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A comparative study on the effects of barite, ilmenite and bentonite on four

suspension feeding bivalves

Maia F. Strachan 1, Paul F. Kingston

School of Life Sciences, HeriotWatt University, Riccarton, Edinburgh EH14 4AS, United Kingdom

a r t i c l e i n f o

Keywords:

Drilling mud

Bivalves

Suspension feeding

Barite

Ilmenite

Bentonite

a b s t r a c t

The impact of drilling mud components on the filtration activity and survival of bivalve molluscs was

investigated by exposing them to suspensions of standard barite, finely milled barite, ilmenite and ben-

tonite in sea water. Introduction of the components stimulated filtration activity in all four bivalves. In

addition, the introduction of standard barite and ilmenite both had lethal effects, with none of the

bivalves surviving the full duration of the experiments. In-vivo observations of the gill surfaces provided

direct evidence of physical damage caused by the administration of barite and ilmenite. A marked differ-

ence between filtration activity and survival of animals dosed with standard barite and fine barite sug-

gests that the observed effects were primarily caused by physical interference with gill function. The

results also suggest that the use of fine barite in offshore drilling may provide a more favourable environ-

mental impact profile than the use of ilmenite.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

Offshore disposal of drilling cuttings produced using any non-aqueous drilling muds has effectively not been permitted since

the turn of the century. The main driver for these regulations has

been the environmental impact of the base fluids used in the

mud formulation. Originally the base fluid was diesel oil. In an at-

tempt to find a solution to the toxic effects and persistence of these

fluids in the cuttings piles, this was later replaced by low-toxicity

mineral oils and then synthetic or vegetable based oils. Currently,

operators using non-aqueous drilling mud must either clean the

cuttings before discharge to

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wall of the borehole to prevent loss of fluids to permeable rock

strata.

Although it is clear that the chemical toxicity of the three mud

additives is very low and is unlikely to have any significant effects

on marine organisms, there is evidence that chronic intermittent

exposure of some species, for example the scallop (Placopecten

magellanicus), to dilute concentrations of operational drilling

wastes, characterised by acute lethal tests as practically non-toxic,

can affect growth, reproductive success and survival (Cranford

et al., 1999). There is also evidence that the presence of fine parti-

cles associated with drilling mud may have a physical impact on

some species of bivalves impairing their feeding mechanisms and

affecting their survival (Barlow and Kingston, 2001).

The work reported here investigates the impact of the drilling

mud components on the filtration activity of representative bi-

valves, by exposing them to prolonged periods in suspensions of

barite, ilmenite and bentonite.

2. Materials and methods

2.1. Experimental animals

Four bivalve species, Modiolus modiolus, Venerupis senegalensis,

Dosinia exoleta and Chlamys varia, were used in the experiments

and all collected from the west coast of Scotland.

M. modioluswas collected from either Loch Linnhe, 5633.8400N

and 524.8250W, or near North Ballachulish, Loch Leven,

5641.2690N and 510.2160W. V. senegalensis was collected from

low shore in South Shian, Loch Creran, 5631.2730N and

524.0060W.D. exoleta was collected from the Cregan Narrows in

Loch Creran, 5632.8390N and 517.2600W.C. varia was collected

from Loch Creran and near North Ballachulish, Loch Leven,

5641.2690N and 510.2160W. After collection the bivalves were

transported to an aquarium and left to acclimatise over several

days. The bivalves were fed the alga Tetraselmis chui daily.

The Mytilus edulis used for some of the in-vivo observations

were collected intertidally from the Firth of Forth, 5559.5360N

and 322.7950W.

2.2. Drilling mud components

Experiments were conducted using freshly milled standard

barite (barium sulphate); fine barite (a more finely ground form);

ilmenite (iron titanium oxide) and bentonite clay (sodium mont-

morillonite). The particle sizes of the different drilling mud compo-

nents were determined using the Mastersizer 2000 Laser Particle

Counter and are shown below. Barite (Fig. 1) contains the largest

particles, followed by ilmenite (Fig. 2), fine barite (Fig. 3) and then

bentonite (Fig. 4), with the smallest individual particles. Fig. 5

shows the particle size distribution of the natural sediment used

in the experiments. The sediment was collected from intertidalmud flats at Torry Bay, Fife, 563.4570N and 334.4040W.

Barite, fine barite and ilmenite have quite sharp and angular

edges whereas bentonite particles vary in shape from rough and

rounded to smooth and angular to small rod shaped particles.

SEM and light photomicrographs of the particles used in theexperiments (including the natural sediment) are shown in the

Supplementary Section Figures S.I.1S.I.7.

2.3. Experimental dosing system

The experiments were conducted in a specially designed recir-

culation system that was capable of delivering predetermined

amounts of fine particulate matter in suspension for extended peri-

ods of time. The flow-through system comprised a test tank, settle-

ment tank, sump, biological filter and pump. The test tank,

10010050cm2, provided suitable living conditions for the

experimental animals with seawater constantly being introduced

by sub-surface dispersal to minimize disturbance to the animals.

A dosing system allowed a precise amount of drilling mud compo-nent to be introduced in the form of a slurry. The slurry, consistingFig. 1. Particle size distribution of standard barite (median particle size 20.1 lm).

Fig. 2. Particle size distribution of ilmenite (median particle size 12.8 lm).

Fig. 3. Particle size distribution of fine barite (median particle size 13.2 lm).

Fig. 4. Particle size distribution of bentonite (median particle size 6.5 lm).

Particle Size Distribution

0.01 0.1 1 10 100 1000 3000

Particle Size (m)

0

1

2

3

4

5

6

Volume(%)

Fig. 5. Particle size distribution of natural sediment (median particle size 16.4lm).

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of 50 l of seawater to 1250 g of drilling material to be administered,

was mixed together in a header tank and kept agitated by means of

a stirrer, set at 700 rpm. A peristaltic pump transferred a metered

amount of slurry into the test tank water inlet at a rate calculated

to provide an even spread of a 2 mm covering over the tank base

over a 24 h period. This was verified by placing sediment traps

on the test tank bottom. Details of the construction of the system

are given byStrachan (2010).

2.4. Experimental procedure

Five individuals of each of the four test bivalve species were

placed in separate cages within the test tank and exposed to a

2 mm daily depth equivalent of the drilling fluid component over

a 28-day period. A daily dose of 2.0 mm depth equivalent per

day was chosen to reflect the approximate level of barite that could

be expected around 100 m from an active drill platform (Barlow

and Kingston, 2001). The dose rate was achieved by injecting

20 ml/min of slurry prepared as described above into the seawater

feed of the test tank.

Control experiments were also conducted in which no sus-

pended material was administered to the test animals, and in

which natural sediment of similar grain size range to the test mate-

rials was used.

2.5. Algal cell counts

Filtration activity of the bivalves was measured by determining

the number of cells of the alga, T. chui removed from suspension

over a period of 1.5 h. Cell counts were carried out using a Master-

sizer 2000 laser particle counter using laser beam obscuration,

each measurement following one minute of ultrasonic displace-

ment to ensure all algal cells were dispersed. The Laser particle

counter was initially calibrated by direct cell counts using a mod-

ified Fuchs-Rosenthal haemocytometer.

2.6. Pseudofaeces compensation

Preliminary observations indicated that the presence of sus-pended material in the water increased the rate of pseudofaeces

production by the bivalves. Pseudofaeces are rejected particles,

usually inorganics, encased in mucus and have not passed through

the gut. Pseudofaeces production is a well documented phenome-

non that results from the animals need to regulate ingestion in

turbid conditions (Velasco and Navarro, 2002). In the present

experiments the consequence of this was the introduction of addi-

tional particulate matter derived from pseudofaeces into the algal

feeding chambers. This caused underestimates of algal cell removal

because the anticipated drop in particle concentration (due to

ingestion of algal cells) was partially masked by the increase of

pseudofaeces particles. In some cases this resulted in an apparent

negative filtration rate. To compensate for this, pseudofaeces pro-

duction resulting from exposure to the suspended materials wasdetermined for each drilling mud component and each species

and a correction factor derived that was applied to the algal cell

readings. The correction factors used are given in Table 1.

2.7. In-vivo gill studies

In-vivostudies were carried out in which direct observation of

the gills was made during exposure to suspended drilling mud par-

ticles by means of fenestration of the shell and mantle.

A square hole of roughly 1.5 cm2 was cut into the shell of the bi-

valve and the mantle tissue was removed. A glass slide, fitted to the

shape of the opening, was attached over the opening using araldite

rapid resin. The bivalves were left for a week to allow acclimation.

Comparison of fenestrated and unfenestrated M. edulis and M.

modiolusused in the tests has shown that filtration activity is unaf-

fected by the procedure (Strachan, 2010). Ten individuals of each

species were used. The bivalves were then placed in the experi-

mental system with the different drilling mud components and

the structure of the gill observed directly and recorded using a Lei-

ca DC300 camera attached to a Leica MZ75 stereo microscope.

Table 1

Correction factors used for compensating for pseudofaeces introduction in algal

uptake determination.

Species treatment M. modiolus V. senegalensis D. exoleta C. varia

Particles/ml added into suspension

Sediment 3615 4620 1506 2063

Standard barite 3471 4131 1330 787

Fine barite 5772 2461 2228 3999

Ilmenite 1987 2362 3215 3689

Bentonite 9095 7806 9368 8246

Fig. 6. Filtration activity as indicated by mean number of algal cells removed from suspension after 1.5 h filter feeding by four bivalve species kept in clear seawater (n= 5).

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3. Results

3.1. Control experiments

The bivalves kept in uncontaminated water followed a similar

filtration activity pattern over each 28-day experimental period

in which there appears to be an irregular cycle of peaks and

troughs in the data for all species (Fig. 6).

A two-way ANOVA indicated a significant statistical difference

between algal uptake on consecutive days (p-value < 0.001).

There was also a significant difference (p-value < 0.001) between

the filtration activity ofC. varia and M. modiolus, and betweenD.

exoleta and V. senegalensis (p-value < 0.001). These differences

probably reflect the range of size of the species used in the

experiments with M. modiolus being the largest and C. varia thesmallest.

3.2. Sediment exposure experiment

Fig. 7 shows the mean filtration rates of the bivalves, over a

28-day period, when exposed to a daily 2 mm depth equivalent

of natural sediment. All four species follow a similar filtration

pattern over the entire experimental period in which there was

an initial decrease in filtration activity of the four species, probably

in response to the initial addition of the sediment. Subsequent

filtration activity became less erratic indicating that the four

bivalve species had become acclimatised to the sediment and

adapted to the influx of the particulate matter. All individuals

survived the full test period.

A two-way ANOVA found a consistent significant difference in

filtration activity between species (p-value < 0.001) after the first

10 days at which point it was possible to roughly rank each speciesin order of filtration activity.

Fig. 7. Filtration activity as indicated by mean number of algal cells removed from suspension after 1.5 h filter feeding by four species of bivalves treated with suspendednatural sediment (n= 5).

Fig. 8. Filtration activity as indicated by mean number of algal cells removed from suspension after 1.5 h filter feeding byM. modiolus(n= 5) treated with barite, fine barite,

ilmenite, bentonite and natural sediment.


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3.3. M. modiolus

M. modiolus showed a considerable variation in its filtration

capacity in the presence of the different drilling mud components(Fig. 8), with a significant difference (p-value < 0.001) between the

treatments. Within the first 7 days of exposure, the filtration

capacity ofM. modiolus was particularly erratic, and no obvious

pattern between treatments. Subsequently filtration capacities

became less erratic, with bentonite stimulating the highest level

of activity, followed by barite, fine barite and sediment, ilmenite

with the controls showing the lowest levels filtration activity. It

should be noted that although bentonite stimulated the highest

filtration rate it also produced the largest fluctuations of the filtra-

tion activity from day to day. The ilmenite exposed M. modiolus

filtered at the closest level to the control individuals.

All animals survived the entire experimental period in the case

of the bentonite, fine barite and natural sediment treatments. In

the case of the ilmenite and standard barite treatments therewas variable mortality with all the individuals dead by day 19.

Survivorship curves for M. modiolus exposed to different drilling

mud components can be found in the Supplementary Section,

Fig. S.I.10.

3.4. D. exoleta

The results of the trials onD. exoletaare shown in Fig. 9. As with

M. modiolus, a statistically significant difference (p-value < 0.001)

was found between the overall filtration capacities of the bivalves

exposed to the different drilling mud components over the whole

test period. However,Fig. 9shows that, as in the previous experi-

ment, within the first 7 days there is a marked short term variation

in the filtration activities, particularly of individuals treated with

bentonite, natural sediment and the controls (no suspended

material added). After day seven the addition of bentonite ap-

peared to stimulate the overall highest filtration over the test

period although great fluctuations in the filtration activity were

still apparent. Also after day seven, the control, sediment and finebarite exposed individuals filtered the algal suspension at the

Fig. 9. Filtration rates as indicated by mean number of algal cells removed from suspension after 1.5 h filter feeding by Dosinia exoleta (n= 5) treated with barite, fine barite,


Fig. 10. Filtration rates as indicated by mean number of algal cells removed from suspension after 1.5 h filter feeding by V. senegalensis(n= 5) treated with barite, fine barite,


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closest levels, although still statistically significantly different

(p-value < 0.001). Ilmenite stimulated an overall higher filtration

rate than barite until the mortality of all individuals under test

by day 11 in contrast to M. modiolus, in which the barite had the

greater effect.

There was a 100% survival rate for the control and sediment ex-

posed D. exoleta. Survival was compromised with the addition of

bentonite, ilmenite and barite. Bentonite was responsible for a

20% decrease in individuals on day 25. By the end of the test period,

on day 28, no more individuals died.D. exoletasurvived for only 10

and 12 days when in the presence of barite and ilmenite respec-

tively. Ilmenite provoked the worst reaction with survival instantlybeing compromised. Only 80% of the individuals remained alive

before completion of the first 24 h of the experimental period.

The first mortalities ofD. exoleta exposed to barite were recorded

on day five. Survivorship curves for D. exoleta over the trial period

may be found in theSupplementary Section, Fig. S.I.13.

3.5. V. senegalensis

Fig. 10displays the filtration capacity ofV. senegalensisexposed

to standard barite, fine barite, ilmenite and bentonite. There was a

statistically significant difference (p-value < 0.001) between the fil-

tration rates of the bivalves between treatments over the entire

test period. As with the previous experiments the first 7 daysshowed wide fluctuations in the recorded filtration rates for all

Fig. 11. Filtration rates as indicated by mean number of algal cells removed from suspension after 1.5 h filter feeding by C. varia (n= 5) treated with barite, fine barite,


Fig. 12. (ab) Gill ofM. edulis kept in clean sea water, (a) lower part of gill, (b) food groove showing algal string. (cd) Gill ofM. edulis exposed to a suspension of standard

barite for 6 days, (c) gill margin showing edge damage, (d) detail of gill margin showing marginal lesion and spanning mucus string.

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treatments, including the controls. After day seven a general pat-

tern is evident with bentonite stimulating the highest filtration

of the algal suspension followed by sediment, and then the fine

barite and controls. The fine barite and control V. senegalensisboth

have similar low filtration capacities from day eight onwards,

although the asynchronous variation resulted in a statistically sig-

nificant difference (p-value < 0.001) between rates. Barite stimu-

lates a higher filtration rate than ilmenite in V. senegalensis.

The control and sediment exposed V. senegalensis both had 100%

survival. The individuals exposed to both fine barite and bentonite

had a 60% survival rate at the end of the 28-day experimental per-

iod. Fine barite and bentonite initially compromised survival on

days five and four respectively. Survival of V. senegalensis in the

presence of ilmenite was compromised on day two and none re-mained alive past day nine. Barite affected survival on day six

and was responsible for 100% death on day 12. Fig. S.I.14 in the

Supplementary Sectionshows the survival rates ofV. senegalensis

when exposed to the different drilling mud components.

3.6. C. varia

Fig. 11 illustrates the filtration activity ofC. variaexposed to the

different drilling mud components. Where survival rates permit-

ted, statistical analysis of the data indicated a significant difference

between the filtration rates recorded for each of the treatments (p-

value < 0.001). As with the three previous species, after day seven,

bentonite stimulated the highest filtration rate. Again there was

great day-to-day variation in filtration rates. The filtration activity

ofC. varia exposed to both barite and ilmenite both decreased fromday one until their mortality on days four and five respectively.

Fig. 13. Gill ofM. modioluskept in clean sea water. (a) Gill surface, (b) gill margin.

Fig. 14. Gill ofM. modiolusexposed to a suspension of standard barite for 6 days. (a) Gill surface showing damage to inter-filamental ciliary junctions, (b) part of fragmented

outer demibranch.

Fig. 15. Gill ofM. modiolusexposed to a suspension of ilmenite for 6 days: (a) outer demibranch showing filament separation, (b) separated and entangled gill filaments.

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There was 100% survival of the controls, sediment, bentonite

and fine barite exposedC. varia. Survival ofC. variain the presence

of both barite and ilmenite was compromised on day three. By day

four none of the barite exposed individuals remained alive and the

ilmenite exposed ones were all dead by day five. Survival rates of

the C. varia exposed to the different drilling mud componentscan be found in theSupplementary Section, Fig. S.I.15.

3.7. In-vivo observations

In-vivoobservations allowed an insight into the natural filtra-

tion mechanism of the gill structure of the bivalve species. Pre-

and post-exposure observations were made on M. edulis and M.

modiolususing standard barite, ilmenite and bentonite

Figs. 12(a-b) and 13(ab) shows photographs of gill filaments of

bothM. edulisandM. modioluskept in uncontaminated conditions

and show regular comb like patterns, filaments of even length with

no evident damage. In Fig. 12b a string of algae can be seen passing

between the tips of the inner and outer demibranch in M. edulis

and inFig. 13b the inter-lamellae junctions ofM. modiolusgill ap-

pear regular, positioned at a similar vertical height between each

filament.

The gills ofM. edulis and M. modiolus both show evidence of

physical damage to the delicate gill tissues when exposed to sus-

pended barite for a period of 6 days. Fig. 12c shows damage at

the edge of M. edulis gill filaments in several locations and

Fig. 12d a detail of the most damaged area with a mucus string

spanning the lesion. Irregularities can be seen in the gill filaments

ofM. modiolus in Fig. 14a and major damage to an outer demi-

branch in Fig. 14b in which the surface area of the demibranch

has been reduced by over a half. Damage to the gill structure of

M. modiolus is also apparent in individuals subjected to suspen-

sions of ilmenite ranging from displaced inter-lamellae junctions

to disassociation and entanglement of the gill filaments (Fig. 15a-

b).Fig. 16shows the gill surface of M. modiolus exposed to sus-

pended bentonite and indicates minimal damage to the gill fila-

ments with occasional displacement the inter-lamellae junctions.

Further photographic evidence of damage to the gills ofM. modio-

lusis shown in the Supplementary Section (Figs. S.I.16S.I.17).

4. Discussion

Control experiments in which the four bivalves species were

kept in uncontaminated seawater and fed daily on the standard-

ized algal diet showed that they had a similar pattern of filtration

activity throughout the 28-day test periods (Fig. 6). There appearedto be an irregular cycle of peaks and troughs in the data for all spe-

cies. Such cycles of activity have been reported for other bivalve

species (Cranford and Gordon, 1992; Navarro and Velasco, 2003;

Vaughn and Hakenkamp, 2001) which may be related to endoge-

nous feeding rhythms related to the tidal cycle (Morton, 1970).

Although the pattern of activity was similar, there was a significant

difference in their overall rates of algal uptake. Filtration rates ofC.

variaandD. exoleta significantly differed from M. modiolus andV.

senegalensis, probably a function of the overall size of the individ-

uals used in the experiments.

The introduction of natural sediment as a suspension in the

water stimulated filtration activity in all four bivalves (Fig. 7). This

is a well documented response reflecting the ability of bivalves to

adapt their feeding behaviour to natural fluctuations of suspended

material in the surrounding water (Bayne et al., 1993; Foster-

Smith, 1975). As well as increasing the filtration activity, the addi-

tion of suspended sediment also increased the day-to-day variabil-

ity observed. A similar and, in most cases greater, increase in both

filtration activity and variability resulted from the addition of the

drilling mud components.

Standard barite and ilmenite both had lethal effects on the sus-

pension feeding bivalves, with none surviving the full duration of

the experiments. Generally, ilmenite provoked the greatest reac-

tion in all four species. M. modioluswithstood the presence of both

barite and ilmenite better than the other three species. C. variahad

the most sensitive reaction with total mortality of all test individ-

uals within 4 days of commencement of dosing.

Comparison of the effects of ilmenite and standard barite on fil-

tration activity was difficult in some species because of the short

time span the test animals survived the experiments. M. modiolus

andV. senegalensis had a higher filtration capacity in the presence

of barite than ilmenite. Linked with the higher filtration capacity is

the production of a larger quantity of pseudofaeces. The correction

Fig. 16. Gill ofM. modiolus exposed to a suspension of bentonite for 6 days: (a)gill surface showing displacedciliary junctions, (b) detailof gill surface showing damageto gill

filaments.

Table 2

Survivorship of four bivalve molluscs kept in suspensions of drilling mud components equivalent to a settlement rate of 2 mm day for a period of 28 days. Survival values given in

days, figs. in parentheses are % surviving (n= 5).

Species No sediment Natural. sediment Standard. barite Ilmenite Fine barite Bentonite

M. modiolus >28 >28 18 18 >28 >28

D. exoleta >28 >28 9 11 >28 >28 (80%)

V. senegalensis >28 >28 11 8 >28 (60%) >28 (60%)

C. varia >28 >28 3 4 >28 >28

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factor used for compensating for pseudofaeces introduction into

the algal suspension showed that both M. modiolusandV. senegal-

ensisproduced a larger quantity of pseudofaeces in the presence of

barite than ilmenite (seeTable 1). The presence of standard barite

and ilmenite produced a different result forD. exoleta.D. exoletafil-

tered the algal suspension at a higher rate and produced more

pseudofaeces when exposed to ilmenite in comparison to barite.

The barite exposedM. modiolus

andV. senegalensis

produces filtra-

tion activity at a closer level to the natural sediment levels with the

ilmenite rate closer to the controls (no suspended material). Again,

D. exoleta followed a different pattern. The D. exoleta dosed with

standard barite filtered the algal suspension at a rate closer to

the control levels with the ilmenite levels closer to those of the

natural sediment.

Pseudofaeces are rejected particles, usually inorganics, encased

in mucus and have not passed through the gut (see Section2.6).

They are produced as a result of the animals need to regulate

ingestion in turbid conditions (Velasco and Navarro, 2002). It is

known that excessive quantities of mucus are produced by bivalves

dosed with relatively high levels of suspended material (Urrutia

et al., 2001)), possibly as a damage limiting response. This is dis-

cussed more fully byStrachan (2010)who also suggests that dam-

aged gills may prevent the separation of the unwanted particles

from the wanted algal cells resulting in the expulsionof both mate-

rials as pseudofaeces leading to an increased filtration activity to

compensate for the loss of uptake of organic material.

In-vivo observations of the gill surfaces provided direct evidence

of considerable damage caused by the administration of barite and

ilmenite (Figs. 1215) and provided strong evidence that both bar-

ite and ilmenite disrupted the structure of the gills, in some in-

stances removing parts of the tissue or causing separation of the

gill filaments and tangling. Although no quantitative measure-

ments were made of the physical damage, it appears that, whilst

both barite and ilmenite treatment was associated with displace-

ment of inter-lamellae junctions; ilmenite appeared to cause

shredding and tangling of the gill lamellae (Fig. 15). This damage

was most likely the cause of the early mortality of animals treatedwith standard barite or ilmenite.

The results of this study indicate that standard barite and

ilmenite can have a demonstrable deleterious effect on the ability

of bivalve molluscs to filter feed and cause major damage to the

ctenidia (gills) which are their main feeding organs. Although in-

vivo observations on the gills of animals treated with fine barite

were not carried out, algal uptake observations indicated that this

material was also disrupting filtration activity, as indeed was nat-

ural sediment. In-vivo observations on bentonite treated bivalves

showed only minor damage in the form of displaced inter-lamellae

junctions (Fig. 16). As already discussed, dosage with ilmenite and

standard barite resultedin early mortality of all test animals with-

in the experimental period. However, the majority of animals trea-

ted with bentonite, fine barite and natural sediment survived theexperiments (seeTable 2).

The marked difference between filtration activity and survival

of animals dosed with standard barite and fine barite is surpris-

ing and suggests that the observed effects were primarily caused

by physical interference with gill function. In this case survival

appears to be related to particle size distribution, the finer the

material, the greater the survival rate. This conclusion is supported

by the observations of Cheung and Shin (2005), working with

natural sediment, who found that mechanical damage caused to

the gill filaments of green-lipped mussels, Pernia viridus, was less

the smaller the size of the particles administered.

Particle size distribution analysis showed that the greatest sim-

ilarity of particle size distributions was between ilmenite and fine

barite (see Figs.2 and 3), with median particlediameters of approx-imately 13lm. The standard barite used in these experiments had

a median particle diameter of 20 lm(Fig. 1), almost twice that of

the fine barite and ilmenite. If physical factors are the prime driver

for the observed effects, then it would be more appropriate to com-

pare the effects of ilmenite with that of fine barite rather than that

of the coarser standard barite. This suggests that the use of fine

barite in offshore drilling operations would provide a much more

favourable environmental impact profile than the use of ilmenite.

Bentonite is a thickening agent added to a drilling fluid to im-

prove its thixotropic properties (see Section 1). It is another major

ingredient in water base drilling mud and was included in this

study mainly for that reason. Filtrationactivity of all animals tested

was the most erratic in suspensions of bentonite, but survival rate

was very good suggesting that bentonite is unlikely to be a limiting

factor in determining extent of effect when drilling muds contain-

ing standard barite or ilmenite are discharged to the sea bed.

Whilst the results of these experiments provide a useful way of

comparing the effects of drilling mud weighting agents on suspen-

sion feeding animals and possibly ranking them as environmental

stressors, caution should be exercised in applying the results to

field conditions. Suspension feeding species are likely to be the

most sensitive to the effects of suspended material (which was

one reason why they were chosen for this study). Also the sus-

pended solid regime to which they were subjected represents a

worst case scenario, simulating conditions at up to 100 m distance

from a point of continuous discharge of drilling waste over a period

of 28 days. In addition, although every precaution was taken to en-

sure the test animals were in good condition, they were artificially

maintained in a laboratory based re-circulating sea water system

that was likely to have placed the animals under additional stress

during the test periods. Preliminary field experiments carried out

by Strachan (2010) in which a layer of barite 4 mm thick was

placed on the sea bed in a closely defined area showed differences

in the benthic communities barely distinguishable from those in

which natural sediment had been administered to the same depth.

Acknowledgements

The authors would like to thank Oil and Gas UK (United King-

dom Offshore Operators Association) for funding this study and

providing valuable advice and help through their Offshore Environ-

mental Monitoring Committee. We are also grateful to Margaret

Stobie and Robert Rennie for their help in the design and construc-

tion of the experimental tank system and Dan Harries for advice

and help, particularly with the field work. We are also grateful to

Dan Harries, John Hartley and Niall Bell for their helpful comments

on the manuscript.

Appendix A. Supplementary data

Supplementarydataassociated with this articlecan befound,in the

online version, at http://dx.doi.org/10.1016/j.marpolbul.2012.06.023.

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