Lake Itasca Research Articles

Deposition and resuspension of particulate organic matter in the sediments of Lake Itasca

Deposition and resuspension of particulate organic matter in the sediments of Lake Itasca

Everyone's country estate: a history of Minnesota's state parks

In 1891 Minnesota established its first state park at Lake Itasca, the headwaters of the Mississippi River. In the century that followed, Minnesotans and tourists from other states have enjoyed hiking, picnicking, fishing, camping, canoeing, and skiing at Itasca and Minnesotan's 64 other state parks. This helpful guide to the past in the parks will be welcomed by people who regularly visit a favorite Minnesota park, people who have set out to visit every park, and people who are newly discovering the parks' wonders.

Two New Species of Larvicidal Nematodes (Mermithidae) from Larvae of Chironomus plumosus (Diptera: Chironomidae) in Minnesota, U.S.A.

Telomermis amphiorchis n. gen., n. sp. and Gastromermis plumosus n. sp. are described from the larvae of Chironomus plumosus in the north arm of Lake Itasca, Clearwater County, Minnesota. Telomermis is distinguished from other mermithid genera in having a terminal mouth, four head papillae, a short S-shaped vagina, no visible cuticular cross-fibers, amphidal opening posterior to head papillae, two simple spicules, and a rounded posterior terminus. The post-parasitic juvenile lacks a larval horn. Gastromermis plumosus is distinguishable from most species of the genus by the mouth opening, amphidal opening and head papillae being at the same level. Four other adequately described species of Gastromermis have those characteristics. The length and shape of the spicule, amphids, and vagina is used to separate G. plumosus from the four species. Octomyomermis itascensis heavily infected the larvae of Chironomus plumosus from 1959 through 1962. In 1969-1970, it was replaced by the two new species. The 1969-1970 condition also existed

Ramalina unifolia sp. nov. from North America

Ramalina unifolia Thorns. is described from North America. At the Lake Itasca Biological Station of the Uni- versity of Minnesota there is a lichen community with an abundance of Ramalina which is conspic- uous when it falls from the deciduous trees on the lower rotting tree limbs and also as detached thalli brought down in the summer rains. One of these species has already been described as R. americana by Mason E. Hale, Jr. (1978), a species with many lobes and straight, small spores, 12-13 x 5-6 Mm. The other was thought to be R. sinensis Jatta, partly because of the abundant nerves on the surface. But unlike that multibranched species this develops as a very broad and distinctively monophyllous thallus with usually abundant laminal as well as lateral and terminal apothecia. It was confused with R. sinensis because of the abundant nerves on the surfaces, the partly decorticate interspaces on the lower surface, and the curved spores. Ramalina sinensis usually has a number of narrower lobes, marginal and ter- minal apothecia, rarely any laminal ones, and often lanceolate processes along the edges and at the tips of the lobes. Ramalina unifolia, in contrast, has a single lamina, usually with abundant laminal as well as marginal and terminal apothecia, and none has been seen with any lanceolate processes at the edges. It is markedly different from its associate, R. amer- icana, which has a very branched thallus and straight, usually smaller, spores.

Effects of Flow Regime and Cyprinid Predation on a Headwater Stream

We used descriptive and experimental approaches on Gould Creek, a first—order tributary of the Mississippi River near Lake Itasca, Minnesota to assess the influence of (1) flow regime on the colonization dynamics and abundance of invertebrates and cyprinids, and (2) cyprinid predation on invertebrates and fishes. Stream flow varied annually during the 3—yr (1984—1986) period. One dry year (1984), with few periods of elevated flow, was followed by two wet years (1985—1986), with prolonged elevated (nonscouring) discharge. The density of benthic and drifting invertebrates increased dramatically with elevated flow. Benthic riffle invertebrates in particular increased, from a maximum of 9000 individuals/m2 in 1984 to 91 000 individuals/m2 in 1985 and 51 000 individuals/m2 in 1986. Larval Hydropsychidae and Simuliidae were the primary groups increasing in abundance during elevated flow. To assess directly the influence of flow regime on benthic insect densities, flow was manipulated in six subsections of a riffle and colonization of natural rock substrates monitored. Total insect abundance was higher under elevated (nonscouring) vs. low flow within 6—8 d; after 24 d insect abundance was three times as high under elevated flow. Insect families responded differently to elevated flow, with the largest increase resulting from a pulse of colonization by larval Hydropsychidae in 6—8 d. Cyprinid density in Gould Creek also increased with elevated flow from 0.1—0.3 cyprinids/m2 in 1984 to 1—2 cyprinids/m2 in 1985 and 1986. The increased fish density was most pronounced during spawning periods in spring (May—June) and consisted primarily of older individuals. All cyprinids, except the creek chub (Semotilus atromaculatus), decreased in abundance 4—5 wk after spring colonization, regardless of flow conditions and invertebrate abundance. All cyprinids in Gould Creek selected pool habitats. Experiments in an artificial stream on Gould Creek indicated cyprinids influenced invertebrate abundance, but the effect of cyprinid predation was variable among habitats. Invertebrate abundance decreased most in structurally complex pools but exhibited little response to cyprinid predation in shallow riffle and raceway habitats. Because predation intensity varied among habitats, pool—dwelling invertebrates such as Chironomidae and Crustacea decreased more in the presence of cyprinid predation than riffle—dwelling Hydropsychidae and Simuliidae. However, if Simuliidae occurred in pool habitats, they were strongly selected by cyprinids, resulting in a significant depression in prey size in pools. Experiments in the artificial stream indicated creek chubs preyed on adult cyprinids, but larger species (adults 70—80 mm) were less susceptible to predation than smaller species (adults 50—60 mm). However, even taxa with small adult size were preyed on at a low rate, and all cyprinids strongly selected pools, with creek chubs having minimal effect on habitat use. These results suggest that (1) the hydrologic regime has broad and pronounced effects on the colonization dynamics and abundance of invertebrates and fishes in headwater streams, and (2) cyprinid predation has weaker but variable effects on the abundance of stream organisms. Predation intensity varies (a) over short temporal scales, because of the dynamic nature of flow regime and the rapid colonizing ability but short post—spawning persistence of cyprinids, (b) over small spatial scales, because of increased abundance of cyprinids in pool vs. riffle habitats, (c) between invertebrate and vertebrate trophic levels because the creek chub is a relatively ineffective piscivore, and (d) between small and large fish because many minnows have a size refuge from creek chubs.

Use of Early Goldenrod (Solidago) Patches by a Stem-boring Dipteran

I examined the occurrence of stem-boring dipteran larvae (Tipula sp.) in 25 discrete patches of early goldenrod Solidago juncea in northern Minnesota. The within-patch proportion of stems containing stem-borers varied from 9-71 %; this proportion was positively correlated with patch area, mean stem diameter and mean interstem distance. Statistical analysis of 311 stems revealed fewer empty stems, more stems containing one larva, and fewer stems with two or more larvae than expected by chance. The deficiency in larval co-occurrences was more marked in large than small patches and is consistent with that expected under a model of intraspecific competition. It is well-known that environments are patchy, and these results stress that an understanding of how populations respond to such patchiness is vital to the understanding of ecological relationships. INTRODUCTION Natural environments are patchy and consideration of patch structure is critical to both the theory and management of populations. The discontinuities that define environmental patchiness can vary on many different scales in time and space and population responses to this variability can involve the proximate physiological or behavioral states as well as the ultimate fitness of individuals (Wiens, 1976). There is no question that environmental patchwork exerts a strong influence on the distribution, interactions and adaptations of organisms in nature (see Ehrlich and Raven, 1965). If the population is to be viewed as an integrated whole, we must understand the spatial scale on which patchiness is expressed and the role of this patchiness on establishing variability in the relative fitness of individuals. There is, however, a general paucity of data on how patchiness is expressed in nature. Herbivorous insects and their favored food plants comprise a system that is particularly sensitive to spatial scales of heterogeneity (Dethier, 1959; Brown and Brown, 1977). One strategy of a plant may be to escape herbivores by being widely dispersed, inconspicuous, or in other ways to exploit dimensionality in space as a sort of refugia (e.g, Tahvanainen and Root, 1972). In response, the insect specializing on such a plant must be fairly vagile (at least during certain stages of its life cycle) and be able to locate and readily recognize host plants. It is clear that this sort of hide and seek dynamic has important implications for the fitness of both the plant and insect. An obvious focal point for studying such a system is the patches of host plants. By measuring attributes of these patches and the number of herbivorous insects within patches, it should be possible to identify some of the factors that influence patch detection, suitability and use by searching herbivores. In this paper I examine the variability in patch structure of the early goldenrod (Solidagojuncea) in N-central Minnesota and how this variability relates to the within-patch occurrence of a stem-boring dipteran (Tipulidae). As with other members of the stemmining guild, adult tipulids lay their eggs on or near the host plant. After emerging, the larvae bore into the pith of the plant and remain in the same stem until metamorphosis into an adult (see Rothschild, 1971; Rathcke, 1976; Stiling and Strong, 1983). STUDY SITE AND METHODS The investigation was made during July and August 1979, in Lake Itasca State Park in northern Minnesota. Twenty-five discrete stands (patches) of Solidagojuncea were sam-

Factors affecting the early life history of yellow perch,Perca flavescens

From 1979 to 1981 we followed the movement, diet, and growth of yellow perch,Perca flavescens, for their first 70 days after hatching in Lake Itasca, Minnesota. Perch spawned inshore during early spring; hatching occurred 10–20 days after spawning. Newly hatched perch were 5.6–6.2 mm total length (TL). Soon after hatching the larvae moved into the limnetic zone where they began feeding. This movement is probably a mechanism to escape intense predation in the littoral zone. Normally the first food of perch was immature copepods, but within a week they incorporated all common zooplankters into their diet. When the perch reached 25 mm TL (about day 40) they returned to the littoral zone, where they ate larger and more abundant prey than was present in the limnetic habitat. There is no correlation between growth rates and zooplankton abundances, which suggests that food quantity is not a limiting factor in the early life history of perch in Lake Itasca.

Niche diversification of Sphagnum relative to environmental factors in northern Minnesota peatlands

The peatlands sampled in this study are located in northern Minnesota and include 32 stands positioned in seven sites in the Red Lake peatland and Lake Itasca areas. Surface water chemistry ranges from pH values of 4.6 to 7.4, with corrected conductivity of 53 to 476 μohms cm−1 and calcium of 6.9 to 44.5 ppm. Six broad physiognomic landscape units are delimited based on quantitative analysis of the vegetation; these range from ombrotrophic ovoid islands to strongly minerotrophic forested fens. These landscape units can best be further divided or themselves characterized by the dominant Sphagnum species. Calculations of niche breadth across eight microhabitat axes and niche overlap for four microhabitat axes (pH, corrected conductivity, height above H2O level, and shade) for 13 species of Sphagnum suggest independent species utilization of the gradients. Among the mire-expanse species, niche breadths become narrower from hollow to hummock along calcium and pH gradients, whereas broadest niche breadths are present for midhummock species along the height gradient. For the taxonomically close species, S. fallax and S. angustifolium, niche overlap is greatest for conductivity and pH, whereas overlap is least along the height gradient. Quantification of niche breadth and niche overlap indicates considerable niche diversification along these microhabitat gradients, with individual species interacting independently to different gradients. These differing niche breadths along several different gradients suggest that these species of Sphagnum are largely equilibrium species, not opportunistic ones. In general, niche overlap is smaller in mire habitats where an abundance of bryophytes coexist with Sphagnum and highest in mire-expanse situations where Sphagnum is dominant.

Bacteria in the Perinuclear Cisternae of the Chrysophyte Chrysosphaerella

Ultrastructural investigation has shown the presence of bacteria in the perinuclear cisternae of Chrysosphaerella brevispina. Transmission of the bacteria was shown to take place during cell division. The presence of intracellular bacteria have been reported in a wide variety of algal classes. The Chlorophyceae until recently have had more reports than any other group of algae (Mishra, 1969; Kochert and Olson, 1970; Burr and West, 1970; Turner and Freidman, 1974; Lee and Kochert, 1976; Dawes and Lohr, 1978; Colombo, 1978). Observations of bacteria in other algal classes involve the Dinophyceae (Silva, 1959, 1962, 1963, 1967, 1978; Gold and Pollinger, 1971; Dodge, 1973; Lucas, 1982), Chloromonadophyceae (Heywood, 1978), and recently the first reports in the Cyanophyceae (Wujek, 1979), Xanthophyceae (Ott, 1979) and the Prasinophyceae (Wujek et al., 1982). The Chrysophyceae rank third in reports of endosymbiotic bacteria documented by electron microscopy (Belcher, 1969; Swale, 1969; Hibberd, 1971; Wujek, 1978; Wujek and Kristiansen, 1978). This investigation reports bacteria in the perinuclear space of Chrysosphaerella brevispina (Chrysophyceae). The only previous record of bacteria associated with the nucleus is Leedale's (1969) observation of endonuclear bacteria in the Euglenophyceae. MATERIALS AND METHODS Chrysosphaerella brevispina Korshikov was collected from ponds in the Lake Itasca, Minnesota region. The species was fixed, prepared, and examined by electron microscopy as previously described (Wujek, 1969). Sections were examined with the Philips EM 300 electron microscope. OBSERVATIONS AND DISCUSSION A number of the cells possessed bacteria in the expansion of the perinuclear cisternae (Figs. 1-3). No swelling of the perinuclear space was observed in This content downloaded from 157.55.39.212 on Thu, 09 Jun 2016 06:24:57 UTC All use subject to http://about.jstor.org/terms VOLUME 87, NUMBERS 1-2 33 o . ;. . . ,, . . .;ii .. , . t,, ', ? ? .. ..,, . -.-. , ril ' J.. ?? r?i ,:I ::i,._', O il, . ,.; , :;.,, o i '? i-. . . .. . ? , !,.,,, 7.l ? , -. . -. , .; ! .. . . : , . Figs. 1-4. Chrysosphaerella brevispina. Bacteria present in the perinuclear space (arrows). Figure 4 illustrates a dividing cell with two nuclei. Scale = 1 m. non-infected cells. Bacteria numbered from one to eight, with the usual number ranging between two to three. Bacteria were not observed in the cytoplasm of the cell. If bacteria were present in cytoplasmic vesicles as ingested particles, they could not be disThis content downloaded from 157.55.39.212 on Thu, 09 Jun 2016 06:24:57 UTC All use subject to http://about.jstor.org/terms 34 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE tinguished. The bacteria present in the perinuclear space showed no signs of being digested and appeared similar in gross morphological features to bacteria observed as endosymbionts in other algae. The presence of bacteria between the cell membrane and siliceous scale covering was not observed. Similar observations were made in a related genus, Mallomonas (Belcher, 1969); however, in a different species of Mallomonas, bacteria were shown to be living in this area (Wujek, 1978). Attempts to observe the bacteria with light microscopy using a variety of bacterial stains were not productive. These attempts included both phase and Nomarski contrast. The transmission of the bacteria from one cell to another appears to take place during cell division (Fig. 4). When the nucleus divides, one or more bacteria move with the daughter nuclei into the new daughter cells. The algae showed no external or internal damage from the bacteria (aside from the swollen perinuclear space). As cells within the same colony were observed with no bacteria, what benefit, if any, was gained from the association was not determined. Because cysts were not observed, it could not be determined if the statospore harbors bacteria during overwintering.

Twenty-Six Years of Change in a Pinus strobus, Acer saccharum Forest, Lake Itasca, Minnesota

PEET, R. K. (Dept. Biol., Univ. N. Carolina, Chapel Hill, NC 27514). Twenty-six years of change in a Pinus strobus, Acer saccharum forest, Lake Itasca, Minnesota. Bull. Torrey Bot. Club 111: 61-68. 1984.-A 0.48 ha permanent plot located in a mesic Pinus strobus dominated forest was remapped after 26 years. Pinus remained the dominant species in basal area and had the lowest mortality rate of any major tree species. The understory was dominated by Acer saccharum, Tilia americana, and Ostrya virginiana, all of which had high mortality rates. Establishment of numerous new stems resulted in a net gain in Ostrya and Tilia density whereas Acer declined substantially. Populations of the shade intolerant Populus tremuloides, P. grandidentata, and Betula papyrifera, which appear to have become established following mild surface fires in the late 1800's, all had high mortality. The shiort-term prognosis is a stable Pinus strobus canopy, loss of Populus and Betula, and a steady enrichment of the understory with Ostrya and Tilia. Acer should remain important but continue to decline in relative density. The long-term outlook is uncertain owing to the increasing probability of catastrophic breakup of the pine canopy.

The Genus Mallomonopsis in the United States

By means of electron microscopy, phytoplankton samples from several localities in the United States were examined for the silica-scale bearing genus Mallomonopsis. Three taxa are newly reported, including one new combination, Mallomonopsis bangladeshica comb. nov. Scales of one undescribed taxon are also illustrated. The genus Mallomonopsis was erected by Matvienko (1941) to hold unicellular Chrysophyceae which are morphologically very similar to species of the uniflagellate genus Mallomonas, but differ in bearing two heterodynamic flagella. The described species and varieties are M. clavata Matv. (1941), M. elliptica Matv. (1941), M. pelophila (Lund) Fott (1954) (=Ochromallomonas pelophila Lund (1942), M. robusta Matv. (1949), M. paxillata Bradley (1966), M. peroneides Harris (1966), M. salina (Asmund and Hilliard) Kristiansen (1976), M. oviformis (Nygaard) Kristiansen (1976), M. ouradion (Harris and Bradley) Harris (1966), M. parvula Diirrschmidt (1983), M. tasmanica Croome and Tyler (1983), and M. bangladeshica (Takahashi and Hayakawa) Wujek and Timpano (this paper). Marvan (1959) described M. deltoidea from Czechoslovakia. From his description and figures, this organism is clearly a The formation of mobile colonies and lack of bristles is atypical for the genus Mallomonopsis, but typical for The scales described and illustrated (with the light microscope) are characteristic of Synura petersenii complex of forms. Marvan himself states I am putting the new species in the genus Mallomonopsis at present, but it is not impossible that the creation of colonies will be more typical so that it would be preferable to put the species in the genus Synura. * Present address: Department of Botany, University of Oklahoma, Norman, Oklahoma 73019. This content downloaded from 157.55.39.254 on Sun, 04 Sep 2016 05:05:21 UTC All use subject to http://about.jstor.org/terms 74 TRANSACTIONS OF THE KANSAS ACADEMY OF SCIENCE Although there have been an increasing number of studies on American scaled chrysophytes, only one report of continental United States freshwater phytoplankton has revealed the presence of the genus Mallomonopsis (Wee et al., 1982). This study was initiated to investigate and report those taxa of Mallomonopsis that appear in the American flora, and introduces the literature of this group to American phycologists. MATERIALS AND METHODS Samples were collected and observed as previously described (Wujek, 1969). Kansas material was collected from a pond northeast of Lawrence and the Minnesota material from the Lake Itasca region (Wujek et al., 1981).

A study of summer mortality factors for natural populations of Chydoridae (Cladocera)

In order to test the hypothesis that predation controls summer population growth of the Chydoridae in Lake Itasca, Minnesota two types of in situ predator-exclusion experiments were conducted. The first series of experiments excluded all predators from chydorid populations by enclosing the chydorids within clear plexiglass chambers suspended in their natural habitat. The second series of experiments excluded fish, but not invertebrates, from chydorids by placing polyethylene exclosures directly on the submerged vegetation. The concomitant abundance of natural chydorid populations was monitored by pattern sampling adjacent vegetation. Populations of Chydoridae that were protected from all predation increased exponentially, while natural populations remaining exposed to predation decreased dramatically. Populations protected from predation by fish but not invertebrates were not significantly different from unprotected control populations. Thus predation by fish and invertebrates combined was sufficient to limit chydorid density. The hypothesis that the predation was due primarily to fish was not supported. It is suggested that because chydorids are smaller and less accessible than preferred prey items, fish are able to significantly utilize them only during periods of high chydorid abundance.

Nocturnal Inshore Movement of the Mimic Shiner (Notropis volucellus): a Possible Predator Avoidance Behavior

Diel seining documented a significant summer nocturnal inshore movement of the mimic shiner (Notropis volucellus) in Lake Itasca, Minnesota. This species initiated inshore movement at sunset, occupied the nearshore area throughout the night, and moved offshore by sunrise. Mimic shiners were collected in less than 1 m of water throughout inshore areas with sparsely developed macroscopic vegetation. Greatest relative abundance was observed at sites possessing the greatest expanse of sparsely or nonvegetated substrate. Relative abundance at inshore sites may be related to habitat characteristics of the sites or to the proximity of feeding habitat. These diel movements differed markedly from those exhibited by the mimic shiner in other lakes. The inshore movement in Lake Itasca may function to reduce crepuscular and nocturnal predation pressure. Our data support the hypothesis that freshwater fishes may possess behavioral plasticity sufficient to alter movement and distribution patterns according to prevailing conditions and that predation pressure may be important in altering prey behavioral patterns.

Male Reproductive Behavior in the Anthomyiid Fly Hylemya alcathoe (Diptera)

On early evenings in June males of the anthomyiid fly Hylemya alcathoe can often be found in large numbers at the edge of woodland clearings. In these locations the males perch on low-lying vegetation and dart out after passing insects. Mating pairs were occasionally seen on leaves. Each male tends to repel intruders from his leaf perch, but individuals rarely spend more than a few hours in any one spot. Some locations are far more attractive to males than others, apparently because of their proximity to conspicuous tree trunks and to overhanging canopy vegetation. The mating system of males of this fly appears to be analogous to that of those aerial swarming insects whose males gather over or by a prominent landmark and wait for receptive females to approach the marker-beacon. INTRODUCTION Males of the anthomyiid fly Hylemya alcathoe Walker form aggregations at certain times of the day. There are many examples of insects whose males become concentrated spatially in places where they may encounter females (reviewed by Downes, 1969; Sullivan, 1981). Certain mosquitoes, midges and other aerial swarming flies are familiar examples. Males of H. alcathoe do not, however, form aerial swarms but instead gather in favorite locations where they perch on leaves fairly close to one another. This paper describes the behavior of perched individuals, their interactions with one another, the daily cycle of aggregation formation and the properties of locations that attract relatively high densities of males. These data help define the mating system of the fly. Some speculations are presented on the adaptive significance for males of this and other insects that gather at visual landmarks to perch or swarm while waiting for females. MATERIALS AND METHODS Flies were observed beside dirt roads in forested locations from 15-22 June in both 1980 and 1981 at Lake Itasca, Minn., at the Biological Station of the University of Minnesota. Members of my classes in field ethology assisted in both years. The vegetation in the study area consisted of few scattered prominent trees and a low ground cover dominated by wild sarsaparilla (Aralia nudicaulis), a favorite perching substrate for the flies. Three sites where flies were consistently numerous were selected for intensive observation. Transects 10 m long by 1 m wide were divided into 10 1 m2 squares. Data were collected by walking past the transects and recording the number of flies in each square at various times of the day from 0700-2030 hr. A total of 50 censuses were conducted from 16-20 June 1981. The census results were used to determine the daily cycle of male perching activity and to establish whether certain squares within the transects were more attractive to perching flies than others. In addition to the census, between 15-18 June, 66 males were marked with dots of liquid paper or acrylic paint placed on the dorsum of the thorax. A few individuals were marked by daubing them while they perched on a leaf; most were netted, removed by hand, marked, then released. Although the flies were not large (ca. 7 mm long), it was possible to capture and mark individuals without damaging them. In the course of census counts, records were made of the location of any marked individuals to determine their site fidelity and longevity. Interactions between perched males were recorded to determine the tolerance of perched individuals to the arrival of an intruder on their leaf. An observer watched an

Temporal and Spatial Patterns of Abundance of the Chydoridae (Cladocera) in Lake Itasca, Minnesota

The chydorid populations of Lake Itasca, Minnesota, were monitored over a 2.5—yr period by pattern sampling. Of the 24 species collected, 15 were sufficiently abundant to allow analysis of population trends. Chydorid species have two basic patterns of abundance. One group of species exhibits only a summer density peak. The second group shows one abundance peak in summer and a second peak in fall or winter. All species experience a precipitous decline in density following the summer peak. Population growth in chydorids is limited by different factors during different times of the year. During periods of ice cover most species are apparently limited by a reduction in primary productivity due to reduced light penetration rather than by water temperature. Spring chydorid populations are limited by temperature. Summer populations, at least for abundant species, are limited by predation. Chydorid species are distributed patchily within Chara beds. Patchiness is moderate in spring, declines to low levels during summer, and peaks in the fall for most species. Patchiness remained about constant during the summer decline in density. Experimentation showed that heterogeneity of the physical habitat is not a necessary cause of patchiness. A Bray—Curtis—type ordination was performed on data from a complete year of monitoring. The primary component of variation extracted from the data matrix is due to temporal differences in abundance among species. The second major source of variation is due to spatial differences in abundance (habitat preference). The most abundant chydorid species tent to have similar habitats and temporal patterns of abundance.

Extraction by Sieving: A Biased Procedure in Studies of Stream Meiobenthos

Extraction by Sieving: A Biased Procedure in Studies of Stream Meiobenthos

Aggregation and foraging behavior of whirligig beetles (Gyrinidae)

Whirligig beetles aggregate in the daytime into dense single-and multispecies groups (‘rafts’) of hundreds or thousands of individuals. On the 22km shoreline of Lake Itasca in northern Minnesota, these aggregations were on the average 0.8 km apart, and they were usually found day after day in the same ocations.

Summer crane-flies of Lake Itasca vicinity, Minnesota

Summer crane-flies of Lake Itasca vicinity, Minnesota

Population regulation of the Chydoridae in Lake Itasca, Minnesota

Population regulation of the Chydoridae in Lake Itasca, Minnesota

Seasonal Abundance and Pattern of Chydorid, Caldocera in Mud and Vegetative Habitats

Pattern sampling (Whiteside and Williams 1975) was employed to study the population changes, habitats and spatial arrangements (pattern) of littoral cladocera in Lake Itasca, Minnesota during 1974. Population dynamics of the 19 most abundant species of chydorids had changes similar to other populations studied in north—temperate lakes. All species had either spring or fall maxima, or both, a mid—summer low in numbers. Population trends were the same for vegetative and mud habitats. In a comparison of Chara vs. mud habitat we found Chydorus piger, Leydigia leydigi, Alona quadrangularis, Pleuroxus aduncus, and Alona circumfibriata to live mainly in littoral mud. Kruzia latissima, Camptocerus rectirostris and Chydorus faviformis were most restricted to the vegetative habitat, while other species were more ubiquitous. Pielou's (1974) terms describing pattern, phase, mosaic, grain, and intensity are discussed. We redefined intensity as describing the visual effect of density. The changes of intensity accompanying changes in density typified 1974 patterns, while no changes in the other parameters were evident. Preliminary experiments exploring the behavior of chydorids suggest that visual stimuli are very important in directing movements, followed by tactile and chemical clues.