OCS Study 

•

MMS 2003-033 

Variability in the accumulation and 

persistence of tar in four intertidal 

communities along the Central and 

Southern California Coast 

Final Technical Summary 

Final Study Report 

U.S. Department of the Interior 

Minerals Management Service

Pacific OCS Region 
3

Variability in the accumulation and 

persistence of tar in four intertidal 

communities along the Central and 

Southern California Coast 

Final Technical Summary 

Final Study Report 

Authors 

Christy A. Roe 

Peter T. Raimondi 

Samantha E. Forde 

Principal Investigator 

Peter T. Raimondi

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 May 2003
4

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 author, 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://coastalresearchcenter.ucsb.edu/SCEI/ 

Suggested Citation 

The suggested citation for this report is: 

Roe, C. A., Raimondi, P. T., and S. E. Forde. Variability in the accumulation and persistence of tar in 

four intertidal communities along the central and southern California coast. MMS OCS Study 2003-

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

California. MMS Cooperative Agreement Numbers 14-35-0001-30761. 44 pages. 
5

Table of Contents 

FINAL TECHNICAL SUMMARY 1 

FINAL STUDY REPORT 5 

 Introduction 5 

 Methods 6 

 Species Descriptions 6 

 Study Area 6 

 Data from long-term monitoring programs 8 

 Visual Surveys 8 

 Permanent Plots 8 

 Experimental Substrate 9 

 Tidal Height 10 

 Temperature 11 

 Results 12 

 Data from long-term monitoring programs 12 

 Visual Surveys 13 

 Permanent Plots 13 

 Experimental Substrate 16 

 Tidal Height 18 

 Temperature 21 

 Discussion 24 

 

 Future Recommendations 30 

 Appendix A 31 

 Acknowledgements 31 

 Literature Cited 32 

 

Modelling the effects of an oil spill on open populations of 

 intertidal invertebrates by S.E. Forde 35 

 

 
Variability in the accumulation and persistence of tar in four intertidal communities 

1

FINAL TECHNICAL SUMMARY 

STUDY TITLE: Effects of an oil spill on multispecies interactions that structure intertidal 

communities. 

REPORT TITLE: Variability in the accumulation and persistence of tar in four intertidal 

communities along the central and southern California coast 

CONTRACT NUMBER: 14-35-0001-30761 

SPONSORING OCS REGION: Pacific

APPLICABLE PLANNING AREA: Central and Southern California

FISCAL YEAR(S) OF PROJECT FUNDING: FY97, FY98, FY99 

COMPLETION DATE OF THE REPORT: September 2002 

COST(S): FY 97 - $40,560, FY 98 - $40,624, FY 99 - $39,269, FY 00 – no cost, FY 01 – no 

cost 

CUMULATIVE PROJECT COST: $120,453 

PROJECT MANAGER: Russell J. Schmitt

AFFILIATION: University of California, Santa Barbara

ADDRESS: Coastal Research Center, Marine Science Institute, University of California, 

Santa Barbara, CA 93106

PRINCIPAL INVESTIGATOR: Peter T. Raimondi 

ADDRESS: Department of Ecology and Evolutionary Biology, University of California, 

Santa Cruz, CA 95460 

KEY WORDS: tar, central California coast, barnacles, Endocladia, Silvetia, mussels, rocky 

intertidal, intertidal zones, intertidal communities, natural tar seeps, experimental 

BACKGROUND AND OBJECTIVES: Our goal was to couple long-term monitoring data 

with experimental studies to better assess the impacts of oil spills. We combined long-term 

data from the Shoreline Inventory Program (currently funded by the Minerals Management 

Service and primarily an inventory of intertidal populations), which includes a number of sites 

in San Luis Obispo and Santa Barbara Counties, with experimental studies of tar 

accumulation and persistence, as part of an effort to improve the ability to assess the impacts 

of potential oil spills in this region. The central California coastline is vulnerable to oil spills 
Final Technical Summary – Roe et al 

2

because of offshore oil drilling and a high amount of tanker traffic. Additionally, the central 

California coastline is characterized by numerous natural subtidal oil seeps, which have been 

documented as far back as 1792 (Foster et al. 1970). The oil from these seeps frequently 

washes ashore and the rocky intertidal in this region often has a large amount of oil (tar) on it. 

These factors make this an ideal location to (1) assess where tar accumulates and persists 

within different species zones and (2) determine what factor(s) are important to the 

accumulation and persistence of tar in the intertidal. To our knowledge no studies have been 

done using natural tar to mimic the effects of oil spills. Variability in tar accumulation and 

persistence may result from both physical and biological factors. Physical factors include 

temperature, tidal height and wave exposure. The major biological factor of importance 

depends on the characteristics of the species tar lands on. These factors not only contribute to 

the weathering (degradation) of tar, but may also have an effect on where the tar accumulates. 

We developed short-term experiments that use naturally occurring tar as a proxy for oil to 

evaluate those species that would potentially be most susceptible to an anthropogenic oil spill 

along the central California coastline. Since our study relies on tar accumulation and 

persistence as a proxy for oil accumulation and persistence, this method simulates a smallscale oil spill. 

DESCRIPTION: Our study sites were in central California from North to South; Point 

Sierra Nevada, Shell Beach and Boat House. Slides from the Shoreline Inventory Program 

were scored to assess where tar was accumulating and to estimate the percent cover of tar in 

permanent plots within different intertidal communities, across many sites and over the course 

of several years. We performed thorough visual surveys in 4 intertidal zones: barnacle 

(primarily Chthamalus), Endocladia (a red alga), Silvetia (a brown, fucoid alga), and mussel 

zones in the fall of 1999 at Point Sierra Nevada, Shell Beach and Boat House. Based on the 

results from the slide data and our visual surveys we established permanent plots in the 

barnacle and Endocladia zones in the Fall of 1999 at Point Sierra Nevada, Shell Beach and 

Boat House to assess the accumulation and persistence of tar in different species zones. The 

permanent plots were sampled twice annually (Spring and Fall) to estimate the percent cover 

of tar in the different zones. Experiments were designed to measure variability in tar patch 

accumulation and persistence by removing the effect of substrate. We made identical casts of 

barnacles with a urethane material that mimics the texture of barnacles which allowed us to 

standardize the substrate. We placed sixty 10x10cm barnacle casts in the intertidal at each of 

our three sites (from north to south: Point Sierra Nevada, Shell Beach and Boat House) in 

April 2000. At each site, we placed fifteen casts into each “zone” (the barnacle zone 

(control), Endocladia, Silvetia and mussel zones for a total of 60 casts per site; 15 per zone). 

The casts were then checked monthly for 18 months (May 2000 to October 2001) for the 

presence or absence of tar. Because our visual surveys showed tar accumulating in higher 

zones, we tested the idea that variability in tar patch accumulation and persistence may be a 

function of tidal height and exposure. In an attempt to determine the relationship between 

temperature and the accumulation and persistence of tar, in March 2001 we measured the 

surface temperature of all the casts, nearby rock and permanent plots in the barnacle,

Endocladia, Silvetia, and mussel zones at both Point Sierra Nevada and Boat House. We then 

compared the mean temperature of the cast vs. rock substrate for each community, at both 

Point Sierra Nevada and Boat House. To insure that we were not making any false 

generalizations, we placed Tidbit temperature loggers at the mean tidal height for each zone 
Variability in the accumulation and persistence of tar in four intertidal communities 

3

(at both Point Sierra Nevada and Boat House) for a full month (April 24, 2001 to May 25, 

2001). 

SIGNIFICANT RESULTS AND CONCLUSIONS: Tar accumulation and persistence 

varied among as well as within sites. Results from the visual surveys suggest tar does not 

persist long in the mussel zone. While we have documented tar stuck to Silvetia, mussels and 

rock in the mussel zone, it does not reside long (observed less than two weeks). Results from 

the permanent plots show that tar accumulates and persists in the barnacle and Endocladia

zones. At Point Sierra Nevada and Shell Beach tar accumulated more in the barnacle zone. In 

contrast, at Boat House more tar accumulated in the Endocladia zone. Results from the 

substrate experiments show that tar is likely to be deposited in all 4 zones. With the exception 

of the mussel zone, once tar accumulated on a standard surface it persisted. We found that the 

permanent plots highest on the shore accumulated the most tar, regardless of site. This 

suggests that in the event of an oil spill, the zones most likely to be affected are those highest 

on the shore. Our results showed that at Point Sierra Nevada, while there was a significant 

difference between the communities there was no difference between the rock and cast 

temperatures within communities. However, at Boat House we did find a difference between 

the rock and cast temperatures within the Silvetia zone. The rock temperature in this zone was 

several degrees cooler than the cast temperature. The results from our Tidbit temperature 

loggers suggest that temperature could play an important role. The results ranked by zones 

from highest temperature to lowest temperature: barnacle, Endocladia, Silvetia and mussels. 

It appears that for different species different mechanisms are important, likely due to 

differences in life history traits. For example, in the Silvetia zone, tar was found on the casts, 

but not on the rock. Biological factors may be more important than physical factors in this 

zone. In the mussel zone we documented (occasionally) tar sticking on the rock, mussels and 

casts. However, tar does not persist long in this zone. In the mussel zone, physical factors 

may be more important than biological factors. For both the barnacle and the Endocladia

zone, tar seems to accumulate and persist roughly the same on the cast and rock substrate. 

For barnacles and Endocladia it may be that physical and biological factors are working in 

concert. It also appears that aside from the natural process of weathering, that barnacles and 

Endocladia have no mechanism for tar removal. Once tar accumulates in these zones, it 

persists. 

Overall, our findings are valuable for predicting the impacts of small to moderate oil spills. 

Moreover, we believe that some of our results may be extendable to a large-scale oil spill. In 

the short-term, the upper zones barnacles and Endocladia are likely to be most affected; our 

results show that once larger sized tar patches accumulate in these zones, they persist. 

However, results from other studies suggest that acorn barnacles are likely to be the species 

most able to recover quickly via recruitment. By contrast, mussels and fleshly algae are more 

resistant (for different reasons) to oiling but are much less likely to recover quickly if 

damaged via an oil spill. 
Final Technical Summary – Roe et al 

4

STUDY PRODUCTS: 

Forde, S.E. 2002. Modelling the effects of an oil spill on open populations of intertidal 

invertebrates. Journal of Applied Ecology 39: 595-604 

Roe, C.A and Raimondi, P.T. 2001. Variability in the accumulation and persistence of tar in 

four intertidal zones. Western Society of Naturalists Meetings, Ventura, CA. 

Forde, S.E. 2000. Assessing the individual and population level effects of a disturbance using 

a dynamic state variable model. Ecological Society of America Meetings, Snowbird, UT. 

 
Variability in the accumulation and persistence of tar in four intertidal communities 

5

FINAL STUDY REPORT 

Introduction

To date much of the information we have regarding shoreline oiling has, been in large part, 

the result of major oil spills, such as, the 1969 Platform A blowout in Santa Barbara, 

California, followed 20 years later by the Exxon Valdez Spill in Alaska. However, it is 

difficult to generalize about the effects of oil spills, due to the spatial and temporal variability 

of spills (Nelson 1982; Foster et al. 1988; KLI 1992) and because information is often 

gathered “after the fact” (Neushul 1970). If a community has not been monitored prior to a 

spill, assessment requires using data collected after the spill and an estimation of the 

community prior to the spill to determine the resulting magnitude of the impact. Typically, it 

is difficult to determine if the condition of the community prior to the spill was estimated 

accurately (Neushul 1970; Nelson 1982; Foster et al. 1983; Jackson et al. 1989; KLI 1992; 

Paine et al. 1996). As a result, postdictive studies of oil spill impacts maybe unable to provide 

estimates of the disturbance caused by oil spills or conclusively demonstrate that the oil spill 

was responsible for any presumed change. Thus, there is need to couple monitoring with 

experimental studies of impacts, particularly those of oil and gas production (Paine et al. 

1996; Forde 2002). 

Our goal was to couple long-term monitoring data with experimental studies to better assess 

the impacts of oil spills. We combined long-term data from the Shoreline Inventory Program 

(currently funded by the Minerals Management Service and primarily an inventory of 

intertidal populations), which includes a number of sites in San Luis Obispo and Santa 

Barbara Counties, with experimental studies of tar accumulation and persistence, as part of an 

effort to improve the ability to assess the impacts of potential oil spills in this region. The 

central California coastline is vulnerable to oil spills because of offshore oil drilling and a 

high amount of tanker traffic. Additionally, the central California coastline is characterized 

by numerous natural subtidal oil seeps, which have been documented as far back as 1792 

(Foster et al. 1970). The oil from these seeps frequently washes ashore and the rocky 

intertidal in this region often has a large amount of oil (tar) on it. These factors make this an 

ideal location to (1) assess where tar accumulates and persists within different species zones 

and (2) determine what factor(s) are important to the accumulation and persistence of tar in 

the intertidal. To our knowledge no studies have been done using natural tar to mimic the 

effects of oil spills. Variability in tar accumulation and persistence may result from both 

physical and biological factors. Physical factors include temperature, tidal height and wave 

exposure. The major biological factor of importance relates to characteristics of the species 

tar lands on. These factors not only contribute to the weathering (degradation) of tar, but may 

also have an effect on where the tar accumulates. Thus, we developed some short-term 

experiments that use naturally occurring tar as a proxy for oil to evaluate those species that 

would potentially be most susceptible to an anthropogenic oil spill along the central California 

coastline. Since our study relies on low levels of tar accumulation and persistence as a proxy 

for oil accumulation and persistence, this method comes close to simulating a small-scale oil 

spill. 
Final Study Report – Roe et al 

6

Methods 

Species Descriptions 

We looked at four communities characterized by the following species: acorn barnacles, 

Endocladia, Silvetia and mussels. Information about natural history is important for assessing 

the potential impacts of oil spills. 

Balanus glandula and Chthalamus fissus/dalli (acorn barnacles) dominate upper intertidal 

zones. Balanus out competes Chthalamus through crowding, but Chthalamus is found higher 

in the intertidal due to increased resistance to desiccation (Ambrose et al. 1995). Balanus

individuals have 2 to 6 broods during the Winter and Spring (Morris et al. 1980). 1,000 to 

3,000 nauplii are released per brood, and peak settlement occurs during Spring and Summer 

(Morris et al. 1980). Chthalamus spp. have up to 16 broods per individual, primarily during 

Spring through Fall (Morris et al. 1980). 200 to 300,000 nauplii are released per brood, and 

settlement peaks in March and February (Morris et al. 1980; Strathman 1992). Juvenile 

barnacles preferentially settle near adults. 

Endocladia is characteristic of the upper intertidal, and may be associated with Mastocarpus. 

Turfweed carpets form a microhabitat for small organisms and an attachment site for juvenile 

mussels (Ambrose et al. 1995). Mussels often displace Endocladia as patches grow, but 

Endocladia can grow on mussel shells. 

Silvetia compressa (formerly Pelvetia compressa) is found in the high intertidal, below 

Endocladia/Mastocarpus associations. Silvetia also serves as shelter for invertebrates and 

algae, and as attachment sites for sessile organisms (Ricketts et al. 1985). Recruitment varies 

depending on adult densities. Studies have shown that dispersal (of new individuals from 

adults) is limited, probably on the order of meters or less (De Vogelaere et al. 1994). 

Therefore, abundance is highly variable, with a maximum in Summer and a minimum in 

Winter (Foster et al. 1991). Population fluctuations are often related to abiotic conditions. 

Mytilus californianus is found in the mid- to low intertidal. Mussel associations provide food 

and shelter for a number of invertebrates and algal species. Mussels spawn year round, with 

peaks in July and December (Morris et al. 1980). Mussels settle preferentially to existing 

beds and growth is variable depending on physical factors (Ambrose et al. 1995). Recovery 

from disturbance varies depending on the size of the clearing and the number of mussels still 

present (KLI 1992; Ambrose et al. 1995). 

Study Area 

Our study sites were determined by the following criteria; 1) sites where long term data had 

been collected under the Shoreline Inventory Program, 2) sites in proximity to natural subtidal 

tar seeps, 3) sites where tar was known to be present on the shore, 4) sites with limited public 

access and 5) sites that would provide good spatial inference. From North to South; Point 

Sierra Nevada (N35° 43.512’ W121° 19.264’) near San Simeon, Shell Beach (N35° 10.118’ 

W120° 41.795’) near San Luis Obispo and Boat House (N34° 33.243’ W120° 36.689’) 
Variability in the accumulation and persistence of tar in four intertidal communities 

7

located on the south base of Vandenberg Air Force Base (Fig. 1). Point Sierra Nevada (Fig. 

2a) has a sloping bench, Shell Beach (Fig. 2b) has a gradually sloping bench and Boat House 

(Fig. 2c) has hogbacks that run parallel to the incoming waves. 

Figure 2. Study sites from North to South 

a) Point Sierra Nevada c) Boat House b) Shell Beach

N

Pt. Sierra Nevada

Shell Beach

Boat House 

Pt. Conception 

Santa Barbara Co.

San Luis Obispo Co.

Figure 1. Location of study sites (Pt. Sierra Nevada, Shell Beach and Boat House) 

With Pt. Conception and the Channel Islands added for reference.

 
Final Study Report – Roe et al 

8

Data from long-term monitoring program 

Slides from the Shoreline Inventory Program were scored to assess where tar was 

accumulating and to estimate the percent cover of tar in permanent plots within different 

communities, across many sites and over the course of several years (Fig. 3). Following this, 

a subset of the slide data was used for the analysis (slides from; 7 biological communities, 6 

sites and 6 years (two seasons (Fall and Spring) per year, totaling 12 seasons). This 

information was also useful for selecting which communities to assess and field study sites. 

Univariate and multivariate repeated measures ANOVAs were run to determine if site, season 

or community affected where tar accumulated and persisted. 

Visual Surveys 

To further assess where tar was naturally accumulating, we performed thorough visual 

surveys in 4 intertidal zones: barnacle (primarily Chthamalus), Endocladia (a red alga), 

Silvetia (a brown, fucoid alga), and mussel zones in the fall of 1999 at Point Sierra Nevada, 

Shell Beach and Boat House. This also helped us select sites for our experiments assessing tar 

accumulation and persistence. 

Permanent Plots 

Based on the results from the slide data and our visual surveys we established permanent plots 

in the barnacle and Endocladia zones in the Fall of 1999 at Point Sierra Nevada, Shell Beach 

and Boat House to assess the accumulation and persistence of tar in different species zones. 

These plots were set up to provide a representative sample of tar accumulation and persistence 

in each zone at each site. Thus, the number of plots and the area sampled varied from site to 

site (2-3 plots per zone; area of individual plots between 5sq meters (Point Sierra Nevada and 

Figure 3. Slides from the Shoreline Inventory Program. Photo’s are of Barnacle plot 4 at Point Sierra 

Nevada. Tar patches are outlined. 

Fall 1998 Spring 2002 
Variability in the accumulation and persistence of tar in four intertidal communities 

9

Boat House) and 20sq meters (Shell Beach)). However, the number of plots per zone and the 

area sampled in each zone within a site were roughly the same (Fig. 4a). The permanent plots 

were sampled twice annually (Spring and Fall) to estimate the percent cover of tar in the 

different zones. [Although, permanent plots were not established in the Silvetia and mussel 

zones, we continued to do visual surveys for tar in these zones twice annually (Spring and 

Fall)]. Using a grid we were able to estimate the size of individual tar patches (Fig. 4b). 

These surveys allowed us to calculate the percent cover of tar in two different zones (barnacle 

and Endocladia), to determine the rate of accumulation in these zones and to do within and 

among site comparisons of tar accumulation and persistence. At each site, the number, size 

and relative freshness of tar patches per marked plot were recorded. Univariate and 

multivariate repeated measures ANOVAs were run to determine if there was an effect of site, 

time (season) or community on tar accumulation and persistence. 

Experimental substrate 

Experiments were designed to measure variability in tar patch accumulation and persistence 

by removing the effect of substrate. We knew from our earlier work that tar accumulates and 

persists in the barnacle zone longer than any other zone. This may be due to a longer 

exposure to air (the tar has a chance to stick) or it may also result in part from the texture of 

barnacles. In contrast, tar does not accumulate and persist in the algal zones, which may be 

due to a shorter exposure period or it may also be biological in nature since many alga 

produce mucilage that may inhibit tar from sticking. To test these ideas we (1) made identical 

casts of barnacles with a urethane material that mimics the texture of barnacles (See, 

Appendix A) which allowed us to standardize the substrate and (2) placed sixty 10x10cm 

a) Permanent Plots b) Sampling Method 

Figure 4. a) shows an example of a permanent Endocladia plot (foreground) and a barnacle plot 

(background) at Point Sierra Nevada, b) shows how the plots are sampled to determine the percent tar 

cover. 
Final Study Report – Roe et al 

10

barnacle casts in the intertidal at each of our three sites (from north to south: Point Sierra 

Nevada, Shell Beach and Boat House) in April 2000. At each site, we placed fifteen casts 

into each “zone” (the barnacle zone (control), Endocladia, Silvetia and mussel zones (Fig. 5a) 

for a total of 60 casts per site; 15 per zone). The casts were then checked monthly for 18 

months (May 2000 to October 2001) for the presence or absence of tar (Fig. 5b). For greater 

resolution in October 2000 we began recording the presence of tar, the number of patches, the 

relative location of the tar on the cast and the diameter of individual tar patches. We only 

measured tar patches ≥ 2.0mm. ANOVAs were run to determine if site, time (monthly) or 

community affected tar accumulation and persistence. Multivariate within subject tests could 

not be run because there were more temporal periods than replicates, hence the GreenhouseGeisser Epsilon (GG) correction on Univariate stats are presented (Quinn and Keough 2002). 

Due to human interference, the barnacle cast experiments at the Shell Beach site were 

abandoned in October 2000. 

 

Tidal Height 

Because our visual surveys showed tar accumulating in higher zones, we tested the idea that 

variability in tar patch accumulation and persistence may be a function of tidal height and 

exposure. Using a tide height computer program we were able to create a model specific to 

the conditions near Boat House. Using 10-minute intervals of all outgoing tides during a oneyear period (January 1, 2000 to December 31, 2000) we were able to calculate the proportion 

b) Sampled monthly for 

presence of tar 

a) 60 casts per site 

15 per zone 

Figure 5. a) shows layout of casts in the field: grey represents 

Endocladia zone, green represents the Silvetia zone and black represents the mussel zone. 

b) photo shows how the casts were sampled.

barnacle zone, brown represents the 
Variability in the accumulation and persistence of tar in four intertidal communities 

11

of time the tide was at a given point on the shore. Using a laser leveler, in February 2001, we 

measured the tidal height of all of the barnacle casts and permanent plots at both Point Sierra 

Nevada and Boat House. We then compared the approximate vertical distributions of the four 

zones at Boat House, within which the barnacle casts were placed. ANOVAs were run to 

determine if tidal height affected where tar accumulated and persisted. 

Temperature 

In an attempt to determine the relationship between temperature and the accumulation and 

persistence of tar, in March 2001 we measured the surface temperature of all the casts, nearby 

rock and permanent plots in the barnacle, Endocladia, Silvetia, and mussel zones at both Point 

Sierra Nevada and Boat House (we used an Omega HH21 handheld microprocessor digital 

thermometer with a type T thermocouple model # TJFT72 special purpose transition joint 

thermocouple with a fine tip probe). Approximately 300 temperature measurements were 

taken at each site during single days while the casts were exposed to air (we followed the tide 

out and back in). We then compared the mean temperature of the cast vs. rock substrate for 

each community at both Point Sierra Nevada and Boat House. To insure that we were not 

making any false generalizations, we placed Tidbit temperature loggers at the mean tidal 

height for each zone (at both Point Sierra Nevada and Boat House) for a full month (April 24, 

2001 to May 25, 2001). The Tidbit temperature loggers were fixed to a PVC plate and then 

bolted to the substrate. The Tidbit temperature loggers recorded the temperature at 10-minute 

intervals. Then, for our analysis we were able to go back and only look at the temperatures 

during times of exposure and daylight (during submersion and night time the temperature 

does not fluctuate between tidal heights). ANOVAs were run to determine if temperature was 

related to where tar accumulated and persisted. 
Final Study Report – Roe et al 

12

Results 

Data from long-term monitoring program 

Results from the multivariate repeated measures analysis suggest that the interaction between 

community type and time (season) was significant (Table 1). The mean percent tar cover 

across all sites for each community over several seasons is presented graphically (Fig. 6). 

Note that Pollicipes plots are only sampled at one site (while the other species are sampled at 

multiple sites), resulting in a lower sample size and thus a higher mean. Additionally, there 

was no data available for the Pollicipes plots in Fall 99. Thus, with the exception of 

Pollicipes, the barnacle plots had the highest percent tar cover followed by Endocladia. In 

contrast, the Silvetia and mussel communities had very little tar cover (Fig. 6). 

Table 1. Results from repeated measures ANOVA testing for the effect of site, 

community type, and season on the deposition of tar in permanent photo plots. 

 

Univariate Analysis Between Subjects 

Source SS df MS F P 

Site 27.699 5 5.534 1.889 0.101 

Community 143.618 6 23.936 8.171 0.000 

 

Error 345.653 118 2.929 

Multivariate Repeated Measures Analysis (Pillai Trace) 

Test of: Value Hypoth. df Error df F P 

Season 0.294 10 109 4.532 0.000 

Season x Site 0.365 50 565 0.891 0.687 

Season x Community 0.919 60 684 2.061 0.000 
Variability in the accumulation and persistence of tar in four intertidal communities 

13

Visual Surveys 

In the fall of 1999 we performed visual surveys at Point Sierra Nevada, Shell Beach and Boat 

House. These surveys allowed us to qualitatively evaluate where tar appeared to accumulate 

within and among sites. Overall, we found more tar at both Point Sierra Nevada and Boat 

House and less tar at Shell Beach (personal observation). At all three sites, tar was found in 

the barnacle and Endocladia zones. During our visual surveys no tar was found in the Silvetia

zone. Additionally at Point Sierra Nevada, four tar patches were found in the mussel zone (in 

contrast, no tar was found in the mussel zones at Shell Beach or Boat House). The four tar 

patches found at Point Sierra Nevada were marked and measured in late October 1999 and 

when we returned to this site in early December 1999 all four of the patches had disappeared. 

Because we were unable to find any tar in the Silvetia and mussel zones no permanent plots 

were established for these zones. However, during sample periods, we continued to search for 

tar in both the Silvetia and mussel zones. 

Permanent Plots 

In the fall of 1999, we established permanent plots at Point Sierra Nevada, Shell Beach and 

Boat House. These plots were surveyed in November 1999, May 2000, November 2000, May 

2001 and November 2001 to: (1) assess the accumulation and persistence of tar within 

different species zones of different tidal heights and, (2) to determine which species would 

potentially be most susceptible to an oil spill. These surveys allowed us to calculate the 
Final Study Report – Roe et al 

14

percent cover of tar in two different zones (barnacle and Endocladia), to determine the rate of 

accumulation in these zones and to do within and among site comparisons. Generally, we 

found more tar at Point Sierra Nevada and Boat House and less at Shell Beach (Fig. 7). The 

interaction between time (season) and community was significant (Table 2). We averaged the 

percent tar cover across all sites and found that over time, tar cover was higher in the 

Endocladia zone (Fig. 8). With the exception of the Endocladia zone at Boat House, we 

found the percent cover of tar to be fairly stable over time in both the barnacle and 

Endocladia zones (Fig. 7). At both Point Sierra Nevada and Shell Beach tar cover was 

consistently higher in the barnacle zone (primarily Chthamalus spp.) compared to the 

Endocladia zone (Fig. 7). This result was consistent with what we expected, based on the 

analyzed slides from our database and previous field observations. In contrast, at Boat House 

the percent tar cover was lower in the barnacle zone compared to the Endocladia zone (Fig. 

7). 

Table 2. Results from repeated measures ANOVA testing for the effect of site, community 

type, and season on the deposition of tar into permanent plots. Results of between subjects 

univariate analysis and multivariate analysis are presented. 

 

Univariate Analysis Between Subjects 

Source SS df MS F P 

Site 47.096 2 23.548 0.957 0.424 

Community 6.606 1 6.606 0.268 0.618 

Site x Community 14.356 2 7.178 0.292 0.755 

 

Error 196.892 8 24.612 

Multivariate repeated Measures Analysis (Pillai Trace) 

Test of: Value Hypoth. df Error df F P 

Season 0.750 4 5 3.756 0.090 

Season x Site 1.073 8 12 1.736 0.187 

Season x Community 0.843 4 5 6.719 0.030 

Season x Community x Site 1.080 8 12 1.760 0.182 
Variability in the accumulation and persistence of tar in four intertidal communities 

15
Final Study Report – Roe et al 

16

Experimental Substrate 

The barnacle casts that were put out at Point Sierra Nevada and Boat House in April 2000 

were in the field for 18 months and were sampled monthly for the presence of tar (Fig. 9). 

While tar did not accumulate on the natural substrate in the Silvetia zone, it did accumulate on 

the standardized substrate in this zone. Additionally, there was little difference in the 

proportion of casts with tar in the barnacle zones compared to the casts in the Silvetia zone 

(Fig. 9). Results from repeated measures ANOVAs showed a strong temporal effect by site as 

well as by community on the deposition of tar (Table 3). Additionally, the interaction between 

site and community type also had a significant effect on the deposition of tar on casts (Table 

3). To date, at Point Sierra Nevada tar accumulated on roughly 90% of the casts in the 

barnacle, Endocladia and Silvetia zones. Less than 7% of the casts in the mussel zone at 

Point Sierra Nevada had tar. At Boat House, tar accumulated primarily in the barnacle and 

Silvetia zone, 80% and 87% respectively. By contrast, only 53% of the casts in the 

Endocladia zone and 20% of the casts in the Mytilus zone (at Boat House) had tar (Fig. 9). 
Variability in the accumulation and persistence of tar in four intertidal communities 

17
Final Study Report – Roe et al 

18

Table 3. Effect of site, community type, and season on the deposition of tar on casts. 

Results shown are from repeated measures analysis where site and community were 

catagorical, fixed variables. Multivariate within subject tests could not be run because 

there were more temporal periods than replicates; hence, GG correction on univariate 

stats are presented. 

 

Between Subjects 

Source SS df MS F P 

Site 0.353 1 0.353 0.409 0.524 

Community 57.026 3 19.009 22.012 0.000 

Site x Community 9.298 3 3.099 3.589 0.017 

 

Error 82.902 96 0.864 

 

Within Subjects 

Source SS df MS F G-G 

Season 56.115 17 3.301 29.713 0.000 

Season x Site 4.739 17 0.279 2.509 0.008 

Season x Community 20.832 51 0.408 3.677 0.000 

Season x Site x Community 6.061 51 0.119 1.07 0.369 

 

Error 181.303 1632 0.111 

 

Greenhouse-Geisser Epsilon: 0.5344 

Tidal Height 

We tested the idea that variability in tar patch accumulation and persistence may be a function 

of tidal height and exposure. Our tidal model predicted that at Boat House, tar (which is 

transported in surface waters) should accumulate at tidal heights around 2.5-3.5 ft above 

MLLW: tidal heights where surface waters are most often in contact with the substrate (Fig. 

10). Hence we predicted that tar should accumulate in the upper tidal zone. In February 2001, 

we measured the tidal height of all of the barnacle casts and permanent plots at both Point 

Sierra Nevada and Boat House. As expected, tidal heights varied as a function of site and 

community (Table 4). The substrate interaction was not included in the analysis because there 

was no difference in the tidal height between the casts and the adjacent rock. Overall, the 

mean tidal height for each community is higher at Point Sierra Nevada than Boat House (Fig. 

11). Based on the tidal model and the mean tidal heights of each zone at Boat House, we 

expected that tar would accumulate in all four zones since the mean tidal height for each zone 

is within the 2.5-3.5 ft above MLLW range. We found no relationship between the mean 

vertical distributions of the four zones (at Boat House), within which the barnacle casts were 

placed to the accumulation of tar (Fig. 12).

 
Variability in the accumulation and persistence of tar in four intertidal communities 

19

Table 4. Results from ANOVA testing for the effect of tidal height as a function of community and site.

 

Analysis of Variance 

Source SS df MS F P 

Site 193.319 1 193.319 295.011 0.000 

Community 61.625 3 20.542 31.347 0.000 

Community x Site 19.755 3 6.585 10.049 0.000 

Error 73.393 112 0.655 
Final Study Report – Roe et al 

20
Variability in the accumulation and persistence of tar in four intertidal communities 

21

Temperature 

The difference between the cast and rock temperatures at Point Sierra Nevada was not 

significant. However, there was a significant difference between the cast and rock 

temperatures at Boat House (Fig. 13). The results from the reduced model ANCOVAs showed 

that the interactions between community and site and between substrate and site were 

significant (Table 5). With the exception of the barnacle zone, temperatures at Boat House 

were warmer than Point Sierra Nevada (Fig. 14). Results from the Tidbit temperature loggers 

ANOVAs show significant interactions between community and site, time and site, and 

between time and community (Table 6). However, there was no significant correlation 

between tidal height and temperature. In contrast, these data also show that overall Point 

Sierra Nevada is warmer, with peak temperatures around 1pm (Fig. 15). Additionally, these 

data show that throughout the day regardless of site, that the barnacle zone is significantly 

warmer (Fig. 16). 

Table 5. Results from reduced model ANCOVA testing for the effect of site, community type, substrate, 

and tidal height on temperature. 

 

Analysis of Variance 

Source SS df MS F P 

Site 1.93996E+02 1 1.93996E+02 1.20508E+01 0.00055 

Community 5.77370E+02 3 1.92457E+02 1.19552E+01 0.00000 

Substrate 1.58892E+02 1 1.58892E+02 9.87024 0.00178 

Tidal Height (ft) 2.72919 1 2.72919 0.16953 0.68071 

Community x Site 8.41364E+02 3 2.80548E+02 1.74274E+01 0.00000 

Substrate x Site 2.16872E+02 1 2.16872E+02 1.34719E+01 0.00027 

Substrate x Community 4.75619E+01 3 1.58540E+02 0.98483 0.39964 

Substrate x Community x Site 6.38997E+01 3 2.12999E+01 1.32313 0.26614 

 

Error 7.63051E+03 474 1.60981E+01 

Table 6. Results from ANOVA testing for the effect of site, community type, and time on 

temperature. 

 

Source SS df MS F P 

Site 135.235 1 135.235 3.916 0.048 

Community 2179.861 3 726.620 21.042 0.000 

Hour 31499.080 9 3499.898 101.355 0.000 

Community x Site 1490.281 3 496.760 14.386 0.000 

Hour x Site 1895.792 9 210.644 6.100 0.000 

Hour x Community 1534.949 27 56.850 1.646 0.020 

Hour x Community x Site 552.135 27 20.449 0.592 0.952 

 

Error 64366.072 1864 34.531 
Final Study Report – Roe et al 

22
Variability in the accumulation and persistence of tar in four intertidal communities 

23
Final Study Report – Roe et al 

24

Discussion 

Tar accumulation and persistence varied among as well as within sites. Intertidal oil cover 

resulting from a spill is patchy in both space and time, resulting in variability in exposure 

duration, dosage, and weathering (Straughan 1982; Foster et al. 1988; Jackson et al. 1989; 

KLI 1992). Therefore there are variable effects on individual species in the community. Data 

from slides also show variability in tar accumulation and persistence among biological 

communities. Like oil, natural tar comes ashore in pulses and thus, not all communities are 

equally affected. Thus far, the slides from the Shoreline Inventory Program have been a 

valuable tool for accessing where tar is accumulating and persisting (at least for large tar 

patches). In the event of an oil spill the slide archive from the SIP will prove to be invaluable. 

Results from the visual surveys also suggest tar does not persist long in the mussel zone. 

While, we have documented tar stuck to Silvetia, mussels and rock in the mussel zone, it does 

not reside long (observed less than two weeks). Since we sampled on a monthly basis (rather 

than more frequently) we do not have a true sense of the shorter periods of accumulation and 

persistence of tar in the mussel zone. In order to get to get a more accurate idea of 

accumulation and loss in this zone, we would need to sample more regularly during periods of 

low tides. In support of our observations, Straughan (1970, 1971) reported that at exposed 

sites oil was removed from the low intertidal zones by wave action within three weeks of the 

Platform A blowout in Santa Barbara. At both Point Sierra Nevada and Boat House the 

mussel zones are on benches with direct exposure to incoming waves. 

Results from the permanent plots show that tar accumulates and persists in the barnacle and 

Endocladia zones. At Point Sierra Nevada and Shell Beach tar accumulated more in the 

barnacle zone. In contrast, at Boat House more tar accumulated in the Endocladia zone. This 

may be a result of the upward shift in species assemblages at this site, which at that point was 

unprecedented (Fig. 17). In 1999, when our permanent plots were initially set up, the 

Endocladia was growing on dead barnacles, by late 2001 the permanent plots in the 

Endocladia zone were reverting back to barnacle plots which indicates a shift in species 

zones. Additionally, it is important to note that not all tar that accumulated persisted. There 

was import and export of tar in these plots. However, once larger tar patches accumulate in 

the upper zones they persist for months to years (Nicholson and Cimberg 1970; Straughan 

1970; Southward and Southward 1978). For example, in our study many of the tar patches 

that were found in October 1999 were still present in October 2001 with little change 

(personal observation). This result is also consistent with the slide data from the Shoreline 

Inventory Program (see Figure 2). The insight gained by determining where tar accumulated 

and persisted allowed us to predict which species would be most affected by an oil spill. 

Results from the substrate experiments show that while tar is likely to be deposited in all 4 

zones, it may not persist long in the mussel zone. Recall, that in our visual surveys very little 

tar was found in the mussel zone and no tar was found on rock beneath Silvetia, although on 

one occasion tar was found stuck to the fronds of Silvetia. This may support the idea that 

texture or biological processes are important (since casts are an abiotic mimic of the texture of 

barnacles). At Boat House, more tar accumulated on the casts (standardized substrate) in the 

barnacle and Silvetia zones than the Endocladia zone. This result was not expected based on 
Variability in the accumulation and persistence of tar in four intertidal communities 

25

our permanent plot data at Boat House that showed that more tar accumulated in the 

Endocladia zone than the barnacle zone. Additionally, not only did tar accumulate on the 

casts in the Silvetia zone at each site, it also accumulated on casts in amounts nearly equal to 

that on casts in the barnacle zone. Further, not only did tar accumulate on the casts in the 

Silvetia zone, it also persisted. It appeared that, with the exception of the mussel zone, once 

tar accumulated on a standard surface it persisted. For example, 7% of the casts in the Mytilus

zone at Boat House had tar in February 2001. Cast number 15 had the largest patch to date. It 

covered half of the plate and a portion of the surrounding mussel bed (390mm x 58.3mm). 

When sampled again in March 2001 the tar was not present. This was also the case in June 

2000 when 7% of the casts in the Mytilus zone at Boat House had tar, but when they were 

sampled again in July 2000 the tar was absent. In support of our permanent plot and slide 

data, our cast data suggest that patches < 2mm degrade rapidly, while larger patches persist 

for many months. For example, Silvetia cast number 7 (at Boat House) had a tar patch that 

first appeared in May 2000 and was initially measured to be 14.7mm (diameter) and in 

October 2001 it was 13.7mm (diameter). 

Figure 17. Slides from the Shoreline Inventory Program, showing the succession of Endocladia plot 1 (Spring 

1993 to Fall 1996) at Boat House. 

Spring 1993 

Fall 1995 

Fall 1993 

Fall 1996 
Final Study Report – Roe et al 

26

We found that, the permanent plots highest on the shore accumulated the most tar, regardless 

of site. For example, at Point Sierra Nevada barnacle plot 3 accumulated the most tar and it 

was also the plot highest on the shore, with a mean tidal height of 7.25ft above MLLW. This 

result was supported at Boat House where Endocladia plot 1 was highest on the shore, with a 

mean tidal height of 4.68ft above MLLW, it was also the plot that consistently had the highest 

percent tar cover. Additionally, when we looked at the average tidal heights of the plots at 

each site, the same pattern was found. At Point Sierra Nevada the average tidal height of the 

permanent; barnacle plots was 6.87ft above MLLW and the Endocladia plots was 5.96ft 

above MLLW. At Point Sierra Nevada the most tar accumulated in the barnacle zone. In 

contrast, at Boat House the average tidal height of the permanent plots in the barnacle zone 

was 3.69ft above MLLW and the average tidal height of the permanent Endocladia plots was 

3.9ft. Once again, at Boat House tar accumulated primarily in the Endocladia zone. Jackson et 

al. (1989), found substantially more oil in the intertidal at tidal heights above MLW. This 

suggests that in the event of an oil spill, the zones most likely to be affected are those highest 

on the shore. 

Our tidal height model for Boat House does not explain the pattern of tar accumulation and 

persistence on the casts. Based on the model we expected that tar would accumulate and 

persist approximately equally in all four zones, and while tar was deposited in all 4 zones, 

most tar accumulated on the casts in the barnacle and Silvetia zones. The model, didn’t 

explain the pattern seen in the Endocladia or mussel zones. The casts were placed throughout 

the zones of each community. The Endocladia zone casts had a mean tidal height of 2.49ft 

slightly lower than the Silvetia zone casts, which had a mean tidal height of 2.59ft. However, 

the difference in tidal height between these two zones does not seem great enough to account 

for the differences in tar accumulation and persistence. Additionally, the Silvetia casts, which 

were lower on the shore accumulated tar at roughly the rate as the barnacle casts, which were 

higher on the shore. The differences seen between Point Sierra Nevada and Boat House may 

be due to the differences in the topography of the sites. Point Sierra Nevada has a much 

steeper vertical profile than Boat House, which is more up and down. Thus, other factors may 

be important. 

If temperature was an important factor determining tar accumulation and persistence, it should 

accumulate in warmer zones, since warm tar is stickier. Additionally, we wanted to determine 

if there was a difference between the rock and cast temperature that could explain the pattern 

we saw in our cast experiments. Our results show that at Point Sierra Nevada, while there was 

a significant difference between the communities there was no difference between the rock 

and cast temperatures within communities. However, at Boat House we did find a difference 

between the rock and cast temperatures within the Silvetia zone. The rock temperature in this 

zone was several degrees cooler than the cast temperature. Recall, that in the Silvetia zone, tar 

was not found on the rock, but was found on the casts. It could be that in this zone the rock 

does not get warm enough for the tar to stick. It may also be that the Silvetia retains enough 

water and provides enough shade, which keeps the rock cooler. Another interesting result at 

Boat House was that there was no difference in the average cast temperature of the barnacle 

zone as compared to the average cast temperature of the Silvetia zone. Recall that these were 

the two zones that accumulated the most tar at Boat House, showing that temperature may be 

important at this site. Additionally, at Point Sierra Nevada, the similarity in rock and cast 
Variability in the accumulation and persistence of tar in four intertidal communities 

27

temperatures suggest that the material of the cast is not significantly affecting the temperature 

in a way that would influence where tar accumulated and persisted in these zones. However, 

our study sites are characterized by different rock types: at Point Sierra Nevada its 

conglomerate and at Boat House the rock is Monterey formation. This may have an effect on 

where tar accumulates and persists since different rock types may have different thermal heat 

capacities (Raimondi 1988). It may also be that conglomerate behaves more similarly to the 

casts, with respect to temperature. However, we measured the substrate surface temperatures 

of the casts and rock at each site, on two different days during the same week. The air 

temperature at Boat House was between 15°C-23°C, while the air temperature at point Sierra 

Nevada was between 15°C-19°C. Overall, the difference was greater at Boat House (it was 

warmer) and this may have affected the results. Thus, data from the Tidbit temperature 

loggers may be more representative of the true patterns. The results from our Tidbit 

temperature loggers suggest that temperature could play an important role. Recall that the 

temperature loggers were placed at the mean tidal height of each zone and at each site where 

they recorded the temperature at 10-minute intervals for a one-month period. These data were 

then averaged over a month. The results ranked by zones from highest temperature to lowest 

temperature: barnacle, Endocladia, Silvetia and mussels. 

Our findings, when coupled with natural history and previous studies, provide insight to 

potential impacts of oil spills along the central California coast. Balanus glandula and 

Chthalamus fissus/dalli (acorn barnacles) dominate upper intertidal zones and our study 

showed tar that accumulated in this zone persisted. Most studies show high mortality of 

barnacles following an oil spill (Foster et al. 1970; Neushul 1970; Nelson 1982; Southward 

1982; Crothers 1983; Bokn 1993). For Chthalamus, there was an observed correlation 

between tidal height and mortality; at higher tidal levels there was higher mortality following 

the Santa Barbara oil spill (Cubit 1970). Further, higher mortality rates are seen in 

Chthalamus than Balanus (Nicholson and Cimberg 1970; Straughan 1970) due to smothering 

by oil. It is suggested that the taller height of Balanus may make it more resistant to oiling 

(Nicholson and Cimberg 1970). Acorn barnacles tend to recover quickly after an initial 

decrease in recruitment immediately following a spill (Bokn 1993, Ambrose et al. 1995). 

However, it appears that Balanus recovers more quickly (2 months) than Chthalamus (10 

months) (Nicholson and Cimberg 1970; Straughan 1970), Nicholson and Cimberg (1970) 

suggested that the presence of a basal plate in Balanus (lacking in Chthalamus) may allow for 

earlier recolonizaion. Forde (2002) showed that recovery of acorn barnacles, which is 

dependent on recruitment, varies considerably due to individual variation in reproductive 

output. Thus, it appears that acorn barnacle recovery is dependent on the intensity of the spill, 

time of year (recruitment) as well as, the reproductive output of the source population (Forde 

2002). 

Endocladia is also found in the upper to mid intertidal zones and appears to be susceptible to 

tarring. In our study we found that once tar accumulated in this zone, it persisted. 

Additionally, there is evidence that Endocladia is susceptible to oiling and may take over 3 

years to recover (KLI 1992). 

While no studies have directly shown the effects of oiling on Silvetia, there is lots of evidence 

for other fucoids. For example, Fucus gardneri showed slow recovery after the Exxon Valdez
Final Study Report – Roe et al 

28

spill due to low recruitment, fewer reproductive adults and high desiccation rates (van 

Tamelen et al. 1997). Hesperophycus harveyanus also exhibited high mortality in the 1969 

Santa Barbara oil spill (Straughan, 1971). Thus, we predict that recovery from a spill would 

be slow due to the high variability in reproduction and recruitment in intertidal fucoid species. 

During our study we occasionally found tar stuck to Silvetia, but it was not observed to stick 

long (less than 2 weeks). However, it seems plausible that in a large-scale spill, where Silvetia

fronds were completely covered by oil, the increase in drag could lead to dislodgement. 

Mytilus californianus is found in the mid- to low intertidal. Some studies have found no 

effect of oil spills on mussels, while others found evidence of mortality (Foster et al. 1970; 

Chan 1972; Southward 1982; Straughan 1982; Foster et al. 1988; Bokn et al. 1993). 

Recovery from disturbance varies depending on the size of the clearing and the number of 

mussels still present (KLI 1992; Ambrose et al. 1995). 

It appears that for different species different mechanisms are important, likely due to 

differences in life history traits. For example, in the Silvetia zone, tar was found on the casts, 

but not on the rock. This provides evidence that biological factors are more important than 

physical factors in this zone. In this case, it is not the lack of input, but rather the lack of 

adherence. We know that Silvetia produces a mucilage that may act to inhibit tar from 

sticking. In addition, Silvetia also regularly sloughs cells, which may also lead to the lack of 

tar accumulation and persistence in this zone. In the mussel zone, we have documented 

(occasionally) tar sticking on the rock, mussels and casts. However, tar does not persist long 

in this zone. In the mussel zone, physical factors may be more important than biological 

factors. We think that as with Silvetia, it is not the lack of input, but rather the lack of 

adherence. Second, accumulation of tar in the mussel zone is lower both on casts and mussels 

– ruling out a biological process of removal (at least one related to mussels). Also the areas in 

which the mussel experiments were done are in open and exposed locations. While we did not 

directly test the effects of exposure we believe that it is an important factor. It is also possible 

that tar is present, but degrades very quickly in the mussel zone (causing our monthly samples 

to be too infrequent to detect the input). For the mussel zone, the mechanism is likely due to 

direct exposure to waves. It seems that in highly exposed areas, the waves regularly scour the 

tar off the rocks. Neushal (1970) suggested that the lower mortality rate of Pollicipes 

polymerus observed after the Santa Barbara spill may have resulted from “exposure to the 

cleansing action of the surf”. 

For both the barnacle and the Endocladia zone, tar seems to accumulate and persist roughly 

the same on the cast and rock substrate. Endocladia neither sloughs cells or produces 

mucous. The barnacle and Endocladia zones are also less affected by wave intensity. For 

barnacles and Endocladia it may be that physical and biological factors are working in 

concert. It also appears that aside from the natural process of weathering, that barnacles and 

Endocladia have no mechanism for tar removal. Once tar accumulates in these zones, it 

persists. 

Overall, our findings are valuable for predicting the impacts of small to moderate oil spills. 

Moreover, we believe that some of our results may be extendable to a large-scale oil spill. In 

the short-term, the upper zones barnacles and Endocladia are likely to be most affected; our 
Variability in the accumulation and persistence of tar in four intertidal communities 

29

results show that once larger sized tar patches accumulate in these zones, they persist. 

However, results from other studies suggest that acorn barnacles are likely to be the species 

most able to recover quickly via recruitment. By contrast, mussels and fleshly algae are more 

resistant (for different reasons) to oiling but are much less likely to recover quickly if 

damaged via an oil spill. 
Final Study Report – Roe et al 

30

Future Recommendations 

There is a great need to investigate recruitment rates at a variety of sites to assess recovery by 

recruitment potential in order to more fully estimate effects of oiling – This work has just 

been partially funded by MMS. Further, post spill impact studies have concluded that surf 

grasses, limpets, gooseneck barnacles, sea star and abalone are all susceptible to oil spills 

(Cubit 1970; Foster et al. 1970; Neushal 1970; Nicholson and Cimberg 1970; Straughan 

1970; Straughan 1971; Chan 1974; Southward and Southward 1978; Bokn et al. 1993; 

Ambrose et al. 1995; Dean et al. 1998). Therefore, we recommend that this project be 

followed with another that extended the survey and experimental work to other taxa including 

surf grass, Lottia, abalone and tide pool organisms (fish and others) that have not yet been 

evaluated for patterns and mechanisms of oil accumulation and persistence. In addition, we 

recommend increasing the spatial scale of the project to encompass sites all along the area of 

risk. We also recommend that the barnacle casts at Point Sierra Nevada and Boat House 

(4/zones, 15casts/zone, 60/site) be left in the field and completely covered with fresh tar (from 

a natural seep) and then sampled monthly to evaluate rates of weathering, recruitment, 

settlement and mortality. We recommend that these casts be followed until the tar completely 

degrades. It would also be interesting to determine the reproductive output (success) of those 

barnacles that settle on the tar compared to barnacles on unfouled adjacent rock. The 

information gained could then be coupled with this study to further our understanding. 
Variability in the accumulation and persistence of tar in four intertidal communities 

31

Appendix A 

Steps for making identical casts; 1) an impression of live barnacles in the intertidal was taken 

with a two part dental compound (often used for making molds of teeth) this became the 

master mold, 2) next using Alginate (another dental compound) 7 identical positives were 

made from the master, 3) next the positives were placed into square tupperware sandwich 

containers and then Synair’s 2868 Por-A-Mold was poured to cover them completely (see 

product guidelines for cure times), 4) once cured the positives were removed from the molds 

5) the 7 identical molds now ready, were sprayed with Synlube 531 (a silicone release agent) 

and then dried with a hair dryer (to expedite the process), 6) finally the molds were filled 

with Synair’s Mark II Por-A-Kast (see product guidelines for cure times) and 7) identical 

casts were removed from the molds and the steps were repeated. It takes approximately 1 

hour (steps 4-6) to make 7 casts. 

Acknowledgements 

We thank the Minerals Management Service, US Department of the Interior, under MMS 

agreement no. 14-35-0001-30761 for supporting this research. Thanks to Dave Lohse for 

advice and for help developing the tidal model. Thanks to Diana Steller and Melissa Wilson 

for comments and advice. Thanks to Keith Skaug for materials advice. Thanks to Trevor 

Meeker for help deploying the casts, help setting up field experiments and all of your support. 

Thanks to Kathleen Donahue, Maya George, Hilary Hayford, Cynthia Hays, Ali Kendall, 

Kristen Kusic, Erin Maloney, Trevor Meeker, Mark Readdie, Marci Roe, Jennifer Switzer, 

Lisa Van Nortwick, Susan Werth and Melissa Wilson for enthusiasm and help in the field. 

The views and conclusions contained in this document are those of the authors and should not 

be interpreted as necessarily representing the official policies, either express or implied, of the 

US Government. 
Final Study Report – Roe et al 

32

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45

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.