BULLETIN OF MARINE SCIENCE, 55(2-3): 631-644. 1994
EFFECTS OF BOTTOM RELIEF AND FISH GRAZING ON
THE DENSITY OF THE GIANT KELp, MACROCYSTIS
M. L. Patton, C. F. Valle and R. S. Grove
ABSTRACT
A series of SCUBA surveys of subtidal reefs in the mainland Southern California Bight
were used to relate the density of giant kelp (Macrocystis), kelp-grazing fish, kelp-fouling
organisms, and sea urchins to bottom relief. On natural and artificial reefs, adult giant kelp
plants were more common on low-relief substrates, i.e., on hard substrates lying less than I
m above the surrounding sand. Conversely, juvenile kelp, kelp-fouling organisms, and kelpgrazing fish were more common on high-relief substrates. There was no statistically significant relationship between the density of sea urchins and bottom relief. The effects of fish
grazing, but not the effects of abrasion and sea urchin grazing, are probably much greater on
kelp plants that are partially encrusted with fouling organisms. To study fish grazing, fouled
sections of adult kelp plants were exposed on high and low relief. To study abrasion and sea
urchin grazing, unfouled sections of adult kelp plants were similarly exposed. On both artificial and natural reefs, fouled sections of kelp plants lost significantly more tissue on high
relief; unfouled sections did not. The results indicated that the relationship of giant kelp
density to bottom relief was produced by differences in fish grazing which were, in turn,
produced by the higher densities of kelp-grazing fish or kelp-fouling organisms on high relief.
The data did not suggest that the relationship between kelp density and bottom relief was
produced by abrasion, sea urchin grazing, or kelp recruitment. Finally, the data suggest that
a reef intended to support giant kelp should be designed to minimize bottom relief and the
number of shelter crevices suitable for kelp-grazing fish,
Giant kelp, Macrocystis, is a large and abundant canopy-forming algae which
grows in extensive, and densely inhabited, beds throughout the subtidal Southern
California Bight (North, 1991). The environmental factors controlling the density
of this algae have been extensively studied (North, 1971; Dayton, 1985; Foster
and Schiel, 1985), but there have been few studies of the relationship between
giant kelp and bottom relief. We felt that kelp density was probably
affected by
bottom relief, because bottom relief has a strong effect on the densities of many
other subtidal organisms (Pequegnat, 1964; Patton and Harman, 1983, 1986; Patton et a!., 1985). Moreover, if kelp density was related to bottom relief, an understanding of this relationship would be essential to the design of artificial reefs
intended to support kelp beds.
During a 1979 baseline study of the mainland Southern California Bight, we
noticed that giant kelp tended to be more dense and more persistent on low-relief
substrates than on high-relief substrates. This observation, which we confirmed
with more localized studies, was interesting because it was counter-intuitive. Both
sediment scour (North, 1971; Dayton, 1985) and dim light (Dayton et aI., 1984;
Dayton, 1985) are harmful to giant kelp. Since there is more suspended sediment
present near low relief (Cook and Gorsline, 1972; Patton and Harman, 1983),
giant kelp should be relatively less dense on low relief.
We studied the effect of fish grazing on giant kelp because fish grazing has
produced low algae densities on high-relief substrates in the tropics (Hay, 1981a,
1981b) and because fish grazing has frustrated several attempts to transplant kelp
to both artificial (Carlisle et a!., 1964; Turner et a!., 1969; Carter et aI., 19R5) and
natural (North and Hubbs, 1968) substrates in the Southern California BigLl. Also,
several relatively large, abundant fish that graze on algae, halfmoon (Medialuna
631
632
BULLETIN
OF MARINE
SCIENCE, VOL. 55, NO. 2-3, 1994
californiensis), opaleye (Girella nigricans), garibaldi (Hypsypops rubicunda), and
sheephead (Semicossyphus pulcher) (Limbaugh, 1964; Quast, 1968; North and
Hubbs, 1968; MEC, 1991), are usually more common in high-relief areas of both
natural (Pequegnat, 1964; Patton et aI., 1985) and artificial (Jessee et aI., 1985;
MEC, 1991) reefs,
We also studied kelp fouling because most kelp-grazing fish prefer fouled plants
(Limbaugh, 1964; Bernstein and lung, 1979; Dixon et aI., 1981) and, therefore,
differences in fish grazing could be produced by differences in the density of
fouling organisms. We expected fouling organisms to be more dense at reef crests
because water motion is faster and more turbulent at reef crests (Pequegnat, 1964;
Patton and Harman, 1983) and because some fouling organisms are more likely
to settle out in such conditions (Bernstein and lung, 1979; Dixon et aI., 1981).
We also examined other factors that affect kelp density and might, therefore,
affect the relationship between kelp density and bottom relief: sea urchin grazing
(Ebeling et aI., 1985; Lawrence, 1975), abrasion (Carter et aI., 1985), kelp recruitment (Reed et aI., 1988; Reed, 1991), and fish grazing on unfouled juvenile
kelp plants (Harris et aI., 1984),
In these studies, we used data from the baseline study and from more intensive
and localized studies. We examined the relationship between bottom relief and
the densities of adult kelp plants, juvenile kelp plants, kelp-grazing fish, sea urchins, and fouling organisms. We used single, fouled kelp blades to measure the
grazing rates of fish. Because fish usually do not feed heavily on unfouled mature
kelp plants (Bernstein and lung, 1979; Dixon et aI., 1981; Harris et aI., 1984),
we were able to use the unfouled tips of mature kelp fronds to measure the effects
of abrasion and sea urchin grazing.
The results suggested that giant kelp is uncommon on high relief because kelp
is more heavily grazed or more heavily fouled in high-relief areas. The data did
not suggest that the relationship between kelp density and bottom relief was produced by abrasion, sea urchin grazing, or kelp recruitment.
METHODS
Baseline Study.- The density of kelp, like most of the phenomena ecologists study (Quinn and Dunam,
1983), is affected by a large number of factors (Dayton, 1985), In the baseline study, therefore, we
examined a large number of sites so that the variation due to factors other than bottom relief would
"average out." The effects of bottom relief were therefore compared with the variation in kelp density
due to "chance" which was not entirely physical randomness but included " .. , the contributions of
the large number of deterministic effects not included in the model." (Quinn and Dunham, 1983).
"Relief" was defined as elevation of the substrate above thc plain of sand covering most of the
southern California subtidal sea floor (Dennis, 1974), We used this definition because the amount of
suspended sediment impinging a hard substrate is probably inversely related to its elevation above a
sand bottom (Cook and Gorsline, 1972; Patton and Harman, 1983), and sediment movements have
profound effects on giant kelp (North, 197 J; Dayton, 1985) and other organisms (Limbaugh, 1955;
Moore, 1977). Earlier work (Patton and Harman, 1983, 1986; Patton et a!., 1985) indicated that
organism densities change little with bottom relief when bottom relief is greater than 1.5 m, so bottom
reliefs greater than 1.5 m were represented as 1.5 m,
We studied 24 different sites in the southern half of the Southern California Bight during 1979 (Fig,
1), Giant kelp and sea urchins were counted with belt transects. Usually, two I m x 30 m transects
were taken at each site on each of two sample dates, Transect locations were haphazard; they were
taken at the point where the divers first reached the bottom, Divers counted the number of sea urchins
and the number of Macrocystis stipes that were longer than 2 m. Usually, divers also made five
measurements of the elevation of the transect above deep, level sand. Roughly half of the sites lay at
depths of 6 m and half at 15 m,
Fish densities were measured at the same sites, Usually two free swim samples were taken on each
of two sample dates, Pelagic and suprabenthie ("reef") fish were counted with a straight free swim
wherein the diver swam 1 m above the bottom at a speed of about 12 m·min ) and counted all the
fish he saw, Swim distance was controlled by limiting swim time, Swimming speed, periodically
PATrON
Topanga
Barge
633
ET AL.: GIANT KELP DENSITY
(TAR)
Rock
Pendleton
(PAR)
~
Oceanside
(OAR)
/'
t
6. •
~6..
PI. Lorna
N
Figure I. Study sites. Black dots mark the locations of the baseline study sites; most dots represent
two sites. Triangles mark the location of the sites of intensive studies: Barge Rock, and Oceanside
Artificial Reef (OAR), the Pendleton Artificial Reef (PAR), and the Topanga Artificial Reef (TAR).
calibrated, tended to remain constant. Since the diver looks both ways, the area searched was twice
the product of the visibility distance and the swim distance; this area was held constant at 700 m2•
Visibility was the average of four measurements of the distance at which a white globe with a diameter
of 20 cm disappeared as a result of turbidity or kelp interference. The globe was suspended I m above
the bottom. Sampling was done when visibility was greater than 4 m, i.e., largely during the clearwater summer-fall season. This technique was designed to measure between-site differences in the
densities of fish species. It does not measure absolute densities because, even under the same conditions, different species are visible from different distances.
Intensive Studies.-Four sites were used: Barge Rock, the Oceanside Artificial Reef (OAR), the Pendleton Artificial Reef (PAR), and the Topanga Artificial Reef (TAR) (Table I). All lie 0.5 to 1.0
nautical miles offshore and all, at the time of the study, supported kelp beds. Barge Rock is composed
of large (> I m) natural boulders and bedrock; OAR, PAR, and TAR are composed of quarry rock.
Most studies were done at Barge Rock and OAR, but since fronts of feeding urchins and terrestrial
runoff largely removed kelp from Barge Rock and OAR before our studies were completed, some
Table I. Characteristics of reefs used for intensive studies
SITE
BASE DEPTH
(m)
CREST DEPTH
(m)
Barge Rock
OAR
PAR
TAR
15
13
13
8.5
10
10
8.5
6
DIMENSIONS
(m)
15 X
15 X
15 X
40 X
25
24
30
130
YEAR
CONS1RUCIED
Natural
1987
1980
1987
634
BULLETIN OF MARINE SCIENCE, VOL. 55, NO. 2-3, 1994
measurements were made on PAR and TAR. Most measurements were made in the first 4 months of
1991.
To relate kelp density and bottom relief on these sites, each diver measured the depth at the base
of each holdfast and the depth at the adjacent sand, Holdfast elevation was defined as the difference
between the holdfast depth and the depth at the sand.
To ensure the holdfasts were selected haphazardly, each diver measured the elevation of the first
holdfast encountered and then determined the bearing between the first holdfast and its nearest neighbor, He then swam 10 m along this bearing, measured the elevation of the holdfast lying closest to
the end of this swim, and repeated the process,
To measure the mean elevation of the reef, divers measured the depth at haphazard points on the
reef and compared these depths with the mean depth at the sand, Each diver measured the depth at
the sand at the point where he first reached the reef, then swam across the reef stopping at random
points to measure the depth of the rock substrate, When he reached the sand-rock interface on the
other side of the reef, he measured the depth at the sand, swam a random number of meters along
the sand-rock interface, and then crossed the reef again, "Random" numbers were obtained from a
random number table,
To measure the effect of fish grazing on fouled kelp, we used single blades that were 30 to 80%
covered with fouling organisms (mainly Membranipora). Cable ties were used to fasten the kelp blades
to sea fans (Muricea calijornica), short algae, or stalked tunicates. Ten blades, spaced I to 2 m apart,
were used in each experiment. They were placed within 1 m (vertical distance) of the reef crest or on
the flat bottom 10 to 15 m away from the reef base, After exposure, the blades were weighed and the
number of scars from fish grazing (Quast, 1968) were counted, In this and subsequent experiments,
blade fouling was measured with a plastic grating of 225 mm2 squares with intersections that were I
mm wide. Grating and blade were placed on a light table and the intersections touching fouling
organisms were counted and expressed as a percent of the number of intersections touching the blade.
Since plants growing close to the reef crests had few blades, the fouling of kelp "floats" or pneumatocysts was measured, Kelp plants were chosen in the same way they were chosen when holdfast
depth was measured. A frond was then selected at random, and a pneumatocyst growing about 0,5 m
above the holdfast was removed, Random numbers were again obtained from a random number table,
Reef crest samples were taken within I m (vertical distance) of the reef crest; reef base samples were
taken within 0,5 m (vertical distance) and 10 m (horizontal distance) of the reef base, After the floats
were cut and flattened, float fouling was measured in the same way that blade fouling was measured
except that the grid had 6.25 mm2 squares with intersections that were 0,5 mm wide, Since the
measurement of pneumatocyst fouling was used as a surrogate for the measurement of blade fouling,
it was necessary to examine the correlation between blade fouling and pneumatocyst fouling using
measurements made on intact blades that were heavily or lightly fouled,
To measure the effect of bottom relief on abrasion and sea urchin grazing, tips of unfouled adult
fronds were exposed on reef crests and reef bases in the same way as the fouled blades were exposed.
Frond tips were one meter long, After exposure, tips were weighed and the number of scars from fish
grazing (Quast, 1968) were counted,
Laboratory measurements of sea urchin grazing were made in two lO-gallon aquaria. Aquaria were
aerated, supplied with a trickle of fresh water, and all fecal material and detritus was removed daily.
Four Strongylocentrotus franciscanus were placed in each aquarium and starved for 2 days, Five
fouled kelp blades were placed in one aquarium and five unfouled blades were placed in the other;
before the experiment was started, all blades were trimmed to a wet weight of 7.0 gm. Fouled blades
were 35 to 36% covered with fouling organisms; unfouled blades were 0 to ] % covered with fouling
organisms. Kelp remaining after 24 h was removed, blotted dry, and weighed. The urchins were then
starved again for 2 days and fouled blades were introduced into the aquaria that previously held
unfouled blades and vice versa, The experiment was repeated twice using the same urchins,
To measure the density of juvenile kelp, the divers used 0.125 m2 quadrats placed haphazardly on
the reef. The first measurement was made at the point where the diver first reached the reef. The diver
then determined the bearing between the two nearest sea fans. He then swam 4 m along this bearing,
measured density again, and repeated the process. Divers counted the number of juvenile kelp plants
between 5 and 20 cm long. Reef crest samples were taken within I m (vertical distance) of the reef
crest. Reef base samples were taken within 0,5 m (vertical distance) and 10 m (horizontal distance)
of the reef base. This technique was also used to determine the proportion of abraded or bitten juvenile
kelp plants.
Prior to ANOV A and t-tests, data were tested for heteroscedasticity and measurements of proportion
were subjected to an angular transformation (arcsin of the square root of the proportion) (Sokal and
Rohlf, 1969).
PATTON
-
3
~
Cl
0
-
~ 2
en
w
a.
..... 1
en
0
0
---
0
635
KELP DENSITY
MACROCYSTIS
C\I
en
z
w
ET AL.: GIANT
00
00
0
R2 = 0.22
0
0
6>
0
0
0
0
1
BOTTOM RELIEF
2
(M)
Figure 2. Macrocystis density vs. bottom relief. Bottom relief defined as the average elevation in
meters of the substrate above the sand. Each point represents the average of two to four 30-m2 transects
taken on a single baseline study site (See Fig. ]).
RESULTS
Baseline Study.-On natural reefs, giant kelp density was significantly greater on
low-relief substrates (Fig. 2). The results of an ANOV A, based on regression
analysis, indicated that the negative relationship between kelp stipe density and
bottom relief was significant (F = 8.121, P = 0.020). It was striking that kelp
density was zero on all high-relief sites.
The results of ANOV As, based on regression analyses, indicated that the densities of several kelp-grazing fish were significantly greater on high-relief sites.
This was true of H. rubicunda (F = 22.67, P < 0.00 I), M. californiensis (F =
8.15, P = 0.009), and S. pulcher (F = 18.01, P < 0.001) (Fig. 3).
The densities of G. nigricans, however, were not significantly greater on high
relief (F = 3.03, P = 0.085) (Fig. 3). Sea urchin densities were not significantly
greater on high-relief sites, either (Fig. 4). This was true for both S. franciscanus
(F = 2.37, P = 0.11) and S. purpuratus (F = 0.03, P = 0.87).
Intensive Studies.-A frequency distribution of holdfast elevation on the Pendleton artificial reef (PAR) and the Topanga artificial reef (TAR) (Fig. 5) clearly
indicated that kelp grew more densely close to the sand. A few holdfasts had
negative elevations; they were growing on rocks lying in small depressions in the
sand. Mean holdfast elevation was significantly lower than mean hard substrate
elevation at both Pendleton (t = 14.5, P < 0.00l) and Topanga (t = 6.5, P <
0.001). Kelp appeared to grow most densely at an elevation of 0.2 m. The examination of videotape taken at Barge Rock and the Oceanside artificial reef
(OAR) indicated that kelp grew most densely at the bases of these reefs, also.
To measure fish grazing, fouled kelp blades were exposed at Barge Rock and
the Oceanside Artificial Reef for 4 to 8 days. The results of one-way ANOV As
indicated that fish grazing was heavier at reef crests than at reef bases at both
Barge Rock (F = 12.6, P = 0.0013) and OAR (F = 16.8, P = 0.003) (Fig. 6A).
Hypsypops rubicunda, M. californiensis, and S. pulcher were observed grazing
on the blades. The hemispherical scars left by fish grazing were common; the
average blade had about three obvious scars.
Necessarily, the blades exposed on reef crests lay in shallower water than the
636
BULLETIN OF MARINE ScrENCE. VOL. 55. NO. 2-3. 1994
150
HYPSYPOP5
UJ
--.J
o
R2 = 0.48
>- a.. 100
I-L
o
-«
lJ)lJ)
z .
UJ I
QlJ)
o
50
0
40
00
o
20
o
2
9
000
00
0
o
o
2
GIRELLA
R2 = 0.27
>-0...
I-L
en ~ 10
R2=0.12
o
o
100
z .
WI
QlJ)
~
l..1...
o
e
200
o
MEDIAL UNA
UJ
--.J
0
o
80
<00
o
o
R2 = 0.44
o
o
l..1...
5EMIC055YPHU5
120
o
o
0
o
o
()
o
o
o
2
1
BOTTOM REL IEF
o
1
BOTTOM RELI EF
2
(M)
(M)
Figure 3. Density of kelp-grazing fish vs. bottom relief. Bottom relief defined in Figure 2. Each
point represents the average of two to four transects taken on a single baseline study site (See Fig. I).
blades exposed at reef bases. To make sure that the apparent effect of bottom
relief on kelp grazing was not really a depth effect, we exposed blades at the
crests of deep reefs and on flat substrate in shallower water so that the blades lay
at the same depth but at different elevations above the sand. Blades were exposed
for 1 to 6 days. The results of one-way ANOVAs indicated that the high-relief
blades were still more heavily grazed at both Barge Rock (F = 58.1, P < 0.00 I),
and OAR (F = 35.7, P < 0.003) (Fig. 6B); this shows that we had not confounded
5. FRANCISCANUS
>-
w
2.0
R2
en
L
w
(/)
0
0.10
-l
I- a..
z
=
«
....•.
.-..
>I-
0
a..
en L
0
06>
1,0
0
00
~
0
0,0
00
0
0
0
z
w
0
0
R2
=
0.001
2.0
«
0
(/)
0
....•. 1.0
~
0
0
0.0
2
BOTTOM REL IEF
S. PURPURA TUS
0
0
0
UJ
-l
3,0
0
1
2
BOTTOM RELI EF
Figure 4. Sea urchin density vs. bottom relief. Bottom relief defined in Figure 2. Each point represents the average of two to four 30-m2 transects taken on a single baseline study site (See Fig. 1).
PATION
100
ET AL.: GIANT
30
PAR
80
TAR
>-
>-
u
u
Z
637
KELP DENSITY
z
60
20
lJ.J~
~~
=>lle
a~
lJ.J
~ ~ 40
a:
MEAN
SUBSTRATE
a:
lJ...
20
o
lJ...
MEAN
SUBSTRATE
ELEVATION
10
ELEVATION
~
-0.2
0.2
0.5
0.8
HOLDFAST
1.1
1.4
0
2.0
1.7
~
0.2
0.5 0.8
HOLDFAST
ELEVATION
(M)
1.1
1.4
ELEVATION
(M)
Figure 5. Frequency distribution of the elevation of kelp holdfasts above the base of the Pendleton
Artificial Reef (PAR) and the Topanga Artificial Reef (TAR). Arrow indicates the mean elevation of
the rock substrate above the base of the artificial reef.
the effects of bottom relief on fish grazing with the effects of depth on fish
grazing.
Since differences in fish grazing could be produced by differences in fouling,
we measured the density of fouling organisms on kelp pneumatocysts at Barge
Rock and, because all kelp temporarily disappeared from OAR following heavy
terrestrial runoff, at the Pendleton Artificial Reef (Fig. 7). The results indicated
that the pneumatocysts taken from reef crests were more heavily fouled than those
taken from reef bases. This was true at both Barge Rock (t = 3.21, P = 0.007)
and PAR (t = 6.08, P < 0.001). Pneumatocyst fouling probably reflects blade
fouling because, in a sample of intact blades collected from PAR, the regression
of blade fouling on pneumatocyst fouling was statisticaJly significant (F = 11.37,
P = 0.04) (Fig. 8).
To separate the effects of abrasion and sea urchin grazing from fish grazing,
we repeated the grazing experiment using the unfouled tips of adult fronds instead
of single fouled blades. During the 6 to 9 days the fronds were exposed, there
was at least one day of heavy surf. The results of one-way ANOVAs indicated
that bottom relief had no effect on tissue loss at either Barge Rock (F = 0.085,
P = 0.771) or OAR (F = 0.313, P = 0.579) (Fig. 9). These results suggest that
abrasion and sea urchin grazing did not change with bottom relief. These results
A.
I-
::r:
~
20
Il!I
LOW RELIEF
HIGH RELIEF
I-
::r:
t!)
t!)
W
lJ.J
~:§
~:§
10
0
10
0
<
<
....J
to
20
....J
co
0
BARGE RK
Figure 6. A. Size
base and the crest
fouled Macrocystis
Oceanside Artificial
on flat substrate in
OAR
0
BARGE RK
OAR
of single, fouled Macrocyslis blades after exposure to grazing for 4-8 days at the
of Barge Rock and of the Oceanside Artificial Reef (OAR). B. Size of single,
blades after exposure to fish grazing for 1-6 days near Barge Rock and the
Reef (OAR). Fronds exposed at the same depth: at the crest of a deep reef and
shallower water. N printed within bars. I SE.
638
BULLETIN OF MARINE SCIENCE. VOL. 55. NO. 2-3. 1994
[?J
III
100
wo 80
60
a: a:
40
O
~aJ 20
0
LOW RELIEF
HIGH RELIEF
10
16
~§
m
BARGE RK
PAR
Figure 7. Percent cover of fouling organisms on Macrocystis pneumatocysts collected from the base
and the crest of Barge Rock and the Pendleton Artificial Reef (PAR). N printed above bars. I SE.
also suggest that fish grazing on unfouled kelp did not change with bottom relief.
Further, there was little evidence of extensive fish grazing; the average blade had
0.5 obvious scars, significantly fewer than the number of scars found on fouled
kelp (t = 8.59, P < 0.001).
To make certain our results were not produced by an unsuspected preference
of sea urchins for fouled kelp, we conducted laboratory feeding experiments. The
results of a two-way ANOVA that used sea urchin "batch" and kelp fouling as
factors showed that fouling had no significant effect on the feeding rate of S.
franciscanus (F = 0.13, P = 0.74) (Fig. 10). Further, neither S. franciscanus (t
= 0, P = 1.0) nor S. purpuratus (t = 0.717, P = 0.482) were more common at
the crest, as opposed to the base, of Barge Rock (Fig. 11) and neither sea urchin
was found at OAR.
Measurements of the density of juvenile kelp at Barge Rock and PAR indicated
that juvenile kelp was more dense on high relief (Fig. 12). The density of juvenile
kelp was significantly greater at the crest of Barge Rock than it was at its base
(t = 4.8, P < 0.001) and there were no juvenile kelp plants at the base of PAR.
100
R
<9
z
:J
:J
f2
80
~
0
2
0.42
0
0
0
0
0
0
60
00
W '-"
0
40
0
~
20
00
W
=
0
0
0
0
20 40 60 80 100
FLOAT FOULING
(%)
Figure 8. Percent cover of fouling organisms on Macrocystis blades vs. percent cover of fouling on
pneumatocysts taken from the same blades.
PATION ET AL.: GIANT KELP DENSITY
200
WJ
1m!
f-
I
CJ
639
LOW RELIEF
HIGH RELIEF
~§ 100
a..
f-
o
BARGERK
OAR
Figure 9. Size of unfouled tips of kelp fronds after exposure for 6-9 days at the base and crest of
Barge Rock and the Oceanside Artificial Reef (OAR). During exposure there was at least one episode
of heavy surf. N printed within bars. 1 SE.
There was no significant difference between the proportion of bitten and abraded juvenile kelp plants at the base of Barge Rock and the proportion at the crest
(t = 0.27, P = 0.79) (Fig. 13).
DISCUSSION
Giant kelp may occasionally grow relatively densely on high-relief substrates.
Such exceptions may be produced by one of the very large number of other factors
that affect kelp density (Dayton, 1985). Also, garibaldi, opaleye, halfmoon, and
sheephead can be less common in colder waters (Patton et aI., 1985; Stephens et
aI., 1984), so that the relationship between relief and kelp density might be less
clear in the northern half of the Bight during cold-water eras. Conversely, the
most important fouling organism, Membranipora (Bernstein and Jung, 1979; Dixon et aI., 1981), is uncommon when the water is too warm (Dixon et aI., 1981),
so the relationship between bottom relief and kelp density could also be obscure
in warm, shallow waters in summer. Finally, vertical substrates form a partial
barrier to urchin movement (Laur et aI., 1986) so that the devastation of an area
by an urchin front can sometimes temporarily reverse the relationship between
kelp density and bottom relief (Patton, pers. obs.).
1.0
(:)
W
~
0.8
=:lC/)..c 0.6
z"""
0.4
81
a..
....J
W
::::.::::
0.2
0.0
HEAVY
LIGHT
FOULING
Figure 10. Effect of organisms fouling Macrocystis blades on the rate at which these blades are
consumed by Strongylocentrotus franciscan us in the laboratory. N printed within bars. 1 SE.
640
BULLETIN OF MARINE SCIENCE. VOL. 55. NO. 2-3. 1994
0.3
;:::
IB
Cf)
z
w
~
(\J
LOW RELIEF
HIGH RELIEF
- 0.2
0 ::2:
-...
Z
I
~
()
0.1
a:
~
0.0
R8J
Figure 11.
PURPLE
Density of sca urchins at thc base and crest of Barge Rock. N printed within bars. I SE.
Our data suggest that the relationship of giant kelp density to bottom relief was
produced by fish grazing on fouled adult plants that was, in turn, produced by a
"preference" of many kelp-grazing fish for high-relief substrates or by differences
in the density of fouling organisms. These data do not suggest that the change in
kelp density with bottom relief was produced by the abrasion of adult or juvenile
kelp plants, sea urchin grazing, fish grazing on unfouled adult or juvenile kelp
plants, or kelp recruitment.
Rates of fish grazing might be affected by fouling, but it is also possible that
the amount of fouling might be affected by fish grazing because sparse kelp is
often more heavily fouled (Reed, ] 990). Thus a decrease in kelp density produced
by fish grazing could cause the remaining kelp plants to become more heavily
fouled.
It is at least possible that fouling could influence kelp density even if fish
grazing were not involved because fouling tends to make kelp fronds brittle and
more susceptible to abrasion (Lobban, ] 978; Dixon et aI., ] 981). Before the blades
become brittle, however, the fouling must be extremely heavy (Lobban, 1978;
Dixon et aI., 1981). Our results show that much fish grazing occurred on blades
that were only partially fouled; this suggests that, on high relief, kelp blades would
probably be destroyed by fish grazing before they became fouled enough to become brittle.
The low kelp recruitment at the reef bases was predictable because there is
20
35
(\J
~
CI)
Z
W
0
::2:
-...
Cf)
I-
Z
JUVENILE
MAC ROCYSTIS
~
l!Ji
10
LOW RELIEF
HIGH RELIEF
«
...J
21
a..
21
0
BARGERK
PAR
Figure 12. Density of juvenile Macrocystis at the crest and the base of Barge Rock and the Pendleton
Artificial Reef (PAR). No juveniles were observed at the base of PAR. N printed within bars. I SE.
PATrON
ET AL.: GIANT
KELP DENSITY
641
100
OC/)
Ww
~::d
~m
0>
::>
cf.-:>
80
60
40
20
o
LOW
HIGH
SOnOM RELIEF
Figure 13. Number of damaged or bitten juvenile kelp plants at the base and the crest of Barge
Rock. Samples had 3-7 plants each. N printed above bars. 1 SE.
more sediment present at low elevations (Cook and Gorsline, 1972; Patton and
Harman, 1983), and small amounts of sediment can inhibit the growth of Macrocystis spores (Devinny and Volse, 1978).
However, the absence of a relationship between relief and grazing on juvenile
kelp was unexpected because Girella and Medialuna, which graze on juvenile
kelp (Harris et a\., 1984), are often more common at high elevations (Pequegnat,
1964; Patton et a\., 1985; Jessee et a\., 1985; MEC, 1991). It was only possible
to study fish grazing on juvenile kelp on one reef. Further studies would appear
to be desirable.
If the natural habitat of giant kelp is low-relief substrate, then it would be
predicted that giant kelp should be adapted to fairly frequent substrate disturbance
because low-relief substrates are disturbed more often than high-relief substrates.
Heavy sediment scour and burial are more likely on low-relief substrates (Cook
and Gorsline, 1973; Dayton et a\., 1984; Patton and Harman, 1983) and small
rocks are more often overturned by storms (Sousa, 1979) or rafted by algae (Haaker and Wilson, 1975; Foster and Schiel, 1985).
In fact, kelp does appear to be adapted to habitat disturbance. Heavy storms
are usually followed by massive kelp recruitment (Tegner and Dayton, 1987;
North, 1988), kelp recruitment is much greater on substrates that have been disturbed by being scoured by sediment, broken, or buried (Harris et a\., 1984;
Ebeling et a\., ]985), and kelp is often one of the first organisms so appear on a
"new" substrate (Foster, 1975). Furthermore, giant kelp has many of the characteristics of organisms specializing in often-disturbed habitats. These organisms
are dispersive, fast-growing, short-lived, and fecund (Hutchinson, 1951; Pianka,
1970). Macrocystis is extremely dispersive during the storms (Ebeling et a\., 1985)
that would produce disturbed substrate. It is fast-growing (Haaker and Wilson,
1975) and usually wins a 'scramble' competition wherein several organisms simultaneously recruit to a 'new' substrate (Tegner and Dayton, ]987). Unlike most
large laminarians, it is fertile all year (Dayton, 1985) and it is more short-lived,
1 to 2 years (North, 1991), than most subtidal algae (Foster, 1975). It is also much
more short-lived than most large, sessile invertebrates found in this area, e.g., the
sea fan Muricea californica (Grigg, ]975) and the corals Astrangia lajollaensis
(Fadlallah, ]982) and Balanophyllia elegans (Fadlallah, 1983).
Our observations suggest that the design of an artificial reef intended to support
642
BULLETIN OF MARINE SCIENCE. VOL. 55. NO. 2-3, 1994
giant kelp should differ radically from that of most artificial reefs previously
constructed in southern California. Almost all of these reefs have been high (> 1.5
m) piles of boulders (Lewis and McKee, 1989). Previous attempts to grow giant
kelp on such reefs may have met with inconsistent success (Carter et aI., 1985)
because the turbulence caused by such reefs stimulates the settlement of kelpfouling organisms or because these reefs form ideal habitat for the Bight's larger
kelp-grazing fish, Hypsypops, Medialuna, Semicossyphus, and Girella. These fish
"prefer" high relief and make use of the many shelter crevices found in such
boulder piles (Feder et aI., 1974; Limbaugh, 1964; Patton et aI., 1985; Patton,
pers. obs.). The best reef for giant kelp would produce relatively little turbulence
and form poor habitat for kelp-grazing fish. Such a reef should be relatively low,
probably less than 1 m, and should have relatively few shelter interstices suitable
for kelp-grazing fish. Such a reef would necessarily be subject to more substrate
disturbance than a high reef. A moderate level of substrate disturbance, however,
should not be injurious to giant kelp, and in the long run, might well benefit this
algae by excluding its more longevous competitors.
ACKNOWLEDGMENTS
Thanks are due to D. Lees, J, Palmer, and J. Elliott for advice and support and to P. Hague and D.
James for diving assistance. This study was supported by Southern California Edison.
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DATE ACCEPTED:
July ]6, 1993.
(M.L.P.) Ogden Environmental, 5510 Morehouse Dr., San Diego, California 921211709; (G.F.V.) Dept. Fish and Game, 330 Golden Shore, Suite 50, Long Beach, California 90802;
(R.S.G.) Southern California Edison, Box 800, Rosemead, California 9/770.
ADDRESSES: