An Analysis of the Sessile, Structure-Forming Invertebrates Living on California Oil and Gas Platforms

Bull Mar Sci. 95(4):583–596. 2019 research paper https://doi.org/10.5343/bms.2017.1042

An analysis of the sessile, structure-forming invertebrates living on California oil and gas platforms

Marine Science Institute, Milton S Love * University of California, Santa Mary M Nishimoto Barbara, California 93106. Linda Snook * Corresponding author email: . Li Kui

ABSTRACT.—Using video transects of oil and gas platform crossbeams off central and southern California, we characterized the structure-forming invertebrates (with a height of at least 20 cm) found around 23 oil and gas platforms at depths between 20 and 363 m. We observed 20,357 individual invertebrates, comprising 19,800 Cnidaria and 557 Porifera of at least 15 species or species groups. Metridium farcimen (Brandt, 1835) was by far the most commonly observed cnidarian, forming 97.6% of all invertebrates catalogued. The alcyonacean, Leptogorgia chilensis (Verrill, 1868), and the scleractinian, Lophelia pertusa (Linnaeus, 1758), were the most commonly observed corals. White vase sponges (most or all in the family Aphrocallistidae) were the most abundant of the sponges (comprising 38.4% observed). We also documented a variety of unidentified foliose, barrel, and other various-shaped sponges. The height of these invertebrates ranged from 20 to 80 cm. Taxa displayed a variety of depth patterns. Some, such as M. farcimen, unidentified white vase sponges, and L. pertusa, were found throughout most or all of the survey depth range, while others (notably the gorgonians L. chilensis, Placogorgia spp., and Acanthogorgia spp.) were found over a relatively narrow range. Invertebrate assemblages tended to be similar Fishes and invertebrates of oil and among many platforms reflecting species similarities over a gas platforms off California broad range of platform depths. Based on these relationships, it is apparent that the assemblages of structure-forming Date Submitted: 4 April, 2017. invertebrates varied by depth rather than geography. Date Accepted: 10 October, 2017.

Oil and gas production from offshore platforms in California began in 1958 and there are currently 26 platforms in these waters. These structures were installed between 1968 and 1989, lie in waters with bottom depths between 14 and 363 m, and are situated from just north of Point Arguello, central California, to off Huntington Beach, southern California (Fig. 1). Below the waterline, jackets of all platforms off California, with the exception of Platform Eureka, are similar in structure. All have a framework of rounded steel crossbeams and tubular vertical sleeves. An exception, Platform Eureka has a series of relatively narrow “skirt pilings” that are attached

Figure 1. Geographic location of the 23 oil and gas platforms surveyed in the present study. Open circles refer to platforms that were not surveyed. to the outside of the jacket. To guide these pilings into the sea floor, large circu- lar guides were constructed at each crossbeam directly above each piling’s location (Love et al. 2010). There is some evidence that a number of California platforms are nearing the end of their economic lives. Once an industrial decision is made to cease oil and gas production, managers go through a decommissioning process to decide what to do with that platform. In California, the decommissioning process will be at least partially governed by the California Marine Resources Legacy Act (CMRLA; http:// leginfo.legislature.ca.gov/faces/billNavClient.xhtml?bill_id=200920100AB2503). The CMRLA lists a number of specific factors that will be considered when making a determination regarding a platform’s ultimate disposition. One of these is that a structure’s net environmental benefit (NEB) be assessed. While NEB remains un- defined in this legislation, the act does state that “The contribution of the proposed structure to protect and productivity of fish and other marine life” must be deter- mined (Chapter 5, Section 6613(c)1). The invertebrates living on the platform structure are one aspect of its fauna. These assemblages, sometimes called “fouling communities,” are composed of a wide range of sessile and motile taxa encompassing nearly every invertebrate phyla. Relatively little research has been conducted on the invertebrate communities associated with the jackets of California platforms. Most of this research has focused either on (1) native invertebrates living either in shallow scuba depths (e.g., Page et al. 2008), (2) invasive invertebrates (Page et al. 2006), or (3) invertebrates living on the legs of platforms, ignoring these structures’ crossbeams (CSA 2005). Here, we focus on the sessile, structure-forming (here defined as having a height of at least 20 cm) invertebrates (including the corals, sponges, and anemones) found over the entire depth range of the platform jackets and throughout the geographical range of the platforms. Our focus is particularly timely, not only because of the likely Love et al.: Sessile, structure-forming invertebrates 585 decommissioning of California platforms, but also because of interest in these invertebrates by the federal government (Hourigan et al. 2015). We first documented the species and their abundances on each of the platforms. Second, we examined the effects of crossbeam depth and platform locations on species distribution. Specifically, three hypotheses were tested: (1) species assemblages varied as depth increased; (2) between central and southern California, there was geographic variability in species assemblages; and (3) species richness was higher at platforms situated in deeper waters reflecting greater surface area on these larger structures and greater depth variability.

Methods

Sampling Protocols.—Large sessile invertebrates on California oil and gas platforms were evaluated by examining existing videos taken during annual fish survey transects. These videos were collected aboard the manned submersibles Delta (Delta Oceanographics) and Dual DeepWorker (Nuytco Research). The surveys we utilized in the present study were conducted from 2000 to 2011, between September and November during daylight hours. The submersible maintained a speed of ap- proximately 0.5 kt and stayed an average 2 m from the platform jacket. During each transect, researchers made their observations from the starboard side of the vessel. Sizes of invertebrates were estimated using a pair of parallel lasers mounted on either side of an external video camera. The projected reference spots were 20 cm apart and were visible both to the observer and the video camera. An environmental monitor- ing system aboard the submarine continuously recorded date and time, depth below the sea surface, and altitude of the submersible above the sea floor. We selected the best quality video at a given platform. Because environmental con- ditions, such as water visibility, varied between surveys and between depths, videos were often selected from different years. Because camera equipment improved in lat- er years, the most recent survey of a depth level was chosen, so long as the crossbeam was in view of the camera for the entire transect, and water clarity and video quality were sufficient to recognize invertebrates of at least 20 cm height. Consequently, at a few platforms, we used videos from different years at different depths (Table 1). Note that for some platforms (Table 1), we do not include the bottom-most crossbeam. This was because bottom water clarity is limited around these structures and we were unable to use the manned submersible near the bottom. Surveys were conducted by making one complete circumnavigation of the outside of a platform along a horizontal crossbeam (and the pilings that intercepted it). Each of the circumferences was defined as a “transect,” which is the experimental unit for the data analyses described below. All horizontal crossbeams on each platform were surveyed. Platforms had between 2 and 10 horizontal crossbeams depending on their depths. Because the sessile invertebrates covering the crossbeams and associated pilings of all California oil and gas platforms are periodically removed down to an average depth of about 15 m, we limited our analyses to those crossbeams at least 16 m below the surface. Each sessile invertebrate with a height of at least 20 cm was identified to the lowest possible taxon. All data can be accessed through the Santa Barbara Channel Marine Biodiversity Observation Network (SBC-MBON; see http:// dx.doi.org/10.6073/pasta/2dc1e7a1ce14e0f3f070076fc4a85e43; Love et al. 2017). 586 Bulletin of Marine Science. Vol 95, No 4. 2019

Table 1. Depths (m) and year(s) of transects conducted at each of 23 oil and gas platforms off southern and central California, including bottom depths and year of installation of each platform. Platforms are listed in alphabetical order. Note that in some instances, the bottom-most crossbeam was not surveyed due to poor water visibility.

Bottom Depths Year Platform Year Location depth (m) surveyed (m) installed A 2011 34°19ˊN, 119°36ˊW 57 22, 33, 45 1968 B 2011 34°19ˊN, 119°37ˊW 58 33, 44 1968 C 2007 34°19ˊN, 119°37ˊW 58 20, 32, 45 1977 Edith 2009 33°35ˊN, 118°08ˊW 49 28, 30, 49 1983 Ellen 2011 33°34ˊN, 118°07ˊW 80 55, 77 1980 Elly 2011 33°35ˊN, 118°07ˊW 77 35, 65, 75 1980 Eureka 2011 33°33ˊN, 118°06ˊW 212 39, 60, 82, 102, 124, 145, 167, 191 1984 Gail 2009 34°07ˊN, 119°24ˊW 224 224 1987 Gail 2011 34°07ˊN, 119°24ˊW 224 52, 73, 96, 117, 141, 167, 196 Gilda 2007 34°10ˊN, 119°25ˊW 62 24, 40 1981 Grace 2008 34°10ˊN, 119°28ˊW 97 94 1979 Grace 2011 34°10ˊN, 119°28ˊW 97 46, 69, 82 Habitat 2007 34°17ˊN, 119°35ˊW 88 25, 41, 65 1981 Harmony 2004 34°22ˊN, 120°10ˊW 363 61, 85, 114, 147, 218, 328, 363 1989 Harvest 2004 34°28ˊN, 120°40ˊW 202 60, 86, 113, 141, 171, 202 1985 Henry 2000 34°19ˊN, 119°33ˊW 52 35 1979 Heritage 2008 34°21ˊN, 120°16ˊW 326 145, 215, 290 1989 Hermosa 2006 34°27ˊN, 120°38ˊW 179 85, 108, 130, 155, 179 1985 Hidalgo 2004 34°29ˊN, 120°42ˊW 129 129 1986 Hidalgo 2005 34°29ˊN, 120°42ˊW 129 57 Hidalgo 2006 34°29ˊN, 120°42ˊW 129 80, 105 Hillhouse 2007 34°19ˊN, 119°36ˊW 58 20, 40 1969 Hogan 2010 34°20ˊN, 119°32ˊW 47 22, 34 1967 Holly 2007 34°22ˊN, 119°52ˊW 60 60 1966 Holly 2011 34°22ˊN, 119°52ˊW 60 51 Hondo 2004 34°23ˊN, 120°07ˊW 255 253 1976 Hondo 2008 34°23ˊN, 120°07ˊW 255 58, 78, 98, 109, 132, 144, 169, 196, 225 Houchin 2000 34°20ˊN, 119°33ˊW 49 35 1968 Irene 2009 34°36ˊN, 120°43ˊW 72 50, 72 1985

Statistical Analysis.—We identified taxa to the lowest taxonomic level possible. Due to the structural plasticity of many sponge species, sponges were usually grouped by morphology. Therefore, our use of the term “species” refers not only to single species, but also to species aggregates. Species density (individuals m−1) was calculated as the number of individuals for a given species or species group on a given transect divided by the length of the transect. To visualize the relationships between species assemblages over depth gradients and across regions, we created two-dimensional, non-metric multidimensional scaling (nMDS) plot using the metaMDS function in the vegan package in R (R Core Team 2016). We used the taxa densities in each transect to illustrate spatial patterns in the nMDS plot. The depths of transects were grouped into five incremental depth categories from 20 to 400 m. The locations of the oil platforms were grouped into three zones: Los Angeles area, Santa Barbara Channel, and north of Santa Barbara Channel. In addition to visualizing species assemblage at the transect level, we also Love et al.: Sessile, structure-forming invertebrates 587 conducted the redundancy analysis (RDA; Legendre and Salvat 2015) to quantita- tively test the effects of the platforms’ maximum depths and their geographic locations (longitudes and latitudes) on species composition. The response variable was a matrix of relative abundance of the species or species groups at a platform level. Hellinger transformation was performed to obtain the similarity of the community composition among the samples (Legendre and Salvat 2015). The independent variables are the maximum depths of the platforms and the longitude coordinators of the platform. As the platforms were distributed from southeast to northwest of the Santa Barbara Channel, the longitude and latitude at the platform were correlated (Pearson correlation coefficient: P < 0.05, correlation = −0.96). Similarly, the temperature was also negatively related with the platform depth (P < 0.05, correlation = −0.84). To determine the importance of abiotic drivers, we then used variation partitioning to separate the effects of two independent variables on species composition (Borcard et al. 1992). RDA analysis and plots were performed using the rda function in the vegan R package. Species richness (the number of species present) was calculated at the platform level and a linear regression model was constructed to test how species richness changed across platforms. First, we constructed an analysis of variance (ANOVA) to test if species richness was affected by the number of years the platforms had been in the water. We then tested species richness in response to surveying effort (the length of the crossbeam) and depth.

Results

We selected 121 transects, conducted between 2000 and 2011, around 23 oil and gas platforms at depths between 20 and 365 m. These structures (with bottom depths ranging from 47 to 365 m) are situated between the northernmost at Point Arguello southward to the southernmost off Long Beach (Fig. 1, Table 1). We observed 20,357 individual invertebrates, comprising 19,800 Cnidaria and 557 Porifera. These comprised at least 15 species or species groups (Table 2). The white anemone, Metridium farcimen (Brandt, 1835; Fig. 2A) was by far the most commonly observed cnidarian and formed 97.6% of all invertebrates catalogued (Table 2). The alcyonacean, Leptogorgia chilensis (Verrill, 1868) (Fig. 2B), and the scleractinian, Lophelia pertusa (Linnaeus, 1758) (Fig. 2C), were the most commonly observed corals (Table 2). Several other species of alcyonaceans comprised most of the remaining Cnidaria. The white vase sponge (most or all in the family Aphrocallistidae; Fig. 2D) was the most abundant of the sponges (comprising 38.4% observed; Table 2). We also documented a variety of foliose, barrel, and other various-shaped sponges that we were unable to identify to a lower taxonomic level. The height of these invertebrates ranged from 20 to 80 cm (Table 2). With the notable exception of M. farcimen and L. chilensis (found on 21 and 14 platforms, respectively), all taxa were observed on <50% of the platforms (Table 3). Taxa displayed a variety of depth patterns (Table 2). Some, such as M. farcimen, white vase sponges, and L. pertusa, were found throughout most or all of the survey depth range, while others (notably the gorgonians, L. chilensis, Placogorgia spp., and Acanthogorgia spp.) were found over a relatively narrow range (Table 2). In general, taxa occupying relatively wide depth ranges were found at wide temperature ranges (Table 2). For instance, M. farcimen, observed at 20–364 m, was found at 7.9–16.6 °C. 588 Bulletin of Marine Science. Vol 95, No 4. 2019

Table 2. Structure-forming sessile invertebrates observed on 23 oil and gas platforms off California at depths of 20–364 m including minimum and maximum depths observed. 1Likely all or mostly Adelogorgia phyllosclera.

Temperature (°C) Max height Phylum/name n Range (m) (cm) Min Max Mean SD Cnidaria Metridium farcimen 19,318 20–364 80 7.9 16.0 10.7 1.5 Leptogorgia chilensis 223 28–75 65 11.1 13.2 11.9 0.6 Lophelia pertusa 198 81–290 40 9.1 10.9 10.0 0.5 Adelogorgia phyllosclera and 30 45–141 20 9.4 11.3 10.2 0.6 unidentified gorgonians1 Placogorgia spp. 16 81–145 60 9.7 10.4 9.9 0.2 Acanthogorgia spp. 10 141–215 25 9.6 10.0 9.8 0.2 Actinaria/Urticina spp. 2 65–226 20 9.0 9.0 9.0 NA Stylaster californicus (Verrill, 1 226 20 9.0 9.0 9.0 NA 1866) Unidentified Ceriantharia 1 191 20 9.5 9.5 9.5 NA Unidentified Hexacorallia/ 1 215 30 Octocorallia Porifera Unidentified vase sponges 214 81–364 40 7.9 10.9 9.9 0.7 Unidentified foliose sponges 165 45–328 60 8.2 11.3 10.3 0.8 Unidentified barrel sponges 99 98–290 52 9.1 10.9 9.9 0.5 Unidentified mound, branching, 76 22–328 20 8.2 13.3 10.1 1.1 and tube sponges Unidentified family Tethyidae 3 328 20 8.2 8.2 8.2 NA

Taxa occupying narrow depth ranges tended to be found over a narrow temperature range (i.e., L. chilensis, 28–75 m and 11.1–13.2 °C). One taxon in particular, L. pertusa, did not follow this pattern. It was found at 81–290 m, but over the small temperature range of 9.1–10.9 °C. Invertebrate assemblages tended to be similar among many platforms (Figs. 3, 4), reflecting species assemblage similarities across the study platforms. Species assemblages varied among depths (Fig. 3) as L. chilensis occurred on the shallow water crossbeams and a number of unidentified sponges resided in relatively deep water (>100 m). Platform depth was the major contributing factor to variability (Fig. 4) ex- plaining 20% of the total variance in our data set, whereas platforms’ geographic location explained only 2%. In particular, L. pertusa, L. chilensis, and the various unidentified sponges were the species or species groups that were most significantly influenced by depth. Based on these relationships, it is apparent that while the assemblages of structure-forming invertebrates varied by depth, there were no consistent geographic differences in species assemblages; the common taxa were geographically widespread. Results of the ANOVA showed that species richness did not vary among platforms (P = 0.77) based on the years they were installed. Note that we have surveyed all the horizontal crossbeams at each platform and the richness in our study represented the whole population for any given platform. The results suggested that species richness increased at any platform with longer crossbeams (P < 0.05) or located in the deeper part of the ocean (P < 0.05). The geographic location of the platform was not Love et al.: Sessile, structure-forming invertebrates 589

Figure 2. Images of four structure-forming invertebrates most commonly observed in this study. A = Metridium farcimen, B = Leptogorgia chilensis, C = Lophelia pertusa, and D = vase sponge. a significant factor P( = 0.3), indicating similar species assemblages across the study area at a given depth.

Discussion

The structure-forming invertebrates that we observed occupy hard substrates characterized by high or low relief. In general, taxa that typified platform assemblages are common on southern and central California natural reefs. Leptogorgia chilensis, for ex- ample, is a major component of low-relief reefs in central California (Hardin et al. 1994), and white vase sponges are very abundant on some southern California reefs (Yoklavich et al. 2011). Perhaps the major difference between platform and natural reef assemblages

590 Bulletin of Marine Science. Vol 95, No 4. 2019

family Tethyidae family

Unidentified Unidentified ------3

californicus

Stylaster Stylaster ------1 sponges sponges

7 5 5 1

Unidentified vase vase Unidentified ------11 15 27

143

barrel sponges barrel

Unidentified Unidentified 2 7 3 3 6 ------

and tube sponges tube and mound, branching, branching, mound,

4 1 7 9 5 ------

Unidentified Unidentified 30 25 foliose sponges foliose

1 2 7 4

Unidentified Unidentified ------12 23

116

spp. Placogorgia

------8 5 3 farcimen

1 9

42 Metridium Metridium 67 - - 111 411 542 503 930 450 485 874 809 201 104

1,992 4,569 2,016 2,257 1,079 1,866

chilensis

Leptogorgia Leptogorgia 2 1 4 8 5 4 ------

- 23 32 15 28 14 45 26 16 Lophelia pertusa Lophelia

2 6 ------

- - 36 53 33 68

gorgonians

and unidentified unidentified and

phyllosclera phyllosclera 1 4 2 6 1 1 - - -

------

Adelogorgia Adelogorgia

Octocorallia Hexacorallia/

------1 Unidentified Unidentified

Ceriantharia

Unidentified Unidentified ------1

spp.

Urticina /

Actinaria ------1 1

spp.

Acanthogorgia ------1 5 3 1 Table 3. Sessile, structure-forming invertebrates observed on 23 platforms off California, listed by platform. Platforms ordered alphabetically. 3. Sessile, structure-forming invertebrates observed on 23 platforms off Table Site A Platform Platform B Platform C Platform Edith Platform Ellen Platform Elly Platform Eureka Platform Gail Platform Gilda Platform Grace Platform Habitat Platform Harmony Platform Henry Platform Harvest Platform Heritage Platform Hermosa Platform Hidalgo Platform Hillhouse Platform Hogan Platform Holly Platform Hondo Platform Houchin Platform Irene Love et al.: Sessile, structure-forming invertebrates 591

Figure 3. Nonmetric multidimensional scaling (nMDS) plot showing that structure-forming invertebrate composition varied over water depths but was similarspatially within the study area. Each point represents the species composition at the transect level and the transect depths (m) were grouped into five depth categories (20–50, 50–100, 100–150, 150–200, 200–400 m). Transects from each of the platforms were grouped into three zones depending on their geographic locations: Zone 1, Los Angela area (squares); Zone 2, Santa Barbara Channel (circles); Zone 3, north of the Santa Barbara Channel (triangles). On this graph, to avoid overlaps, some species or species groups were removed. Specifically, unidentified Ceriantharia, Adelogorgia phyllosclera, and unidentified gorgonians were clustered with Metridium farcimen and unidentified vase sponges and unidentified mound, branching, and tube sponges were clustered with Lophelia pertusa.

Figure 4. Redundancy analysis (RDA) plot demonstrating the effects of two potential abiotic factors on the invertebrate composition among platforms. The dependent variable was the matrix of species density at each of the platforms. The depth was the maximum depth for the platform and it explained 20% of the variance in the data. The location is the longitude coordinate of the platform (latitude was correlated to longitude). Location explained only 2% of the variance, indicating species composition was not driven by platform locations. 592 Bulletin of Marine Science. Vol 95, No 4. 2019 was the ubiquity of M. farcimen at most of the platforms. While this anemone was the dominant species on many platforms, on natural reefs in California it occurs only spo- radically and usually in low densities (Lissner and Dorsey 1986, Hardin et al. 1994, Pirtle 2005, Bright 2007, Yoklavich et al. 2011). Our surveys of natural reefs indicate that this species is most abundant on hard substrata not subject to sediment deposition— precisely a characteristic of many platform jackets. On the other hand, a number of taxa that might be expected to live on platforms (based on occurrences on California natural reefs and at appropriate depths) were not present. These taxa included a number of priminoid [e.g., Plumarella longispina Kinoshita, 1908, and Parastenella ramosa (Studer, 1894)], plexaurid (e.g., Swiftia spp.), and antipatherid (i.e., Antipathes dendrochristos Opresko, 2005) corals (Bright 2007, Yoklavich et al. 2011). There are several possible explanations for these absences. First, we may have missed these taxa if they were rare. In addition, even the oldest of these platforms has only been present for about 50 yrs. If larval dispersal of these taxa is limited by current flow or planktonic larval duration or if successful recruitment is only episodic, these structures may not have been present long enough to have these corals settle out and survive. Furthermore, these taxa may have specific environmental re- quirements that platforms do not meet. Lastly, competitors for space on platforms may overpower the settlement of these taxa. Interplatform assemblage differences were driven by platform depth. Because platforms cover the entire water column, deeper platforms “added” more species as depths increased (Fig. 5). In addition, the invertebrate assemblages did not vary geographically. This was somewhat surprising as four platforms (Irene, Hidalgo, Harvest, and Hermosa; Fig. 1) are situated in a cold-water mass north of the Southern California Bight. Apparently, these taxa have a wide geographic distribution and are not limited to one geographic area. However, note that because many of the “taxa” we documented may be composed of multiple species, there could be underlying geographic structure that is masked by these groupings. A previous study (CSA 2005) had surveyed the overall invertebrate assemblages on eight platforms off central and southern California—all platforms that we have surveyed. Using a remotely operated vehicle, the research focused only on platform legs (rather than the crossbeams and legs as in our study) and its conclusions regarding sessile, structure-forming invertebrates varied somewhat from ours. Similar to our study, CSA (2005) did find that M. farcimen was the dominant taxa on many platforms. It also found that vase sponges were characteristic of a depth range of about 100–200 m. However, with the exception of vase sponges, CSA (2005) documented few other large sponges (e.g., no barrel and few foliose sponges), almost no gorgonian corals, and did not record any L. pertusa. We suspect there may be two reasons for this discrepan- cy. First, our surveys covered far more of the platform structure than did that of CSA (2005). Thus, it might be expected that our study would stand a better chance of observ- ing the less common taxa. Second, it is possible that many of these larger taxa are not adapted to living on the sheer vertical faces of jacket pilings, faces that may be subject to intense, sheering currents. It is difficult to compare our data on sessile structure-forming invertebrates with those from platforms in other parts of the world. Many of these platforms are either in warmer water masses (Scarborough Bull and Kendall 1994, Ponti et al. 2002, Stachowitsch et al. 2002, Yan et al. 2006, Friedlander et al. 2014, Sammarco et al. 2014) or, as in the case of North Sea structures, are sited in waters far shallower than most of Love et al.: Sessile, structure-forming invertebrates 593

Figure 5. The relationship between the number of species or species groups of sessile, structure- forming invertebrates and maximum platform depth. Platforms are ordered on the x-axis from deepest to shallowest. the platforms we surveyed (van der Stap et al. 2016). However, on some of the deepest of the North Sea platforms surveyed (to depths of about 160 m), three species of structure-forming invertebrates—Metridium dianthus (Ellis, 1768); the soft coral, Alcyonium digitatum Linnaeus, 1758; and L. pertusa—are major components of the assemblage at depths starting (depending on species and platform) as shallow as about 20 m and end- ing near the bottom (Roberts 2002, Whomersley and Picken 2003, Cordah 2011, van der Stap et al. 2016). Unlike off California, North Sea platform invertebrate cover appears to decrease and become sparse near the bottom on at least some platforms, perhaps the result of increasing siltation near the bottom (Cordah 2011). The presence of L. pertusa on both California and North Sea platforms attests to the apparent ubiquity of this species. This scleratinian coral appears to be circumglobal (Rogers 1999) and, worldwide, is found at depths of 30–3383 m (Freiwald et al. 2004). On North Sea structures, it has been found in waters as shallow as 40 m, although most colonies live at depths >70 m “below the seasonal thermocline in the northern North Sea” (Roberts 2002). The shallowest colony we observed was in 81 m. It is unknown whether L. pertusa in the eastern Pacific Ocean has physiological limitations similar to those in the North Sea. However, worldwide, this species has been found in 4–12 °C and at around 8 °C on North Sea platforms (Roberts 2002), the latter being similar to the 9.1–10.9 °C waters it inhabited off California. In general, California platforms harbor a greater diversity of large sessile invertebrates than are found around North Sea platforms (LA Henry, University of Edinburgh, pers comm). The maximum size of the three largest taxa on North Sea platforms, M. dianthus, A. digitatum, and L. pertusa are 60 cm height, 20 cm height, and almost 2 m width, respectively (Hayward and Ryland 1995; LA Henry, University of Edinburgh, pers comm); no other taxa >20 cm have been observed. In contrast, on California platforms, there are a number of taxa that are 20 cm or larger and nearly half are 40 cm and greater (Table 2). In particular, while large sponges are a major part of the California platform fauna, they are not found on North Sea platforms. Rather, low-lying taxa, such 594 Bulletin of Marine Science. Vol 95, No 4. 2019 as Leucosolenia variabilis Haeckel, 1870, Leucosolenia botryoides (Ellis and Solander, 1786), Suberites ficus (Johnson, 1842), and the encrusting Halichondria panicea (Pallas, 1766), have been documented (Cordah 2011; J Coolen, Wageningen Marine Research, pers comm). Low-lying sponges occur on California platforms, but we did not docu- ment the presence or abundance of this community. We note that the somewhat larger Haliclona oculata (Linnaeus, 1759) has been observed on North Sea shipwrecks and may live on platforms in that area (J Coolen, Wageningen Marine Research, pers comm). Lastly, while our surveys encompassed substantial portions of each platform, they were by no means complete. First, we did not examine the shallowest crossbeams. In addition, we surveyed neither the risers that bring oil or gas to the surface nor the in- ner side of the crossbeams or pilings. Lastly, only those portions of the pilings that intersected the crossbeams were surveyed. Thus, it is possible that some relatively rare taxa were not recorded or that we have underestimated the importance of some taxa. Nevertheless, we believe this study provides a useful documentation of the sessile, structure-forming macro-invertebrates on California offshore platforms.

Acknowledgments

This research was conducted aboard the RV Velero and we thank Captain I Leask and all of its crew for their help. We thank submersible pilots J Heaton, D Slater, J Lilly, and C Ijames for their great work. LA Henry and J Coolen provided substantial information regarding North Sea platform invertebrates. This research was funded by MMS contract numbers 1435-01-03-CA-72694 and BOEM M10AC2001 and by BOEM Awards M11AC00008 and M15AC00014 and by the National Aeronautics and Space Administration Biodiversity and Ecological Forecasting program (NASA Grant NNX14AR62A), the Bureau of Ocean and Energy Management Ecosystem Studies program (BOEM Award MC15AC00006), University of Southern California Sea Grant, and NOAA in support of the Santa Barbara Channel Biodiversity Observation Network.

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