ICES]. mar. Sci.,49: 127-143. 1992

The benthic infauna of the North Sea: species distribution and assemblages

A. Kiinitzer, D. Basford, J. A. Craeymeersch, J. M. Dewarumez, J. Dorjes, G. C.A. Duineveld, A. Eleftheriou, C. Heip, P. Herman, P. Kingston, U. Niermann, E. Rachor, H. Rumohr, and P. A. J. de Wilde

D.. I. 1..~ Kiinitzer.. ~~.A.. Basford. , . Craevmeersch.~2~~~~~~~ ~. A.. Dewarumez.~~~~~~~~, 1. M.. Diiries.,~~, , Duineveld, G. C. A,, Eleftheriou,A., Heip, C., ~ermin,P., Kingston, P., ~,ermann.U., Rachor, H., Rumahr, H., and de Wilde, P. A. 1.1992. The benthic infauna ofthe North Sea: species distribution and assemblages.lCES I. mar. Sci., 49: 127-143

In 1986 participants of the Benthos Ecology Working Group of ICES conducted a synopticmapping ofthe infauna ofthe southernand central North Sea. Together witha maooine.. ofthe infauna ofthe northern North Sea bv Eleftheriou and Basford (1989) this nrnvides thedatabase for thedescrintionofthe benthicinfaunaofthe whole~orth~eain

Assemblages were further separated by the 30,50 m and 100 m depth contour as well as by thesediment type. In addition to widely distributed species, cold water speciesdo not occur further south than thenorthernedgeoftheDogger Bank,whichcorrespands to the 50m depth contour. Warm water species were not found north of the 100m depth contour. Some species occur on all types of sediment but most are restricted to a special sediment and therefore these soecies are limited in their distribution. The factors struc- (tirlng ,p,..~:i clslnhufion\ 2nJ a,semhl.tge~ <:em ld h: rr.nlprrdtL.rc. thc ~rlrlurn;~ .,I d~ticrcnln.4tr.r nur\cr. c &:. :\llaiit~cu.dcr thc t\pr. ol cr.iimznl anJ the id01

Key words: benthos, North Sea, assemblages

Received 21 March 1991; accepted 9 January 1992

A. KCnitrer: Alfred- Wegener-tnrtitul fir Polar- undMeertiforrchu,zg. Columbusstra&, D-2850 Brfmerhovm, Germany. Present address: Umwellburzdesaml, Bismarckplofz I, D- 1000 Berlin 33, Germany. D. Bosford: Marine Laboratory, P.O. Box 101, Victoria Road, Aberdeen AB9 808,Seotland. J. A. Croeymeersch: Delta Inslituul voor Hydrobiologisch Onderzoek, Vierstran1 28, NL-4401 EA Yerseke, The Netherlands. J. MM. Dewarumez: Stalion Marine, BP 41 Wimereux, F-62930, France. J. Ddrjes: lnslitut fir Meererhiologie und Meerergeologie "Senkenberg", Schleusenstr. 39A, D-2940 Wilhelmshaven, Grrmony. G. C. A. Duineveld & P. A. J. de Wilde: Nederlands Insriruur voor Onderroek der Zee, PO.Box 59. NL-1790 AB Den BurglTerel, The Nerherlonds. A. Elejiherioli: Marine Lnboralory, P.O. Box 101, Victoria Rood, AberdeenAB9 8DB. Seotland. Presenr address: Depcrtment ofBiology, U~iversilyof Crete, P.O. Box 1470, Heraklion 71110, Crere, Greece. C. Heip: Insticut voor Dierkunde. Ledegoncksrroar 35, 8-9000 Gent, Belgium. Present addrers: Della lnsliruul voor Hydrobiologirch Onderzoek. Vier.slraa1 28, NL-4401 EA Yerseke, The Netherlands. P. Herman: Delta Insti~uulvoor Hydrobiologisch Onderzoek, Yier~rroot28, NL-4401 EA Yerseke. The Netherlands. P, Kingston: Heriot- Walt Univer,~iry,Insrilule of Offshore Engine@*. Research Park Ricearton, Edinburgh EH14 4AS. Scotland U. Nirrmann: Biologiscke Anstan Helgoland, Notkestr. 31, 0-2000 Hamburg 52. Germany. E. Raehor Alfred-Wegener-lnstifuf fir Polar- und Meeresforschung, Columbuss~raJe,0.2850 Bremerhauen, Germany. H. Rumohr: Insfilur fsr Meereskurrde, Diisternhrooker Weg 20.0-2300 Kiel. Germany.

Introduction describe and quantify areas with similar fish food. During the following 20 years several areas in the North Sea were At the beginning of this century Petersen (1914) devel- investigated for their benthic infanna which were classi- oped the concept of infaunal communities in order to fied into certain communities or assemblages. Most of 128 A. Kunitzer et al.

Figure I. Stations sampled by participants of the North Sea Benthos Survey (NSBS) in 1986. Stations in the northern North Sea (Aberdeen) were sampled between 1980 and 1985. these surveys were carried out in coastal areas, not Sea from 60 to 100 m depth; and the northern North Sea further north than the Dogger Bank area. A review of the from 100 to 200 m depth. The assemblages of these zones infaunal assemblages of the North Sea was given by are further structured by sediment composition. Before Kingston and Rachor (1982), showing the low number of the North Sea Benthos Survey (NSBS) it was not possible benthic surveys in the central and northern North Sea. to confirm or reject Glemarkc's concept of the zones in the Investigations in the vicinity of oil platforms suggested North Sea. that infauna assemhlagesnorthofthe Dogger Bankmight Since in most earlier surveys sampling of bottom fauna be similar to those south of it. On the other hand, was done with different gears, and surveys were spread Glemarkc (1973) developed a concept of three different over more than half a century, during which major zones of benthic assemblages along the European North changes occurred in the benthic fauna (Kroncke, 1990), Atlantic Continental shelf, reflectingdifferencesin annual the results of these surveys are not comparable. Partici- variation of temperaturein bottom waters. He divided the pants of the Benthos Ecology Working Group of ICES North Sea into three zones: the southern North Sea up to therefore decided to map the henthic fauna of the whole the northern edge of the Dogger Bank; the central North North Sea during the same period of the year and with Benthic infauna of the North Sea

Figure 2. Classification of stations by TWINSPAN, using only species presence/absencedata.

1st dlshotomy depth < 70m depth > 70m

2nd dichotomy 1 2 3 4 depth mu* <30m 30-70m 70-loom mainly >lorn deposit coarser sediment finer sediment liner sediment

3rd dichotomy It 1 b 2a 26 3 a 3b 4a 4 b depth 30.70m 30-50m > loom depo~~ fine sand muldy finesand fine sedimenf coarse sedimem

Figure 3. Scheme of TWINSPAN classification (species presence/absence data), showing the environmental parameters of the assemblages. Figure 4. Classification of stations by TWINSPAN, incorporating species abundances.

standardized gear. This survey, the North Sea Benthos far, only the distribution of echinoderms (Ursin, 1960), Survey (NSBS), was carried out during April 1986. The polychaetes (Kirkegaard, 1969) and bivalves (Petersen, area north of 58"Ncouldnot be sampledduring theNSBS 1977) has been described for the central and southern but the benthic fauna of that area had been sampled North Sea. Almost nothing is known about the dis- directly between 1980 and 1985 by Eleftheriou and tribution of , such as amphipods and Basford (1989) with the same gear, and these data have cumaceans. The above mentioned authors found that the been included in the analysis. The combined data of the fauna of the North Sea was a mixture of arctic-boreal NSBS and Eleftheriou and Basford (1989) provide infor- (North Atlantic-North Pacific elements) and lusitanean- mation on theinfauna of the whole North Sea and should boreal species (Tethys elements), the first being dominant act as a basis for assessing natural and anthropogenic north of the Dogger Bank, the latter being important changes in the benthos. south of it. This pattern of distribution was demonstrated Besides the classification of the infauna into assem- for echinoderms and bivalves, the distribution of which blages, the NSBS enables us to describe the distrihution was related to temperature and water bodies, but was less of single species within the whole North Sea area. So clear for polychaetes, which seemed to be related more to Benthic infauna of the North Sea

1st dichotomy depth < 70m depth > 70m

2nd dichotomy I I1 111 Iv depth manly <30m 30-70m

3rd dlshotomy la I b 118 Ilb llla depth 30-5Om 50.70m 70.100m deposit muday hnesand line sand indicator specles ~ephf~rc ~onioesp ~uculao. Opnelfa D. MlnoSDio c Ophe!t* 0. PmfldowYia C E~~nocaroiumcPfiomcephaius h Ca!fieoassa s NBphrys ,. rnyaaiia so. fmgone n. Umdioep PMOne r EUdoieNa i /viabe8 c sprophaner b.

Exogone v. POWCiCiS SP number of staions 52 19 40 61 46 41 12 2 number of species 2718 29f9 44i9 43i10 54f16 51113 44f12 np(H') 12f4 1415 14-5 28r5 24+10 25*7 23+5 1nam2 805 + 728 873+ 623 1995 + 1499 1093 f 686 1224 1 1233 2863i18M 1775f1114 bianass (g AFDW.m.9 9.5+9.9 4354.3 12.61-75 7616.5 7.4f7.0 3.5? 3.7 3.8 f 2.2 ...... southern North Sea ------central North Sea --- ...... northern North Sea ------

Figure 5. Scheme of TWINSPAN classification (species abundance data), showing the environmental and biotic parameters (meanis.d.)ofthc assemblages and the indicator species. the type of sediment. The present paper tries to combine 1988) where invited experts on certain taxonomic groups the main pattern of species distributions with the checked the identifications of species, the species distribution of infaunal assemblages in the North Sea. list contained a lot of synonyms (e.g. Paramphinome jeffreysi P. pulchella, Pectinaria uurrcoma = Amphictene auricoma). Spelling and synonyms were checked usmg the Materials and methods Marine Conservatton Societv soecies directow.,. a coded checklist of the marine fauna and flora of the British Isles Sampling and its surrounding seas (Howson, 1987). The directory includes most of the species recorded during the NSBS. Species which do not really belong to the macrobenthos and hence, were badly sampled (e.g. fishes, larval stages, and Basford (1989) between 1980 and 1985, always in epibenthic species) were removed. During a final work- spring and early summer. The grid is of 30 x 35 nm with shop on Texel in 1989 the identification of all species four stations at the angles of the rectangle and one in the (including those of the northern North Sea) was checked centre. At each station the infauna was sampled either by among participants and uncertain identifications were box sampler (usually three cores) and one van Veen grab amalgamated into wider groupings. or during bad weather conditions only by van Veen grab For the final workshop a list of "suspect" species was (usually three grabs). The numbers of samples per station drawnup by calculating an indexofparticularity, express- and per participant are given in the fifth report of the ing the degree to which species were found exclusively by ICES-Benthos Ecology Working Group (Anon., 1986). one or a few laboratories. Depending on the number S,,, Samples were sieved over a I-mm mesh (a 0.5-mm mesh of stations in which a species is found, one can calculate in the northern North Sea), preserved in 5% buffered the number LC,, of laboratories that should have found formalin and sorted and identified by each participant. the species, if the latter were homogeneously distributed over the whole North Sea Taxonomic problems

Although many taxonomical problems were solved during a special workshop on of North Sea where S,,,=total number of stations, &=number of benthos held on Helgoland in 1988 (Heip and Niermann, stations sampled by laboratory i, L,,=total number of A. Kiinifzeret al.

Figure 6. Median grain size of the sediment at each station (analysed by inon, Wilhelmshaven and Basford, Aberdeen) and depth distribution in the North Sea. laboratories. The index of particularity describes the and were restricted to a few laboratories due to change or degree of digression from this hypothesis of homogeneity, due to their geographical distribution, several tens of by calculating species in the list turned out to be identified to different levels by different laboratories, or simply given different PISP = LC,,- Lobs: names following different taxonomical handbooks. This where L,,,=numher of laboratories that have found the source of error was remedied as much as possible in the species. The assumption of homogeneous distribution discussions. over the North Sea is in itself nonsensical but the index Species identified to different levels had to be lumped provided a basis for a thorough discussion of taxonomy together. Three examples are given. First, different used between the participants. An auxiliary basis for this laboratories used different identification keys and not discussion was a computerized atlas showing the spatial every key differentiated all species. Pholoe pallida of distribution of all species, genera, families and phyla. Chambers (1985), for instance, is not mentioned in other The list of "suspect" species proved very useful. keys (e.g. Hartmann-Schroder, 1971; Fauvel, 1923) and Although many species were recognized by everybody, there is confusion between this species, P, inornata and Benthic infauna of the North Sea

Pholoe SP.

Frequency: 188 Frequency: 183

o& 2

ANNELIDA ECHINODERMATA Slgalionidae Pholoe Gonlodo mocuiofo

I Frequency: 185 Frequency: 217 o* o* I

ANNELIDA Goniodidae L Gonioda

Figure 7. Distribution and density of species with a wide occurrence in the North Sea. (See Methods for further explanation,)

P.minuta. Consequently, they were all lumped at the into account. At each division indicator species are given. level. Among others, species of the genera Ensis, In TWINSPAN, indicator species are those species which Lunatia, Thyasira, Magelona, Myriochele and Tharyx occur at most stations of the one group but at nearly no were also lumped at the genus level. Second, some stations of the other group, and species which occur at laboratories identified taxa as e.g. the sipunculids, the higher densities in one group. holothurioids, the anthozoans and the nemerteans to the species level, others did not. Third, even well-established Diversity laboratories have different opinions on the taxonomy of some species. It was felt that some taxa (e.g. the capitel- As a measure of faunal diversity exp (H') was used where lids) need a review before accurate identification can be H' is the Shannon-Wiener diversity calculated with made. natural logarithms (Hill, 1973). The revised species list finally contained 954 different taxa. Before, there had been 1270 taxa. The TWINSPAN analysis has been carried out on the basis of 709 taxa. Biomass Depending on the institute, the biomass was measured TWINSPAN analysis eitherdirectly asash-freedry weightorcalculatedfromwet weight using appropriate conversion factors (Salmedel Stations were grouped according to their similarity in etal., 1985; Rumohr eta/.,1987). species composition using TWINSPAN analysis (Hill, 1979). This Fortran program divides the ordinated Maps of species distribution stations into two groups (first dichotomy) and proceeds by dividing each group into two further groups (second On the maps of species distribution (see Figs 7 to 16) dichotomy) and so on. This analysis was run twice, first, occurrences of species are indicated by circles. The radius solely with binary (presence or absence of a species) data of the circle is proportional to the log-transformed and second, taking the actual abundances of the species abundance of the species. The box and whisker plot Frequency: 108

043-2

ECHINODERMATA Spatongidoe Echinocordiurn

Frequency 85

2

MOLLUSCA Montocutfdoe Tell~myo

,, , Figurc 8. Distribution and density of species with a soulhcrn occurrence ii~the North Sea

expresses the frequency distribution of the non-zero absence data. The infauna assemblages of the North Sea observations on a logarithmic scale. The numbers of are determined by the depth and by the sediment type. outliers are given by the numbers to the right and left of At the first dichotomy most stations north of the the box and whisker plots. Dogger Bank (indicator species: Spiophanes kriyeri, Myriochele sp., Minuspio cirrifera, Aniulis entalis) were separated from the stations south of the 70-m depth Results contour (indicator species: MageNona sp., Echinocardium Assemblages cordatum). The benthic fauna of the deeper northern half of the North Sea is different from the fauna of the Differences in the benthic fauna of the North Sea were shallower southern half. elaborated by running theTWlNSPAN analysisfirst with At the second dichotomy the stations south of the 70-m binary (presence or absence of species) data and second, depth contour were divided along the 30-m depth contour taking the actual ahundances of species into account. into those with coarser sediment mainly shallower than Figure 2 shows the stations of similar speciescomposition 30m (group 1, no indicators) and those with a sediment based on presence/absence data and Figure 4 shows the of fine sand and with a depth generally greater than similarity in species composition when densities are taken 30 m (group 2, indicators: Amphiurafrliformis, Phoronis into account. The areas inhabited by certain assemblages sp., Pholoe sp., Mysella bidentata, Nephtys hombergi, remain more or less the same in both cases. The type of Cylichna cylindracea, Harpinia antennaria). The stations sediment at each station is shown in Figure 6. north of the 70-m depth contour were divided mainly by the 100-m depth contour into those in the central North Assemblages based on presence or absence of Sea (group 3, indicators: Mysella bidentatu, Scoloplos armiger, Chaetoderma nitidulum) and those in the species northern North Sea (group 4, indicators: Eragone Figure 3 gives the scheme of TWINSPAN classification werugera). for the eight different assemblages which are shown At the third dichotomy stations near the English in Figure 2. The classification was based on presence/ Channel (group lh, indicators: Glycera lapidurn, Benthic infauna of the North Sea

Ophiuro offinis

Frequency: 63

.-++2

ECHINODERMATA Ophiolepidoe Ophiura Montacuio substrbfo

Frequency: 35

o* 2

MOLLUSCA Montocutidoe - - - - M0ntaC"ta Figure 9. Distribution and density of species with a northern occurrence in theNorth Sea

Po1,ycirrusmedusa) were separated from the other stations are shown in Figure 4. This classification gave about the on coarse sediment in the southern North Sea (group la, same results as the analysis with presence/absence of indicators: Fabulina fabula, Lunatiapoliana). The stations species. In comparison to the analysis described before, on fine sand were further divided into stations on muddv the assemblages of the northern and central North Sea fine sand south of the Dogger Bank (group 2b, indicators: seem to bemore similar. The borderlinehetween southern Eudorella fruncatula, Callianassa subterranea, Ampelisca and northern assemblages is shifted slightly towards the tenuicornis, Nucula nitidosa, Harpinia antennaria, north. Chaetopterus variopedatus) and those on clean fine sand in At the first dichotomy stations wereseparatedalong the the central North Sea mainly north of the Dogger Bank 70-m depth contour into stations to the north (indicator and on the Dogger Bank (group Za, no indicators). In the species: Spiophaneskroyeri,Minuspiocirrifera, Myriorhele central North Sea some stations along the English coast sp.) and stations to the south of it (indicator species: (group 3b, indicators: Clycera lapidum, Lepfochifon Echinocardium cordatum, Magelona sp., Bafhyporeia asellus) have a different fauna than the other stations elegans). (group 3a, no indicators). Within the northernNorth Sea, At the second dichotomy among the stations south of stations alone- the Scottish coast including- the Orknevs the 70-m deothcontour. those on coarser sediment (erou~.- . and Shetlands, being mainly shallower than lOOm I, no indicators) were separated from those on fine sand depth and with coarse sediment (group 4h, indicators: (group 11, indicators: Amphiura filiformis, Pholoe sp., Spaerosyllis bulbosa), were different from those deeper Phoronis sp., Mysella bidentafa, Harpinia antennaria, than l00m on muddy fine sand (group 4a, indicators: Cylichna cylindracea, Nephtj's hombergr). Among the Thyasira sp.). northern stations those along the Scottish coast on coarse sediment (group IV, indicators: Sphaerosyllis bulbosa, Hesionura elongafa) were separated from the other Assemblages based on abundance of species stations in the central and northern North Sea (group 111, indicators: Levinsenia gracilis, Thyasira sp.). Figure 5 gives the scheme of TWINSPAN classification, At the thirddichotomy stationsnorth-west ofDenmark based on species abundances, for the assemblages which (group Ib, indicators: Aonides paucibranchiata, Phoxo- Uu Frequency 43

0 c

I

MOLLUSCA Maciridae Spisulo p- Pkione remofo

odFrequency: I

ANNELIOA Psionidoe Ptsione

Figure 10. Distribution and density ofspecies with a wide occurrence on coarse ssdlments in the North Sea

cephalus holbolli, Pisione remota) were separated from the highest in the assemblages in areas deeper than 70m other stations on coarser sediment (group Ia, indica- (group IIIa, IIIh). Towards the Scottish coast (group IV) tors: Nephzjis cirrosa, Echjnorard~umcordaturn, Urothoe species number and diversity decrease again. poseidonis). The stations on fine sand were divided into The variation in densities is too high to show clear those on muddy fine sand south of the Dogger Bank differences between assemblages. Densities seem to he (group IIa, indicators: Nucula nitidosa, CaNianossa lower in the assemblages on shallow coarse sediment subterranea, EudoreNa truncatula) and those on fine sand (group la, Ih). They seem to he highest ingronpIIIb hut at in the central North Sea at 5&?0-m depth (group stations in this group a finer mesh of 0.5 mm instead of IIb, indicators: Ophelia borealis, Nephtys longosefosa). 1.0 mm was used. Also at the stations of group IV the Stations deeper than ?Om were divided along the 100-m 0.5-mm mesh was used, and therefore densities are higher depth contour into those of the northern North Sea than they would have been by using a l-mm mesh and are (group IIIb, indicators: Minuspio cirrifera, Thyasira sp., not directly comparable to the densities in group I and 11. Aricidea carherinae, Exogone verugera) and those of the The variation in biomass is also very high. The mean central North Sea at about 70-100-m depth (group IIIa, biomass per assemblage is lowest in the northern North no indicators). Sea (groups IIIb and IV). The biomass increases towards the shallower southern North Sea and reaches highest values south of the Dogger Bank (group Ia, IIa). Species number, diversity, density, biomass

Thedifferenceinhioticparametersamongtheassemblages Species distribution is shown in Figure 5 for the species number, diversity, density and biomass. Sinceit isimpossible to show the distribution ofall species Species number and diversity gradually increase from in theNorth Sea, only a few examples will be given here to the assemblages shallower than 30m (group Ia, Ib) to the show the main patterns. These species were chosen, assemblages in the 3&70-m depth(group IIa, IIh) and are because they were shown to be typical of individual Benthic infauna of the North Sea 137

Frequency I2 6

10

ANNELIDA Syllidae --- Sphaerosyllis Phoxocepholus holbolli

I Frequency 21 Frequency: 20

ARTHROPOOA Phoxocepholidoe 11Phoxocepholus - --- ~ -~ 17igurc I I. Distribution and density of species with a re .ricted occurrence on coarse sediments in the North Sea.

Aricideo m,i)ufo

Bathyporeio elegons Frequency: 4i

o& I Frequency:

.iij ANNELIDA Poroanidoe 2 .Aricideo -- - Figure 12. Distribution and density of species with a wide ARTHROPODA occurrence on fine sand in the North Sea. Houstoii$doe Bathyporeia 138 A. Kiinitzer et al.

Frequency: 54 Frequency: 68

2 438 2 0

ARTHROPODA MOLLUSCA Haustoriidoe Tellinidoe Bathypore0 Fabulino

Sigoiion mothildoe

Frequency: 45 1 Frequency: 37

.-&-o 2 757 2 32 0 0

ARTHROPODA ANNELIDA Haustoriidae Sigoiiondoe Urothoe - Sgoiion

Figure 13. Dislributlon and density of species with a restricted occurrence on fine sand in the southern North Sea. Horpinio onfennar,~

Frequency: 05

o* 2

Eudoreilo ARTHROPODA truncatuio Phoxocepholidoe Horpino

~ ... Glycinde nordmonni o- o-

2 Frequency: 79

I o& ARTHROPODA Leuconldoe 2 Eudarello

Figure 14. Distrihuiion and density of species with a wide ' ANNELIDA occurrence on muddy fine sandin the North Sea. Goniadidoe .Glyc~nde Benthic infauna of the North Sea 139

Callionosso subterroneo Frequency: 47 oaFrequency: 43 2

ARTHROPOD@ Collionassidae -- CCl110n0ss0 Chaetopferus Synelmk Ivar;apedotus kloffi Frequency: 32 Frequency: 31

o-& 062 2

ANNELID@ ANNELIDA Chaetopteridae Pilorgiidoe Choetopterus _--- Synelmis Figure 15. Distr~hutionand density ofspecies with a restricted occurrence on muddy finc sand in the southern North Sea

assemblages identified by the TWINSPAN analysis. Species with a northern distrihution were usually never Typical for an assemblage means that the species occur at found south of the 50-m depth contour, e.g. Ophiura m3il ,tatlonr ofthls :~~scolhl~gthut it nc.:irl! no sl.ltloni ofltli<. .M~JIII~I~UILIu~~.~I~I.IIu. A!~l~/~.~~~!~l~~/ti:in~l .\f!nu.%p!,, oi lhr. .1<11acznla>rc.mhl.ier',. Not :ill inJ~

Frequency Frequency: 6

o& o* I0 3

ANNELiDA ARTHROPODA Spionidae Leuconidoe -Loonice Leucon Molgulo Thyosira SP. ferrugineo

Frequency: 23 Frequency: 22

04. 10 0 3 130 0 0 0 CHORDATA MOLLUSCA Molgulidae Thyasiridoe Moiqulo Thyosiro Figure 16. D,strihut~onand density of species with a restricted occurrence on mudd) fine sand in the northern North Sea.

CaNianassa subferranea, Nucula nitidosa, Chaeropterus TWINSPAN analysis which included species abundances variopedatus and Synelmis klatti are restricted to the (Fig. 4). To avoid thispossibleerror, theclassification was southern North Sea (Fig. 15) and Leucon sarsi, Thyasira carried out first only with presence/ahsence data where ferruginea, Laonice sarsi and Molgula sp. are restricted to densities were excluded from the analysis (Fig. 2). In both the northern North Sea (Fig. 16). classifications the assemblages in the northern half of the North Sea were divided by the 100-m depth contour. The distribution of individual species also confirmed that Discussion three different assemblages exist in that area: one in Scottish coastal areas, one in offshore areas deeper than Only the main patterns of species distributions have been 100 m and one in the deeper part of thecentral North Sea. described in this paper. They show that the bottom fauna Jones (1950) reviewed the literature on marine bottom oftheNorth Seaiscomposed ofnorthernelements that do communities. He found that most authors were in agree- not extend further south than the north of the Dogger ment that the communities, in the sense used by Petersen, Bank, and southern elements going not further north than were realities. All authors were agreed that there is a the 100-m contour. Northern and southern species there- correlation between the distribution of the com- fore mix in the central North Sea and northern and munities and certain physical factors. The biological southern assemblages overlap along the 70m contour. factors, like relationships with other organisms, the The occurrence of cold water species north of the Dogger presence of suitable food for predators, para- Bank and of warm water species in the southern North sitism, commensalism, etc. seemed to he of secondary Sea was already recognized by Ursin (1960), Kirkegaard importance. Biological factors are of importance regard- (1969) and Petersen (1977). None of these authors, how- ing the persistence of an assemblage. The persistence in ever, showed that the southern species occurred as far the species composition of an assemblage is a function of north as the 7&100-m depth contour. the biological factors. Josefson (1981) showed that in The methodological difference in the mesh size of the a benthic community at 300m depth in the Skagerrak sieve (0.5 mm at stations in the northern North Sea, 1 mm the persistence was lower than at a shallower 100-m at all other stations) might have had an effect on the community. At 300 m depth about 30% of numbers and Benthic infauna of the North Sea 141 biomass were replaced over a 5-year period. Parts ofthese bottom waters is high (> 10°C) (Tomczak and Goedecke, ~hangescould he interpreted in terms of amensalism and 1962). while in the stratified areas north of the Dogger commensalism between trophic or functional groups of Bank summer temperatures are less than 7°C. In winter species. The persistence of a 80-mcommunity off the coast the southern North Sea is colder (4°C) than the rest of ofNorthumberland has been shown to be lower than that the North Sea (5-7°C). These differences in temperature of a 55-m community due to sequential changes in the north and south of the Dogger Bank might explain why dominant species which were regarded as evidence of cold water species do not go further south than the biological interaction (Buchanan and Moore, 1986). The Dogger Bank. The explanation for why warm water significant environmental factors determining assem- species are not found below 7&100m depth, although blages were shown by Jones (1950) to be temperature, they survive the cold summer temperatures in the central salinity and the nature of the bottom deposit. Within the North Sea, might lie in the general current pattern. Atlantic boreal region Jones divided the benthos by the A third factor which may cause the differences among temperature range and salinity range into shallow water the assemblages is the availability of food. Large stocks communities, offshore communities and deep communi- of copepods develop only in the northern North Sea. ties. The latter have their upper limits of distribution at They consume the summer production of phytoplankton 70 m depth. These communities were further structured (Fransz and Gieskes, 1984). The faecal pellets do not by the sediment. reach the deep water, being recycled higher in the water In his model of three infaunal ktages in the North column (Krause, 1981) so limiting this source of food to Sea, Glemarec (1973) demonstrated the 50- and 100-m the benthos in the summer months. This could explain contours as being important structuring borders for the the low biomass of infauna in the northern North Sea. assemblages in the North Sea. The depth contours by Further south, main parts of the phytoplankton produc- which assemblages were separated in our analysis are the tion reach the bottom, resulting in better food supply to 30-m, 50-m, 70-m and 100-m contours. As shown in this the benthos, especially in summer months. Buchanan paper, the zone deeper than 100m is inhahited by cold (1963) has stated already that a relevant ecological factor water species; the one shallower than 50 m is inhabited by for benthic assemblages can he found in the quality of the warm water species; and theintermediatezone between 50 suspended matter together with the speed and nature of its and l00m depth is inhabited by cold as well as warm flow over the bottom. water species. The separation of benthic assemblages along the 30-m The classification of assemblages in this paper shows depth contour can be caused by several environmental that there are more than two depth contours structuring factors. No thermal stratification of the water column the benthos and that the 70-m and 30-m contours are develops in summer months in the shallow coastal areas, more important for the distribution of assemblages than whereas below 30m depth a stratification may develop the 50-m depth contour. The separation of the fauna into (Tomczak and Goedecke, 1964). Strong tidal currents a northern and a southern one along the 70-m contour exist in the shallow coastal zones and the wave action might he a result of the current pattern in the North Sea. reaches the bottom, stirring up fine particles of sediment Most of the Fair Isle4rkney inflow of Atlantic water and organic matter. These areas therefore consist of sand moves eastwards at about 5T30'N and only part of and gravel, while in areas of 30 to 50 m depth the deposit it travels southwards down the coast of England (Lee, usually consists of muddy fine sand. As a consequence of 1980). The shallow southern North Sea is, in contrast these environmental differences, the food availability to the deeper northern areas, influenced by the English must be different, resulting in different feeding types. Channel inflow which extends up to the Dogger Bank. Besides depth, the sediment structures the distribution Thenorthern North Sea and part of the central North Sea of the assemblages. Depth and sediment are interrelated is thereforeinfluenced by a differenttypeofwater than the since coarser sediments usually occur in shallower areas. rest of the North Sea. Among plankton communities As shown in this paper, several species occur on all types those of the northern and central North Sea are similar of sediment while other species are restricted to sedi- but neritic species are more numerous in the central North ments of a certain grain size. This holds for all groups: Sea (Adams, 1987). The distribution of larger epifauna polychaetes, molluscs, echinoderms and crustaceans. has been shown to he likewise determined by these two Kirkegaard (1969) found the sediment to he more import- different water masses (Frauenheim efa/., 1989). The epi- ant than the depth for determining the distribution of fauna north of the Dogger Bank is different from the one polychaetes in the North Sea. Also, in the German Bight in the southern North Sea. associations are mostly tied to different types of bottom Another factor determining the distribution of (Salnvedel el a/., 1985). The bottom fauna communities assemblages is the annual variation of temperature in off the coast of Northumberland, on the other hand, are hottom waters. Large areas of the southern North Sea poorly correlated with the texture of the bottom sedi- are not stratified during most of the year (Tomczak and ments (Buchanan, 1963). In the latter area the sediment Coedecke, 1964) and therefore the summer temperatureof conditions of the various communities overlap broadly ier et al. and different communities are found in apparently similar Fauvel, P. 1923. Polychetes errantes. Fauna de France, 5: sediments. The present results show that northern and 488. Fransz, H. G., and Gieskes, W. N. C. 1984. The imbalance of southern assemhlages meet in the area off the coast of phytoplankton and copepods in the North Sea. Rapports et Northumberland. Since these assemhlages seem to be Prods-Verbaux des Reunions du Conseil International pour determined by different water masses, the sediment might l'Exploration de la Met, 183: 218-225. be of less importance in structuring the assemhlages. In Frauenheim, K.,Neumann, V., Thiel, H., andTiirkay, M. 1989. 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