Analysis of the Effects of Off Shore Mining

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284 Mar Ecol Prog Ser 122: 237-306, 1995

HEIDRUN 1988 SNORRE 1989

Fig. 4 . Biplot of DCA fauna1 ordinatio ns and th e 8 most importan t environmen tal variabl es within each field (base d on forwardselection in CANOCO).For Heidrun and Snorre fields the statlons are given as numbers, whlle for Togi and Mime fields the sta-tions are given as b ea nn g and direction from the platform (e .g. 180/500 is the station along the 180" bearing 500 m away from theplatform) . Not all stations are plotted to avoid overc rowded fi gures. All stations are given in the cor respondi ng MDS plots In Fig. 5

m--O

sition and the environmental variables were in most a more random rank order in the 2 methods due to

cases weak, there was a high deg ree of consistency in similarly low values.

th e 2 methods applied, in both weight an d rank within

fields. At the Snorre field depth and related variables

played a n important role, despite the fact that the varia- Development of contamination

tion in depth was relatively small (288 o 340 m) . At theMime field the TOC content was the principal environ- Contamination is usually regarded as a significant

mental variable, whereas the other variables showed increase in concentrat ion of a chemical over a back-

MIME 1990

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290 Mar Ecol Prog Ser 122:277-306, 1995

contaminated. By 1988 the area contaminated had

increased an d included station groups b, c an d d

(Fig. 6, Table 4) . By 1991 all stations differed from the

reference stations suggesting that the whole field was

contaminated out to 6000 m on all axes.

It is of interest to find which environmental vari-

a b l e ( ~ ) ere responsible for the observed grouping of

stations, especially that leading to contamination at the

outermost group b compared with the reference

group a (Fig.6) .Barium is a good tracer for dispersal of

drill-cuttings. In order to illustrate relations between

concentrations of Ba in the sediment an d the obtained

station groups (Figs. 6 to 8 ),values from the individual

stations, each containing 3 replicates, were plotted as

means with 95% confidence intervals (Fig. 9) . Th e

plots clearly show, sav e for Veslefrikk in 1990,a signif-

icant increas e in the concentrations of Ba towards the

platforms and that the outermost, uncontaminated

group a have significantly lower concentrations of Ba

in the sediment.

Yet it is not Ba alone that ch anges along the gradient.

Inspection of the environmental variable data-matrix,

and a superimposition of the environmental variables

(not shown) on the PC A plot, showed that the variables

-VALHALL 1991- f -

1 -- -- -- I -6

a b c d e f

VESLEFRIKK 1990

1

Stat ion groups from classif ication and PCA ordination

Fig. 9. Sediment concentrations of barium (Ba) with ~nhe station groups from classification and PC A ordination at the Valhall,

Gyda and Veslefrikk fields. Values given as m ean with 95% confidence intervals





29 2 Ma r Ecol P rog S e r 1 2 2 : 277-306. 1995

VALHALL 1985

VALHALL 1988

VALHALL 1991

S-,?,,.!-O lW k ,

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b

Fig. 10. Class ificat ion (left ) of s ta t~ on s t the Valhal l f ield accor ding to fauna1 composi t ion, MDS ordinat ion plots (ce ntre )with the

groupings obta ined f rom c l a s s if i cat i on, a nd s upe r impos it i on of t hes e s t a t ion groups o n the or ig ina l s ampl ing s t a t ions ( ngh t ) fo r

the yea rs 1985 , 1988 and 1991

Comparison of the pa ttern s of effects on the fauna

(Figs. 10 to 12) with the patte rns of co ntamination

(Figs. 6 to 8) showed that there were remarkably few

differences. In general the effects, with 4 categories,

closely rn~micked ontamination. In terms of ar ea , at

Valhall 1991 only 2 contaminated stations had u n-affected fauna (344"/4000 m and 6000 m) .At Gyda in

1993 only 1 station, station 225"/4000 m , was contami-

nated but h ad an unaffected faun a, wherea s at Vesle-

frikk in 1993, 2 stations were contaminated (260°/

1000 m and 330°/1000 m) and t he fauna was un-

affec ted. Effects of c ontaminatio n thu s closely followed

contamination and the areas affected were nearly

identical.Table 5 illustrates results from forward selection of

environmental variables in CANOCO and from the



Olsgard & Gray: Effects of offshore oil and gas activities on benthic communities

GYDA 1987

GYDA 1990

GYDA 1993

Fig. 11. As for Fig. 10. but for the Gyd a field in the years 1987, 1990 and 1993

BIOENV analyses for the Valhall, Gyda an d Veslefrikk

fields over time. Table 5 shows clearly that whereas t he

fauna at Valhall was already affected by oil-related

contaminants (Sr, Ba an d THC) in 1985, this was not so

at Gyda and Veslefrikk. At the latter 2 fields the sur-

veys done in 1987 correspond closely to baseline sur-

veys with low percentage values from the forward

selection in CANOCO and low r, values from BIOENV.Furthermore, the most important variables (Cu at

Gyda an d dept h at Veslefrikk) were not those typically

associated with oil-related effects such as Sr, Ba an d

THC, as a t Valhall in 1985.

In 1990 both th e Gyda a nd Veslefrikk fields showed

clear relationships to variables associated with drill-

cuttings (THC, Ba, and Zn) and both the p ercenta ge

variance values from forward selection and the r, val-

ues increased greatly from 1987 to 1990 at the 2 fields,

indicating clear relationships betwee n fauna1 composi-tion and sediment contaminat ion in 1990. In 1993 the

Gyda field had variable loadings comparable to those



29 4 Mar Ecol Prog Ser 122: 277-306. 1995

VESLEFRIKK 1987

VESLEFRIKK 1990

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Fi g 12. As for Fig. 10, but for th e Veslefr ikk f i e ld in the years 1987, 1990 an d 1993

Iis-' '

I

of Valhall i n 1988 and Veslefrikk i n 1990 with THC,

Ba and Sr as the most important variables in both

CANOCO and BIOENV

In 1991 at Valhall (Table 5a) the CAN OCO forward

selection suggested TOC, Zn, Pb, THC an d Sr a s the

most important variables related to the fauna whereas

BIOENV ranked the variables: THC, Sr, Zn, Ba andTOC. Overall the same 5 variables were ranked signif-

icantly, and all showed a high de gree of correlation

' j j r ~ i s ~ t ~ ;I U U U

a b c d e

with the f aunal composition. This shows that the differ-

ences between methods were small, and that in an

area clearly affected by drill-cuttings all the variables

related to the discharges were highly intercorrelated.

In 1993 at Veslefrikk (T able 5c) the key vari ables asso-

ciated with faunal changes were the heavy metals Cd,

Cu, Zn, Pb and the TOC in CANOCO and Fe, Cd, Cu,Pb an d Zn in BIOENV. Of t he 5 most signific ant only

TOC and Fe were not selected in both methods, again





296 Mar Ecol Prog Ser 122. 277-306, 1995

Table 6 Relat ~onshipsbetween envlronmental variables and fauna for 3 non-baseline surveys using forward selection in

CANOC O (left) and BIOENV in PRIMER ( ngh t) .Explanation as In Table 3

Oseberg S 1989 Gullfaks A 1989 Hod 1991 Oseberg S 1989 Gullfaks A 1989 Hod 1991

Ba 15 7*0 " TH C 29 9°u " THC 16 8"o" Ba r= O3 40 TH C r =O 5 0 1 B a r =0 4 1 5

THC 1 45' 1' ' Ba 25 9% " Ba 160'7" THC r = 0 338 Ba r - 0 492 Sr r = 0 373

S r 1 2 2 " , " Cu 24 240'' Sr 15 2@0" Sr r =O 1 9 1 C u r =O 3 8 9 TH C r = O 3 2 2

Depth 6 5'0"' Zn 225"/ 0" Pb 14 4"0" Depth r = 0 073 Fe r = 0 292 Cd r = 0 321

<63 pm 6 1%"' Pb 225% " Cd 12 0%" Zn r = 0 039 Zn r = 0 292 Pb r = 0 291

Max corr Max corr Max corr

Ba THC Depth THC Ba TOC Ba Sr THC Cd

r, = 0 373 r,= 057 1 Zn Mdq

r, = 0 552

VESLEFRIKK 1990 1;ymS

VESLEFRIKK 1993

I

Fig. 13. Correlation between environmental variables, as

shown by a PCA ordination plot of the envl ronmental vari-

able s measure d at the Veslefrikk field in 1987, 1990 and 1993.

Arrows that point in the same direction indlcate positively

correlated variables, perpendicular arrows ~ndl cat e ack of

correlation and arrows pointing in the opposlte direction indi-

cate negat~vely orrelated variables. In 1987 the expla~ned

variance along Axis 1 and 2 was 37.8% and 18 6% , respec-

tively The corresponding values in 1990 were 45.5% a nd

22.2%, and in 1993, 68 4% and 15.5%, respectively

that is often raised is are there species that cha ractense

the initial stag es of effects of pollution, an d ar e these

species simdar over large geographical scales, i.e. uni-

versal indicators of initial distu rban ce? Using the SIM-

PER program in PRIMER the species that charactense

the difference betwe en the unaffected group a an d the

just-affected grou p b were determi ned. Table 7 shows

lust the 3 species highest ranked in SIMPER that show

highest reduction in abundance from the uncontarn-nate d to the just-c ontami nated grou ps of stations

(althoug h there a re in fact many more th an 3 species

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Analysis of the Effects of Off Shore Mining

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