Superior photosynthetic performance of the invasive kelp Undaria pinnatifida may contribute to continued range expansion in a wave-exposed kelp forest community

In New Zealand, the kelp Undaria pinnatifida has colonized nearly all major coastal ports and has spread to colonize diverse habitats over a range of hydrodynamic environments. We report one of the earliest surveys of the distribution of this species in Macrocystis pyrifera dominated kelp forest habitats in New Zealand alongside experiments comparing the photosynthetic physiology of U. pinnatifida to nine native co-occurring macroalgal species. Undaria pinnatifida was identified at four locations that had previously been reported to be uninvaded and was found throughout the depth range of the kelp forest. Highest densities were observed at shallow depths, reflecting the commonly accepted mode of invasion via sheltered rock pools. Photosynthesis versus Irradiance (P vs. E) curves were constructed for 10 dominant brown macroalgal species. Undaria pinnatifida exhibited greater photosynthetic ability than native macroalgae with maximal rates of photosynthesis at least 1.6-times higher than the habitat forming native kelp species Macrocystis pyrifera and Ecklonia radiata and more than 2.5 times higher than eight other native species. The photosynthetic efficiency of U. pinnatifida under low light was at least as high as macroalgae common on deeper sections of surveyed reefs (e.g. M. pyrifera and E. radiata). Collectively, this information shows that U. pinnatifida has superior photosynthetic performance across all ecologically relevant light intensities. Our data show that the photosynthetic physiology of U. pinnatifida is not likely to limit invasion into deeper sections of reef. This trend could continue, with the potential to fundamentally alter the structure of these wave-exposed kelp communities.