for macrophytes and 110 sites for sediments. cRank: taxonomic groups ~anked by frequency of occurrence; oqly 5 most abundant groups were ranked, """N d --Denotes t3.<0nomic group representing less than 0.1% of total number. 43 2 and ln late September ( 1, 962 individuals/m ). Lovl standing crops were 2 observed in June 1977 (243 inclividuals/m ) when macrophyte sampling was resumed. Invertebrate biomass values fluctuated with standing crop 2 values. Highest biomasses (6.557 g/m) were recorded in mid-July 1977. 2 The lowest biomasses (1.007 g/m ) occurred after macrophyte senescence in early October. Seventy-seven taxa were identified from the 15,654 macroinvertebrates collected from dredge samples at Site II. Similar to the Au..t)wudu.s at this site, Hirudinea were most frequently collected and occurred in 98.2% of the samples. They were not, however, the most abundant group (21.3%); Diptera ranked second in occurrence (97.3%) and were less abundant (9.4%). Nembers of the class Oligocheata ranked third in frequency of occurrence (96.4%) and were the most abundant invertebrate group in the sediments ( 40. 5%) • Isopods and amp hipods vrere observed less frequently (90.0% and 88.2%, respectively), and amphipods were less abundant (8. 3%) than isopods (13. 4%). These 5 macroinvertebrate groups comprised approximately 93% of all invertebrates from sediment samples at this site. Site III -· Nymphaea tubeJw The low initial surface area of Nymphaea tubeJw Aut)wu.e.-h6 standing crops >vere highest during the last two weeks in July 1976 (Table 6), \,rhich generally corresponded to the attainment of maximum macrophyte surface area. During this time, mean standing crop 44 2 2 Table 6. Mean standing crop (individuals/m and biomass (g/m ) of macroinvertebrates for Au6vJuc.h./.) and benthos, Site III, Nymphae_a tubeJwoa, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River. Macrophytes Sediments Date Standing crop Biomass Standing crop Biomass 2 (individuals/m ) (g/m2) (individuals 1m2) (g/m2) 061676 53 0.047 940 4.548 062lf 76 107 0.140 1280 4.983 070176 184 0.161 711 4.275 071676 527 0.430 2491 7.288 072276 954 0. 772 2160 6.095 070376 956 0.513 1729 7.159 080576 517 0.237 1123 6.398 081276 463 0.340 2146 8.361 081976 448 0.117 2451 6.817 082676 499 0.121 1869 6.203 090376 518 0.090 1729 4.880 091076 667 0.115 1275 7.690 091776 911 0.?.64 1128 5.190 092676 811 0.185 2334 7.092 100176 1800 4.477 101576 1707 5.577 102976 2175 10.986 111276 1562 5.065 061277 188 0.093 375 4.023 061877 183 0.111 346 3.692 062577 226 0.655 558 3.370 070277 546 0.258 1121 3.384 070977 737 0.308 1848 4.332 ------45 2 was 954-956 individuals/m . A similar peak was discernable in mid 2 September (911 individuals/m ). Low invertebrate standing crops were 2 recorded during June 1976 (53 individuals/m ) and June 1977 (183 2 individuals/m ). Biomass values fluctuated with standing crop values within this site. The highest biomasses were recorded in late July 2 (0.772 g/m ), this corresponded to high invertebrate standing crops at 2 that time. The lowest biomasses (0.047 g/m ) were observed in mid-June and corresponded to the lowest invertebrate standing crops on that date. The 5,503 macroinvertebrates harvested from petioles and floating leaves of Nyrnphae.a.. represented 63 taxa. Relative abundance, frequency of occurrence, and rank of the 17 taxonomic groups are represented in Table 7 Diptera was the most frequently occurring taxonomic group (84.9%) and was also the most abundant group (26.1%). Amphipods and gastropods occurred in most samples (77.4% and 77.4%, respectively), and the Amphipoda were more abundant (19.4%) than the Gastropoda (7.7%). Lepidoptera ranked third in occurrence (66.7%) and was the second most abundant (20.5%) invertebrate group in all samples .. Oligochaeta ranked fourth in frequency of occurrence (61.3 %) with a relative abundance of 8.2%. Leeches (Hirudinea) were taken from 52.7% of all samples, but they represented only 5.2% of the entire Au6WuQh6 community in Site III. These 6 groups constituted approximately 90% of the Aut)WUQho community. Benthic invertebrate standing crops were variable within this site (Table 6 ). Lowest standing crops occurred from June 1976 (940 2 2 individuals/m ) and June 1977 (346 individuals/m ). Peak standing crops 2 occurred in mid-July (2,491 individuals/m ), the date of highest recorded Au6wu.c.lll> standing crop at this site. Another peak \vas noted in mid- a b c Table 7. Relative abunda'lce , frequency of occurrence , rank , and total '1umbers of each taxonomic group sampled in the Nymphaea. tu.be!to6ct stand (SHe III), Lake Onalaska, N~vlgation Pool No. 7, Upper Mississippi River, 1976-1977. Re1ative abunda.nce Frequency of occurrence Rank Total numbers Taxonomic group Macrophyte Sediment Macrophyte Sediment Macrophyte Sediment Macrophyte Sedirr.ent A::1phipoda 19.4 10.!, 77.4 82.1 2 5 10&5 2084 Co1eopte~a 0.5 0.5 5.4 19.6 28 91 D.iptera 26.1 34.2 84.9 99.1 1435 6870 Ephe~eroptera 0.1 0.1 5.4 7 .1 17 Gastropoda 7.7 6.1 77.4 92.0 2 4 425 1223 Hc!:liptera 1.6 C.l 30.1 8.9 89 22 Hirudinea 5.2 7.7 52.7 93.8 5 3 288 1560 Hydracarina 2.7 0.1 35.5 9.8 150 20 Isopcda 0.2 3.2 6.5 33.9 12 645 d Lepjdoptera 20.5 -- 66.7 7. l 3 1126 10 l!ega1optera 0 -- 0 4.5 0 tlema toda 0.1 0.4 7.5 30.4 7 87 Odonata 0.5 0. 2 17.2 14.3 28 32 Oligcchaeta 8.2 30.9 61.3 96.4 4 2 450 6207 Pe1ecypo Trichoptera 5.5 2.8 44.1 48.2 304 572 Turbellaria 1.6 0.3 30.1 24.1 89 54 total 5503 20069 aRelarive abundance: 7. of total number represented by each taxonomic grouo. bfre~uency of occurrence: % of sites at which each taxonomic group was collected; based on 93 sites for macrophytes and 110 sites for sediments .f:- cRank: taxonomic groups ranked by frequency of occurrence; only 5 most abundant groups were ranked. 0\ d -- O 2 August (2,451 individuals/m ) and in late September (2,334 indivi- 2 duals/m ), before macrophyte senescence. Benthic biomass values were variable during the year at this site. The highest recorded value 2 was 10.906 g/m during late October, which was several weeks prior to ice cover. The lowest values were noted in late June and early July of 1977. A total of 79 taxa were identified from the 20,069 macroinverte- brates collected from sediments in Site III (Nymphaea tubeto~a). Relative abundance, frequency of occurrence, and rank of the 17 taxonomic groups for this site are represented in Table 7. Diptera occurred in 99.1% of all samples with a relative abundance of 34.2%. Oligochaeta were also prevalent and ranked second in frequency of occurrence (96.4%); they had a relative abundance of 30.9%. Hirudinea were commonly collected from dredge samples (93.8%) and comprised 7.7% of the total number. Gastropoda (92.0%) and Amphipoda (82.1%) ranked fourth and fifth in frequency of occurrence, respectively. Amphipods were more abundant in sediments (10.4%) than were gastropods (6.1%). These 5 taxonomic groups represented almost 90% of all macroinvertebrates found in sediments at Site III. Site IV - Control During most of the year, invertebrate standing crops reillained relatively stable with only slight fluctuations (Table 8 ). Standing crops of macroinvertebrates from the sediments at this site were low 2 in mid-June 1976 (729 individuals/m ) and late June 1977 (503 indivi- 2 duals/m ) . Haximum standing crops occurred in early November, t'vo 2 weeks prior to ice cover during 1976 (5,206 individuals/m ). There was a sharp increase in standing crops beginning in late September. 48 Table 8. Mean standing crop (individuals/m2) and biomass (g/m2) of macroinvertebrates, Site IV, Lake Onalaska, Navigation Pool No. 7, Upper Hississippi River. Standing crop Biomass Date (individuals/m2) (g/m2) 061676 729 3.121 062476 1145 4.170 070176 999 6.172 071676 1176 4.839 072276 863 2.429 073076 1187 4.438 080576 1226 2.739 081276 1263 3.747 081976 1282 3.697 082676 1095 3.738 090376 977 3.217 091076 1106 6.757 091776 1333 5. 927 092476 2596 8.197 100176 2117 6.454 101576 3066 12.385 102976 4095 8.960 111276 5206 10.828 112676 2079 8.962 121076 2124 8.044 010777 704 4.919 020477 1935 10.923 030477 3229 5.935 0Lf0177 2386 14.067 0429 77 1555 7.793 052777 1492 6.483 061177 1964 9.215 061877 932 2.941 062577 503 2.137 070277 728 2.211 070977 946 2. 311 49 This phenomenon corresponded to the approximate time of macrophyte senescence. After the high November value, standing crops decreased 2 to 704 individuals/m during June 1977. Benthic invertebrate biomass was variable during the sampling period. Low values were recorded from samples in July 1976 (2.429 2 2 g/m) and July 1977 (2.211 g/m ). The highest observed biomass 2 occurred in early March 1977 (14.067 g/m ). Biomasses were generally higher during the period of ice cover than at other times during the year. A total of 79 taxa were identified from 34,297 macroinvertebrates collected in samples from the control site. Relative abundance, frequency of occurrence, and rank of the 17 taxonomic groups for this site are represented in Table 9. Oligochaeta and Pelecypoda occurred in almost all samples (97 .0%) >vith oligochaetes more abundant than clams (18.4% and 4.8%, respectively). Diptera ranked second in occur- renee (93.2%) and comprised 20.7% of all invertebrates sampled. Amphi- poda and Gastropoda occurred at the same frequency (89.4%), but amphi- pods were more abundant (21.5%) than gastropods (17.5%). Numerically, amphipods were the most abundant invertebrate in the control site. Isopoda ranked fourth in occurrence (79.5%) and represented 9.3% of the community. Hirudinea were the fifth most frequently encountered invertebrate (78%) but were not very abundant (2.2%). Collectively, these 7 taxonomic groups comprised about 95% of the entire benthic macroinvertebrate com.rnunity in the non-vegetated site. The number of taxa represented ?Y Au{pJuc.h.J.> and benthic macroinver- tebrates for each si.te is summarized in Table 10. Although the five 8 Table 9. Relative abundance • frequency of occurrenceb, rankc, and total numbers of each taxonomic group sampled in the control site (Site IV). Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977 T.:~.xcnornic .rronp Relative abundnnce Frequency of occurrence Rank Total numbers r\c.J?~ i p ...•d3 2!. 5 89.4 3 7373 Coleoptera 0.1 2.9 41 Dlptt!ra 20.7 93.2 7102 £rh~m ...·roptera 0.1 13.6 35 Gds~ropvJ:rt 17.5 89.4 3 6000 d Ht"~iptt:>ra 2.3 18 H !_ rud 1 nt~a 2.2 78.0 5 768 HyCracc~r:ina 0.3 28.0 93 !SO;JtJda 9.3 79.5 4 3205 LqJiCL.,:··tera 0.1 3.8 38 ~~t'gJ!O?teca 2.3 ~e:J.iJ(L)l!.3 0.6 49.2 209 llJ,.nl.l!.a 0.5 31.1 164 O!! ~c1~ ~t.:le r a 18.4 97.0 6326 P-.:-l(·CYtlOdR 4.8 97.0 1649 Tr:fchoptera 4.7 73.5 1612 Turbclltrla 1.0 25.7 351 co tal 342Y7 '·Re!J~!ve abundance: : of total number represented by each taxonomic group. hFrc:qut"ncy o! occur-rence: :: of sites at which each taxonomic group was collected; baRed on 150 sites. cR.:1n\: taxonomic groups ranked by frequency of occurrPnce; only 5 most abundant groups were ranked. V1 0 d __ cl·notes taxonomic group representing less than 0. !% of total number. a T'! ~ lc- 1 (). ~;unber of taxa repre~ented by Au6(tlf..lch6 and benthic macroinvertebrates for 5 ranked taxonomic groups at each site, Lake Onalaska, Navi~aticn Pool ~o. 7, Up~Pr Mississippi River. 1976-1977. Site I Site II Site III Site IV R2nf. of ~;o. No. No. No. No. No. No ta.:':O:"!Cl'!liC ~v:rophyte of Sediment of Macrophyte of Sediment of Macrophyte of Sdiment of Sediment of group taxa taxa taxa taxa tRX8 taxa taxa Oligochaeta 1 Oligochaeta 1 Hirudinea 9 Hirudinea 6 Diptera 20 Diptera 24 Oligochaeta 1 Pelecypoda 4 Hirudinea 7 Hirudinea 6 Isopoda 1 Diptera 30 Amphipoda 1 Oliguchaetn 1 Diptera 27 Gastropoda 6 ) Diptera 12 Amphipoda 2 Diptera 17 Oligochaeta 1 Lepidoptera 1 Hirudinea 7 Amphipoda 1 Gastrupoda 10 Isopod a 1 DJptera 29 Amphipoda 1 Isopoda 1 Oligochaeta 1 Gastropoda 8 Isopoda Gastropoda 7 Isopod a 1 Gastropoda 6 Amphipoda 2 Hirudinea 7 Amphipoda 1 Hirudinea 8 total for ~ groups 2R 39 34 40 36 41 52 t(1tal for all grou~s 52 85 56 77 63 79 79 -- a Taxonomic groups ranked by frequency of occurrence. V1.... 52 ranked taxonomic groups represented approximately 90% of all inverte brates from all sites, a comparison with the total taxa from all sites indicated that only about one-half of the total population was included. There was not a site specific difference. The most frequently encountered taxonomic group for each site as Aunwucho was also the most frequently encountered taxonomic group in the underlying sediments. Approximately the same number of taxa were represented by each group on macrophytes or in sediments, or if a change in rank of taxonomic group had occurred between macrophyte and sediment. The Diptera were an exception. At Sites I and II, there were approximately one-half as many dipteran taxa on the macrophytes as compared to sediments. At Site III, the ratio was more equivalent. Lepidoptera was a frequently occurring Aunwucfu taxonomic group at Site III, was absent from Sites I and II. It was also absent from the adjacent sediments at Site III. Pelecypoda was a frequently occurring taxonomic group at Site IV, occurring as frequently as oligochaetes, but i>Jas absent from sediments at Sites I, II, and III. Differences in the distribution of major taxonomic groups of invertebrates were noted between the experimental and control areas. The distribution characteristics of each of these major groups are summarized belovl. Oligochaeta Oligochaetes v1ere frequently encountered Au6Wt{d~5 at Sites I and III. They were also frequently encountered benthic invertebrates at all sites. They were the most abundant invertebrates on macrophytes at Site I and in sediments at Sites I and IV. Because of taxonomic 53 problems with specific genera and mechanical problems in preparation, class level was maintained in identification. Hirudinea Leeches were frequently encountered invertebrates at all sites on macrophytes and in sediments. The taxonomic composition of this group is indicated in Table 11. On macrophytes, Helobdetia ~tagn~ was the most abundant species at Sites I (86%), II (81.5%), and III (67.7%). H. ~tagn~ also dominated the sediments at Sites I (74.9%) and II (78.9%), but at Site III, H. uonga:ta became abundant (81. 7%) and H. -6tagna.LL6 was absent. At Site IV, H. ~tagn~ was again the most abundant member of Hirudinea (76.7%). A single species, Ptaeobdetia onna:ta was collected only from macrophytes at Sites I, II, and III, and never from underlying sediments. Isopoda Isopods occurred frequently as Aunwucho at Sites I and II, and as benthic invertebrates at Sites I, II, and IV. This group was represented by a single genus in the study area, Meti~ spp. Amphlpoda Amphipods were frequently occurring invertebrates in the sediments at Sites I, II, III, and IV, and as Aunwuch6 at Site II and III. Two genera >vere represented by amphipods in the sediments at Sites I and II; only one genus was represented at all other sites. Hyaliefa azteca was represented at all sites (100%), except in the sediment at Sites I (99. 4%) and II (99. 3%). At these tvJO locations, GammaJr.Li6 spp. was a % of total number represented by each taxon. b V1 Blank spaces indicate taxon not present. .!:'- 55 present (0.6% and 0.7%, respectively). Lepidoptera Lepidoptera were commonly encountered invertebrates only on the macrophytes at Site III; they were represented by the genus Nymphuta spp. Although 3 genera of Lepidoptera (Nymphuta, PaJw.gyJLac.:tW, and Panapoynx) were identified from Site III, their taxonomy is so closely allied that various authors place them in the genus Nymphu.i..a. Diptera Dipterans occurred frequently at every site in sediments and on macrophytes. They were represented by 12-20 taxa on macrophytes and 27-30 taxa in the sediments. The taxonomic composition of this group is represented in Table 12. Palpomy~a spp. were the most abundant Aul)wuc.hc. at Sites I (39. 9%) and II (51.1%) and Potype.cLU.wn spp. dominated at Site III (67.6%). }funy taxa were collected only once or twice during the course of this investigation. This \vas especially true for members of the Cyclorrapha in the sediments at Sites I, III, and IV. Overall, the Brachycera and Cyclorrapha were poorly represented in the study area with the possible exception of Cf~y~op~ spp. Gastrcpocla Gastropods were frequent Aunwuc.h...0 at Sites I, II, and III, and frequent benthic invertebrates at Sites III and IV. The taxonomic composition of this group is presented in Table 13. Gy'W.ui.Ull spp. were the most abundant snails on macrophytes at Site I (50%); whereas, Phyc.a spp. were the n'ost abundant snails on macrophytes at Sites II 56 1 table 12. TaX('lWmk compnsttlon' of Otptt._•rn nt t.•ach ~Itt>, Lnkl' On.ll.:t~ka, N.-tvlg.ttlon Pool No.7, Uppt•r ?-11!'-ltdHslppl River, 1976-1977. ------ Site I Site II Sit~ III Site IV Taxon ----- MacrophytP St.•d iment Hacrophytt.• SL•d imL'tlt ~1.J.Cnlphyte Sed im~.•nt s,•d imt.•nt Srachvcera 0df•t1fl,'tll(i.t1 S~r.'· O.J 0.1 0.6 0.3 0.4 <0.1 b Cl1 ":.rf .\cp~ spp. 2.9 2.5 0.1 <0.1 Tabiuml spp. 0.1 0.3 <0.1 <0.1 Cyclorrapha Ephyd"r.a s;;p. 0.1 /itjdteU£a ;pp. <0. 1 Lcmnapltitct spp. <0.1 Noupl~ta s)p. O.t <0.1 Re:wcc,•.. c. srp. 0.3 1.6 <0.1 <0.1 Sepec!cn spp. 0.1 0.4 Ewt,:tW spp. <0.1 Nematocera Bczzia. sr0· 1.7 4.2 1.0 4.2 3.2 0.3 0.1 Pai.pomlj~'l S?P. 39.9 3.0 51.1 1.6 9.4 4.1 3.2 ChL'tatwrr~;~ "?P· l.r) 8.2 0.8 16.7 0.4 63.8 12 .o CJtyp.tccitl/tcH.~~' 1 U~ SIJP· 0.7 0.5 3.3 2.2 CJtypt.oci.adc.•.~c( •% of t;lt;cd ·;,•Jr•l-x·r !'C?r-L·:.;('td·ed t.·; .:."lLh t:1xo•1. bP.lank :-.;l~~·cs ind hat;· ta:vm n1,t pr£'sc:nt. a Table 13. Taxonomic composition of Gastropoda at each site, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. Site I Site II Site III Site IV Taxon Macrophyte Sediment Macrophyte Sediment Macrophyte Sediment Sediment AmnJ..eol!..a. spp. 3.4 10.5 5.0 13.2 44.5 69.4 68.0 Vaf!_vata spp. 1.7 1.4 b 0.2 14.2 21.9 Co.rnpe.. eomct spp. 0.1 0.1 Uopiax spp. 0.1 VJv-tpcvr.u,~ spp. 0.5 0.1 0.2 t: • . • eJt..IU.1>1>-U1 spp. 3.4 1.0 0.6 1.8 PhyM spp. 25.9 41.1 45.0 37.9 44.5 1.3 5.9 Gy:uwJ'.LL!> spp. 50.0 39:7 29.0 45.2 9.4 1.9 1.5 FcU~oma srp. 6.9 1.9 8.0 1.8 1.1, 0.4 0.5 Mene-trL-6 spp. 8.6 4.8 12.0 1.8 0.2 0.1 8 % of total number represented by each taxon. b Vl Blank spaces indicate taxon not present. -...) 58 (45%) and III (44.5%). AmniQola spp. was also abundant on macrophytes at Site III (44.5%). In the sediments, Phy~a dominated at Site I (41.1%) and Gy~auluo at Site II (45.2%). However, their relative abundance was similar at both sites: at Site I, Phyut was 41.1% and GyMuluo was 39.7%; at Site II, Phy~a was 37.9% and GyM~fuo was 45.2%. At Sites III and IV, AmVUQOM was very abundant (69.4% and 68%, respectively). Members of the Viviparidae (Campefoma, Uoptax, and Viv.{pMuo) were collected only occasionally from the sediments at Sites I, III and IV. Pelecypoda Pelecypoda (four species) were frequently encountered in the sediments only at Site IV. Sphae!Uum spp. was the dominant repre sentative (93.4%) at this site. 1'-..vo other genera, M~!JQ~Wn spp. (5.1/~) and PiMicU.1m1 spp. (1.4%) were infrequently collected, and P~op.tvw. spp. was collected only once during the study (Table 14). a Table 14. Taxonomic composition of Pelecypoda at each site, Lake Onalaska, Navigation Pool No. 7, Upper Mississippi River, 1976-1977. Site I Site II Site III Site IV Taxon Macrophyte Sediment Macrophyte Sediment Macrophyte Sediment Sediment Unionidae P![op:teJ:a. spp. b 0.3 0.2 0.1 Sphaeridae Mu.ocuUwn spp. 5.9 5.1 3.8 3.4 P.£1>6)_cU_um spp. 0.5 O.B Spfw_e;Uu.m spp. 9}, 7 94.9 95.5 95.7 a i. of total number represented by each taxon. b Blank spaces indicate taxon not present. \.J1 \0 60 DISCUSSION A macroinvertebrate survey of vegetated and non-vegetated areas revealed the presence of 17 major taxonomic groups represented by 131 taxa. Among these groups, 90%-93% of the invertebrate community was dominated by only 8 taxonomic groups. Based on relative abundance and frequency of occurrence of these 8 groups, only 75 taxa were represented. Therefore, approximately one-half of the total taxa present comprised only 10% of the community. This agrees with the commonly observed phenomenon that undisturbed environments have relatively few taxa with large numbers of individuals and large numbers of taxa with few individuals. The 8 invertebrate groups dominating the community were Oligochaeta, Hirudinea, Isopoda, Amphipoda, Lepidoptera, Diptera, Gastrqpoda, and Pelecypoda. The study sites represented a gradient with respect to water depth. Temperature, pH, dissolved oxygen, and alkalinity exhibited seasonal fluctuations, but were relatively constant among all sites throughout the sampling period. All physical and chemical variables measured were within environmental tolerances of the i'1vertebrate groups that were identified. It is difficult, therefore, to identify one physical chemical variable or a combination of variables uhich effect distribution. This is particularly appa.rent when examining dissimilar groups that have ider1ticaJ or similar tolerance ranges and similar groups that show marked distributional changes in the same tolerance range. A gradient also existed for aquatic macrophytes of the littoral area. The 61 three macrophyte stands were not spatially separated, but they had contiguous borders with similar annual growth regimes. However, growth patterns of emergent and submersed leaf petioles were different; this would determine the particular macroinvertebrates utilizing macrophytes as substrates. SpaJtgani_wn e.uJtyc..aJtpwn and Sagil:tafU._a ia.U6o..Ua possess long, narrow, overlapping leaves diverging basally from a rootstock, and grow in relatively dense stands. Nymphae.a t:ubeJLo).)a has rounded leaf petioles that arise from an underground horizontal rhizome. In addition, the petioles are more interspersed in open water with the air-water interface occupied by the leaves. These growth patterns created.habitat differences which resulted in differences in the Aufivuc.lu, corrmmnities of the submersed sterns, and the benthic communities underlying macrophytes and in the non-vegetated areas. These differences in distribution are discussed below with respect to composition of the 8 prominant taxonomic groups and includes their life histories and nutritional roles. Oligochaeta One major component of the benthic and Au6wucJu rnacroinvertebrate fauna of the study was represented by members of the Oligochaeta. Elstad (1977) reported that oligochaetes were ubiquitous and the most abundant and dominant rnacroinvertebrates of Navigation Pool No. 8. In the present study, oligochaetes attained greatest densities in sediments 2 Of vegetated areas; t h e num b er o f 1n. d lVl. . d ua 1 s,m! were generally lower on submersed stems of macrophytes. Haxirnum densities occurred in summer and fall months, usually with a peak in the n'Jrnber of individuals/ m2 1n . July and late September of early October ( Figs. 5, 6, and 7). According to Hetzel (1975), this periodicity in breeding is common, 2 Fig. 5. Number of Oligochaeta/m sediment at SpCULganiwn site (l) and SagilicuU..a site (II) bet\.;reen 1976 and 1977. 62 ------ ....c-______------___ I I ·---~--, g 8 ~ 8.. ~ £ iN ti"'4t0.1'S ln 2 Fig. 6. Number of Oligochaeta/m sediment at Mymphae_a site (III) and non-vegetated site (IV) between 1976 and 1977. " "''" z ?, I-~-,----~--·---., -, --~-.. -,~·---,---..__ ..,--r----..,-- r) 0 (.1 ~ g ~ § ~ ~ ~ ~ ~ 2 Fig. 7. Number of Oligochaeta/m macrophyte (Au£wu~hs) at the three vegetated sites between 1976 and 1977. ...1 .,:::> r- z .,:::> "Ol r" co ...."Ol .... u 0 - :;: <( :! :2 LIJ 0:: <( z a. <( UJ ::c ~ (I) a. 0 t .... 0:: ~ 0 <.( >- <( a. z (/) (I) t 0 :::> ( <( r- I I I I I I I ! I ~-_j_~---~~0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 M <'I ... M N M N ..- 31AHd0UJ'I11'l z~ I VBVH:J081l0 ::10 l:l31:WWN although oligochaetes lack distinct age classes with production often continuous. Generally, the life cycle of macroscopic species is 1 to 2 years. Oligochaetes are preyed upon by dipteran larvae, other aquatic insects, leeches, and in the case of Chaetogalt~. by other oligochaetes. Most derive the bulk of their nutritional needs from the ingestion of bacteria aad amorphous organic matter in sediments and soils. Many species, particularly in the Naididae are herbivorous or graze diatoms and other algae that grow epiphytically on aquatic macrophytes. Hirudinea Members of the Class Hirudinea constitute a major portion of the mactoinvertebrate fauna of most freshwater habitats. Elstad (1977) cited the general distribution of leeches in Pool No. 8 as favoring eutrophic, shallow water habitats with abundant debris and plant material. According to Sefton (1976) these areas also supported the largest total macrophyte biomasses. In this study, leeches as a group attained highest concentrations in vegetated sites and the number of 2 individuals/m was generally lower in non-vegetated sites (Figs. 8 and 9 ). Leeches displayed seasonal changes in population densities; generally, 2 the lowest number of individuals/m occurred in the summer and fall months. According to Sawyer (1972), this is a cow~only observed life cycle pattern as many have an annual life cycle and die after breeding. Furthermore, in life cycles that extend beyond a year, the species burrow into the sediment and aestivate during the winter months below the frost line. The Hirudinea in the study area belong to two major freshwater Fig. 8. Number of Hirudinea/m2 sediment at the vegetated and non-vegetated sites between 1976 and 1977. 00 ------··------·.------...... 0 "' sw z C> w .,< > I % II z0 ...<> 0 z ~ ----- ;; I -,- I ,- I I I I I I I I I I 0 0 0 0 0 0 0 0 0 0' 8' 8 g 8 0 g g 0 g g g $' ~ ~ .. ~ ..."' ~ s ~ ~ . ~ " "' jQ tUC!>'HlN JN1t4HO~S l"' 'Y:O:t-l:OrllJlH 2 Fig. 9. Number of Hirudinea/m macrophyte (Aunwu~ho) at the three vegetated sites bet\·leen 1976 and 1977. 3000- NYMPHAEA 2000- IJJ f > 1000- :c 0.. 0 c: (.) < I =::::-:::=:.. ::::: -~~~==~~--~~~~~ 3000-~ Sf\GITTARIA N :! ...._ 2000- < w z 0 :;) 1000- cr: :J: ~ Lt. 0 cr: 3000-1 w SPARGANIUM r:o ~ :;) 2000-~ z ":000- _____...-- JUN JUL AUG SEP OCT JUN JUL 1976 1977 (J'\ -....J 68 families (Table 11). The Glossiphoniidae included the genera Hetobdeita, GioMipho;ua, Piac.obdeita, and Bat!tac.obdetia and the Erpobdellidae included the genus E'Lpobdetia. The distribution of particular species differed among and within each specific site. Heiobdetia ~tagn~, the most common North American leech, was the dominant species on sub mersed vegetation of SpMganium, SagLttaJUa, and Nymphaea. It was. also the dominant species in sediment underlying SpcV1.gartiu.m and Sagi:ttcvz.ia and the non-vegetated area, but was absent from sediment underlying Nymphae_a, where H. eiongata dominated. Within the vegetated sites, H. eiongata exhibited a general preference of sediment as a substrate. The substrate preference of Enpobdetia punc.tata for macrophytes is evident at Sites I and II, though not at Site III. Substrate preference is related to the functioning of the leech sucker, >vhich is used in feeding, reproduction, and location and requires a solid substrate to achieve a strong suction. In addition, solid substrates are also usually necessary for deposition of cocoons. Generally, the vegetation at Site III was do1ninated by three species of Hirudinea: H. iineata, H. ~ta.gnct.UJ.:,, and E. punc.tata >vhile the other vegetated sites were dominated by a single species, H. ~tagnct.UJ.:,. Four species of parasitic leeches were collected from the study area. Bat!Lac.obde_iia phaleJta is a parasite of amphibians, Piac.obdcila mo1ttit)eJLa a parasite of fish, and Piac.obdell.a pa.JtM.itic.a and P. oJtnata parasites of reptiles. P. oJtnata occurred only on submersed vegetation, never in sediments. This species is a parasite of turtles, and leaves the host only during the breeding season (June to August) to deposit eggs on a solid substrate. 69 The distribution of most leech species is affected by a number of physical, chemical, and biological factors, the most important of which include the availability of food organisms and nature of the substrate (Sawyer 1972). Although not generally host specific, certain leech groups tend to restrict their ingestion of food items to particular groups. For example, Hetobdelta spp. feed predominantly on tubificids and related oligochaetes, chironomid larvae, and various smaller crustaceans, while H. ~tagn~ and the erpobdellids feed on aquatic insect larvae and oligochaetes. In this study, leech abundance was highly correlated (aO.Ol) with oligochaete abundance on vegetation. Leech abundance was also highly correlated (aO.Ol) with amphipod and isopod (Crustacea) abundance on SpoJtgaiUwn and Sag~a, with dipteran abundance on Sag~~ania, and with coleopteran, ephemeropteran, and gastropod abundance on Nymphaea. In the sediments of vegetated areas, leech abundance \vas highly correlated (aO.Ol) v1ith dipteran abundance underlying SpoJtgaiUwn and Sagittania, and with oligochaete, nematode, and turbellarian abundance underlying Nymphaea. The change in correla tion of specific food groups from sediment beneath Nymphaea may reflect the change in dominant leech species, however, little information is available concerning the biology of H. c.£ongata. 'Generally, the relative. abundance of the host organism dctennines the distribution of leech species, and any factor which alters this pattern will directly effect the leech population. Isopoda The Isnpoda or pillbugs can constitute significant members of benthic organisms. lsopods tend to occur in shallow water areas of 70 intermediate trophic status in Navigation Pool No. 8 (Elstad 1977) and were particularly abundant in areas of submerged VaLteonen~a am~cana and CCJultophyilum demeMwn. In this study, Metfu.o spp. was the only taxon represented by Isopoda. According to Pennak (1978), it is not unusual to find a single species and is very unusual to find two species in the same habitat. Generally, Metfuo spp. reached maximum densities in sediments of vegetated sites; the numbers of individuals/m2 were relatively higher than non-vegetated sites (Figs. 10 and 11). Significant 2 numbers/m were also recorded for submersed portions of SpMgaruum and Sagittahia. Isopods are best characterized as scavengers and usually remain secreted under vegetation and debris, becoming active at night. Peak concentrations occurred in fall months, usually during October. Maximum densities occurred in the spring months in the non-vegetated areas. Reproductive habits of Isopoda have been recorded for only a few species, but it is thought that breeding occurs throughout the year. The life span of most isopods is about one year or less with a genera- tion time of 8-12 months, but it is variable. Amphipoda Amphipoda or scuds are also members of the subclass Crustacea. Although primarily marine and estaurine organisms, the few species that occur in freshwater are chiefly benthic on sediments. Elstad (1977) reported that abundant populations of amphipods occurred in eutrophic areas of high aquatic macrophyte production in Navigation Pool No. 8. In the present study, amphipods sustained greater densities in the non- vegetated area (Figs. 12 and lJ). Within the vegetated areas, amphipod 2 Fig. 10. Number of Isopoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977. ------·----r----- 0 "' ~ ..."'CJ > I z 0z --,--1 ·-,-·--, _ _. I I I I I I I 8 § § ~ ~ ~ ~ ~ ~ ""' ~ ~ M ~ 2 Fig. 11. Number of Isopoda/m macrophyte (Au.fpJu.c.hJ.J) at the three vegetated sites between 1976 and 1977. 3000- NYMPHAEA 2000J 1000_1 U1 I ! > ::;: 0.. 0 c:c 3000- {) ~- < a 0 1000- 0.. 0 (f) u. 0 c: SPARGANIUM w OJ 2000- ...!! ::;) z I 1000-, i ~ JUN JUL OCT JUN JUL 1976 1977 ...... t-.. 2 Fig. 12. Number of Amphipoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977. ·--~------·------·------~ ii'"' ;!,_ ..0 ~ r::: "' ~ ~ ;! 2 Fig. 13. Number of Amphipoda/m macrophyte (Au6wuehh) at the three vegetated sites between 1976 and 1977. 74 z ,:::> "Ol,... U) ".,...en 1- 0 0 :::! - <{ 0. z ~ ~ (!) :::> < ..J ,:::> z ,:::> I l I I I I I I 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 (') N ... (') N ... M N .... 75 abundance was higher in areas of Nymphaea growth. According to Pennak (1978), amphipods are cold stenotherms, strongly thigmotactic, and react negatively to light. Consequently, during daylight they are found in vegetation or under and between debris and stones. This probably accounts for high amphipod abundance in vegetation fron other studies, while the present study may have actually represented their normal distribution. Amphipods showed seasonal growth patterns with the greatest 2 number of individuals/m occurring in summer and fall months and peak densities occurring in August and November (Figs. 12 and 13). Most amphipod species have an annual life cycle, breeding between February and October. Depending on \vater temperature maximum densities usually occur in August (Hargrave 1970). The Amphipoda were represented by two genera in this study. Hya11..ua was the dominant genus, with a fe\v individuals of Gammcuu.L6 spp. found on Sp~~ganium and Sag~. Scuds in general are omnivorous, general scavengers, ingesting bacteria, algae, and particulate detritus of animal and plant remains. Pennak (1978) also reported that species occurring on aquatic vegetation brm~se the film of periphyton. Lepidoptera TI1e Lepidoptera are primarily terrestrial insects with some genera occupying the aquatic environment during immature life stages; the adults are aerial and terrestrial. The North American fauna is widely distri buted and when environmental conditions are favorable, they are usually associated with specific aquatic macrophytes. Lepidoptera occurred in maximum densities in areas of NympfLac..a tubeAo/sa gro1vth (Fig. 14). The 2 Fig. 14. Number of Lepidoptera/m sediment at the three vegetated sites and number/m2 macrophyte (AubWU~~) at the Nympha~a site (III). 76 < ii' < 1- 1- i3 ~ II "' ..... i ~ ..."' ~ b I § 0 .. 5! ~ lNl~IO 1$ zW '1~iJJd001d]1 :10 H'J9YHlH 77 submersed stems of Sp~gQnium and Sag~a were depauperate of Lepido ptera and few larvae were collected from sediments at other vegetated or non-vegetated areas. Maximum densities occurred in fall, with a 2 peak in number of individuals/m observed in September. Pennak (1978) stated that all species are thought to over-winter as immature larvae, however, little information is availalble on the life history of the North American species. The few species of Lepidoptera that have larval stages are primar ily members of the family Pyralidae and occur in shallow water overgrown by aquatic vegetation. The Lepidoptera in this study were dominated by the genus Nymphul.a, although P~gljMc.ti.o and P~aponljX were also iden tified. P~gy~ac.ti.o spp. ingests mainly diatoms and other algae and Nijmphula spp. and P~ponljX spp. are shredders of aquatic vegetation. Diptera The Diptera are major components of invertebrate communities in both lotic and lentic ecosystems. They have extreme variability in morphology, reproductive biology, and ecology; however, some general comments concerning the group can be stated. Adults are aerial or terrestrial Hith aquatic immature stages. Larval stages last from several \Veeks to 2 years and many over-winter as larvae. Most life cycles are less than 2 years, during which time the larvae usually pass through 3-4 jnstars. Because rearing of larval fonns Has not accomplished and emergence traps \vere not utilized, comments concerning the seasonality of life cycles or emergence patterns from the data collected are inappropriate. Generally, non-vegetated sites produced the greatest densities of I'd 2 dipterans; the number of individuals/m was lower in vegetated areas (Figs. 15 and 16). However, with regard to the production of larvae, sediments underlying Nymphaea were similar to sediments of non-vegetated areas. The lowest densities occurred on submersed vegetation; the 2 number of individuals/m was higher for sediments (Fig. 17). The study area was dominated by the primitive dipteran families included in the suborder Nematocera. The more advanced forms of the suborders Cyclorrapha and Brachycera were dominant in the area of Spanganium growth (Site I). All three suborders are dominant as benthic invertebrates, but were represented in smaller numbers in the Aut)wuc..Yv.s. The Brachycera are represented by the Stratiomyiadae (soldier flies) and the Tabanidae (horse flies). Soldier fly larvae are predominantly herbivorous, feeding on algae, decaying vegetable matter, and small microorganisms. Horse fly larvae are generally aquatic, but Taban~ spp. and Cfmy~op~ spp. must extend their caudal spiracles out of the water. Horse flies are predaceous, but C~y~op~ spp. probably feeds on vegetable material. The Cyclorrapha were represented by 4 families in the study (Table 12). The Ephydridae (shore flies) included the genera Ephycl!La, Hydttei.i.ia, and L.c.mnapWa. Ephydrids are generally leaf miners, deriving the bulk of their food from diatoms and other algae. The Muscidae, a predaceous family, \vas represented by the single genus NotipiU..Ut. The Sciomyzidae (marsh flies) was represented by Re.noc..VUt spp. and Se.pe.don spp. These larave are chiefly predaceous on aquatic snails. The last family of Cyclorrapha observed in thi.s study was the Syrphidae or flower flies. It \vas represented by a single genus, Ew.:taLi_.,s, the only connnon aquatic • LL61 put? 9L61 uaati:Flq (II) a:ns vflil'Y.}4-?6tJS put? (I) 3118 WTIYJtJ6UVdS 1B 1U3ill1pas zmjEJ31dTa JO JaqmnN ·~1 ·2Td 79 J.Nli"ll!Cl$ tf\1 \''l:.:lJldiO JO l:i:liU'HHi 2 Fig. 16. Number of Diptera/m sediment (AutWU~Iu) at the Nymphaea site (III) and the non-vegetated site (IV) between 1976 and 1977. ou \ ,_0 ,_..'" UJ"' w "'J: 0. :;; ">"' > I ... z z ... 0 \!' z: "'... ~ -~ --~- :;:E I I I I ·--~---~~--I I I I I I I I I I 0 0 0 0 0 0 0 8 8 0 0 0 8 g 8 g 0 ~ g ., .... 8 ~ ~ ~ 8 8 ~ ~ "' .. .. "' "' ~ ~ " Fig. 17. Number of Diptera/m2 macrophyte (Att6Wuc..h6) at the three vegetated sites between 1976 and 1977. 81 ..J :::>.., z ..,:::> ,....,... (l)... ~ .....(l) .... 0 0 :;: ct ~ :::> UJ a: a.. ct ct z UJ :r: 1- c:( (/) 0 0.. 1- a: ~ 0 ct >- c:( 0.. z (/) (/) (!) ::;) ct ..J ..,::;) z ..,::;) ~~·~--~~...... _.I I l I I I I j 0 0 0 0 0 0 0 C'l 0 0 0 0 0 0 0 0 8 0 0 0 0 0 0 0 0 M <'II ..- M N .- M N """ 3lAHd01:J:)V~'4 zV4 I VH3ldl0 :10 H3l.H"JflN 82 genus of the United States. It generally feeds on decaying organic matter. The contribution of the Cyclorrapha and Brachycera to the study area was generally small. These suborders appear to be poorly re represented in the littoral zone. The Nematocera was represented by 4 families and were the dominant dipterans in all study sites (Table 12). The Ceratopogonidae (biting midges) was represented by 2 genera, Bezzia spp. and Pafpomyia spp. Some of these midges are herbivorous and some are carnivorous on each other, Crustacea, and small oligochaetes; however, they generally live as predators and scavengers in mud or sand. Pafpomyia spp. was the dominant dipteran taxa of the submersed stems of Spa4ganium and Sagit~. It also exhibited a significant preference for macrophytes as a substrate; this was evidenced by the number of benthic individuals being considerably less. Tipulidae (crane flies) rtJas represented by Enopte.!La spp., Hexa.toma spp., and Umn.opft-U_a spp. The family is characterized by carnivorous and herbivorous detrital feeders. Larval duration last from 6 weeks to 4 years, but most have a 2-year life cycle. Hex.atoma spp. predominated among the crane fly genera identified. It was benthic, predominating beneath Spa..,'Lgan[um and to a lesser extent under Sag.{;tfJVU.a... No crane fly genera were observed from non-vegetated areas or submersed stems of vegetation. The Culicidae or mosquitoes Here represEnted by Anophuv.:, spp. and Culex spp. Larvae feed on small invertebrates, algae, and detrital particles that are collected by their mouth brushes. They are usually suspended in various positions from the v;ater surface except when 83 disturbed or feeding. They were an insignificant group in this study were probably incidentally collected in bottom samples. The remaining 25 genera were all representatives of the Chironomidae true midges. This is the largest aquatic dipteran family and includes approximately 2,500 species. Chironomid larval stages range from 2 weeks to 4 years with one generation in two years to several generations per year. Aestivation is rare in this family, but some species over winter in larval hibernation cocoons, emerging the following year as soon as ice melt. Larvae are generally herbivorous and feed on diatoms, other algae, higher aquatic plants, and organic detritus. There is, however, a complete rAnge of feeding havits with some groups being predaceous on other midge larvae, Crustacea, and small >vorms. The Chironomidae in the present study exhibited a variable composition of specific genera s t specific sites; hovlever, three genera tended to be dominant at specific sites. Generally, Polypeditwn spp. dominated the submersed petioles of Nympha.ea. and to a lesser extent those of Spcur.garuum. In all vegetated areas, it exhibited a preference for macrophytes as a substrate, VicJwtendipv., Has generally benthic; the largest number of individuals occurred in non-vegetated areas where it was the dominant dipteran taxon, Clv.JWVtOillU;S spp. was the dominant midge on sediments underlying Sa.g.Lt:ta.w and Nympha.ea.. At these locations, it dominated all other chironomid taxa and to a lesser extent in the non-vegetated area. Some midge larvae, such as C!Lyptoc.hJAonomu.J.:. spp. and Cunota.nypuo spp., exhibited an exclusive substrate preference for sediments. Only one taxon, Eui2/Lr.6fle/t-[el£a spp., was exclusively associated with vegetation; it was found only on submersed Nymphac_a. petioles. Two taxa, ()ldhocl.adiuo spp. and Tanypuo spp., were benthic only in the non vegetated area (Site IV). The remaining groups, such as Laut~bo~nietta spp., showed variable site specific substrate preferences or exhibited little preference; the numbers of individcals on macrophytes and on sediments being similar. Gastropoda The freshwater Gastropoda are part of the large group of molluscan fauna. Generally, they are widely distributed; however, families, genera, and species exhibit restricted geographical ranges. Gastropod densities were correlated with shallow, hard water of intermediate trophic status in Navigation Pool No. 8 (Elstad 1977), however, almost all species of freshwater gastropods have extreme environmental tolerances. In the present study, non-vegetated areas produced substantially greater densities of gastropods (Figs. 18 and 19). As a group, the Gastropoda exhibited seasonality in population dynamics; generally, the greatest 2 number of individuals/m occurred in Slli~er and fall months, with peak densities occurring in July and October. The smaller species of North American fres1n-;rater snails tend to have an annual life cycle (9-15 months) with reproductive periods occurring in spring or fall or over the course of a summer. Over-wintering of juveniles and adults in species with longer life cycles is accomplished by aestivation in the sediment. Some species migrate to deeper water, but some studies indicate that winterkill extensively destroys gastropod populations (Bickel 1965, Mag rude 19 34, Ho1f er t and Ilil tun en 1968) . Two large gas tropocl subclasses \..rere represented by the 10 genera collected from the study area. The Prosobranchia are characterized by 2 Fig. 18. Number of Gastropoda/m sediment at the vegetated and non-vegeta sites between 1976 and 1977. ·- .,5 z \ ~ ;l - >- ""2 - .."' "" - "'.. 2 1- .. ..."' ,__ "' !\ .. I ,____:"' ,.._" ~ () w 0 r- > 0 z ,__ .... () 0 - .. "' - { :::>" ) " - ( _, :: I - ~ ";.; I I I I I I -~-·---·-· I I I I 0 0 0 0 0 § § (' 0 , ., 0 "' ... ~ ~ ~ ;,: ~ ~ ~ ~ ~ 2 Fig. 19. Number of Gastropoda/m macrophyte (Au6wueh6) at the three vegetated sites between 1976 and 1977. 86 -' ,:::> I-- z ,::> ...... ,..Ol .....ID ,..Ol ... 0 0 ~ Q. Cl ::> z ::>-, T---~~~-"'"'1--i~~,..---T~-~-· 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 f:') N l"l N •- M N :lV.HdOH:>Vl\1 z;i'4 I VGOdO~lSV:J ::!0 H381'lnN U/ baving an operculum, ctenidia, ponderous shells and being slow-moving and include Am~~ola spp., Valvata spp., Campeloma spp., Liopfax spp., and Vivipan~ spp. (Table 13). The Pulmonata are non-operaculate, have vascularized mantle cavities, light-~veight shells and are active and include FeJt/u~Mia spp., PhLjJ.>a spp., Gy!taul~ spp., Helif.>oma spp., and Menet~ spp. PhyJ.>a spp. and Gy!ta~t~ spp. were dominant gastropods on submersed stems of SpMga~~ and Sagi.t-ta!tia and PhyJ.>a spp. and Am~~ola spp. dominated the leaves and petioles of Nymphaea. In the sediments, PhyJ.>a spp. and Gy.Jtaul~ spp. dominated beneath SpMgani~ and Sagi:t.:ta!Ua. The sediment underlying Nymphaea was dominated by Am~~ola spp. and to a lesser extent by Valvata spp. This same distribution of species occurred in the non-vegetated areas. Gy.Jtaul~ spp. and Helif.>oma spp. preferred macrophytes as a substrate while Amni~ola spp. showed a preference for sediment. Generally the Gastropoda are omnivorous although the quality of food seems to be important in reproduction and growth. Living algae are the principle food items, but detritus is also ingested. Certain species are thought to devour the egg masses contained in cocoons of Hirudinea. This is an antagonistic relationship because gastropods are also prey items for many of the same leech species. Pelecypoda The Pelecypoda (Clams and mussels) form the second major group of freshwater molluscan fauna. The North .~erican native species consist of t>.;ro superfa:nilies, the majority being Unionacea or freshwater mussels aud the Sphaeriacea or pill and fingernail clams. The Pelecypods are widespread in Navigation Pool No. 8 with little habitat preference, 00 but restricted from stagnant marshy openings (Elstad 1977). In this study, clams as a group had their maximum densities in the non-vegetated 2 sites; the number of individuals/m was generally lower in vegetated sites (Fig. 20). Occasionally, they are observed on submersed stems, but this is a consequence of the basal petiole growth of these plants. Although Pelecypoda are found generally in non-vegetated habitats,. the Sphaeridae are less specific in habitat preference and are found in all types of bottom. Clams exhibited seasonal changes in population dynamics. Peak densities occurred in summer and fall months especially in the vegetated areas, and a maximum was observed during the winter in non-vegetated areas. }lliximum biomass per area occurred in early summer, with maximum densities in late summer. Being uniformly hermaphroditic, reproduction in the Sphaeridae is continuous throughout the year with few young being released during the winter. Growth in the Sphaeridae extends from April to September, but it is thought that clams live no longer than 12-18 months. Some species of Pelecypoda migrate to littoral areas in spring and early su.111mer and return to deeper water during the win.ter; hmvever, small clams may remain for relatively long periods deep in the substrate in a resting state. Clams burrow well below the sediment surface, therefore, their food consists primarily of organic detritus and zooplankton. Although 8 taxonomic groups comprise the majority of the inverte- brate conmmnity sampled, only 6 groups vere actually dominant. Eased on their frequency of occurrence, Lepidoptera and Pelecypoda occurred in ouly 2 site,, Hhere they att.1.ined sufficient densities to warrant 2 Fig. 20. Number of Pelecypoda/m sediment at the vegetated and non-vegetated sites between 1976 and 1977. 89 ------r------r------\ 0 :E "'... < ::> ii' z 1-"' < < ... a: "' ... "< ""'> c.. I ... z I ""' "' ... 0 "' ~ z ...,.... --·---I I I I I I I --.--~---~- I i I I 0 0 0 0 c 0' 0 0 0 0 0 ' 0 0 0 0 0 g 8 0 g g 0 g 0 0 0 0 0 ~ ,., 8 8 "~ N ~ ... ~ M ~ .. " .. . "' 1NJmaJ!:: zrt I Vt10dA:>11J.d .1(1 t:llOo~nN 90 inclusion as dominant groups. Hirudir.ea and Diptera were ubiqui tous in vegetated and non-vegetated sites. Oligochaeta and Amphipoda occurred as dominant groups in all sites except as Au6wudU. on Sagil;taJU..a for oligochaetes and SpaJLganiwn for arnphipods. Isopoda were not dominant at Site III, and Gastropoda were not dominant as a benthic group at Sites I or II. Therefore, only 6 groups comprised the majority of the inverte brate community and exhibited only site specific changes in rank of occurrence and abundance. Previous studies of littoral macroinvertebrates indicate that production in vegetated areas is usually higher than in non-vegetated areas. With some qualification, data from the present study generally agreed with these obs ervat1ons.· Production ranged from slightly less 2 than 3,000 individuals/m2 to greater than 14,000 individuals/m during early su~~er in vegetated areas (Figs. 21, 22, 23 and 24). During this same time, production in non-vegetated areas was approximately 4•500 to 5, 500 individuals/rn2 The production in vegetated ar-eas ~;vas' there fore, greater than in non-vegetated areas during the summer' but only for a 3-4 month duration. The initial autumnal increase in non-vegetated areas corresponded to the senescence of macrophytes in late September:' and the peak in November occurred several weeks after winter freeze in the vegetated areas. Much of the November peak was due to the presence of l.:rge numbers of Diptera in the non-vegetated areas. It is also the result of an increase in the number of Amphipoda. Although the increase in amphipods n coL!ort t by late year my represent t rep 1 acemen breed;ng paJ"rs, jt · "' 1. bl h . . d" enuu1·gration from ~ -- · 1 8 ·~re proua e t at lt 1n 1cates the ve.geta.ted aTeas. F • the Diptera. •urthermore, aestivation is rare :.LD ' - 2 Fig. 21. Areal production (total number of individuals/m ) from the associated sediment underlying Spa.Jtgan{.wn e.WLyCLUtpum between 1976 and 1977. 91 ____s ------ I I I I I I g § g 8 § 0 ~ M "' "' ~ .tN1WHI:J'S zr4 $11'00!/I!ONI JO ~3ortnff 1'f101. 2 Fig. 22. Areal production (total number of individuals/m ) from the associated sediment underlying Sag-Lttahia tatinolia between 1976 and 1977. 92 < ir < I I- < ""' ,. 0z 1- <.> 0 ------=--. -======I I I I I I I 8 § 8 8 .. ~ ~ "' ~ .. J.H'"J~IOJS tf'll I S1YOO () "'c > 0z ~ I I I I I I I I -----I I I I I 0 ' 0 0 0 0 0 0 0 § C' 0 ~ 8 8 0 8.... 8 ~ 8 g ~ ~ ~ ~ .. "' "' .. ~ ~ --- 2 Fig. 24. Areal production (total number of individuals/m ) from the sediment at the non-vegetated site between 1976 and 1977. If- a:: < 2 .. "'.. % .,::> IL I I I --~- I I I I I I I ---~-·· I I 0 0 0 b 0 g 8 8 ...8 8 2 ... ~ :: ~ 2 ~ ~ "' ~ "' ~ " ~ ~ l.Hlri1011S liiJI;l I S1Vn0! 1\IQNJ •o tjJtH"ON WiO! 95 Migration or emmigration of macroinvertebrates to these areas is, however, entirely speculative. Other macroinvertebrate groups were not well represented in the November peak. Most other groups over-winter as eggs or small immature organisms, or they aestivate in the sediments below the frost line. In addition, it was noted that most macroinvertebrates have an annual life cycle with breeding in spring or summer or continuous throughout the year and mortality in the fall or winter months. Hence, it should not be unusual to find individuals whose life cycles extend beyond a year but do not aestivate in the non-vegetated areas during ice cover. Further more, the non-vegetated area was in close proximity to the vegetated areas, and winter freeze did not extend into the sediments at this site. The most commonly measured physical and chemical variables of lentic environments do not seem to affect the distribution of macroinvertebrates, but some other factor or factors are involved. Fe>.;r significant corre lations bet,veen invertebrate groups and macrophyte surface area or physical and chemical variables were noted. Moreover, there were few 2 correlations between numbers of individuals/m of benthic macroinver tebrates and numbers of individuals/r/ of the Au.&Du.c.h.,!:J cmmnunities. This study and other studies in the literature indicated that the particular habit and growth form of the macrophyt.e determines taxonomic composition of the invertebrate conununity. The present study also indicated that the presence or absence of aquatic macrophytes also determines taxonomic composition. It is more probable. that the macrophytes do not directly determine composition, rather they affect particular oviposition or breeding behavior of the invertebrates. Benthic macroinvertebrate communities of vegetated and non-vegeta ted areas were distinct based on areal production. Generally, vegetated areas were composed of an Oligochaeta-Hirudinea-Isopoda complex. These 2 taxonomic groups tended to maintain maximum numbers of individuals/m of sediment when vegetation was present. The non-vegetated areas were dominated by an Amphipoda-Diptera-Gastropoda-Pelecypoda complex. These 2 taxonomic groups tended to maintain maximum numbers of individuals/m of sediment when vegetation was absent. Production of the Au-Qwuc..h.-6 communities on macrophytes was lower than production of the benthic communities inhabiting underlying 2 sediments. Generally, Nymphae.a supported more invertebrates/m than 2 Sag~, and Sag~a supported more invertebrates/m than Sp~ganit~ (Fig. 25). Furthermore, each macrophyte stand had a unique macroinvertebrate taxonomic composition. Standing crops of Oligochaeta were higher on Sp~ganium and Nymphae.a_ than on Sag~. Sag~ahia stands, however, supported more isopods and leeches than either Sp~ganium or Nymphae.a. In·addition, the specific taxonomic composition of the Hirudinea was similar, e.g., Hel.obdella .6:tagncct.U, was the dominant species of all three macrophyte species. Nymphae.a stands supported greater numbers of Amphipoda, Gastropoda, Diptera, and Lepidoptera. Dissimilarities in taxonomic composition of the Au6wu.dg on macrophytes are evident for the last three groups. The specific genera of gastropods were different for each plant stand. Spa!Lgavu.wn was dominated by Gyr...a.u.f.U-6 spp. and to a lesser extent by PhLjJ.:,a spp. SagLUcuU.a was dominated by PhyJ.:,a spp., and to a lesser extent by Gyiz.auXu.J., spp. Nymplw.e.a -v:as domina ted by Phy.6a spp. and 2 Fig. 25. Areal production (total number of individuals/m ) from the macrophytes at the three vegetated sites bet\veen 1976 and 1977. 97 NYMPHAEA 5000- ....w >-:r D.. 0 a: 0 SAGITTA RIA <( :::0 5000- N :::€ ..... (/) 4000- ..J <( ::> 0 > 300o- i5 :::; t.L 0 2000 a: w Ill ::;: z::> ..J ~ 0 1- SPARGANIUM 4000- 3000- 2000- OCT Amnic.ofa spp. in equal numbers. With regard to the Diptera, SpaJtganiwn Sagit1aA~ were dominated by Pafpomyia spp., whereas Nymphaea was dominated by Pofyped.Ltum spp. Differences were more obvious v1ith the Lepidoptera. Only Nymphaea was associated with this taxonomic group. No Lepidoptera were observed on submersed stems of either SpiVtganium or Sag ).;t;taJv[a. • An analysis of the benthic communities underlying each macrophyte indicated the same taxonomic group that dominated the Auawuc.ho communi ties dominated the sediments. For example, the Au.t)wudu and benthic communities of SpaJtganium were dominanted by the Oli.gochaeta. The tax onomic composition of the benthic fauna was different from that of the Aut)WtLdu, and at all sites, areal production was greater for the benthic communities. In the Hirudinea, Huobd~ .6ta.gm:0f._{A was the dominant benthic species on SpaJtganium and Sagd;ta_;Ua, but H. uongata. was dominant on Nymphaea. In the Gastropoda, Phy,oa spp. and GyMufLU~ spp. dominated the benthic community beneath SpiVtganiwn, G!:fMLii_U-6 spp. ·dominated beneath Sag--i..t:ta!UIL, and Amn--i..c.ofa spp. dominated beneath Nymphaea. This \vas similar to the Au.t)wu.du communi ties of the macrophyte stands. The difference in taxonomic composition was evident for the Diptera. The benthic community beneath SpCULgcouum w·as composed primarily of Hexa..toma spp.; hO\vever, SpcV1.ganiwn possessed the most diverse dipteran benthic community. The benthic community of SagJ..tta.r~.--i..a was co-dominated by CIU!r.ononnv!. spp. , V--i.c.JLo.te.ndJ.pe-6 spp. , Lcu.LteJtbotLniuta spp. , and He.xa.toma spp. The benthic community of Nymplw.ea was dominated by the single genus Chuwnomu,o spp. 99 An analysis of the benthic conununities beneath macrophytes indicated dominance of the same groups that dominated as Au6wuc.h6. The species composition of some of these groups was, however, different, 2 and the benthic production (number of individuals/m ) was generally higher. The presence of representatives typical for one cow~unity, such as the Au6wuc.h6 in the benthic conununity (for example, in the Gastr.opoda) may have been the result of invertebrates moving from sediment to macrophyte or from macrophyte to sediment. Nevertheless, there were 2 differences in taxonomic composition and numbers of individuals/m associated with macrophytes and sediment; therefore, they are distinct communities. J.VV SUMMARY AND CONCLUSIONS A survey of the Aut)wuc.ho and benthic macroinvertebrate communities of vegetated and non-vegetated areas was conducted during 1976-1977. A total of 93,800 invertebrates were haPJested from 785 ponar dredge samples and severed plant petioles. Seventeen major taxonomic groups representing 131 taxa were weighed, identified, and enumerated. In addition, the surface areas of stems and leaves of three species of aquatic macrophytes were determined. The following conclusions can be made based on the results of this study: 1. Eight taxonomic groups comprised 90%-93% of all macroinverte brates from all sites. These taxa were Oligochaeta, Hirudinea, Isopoda, Amphipoda, Lepidoptera, Diptera, Gastropoda, and Pelecypoda. 2. The most frequently encountered taxonomic group as Aut)wuc.ho was also the most frequently encountered benthic group from asso ciated sediments at each site. There were not site specific differences. 3. Lepidoptera and Pelecypoda attained sufficient densities to be included as dominant taxonomic groups only in the Nyrnphae.a tube/W-6a stand and the non-vegetated site, respectively. 4. Hirudinea and Diptera were ubiquitous in vegetated and non vegetated sites. Site specific substrate· preferences were noted \-Jithin the ;-\mphipoda, Gastropoda, Diptera, and Lepidoptera. 5. Oligochaeta and Amphipoda were dominant groups at all sites and on all substrates except as Aunwuc.ho on Sag.Ltta..!U_a f!_a;t[t)oua for oligochaetes and Spc.it.gcuuurn ewr..yc.a/l.pu.m for amphipods. 6. Macroinvertebrate production \vas greater in vegetated areas as compared to non-vegetated areas during a 3-4 month sunuu2r period. 7. An increase in the number of benthic invertebrates/m2 was noted in fall at all vegetated sites; this phenomenon corresponded to rnacrorhyte senescence. 101 8. An increase in the number of benthic invertebrates in non vegetated areas occurred during early winter freeze and during the period of ice cover in vegetated areas. This increase was the result of large numbers of Amphipoda and Diptera, two groups that do not aestivate during winter. 9. Physical and chemical variables were constant among all sites, but exhibited seasonal fluctuations. These variables were within the environmental tolerance ranges of the invertebrate groups identified; consequently, they did not effect distri bution. 10. Few significant correlations were obtained between surface areas of macrophytes and numbers of invertebrates. In addition, few correlations were observed between numbers of invertebrates/m2 of the benthos and number of invertebrates/ m2 of the Aui)wuc.ho. 11. Vegetated areas were composed of an Oligochaeta-Hirudinea Isopoda complex. These taxonomic groups maintained maximum densities when vegetation was present. 12. 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