A demographic and experimental study was conducted on the laminarian alga Ecklonia radiata (C. Ag) J. Agardh. near the limit of its range in southern New Zealand at sites in Akaroa Harbour, Banks Peninsula, and Tory Channel in the Marlborough Sounds. These populations of Ecklonia are unique; their occurrence is shallow, yet their light environment is poor due to the presence of a Macrocystis canopy. Growth was slow compared to northern New Zealand and Australian populations, and productivity was also much lower. Population dynamics were explored in terms of both plant age and plant size: plant survival was closely linked to plant age, sori area was closely linked to plant biomass. Significant morphological variation was found at scales of 3km within Akaroa Harbour, and scales of 300km between Akaroa Harbour and Tory Channel. Ecklonia in Ohinepaka Bay (Akaroa Harbour) had the longest stipes and greatest biomass of any site. Tory Channel plants had shorter stipes and longer lamina compared to plants from Akaroa Harbour. These morphological differences are discussed in relation to environmental differences between sites. Orthogonal Ecklonia and Macrocystis manipulations tested the effect of both canopies on juvenile recruitment and growth. Canopy removal did not affect the recruitment of laminarians, but appeared to enhance survival of pre-existing recruits. Growth of understorey E. radiata generally increased following the removal of overlying canopy layers, but this was tempered by a high rate of lamina erosion in treatments where both canopies were removed. This was hypothesised to be a result of increased sedimentation in these treatments. Dense stands of juvenile E. radiata were examined in relation to models of density dependent regulation. Findings suggest that plants do not experience large density effects, but these may become more severe as stands mature. The effect of herbivorous invertebrate abundance was investigated in relation to macroalgal recruitment. Enclosure/exclosure cages suggested that an abundant and large herbivorous gastropod (Cookia sulcata) enhanced the recruitment and growth of macroalgae, possibly through consumption or redistribution of sediment. The main finding of this study were that these southern New Zealand populations of E. radiata have constrained life-history characteristics. This limitation is mainly through growth suppression of recruits by overlying canopies, but the interactive effects of other factors, such as grazers, sediment, and larger-scale hydrodynamic processes cannot be discounted.