Relating effects of nonaqueous drilling fluid, wat - CyberLeninka

Marine Pollution Bulletin 110 (2016) 520–527

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Benthos response following petroleum exploration in the southern Caspian Sea: Relating effects of nonaqueous drilling fluid, water depth, and dissolved oxygen R.D. Tait a, C.L. Maxon b,⁎, T.D. Parr c, F.C. Newton III a b c

b

Exxon Mobil Corporation, 22777 Springwoods Village Parkway, Spring, TX 77389, USA Maxon Consulting, 2546 San Clemente Terrace, San Diego, CA 92122, USA Terry Parr Environmental Consulting Services, 7924 Grape Street, La Mesa, CA 91924, USA

a r t i c l e

i n f o

Article history: Received 16 January 2016 Received in revised form 25 February 2016 Accepted 26 February 2016 Available online 11 July 2016 Keywords: Hypoxia Benthic macrofauna Nonaqueous drilling fluid (NADF) Linear alpha olefin (LAO) Caspian Sea Drill cuttings

a b s t r a c t The effects of linear alpha olefin (LAO) nonaqueous drilling fluid on benthic macrofauna were assessed over a six year period at a southern Caspian Sea petroleum exploration site. A wide-ranging, pre-drilling survey identified a relatively diverse shelf-depth macrofauna numerically dominated by amphipods, cumaceans, and gastropods that transitioned to a less diverse assemblage dominated by hypoxia-tolerant annelid worms and motile ostracods with increasing depth. After drilling, a similar transition in macrofauna assemblage was observed with increasing concentration of LAO proximate to the shelf-depth well site. Post-drilling results were consistent with a hypothesis of hypoxia from microbial degradation of LAO, supported by the presence of bacterial mats and lack of oxygen penetration in surface sediment. Chemical and biological recoveries at ≥200 m distance from the well site were evident 33 months after drilling ceased. Our findings show the importance of monitoring recovery over time and understanding macrofauna community structure prior to drilling. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Studies on impacts to aquatic environments associated with oil and gas exploration and production have been conducted over the past few decades, with many focusing on benthic community effects from discharge and accumulation of drill cuttings and nonaqueous drilling fluid (NADF) on the seafloor (Kingston, 1992; Neff et al., 2000; Schaanning et al., 2008). Effects include physical smothering and acute toxicity, as well as habitat alteration, such as organic enrichment, changes to sediment particle size, and increased oxygen demand. The most widely used synthetic NADFs are esters, poly alpha olefins, and olefin isomers, including linear alpha olefin (LAO). These fluids tend to be less toxic and degrade faster in aquatic environments than their petroleum-based predecessors and hence, should present less risk to the receiving environment. The short-term toxicity of synthetic drilling fluids is well characterized in laboratory studies using standard test organisms (Still et al., 2000; Munro et al., 1998), as are in situ effects of ester-based drilling fluid discharged in offshore environments (Roberts and Nguyen, 2006). However, information is lacking on the

⁎ Corresponding author. E-mail addresses: [email protected] (R.D. Tait), [email protected] (C.L. Maxon), [email protected] (T.D. Parr), [email protected] (F.C. Newton).

effect mechanism and subsequent recovery over time of benthic macrofauna from LAO-based fluids and their associated drill cuttings. This study assesses effects on macrofauna associated with discharged cuttings with adhered LAO at a single well site located at 145 m water depth, approximately 45 km off the Azerbaijan coastline in the southern Caspian Sea. Exploration drilling commenced in July 2001 and ended in January 2002. The well was drilled to a target sub-seafloor depth of approximately 6700 m using water-based drilling fluid in the top sections and LAO fluid for the deeper well sections. Barium, in the form of barite, was used in both sections as the weighting agent. Results are examined from an August 1998 regional pre-drilling survey and two focused postdrilling surveys conducted in September 2002 and November 2004 (Fig. 1). The pre-drilling macrofauna community is described in Parr et al. (2007) as a relatively diverse macrofauna, numerically dominated by amphipods, cumaceans, annelid worms, and gastropods at shelf depths (b150 m), compared with increasingly hypoxic habitats dominated by a few species of annelid worms and crustaceans at deeper depths. Results for the 2002 post-drilling survey, conducted eight months after well completion, are reported in Tait et al. (2004). The present study expands on the 1998 regional pre-drilling and 2002 post-drilling studies by including 2004 results to examine LAO degradation and macrofauna recovery 33 months after drilling ceased, and to propose and evaluate the hypothesis that the primary effect mechanism on macrofauna at the well site was hypoxia from the microbial degradation of LAO.

http://dx.doi.org/10.1016/j.marpolbul.2016.02.079 0025-326X/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

R.D. Tait et al. / Marine Pollution Bulletin 110 (2016) 520–527

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Fig. 1. Regional and survey area maps showing location of Nakhchivan contract area (left), 1998 pre-drilling stations (middle), and 2002 and 2004 post-drilling station locations (right). Shallow (120–170 m) pre-drilling stations used in post-drilling comparisons are identified (□, middle).

2. Study design and methods 2.1. Study design The 1998 pre-drilling survey sampled sediments 30 months before the start of drilling in the northern half of the Nakhchivan contract area, which included the future well site location (Fig. 1). Fifty-six sediment samples were collected over a depth range of 67–692 m at 36 stations, including six stations with 3–5 field replicates to establish small-scale (within-station) estimates of variance to aid post-drilling sampling design. The well site location, which was designated later, was not sampled. Post-drilling studies were designed to quantify spatial changes in macrofauna, sediment chemistry, and sediment particle size in relation to drilling operations and discharged cuttings. The post-drilling design was based on the spatial variability of macrofauna estimated from the 1998 regional dataset and hydrodynamic modeling (Nedwed et al., 2004), which predicted that LAO retained on drill cuttings would settle within 400–600 m of the well site, without strong directional bias due to persistently weak subsurface circulation. Comparison of pre-drilling mean results for nine key macrofauna variables showed no statistical differences (one-way ANOVA, p N 0.05) and homogeneity of variance (Levene, 1960) (p N 0.05) between tightly grouped station replicates and more distant non-replicated stations sampled within the same depth range as the well site (data not shown). Therefore, stations located at 800–1000 m from the well site at depths of 120–170 m could be used as a within-survey reference group to evaluate effects from the drilling-related discharge. Twenty-five post-drilling sediment samples were collected in 2002, and 15 of those stations were re-sampled in 2004 (Fig. 1, right). In 2002, five stations each were located along five transects (N, NE, SE, SW and NW) at distances of 50 m (well site), 200 m, 400 m, 600 m, and 800 m (reference) from the well site. In 2004, only the N, SE, and SW transects were re-sampled, following 2002 results indicating near omnidirectional settling of discharged cuttings. The design was modified slightly in the 2004 post-drilling survey, resulting in the replacement of an 800m station located at a depth of 192 m (station 17), with a 1000-m station located at a depth of 135 m (station 24), similar to the well site depth. Station 17 results were excluded from the 2002 dataset. In 2000, 2002, and 2004 dissolved oxygen (DO) measurements from the water surface to within 5–10 m above the seafloor were profiled at 29 locations, ranging from 0 to 80 km of the well site. Near-bottom water DO at the 145-m deep well site was above the hypoxia upper limit of 2 ml l− 1 (Diaz and Rosenberg, 1995), ranging from 4 to

6.5 ml l−1. Concentrations at or below 1 ml l−1, the lower limit of hypoxia (Levin, 2003), were measured in near-bottom waters N600 m. Combined results showed stable near-bottom concentrations ranging from N7 ml l−1 DO at shallow stations, grading into hypoxic conditions at stations deeper than 400 m (Fig. 2). Regional pre-drilling and focused post-drilling sediments were analyzed for grain size, sediment organic carbon, macrofauna, 41 polycyclic aromatic hydrocarbons (PAH), nC9–nC44 saturated hydrocarbons (SHC), and 14 metals. Sediment profile imaging (SPI) photographs and analysis of LAO were conducted on post-drilling sediments only. SPI provided information on the horizontal and vertical extent of settled cuttings, evidence of bacterial mats, and estimates of reduction–oxidation potential discontinuity (RPD), a surrogate for the depth of oxygen penetration in surface sediment (Rosenberg et al., 2001). Except for total PAH, post-drilling chemical results, including sediment organic carbon,

Fig. 2. Dissolved oxygen concentrations in near-bottom water measured in the southern Caspian Sea over a four year period. Hypoxia upper limit (2 ml l−1) predicted at ~400 m bottom depth.

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Fig. 3. SPI photos of representative 2004 reference sediment (left) and post-drilling sediment at 200 m from the well site in 2002 (middle) and in 2004 (right). Dashed lines represent the average apparent RPD layer. White bacterial mats on surface of settled cuttings are visible in 2002 photo.

were commensurate with pre-drilling results at similar depths, and therefore, excluded from the present study. Summed results (e.g., total PAH = sum of 43 analytes, LAO = sum of 6 analytes) were evaluated, reducing the original data set from over 400 variables to ten macrofauna and two chemical variables (Fig. 4). Statistical analyses of macrofauna were limited to general community indicators of total abundance, species richness (no. of taxa per 0.1 m2 grab sample), dominance (total no. of taxa with the highest abundances that comprise N50% of total abundance per 0.1 m2 grab sample), and abundances of higher taxonomic groups, rather than genus or species.

3. Results 3.1. SPI photographs Estimated RPD depths were absent or extremely shallow (≤0.5 cm) in well site sediments, reaching reference depths (2.5–3.4 cm) at distances of 400 m in 2002, and generally at 200 m in 2004. Colonies of a dense white bacterium, resembling Beggiatoa sp., observed in all 2002 well site images and three 200 m images, were completely absent in 2004. Differences between 2002 and 2004 sediment conditions at ≤ 200 m from the well site are represented in photos taken at postdrilling station 4 (Fig. 3). White mats covering black, refractive cuttings and adhered muds in 2002 transitioned to 1–4 cm deep deposits of light brown sediment in 2004. The majority of 2004 SPI photographs at ≥200 m from the well site displayed evidence of active bioturbation in the top 4–5 cm, with burrows extending beyond a depth of 5 cm in several images, most likely from the polychaete Nereis diversicolor, a relatively large deeper burrowing organism.

2.2. Field and laboratory methods Sediment collection methods were consistent across surveys, and are described in Parr et al. (2007), which presents results for macrofauna (0.1 m2 grab, 500 μm sieve), sediment particle size, and organic carbon from the 1998 pre-drilling study. Laboratory methods are described in Tait et al. (2004) for all parameters except LAO, which was quantified as C14–C18 parent compounds and the sum of corresponding isomers by GC with a flame ionization detector using a modification of the SHC method described in Douglas et al. (1994). Benthic macrofauna were identified to the lowest practical taxonomic level (usually genus or species) and enumerated. Species vouchers (preserved in 70% ethanol) are held by the Benthic Invertebrate Collection, Scripps Institution of Oceanography, University of California, San Diego.

3.2. Sediment LAO and PAH Mean LAO (log-transformed) was significantly elevated at the well site and at the 200 m station group compared to reference stations in 2002 and 2004 (Table 1). LAO concentrations ranging from 3740 to 25,200 μg g−1 decreased exponentially from the well site to b15 μg g− 1 at 600–800 m in 2002 and to b2 μg g−1 (the limit of

Table 1 Comparison of post-drilling data grouped by distance from the well site. Only significant (p b 0.05) ANOVA results shown. Groups with different letters are significantly different from Duncan's test (Duncan, 1955). No significant difference in homogeneity of variance (Levene, 1960) indicated by ‘ns’. Key parameter

50 m

200 m

400 m

600 m

800 m reference

ANOVA F-ratio

ANOVA p

Levene p

2002 post-drilling RPD deptha Total PAHb LAOb Amphipoda Cumacea Oligochaetea Polychaetea Total abundance Species richnessa

C A A B B C B B B

BC AB A A A BC AB A A

AB B B A A AB AB A A

A B B A A A A A A

A B B A A AB A A A

9.77 9.66 18.1 9.97 0.23 9.00 8.97 0.41 18.7

b0.01 b0.01 b0.01 b0.01 0.04 b0.01 b0.01 0.02 b0.01

b0.01 ns ns b0.01 ns b0.01 b0.01 ns b0.01

2004 post-drilling RPD depth Total LAOa,b Total PAHa,b Species richness

B A A B

AB B B AB

AB BC B AB

A BC B A

A C B A

6.84 17.7 8.29 4.44

0.01 0.01 b0.01 0.03

ns b0.01 0.03 ns

a b

Welch (1951) ANOVA for unequal variances. Log-transformed data.


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Fig. 4. Mean results (+std. dev.) for LAO, PAH, and key macrofauna parameters for 1998 shallow pre-drilling (SP) stations (120–170 m depth), 1998 deep pre-drilling (DP) stations (≥600 m depth), and 2002 and 2004 post-drilling stations grouped by distance (m) from the well site. NM = not measured.

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detection) at 400–800 m in 2004 (Fig. 4). The post-drilling spatial distribution of LAO in surface sediment around the well site is shown in Fig. 5. Total PAH (log-transformed) was significantly elevated at the well site compared to all other post-drilling station groups (Table 1). Concentrations of total PAH based on 41 analytes ranged from 78 to 9000 ng g−1 across post-drilling surveys (Fig. 4), with lower concentrations reported at all but a single well site station (4W). Sediment from all post-drilling stations, except for 4W, had relatively uniform distributions of individual PAH analytes, with near equal concentrations of low (2–3 ring) and high (4–6 ring) molecular weight compounds. Other than at the well site, analyte distributions were similar to those found in the majority of 1998 pre-drilling sediment, indicating combustionrelated sources from the prominence of 4- and 5-ring non-alkylated PAH (LaFlamme and Hites, 1978). Sediment at station 4W had a much higher proportion of 2- and 3-ring PAH, dominated by alkylated naphthalenes (i.e., nC2- to nC4-naphthalenes), alkylated phenanthrene/anthracenes, and to a lesser extent alkylated fluorenes, possibly from degraded petroleum (Steinhauer and Boehm, 1992) or leached natural gas condensate. Although the LAO formulation used at Nakhchivan was 0.0005% total PAH by weight, it was comprised mainly of acenaphthene (76%), which was low (b2% of total PAH) in postdrilling well site samples, indicating a PAH source to sediment other than drilling fluid. For this reason, and because concentrations were elevated only at one well site station and a single 200 m station in 2002, total PAH was removed from further analysis. 3.3. Benthic macrofauna In general, abundances of major taxonomic groups and community indicators of total abundance, species richness, and dominance were reduced at the well site compared to shallow (120–170 m depth) predrilling stations (n = 22) and 800-m reference stations (n = 9) (Fig. 4). In 2002, amphipod and cumacean abundance, species richness, and dominance were significantly lower at the well site compared with the 800-m reference group (Table 1). In 2004, only well site species richness remained significantly depressed compared to reference. In 2002, ostracod crustaceans comprised 51% of total abundance at 50–400 m from the well site compared with only 1.6% in 2004. Ostracod colonization of sediments has been reported from natural saline environments where periodic or episodic hypoxia occurs (Gamenick et al., 1996; Modig and Olafsson, 1998), and may be an effective sentinel of eutrophication, organic loading, and anthropogenic impact (Ruiz et al., 2005). The post-drilling dominance of ostracods was limited in space and time to a 400 m radius around the well site in 2002. Numbers were dramatically reduced in 2004, trending toward shallow pre-

drilling abundances. The pelecypod Didacna profundicola, which was found at relatively low abundance in shallow pre-drilling sediment (avg. 27 m−2), was the only other post-drilling taxa with abundance ≥600 m−2 at the well site (2002 only). Excluding well site stations, average abundances of polychaetes and cumaceans generally were higher in 2002 compared with both 2004 and shallow pre-drilling abundances (Fig. 4). In contrast, average abundances of amphipods and gastropods were lower in most 2002 groups. Differences tended to be greatest between 2002 and 2004 reference stations, indicating naturally high temporal variability in the four aforementioned groups. Only three of ten macrofauna parameters differed statistically between shallow pre-drilling and post-drilling reference groups (Table 2). Hypania invalida, a highly eurytopic species, was the dominant polychaete in all surveys, with the highest relative abundance recorded in 2002 (74% of all polychaetes). It is present in freshwater rivers, reservoirs, and brackish habitats throughout Europe, having been introduced into the Caspian in the last 50 years through its canal connection with the Black Sea (Karpinsky, 1992). Whereas amphipods were completely absent at the well site in 2002, four species were recorded in 2004, with a collective average abundance of 130 m−2. Extending out to 200 m from the well site, average amphipod abundance increased to 1713 m−2, similar to 800 m reference sites. Molluscan gastropod (snail) species, mainly from a single genus, Turricaspia displayed significant increases in average abundance between post-drilling surveys; however, there were no significant within-survey differences. 4. Discussion Several lines of evidence indicate hypoxia from microbial degradation of LAO drilling fluid as a probable factor in observed changes in post-drilling benthic community structure. Lines of evidence fall into three major categories: 1) dense white bacterial mats coupled with absence of a discernible RPD layer in well site surface sediments, 2) degradation of the LAO chemical signature over time, and 3) changes in postdrilling macrofauna community structure with increased LAO that paralleled depth/DO related changes in the pre-drilling community. 4.1. Hydrocarbon-degrading bacteria Beggiatoa sp. is found in Caspian Sea sediments (Salmanov, 2006) and is an effective petroleum hydrocarbon degrader under aerobic and anaerobic pathways (Atlas, 1981). In 2002, dense white filamentous bacterial mats, resembling Beggiatoa sp., were observed at all but

Fig. 5. Spatial distribution of LAO in surface sediment around the well site in 2002 (left) and 2004 (right) post-drilling surveys.


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Table 2 Comparison of shallow pre-drilling and post-drilling reference data. Only significant (p b 0.05) ANOVA results shown. Groups with different letters are significantly different from Tukey's HSD test. No significant difference in homogeneity of variance (Levene, 1960) indicated by ‘ns’. Key parameter

Shallow predrilling n = 7

2002 reference n = 4

2004 reference n = 4

ANOVA F-ratio

ANOVA p

Levene p

Cumaceaa Total abundance Species richness

A A A

B B A

A AB B

23.3 5.08 6.83

b0.01 0.02 0.01

.04 ns ns

a

Welch (1951) ANOVA for unequal variances.

two stations located within 200 m of the well site. The densest mats were observed at four well site stations with the highest LAO (11,513– 23,230 μg g−1) measured in 2002. Stations with LAO ranging from 1024 to 7130 μg g− 1 had patchy occurrences of less dense mats. In 2004, well site sediments had no visible mats, and LAO concentrations at ≤200 m from the well site were lower (3.17–3740 μg g−1) at all but well site station 4W, which was 2000 μg g−1 higher than in 2002, noting an 18 m offset in station location between surveys.

4.2. Degradation of LAO in post-drilling sediment Fresh LAO consists primarily of normal straight-chain alkanes with a terminal olefin, with minor contributions of alkylated (branched-chain) isomers for each C14 to C18 constituent. As LAO degrades, relative concentrations of alkylated isomers (e.g., C14 isomers) increase, and the corresponding parent compound (e.g., C14) decreases under aerobic or anaerobic bacterial pathways (Atlas, 1981; Sauer et al., 1993). This pattern was observed between 2002 and 2004 post-drilling sediments with N50 μg g−1 LAO (Fig. 6). Between post-drilling surveys (ca. 8 months), there was a 42% average decrease in sediment LAO at five stations (avg. distance = 78 m) with the highest initial concentrations. This result is fairly consistent with an average decrease of 48% over 7 months reported for stations located at 75 m distance from a North Sea well site drilled using esterbased fluid (Daan et al., 1996). The rate of decrease in LAO is somewhat unexpected, because slower microbial degradation rates for olefin versus ester-based drilling fluid formulations in fine-grained sediment are reported from both laboratory (Mille et al., 1988) and field studies (Roberts and Nguyen, 2006), which cite relative field degradation rates of synthetic NADFs as: ester N LAO N acetal N ether ≫ mineral oil. High populations of indigenous hydrocarbon-degrading bacteria (Salmanov, 2006) aided by hypoxia tolerant, bioturbating tubificid worms (Parr et al., 2007; Karpinsky, 1992) may have contributed to the relatively rapid LAO degradation rate observed at the Caspian Sea well site.

Fig. 6. Percent distribution of LAO analytes measured in the initial LAO drilling fluid (prior to discharge) and in 2002 and 2004 post-drilling sediment samples collected from well site station 1W. The relative increase in alkylated isomers and corresponding decrease in the associated parent compound over time denotes a typical hydrocarbon microbial degradation pattern.

4.3. Evidence of LAO-induced hypoxia There were statistically demonstrable declines in species richness and dominance with increased bottom depth (decreased DO) in the pre-drilling survey, and with increased LAO in combined post-drilling surveys. These relationships were strongest with species richness (Fig. 7), and insignificant with total abundance (p N 0.05). The diminished response of post-drilling total abundance was partially due to conflicting trends between fauna, in which abundance of most groups decreased significantly with increasing LAO, but several actually flourished. Ostracod crustaceans, the pelecypod D. profundicola, and two species of polychaete worms (N. diversicolor, H. invalida) were the only fauna with abundances greater than 600 m−2 (the highest average abundance for any shallow pre-drilling species) found in sediment with N5000 μg g−1 LAO; and ostracods were the only enriched fauna (N2000 m−2) in sediment with N 15,000 μg g−1 LAO. The decline in average ostracod abundance from 2002 to 2004 (1046 to 88.6 m−2) was accompanied by an approximate 70% decline in LAO from the well site out to 400 m distance. Dominance of certain fauna in sediment with high concentrations of LAO mimics the response of hypoxia-tolerant polychaetes (Woulds et al., 2007) and motile ostracods (Noguera and Hendrickx, 1997) to organically enriched sediment. The enhanced number of ostracods at a mud-volcano site sampled in the Nakhchivan Contract Area in 1998, and their overwhelming numerical dominance at post-drilling well site stations, indicate a strong chemo-sensory capability as opportunist foragers. Didacna sp. also tolerates moderate levels of organic loading

Fig. 7. Regression results (±95th conf. interval) for post-drilling log LAO ( ) vs. species richness, using only sediment samples N2 μg g−1 total LAO (n = 22). Results for predrilling station bottom depth ( ) vs. species richness plotted for comparison. Upper and lower hypoxia limits predicted using Fig. 2 regression results.

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and low oxygen, and was the only genus of pelecypod molluscs from pre-drilling sediment at depths greater than 600 m, while also commonly occurring at shallower depths. The normal shallow water (120–170 m) pre- and post-drilling benthos resides in a stable environment of sufficient bottom water DO (N4–7.5 ml l− 1), with the highest values of species richness (17–41 taxa 0.1 m−2 grab) and dominance (8–19 species) reported across surveys. In general, the shallow benthos was distinguished by a relatively abundant and diverse pericarid crustacean fauna, dominated by amphipod families Gammaridae, Corophiidae and Haustoriidae. Abundance of gastropods (snails) and molluscan pelecypods (clams) also were highest in this depth range. This community structure was observed in all post-drilling sediments with ≤100 μg g−1 LAO. The benthic community found at depths of 400–600 m (with bottom water DO predicted at 1–2 ml l−1) in the pre-drilling survey and at postdrilling stations with LAO N 100–500 μg g−1 was less diverse (≤15 taxa 0.1 m−2 grab) than the normal shallow water community. This transitional community was increasingly dominated by the hypoxia tolerant ampharetid polychaete H. invalida, a few species of tubificids (an oligochaete family that is also tolerant of hypoxia), 2–8 cumacean groups (most capable of leaving the sediment to enter the water column), and 4–7 groups each of amphipods, gastropods, and pelecypods. Post-drilling stations located within 50 m of the well site and at a single 200 m station with high LAO (2000–10,000 μg g−1) in general, had community characteristics similar to those found at pre-drilling stations sampled at ≥ 600–692 m depth, where hypoxic conditions prevailed. This oxygen deprived community was dominated by H. invalida, with patchy occurrences of several motile cumaceans, and the bivalve D. profundicola. Other than relatively high abundances (N 850 m−2) of an unidentified ostracod species within 50–200 m of the well site, post-drilling sediments with N 10,000 μg g−1 LAO had low individual abundances (10–150 m−2) of only a few hypoxia-tolerant annelid worms (H. invalida, N. diversicolor, Psammoryctides deserticola) and two species of Cumacea (Schizorhynchus eudorelloides, Stenocuma diastyloides). A similarly depauperate community was not found in the pre-drilling survey that had a maximum sampling depth of 692 m with a predicted bottom water DO of 0.72 ml l−1 (below the lower limit of hypoxia) (Fig. 7). 5. Conclusions Our findings show the importance of monitoring recovery over time and understanding natural changes in macrofauna community structure related to water depth and oxygen availability prior to drilling. Although direct causal relationships were not established, the independent, parallel relationship between LAO and bottom depth (as a surrogate for dissolved oxygen) with species richness indicate hypoxia as the most likely cause of the observed taxonomic shift from a relatively diverse macrofauna to a less diverse assemblage dominated by a few fauna. At the shelf-depth well site in 2002, eight months after drilling ceased, dense bacterial mats and dominance by an unidentified ostracod lead us to hypothesize that LAO is a strong attractor of certain chemo-sensory fauna. In 2004, recolonization around the well site was well underway with recruitment cues approaching normal at a threshold of approximately 100 μg g−1 LAO. Abundances of six out of seven taxonomic groups at 200–600 m from the well site were commensurate with shallow predrilling and 2004 post-drilling reference results, as were the more conservative indictors of total abundance, species richness, and dominance. Thus, the area of maximum observable effects, 33 months after well completion, was contained to a relatively uniform area less than 0.12 km2, centered near the well site. Our results have implications for predicting impacts to macrofauna from drilling discharges containing low-toxicity, biodegradable synthetic NADF. These discharges should have relatively little impact on a macrofauna that resides beyond shelf depths and is naturally impoverished due to oxygen deficiency. In contrast, oxygen-sensitive,

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