ROLE OF NUTRIENT FLUCTUATIONS AND DELAYED DEVELOPMENT IN GAMETOPHYTE REPRODUCTION BY MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) IN SOUTHERN CALIFORNIA1

Organisms occurring in environments subject to severe disturbance and/or periods of poor environmental quality that result in severe adult mortality 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 Niño. 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 experimentally examined the environmental conditions responsible for regulating delayed development of the microscopic stages of M. pyrifera from Southern California, USA. Nutrients controlled the delay and resumption of gametophyte growth and reproduction, perhaps linked to the large fluctuations in nutrients occurring seasonally and interannually in this region. Although growth of gametophytes proceeded in the virtual absence of nitrate, both nitrate and other trace nutrients were necessary for gametogenesis. Upon exposure to elevated nutrients, delayed gametophytes produced sporophytes more quickly (5–20 d) and at smaller sizes (10–200 μm) than gametophytes that had never been delayed (18–80 d, 80–400 μm, 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.     

THE EFFECTS OF HIGH IRRADIANCE ON THE SETTLEMENT COMPETENCY AND VIABILITY OF KELP ZOOSPORES1

Elevated irradiance has a profound effect on the successful dispersal and establishment of kelp zoospores, affecting their physiology and viability. The research to date, however, has been on zoospores localized near the benthos, with little attention on the importance of vertical transportation and subsequent exposure to increased irradiance. Therefore, we wanted to investigate the effects of exposure to high irradiance on the reproductive planktonic life‐history stages of kelps Macrocystis pyrifera (L.) C. Agardh and Pterygophora californica Rupr. Zoospores of both species were exposed to different irradiances (75, 275, 575, 1,025 μmol photons · m^?2 · s^?1) over varying durations (1, 2, 4, 8, 12 h) and subsequently monitored for settlement competency, gametophyte development, and reproductive viability. Settlement success for M. pyrifera was uniform throughout all irradiance × time treatments, while settlement for P. californica decreased with increasing exposure time but not irradiance, although settlement was generally reduced at the highest irradiance level. Following zoospore settlement, germ tube development was visible in the gametophytes of both species within 1 week, although a significant decline of germ tube density in P. californica was observed with increasing irradiance. Similarly, a decrease in germ tube development with increasing exposure was observed across all irradiance levels for M. pyrifera, but irradiance itself was not significant. Further development into embryonic sporophytes was remarkably similar to gametophyte development, suggesting that the effect of exposure of kelp zoospores to high irradiance on subsequent sporophyte production is mediated through gametophyte development as well as zoospore survival.     

ROLE OF SETTLEMENT DENSITY ON GAMETOPHYTE GROWTH AND REPRODUCTION IN THE KELPS PTERYGOPHORA CALIFORNICA AND MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

Laboratory studies were used to examine how variation in the density of spore settlement influences gametophyte growth, reproduction, and subsequent sporophyte production in the kelps Pterygophora californica Ruprecht and Macrocystis pyrifera (L.) C. Ag. In still (non-aerated) cultures, egg maturation in both species was delayed when spores were seeded at densities 300 · mm^?2. Although the density at which this inhibition was first observed was similar for both species, the age at which their eggs matured was not. P. californica females reached sexual maturity an average of 4 days (or ～ 30%) sooner than did M, pyrifera. As observed previously in field experiments, per capita sporophyte production was negatively density dependent for both species when seeded at spore densities of 10 · mm^?2. Total sporophyte production (i.e. number · cm^?2) for both species, however, was greatest at intermediate densities of spore settlement (～ 50 spores · mm^?2). In contrast, total sporophyte production by P. californica steadily increased with increasing spore density in aerated cultures; highest sporophyte density was observed on slides seeded at a density of 1000 spores · mm^?2. Preliminary experiments with P. californica involving manipulation of aeration and nutrients indicate that inhibition of gametophyte growth and reproduction at higher densities of spore settlement in non-aerated cultures was probably caused by nutrient limitation.     

ARRESTED DEVELOPMENT OF GIANT KELP (MACROCYSTIS PYRIFERA,PHAEOPHYCEAE) EMBRYONIC SPOROPHYTES: A MECHANISM FOR DELAYED RECRUITMENT IN PERENNIAL KELPS?1

Delayed recruitment of microscopic stages in response to cyclical cues is critical to the population dynamics of many annual and seasonally reproducing perennial seaweeds. Microscopic stages may play a similar role in continuously reproducing perennials in which adult sporophytes are subject to episodic mortality, if they can respond directly to the unpredictable onset and relaxation of unfavorable conditions. We experimentally evaluated the potential for temporary reduction in limiting resources (light, nutrients) to directly delay recruitment of giant kelp (Macrocystis pyrifera (L.) C.A. Agardh) gametophytes and embryonic sporophytes. Laboratory cultures were subjected to limiting conditions of light and nutrients for 1 month and then exposed to nonlimiting conditions for 10 days. Gametophytes in all treatments failed to recruit to sporophytes after 2 weeks, suggesting they are not a source of delayed recruitment in giant kelp. Sporophytes in light‐limited treatments, however, survived and grew significantly slower than non–light‐limited controls. When stimulated with light, light‐limited sporophytes grew from 2 to>10 times faster than unstimulated controls depending on nutrient availability. These results suggest that limiting resources can delay recruitment of embryonic giant kelp sporophytes for at least 1 month. Flexible timing of recruitment from embryonic sporophytes may enhance persistence of continuously reproducing perennial species when mac‐ roscopic adults are subject to episodic large‐scale removals.     

DISCOVERY OF A FERTILE PELAGOPHYCUS×MACROCYSTIS(PHAEOPHYTA) PUTATIVE HYBRID AND SUBSEQUENT PRODUCTION OF F2 SPOROPHYTES IN THE LABORATORY1

A fertile putative hybrid of Pelagophycus porra (Lem.)Setch. × Macrocystis pyrifera (L.) was discovered in Big Fisherman Cove, Santa Catalina Island, California, in March 1986. The plant possessed a single, solid primary stipe that bifurcated into two secondary stipes, each with a hooked-shaped pneumatocyst. A total of 15 blades, 14 with sori, were produced on two to three dichotomies above and below each pneumatocyst. Gametophyte development of spores released from sori and the resulting early sporophtes (2–3 mm) were typical of Laminariales. Gametophytes appeared within a day of spore germination, 500-cell stage sporophytts within 3–4 weeks, and 2–3 mm sporophytes within 5–6 weeks. The cultures expired before branching patterns could be determined.     

SPORE SUPPLY AND HABITAT AVAILABILITY AS SOURCES OF RECRUITMENT LIMITATION IN THE GIANT KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE)1

The causes of spatial variation in the recruitment of benthic marine algae are frequently misunderstood because of difficulties in distinguishing among the many factors that influence the supply and establishment of microscopic propagules. We used the recently constructed San Clemente Artificial Reef (SCAR) experiment to examine the roles of dispersal distance, size of spore source, and habitat availability as sources of variation in the recruitment of the giant kelp Macrocystis pyrifera (L.) C. Ag., a species whose recruitment has often been considered to be dispersal limited. Sparse colonization on SCAR by adult Macrocystis occurred within 6 months after reef construction via drifters (i.e. individuals from neighboring kelp beds that became dislodged and set adrift). The abundance of drifters on SCAR declined exponentially with distance from the nearest source population (San Mateo), suggesting that San Mateo was the likely source of drifters. Dense recruitment of small Macrocystis sporophytes was observed within 8 months of reef construction. The density of recruits on SCAR showed an initial increase with distance from San Mateo before declining exponentially. Nonetheless, substantial recruitment was observed at the most distant locations on SCAR located 3.5 km from San Mateo. In contrast to drifters, the density of recruits was positively correlated to the bottom cover of artificial reef substrate. Importantly, no correlation was found between the local density or fecundity of drifters and the local density of kelp recruits suggesting that recruitment on SCAR resulted from widespread spore dispersal rather than from the local dispersal of spores from drifters.     

REPRODUCTIVE LONGEVITY OF DRIFTING KELP MACROCYSTIS PYRIFERA (PHAEOPHYCEAE) IN MONTEREY BAY,USA1

Drifting Macrocystis pyrifera (L.) C. Agardh sporophytes have long been viewed as the primary long‐distance dispersal vector; yet, few data exist that support the ability of reproductive viable sporophytes to actually travel the presumed hundreds to thousands of kilometers. This study addressed the reproductive longevity of experimental and naturally occurring M. pyrifera drifters. Temporal variability in sporophyte size and reproduction was estimated for experimental drifting sporophytes that were tethered to surface buoys and compared with attached plants (controls). Reproductive viability was also studied for beach‐cast drifters (BCD), and naturally drifting sporophytes observed during field surveys in Monterey Bay. Detached drifting sporophytes were tracked with radio transmitters to follow drifter trajectories and to measure drifting speed. Experimental drifters (ED) experienced a 74% reduction in frond length after 35 days, a 76% reduction in average frond number after 70 days, and a reduction in average sorus area by 83% after 28 days. Although zoospore production was reduced following detachment, sporophytes remained fertile with high zoospore germination success as long as sori were present (125 days). Zoospore production and germination success for natural and BCD was similar to ED. The average displacement of radio‐tagged drifters was 7.12 km·day^?1, suggesting that a sporophyte adrift for 125 days disperses viable propagules (zoospores) over 890 km (±363). Dispersal of propagules is important for population restoration, distribution, and genetic diversity. Such dispersal distances are long enough to connect potentially all Northern Hemisphere Macrocystis populations across a generational timescale and may facilitate inter‐hemispheric gene flow.