LONG-TERM e f f e c t s o f o b m c u t t in g d is c h a r g e s IN THE SANDY EROSION AREA OF THE DUTCH CONTINENTAL SHELF

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1 NIOZ-RAPPORT LONG-TERM e f f e c t s f b m c u t t in g d is c h a r g e s IN THE SANDY EROSION AREA OF THE DUTCH CONTINENTAL SHELF R. Daan, M. Mulder N ederlands Instituut vr O nderzek der Zee Beleidsgericht Wetenschappelijk Onderzek NIOZ (BEWON)

2 1994 This reprt is nt t be cited withut acknwledgement f the surce. Netherlands Institute fr Sea Research (NIOZ) P.O. Bx 59, 1790 AB Den Burg, Texel, The Netherlands Nrth Sea Directrate Ministry f Transprt and Public Wrks P.O.Bx 5807, 2280 HV Rijswijk (Z-H) The Netherlands ISSN Cver design: H. Hbbellnk

3 LONG-TERM EFFECTS OF OBM CUTTING DISCHARGES IN THE SANDY EROSION AREA OF THE DUTCH CONTINENTAL SHELF R. Daan, M. Mulder This study was cmmissined by the Nrth Sea Directrate (RWS) and als carried ut in the framewrk f the Dutch cllabratin prgramme 'Plicy Linked Eclgical Research Nrth Sea and Wadden Sea' (BEON) NETHERLANDS INSTITUTE FOR SEA RESEARCH Beleidsgericht Wetenschappelijk Onderzek NIOZ

4 1 SUMMARY AND CONCLUSIONS During drilling activities in the Nrth Sea there has been an extensive use f il based drilling muds (OBM) in the 80's and early 90's. With the discharges f drill cuttings substantial amunts f il reached the seabed in thse years. Research n the envirnmental impact f these discharges, based n the assessment f effects n the benthic fauna cmmunity, has been carried ut n the Dutch Cntinental Shelf since Frm 1993 there is a cmplete ban n dumping f OBM-cntaminated cuttings. Hwever, research n the lng-term effects f frmer discharges and, in particular, n the spatial extent f these effects was cntinued t assess pssible recvery f sediment quality and benthic fauna. In 1993 a study was carried ut at lcatin Pb in the sandy ersin area in the suthern part f the Dutch Cntinental Shelf. At platfrm Pb 4 wells had been drilled. The first well was drilled with OBM in 1985 and a first envirnmental survey was carried ut in September 1985, 2 mnths after cmpletin f the well. Later in 1985 and in mre wells were drilled with water based muds (WBM) and the furth well was drilled in 1987 using OBM again. This reprt presents the results f the 1993 survey, which tk place years after drilling f the last well. Sediment sampling was mainly cncentrated alng a transect in the residual current directin f Pb. Sampling statins were chsen at distances ranging frm 25 t 5000 m frm the platfrm. Analyses included sediment grainsize characteristics, chemical analyses and faunistic descriptins. A few statins were als sampled in perpendicular directins, but nly analysed fr sme physic-chemical characteristics and numbers f sea urchins (Echincardium crdatum, specimens >15 mm). In the residual current directin the chemical analyses revealed substantially elevated il cncentratins up t 100 m frm the platfrm, but traces f il were visually bserved up t 250 m. Cncentratins f Ba were abve backgrund level up t 250 m t. At 100 m elevated cncentratins f il and Ba were fund t a depth f at least 20 cm in the sediment. There hardly seems t be any degradatin f il in the deeper anaerbic sediment layers. At distances >500 m and at statins in perpendicular directins at distances >250 m n traces were fund f the frmer discharges. Very clse t the platfrm (at 25 m) il and Ba cncentratins were smewhat elevated, but lwer than at 100 m. A depressin in the seabed at this statin and the presence f large amunts f ld shell fragments indicated that the sandy tp layer had been remved by increased turbulence and ersin f the seabed arund the platfrm legs. Increased ersin might als have resulted in transprt f discharged material away frm the platfrm. In the benthic macrfauna cmpsitin an accumulatin f effects was fund at 100 m and 250 m (residual current directin). At these statins the fauna was cnsiderably impverished. At 500 m nly the absence f adult E. crdatum indicated a lngterm effect f the frmer discharges. In fact the absence f E. crdatum at this statin shuld prbably be cnsidered as the lng-term cnsequence f a shrt-term effect (i.e. exterminatin f the species due t OBM cntaminatin f the sediment in the perid immediately after drilling) and nt as an actual effect f cntaminatin at the lng term, since the surce f disturbance culd n lnger be demnstrated. In view f the gradient bserved between 100 and 500 m, the fauna cmpsitin at 25 m frm the platfrm was anmalus. At this statin an unexpectedly high number f taxa was fund and, in fact, nly the absence f sme OBM sensitive species was indicative f an envirnmental effect. Hwever, the species cmpsitin was different frm that at all ther statins, prbably due t the anmalus sediment cnditins at this statin. The recvery ptential f the macrfauna is discussed with special reference t the life cycle f the sensitive indicatr species E. crdatum. Based n this discussin it is suggested t plan a fllw-up survey at Pb in The results and cnclusins f this study may be summarized as fllws: 1. Elevated il cncentratins (up t 300 mg-kg"1 dry sediment) were fund up t 100 m (residual current directin) frm the platfrm. At 250 m the chemical analyses did nt shw elevated cncentratins, but traces f il were visually bserved during the fieldwrk. Elevated Ba cncentratins cnfirmed the presence f discharged material at this statin. 2. At 100 m il was fund t a depth f at least 20 cm in the sediment. At 250 m Ba cncentratins were highest in the upper 10 cm, but als at cm the cncentratins were substantially elevated, indicating that much f the discharged material is stred in the deeper sediment layers. 3. In the residual current directin n traces f discharged material were bserved at distances >500 m. In perpendicular directins n traces were fund at distances >250 m. 4. An accumulatin f bilgical effects was fund at 100 m and 250 m (res. curr. directin) frm the platfrm. The fauna was substantially impverished at these statins, even at relatively lw il cncentratins (250 m). 5. Adult Echincardium crdatum were absent up t 500 m frm the platfrm.

5 2. At 25 m frm the platfrm the fauna cmpsitin was different frm all ther statins, but nt really impverished. It is suggested that increased turbulence arund the platfrm legs may have caused increased ersin f the seabed s that ld banks f shells and shell fragments were uncvered, which were clnized by a macrfauna cmmunity with a different cmpsitin. 7. Based n the abve findings it is cncluded that envirnmental effects arund an OBM lcatin in the ersin area f the Dutch Cntinental Shelf were still detectable years after terminatin f the discharges f OBM cntaminated drill cuttings.

6 3 SAMENVATTING EN CONCLUSIES In de 80-er jaren werd p de Nrdzee bij lie- en gasbringen veelal liehudende brspeling (OBM) gebruikt. Restanten daarvan werden met het pgebrde gruis p de zeebdem gelsd. Sedert 1985 wrdt p het Nederlands Cntinentaal Plat nderzek verricht naar de milieueffekten van deze lzingen, in het bijznder naar de effekten p het benthische systeem rnd de lkaties waar deze spelingen zijn gebruikt. Met ingang van 1993 is een vlledig verbd ingeverd p het lzen van liehudend brgruis p het NCP. Het nderzek naar lange-termijn effekten, en met name naar de ruimtelijke mvang daarvan, gaat echter dr m een mgelijk herstel van sedimentcndities en bdemfauna vast te stellen. In 1993 is een veldnderzek uitgeverd bij lkatie Pb in het zandige ersiegebied in het zuidelijke deel van het Nederlands Cntinentaal Plat. Op lkatie Pb zijn in ttaal 4 bringen verricht. De eerste bring vnd plaats in 1985 en werd uitgeverd met OBM. Een eerste veldsurvey werd in september van hetzelfde jaar uitgeverd, 2 maanden na de bring. Later in dat jaar en in 198 werden ng eens 2 bringen verricht, nu met gebruikmaking van uitsluitend brspeling p waterbasis (WBM). De laatste bring vnd plaats in 1987 en hierbij werd weer gebruik gemaakt van OBM. In dit rapprt wrden de resultaten van de in 1993 uitgeverde survey gepresenteerd, die jaar na de laatste lzing plaatsvnd. Bdembemnstering was in hfdzaak gecncentreerd p een aantal statins langs een raai in de reststrmrichting van Pb. De statins werden gekzen p afstanden variërend van 25 tt 5000 m van het platfrm. De bdemmnsters werden geanalyseerd p krrelgrtte-samenstelling, lie- en Bariumcncentraties en makrfaunasamenstelling. In dwarsstrmse en tegenstrmse richting werden k enkele statins bemnsterd. Van deze mnsters vnd slechts een beperkte fysisch-chemische analyse plaats en werden alleen de aantallen zee-egels (Echincardium crdatum, exemplaren >15 mm) in het veld geteld. Uit de chemische analyses bleken in de reststrmrichting verhgde liecncentraties vr te kmen tt p 100 m van het platfrm, maar tijdens het veldwerk waren k liespren te zien p 250 m. Barium-cncentraties p dit statin waren k hger dan natuurlijke achtergrndwaarden. Op 100 m werden verhgde lie- en Ba-cncentraties vastgesteld tt p minstens 20 cm diep in het sediment. In de diepere anaerbe sedimentlagen lijkt nauwelijks afbraak van lie plaats te vinden. In de reststrmrichting werden vanaf 500 m geen restanten van gelsd materiaal meer aangetrffen, in andere richtingen al niet meer vanaf 250 m. Vlak bij het platfrm (p 25 m) werden weliswaar enigszins verhgde lie- en Ba-cncentraties gevnden, maar deze waren lager dan p 100 m. Op dit statin bleek een srt uithlling in de zeebdem vr te kmen en de aanwezigheid van grte heveelheden ud schelpengruis in de mnsters det vermeden dat de zandige tplaag hier is verdwenen, kennelijk ais gevlg van turbulente strmingen rnd de pten van het platfrm, waardr de zeebdem ter plaatse sterk kan zijn geërdeerd. Deze plaatselijk tegenmen ersie kan k tt gevlg hebben gehad dat gelsd materiaal uit de direkte mgeving van het platfrm weg is gespeld. Aan de hand van de samenstelling van de benthische fauna kn een accumulatie van effekten wrden vastgesteld p zwel 100 m ais 250 m (reststrmrichting). De fauna was hier aanmerkelijk verarmd. Op 500 m wees alleen ng het ntbreken van vlwassen E. crdatum p een lange-termijn effekt van de vregere lzingen. De afwezigheid van E. crdatum p dit statin met echter gezien wrden ais een gevlg p de langere termijn van een vreger pgetreden effekt, nl. sterfte nder deze srt ais gevlg van verntreiniging van het sediment met OBM. Van een latent ptredend effekt lijkt geen sprake, aangezien de aanwezigheid van de brn van verstring (OBM) p dit statin niet meer kn wrden aangetnd. Gezien de duidelijke gradient die tussen 100 en 500 m in de faunasamenstelling werd waargenmen, was te verwachten geweest dat het 25 m statin een wellicht ng armere macrfauna te zien zu hebben gegeven. De faunasamenstelling was hier echter wel sterk afwijkend, maar niet bij uitstek arm. In feite werd zelfs een nverwacht grt aantal taxa aangetrffen en was alleen het ntbreken van enkele zeer gevelige srten indicatief vr een effekt van de lzingen. Kennelijk heeft de afwijkende bdemsamenstelling ter plaatse geleid tt een specifieke habitat die dr een fauna met een afwijkende samenstelling is geklniseerd. De mgelijkheden vr herstel van de bdemfauna en de snelheid daarvan (met name p enige hnderden meters afstand) zijn bediscussieerd, met name aan de hand van de levenscyclus van de gevelige indicatrsrt E. crdatum. Op basis van deze discussie wrdt aanbevlen een eventuele vervlgsurvey bij Pb rnd 1997 uit te veren. De resultaten en cnclusies kunnen ais vlgt wrden samengevat: 1. Verhgde liecncentraties (tt 300 mg-kg'1 drg sediment) werden aangetrffen tt p 100 m (reststrmrichting) van het platfrm. Op 250 m tnden de chemische analyses geen lie aan, hewel tijdens het veldwerk in een aantal mnsters wel degelijk liespren werden waargenmen. Verhgde Ba-cncentraties bevestigden dat p dit statin ng steeds restanten van het gelsde materiaal aanwezig waren. 2. Op 100 m van het platfrm (reststrmrichting)

7 4 werd lie in het sediment aangetrffen tt p tenminste 20 cm diep. Op 250 m waren Ba-cncentraties het hgst in de bvenste 10 cm van het sediment, maar k p cm werden ng aanmerkelijk verhgde cncentraties aangetrffen, hetgeen er p duidt dat een belangrijk deel van het gelsde materiaal ng aanwezig is in diepere sedimentlagen. 3. In de reststrmrichting werden vanaf 500 m van het platfrm geen lie f andere restanten van gelsd materiaal aangetrffen. In tegengestelde en dwarsstrmse richtingen was dit al vanaf 250 m het geval. 4. Een accumulatie van bilgische effekten werd aangetrffen p zwel 100 m ais 250 m van het platfrm. Op deze statins was sprake van een sterk verarmde macrfauna en dat met name p 250 m bij relatief lage liecncentraties. 5. Grte Echincardium crdatum ntbraken tt p 500 m van het platfrm.. Op 25 m van het platfrm werd een fauna-samenstelling aangetrffen die sterk afwijkend was van die van alle verige statins, maar niet bij uitstek arm. Mgelijk zijn turbulente strmingen rnd de pten van het platfrm er de rzaak van geweest dat hier een hlte in de zeebdem is uitgeschuurd, waardr dieper gelegen banken van schelpengruis blt zijn kmen liggen, die dr een fauna met een afwijkende samenstelling is geklniseerd. 7. Op basis van bvengenemde resultaten kan gecncludeerd wrden dat zes jaar na een OBM-bring in de ersiezne van het Nederlands Cntinentaal Plat milieueffekten van lzingen van met OBM verntreinigd brgruis rnd de betreffende lkatie ng steeds knden wrden aangetnd.

8 5 1 INTRODUCTION 1.1 GENERAL PART Oil based drilling muds (OBM) have been extensively used during drilling activities in the Nrth Sea in the 80's and the early 90's. Althugh drill cuttings frm the wells bred generally passed ne r mre treatment facilities t separate mud frm the cuttings befre these were discharged, there were always substantial amunts f adhering residuals f base il that reached the seabed in this way. Cncern abut the envirnmental risk f these dumpings has led t benthic mnitring studies in all Nrth Sea sectrs. OBM are still in use but the extent f discharges has cnsiderably decreased. Due t agreements between industry and natinal authrities r t natinal regulatins, there are n lnger discharges f OBM cuttings in the Nrwegian, Danish, German and Dutch sectrs since 1 January 1993 (An n y m u s, 1994). When wells are drilled with OBM at installatins in these 5 55* L4a V F18-9 L3-3 Fig. 1. Psitin f platfrm Pb. Open circles are drilling lcatins investigated in previus studies. Slid line: brder f the Dutch part f the Cntinental Shelf. sectrs, the drill cuttings are brught ashre fr treatment and dispsal. Only in the UK sectr dumping f OBM cuttings is still ging n, but varius systems were develped t reduce the il cntent f the material dumped. Cuttings are treated dwn t an il cntent f 5-% nw befre being discharged (Annym us, 1994). With the terminatin f OBM cutting discharges, further investigatins n the assciated shrt-term effects have cme t an end in the Dutch sectr. Hwever, in view f pssible future clean-up measures fr the seabed arund abandned well sites, the lng-term effect f OBM cutting discharges is still subject f interest. The Dutch sectr can be rughly subdivided in a sandy ersin area in the suth, an area f net sedimentatin in the nrth and an intermediate transitin zne in between. A frequent mnitring prgramme has been running since 1985 at lcatin K12a in the transitin zne. The last field survey at this lcatin was carried ut in 1992, 8 years after dumping f OBM cuttings (D aan & M u ld e r, 1993). The results f this lng-term study indicated that there was a decrease in the spatial extent f envirnmental effects, with clear signs f recvery f sediment cnditins and macrfauna at 500 m frm the platfrm and beynd that distance. Hwever, clser t the platfrm elevated il cncentratins were still bserved, particularly in the deeper sediment layers. Bilgical effects culd still be identified and at 100 m frm the platfrm an accumulatin f effects became manifest by a severely impverished benthic macrfauna. Up t nw, data n lng-term effects are lacking frm lcatins in the ersin area and the sedimentatin area. Because f the hydrgraphical differences in the three areas, it is cnceivable that there may be als differences in (re-)distributin and degradatin rates f il in the sediments and in the persistence f assciated effects n the benthic infauna. Therefre, the attentin has mved t lngterm studies in these areas t estimate the extent f il cntaminated areas arund frmer OBM discharge sites and t assess the degree f bilgical deteriratin, c.q. f pssible recvery f the benthic macrfauna cmmunities. This reprt presents the results f a study at lcatin Pb in the ersin area, six years after the last OBM cuttings were dumped at this lcatin. Platfrm Pb is situated in the suthern part f the Dutch sectr, in ±30 m waterdepth (Fig. 1). The sediment cnsists f fine and carse sand, whereas the silt fractin (<3 pm) is less than 1% (Kuiper & G r e n e w u d, 198). At Pb fur wells have been drilled, tw f them with OBM based n lw-tx il (Table 1). All drill cuttings were discharged n the seabed. Fig. 1 shws als the ther lcatins that have been studied in preceding years. A first field survey, aimed t assess shrt-term effects, has been carried ut already in September 1985, tw mnths after cmpletin f the first well.

9 Psitin Area Drilling activities TABLE 1 Infrmatin n drilling lcatin Pb. Emissin 1st OBM drilling 2nd OBM drilling Platfrm 52 44'17" N 03 48'18" E Ersin zne; fine and carse sand; Silt<1%; depth appr. 30 m. June OBM drilling Oct W BM drilling Jan. 198-W BM drilling Aug OBM drilling 184 tnnes f lw-tx il 104 tnnes f lw-tx il Present Frmer effect study: Survey Sept (Mulder eta!., 1987; Kuiper & Grenewud, 198). The chemical sediment analyses f that survey revealed high il cncentratins in the sediment up t 250 m frm the platfrm (residual current directin) and slightly elevated cncentratins up t 1000 m (Kuiper & G r e n e w u d, 198). Bilgical effects, in terms f reduced species richness and macrfauna abundance, were detectable up t 750 m (M uld er et al., 1987). Hwever, the abundance patterns f the sea urchin Echincardium crdatum and the bivalve Mntacuta ferruginsa, tw species that later have been shwn t be very sensitive t OBM cntaminatin, indicated that there were effects up t >1000 m. The present survey was carried ut in August 1993, i.e. eight years after the first survey and six years after the last discharge f OBM cuttings. The results f this survey will be cmpared with thse f the first survey f ACKNOWLEDGEMENTS This study was perfrmed under cntract with the Nrth Sea Directrate f the Ministry f Transprt, Public Wrks and Water Management (RWS, Nrth Sea Directrate) and als financed by the Ministry f Ecnmic Affairs (EZ) and the Netherlands Oil and Gas Explratin and Prductin Assciatin (NOGEPA). The study was carried ut in the framewrk f the Dutch cllabratin prgramme 'Plicy Linked Eclgical Research Nrth Sea and Wadden Sea' (BEON). The prject was crdinated by the wrking grup 'Mnitring Offshre Installatins', in which participated: Dr. W. Zevenbm (RWS, Nrth Sea Directrate), chairwman Drs. J. Asjes (RWS, Nrth Sea Directrate), secretary Ing. M. de Krieger (RWS, Nrth Sea Directrate) O 250 Residual current transect (12 ) O 5000 Fig. 2. Psitins f the sampling statins alng a crss-shaped transect. Slid circles: samples analysed fr macrfauna.

10 7 Drs. K. Meyer (VROM) Ir. L. Henriquez (EZ, State Supervisin f Mines) Drs. P. Seeger (EZ) Drs. W. Vnck (RWS, Natinal Institute fr Castal and Marine Management (RIKZ) Dr. D. Vethaak (RWS, RIKZ; frm April 1994 nwards) Drs. J.M. Marquenie (NOGEPA) H.J. van het Grenewud (TNO) M. Mulder (NIOZ) Dr. R. Daan (NIOZ) Thanks are due t captain, crew and the emplyees f RWS-Nrth Sea Directrate n bard f the R.V. Mitra fr their assistance in the fieldwrk. The chemical and grainsize analyses were perfrmed by IMW- TNO den Helder and are reprted als separately (G r e n e w u d, 1994). J. van der Meer advised n statistical analyses (lgit regressin). Thanks are als due t M. van Arkel fr his rganising helpfulness. 2 METHODS 2.1 SAMPLING The survey at Pb was carried ut in the 3rd week f August Sampling statins were chsen alng a crss-shaped transect, the main axis running parallel t the residual current directin (Fig. 2). The 100-m and 250-m statins in the residual current directin were apprached twice and bth times 5 grab samples (Van Veen grab, 0.2 m2) were cllected. At each f the ther statins 10 samples were cllected. Frm each sample small duplicate sediment cres (diameter 28 mm, depth 10 cm) were taken fr chemical and grainsize analyses. The pled sediment subsamples f each statin were thrughly hmgenised and immediately frzen at -20 until later analysis in the labratry. The cntents f the grab were washed thrugh a sieve (mesh size 1 mm). During sieving the numbers f Echincardium crdatum (specimens >15 mm) were cunted in 8 samples at each statin. The remaining macrfauna was preserved in a % neutralized frmaldehyde slutin. At the 100-m statin an additinal bxcre sample was cllected t assess vertical prfiles f il and Ba cncentratins in the sediment. Subsamples were taken frm the sediment layers 0-2 cm, 2-10 cm and cm and further treated in the same way as the rutine sediment samples. 2.2 LABORATORY ANALYSES In fact, field samples were cllected in excess and nt all samples were analysed. Table 2 gives an verview f the analyses that were applied t the samples f each statin GRAINSIZE ANALYSIS Grainsize analyses were perfrmed t verify if the natural sediment cmpsitin is mre r less hmgeneus in the area investigated. The analytical prcedures are described in detail by G r enew u d & S c h ö lten (1992a) BARIUM ANALYSIS Barite is a substantial cnstituent f drilling muds. Because f its inertia Barite prvides a gd indicatr fr the dispersal f discharged material, in particular f the smaller grain size fractins. Cncentratins f Ba in the sediment were determined as fllws: Abut 10 grammes f sediment were dried fr 2 TABLE 2 Schedule f analyses f the samples cllected at Pb.Gralnslze = analysis f grainsize distributin. Oil cncentratin = analysis f il cncentratin in the sediment. Ba cncentratin = analysis f Ba cncentratin in the sediment. E. crdatum = n bard cuntings f Echincardium crdatum, specimens >15 mm. Fauna analyses = cmplete fauna analyses ( samples per statin). Transect Statin Distance Grainsize OU cncentratin Ba cncentratin Vert, prfiles E. crdatum Fauna analyses m X X X X X 100 m 2X 2X 2X X X X 250 m 2X 2X 2X X X 500 m X X X X X 750 m X X X X 1000 m X X X X 2000 m X X X 5000 m X X X X m X X 5000 m X X m X X 5000 m X X m X X 5000 m X X

11 8 hurs at 105 C. Then 2 grammes were hmgenized and destructed by means f sulphuric acid and hydrgen perxide. After settling, the barium cntent f the destruate was determined using inductive cupled plasma atmic emissin spectrmetry (ICP-AES) OIL ANALYSIS Oil analyses f sediment samples were perfrmed using the gas chrmatgraph mass spectrmeter (GCMS) technique. Cncentratins f alkanes (C i0 - C30), unidentified cmplex matter (UCM) and 'ther cmpnents' were quantified. The analytical prcedures are described in detail by G r enew u d & S c hö lten (1992a) FAUNA ANALYSIS Macrfauna analyses were perfrmed n samples f each f 8 statins at the residual current transect. Rutine methds f srting and identificatin are described by M ulder et al. (1988) STATISTICAL PROCEDURES Pssible shifts in the macrfauna cmmunity were tested by cmparing the relative abundance f all identified species at each f the statins (ANOVA). This methd is als described in detail by Daan et al. (1990). Pssible gradients in the distributin patterns f individual species were tested by lgit regressin (see e.g. Jngma n et al., 1987). The regressin was applied t thse species f which at least 20 specimens were fund. The methd was als used in frmer studies and mre details abut its applicatin are given in Daan et al. (1990). Hwever, a fundamental imprvement was intrduced cmpared t the prcedure applied in frmer studies. Details f the cmplete prcedure as perfrmed nw are given in the appendix, but the principles are shrtly utlined here. In fact, the usual prcedure prvides a test f the Hypthesis H0 that the prbability (it) f a species being present in a sample des nt depend n distance t the platfrm against the alternative hypthesis Ht that there is a systematic increase r decrease f p with increasing distance frm the platfrm. In ther wrds mdel (0): lgit(rc) = ln(7i/1-7t) = exp(b0) (1 ) is tested against mdel (1): lgit(jt) = ln(rc/1-n) = exp(b0+bi*d) (2) where d is distance t platfrm and b0 and b-, are mdel parameters. Fr bth mdels the mdel parameters are estimated accrding t the maximum likelihd principle, D is ta n c e (m ) Fig. 3. Depth prfile alng the residual current transect (uncrrected fr tidal differences). fllwing an iterative prcedure. The gdness f fit f bth mdels can nw be cmpared n the basis f their lg-likelihd. If the difference in lg-likelihd f mdel (1) and mdel (0) exceeds a certain critical value H0 is rejected in favur f Ht and it is decided that the frequency f ccurrence f the species significantly depends n distance t platfrm. This was the primary prcedure and is perfrmed here t. Hwever, acceptance f H, des nt necessarily mean that Mdel (1) gives a perfect fit. There still may be a cnsiderable deviatin f the bserved values and the fitted values. This may be due t verdispersin in the data, i.e. the assumptin f binmial variance is unrealistic and the variance in the data is greater than predicted by the binmial mdel (M c C u llag h & N eld er, 1983). Therefre mdel (1) is further tested against the full mdel: lgit(rc) = ln(7t/1-7t) = exp(b ) where b is directly estimated frm the relative frequency f ccurrence f the species at the i,h statin. When the difference in lg-likelihd between the full Mdel and Mdel (1) is large this may be reasn t decide that a pssible significant gradient in frequency f ccurrence as established by the first test is due t ver-dispersin in the data. 3 RESULTS 3.1 SEABED CHARACTERISTICS Depth recrdings (uncrrected fr tidally induced differences) shwed that there was a depressin in the seabed within 100 m f the platfrm, fllwed by an elevatin at m (Fig. 3). The depressin might have been caused by ersin due t turbulent currents arund the platfrm legs. At the 25 m statin the

12 9 % h in CM E I lz j r T T O O LO LO ^ C\J C\J Z - LO C\J a CM CM 0 5 CO t - CM O O i t CM CM milii $ z? I CM ÇM O CO O O O Q O ) t T- f - CM IO O O s t t 0 % > 1000 pm % > 500 pm % > 300 pm m % > 180 pm % > 125 pm % > 90 pm H % > 3 pm m % < 3 pm Distance (m) Fig. 4. Grainsize distributin alng the residual current transect (data frm G renew ud, 1994). The size fractins 3-90 mm and < 3 mm are nt visible, because they were extremely lw. Fr details see Table 9. samples cntained large amunts f shell fragments, mst remnants f ld Dnax and Spisula banks, but als shells f Mytilus edulis. The latter must have been living attached t the legs f the platfrm, since this species des nt ccur n the seabed by nature in this area. It is cnceivable that ersin has remved the finer material f the superficial sediment layers and that, as a result, deeper layers cntaining much ld shells were uncvered. The sediment alng the residual current transect cnsisted mainly f fine t carse sand (Fig. 4). The grainsize fractin pm cntributed 70-80%. Carser material (grainsize >300 pm) was cnsistently present in all samples and cntributed 10-25%. Particularly at 250 m the carse fractin was relatively large. Very carse material (>1000 pm) was fund In the vicinity f the platfrm and at 2000 m. This material cnsisted prbably f shell grit. The silt fractin (<3 mm) was far belw 1% at all statins and des nt appear therefre in the figure. Hwever, the data listed in Table 9 shw that the silt fractin was smewhat elevated at 25 m and 100 m cmpared t the ther statins. 3.2 BARIUM CONCENTRATIONS IN THE SEDIMENT The Ba cncentratins in the sediment arund the platfrm are listed in Table 3. Interpretatin f the data shuld take int accunt that Ba is usually present in the sediment in lw backgrund cncentratins. G r en ew u d & S c hö lten (1992) have shwn that the natural backgrund cncentratins are strngly related t the silt (fractin <3 pm) cntent f the sediment. Because the sediment at Pb is deficient in silt, the natural Ba cncentratin may be expected t be very lw. The mean backgrund cncentratin at silt cncentratins <1% can be estimated frm a plt f Ba against silt given in Fig. 2 f Daan & M uld er (1993). This cncentratin will be in the rder f 17 mg-kg-' dry sediment and nt exceed TABLE 3 Ba cncentratins in the sediment arund Pb (data frm Grenewud, 1994). Statin Sa m g/kgdry weight 25 m m m 740/ m m m m m 13, m 18,8

13 10 >* 800 O) 00 O) O) í >> T> O) JÉ D> E <D m IO c\i IO IO IO IO 0-2 cm 2-10 cm cm D is ta n c e (m ) Fig. 5. Ba cncentratins alng the residual current transect (data frm G renew ud, 1994). Depth Fig.. Vertical prfile f Ba cncentratins in the sediment at 100 m (residual current transect, data frm G renew ud, 1994). a maximum level f =35 mg-kg'1. Fig. 5 shws that, alng the residual current transect, the cncentratins were substantially higher than maximum backgrund level at the statins up t 250 m frm the platfrm. At all ther statins the cncentratins were belw the maximum backgrund level and generally in the rder f the expected backgrund level (see als Table 3). Only at the statin 250 m-192 the cncentratin was apprximately at the maximum backgrund level, but, in terms f elevated Ba cncentratins, traces f the discharged cuttings culd nly be detected up t 250 m frm the platfrm in the residual current directin. A vertical prfile f Ba in the sediment at 100 m (Fig. ) shws that the highest cncentratins were fund in the upper 10 cm, but als at cm depth TABLE 4 Oil cncentratins at sme statins at the residual current transect (data frm Grenewud, 1994). Statin il ene. mg.kg-1 dry weight 25 m 38, m 90,2 100 m 87, m 3,8 250 m 1,7 500 m 1, cm 94, m 2-10 cm 301, cm 8,73 the cncentratins were high, indicating that much f the discharged material is stred in the deeper sediment layers. c 0) E T3 (D ÖJ >* k. TD ' 7 5 ' O) 50- O) E JBa2L tasjsjffiuod JSÏSL. X T T T Distance (m) Fig. 7. Ttal il cncentratins at Pb (data frm G renew ud, 1994).

14 BIOLOGICAL FEATURES ^ 400 c «D E "D (O >* T3 O) I 200- ai E cm 2-10 cm cm Depth Fig. 8. Vertical prfile f il cncentratins in the sediment at 100 m (residual current transect, data frm Grenewud, 1994). 3.3 OIL CONCENTRATIONS IN THE SEDIMENT Oil cncentratins were determined at a few statins alng the residual current transect nly (Table 4, Fig. 7). Elevated cncentratins were fund at 25 m and at 100 m, particularly at the latter statin. A vertical prfile f il in a bxcre sample at this statin shws that the cncentratin was highest in the sub-superficial sediment layer (2-10 cm depth), but als the superficial and deeper layers were bviusly cntaminated (Fig. 8). Visual bservatin during fieldwrk suggested that il cntaminatin was severest at apprximately 20 cm depth. At 250 m the analyses did nt reveal elevated il cncentratins in the sediment. Althugh in ne f the 2 sets f pled samples a ttal cncentratin was fund that was almst 3 times as high as in the 500-m sample (see Table 4), the value f 3.8 mg-kg'1 dry sediment is well belw the maximum backgrund level f 7 mg-kg"1 adpted fr the Dutch sectr till nw (Z even b m et al., 1992). Nevertheless sme traces f il were visually bserved in 4 f the riginal grab samples, immediately after they were cllected n bard f the research vessel. The absence f il at 500 m as assessed by the chemical analysis was in crrespndence with the visual bservatin that the sediment was clean at this statin and did nt shw any trace f il. Als the samples f the ther statins (nt analysed) all lked clean FIELD OBSERVATIONS The n bard cuntings f Echincardium crdatum revealed that large specimens (>15 mm) were absent in the samples up t 500 m in the residual current directin frm the platfrm (Fig. 9). At all ther statins, including thse in upstream and perpendicular directins, large specimens were fund in 1 r mre f the samples, but densities were generally lw ( ind-rrf^). Althugh the absence f large E. crdatum at the residual current statins up t 500 m frm the platfrm was indicative f a lng-term effect f the frmer discharges, lgit regressin did nt reveal a significant increase in frequency f ccurrence f the species in the samples with increasing distance frm the discharge site. The individual size f the animals ranged between 32 and 51 mm. The large size f these adult animals is characteristic f the sandy areas in the Suthern part f the Nrth Sea (Du in eveld & J en n e s s, 1984) GENERAL FAUNA DESCRIPTION The labratry analyses yielded 7 identified species. In Table 5 their percentual ccurrence in the samples is summarized. The riginal data are listed in Table 13 (Appendix). The fauna in the area was numerically dminated by juvenile Echincardium crdatum, which accunted fr 53% f the ttal fauna numbers. The species was nt hmgeneusly distributed alng the residual current transect and shwed high numbers beynd 500 m frm the platfrm, whereas it was almst absent at 100 m. Fig. 10 shws a clear gradient in the abundance pattern f juvenile E. crdatum and lgit regressin cnfirmed that there was a significant (5% level) increase in frequency f Q. E c v> Distance Fig. 9. Abundance f Echincardium crdatum (specimens > 15 mm) arund Pb.

15 12 TABLE 5 The benthic fauna at Pb, August Percentage f ccurrence f each species in the ttal number f samples (48). POLYCHAETA Orchmenella nana 8 Sthenthe marina 2 Lepidepecreum lngicrne 2 Sthenthe spec. 2 Harmthe lngisetis Leucthe incisa 4 Urthe pseidnis 88 Sthenelais limicla 2 Mntacuta ferruginsa 8 Bathypreia guilliamsniana 77 Pisine remta 2 Dnax vittatus 38 Bathypreia elegans 9 Etene lactea 13 Mactra crallina 2 Periculdes lngimanus 4 Anaitides maculata 13 Spisula elliptica 15 Synchelidium haplcheles 15 Anaitides spec. juv. 4 Spisula spec. juv. 2 Ara typica 2 Eumida sanguinea 2 Tellina fabula 42 Nephtys hmbergii 19 Ensis ensis 21 ECHINODERMATA Nephtys cirrsa 100 Ensis spec. juv. 4 Nephtys caeca 10 Thracia phaselina 2 Asterias rubens 4 Glycera capitata 2 Trnus subcarinatus 2 Ophiura texturata 2 Glycera spec. juv. 2 Natica alderi 31 Ophiura albida 38 Glycinde nrdmanni 2 Ophiura spec. juv. 83 Gniada maculata 44 CRUSTACEA Echincardium crdatum 23 Sclpls armiger 50 Echincardium crdatum juv. 90 Aricidea jeffreysii 2 Crangn allmani 2 Echincyamus pusillus 23 Aricidea minuta 29 Prcessa parva 27 Paranis fulgens 4 Pntphilus trispinsus 4 OTHER TAXA Pecilchaetus serpens 10 Pntphilus spec. juv. 2 Spi filicrnis 0 Pagurus bernhardus 10 Nemertlnea 71 Spiphanes bmbyx 98 Macrpipus spec. juv. 27 Nematda 4 Sclelepis bnnieri 4 Pinntheres pisum 2 Amphixus 13 Magelna papillicrnis 10 Thia scutellata 13 Turbellaria 2 Chaetzne setsa 13 Crystes cassivelaunus 2 Phrniden 2 Ophelia limacina 10 Decapda larven 13 Harp, cpepda 8 Euznus flabelligerus 2 Gastrsaccus spinifer Oligchaeta Travisia frbesii Schistmysis rnata 8 Medimastus gracilis 2 Iphine trispinsa 15 Lanice cnchilega 25 Diastylis bradyi 23 Megalurpus agilis 10 MOLLUSCA Atylus swammerdami Atylus falcatus 4 Arca lactea 2 Hippmedn denticulatus 4 ccurrence f juvenile E. crdatum in the samples with increasing distance t the platfrm. T a lesser extent the amphipd Bathypreia elegans was als dminant, attributing 1% f ttal fauna numbers. This species did nt shw a cntinuus gradient in its abundance pattern, but just a lcal minimum in the zne m. There were nly 5 ther species that were mre r less abundant (mean density >10 ind-m'2). Nne f them shwed a clear gradient, but 3 species displayed a similar trend as Bathypreia elegans. Nephtys cirrsa, Spiphanes bmbyx and Bathypreia guilliamsniana ccurred in relatively lw densities in the zne m. In cntrast, Spi filicrnis shwed a maximum in this zne. The ttal fauna abundance was lw at 100 m and 250 m and gradually increased with increasing distance t the platfrm (Fig. 11). The abundance at 25 m seemed nt t fit in this pattern. Hwever, the relatively high abundance at this statin was largely due t high numbers f the plychaete Lanice cnchilega. When this species is excluded frm the calculatin, the ttal numbers ranged between 200 and 400 ind-m"2 in the zne m. The high numbers utside this zne were largely caused by the abundance f juvenile E. crdatum and Bathypreia elegans. When these species are left aside, the resulting ttal fauna numbers ranged between 190 m'2 at 100 m and 250 m and 410 m'2 at 5000 m. Analysis f variance n lg-transfrmed densities revealed that nly the 100-m and 250-m statins had significantly lwer fauna numbers than the statins further away. Frm the survey carried ut in 1985 (M uld er et al., 1987) it was already knwn that the species richness in the area where Pb is situated is lw cmpared t the mre silty sediment in the nrthern part f the Dutch sectr, i.e. the number f species per sample is lw. At the varius statins sampled alng

16 13 ff) O) ô <DQ. 30 -, H Survey '85 Survey '93 (0 * Ō 20 - k_ <D n E 3 C O C 200C 5000 t n C M m i c m CM IO D is ta n c e (m) D is ta n c e (m ) Fig. 10. Abundance pattern f juvenile Echincardium crdatum alng the residual current transect. the residual current transect in 1985, the number f species per sample ranged between 14 (near the platfrm) and 2 (at >1000 m), whereas in the sedimentatin zne ne grab sample usually yields ver 30 species. During the 1993 survey at Pb the number f species per sample ranged between 9 (at 100 m) and 1 (at 1000 m), which is even less than in 1985 (Fig. 12). At distances between 750 and 5000 m the numbers fluctuated between 13 and 1. On apprach f the platfrm there was a cntinuus decrease and analysis f variance revealed that the 100-m and 250-m statins had significantly less species per sample than the statins at larger distance. At 25 m this trend was interrupted and the mean Fig. 12. Number f identified species per sample at Pb (residual current transect, surveys 1985 and 1993). number f species per sample at this statin was abut the same as at the remte statins. The ttal number f species fund (in samples) at each statin shws a similar pattern (Fig. 13) PRESENCE-ABSENCE DATA: LOGIT REGRESSION Pssible gradients in the spatial abundance patterns f 17 individual species were tested by lgit regressin. The results are listed in Table and shw that, accrding t the uncrrected test, 3 species shwed a significant (5% level) gradient in their spatial frequency f ccurrence. All 3 species tended t ccur less frequently in the samples clse t the platfrm than at the remte statins (slpe f the gradient psitive). In the plychaete Aricidea minuta the gradient 40 - in r n F a> E C <D O C c T3 C 3 n m 20 - n E3 Z S D istance (m) Fig. 11. Ttal macrfauna abundance at Pb (residual current transect). 0 -F - M----- M M M M M V D is ta n c e (m ) Fig. 13. Numbers f identified species per statin ( samples) alng the residual current transect at Pb.

17 14 TABLE List f species fr which gradients in frequency f ccurrence were tested by lgit regressin. Sign f the gradient (+/-) and significance level are indicated: += increasing frequency f ccurrence away frm the lcatin; -= decreasing frequency f ccurrence away frm the lcatin; 0= n gradient; n.s. = nt significant. sign uncrr. test sign, level (%) crr. test Nephtys cirrsa Gniada maculata - n.s. n.s. Sclpls armiger + 5 n.s. Aricidea minuta + 1 n.s. Spi filicrnis - n.s. n.s. Spiphanes bmbyx + n.s. n.s. Lanice cnchilega - n.s. n.s. Dnax vittatus + n.s. n.s. Tellina fabula + n.s. n.s. Natica alderi + n.s. n.s. Prcessa parva + n.s. n.s. Urthe pseidnis + n.s. n.s. Bathypreia guilliamsniana + n.s. n.s. Bathypreia elegans + n.s. n.s. Ophiura albida - n.s. n.s. Echincardium crdatum + n.s. n.s. Echincardium crdatum juv. + 5 n.s. was als significant at the 1% level. The number f rejectins f H0 (i.e. frequency f ccurrence is nt dependent n distance t platfrm) Is lw, but still appears t be significantly (5% level) higher than shuld be expected if H0 were true fr all species. This Implies that the prbability that all 3 rejectins f H0 were statistical Type-1 errrs is less than 5%. Hwever, after crrectin f the test fr ver-dispersin it appeared that there was n species shwing a significant gradient, indicating that the suppsed significances as established by the uncrrected test might be due t ver-dispersin RELATIVE MACROFAUNA ABUNDANCE A plt f the relative macrfauna abundance, calculated as the mean rank f all species at each statin (Fig. 14), shws that the mean rank was lw at 100 m and 250 m. There was a gradual increase with increasing distance t the platfrm. At 25 m the relative abundance was unexpectedly high. Analysis f variance revealed highly significant (0.1% level) differences in the mean ranks f the different statins. An LSD-test, additinally applied t test the significance f differences between individual statins (Table 7), shwed that the relative abundance at 100 m and 250 m was significantly lwer than at the statins between 1000 m and 5000 m, and als lwer i m 100 m 250 m 500 m 750 m 1000 m 2000 m 5000 m Distance t platfrm Fig. 14. Relative macrfauna abundance at Pb (mean ranks ±95% cnfidence limits).

18 15 TABLE 7 Statistical significance (LSD-test) f differences in relative abundance between statins at the residual current transect. 25 m 100 m 250 m 500 m 750 m TÖÖÖ m X 100 m 0,5 X 250 m 0,1 n.s. X 500 m 1 n.s. n.s. X 750 m n.s. n.s. 5 n.s. X 1000 m n.s. 1 0,5 5 n.s. X 2000 m n.s. 0,5 0,1 5 n.s. n.s m n.s. 5 1 n.s. n.s. n.s. than at 25 m. At 500 m the relative abundance was still lw, but nt significantly different frm that at the 5000-m reference statin ABUNDANCE PATTERNS OF OBM SENSITIVE AND OPPORTUNISTIC SPECIES In Table 8 a number f species is listed, which in earlier studies have shwn t be susceptible t OBM cutting discharges (see Daan et al., 1990). Fur pprtunistic species are als excluded. The abundance patterns f all these species were inspected fr the presence f pssible gradients at Pb in The table shws that mst species were nt fund r in t lw numbers t recgnize any pattern in their abundance. Amng the species listed there were nly 5 f which >20 specimens were fund. Of these species Echincardium crdatum and Tellina fabula seemed t ccur in reduced densities in the vicinity f the platfrm, whereas Lanice cnchilega was especially abundant at 25 m. Increased abundance f pprtunistic species in the vicinity f the platfrm was nt bserved. 4 DISCUSSION The chemical analyses revealed n traces f discharged material at distances >250 m in the residual current directin. At 250 m the presence f discharged material culd be detected nly by elevated Ba cncentratins, but traces f il were visually bserved. This nce mre illustrates the patchy distributin f cntaminants in the sediment, even within grab samples. In ther directins such traces were nt even fund at 250 m. There seems t be an area f limited extent where the sediment is cntaminated. "D O <D C\J O (5 i s QC\4 O C Q»O 3.4. EFFECTS IN RELATION TO OIL CONCENTRATIONS The bilgical effects bserved at each f the statins investigated are illustrated in Fig. 15 in cmbinatin with the il cncentratins at these statins. An accumulatin f effects was bserved at bth the 100-m and the 250-m statins. This seems remarkable particularly fr the 250-m statin, since the chemical analyses did nt reveal il cncentratins that were significantly elevated abve backgrund level at this statin, althugh traces f il were psitively bserved. The ccurrence f bilgical effects at 500 m shuld prbably be explained as a lng-term cnsequence f disturbance f sediment cnditins in previus years, because traces f cntaminatin were nt bserved here any mre. At 25 m the number f effects was lw cmpared t the 100-m statin, but the chemical analyses as well as the field bservatins indicated that the il cncentratins at this statin were indeed lwer than at 100 m ???? species richness reduced mg il.kg dry sediment relative macrfauna abundance reduced >50% f the abundant sp. shw reduced numbers verall macrfauna abundance reduced dminant species ccur in reduced numbers sensitive species ccur in reduced numbers Fig. 15. Effects bserved at the residual current transect at varying levels f sediment cntaminatin.

19 1 TABLE 8 Evaluatin f the abundance patterns f 37 species sensitive t OBM cntaminatin and 4 pprtunistic species. tendency:+ = tendency fr higher abundance away frm the platfrm - = tendency fr lwer abundance away frm the platfrm 0 = n tendency fr a spatial gradient (?) = ttal number f specimens fund < 20 (Nte that the qualificatins are based n the abundance patterns f the individual species and nt n presence-absence data as used In lgit regressin). tendency A. Species vulnerable t OBM cntaminatin Mntacuta ferruginsa 0 (?) Scalibregma inflatum species nt fund Phle minuta species nt fund Amphiura filifrmis species nt fund Echincardium crdatum (>15 mm) + Mysella bidentata species nt fund Nephtys hmbergi 0 (?) Lumbrineris latreilli species nt fund Chaetzne setsa 0 (?) Owenia fusifrmis species nt fund Nucula turgida species nt fund Gattyana cirrsa species nt fund Harpinia antennaria species nt fund Lagis kreni species nt fund Glycinde nrdmanni 0 (?) Cylichna cilindracea species nt fund Harmthe lngisetis 0(?) Callianassa subterranea species nt fund Magelna papillicrnis 0 (?) Tellina fabula + Natica alderi 0 Spiphanes bmbyx 0 Ophidrmus flexusus species nt fund Ntmastus latericeus species nt fund Lumbrineris fragilis species nt fund Amphiura chiajei species nt fund Leucthe incisa 0 (?) Chaetpterus varipedatus species nt fund Tharyx marini species nt fund Ophiura albida 0 Gyptis capensis species nt fund Lanice cnchilega - Periculdes lngimanus 0(7) Diplcirrus glaucus species nt fund Abra alba species nt fund Turritella cmmunis species nt fund Sthenelais limicla 0 (?) B. pprtunistic species Nereis lngissima species nt fund Capitella capitata species nt fund Spi filicrnis 0 Anaitides grenlandica species nt fund A depth prfile at 100 m shwed that il culd be fund up t at least 20 cm in the sediment. It seems nt unlikely that deeper layers are als cntaminated, but the sampler did nt penetrate deeper than 20 cm in the sediment. The il cncentratins at 25 m and 100 m were cnsiderably lwer than during the first survey in 1985 at 25 m, when an extremely high cncentratin f 11,300 mg il-kg'1 dry sediment was fund (K u iper & G r e n e w u d, 198). Hwever, the data are nt cmpletely cmparable because analytical techniques have been cnsiderably imprved after Mrever the high cncentratin f 1985 was fund in the tp layer f the sediment (up t 8 cm depth), whereas deeper layers were nt sampled. It is nt clear, therefre, whether the il has been degraded, redistributed r stred in deeper sediment layers. Particularly the presence f il in deeper sediment layers may be lng-lasting, since the anaerbic cnditins in the deeper layers are unfavurable fr bidégradatin. A clear gradient f decreasing effects culd be bserved frm 100 m t 500 m, which was in crrespndence with decreasing cntaminatin levels ver this part f the transect. The 25-m statin did nt fit in this gradient and revealed an unexpectedly high number f taxa. It seems remarkable that there were 10 species, which were fund exclusively at this statin and nne f them was a knwn pprtunist. At all ther statins there were tgether nly 13 species that were uniquely fund at ne statin. Per statin that number ranged between 1 and 4. Apparently the 25-m statin represented a different fauna cmpsitin. This has t be explained mst likely by the different structure f the sediment at this statin. The depressin in the seabed as revealed by the depth recrdings and the presence f large amunts f ld shells strngly indicate that increased ersin f the seabed clse t the platfrm has remved the sandy tp layer f the sediment, s that banks f ld shells lying deeper in the sediment were uncvered. Ersin might als have resulted in transprt f discharged material away frm the platfrm, which culd explain the relatively lw il cncentratins bserved. As a cnsequence the changed seabed structure culd prvide a particular habitat, that was reclnized by macrfauna with a different cmpsitin. Adult specimens f Echincardium crdatum were relatively rare in the whle area and due t the lw numbers in the samples a pwerful statistical analysis f its abundance pattern was hampered. It is nt surprising therefre that lgit regressin detected n significant relatin between its frequency f ccurrence in the samples and distance t platfrm. Nevertheless it is remarkable that adult specimens f the species were absent up t 500 m in the residual current directin. During all previus surveys arund OBM lcatins in the Dutch sectr, where cuntings f E. crdatum included the assessment f size-frequency distributins, there was clear evidence that the radius t where specimens were absent was the largest in

20 17 the residual current directin and increased fr the larger size classes. A shrt-term study at lcatin L5-5 in 1990 (1.5 year after drilling, see Table 12) revealed the absence f the species up t 250 m, whereas small specimens (size class mm) turned up in the samples at 500 m and large specimens (>25 mm) nly ccurred at statins at >1000 m. In 1991 (3 years after drilling) juveniles had returned at 250 m, but specimens >25 mm nly ccurred at >750 m. At lcatin K12a (OBM drilling in 1984) E. crdatum were cunted during surveys between 1985 and Of the first three surveys ( ) nly data n numerical abundance are available, but fr the surveys f 1988, 1990 and 1992 size frequency distributins were als assessed (see Table 12). During all surveys E. crdatum was either absent r ccurred in cnsiderably reduced numbers at 100 m frm the platfrm. Between 1988 and 1992 it appeared that specimens fund at 100 m were always juveniles <10 mm and undubtedly the result f the current year's spatfall. At 250 m the species seemed t be almst absent in 1985,198 and 1990, but in 1987, 1988 and 1992 ttal numbers per n r were in a similar range as at the statins further away frm the platfrm. Hwever, specimens larger than 15 mm were hardly fund. Up t 1990 the largest size class (25-50 mm) was nly bserved at the 5000-m reference statin. This size class had returned in the area between 500 and 1000 m in 1992, 8 years after the discharges f OBM cuttings. Because f the lw numbers f large E. crdatum in the samples at K12a and Pb it is nt pssible t accurately define the distance t where the species was affected after 8 years and years respectively, but at bth lcatins this distance seems t be in the rder f 500 m. A mre detailed cmparisn f the 1985 and 1993 data n species level is hampered by the fact that the species cmpsitin was quite different between bth years due t strngly different average abundance levels f individual species. Since the majrity f benthic infauna species have a life-span that is prbably <1 year these differences shuld mst likely be explained by natural year t year fluctuatins in settlement and survival f new generatins. Hwever, a clear difference between the 1985 and the 1993 results can be fund in the number f significant gradients in individual species as detected by lgit regressin. In its basic frm lgit regressin detected significant gradients in 23 species in 1985, i.e. 72% f the ttal number f species tested (see Table 11). After crrectin f the test statistic fr pssible verdispersin there were still 8 species (25% f the ttal number tested) that shwed a significant gradient. In the present study nly 3 species (18% f the species tested) displayed a significant gradient accrding t the uncrrected test. After crrectin f the test statistic fr ver-dispersin there was nt any species fr which a gradient was significant. It is nted that verdispersin may bscure gradients and it shuld nt necessarily be cncluded that the absence f significant gradients indicates that the survival rates f individual species are n lnger affected by the riginal surce f disturbance, which wuld explain that there is n significant relatin between frequency f ccurrence f individual species and distance t platfrm. Nevertheless, the difference in the numbers f significant gradients in 1985 and 1993 can be cnsidered a clear indicatin that the impact n individual species has decreased during the years after the discharges were terminated. On the ther hand, particularly the very lw mean relative macrfauna abundance at 100 m and 250 m unmistakably indicates that there are still clear effects n the cmmunity as a whle. Cmpared t 1985 the extent f the area that was affected seems t have decreased. In 1985 bilgical effects were detected up t m (M ulder et al., 1987). In 1993 an accumulatin f effects was nly bserved at 100 m and 250 m. At 500 m nly the absence f large E. crdatum was indicative f a lng-term effect. Because n traces f discharged material were fund at this statin, the surce f disturbance, which in previus years may have eradicated the adult ppulatin f E. crdatum, seems t have disappeared. Nevertheless, it still may take several years befre such adult ppulatins will have recvered. Accrding t D u in e v e ld & J enness (1984) the age at which individuals reach a size f 30 mm shuld be estimated at =4 years. Therefre, the generatin f juveniles that was fund at the 500-m statin in 1993 (and which represented undubtedly recruitment f that year's spatfall) will reach that size nt befre Therefre, if a future fllw-up survey wuld be cnsidered, it might be advisable t plan such a survey in 1997, since recvery f adult ppulatins shuld nt be expected t ccur befre that year. 5 REFERENCES Annymus, Summary recrd f the 18th meeting f the wrking grup n il pllutin. The Hague, 8-11 Febr GOP-18/13/1-Rev.1-E. Daan, R., W.E. Lewis & M. M ulder, Bilgical effects f discharged il-cntaminated drill cuttings in the Nrth Sea. Brspeling lll-iv, NIOZ-rapprt , NIOZ, Texel, The Netherlands: Daan, R., W.E. Lewis & M. Mulder, Bilgical effects f washed OBM drill cuttings discharged n the Dutch cntinental shelf. Brspeling V, NIOZ-rapprt , NIOZ, Texel, The Netherlands: Daan, R., M. Mulder & W.E. Lewis, Lng term effects f discharges f washed and unwashed OBM drill cuttings n the Dutch Cntinental Shelf. Brspeling VI- VII, NIOZ-rapprt , NIOZ, Texel, The Netherlands: Daan, R. & M. Mulder, Lng term effects f OBM cutting discharges at a drilling site n the Dutch Cntinental Shelf. NIOZ-reprt : Duineveld, G.C.A. & M.l. Jenness, Differences in grwth rates f the sea urchin Echincardium crdatum as estimated by the parameter w f the Bertalanffy

21 18 equatin applied t skeletal rings. Mar. Ecl. Prg. Ser. 19: G renew ud, H. van h et, Mnitring the lng-term envirnmental impact f OBM drill cuttings discharged n the Dutch Cntinental Shelf, 1993: Sediment analysis. TNO-reprt (in prep.). G renew ud, H. van het & M. Schölten, Mnitring the lng-term envirnmental impact f washed and unwashed OBM drill cuttings discharged n the Dutch Cntinental Shelf, 1990: Sediment analysis and biaccumulatin. TNO-reprt IMW-R 92/412:1-40. Jngman, R.H.G., C.J.F. T er Braak & O.F.R. van Tngeren, Data analysis in cmmunity and landscape eclgy. Pudc, Wageningen, The Netherlands: Kuiper, J. & H. van het G renew ud, 198. Mnitren van de verspreiding en de bilgische effecten van lie rnd platfrms in de Nrdzee. TNO-reprt R85/350a: McC ullagh, P. & J.A. N elder, Generalized linear mdels. Chapman and Haii, Lndn, Mulder, M., W.E. Lewis & M.A. van Arkel, Effecten van liehudend brgruis p de benthische fauna rnd mijnbuwinstallaties p het Nederlands Cntinentaal Plat. Brspeling I, NIOZ-rapprt NIOZ, Texel, The Netherlands: 1-0. M ulder, M W.E. Lewis & M.A. van A rkel, Bilgical effects f the discharges f cntaminated drill cuttings and water based drilling fluids in the Nrth Sea. Brspeling II, NIOZ-rapprt NIOZ, Texel, The Netherlands: Z evenbm, W., M. Rbsn, L. M assie & L.O. R eiersen, Envirnmental effects f discharges frm ffshre il and gas industry in the Nrth Sea. GOPexpert grup reprt as a cntributin t the 1993 Quality Status Reprt f the Nrth Sea, 1-38.

22 19 Appendix PROCEDURE LOGIT REGRESSION The different variables and parameters are dented by the fllwing symbls: N = numbers f statins sampled = prbability f the species being present in a sample (0<rc <1 ) at the i,h statin (i = 1...,N) d = distance f the i,h statin t the platfrm n = number f samples at the ilh statin y = bserved number f samples in which the species is present at the i,h statin b0, b-, and b are mdel parameters L is the lg-likelihd f a mdel as a functin f the 7t 's and L = Z[(n -y )*ln(1-jti) + yi*ln(jtj)] Mdel (0) is given by n = [exp(b0)]/[1 +exp(b0)] Mdel (1) is given by jt = [expfb+b, *d )]/[1 +exp(b0+b1*d )] We nw calculate the chi-square statistic X2 = 2*(Lr L) where Lq is the maximum lg-likelihd fr mdel(o) and L, the maximum lg-likelihd fr mdel(1). The result is cmpared with the critical x2 value (a = 0.05, v = 1) t decide whether mdel (1) fits significantly better than mdel (0) r nt. The secnd part f the prcedure prvides a crrectin f the x2 statistic as defined abve fr pssible verdispersin in the data. T that end this statistic is divided by the dispersin parameter <D= 2*(L2-L1)/(N-2) where L2 is the maximum likelihd fr the full mdel. The resulting statistic is cmpared with the critical x2 value (a = 0.05, v = 1), t decide whether mdel (1) fits significantly better than mdel (0) r nt. It is nted here that < >is estimated frm the fit f the bserved values t the full mdel and mdel (1) and that the crrectin in fact intrduces additinal uncertainty f the test parameter than was initially present. All calculatins were perfrmed in SYSTAT.

23 20 Table 9. Grainsize distributin f the sediment at Pb (data frm Grenewud, ). Distance (m) %< 3 /m %> 3 ^m %> 90 <ml%> 125 A/m %> 180 pm %> 300 /m %> 500 /vm %> 1000 prr\ cm cm cm

24 21 Table 10. C ncentratins f il cmpnents at Pb (mg/kg dry sediment), data frm Grenewud, Statin 25 m 100 m 100 m 250 rr Cm pnent C C Naphtalene C C C C C C Pristane C Phytane C C C C C Other peaks (incl. UCM) Ttal m 500 m 100 m 0-2 cm 2-10 cm cm

25 22 Table 11: Lgit regressin: Values f maximum likelihds and test statistics befre and after crrectin fr verdispersin. A: Pb - September 1985 uncrrected test crrected test 2(L 1-4-0) sign sign.(%) M l 2 2(L 1-L-0 )/ <t> sign. (%) Etene lnga ns Anaitides maculata ns Nephtys cirrsa ns Glycinde nrdmanni Sclpls armiger ns Aricidea jeffreysi ns Pecilchaetus serpens ns Spi filicrnis Spiphanes bmbyx Sclelepis bnnieri ns ns Magelna papillicrnis ns Chaetzne setsa ns ns Ophelia limacina ns Travisia frbesi Owenia fusifrmis ns Lanice cnchilega ns ns Mntacuta ferruginsa Tellina fabula ns Natica alderi ns Prcessa parva ns Pntphilus trispinsus ns ns Iphine trispinsa ns Megalurpus agilis ns ns Atylus swammerdami ns ns Leucthe incisa Urthe pseidnis ns ns Bathypreia guilliams ns Bathypreia elegans Periculdes lngimanus ns ns Ophiura albida 0 + ns Echincyamus pusillus ns Echincardium crdatum B: Pb - September 1993 uncrrected test crrected test 2(L-, -L0 ) sign sign (%) M l 2 $ 2(L 1 -L0 )/ $ sign. (%) Nephtys cirrsa a ns Gniada maculata ns ns Sclpls armiger ns Aricidea minuta ns Spi filicrnis ns ns Spiphanes bmbyx ns ns Lanice cnchilega ns ns Dnax vittatus ns ns Tellina fabula ns ns Natica alderi ns ns Prcessa parva ns ns Urthe pseidnis ns ns Bathypreia guilliams ns ns Bathypreia elegans ns ns Ophiura albida ns ns Echincardium crdatum ns ns E. crdatum juv ns

26 23 Table 12. Densities f Echincardium crdatum (numbers per m2) at the residual current transect and a perpendicular transect fr 3 lcatins sampled between 1985 and (-= statin nt sampled).» L5-5 year distance: (m) r.c. transect p. transect ttal ttal > > > > ttal > > > > K12a year 1985-ttal ttal ttal ttal >10 mm >15 mm >20 mm >25 mm ttal >10 mm >15 mm >20 mm >25 mm ttal >10 mm >15 mm >20 mm >25 mm Pb year 1985-ttal ttal > > > >

27 24 Table 13. Data platfrm Pb, survey August Mean densities (n.m-2) Number f samples ( ) in which species are present. Tt. number f ind. per m2 per statin. Number f identified species. Distance t platfrm (m) Number f analysed samples POLYCHAETA Harmthe lngisetis 0.8 (1) 0.8 (1) 0.8 (1) Sthenelais limicla 0.8 (1) Pisine remta 0.8 (1) Etene lactea 2.5 (3) 3.3 (3) Anaitides maculata 3.3 (4) 0.8 (1) 0.8 (1) Anaitides spec. juv. 0.8 (1) 0.8 (1) Eumida sanguinea 0.8 (1) Nephtys hmbergii 2.5 (3) 5.0 (3) 0.8 (1) 0.8 (1) 0.8 (1) Nephtys cirrsa 71.7 () 48.3 () 40.8 () 30.0 ().7 () 53.3 () 80.8 () 90.8 () Nephtys caeca 2.5 (2) 0.8 (1) 0.8 (1) 0.8 (1) Glycera capitata 0.8 (1) Glycera spec. juv. 0.8 (1) Glycinde nrdmanni 0.8 (1) Gniada maculata 5.8 () 2.5 (3) 0.8 (1) 4.2 (2) 3.3 (4) 0.8 (1) 1.7 (2) 2.5 (2) Sclpls armiger 0.8 (1) 8.3 (5) 8.3 () 10.8 () 0.8 (1) 9.2 (5) Aricidea jeffreysii 1.7 (1) Aricidea minuta 2.5 (3).7 (4) 0.8 (1) 1.7 (1) 11.7 (5) Paranis fulgens 0.8 (1) 0.8 (1) Pecilchaetus serpens 4.2 (3) 0.8 (1) 0.8 ( 1 ) Spi filicrnis 7.5 (4) 5.8 (4) 50.8 () 80.0 () 1.7 (2) 1.7 (2) 0.8 (1) 4.2 (4) Spiphanes bmbyx 95.8 () 74.2 () 11.7 () 20.8 () 34.2 () 40.0 (5) () () Sclelepis bnnieri 0.8 (1) 0.8 (1) 1 Magelna papillicrnis 0.8 (1) 0.8 (1) 1.7 (2) 0.8 (1)B Chaetzne setsa 4.2 (3) 2.5 (3) Ophelia limacina 1.7 (2) 1.7 (2) 2.5 (1) Euznus flabelligerus 0.8 (1) Travisia frbesii 2.5 (1) 0.8 (1) 0.8 (1) Medimastus gracilis 4.2 (1) Lanice cnchilega (5) 3.3 (3) 3.3 (4) MOLLUSCA Arca lactea 0.8 (1) Mntacuta ferruginsa.7 (3) 0.8 (1) Dnax vittatus 0.8 (1) 1.7 (2) 7.5 (4) 3.3 (3) 0.8 (1).7 (5) 1.7 (2) Mactra crallina 0.8 (1) Spisula elliptica 4.2 (4) 0.8 (1) 1.7 (2) Spisula spec. juv. 0.8 (1)

28 25 Table 13. cntinued. Distance t platfrm (m) Number f analysed samples Tellina fabula 0.8 (1) 2.5 (2) 0.8 (1) 4.2 (4) 18.3 () 5.8 (4) 4.2 (2) Ensis ensis 0.8 (1) 1.7 (1) 4.2 (4) 1.7 (2) 1.7 (2) Ensis spec. juv. 0.8 (1) 0.8 (1) Thracia phaselina 0.8 (1) Trnus subcarinatus 0.8 (1) Natica alderi 5.0 (4) 0.8 (1) 0.8 (1) 3.3 (2) 2.5 (3) 2.5 (2) 5.0 (2) CRUSTACEA Crangn allmani 0.8 (1) Prcessa parva 3.3 (1) 5.8 (3) 15.8 (4) 28.3 (5) Pntphilus trispinsus 1.7 (2) Pntphilus spec. juv. 0.8 (1) Pagurus bernhardus 3.3 (3) 2.5 (2) Macrpipus spec. juv. 4.2 (3) 7.5 (2) 0.8 (1) 2.5 (3) 1.7 (2) 1.7 (2) Pinntheres pisum 0.8 (1) Thia scutellata 2.5 (3) 0.8 (1) 0.8 (1) 0.8 (1) Crystes cassivelaunus 0.8 (1) Decapda larven 1.7 (2) 0.8 (1) 1.7 (1) 0.8 (1) 1.7 (1) Gastrsaccus spinifer 5.8 (3) Schistmysis rnata 5.8 (4) Iphine trispinsa 2.5 (3) 0.8 (1) 0.8 (1) 0.8 (1) 0.8 (1) Diastylis bradyi 0.8 (1) 0.8 (1) 3.3 (2).7 (4) 3.3 (3) Megalurpus agilis 0.8 (1) 0.8 (1) 1.7 (2) 0.8 (1) Atylus swammerdami 1.7 (1) 1.7 (2) Atylus falcatus 2.5 (2) Hippmedn denticulatus 1.7 (2) Orchmenella nana 0.8 (1) 2.5 (1) 2.5 (2) Lepidepecreum lngicrne 0.8 (1) Leucthe incisa 2.5 (2) Sthenthe marina 0.8 (1) Sthenthe spec. 3.3 (1) Urthe pseidnis 40.0 () 9.2 (5) 3.7 () 22.5 () 15.0 (4) 17.5 (5) 5.0 (4) 31.7 () Bathypreia guilliamsniana 15.8 (5) 12.5 (5) 4.2 (3) 3.3 (3) 18.3 () 7.5 (5) 9.2 (5) 10.8 (5) Bathypreia elegans () () 1.7 (4) 41.7 () 3.7 () 19.7 () () () Periculdes lngimanus 0.8 (1) 0.8 (1) Synchelidium haplcheles 0.8 (1) 0.8 (1) 1.7 (2) 0.8 (1) 0.8 (1) 0.8 (1) Ara typica 0.8 (1) ECHINODERMATA Asterias rubens 0.8 (1) 0.8 (1) Ophiura texturata 0.8 (1) Ophiura albida 2.5 (2) 2.5 (2) 5.0 (3) 0.8 (1) 10.8 (2) 2.5 (3).7 (4) 8.3 (1)

29 2 Table 13. cntinued. Distance t platfrm (m) Number f analysed samples Ophiura spec. juv (5) 2.5 (2) 15.0 () 0.8 () () 40.0 (5) 50.8 (5) 5.0 (5) Echincardium crdatum 4.2 (4) 3.3 (4) 1.7 (1) 1.7 (2) Echincardium crdatum juv () 0.8 (1) 81.7 () () 00.0 () 85.8 () () () Echincyamus pusillus 0.8 (1) 1.7 (2) 0.8 (1) 5.0 (4) 2.5 (2) 0.8 (1) OTHER TAXA Nemertinea P () P (3) P (4) P (3) P (4) P (4) P (5) P (5) Nematda 0.8 (1) 0.8 (1) Amphixus 9.2 (4) 0.8 (1) 0.8 (1) Turbellaria 1.7 (1) Phrniden P (1) Harp, cpepda 0.8 (1) 1.7 (2) 0.8 (1) Oligchaeta 2.5 (2) 1.7 (1) Ttal nr. f individuals Nr. f identified species P=present (nt cunted)

30 NIO Z-RAPPO RT CONTENTS Summary and cnclusins... 1 Samenvatting en cnclusies Intrductin General p a r t Acknwledgements... 2 Methds Sampling Labratry a n a lyses Grainsize a n a ly s is Barium a n a ly s is Oil analysis Fauna analysis Statistical prcedures Individual species (lgit regressin) Macrbenthic cmmunity (relative abundance) Results Seabed characteristics Barium cncentratins in the sediment Oil cncentratins in the s e d im e n t Bilgical features Field b servatins General fauna d e scrip ti n Presence-absence data: lgit regressin Relative macrfauna abundance Abundance patterns f OBM sensitive and pprtunistic sp e cie s Effects in relatin t il cncentratins Discussin R eferences...17 Appendix...19