Modelling potential effects of petroleum exploration drilling on northeastern Georges Bank scallop stocks

Ecological Modelling 166 (2003) 19–39

Modelling potential effects of petroleum exploration drilling on northeastern Georges Bank scallop stocks Peter J. Cranford∗ , Donald C. Gordon Jr., Charles G. Hannah, John W. Loder, Timothy G. Milligan, D.K. Muschenheim, Y. Shen Department of Fisheries and Oceans, Maritimes Region, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada Received 6 November 2001; received in revised form 5 August 2002; accepted 19 February 2003

Abstract A set of numerical models was used along with laboratory and field observations to evaluate the potential risk of exploratory oil and gas drilling on northeastern Georges Bank to sea scallop (Placopecten magellanicus) stocks. The models were used to predict the drilling waste zone of influence and the impact of chronic exposure on scallop growth and reproduction. Growth and reproduction are generally considered to be the most important sublethal effects of chronic contaminant exposure. The highest near-bottom concentrations of drilling waste (water-based mud) from a hypothetical 92-day exploration well was predicted to occur along the side of the bank (>100 m depth). Laboratory information on drilling mud toxicity threshold concentrations indicated a potential for 0–48 days of growth inhibition depending upon the site, settling velocity of the mud, and area over which results are averaged. Scallop stocks on the side of the bank are relatively sparse, but dense aggregations are found in some areas and it is possible that changes in reproductive output could have detectable effects at the population level. Growth inhibition in the tidal front region, which has the densest scallop stocks, was predicted to be more localised and confined to a range of 0–15 days. Growth loss in the central mixed region (<65 m) was predicted to be negligible (<2 days). These results illustrate the importance of site location and waste settling velocity on potential effects. The magnitude of effects predicted at each site was closely related to bottom stress (u∗ ) as this determines how rapidly drilling mud reaching the seabed are redistributed and diluted both horizontally and vertically. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Offshore oil and gas; Drilling wastes; Sea scallop; Growth; Reproduction; Georges Bank

1. Introduction A long-standing environmental concern related to offshore oil and gas extraction is the impact of chronic, low-level stresses on marine ecosystems associated with the discharge of operational wastes (Boesch et al., 1987). The most important sublethal effects on adult ∗ Corresponding author. Tel.: +1-902-426-3277; fax: +1-902-426-2256. E-mail address: [email protected] (P.J. Cranford).

organisms exposed to chronic waste discharges, from both ecological and fisheries perspectives, are the impairment of growth and reproduction. Numerous studies on the potential hazard of operational drilling wastes to marine organisms have been conducted (Neff, 1987; GESAMP, 1993) but relatively few have addressed the actual risk of impacts occurring as this requires knowledge of the expected environmental concentration of wastes. Numerical models are valuable tools for evaluating the potential environmental impact of drilling activities as they provide a

0304-3800/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0304-3800(03)00100-5

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quantitative framework for integrating knowledge on the intrinsic physico-chemical properties of the different contaminants and the extrinsic processes that control their transport and fate in the environment. Applications of a set of models are presented which predict potential impacts of exploration oil and gas drilling on commercial sea scallop (Placopecten magellanicus) stocks on northeastern Georges Bank. Georges Bank, which straddles the United States–Canadian boundary, is one of the most productive fishing banks in the North Atlantic Ocean and supports the largest offshore scallop fishery in the world. Exploration drilling on Georges Bank is currently prohibited under moratoria in both countries. However, as hydrocarbon resources are believed to be extensive, further reviews of this policy are anticipated. High primary production on Georges Bank supports large populations of benthic suspension feeding invertebrates (Horne et al., 1989; Thouzeau et al., 1991). In the Canadian sector of Georges Bank (the Northeast Peak), bottom-dwelling invertebrates account for up to 70% of the total landed value of all resource species harvested, and the single most valuable fishery resource is the sea scallop. Scallops were targeted for this impact assessment because of their economic importance, and the availability of information on their sensitivity to drilling wastes. Benthic invertebrates have generally been the focus of studies on the potential impacts of drilling fluids (muds) and well cuttings as the bulk of these wastes sediment rapidly. Scallops are sedentary after the juveniles settle on the seabed and could be exposed to contaminants over the entire drilling period. As filter-feeders, scallops obtain their food particles (phytoplankton and detritus) from the benthic boundary layer (BBL). Resuspension of bottom sediments is ubiquitous on Georges Bank (Amos and Judge, 1991; Muschenheim et al., 1995), causing frequent reductions in the nutritional value of the near-bottom suspended particulate matter (Grant et al., 1997). Sea scallops partly compensate for this dilution of the food resource by exploiting the resuspended organic matter (Grant et al., 1997). Resuspension/deposition processes concentrate drilling waste particles in suspension near the seabed (Muschenheim and Milligan, 1996), where scallops could be affected by the chemical toxicity of contaminants, physical disturbance to feeding processes, and/or the presence of non-nutritious materials in their diet.

The high sensitivity of adult P. magellanicus to different types of used drilling muds and major constituents was shown in chronic exposure studies in which cohorts were exposed to low-levels of suspended wastes for up to 72 days (Cranford, 1995; Cranford and Gordon, 1992; Cranford et al., 1999). These laboratory studies showed that low-levels of drilling wastes can influence food utilisation, growth, reproduction and survival. Threshold concentrations of drilling mud causing significant impacts on somatic and reproductive tissue growth varied between 0.5 and 10 mg l−1 , with the greatest sublethal effects observed for a used “low-toxicity” mineral oil-based mud (Gordon et al., 2000). While this base-oil appears to exhibit chemical toxicity, drilling waste particles (primarily bentonite and barite) also physically interfere with feeding/digestion processes, resulting in growth inhibition (Cranford and Gordon, 1992). Well cuttings particles, which tend to be larger than drilling mud particles, had a relatively low impact on scallops. Exposures during gametogenesis (gonad development) tended to show a selective impact of drilling wastes on reproductive effort as opposed to somatic tissue growth. As part of a multidisciplinary program to improve scientific understanding of the fate and effects of operational drilling wastes, numerical circulation, waste dispersion and biological effects models were developed that can be used to predict the spatial and temporal extent of environmental impact zones around specific drilling sites on the continental shelf (Gordon et al., 1992). For the present application, numerical models for the transport and dispersion of suspended materials in the BBL (Hannah et al., 1995, 1996, 1998) were enhanced to include physical oceanographic information for Georges Bank, the results of field and laboratory studies on drilling waste particle dynamics, and information on operational discharge practices (Gordon et al., 2000; Loder et al., in preparation). This paper attempts to quantify the potential risk to the production of Georges Bank scallop stocks by integrating numerical model predictions of near-bottom waste concentrations with laboratory observations of sublethal effects of drilling wastes, and information on scallop stock distribution. Model predictions were used to explore how different oceanographic regimes contribute to the potential spatial and temporal extent of impact zones.

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2. Materials and methods Our procedure for modelling the potential effects of drilling wastes on scallops requires a model for predicting the concentrations of the drilling wastes and a model for converting the concentrations into effects on scallops. These models have data requirements. The required data include a drilling waste discharge scenario, an estimate of the fraction of the drilling waste that reaches the BBL, ocean currents, and exposure–response information for scallops. The models and the data are described here. 2.1. Modelling near-bottom drilling waste concentrations A set of models, referred to as benthic boundarylayer transport (bblt) models, have been developed to study the dispersion and transport of suspended sediment in the BBL of continental shelf environments. Basic model concepts, assumptions and exploratory applications are described by Hannah et al. (1995, 1996, 1998) and Loder et al. (in preparation). A brief overview is given here. Estimates of the current profile and bottom stress are combined with estimates of the vertical profiles of sediment concentration and vertical mixing to generate estimates of drift and dispersion. The sediment load is partitioned into discrete pseudo-particles or packets each with mass m and settling velocity w. The packets are advected horizontally and mixed vertically. Vertical mixing is represented by random exchange (shuffling) of the packets, which is controlled by a specified mixing time scale tm . The overall (horizontally averaged) vertical distribution of the sediment is assumed to be governed by an equilibrium concentration profile which is used to derive a probability density function for the vertical position of the packets. The concentration profile, c(z), is taken as c(z) = ca (a/z)p (Rouse, 1937), where z = 0 at the sea floor and is positive upwards, ca is the concentration at the reference height z = a, p = w/(κu∗ ), w is the settling velocity and the von Karman constant κ = 0.4. The friction velocity u∗ = (τb /ρ)1/2 where τ b is the magnitude of the bottom stress and ρ is the density of water. The bottom stress was based on a quadratic drag law, τb = Cd |ub |2 , where ub is the near-bottom current and Cd is a drag coefficient whose value depended on the height above the bottom

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of the near-bottom current (Cd = 0.005, 0.0025 for currents 1 and 10 m above bottom). The critical shear stress is take to be zero and the material is always in suspension. In addition, the Rouse profile was derived for an unstratified fluid and modifications have been made to allow the specification of a maximum height of the profile (hmax ) to account for the limiting influence of the water depth and seasonal stratification. The version of bblt used herein was the ‘local’ version which neglects spatial variability in the physical environment around the discharge site, but includes the effects of the horizontal currents, their vertical shear, and vertical mixing that are the primary factors in short-term dispersion and transport at the release site. The horizontal dispersion is generated by the interaction of the vertical shear and the vertical mixing. There is no explicit horizontal mixing in the model. This local version can be forced by either a measured time-varying current profile or profiles from 3-D circulation models. Preliminary applications of a spatially explicit version of bblt to Georges Bank showed that the horizontal current shears and the spatially variable bottom friction are important to the drift and dispersion of material on Georges Bank (Xu et al., 2000). However, the local version was shown to provide a good description of the drift and dispersion over the first few days and an accurate characterization of the different regimes on the bank. In addition, the spatially explicit version does not allow for time-varying sediment release and the current forcing only includes the seasonal mean and M2 tidal currents which does not contain the full spectrum of time variability of the currents and their vertical shears. Local bblt was used to predict the average concentration of wastes in the bottom 10 cm of the water column, the approximate layer from which scallops obtain their food particles, around nine hypothetical drilling sites (Fig. 1). Application sites were selected to represent the different summertime oceanographic regimes on Georges Bank (Loder et al., in preparation) and include the Hunky Dory and Growler sites identified in a 1987 drilling proposal by Texaco Canada Resources Ltd. One site is in the area on the top of the Bank (less than 65 m depth) that is vertically well-mixed year-round. Three sites are located in the area on the side of the Bank that is stratified during summer (greater than 100 m). Five sites are in

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Fig. 1. Maps of Georges Bank showing model application sites, bathymetry and oceanographic zones. Site names correspond with moored current meter stations (䊉) and calculation nodes from the 3-D circulation model (䊏).

the transition zone (tidal-mixing front) between the mixed and stratified side regions (65–100 m). Moored current measurements, collected at multiple vertical positions from six sites on the Northeast Peak (Fig. 1) during 1988–1989 (Loder and Pettipas, 1991; Loder et al., 1993) provided data used to force summertime bblt applications listed in Table 1. Year-round current records from 1994 to 1995 at the NEP site (Smith, personal communication) provided forcing data during summer and winter (Table 1).

Different start days were chosen reflecting alternative neap stages (minimal dispersion) of the fortnightly and monthly tidal modulation cycles. A 3-D seasonal and M2 tidal circulation model (Naimie, 1995, 1996) was used to provide current forcing for additional bblt model applications (Table 2). For each model application a series of sampling stations were located as follows. A preliminary bblt run was performed to assess the primary drift direction inherent in that time series. Sampling stations were then located at 0, 2, 5,

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Table 1 Summary of local bblt applications using observed currents and the hypothetical waste discharge scenario Region

Site

Sections 1–4 (first 62 days) Side GBFS1 Frontal

Water depth (m)

Season

Start day

f

Drift (◦ T)

155

Summer Summer Summer Summer Summer Winter Summer Summer

189 217 189 217 208 8 189 217

0.2 0.2 0.4 0.4 0.8 1.0 1.0 1.0

60 50 140 160 225 200 180 180

Summer Summer Summer Winter

189 189 208 8

0.2 0.8 1.0 1.0

50 140 225 200

GBFS2

67

NEP

73

GBFS4

63

Section 5 (last 50 days) Side GBFS1 Frontal GBFS2 NEP

155 67 73

Mixed

Each was run at two effective settling velocities (1 and 5 mm s−1 ). Oceanographic regions and site locations are indicated in Fig. 1 and daily releases of mud are summarised in Fig. 2. Start day is Julian day. The parameter ‘f ’ represents the fraction of wastes released at 10 m below the sea surface in sections 3–5 that is assumed to reach the BBL. Drift indicates the net direction of the sediment plume during the simulation.

and 10 km along four orthogonal directions including the drift line and up to eight additional locations were chosen at greater distances along the drift line and along a secondary axis based on drift patterns observed in preliminary runs. The average waste concentration in the bottom 10 cm was recorded every 30 min. The vertical shuffling time scale at the different sites was estimated using observations and models of the vertical mixing rates (Loder et al., in preparation) and the values of tm ranged from 3 to 8 h, where tm = 3 h maximises the dispersion due to M2 tidal currents

(Hannah et al., 1995). The maximum profile height, hmax , was estimated from observations and models of the bottom boundary-layer heights and ranged from 17 m at the deep sites to the full water column over the crest of the bank (Loder et al., in preparation). The hypothetical drilling waste discharge scenario used as input to bblt was prepared with the assistance of Texaco Canada Petroleum Ltd. In this scenario, the exploration wells are drilled using water-based muds in which the major solid components are bentonite clay and barite (barium sulfate), and a total of 468 Mt

Table 2 Summary of local bblt applications using currents predicted by 3-D model and the hypothetical waste discharge scenario Region

Site

Water depth (m)

Season

f

Drift (◦ T)

Side

GBFS1 GBFS1 Growler Hunky Dory

126 126 147 107

Summer Winter Summer Summer

0.2 0.4 0.2 0.2

65 60 190 5

Frontal

GBFS2 GBFS2 GBFS6 NEP ENEP SNEP

74 74 80 72 91 91

Summer Winter Summer Summer Summer Summer

0.4 1.0 0.4 0.8 0.2 0.2

145 90 161 200 148 253

Each application was run at two effective settling velocities (1 and 5 mm s−1 ). These simulations cover wastes released during the first 62 days only. Oceanographic regions and site locations are indicated in Fig. 1 and daily releases of drilling mud are summarised in Fig. 2. The parameters ‘f ’ and ‘drift’ are as in Table 1.

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Fig. 2. Daily drilling mud release used in Georges Bank model applications. This hypothetical waste discharge scenario represents an exploration well drilled using water-based mud.

of mud is released into the marine environment over a period of approximately 3 months. The amount of mud discharged in the scenario is in the range reported for the eight exploratory wells drilled on the US sector of Georges Bank in 1981–1982 (Neff, 1987). Drill cuttings were not included in the model simulations reported here because, as discussed before, these larger particles were observed to have low impact on scallops (Cranford et al., 1999). The drilling scenario is broken down into five separate sections (Fig. 2). During the first two sections (0–850 m depth), drilling muds were discharged directly at the seafloor. During the deeper three sections (850–4600 m), material was released at a depth of 10 m below sea surface. The largest discharges took place during the first week, but substantial bulk dumps occurred at the end of the final two sections. The discharge density generally was held at 1.075 kg m−3 for sections 1–4 and at 1.230 kg m−3 for section 5. A 50/50 mixture of bentonite and barite was assumed in all bblt simulations. Owing to the limited duration of some current records and computational considerations, waste drift and dispersion for the first 62 days (well sections 1–4 except for bulk discharge at end) and the last 25 days (end of section 4 plus section 5) were modelled separately using the same current time series. Since at least two-thirds of the discharge

in the BBL occurred during the first 62 days of the scenario, the focus of the impacts evaluation was on this period. During the first two sections of the well the mud is discharged at the sea floor and all of it is available for transport by the bblt model. However, during the final three sections the discharges are at 10 m depth. The fraction, f, of the discharged mud that reaches the BBL during these sections, was estimated as follows. Simulations were carried out using industry standard convective descent models to determine the depth of descent of the waste discharge plume under different discharge conditions, densities and environmental conditions (Andrade and Loder, 1997). The factors which significantly affected f were the mud density, water depth, BBL depth, initial downward volume flux of the discharge, current strength, and water column stratification. Estimates of f for the different application sites are given in Tables 1 and 2 and range from 0.2 in the stratified side region to 1.0 in the vertically mixed central region. The f value was assumed to be higher during winter based on the assumption that, in the absence of stratification, the BBL extends higher into the water column. Preliminary runs of bblt demonstrated that nearbottom waste concentration was highly dependent on

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particle settling velocity (w) (Hannah et al., 1995). This arises from the assumption of a Rouse-type balance between boundary-layer turbulence, parameterised by u∗ and w. While local variations in u∗ values are generally constrained to within an order of magnitude, the range of possible settling velocities for fine particulate matter can vary over three orders of magnitude. A major concern was whether barite and bentonite discharges, because of their significantly different physico-chemical properties (e.g. density, particle size spectra, ion exchange capacity) which can influence w, should be modelled separately or as a combined discharge. In choosing a representative range of w to use in these applications, we relied on (1) laboratory studies of drilling waste settling rates in both unflocculated and flocculated states (Milligan and Hill, 1998); (2) published literature values of in situ observations of settling rates of naturally flocculating material (Hill et al., 1998); (3) Stokes’ settling velocities for individual, unflocculated barite grains (Gibbs et al., 1971); and (4) field observations of drilling waste vertical distributions at the Cohasset and Panouk oil developmental sites on Sable Island Bank made between July 1991 and September 1993 (Muschenheim et al., 1995; Muschenheim and Milligan, 1996). From the field observations it was concluded that the settling behaviour in a combined suspension would be determined by flocculation processes dominated by the fine bentonite particle fraction. The settling velocity range for both the barite and bentonite fractions, therefore, becomes similar with an effective settling velocity of a 50/50 mixture of bentonite and barite under tidally-energetic conditions falling in the range of 1–5 mm s−1 (Gordon et al., 2000). As a result, total mud discharge was used as input to the bblt model, and each application was run twice using settling velocities of 1–5 mm s−1 to bracket the expected range. 2.2. Potential effects on scallop growth The basis for the biological interpretation of total drilling waste concentrations predicted by bblt are the results of laboratory toxicity experiments reported by Cranford and Gordon (1992), Cranford (1995) and Cranford et al. (1999). These experiments exposed sea scallops to different concentrations of various drilling

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wastes in raceway tanks and determined chronic lethal and sublethal effects, including effects on reproductive and somatic tissue growth. Considering that mineral oil-, and synthetic oil-based drilling fluid discharges are effectively prohibited in the offshore regulatory region including Georges Bank, and observations of the relatively low impact of cuttings particles on scallops, only the bentonite and barite biological effects data are relevant to the model applications in this study. Two kinds of effects thresholds were estimated from the bentonite and barite exposure data. The first is the zero growth concentration (C0 ) threshold, at or above which there is no tissue growth. The second is the no observed effects concentration (C1 ) threshold, at or below which there is no significant effect on scallop growth. For pure bentonite, zero growth was observed at 10 mg l−1 and no effect was detected at 2 mg l−1 (Cranford and Gordon, 1992). The effects thresholds had to be estimated for barite as laboratory experiments observed zero growth at 0.5 mg l−1 , the lowest concentration tested. However, this value was used as an estimate of C1 , as observed effects on the physiological components of growth indicated that growth would occur at barite concentrations below 0.5 mg l−1 (Cranford et al., 1999). The no effects concentration for barite was estimated at 0.1 mg l−1 by assuming the ratio C1 /C0 was the same as observed for bentonite. Recent unpublished laboratory observations of drilling waste impacts on scallops confirm this assumption (Cranford, unpublished data). The bblt simulations report the concentration of a composite drilling waste variable. Thus, the first step was to convert this into time series of barite and bentonitite concentrations. Based on the drilling waste scenario, the material was partitioned 50/50 into barite and bentonite components. Although the scenario indicates some variation in the proportion of each component with time, the variations were small and the use of a variable proportion of waste constituents would require additional model refinements. Sublethal effects on scallops at each site were estimated as the total time (days) that growth (somatic and reproductive) is inhibited. The calculation was based on a linear growth response between C0 and C1 , as was observed in laboratory experiments (Cranford et al., 1999), and additive effects of barite and bentonite. For a particular drill mud component x, the growth

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reduction index Rx due to that component was modelled as Rx = ax Cx + bx ; ax = (C1x − C0x )−1 ;
 C1x −1 x b = 1− x C0 where Cx is the concentration of component x, C0x is the no effects concentration for that component, and C1x is the no growth concentration. Rx is restricted to have values in the range [0, 1]; values less than 0 are set to 0 and values greater than 1 are set to 1. The total growth reduction index R = Rx and R is again restricted to the interval [0, 1]. The time-varying concentrations from bblt give rise to a time-varying R. The total potential growth days lost during a simulation of N time steps with M time steps per day is N Ri PGI = i=1 (1) M where M = 48 for the 30 min time steps used here. Potential growth inhibition (PGI) was computed separately for high (5 mm s−1 ) and low (1 mm s−1 ) effective settling velocities. Given the equal masses of barite and bentonite in the discharge scenario and the much lower value of C0 for barite, the PGI index is dominated by the barite in the simulations reported. However, the formulation presented here is quite general and can be used for more than two constituents. 3. Results 3.1. Near-bottom drilling waste concentrations To illustrate the predicted horizontal distribution of drilling mud discharges for the major oceanographic regions of Georges Bank, several contour plot ‘snapshots’ of near-bed concentration (mg l−1 ) were selected from among the model applications listed in Tables 1 and 2. More detailed results are available in Loder et al. (in preparation). Fig. 3 shows waste distributions on Day 12 at the end of the second well section after two large waste dumps at the seabed (Fig. 2), assuming a ws of 5 mm s−1 . The magnitude of predicted near-bottom concentrations of drilling waste was highly dependent upon the geographic location of the release (Fig. 3). Both the observed and

model current applications indicate that the predicted mean drift of the near-bottom drilling waste plume is generally along depth contours except over the Bank’s side where more variability in near-bottom drift direction occurs (Fig. 3, Tables 1 and 2). This pattern is consistent with the residual circulation. Results for the Growler site indicated that drift from the side of the Bank up into the frontal zone is possible under some conditions (Fig. 3, Table 2). The spatial patterns and near-bottom concentrations of drilling mud predicted by observed and modelled currents were similar in most cases (Fig. 3). This demonstrates that the 3-D circulation model used to force bblt at some sites and seasons provides reasonable current predictions. To simplify interpretation of the 528 time-series plots of near-bed concentrations from the 44 model applications listed in Tables 1 and 2, an index of the total potential “exposure” of benthic organisms to drilling mud was computed for each sampling location as the time-integrated  concentration over the duration of the simulation ( C t). Model results for sampling stations located along the primary drift line and the first 62 days of discharge are shown based on observed current forcing (Fig. 4) and 3-D model current forcing (Fig. 5). In general, the waste exposure index decreased rapidly over distances of 2–10 km from the release point and was very sensitive to the effective settling velocity selected for the discharged drilling mud (Figs. 4 and 5). There is some similarity of the spatial pattern and temporal variability in exposure concentrations predicted using ws values of 1 and 5 mm s−1 . However, the higher velocity resulted in concentrations that are about an order of magnitude greater than those at the lower velocity (Figs. 4 and 5). At the higher settling velocity, benthic organisms at the GBFS1, Growler and Hunky Dory drilling sites on the side of the Bank (>100 m depth) were predicted to be exposed to substantial waste concentrations as far as 20–40 km from the release point (Fig. 5). In the frontal region, exposure to high concentrations was more localised (within 2 km of the release point) and the sites with the highest potential exposure are NEP, GBFS2, and SNEP. Predicted near-bottom waste exposure was lowest in the mixed region of the Bank. Waste exposure predictions for different summertime starting dates (Julian Days 189 and 217) were generally similar at all sites (Fig. 4). Model-forced applications for sites in the frontal (GBFS2) and side

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Fig. 3. Snapshots of drilling mud concentrations (mg l−1 ) averaged over the bottom 10-cm of the water column on Day 12 (end of well section 2) of the hypothetical drilling waste release scenario (ws = 5 mm s−1 ). The release is at the origin for each site, and time-series ‘sampling’ positions are shown for each site as open circles. Application sites were chosen to be representative of the mixed, frontal and side regions of Georges Bank (Fig. 1) during different times of the year.

(GBFS1) regions of Georges Bank indicate that waste concentrations in winter are lower than in summer (Fig. 3). However, waste concentrations at NEP, where bblt was forced by observed currents, were higher in winter than summer (Fig. 4), in part due to reduced drift across the Northeast Peak in winter. The reduced winter concentrations at the GBFS2 site reflect the increased boundary-layer thickness associated with reduced stratification and increased vertical mixing in winter and are also expected for other frontal sites near the bank edge. The reduced winter concentrations at the GBFS1 site are associated with the stronger tidal currents in winter predicted by the 3-D model, and

are of uncertain reliability (although reduced concentrations are expected from amplified wave and wind influences in winter). 3.2. Potential effects on scallop growth The results of the model applications can be summarised as follows, grouped according to the physical oceanographic zone on the Bank in which the site is located (Fig. 1). The results were expressed as the total number of days of PGI during drilling of the exploration well sections assuming that scallops are present everywhere in the model domain and that growth is

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Fig. 4. Time-integrated near-bottom concentrations (total potential exposure) at various distances along the primary drift line at bblt model application sites (Fig. 1) forced by observed currents (Table 1) for the first 62 days of the drilling waste release scenario (Fig. 2). Results are shown for settling velocities (ws ) of 1 mm s−1 (left) and 5 mm s−1 (right). The starting date (Julian) of each bblt run is given in parenthesis by the site name. Note the different scales on the vertical axis.

continuous throughout the year. In actuality, as discussed later, scallops are very patchy in distribution and growth rate varies seasonally so that the predicted impacts of a given case depends upon the location of scallop beds relative to the release point and the tim-

ing of drilling. PGI estimates at sampling sites along the primary drift direction are shown for cases forced by observed (Fig. 6) and 3-D model currents (Fig. 7), and a ws value of 5 mm s−1 . PGI for each site and mud settling velocity, averaged for all sampling locations

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Fig. 5. Time-integrated near-bottom concentrations (total potential exposure) at various distances along the primary drift line at bblt model application sites (Fig. 1) forced by 3-D model currents (Table 2) for the first four sections (62 days) of the drilling waste release scenario (Fig. 2). Results are shown for settling velocities (ws ) of 1 mm s−1 (left) and 5 mm s−1 (right). Sites are identified by node numbers from the 3-D model and names are given in parenthesis. Note the different scale on the vertical axis.

within different distances from the release point, are given in Tables 3 and 4. The model applications for the mixed region on the top of the Bank (GBFS4) were forced by current meter data and the net drift of the sediment plume was to the south (Table 1). There was just one location where

growth inhibition exceeded 1 day and that was at the release point and this was indicated only at the higher settling rate (Fig. 6, Table 3). There was no growth inhibition predicted at any of the sites in the frontal region at the lower settling rate (Tables 3 and 4). At the higher settling rate, PGI

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Fig. 6. Summary plots of the total days of PGI along the primary drift axis during exploration drilling at the model application sites. Potential impacts are shown for the first 62 days (upper) and remainder (lower) of the drilling waste release scenario (ws = 5 mm s−1 ). Model forcing was by observed currents and the start day (Julian) is shown in parenthesis.

during the first 62 days of the drilling scenario ranged on the order of 2–18 days at the release point and 0–5 days within a 10 km radius (Tables 3 and 4). Growth lost during the remainder of the scenario was ∼35–45% of the 112 days total PGI (Table 3). The

area in which scallop growth is potentially inhibited for more than 10% of the 112-day discharge period was approximately 2 km2 for all frontal sites (Fig. 1) except at SNEP during summer and NEP during winter, which have potential impact zones (>10% PGI) of

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Fig. 7. Summary plots of the total days of PGI along the primary drift axis during exploration drilling at the model application sites. Potential impacts are shown for the first 62 days of the drilling waste release scenario (ws = 5 mm s−1 ). Predictions are shown for model sampling locations along the primary drift axis. The bblt model was forced by summer (September and October) and winter (January and February) currents predicted by the 3-D circulation model.

approximately 12 and 78 km2 , respectively. Unlike the NEP site, PGI at GBFS2 was lower in winter than summer (Table 4). Model applications for sites located in the stratified side region indicated that scallop growth inhibition may occur even if settling velocity is relatively low. For ws = 1 mm s−1 , PGI at the release point during the first 62 days of the drilling scenario ranged between

0.2 (Hunky Dory) and 9.5 (GBFS1) days. For ws = 5 mm s−1 , the predicted impact for this same area and drilling period was much higher and ranged between 14 and 32 days or 22–52% of the discharge period (Tables 3 and 4). Growth effects generally dropped with distance from the release point, but PGI estimates based on the higher settling velocity are on the order of 10 days as far as 40 km from the drilling site along

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Table 3 Total days of PGI for sea scallops (P. magellanicus) calculated from output from the local bblt model with observed current forcing Region

Site

Start day

PGI (days) 0 km

Sections 1–4 (first 62 days) ws = 1 mm s−1 Side GBFS1 GBFS1 Frontal GBFS2 GBFS4 GBFS2 GBFS4 NEP NEP ws = 5 mm s−1 Side Frontal

GBFS1 GBFS1 GBFS2 GBFS2 GBFS4 GBFS4 NEP NEP

189 217 189 189 217 217 208 8 189 217 189 217 189 217 208 8

Sections 1–5 (full 112-day discharge scenario) ws = 1 mm s−1 Side GBFS1 189 Frontal GBFS2 189 NEP 208 NEP 8 ws = 5 mm s−1 Side Frontal

GBFS1 GBFS2 NEP NEP

189 189 208 8

∗ 9.5

2 km

5 km

10 km

Drift line

0.3 0 0.7 0 0 0

5.8 3.7 0 0 0.3 0 0 0

4.7 2.7 0 0 0.2 0 0 0

3.4 2.0 0 0 0.1 0 0 0

3.2 1.7 0 0 0.1 0 0 0

∗ 32.5

∗ 19.8

∗ 15.7

∗ 12.7

∗ 12.8

3.7 4.0 0.5 1.9 1.9 ∗ 9.0

2.5 2.9 0.3 1.4 1.4 ∗ 6.8

1.9 2.3 0.2 1.1 1.1 5.4

2.7 3.5 0.2 1.0 1.4 ∗ 6.3

0.5 0 0

7.1 0 0 0

5.7 0 0 0

4.1 0 0 0

3.9 0 0 0

∗ 52.1

∗ 29.7

∗ 22.6

∗ 18.2

∗ 17.8

∗ 11.0

6.4 3.2 ∗ 16.4

4.3 2.3 ∗ 12.3

3.3 1.8 9.6

4.4 2.3 10.6

∗ 7.5

∗ 30.0 ∗ 10.7 ∗ 10.8

1.8 4.7 6.0 ∗ 18.4

∗ 11.7

∗ 18.6 ∗ 32.9

∗ 14.5

∗ 10.6

∗ 8.4

∗ 8.3

The different application site locations are indicated in Fig. 1. PGI for each application was averaged over a radius of 0, 2, 5 and 10 km from the discharge site, and along the primary drift line out to 30–50 km. An asterisk indicates that growth inhibition was greater than 10% over the simulated period.

the primary drift line at GBFS1, Growler and Hunky Dory during drilling of the first four sections of the well (Fig. 7). The spatial extent of sublethal effects at GBFS1 is smaller during winter (Fig. 7) and for the final well section (Fig. 6). When PGI estimates for GBFS1 are totalled for the two time periods and all values are averaged over sampling locations along the 50 km primary drift line, a total of 18 days of growth inhibition is predicted during the 112-day drilling scenario. Stations located off the drift line indicated a similar impact on growth out to a radius of about 2 km. Therefore, assuming the higher settling velocity, the potential area in which scallop growth is inhibited for

18% of the exposure period may be on the order of 150 km2 (π × 2 km × 25 km).

4. Discussion 4.1. Potential impacts on Georges Bank scallop growth The interpretation of the results from these bblt applications on Georges Bank depends upon several factors which include the location of the release site, the distribution of scallop stocks and the time of year

P.J. Cranford et al. / Ecological Modelling 166 (2003) 19–39

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Table 4 Total days of PGI for sea scallops (P. magellanicus) calculated from output from the local bblt model with 3-D model current forcing Season

Region

Site

PGI (days) 0 km

2 km

5 km

10 km

Drift line

1 mm s−1

ws = Summer

Side

Frontal

Winter ws = 5 mm s−1 Summer

Side Frontal Side

Frontal

Winter

Side Frontal

GBFS1 Growler Hunky Dory GBFS2 NEP ENEP SNEP GBFS6 GBFS1 GBFS2 GBFS1 Growler Hunky Dory GBFS2 NEP ENEP SNEP GBFS6 GBFS1 GBFS2

2.2 4.1 0.2 0.1 0 0 0.1 0 1.9 0 ∗ 23.0 ∗ 26.2 ∗ 14.1 ∗ 12.0

4.6 5.5 ∗ 13.8 ∗ 8.4 ∗ 13.2 5.4

0.5 1.8 0 0 0 0 0 0 0.5 0 ∗ 9.9

∗ 14.8

∗ 8.8

4.2 1.3 2.3 ∗ 7.9 3.0 3.7 1.3

0.3 1.2 0 0 0 0 0 0 0.3 0 ∗ 7.3

0.2 0.8 0 0 0 0 0 0 0.2 0

0.3 1.2 0 0 0 0 0 0 0.4 0

5.9

∗ 12.9

∗ 12.4

∗ 10.6

3.0 0.9 1.6 6.0 2.1 2.5 0.8

5.4 2.4 0.7 1.3 4.7 1.6 2.0 0.6

∗ 6.7

∗ 21.0 ∗ 10.8

4.7 1.0 2.3 ∗ 7.1 2.6 3.7 0.7

Predictions are for the first 62 days of the hypothetical discharge scenario at different applications sites indicated in Fig. 1. PGI for each application was averaged over a radius of 0, 2, 5 and 10 km from the discharge site, and along the primary drift line out to 30–50 km. An asterisk indicates that growth inhibition was greater than 10% over the simulated period.

at which the wastes are released. Release sites that were closest to high scallop densities tended to have a greater chance for impacts, but the impacts also depended on waste concentration and net drift direction. The three application sites on the side of the Bank, where bottom stress (and hence suspension) and dispersion are lowest, have the highest predicted near-bottom drilling waste concentrations, and therefore the highest potential for scallop growth impairment (Tables 3 and 4). The GBFS1 site is in an area of low scallop abundance (Fig. 8) and the net drift of the near-bottom discharge plume was predicted to be north east (Tables 1 and 2), generally away from the scallop beds. However, inclusion of spatial variation in the currents and depth at this site using a spatially variable version of bblt (Xu et al., 2000) indicates that drift and dispersion onto the bank may be underestimated with local bblt. Growler is also located in an area of low scallop abundance, but net drift at this site was predicted to be onto the Bank (Table 2) so some dis-

tant effects are possible. Hunky Dory is located in an area of moderate scallop density and wastes released at this site have a much greater potential of coming into contact with scallop stocks and affecting growth. Assuming a waste settling velocity of 5 mm s−1 , scallop growth could be inhibited for up to 17% of the drilling period at a distance of 40 km along the primary drift line. Given that the waste stream at this site can be up to 10 km wide (Fig. 3), this sublethal impact zone may exceed 200 km2 . The greatest scallop densities are located throughout much of the frontal zone with maximum catches recorded in the northern area near GBFS2 and GBFS6 (Fig. 8). High to moderate densities are also found near the ENEP and SNEP sites, but catches have been relatively low in the area around NEP. Potential scallop growth inhibition by drilling wastes within the frontal region appeared to be confined primarily to the release point (Figs. 6 and 7), where a maximum of 19 days of growth inhibition was predicted for the

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P.J. Cranford et al. / Ecological Modelling 166 (2003) 19–39

Fig. 8. Location of the bblt application sites on Georges Bank in relation to scallop stocks. Stock distributions were calculated based on average catch statistics from 1993 to 1997. The net direction of the sediment plume at each site during summer is indicated by the arrows.

GBFS2 site for the entire scenario (Fig. 6, Table 3). The results suggested less than 10 days of growth inhibition at distances of 10 km along the drift line at SNEP and NEP. Despite the presence of moderate scallop densities at GBFS4, this site on the top of Georges Bank (Fig. 8), has the lowest potential for impacting scallop stocks due to the high bottom stress and strong shear dispersion that rapidly redistribute and dilute, both horizontally and vertically, the wastes. 4.2. Confidence in model results There is a moderate to high degree of confidence in the reliability of bblt’s representation of the important physical processes which control sediment dispersion and transport. Realistic representations of spatial and temporal variability in bottom shear stress, horizontal currents and associated vertical shears, and vertical mixing are included (Hannah et al., 1995, 1998;

Loder et al., in preparation). The 3-D circulation model has been shown to provide a good representation of the predominant seasonal-mean and tidal flows on Georges Bank (Lynch and Naime, 1993; Naimie, 1995; Horne et al., 1996; Naimie, 1996). The present applications use the local version of bblt in which the physical conditions are uniform over the entire model domain, which can extend out from the release point as far as 50 km. In reality, physical conditions on Georges Bank can change markedly over distances of just a few kilometres, so confidence in model output drops with increasing distance from the release point. Evaluations using the spatially variable version of bblt indicate that local bblt generally tends to underestimate dispersion rates, and hence overestimate waste concentrations (Hannah et al., 1998; Xu et al., 2000). The additional influences of horizontal variability can result in reduced or increased concentrations (such as the increased on-bank drift noted for GBFS1) depending on the site.

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The effect of spatial variability in physical conditions around a particular site on the predicted near-bottom concentrations is considered to be relatively small compared to those from settling velocity. Laboratory experiments indicate that drilling wastes flocculate rapidly in seawater, and therefore have high effective settling velocities (Milligan and Hill, 1998). Observations at the CoPan production site on Sable Island Bank (34 m depth) indicate that drilling waste flocs are most concentrated near the seabed and can be seen as far as 8 km from the release point during development drilling (Muschenheim and Milligan, 1996). The expected range of settling velocities was estimated using measured drilling waste concentration profiles around the CoPan site, but it appears that these measurements did not fully resolve the dense mats of flocculated drilling wastes seen in video images. Thus, higher effective settling velocities (>5 mm s−1 ) are possible, which would increase near-bottom concentrations and scallop growth impacts by several fold above the present model predictions. However, this is unlikely to occur under the tidally-energetic conditions on Georges Bank as the size of drilling mud flocs would be limited by the relatively high turbulence levels (Milligan and Hill, 1998). The biological effects thresholds are based on the results of extensive laboratory experiments conducted with adult scallops. The zero growth threshold for barite had to be estimated but is reasonable based on measured physiological effects (Cranford et al., 1999) and growth measurements from recent chronic exposure experiments with barite (Cranford, unpublished data). An implication not considered here is how the suspension of natural sediments, which contains organic matter that may be exploited as food by sea scallops (Grant et al., 1997), interacts with the drilling wastes. There is also some uncertainty whether flocculation, which was limited in the laboratory experiments, influences the toxicity of drilling wastes. Considering that the larger water-based mud cuttings had a much lower impact on scallops than bentonite and barite, natural aggregation processes may mitigate the effects of fine particulate wastes on scallop feeding behaviour. This is suggested by observations that sea scallops exposed to aggregated bentonite in the laboratory did not reduce feeding rate (White, 1997) as was observed for scallops feeding on disaggregated bentonite (Cranford and Gordon, 1992). However,

35

field observations of sea scallops feeding on flocculated suspensions (Cranford et al., 1998) showed that the natural flocs were more fragile than the flocs prepared in the laboratory and are easily disrupted by the animal’s feeding processes. Once the flocs are disaggregated, the scallop would be exposed to a similar size spectra of particles as was presented in the laboratory experiments, and similar results are anticipated. As the ‘no observed effects’ threshold concentration for bentonite (2 mg l−1 ) was greater than the zero growth threshold for barite (0.5 mg l−1 ), all growth impacts predicted by the present model applications are attributed to barite concentration. Although barite was assumed to comprise 50% of the mass of the drilling mud, the proportion of barite can exceed 80% during drilling of the final well sections. The predicted impact on scallop growth during drilling of the final well section is, therefore, underestimated and could exceed that predicted for the first four sections. In summary, the greatest uncertainty in predicting impacts on scallop stocks is related to the vertical distribution of the drilling mud and the parameterisation of its settling velocity. The predicted effects on scallops that assume a settling velocity of 5 mm s−1 should be viewed as conservative. Such a conservative approach was recommended by Gray and Bewers (1996), who proposed that marine environmental risk management based on the Precautionary Principal should have a scientific foundation, with risk assessments formulated on adequately pessimistic assumptions regarding uncertainties in the prediction of effects. 4.3. Applications of bblt models and results The bblt models, coupled with biological effects data, are valuable quantitative tools that may be applied in environmental impact assessments, for designing environmental effects monitoring programs, to identify major processes controlling the zone of influence of drilling wastes, and for exploring the effectiveness of different impact mitigation options (e.g. changing discharge depth or drilling mud formulation). The model applications presented here demonstrate the usefulness of local bblt to address specific questions related to exploratory drilling at specific sites. The models have also been applied to assess the environmental impacts of oil and gas production drilling at offshore sites on the Scotian Shelf

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P.J. Cranford et al. / Ecological Modelling 166 (2003) 19–39

Fig. 9. Number of days of potential scallop growth inhibition (PGI) averaged at the discharge point for all application sites and times on Georges Bank as a function of bottom friction velocity (u∗ ). Friction velocity was averaged over the first 12 days of the exposure as the majority of impacts potentially occur during this period. The horizontal lines from each data point show the maximum range of u∗ values at the site over this period. The curve was fitted by eye.

(Hannah et al., 1995; MacLaren Plansearch, 1997) and have been implemented by industry as a generic tool for assessing the potential effects of exploration drilling on the Scotian Shelf, Slope and St. Pierre Bank (LGL, 1999). The models can also be used to explore other important unanswered questions such as the potential cumulative impacts of different effluent streams discharged from single or multiple drilling platforms. Potential model applications are not limited to petroleum extraction issues but have potential for use in assessing impacts from other activities, including the dumping of dredge spoils and seabed aggregate extraction, if information is available on flocculation, effective settling rate, and contaminant toxicity for the desired target species. Model results suggest that geographic information on bottom stress could be useful to environmental managers for screening drilling applications as these data would provide an indication of the potential risk of benthic impacts. To illustrate this, mean (over the tidal cycle) friction velocity (u∗ ) estimates for each model application site are plotted against the predicted impact on scallops (PGI at discharge site assuming ws = 5 mm s−1 ) in Fig. 9. Sites with high u∗ have a low potential for benthic impacts even when settling velocity is high (Fig. 9), as the drilling mud is pre-

dicted to be mixed high into the water column by the high bottom stress. The severity of impacts to benthic fauna is predicted to rapidly increase with decreasing u∗ values, but would decline once u∗ falls below the critical value for resuspension of drilling wastes (Fig. 9). A value of u∗crit for resuspension of sandy sediment from GBFS2 was estimated to be 2.14 cm s−1 (Grant et al., 1997). Although mean u∗ values for this site are less than u∗crit (Fig. 9), u∗ often exceeds this value during each tidal cycle, resulting in the frequent resuspension of sand and the silt-clay fraction (including drilling mud) accumulated between sand grains. Geographic variations in current regime may help explain why no correlation was observed between the measured spatial extent of benthic fauna effects around oil and gas fields and the amount and toxicity of drilling wastes discharged (Olsgard and Gray, 1995). 4.4. Implications of growth inhibition to scallop populations Interpretation of the predicted growth impacts at the population level requires knowledge of annual growth trends and the life history of sea scallops on Georges Bank. Scallops on Georges Bank display a semi-annual reproductive cycle, with spawning occurring in May–June and September–October (DiBacco et al., 1995). The autumn spawn is larger than the spring spawn and, while only mature gametes are released during the spring, the scallops are reproductively spent after the fall spawn. Gametogenesis is immediately reinitiated after spawning in the fall. Somatic weight tends to decrease during gametogenesis as accumulated energy reserves are utilised to support gonad growth, but increases outside the reproductive period and when food is abundant. Sea scallops appear to invest surplus energy primarily into the production of gametes such that reproductive effort increases only when conditions are favourable. As a result of this conservative strategy of controlled growth and opportunistic reproduction, interannual variations in environmental conditions greatly alter the timing (semi-annual or annual) and nature (synchronised or protracted) of spawning events on Georges Bank (DiBacco et al., 1995). Contaminant stress during gametogenesis, resulting from the presence of non-nutritious and/or toxic

P.J. Cranford et al. / Ecological Modelling 166 (2003) 19–39

drilling waste particles in the diet, can result in reduced gonad growth rates (Cranford and Gordon, 1992; Cranford et al., 1999) that would result in the production of fewer gametes (lower fecundity) and/or smaller ova having a reduced energy content. More severe stress can result in the resorption of gametes and impacts on somatic tissue growth (Cranford et al., 1999). It is likely that drilling wastes would have more effect on the scallop fishery through changes in fecundity (an impact not apparent in the fishery until reduced recruitment in future years) than in muscle size. However, impacts on somatic tissue growth can also affect reproductive success as the accumulation of carbohydrate and lipid energy reserves in the muscle and digestive gland is believed essential for the initiation of gametogenesis and the later maturation of gonad (Robinson et al., 1981). Considering that gametogenesis is nearly continuous on Georges Bank, exposure to drilling wastes would have some impact on fecundity and egg viability regardless of the time of drilling. The spring and summer are of greatest concern as the majority of annual gonad production occurs between March and August. The viability of eggs in adults exposed to drilling wastes may also be of concern as the potential consequences to larval survival could impact future year class strength. It is unlikely, however, that the scallops would release non-viable eggs, but would resorb and utilize the high nutritive content of some gametes to allow others to reach the critical size for spawning (DiBacco et al., 1995). Scallop populations from regions characterised by nutritive stress were observed to produce viable gametes even though reproductive effort was low (MacDonald and Thompson, 1986). Given a primary gonad growth period of 6 months, we estimate that the loss of 10–20 days of gonad growth during drilling of a single exploration well could reduce scallop fecundity by 5–10%. Although McGarvey et al. (1993) have demonstrated a correlation between egg production and recruitment for scallops on the Northeast Peak, it is unlikely that a 10% reduction in fecundity would be detectable in future stocks unless it occurred over a very large area in a region of abundant scallop stocks. The only region in the Canadian sector of Georges Bank that appears to meet these criteria is located on the eastern side of the Bank near the Hunky Dory application site (Fig. 8). Although it is difficult to predict how this level of

37

growth impairment would alter recruitment to the Georges Bank scallop fishery because of natural variability, the prediction that the sublethal effects impact zone may exceed 200 km2 at this site suggests that effects might be detectable at the population level.

Acknowledgements Funding for this modelling project was provided by the federal Program on Energy Research and Development (PERD) and the Department of Fisheries and Oceans. We thank the many members of the Georges Bank Steering Committee who provided input over the 6 years of the project. G. Tidmarsh of Texaco Canada Petroleum Ltd. provided the data upon which the drilling waste release scenario was based. These biological applications are dependent upon bblt and we thank E. Gonzalez and Z. Xu for their role in its development and for carrying out the Georges Bank applications. We thank S. Armsworthy for assistance with the biological effects calculations and G. Robert, and G. Black, for providing scallops stock distribution data. And finally, we thank B. Hargrave and P. Boudreau for constructive comments on the manuscript. References Amos, C.L., Judge, J.T., 1991. Sediment transport on the eastern Canadian continental shelf. Cont. Shelf Res. 11, 1037–1068. Andrade, Y., Loder, J.W., 1997. Connective descent simulations of drilling discharges on Georges and Sable Island Banks. Can. Tech. Rep. Hydrogen Ocean Sci. 185, vi, 83. Boesch, D.F., Butler, J.N., Cacchione, D.A., Geraci, J.R., Neff, J.M., Ray, J.P., Teal, J.M., 1987. An assessment of the long-term effects of U.S. offshore oil and gas development activities: future research needs. In: Boesch, D.F., Rabalais, N.N. (Eds.), Long-Term Environmental Effects of Offshore Oil and Gas Development. Elsevier Applied Science, New York, pp. 1–53. Cranford, P.J., 1995. Relationships between food quantity and quality and absorption efficiency in sea scallops Placopecten magellanicus (Gmelin). J. Exp. Mar. Biol. Ecol. 189, 123–142. Cranford, P.J., Gordon Jr., D.C., 1992. The influence of dilute clay suspensions on the sea scallop (Placopecten magellanicus) feeding activity and tissue growth. Neth. J. Sea Res. 30, 107– 120. Cranford, P.J., Emerson, C.W., Hargrave, B.T., Milligan, T.G., 1998. In situ feeding and absorption responses of sea scallops Placopecten magellanicus (Gmelin) to storm-induced changes in the quantity and composition of the seston. J. Exp. Mar. Biol. Ecol. 219, 45–70.

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magellanicus. M.Sc. thesis, Dalhousie University, Halifax, Nova Scotia. Xu, Z., Hannah, C.G., Loder, J.W., 2000. A 3D shear dispersion model applied to Georges Bank. In: Spaulding, M.L., Blumberg, A.F. (Eds.), Estuarine and Coastal Modeling. Proceedings of the Sixth International Conference, ASCE, New York, pp. 581–596.