Recovery of benthic megafauna from anthropogenic disturbance at a hydrocarbon drilling well (380 m depth in the Norwegian Sea)

Abstract

Recovery from disturbance in deep water is poorly understood, but as anthropogenic impacts increase in deeper water it is important to quantify the process. Exploratory hydrocarbon drilling causes physical disturbance, smothering the seabed near the well. Video transects obtained by remotely operated vehicles were used to assess the change in invertebrate megafaunal density and diversity caused by drilling a well at 380 m depth in the Norwegian Sea in 2006. Transects were carried out one day before drilling commenced and 27 days, 76 days, and three years later. A background survey, further from the well, was also carried out in 2009. Porifera (45% of observations) and Cnidaria (40%) dominated the megafauna. Porifera accounted for 94% of hard-substratum organisms and cnidarians (Pennatulacea) dominated on the soft sediment (78%). Twenty seven and 76 days after drilling commenced, drill cuttings were visible, extending over 100 m from the well. In this area there were low invertebrate megafaunal densities (0.08 and 0.10 individuals m(-2)) in comparison to pre-drill conditions (0.21 individuals m(-2)). Three years later the visible extent of the cuttings had reduced, reaching 60 m from the well. Within this area the megafaunal density (0.05 individuals m(-2)) was lower than pre-drill and reference transects (0.23 individuals m(-2)). There was a significant increase in total megafaunal invertebrate densities with both distance from drilling and time since drilling although no significant interaction. Beyond the visible disturbance there were similar megafaunal densities (0.14 individuals m(-2)) to pre-drilling and background surveys. Species richness, Shannon-Weiner diversity and multivariate techniques showed similar patterns to density. At this site the effects of exploratory drilling on megafaunal invertebrate density and diversity seem confined to the extent of the visible cuttings pile. However, elevated Barium concentration and reduced sediment grain size suggest persistence of disturbance for three years, with unclear consequences for other components of the benthic fauna.

Figure 1. The Morvin survey design.

The 2009 video transect survey is shown in red. Previous surveys were at the same location with 100 m video transects radiating from the well and are shown in green. The location of the Morvin field in the Norwegian Sea is shown as an inset.

Figure 2. Examples of the megafaunal taxa observed at Morvin.

A: Mycale sp., B: Hymedesmia sp., C: Alcyonium sp., D: Cerianthus sp., E: Pennatula phosphorea, F: Kophobelemnon stelliferum, G: Funiculina sp., H: Bolocera sp., I: Porania sp., J: Asterias rubens, K: Henricia sp., L: Echinus sp., M: Parastichopus tremulus, N: Munida sp., O: Lithodes sp., P: Geryon sp., Q: Molva molva, R: Sebastidae, S: Glyptocephalus cynologus. Scale bar on images represents 50 mm.

Figure 3. Habitat heterogeneity and species diversity at background sites.

The relationship between the number of rocks observed in video transects and two indices of megafaunal invertebrate species diversity for the 2006 Pre-drill video survey and 2009 Reference sites (randomly selected undisturbed locations) (left; rarefied species richness ES₍₅₀₎, right; Shannon-Weiner Index H′).

Figure 4. Physical disturbance at Morvin.

Representation of the visible horizontal extent of drilling disturbance at Morvin: a) Post 1, b) Post 2, c) Post 3. The filled black circles in the centre represent well position, dark grey = complete coverage of sea bed with drill cuttings; light grey = partial coverage.

Figure 5. Mean (±sd) megafaunal invertebrate density (individuals m⁻²) at Morvin.

(a) soft sediment, (b) hard substrate and (c) generalist megafauna. Background sites are shown in white, visibly disturbed areas in dark grey and areas beyond disturbance are shown in light grey. Filled circles in hard substratum chart present show the density of rocks in the transects.

Figure 6. Multidimensional scaling ordination of megafaunal assemblages under different disturbance conditions.

Based on Bray Curtis similarity of pooled invertebrate megafaunal density data for the disturbance zones in 2006 (Pre, Post 1 and Post 2) and 2009 (Post 3, R). For each survey the transects have been divided into Background, Visible Disturbance and Beyond Disturbance according to the coverage of the sediment by drill cuttings, notable groups are labelled. Similarity levels from cluster analysis.

Figure 7. Response Y recovery index in comparison to pre-drill.

Shown for Rarefied species richness (ES₍₅₀₎), Species evenness (J), Shannon-Wiener Index (H′), Bray Curtis similarity, total sessile organism density and total motile organism density. Unfilled circles indicate disturbed zones and filled circles indicate distance from disturbance. Dashed lines indicate zero. Error bars = standard deviation.

Figure 8. Mean density of decapod burrows (±sd) in the Post 3 survey at Morvin.

White = Background, dark grey = Visible Disturbance, light grey = Beyond Disturbance.