OCS Study 

•

MMS 2005-003 

Spatial and Temporal Variability in 

Early Successional Patterns of an 

Invertebrate Assemblage at an 

Offshore Oil Platform 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 

 OCS Study 

 MMS 2005-003 

Spatial and Temporal Variability in 

Early Successional Patterns of an 

Invertebrate Assemblage at an 

Offshore Oil Platform 

Final Technical Summary 

Final Study Report 

Authors 

Jason Bram 

Principal Investigators

Henry M. Page 

Jenifer Dugan 

Prepared under MMS Cooperative 

Agreement No. 14-35-0001-30761 

by 

Southern California Educational Initiative 

Marine Science Institute 

University of California 

Santa Barbara, CA 93106 

U.S. Department of the Interior 

Minerals Management Service Camarillo

Pacific OCS Region January 2005
Disclaimer 

This report has been reviewed by the Pacific Outer Continental Shelf Region, Minerals Management 

Service, U.S. Department of the Interior and approved for publication. The opinions, findings, 

conclusions, or recommendations in this report are those of the authors, and do not necessarily reflect 

the views and policies of the Minerals Management Service. Mention of trade names or commercial 

products does not constitute an endorsement or recommendation for use. This report has not been 

edited for conformity with Minerals Management Service editorial standards. 

Availability of Report

Extra copies of the report may be obtained from: 

U.S. Dept. of the Interior 

Minerals Management Service 

Pacific OCS Region 

770 Paseo Camarillo 

Camarillo, CA 93010 

Phone: 805-389-7621 

A PDF file of this report is available at: 

http://www.coastalresearchcenter.ucsb.edu/SCEI/ 

Suggested Citation 

The suggested citation for this report is: 

Page, H., J. Dugan, and J. Bram. Spatial and Temporal Variability in Early Successional Patterns of 

an Invertebrate Assemblage at an Offshore Oil Platform. MMS OCS Study 2005-003. Coastal 

Research Center, Marine Science Institute, University of California, Santa Barbara, California. MMS 

Cooperative Agreement Number 14-35-0001-30761. 39 pages. 
Table of Contents 

FINAL TECHNICAL SUMMARY....................................................................... 1 

FINAL STUDY REPORT...................................................................................... 7 

 Introduction....................................................................................................... 7 

 

Materials and Methods..................................................................................... 8 

 Study Site ...................................................................................................... 8 

 Development of the assemblage ................................................................... 9 

 Biotic interactions ......................................................................................... 10 

 Statistical analysis ......................................................................................... 11 

 

Results................................................................................................................ 11 

 Development of the assemblage ................................................................... 11 

 Longer-term patterns..................................................................................... 16 

 Biotic interactions ......................................................................................... 18 

 Accumulation of invertebrate biomass on the platform structure................. 19 

 Composition and rate of faunal litterfall from platform structures............... 21 

 

Discussion........................................................................................................... 22 

 Development of the assemblage ................................................................... 22 

 Effect of depth and year on assemblage development.................................. 24 

 Patterns of succession at the platform........................................................... 25 

 

References.......................................................................................................... 28 

Acknowledgements............................................................................................ 32 

 
List of Tables

Table 1. List of sessile invertebrates on the settlement plates during the study ..... 12 

 

List of Figures 

Figure 1. Schedule for deployment and retrieval of tiles in the field...................... 9 

Figure 2. Mean percent cover of colonial tunicates, encrusting bryozoans, and 

 amphipod tube mat......................................................................................... 13 

Figure 3. Mean percent cover of barnacles and sponges ........................................ 14 

Figure 4. Mean density of barnacles ....................................................................... 15 

Figure 5. Mean percent cover of selected taxa........................................................ 16 

Figure 6. Mean percent cover of open space .......................................................... 17 

Figure 7. Mean percent cover of selected taxa on tiles submerged in April........... 18 

Figure 8. Mean percent cover of tubiculous amphipods, colonial tunicates, encrusting 

bryozoans, barnacles, and open space............................................................ 19 

Figure 9. Mean accumulation of biomass ............................................................... 20 

Figure 10. Mean biomass of invertebrates on ceramic tiles at depths of 6,12,18m.. 20 

Figure 10. Accumulation of invertebrate biomass on tuffys..................................... 21 

Figure 11. Mean biomass of invertebrate taxa in faunal litterfall............................. 22 

Plate I. Photographs of ceramic tiles submerged for 24 months.............................. 27 
Spatial and Temporal Variability in Early Successional Patterns 

1

FINAL TECHNICAL SUMMARY 

STUDY TITLE: Early Development of Fouling Communities on Offshore Oil Platforms 

REPORT TITLE: Spatial and Temporal Variability in Early Successional Patterns of an 

Invertebrate Assemblage at an Offshore Oil Platform 

CONTRACT NUMBER: 14-35-0001-30761 

SPONSORING OCS REGION: Pacific 

APPLICABLE PLANNING AREA(S): Southern California 

FISCAL YEAR(S) OF PROJECT FUNDING: FY 99, FY 00, FY 01 

COMPLETION DATE OF REPORT: January 2005 

COSTS: FY 99 - $83,695; FY 00 - $75,426; FY 01 - $39,819; FY 02 - no cost 

CUMULATIVE PROJECT COST: $198,940 

PROJECT MANAGERS: Russell J. Schmitt

AFFILIATION: Coastal Research Center, Marine Science Institute, University of 

California, Santa Barbara, CA 93106-6150 

ADDRESS: University of California, Santa Barbara, CA 93106 

PRINCIPAL INVESTIGATORS: Henry M. Page, Jenifer Dugan 

ADDRESSES: Marine Science Institute, University of California, Santa Barbara, CA 93106-

6150 

KEY WORDS: Succession, invertebrates, fouling assemblage, oil platform, artificial habitat 

BACKGROUND: The subtidal portions of offshore oil platforms provide a hard attachment 

site for an assemblage of invertebrates that can reach tens of centimeters in thickness. Many 

of the species inhabiting platforms off the coast of California are also typically found in local 

rocky intertidal and subtidal habitats. Interest in the biotic assemblages associated with 

platforms has increased recently because of the “rigs to reefs” concept. The potential function 

of such structures as artificial reefs may vary greatly with location, depth and configuration. 

However, there is a general lack of published information on factors affecting patterns of 

development or processes that can influence the rate of development and composition of the 

invertebrate assemblages on offshore platforms. 
Final Technical Summary – Page and Dugan 

2

OBJECTIVES: The objectives of this study were primarily to: 1) test the effects of water 

depth and time, including time of year and year, on the rate of colonization, species 

composition, and cover of the invertebrate assemblage at an offshore platform through the 

manipulation of artificial substrata (tiles) attached to the structure, 2) explore the potential 

influence of selected taxa that occur early in the successional sequence on the relative 

abundance of other colonists, and 3) evaluate the importance of depth and time on the 

accumulation of invertebrate biomass on the platform structure.

DESCRIPTION: This study was conducted at the oil and natural gas production platform 

“Houchin” (Pacific Operators Offshore, Inc.), located 6.6 km offshore of Carpinteria, 

California, in the Santa Barbara Channel (34°20'N, 119°33'W) in a water depth of ~49 m. 

Just prior to the initiation of this study, invertebrates attached to the platform structure were 

removed to a depth of 18 m during routine maintenance operations. We used unglazed 

ceramic tiles to experimentally investigate the effects of depth, time of year, and year on 

patterns of development of the invertebrate assemblage. 

The tiles (15 x 15 cm) were attached to the platform at three depths (6, 12, 18 m), randomly 

assigned to a length of deployment of either 2, 4, 6, 12, or 24 months, and replaced following 

retrieval, thus allowing data gathered for each length of deployment to be independent 

through time. Deployment of tiles was initiated in April 1999 and terminated in April 2001. 

To explore the effect of selected early colonists and depth on patterns of development of the 

invertebrate assemblage, we conducted a second experiment from June 2002 through June 

2003 in which selected taxa were routinely removed from experimental tiles. In addition, 

scrapings were collected from the recently cleaned platform surface after designated intervals 

(2, 4, 6, 12 months) at three depths (6, 12, 18 m) from September 1999 to July 2001, and the 

more structurally complex tuffys were deployed monthly from May 1999 to April 2001. At 

an adjacent platform, rates of faunal litterfall of invertebrates from the platform to the benthos 

were measured. 

SIGNIFICANT CONCLUSIONS: The invertebrate assemblage that developed on the tiles 

submerged at Platform Houchin was composed of taxonomic groups commonly reported from 

other subtidal hard substrate habitats, including coastal rocky reefs in the Santa Barbara 

Channel. There was, however, a notable absence of macroalgae on the tiles. 

Colonial tunicates were predictable early colonists of the tiles. Colonial tunicates are often 

considered “opportunistic” and poor competitors and our data suggest that later colonists 

negatively influenced the rates of recruitment, growth, or survival of tunicates as submersion 

time increased. Encrusting bryozoans were also early colonists of the tiles, but the cover of 

this taxon varied less predictably over time than tunicates. With longer submersion times, the 

overall cover of bryozoans decreased, suggesting that later colonizing taxa also inhibited the 

recruitment and/ or growth of bryozoans. Other taxa, including tubiculous amphipods, 

barnacles, and sponges occurred on the 2-month tiles in variable and generally low cover. 

Among these taxa, the cover of tubiculous amphipods (as amphipod tube mat) was highest, 

but generally much less than 20%. Since open space was available on the tiles for up to 6 

months, the availability of this resource did not appear to limit amphipod recruitment or 

growth. The mean cover and variability in cover of amphipods generally increased with 
Spatial and Temporal Variability in Early Successional Patterns 

3

increasing submersion time up to 6 months, suggesting that the increase in surface complexity 

over time provided by other attached organisms could have facilitated the recruitment and/or 

survival of amphipods. However, in our study, the removal of tunicates, bryozoans, or 

barnacles did not result in a significant decrease in the cover of amphipods, suggesting that 

factors other than these early colonists influenced the cover of amphipod tube mat. 

Cover of barnacles and sponges on the 2-month tiles was typically very low. Open space was 

still available after 2 months and there was no evidence to suggest that barnacles or sponges 

were inhibited from settling or were overgrown by taxa already on the tiles. It is likely that 

the cover of barnacles and sponges was determined by factors such as larval supply, postsettlement survival, and/or growth rate. 

Depth greatly influenced the cover of bryozoans, amphipods, barnacles, sponges, and mussels 

on the tiles. However, the effect of depth on cover varied among taxa. For example, 

barnacles appeared earlier (after 2 months) and were present in significantly higher cover at 6 

m (55% after 24 months) compared with the deeper depths (<28% after 24 months). In 

contrast, bryozoans were present in higher cover and persisted longer on tiles at deeper 

depths. The data from our removal experiment also suggest that depth-related variation in 

the distribution and cover of species occurring early in the successional sequence (i.e., 

bryozoans, amphipods, barnacles) was not the result of interspecific interactions. Our results 

suggest the alternative explanation that the distribution and abundance of these taxa was 

related to variation in the density of larvae (or juvenile amphipods) available for settlement 

among depths. 

Mussels appeared later in the successional sequence. Mussel recruitment was low during this 

study; mussels occurred only at 6 m on the 12 and 24-month tiles in 2000-2001 (Figs. 3f, 6d). 

The lack of mussels on tiles at deeper depths (12 and 18 m) may indicate that mussel larvae 

were not present. However, mussels have been shown to recruit and grow on platforms in the 

Santa Barbara Channel to depths exceeding 18 m. The higher cover of barnacles at the 

shallowest depth (6 m) may have facilitated mussel attachment at this depth by providing a 

more rugose substrate, whereas the low density of barnacles at 12 and 18 m may have 

retarded mussel recruitment at those depths. 

Some taxa (sponges, barnacles, mussels) occurred in much higher cover (and density for 

barnacles) on the tiles in 2000-2001 compared with 1999-2000. Rates of recruitment of these 

taxa may have been influenced by differences in oceanic conditions between the two years. A 

record coastal upwelling event occurred in Spring 1999 and coastal upwelling continued in 

the Southern California Bight through Summer 1999. 

The development of the invertebrate assemblage on the tiles submerged at Platform Houchin 

could not be categorized as classical succession, but was clearly not random. Development of 

this assemblage appears to fit the pattern where a combination of life history of the constituent 

species, settlement preferences, and responses to spatial and temporal environmental 

variability create a pattern of early succession that was largely predictable in terms of the 

sequence of occurrence of dominant species, but variable in rate, depending on time of year or 

depth of tile submersion. 
Final Technical Summary – Page and Dugan 

4

STUDY RESULTS: A total of 41 invertebrate genera from 6 phyla and unidentified taxa of 

a seventh phylum (Porifera) colonized the tiles. Colonial tunicates and encrusting bryozoans 

were consistent early colonists on the tiles across all depths. Tubiculous amphipods were also 

early colonists of the tiles, but highest cover of this taxon generally occurred after 6 or 12 

months. Between 12 and 24 months, the cover of amphipod tube mat remained unchanged or 

declined slightly at all depths. Barnacles could occur on the tiles at 6 m after 2 months and 

generally increased in cover over time at all depths, with a much slower increase occurring at 

12 and 18 m compared with 6 m. Cover of barnacles was higher on tiles at 6 m compared 

with 12 and 18 m. Sponges were largely absent from tiles submerged for <12 months, but 

greatly increased in cover from 12 to 24 months at all depths, with significantly higher cover 

at 12 and 18 m compared with 6 m at 24 months. Mussels were also later colonists, occurring 

on the tiles only at a depth of 6 m after 12 months. 

In the removal experiment, colonial tunicates, encrusting bryozoans, amphipod tube mat, and 

barnacles comprised 82-88% of total cover on the controls at each depth after 12 months. In 

the tunicate removal treatment, the mean cover of amphipod tube mat, encrusting bryozoans, 

barnacles, and open space did not differ from the control; there was a trend of higher cover of 

bryozoans in the removal treatment. In the bryozoan removal treatment, only the mean cover 

of open space was higher than the control. In the barnacle removal treatment, only the cover 

of bryozoans was significantly higher than the control. A significant effect of depth on the 

cover of the tile assemblage was found in the barnacle removal treatment where the percent 

cover of amphipod tube mat decreased with increase in depth. There was a marginally 

significant increase in the cover of bryozoans in the tunicate removal treatment with increase 

in depth. 

The standing crop biomass of the invertebrates varied significantly with depth, year, and time 

from scraping. The accumulation of biomass always increased with increased exposure time, 

was highest at a depth of 6 m, and was greater in 2000-2001 compared with 1999-2000 on 

both the tiles and on the scraped platform surfaces. The standing crop biomass of the 

invertebrates on the tuffys also varied significantly with depth. The accumulation of biomass 

was highest at a depth of 6 m and lowest at the 18 m depth. A marked increase in biomass 

occurred between April and June of 2000 at all three depths. Most of the biomass found on 

the tuffys (>99%) was made up of gammarid and caprellid amphipods. 

Mussels, hydroids, bryozoans, sponges, and barnacles accounted for nearly all (>99%) of the 

invertebrate biomass in the litterfall traps. Mussels accounted for the greatest biomass of any 

taxa in all but 3 months. Faunal litterfall tended to be highest in the spring and summer 

months and lowest in the winter months. 
Spatial and Temporal Variability in Early Successional Patterns 

5

STUDY PRODUCTS: 

Presentations: 

Bram, J., H. M. Page, and J.E. Dugan. 2001. Early development of subtidal invertebrate 

assemblages on offshore oil platforms in the Santa Barbara Channel. Western Society 

of Naturalists, Ventura. 

Page, H. M. 2001. Ecology of offshore oil platforms and mussel mounds in the Santa 

Barbara Channel. Invited seminar speaker. Bodega Marine Laboratory. 

Publications: 

Bram, J. B., 2003. Spatial and temporal variability in early successional patterns of an 

invertebrate assemblage on an offshore oil platform. Masters Thesis, University of 

California, Santa Barbara. 

Bram, J. B., H. M. Page, and J. E. Dugan. Spatial and temporal variability in early 

successional patterns of an invertebrate assemblage at an offshore oil platform. 

Journal of Experimental Marine Biology and Ecology, in press. 
Final Technical Summary – Page and Dugan 

6
Spatial and Temporal Variability in Early Successional Patterns

7

FINAL STUDY REPORT 

INTRODUCTION 

The process of change in species composition during the colonization of open space is 

defined as succession (Horn, 1974; Connell and Slatyer, 1977). Classical succession is 

considered to be an orderly and directional process that results from the alteration of the 

physical environment by the developing community, eventually leading to a climax 

community (Clements, 1936; Odum, 1969; Colinvaux, 1973). Classical successional 

theory was originally derived from studies of terrestrial plant associations, and has now 

been applied to many other types of assemblages, including those in the marine habitat 

(e.g. Drury and Nisbet, 1973; Connell and Slatyer, 1977). The concept of succession has 

been used to predict how an assemblage will change over time and to understand 

processes responsible for these changes. However, in many systems, no single 

successional model has successfully accounted for changes in community structure over 

time and descriptions of succession may often involve more than one model (Connell and 

Slatyer, 1977; Dean and Hurd, 1980, Walker and Chapin, 1987; Farrell, 1991). 

The development of epibenthic assemblages on hard substrates in the marine 

environment has been the subject of numerous investigations (e.g., Scheer, 1945; 

Sutherland and Karlson, 1977; Greene and Schoener, 1982; Underwood and Anderson, 

1994). These assemblages provide useful systems for ecological study because of the 

presence of an easily defined limiting resource (space), the relative ease of experimental 

manipulation, and high species turnover over short time spans (Dean and Hurd, 1980). 

Several studies have found evidence supporting the idea of succession occurring through 

a predictable progression of species (Scheer, 1945; Sousa, 1979ab; Vance, 1988; Farrell, 

1991). However, results from other studies have suggested that succession may be less 

predictable. For example, patterns of succession in hard-bottom assemblages were found 

to vary both temporally (Sutherland and Karlson, 1977; Turner and Todd, 1993; 

Underwood and Anderson, 1994; Chapman, 2002) and with water depth (Ojeda and 

Dearborn, 1989; Smith and Witman, 1999; Garrabou et al., 2002). Thus, the 

predictability of successional patterns may vary with the specific habitat or assemblage 

being studied (Underwood and Anderson, 1994). 

The subtidal portions of oil platforms off the coast of California provide habitat for an 

assemblage of invertebrates that can reach tens of centimeters in thickness (Wolfson et 

al., 1979; Page et al., 1999). Many of the species attached to platforms are also found in 

local rocky intertidal and subtidal habitats. However, the physical and biological 

environment at offshore platforms differs from that of inshore rocky intertidal and 

subtidal reefs. Platforms in the Santa Barbara Channel are located offshore (>3 km) on 

soft-bottom in deeper water (>30 m depth). The platform invertebrate assemblage 

experiences fully oceanic conditions of water temperature, salinity, and current flow, and 

invertebrate growth rates are rapid compared to inshore habitats (Page, 1986; Page and 

Hubbard, 1987). In addition, the densities of grazers (e.g., urchins, gastropods), which 

can reach 10-17 individuals m-2 on inshore subtidal rocky reefs (Breitburg, 1985), are 
Final Study Report – Page and Dugan 

8

much lower on platforms (Page, unpubl. data). Disturbance in the form of storm swell 

and maintenance operations opens up space for colonization on the platform structure 

through the dislodgement of invertebrates (Page et al., 1999). 

There is little published information on factors affecting patterns of development or 

processes that can influence the rate of development and composition of the invertebrate 

assemblage on platforms. However, two studies in natural habitats in the southern 

California region provide alternative scenarios that could occur during succession on 

offshore platforms in the Santa Barbara Channel. Vance (1988) found predictable 

patterns of succession of macroalgae and sessile invertebrates on a subtidal vertical rock 

wall (~15 m depth) near Santa Catalina Island (30 km south of Los Angeles, California), 

which with some variation was repeatable from season to season and year to year. In 

contrast, Breitburg (1985) found species succession to be dominated by macroalgae, 

particularly encrusting coralline algae, and strongly influenced by grazers on 

experimental plates deployed at a subtidal rocky reef (~15 m depth) near Santa Barbara, 

California. 

Based on the physical conditions present offshore, and the scarcity of grazers that could 

influence the course of succession, we predicted that development of the platform 

assemblage might conform more closely to the successional pattern found by Vance 

(1988) than the pattern found on local rocky reefs by Breitburg (1985). In addition, early 

colonists may influence the rate of recruitment or survival of subsequent colonists 

through inhibitory or facilitative interactions (e.g., Connell and Slatyer, 1977; Sutherland, 

1978; Dean and Hurd, 1980; Breitburg, 1985); these interactions could explain temporal 

and spatial variation in patterns of succession. 

In this study, we tested the following hypotheses through the manipulation of artificial 

substrata (tiles) attached to an offshore platform: 1) development of the platform 

assemblage occurs through a predictable succession of species or taxa, 2) the rate of 

development, cover, and composition of the assemblage varies with date, year, length, 

and depth of submersion, and 3) biotic interactions can modify the cover and composition 

of the developing assemblage.

MATERIALS AND METHODS

Study Site 

This study was conducted at the oil and natural gas production platform “Houchin” 

(Pacific Operators Offshore, Inc.), which is located 6.6 km offshore of Carpinteria, 

California, in the Santa Barbara Channel (34°20'N, 119°33'W) in a water depth of ~49 m. 

The submerged portion of the platform consists of vertical, oblique, and horizontal 

cylindrical cross members, which provide support to the structure, and conductor pipes, 

through which wells are drilled. The assemblage of sessile and semi-mobile invertebrates 

that colonize the support members and conductor pipes of Platform Houchin are also 

found on other platforms in southern California (Simpson, 1977; Wolfson et al., 1979; 
Spatial and Temporal Variability in Early Successional Patterns

9

Page et al., 1999). Invertebrates that are prominent space-holders on the platform at 

shallow depths (to ~6 m) include mussels (Mytilus californianus, Conrad, 1837, M. 

galloprovincialis, Lamarck, 1819), barnacles (e.g. Megabalanus californicus, Pilsbry, 

1916, Balanus trigonus, Darwin, 1854) and the strawberry anemone (Corynactis 

californica, Carlgren, 1936), with encrusting bivalves (Crassadoma gigantea, J. E. Gray, 

1825) and the white plumed anemone (Metridium senile, Linnaeus, 1761) more abundant 

deeper. Just prior to the initiation of this study, invertebrates attached to the platform 

structure were removed to a depth of 18 m during routine maintenance operations. 

Development of the assemblage 

We used unglazed ceramic tiles (15 x 15 cm) attached in groups of five to experimental 

frames to test the first two hypotheses. Each frame (15 x 105 cm) was constructed of 

PVC pipe reinforced with rebar. Individual frames, containing five tiles each, were 

attached to four adjacent vertical conductor pipes at depths of 6, 12, and 18 m (n=4 

frames for each depth). Adjacent conductor pipes were located ~1.5 m apart. Each tile in 

a frame was randomly assigned a submersion time of 2, 4, 6, 12, or 24 months and 

replaced at the specified time with a new tile (except for the 24 month tiles) (Fig. 1). 

Each tile was only censused once. Thus, over the 24 month duration of the experiment, 

data were obtained from 12 sets of tiles submerged for 2 months, 6 sets for 4 months, 4 

sets for 6 months, 2 sets for 12 months, and one set for 24 months. Since tiles were 

replaced with new tiles following retrieval, data gathered for each length of submersion 

were independent through time (Underwood and Anderson, 1994). The smooth side of 

each tile was used as the sampling surface and oriented outward from the platform. 

Figure 1. Arrows indicate the schedule for the deployment and retrieval of tiles submerged in the field for 

periods of 2, 4, 6, 12, or 24 months at depths of 6, 12, or 18 m. Retrieved tiles were replaced with new tiles 

(n=4 for each depth). 

 

Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr

1999 2000 2001

24

Submersion time (months)

12

6

4

2

Date (month)

Apr Jun Aug Oct Dec Feb Apr Jun Aug Oct Dec Feb Apr

1999 2000 2001

24

Submersion time (months)

12

6

4

2

Date (month)
Final Study Report – Page and Dugan 

10

During the course of the study, a few large individuals of the anemone, Metridium senile, 

moved onto a few of the tiles from the adjacent platform structure, thereby occupying 

space on the tile and potentially inhibiting the recruitment of other invertebrates. Once a 

month, SCUBA divers inspected the tiles and all large anemones that had moved onto the 

tiles were removed. On collection, the tiles were carefully placed in plastic bags in situ

and transported to the laboratory. In the laboratory, the tiles were photographed to aid in 

species identification, and then frozen for later analysis. Tiles were first deployed in 

April 1999 and the experiment was terminated in April 2001. 

 

We identified and measured the percent cover of invertebrates on each tile using pointcontact sampling (Underwood and Anderson, 1994). A pin (1 mm diameter) was inserted 

through each of 50 holes spread uniformly throughout a clear fiberglass grid situated 

above the tile. All organisms touched by the pin were identified and recorded. The 

percent cover of each species was determined by multiplying the number of contacts by 

two. Due to the layering of species on the tile, the pin could contact more than one 

species and the total cover could exceed 100%. Non-sessile species, such as brittlestars, 

were removed prior to point contact analysis. Tubiculous amphipods were quantified 

using percent cover of their tube mat. Due to the difficulty in identifying some of the 

organisms to species level, data were combined into higher taxonomic categories for 

statistical analyses.

We also quantified barnacle density by counting individuals on each tile. Four species of 

barnacles were found during the study (Megabalanus californicus, Balanus trigonus, 

Balanus pacificus, Pilsbry, 1916, Balanus regalis, Pilsbry, 1916). For statistical analysis, 

all the barnacle data were pooled due to the difficulty of accurately determining the 

species of new recruits. 

Biotic interactions 

To test the effect of early colonists and depth on assemblage development, we conducted 

a second experiment from June 2002 through June 2003 in which selected early 

colonizing taxa were routinely removed from experimental tiles. Individual frames, each 

with four unglazed ceramic tiles (15 x 15 cm), were attached to four adjacent conductor 

pipes at depths of 6, 12, and 18 m (n=4 frames for each depth). Each tile in a frame was 

randomly assigned to one of four treatments in which either 1) colonial tunicates, 2) 

encrusting bryozoans, or 3) barnacles were removed, or 4) no taxa were removed 

(control). The taxa selected for removal were chosen based on their appearance early in 

the successional sequence in the previous experiment and their potential to affect the rate 

of colonization, species composition and cover of the developing assemblage. Colonial 

tunicates can grow rapidly and have been reported to inhibit barnacle recruitment (Young 

and Gotelli, 1988). Encrusting bryozoans can also inhibit the recruitment and growth of 

other species (Sutherland and Karlson, 1977). Barnacles have been reported to inhibit the 

recruitment of tunicates, but facilitate the recruitment of mussels (Dean and Hurd, 1980). 

As in the preceding experiment, the smooth side of each tile was used as the sampling 

surface and oriented outward from the platform. 
Spatial and Temporal Variability in Early Successional Patterns

11

 

Every 4 weeks, SCUBA divers inspected the tiles and carefully removed the appropriate 

taxon from each treatment using a small spatula. As described earlier, individuals of the 

anemone, Metridium senile, which migrated onto the tiles, were removed. On 

termination of the experiment, the tiles were carefully placed in plastic bags in situ and 

transported to the laboratory. The tiles were then frozen for later analysis. We identified 

and measured the percent cover of invertebrates on each tile using the point-contact 

sampling method described above. 

Statistical analysis 

All statistical analyses were conducted using Systat 10 (SPSS). The main effects of date 

and depth for each length of submersion (first experiment) or the removal of taxa from 

the tiles and depth (second experiment) on the percent cover of selected taxa (and also 

density for barnacles) was explored using Analysis of Variance (ANOVA). We tested 

the main effects using a split plot model where all the tile treatment combinations were 

present within each “whole plot” (pipe x depth combination); error terms were calculated 

as in Hicks (1999). The percent cover data were arcsine transformed (x’=arcsine √x) and 

the barnacle density data were log transformed (x’= log10 (x +1)) prior to analysis (Zar, 

1999). 

RESULTS 

Development of the assemblage 

Invertebrate species from 7 phyla colonized the tiles (Table 1). Invertebrate taxa 

important as space holders included a colonial tunicate (Diplosoma listerianum, 

Macdonald, 1859), encrusting bryozoans (Dendrobeania laxa, Robertson, 1905,

Membranipora serrilamella, Osburn, 1950), tubiculous amphipods (Ericthonius sp., 

Milne-Edwards, 1830), barnacles (Balanus trigonus, Balanus pacificus, Megabalanus 

californicus), and sponges. No macroscopic algae colonized the tiles. 

Colonial tunicates (primarily Diplosoma listerianum) were consistently among the first 

taxa to colonize the tiles. Mean cover of tunicates on the 2-month tiles was highly 

variable across sampling dates, with a significant date x depth interaction (P<0.001, 

F=3.664, df=22, 66, ANOVA) (Fig. 2a). Cover of this taxon on the 2-months was 

highest (62-78%) in late summer and fall (August, October) and lowest (<5%) in spring 

and early summer (April, June). With increased submersion time, temporal variation in 

the cover of tunicates became less evident (Fig. 2b), and was not significant (P>0.1) on 

the 6-month tiles where cover was <25% at 12 m and <10% at 6 and 18 m (Fig. 2c). 

There was significant effect of depth on the mean cover of colonial tunicates only for the 

6-month tiles (P=0.003, F=17.953, df=2, 6, date x depth interaction, P>0.1). 
Final Study Report – Page and Dugan 

12

Encrusting bryozoans (primarily Dendrobeania laxa) were also early colonizers of the 

tiles (Fig. 2d). There was a significant date x depth interaction in the mean cover of 

bryozoans on the 2-month tiles (P<0.001, F=3.558, df=22, 66, ANOVA) with a 

suggestion of seasonality at 18 m where highest cover occurred in late summer-early fall. 

Table 1. List of sessile invertebrates on the settlement plates during the study. 

Phylum Species 

Porifera Sponges Unidentified 

Cnidaria Anemones Corynactis californica 

 Epiactis prolifera 

Hydrozoans Aglaophenia latirostris 

 Plumularia sp. 

 Tubularia crocea

Ectoprocta Bryozoans Bugula neritina 

Celleporella brunnea 

Celleporella hyalina 

Crisulipora occidentalis 

Dendrobeania laxa 

Filicrisia franciscana 

Membranipora serrilamella 

Rynchozoan rostratum 

Tubulipora pacifica 

Tubulipora tuba 

Chordata Tunicates Botrylloides diegensis 

Diplosoma listerianum 

Styela truncata 

Mollusca Bivalves Crassodoma giganteus 

Hiatella arctica 

Kellia suborbicularis 

Mytilus californianus 

Mytilus galloprovincialis 

Gastropods Megatebennus bimaculatus 

Annelida Polychaetes Halosydna brevisetosa 

 Serpula vermicularis 

 Spirorbis sp.

Arthropoda Amphipods Caprella californica 

Caprella equilbra 

Caprella verrucosa 

Dulchiella sp. 

Elasmopus sp. 

Ericthonius sp. 

Gammeropsus sp. 

Podacerus sp. 

Pleustimptes sp. 

Stenothoe sp. 

Barnacles Balanus pacificus 

 Balanus regalis

 Balanus trigonus

 Megabalanus californicus 
Spatial and Temporal Variability in Early Successional Patterns

13

Unlike the pattern for colonial tunicates, overall cover and temporal variability in the 

cover of bryozoans did not decrease with increase in submersion time from 4 to 6 months

(Fig. 2e, f: P>0.1). However, the increase in cover of encrusting bryozoans with depth 

became pronounced on the 6-month tiles (depth: P<0.001, F=40.861, df=2, 6; date x 

depth interaction: P>0.1, ANOVA). 

Tubiculous amphipods could also colonize tiles early but, if present, generally occurred 

in low cover (<10% on 2-month tiles: Fig. 2g). Exceptions were observed in December 

1999 at 6 m and in December 2000 at 12 m when the mean cover of amphipod tubes was 

~30% on the 2-month tiles. There was a significant date x depth interaction in the mean 

cover of amphipod tube mat on the 2, 4, and 6-month tiles (date x depth interaction, 

P<0.001, ANOVA) (Fig. 2g-i); peaks in cover occurred in the spring on the 4 and 6-

month tiles. The cover of amphipod tube mat increased with time of submersion from 2 

to 6-months, and generally ranged from 15-20% in the fall (October) to 40-60% in the 

spring (June) after 6 months (Fig. 2i). 

Figure 2. Mean percent cover (±1SE, n=4) of colonial tunicates (a-c), encrusting bryozoans (d-f), and 

amphipod tube mat (g-i) after 2, 4, and 6 months of submersion at 6 (□), 12 (▲), or 18 (○) m. 

Bryozoans

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(e) 4 months

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(f) 6 months

Time (month)

May Sep Jan May Sep Jan May

0

20

40

60

80

100

Tunicates

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(b) 4 months

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(c) 6 months

May Sep Jan May Sep Jan May

Percent cover

0

20

40

60

80

100

Amphipods

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(h) 4 months

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(i) 6 months

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(a) 2 months (g) 2 months (d) 2 months

1999 2000 2001 1999 2000 2001 1999 2000 2001
Final Study Report – Page and Dugan 

14

The cover of barnacles was consistently low (generally <10%) on the 2-month tiles at all 

depths and well-defined seasonality in the cover of this taxon was not evident (Fig. 3a). 

However, as submersion time increased, the cover of barnacles also increased, 

particularly during the second year (Fig. 3a-c). Cover of barnacles also varied among 

depths; highest cover was observed at 6 m on the 4 and 6-month tiles. 

Figure 3. Mean percent cover (±1SE, n=4) of barnacles (a-c) and sponges (d-f) after 2, 4, and 6 months of 

submersion at 6 (□), 12 (▲), or 18 (○) m. 

Spatial and temporal patterns in the density of barnacles were generally similar to those 

for percent cover with highest densities at 6 m on the 4 and 6-month tiles (Fig. 4b, c). 

Very little barnacle recruitment occurred in the first year. For example, on the 2-month 

tiles at 6 m, there was a mean of 1.9 individuals 225 cm-2 in the first year compared with 

23.1 individuals 225 cm-2 in the second year (Fig. 4a). In the second year, barnacle 

density was higher at 6 m than 12 and 18 m on the 12-month tiles (P=0.008, F=17.775, 

df=2, 6, ANOVA, P<0.05, Tukey post hoc test). (Mytilus californianus, M. 

galloprovincialis) were later colonists, occurring on the tiles in low cover (<10%) only at 

a depth of 6 m after 12 months (Fig. 5f). 

Barnacles

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(b) 4 months

May Sep Jan May Sep Jan May

Percent cover

0

20

40

60

80

100

(c) 6 months

May Sep Jan May Sep Jan May

0

20

40

60

80

100

Sponges

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(e) 4 months

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(f) 6 months

Time (month)

May Sep Jan May Sep Jan May

0

20

40

60

80

100

(a) 2 months (d) 2 months

1999 2000 2001 1999 2000 2001
Spatial and Temporal Variability in Early Successional Patterns

15

Figure 4. Mean density (±1SE, n=4) of barnacles after 2, 4, 6, and 12 months of submersion at 6 (□), 12 

(▲), or 18 (○) m. 

There was no difference in the percent cover of colonial tunicates, encrusting bryozoans, 

or amphipod tube mat between 12-month tiles submerged in April 1999 and April 2000 

(P>0.1, ANOVA: Fig. 5). There was also no difference (P>0.1) in the cover of mussels 

on the 12-month tiles between years (Fig. 5f). In contrast, barnacles and sponges 

occurred in significantly higher cover on 12-month tiles submerged in 2000 compared 

with 1999 (barnacles, P=0.008, F=38.646, df=1, 3; sponges, P=0.005, F=55.359, df=1, 3, 

year x depth interaction for both taxa, P>0.1: Fig. 5). In addition, there was a significant 

effect of depth on the cover of barnacles (P=0.009, F=11.231, df=2, 6). 

The mean cover of uncolonized or open space on the 2-month tiles was highly variable, 

ranging from peaks of 60-90% in spring and early summer (April, June) to 15-40% in the 

late summer and fall (August, October) with a significant date x depth interaction 

(P<0.001, F=5.061, df=22, 66, ANOVA: Fig. 6a). The cover of open space decreased 

with increasing submersion time to generally <20% at all depths on the 6-month tiles 

(Fig. 6c). There were no significant date, depth, or date x depth interaction effects on 

cover of open space for the 4-month tiles and a significant time effect only for the 6-

month tiles (P=0.009, F=7.257, df=3, 9). Cover of open space on the 12-month tiles was 

<5% (not shown). 

(a) 2 months

May Sep Jan May Sep Jan May 

Mean Density (No./225cm2

)

0

20

40

60

80

100

120

140 (b) 4 months

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

120

140

1999 2000 2001 1999 2001 2000

(c) 6 months

Time (month)

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

120

140 (d) 12 months

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

120

140
Final Study Report – Page and Dugan 

16

Figure 5. Mean percent cover (±1SE, n=4) of selected taxa after 12 months of submersion at 6 (□), 12 

(▲), or 18 (○ ) m in 1999-2000 and 2000-2001.

Longer-term patterns 

We examined the succession (timing of occurrence and percent cover) of taxa over longer 

time scales using data from tiles that were submerged in April 1999 and retrieved after 2, 

4, 6, 12, and 24 months. Colonial tunicates and encrusting bryozoans were early 

colonists on the tiles across all depths, but declined to <15% cover each on the 24-month 

tiles (Fig. 7). Tubiculous amphipods were also early colonists of the tiles, but highest 

cover of this taxon generally occurred after 6 or 12 months (Fig. 7). Between 12 and 24 

months, the cover of amphipod tube mat remained unchanged or declined slightly at all 

depths. 

(b) Bryozoans

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

(a) Tunicates

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

(c) Amphipods

May Sep Jan May Sep Jan May 

Percent cover

0

20

40

60

80

100

(e) Sponges

Time (month)

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

(d) Barnacles

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

May Sep Jan May Sep Jan May 

0

20

40

60

80

100 (f) Mussels

1999 2000 2001 1999 2000 2001
Spatial and Temporal Variability in Early Successional Patterns

17

Figure 6. Mean percent cover (±1SE, n=4) of open space after 2, 4, and 6 months of submersion at 6 (□), 

12 (▲), or 18 (○) m. 

Composition of the invertebrate assemblage changed between 12 and 24 months with 

barnacles, sponges, and mussels becoming the dominant space holders. Barnacles 

occurred on the tiles at 6 m after 2 months, albeit in very low cover (~1%) in 1999, and 

generally increased in cover over time at all depths, with a much slower increase 

occurring at 12 and 18 m (<10% until 24 months) compared with 6 m (>20% after 6 

months) (Fig. 7). Between 12 and 24 months, cover of barnacles increased 1.6-fold at 6 

m, 4-fold at 12 m, and 9-fold at 18 m. Cover of barnacles was higher on tiles at 6 m 

compared with 12 and 18 m (P=0.006, F=9.786, df=2, 9, One-way ANOVA, P<0.05 

Tukey post hoc test) on the 24-month tiles. Sponges were largely absent from tiles 

submerged for <12 months, but greatly increased in cover from 12 to 24 months at all 

depths, with significantly higher cover at 12 (80%) and 18 m (68%) compared with 6 m 

(39%) at 24 months (P=0.011, F=7.751, df=2, 9, One-way ANOVA, P<0.05, Tukey post 

hoc test). Cover of mussels also increased greatly (from 7 to 36%) between 12 and 24 

months, but only at a depth of 6 m (Fig. 7). 

 (a) 2 months

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

(b) 4 months 

May Sep Jan May Sep Jan May 

Percent open space

0

20

40

60

80

100

(c) 6 months

May Sep Jan May Sep Jan May 

0

20

40

60

80

100

Time (month)

1999 2000 2001
Final Study Report – Page and Dugan 

18

Figure 7. Mean percent cover (±1SE, n=4) of selected taxa on tiles submerged in April 1999 at depths of 

6, 12, and 18 m after 2, 4, 6, 12, and 24 months of submersion.

Biotic interactions 

In the removal experiment, colonial tunicates, encrusting bryozoans, amphipod tube mat, 

and barnacles together comprised 82-88% of total cover on the controls at each depth 

after 12 months (Fig. 8). There were no significant interactions (P>0.1) between removal 

treatment and depth on the cover of any taxon. In the tunicate removal treatment, the 

mean cover of amphipod tube mat, encrusting bryozoans, barnacles, and open space did 

not differ from the control (P>0.05, ANOVA). In the bryozoan removal treatment, only 

the mean cover of open space was higher than the control (P=0.008, F=38.513, df=1, 3, 

6 m

May Sep Jan May Sep Jan May

0

20

40

60

80

100 6 m

May Sep Jan May Sep Jan May

0

20

40

60

80

100

12 m

May Sep Jan May Sep Jan May

Percent cover

0

20

40

60

80

100 12 m

May Sep Jan May Sep Jan May

0

20

40

60

80

100

18 m

Time (months)

May Sep Jan May Sep Jan May

0

20

40

60

80

100

bryozoans 

tunicates 

amphipods

18 m

May Sep Jan May Sep Jan May

0

20

40

60

80

100

barnacles 

mussels

sponges

1999 1999 2000 2001 2000 2001
Spatial and Temporal Variability in Early Successional Patterns

19

ANOVA). In the barnacle removal treatment, only the cover of bryozoans was 

significantly higher than the control (P=0.005, F=52.606, df=1, 3). 

 

A significant effect of depth on the tile assemblage was found in the barnacle removal 

treatment where the percent cover of amphipod tube mat decreased with increase in depth 

(P=0.02, F=4.994, df=2, 6, ANOVA: Fig. 8). There was a trend of increase in the cover 

of bryozoans in the tunicate removal treatment with increase in depth (P=0.055, F=4.902, 

df=2, 6: Fig. 8). 

Figure 8. Mean percent cover (±1SE, n=4) of tubiculous amphipods, colonial tunicates, encrusting 

bryozoans, barnacles, and open space on tiles submerged at depths of 6, 12, or 18 m in which either (a) 

tunicates, (b) bryozoans, or (c) barnacles were removed monthly for 12 months. 

Accumulation of invertebrate biomass on the platform structure 

Biomass on conductor pipes and tiles consistently increased with exposure time, was 

highest at a depth of 6 m, and was higher in 2000-2001 compared with 1999-2000 (Figs. 

9, 10). We found no effect of orientation (down current vs. up current) on the 

accumulation of biomass at the 3 depths. 

Bryozoans removed

Category

Amp

Tun

Barn

Open

Tunicates removed

0.0

0.2

0.4

0.6

0.8

1.0

12 m

Percent cover

0.0

0.2

0.4

0.6

0.8

1.0

18 m

Amp

Bry

Barn

Open

0.0

0.2

0.4

0.6

0.8

1.0

Barnacles removed

Treatment

Control

Amp

Bry

Tun

Open

6 m
Final Study Report – Page and Dugan 

20

Figure 9. Mean (x ±1SE, n=8) accumulation of biomass at 6 (○), 12 (▲), and 18 m (□) on the surface of 

conductor pipes on Platform Houchin. Surfaces were initially scraped to remove attached organisms in 

July 1999. 

Figure 10. Mean biomass (x ±1SE, n=8) of invertebrates on ceramic tiles deployed at depths of 6 (○), 12 

(▲), and 18 m (□) for 2, 4, 6 or 12 months at Platform Houchin. The same tiles were used for percent 

cover estimates (Figs. 2, 3). 

(a) 2 months

Mean Biomass (g/400cm2

)

0

20

40

60

80

100

120

140 (b) 4 months

0

20

40

60

80

100

120

140

(c) 6 months

Time (month)

0

20

40

60

80

100

120

140 (d) 12 months

0

20

40

60

80

100

120

140

July Nov Mar July July Nov Nov Mar July Mar July Nov Mar July

July Nov Mar Nov Mar July July July July July Nov Mar Mar Nov

1999 2000 2001 1999 2000 2001

(a) 1999 - 2000

2 4 6 12

0

20

40

60

80

100

120

140

(b) 2000 - 2001

Time (month)

2 4 6 12

Mean biomass (g/400cm2)

0

20

40

60

80

100

120

140
Spatial and Temporal Variability in Early Successional Patterns

21

Figure 11. Accumulation of invertebrate biomass in tuffys (±1SE, n=4) after 1 month of exposure at 

depths of 6, 12, and 18 m. 

Date

6/1/99 10/1/99 2/1/00 6/1/00 10/1/00 2/1/01 6/1/01 

Mean biomass (g)

0

2

4

6

8

10

12

6 m 

12 m 

18 m 

The standing crop biomass of the invertebrates on the tuffys varied significantly with 

depth (P<0.001, F = 30.110, df = 2, One-way ANOVA). The accumulation of biomass 

was highest at a depth of 6 m and lowest at the 18 m depth (Fig. 10). A marked increase 

in the rate of biomass accumulation occurred between April and June of 2000 at all three 

depths. Most of the biomass found on the tuffys (>99%) was made up of gammarid and 

caprellid amphipods. 

Composition and rate of faunal litterfall from platform structures 

Mussels, hydroids, bryozoans, sponges, and barnacles accounted for nearly all (>99%) of 

the invertebrate biomass in the litterfall traps (Fig. 11). Mussels accounted for the 

greatest biomass of any taxa in all but 3 months. Faunal litterfall tended to be highest in 

the spring and summer months and lowest in the winter months. 
Final Study Report – Page and Dugan 

22

Figure 12. Mean biomass (±1SE, n=8) of invertebrate taxa in faunal litterfall traps deployed at Platform 

Hogan for 1 month at a depth of 18 m. 

Date

12/1/1999 4/1/2000 8/1/2000 12/1/2000 4/1/2001 

Faunal litterfall (g wet weight month-1)

0

200

400

600

800

Mussels 

Hydroids, Bryozoans, Sponges 

Barnacles 

Other 

DISCUSSION 

Development of the assemblage 

The invertebrate assemblage that developed on the tiles submerged at Platform Houchin 

was composed of taxonomic groups commonly reported from other subtidal hard 

substrate habitats (e.g., Sutherland and Karlson, 1977; Turner and Todd, 1993; Vance, 

1988), including coastal rocky reefs in the Santa Barbara Channel (Breitburg, 1985). 

There was, however, a notable absence of macroalgae on the tiles. Macroalgae are often 

a conspicuous component of hard substrate assemblages (Sousa, 1984; Breitburg, 1985); 

however, Bascom et al. (1976) reported that macroalgae were sparse on a platform 

located ~6 km from our study site. The low abundance of macroalgae on our tiles 

suggested the possibilities that spores in the plankton were flowing through the platform 

without settling (Bascom et al., 1976) or that shading provided by the overlying platform 

superstructure and/or vertical orientation of the conductor pipes negatively influenced 

algal survival and growth (Baynes, 1999). While we have no data on the availability of 

algal spores, we have observed filamentous red macroalgae on support members exposed 

to more direct sunlight around the periphery of the structure, which tends to support the 

later hypothesis. Forteath et al. (1982) also suggested the importance of light level in 

influencing the distribution of macroalgae on the legs and support members of a platform 

(Montrose Alpha) in the North Sea. 
Spatial and Temporal Variability in Early Successional Patterns

23

The succession of taxa on our tiles generally occurred in a predictable sequence with 

colonial taxa appearing first. Colonial tunicates occurred in highest cover early in 

assemblage development (2-month tiles) at all depths (Fig. 2), but comprised only a 

minor component of the assemblage (<10% cover) on tiles submerged for 12 months. 

Colonial tunicates are often considered “opportunistic” and poor competitors (Greene et 

al., 1983, but see Dean and Hurd, 1980; Nandakumar, 1996) and our data support this 

view by suggesting that later colonists negatively influenced the rates of recruitment, 

growth, or survival of this taxon as submersion time increased. Interestingly, the cover of 

tunicates did not increase in response to the experimental removal of potential 

competitors (encrusting bryozoans, barnacles) (Fig. 8). However, the removal 

experiment was terminated in June, a month of low tunicate cover the previous two years. 

Since tunicates occurred in highest cover in the fall and winter months, interactions 

between this taxon and other early colonists may be greatest in those months when 

tunicate recruitment/growth was highest. Our experiments thus suggest that season was 

important in mediating interactions among early colonists on the tiles. 

Encrusting bryozoans were also early colonists of the tiles, but occupied higher cover 

longer than tunicates. This pattern suggested that bryozoans were better competitors for 

space than tunicates in young assemblages. However, the higher cover of bryozoans with 

increasing depth on the 6 and 12-month tiles suggests that competitive interactions were 

important in limiting the cover this taxon at the shallowest depth (6 m) in older 

assemblages. Results from the removal experiment, in which cover of encrusting 

bryozoans in 12 month old assemblages was higher than control values when barnacles 

were removed, supports this conclusion (Fig. 8). 

Other taxa, including tubiculous amphipods, barnacles, and sponges also occurred early 

in assemblage development (on the 2-month tiles), but in variable and generally low 

cover. Among these taxa, the cover of tubiculous amphipods (as amphipod tube mat) 

was highest, but generally much less than 20%. Since open space was available on the 

tiles for up to 6 months, the availability of this resource did not appear to limit amphipod 

recruitment or growth. The mean cover and variability in cover of amphipods generally 

increased on tiles as submersion time increased for up to 6 months, suggesting that the 

increase in surface complexity over time provided by other attached organisms could 

have facilitated the recruitment and/or survival of amphipods. For example, Bros (1987) 

reported that the abundance of the tubiculous amphipod Ericthonius brasiliensis, was 

positively associated with the increased habitat heterogeneity provided by barnacle tests. 

However, in our study, the removal of tunicates, bryozoans, or barnacles did not result in 

a significant decrease in the cover of amphipods (Fig. 8), suggesting that factors other 

than these early colonists influenced the cover of amphipod tube mat. 

Barnacles and sponges, which reached highest cover later in succession, were also 

present on the 2-month tiles, but in low cover (<10%). The availability of open space did 

not appear to limit the recruitment of these taxa because open space was still available 

after 2 months. In addition, there were no observations to suggest that barnacles or 

sponges were inhibited from settling or were overgrown by taxa already on the tiles (e.g., 

Young and Gotelli, 1988; Nandakumar, 1996). The cover of barnacles (Fig. 8) and 
Final Study Report – Page and Dugan 

24

sponges (unpublished data) did not increase in response to the experimental removal of 

early colonists, also suggesting that inhibitory or competitive interactions did not 

negatively affect the recruitment or growth of these taxa during the experiment. More 

likely, recruitment of barnacles and sponges was limited by the availability of larvae in 

the water column. The higher density and/or cover of barnacles and sponges on tiles in 

2000-2001 compared with 1999-2000 supports this conclusion (Fig. 4). 

Mussels colonized the tiles later (>12 months). Although mussels are abundant on oil 

platforms in the Santa Barbara Channel (Bascom et al., 1976; Page and Hubbard, 1987), 

mussel recruitment was low during this study. Mussel pediveligers recruit preferentially 

onto rugose or filamentous surfaces (Caceres-Martinez and Robledo, 1994; Marsden and 

Lansky, 2000). The recruitment of mussels can be facilitated by other species, but early 

colonists are not essential for recruitment (Dean and Hurd, 1980). Thus, we hypothesize 

that mussel recruitment was low in this study, at least in part, because of a paucity of 

larvae in the water column (also, see below). 

Effect of depth and year on assemblage development 

Depth affected the intensity of recruitment, and the distribution and cover of later 

colonizing species, but not the sequence of early development. Barnacles appeared 

earlier (after 2 months) and were present in significantly higher cover at 6 m (55% after 

24 months) compared with the deeper depths (<28% after 24 months) (Fig. 7). Mussels 

were also only found on the tiles at 6 m, whereas sponges occurred in highest cover (70-

80%) after 24 months at the deeper depths (12 and 18 m). 

 For barnacles, depth-related variation in density and cover may be related to variation in 

the density of larvae available for settlement. For example, Grosberg (1982) found that 

the vertical distribution of adult barnacles on pier pilings and of newly settled spat on 

settlement plates, reflected the vertical distribution of barnacle cyprids in the water 

column. The absence of mussels on tiles at deeper depths (12 and 18 m) may indicate 

that mussel larvae were not present. However, mussels recruit and grow on platforms in 

the Santa Barbara Channel to depths exceeding 18 m (Page and Hubbard, 1987; Page, 

unpublished data). Barnacles can enhance mussel recruitment by increasing habitat 

heterogeneity (Dean and Hurd, 1980; Berlow, 1997). Thus, the higher cover of barnacles 

at the shallowest depth (6 m) may have facilitated mussel attachment at this depth, 

whereas the low density of barnacles at 12 and 18 m may have retarded mussel 

recruitment at those depths. Unfortunately, this possibility was not adequately tested by 

the removal experiment because of generally low mussel recruitment onto tiles during the 

experiment. In addition to larval supply, depth-related variation in post-settlement 

mortality of later colonists, associated with predation or other factors, could also 

contribute to the observed patterns (e.g., Keough and Downes, 1982). However, 

predators were not observed feeding at our tiles. 

The sequence of appearance of species on the tiles was similar between years, but the 

cover of barnacles and sponges was much higher (and density for barnacles) on the tiles 
Spatial and Temporal Variability in Early Successional Patterns

25

in 2000-2001 compared with 1999-2000. Rates of recruitment of these taxa may have

been influenced by differences in oceanic conditions between the two years. A record 

coastal upwelling event occurred in Spring 1999 and coastal upwelling continued in the 

Southern California Bight through Summer 1999 (Schwing et al., 2000). These “La 

Nina” conditions persisted through Spring 2000, with ocean temperatures cooler than 

normal (Bograd et al., 2000). Although April 2000 to April 2001 was also a La Nina 

year, sea surface temperature measurements indicated that oceanographic conditions were 

closer to normal than in the preceding year (Durazo et al., 2001). 

Water temperature per se can affect the development of marine invertebrates and lower 

water temperatures in 1999-2000 compared to 2000-2001 could have affected the density 

of larvae in the water column through effects on the reproductive output of adults 

(Schwing et al., 2000). In addition, upwelling events associated with La Nina conditions 

could have transported planktonic larvae offshore reducing the supply of larvae available 

for settlement (Roughgarden et al., 1988; Schwing et al., 2000). It is also possible that 

larval and post-settlement mortality were higher during La Nina conditions (Schwing et 

al., 2000). 

Patterns of succession at the platform 

The absence of macroalgae, including crustose coralline algae, on the tiles at Platform 

Houchin, and a scarcity of grazers, provided for a very different pattern of succession 

than found by Breitburg (1985) at an inshore subtidal reef in the Santa Barbara Channel. 

At the reef site, many sessile invertebrates were inhibited from settling by crustose 

coralline algae, and were also much more abundant when large grazers were 

experimentally excluded (total invertebrate cover on grazed plates <2%). Macroalgae 

were present on the rock wall study site of Vance (1988), but grazers did not appear to 

influence the pattern of succession, which was generally predicable from season to 

season and year-to-year and, in this regard, more similar to the results found in the 

present study. Vance (1988) attributed this regularity to a relatively stable physical 

environment, in terms of depth, current speed, temperature, and salinity, which could lead 

to less variable patterns of larval recruitment. Since the offshore physical environment at 

the platform is more similar to the vertical rock wall site of Vance (1988), than to the 

inshore reef site of Breitburg (1985), a similar explanation may well apply, at least in 

part, for patterns of succession found in the present study. 

In addition, our results differed from those of other studies (Sutherland and Karlson, 

1977; Osman, 1977; Underwood and Anderson, 1994) where strong seasonality in the 

timing and intensity of recruitment of different species contributed to variation in the 

pattern of succession, which depended on the time-of-year artificial surfaces were 

deployed. In the present study, only two taxa (both colonial) dominated early succession 

and these taxa recruited to the 2-month tiles year round (although the abundance of 

tunicates varied seasonally). Oceanographic climate may play an important role in 

influencing the rate of succession since the recruitment of later successional taxa 

(barnacles, sponges) differed considerably between the two years of the study. 
Final Study Report – Page and Dugan 

26

In conclusion, development of the invertebrate assemblage on the tiles submerged at 

Platform Houchin could not be categorized as classical succession (sensu Clements,

1936) because some species/taxa that were present in high cover after 12 and/or 24 

months (e.g., barnacles, sponges) could also colonize the tiles early in the successional 

sequence, albeit in low cover, and did not appear to require the presence of earlier taxa 

for recruitment. Rather, development of this assemblage appeared to fit a pattern where 

differences in life history traits among species colonizing the tiles (e.g., production of 

propagules, growth, longevity), competitive ability, depth-related settlement preferences, 

and responses to oceanographic climate, create a pattern of succession that was largely 

predictable in terms of the composition and sequence of occurrence of dominant taxa, but 

variable in rate of development, depending on date of submersion, year, and depth. 
Spatial and Temporal Variability in Early Successional Patterns

27 

Plate I. Photographs of ceramic tiles submerged for 24 months at depths of 6, 12, and 18 m at Platform 

Houchin.

6 m 

12 m 

18 m 
Final Study Report – Page and Dugan 

28

REFERENCES 

Bascom, W.A., Mearns, J., Moore, M.D., 1976. A biological survey of oil platforms in the 

Santa Barbara Channel. J. Petrol. Technol. 28, 1280-1284. 

Baynes, T.W., 1999. Factors structuring a subtidal encrusting community in the southern 

Gulf of California. Bull. Mar. Sci. 64, 419-450. 

Berlow, E.L., 1997. From canalization to contingency: historical effects in a successional 

rocky intertidal community. Ecol. Monogr. 67, 435-460. 

Bograd, S.J., DiGiacomo, P.M., Durazo, R., Hayward, T.L., Hyrenbach, K.D., Lynn, R.J., 

Mantyla, A.W., Schwing, F.B., Sydeman, W.J., Baumgartner, T., Lavaniegos, B., 

Moore, C.S., 2000. The state of the California Current, 1999-2000: forward to a new 

regime? Calif. Coop. Oceanic Fish. Invest. Rep. 41, 26-52. 

Bram, J.B., 2003. Spatial and temporal variability in early successional patterns of an 

invertebrate assemblage on an offshore oil platform. MA Thesis, University of 

California, Santa Barbara. 

Breitburg, D.L., 1985. Development of a subtidal epibenthic community: factors affecting 

species composition and the mechanisms of succession. Oecologia 65, 173-184. 

Bros, W.E., 1987. Effects of removing or adding structure (barnacle shells) on recruitment to 

a fouling community in Tampa Bay, Florida. J. Exp. Mar. Biol. Ecol. 105, 275-296. 

Caceres-Martinez, J., Robledo, J.A.F., 1994. Settlement and post-larvae behavior of Mytilus 

galloprovincialis – Field and laboratory experiments. Mar. Ecol. Prog. Ser. 112, 107-

117. 

Chapman, M.G., 2002. Early colonization of shallow subtidal boulders in two habitats. J. Exp. 

Mar. Biol. Ecol. 275, 95-116. 

Clements, F.E., 1936. Nature and structure of the climax. J. Ecol. 24, 252-284. 

Colinvaux, P.A., 1973. Introduction to ecology. Wiley, New York. 

Connell, J.H., Slatyer, R.O., 1977. Mechanisms of succession in natural communities and 

their role in community stability and organization. Am. Nat. 111, 1119-1144. 

Dean, T.A., Hurd, L.E., 1980. Development in an estuarine fouling community: the influence 

of early colonists on later arrivals. Oecologia 46, 295-301. 

Drury, W.H., Nisbet, I.C.T., 1973. Succession. J. Arnold Arboretum 54, 331-368. 
Spatial and Temporal Variability in Early Successional Patterns

29 

Durazo, R., Baumgartner, T.R., Bograd, S.J., Collins, C.A., Campa, S., Garcia, J., GaxiolaCastro, G., Huyer, A., Hyrenbach, K.D., Loya, D., Lynn, R.J., Schwing, F.B., Smith, 

R.L., Sydeman, W.J., Wheeler, P., 2001. The state of the California Current, 2000-

2001: A third straight La Nina year. Calif. Coop. Oceanic Fish. Invest. Rep. 42, 29-60. 

Farrell, T.M. 1991. Models and mechanisms of succession: an example from a rocky 

intertidal community. Ecol. Monogr. 61, 95-113. 

Forteath, G.N.R., Picken, G.B., Ralph, R., Williams, J., 1982. Marine growth studies on the 

North Sea Oil Platform Montrose Alpha. Mar. Ecol. Prog. Ser. 8, 61-68. 

Garrabou, J., Ballesteros E., Zabala, M., 2002. Structure and dynamics of North-western 

Mediterranean rocky benthic communities along a depth gradient. Estuar. Coast. Shelf 

Sci. 55, 493-508. 

Greene, C.H., Schoener, A., 1982. Succession on marine hard substrata: a fixed lottery. 

Oecologia, 55, 289-297. 

Grosberg, R.K., 1982. Intertidal zonation of barnacles: the influence of planktonic zonation of 

larvae on vertical distribution of adults. Ecology 63, 894-899. 

Hicks, C.R., 1999. Fundamental concepts in the design of experiments, 5th ed. Oxford 

University Press, New York. 

Horn, H.S., 1974. The ecology of secondary succession. Annu. Rev. Ecol. Syst. 5, 25-37. 

Kay, A.M., Keough, M.J., 1981. Occupation of patches in the epifaunal commnnities on pier 

pilings and the bivalve Pinna bicolor at Edithburg, South Australia. Oecologia, 48, 

123-130. 

Keough, M.J., Downes, B.J., 1982. Recruitment of marine invertebrates: the role of active 

larval choices and early mortality. Oecologia 54, 348-352. 

Marsden, J.E., Lansky, D.M., 2000. Substrate selection by settling zebra mussels, Driessena 

polymorpha, relative to material, texture, orientation, and sunlight. Can. J. Zoo. 78, 

787-793. 

Nandakumar, K., 1996. Importance of timing of panel submersion on the competitive 

outcome and succession of sessile organisms. Mar. Ecol. Prog. Ser. 131, 191-203. 

Odum, E.P., 1969. The strategy of ecosystem development. Science 164, 262-270. 

Ojeda, F.P., Dearborn, J.H., 1989. Community structure of macroinvertebrates inhabiting the 

rocky subtidal zone in the Gulf of Maine: seasonal and bathymetric distribution. Mar. 

Ecol. Prog. Ser. 57, 147-161. 
Final Study Report – Page and Dugan 

30

Osman, R.W., 1977. The establishment and development of a marine epifaunal community. 

Ecol. Mongr. 47, 37-63. 

Page, H.M. 1986. Differences in population structure and growth rate of the stalked barnacle 

Pollicipes polymerus between a rocky headland and an offshore oil platform. Mar. 

Ecol. Progr. Ser. 29, 157-164. 

Page, H.M., Hubbard, D.M., 1987. Temporal and spatial patterns of growth in mussels 

Mytilus edulis on an offshore platform: relationships to water temperature and food 

availability. J. Exp. Mar. Biol. Ecol. 111, 159-179. 

Page, H.M., Dugan, J.E., Dugan, D.S., Richards, J.B., Hubbard, D.M., 1999. Effects of an 

offshore oil platform on the distribution and abundance of commercially important 

crab species. Mar. Ecol. Prog. Ser. 185, 47-57. 

Roughgarden, J., Gaines, S., Possingham, H., 1988. Recruitment dynamics in complex lifecycles. Science 241, 1460-1466. 

Scheer, B.T., 1945. The development of marine fouling communities. Biol. Bull. 89, 103-121. 

Schwing, F.B., Moore, C.S., Ralston, S., Sakuma, K.M., 2000. Record coastal upwelling in 

the California Current in 1999. Calif. Coop. Oceanic Fish. Invest. Rep. 41, 148-160. 

Simpson, R.A., 1977. The biology of two offshore oil platforms. Inst. Mar. Res, Univ Calif, 

San Diego, IMR Ref. 76-13. 

Smith, F., Witman, J.D., 1999. Species diversity in subtidal landscapes: maintenance by 

physical processes and larval recruitment. Ecology 80, 51-69. 

Sousa, W.P., 1979a. Experimental investigations of disturbance and ecological succession in a 

rocky intertidal algal community. Ecol. Monogr., 49, 227-254. 

Sousa, W.P., 1979b. Disturbance in marine intertidal boulder fields: the nonequilibrium 

maintenance of species diversity. Ecology 60, 1225-1239. 

Sousa, W.P., 1984. Intertidal mosaics: patch size propagule availability, and spatially variable 

patterns of succession. Ecology 65, 1918-1935. 

Sutherland, J.P., 1978. Functional roles of Schizoporella and Styela in the fouling community 

a Beaufort, North Carolina. Ecology 59, 257-264. 

Sutherland, J.P., Karlson, R.H., 1977. Development and stability of the fouling community at 

Beaufort, North Carolina. Ecol. Monogr. 47, 425-446. 
Spatial and Temporal Variability in Early Successional Patterns

31 

Turner S.J., Todd, C.D., 1993. The early development of epifaunal assemblages on artificial 

substrata at two intertidal sites on an exposed rocky shore in St. Andrews Bay, N.E. 

Scotland. J. Exp. Mar. Biol. Ecol. 166, 251-272. 

Underwood, A.J., Anderson, M.J., 1994. Seasonal and temporal aspects of recruitment and 

succession in an intertidal estuarine fouling assemblage. J. Mar. Biol. Assoc. U.K. 74, 

563-584. 

Vance, R. R., 1988. Ecological succession and the climax community on a marine subtidal 

rock wall. Mar. Ecol. Prog. Ser. 48, 125-136. 

Walker, L.R., Chapin, F.S., III, 1987. Interactions among processes controlling successional 

change. Oikos, 50, 131-135. 

Wolfson, A., VanBlaricom, G., Davis, N., Lewbel, G.S., 1979. The marine life of an offshore 

oil platform. Mar. Ecol. Prog. Ser. 1, 81-89. 

Young, C.M., Gotelli, N.J. 1988. Larval predation by barnacles: effects on patch colonization 

in a shallow subtidal community. Ecology 69, 624-634. 

Zar, J., 1999. Biostatistical analysis, 4th ed. Prentice-Hall, Upper Saddle River, New Jersey.
Final Study Report – Page and Dugan 

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ACKNOWLEDGEMENTS 

We thank B. Johnson (Pacific Operators Offshore) for assisting with access to Platform 

Houchin, and to the Minerals Management Service for providing funding. Thanks also to S. 

Gaines and J. Connell for comments on the manuscript, S. Schroeter for statistical advice, H. 

Chaney and D. Cadien for taxonomic assistance, and to the numerous divers and assistants, 

including but not limited to B. Mardian, C. Culver, A. Willis, A. Engen, and B. Wolcott. 
Spatial and Temporal Variability in Early Successional Patterns

33 

The Department of the Interior Mission

As the Nation's principal conservation agency, the Department of the Interior has responsibility for most 

of our nationally owned public lands and natural resources. This includes fostering sound use of our 

land and water resources; protecting our fish, wildlife, and biological diversity; preserving the 

environmental and cultural values of our national parks and historical places; and providing for the 

enjoyment of life through outdoor recreation. The Department assesses our energy and mineral 

resources and works to ensure that their development is in the best interests of all our people by 

encouraging stewardship and citizen participation in their care. The Department also has a major 

responsibility for American Indian reservation communities and for people who live in island territories 

under U.S. administration. 

The Minerals Management Service Mission 

As a bureau of the Department of the Interior, the Minerals Management Service's (MMS) primary 

responsibilities are to manage the mineral resources located on the Nation's Outer Continental Shelf 

(OCS), collect revenue from the Federal OCS and onshore Federal and Indian lands, and distribute 

those revenues. 

Moreover, in working to meet its responsibilities, the Offshore Minerals Management Program

administers the OCS competitive leasing program and oversees the safe and environmentally sound 

exploration and production of our Nation's offshore natural gas, oil and other mineral resources. The 

MMS Royalty Management Program meets its responsibilities by ensuring the efficient, timely and 

accurate collection and disbursement of revenue from mineral leasing and production due to Indian 

tribes and allottees, States and the U.S. Treasury. 

The MMS strives to fulfill its responsibilities through the general guiding principles of: (1) being 

responsive to the public's concerns and interests by maintaining a dialogue with all potentially affected 

parties and (2) carrying out its programs with an emphasis on working to enhance the quality of life for 

all Americans by lending MMS assistance and expertise to economic development and environmental 

protection.