Population dynamics and stage structure in a haploid-diploid red seaweed, Gracilaria gracilis

1 Many red seaweeds are characterized by a haploid-diploid life cycle in which populations consist of dioecious haploid (gametophyte) and diploid (tetrasporophyte) individuals as well as an additional diploid zygote-derived sporangium (carposporophyte) stage. A demographic analysis of Gracilaria gracilis populations was carried out to explore and evaluate the population dynamics and stage structure of a typical haploid-diploid red seaweed. 2 Four G. gracilis populations were studied at two sites on the French coast of the Strait of Dover, Survival, reproduction and recruitment rates were measured in each population for up to 4 years. Eight two-sex stage-based population projection matrices were built to describe their demography. 3 All four populations were characterized by high survival and low recruitment rates. Population growth rates (lambda) were similar between populations and between years and ranged from 1.03 to 1,17, In addition, generation times were found to be as long as 42 years. 4 Sex and ploidy ratios were variable across populations and over time. Female frequencies ranged from 0.31 to 0.59 and tetrasporophyte frequencies from 0.44 to 0.63. However, in most cases, the observed population structures were not significantly different from the calculated stage distributions. 5 Eigenvalue elasticity analysis showed that lambda was most sensitive to changes in matrix transitions that corresponded to survival of the gametophyte and tetrasporophyte stages. In contrast, the contribution of the fertility elements to lambda was small. Eigenvector elasticity analysis also showed that survival elements had the greatest impact on sex and ploidy ratios.