the effects of harvesting macrocystis pyrifera on the algal assemblage in a giant kelp forest
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The effects of harvesting Macrocystispyrffera on the algal assemblage in a giant kelp forest
R. S . Kimura' & M . S. Foster
Moss Landing Marine Laboratories, P.O. Box 223, Moss Landing, CA 95039, U.S.A .
I Present address : TERA Corporation, Marine Studies Group, P.O. Box 400, Avila Beach, CA 93424,
Keywords: seaweed, Macrocystis, harvesting, understory algae, light, recruitment, central California
It is commonly observed that the abundance of
understory algae, including juvenile stages of sur-
face canopy kelps, is reduced within stands of adult
Macrocystis pyrifera (Dawson et al. 1960; Neushul
1965, 1971 ; Rosenthal et al. 1974; Foster 1975,
1982). Surface canopies ofM. pyrifera can reduce
bottom light to < 1 % of the surface intensity (Neu-
shul 1971 ; Reed & Foster 1984), and experimental
removal of this canopy can result in increased
abundance of understory plants (Pearse & Hines
1979; Reed & Foster 1984) .
Commercial harvesting removes M. pyrifera can-
opy tissue and thus increases bottom irradiation
(Neushul 1971). Besides altering the understory flo-
ra, including the abundance of M. pyrifera recruits,
the continued removal of photosynthetic tissue may
affect the survivorship of adult M. pyrifera, particu-
larly if holdfast growth is reduced (Miller & Geibel
1973) . North (1957) observed the bottom vegeta-
tion in an area one year after commercial harvest-
ing, and noted no differences in plant abundances
compared to adjacent, uncut areas . However, in a
nearby area where plants were cropped continuous-
ly for two years by boat propellers, he found more
juvenile M. pyrifera and foliose red algae . Miller &
Geibel (1973) hand-cut the same giant kelp plants
five times in 14 months . This resulted in the com-
plete loss of these plants the following winter, but as
in North's study above, this repetitive removal of
surface canopy also enhanced the abundance of
juvenileM. pyrifera and foliose red algae relative to
adjacent, uncut areas .
Portions of Carmel Bay kelp forest have been
Hydrobiologia 116/117, 425-428 (1984) .
Dr W. Junk Publishers, Dordrecht . Printed in the Netherlands .
harvested once per year since 1973 (R . McPeak,
Kelco Co., pers . commun .). Harvesting normally
occurs in summer when canopies are most luxu-
riant and seas are calm . In this study, we compared
changes in understory algal abundance and re-
cruitment in an unharvested area with those in two
other areas harvested at different times of the year .
The Carmel Bay kelp forest (36 33'N, 121 0 51 'W)
has a total summer surface area of approximately
100 ha with M. pyrifera frond densities of 2-3 m -2
(Kimura 1980). Algal abundance was determined
during one year (June 1975 to May 1976) in three
30-m radius (- 0.3 ha) study areas arranged in a
triangle so the centers were - 100 m apart . These
sites were established in depths ranging from 15 to
18 m, and in a location known to have not been
previously harvested. One area was harvested in
summer (June) by a commercial harvesting ship .
Divers harvested a second area in fall (October) by
hand to a depth of 1 .2 m below the surface, to
simulate the effects of commercial cutting . A third
unharvested area was used to compare natural
trends with those in the harvested areas . The swath
cut by a commercial ship is 8 m wide, but the
number of adjacent swaths cut is highly variable
(D .C . Barilotti, Kelco Co ., pers. commun .). Thus,
our areas exceeded the minimum possible width,
and were much larger than the 20-m width suggest-
ed by Barilotti (pers . commun.) as necessary to
eliminate shading by the surrounding plant canopy .
Within each area, the densities of Macrocystis
pyrifera, Pterygophora californica, and other large
algae that could be distinguished as individuals,
were determined at 1- to 3-month intervals in 20 to
23 permanently marked 10-m 2 quadrats initially
picked at random. Juvenile M. pyrifera (< I m
long) and P. californica (lacking sporophyll scars)
were distinguished from adults . Percent cover of
these and other macroalgae was measured during
each sampling period using randomly distributed
point quadrats . A single quadrat consisted of a line
3 m long with ten randomly spaced weights. All
plants intersecting an imaginary line perpendicular
to the substratum through a weight were noted .
Sixty to 120 quadrats were sampled during each
survey in each study area . Differences in abundance
within the harvested and control areas were tested
over time (assuming independence) for significance
(p < 0.05) using Kruskal-Wallis tests (Sokal &
Rohlf 1969) followed by Newman-Keuls multiple
comparisons (Zar 1974) .
Percent cover and overall configuration of the
M. pyrifera surface canopy over the entire kelp
forest from 1973 to 1979 were estimated by eye from
uniform-size projections of infrared aerial photo-
graphs supplied by the State of California Depart-
ment of Fish and Game. The canopy cover was
greatest during summer 1975, and was considered
to be 100%. All other estimates were made relative
to the cover on this date .
Within the study areas, a total of 27 algal taxa
were encountered (Table 1) . All species in the har-
vested areas were also found in the control area .
With the exception of juvenile M. pyrifera and
Pterygophora californica, seasonal changes in plant
density and percent cover for all taxa and devel-
opmental stages were slight, and were similar be-
tween treatments. Figure 1 illustrates this for the
combined category of total understory cover ; data
for individual species are given in Kimura (1980) .
Seasonal changes in densities of juvenile M. py-
rifera and P. californica in each study area are
shown in Figure 2 . Some of the original quadrats
marked in each area could not be relocated, so the
number of replicates varied between, and once
within, areas . Natural recruitment of these kelps
occurred in spring and early summer . In the area
harvested in summer, there was an additional and
Algal taxa encountered in the study areas . Identifica-
tion and authorities from Abbott and Hollenberg (1976) .
Derbesia marina (Lyngb .) Sol .
.) C. Ag
Desmarestia ligulata (Lightf.) Lamour . var . ligulata
Desmarestia ligulata var. firma (C . Ag .) J . Ag.
Dictyota binghamiae J . Ag .
Macrocystis pyrifera (L .) C . Ag .
Pterygophora californica Rupr .
Bossiella californica (Dec .) Silva ssp. schmittii (Manza)
Botryoglossum farlowianum (J .Ag.) De Toni
Calliarthron tuberculosum (Post . & Rupr.) Daws .
Callophyllis flabellulata Harv .
Fauchea laciniata J. Ag .
Fryeella gardneri (Setch .) Kyl.
Laurencia spectabilis Post. & Rupr .
Laurencia subopposita (J. Ag .) Setch .
Nienburgia andersoniana (J. Ag.) Kyl.
Opuntiella californica (Fart .) Kyl .
Plocamium cartilagineum (L .) Dix .
Polyneura latissima (Harv .) Kyl .
Polysiphonia spp .
Prionitis lanceolata (Harv.) Harv .
Rhodoptilum plumosum (Harv . & Bail .) Kyl .
Rhodymenia spp .
Schizymenia pacifica (Kyl.) Kyl .
Weeksia reticulata Setch.
J A S 0 N D J F M A M
Fig. 1 .Total understory algal cover in the three study areas .
Arrows indicate time ofMacrocystis pyrifera harvest . Data are
mean cover per point quadrat ; n = 60-120 per survey .
SUMMER HARVEST N 20
FALL HARVEST N2 1
A S 0 N D J FM A M
Fig. 2 . Mean density 95% confidence intervals of Macrocystis
pyrifera (solid line), and Pterygophora californica (broken line)
juveniles in the three study areas . Arrows indicate time of har-
vest ; *, n = 5 .
significantly greater recruitment of both species the
following winter. However, many of these plants
were soon lost during winter storms . In the area
harvested in fall, recruitment times of both species
were also altered, but not so dramatically ; juveniles
appeared approximately one month earlier than
those in the unharvested area (Figure 2) .
Seasonal changes in M. pyrifera surface canopy
cover are shown in Figure 3 . As in most other cen-
tral California kelp forests (Miller & Geibel 1973 ;
Gerard 1976; Foster 1982), the canopy in Carmel
Bay undergoes an annual cycle of summer peak
abundance, followed by winter reduction due to