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,

    U.S.A .

    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

  • 426

    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

    Table 1.

    Algal taxa encountered in the study areas . Identifica-

    tion and authorities from Abbott and Hollenberg (1976) .


    Derbesia marina (Lyngb .) Sol .


    Cystoseira osmundacea(Turn

    .) 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)

    Johans .

    Bossiella spp.

    Botryoglossum farlowianum (J .Ag.) De Toni

    Calliarthron tuberculosum (Post . & Rupr.) Daws .

    Callophyllis flabellulata Harv .

    Coralline crust

    Fauchea laciniata J. Ag .

    Fryeella gardneri (Setch .) Kyl.

    Laurencia spectabilis Post. & Rupr .

    Laurencia subopposita (J. Ag .) Setch .

    Nienburgia andersoniana (J. Ag.) Kyl.

    Noncoralline crust

    Opuntiella californica (Fart .) Kyl .

    Pikea californicaHarv.


    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

    975 1976

    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 .

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    1 .0-










    1 .0-



    1 .0-








    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








    W 1973--1974--1975--1976--1977--1978--1979