Drilling platforms as artificial reefs: distribution of macrobenthic ...

ICES Journal of Marine Science, 59: S316–S323. 2002 doi:10.1006/jmsc.2002.1225, available online at http://www.idealibrary.com on

Drilling platforms as artificial reefs: distribution of macrobenthic assemblages of the ‘‘Paguro’’ wreck (northern Adriatic Sea) Massimo Ponti, Marco Abbiati, and Victor Ugo Ceccherelli Ponti, M., Abbiati, M., and Ceccherelli, V. U. 2002. Drilling platforms as artificial reefs: distribution of macrobenthic assemblages of the ‘‘Paguro’’ wreck (northern Adriatic Sea). – ICES Journal of Marine Science, 59: S316–S323. Offshore drilling raises the issue of disposal of platforms at the end of their productive cycle and re-use as artificial reefs has been proposed. The wreck of the ‘‘Paguro’’ drilling platform, which sank in the northern Adriatic Sea in 1965, offers the opportunity to study the performance of offshore structures as artificial reefs in a region where this solution has not been explored before. We provide a description of the macrobenthic assemblages present at the wreck at different sites and at different depths, based on destructive and photographic sampling. Results show that the wreck has been colonized by a rich and diversified fauna. Primary space has been dominated by mussels and oysters, which provide suitable habitats for a variety of benthic invertebrates. Assemblages vary among sites, species richness is greatest at those sites facing prevailing currents. Distribution patterns vary vertically, evenness (Hill’s N10) decreasing significantly with depth. It is argued that the results contribute to the background knowledge required in rigs-to-reefs programmes.  2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Keywords: artificial reef, benthic fauna, decommissioning, northern Adriatic Sea, offshore platform. Accepted 31 January 2002. M. Ponti, M. Abbiati, and V. U. Ceccherelli: Centro Interdipartimentale di Ricerca per le Scienze Ambientali, Universita` degli Studi di Bologna, Via Tombesi Dall’Ova 55, I-48100 Ravenna, Italy; tel: +39 0544 215126; fax: +39 0544 31204; e-mail: [email protected]

Introduction Building artificial reefs on subtidal soft bottoms is among the actions envisaged in several research programmes and management projects aimed at enhancing the value of coastal areas worldwide (Baine, 2001). Artificial reefs are usually created to increase fish production and yield, to support recreational activities (diving and angling), and to promote nature conservation and coastal protection (Bohnsack and Sutherland, 1985; Baine, 2001; Svane and Petersen, 2001). These reefs should increase the surface available for settling of sessile organisms (Relini et al., 1994), offer shelter and refuge to a variety of plants and animals, and provide habitats for reproduction and oviposition (Frazer and Lindberg, 1994; Lozano Alvarez et al., 1994; Bombace et al., 1997). Moreover, many represent an important tourist attraction for both divers and anglers, because they host an interesting benthic flora and fauna and act as fish aggregating devices (Bombace 1054–3139/02/0S0316+08 $35.00/0

et al., 1994; McGlennon and Branden, 1994; D’Anna et al., 1994). Reefs may also be used as an impediment to trawling (Bombace, 1989). A recent review on artificial reef performance, however, showed that only 50% of the structures met the objectives for which they were promoted (Baine, 2001), their success depending largely on appropriate prior planning and ongoing monitoring and management. Artificial reefs are commonly made of a variety of building and waste materials (Branden et al., 1994; Hatcher, 1997). Offshore gas and oil production poses the issue of identifying decommissioning alternatives for the platforms, including their possible use as artificial reefs. Most rigs-to-reefs research focuses on their efficiency as fish aggregating devices and on their use for recreational activities (Kasprzak and Wilson, 1994; Bull and Kendall, 1994; Stephan and Osburn, 1994). Conversely, little is known about the potential impact on the broader environment or, specifically, about the composition and patterns of distribution of macrobenthic

 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Drilling platforms as artificial reefs

30

S317

45°

Agostino B

Italy Garibaldi A-T-C

30' Porto Corsini

PCW B-C

40°

Sarom 3 20 Armida

Garibaldi B Sarom 4

10°

15°

P.C. 80bis Amelia A

20

Flu mi Uniti

P.C. 26

Antares

30

"Paguro" 44°20' N 12°15'

30'

45'

Figure 1. Bathymetric chart with location of Agip offshore platforms and the ‘‘Paguro’’ wreck (northern Adriatic Sea; 4423 13 N 1234 57 E).

assemblages (McGurrin and Fedler, 1989; Baine, 2001; Svane and Petersen, 2001). Decommissioned structures have been used for reef construction in the Gulf of Mexico (Bull and Kendall, 1994). This option is also being considered for the North Sea (Aabel et al., 1997), but total removal of decommissioned platforms is still the main disposal strategy adopted in Europe. In the northern Adriatic Sea, a long history of offshore gas extraction is now also posing the disposal problem. In this context, the wreck of the ‘‘Paguro’’ drilling platform, which sank as a result of an accident in 1965, offers a unique opportunity to investigate its effectiveness as an artificial reef (Ponti et al., 1998; Giovanardi and Rinaldi, 1999). The area around the ‘‘Paguro’’ has been declared a Marine Protected Area, where all fishing activities are strictly prohibited. Access is restricted to recreational diving and scientific activities, under the control of local authorities and a management institution. About 4000 divers visit the area annually, indicating that this type of environment is attractive and therefore has economic value. Here, we investigate the species composition of macrobenthic assemblages that have colonized the wreck to evaluate their potential ecological and economic value. Distribution patterns of species are analysed in relation to depth and orientation of the substrate.

Material and methods Study site The AGIP drilling platform ‘‘Paguro’’ sank 12 nautical miles offshore Ravenna on 29 September 1965 (Figure 1)

–8 m Ta 1–2 Site

A

Heliport

–11 m

Site

Lodging module

B

–13 Tc

Tb 1–2

Pontoon

–24 –24 m Dominant current's direction

–34 m

N

Figure 2. Three-dimensional reconstruction of the wreck showing relevant structures and their orientations (modified after Ponti et al., 1998), as well as sampling sites (A, B) and photographic transects (Ta1, Ta2, Tb1, Tb2, Tc).

as a result of a fire caused by methane leaking from the well. The platform leans on the seabed with the starboard side buried several metres deep in the sediment (Ponti et al., 1998). The main identifiable structures are the pontoon, the lodging module, including five decks, and the leg of prow connecting the cylindrical base to the remains of the heliport (Figure 2). The bottom consists of a sandy silt (Brambati, 1992) and had an average depth of 24 m, but the gas eruption excavated a crater to a depth of about 34 m. The wreck extends over a depth range between 10 and 34 m (Ponti et al., 1998). Water temperatures in the area vary between 28C and 30C in summer and 6C and 8C in winter (Ponti et al., 1998). Vertical profiles show a sharp seasonal thermocline that reaches an average depth of 16 m in

Table 1. Mean density (and s.d.) per sample (2020 cm) of faunal taxa identified at sites A and B and at 14 and 21 m depth. A14 Mean Calcispongiae Sycon sp. Demospongiae Cacospongia scalaris Schmidt Cacospongia mollior Schmidt Laxosuberites rugosus (Schmidt) Demospongiae indet. Anthozoa Epizoanthus arenaceus (Delle Chiaje) Actiniaria indet. Turbellaria indet. Nemertea Nemertea indet. 1 Nemertea indet. 2 Gastropoda Melanella sp. Bowdich Odostomia conoidea (Brocchi) Phanerobranchia indet. Bivalvia Modiolus barbatus (L.) Musculus costulatus (Risso) Mytilus galloprovincialis Lamarck Barbatia barbata (L.) Arca noae L. Chlamys varia (L.) Anomia ephippium L. Ostreidae indet. Mysia undata (Pennant) Hiatella arctica (L.) Gastrochaena dubia (Pennant) Polychaeta Harmathoe¨ spinifera (Ehlers) Syllis gracilis Grube Sphaerosyllis hystrix Clapare`de Autolytus sp. Grube Ceratonereis costae (Grube) Marphysa sanguinea (Montagu) Lumbrinereis fragilis (O.F. Mu¨ller) Polydora ciliata (Johnston) Spio multioculata (Rioja) Cirratulidae unident. 1 Cirratulidae unident. 2 Serpula vermicularis L. Hydroides sp. Gunnerus Pomatoceros triqueter (L.) Sipunculida indet. Cirripedia Verruca stroemia (O.F. Mu¨ller) Balanus trigonus Darwin Malacostraca Alpheus macrocheles (Hailstone) Thoralus cranchii (Leach) Pisidia longimana (Risso) Pilumnus hirtellus (L.) Janira maculosa Leach Corophium sextonae Crawford Microdeutopus similis Myers Gammaridea indet. 1 Gammaridea indet. 2 Ofiuroidea Ophiothrix fragilis (Abildgaard) Amphipholis squamata (Delle Chiaje) Echinoidea Paracentrotus lividus (Lamarck)

A21 s.e.

0

—

0.3 0.3 0.7 0

0.6 0.6 0.60 —

263 236 7.0 9.6 0 —

B14

Mean

s.e.

Mean

0

—

0

— 0.6 0.7 —

0 — 0 — 0.6 1.0 0.3 0.6

0 0.3 — 0

— —

0 0

— —

0 0

0 0.7 0

— 1.2 —

0 0 0

— — —

0 0.3 1.0 0 0 0 1.7 0.7 0.7 12.7 1.3

— 0.6 1.7 — — — 2.1 0.6 1.2 7.1 1.2

2.3 5.7 0 1.3 0 0 9.3 2.7 0 13.7 4.0

3.2 5.5 0 1.2 — — 7.6 0.6 — 7.5 3.6

0 36.0 8.7 0 3.0 0 0 5.7 0.7 2.3 6.3 3.0 0 9.7 1.3

— 26.1 7.6 — 2.0 — — 7.4 0.6 4.0 6.5 3.0 — 11.7 1.2

0.3 3.3 0 0 7.7 0.7 0 0 0 0 0 7.0 0.3 7.7 3.7

0.6 2.1 — — 2.9 1.2 — — — — — 8.9 0.6 10 3.5

1.0 97.0 7.3 0.7 2.7 0 0 9.3 0.3 4.0 19.7 1.3 0 24.7 1.0

11.0 0.7

10.4 0.6

— 0 — 0 0.6 0 0.6 0.3 — 8.7 6.8 164 — 2.7 — 0.3 — 0.3

4.16 1.2

0 0 1.0 0 10.3 106 3.3 0 0

— — 1.7 — 7.6 78 5.8 — —

0 0 0.3 0.3 0 19.3 0 0 0

13.3 3.7

11.5 1.5

8.7 3.7

14.2 3.5

—

0

—

0

s.e.

—

Mean

s.e.

0.3

0.6

0 0.3 0 0.3

— 0.6 0.6

713 222 132 88.5 117 103 0 — 30 14.5 56.7 43.9 0.3 0.6 2.0 1.0 2.3 4.0

0 0

4.3 1.7

B21

— —

1.0 4.7

1.73 6.4

0.3 0.6 1.3 1.5 0 —

0 2.0 0.7

0 2.7 0.6

1.0 0.0 0 — 1.3 2.3 0 — 0 — 1.0 0.0 2.3 2.5 2.7 3.8 0.3 0.6 9.0 1.7 1.7 2.9

0.7 3.0 2.7 0 0.3 7.7 8.7 13.7 3.0 26.3 10.7

1.2 2.7 0.6 — 0.6 12.4 10.8 14.8 1.7 15.0 7.6

1.0 61.2 8.4 1.2 2.9 — — 8.0 0.6 6.9 17.0 2.3 — 20.4 1.7

1.7 71.7 7.0 0 12.0 1.0 0.3 28.3 4.3 14.7 3.0 5.0 0 56.3 4.0

1.5 28.3 12.1 — 9.0 1.0 0.6 24.5 4.5 15.6 1.0 6.2 — 59.0 2.0

1.0 1.0 21.3 20.0

24.0 16.0

41.6 15.4

— 0.7 0.6 — 1.3 2.3 — 0.3 0.6 0.6 0.7 0.6 5.1 15.3 22.2 53 410 463 4.6 4.7 3.2 0.6 0.7 1.2 0.6 0 —

40 10.5 12.3 9.3 0

—

29.0 18.3

4.6 20.5

0.3

0.6

Drilling platforms as artificial reefs

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–1

S (sample )

Species richness 40 30

A14

20 10

A14

Heterogeneity diversity –1

B21

B14 A14 B14

0

N1 (sample )

B14

B21

12 A21

A21

8

B21

A21

4 0

–1

N10 (sample )

Evenness 0.4

Figure 4. nMDS plot (stress=0.10) on 4th root transformed densities in samples (3 replicates) from different sites (A and B) and different depth (14 and 21 m).

0.3 0.2 0.1 0.0

A14

A21 B14 Site-depth

B21

Figure 3. Mean species richness (number of species, S), heterogeneity diversity (Hill’s N1) and evenness (Hill’s N10) per sample (+SE, n=3) at two sites (A and B) and at two depths (14 and 21 m).

August. Below the thermocline, the temperature never exceeds 14–16C. In spring and autumn, turbid plumes from Po river floods dilute surface waters, reducing the salinity and raising nutrient concentrations (Fonda Umani et al., 1992; Vollenweider et al., 1992). These phenomena contribute to dystrophic conditions that occasionally cause anoxic crises in the area of the wreck. The northern Adriatic geostrophic circulation has a complex and variable pattern, mainly in the summertime and in the surface layer, owing to the baroclinic component (Franco et al., 1982; Artegiani et al., 1997). Nevertheless, prevailing currents around the wreck are aligned in a SE direction.

Sampling and analyses The macrobenthic assemblages were investigated during the summer of 1994 by means of destructive as well as photographic sampling. Three replicate samples were collected at two depths (141 m and 211 m) on vertical walls at site A (pontoon exposed to SE) and site B (lodging module exposed to NE; Figure 2). SCUBA divers scraped off areas of 2020 cm using a hammer

and chisel and collected the organisms in a net supported by an aluminium frame; the organisms were transferred within a few hours to the laboratory. Samples were sieved using a 0.5 mm mesh and preserved in 4% formaldehyde. The endobiontic organisms were removed from the calcareous oyster shells by dissolution in 4% hydrochloric acid. Species were identified to the lowest possible taxonomic level and their abundance was estimated as number of individuals per sample. For each sample, species richness (number of species, S), heterogeneity diversity (Hill’s N1) and the corresponding evenness component (Hill’s N10) were calculated (Gray, 2000). Abundance data were analysed after fourth root transformation using non-metric multidimensional scaling (nMDS) based on the Bray–Curtis similarity index (Clarke, 1993). Differences between assemblages at sites A and B (fixed factor) and at depths of 14 and 21 m (fixed factor) were assessed by non-parametric multivariate analysis of variance (NPMANOVA) performed by unrestricted permutation of raw data (Anderson, 2001). The similarity percentage procedure (SIMPER; Clarke, 1993) was used to determine which species mostly contributed to the dissimilarity between sites and depths. Differences in densities of the most abundant species and differences in species diversity indices (S, N1 and N10) at different sites and depths were tested by two-way ANOVA. Cochran’s C test was used to verify the assumption of homogeneity of variances, and appropriate transformations were applied to the data when necessary (Winer, 1971). Photographic samples were taken out along five vertical transects (depth range 13–30 m). Transects Ta1 and

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M. Ponti et al. Table 2. Non-parametric multivariate analysis of variance (NPMANOVA) of density data (transformed to fourth root, no standardization) at two sites and two depths (analysis based on Bray–Curtis dissimilarities, permutation of raw data; PNP: possible number of permutations; D MS: denominator mean square). Source

d.f.

SS

MS

F

p

PNP

D MS

Site Depth Sitedepth Residual Total

1 1 1 8 11

2702.3 1390.3 1136.3 4340.3 9569.2

2702.3 1390.3 1136.3 542.5

4.98 2.56 2.09

0.001 0.063 0.074

462 462 15 400

Res Res Res

Ta2 were located on the SE wall of the pontoon (site A), while the others were located on the lodging module: Tb1 and Tb2 faced NE (site B); Tc faced NW (Figure 2). Photographs were taken at 1-m intervals along a tape measure using a Nikonos V underwater camera with a wide-angle lens (20-mm focal length) and colour slide films (ISO 100). The area covered by each slide was 0.1 m2 (4027 cm). Percent cover of identifiable organisms was estimated by means of an image processing system that corrected for unreadable slide areas. Percent cover values measured at the same depth along the two transects at site A (Ta1 and Ta2) and at site B (Tb1 and Tb2) were averaged. Data were analysed by using cluster analysis and nMDS after fourth root transformation using the Bray–Curtis similarity index.

Results Destructive sampling Overall, 53 invertebrate taxa were identified (Table 1). Algae were notably absent. Sponges included four encrusting species, among which Laxosuberites rugosus was the most frequent. The anthozoan Epizoanthus arenaceus was the most abundant taxon, reaching the highest density at the deeper site on the pontoon (A21). Fourteen species of molluscs were found, Hiatella arctica being the most abundant bivalve. Polychaetes were represented by 14 species, among which Syllis gracilis, Ceratonereis costae, Pomatoceros triqueter, and Serpula vermicularis were present at all combinations of sites and depths. Among 11 species of crustaceans, the barnacles Verruca stroemia and Balanus trigonus, and particularly the amphipod Corophium sextonae, were the most common. The most abundant echinoderm was Ophiothrix fragilis. Endobiontic species found inside oyster shells include the bivalve Gastrochaena dubia, the polychaete Polydora ciliata, and some cirratulids and sipunculids. Most species collected typically occur on subtidal hard bottoms or in oyster and mussel beds. However, some soft-bottom species (e.g. the polychaetes Lumbrinereis fragilis and Spio multioculata) were also found.

Species richness, heterogeneity diversity, and evenness were significantly higher (ANOVA, p0.05 (none)

0.61

p>0.05 (sqrt)

0.54

p>0.05 (sqrt)

0.63

p>0.05 (none)

0.70

p>0.05 (Ln(x+1))

0.43

p>0.05 (none)

F=0

9.6 1.9 0.0

* n.s. n.s.

0.1 7.2 0.4

n.s. * n.s.

7.6 0.0 3.5

* n.s. n.s.

5.7 0.2 0.1

* n.s. n.s.

10.2 1.3 3.9

* n.s. n.s.

14.3 1.6 0.3

** n.s. n.s.

one exception, to samples collected along transects exposed to the SE. Distances among the clusters identified suggest that differences among assemblages along different transects increased with depth, which is consistent with the results from destructive sampling.

Discussion Studies carried out in the Adriatic Sea in the 1970s and 1980s showed that artificial reefs located in soft-bottom

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M. Ponti et al. 14

SE NE NW

14 15

13

13 16 17 13 20 15 18 17 15

14 18

21 19 22

20 22 23

22 30 27 24 26

23 21

20

19

Figure 5. nMDS plot (stress=0.11) on percent cover data of the assemblages: site orientations are indicated by symbols; depth in m is given by inner number; dotted lines show clusters obtained at an arbitrary similarity level of 80%.

areas near the coast allowed settlement of dense assemblages of hard-bottom species dominated by algae and filter-feeding animals (Bombace et al., 1997). The offshore wreck of the ‘‘Paguro’’ drilling platform also hosted a rich fauna, presumably reflecting a high structural complexity as well as its wide bathymetric range. Dense mussel beds covered the structures down to 12–13 m, but were replaced by oysters at greater depth. Mussels and oysters provided a suitable habitat for settlement of sessile filter-feeders and endobiontic species, as well as refuges for crustaceans and brittle stars. Moreover, the sediment trapped among the shells hosted a fauna typical of soft bottoms, including predators and detritivorous species. In contrast to inshore reefs, macroalgae were notably absent from all samples taken from the wreck. Sporadic observations at other times of the year indicate a consistent pattern in this respect that may be related to algal development being limited by high turbidity in the area (Airoldi and Cinelli, 1997). Major differences in the distribution of species were observed across sites with different orientations and across depths. Overall, assemblages at sites exposed to the NE were richer and more diversified compared to those at sites exposed to the SE. Moreover, assemblages were characterized by higher evenness in shallower areas. Differences among sites increased with depth and become particularly pronounced below 20 m. The patterns observed may have been related to a variety of factors. Site orientation, for example, may be important because it determines exposure to prevailing currents (Glasby and Connell, 2001). Thus, assemblages exposed to the NE may have benefited from increased

inputs of particulate food, larger oxygen exchange, and greater supply of larvae. Similarly, differences in depth distribution may reflect water stratification during summer and the potential establishment of dystrophic conditions near the bottom. Whatever ecological factors are involved, the results show that surface orientation influences the composition of and abundance patterns within the macrobenthic community. This is important in light of potential re-use of decommissioned platforms as artificial reefs in the northern Adriatic Sea, because an appropriate disposal strategy should take into account these effects. The Italian government recently authorized the sinking of parts of decommissioned platforms inside the ‘‘Paguro’’ area. According to the results of the present study, structures were cut and positioned to rise at least 10 m above the bottom and with the more extensive surface facing prevailing currents. Further studies on the effects on the local soft-bottom fauna and on the fish assemblages attracted to this type of artificial reefs, as well as comparisons of the communities of natural and artificial hard substrata, are needed if we are fully to understand their potential positive and negative impacts in the Adriatic Sea. Nevertheless, the results clearly show that platform wreck may even accidentally allow the settlement of rich faunal assemblages.

Acknowledgements We thank Dr B. Calcinai for identifying the Porifera and Dr M. J. Anderson for his advice on multivariate analyses. The manuscript was improved by Dr L. Airoldi, who provided critical comments and suggestions. Agip Petroli (ENI group) allowed access to unpublished data on layout of the ‘‘Paguro’’ platform; the ‘‘Daphne’’ Survey & Study Organization (ARPA Regione Emilia Romagna) supplied hydrographical data; Ravenna’s Harbour Authority and the ‘‘Paguro’’ Association provided logistical support.

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