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 wh ich populations consist of dioecious haploid (gametophyte) and diploid (tet rasporophyte) individuals as well as an additional diploid zygote-derived s porangium (carposporophyte) stage. A demographic analysis of Gracilaria gra cilis populations was carried out to explore and evaluate the population dy namics and stage structure of a typical haploid-diploid red seaweed. 2 Four G. gracilis populations were studied at two sites on the French coas t 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 p opulation projection matrices were built to describe their demography. 3 All four populations were characterized by high survival and low recruitm ent rates. Population growth rates (lambda) were similar between population s and between years and ranged from 1.03 to 1,17, In addition, generation t imes were found to be as long as 42 years. 4 Sex and ploidy ratios were variable across populations and over time. Fem ale frequencies ranged from 0.31 to 0.59 and tetrasporophyte frequencies fr om 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 c hanges in matrix transitions that corresponded to survival of the gametophy te and tetrasporophyte stages. In contrast, the contribution of the fertili ty elements to lambda was small. Eigenvector elasticity analysis also showe d that survival elements had the greatest impact on sex and ploidy ratios.