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Login: Register窗体顶端Athens/Institution LoginNot Registered?UserName:Password: RemembermeonthiscomputerForgottenpassword?窗体底端窗体顶端Athens/Institution LoginNot Registered? User Name: Password: Remember me on this computer Forgotten password? 窗体底端HomeBrowseSearchMy SettingsAlertsHelp窗体顶端Quick SearchTitle,abstract,keywordsAuthore.g.j s smith Journal/booktitleVolumeIssuePage窗体底端59 of 370Ecological Modelling Volume 166, Issues 1-2 , 1 August 2003, Pages 19-39 This DocumentSummaryPlus Full Text + Links Full Size ImagesPDF (552 K) ActionsCited BySave as Citation AlertE-mail ArticleExport Citationdoi:10.1016/S0304-3800(03)00100-5 Copyright 2003 Elsevier Science B.V. All rights reserved. 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 and 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; revised 5 August 2002; accepted 19 February 2003.; Available online 21 May 2003. AbstractA 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 (100m depth). Laboratory information on drilling mud toxicity threshold concentrations indicated a potential for 048 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 015 days. Growth loss in the central mixed region (65m) 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. Author Keywords: Offshore oil and gas; Drilling wastes; Sea scallop; Growth; Reproduction; Georges Bank Article Outline1. Introduction 2. Materials and methods 2.1. Modelling near-bottom drilling waste concentrations 2.2. Potential effects on scallop growth3. Results 3.1. Near-bottom drilling waste concentrations 3.2. Potential effects on scallop growth4. Discussion 4.1. Potential impacts on Georges Bank scallop growth 4.2. Confidence in model results 4.3. Applications of bblt models and results 4.4. Implications of growth inhibition to scallop populationsAcknowledgements References1. IntroductionA 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 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 and 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 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 StatesCanadian 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 and 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 and 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 and 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 10mgl1, 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, Hannah et al., 1996 and Hannah et al., 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 and 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. 2. Materials and methodsOur 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 exposureresponse information for scallops. The models and the data are described here. 2.1. Modelling near-bottom drilling waste concentrationsA set of models, referred to as benthic boundary-layer 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, Hannah et al., 1996 and Hannah et al., 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/(u*), w is the settling velocity and the von Karman constant =0.4. The friction velocity u*=(b/)1/2 where b is the magnitude of the bottom stress and is the density of water. The bottom stress was based on a quadratic drag law, b=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 of the near-bottom current (Cd=0.005, 0.0025 for currents 1 and 10m 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 10cm 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 65m 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 100m). Five sites are in the transition zone (tidal-mixing front) between the mixed and stratified side regions (65100m). (26K) Fig. 1. Maps of Georges Bank showing model application sites, bathymetry and oceanographic zones. Site names correspond with moored current meter stations () and calculation nodes from the 3-D circulation model (). Moored current measurements, collected at multiple vertical positions from six sites on the Northeast Peak (Fig. 1) during 19881989 ( Loder and Pettipas, 1991 and 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 and Naimie, 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, and 10km 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 10cm was recorded every 30min. Table 1. Summary of local bblt applications using observed currents and the hypothetical waste discharge scenario Each was run at two effective settling velocities (1 and 5mms1). 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 10m below the sea surface in sections 35 that is assumed to reach the BBL. Drift indicates the net direction of the sediment plume during the simulation.Table 2. Summary of local bblt applications using currents predicted by 3-D model and the hypothetical waste discharge scenario Each application was run at two effective settling velocities (1 and 5mms1). 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.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 8h, where tm=3h 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 17m 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 i