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Natural fluctuations subtidal community Fjord (Norway)

• III

• III

a the

rocky Oslo

Cycles Predictions

Scll1ina Predation

Rocky-bottom

Cycles Predictions

Colonisation Prtdation

Substrat rocheWl

A BSTRACf

RÉSUMÉ

Hartvig Christie Inst ilute for Mari ne Biology. P.O . Box 1064. Bli ndern. Oslo 3. Norway.

As part of a programme aimed at dcveloping methods for ecologjcal monitoring, a detailed analysis of a rocky community at 12 m depth was started. The study was based on nondestructive sampl ing using stereophotography, and add itiona] expcriments were done with settlemen t panels. predator exclusion and scraping of substrate to oblain rurther information about the dynamics of the communi ty. Natural variations were round to be gaverne<! by key species which structuroo the community. or more lhan 60 spccies round, ca. 10 were important in terms o f abundance. Very few species exhibited stable abundance, and the community was dominated by an nuai cycles in populations mainly provided by seasonal settlement followed by predation and grazing, notably by Asterias rllbens. Excl usion of th is species lead to d ramatic structural changes on arti fi cial panels. Varia tion in abundance was analyzcd usi ng harmonie series plo ts. Models were produced for the most importan t species, and gave acceptable predictions when teste<! agai nst the followi ng year's data. Cycl ic patterns appear annually, bu't suggestions of long tenn trends were found to slightly a lter this annual cyclic stability.

Oceanol. Acta, 1983, Proceedings 17th European Marine Biology Symposi um, Brest, France, 27 September· l October, 1982, 69·73.

Variations naturelles dans une communauté infralittorale S\lr substrat rocheux du fjord d'Oslo, Norvège

Une analyse détaillée d'une communauté sur substra t rocheux. à 12 m de profondeur, a été entreprise; elle fait partie d'un programme visant au développement de méthodes de surveillance écologique. L'étude a été basée sur une méthode d'échanti llo nnage non destructrice (la stéréophotographie) ; des expériences complémentaires ont également été effectuées, a fi n d'obtenir davantage d' infonna tion sur la dynamique de la commu· nauté : colonisation de panneaux, exclusion de prédateurs. dén udation de substrats.

Il a été constaté que les variations naturelles semblent être gouvernées par les espèces· clés qu~ de pl us, structurent la com munauté. Parmi les quelque 60 espèces tro uvées, environ JO sont importantes en termes d 'abondance. T rés peu d'espèces présentent une abondance stable, et la dynamiq ue de la communauté est dominée par les cycles annuels q uanti ta ti fs des populations, principa lement détenninés par une colonisation saison· nière suivie de prédation et de broutage.. notamment par Asterias rubens. L'exclusion de celte espèce entraine des changements structuraux très importants sur les panneaux artificiels.

Les varialÎons d 'abondance ont été analysées à l'aide de séries hannoniques. Des modèles sont présen tés pour les espèces les plus im portan tes; ils on t fou rni des préd ictions acceptables si on les compare a ux données de l'année suivante. Des variations cycliques sc produisent annuellement, mais il est suggéré q ue des tendances à long terme existent également et mod ifient légèrement cette stabilité cyclique annue lle.

Oceanol. Acta, 1983, Actes 17e Symposium Européen de Biologie Marine, Brest, 27 sep-­tembre-- I er octobre 1982, 69·73.

69

H. CHRISTIE

INTRODUCTION

Aimed at developing a survey programme for ecological mo nitoring, investigation on natural fluctuations and their structuring factors was started in 1978. Lewis (1976) suggested that il was necessary to know the causal mechanisms in a comm unity 50 that one mighl distinguish the natural variat ions from changes caused by human act ivities(e.g. pollution).

Resuhs obtained from 3 to 4 years of detailcd investiga­tion on a rocky subtidal community are presented here. ACter one year of study, C hristie (1980) suggested that the observed seasonal vâriations were governed by a summer settlement followed by predation. Seasonal variations are the most common natural variations. The mechan isms giving rise to the seasonal variations and the communities' predictability (if the same seasonal cycles relUrned annually) are important factors in a monitoring contex!. Attention is paid 10 the most abundant species and (( key species» (see Paine, 1969 ; Dayton, 1972; Lewis, 1 976~ but longterm trends are 100ked for as far as the data permit.

If the same seasonal events accur year afl er year, the community would have high stability. Sutherland and Karlson (1977) bclieved from their study that fo uling communily structure would undergo differen t changes fro m year to year, resulting in a community either changing dynamically or changing stepwise with mul­tiple stable points (Sutherland, 1974). However, stabi­lit y and predictability of rocky communities are depen­dent on organizing factors, for example com munities organized by predators which then provide space for new settlement (Paine, 1966 ; Dayton, 1971 ; Menge, 1976; Karlson, 1978 ; Pcterson, 1979). Where the role of predators are unimportant (Keough, Butler, 1979~ competition will normally govern the community structure. Exclusion of predators (both natural and by design) will in sorne cases lead to total dominance by a dominant competitor (Paine, 1966; Menge, 1 976~ a stable point, such as the populations resistant to recruit­ment found by Sutherland (1974 ; 1978). Naturallong lerm changes in abundance of important species could lead to very unpredictable community structures, and must also bc deall with bcfore one can distinguish pollutants as a positive organ izing factor.

METHODS

The community studied is on a smooth, nearly vertical rocky wal~ at 12 m depth at Drebak in the Oslofjord. Seasonal changes in temperature were fo und to vary bclween 0 oC in winter and ca. 15 OC in summer. SaJinity ranged between 24 and 34 °/

00,

A fixed site was permanently marked off with plugs (see Christie, 1980). By using a reference system, the exact same area was sampled nondestruct ively by the underwater stereophotography technique introduced by Lundiilv (1971 ). The equipment we used was des­cribed in detail by Green (1980). A comprehensive survey programme connected to the stereophotographic sampling is descrîbed by Christie el al. (in press). In the

70

presen t slUdy 2 ml of the boltom were used as a control area, and another 0.5 ml was scraped at the start in 1978. A seulement experiment started in 1978 with ceramic plates (15 x 15 cm) submerged for 2 months. Fou r PVC plates (40 x 40 cm each) were put out in March 198 1 and another plate (100 x 35 cm) was hung on a rope ca. 2 m above the bottom to avoid benlhic predalors.

Ali sampling wascarried out by SCUBA divins. approxi. mately once a month.

Ali stereoslides are analysed with a stereoscope (see Green, 1980 ; Christie el al., in press) by counting colonies or solitary individuals or by estimating abun· dance as percentage cover by point sampling (Christie, 1980). Settl ing plates were qualîtatively analysed by presence/absence of species.

ln calculating curve li tting models of fluctuations, harmonic series analysis (Fourier series) was used (Bliss, 1970, p. 219 ; G ray, 1981, p. 75).

RESULTS

Ahhough more than 60 species were round to occur in the community (Christie, 1980~ only the mosl important species are treated in tbis paper. The species selected are those of greatest abundance or those of greatest importance in structuring the communit y.

Figure 1 shows data obtained through 1978-1980 of frce space (unoccupied by organisms) and of impor­tan t organisms. By using the method of Bliss (1970, p. 219) to creatc curvcs with the bcst fit to the scasonal nuctuations, the observed data from 1978 and 1979 pro­duced cyclical cu rves which, as a series of sine functions. serve as predictions for the seasonal variation pauerns of the different species for 1980. In Figure 1 the o bserved fluctuations from 1980 are compared to the predictions.

For ail species the comparison shows a similar pattern of variation through 1980 as the pred icted mode!. However, the models consider only annual patterns, and not long-term trends which accur in most species or as a phase displacement compared to annual cycles as seen in Figure 1 G. Usi ng the data for three or fo ur years, models can bc calculated taking into considera­tion both linear increase or decrease and eventually phase displacement. The curves with the best mathe­matical lit are drawn in Figure 2. Ali species presen ted nuct uate in a 12 month cycle. except the ereet red algae (Fig. 2 G) which had the best fil to a pcriodici ty of 10.2 months. However, this species shows a stable trend in abundance, whi le the hyd roid L. longissima (Fig. 2 H) decreased and the others showcd an increase. Other patterns of long-term trends are not assessable after only 3 or 4 yearsofstudy.

A few species occu rred as stable populations throughout the sampling period. The most important are the encrusting Lithothamnioll sp. and Pomaloceros Iriqueter (F ig. 3). Spccies o f less importance (abunda nce) but al50 occurring in stable populations a il through the sampl­ing pe:riod are represented by AIcyonium digitatum and T ealia fe/i /w.

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Seusonll/ cycles of free spou and importum ~pecies. The obU'".ed datu f rom /978 and 1979 (0) ure bu.fisfor ,hl' cu/cll/ulions of (mnWl/ cJ'clic cunV'fiuing ",ode/s ( salid lin f') whirh U/5O ure p/alled as prediction5 fur 1980. The ubse,..ed fluctuulions from 1980 ( . -- . ) urt' plol/ell for compurison la the pfcllicled cur\"eS. Ahundlmce in num/}('rs m - 1 (n) . or in percenl caver (%) . A) Tubularia sp .. 0) Laa medc.a long's.sima.- C) fr« spou. 0) Asterias rubens: E) Psamechinus miliaris: F) Acmaea .Ip.: G) l'rl'CI red algue.

Although they ex hibit unpredictable van ations, asei­dians arc also characteristic of these rocky communities. Of these, the most abundant were Baylryl/aides /eachi, Slyda sp. and Corella paralJelagramnw.

The most important patterns on the scraped area are shown in Figure 4. Cycles of bot h frcc spacc (A) and A. rubens (8 ) have higher pcaks compared 10 the conlrol area. On the scrapcd area the sett lcd organisms were more effeclively rernoved by prcdators, and in the win 1er time the covering organisms (as the inverse 10 free space) are only represenled by Ihe stable popula­tions of LilllolllllllmiOIl sp. and PomalOceras trÎqlleter. ln Figure 4 C results of the settlemenl plate experimen t a re plottcd. Thcre was a signilicant posit ive correlation betwecn density of A. rubells and the number of species sell lcd and a negati vc correlation between Ihese two and the frec spacc (Fig. 4). This pallern Îndicates that many organisms seille and occupy spacc throughou t the summer, and that A. rubens was al1ractcd by the food supply available, but would leave Ihe arcas when organisms were removed, as on the scraped area. From the various specics fo und on sClIling plates (Fig. 4 C). most dense spatfall Îs found among the hydroids (Lao­medea longissima, TI/bularia sp.), bivalves (M y ti/lis edulis, M adia/us mOdiolus, Hiatella arc/ka) and the

71

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Figure 2

NATURAL FLUCTUATIONS IN A ROCKY COM MUNITY

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Culculalell modds for urUJIIll/ cycles addcll IO,lg /trm 'rends. Same specics und ubunlkmre liS in Figure 1.

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Figure 3

Pl'rcent CMer of LithOlhamnion sp. (. ) und Pomatoceros triqueter (0) during Ille $llmp/ing Mriod.

polychaete Pomatoceras trÎqlleter. Eggs and juven iles of the nudibranch CoryplJellll sp. are found in high dcnsities in the hydroid colonies. Sorne authors believe that grazing by thÎs specie is an important factor in rernoval of the hydroids (Thompson, Brown, 1976 ; Christie, 1980). The elTeet of predator exclusion is shown in Figure 5. Although prcdators (mainly A. rubells) 10 sorne extent invaded the panel, a monthly removal of predators leads 10 dramatic changes. The exclusion panel acquired

H. CHFIISTIE

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Figure 4 Comporium of perunt cOI'er of frfe spa('e ( A) und ASleri ll.s rubc:ns ( 8 ) on ('ontrol arta ,.....) and on scropt'd area (0- - -0). ( C) N~m­bers of species stl/led 011 stlllingplilits.

a dense (ca. 90 %) cover of organisms. ti rsl Ihe hydroids senled, but Ihe panel was Ialer 10lally dominaled by M yI UliS edulîs. The control panel followed the same pat tern as the scraped area (Fig. 4 A). a dense settlement through the summer and a subseq uent disappearance of organisms (due to predation).

DISCUSSION

Of the few species important for structuring the commu­nity. two OCCUT conlinuously(Fig. 3). whilst the rest show seasonal oscillations (Fig. 1. 2). The an nuai panerns are predictable (Fig. 1). but maximum abundance varies from year to year. These longer-term changes were not random variations., but followcd increasing or decreas­ing trends du ring the 34 yearsofstudy, and are probably an example of one of the long term cycles described by Gray and Christie (1983).

The communi ty is dominated by a summer recruitment (Fig. 4 C) followed by removal o f scnled organisms by predators and grazers. The most important predator, A sterias rllbens, kept the bivalve populations down 10 almost zero abundance (also found by e.g. Dare, 1982).

72

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Totul percent CO\'er of orgunisms on stirling fNJne/s ( e - e ) , ('on/roi J1<Inefs (0--01, prl.'dmor exclusion fNJnl.'!s .

Excl usion of A.~·teria~· resulted in total dominance by My/UliS edulî.\· (Fig. 5). <lS <llso fo und by l'.g, Paine (1966) wi th M. calijorniol/as on the US West coast. Hydroid population a lso exhîbi ted annual rapid increases (Fig. 1. 2. A and H). the lauer mainly due ta cropping by Coryphella sp. (Christ ie, 1980). Impon ant is also the grazing by PS(lmecllil1l1S miliaris and Acmaea sp. on other seu led organîsms. This pattern demonstrates that predators and grazers are the kcy structu ring species. The trend of increased abundance of predators was inversely related to the decreased dominance of sessile orga nisms (Fig. 2 Cl. and long tenn trends in thcsc species must influence community structure and sta­bility. Rocky com munities have often becn rou nd to be controlled by predator activity (e.g . Paine, 1966 ; Dayton, 1971; Karlson, 1978).

Predators are dependent on new recruits for food source and recrui tment is only possible where space is provided by prcdators. These factors seem to be in a seasonal equilibrium. If sampled once a year and at the same time (season). the community would appear to bc constantly at the same poin t. However, with mont hly sampling the comm unity was fou nd to be governed by con tin uous pert urbations providing spatial and temporal changes in species composi tion. As the influence of the species (and factors) occurs more or less at the same lime each year, the community is sug· gested as being in an " annual stability ,. or an .. ann ual pattern of stabil ity ", Dramatic changes in the popu la­tions of key species which prod uce this pattern might change this annual stability and the community might develop toward a stable point (Sutherland, 1974) as shown by the predator exclusion expe:riment, which resulted in mussel domi nat ion. Similar mussel-dominat­ed communities studied by Dayton (197 1). Paine (1966 ; 1974) and Menge (1976) arc described by Suther· land (1981) to be stable over many years.

When predation is the main perturbation, following a disaster event (studied by the scraping experiment) the community must be taking longer than three years to return to normal conditions.

ln a monitoring contexl. non destruct ive stereophoto-

graphie sampl ing of rocky surface communit ies has been shown to be a good method for fo llowing dyna­mical changes in community structure. Knowledge of natura1 fluctuations, change in communily Slructu re and swbi1ity must be delermim.>d bcfore il is possible to dist inguish changes induced by anthropogenic act i­vit y from the natural ones.

Curve fi lt ing models (Fourier series) of fl uctuations a re

REFERENCES

Bliss C. 1., 1970. Sla/is/les in bi%gy. Vol. 2. McGraw-Hill. New York. 639 p.

Christi" H .. 1980. Methods for eçologica! monitoring : biologieal interactions in a rocky subtidal community. Helgol. M eeresumers_ 33.473-483.

ChrisTie H .• Enns R. A .. Sllndnes O. K .. in press. Field melhods for În sÎw subt idal hard OOltom studies. in Umt.'rll'o/cr pllO/ograplty und /t'1t-"i5ionjor srfm/is/s. edited by J. D. George and E. Lythgoe. Oxford Univ. Press.

Dare P. J ., 1982. Notes on the swarming bchavior and population density of As/erias robens L. (Echinodermata : Asteroidea) feed ing on the mussel. M y/i/us ed .. lis L, J . Cons. lm. Exp/or. M er. 40, 11 2- 11 8.

Da)-Ioo P. K., 1971. Competi tion disturbanœ, and eommunity orga­nisation : the provision and subsequent utilization ofspace in a rocky intenidal communilY, Ecol. Mooogr_ 41, 351-389.

Da)1on P. K" 1972. Toward an understanding of community resi ­licnce and the potential efTeçts of enriehmenl 10 the benlhos al McMundo Sound, Antarclica, in : Proceedings of the colloquium on COllSU\'u/iOlr problems in AmarcrleQ. edi ted by B. C. Parker. Allen Press, Lawrence. Ks, 81·95.

Gray J . S .• 198 1. The ecology of I7Ulrine sedimem s. Cambridge Uni· .\'crsi ty Press, 185 p.

G ray J . S., ChriSi ie H., 1983. Predieting long·lerm changes in marine benthic communities. Mur. Ecol. Progr. Sa., 13,87-94.

Grl'l'll N. W., 1980, Underwater stcrcophOlography applied in ecolo­gical monitoring. Repons l. Methods and preliminary evalualion. Norwegian Institute for Waler Research. Oslo. Report OF-806lJ 1-99.

Karlson R., 1978. Predation and spaee utiliunion patterns in a marinc epifaunal community. J . Exp. Mur. 8 iol. Ecol. 31. 225-239.

Keough M. J .• Butler À . J o, 1979. The role of Asteroid predators in the organization of a sessile community on pier pilings. Mur. 8iol, SI,167- 177.

73

NATURAL FLUCTUATIONS IN A ROCKY COMMUNITY

a useful tool in making pred ictions and in selecl ing the bcsl limes fo r sampling.

Acknowledgements

1 am very gra leful to Pro John S. Gray for scienlific support and 10 Âge Larsen for his help with harmon ic series ana lyses.

Lewill J . R., 1976. Long-term ecological surveillance practieal rclaties in the rocky littoral Ou mrogr. M ar. Biol~ 14, 37 1-390.

Lundi lv T., 1971. Q uantitative sludies on rocky-bollom biocoenoses by undcrwater photogrammetry a mcthodjeal study. Thalauia J ugos/. 7, 201·208.

Menge R. À •• 1976. Organiullion of the New England rocky intcrt idal community : role of predation. competi tion. and envi ronmemal hcterogenity. Ecol. Monogr. 46, 335·393.

Paine R. T., 1966. Food web campluity and spodes diversi ty. Am. Nal_ 100, 65-75.

Paine R. T., 1969. The Pisastcr-Tegula interaction : prey patches. predator food preference. and interlidal community structure. Eco­/ogy. 50. 950-961.

Paine R. T., 1974. Intert idal community structure : expcrimental studies on the rela tionship between a dominant competitor and its principal predator. Oeco/ogia, 15,93-120.

Peter.;on o., 1979. The importance of predation and competition in organiring the intertidal epifaunal communities of Barnegat In(et, New Jersey. Oecologia, 39, 1-24.

SuIMriand J. P., 1974. M ultiple stable points in na tu ral communities. A m. Nal. I08, 859-873.

Sutherland J . P .. 1978. Funct ional raIes of Sehitoporel/a and Sifela in the fouling community at Beaufort. North Carolina.. Eco/ogy. 59, 257-264.

Sulht'rland J . P., 1981. The fo uling communi ty at Beaufort. Nort h Carolina : a study in stability, Am. N UI . 118, 4. 499-519.

Sutherland J . P., Karlson R. H., 1977. Developmem and stabilily of the rouling community at Beaufort North Carolina. E.eol. M emogr, 47.425-446.

Thompson T. E.., Brmm G. H .. 1976. British apisrobNmeh mol/uses, Acad. Press. London. 203 p. (synop$CS or the British fa una No. 8).