The biology of kelp gametophyte banks in a southern California kelp forest.

摘要

Organisms occurring in heterogeneous environments may survive periods of unfavorable conditions if one stage of their life cycle (i.e. haploid gametophyte) can delay development as a resistant form. Delayed development may be important for kelp species that experience seasonal and inter-annual periods of poor conditions that result in more than several months of adult absence. For example, adult absences of the giant kelp, Macrocystis pyrifera, may persist for at least seven months along some coastlines in the northeast Pacific after El Nino events. To investigate the potential role of delayed gametophytes in kelp populations, I experimentally examined the cues responsible for regulating delayed development of M. pyrifera. I also assessed if these stages could increase the success of adult sporophyte production in the laboratory and within a natural population in southern California, USA. I found that nutrients, and specifically nitrate, regulated the resumption of gametophyte development more strongly than irradiance, perhaps linked to the large fluctuations in nutrients sustained year-round in southern California. Upon exposure to elevated nutrients, delayed gametophytes produced sporophytes more quickly (5-20 days) and at smaller sizes (100 mum) than non-delayed gametophytes (17-80 days, 100-400 mum, respectively). Thus, delayed gametophytes reduced the effects of density-dependence on sporophyte production (Chapter 2). Gametophytes delayed development as 1-2 cells (50 mum) for seven months in the laboratory and produced sporophytes within five days once nutrients were increased. This suggests that delayed gametophytes have the potential to regulate recovery after El Nino disturbance where adults have been removed. This rapid response was shared by several other kelp species tested from southern California and in all cases, fecundity did not decrease with delay duration, suggesting that there is no cost of delayed development for kelps as has been demonstrated for other organisms (Chapter 3). In the field, removing the microscopic stages from natural rock prevented the immediate recruitment of new kelp sporophytes, which occurred within one month where microscopic stages were left intact. This rapid response may confer a competitive advantage for the resulting sporophyte that will reach a larger size earlier than its neighbors. Using a genetic-based parentage analysis, I estimated that a portion of the M. pyrifera sporophytes that recruited into the field sites originated from zoospores that dispersed >100 m from their parents and had delayed as gametophytes for at least 7 months. Further, genetic diversity was high (∼0.9 HE) for adults and for juveniles that arose from delayed stages (Chapter 4). Thus, banks of M. pyrifera gametophytes may rapidly regulate the recovery of genetically diverse populations after severe disturbances such as El Nino. They may also reduce population genetic subdivision and self-fertilization rates for intact populations by promoting the reproductive success of zoospores that are dispersed >10 m and during environmental conditions that are insufficient for sporophyte production.