Population dynamics of giant kelp Macrocystis pyrifera along a wave exposure gradient

Sporophyte recrultment, holclfast growth and mortality of giant kelp Macrocyst~spynfera were measured seasonally on permanent transects at 3 sltes [protected Intermediate and exposed) along a wave exposure gradient on the Monterey Peninsula, c e n t ~ a l Cal~fornia (USA) between 1988 and 1991 The constant presence of cold, nutnent-nch water and the relative absence of o t h e ~ kelps and large grazers allowed the dynanucs of M pynfera populations to be examined under conditions in which wave exposure was highly vanable and ~nfluences of other abiotlc and biotic factors were minimized Recovery of M pynfera populations fiom decreased adult density (presumably due to stormlnduced mortality, adult density was negatively correlated with storm activity) was a 2-stage process requiring the establishment of luvenile populations and condit~ons uitable for ]uvenile growth to adult size Sporophyte recrultment was negatively correlated with M pynfera canopy cover, and thus appeared to be related to irradiance Recruitment was low and continuous under a temporally stable M pyrifera canopy at the protected slte At the intermediate and exposed sites, canopy cover was more variable, canopy loss was greater, and durdt~ons of low canopy cover were longer than at the protected site resulting in episodic sporophyte recruitment These distinct patterns in sporophyte lecruitment resulted In continuous luvenile populations at the protected site and intermittent luvenlle populations at the intermediate and exposed sites Growth of luveniles to adult size required additional i r rad~ance probably due to gleater light requirements for luvenlle growth than for sporophyte recrultment We observed that luveniles grew to adult slze when canopy cover was low and adults were below a threshold denslty estimated at -10 plants 100 m 2 , but ~uveni les also occasionally grew to adult size following periods of low canopy cover only Ep~sodic sporophyte recruitment at the intermediate and exposed sites resulted in decreased coincidence of the 2 recovery stages, adult dens~t ies were often decreased below threshold piior to the establishment of luvenile populations Recovery time, that required by populations to return to densities above threshold was equal to the time lag between occurrence of the 2 recovery stages and was therefore greatest at the more exposed sites Compansons between central and southern Cal~tornia M pyl~fera populat~ons suggest that by altering recovery time variable frequency and magnitude of storm disturbance may result in different periodlclties of adult population cycles K E Y WORDS. Glant ke lp . Macrocyst~spyr~fera . Wave exposure. Demography. Recruitment. Recovery INTRODUCTION water motion, temperature, nutrients, and light) and biotic ( e .g . competition, grazing, and self-shading) facRecent investigations of the population dynamics of tors on the different life history stages of the species giant kelp Macrocystispyrjfera (hereafter Macrocystis) (Foster 1982, Dayton et al. 1984, 1992, Foster & Schiel have addressed the effects of various abiotic (e.g. 1985, Dean & Jacobsen 1986, Deysher & Dean 1986, Dean et al. 1989, Reed 1990). Most of these studies, however, have been done in regions where several 'Present addressUn~versity of California San Diego, Scripps Institution of Oceanography 0208, La Jo]la, California limiting naturally CO-vary. For example, tern92093, USA. E-mail mgraham@ucsd edu perature and nutrient levels CO-vary in southern Cali"Addressee for reprints. E-mail: charroId@mbayaq.org fornia (Jackson 1977, Zimmerman & Robertson 1985), O Inter-Research 1997 Resale of full a r t~c l e not pe rm~t t ed Mar Ecol Prog Ser 148: 269-279, 1997 and their effects on Macrocystis during episodic El Nifio events often are confounded by increased frequency and magnitude of storm disturbance (Tegner & Dayton 1987). Under such circumstances, it may be impossible to isolate the effects of individual factors or their interactions on Macrocystis populations. Although many studies have addressed the effect of wave exposure on macroalgal populations, most have done so with qualitative estimates of wave exposure. Quantitative estimates may provide both an unbiased assessment of among-site differences in wave exposure, as well as an accurate temporal record of wave activity (i.e. storms). Seymour et al. (1989) were the first to quantify wave exposure for estimating mortality of Macrocystis due to waves. They observed that increased horizontal orbital velocities corresponded with increased mortality of Macrocystis in southern California. Along the Monterey Peninsula, where a gradient in storm disturbance is thought to have a strong role in regulating macroalgal community structure (Foster 1982, Kimura & Foster 1984, Reed & Foster 1984, Harrold et al. 1988), quantification of temporal and spatial variability in wave activity may provide a good correlative test of the effect of wave exposure on Macrocystis populations. The nearshore environment of the Monterey Peninsula offers a unique setting to study the effects of water motion on population dynamics of Macrocystis. Large magnitude storm disturbance occurs frequently in central California; some regions experience high orbital velocities (large, high frequency waves) during much of the year. This is in sharp contrast to the large magnitude but low frequency storm disturbances described for southern California (Seymour et al. 1989). Furthermore, strong upwelling and tidal flushing from nearshore submarine canyons result in the presence of cold, nutrient-rich water throughout the year (Traganza et al. 1981, Breaker & Broenkow 1994). Temperatures along the Monterey Peninsula rarely exceed those in southern California which were observed to be detrimental to Macrocystis reproduction (Deysher & Dean 1986), juvenile growth (Kopczak et al. 1991), or adult growth (Zimmerman & Robertson 1985). Also, since the re-establishment of sea otter populations along this coast in the late 1960s, the effect of sea urchin grazing on Macrocystis populations along the Monterey Peninsula has been minimal and isolated, the last documented episode occurring in 1986 (Watanabe & Harrold 1991). Finally, Harrold et al. (1988) observed that the understory stipitate kelp Pterygophora californica, a common competitor with Macrocystis for light and space (Dayton et al. 1984, 1992, Reed & Foster 1984), was sparsely distributed along the north coast of the Monterey Peninsula. In this context, only intra-specific competition and storm disturbance remain as the primary factors regulating Macrocystis populations in this region (Kimura & Foster 1984, Harrold et al. 1988). Our objectives were (1) to examine the relationship between wave exposure and population dynamics of Macrocystis by comparing recruitment, holdfast growth, and mortality of its macroscopic life history stages at 3 sites along a wave exposure gradient on the Monterey Peninsula, central California, and (2) to study the response (i.e. recovery) of Macrocystis popula t ion~ to decreased adult density. Lastly, we compared our results to previous work on Macrocystis in southern California and offer a potential explanation for the divergent patterns in population dynamics observed among central and southern California Macrocystis populations.