ACCESS NUMBER: 85244 

STUDY TITLE: ROLE OF GRAZERS ON THE RECOLONIZATION OF HARD-BOTTOM 

COMMUNITIES IN THE ALASKA BEAUFORT SEA

REPORT TITLE: ROLE OF GRAZERS ON THE RECOLONIZATION OF HARD-BOTTOM 

COMMUNITIES IN THE ALASKA BEAUFORT SEA

CONTRACT NUMBER: 1435-01-98-CA-30909; TO: 85244 

SPONSORING OCS REGION: Alaska 

APPLICABLE PLANNING AREA: Boulder Patch, Beaufort Sea 

FISCAL YEARS OF PROJECT FUNDING: 2002, 2003, 2004, 2005 (no cost extension in final 

year) 

COMPLETION DATE OF REPORT: May 2006 

COSTS: 2002: 52,850.96 

 2003: 66,247.81 

 2004: 74,416.24 

 2005: 43,705.11 

PROJECT MANAGER: Brenda Konar 

AFFILIATION: School of Fisheries and Ocean Sciences, University of Alaska Fairbanks 

ADDRESS: P.O. Box 757220, Fairbanks, Alaska 99775 

PRINCIPAL INVESTIGATOR: Brenda Konar 

KEY WORDS: Boulder Patch, disturbance, recolonization, recovery, macroalgae, invertebrates 

BACKGROUND: 

Alaska’s Beaufort Sea shelf is typically characterized by silty sands and mud and as 

having an absence of macroalgal beds and associated organisms (Barnes and Reimnitz 1974). 

In 1971, a diverse kelp and invertebrate community was discovered near Prudhoe Bay in 

Stefansson Sound, Alaska (Figure 1). This area was named the Boulder Patch by the U.S. 

Board of Geographic Names. Since its discovery, the Boulder Patch has been subject to much 

biological and geological research (Dunton et al. 1982, Dunton and Schell 1987, Dunton and 

Jodwalis 1988, Dunton 1990, Martin and Gallaway 1994, MMS 1996, 1998, Dunton and 

Schonberg 2000 and others). 

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ACCESS NUMBER: 85244 

Figure 1. Chart of Steffanson Sound within the Beaufort Sea showing the 

location of DS11 within the Boulder Patch. Hatched polygons are areas with 

high boulder/cobble density. Figure from Dunton and Schonberg (2000).

The Boulder Patch contains numerous cobbles and boulders that provide substrate for 

attachment for many invertebrates and several species of red and brown algae. The 

invertebrate assemblage that inhabits the rocks and associated kelp beds has representatives 

from every major phylum (Dunton 1985). The dominant alga is the brown Laminaria solidungula, 

which constitutes 90% of the brown algal biomass (Dunton et al. 1982). This alga is an 

important food source to many benthic and epibenthic organisms (Dunton and Schell 1987). 

Differences in infaunal abundance and biomass between the Boulder Patch and peripheral 

sediment areas demonstrate the importance of this unique habitat, with algal and epilithic 

invertebrates covering nearly all exposed substrate, except recently upturned rocks (Dunton and 

Schonberg 2000). 

Boulder fields are very dynamic systems because of physical disturbance (Sousa 1979, 

1980, vanTamelen 1987). In the high arctic, ice gouging can overturn large boulders and 

cobbles and hence expose new substrate (Dunton et al. 1982, Conlan et al. 1998). In many 

areas, currents can overturn smaller rocks that have epibenthic organisms attached to them 

(Dunton et al. 1982). These organisms cause resistance to the currents, which results in rocks 

moving and turning. The existence of dead (white) corallines on the underside of many small 

rocks suggest that these processes are common in the Boulder Patch (Dunton et al. 1982). 

When a boulder is overturned, algae and sessile invertebrates can be killed in whole or part by 

a combination of grazing, anoxia, low light levels, or mechanical damage caused by crushing or 

abrasion (Sousa 1980). Algal and some invertebrate populations recolonize cleared surfaces 

through either vegetative regrowth of surviving individuals or by recruitment from spores (Sousa 

1979). 

Recolonization experiments in the Boulder Patch have shown that recovery of denuded 

areas is slow (Dunton et al. 1982). In temperate systems, algal communities can recover to 

previous densities within one year of denuding (Foster 1975). Many studies on coralline algal 

recruitment have shown that although their growth is remarkably slow, they will settle to a visible 

size in a few months (Johanson and Austin 1970, Colthart and Johanson 1973, Adey and 

Vasser 1975, Matsuda 1989, Konar and Foster 1992, Konar 1993, Dethier and Steneck 2001). 

In contrast to temperate systems, 50% of the substrate in the Boulder Patch was still bare three 

years after an initial disturbance (Dunton et al. 1982). One reason suggested for this slow 

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ACCESS NUMBER: 85244 

recolonization is invertebrate grazing. Motile herbivorous, omnivorous, and carnivorous 

invertebrates such as chitons, snails, seastars, and polychaetes have been frequently observed 

in the Boulder Patch (Dunton et al. 1982). Many studies have shown that grazers can be very 

important in structuring communities (Johnson et al. 1997, Worm and Chapman 1998, Jenkins 

et al. 1999, Ojeda and Munoz 1999, Morton 1999, Wilson et al. 1999, Konar 2000). 

The reason that grazing has been suggested to cause the slow recovery in the Boulder 

Patch is that polystyrene floats (suspended 1 m above the bottom) that were used to mark sites 

in a study by Dunton (1985) recolonized much more quickly than denuded substrates. When the 

floats were first examined by Dunton (12 months after initial deployment), all were covered with 

hydroids, bryozoans, red algae, and polychaete worm tubes. This is the same suite of species 

that eventually recruited onto the denuded experimental boulders; however, recruitment onto 

the boulders was much slower. Since the floats were not intended to be experimental 

substrates, no quantitative comparison was made to non-floating experimental substrates. 

Dunton (1985) did suggest that these floats may have recovered faster than the experimental 

substrates because the floats were not subject to grazing or predation pressures by benthic 

animals. Recruitment onto the floats may have been due to larvae and spores exhibiting a 

substrate preference for polystyrene; however, Dunton concluded that this was unlikely because 

all epilithic species probably do not possess a common selectivity for this artificial substrate. 

OBJECTIVES: 

To determine if grazing is associated with the slow recruitment in the Boulder Patch, 

various comparisons using exclusion cages, cage controls, and natural rock were employed. 

The specific hypothesis tested was that there would be no significant difference between 

recruitment of sessile organisms on cleared boulders with and without cages, which were 

employed to exclude macrograzers. Concurrent with this recruitment study, community 

observations and surveys were performed to determine which macrograzers were the most 

conspicuous on the rocks, if space was limiting (which would be suggested by a paucity of open 

space on rocks), and if the dominant alga, Laminaria solidungula, was reproductive during the 

study period. 

DESCRITION: 

Methods 

Cages

To address the hypothesis that there is no significant difference between recruitment of 

sessile organisms on cleared boulders with and without cages, cages were deployed at each of 

the three sites within DS11 at the Boulder Patch in August 2002 (Figure 1). Cages are 

commonly used for inclusion and exclusion manipulative experiments (see Vadas 1985, Coyer 

et al. 1999 for overviews). The boulders used in this study were collected from DS11. Because 

of the difficulty in removing living material from rocky substrata underwater, all boulders were 

brought to the surface to be cleared. Care was given not to chip boulders so as to expose new 

surface because this leaching and weathering of newly exposed surfaces may affect recruitment 

(Dayton 1971, Reed and Foster 1984). After five days, denuded rocks (with and without cages 

and cage controls) were placed back into the field. 

Rocks that were cleared of sessile organisms were also surveyed for overlapping 

borders between organisms. These surveys were supplemented by other random rocks that 

were surveyed in the field to increase the general sample size of organisms observed. The data 

obtained by these observations are presented in the appendix as a note, which is in press with 

Polar Biology (Appendix 1). 

Six cages were deployed to exclude large macrograzers at each of three locations within 

DS-11, totaling 18 cages. The cages were 25 cm on a side, 10 cm tall, with a 1 cm mesh size 

and constructed of stainless steel mesh (Figure 2). All cages were coated with a non-toxic 

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ACCESS NUMBER: 85244 

antifouling compound to inhibit sessile invertebrate and algal growth (Coyer et al. 1999). 

Eighteen cage controls also were deployed to control for any artifacts caused by the cages, 

such as decreased light levels or increased sedimentation. These cage controls, which were 

cages with holes cut into the sides so that grazers could easily pass through them, are 

commonly used (Russ 1980, Breitberg 1985, Stocker 1986). For comparison, 18 cleared rocks 

were deployed with no cages to determine natural recruitment. As a control for natural 

community changes, 18 non-cleared boulders also were monitored. 

 Visual estimates were used to determine percent cover of sessile organisms on all 

boulders. All algae (except for corallines) were identified to genus while all other organisms 

(sponges, bryozoans, hydroids, and tunicates) were combined into larger taxonomic groups. 

Care was taken not to disturb boulders while sampling so as not to affect mobile organisms 

under and around the boulders (Chapman and Underwood 1996). 

To determine if there was significant light reduction due to cage shading, light 

measurements were taken under and adjacent to cages and cage controls with a Li-Cor Model 

LI-185A Quantum Light Meter on three different days in 2002 (Foster et al. 1985, Ramus 1985). 

Light has been shown to influence algal recruitment (Reed and Foster 1984, Graham 1996, 

1997, Edwards 1998, Dayton et al. 1999, Huovinen et al. 2000) and to a limited extent 

invertebrate settlement (Duggins et al. 1990). 

Community surveys

Surveys were conducted in 2004 to determine community composition (grazers, sessile 

organisms, macroalgae, and bare rock), and reproductive status of the dominant alga, 

Laminaria solidungula at DS11. To determine potential grazers, random boulders were collected 

from fifteen 0.25m2

 quadrats from which grazer densities were calculated. Percent cover 

estimates of sessile organisms (invertebrates and macroalgae) and bare rock were used to 

access substrate cover and available open space along eight randomly placed transects. Along 

each transect, five one-meter long random point contact bars (similar to those described by 

Foster 1975, Coyer et al. 1999) were randomly placed. These bars had a string attached at 

each end, with five knots along their length. Knots randomly placed on the line were pulled tight 

away from the bar and placed on the substrate, and the identity of the alga or invertebrate was 

recorded. If no organism was present at the knot, the type of substratum was noted. By moving 

the string from one side of the bar to the other, 10 points were sampled per area. Macroalgal 

biomass estimates were completed by collecting boulders from five randomly placed 25m2

quadrats. Boulders were brought back to the lab where all macroalgae were removed, identified, 

and weighed for damp biomass. To determine if the dominant alga, Laminaria solidungula was 

reproductive at DS11, one random 10m2

 area was surveyed along eleven randomly placed 30m 

transects. All L. solidungula in each area were enumerated and the state of reproduction was 

recorded. 

SIGNIFICANT CONCLUSIONS: 

Monitoring cleared boulders (caged, and uncaged) for three years resulted in no growth 

of any sessile organism for the first two years. In the third year, new invertebrate and coralline 

growth was seen on 79% of the cleared boulders, with significantly more growth on caged 

boulders. Uncleared control rocks showed no significant temporal variation. Concurrent surveys 

and observations suggested that in this arctic community 1) there are two primary types of 

macrograzers on rocks, chitons and seastars, 2) space is limiting, 3) the dominant alga, 

Laminaria solidungula was reproductive during the study period. This study strongly suggests 

that any perturbations causing scouring of hard substrate in the Beaufort Sea will result in very 

slow community recovery. 

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ACCESS NUMBER: 85244 

STUDY RESULTS: 

Cages

 Community composition on the control, uncleared boulders did not significantly vary over 

time (ANOVA, F3,59=0.626, p=0.6, Figure 2). The majority of uncleared boulders were covered 

with encrusting coralline algae and foliose algae, while very little (a mean of 2% or less) was 

bare. Foliose algae were primarily Phycodrys rubens, averaging 21% over the three year period. 

Other foliose algae included Phyllophora truncata (synonym Coccotylus truncates), Dilsea 

socialis, and Odonthalia dentata. 

0

20

40

60

80

bare rock coralline total foliose

algae

total

invertebrates

percent cover

2002

2003

2004

2005

Figure 2. Mean percent cover (±1 s.e.) of bare rock, encrusting coralline 

algae, total foliose algae, and total invertebrates on uncleared control boulders 

in 2002, 2003, 2004, and 2005. 

In the first two years of this study, no growth was observed on any experimentally 

cleared rock. In the third year, 79% of the cleared rocks showed some growth. While growth 

was limited on all cleared rocks (never exceeding 10% and averaging less than 2%), the rocks 

in cages had significantly more growth than the other two treatments (cage control and 

uncaged; ANOVA, F2,39=10.599, p=0.0002). All growth seen in the third year was relatively small 

in size (typically less than 0.5cm) and included bryozoans, hydroids, spirorbids, barnacles, 

sponges, and encrusting corallines (Figure 3). No growth of brown or red foliose algae was seen 

on any cleared boulder in any year. 

 Cages and boulders were examined for light effects. While boulders in open areas did 

receive significantly more light than boulders in cages and in cage controls (ANOVA, 

F2,6=12.924, p=0.0067), there was no significant light difference between cages and cage 

controls (Figure 4). This suggests that differences found between results occurring within cage 

boulders and cage control boulders were not due to a lighting effect. 

5
ACCESS NUMBER: 85244 

0

0.5

1

1.5

2

2.5

3

3.5

corallines

bryozoans

hydroids

spirorbidsbarnacles

percent cover

cages

cage controls

cleared

Figure 3. Mean percent cover (±1 s.e.) of encrusting coralline algae, 

bryozoans, hydroids, spirorbids, and barnacles on each of the treatment 

boulders in 2005. 

0

5

10

15

20

25

30

35

40

cage partial open

light attenuation

Figure 4. Mean percent light attenuation (±1 s.e.) under cages, in cage 

controls, and adjacent to cages in 2002.

Community surveys

Grazers that were found in the DS11 surveys consisted of two chitons, Amicula vistita, 

and Ischnochiton albus, and seastars of various genera (Figure 5). There were significantly 

more I. albus than there were seastars (ANOVA, F2,24=3.264, p=0.01) but no differences were 

seen between Amicula densities and densities of the other two grazers during this survey.

In general, substrate (as measured by percent cover) at DS11 was dominated by 

encrusting corallines (Figure 6). Red holdfasts were also fairly abundant, covering 

approximately 25% of the substrate. Bare rock was found during these surveys, however it was 

in very small amounts (typically less than 5%). Overall, significant differences were found in the 

percent cover of various sessile organisms (ANOVA, F5,234=103.893, p<0.0001). In particular, 

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ACCESS NUMBER: 85244 

macroalgae were always significantly more abundant than any of the invertebrate groups, with 

the encrusting corallines also being more abundant than the red holdfasts (Table 1). 

Biomass of macroalgae at DS11 in 2004 was dominated by the foliose red algae, 

particularly Phycodrys rubens, Phyllophora truncata, and Dilsea socialis (Figure 7). Other reds 

found in these surveys included Odonthalia dentata, Rhodomela confervoides, and Ahnfeltia sp. 

Brown macroalgal biomass was dominated by Laminaria solidungula. Laminaria saccharina also 

was found but in very small amounts. Another brown macroalgae, Alaria esculenta, was not 

found during these surveys but was seen adjacent to some transects. Overall, some significant 

differences were found among the macroalgae for biomass (ANOVA, F7,32=2.013, p=0.0841, 

Table 2). These differences typically involved Phycodrys and Phyllophora being significantly 

more abundant than the other macroalgae. 

Laminaria solidungula were reproductive during this study. Divers noticed this alga in a 

reproductive state during all years, and a quantitative survey was completed in 2004. From this 

survey, 115 plants out of 178 observed were reproductive, approximately 70% of the population. 

0.00

0.20

0.40

0.60

0.80

1.00

1.20

Amicula Ischnochiton seastars

#/0.25m

2

Figure 5. Mean density (±1 s.e.) of grazers at DS11 in 2004. Estimates are 

from searches done on fifteen randomly placed 0.25m2

 quadrats. 

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ACCESS NUMBER: 85244 

0

10

20

30

40

50

60

70

hydroid

bryozoan

sponge

enc

coralline

red

holdfast

bare rock

percent cover

Figure 6. Percent cover (±1 s.e.) of sessile organisms and bare rock at DS11 

in 2004. Estimates are from five random point contacts placed along eight 

randomly placed transects. 

-.843 6.623 .8021

-1.235 6.623 .7136

-23.030 6.623 <.0001 S

-61.340 6.623 <.0001 S

-4.593 6.623 .1731

-.392 6.623 .9073

-22.186 6.623 <.0001 S

-60.497 6.623 <.0001 S

-3.750 6.623 .2658

-21.794 6.623 <.0001 S

-60.105 6.623 <.0001 S

-3.358 6.623 .3188

-38.311 6.623 <.0001 S

18.436 6.623 <.0001 S

56.747 6.623 <.0001 S

Mean Diff. Crit. Diff. P-Value

hydroid, bryozoan

hydroid, sponge

hydroid, red holdfast

hydroid, encrusting coralline

hydroid, bare rock

bryozoan, sponge

bryozoan, red holdfast

bryozoan, encrusting coralline

bryozoan, bare rock

sponge, red holdfast

sponge, encrusting coralline

sponge, bare rock

red holdfast, encrusting coralline

red holdfast, bare rock

encrusting coralline, bare rock

Table 1. Fisher’s PLSD for percent cover of sessile organisms and bare rock 

at DS11 in 2004. S=Significance level of 5%. 

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ACCESS NUMBER: 85244 

0

5

10

15

20

25

30

35

40

Phycodrys

Phyllophora

Dilsea

Odonthalia

Rhodomela

Ahnfeltia

Laminaria

solidungula

Laminaria

saccharina

biomass (g)/0.25m2

Figure 7. Biomass (±1 s.e.) in grams of macroalgae at DS11 in 2004. 

Estimates are from five 0.25m2

 randomly placed quadrats. 

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ACCESS NUMBER: 85244 

2.420 20.397 .8106

12.240 20.397 .2305

24.000 20.397 .0226 S

24.020 20.397 .0224 S

24.020 20.397 .0224 S

17.040 20.397 .0985

23.640 20.397 .0245 S

9.820 20.397 .3341

21.580 20.397 .0388 S

21.600 20.397 .0386 S

21.600 20.397 .0386 S

14.620 20.397 .1540

21.220 20.397 .0419 S

11.760 20.397 .2489

11.780 20.397 .2481

11.780 20.397 .2481

4.800 20.397 .6349

11.400 20.397 .2634

.020 20.397 .9984

.020 20.397 .9984

-6.960 20.397 .4920

-.360 20.397 .9715

0.000 20.397 •

-6.980 20.397 .4908

-.380 20.397 .9700

-6.980 20.397 .4908

-.380 20.397 .9700

6.600 20.397 .5145

Mean Diff. Crit. Diff. P-Value

Phycodrys, Phyllophora

Phycodrys, Dilsea

Phycodrys, Odonthalia

Phycodrys, Rhodomela

Phycodrys, Ahnfeltia

Phycodrys, L. solidungula

Phycodrys, L. saccharina

Phyllophora, Dilsea

Phyllophora, Odonthalia

Phyllophora, Rhodomela

Phyllophora, Ahnfeltia

Phyllophora, L. solidungula

Phyllophora, L. saccharina

Dilsea, Odonthalia

Dilsea, Rhodomela

Dilsea, Ahnfeltia

Dilsea, L. solidungula

Dilsea, L. saccharina

Odonthalia, Rhodomela

Odonthalia, Ahnfeltia

Odonthalia, L. solidungula

Odonthalia, L. saccharina

Rhodomela, Ahnfeltia

Rhodomela, L. solidungula

Rhodomela, L. saccharina

Ahnfeltia, L. solidungula

Ahnfeltia, L. saccharina

L. solidungula, L. saccharina

Table 2. Fisher’s PLSD for macroalgal biomass at DS11 in 2004. 

S=Significance level of 5%.

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ACCESS NUMBER: 85244 

STUDY PRODUCTS: 

 Refereed Publications 

Konar, B and K Iken. In Press. Competitive dominance among sessile marine 

organisms in a high Arctic boulder community. Polar Biology. (see Appendix 1) 

 Technical Reports 

Role of grazers in the recolonization of hard-bottom communities in the Alaska 

Beaufort Sea. Annual Reports for 2003 and 2004. 

 Presentations and Interviews 

March 8, 2005: Presentation at Coastal Marine Institute Annual Meeting 

February 16, 2004: Presentation at Coastal Marine Institute Annual Meeting 

November 7–10, 2003: Symposium Speaker: Western Society of Naturalists, 84th

Annual Meeting in Long Beach, Cal 

August 22, 2003: Interviewed for KUAC on Boulder Patch project 

August 10, 2003: Presentation for Endicott Island BP workers 

August 5, 2003: Interviewed for series on Boulder Patch project for KUAC 

February 19, 2003: Presentation at Coastal Marine Institute Annual Meeting 

November 8–11, 2002: Poster presented at the Western Society of Naturalists 

83rd Annual Meeting in Monterey California 

October 2002: Interviewed by Arctic Science Journeys radio program 

August 1 and 8, 2002: Presentation for Endicott Island BP workers 

 

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