Artificial substrates which release nutrients : Effects on periphyton andinvertebrate succession

G. Winfield Fairchild' & Rex L . Lowe 2'Department of Biology, Central Michigan University, Mount Pleasant, MI 48859, U.S.A .Present address: Biology Department, West Chester University, West Chester, PA 19383, U.S.A .2Department of Biology, Bowling Green State University, Bowling Green, OH 43403, U .S.A .

Keywords: periphyton, artificial substrate, phosphate, nitrate, chlorophyll, succession

Abstract

Nutrient-diffusing substrates for periphyton were made from clay flower pots (O.D . = 8.8 cm), sealed withplastic petri dishes, and filled with 2% agar and specified nutrients . When placed in water, the nutrients slowlydiffuse through the agar and clay walls of the pots, becoming available to organisms colonizing the outersurface .

Forty-eight pots, 16 containing 0 .1 M KH2PO4, 16 with 0 .1 M NaNO 3 , and 16 with no added nutrients,were placed at 0 .5 m depth in Douglas Lake, Michigan . Four pots of each nutrient treatment were sampledfor algal periphyton and invertebrates after 7, 14, 25, and 36 days . A total of 72 algal species were enumerated .Of these, Epithemia adnata (Ktitz .) Breb ., Rhopalodia gibba (Ehr.) O. Mull, and Anabaena sp . experiencedstrong growth stimulation in response to phosphate addition. No significant effects of nitrate addition werenoted. Measures of algal community structure also reflected the impact of phosphate addition. Final algalbiomass was enhanced 10-fold, successional change was prolonged, and species diversity declined relative tonitrate and control pots . Chironomids, chydorid Cladocera, and gastropods dominated the invertebratefauna found on the pots . Densities of the small, algivorous chironomid Corynoneura nr. lobata Edwardswere highest on phosphate pots at the end of the study, in apparent response to the increase in periphytonbiomass .

Introduction

Because of their attached habit, rapid growth,and ability to integrate short term fluctuations intheir chemical environment, the benthic algae orperiphyton have long been considered a useful bio-logical tool for monitoring point sources of nut-rients (Patrick, 1973) . The abundances of certainindicator species have been successfully correlatedwith known concentrations of particular ions (e.g.,Stevenson & Stoermer, 1982) . Nutrient-inducedchanges in algal community structure (e.g., totalbiomass, productivity, species richness or diversity)have also been recognized (Weitzel, 1979; Marcus,1980) .

Studies of nutrient impacts upon the periphyton

Hydrobiologia 114, 29-37 (1984)©Dr W. Junk Publishers, The Hague . Printed in the Netherlands .

have frequently depended upon "natural experi-ments," situations in which a range of ambient ionconcentrations can be measured and correlatedwith algal samples collected nearby (Douglas, 1958 ;Stockner & Armstrong, 1971 ; Evans & Stockner,1972; Lowe & McCullough, 1974 ; Marcus, 1980) .Studies showing seasonal changes in the periphytonhave similarly associated these changes in part withparticular ions (Round, 1960; Castenholz, 1960 ;Williams et al, 1973 ; Hodgkiss & Tai, 1976 ; Hooper-Reid & Robinson, 1978) . Frequently, however, theinterpretation of nutrient effects is complicated bythe covariance of other factors (e.g ., temperature,light, current velocity), which may accompanychanging nutrient availability . Because of the po-tential for subtle effects of extraneous variables,

30

an in situ experimental design which manipulatesnutrients alone while keeping other conditions asuniform as possible is highly desirable . Recent en-closure studies (Twinch & Breen, 1978 ; Moss, 1981)are good examples of attempts to manipulate nut-rients in situ .

This paper describes the use of artificial sub-strates, constructed from clay flower pots, whichslowly release specified nutrients and thus promotethe establishment of particular periphyton com-munities on their outer surfaces . The paper furtherdetails the effects of phosphate and nitrate releasedfrom pots placed in Douglas Lake, Michigan on thecolonization and growth of periphyton and inverte-brates over a 36 day period . The experimental de-sign thus provides a test of current theory concern-ing the effects of critical nutrients upon the succes-sional process .

Materials and methods

Flowerpot substrate construction

Clay flower pots, with an outer diameter of 8 .8cm, 8.0 cm in height, and with an internal volume of245 cm3, were each sealed with a plastic petri dish,to which a #10 rubber stopper and a 20 cm length of0.63 cm wooden dowel were glued (Fig . 1). Potswere then filled through the smaller aperture with a2% agar solution in distilled water . The agar pouredinto phosphate (P) pots also contained 0 .1 MKH2PO4 . Nitrate (N) pots contained 0 .1 M NaNO 3 ,while control (C) pots contained no added nutrients .The smaller aperture was then closed with a #000rubber stopper, and the agar permitted to gel . Al-though flux rates of nitrate and phosphate from thepots have yet to be quantified fully under varyingconditions, preliminary laboratory experimentssuggest that both ions are released from the pots formany weeks (Fairchild et al, in prep .) .

Periphyton colonization study

The in situ study of periphyton succession wasperformed at the University of Michigan BiologicalStation on Douglas Lake in northern lower Mi-chigan. The lake is moderately large (15 .2 km 2 sur-face area), with extensive sandy shoals and multipledepressions of ice-block origin . Portions of the

ue 9 9 C3 P 9 96

C7 P7 N7 C7 P3

P25 N25 C25

N25

C7 P7 6 0 E0 6P7

P36 9 @ O P2 OFig. 1 . Construction and placement of substrates, DouglasLake, Michigan. Each substrate consisted of an inverted clayflower pot, sealed on the bottom with a plastic petri dish, andfitted with rubber stoppers (#000 and #10) on top and bottom.Substrates were filled with 0 .1M KH 2PO4 (P), 0.1 M NaNO3(N), or no added nutrients (C), then placed into the lake bottomto the base of a 20 cm length of wooden dowel. Four pots of eachtreatment were sampled on each of days 7, 14, 25, and 36 .

abundant published information concerning thelimnological characteristics of the lake are summar-ized in Lind (1978). Studies of littoral periphytonand their herbivores in the lake include work byYoung (1945), Eggleton (1952), Lyman (1956),Clampitt (1970), and Andresen & Stoermer (1978) .Sixty pot substrates were constructed on 3-4 July1981 . Twenty were filled with 2% agar and 0 .1 mKH2PO4 , 20 with 2% agar and 0 .1 M NaNO3 and 20with just 2% agar . The pots were then placed ap-proximately 10 m from shore at 0 .5 m depth in auniform, sandy portion of South Fishtail Bay, nearthe Biological Station's Stockard Laboratory . Thewooden dowel of each pot was inserted into thesand to the base of the rubber stopper, the pot itselfthus extending from 3-11 cm above the sand . For-ty-eight of the pots were arranged, 50 cm apart, in

the lattice pattern shown in Fig . 1 . Of these, 16represented each nutrient treatment . Four pots ofeach treatment were sampled after periods of 7, 14,25, and 36 days .

Sampling was performed by placing a plastic 400ml beaker over each pot, so that the top of thebeaker fit snugly against the expanded lip of thepot. The beaker thus enclosed all of the pot abovethe lip (surface area = 138 cm 2) and 255 ml ofsurrounding water . The pot was then removed fromthe sediments, inverted, lifted from the waterbeaker downward, and carried to a nearby boat .The water in the beaker and detached organismswere first emptied into a 1 liter widemouth samplejar. Attached organisms were then scraped from allsurfaces of the pot above the lip with a toothbrush .Washings of the toothbrush and cleaned surfaces ofthe pot were also added to the sample jar . Sampleswere preserved in Lugol's Solution, allowed to set-tle for 5 days, and then reduced to 100 ml by decant-ing .

Each sample was then well shaken, and two 2 .5ml aliquots were extracted and combined for enu-meration of algal periphyton . The resulting 5 mlsubsamples were initially scanned to allow familiar-ity with the algal flora prior to enumeration . Aportion of each subsample was then removed bypipette while constantly agitating the vial to ensureuniform suspension of algal cells . The pipetted por-tion was transferred into a Palmer-Maloney nan-noplankton counting chamber (capacity = 0 .1 ml)and allowed to settle for 5 minutes . The algal cellswere then enumerated at 450X with a Bausch andLomb research light microscope . Three replicatesfrom each subsample were counted using a singlehorizontal scan across the counting cell . A volumeof 9.6 X 10 -3 ml was analyzed from all subsamplesexcept those from P pots on day 36, for which 6 .0 X10-3 ml was examined . The number of cells countedranged from 108 to 1386 . All algae were indentifiedto genus in the counting chamber except for somenaviculoid pennate diatoms not easily identifiablefrom wet mounts . Cell dimensions were determinedfor 10 randomly chosen cells, and mean cell volumescalculated for each of the algal taxa . Mean cellvolumes were then multiplied by cell numbers toobtain biovolumes for each taxon . Species of rela-tively abundant diatoms were determined fromstandard Hyrax® mounts (Patrick & Reimer, 1966) .

The remaining 95% of each sample was retained

3 1

for total counts of invertebrate taxa. Samples wereinitially concentrated using 70 tm Nitex screening,then placed in a Bogurov counting chamber (Gan-non, 1971) for sorting and enumeration with a dis-secting microscope. M icrocrustacea and chironom-ids were removed and mounted in lactophenol foridentification using a Zeiss research light micro-scope .

Twelve additional pots, 4 of each treatment, wereplaced in the lake directly adjacent to the samplinggrid. These were sampled on day 37 of the study forchlorophyll-a analysis . Pots were collected andscrubbed as previously described . Ten ml subsam-ples were then filtered (Millipore®, 0 .45 µm), dis-solved in buffered 90% acetone and analyzed usinga Turner 111 fluorometer . Chlorophyll-a valueswere corrected for phaeophytin (Holm-Hansen etal, 1965) .

All statistical comparisons of treatment effectson the densities of particular taxa were performedusing 1-way Analysis of variance (ANOVA) . Thedata were first log-transformed where necessary tomeet the assumptions of the procedure . Subsequenta posteriori comparisons utilized the Scheffe's SMethod (Scheffe, 1959) .

Results

Chlorophyll-a concentrations taken on day 37 ofthe study are shown in Table 1 . The N and C potsdid not differ significantly, with 1 .28 (SE 0 .37) and1 .07 (SE 0 .17) mg . m-2 chlorophyll-a, respectively .The P treatment had a chlorophyll-a concentrationof 14.73 (SE 2 .27) Mg . M-2, significantly higherthan either the C or N treatments (p < .001) .Phaeopigments were found in small quantities . Theratios of phaeopigments to chlorophyll-a were 0 .05(SE 0.02), 0 .08 (SE 0.05) and 0.16 (SE 0 .10) for P,

Table 1 . Means and (SE) for total biovolume on day 36, andchlorophyll-a adjusted for phaeopigments on day 37, forphosphate (P), nitrate (N), and control (C) substrates . The ratioof chlorophyll-a/ biovolume is expressed as (µg . µm-3 ) 10-8 .

Treatment Biovolume

Chl-a(µm3 106 . cm 2) (Mg . M -2)

Chl-a/ Biov.((µg . µm-3) 10-8 )

P 84 .52(8 .56) 14 .73(2 .27) 1 .74N 7.89(0 .85) 1 .28(0 .37) 1 .62C 7 .42(1 .51) 1 .07(0 .17) 1 .44

32

a0U)FZ

U)N

100 -

10

NAM7-14

DAY

Fig. 2a-d. Estimates of (a) total biovolume, (b) succession rate,(c) taxon richness (S), and (d) taxon diversity (H'B) based onbiovolume proportions for phosphate (P), nitrate (N), and con-trol (C) pots . Vertical lines denote I SE on each side of the mean.

N, and C pots respectively . Ratios of chlorophyll-ato total biovolume, expressed as (µg. µm-3 ) 10-8 ,were quite similar for all three treatments, despiteprobable differences in nutritional state and effectsof differing species composition, and both chloro-phyll-a and total biovolume are thus in fairly closeagreement as estimators of total biomass .

Accumulation of biovolume (Fig . 2a) was rela-tively slow on all three substrate types through day14, and ranged from4.5 X 10 6 to 7.1 X 106 µm3 - cm-2for the three treatments. By day 36 total biovolumesof the C and N substrates had leveled off at about 8X 10 6 µm3 . cm-2 . On the P substrates algal biovol-ume continued to accumulate rapidly after day 14and by day 36 had reached 8.4 X 10 7 (SE 1 .7 X 10 7 )µM3.cm-2 .

14-25

14

25

25-36

It

36

Two indices were used to determine nutrient ef-fects upon periphyton community structure overtime: a measure of succession rates (SRI), and tax-on diversity values (HB) . The data used for bothmeasures were the biovolumes for each taxon rath-er than cell numbers to account for large cell sizedifferences between taxa and emphasize the meta-bolic responses of particular taxa to the nutrientadditions .The Succession Rate Index SRI (Jassby &

Goldman, 1974; Williams & Goldman, 1975) wasused to evaluate rates of successional change im-posed by the three nutrient regimes :

SRI (a, b)=(I((fib-fia)/b-a))2)i 2

(1)i=lwhere a and b denote sampling times (in days) . Theterms fi a and fib represent the fraction of total biovo-lume diversity contributed by the ith taxon on the 2sampling dates (Armstrong, 1969), and are calcu-lated using the equation :

nfi = (B ilog2(B i / B))/ (-I B i log2(Bi/ B))

(2)i=1

where bi = the biovolume of the ith taxon, and Brepresents total biovolume . The SRI is thus anindex of aggregate change per day in the relativeproportions of the algal taxa . It is most sensitive tochanges in taxa comprising large portions of totalbiovolume. Each of the replicates for a nutrienttreatment was randomly paired with a replicate ofthe same treatment obtained on the following sam-pling date, yielding 4 independent SRI estimateswhich were then averaged .

Values for SRI were initially quite similar for all3 treatments, together averaging 0.039 betweendays 7 and 14 (Fig. 2b) . Subsequently, values on theP pots rose 0 .070 (SE 0 .020) between days 14and 25, then declined to low values between days 25and 36 as community composition stabilized . Incontrast, values of SRI generally declined onthe N and C pots throughout the study. Differ-ences between treatments were not significant forany of the time intervals .

A total of 72 algal taxa were enumerated . Fifteento 30 taxa were typically found in counts of individ-ual pots, and had achieved near maximum valuesby day 14 (Fig . 2c) . Taxon diversity using relativebiovolumes (HB) was also calculated for each pot asadvocated by Wilhm (1968) :

m0

X

IW

nHB = - I (B;/ B) log2 (B,/ B)

(3)=1

where again Bi/ B = the proportion of total biovol-ume represented by the ith taxon . Near maximumvalues of H B were achieved by day 7, and subse-quently changed very little on the N and C sub-strates, but declined on the P substrates (Fig . 2d) .

Diatoms (Bacillariophyceae) were the most abun-dant group of algae on all substrates, although therelative abundances of dominant taxa varied amongtreatments . Green algae (Chlorophyceae) were rela-tively insignificant in all treatments while blue-green algae (Cyanophyceae) were most abundanton the P substrates . Individual taxa comprising thegreatest percentage of total biovolume are dis-cussed below. Mean cell volumes are also noted foreach .

100-

10-

.01

to-b. Rhopalodia gibba

1 .0-

.01

100

10-

1.0-

.01

.1-

a . Epithemia adnata

c. Gomphonema tenellum

7

14 25

Most of the relatively large increase in algal bio-volume seen on the P substrates can be attributed toEpithemiaadnata(Kutz .) Breb. (Fig . 3a) . This rela-tively large (1400 µm3 ) diatom increased in bio-volume from 7 X 10 5 µm3 . CM-2 on day 14 to 7.5 X107 µM3.cm -2 on day 36, accounting for 89% of thetotal biovolume at the end of the study . Densitieson days 25 and 36 were significantly higher (p <.001) than for either the C or N substrates, whichsupported comparatively small populations of Epi-themia . The amount of surface occupied per cellwas estimated to be 350 µm 2 , and a maximum of18% of available surface may thus have been occu-pied by Epithemia on the P pots by day 36 .

Rhopalodia gibba (Ehr.) 0 . Miill . (Fig. 3b),another relatively large diatom (2550 µm 3), alsohad significantly larger populations on the P pots

1 .0

.1

.01-

.001

10

1 .0

.1

.01

10

1 .0

d. Anabaena sp .

e. Achnanthes minutissima

i

f. Naviculoids

.136

7

14

25

36

DAY

33

Fig. 3a-f. Cell biovolume (µm3 10 6 )cm-2 for phosphate (dash-dot lines), nitrate (solid lines) and control substrates (dashed lines) . Verticallines denote l SE on each side of mean .

34

than on either the C or N pots on day 25 (p < .01)and was significantly more abundant than on the Npots (p < .05) on day 36 . Rhopalodia, like Epithe-mia, showed rapid growth on the P pots until day25, then began to level off between days 25 and 36 .

The filamentous blue-green alga Anabaena sp .(5 .8 µm3) (Fig. 3d) showed significantly greatergrowth on the P substrates than on the N or Csubstrates on day 25 (p < .05) and 36 (p < .01) .Although initially a small portion of total biovol-ume, Anabaena continued to grow rapidly on the Psubstrates at the end of the study when other taxahad begun to experience growth declines .

The biovolume of the stalked diatom Gompho-nema tenellum Ktitz. (300 µm3 ) showed similar in-creases throughout the study in all 3 treatments(Fig . 3c) . Achnanthes minutissima KUtz. (40 µm 3 )also experienced no significant growth response toP addition, but did show consistently higher biovo-lumes on the N pots vs . control pots on all 4 sam-pling dates (Fig . 3e) . Miscellaneous naviculoid dia-toms (Fig . 3f) were composed primarily of severalspecies of the genus Navicula that were indistingui-shable in the Palmer cell. This group experiencedrapid growth until day 14, when populations eitherleveled off or declined. As with Achnanthes, thebiovolume of these naviculoid diatoms was consist-ently higher on the N rich substrates .

Of the invertebrates which colonized the pots, thechironomids were numerically dominant . One spe-cies, Corynoneura nr . lobata Edwards (Fig . 4a),constituted more than 50% of all chironomids on all4 sampling dates . Corynoneura increased markedlyon the P pots on day 36, with significantly higherdensities than on N and C pots (p < .01). Numberson N pots declined to 0 on day 36 . Other chironom-ids (Fig. 4b) were primarily early instar Orthocla-diinae and Tanypodinae .

The chydorid Cladocera (Fig . 4c) had attaineddensities averaging 10 individuals per pot by day 25 .Acroperus harpae Baird and Alona spp . were dom-inant . Gastropods (Fig . 4d) consisted chiefly of veryyoung individuals, not identifiable to species, whichmay have developed from eggs deposited on the potsearly in the study . Individuals of Goniobasis lives-cens (Menke), Physa parkeri (Currier), P. integra(Haldeman), Campeloma decisum (Say), Helisomaanceps (Menke), and Amnicola limosa (Say) werealso present . A diversity of other taxa were found,but at low densities relative to those shown .

50

40

10

10

I-0a

WmDz

5

20

10

10'

c. Chydorid

_{Cladocera

t

d . Gastropods

5 .

,}off7

14

25

36

DAY

Fig. 4a-d. Densities of major invertebrate taxa on pots contain-ing phosphate (dash-dot lines), nitrate (solid lines), and noadded nutrients (dashed lines) . Vertical lines denote I SE oneach side of the mean .

Discussion

Most true phytoplankton reached the pots rapid-ly, but experienced little benthic growth . For ex-ample, Cyclotella comensis Grun. and C. michiga-niana Skv. declined from 1 .12 X 105 µm3 .cm-2(5.1% of total biovolume) on day 7 to 1 .08 X 10 5µM3.cm-2 (0.3%) on day 36 .

Members of the periphyton, whose entrance intothe seston can be facilitated by storms, human re-creation, and similar disturbances, were probablyderived principally from the surrounding epip-sammic community, from the epiphytic algae ofnearby stands of Vallisneria americana Michx. andPotamogeton spp., and from epilithic algae of wa-ve-swept rocky shoals in the lake . Current patternsin Douglas Lake have been well described (Gannon& Brubaker, 1969; Gannon & Fee, 1970), and sug-gest the potential for transport of suspended algaethroughout South Fishtail Bay . Young (1945) has

also demonstrated the importance of wave action inscouring epiphytic algae from culms of Scirpus va-lidus Vahl. on an exposed, sandy shoal just north ofthe study site. We feel that initial colonization of thenew substrates by the algae was passive, and that allflower pots were equally accessible to algal colo-nists .

Invertebrates which colonized the pots were alsopresent in the sediments and upon nearby vegeta-tion, and arrival on the pots probably involvedprimarily substrate-associated movement or weakswimming over short distances . Although the mi-crodistribution of most benthic invertebrates isknown to be very patchy as a consequence in part ofsubstrate depth and location (Fairchild, 1981), theseparation of replicate pots, the constant depth andapparent uniformity of the sandy bottom supportthe contention that pots of all treatments wereequally accessible to invertebrate colonists as well .

Despite evidence that invertebrate grazers maystrongly influence periphyton biomass in some si-tuations (Cattaneo, 1983), field studies which haveincluded estimates of invertebrate densities haveremained few . In our study, only the chironomidCorynoneura nr . lobata showed a significant nu-merical difference among nutrient treatments . Co-rynoneura is a small, free-living, and highly mobilechironomid which appears to feed almost continu-ously on algal and detrital material . Kesler (1981)has demonstrated mean grazing rates of 0 .025 µgdry weight algae per minute by 4th instar individu-als of C. scutellata W innertz, and has estimated that3-15% of daily algal accumulation was removed bythis species in Nonquit Pond, Rhode Island . Weobtained maximum densities of primarily early in-star Corynoneura on the phosphate pots, in appar-ent response to the greater algal accumulations .

Densities of other invertebrates differed very lit-tle between treatments . Most abiotic factors knownto influence periphyton succession (e .g ., currentvelocity, temperature, light, pH, substrate texture-[Patrick 1977]) similarly can be expected to haveaffected all pots equally . Differences in algal suc-cession are thus most reasonably ascribed to nu-trient-driven differences in algal growth .

The only taxa to respond significantly to P addi-tion, Epithemia adnata, Rhopalodia gibba, andAnabaena sp ., appear to be especially suited to highP and low N environments . Anabaena is a known,heterocystous nitrogen fixer. Drum & Pankratz

35

(1965) and later Floener & Bothe (1980) haveshown that R. gibba possesses symbiotic coccoidblue-green inclusions capable of N-fixation . Final-ly, cells of E. adnata in our samples were also foundto contain a coccoid blue-green symbiont, also not-ed by Geitler (1977), and an investigation of itscapacities of N-fixation are now in progress . Con-sideration of Douglas Lake simply as a P-limitedsystem may thus be incorrect .

Further use of the flower pot substrates can thusbe expected to identify biological indicator speciesdiagnostic of particular nutrients in a variety of laketypes (e.g., Lowe, 1974; Collins & Weber, 1978 ;VanLandingham, 1982) . For example, the sub-strates should prove useful in clarifying the role ofaquatic vascular plants in supplying inorganic nut-rients to their epiphyes, as both surfaces are capableof releasing highly localized nutrients from internalpools . Release rates may also be highly seasonal, asevidenced by the disparate results of Carignan &Kalff (1982) and Landers (1982) concerning phos-phate release by Myriophyllum spicatum L . BothEpithemia adnata and Rhopalodia gibba are fre-quently epiphytic (Eminson, 1978 ; Patrick & Rei-mer, 1975), and their distribution might thus beused as evidence of phosphate release by particularplant species or growth stages .

The study provided little evidence of competitiveeffects exerted by the dominant species upon othertaxa . Taxon diversity based on biovolume propor-tions declined in the P treatments relative to C andN treatments, but this is seen as a mathematicalconsequence of the increased growth of the P-stim-ulated species . Competitive exclusion apparentlydid not occur, as evidenced by the uniformity oftaxon richness estimates on the P, N, and C sub-strates (Fig . 2c) . Some taxa did appear to showslight growth inhibition on the P substrates . Densi-ties of Achnanthes minutissima, a frequent domi-nant in a wide range of environments (e.g. Allan-son, 1973; Brown & Austin, 1973 ; Eminson, 1978 ;Kuhn et al., 1981) were slightly depressed on P potsrelative to N and C substrates on all 4 samplingdates. The findings of Williams et al. (1973) that A .minutissima showed enhanced growth in responseto nitrate are supported here (Fig . 3e). The navicu-loids, generally motile and easily dispersed formswhich comprised 15 .1% of total biovolume on day7, similarly experienced slightly less growth on theP substrates . Competitive inhibition leading to spe-

36

cies replacements during succession is a major tenetof successional theory (Horn, 1974 ; Tilman, 1982),and has been inferred for the periphyton (Brown &Austin, 1973; Eichenberger & Wuhrmann, 1975 ;Marcus, 1980) . The absence of strong competitiveeffects in our study may be, in part, a consequenceof the short time span involved .

The growth response of Epithemia, Rhopalodia,and Anabaena affected not only the accumulationof biomass, but also the duration of successionalchange in species composition on the P substrates .The approximately 10-fold increase in biomass to15 .5 mg - m -2 chlorophyll-a is seen as a relativelysmall perturbation of the periphyton community incomparison to the often much larger algal re-sponses to natural sources of nutrient loading(Cushing, 1967; Wilhm et al, 1978). Moss (1968)provides a comprehensive review of earlier work onthe subject, citing periphyton accumulations ex-ceeding 1000 mg . m -2 chlorophyll-a under extremeconditions .

Whereas maximum biomass values were reachedby day 14 on the N and C substrates, biomassaccumulation was prolonged on the P substrates .Associated with this continued growth, especiallyfrom days 14 to 25, were high SRI values indicatingrapid shifts in the relative dominance of the P-stim-ulated taxa. Factors actually limiting further growthon each of the 3 substrate types at the end of thestudy remain unclear .

Acknowledgements

The authors wish to thank W . Richardson, M .Kaufman, B . Glover and J . Warriner for field andlaboratory assistance, and D . Oliver and H . Van-derSchalie for identification of some of the chiro-nomids and gastropods . Assistance in computerprogramming by P . Fairchild and criticisms of themanuscript by D . King, D. Wujek, and J. Lehmanare greatly appreciated . The field research wasmade possible by the generous technical support ofthe University of Michigan Biological Station,where both authors were summer faculty . Fundingfor the project was supplied by a faculty researchgrant to W . Fairchild from Central Michigan Uni-versity.

References

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Received 17 August 1983 ; in revised form 12 December 1983 ;accepted 22 December 1983 .