The Distribution of Fresh-waterProtozoa on a RelativelyHomogeneous Substrate

by

JOHN CAIRNS, JR.*Department of Zoology University of Kansas

WILLIAM H. YONGUE, JR .*West Charlotte Senior High School Charlotte, North

INTRODUCTION

Carolina

CAIRNS (1965) has shown that most species of fresh-water proto-zoans have a very low frequency of occurrence . For example, in astudy of 202 areas of streams 75% of the species recorded were foundless than 2% of the time. This is not surprising in view of the vastnumber of species theoretically available for colonization, and thefact that there is usually a continual succession of species within afresh-water protozoan `community' . The species present in a habitatat any given time may depend upon (1) invasion rate, (2) biologicalinteractions, and (3) environmental conditions . Although any ofthese may be critical it is most likely that the final result usuallyrepresents an interaction of these factors. The conceptual problemsinvolved in species - niche relationships for small planktonic organ-isms have been beautifully explored by HuTCHINSON (1961). HuT-CHINSON notes the paradoxical situation of many species in a com-paratively homogeneous environment presented by the multi-specificassociations of autotrophic phytoplankton in the freely circulating up-

* This work was carried out while the authors were respectively VisitingLecturer and Teaching Assistant at the University of Michigan BiologicalStation, Pellston, Michigan .

Received October 18th 1966 .

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per waters of lakes and oceans in which most species must be com-peting for the very limited supply of a very small number of mineralnutrients in an environment that does not provide much possibilityof geometrical fractionation corresponding to niche specificity. Thosefresh-water protozoan species associated with a homogeneous sub-strate probably have similar relationships although some variations indetail seem inevitable . The purpose of this paper is to determine andevaluate the species distribution within such a system .

METHODS AND PROCEDURES

The water used in these experiments came from an intake pipe inDouglas Lake placed about one foot from the bottom, about 35 feetfrom shore at a depth of about 17 feet . This was pumped directly toa reservoir tank located in the Aquarium Building of the BiologicalStation, and from there by gravity feed to various outlets within thebuilding. Since the storage tank held only about 200 gallons of waterand since this was used for a variety of purposes, it is unlikely thatthe average retention time was over four hours . Water from theoutlets was taken through rubber tubing to three plastic troughs 48inches long, 221 inches wide, and 3? inches deep (Fig . 1) . A baffle wasplaced at the `upstream' end to insure a fairly uniform flow and a

+41A* °

Fig. 1

second baffle was placed near the outlet to produce a constant depthof 4 inch. Three of these plastic troughs were set up each with adifferent volume of water flowing through . The first received 62 .5 mlper second; the second, 31 .3; and . the third, 13 .3. A standard 100 wattlight bulb in a goose neck lamp was placed 18 inches above the midpoint of each trough and left on for 12 hours daily . The light intensitywas not measurable with a standard Weston light meter during theremaining 12 hours . Water temperature in the test units was 2V± 2°Cand the pH was 7-8 throughout the test period . The chemicalcharacteristics of Douglas Lake water (Table 1) are for the lake ingeneral although presumably the water at the intake would be

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TABLE I

Chemical anti Physical . Characteristics of

Douglas Lake Water, June 26, 1964

Light Extinction Coefficients : Secchi disc 0 .44Photo-meter 0 .58

comparable in gross characteristics . In any case the important factorsare the uniform distribution of water within a single system and thecontrasting volumes of water of comparable quality passing throughthe three systems . The test units were started two days apart to allowample examination time at the end of the experiment . Protozoanpopulations were allowed to develop and stabilize in the units for aperiod of 16 days . During this time daily examinations were madeof the species in areas slightly `downstream' from the proposedexamination area in order that we might have ample time to examinespecies common to the system and learn to identify them readily .Although succession did occur during the 16-day period the speciescomposition was remarkably stable and the familiarity with theestablished species developed during the preliminary examinationswas exceedingly helpful. At the end of this period ten samples weretaken in the middle of each trough at equidistant points in a transectat right angles to the direction of flow and extending nearly to theedges of the trough . One ml of fluid was removed with a standard

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Depthin

Temp.Weters

°C pH

D.0 .*in

o .p .m,**

Conductivity

Free***C02 inP.P .m .

Bicarbonatecarbon inP .o .m .

TotalAlkalinityin o.p,m,

0 .0 19 .80 8 .42 10 .43 252 1 .08 30 .01 126 .21 .0 19,782 .0 19 .753 .0 19 .735 .0 19 .646 .0 18 .997 .0 18 .93 8 .44 8 .16 244 0 .036 31 .35 130 .6

10 .0 18 .7712 .0 17 .61 8,12 7 .37 258 12 .4 30 .06 127 .614 .0 13 .33 8 .01 6 .64 262 2 .57 29 .8016 .0 8 .49 7 .59 2 .60 278 8 .57 30 .02 126.418 .0 6 .00 7 .61 2 .14 276 7 .33 29 .90 124 .619 .0 5 .77 7 .55 0 .4 278 8 .40 29 .44 123 .019 .5 Bottom

* Dissolved oxygen concentration

** in micromhos corrected to 25 °C

*** Calculated

pipette from each of the ten sampling sites and an attempt was madeto identify and record the density of each species using a systemcomparable to that suggested by SRAMEK-HUSEK (1958). The ratingsystem for the number of individuals of each species per drop ofwater was :

Estimates of density of various species in a diverse mixed popu-lation on slides containing algal strands and pieces of debris are atbest educated guesses! Examination time was limited to two days foreach of the ten-unit transects . In this way examinations could bereasonably complete within a time period in which major changes inspecies composition were unlikely . Identifications were made, usuallyfrom living specimens, with standard references and with appropriatestains where necessary .

There is little doubt that relatively large size and certain types ofmovement make some species more prominent than others which, inturn, will skew the population estimate . Although fixing the materialhas some advantages for counting, it frequently distorts and evendestroys the more fragile species producing problems which in ouropinion result in a greater error than working with living material . Itis quite likely that freezing techniques, which were not available tous at the Biological Station, might produce superior results .

RESULTS

The results are given in Tables 2 through 4. As might be expectedspecies with high population densities were more likely to occur at

TABLE 2

The number of species at each of ten sampling locations after16 days operation of each of the three systems .

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Sampling Location 1 2 3 4 5 6 7 B 9 10 Average

System #1 23 17 20 15 19 25 25 25 20 22 21 .1

System #2 25 33 31 32 30 33 26 23 23 22 27 .9

System #3 14 27 22 19 26 27 20 23 27 37 24 .2

1=1 4= 8- 16 7>+1002 = 2-4 5 = 16- 323 = 4-8 6 = 32-100

TABLE 3

The number of species at each of three sampling locations after26 and 34 days operation of each of the three systems .

a large number of sampling locations than low density species .Generally there were varations in individual species density atvarious sampling points in a transect although, as was the case in

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TABLE 4

The number of locations at which each species occurred in eachsystem after 16 days of operation

Number of areas atwhich species occurred

SystemNo .1

SystemNo .2

SystemNo.3

1 22 24 22

2 3 14 10

3 3 8 10

4 7 5 6

5 2 6 8

6 4 3 2

7 2 3 3

8 4 2 6

9 4 4 2

10 3 6 1

Total number ofspecies present 54 75 70

26 days Operation 34 days Operation

Sampling Location 1 2 3 Average 1 2 3 Average

System #1 25 26 17 22 .6 23 25 27 25 .0

System #2 23 15 17 18 .3 29 25 20 24 .7

System #3 23 25 18 22 .0 26 39 23 29 .3

species diversity, no pattern was evident. Since an estimate of thedensity of individuals whose movements were restricted only bymethyl cellulose is quite subjective the data has not been included inthis paper. However, gross differences such as 0 and 6 or 7 occurredin adjacent sampling points several times and differences of threeunits between adjacent sampling locations were not uncommon . It isquite evident that variations in density occur regularly which was theonly point of importance in this particular study.

DISCUSSION OF RESULTS

The results of the sampling summarized in Table 2 suggest thatthe distribution of species is far from uniform . A t-distributionanalysis of the results obtained for each of the systems in table 2indicates that the variation in species diversity is greater than randomfor a probability of 0.05. System 1 had five samples (out of ten) .System 2 six samples, and System 3 three samples outside the expec-ted range for this level of confidence . One might suspect that thiswas due to micro variations in flow within the system . In this regardit is interesting that the greatest diversity of species in system 3 after16 days operation occurred at location 10 (37 species) and the smallest(14 species) at location 1 . These were presumably similar locationsnear opposite sides of the trough . The diversity of species at each ofthe sampling locations in each of the three systems shows no patternthat suggests a linear gradient within the system .

The average number of species was lowest in the system with thegreatest flow, next highest in that with the least flow, and highest inthat with an intermediate flow. This same pattern was evident for thetotal number of species for each system (Table 4) . It is possible thatthis represents a balance between a current too swift to permit certainspecies to become established and a current too slow to bring suffici-ent nutrients past a given point to sustain other species . However,the less extensive results for 26 and 34 days of operation summarizedin Table 3 indicate that random variation in diversity is the mostlikely explanation .

Since the estimates of density were, at best, exceedingly roughthey were not tabulated . However, if one conceeds that they havesome value, two indications are worth noting: (1) a single speciesmight vary from a density of 0 (not recorded) to 5 or 6 in a transectand (2) there was, as one might expect, a correlation between densityand frequency of occurrence in a particular transect .

It is worth noting that the species common in the Douglas Lakeplankton at various depths were regularly found in the three systems

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but the majority of species recorded were not found in the plankton .The latter may have been low density or `fugitive' species in the openwater or more likely associated with the mud substrate at the bottom .

The results of this study suggest that within a presumably homo-geneous environment the individuals of a species are not uniformlydistributed . No patterns were evident to suggest a cause for theaggregations observed .

It would appear from these results that the conceptual problemsinvolved in applying the niche concept to planktonic organisms arequite similar for protozoans inhabiting an apparently homogeneoussubstrate despite the apparent abundance of certain nutrient materials .

SUMMARY

The distribution of fresh-water protozoan species on an artificialuniform substrate was studied in plastic troughs with continuousflow of unfiltered lake water. Ten areas were sampled at the sametime in each trough and the diversity of species per area determined .Estimates were made of the density as well, but since some specieswere too delicate to preserve without destroying certain taxonomiccharacters the primary emphasis was placed on the diversity ofspecies. No patterns were evident which suggested anything otherthan a random distribution of both species and individuals withineach system. The situation appears conceptually similar to thatoriginally pointed out by G . EVELYN HUTCHINSON in `The Paradoxof the Plankton' in which he notes the difficulty of applying the nicheconcept to the multi-specific association of autotrophic phyto-plankton in the freely circulating upper waters of lakes and oceans .

ACKNOWLEDGEMENTS

We are grateful to Dr . ALFRED H. STOCKARD, Director of theBiological Station for information and advice about Douglas Lake .Dr . GEORGE W. SAUNDERS, University of Michigan, provided thedata given in Table 1 . Dr . DENNIS STRAWBRIDGE, Michigan StateUniversity made several helpful suggestions regarding the analysis ofthe data .

REFERENCES

CAIRNS, JOHN, JR ., - 1965 - The environmental requirements of fresh-waterProtozoa. Third Seminar August 13-17, 1962 on Biological Problems

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in Water Pollution. U. S. Dept. Health, Education, and Welfare, pp48-52. (Abstract pp 385-386) .

HUTCHINSON, G . E., - 1961 - The paradox of the plankton . Amer. Naturalist,95 (882): 137-145 .

SRAMEK-HusEK, R., -1958 - Die Rolle der Ciliaten-analyse bei der biologischenKontrolle von Flussverunreinigungen . Verh . int. Ver. Limnol., 13 :636-645 .

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