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I.ondtin lil 4NS ('opyrhht. 0 1989 Rlackwcll Srlrntihc I'uhlicatinns Aulhoriration to photwtip) itc-mr kir inlrrnal or pcrsonal UK. or thr iiitcrnal or pcrsinal UN of spcrihe rlirnts. ir arantrcl h) Hlarkwrll Sciriitilir l'uhlications for Iihrarirs and othcr uwrs rrgistcrcd with thc ('opyriahi Vlrarancr ('rnlrr (1.1 .I L Transactional Rcpurtina Scrvicc. providrd that thc statrd fcc of $01 (MI prr cop) is paid dtrrrll\ to Ihr (7 'I.. 27 ('ungrrcss Sttrct. Salem. MA lllV7V. I1 S A Spc'rial rrqncsts should hr addrrsrrd 111 thr Cditor. IM)46,-5070189 SO 1 MI. Periphyton responses to invertebrate grazing and riparian canopy in three northern California coastal streams JACK W. FEMINELLA, MARY E. POWER' and VINCENT H. RESH Department of Entomological Sciences, and 'Department of Zoology, University of California at Berkeley SUMMARY. I. Field experiments were conducted to examine the impact of grazing invertebrates on periphyton biomass in twenty-one pools across three northern California coastal streams (USA.): Big Sulphur Creek, the Rice Fork of the Ecl River, and Big Canyon Creek. Periphyton accrual on artificial substrate tiles was compared in each stream hetween two treatments: thosc elevated slightly above the stream bottom to reduce access by grazers (=platforms) and those placed directly on the stream bottom to allow access by grazers (=controls). 2. Crawling invertebrate grazers (cased caddisflies and snails) were numerically dominant in each stream (86% of all Brazers in Big Sulphur Creek. 61% in the Rice Fork, 84% in BiB Canyon Creek). Platforms effectively excluded crawling grazers. but were less effective in excluding swimming mayfly grazers (Baetidae). 3. Periphyton biomass (as AFDM) on tiles was sienificantly lower on controls compared to platforms for the Rice Fork, an opencrnopy stream, and Big Sulphur Creek, a stream with a heterogeneous canopy. In con-' trast, no grazer impact was found for Big Canyon Creek, a densely shaded stream. Here, extremely low periphyton biomass occurred for both treat- ments throughout the CIO day study. 4. The influence of riparian canopy on periphyton growth (i.e. accrual on platforms), grazer impact on periphyton, and grazer abundance was examined for Big Sulphur Creek. As canopy increased (15-98s cover), periphyton hiomass on platforms decreased. In contrast, canopy had little influence on periphyton accrual on controls; apparently, grazers could maintain low periphyton standing crops across the full range of canopy levels. The abundance of one grazer species. the caddisfly Gumaga nigrirda, was highest in open, sunlit stream pools; abundance of two other prominent grazers, Ifdimpsyche boreah (Trichoptera) and Ccnrroprilum CORVFIU~?l (Ephemeroptera), however, was unrelated to canopy. Corrrvpnndcnrc: Dr 1 W Frminclla. Ikpartmcnt or Fishcries and Wildlilc Ecology (tntcr. lltih Slatc 445 University. Lxignn. Ul'R4SZZ-SZIO. 11 S A 446 1. W. Fmnirtrlla. M. E. l'owrr and V. If. Rrsh Srrrarn prriphyton. grazing invcrtrbratcs, ond ronopy 447 Introduction In recent years, several studies have shown that grazing aquatic animals strongly influence the production and successional dynamics of stream periphyton (I.amherti & Mtme, IW; Mat- thews. Stewart & Power, I9H7). Grazers may depress periphyton standing crops (Power & Matthcws. lYR3; McAuliffe, 19R4a; Jacoby. were used only in small segments of stremir. Recause of these restrictions in exlierinicnt;tl design. such studies cannot acleclii;ttely ;tccout~t for within- or hetween-stream variation in sub slrate. current, nutrients, light, and grazing. which in various combinations may influence the outcome of grazing experiments. Research on the role of grazers on stream periphyton at spatial wales across large stream IYRS; Hill & Knight. 19R7). alter ratesol primary Droduction (Cirecorv. I9R3: lamherti & Resh. segments (e.& km reaches) have centred only on temperate (Power & Matthcws, lM31and tropi- . ".. . 19R3; Power, 1984). and influence periphyton species composition (Hart. IW5; Colletti ct ol., 19R7; bmberti ct a/.. 1987a; Steinman ct ol., 1987; Power, Stewart & Matthews, IW). Results from previous experimental investiga- tims of grazing invertebrate effects on periphyton have had several important limita- tions. Many studies involved the use of cages or artificial streams that were stocked with only OM' or a few speciesof grazers, which de-emphnsizcd the collective role til grazer rssemhlagcs (as well as tither invertebrates) in influencing periphyton communities. Other studies involved the use of in ritrr trcatment/control plots that were poorly replicated (see Ilurlhert. 19R4). confined to specific hnbitats such as single pils or riffles. or cal (Power,' 1984) grazing fishes. Unf;wtur,ately. no similar attempts at this larger spatial scale have been made to assess the effects of grazing invertebrates on periphyton. despite the donii- nant role of invertehrates as primary consumers in many stream ecosystems (Lamherti & Resh. 19R3; McAulifle, 1984a; tlart, IWS; Pringle. 198s; Dudley, Cooper & Ilemphill. 19th; Jacohy, 19R7). In this study. nur primary ohjective was to examine the impact of grazing invertebrates on periphyton biomass across twenty-one pools (4-4R m long) within three streams in northern California. U.S.A. A secondarycibjective was to examine the influence of riparian canopy. a lac- tor that regulates incident IiRht that reaches 'TABLE I. (a) General physical features. and CuTrent velocity and depth measurcmcntc at eipcrimenlal plotsrithinstudypnnlsin BigSulphur Creek, RigCanyonCreek. and the Rice Fork Current vclcrcity was mearurcd eithrr with a Pygmy-Gurlcy meter held S-lOcm ahove tile surfacer. or hy liming the movement 01 Rhodamine-B dye acrcm tiles. Discharge was measured 27 June 1986 Measurements lor all other Ieatures reflect average mnditions during August-Oeloher I W. (h) Water chemistry's ranges are values recorded during winter (=W) and summer (=S) haw flows. Feature (a) General: Altitude (m s I ) Gradient (m km 1) Water temperature ("C) (range) Discharge (m 1s 1) Experirncntal plots Current (cm s 1) Depth (cm) (W pi1 (units) C)rlhivhihmphate 0% I I) Nilrate nttroEen (lig I I) Ammonia nitrngen I I) lntal alkalinity (mg I 1 CaCO,) I otnl hardnccs (m8 I 1 CaCO,) ('hlordc (m8 I I) Free Ul, (mr I 9 Stream Big Sulphur Creek tl7u 47 19-27 0 044 4s 22 11.0 IS (Wl-32 (S) 6 (S)-lS (W) 220 (W)-252 (S) 248 (Wb274 (S) I2 (W)-I3 (S) 2.0 (S) 64 (W)-97 (S) Big Canyon Creek 57u 61 14-19 0.1015 6.0 22 Rice Fork 610 S 21-2s 0.215 3.6 24 streenis. on periphyton accrual. grazer ahitncl;incr. ant1 firwrr impact on priphytoii. Methods .Trudy arra Sites within three permanent streams of the Northern California ('oar1 Range, U.S.A.. were chosen for study: RiR Sulphur ('reek (31P47'N. 122O47.W. Sonoma (:ounty). a semd-order tributary of the Russian River, the Rim Fork (3T20'N. 122"SZ'W. Mendocino County), a third-order tributary of the Eel Rivet; and Rig ('anyon Creek (IR"51'N. 122"41'W. Lake ('ounty). a second-order tributary of the Sacra- ntcnto River. Physica! characteristics for each stream are listed in Table I(a). Hig Sulphur Creek flows northwest through the Mayacama Mountains and The Geysers Known (ieothermal Resources Area (KGRA). Ihe study sites are located in a reach that receives variable shading from riparian vegeta- tion and steep canyon slopes. Riparian vcgeta- tion is predominantly white alder (Alnuc rhr~nihr/olin Nutt.). oaks (Qrtcrrut spp.), ('alifornia bay laurel (Umbelloria californira Nittt.), I)ouglas fir (I?crudoau~a mmzicsii (Mirh.) Franrol. and sedges (C'urrx sp.). The substratum consists mtly of sand, gravel, and cobbles with occasional hnulders and hedrock. lhe Rice Fork, located 72 km northwest of Rig Sulphur Creek, flows northwest through the Mendocino National Forest. Because the streambed width exceeded XI m in most places, the stitdy reach was open to direct sunlight dur- ing most of the day. Riparian vegetation consists mostly of alder and willow (Solit sp.), and the predominant upslopc vegetation is oaks, and ponderosa (Pinw pondrroso Laws.) and sugar (1'. lamkrriona Douglas) pines. Predominant substratum ranges from gravel to large cobble. with wme hcntlders. HigCanyon('reek. Itmted I I km northeast of Rig Sulphur ('reek, also flows through the KGRA. The study sites were located approx- imately I km downstream from the spring source. All sites within Rig Canyon Creek are heavily shaded by riparian vegetation consisting mostly of alder and oaks. The sithstratum is pri- marily mixed cobble and hiulder. I'ctrther site descriptions of Rig Sulphur and Hig C'anyrin ('reeks are bund in Lamherti & Resh ( I9R3) and McElravy & Resh (19137). Analysis of water chemistry for the three streams indicated similar orthophosphate con- centrations and pH, but different levels of total hardness and alkalinity. and ammonia and nitrate nitrogen (Tahk Ih). Water srmplesfrom Big Canyon Creek were consistently low in ammonia and nitrate nitr~n;concentrationsof the latter were close to zero in this stream during both winter and summer base flows (Table Ib). Expcrimcntal design Six to nine pools were chosen in each stream to contain experimental plMs (Tahle 2). Four plots were established in each pool (ei~hty-four plots within all three streams); each plot con- sisted of two set1 of artifial substrate tiles (unglazed red clay tiles, 7.6~7.6 cm each, lour to five tiks per set). Artificial substrate tiles have heen shown to accumulate levels of periphyton and invertebrates that are similar to stream rocks (bmberti & Resh. IWS) and. in addition, tiles may increase sampling precision Over that of natural substrate sampling (Rosen- herg & Resh. 19R2). Before being placed in the stream, tiles were kached in running tap water for 24 h. To redmi the amss of inverfehrate grazers, one of the tw sets of tiles was elevated .%IS cm off the stream hottom using an 18x 18 cm square Plexiglas platform supported by an emergent, inverted J-shaped aluminium stake (see Iam- berti & Resh (1983) or this issues' cover for photograph of a similar design]. To prevent dis- plncement of tiles from platforms hy stream cur- rent. all tiles were glued directly to platforms using an inert aquarium adhesive. The semnd set of tiles was placed directly on the stream hottom in areas that were adjacent to platforms: these bottom tiles were used as controls to which grazers had complete access. No adhesive was necessary to maintain the position of control tiles on the rtrcam hottom. Amount of riparian canopy (as percentage cover) was estimated for each of the experimen- tal plots using a spherical densiometcr (Lcm- mon, 1957; Power, IW). In a study designed tn examine the relationship between densiometry and light radiation for two California coastal streams that were close to our study sites, Ligon (1986) reported that canopy density accounted Stream prriphyron. grazing invrrrebrares. and ranopy 44Y 44U 1. W. Fbninrlla. M. E. Powrrand V. H. Rrsh TARLE 2. Characteristics ol experimental conditions in Big Sulphur Creek. Iltg ('anyon ('reek. and Ihc Rice Fork Fealurc Irngth (m) 01 study reach No nl experimental pocl Length (m) nf pols (range) Width (m) of pdr (range) No. of experimental p)ots/puol 9h Cannpy/plot (f fSD) (N) Experimental period (1%) (range) Stream Big Sulphur Creek 9 411 J(M2 4 b2+3R (36) 1.5-98 I I June-16 Aug . -. 7n0 Rig Canyon ('reck IN1 6 61 I 1 0-4 (I 4 R2+ 14 (24) 37-W I I June-I8 Aug Rice Fork (1(1l fl 2.S4H 5612 7 4 3R+W (24) 2-94 IS June IY Aug for 90% of the total variation in light (P7.5% cover, N=I5). moderately shaded (2S-75% cover. N= I I). and open (~25% cover, N= IO), and we used a two-factor. repeated-measures ANOVA to determine the probability of separate and interactive effects among groups (platforms. controls). canopy levels (shaded. moderately shaded. open). and time (five intervals). To assess grazing pressure among plots with dif- ferent canopies, means ol log-transformed AFDM on control tiles were compared using ANOVA and an a-postrriori Studcnt-Newman- Keuls multiple range test. Because 01 their close proximity (<0..5 m apart) platform and control tiles within each plot werc assumed to have equivalent canopy levels. 'I i Hiq (.onV time kbyrl FIG. I. DemitiCr d pzin~ imcrlchratcs (per 14110 cm*; if9S8 C.1.) thml mkmized tiks m raised plat- lornn (did li) and ccmtrd tiks m the stream hotturn (dashed liner) in Big Sulphur Crcek. the Rice Fork, and Ri Canyon Creek. Samplc sites ranvd lrom 3.S.H lor Rig Sulphur ('reek. IS-24 lor the Rkc Fork. and 22-24 lor Big Canyon Creek. &pendin( upon sampling date. I>rnity (iiamtier~ pcr 111l CIII:) Stream (irazer species Platkirm ( 'tintriil Big Sulphur ('reek Rice liwk firhcnpryrhr hnrrah (Ilagcn) (C) (;umnRo nizrirula (Mcl. ) ((') I Irlirprvrhr hnrrahs (C') 3 27 t 5 mi (;umaEa ntRrirsla (C) 0 051 0 u, OIM+I 73 Harrir rriroudarus Ddds (S) I .3Xt 2.Xh I.flVI2 23 Big ('anyon Creek (~lo.i~osnma orrgonrnrr Ling (C) I1 I 2'?+_2 (15 Ilorric rricaudorw (S) 0 JHt I IIX II IV?tl33 It OM I I1 22 ll.l2!ll 7fi 13.113 ! Ih zv 2 wtt 7 hl (inrroptilum ronivram Idc (S) I 10! 2 21 2 27f2.HI 0 IR t II 54 f'hysclla Ryrina Say (C) It 0311 (I IV 0 4hfll MI swimming haeticl mayfly nymphs. The increase in grazer density on control tiles in the two streams was principally the result of recruitment and colnnizalion hy the cased caddisflies Ifrlico- psyche borealis and Gumaga nigrirula. In contrast. platforms were less effective at reducing grazer densities in Big Canyon <*reek: densities on platforms were depressed only slightly below thcne of controls over all dates (Fig. I). Platforms were less effective in this stream hecause the small. swimming grazers dominant here (mostly Baefis rricaudarur) were hetter ahle to colonize tiles supported ahove the streambed (Tahle 3). All grazers were consider- ahly less ahundant and more spatially variable in RigCanyonCreek than they were in BigSulphur Creek and the Rice Fork. Arcrual nf prripltyfm periphyton hiomass (as AFDM) increased over time in each stream during the study (Fig. 2. sce also Tahle 4). I lawever, the magnitude of periphyton hiomass on platform tiles varied greatly among streams. as did the degree of dif- lercnce hetween AFDM on the platform and the contrtd tiles (Fig. 2). Platform tiles accrued sig- nificantly higher hiomass than control tiles in holh big Sulphur ('reek and the Rice Fcirk (ktween-group dilferences: /'M levels hetween platform ancl control croup wcrc statistically indistinguishahle (/'>O.tI5; Table 4). .~rparation nf grazer infltirncc on pcriphvron front rovariarion in .strrant drprh and ctrrrent idority Although the use of ii platform/control tile set for each experimental plot was unaffected hy canopy (i.e. canopy levels were equivalent within a given plot hut canopy hetween plots could vary). other physical conditions, such as stream depth and current velncity within plots were not equivalent. For example. in Hig Sul- ohur ('reek itnd the Rice Fork. holh of which showed hieh hetween-group differences in Iwriphytoii hionless (-l'dde 4). plnlfwioi tiles had sipiiiliciintly lower depths and higher current vclirities Ihan corrcsponcling control tiles ('l'iible 5). 'lliercfore, iin iiclclilional nianipiilw litin was ncccssitry lo separate the influcnce of gr;i7ing lrtini dcptli and current on periphyton Iiiiinwss. Siiloincrsed hricks iintl stream rtrks were used to raise several control plots off the stream hottom to depths that were similar to adjacent platforms, therehy minimizing hctween-group variation in depth and current but maintaining differences in grazer densities (/D0.05; repeated-measures ANOVA). Ilow- ever. this kiw statislicit1 asstriation (i.e. high within-grwpvariawc) wasactually a function of species-specific cliffercnccs in degree of relatitin- ship with canopy; when iiiialywd separ;itely. relationship with canopy emerged lor some grazer spcics hut iiol aithers. I lriiip correl;ititiii analysis for cnnopy ;iiitl ihc three iiuntcric.iilly dominml grilzcrs. log.~r;inskirnicc( \t;incline hi- mass and density of two of the three species (Ifelirop.ysrhr horeali.r am1 (iniroptilurn row wxitm) showed nu consislent relationship with canopy (Tahle 8). In mntrast. standing hiomass and density of thc third species ((;urnago rtigricuku) showed strong negative correlations with canopy over mcat dates (days 31.45 andho; Table 8). Discussion Wiens (IUXI. IYHh) iind McAuliffe (IWh) tkmonstriitctl the iinporl;ince of spttial iind tem- pird scales of cihscrviilioii in tktecting hiolog- irally relevant patterns in natural communities. lixprimentr Lksig~tl lo elucklate causal pro- cesses for such patterns ala) may he suhject to scale hiases. For example. the outcome of experiments conducted 111 small spatial/tcm- piral scales may: (I ) reveal location- or time- specific picresss that arc not operating to pro- duce patterns at larger scales. or (2) fail to reveal relevant prtresses at all. One apprnach to cir- cunivenl this bias is to ohtain results at one wale (e.g. within a single stream ptwol) and move to higher rides (e,g. acrcss sever;il pc~il~ over a streiim reach) to assess whether thew results arc similar. In the present sludy. we compwed pcriphytoii accruiil on tiles a1 reduced (i.e. on platlorms) and amhieiit (i.e. on the stream hot- tom) gritzing pressures and replicated these cxpcriments across reaches of three different strciinis. at a tinte (suninirr) when periphyton growth often re;ichcs its highest aniiual rates. Using this iipprmch. we overriiiiie tiiiiiiy of the sliiitiiil/tenipiral lii;iscs inherent in prcviotrr xtiitlics til invcrtehralc grwiiig. lo allc~w it more general appraisal of the impact of grazers on strewn periphyton. In two of the three stream examined (Big Sulphur Creek and the Rice Fork), grazing invertehratcs were instrumental in maintaining low periphyton standin8 crop. Iltwevcr. hecause platforms were effective only mt reduc- ing crawling grazers (cased caddisflies mnd snails) and hd tittk effect on swimming grnzcrs (hnetid mayflies). periphyton that mccumulalecl on platforms wns influenced to ame degree hy the grazing activities of these latter inverte- hratcs. lhere is me controversy over the rclr- live effects of Brazing mayflies compared to caddisflies and snails. which may relate to m!wthpart murphologier that are specialized to harvest periphyton assemhlaps with different physiogmmics. Miyflia may have mnsiderahly kss impact on depessinR periphyton than cad- disflies andsnnih(Jnc0by. I9R7; lamhertirial.. IW7a; hut KC Knhler, 19RS;Cdlctti eta/.. IW7; and tlillB Knight. 1987, for contrastingviews). Thcre is general agreement. however. that graz- ing mayflies can alter local periphyton Ieatures. such as increasing periphyton export ( Lamherti rf a/., IW7a) and selectively rcnioviiig species of some diatoms (Jacoby. 1987). activities that wcwld influence periphyton residues that poten- tially are measurahk at larger spatial wales. In Ihe context of the present study. it is likely that with he alwcncc of mayfly grazers (ix. under conditions of compkte grazer exclusion). dif- lcrences in periphylon hctwccn conlrols and platforms. hem the effects attrihutahle to graz- ing. wcwld have hecn even more proiiounccd. Several studies conducted at sloalial wiiles smaller lhan thw used in this study hnve sug- geslcd that depression of periphyton hy stream griizers results in fcnnl-limited rr)ntlitions. which iii;cy iiiflueiice grwct ;ihiiiidmce (McAulifIe, lOX4a) ;tiid iIeci~*:ise iiicliviclii;il gra7cr growth riitr (Ilill XI Ktiiglit. IYX7) iid pupal si7e (lliirt. 10x7). In ;idditioii, rewlts (if previous experi- ments coiicluctetl over several years in Rig Sul- phur ('reek h;ive demonstrated depression (of periphyton hitonims hy grazers 1e.g. 1979 and IYH(t. I.amherti A Hcsh (198.3); IW: Lamherti. I'eminella & Rcsh( IYH7h): IVHhand IW7: Fem- inella (IYXY)I ;ind alui suggest that such ftwd liniitations may affect spatial distrihution (Lam- herti & Resh. 1982). growth (Lamherti ci a/., IY87h). and even fecundity (Ferninella. 19XY) ol grazers. C'ollectively, results from these studies comprise ntulti-year and now multi-site data (i.e. collected concurrently from several sites over two streiim reaches) that demonstrate strung regulalioii of periphyton standing crops hy stream grazers. <;razing iiivertehrates were kound to have little influence oii periphyton in Big ('anyon ('reek. the stream that had the densest riparian canopy and lowest grazer densities. The mcist plausihle explanation for uniformly low periphyton hiomass on hoth control and plat- form tiles in this stream was that nitrogen iivailahility. indicated hy low amhicnt nitmgen rnncenttations (Tahle Ih). rather than light or grazing. limited periphytiin. Using linear cor- relation, WE found no consistent relationship (FzO.ft5) hetween periphyton hiomass (as AFDM) and canopy. or periphyton and grazer density. patterns that would have heen expected il light was limited or if grazers were important in reducing periphyton stantling crops in this stream. The presence of low dissolved ammonia ;ind nitrate-nitrogen levels in Big Canyon ('reek may have slowed periphyton growth on all tiles. iind detection of grazing impact may have heen ohscrvahle only at a time peritd lctnger than (II) days. Nitrogen has heen reported tu hc a limiting macronutrient in other streamsol western North America (c.g. (irimm & Fisher. IYM). although the influence of such nutrient limitations tin stream grazrr/periphyton interactions has received attention only recently. In one of the first in sim experimental studies involving nutrients. praters. and periphyton, Stewart ( IW7) lound that nutrient supplements increased priniwy prtnlirctivit y in a prairie-mitr- pin streani hut did iiol increase periphyton standingcrops. which were controllecl hy pzing minnows over ;dl nutrient levels testetl. Inlltwiti.i* 01 iwmp IVI Iwrililt lvwi. ,yrflzitl~ iitipwi. itticl griczrrr Hipariiin r;inopy. ;iiicl it\ rllrcts oii iiiciclrirt light levels that rcech strc;inis. is ;I major I;ictot influencing henlhic coniiiitiiiities (('unittiitis. IY74; Minshtill. IY7X; Murphy, 1l;iwkins Xr Anderson. IYXI: Ihwkins. Murphy & Anclcr- son. 19821. Stream section\ with opcii canopics have heen shown to have higher Iwriptiytron standing crop and primal y prcduction thiio more shaded sectiims (Lyford A (iregory. lY75; llawkins rf ul.. IYX?; Murphy. IYHJ; Power. IYR4: luller. Roelcils & Fry. IYXh), which C;III result in higher food iiv;iil;ihility (as periphyton) for grazing insects (Rehmer & I lawkins. 19Xfo) and. in turn, their preclatiirs (Miiiphy (G 11:iIl. IWl). Accrual of periphyton oii platfornir (i.c. reduced grating pressiircr) in Hig Sullotii1r ('reek, the streani with Iirtcrogciirous canopy. suppirts this view. Ilerc. light wasclenrly;~ rate- limiting reaiurce for periphyton. ;is :scru;it OII platforms was fastest in open. simlit plots (I:ig. 3). Similar results tlemonstrating ;I strong inh- encc of light on periphyton protluctivity were reported for a coastid Al:tsk;tn freshw;tter streiim and its connrctctl interticl;d re:ich Ivy Murphy (IVH4). Despite hiRher periphyton acciiniiil;ition on platforms in low-canopy plots. Iwriphytoii 011 control tiles (Le. ;imhient gr;viiig Ioreswres) remained low under the full range of caiitypy cover (Fig. 4). Simil:ir low perilohytoii (mc;l\- wed as chlorophyll it) hiomas5 in grazed treat- ments were found itcrow :I r;inge of shading regimes hy Iliiwkitis Xr hriiish (IYX7. p. 213. Fig. 2). Appweritly. this p~itlcrn was c;iii~ecl hy dilfetcntiiil priiriiig 01 tlie sniiil JrcXtt srltc ttht ((jould). whow ahuiidance p;ir;illelecl t1i;it of alpl hicmass. In Oig Sulphur ('rcrk, the cecl- disflies f iutnagu itigrii rrlu iind f lulicoipsydtr borealis duminated the invertehrate grazer hio- mass, but only triRricrtlc showed a strong inverse relationship with citnopy (Tiihle 8). heing most ;ihuncl;iiit at opi plots where periphyton accrual rates were hstest . This pat- tern was similar tookrviitions in B I';in;inimii;tn stream with heterogeneous ciinopy (I'owcr. IYfM). where hetween-pool variation in algid accrual rates was tr;rkecl quantil;ilively hy ;iIgae-gr:izing armoured catfi\h 111 ctoiitrast . tlic iihundiince of /I. horiwli.r iii lrie Sulphur C'rcek was unrelatctl to ciiiiopy. N;itcir;il tlensities of hvth (;. ttiXric'ulrt (Iiwiiiielh. 19x9) ancl If. lwrc*akx (Imnloerti iv id . IVX7h) ciin depress piiphytoii. ant1 the coiiihiiictl ;ictioii ol these iiiid oossihty tither graters (e.g. (infritpfihon conivrunt) could acccount lor low periphyton ahuiidance in sites across a wide range of canopy regimes. I hplanatioiis for strong negative associations hetween gr;izer ahunclance aid canopy density may itivolve specilic orientation patterns to liar- ticiilar periphyton assenihlages that result from diflrrerit light regimes or specific light regimes themselves. For example. periphyton com- munities in open sites are often dominated by upright filamentous algae, and more shaded sites hy epilithic diatoms (Lowe. Golladay & Wehster. 19Hh). If grazers were ahle to detect clilferences (either physiognoniic or phototactic) among sites of contrasting canopy regimes and cttiilcl move to preferred areas through hehavioural means. such as hy larval drifting (Kohler, IY85) or crawling (Richards & Min- shall. 1988). their abundances would eventually hecome highest in the most favourahlc sites. This may account lor the disproportionately high larval densities of G. nigriculn in open sites in Rig Sulphur ('reek. I mvae of this species are highly mohile ancl have shown a strong preler- cnce lor filanientt~irs algae 1c.g. f'ladophora X/iintcrafa (I,.) Kul7.( compared to epilithic tli;itonis that occur niost frequently in sites under moderate to high canopy cover (Ferninella. IYXY). An alternative explanation fair this pat- tern may involve phototactic orientation hy aerial adult insects to open. siinlit areas. and suhsequenl oviposition hy lemales in these sites. I,ow correspondence hetween canopy density and ahuncfance of cither species ol grazers (e.g. If. hnrrfl/is. <'. cortvrrum) in Rig Sulphur Creek may suggest that grazers avoid habitats con- taining competitively superior grazer species (see Ilawkins& Furnish. IY1(7),or their distrihu- lions are limited hy physical constraints (c.g. frequent hydrologic disturhance, depth. suh- strate). While we cannot directly assess the rela- tive impact ol spcies interactions and physical Factors such as depth and suhstrate on grazers. hydrologic disturhance. which in other streams has heen shown to have a prolound influence on periphyton (Power A Stewart, 19x7; Rohiiison &i Hiishfiirth. lYX7) and iisscK-iatecl invertehrates (Kohinson x1 Minshall. IYXfo). prohahly had niiiiiiiiiil effects on grazer distrihutions during thisstudy. Recause coastal ('alifornia has a Mecl- iterranean climate characteriied hy a long. rain- lree pcrind Irom spring to auiumn (McElravy, Lamherti & Rcsh. IYKY; Resh. Jackson & McElravy, IWY), summer pnpulations of henthic invertehrates in these streams are not alfectcd by hydrologic disturhance. Comlusiom Grazers, nutrients, and light can all limit rates of periphyton accrual in aquatic environments. Our experiments indicated that grazing invcr- tehrates strongly suppressed new periphyton growth on tiles introduced into two of the three streams examined during the summer low-flow period. These experiments, however, do not address the ahility of invertebrate grazers io con- trol estahlished periphyton, which may develop in the absence of grazers and may persist either hy growing to a large, invulnerable size or by rapid turnover. Algal persistence hy means ol large size in both Bit Sulphur Creek and the Rice Fork is unlikely because one grazer. <;itmago nigriruka, Which is important in hoth streams. can clip md ingest filamentous algae ol all sizes (Ferninella. IW). Algal persistence hy means of rapid growth also appears unlikely. given the cyles of algal growth we have observed in northern California streams. During the early part 01 the low-flow season. prior to recruitment of grazers (April-June), conspicuous hlooms of filamentous algae develop in sunlit reaches uf many streams, including all three streams in this study. Sukquent IOU olperiphyton hiomass as grazer densities increase suggest that grazing may limit periphyton in these streams even after considerahle biomass has been estahlished (see also Lamberti 8 Resh. I9R3). Ilowever, mrrela- tive ohservations alone cannot eliminate dcclin- ing nutrient availability, the development of unfavourable temperature regimes. or other physical conditions. as lactors that also may con- tribute to reductions in periphyton biomass. The accrual of suhstantial periphyton on tiles with reduced grnzin8 in Big Sulphur Creek and the Rice Fork hy day fill, however, suggests that physicochemical factors were not severely hit- ing to periphyton. and that grazing on its ow11 can he a no tent limiting force during low-flow contlitions 456 J. W. l+ntirirlla, M. E. Poww attd V. 11. Hrdt Acknowledgments We thank C'. llills for access to the sludy site at Big (.;inyon ('reek. E. McElravy. I). Jensen, F. I.igon. I. Phillips and 1.. Stephens for field or lahixitory assistance. R. Williamson for water chcmical analysis. and R. Wrubel for stalislical advice. This rese;irch was supported in part hy a Visilittg Profcssorship for Women grant (to MEP) from the National Science Foundation (RII-WMMl I). Rdwoner Andenon N,li & Cummins K.W. (1979) Influences td diet on thc lilc histories til aquatic insects Jour- nal nfrhr I;ithrrirr Rrcrarrh Roard of Canada. 36, 33.5-142. Anttiinc S E. & Renwn-Evans K. (19R3) Thc clfecl ol lipht intcmity ad quality nn the growth 01 henthic algae. 1 Phyttlpigmcnl variatbns. Arrhiv flir llvd~iihruln~ir, 98, 299-.%. Rcak S L & ApFlcman D. (1971) Chhrophyll syn- thesis in ('hlnrr/la Regulation hy degree 01 light limitation til growth. Flanr PhysiolnRy. 47, 230- 235. khmer D J. & Ilawkins (I.P (1%) Elfccts til over- head canopy on macroinvertchratc prucluclicm in a Utah slrcnm Frrrhwwrrr RinloRy. Ib. 2Rl-.WMl. Brown I.E. & Richnrdstin F.L. (IW) lhc cllccl of growth environmenl nn the physinbgy ail algae: light intensity. Journal of Fhyrology. 4,3R54. Cdlclti P.J.. Blinn D.J., Pickart A. & Wagner V T. (Iw17) lnfliielmofdilfcrent dcndtiesol he mayfly grazer Hcpragrnia rriddlri m lntic diatm mm- munitics. Journal of rhr North Ameriran Benrholol(ira1 Soriery. b, 2lfL28Il. Cummins K.W. (1974) Struclure and function ol stream ccosystems. Biosrirnrr. 24,63144I. Dudley .TI... Cnq~r S.D. & liemphih N. (IW) Ellects ol macroalgae m a stream invertehrate mmmunity. J~mrnal of fhr North Amrriran Rrnfholn,qiral Sorirfv. 5. 9.3-lIK. Fcminclla J W. (IW) lnlcractiuns hetwecn grnzing aquatic invcrtchralcs and their Imd resources in thrcc mirthern ('alilornia streams and a lrcshwaler manh. Ph.1). diswrlnlion. IJnivcrsity 01 (.aliltir- nia. Rcrkclcy. ('aliftirnia Fullcr R I... Hcdtilr J.L. & Fry 'I.J. (1%) The importance til alg~ to stream invcrtchratcs. Jrirrr. ne1 nfthr Nnrfh Ainrriran RrnfholuRird Snrirfv. 5, 2Yw2Yfl. (ircgiry S V. (IVW3) Plant-hcrhivnrc interactbins in stream systems In: Sfrrum Eridqy: Applirarinn and lirfinR of (;rnrral F;rnlnRic.al Thrnry (Etls J R Harnes ancl (i W Minshall). Plenum Puh- liihinp (.iirpnr:itim. Ncw Ynrk (irimni N I4 R Fi\hcr S (i ( IVW) Nitrngen limitation in ii kintiran Ik\crt stream Jrnimul o/ rhr Nrirrh Arrri~rrr ctn Rrnrhiibipcul .%M rr'fy. 5. 2-15 Ilart I) I). (IYW) (irwiiip iiiwctr inrcliatc ;ilpil inter- actions in a ~trc~iiii Iicnthic ciininiunity Oikiii, 44. 4lc 411 Ilart 0 I). (19x7) I:.cpctirnciit;il slii~lics~ilcn~~liiit;ilivc cwipctiticin in ;I pr;iiin~: stream imcct Oi~idiixiu (Rrrlm). 73.4 I - 47 Ilawkins C'.P. & Furnish J.K (1'4x7) Arc snails impatant compctittirs in strriim ecosyslenir" Oikor. 49, 20% 2211 Ilawkins C.P, Murphy M I. & Andcrsiin N II (IW2) Ellccts til unopy. \uhstratc cornpositinn. and gradicnt on the \triictiirc of macroinvcrtchralc communiticsin C'auade Hange strcamsolOrcgcm &rfi/u~y. 63, IH4tLIH5h llill W.R & Knighl A W (19x7) L':xpcrimcnlal ana- lysis til the grating interaction hclwcrn a mayfly and stream algae hdriRy. (3, IWFIWiS tlurlhert S II. (IW) Pscudorcplication and the design oil ecological licld crpcrimcnts ErnlriRrral MnnoRraphs. 51. I17 21 I Jacnhy J M (IVHS) (irahg eflcctr nn prriphyttiii by I'hrridnrirr fluviatilir ((instriqnida) in :I Iowlaiid stream Jtiurrialof Frrrliwafrr ErnIfiRv. 3,265--274 Jacohy J.M (1'487) Allcr;tticins in periphyton charar- tcristinduc tugraringin a('ascwdc Icwthill stream Frr,rhwatrr Binlogv. 18. 495-SIWl. Kcihler S.I.. ( IYKS) Iclcnlilication til strc:im drill mcchankms: an crpcrimcntal and ohscrvationiil npprtiach. &rnkl~)'. 66. 174Y-1761 I.nmhcrti (;.A. & Mcnirc J.W (IW) Aqiiatic inrcls as primary ainsumcrs. In: Thr El ii1nR.v 111 Aqfiuw lncrrfc (Eds V. II Rrsh ancl I) M HcwnhcrR). Praeger PuMishinC ('nmpany. New York bmhcrti (;.A R Rcsh V tl (IM?) Strciim pcriphytnn and inwct hcrhivtires: an cxperimental study 01 yrazing hy a caddisfly population. hn/- Lamherti G.A. & Hcsh V II (IURZ)('omparahility1if intrdmd tiles and natural suhstrarcs lor sampling btic hactcria. algac. and mncroinvcrtchratcs. Fwshwarrr BioloRv. IS, 21-U). Lamherti G.A.. Ashkcnas LR., (ircgtiry S.V (I Stcinman A.1) (IVRla) Efkcts olthrcc hcrhivcircs on periphyton ctimmunities in lahiriitiiry streams. Jnwrnal nf fhr Nurfh Amrra-en Rrnfhn/rtRfc.u/ Snrirfv. 6,92-104. Lamherti(i.A..FcininellaJ.W. & RcshV.11. (1YN7h) Hcrhivtiry and intrarptcilic ccwnpctititin in :I stream caddisfly populatitin. Orcri/riRra f/lrrlfnJ. 73, 7MI. Lcmmun P.E. (1957) A new instrumcnl lnr mcawring Itirest cwerstory cincvy. Jnarnal 111 Frirr\fry, 55. flfl74lflIl. Lignn. F K. (l%)l'hc reyninsc til iilpal cnnimunitics in strcams to timhcr harvest activiiic\ Mailer's thesis. University nf C'a1iliirni;i. Hcrkclcy. Calilornia. lnwe R I... ~iolladay S.W. R Weh\ter J H (IYIUI) Periphyton rcspunw tu nutrient m;ini~iul;iliciii in itrcams draining clcarcut and lorcstcd w;itcr\hcds Jnrirnal ,if rhc Nnrrh AriirrrIrwi Rrnfhiilii~rr ul Sorirfv. 5. 221 22'4 I.ylnrcl J I1 , Ir d t ircpry S V I IV75) I he tlyn;tniics and structure 01 Irriphyttin coninnmitie\ in tlircc I#)'. b4, 1124-1135 ( ..lw. . .i\e Mountain \tirains. Infrmrrriiinrrlr Vrr- r~iiir~i~rr~ /fir lhirirrtrrr~hr irnrl ~~~t*II'fmf/f~ /.rm- nrrlwir ~~~rliuii~lltiii~rri. 19. Ihlll- Ihlh. M;itthcw\ W J . Stewart A.J R Powci M.E. (1W7) (;rannp lishcs iis conipincnti 01 Niirth American wcmi ccwystcms: ellccts ail (ampri.rfomu ,mrwrinliirn In: ('~immioiify arid Evrhtirmary Ern- IoKv ,,/ North Awirriran Sfrrum Ashrr (Ed- W. J Matthcws and I). C. Ilcim) University of Oklahoma Press. Normnn. Oklahoma. McAulillc J H. (IWa) Hcsourcc ikprcrsiion hy a stream hcrhivore- clfccts on distrihuticms and ahunclances of othcr grnicrs. 0ikii.r. 42. 321-333. McAulillc J H. ( IWh) ('ompclilbm kir spncc. diq- itiih;incc. and thc structure 111 a hcnthic stream ciininninity. tc iilnRy, 65, "%VIR. Mcl:lr:ivy I:. P & Resh V.II (IW7) I~ivenily. sea- sonality. ancl annual variahility of caddiifly (-Iri- chciptcra) adults Irtnn two streams in Ik Califcnnia ciwt ranp. Fun- I'arifir Enf~moln~i.rf. (3, 75-91 McIlravy 1i.P . I.amhcrti (i A R Hcqh V.11. (IW) Year-tit ycar variation in the aquniic m.acroinvcrtc- hratc launa ail n northern ('aliltirsia stream. Jour- nalrilfhr Nnrfh Amrric.un ~mfho/f~Rira/.~nrirr)', I, TI 61. Minshall (i.W. (IV7R) Autotrophy in stream eco- systems. 8in.rrirnc.r. 21). 767-771 Moss 13. ( M7a) A spcctrc~hotomctric mclW kn Ihc estimation 01 pcrccntage ckgradatim ol chkno- phyllr In phervigmcntr in cnlracts til algae. I.im- riolngy and OcrunriRruphy. 12, 33.5-340. Mcw R. (IuCSlh) A note on the cstimatirin ril chkiro- phyll a in freshwatcr aha1 nimmunilics. I.imnnl- IIRV and Orrann~raphy. 12. .uO-.142 Murphy M I. (IW) Primary prtnluctinn and ginzing in freshwater and intcrtidnl reaches of a coastal slrcam. S0utk;nt Alaska. I.imnolnl(y and fkrnnn~raphv. 29, IYlS-HlS. Murphy M.I.. & llnll 1.1) (IWI) Varicd cflects of clew-cut kqging tm prcdatcnt and their hahitat in small streams til thc Cascade Mnuntnim. (hegon. ('unudian Jriiwnal of Frrhrrrri nnd Aquufir SI i- rnrrr, .W, 137-145 Murphy M I... Ilswkins (' P & Anclerrm N II. ( lWl) Elkct~~llc;tm~1y mtnliliiatiim andnccumii- I;itccl scclimcnt on strenm ecnnmunities. 7ran.w- IIIIII.~ nf rhr Amrriran Fi.thrrirs Snrirfy. I IO, 469- 47x Power M E. (IW) Ilahitnt qunlity and thc diatrihu- iiiin til algac-graring catfish in a Panamanim \ire;iiii. Jorrrnrl I/ Animal 1:cnlriRy. 53, 157-374. Power M E. R Matthews W J. (I"?) Algae-grnzing minnows (( brnpiirrrimn anrmrdrtm). piwivainius bass (Mwrolifrriir spp.). and the dislrihulion til aitachcal alpe in n pr;iiric.m;irFin strcam. 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RcA+nsnn C.T. & Mimhnll 0.W. (1%) Elfccls til disturhana Irequcncy tn slrcam hcntht cnm- munity slructure in relitinn to camvy ctivcr sntl seasnn. Journa/ nfrhr North Amrrican BrnrhnlnRi- Rohinm C.T. & Rushlorlh S.R. (ICm7) Elkcts ail phyqical disturbance and camrpr cnvcr iin attnched dintnm mmmunity structure in an Idnho strcam. Ilydritbidogia. 154. 4-59, Ron1 R.M. (IY61)lhc nichc c~plcntationpnttcrnol the Blue-war Gnatcatcher. ~:rr~lr~nKaI Mimnnraiihr. Cd SlKkfy. s, 237-24R. a1 suhstrrtcs in the study nl henthic mwriiin- vcrtchralcr. In: Ampria1.Suhsrrarr.r (Ed 1. Cairns. Jr). Ann Arhur Scicntilic Puhlkhcrs. Ann Arlnir. Michigan. Rorcll J.(;. & Wallen D.E. (1976) Analysing clnln with repeated ohrmatinm cm cach cxperinrntal unit. Journal o/Al(riru/fura/ Srirnrr f('amhridl(r). 87, 425432. Steinman A.D.. Mclnlire C I) . (iregnry S.V.. lam- hcrli O.A. & AshLena L.R (Iw17) Ellecls til hcrhivnre type and density n tarnnomic struelure nnd physiogmnny n( algal aswmhlnges in Inhtirn- tory stream. Journal nf fhr North Anirriiun Renfho/oRira/ .hriNv. b, I7%Im. Stewart A.J. (I9R7) Reyrome ol stream algae tci graz- ing minnms nnd nutricntr: a licld test Ian intcrac- lions. CJrrhRia (BrdinJ. 72, 1-7. Strickland J.0.H. & Parwns 'I.R. (IW) A Frarrbul Ilandhoalr of .k.wawr Analysis. Fishcrier Research Board 01 Canada. Ottawa. W~t~l~nd J R.. Waaland S.D. & Rates 0. (1974) t'hhnopla-t structure ad pigment nnnpnitinn in thc red alga Griffithsia prifrr: rcgulatbm hy light intensity. Phvrnhgv. IO. I9FIW. Wicns J.A (IWIl) Lak prnMcmr in nvian ccnw\iiig Sfudirs in Avian Ricd~gy. b, 5IVS2l Wicns J.A. (IW)Sp.lialscalc nndtcm(rnalvnriali~in in st&s til shruhsteppc hirch In: Cnnimrinrfv Ei&#y (13s J I)iamml and.1'. J ('ax). llarpcr R Row. New Vcsk. Wincr 11 1. ( IWl) Siacicfirnl Prirrc rplri in Erprrirnipn tu1 /)rtil(n. 2nd cdn. Mdirnw-llill ('timpany. New York Zar J.II. (1'474) Biiirfarirfirnl Antrlviv lDrcnlicr!~lla!~ Inc., Englewcrd <'tills. Ncw Jcrscy Notice to contributors hlanttwripts. 411 iri;tntirci tiit\ 11wai coptr\) \hcirtlcl IN \cnl lit I )I 4 (i 1!11thrw. St'htnd 111 l1i~~lo~t~itl '4 trtit r\. ()iiccii hlar) k Wc\tlirld ( dcgr Mtlr I ttd Kiiad. I.tindm I I JNS. II K . r'rcrpt that ittithwr tit tlrr Soutlrri~r Ilmrirphrir \liotild \end thctt irrnnuwriptr tu I'riilrrwir (' M .I tiwnrentl. lkparttircnt til L~tiltigy. I Intvcrsity 01 t lta~ti. I' I P Ihix %I. I )unrclttr. Nrw Lcaland. 1 hr wtp.inirl drawings should not hc sent until thr I dttor rrqiirstr thrnt. lhr text should hc typd on onr sidr 01 tlrc paprr. dcruhlc spacrd. with amplr marsins. (I'iirspccttvr authors arc rclcttrd to thr l~ditoital ti1 I:ri.\hrvurr*r flrdqqi.. 9. 197-4112, atid lo lhr Hlarkwrll puhliration of I I~III~V tr~ I otttthttm trr /hi* ./,Jwnrulr ,I/ tAc* ll?ll~\h l,lfllll~l,lll .sol l('lt' ) (a) / irk* pqi' 1 his shtiiild includr the titlr. list 01 authors' namrs. in\titutr nr lahratury rif iriigin. nanrr and atltlrrrr til thr author to whom ~IIHI~S should hc srnt. and an ahhrcviatcd litlr Itrr itrr as a running hmd Iinr. (h) .Stiiiiiti~iit~ All paprrr should includr a stinttriarv. in short nritrthrrrcl parasraphr. Iinr- ttrd to ahout 1% 01 thr lrirgth til thr (rat. lhis \h~ittld prwtdc a ctinciw rtatrntcnt 01 thr rcopr nl thr wnrk and itr principal hndings and k lully intrlhgihlc wtthciut trlcrmcc tit thr main #rat. (r) /rtwdiitmiti 1 his should contain a clrar statcnirnt of thr rraunr liir doing the work. outltntirg cssrnlial harkgrnund infnrmalton hut th~ruld not includc ctthrr Ihr results ur cunclu- tinnr I~II blerc*rrds urtd 41/ihd% lhis should hc rtitrrtsr hut providr rullirirnl clrlails to allow thr work 10 hc repralrd IC) Hiwlrr 1 hir should not include niatcrial eplirtilirtatr to thr Ihrcustirin (I) /)iu ii\\i~irt. I his should highlight thr signi- liranrr of thc rrrultr and plarc thrm in the rtintrxt of iilhrr work (R) 11 In~~ii~lr~i1,~riit~arr (h) Hi*k'ti.trt I? li) l;ihli*s (t) fllurrmrrrmi (1) /'lkWrl' h*.~lWd\ Srlrntlllr nanw. I hr ctimplrtr wirntific namc (gmus. spcrirs and authority) should br cilrd ftrr rvrry ttiynisnt whrn first mrntioned. lahlcsarc dirn trwlul in rnllatitig rprrthr nnmrs and. if iisrcl in this wa). should hc rrfrircd tn rarly in Ihr tcxt. Suhrrqurnt to it? Arst appcarancc in thr tcxt. thr grnrrir tranir may hc ahhrcviatcd tu an initial rxrrpt whrrr tnlrrvcntng rckrcncer to ntlirr grnrra wnulcl catisr confusion, ( .rimmon nairtrr 01 or~anisnrs. il urd. IIIUSI hc arcnmpan- ird hy thr rotrrrt scicntilic namc nn lir\t irtcirtttrn Latiti namrs shriukl hc trndcrlinrd. Ahbrrvlath- and mltr. I ull natirr\ with itnecmr- tt~tiin alihrrviations ntuqt hr gt\cti wtth tlrr hrqt tiirntitin: nrw alilrrrvi~ttt~itt~ rhiittlcl In. coinrd oitly fiir iinwicldly names and shiiuld ntit lw uwd iit a11 titrlrss thr natrrrs wcur Irrqiicntly. In thr title :incl sunriirary uiiiirual nhhrrvtatiotrr should lw tdrnitlircl. in thr itrtr~idtirtion and dtr~urrititi thry rhoulcl hc ri\ctt \partn@I) SI ittiit\ air prclivid ( iiiitittiutiiis \Iiiittltlc.~i~i~ult thr Mii~;il Sfi