Initial Colonization of Artificial Substrate: Community ...

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Initial Colonization of Artificial Substrate: Community Development end Structure Studied by Scanning Electron MMieroseopy

HUDOP;,C., AND E. I3ou~c;e.r. 1981 . Initial co!onization of artificial substrate: community development and strelcture studied by scanning eEectrcrn naicroscopy. Can. J. Fish. Aquat. Sci. 38: 137 1 - 1384. The initia! phases sf subtidal and intertidal con~rnunjitydevciopment were observed using scanning electron microscopy on samples from plastic panels lmnnersed monthly in the St. Lawrence Estuary. Bacteria and cliatona populaticdns were quantitatively evaIuated on sarnpIes collected fr'ffon-~ May to Novcrnber, 1978. The gattern sf immersion andlor periodical emersic~naccording to the Ievel greatly influenced thc cc3mmunity structure. Subtidal panels I-5.0 m) were rapidly colonized by bactcria while diatoms settled 2-7 wk later, depending on the season. 6,'occonebs spp. arnd A~~aplzordt spp. were the major diatom colonizers until mid-August. In September. Syncdra ruhrtCcssta settled on the panels. Until mid-September. all dominant species formed well-defined, generally monospecifkc clunaps. In contrast with C'ocs-oneis spp. and Ar~fj~laora spp., which lie horizontal!y on the slafi~~ce. S. fcrblilcrt~s,which is needlelike in shape, fore~aederect fan-shaped coIonEes. Other late invaders pc~ssesseda mucus stalk, raising themselves from the surface and thus better utilizing the vertical dimension. Clump overlap and increased species interactions occurred with higher cell densities. In the intertidal zone bacteria settled after 8- 12 wk while AcBzncsn~kesbrevipw var. [ B ( P Y V U appeared ~ after 20 wk, the only cliatom species able ?cd resist se~raidiurnalemersion. The ability of the panels to retaia water through detritus and incg~~larities is probabfy the n~ainafactor allowing the development of this cornmaanity. Panels ernersed only at spring tides Imc~nthly)were rapidly calsraized by bacteria, and herrvy diatom settlement occurred within 4 wk. Successive monthly ernersions eliminated or strongly reduced diatom popuiations, which were replaced by filamentous (Ectocayaceae) algae. The three types of csrnrnunities arc compared and the strategic advantage of upright forms is discussed In relation to population density. light availability, and detrital cover.

Key bvorc!s: St. Lawrence Estuary, artificial substrate, community structure, community developanent. Cot-corzeis spp., Syaredra tacblarrbczta. An~f~hom spp. diatoms

HUDON,C.,

AND E. BOURCE'B.1981. Initial colonization of artifisiai substrate: community development and structure s t ~ ~ d l ebyd scanning electre~nmicroscopy. t a n . 5. Fish. Aquat. Sci. 38: 1371 - k384.

Lc ddveloppeanent initial des comn~unaut6sinfralittoralcs et intertidales a dti. observe en microscopic Clectronique 2 baiayage sur des plaques de piastique immergdes dans l'estuaire du St-Laaarent. Les populaticsrzs de bactkries eb de diatsn16es orat kt6 CvaluCes qktantitativemeplt entre mai et novembre 1978. Ees patrons de coloniszation varient seiora que Iq6nnersion est semi-dium-ae, mensuelle ou absente. Dans i76tageinfralittoral, les bactkries colonisent rapidement les plaques, tandis que les diatorndes sc fixent de 2-7 sernaines plus t a d . selon la saison. Cwc-oneisspp. et Atvhczm spp. donsinent la flore jusqu'h !a mi-aoiat. En septernbre, Sj1ntjdra rcsl~i~bcstcs colonise les plaques. Durant ceete pkriode, les espEces dorninarates fornrent des agrCgats monosp6cifiques distancts, gdnkralernernt isslks. Coanirairernent aux ecpkces pionnikres, qui reposeant hcxizontalernent sur Ic substrat. S . t~ahulrztcaforme des colonies dresskes en forme d9dventails.alors quc d'aukres csp2ces utiliscnt un pdd'ncelle rnuqueux pour se souIevcr au dessus du substrat. La superposition des cspkccs survient avec B'aargrnentatlon tie densitk. Dans 196tageintertidal, les bactkries se fixcnt 8- 12 sernaines aprks I'imrnersion, tandls qu9,4c.hrzanfhesbrevipex var. pe~rr~tlcl, la seule diatomde rksistants il 19&metsionsemi'Cormlriblatianto the program of GIROQ IGrsupe htemniversitaire de Recherckes OcCanograghiques du Qukbec).

Printed in Canada (J6 185) Imprimk au Canada (J4 195)


CAW. J . FISH AQLrAT. SCI., VOL. 38. I981

diume, n'apparait qu'agrks 20 scmaiaaes. La prdscnce de dCtrittrs sur Ee substrat est l'un des principaux fxteurs dc rCtentiun d'eala, permestant indirecterncnt la croislpance des diatsmkcs. Les plaques 6nrergf5es aux naarCes de vives-earax seulcrnent Irnensarcllement) sont rapidemcnt colonisCes par les bacterics et Ies diatom6es. Par contre, Ics 6rnersions succecsives rkduisent. voire 6Hialinent les diatornCes. qui sont remplacdes par les cctocarpales. t c s trois types :kc commurrautks sorat co~raparkhet I'avantage stratCgiqlac du d6velksppernent vertical est examin6 en relatiurs aacc la densir6 deb populationh, la disponibilitk de BumiCrc et la presence dcs dktriteas. Received Atagust 15, 1980 Accepted June 9, B 98 1

IN most studies of eariy phases of colonization of solid skilastrates, bacteria and diatoms are considered separatejy mainly because of the differing requirements of anethods~lcrgicallappoach. Bactce.ia are usually staiated and examined in Bight or epifluoa-escencc microscopy whereas diatoms are usually scrapcd off their substrate and later exarnincd using light n-aicroscopy. Although of considerable interest, such studies ailow casnsideration elf onBy the pl~ysicochemica!tolerances and tias seasonal variations of species abundance. Because they ignore spatial relationships, slaey result in a very iinconnplete view of the t~rganizationof community. An asveral! view of the taxa present without altering their original position and disrributia~lrecan be nbtaincd using light microscopy of intact popuIations settlecl on glass slides (Belanger and Cardinal 1977). However, this technique is restricted by the wbstrate nature and rnultilayered cell cover. These limitations were ccdverccrme by using scanning electron microscopy kSEM) to s t d y the early phases of settlenrene on a number of natural and artificial substrates. The flora associated with the surface of various aquatic macrophytes. such as As~.c>~hyvlvl~rn nnsa'osntm (CuncieEl et aI. 19771, Claart~sp. and Bott~rnog~aon Panfacans ( AIBanson k 973b, Sprarrinc~u~aernhflorw (Gessner et al. 1972). and Zoster-a mcrrisu (Sieburth and Thomas I C)73), was studied. Surface colcrnizaaion of n-earine pjaetts has been reviewed by Sicburth ct al. ( 1974). Nc~nliving organic srnbslrates such as driftwood were also investigated using SEM (Brooks et a!. 19-72?;Cundell and hfitchell 1977) and a comparison between natural and artificial substrates was made by Paul et al. (1977'), using sycamore leaf and p l y urethane foanm. Investigations of vdrious inert natural surfaces such as rock (Perkins and Kaglan 19-78) and sand grains [Weise and Wheinheimer 1978) indicated preferential settfement and development in cracks and holes of the substrate surface. Di Salvo and Daniels 41975) used glass slides to examine bacterial attachment. Although these studies pave irnporLant informatic~nabout the corn~nunitystr~ctureand organization, they prt~videdno quantitative analysis of the sequence s f colm~izoltionby bacteria and dicttoms. As pointed out by Mclntire and Mcsa~re (1977), few studies give a caanprehe~~sivc view of the biological interactions occaarring among diatom taxa as ~vcllas bctween the diatoms and sther groups of piants rind animals. Accordingly, this study examines quantitatively thc cornmnuniay developn~~ent and biological interactions occurriasg in a slow-growing, low-density priphytic assemblage.

Study Area The study area, a 90-knn section of the south shore of the

kowcr St. Lawrence Estuary (Fig. I ), is in!luenced by strong scmidiurnal tides ranging from 1 . I to 4.6 m (Canaciian Hydrographical Service 1478). Salinity and temperature showed no strong spatial variation or gradients ehet~ugholatehc area. .At any given time, salinity and tennperature ncvcr varied more than 2c:0,and 3°C between any two stations of similar depth. Water transparency ranged from 2 to 5 an (Nota and Loring 1964], clepcndirag on nnixing. Seasonal variations of salinity armd temperature occurring during the sampling period were nluch more pronouncecf. From early June to early November, salinity steadily increased from 24 to 29c~m. Surface water tenaperature increased from 9°C in early June to a maxilnu~mof 15°C in early August, and thereafter decreased gradually to 2°C in late October. At 5-rn depth, water tcmgenature varied frtm 3 8s 9°C in August, then decreased to 2OC in late October.

&laBeriaI and Methods Sampling stations were established at teggatt's Point, Pointe Mitis. and Les Lkts in the intertidal and subtidal zones (Fig. 1 ) . The first two stations were Incated on the exposed and protected sicles, respectively, c~f the same tleadland whereas at Les Blets the sheltered side s f a pier was based. Black. flexible, and nontoxic Conoflexo' (Picanner Plastic, diivisic~nof t o f Plastic k c . ) plastic sheets were used as subthe strate throughout the study. This material is resistant various solvents used as well as to tow temperatures. Its sudkee contour shows fine ridges about 15 p n wide and 3 pm deep which are very suitable idPr initial colonizers. were placed vertiFifty-seven 400-cm2 Conoflex '"panels cally on the various coi~echors.B~lterZidaicollectccdrs were made of Harge plywood sheets attached either to a vertical s u p p ~ ~ r t anchored in thc tidal zone as in Bourget and Lacraix (1971) or directly onto the side of a pier. SubtndaI collectors were made out of welded iron bars which held the panels 0.5 n.g above the bottom. The subtichi collectors were sarmaplcd by SCUBA diving. One interticla! and one slabtidal collector were placed at keggatt's Point and Pointe hMitis stations. Les EEeth pier held a vcrtica! row of pancis from the Bow-water levcE (chart datum) and above. The vertical position of each collector is given in Table 1. Each collector contained a series of panels which remained in position for periods ranging from 3 to 22 wk. At eacl~site, four panels were present during the entirc sampling period. 'Fhree other panels were irnmerscd at monthly intervals until the last collection in November, and extra panels were also added irregularly. Subtidal stations were sampled at fort-


HUIZON AND BUURGET: CCPLONIZATIONOF A R T I F I C ~ ASUBSTRATE I,

PIG. 1

STUDY

AREA

Localkon 01 thc staticjns lam the lower St I J a ~ r e n c eEstuary.

nightly intervals ;and all others were sampled at weekly intervals. Table I gives details of total number of panels and sarnples at each station. All samples were obtained by punchiaag out 2 X 2 crn frdgrnents from the panels and each sample thus represented 1% of the panel area. Sarr-Eplesize was linnitecl by the size of the microscope chzatnker. Because during the field work a strong iinllucnce of tidal level was observed on distribiation, all thc panels were sampled in the top, middle, and lower portions. The exact Bocation of each sample at random within a given portion was noted using a coordinate grid. SEM prclirninary observations of intertidal panels showed that marked variations of the ppulatic~n~s occurred within a verticm. Thus, all samples used I'asr cal range as snnall as I temporal comparisons ial the Intertidal came from the same level. Such spatial variation was not observed in the subtidal zone, so there, all three replicate sanaples were used to describe populations or1 a given panel. Procsdua-c was as folItaws. Immediately after collection, all samples were immersed in filtered seawater and taken to the laboratory. They were then fixed for the SEM using phosphate-buffered (pH 7.2) gluttaraldehyde 2.5% for 2 h and dehydrated in a graded series of ethanol soIutions (25%, 50%- 75%). Samples were kept in 75% ethanol until final SEM prepamtion. Whole panels and additional duplicate samples were directly fixed in 95% ethaana~lexcept when coionizcd by invertebrate larvae. The latter were prepared for the SEM as indicated abc~ve. Before SEM observation, samples were transfered to 99% ethanol, freeze-dried for 12 En, rnounted witk nail polish on aluminium stubs, and coated with gold-palladium. BbserT A B P1. ~

8 373

vations were carried out ~ i t ah SEM ETK-AUTCISCAN at 30 kV and 8" angle bean-a. Thc fixation and drying procedures were selected for their reliability for the preservation of surface-associated anicroorganisnas (Garland et al. 19'99).Cell enumeration in the residual rinsing solutions shtawed ara insngnificant number s f cells lost due to manipulations. lBry weights of total organisms colonizing individual panels were obtained by scraping freeze-draed samples. The scraped material as then wcaghed on a Cahn balance to the nearest 0.0l mg. Samples were anaiyzed as follows. Ali diatom taxa were individually characterized durlng the counting procedure. Taxa observed in less &an 5% of the samples were not considered further in the analysrs The diatoms were idenltifned with characteristics seen with the SEM and light anicroscog~eusing naonc~graphsof Boycr ( H 927a, b). Musted d 1930), CBcve-Euler ( E 95 1 - 551, Hendey ( 19691, and Patrick and Reirner B 1966). AEI identifications were later verified agalnst collections at the Philadelphia Academy asf Sciences rand checked by Dr Ruth Patrick. A statistically valid estimate of bacteria and diatom populations present 081 a sample was obtained by counting indivialraals at 2tBB)OX (1000 pn') for bacteria and at 5OOX (38 000 pn') for diatoms. 'I'he distribution of organisrlas on each sample was determined by fitting a theoretical rn~odel(pcssitive binomial, Poisson series, or negative binomial) to the counts of randornly selected nnicroscopic fields at the appropriate magerification. For each sample, bacteria were counted on 5 - 1 0 quadrats, depending on density. For diatoms. up to 250 microscopic replicate observations were necessary to give an accurate count (precision on the mean ,8096).

SPECIES~ O ~ / B P ~ P O S HAND $ I OSH'ATIAL N DKTTRBDVTICIN We considered bacteria to include all rods and cocci forms. and we lanadc no attempt at strain identification. 'The cells were e~isilyvisible on the substrate and rao slime formation was observed. The distribution of bacteria was tested with a Molmogorov - Srlalrnov test appIled to the quadrat counts. Thc results given in Table 2 indicate a random distribution of bactcria on intertidal and subtidal panels at both high and low densities. Wc identified 20 tawa of diatoms belonging to 17 genera (Table 3). Major species abundance and composition were quite silnilar at the two subtidal stations (Leggatt's Point and Poinre Mitis). It can also be observed that the lower intertidal station of Ees Blets had Inore species in common with the srrbeidal stations than the other intertidal stations. 'l'ke total number of species diminished witk increasing tidal height.

Description of tRs stations and sample characteristics.

Station K,mation of collector Vertical range (rn) Total no. of panels imrilerhed Total no. c ~ fsamples Nap. of sampling kaeric~ds

Lepgatr'~Point

Pointe Mitis

I,es Ilzts

Intertidal

Subtidal

Intertidal

Subtidal

Pier side

+2.7 to 2.3 9 B 45 14

-5.0

s1.6 ea, 1.1

13 143 6

13 E 44 I4

-5.0 12 82 6

+2.7 to 0.3 10 23 1 12

CAN. J. FISH. AQUAT. SCI., VOL. 38, I981

TABLE2. Examples of random distribution sf bacteria from intertidal and subtidat samples of different densities, by mean of Kolrnr>gorov-Srnimc3\9 test. The 1%' replicate counts were c~btained from 2000X (lC@O p.rn2) enlargements using the $EM. **P < 0.08 1 .


Intertidal Subtidal

Densities

N

Minimum

Maxinaurn

Mean

High Low High LOW

25 25 29 30

208 88 338 67

846 599 848

554.2 328.56 559.0 111.7

187

Standard enor

Maximum deviation

65.7 44.9 48.8

0.I@** 0.073"" 0.092** 0.058""

18.9

TABLE3. Presence (%) of the different mxa caf diatoms f w n d at Leggatt's Point, P~rintehfitis, Les Jlets, and the intertidal collectors of the Fame stations. Percentages of presence were calculated from numbers of sampIes at each station where diatoms were present." Subtidal

Interfidal

Low ( -t 0-3 to+0-7rn)

High (>+l.Om)

Points Mitis

Les Elets

Leggatt's Point Ptainte Niris k s Ilets

%

%

%

(-5.0 m)

Taxa

&eggart's Point 96

Coc'coneis c'osfaP4 Synedra P Q ~ U ~ ~ F Q Acknuratl~espseu*roen/czcmdica ,4mpBac>rcrpusio Synedra inve~sri~ns Gomphora~makamrsc8rabicum BiddubpFnica afdtcr WVavicm4Q spp. Coccaaneis st-are!lum Amphora cosfQdks Diplorzsis spp. &'ossipaodiscu~ spp. Rkoiccrsp!zeniaz curvrzfa Pdeavicrda criasicula

"Species present on less than 10% of the samples bearing diatoms, at the two subtidal stations: Rhabdonema arc-ndatunt, F'rclgilaria ccr>irsrrecens,Nitschia parksdoxa, Gyrosi,qma faiasc-ids, Thusassiosira nordenskiolsfii. Chan6aeroccros asianaicus, and Sksbefonema c.osfarimm. The last three were damaged planktonic species.

(5%) of dominance of the major diatom species found at Leggatt's Point, TABLE4. Preque~~cy Poipate Mitis, and Les Hieas. The percentage of dominance was computed from the number of sampies where the species was present. A species W ~ considered S dominant when it reached at least 40% 'ef the number of diatom cells present. Subtidal

Taxa

Low intertidal

High intertidal

Leggatt's Point

Pointe Mitis

Les Hlets

Leggatt9s Point Po'ante Mitis Les Ilets

57 $7

32 50 24

33

-

-

-

Cocconeb c o s l ~ f a Sywdra tabulafa Amp~brorapid.cio Synedra ink1@.~fiens Achnnnti~esbrevipes var. pa!vula

-

-

14

-

-

-

88

Total no. of sanlptes analyzed Tdal no. of species No.of samples with diatoms No. sf dominant specks

50 23 43 2

25 18 21 3

22 E4 8

51 7 20

2

1

HUDON AND BOWRGET: COtONlZATION OF AWTlFiCEAL SUBSTRATE

T~s1-s5 . Results sfthe gor3darttss of fit test applied to the Poisscmn series and negative binc~rrriaral distribution for two densities of the dominant diatoms species. **P < 0.01: ***P < 0.005.


N

K

x

Poisson series

Negative binomial

"$niy lsw densities were obsemed

fie(%.Clump growth combined with the recruitment of new Only Achncd~afhe.~ brevipes var. ~ Q T V L &was. ! ~ limited to intercells later produced complex assemblages, as shown on Fig. tidal stations. where it was very common. Five dominant species were observed (Table 4). Coc-coreeis 2E-F. The spatial distribution of A. b. yare pas-vula, the cos~drt~~ was present at all stations (Table 3) and was dominant dominant species in the intertidal zonet is shown in Fig. 4B. at lower sampling sites ( - 5 , 0. and +0.5 RI). Synedra takdba- It did not show any preferred orientation or pattern. The cells and A~nphoreepusio, two other common species (Table were usually Hying randomly among the detritus. the raphe 31, were dominant at -5.0 m only. Sy~zeCdB.ai ~ ~ v e s f i ean ~ ,side hcing the substrate. This species was occasisnally seen species common at all stations, reached dominance in the low with a shoa stalk. In summary. these species of diatc~ms intertidal zone 4 +$.5 m at Les Ilets) while A. b. var. yarv~da employ different strategies of spatial utilization. dominated higher level sites. Diatom distibution was first examined using counts from random microscopic field areas at 500x. Highest and lowest The settlement of organisms on panels immersed subtidally densities of four dominant species of diatr~mswere fitted 80 ( - 5 rn) at monthly intervals at Leggatt's Point and Pointe the Poissc~nand negative binomial theoretical distributions. Mitis was examined. The first samples, taken 15 -2 1 d after The results given in Table 5 indicate that the negative binorniianmerslon, were always colonized by bacteria (Fig. 3A, Bb. al model fits the data best, thus indicating a strong and conThe numbers were low initially and gradually increased with stant sc~ntagiousdistribution of cells on the substrate. The time. On all Pointe Mitis panels and on those immersed in precision on the estimated population mean was maintained at August at Leggatt's Point the bacterial counts stabilized bea tolerable enor of 2@%, using the following equation to tween 20 x 10' and 40 X 1 O4 cells mrn '. High counts (e.g. determine the appropriate number of subsamples: 150 X 10"cells rnm ') were always fcAlowed by a sharp decrease. Diatoms always appeared later than bacteria on the plates. C. costaf~ausuaIIy as the first species to be observed. The extent of time prior to diatom csBonizatlon ranged from where N is the number of replicate observations necessary to 4 to 6 wk depnding on whether the panels were immersed in attain a fixed percentage of confidence ( D = 80%) on the July or in June (Table 6 ) . The acceleration of diatom colosample's mean. A prelimina~grsampling provides estimate nization later in the season was very pronounced at both ) of the second parameter of the kggzltt's Point and at Pointe Mitis. Panels immersed in Jtaly values of the mean (iand negative binomial distribution ( K ) (Elliott 19'7 1). at this latter station showed colonization by C . C - O S M ~ after Q as Characteristic aggregated distributions of dominant species little as 2 wk (column 4, Table 6). Although C. costafa was of diatoms are shown in Fig. 2. At first, all clumps were the first species to settle its density remained low and only a. strongly monospcific, separated by luge bare areas. slight increase during the season was observed. Highest wumCocc*onePscostafa ((Fig. 2A) and Ar~~pkorra spp. were usually k r s on subtidal panels were observed in July, when 364.5 lying with the wphe facing the substrate and had a tendency ceHs mna were counted. to occupy the ridges in the substrate. The cells of S. investhcrrs $:k?~leha investi~nsand A. pusio were other species uband S. tab31gdafa were both similar in shape but they showed served at the beginning of the colonization period. The former very different patterns of aggregation. Synedrl2 invsstiens species rarely reached densities sufficient to be quantified, sells were separated from one another and were lying in the Amphorn pusis was observed sianultaneously with C , costata ridges of the panels (Fig. 2B) whereas those of S. l~tbulafa at Pointe h4itis but occurred later at Leggatt's Point. As~phuva formed regularly arranged fan-shaped colonies (Fig. 2C). pusio showed very characteristic growth patterns on all panels These colonies were seen t c ~erect as the density of thc com- immersed throughout season. initially, there was a latent munity increased. Ae.hlaanthes pseud~groenlc~ladicu (Fig. 2Db phase of about 5 wk during which the number uf cells fluctuand Gomphonema kam%sc*8zczticumwere frequently observed ated but remained below 100 cells mm '. This was followed on the panels. Both species were capable of forming a mucus by a growth phase at the end of September, when the p o p peduncle, which in G . h m % s ~ ~ I ~ a t Pcould ' ( ~ ~ lbecome m mmi- laeion reached 236-675 cells = ~ n r n - ~ -

-

a

-

CAN. J. FISH. AQBTAT. SCI , C'OE,. 38, 1981


1 376

Frc;. 2. Monospecific aggregates of doaninaamt diatom species fcjund subridally. All micrographs were taken h,m the srrrne barnpie. (A) Cucccvwis costcatu (C.C.)laying with the raphe-bearing valve facing the substrate. (B) ,Tynedra i~~r,esneris cells laying horizontally. { C ) Erect fdn-shaped coirpnics of ,YynetPj*lal~lPzu/oru(S.f.1. (D) Achnanthe.~ps~radf)grnesml4lndica ( A .p.) cells bearing a short peduncle. (E) The subtidal community as cfiserveal at -5 rn tit the end of Septernher. (A-E) Scale bar indicates 100 k m . (F)Absetnblagt: aaf the dominant diakrprn s ~ c i e s fottnd subtadally. Note how peduncle pemsits the cells of A . psriado~roc~nltz~adica L4.p.) to escape smothering. S a t e bar indicates 10 pm.

HUDQPN AND BOUWGET: COLONIZATION OF ARTIFICIAL SUBSTRATE

1377


P o i n t s # i t i s % - 5,Om 1 k3

June

F ~ G .3. Variations of abundance of bacteria and diatoms on five panets immersed at -5.0-m depth at keggatt's Point (A) and Poipate Mitis (B) stations. The date of immersion s f a panel is indicated by an arrow. Bacteria (a), and the diatoms C'a~cconaiscosfafa(A), Syncdra invesdiens (0). Aehnunfl~espseudogroenlandica (El), Ampheam pusio C l), and Svsacdra tabuEata 10). Values on the akscisba indicate the presence of less than 20 cells nm-'. Vertical lines represent the confidence intervals. Scale bars indicate 10 yrn ( A , B , D. F) or 1110 prn (C).

-

detritus accumulated on panels for a period of $ wk (Fig. 4A). Then, in early August bacteria appeared, 6 wk later than in the subtidal zone (Fig. 4B). Simul&neously, filamentous Ectscarpaceae began to grow and reached a maximum abundance 2 wk later (Fig. 4C).Thereafter, their abundaeace declined and the bacterial density increased until early September as the detritus continued to accumulate. Ach~anthes&re\'G'$es vae. pauvula, the only species that showed important development on intertidal panels (Fig. 4D), was first seen in September. At first, cells occurred only in the depressions of the substrate, but iater they spread wherever detritus accumulated, reaching densities as high as 2194 cells mm-'. No evident relationship could be found between the abundance of A . b. var. parvula and tidal height. In view of the marked differences between subtidal and intertidal colonization, we decided to examine the bacterial and diatom settlement at intermediate leveIs varying from cssntinuously immersed to semidiurnally emsrsed. At Ees SETTLEMENT IN THE INTERTIDAL AND SUBLITTORAL FRINGE Ilets, three panels were placed at f0.3, C0.5, and C0.7 rn above low water of spring tides on May 26, shortly after the Intertidal panels placed between + 1.(2 and 4-2.5 rn differed spring tides, and were only ernersed for short periods during markedly from subtidal panels in basth the species present and each complete lunar cycle. the pattern of colonization. After immersion in June, only The fluctuations of the t h e major types of colsnizers

Settlement of C. csostatcz,S . irsvesfieas, and A . pusbo was usually followed in August by colonization sf Synedra tabulafa. The density of this species remained under 100 cells mm-' during the whole summer but, at the end of September, cell density reached 641 - 1485 cells mm-' and the species comp8ete8y covered the other species present (Fig. 3). Other species (e.g. A . psei4dogroenh~tdicca and G . kamtschaeicnrm) were frequently observed but their numbers rernained low, Generally, on these subtidal panels numbers of diatom species increased as the season progressed, reaching a maximum number (between 15 and 20 depending on the panel) at the end of September. At first, detritus occumed as minute organic particles. Both the size and the quantity of the detritai particles graQuaIly increased with time sf immersion. The detritus was often associated with diatasm patches, suggesting mechanical retention by the cells (Fig. 2E).

-

-

CAN. J. RSH. AQUAT. SCI.. VBE. 38, 1981


TABLE6. Time lag between immersion of panels and setttement of Cocc~neis costafa at different periods of the season at the (-5-m) Leggatt's Point and Point Mitis subtidal stations.

Station Leggatt's Point

Point Mitis

Time between the first oksemation and an average of 65 ind.!mnn2 (wk)

Dare of immersion of plate

Time lag before colonization (wail

June 10 July 9 August 6

6

7

4

3.5 >I3

June July

18

I4

3-5

5 4

>22 2

FIG. 4. Community development on a panel immersed in early June and sernidiumaiiy exposed to air. (A) Detritus {Deb accun~ulatgdafter 6 wk (July 5 ) of immersion. (B) Detritus (De) and bacterial colonization after 18 wk (August I ) of immersion. (62)Dense filanrentous Eceocavaceae cepionization after I % lWk(August 16) of irnmersi~~n. (Dl CoIonization by Achn66rrnfhes brevipss var. ~ ( L Y V U ~after C ~ 18 wk of immersion (September 26). Scale bars indicate Bb) prn (A, B , Dl or 100 pnl (C).

(bacteria, diatoms, and filmentous Ectc~cnrpaceae)on the panels are shown in Fig. 5. Bacteria were present on the panels throughout the summer at all three levels: they showed synchronous growth patterns with density peaks occurring in late July and the end of September. Their abundance remained

about 10 X lo4 to 20 X 10"eells rnm- ', Bower than the densities observed on the subtidally immersed pIaecs.At all levels, diatoms were present on the panels at the first sarnpiing date, 4 wk after immersion. The species were common subtidal forms (Table 38, C. costa861 and S. ilavesrisns achieving a


HUDON AND BOURGET: COLCBNIZATION OF ARTlFIClhL SUBSTRATE

Fro. 5 . Variation of abundance of bacteria (@I, filamentous brown algae ("), and number s f diatom species (shaded area) on panels at station Les %letsbetween June and November. Diatom abundance is not shown as reliable estinlates could not be obtained at all times due to the presence of filarnentoaasbrown algae. The symbol associated to the number of diatoaaa species indicates the dominant species: Cocconeis cossata (A),Synedra it~~e~sien~ (A),no dominance noted ((0). Encircled letters refer to one of the micrographs below. Hlustrltting the co~esponding successioneaa8 stage (see text). Scale bars indicate 10 pm (A, C ) or 100 p,m (R).

f 380

CAN. J . FISH. AQUAT. SCI., VOL. 38, 1981

doanniplance. The fonner occumd in low densities at +0.7 an (56 cells mm-') but reached a density of 1080 cells mrn ' at +0.5 rn. It was dominant at both Eevels B +0.7, +8.5 m) sampled in midJune. However, in June at both levels a large proportion of cells were damaged or dead and covered with bacteria (Fig. 5A). Because the fixation procedure was identical for all samples and bacteria wcre ncver observed on diatoms from subtidal samples nor from intertidal samples prior to the June sampling, these cells were presumed to be dead or in a poor physioEoglca1 state. Sj~nedr-ainivstiens, the other species frequently observed, reached only 47 cells ' mm-=. Two weeks later, at mid-July. diatoms had completely disappeared frcm the +8.5-m level and had considerably diminished at the +0.5-m level, which was now dominated by S. invesfims. In contrast, at the +$B.3-m level, @. eosfata cells appeared in good condition and a density of 246 cells = mm-' was observed (Fig. 2A, 5A). Filamentous Ectocarpacae were first observed in mid-July growing in small clumps on the Coccs6aei.~-coveredsubstrate(Fig . 5B). In early August, diatoms were no longer present at the +0.5-m level and were in Iow numbers at the +0,3-rn level. Simultaneously, filamentous Ectocapaceae had reached their maximum abundance and detritus was abundant. Later on, Estocapaceae decreased and they had disappeared by midSeptember. Diatoms, especially S . i~rr:er;ehens,occuned in low numbers until November at the +0.3-m level. Furthermore, during the autumn the cells appeared unhealthy (Fig. 5C). At aI1 stations subjected to smersion, filamentous Ectocarpaceae and detritus were abundant. %Jnfmtun~teIy these two ce~mponentswere difficult to evaluate quantitatively using the SEM. As an indication only, Fig. 6 compares the variations of the dry weight of total colonization in monthly (+6). 3 rn) and scmidiurnlally ( + % -3 rn) ernersed conditions. There were significant variations of the dry weight at +0.3 rn during the summer. The highest value (122.5 X BB " g cm- ') coincided with maximum growth of filamentous algae, which formed a 3-mm-thick olive-brown layer on thc plates. Such a large change of biomass greatly overcomes the error involved in the diatom removal procedure. Colonization s n the plates at the + B .3-m level was less csnspieuous, more gradual, and led to the formation of a thin film in September. No confidence interval can be drawn for the values of the two curves in Fig. 6, as each point represents a single sanmple, made unavailable to SEM observation. Nevertheless, the curves give a good view of the general colonization patterns observed, contrasting the marked iluctuations at +0.3 rn with the progressive growth at + f - 3 m.


-

-

FIG. 6 . Seasonal variation of the dry weight of material on panels emersed monthly ( + 8 . 3 m) at Les Elets station ( A )and selraaidiumrai8y (+ 1.3 m) at Poirite Mitis (6%).

was achieved by Bklanger and Cardinal ( 1957), who observed undisturbed populations settled on glass slides using light microscopy. However. the three-dimensisnak aspect of cornanunity development could not be examined using this technique. Even with the SEM, which we have used, it is not easy to quantify interactions, but because we have documented the localization and orientation of the various h x a as well as the "'smothering" growth pattern of S. f~zhidata,we feel confident about proposing some biological interactions. En the following discussion, in addition to examining the influence of physicochemical casnditi~ns,we attempt to explain how biological interactions might influence the structure of benthic diatom communities.

INFLUENCE OF PHYSICWHEIMICAL FACTORS ON INITIAL SETTLEMENT AND GROWTH OF DIATOMS

In our study, the physicochemlcal hctors may have influenced the dme of settlement and abundance of many species. Rapid growth of bacteriaH population observed on panels immersed later in the summer can probably be explained by the summer increase of water temperature (Bott 1975). According to Cleve-Euler (1953), A. PUSB'O is slightly haloplzilic. Thus, it was not surprising that the abundance of these species remained Eow during the summer while salinities were low (24-27'/,), and only increased later in the summer as the Stladies on diatom cslsnizatioma of hard substrates using salinity semhed 28 - 29"Ioo. The combination of temperature glass or Plexiglas panels and light rnicroscapy are common. and salinity could also be of some importance. Optimal This technique provides an accurate view of species com- gsc~wthof C. rosfafo, laat arctic boreal species BCleve-Euler position, abundance, and diversity of the assen~blages 19553).was obscmed between 5 and 14°C and at 26-27% in (Mclntire and Overtoen 18'71; Bacon and Taylor 1996; Main the Gulf of St. Lawrence (Bellanger and Cardinal 1977). and McIntire 1954; Moore md McIraeire 1977). However. the These prdere'nses could explain the settlement of this species technique has limitations as direct observations of spccics in early summer. Our results also suggest that high light ininterrelations and of effect of sediment accumulation are not tensities may influence the abundance of this species as the possible. An intemediate step to overcome surface scraping largest numbers were observed in shallow waters (e.g. station

HUDON AND BOCTRGET: COtOWiZATION OF ARTIFICIAL SUBSTRATE


Les Ilets). Emersion also was an important factor controlling the abundance of this species as it was not very cornnlosa on the semidiurnally emersed plates and was eliminated from n~onthlyemersed panels following their exposure at neap tides. The only species resistant tcd seanidiurnai crnersion, A. &. var. gzarvubca, was commonly observed from February to March on partially exposed rocky sur%aces (Hendcy 1964) of the English coast. These conditions are very similar to those in which this species was observed in our study. The two other commonly observed species. A. ps.~udogroen!~ndiccband 6;. ikamtschaticum, arc Arctic marine species (Boyer 1927 a, b; Hendey 1964) and. as expected, their maximuan growth was observed in early November, when temperatures were Iowest (2°C) rind salinities maximum (28-29'1%). The influence of tidal level on the hard substrate algal cesmnaunity has been investigated by many authors (Castenhctlz 1963; McIntire and Wulff 1969; Doty 19464) and our results arc in agreement with previous observations. In our study, colonization was reduced at higher levels: the settlenmt of bacteria took up to 14 wk, the number of diatom taxa was reduced to a few tolerant species, and the abundance of Ectocarpacae was lower. Bacterial colonization occurred earlier on continuously imar-aersed subtidal panels than on intertidal panels. At increasing depth (> 18 rn in the Estuary) diatom growth decreased and became negligible. Sianilarly, shallow-water panels were colonized faster than deep-water panels. Comparison of panels immersed in July at E 5-, and 10-rn depths at Leggatt's Point ( I and 10 m, unpublished data) showed a time lag of 1,4, and >7 wk. respectively, before diatom settlement. Here, perhaps higher light availability or water temperatures accelerated settlement. as temperatures were 12, 9. and 5"C, respectively, at these three depths. At 5-m depth, the time lag before diatom colonization also decreased from 6 to 4 wk on panels immersed in June (9- 10°C) and August ( 15'C). Further studies are needed to document tlris aspect. The lcow number of species enumerated in this study can he explained in many ways. First. it must be stressed that only the major species, occuff~inpon at least 5% cof the samples, were included in this study. There is also no doubt that a nonliving substrate is less hvorable to diatom growth than1 macroalgae or marine angiosperms (Round 1971; Main and McIntire 1974). Furthemore, because the &st growth depth for epiphytic diatoms is well above 5 rn (Round 1961: Sladeckova E962), we can assume that the communities described in this work were all limited to various extents by various physicochemicaI stresses. mainly high turbidity (low Eight intensity) and low temperature at 5-m depth and desiccation in the intertidal. The annual range of salinity for this area is %6-28"Iw and souPd also partially explain the simplified community observed here. However the reduction of light to 1-5% at 5-m depth is the hctor most likely to be limiting. All these factors combined result in a small species poc~l(Patrick 1969) and provide the best explanaticsn for the simplified communities and interactions observed here. The effect of grazing on attached diatom ccommunities has been dcocumented by many authors (Dickrnan 1968; Patrick 1972; Nicotri l997), who showed the selective effect of predation. Jn our study, littorinid gastropods and sea urchins were oceasiomally observed on the panels but the distinctively denuded '?racks" visibIe behind them were dis-

111

T ~ m s

-.

. 7 . Schematic representation of colonization in the subtidd (A), intertidal (B), and subtittorai fringe ( C ) . Arrows indicate the various possible interactions between community components. The relative abundance sf an c~rganismis given by the size of the enclosure, and shaded ones indicate an eventual seasonal favorable factor; (dl detritus. Vertical Iines refer to spring ride monthly ernersion EmersEon disturbances are shown by a zigzag line. I

carded from the sampling. The effect of grazing by amphipods and harparticoid copepods was investigated separately and this daea will be published later. Thus, the communities described here arc not only thc result of biological interactions and physicochemical stmss but also of grazing pressures. In summary, the communities observed in this study can be characterized by their very slow development and Iow diversity. SUCCESSION IN THE LITTORALZONE

As indicated above, stations of similar level had similar and characteristic patterms of temporal successicdn. In the subtidal area, the pattern of colonization can be divided in three distinct phases, shown on Fig. 7A. During the first phase. only bacteria settled on eke panels. As previously shown. the &aration of this period varied with physicochen~ical factors. During the second phase, settlement of diatoms occumed and the cells remained grouped in separate clumps. S L B simultaC~ neous growth in separate clump4 indicates that the presence of erect species is not conditior~edby the earlier growth of flatlaying species, thus ruling out the idea of dependency toward the preceding colonizes currently used to identify "true succession." At first, the number of species and their abundance were low but they gradually increased with time. Detritus

TABLE7.

Setrnmary of previous observations on sequences of diatom colonization.


Oksemations

Substrate

'9ndividuais sf motile pnnate diatoms which multipiied to Panels and plates of glass, form a film bound to the substrdtum by mucous secretions." Plexiglas, vinyl acetate. ""Thase pennate diatoms. . . Eater on secrete gelatinous tubes wood, and iron inside which they arrange themselves and multiply as to give the appearance of branched filaments"

1$TQiicu!6tspp. followed by ,Fplnpcdra spp. and Maridion spp.

Class slides

Bacteria followed by unicelIular diatoms, then by Lllothrir. and filamentous and coionial diatoms (e.g. inchnassthw spp. Naviru!~,Sc*hizunemsaspp.) and later on by Bctoc.arpees and Emtercamorpha

Test plates treated with anti-fouling preparation

Location

Authors

La Joila. CA

Aleem ( 1957)

Prater's creek, SC

Reisen and Spencer 4 1970) Wrtssell (197 I )

A cmst of Ccre-eca~zebs scldeetllrna (tlat-laying species) was obsewed first, described "'as an amorphous pavement of broken fmstules and detritus (which) supports stalked and colonial diatoms, Now seiective colonization by. . . ribbons ~ n , Ach~asantheslcwigipas, of Rkabdonema a d ~ - i a f i c ~stalked Nitzchiez sp. (etc,) followed"

Peetaquamscutt River, R1.

Sieburtk and Thomas ( 1973)

( I ) Bacteria foliowed by (2) pennate diatoms and later on by [ 3 ) stalked diatoms

Fort Lauderdale, EL,

Cundeil and Mitchell Q 1977)

Bklanger and Cardinal d 1977)

' % At

first, the cornnlunity is a two dimensicsnnal one. . . When the growth becomes relatively heavy many sf the species that formerly laid War on the substrate now wili stand upright because of jelly pads or their ability to produce jelly stalks. Thus a three dimensisnnal community will be produced. . ."

The last succession phases consisted always of long and spiny cells; at one station a broom of many species starled after the end of observations- At the two other stations, however, patches of Eneel-csmoaphcz and blue-green algae were appearing at the end of axperil~~ents

Sediments, under Eaboratosy conditions

Uniceilular forms of benthic algae like Cocconeis c.osFatn are follcawed later by Cocconeds srvuteilfmand finally by Faagilavia ccmsfwuens, A~nphoraspp. and Narlic~tlaspp.

Glass slides

Gulf c~fSt. Lawrence

" T h e first diatonas found attached were unicelluiar but after one or two months colonial forms were found. . ."

Rocky intertidal

Ria of Vigo Nieil ( 1979) (Noahwestem Spain)

accumulated in the diatom aggregations. The third phase started when the clumps began to overlap following the multiplication and recruitment of new diatom cells. During this period the number of species and the cell density reached their peaks. Dominance by one species was soon observed and all the substrate available was occupied by diatoms which were patially covered by detritus. This period marked the beginning of potential species interaction such as c o m ~ t i t i o nfor space (see Sicbbarth 1968; Patrick 8977). In the semidiumalIy emersed intertidal zone, community developnaent was very slow. Figure 7B irlustrates the probable factors structuring the community in this zone. First7 the detritus (d) progressively accumulated on the panels for many weeks before bacteria developed. Then, filamentous brown alga settled and grew extensively. Forlowing their subsequent elimination, the substrate became covered with large amounts of detritus and bacteria. Diatoms then settled (onIy one tolerant species, A. b. var. parvuCa, was commonly observed) and their density increased slc~wly. Colonization by diatoms probably required the water retentinn provided by detritus (see Hopkins 19641, as in its absence no colonization was ob-

sewed. 'Phis situation is the best example of ""conditioning" observed in this study. The sublittoral fringe shares characteristics of both subtidal and intertidal zones. However, biological interactions are overshadowed by the monthly ernersions. Figure 7C iElasstraees this pattern of colonization. As in the subtidal, bacteria and diatoms settled early and reached high numbers. At first, the species composition resembled that of the subtidak area, except for the presence of A. 1%. var. pa8-vu6a9the dominant species of the iratertidal zone. Following the first monthly emersfm period, the community became similar to that of the intertidal one. Diatoms were reduced to a few tolerant species or were completely eliminated, and detritus became very abundant. Bacterial growth and detrital ~ccumbalationappeared cancaamently with the decay of filamentous algae md diatoms, The maintenance of vestigial diatom populations at the lower (4-0.3-m) level (station Les $lets) may have resulted from the joint action of minima1 shading and sufficient rraoistanre retention by filamentous brown algae and detritus. Our observations suggest that the sublittoral fringe community was more perturbed by emersion than the semidiurnally emened coanmunity.

HUDON AND BOURCET: COLONIZATION OF ARTIFICIAL SUBSTRATE

ARE THEREDIFFERENT ~ O E C ) N I Z A T I C ~STRATEGIES N FOR BENTHICDIATOMS?


For horizontally positioned species (Coc*cwrmeis spp., A~nfzhoraspp.). the ability to move using the raphe probably allows spacing of cells after multiplication, as well as movements to crevices of the substrate for increased protection. Synedra fabuC~fa and A . p.~eudogr~~e~alandica were obscrved to use all three spatial dimensions, thus allowing an increase in their density. The fact that these species appeaahed late in the succession when densities were high suggests that they are well adapted for colonizing in conditions of limited space, their mode of attachment allowing them to avoid the reduced illumination due to "'smothering' ' by other species or detritus. Similar observations were made on 6;. ka~ntschatb'cum,a Qendritic colonial form which achieved an almost complete indepndence sf the substrate and reached dominance when overall densities were high (unpublished data). In fact, although these colonial forms are well adapted to occupy limited space and avoid smothering, they could have a detrimental effect on the lowee canopy level by reducing available light and nutrients. However, because heterotrophy is common among benthic diatoms (Hellebust and Eewin 1970), further investigation is needed to confirm this possible detrimental effect. Such occupancy of the vertical dimension is probably achieved at the expense of community stability as adhesion to the substrate is limited and the dominant species become more exposed to grazing and wave conditions. If strategies of substrate occupation are of some ecslogicd importance, a recurrent sequence of diatom colonization, eventually leading to the use of the vertical dimension, should be observed. Thus we suggest that dominance is most likely to shift in time from species laying flat on the substrate to progressively more erect species, except perhaps in high energy habitats where flat-laying species might remain dorninant. Although the occurrence of stalked f s m s Is apparently not dependent on the presence of flat-laying species (see Succession in the littoral zone scctlon), the sequence sf dominance always proceeded toward vegaicak species. A sunrey of the literature available (Table 7) also indicates that such a succession of strategies is indeed observed, no matter which type of substrate is examined. Furthennore, a qualitative description of this phenomenon was made by Patrick and Roberts ( i 959). Because similar vertical stratification has also been observed in keIp beds (Dayton 1975) and forests (Whittaker 1972), there probably is an analogous response to problems of space and light availabiHity in all vegetal com~nunities.

Acknowledgments

We thank Mr Sean-Pieme Ricbourg for assistaa~sewith the scanning electron microscopy and computing, which were carried out at Enstitaat National de Recherche Ssiemtifique (INWS) - PCtmle. Br R. Patrick's comments on the manuscript and help with the identification of diatoms were very appreciated. A. Cardinal. P. Morisset, B. Hjmmelman, H. GuderIey, J. C. Auclair, S. Brault, and E. W. Haatchinson provided both helpful discussion and critical review of the manuscript. This study was supported by grants f iam the Natural Sciences and Engineering Research Council Canada to E. B. (A 05 1 11, f o m Fisheries and Oceans, from the Bepafiment of Education of QuCbec to GIRO($, and from Lava! University.

1383

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