ROLE OF NUTRIENT FLUCTUATIONS AND DELAYED DEVELOPMENT IN GAMETOPHYTE REPRODUCTION BY MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) IN SOUTHERN CALIFORNIA 1 Laura T. Carney 2,3 and Matthew S. Edwards Department of Biology, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182, USA Organisms occurring in environments subject to severe disturbance and ⁄ or periods of poor envi- ronmental quality that result in severe adult mor- tality can survive these periods by relying on alternate life stages that delay their development in a resistant state until conditions improve. In the northeast Pacific, the forest-forming giant kelp Macrocystis pyrifera (L.) C. Agardh periodically experiences widespread adult mortality during extended periods of extremely low nutrients and high temperatures, such as those associated with El Nin ˜o. Recovery following these periods is hypothesized to occur from microscopic life stages that delay their development until the return of favorable conditions. In the laboratory, we experi- mentally examined the environmental conditions responsible for regulating delayed development of the microscopic stages of M. pyrifera from South- ern California, USA. Nutrients controlled the delay and resumption of gametophyte growth and repro- duction, perhaps linked to the large fluctuations in nutrients occurring seasonally and interannually in this region. Although growth of gametophytes pro- ceeded in the virtual absence of nitrate, both nitrate and other trace nutrients were necessary for gametogenesis. Upon exposure to elevated nutrients, delayed gametophytes produced sporo- phytes more quickly (5–20 d) and at smaller sizes (10–200 lm) than gametophytes that had never been delayed (18–80 d, 80–400 lm, respectively), reducing negative density-dependent effects. This finding demonstrates that delayed gametophytes of M. pyrifera rapidly utilize increased resources to consistently produce sporophytes. Further work is needed to assess their potential role in population recovery following periods of poor environmental quality. Key index words: delayed growth and reproduction; density dependence; kelp gametophytes; Macro- cystis pyrifera; nitrate; nutrients Abbreviations: ENSO, El Nin ˜o Southern Oscilla- tion; PES, Provasoli enriched seawater; THSD, Tukey’s HSD post hoc Many organisms occur in environments that vary both spatially and temporally in habitat quality and experience periodic severe disturbances. These organisms may survive these periods that often coin- cide with severe to complete adult mortality by delaying their development as a resistant ‘‘resting’’ stage and then resuming development when condi- tions improve (Cohen 1966, Venable and Lawlor 1980, Levin et al. 1984, Philippi and Seger 1989, Hairston and Ca ´ceres 1996). Perhaps some of the best-studied examples of delayed development come from terrestrial plants that form long-lived seed or seedling banks during unfavorable periods (e.g., rainfall and ⁄ or chemicals associated with fires in ter- restrial plants; Keeley 1987) and resume develop- ment when resources are renewed (Bewley and Black 1994, Grime 2001, Makana and Thomas 2005). Although examples of delayed development have been observed across numerous taxa (reviewed by Carney and Edwards 2006), one commonality among species that rely on them is that the delay is generally initiated by the onset of unfavorable envi- ronmental conditions. Marine copepods (Alldredge et al. 1984) and microalgae (Hollibaugh et al. 1981) produce dormant stages in response to food scarcity and conditions limiting to growth, respectively, that may persist for decades or until conditions become sufficient for vegetative development and adult survival (McQuoid et al. 2002). The environmental conditions responsible for resuming development have been identified for some species but remain less understood for other organisms. Delayed development is likely important for the recruitment of several macroalgal species along tem- perate coastlines in the eastern Pacific (Edwards 2000), and while evidence for banks of delayed stages in seaweeds is growing (reviewed by Hoffmann and Santelices 1991, Carney and Edwards 2006), the environmental conditions that regulate delayed development in macroalgae remain unclear. For example, many annual kelps (order Laminari- ales) appear to rely on microscopic life stages to delay their development during poor conditions 1 Received 21 July 2009. Accepted 24 March 2010. 2 Present address: Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093, USA. 3 Author for correspondence: e-mail lcarney@ucsd.edu. J. Phycol. 46, 987–996 (2010) Ó 2010 Phycological Society of America DOI: 10.1111/j.1529-8817.2010.00882.x 987