THE EFFECTS OF COMPETITION FOR LIGHT AND SPACE ON THE PERENNIAL ALGAL ASSEMBLAGE IN A GIANT KELP (MACROCYST!$ PYRIFERA) FOREST A thesis submitted to the faculty of San Francisco State University in partial fulfillment of the requirements for the degree Master of Arts by DANIEL CLIFFORD REED San Francisco, California May 1981 THE EFFECTS-OF COMPETITION FOR LIGHT AND SPACE ON THE PERENNIAL ALGAL ASSEM-BLAGE IN A G KELP (MACROCYSTIS PYRIFERA FOREST Daniel Clifford Reed San Francisco State University 1981 The algal assemblage in the sheltered giant kelp forest at 1·1- water Cove in Carmel Bay, California, consists of a Macrocystis £Y!j_­ fera surface canopy, a dense subsurface canopy of pterygophor~ califor­ nica and an understory of articulate and encrusting coralline algae. Shading by the kelp canopies can reduce light reaching the bottom an order of magnitude. To determine the effects of the P. californica canopy on algal recruitment and growth, half of a mature stand of P. cal i forn ica was removed in May 1978 and changes were fo 11 owed in per­ manent quadrats established in this area as well as in an undisturbed control area. In the experimenta 1 area, high recruitment of fi. ~­ fera and P. californica and low recruitment of Desmarestia ligulata ssp. ligulata occurred later that No algal recruitment occurred in the control site. When M. pyrifera and .E_. cal ifornica ts were continually removed in same area the next year,~· ligu1ata became more abundant. Comparison quadrats cleared of the ght coralline understory indicated the f.. ca1ifornica canopy also inhibi- ted algal growth as the re-establishment of the line understory was significantly lower in quadrats located under the P. californica canopy. Macrocystis pyrifera displays a greater flue on in canopy density and cover than P. cal ifornica, s lar experimental clear- ings of the surface canopy suggest it is also capable of suppressing algal recruitment and growth. Inhibition by the dense coralline understory was observed as highest rates of algal colonization were found in upright coralline removal quadrats located in the Pterygophora californica removal site. In these areas, there was high recruitment of~· californica, Macro­ cystis pyrifera, Desmarestia ligulata and moderate to low recruitment of .Q_. kurilensis, Nereocystis luetkeana and fleshy red algae. Signi­ ficantly fewer recruits were counted in coralline understory control quadrats. Slow-growing coralline algae recruited year-round, but these small plants had 1ittle effect on faster-growing brown and fleshy red algae which recruited in the spring. Competition for space does not appear important in this system, as lowest rates of algal colonization were recorded in quadrats scraped to bare rock. These results suggest that competition for light has a strong .influence on algal recruitment and growth in the kelp forest under­ story. Many species which coloni.zed following the removal of dominant kelps and corall ines are common to the understories of nearby forests subjected to higher and more frequent levels of disturbance. Without continual di:sturbance, the dominant perennials would soon become re­ established, as their recruitment was also high. The importance of / competition in structuring thi.s perennial algal community appears to result from the relatively low rates of both biological and physical disturbance at Stillwater Cove. --~--------------------------- ACKNOWLEDGEMENTS I thank my major advisor, ~~ichael S. Foster, for his friendship, advice and enthusiastic support, and for instilling in me an apprecia- tion for marine algae. His sharing of ideas on marine communities has been a most stimulating and valuable experience. The other members of my committee, Michael N. Josselyn and John S. Oliver, provided con­ structive comments which considerably improved drafts of the manu­ script. Discussion with J. Barry, J. Estes, C. Harrold and A. Hurley helped to clarify my thinking on the presentation of the data. I would also like to thank R. Ogren for sharing his work and ideas on the ecology of Pterygophora californica. The field work could not have been accomplished without the help of diving partners D. Rose and R. VanWagenen. P. Bentivegna, J. Heine, G. Ichikawa, M. Kelly and R. Walker also provided invaluable assistance in the field. M. Kellogg identified the invertebrates associated with the coralline mats. Special thanks to J. Barry for generously providing the painstaking knowledge of computers. R. Stelow typed the manuscript and L. Me Masters prepared the figures. I am grateful to the Pebble Beach Association for allowing access to Stillwater Cove. Most of all, my thanks go to Director John H. Martin and the faculty and staff of Moss ' Landing Marine Laboratories for providing such a unique and stimu- lating environment in which students can gain a working knowledge of the marine sciences. Support for the field research was provided by Moss Landing Marine Laboratories and a gradute research grant from The David and Lucile Packard Foundation. v TABLE OF CONTENTS Page INTRODUCTION. 1 STUDY SITE. 4 METHODS. 6 RESULTS .. • 10 Pterygophora Removal. 10 Cora 11 i ne Understory Treatments. 11 Yearly Fluctuations. 14 Canopy Effects of Light Reduction. 15 Seasonal Effects .................... 16 DISCUSSION. 19 LITERATURE CITED. 28 APPENDIX I ..... 33 APPENDIX II. 35 FIGURES. • • 36 TABLE 1. 56 vi .J..-------------------------- LIST OF FIGURES Figure 1 Location of study site. 36 2 Diagrammatic representation of experimental manipu- lations. 38 3 Initial algal percent cover. 40 4 Algal percent cover 270 days after Pterygophora removal ....... 42 5 Percent cover of algal understory prior to coralline removal ....... 44 6 Percent cover of algal understory 130 days after coralline removal. 46 7 Seasonal variation in light reduction. 48 8 Percent cover of Desmarestia 1 igulata. 50 9 Percent cover of Desmarestia kurilensis. 52 10 Percent cover of Galliarthron tuberculosum ....... 54 vii .J..------------------------- INTRODUCTION Light is a major limiting resource in many terrestrial plant communities, as plant shading may cause light levels to fall well below the compensation point (Harper, 1977). This often has pro­ nounced effects on the understory assemblage. Because of their I striking vertical stratification, giant kelp forests are often com­ pared to terrestrial forests (Kuhnemann, 1970; Neushul, 1971; Foster, 1975a) and correlative studies suggest shading may effect understory algae in a similar way (Kitching, et ~., 1934; Dawson, et ~., 1960; Mclean, 1962; Kain, 1966; Foster, 1975b). The effects of light may be even more dramatic in marine systems, as its intensity and quality are also altered by water. When a resource such as light becomes in short supply (i.e., due to shading), competitive interactions for this resource may occur. Dayton (1975a) demonstrated a high degree of competition among three kelp canopy guilds by experimentally removing canopy species in an Alaskan kelp corrmunity. Similarly, Pearse and Hines (1979) observed an increase in understory algal cover following an experimental re­ moval of the surface Macrocystis pyrifera canopy in a giant kelp for­ est in central California. The hypothesis that competition for pri­ mary space is important in structuring marine communities has been well documented for animal and plant-animal interactions (Connell, 196la, b; Paine, 1966, 1979; Dayton, 1971; Lubchenco, 1980), yet evi­ dence that space competition among algal species is important in structuring plant populations is weak. 1 ...~----------------------------- 2 The intensity and rate at which a system is disturbed also play an important role in regulating the algal assemblage. Barrales and Loban (1975) examined several Macrocystis pyrifera forests off Chubut, Argentina and correlated distinct differences in al compo- si on to the degree of wave exposure. The algal communities ex- posed forests were characterized by a rich red algal flora, whereas \ understory in more sheltered areas was dominated by the green al Codium vermilara. In areas of moderate wave exposure, the brown gae Lessonia sp. and Zonaria sp., along wtth the articulate coralline Corallina officinalis, prevailed. Similarly, biological disturbance in the form intense grazing (primarily by sea urchins) has been reported to restrict the local distribution of algae in a wide vari of subtidal algal assemblages (Leighton, et ~., 1966; Jones and Kain~ 1967; Ogden, et ~., 1973; Breen and Mann, 1976). Results from these investigations and others (Aleem, 1956; Neus et ~·, 1967; Divinny and Kirkwood, 1974; Divinny, 1978) suggest that many factors may be involved in the regulation of unders 1 associations in giant kelp forests. Light, nutrients and suitable space to settle and grow are essential for plant growth, and inter­ actions among the plants for these resources probably pl a s fi- cant role in the regulation of algal communities. The present study considers the effects of light and space on the perennial algal assemblage in a sheltered ant p forest i few sea urchins. The system is characterized a surface of Macrocystis pyrifera, an understory canopy of the lked kelp, Pterygophora californica, and a bottom cover composed of articulate 3 and encrusting coralline algae. Light was manipulated by selectively removing the two kelp canopies and upright portions of the coralline understory. Subsequent changes were followed and compared with appro­ priate controls. The effect of newly created space on algal community structure was examined