The effects of harvesting Macrocystis pyrifera on the algal assemblage in a giant kelp forest

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<ul><li><p>The effects of harvesting Macrocystispyrffera on the algal assemblage in a giant kelp forest</p><p>R. S . Kimura' &amp; M . S. Foster</p><p>Moss Landing Marine Laboratories, P.O. Box 223, Moss Landing, CA 95039, U.S.A .</p><p>I Present address : TERA Corporation, Marine Studies Group, P.O. Box 400, Avila Beach, CA 93424,</p><p>U.S.A .</p><p>Keywords: seaweed, Macrocystis, harvesting, understory algae, light, recruitment, central California</p><p>Introduction</p><p>It is commonly observed that the abundance of</p><p>understory algae, including juvenile stages of sur-</p><p>face canopy kelps, is reduced within stands of adult</p><p>Macrocystis pyrifera (Dawson et al. 1960; Neushul</p><p>1965, 1971 ; Rosenthal et al. 1974; Foster 1975,</p><p>1982). Surface canopies ofM. pyrifera can reduce</p><p>bottom light to &lt; 1 % of the surface intensity (Neu-</p><p>shul 1971 ; Reed &amp; Foster 1984), and experimental</p><p>removal of this canopy can result in increased</p><p>abundance of understory plants (Pearse &amp; Hines</p><p>1979; Reed &amp; Foster 1984) .</p><p>Commercial harvesting removes M. pyrifera can-</p><p>opy tissue and thus increases bottom irradiation</p><p>(Neushul 1971). Besides altering the understory flo-</p><p>ra, including the abundance of M. pyrifera recruits,</p><p>the continued removal of photosynthetic tissue may</p><p>affect the survivorship of adult M. pyrifera, particu-</p><p>larly if holdfast growth is reduced (Miller &amp; Geibel</p><p>1973) . North (1957) observed the bottom vegeta-</p><p>tion in an area one year after commercial harvest-</p><p>ing, and noted no differences in plant abundances</p><p>compared to adjacent, uncut areas . However, in a</p><p>nearby area where plants were cropped continuous-</p><p>ly for two years by boat propellers, he found more</p><p>juvenile M. pyrifera and foliose red algae . Miller &amp;</p><p>Geibel (1973) hand-cut the same giant kelp plants</p><p>five times in 14 months . This resulted in the com-</p><p>plete loss of these plants the following winter, but as</p><p>in North's study above, this repetitive removal of</p><p>surface canopy also enhanced the abundance of</p><p>juvenileM. pyrifera and foliose red algae relative to</p><p>adjacent, uncut areas .</p><p>Portions of Carmel Bay kelp forest have been</p><p>Hydrobiologia 116/117, 425-428 (1984) .</p><p> Dr W. Junk Publishers, Dordrecht . Printed in the Netherlands .</p><p>harvested once per year since 1973 (R . McPeak,</p><p>Kelco Co., pers . commun .). Harvesting normally</p><p>occurs in summer when canopies are most luxu-</p><p>riant and seas are calm . In this study, we compared</p><p>changes in understory algal abundance and re-</p><p>cruitment in an unharvested area with those in two</p><p>other areas harvested at different times of the year .</p><p>Methods</p><p>The Carmel Bay kelp forest (36 33'N, 121 0 51 'W)</p><p>has a total summer surface area of approximately</p><p>100 ha with M. pyrifera frond densities of 2-3 m -2</p><p>(Kimura 1980). Algal abundance was determined</p><p>during one year (June 1975 to May 1976) in three</p><p>30-m radius (- 0.3 ha) study areas arranged in a</p><p>triangle so the centers were - 100 m apart . These</p><p>sites were established in depths ranging from 15 to</p><p>18 m, and in a location known to have not been</p><p>previously harvested. One area was harvested in</p><p>summer (June) by a commercial harvesting ship .</p><p>Divers harvested a second area in fall (October) by</p><p>hand to a depth of 1 .2 m below the surface, to</p><p>simulate the effects of commercial cutting . A third</p><p>unharvested area was used to compare natural</p><p>trends with those in the harvested areas . The swath</p><p>cut by a commercial ship is 8 m wide, but the</p><p>number of adjacent swaths cut is highly variable</p><p>(D .C . Barilotti, Kelco Co ., pers. commun .). Thus,</p><p>our areas exceeded the minimum possible width,</p><p>and were much larger than the 20-m width suggest-</p><p>ed by Barilotti (pers . commun.) as necessary to</p><p>eliminate shading by the surrounding plant canopy .</p><p>Within each area, the densities of Macrocystis</p></li><li><p>426</p><p>pyrifera, Pterygophora californica, and other large</p><p>algae that could be distinguished as individuals,</p><p>were determined at 1- to 3-month intervals in 20 to</p><p>23 permanently marked 10-m 2 quadrats initially</p><p>picked at random. Juvenile M. pyrifera (&lt; I m</p><p>long) and P. californica (lacking sporophyll scars)</p><p>were distinguished from adults . Percent cover of</p><p>these and other macroalgae was measured during</p><p>each sampling period using randomly distributed</p><p>point quadrats . A single quadrat consisted of a line</p><p>3 m long with ten randomly spaced weights. All</p><p>plants intersecting an imaginary line perpendicular</p><p>to the substratum through a weight were noted .</p><p>Sixty to 120 quadrats were sampled during each</p><p>survey in each study area . Differences in abundance</p><p>within the harvested and control areas were tested</p><p>over time (assuming independence) for significance</p><p>(p &lt; 0.05) using Kruskal-Wallis tests (Sokal &amp;</p><p>Rohlf 1969) followed by Newman-Keuls multiple</p><p>comparisons (Zar 1974) .</p><p>Percent cover and overall configuration of the</p><p>M. pyrifera surface canopy over the entire kelp</p><p>forest from 1973 to 1979 were estimated by eye from</p><p>uniform-size projections of infrared aerial photo-</p><p>graphs supplied by the State of California Depart-</p><p>ment of Fish and Game. The canopy cover was</p><p>greatest during summer 1975, and was considered</p><p>to be 100%. All other estimates were made relative</p><p>to the cover on this date .</p><p>Results</p><p>Within the study areas, a total of 27 algal taxa</p><p>were encountered (Table 1) . All species in the har-</p><p>vested areas were also found in the control area .</p><p>With the exception of juvenile M. pyrifera and</p><p>Pterygophora californica, seasonal changes in plant</p><p>density and percent cover for all taxa and devel-</p><p>opmental stages were slight, and were similar be-</p><p>tween treatments. Figure 1 illustrates this for the</p><p>combined category of total understory cover ; data</p><p>for individual species are given in Kimura (1980) .</p><p>Seasonal changes in densities of juvenile M. py-</p><p>rifera and P. californica in each study area are</p><p>shown in Figure 2 . Some of the original quadrats</p><p>marked in each area could not be relocated, so the</p><p>number of replicates varied between, and once</p><p>within, areas . Natural recruitment of these kelps</p><p>occurred in spring and early summer . In the area</p><p>harvested in summer, there was an additional and</p><p>Table 1.</p><p>Algal taxa encountered in the study areas . Identifica-</p><p>tion and authorities from Abbott and Hollenberg (1976) .</p><p>Chlorophyta</p><p>Derbesia marina (Lyngb .) Sol .</p><p>Phaeophyta</p><p>Cystoseira osmundacea(Turn</p><p>.) C. Ag</p><p>Desmarestia ligulata (Lightf.) Lamour . var . ligulata</p><p>Desmarestia ligulata var. firma (C . Ag .) J . Ag.</p><p>Dictyota binghamiae J . Ag .</p><p>Macrocystis pyrifera (L .) C . Ag .</p><p>Pterygophora californica Rupr .</p><p>Rhodophyta</p><p>Bossiella californica (Dec .) Silva ssp. schmittii (Manza)</p><p>Johans .</p><p>Bossiella spp.</p><p>Botryoglossum farlowianum (J .Ag.) De Toni</p><p>Calliarthron tuberculosum (Post . &amp; Rupr.) Daws .</p><p>Callophyllis flabellulata Harv .</p><p>Coralline crust</p><p>Fauchea laciniata J. Ag .</p><p>Fryeella gardneri (Setch .) Kyl.</p><p>Laurencia spectabilis Post. &amp; Rupr .</p><p>Laurencia subopposita (J. Ag .) Setch .</p><p>Nienburgia andersoniana (J. Ag.) Kyl.</p><p>Noncoralline crust</p><p>Opuntiella californica (Fart .) Kyl .</p><p>Pikea californicaHarv.</p><p>.</p><p>Plocamium cartilagineum (L .) Dix .</p><p>Polyneura latissima (Harv .) Kyl .</p><p>Polysiphonia spp .</p><p>Prionitis lanceolata (Harv.) Harv .</p><p>Rhodoptilum plumosum (Harv . &amp; Bail .) Kyl .</p><p>Rhodymenia spp .</p><p>Schizymenia pacifica (Kyl.) Kyl .</p><p>Weeksia reticulata Setch.</p><p>100</p><p>2001</p><p>0</p><p>200</p><p>100</p><p>200l</p><p>1001</p><p>0</p><p>J</p><p>CONTROL</p><p>SUMMER HARVEST</p><p>FALL HARVEST</p><p> 1</p><p>J A S 0 N D J F M A M</p><p>975 1976</p><p>Fig. 1 .Total understory algal cover in the three study areas .</p><p>Arrows indicate time ofMacrocystis pyrifera harvest . Data are</p><p>mean cover per point quadrat ; n = 60-120 per survey .</p></li><li><p>NE</p><p>O</p><p>3.0</p><p>2.0</p><p>1 .0-</p><p>0</p><p>9.0</p><p>810-</p><p>7.0-</p><p>6,0-</p><p>5.0-</p><p>4.0-</p><p>3.0</p><p>2.0-</p><p>1 .0-</p><p>0</p><p>2.0-</p><p>1 .0-</p><p>0</p><p>50</p><p>l</p><p>CONTROL N23</p><p>SUMMER HARVEST N 20</p><p>FALL HARVEST N2 1</p><p>4</p><p>A S 0 N D J FM A M</p><p>19751976</p><p>Fig. 2 . Mean density 95% confidence intervals of Macrocystis</p><p>pyrifera (solid line), and Pterygophora californica (broken line)</p><p>juveniles in the three study areas . Arrows indicate time of har-</p><p>vest ; *, n = 5 .</p><p>significantly greater recruitment of both species the</p><p>following winter. However, many of these plants</p><p>were soon lost during winter storms . In the area</p><p>harvested in fall, recruitment times of both species</p><p>were also altered, but not so dramatically ; juveniles</p><p>appeared approximately one month earlier than</p><p>those in the unharvested area (Figure 2) .</p><p>Seasonal changes in M. pyrifera surface canopy</p><p>cover are shown in Figure 3 . As in most other cen-</p><p>tral California kelp forests (Miller &amp; Geibel 1973 ;</p><p>Gerard 1976; Foster 1982), the canopy in Carmel</p><p>Bay undergoes an annual cycle of summer peak</p><p>abundance, followed by winter reduction due to</p><p>a</p><p>W</p><p>&gt;0</p><p>0</p><p>W</p><p>Q</p><p>FWSSFWSSFWSSFWSSFWSSFWS</p><p>W 1973--1974--1975--1976--1977--1978--1979</p><p>W</p><p>Fig. 3 . Seasonal changes in the percent cover of the Macrocystis</p><p>pyrifera surface canopy over the Carmel Bay kelp forest. FW WS</p><p>= autumn, winter, spring, summer .</p><p>removal of fronds and plants during storms . There</p><p>have been no obvious changes in the size or con-</p><p>figuration of the canopy since 1973 .</p><p>Discussion</p><p>The results of this one-year study suggest that the</p><p>once per year partial harvest of the M. pyrifera</p><p>canopy in the Carmel Bay kelp forest has no short-</p><p>term direct or indirect effects on the algae other</p><p>than enhancing kelp recruitment. The variations in</p><p>recruitment times in the harvested areas suggest</p><p>that M. pyrifera and Pterygophora californica</p><p>recruitment is greatly affected by light, and that</p><p>spores and/or `dormant' gametophytes are prob-</p><p>ably available throughout the year . These effects on</p><p>the recruitment of M, pyrifera are consistent with</p><p>the harvesting study of Miller &amp; Geibel (1973), the</p><p>boating channel observations of North (1957), and</p><p>the kelp canopy removal experiments by Pearse &amp;</p><p>Hines (1979) and Reed &amp; Foster (1984). In addi-</p><p>tion, North (1957), Rosenthal et al. (1974), and</p><p>Gerard (1976) found that, under natural condi-</p><p>tions, M. pyrifera recruitment occurred only after</p><p>surface canopies were thinned by storms .</p><p>I n the present study, the most dramatic effect was</p><p>observed in the area harvested in summer where, by</p><p>the following winter, there was an additional and</p><p>large recruitment of kelp . Thus, harvesting imme-</p><p>diately enhanced recruitment, but later removal by</p><p>storms reduced the differences in kelp abundance</p><p>between harvested and nonharvested areas . Fur-</p><p>thermore, harvesting as currently done in this kelp</p><p>forest has not altered either the seasonal cycle of the</p><p>canopy or the area and configuration of the entire</p><p>forest. Rosenthal et al. (1974) suggested that peri-</p><p>odic recruitment combined with high survivorship</p><p>creates a more multi-aged M. pyrifera population</p><p>that may be less susceptible to catastrophic loss</p><p>during storms . Winter storm intensity varies in cen-</p><p>tral California (Foster 1982), and it is possible that</p><p>the winter recruits in the summer-harvested areas</p><p>may survive during certain years . If so, this may</p><p>contribute to a more varied age structure of the M.</p><p>pyrifera population .</p><p>These results suggest that the adult mortality of</p><p>M. pyrifera and subsequent increases in the red</p><p>algal understory observed by Miller &amp; Geibel</p><p>(1973) probably reflect these authors' admittedly</p><p>427</p></li><li><p>428</p><p>high harvesting rates . The differences in understory</p><p>red algal response between our study and that of</p><p>Miller &amp; Geibel (1973) may also be due, in part, to</p><p>time and site differences . The studies were done in</p><p>different years, and the results may partially reflect</p><p>some unknown, long-term variation in algal re-</p><p>cruitment and growth . In addition, the latter study</p><p>was done at a shallower site without an understory</p><p>canopy of P. californica . The increased irradiation</p><p>as a result of harvesting in shallower water may</p><p>have been greater at Miller &amp; Geibel's (1973) site .</p><p>As suggested by Foster (1982) and shown by Reed</p><p>&amp; Foster (1984), light reduction by P. californica</p><p>can also greatly reduce the recruitment and growth</p><p>of bottom cover red algae, even in the absence of a</p><p>M. pyrifera surface canopy . Therefore, site-specific</p><p>differences in community structure, as well as har-</p><p>vesting extent, rate, and timing, should be consid-</p><p>ered in assessing the effects of harvesting on par-</p><p>ticular kelp forest communities .</p><p>Acknowledgements</p><p>We thank D. Hunt, J. Houk, K. McCleneghan,</p><p>R . McPeak, D. Miller, and T . Thompson for their</p><p>advice and assistance . J . Heine and D. Schiel made</p><p>helpful comments on the manuscript . The state of</p><p>California Department of Fish and Game gener-</p><p>ously provided field support . Financial support for</p><p>the research was provided by NOAA, National Sea</p><p>Grant College Program, under Grant No . 04-6-158-</p><p>44021, Project No. R/CZ-21, California Sea Grant</p><p>College Program, and for presentation of the paper</p><p>by Kelco Co .</p><p>References</p><p>Abbott, I . A . &amp; G . J . Hollenberg, 1976 . Marine Algae of Cali-</p><p>fornia . Stanford University Press, Stanford, 827 pp .</p><p>Dawson, E . Y ., M . Neushul &amp; R . D. Wildman, 1960 . Seaweeds</p><p>associated with kelp beds along southern California and</p><p>northwestern Mexico . Pac. Nat . I : 1-81 .</p><p>Foster, M . S ., 1975 . Regulation of algal community develop-</p><p>ment in a Macrocystis pyrifera forest. Mar. Biol. 32 :</p><p>331-342 .</p><p>Foster, M . S., 1982 . The regulation of macroalgal associations in</p><p>kelp forest . In L. Srivastava (ed .), Synthetic and Degradative</p><p>Processes in Marine Macrophytes. Walter de Gruyter &amp; Co .,</p><p>Berlin : 185-205 .</p><p>Gerard, V. A ., 1976 . Some aspects of material dynamics and</p><p>energy flow in a kelp forest in Monterey Bay, California .</p><p>PhD Thesis, Univ. Calif., Santa Cruz, 173 pp.</p><p>Kimura, R. S ., 1980 . The effects of harvesting Macrocystis</p><p>pyrifera on understory algae in Carmel Bay, California . MA</p><p>Thesis, Calif. St. Univ ., Fresno, 108 pp .</p><p>Miller, D . J . &amp; J . J . Geibel, 1973 . Summary of blue rockfish and</p><p>lingcod life histories; a reef ecology study; and giant kelp,</p><p>Macrocystis pyrifera, experiments in Monterey Bay, Cali-</p><p>fornia . Fish Bull . 158, St . Calif. Dept . Fish &amp; Game, Sacra-</p><p>mento, 137 pp .</p><p>Neushul, M ., 1965. 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Dayton, 1974</p><p>. Ecology</p><p>and natural history of a stand of giant kelp, Macrocystis</p><p>pyrifera, off Del Mar, California. Fish. Bull . 72 : 670-684 .</p><p>Sokal, R . R . &amp; F. J . Rohlf, 1969 . Introduction to Biostatistics .</p><p>W. H. Freeman &amp; Co . Press, San Francisco, 375 pp .</p><p>Zar, J . H ., 1974 . Biostatistical Analysis . Prentice-Hall, Inc .,</p><p>Englewood Cliffs, 620 pp .</p><p>page 1page 2page 3page 4</p></li></ul>


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